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UBC Theses and Dissertations

Compressive stress relaxation of wood pulps with particular reference to pulp chemistry Kirbach, Eberhard 1973

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\5\7S- • ' COMPRESSIVE STRESS RELAXATION OF WOOD PULPS WITH PARTICULAR REFERENCE TO PULP CHEMISTRY By EBERHARD KIRBACH Diplom-Holzwirt, University of Hamburg, 1966 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of Forestry We accept this thesis as confirming to the required standard. The University of British Columbia April 1973 i In p resenting t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission for e xtensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Department of The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada Date ABSTRACT Stress relaxation responses in compression following steeping either in distilled water or 18.6% NaOH were examined on wood pulp mats with the objective to relate quantitative, physical, as well as structural features of pulp polymers to mat rheology. The study included 32 commercial and and laboratory prepared pulps (groundwood, holocellulose, paper grade, viscose, acetate and alpha-cellulose pulps) which provided wide variation in chemical characteristics. Rate of stress relaxation was found to differ considerably between individual pulps, indicating that each pulp possesses a different ability to dissipate stress. The stress relaxation traces, following quasi steploading (1.0 - 1.5 s e c ) , suggest that pulp mat relaxation is governed by two mechanisms (Mj and M^). M^ , dominating between 0.0 to 1.0 min, is believed to involve primarily inter-fibre processes, while M^ , controlling the response there-after, comprises intra-fibre, essentially molecular processes. Swelling media were found to influence pulp mat rheology. Considerably higher rates of stress dissipation were observed with caustic (18.6% NaOH) than with water swollen mats. This indicates that intra-crystalline swelling, in addition to inter-crystalline, enhances time dependent responses. The:effect of any swelling medium on pulp mat rheology appears to be highly dependent on length of swelling treatment, since i t was observed that the capacity of pulps to dissipate stress changes inversely with length of steeping time. This phenomenon suggests inter-changeability of chemical and physical stress systems. Evidence presented in the study shows that pulp chemistry controls time dependent: behavior of pulp mats. Quantitative and structural characteristics of hemicelluloses appeared to be most important for rheological processes. In the caustic swollen state, decrease in hemi-cel lulose caused proportional changes in stress relaxation. The same observation was made for water swollen groundwood, holocellulose and paper pulps. In water saturated low yield pulps, however, the viscoelastic i i i e f f e c t of h e m i c e l l u l o s e s was r e v e r s e d , due to degradat ion and r e d e p o s i t i o n phenomena. Here , r e s i d u a l hemice l lu loses were found to re ta rd s t r e s s d i s s i p a t i o n . From these o b s e r v a t i o n s , i t was deduced that t h i s group of wood polymers func t ions as an important l inkage i n s t r e s s d i s t r i b u t i o n and d i s s i p a t i o n systems of the undegraded or only s l i g h t l y degraded l i g n i n - c a r b o h y d r a t e system. Low DP and high degree of branching make b e m i c e l l u l o s e s h i g h l y su i ted to d i s s i p a t e s t r e s s i n the wood of l i v i n g t r e e s , as w e l l as ground-woods and h o l o c e l l u l o s e pulps and to a l e s s e r extent i n other pulp types . C e l l u l o s e was found to account f o r most of the d i s s i p a t e d s t r e s s i n pulp polymeric systems. I ts q u a n t i t a t i v e c o n t r i b u t i o n to pulp rheology appeared to vary l i t t l e between pulps of no or l i m i t e d c e l l u l o s e degradat ion . Severe degrada t ion , e . g . i n low y i e l d p u l p i n g , however, enhanced the time dependent response . T h i s was demonstrated f u r t h e r by r e l a x a t i o n t e s t s on v i s c o s e pulps degraded by r a d i a t i o n . L i g n i n appeared to be of only subordinate importance f o r pulp mat s t r e s s r e l a x a t i o n i n compression, p r i m a r i l y a t t r i b u t a b l e to o r i e n t a t i o n or layer e f f e c t s and to the dominant r o l e of h e m i c e l l u l o s e s . Re laxat ion measurements on wood pulp products are suggested as u s e f u l t o o l s f o r p r e d i c t i n g and est imat ing pulp and paper p r o p e r t i e s , such as p r e s s i n g behavior and a l k a l i s o l u b i l i t y of v i s c o s e p u l p s , pulp beat ing response , r u n n a b i l i t y and p r i n t a b i l i t y of paper. i v TABLE OF CONTENTS TITLE PAGE * ABSTRACT 1 1 TABLE OF CONTENTS i v LIST OF TABLES v i i LIST OF FIGURES v i i i ACKNOWLEDGEMENTS .. x i i 1.0' INTRODUCTION...... ' 1 2.0 BACKGROUND TO THE STUDY 3 2 .1 Morphology and Composition of Wood C e l l W a l l s . . . . . 3 2 . 1 . 1 Major wood c e l l types 3 2 .1.2 C e l l wall o r g a n i s a t i o n 3 2 .1.3 C h a r a c t e r i s t i c s of wood chemical c o n s t i t u e n t s . . . . . . . . . . . . D 2 .1.3 .1 C e l l u l o s e ; 6 2 . 1.3 . 1 . 1 Supermolecular arrangement 7 2 .1.3 .1.2 The c r y s t a l s t r u c t u r e 8 2.1.3.2 H e m i c e l l u l o s e s 9 2 .1.3.3 L i g n i n 1 0 2.2 C h a r a c t e r i s a t i o n of Various Pulp Types *2 2.2.1 Mechanical pulps *3 2.2.2 H o l o c e l l u l o s e pulps t. ^ 2.2.3 S u l p h i t e pulps ^ 2.2.4 Sulphate pulps 1 7 2.2.5 Bleached pulps 2 0 2.2.6 A l p h a - c e l l u l o s e p u l p s . . 2 1 V Page 2.3 Time Dependent Mechanical Properties of High Polymers, such as Wood and C e l l u l o s i c s 22 2.3.1 Molecular approach to v i s c o e l a s t i c i t y . 23 2.3.2 Phenomenological study of v i s c o e l a s t i c i t y 26 2.3.3 V i s c o e l a s t i c behavior of c e l l u l o s e e f i b r e mats 29 2.3.3.1 Rheology of paper 29 2.3.3.2 Rheology of wet f i b r e mats i n compression 32 2.A Compressibility of C e l l u l o s i c Fibre Mats 35 2.4.1 Mechanisms i n compression of wet c e l l u l o s i c f i b r e mats.... 36 2.4.1.1 Fibre bending..... 36 2.4.1.2 Fibre r e p o s i t i o n i n g 37 2.4.1.3 Compression at points of contact 38 2.4.2 Factors determining compression c h a r a c t e r i s t i c s of c e l l u l o s e f i b r e mats 38 2.4.2.1 Fibre morphology. 38 2.4.2.2 Fibre properties 40 2.4.2.3 Structure of f i b r e mats 42 3.0 MATERIALS AND METHODS 43 3.1 Pulp Samples 43 3.1.1 Ho l o c e l l u l o s e pulps.... 43 3.1.2 Alpha-celluloses 44 3.2 Physical Testing 4 6 3.3 Determination of Pulp Constituents 47 3.3.1 Carbohydrates 47 3.3.2 Lignin 50 3.3.3 Other measurements 50 vi 4.0 RESULTS AND DISCUSSION..... 52 4.1 Fractional Stress Relaxation Tests 52 4.2 Effect of Steeping Media (Water vs. Caustic) 56 4.3 Effect of Residual Chemical Constituents 61 4.3.1 Lignin 6 2 4.3 .2 Hemicelluloses • 65 4 .3 .2 .1 Mechanical pulps 67 4 .3 .2 .2 Holocellulose pulps... 69 4 . 3 . 2 . 3 Sulphite and sulphate paper pulps 71 4 . 3 . 2 . 4 Viscose and acetate pulps 74 4 . 3 . 2 . 5 Alpha-cellulose pulps 7 5 ^ 4 . 3 . 3 Cellulose 7 6 4.4 Interchangeability of Stress Systems 8 0 4.5 Application of Stress Relaxation Measurements for Characterising Pulps 8^ 4.5.1 Estimation of viscose pulp alkali solubility S3 4.5 .2 Predicting viscose pulp pressing characteristics 8 ^ 4 .5 .3 Predicting fibre response to machining 8 ^ 4 .5 .4 Predicting paper printability. 8 5 5.0 CONCLUSIONS , 8 7 6.0 LITERATURE CITED 90 v i i LIST OF TABLES Table " Page 1. The chemical composition of woods from three angiosperms and three conifers 124 2. Description of pulps used in the study 125 3. Summative data (in per cent of extractive-free wood) for chemical composition of pulps tested in the study 127 4. Summary of fractional stress relaxation results on pulps tested in the study as read after 35 min relaxation time (1 -£"(35 min)/^"(o)) following steeping in water or caustic 135 5. Properties of viscose pulps treated with various doses of gamma-radiation 141 6. Multiple curvilinear covariance analysis for the relationship between 10% NaOH solubility and fractional stress relaxation of irradiated and untreated viscose pulps 142 v i i i LIST OF FIGURES Figure Page 1. Schematic representation of the cell wall organisation in a coniferous tracheid and/or angiospermous wood fibre, with respective microfibril orientation 144 2. The, unit cell as proposed by Meyer and Misch 145 3. Stress relaxation under constant strain 146 A. Steeped pulp specimen mounted between glass plates for testing of relaxation in compression.......... 147 5. Gas chromatogram of the acetylated hydrolysates of western hemlock groundwood pulp No. 10-2 (brightened).... 148 6. Gas chromatogram of the acetylated hydrolysates of western cottonwood groundwood pulp No. 10-4 (brightened) 149 7. Gas chromatogram of the acetylated hydrolysates of western cottonwood peracetic acid holocellulose No. 9-4 150 8. Gas chromatogram of the acetylated hydrolysates of unbleached coniferous sulphate pulp No. 7-1 151 9. Gas chromatogram of the acetylated hydrolysates of bleached coniferous sulphate pulp No. 7-2.. 152 10. Gas chromatogram of the acetylated hydrolysates of bleached predominantly angiospermous sulphate pulp No. 8-2 153 Figure . Page 11. Gas chromatogram of the acetylated hydrolysates of coniferous viscose pulp No. 1-2.................... 154 12. Gas chromatogram of the acetylated hydrolysates of alpha-cellulose pulp No. 0-3 155 13. Gas chromatogram of the acetylated hydrolysates of bleached coniferous sulphite pulp No. 6-1 156 14. Fractional stress relaxation curves for eround-wood pulps following water or caustic steeping (n=5) 157 15. Typical fractional stress relaxation curves for holocellulose pulps following water or caustic steeping (n=5)... 158 16. Fractional stress relaxation curves for paper pulps following water or caustic steeping (n=5) 159 17. Fractional stress relaxation curves for acetate and alpha-cellulose pulps following water or caustic steeping (n=5).. 160 18. Typical fractional stress relaxation curves for three viscose pulps following water or caustic steeping (n=5) 161 19. Correlation between fractional stress relaxation of 14 viscose pulps read after 35 min relaxation time following steeping in water or caustic and pulp bemicellulose content............................... 162 Relationship between f r a c t i o n a l stress r e l a x a t i o n (n=5) and l i g n i n contents as observed on water steeped samples of various pulp types a f t e r 35 min re l a x a t i o n time... ..• Relationship between f r a c t i o n a l s t r e s s r e l a x a t i o n (n=5) and l i g n i n contents as observed on c a u s t i c steeped (18.6% NaOH) samples of various pulp types a f t e r 35 min r e l a x a t i o n time Relationship between f r a c t i o n a l stress r e l a x a t i o n (n=5) and hemicellulose contents as observed on water and cau s t i c (18.6% NaOH) steeped samples of various pulp types a f t e r 35 min re l a x a t i o n t ime Relationship between f r a c t i o n a l stress r e l a x a t i o n (n=5) and c e l l u l o s e contents as observed on water steeped samples of various pulp types a f t e r 35 min rel a x a t i o n time. Relationship between f r a c t i o n a l stress r e l a x a t i o n (n=5) and c e l l u l o s e contents as observed a f t e r 35 min re l a x a t i o n time on ca u s t i c steeped (18.6% NaOH) samples of various pulp types........ Short and long time response i n f r a c t i o n a l s t r e s s r e l a x a t i o n (n=5) of two viscose pulps exposed ( a i r - d r y ) to d i f f e r e n t gamma-radiation l e v e l s and then steeped i n 18.6%. NaOH (30 sec, 22°C) C o r r e l a t i o n between f r a c t i o n a l stress r e l a x a t i o n (n=5) of i r r a d i a t e d and untreated.viscose pulps read a f t e r 35 min re l a x a t i o n time following steeping i n 18.6% NaOH and pulp c a u s t i c s o l u b i l i t y xi Figure . Page 27. Relationship between caustic solubility and relative amount of hemicelluloses of 14 viscose pulps..... 170 28. Fractional stress relaxation results (n=5) on seven viscose pulps as read after 35 min relaxation time following short and long time steeping in water or caustic............. 171 29. Effect of steeping time in 18.6% NaOh (22°C) on residual stress relaxation (n=5) of viscose pulp No. 3-2 172 30. Correlation between fractional stress relaxation (n=5) of 14 viscose pulps read after 35 min relaxation time following steeping in water or caustic (18.6% NaOH) and caustic solubility 173 x i i ACKNOWLEDGMENTS The author wishes to express his most sincere thanks to Dr. J.W. Wilson, Faculty of Forestry, University of British Columbia, for introducing him to the fields of wood and pulp rheology, and for his constructive criticisms and understanding guidance throughout the study. The author would also like to acknowledge with gratitude his appreciation to Dr. L. Bach, Department of Civil Engineering, Technical University, Copenhagen, Dr. G.G.S. Dutton, Department of Chemistry, Dr. F.E. Murray, Department of Chemical Engineering, Dr. L. Paszner and Dr. R.W. Wellwood, Faculty of Forestry, for their generous counsel, helpful advice and reading of the manuscript. Special acknowledgement must be extended to Dr. A. Kozak, Faculty of Forestry, for his advice on statistical analysis and computer programming. Grateful appreciation is also due to the National Research Council of Canada and to the University of British Columbia for the financial support extended through the academic program. 1.0 INTRODUCTION When mechanical and chemical excitation systems are applied to wood' or wood derived c e l l u l o s i c materials the ligno-cellulosic polymer systems undergo elastic, vlacoelastic and viscose deformation. These rheological processes are known or can be expected to depend on the molecular structure and compositional characteristics of wood polymers, as well as on other factors such as magnitude and nature of the external force, stress or strain history, temperature and degree of swelling. Stress relaxation studies in tension on wood tissues using infrared polarisation techniques provided evidence that wood rheological behavior is a combined response of the three structural polymers: cellulose, hemicellulose and lignin (48). It was shown that the original response is disturbed by removing wood constituents. Thereby viscoelastic behavior of the remaining complex was drastically changed. Based on this observation and other studies (95,273,283) showing dependence of mechanical properties on compositional characteristics a postulate was put forward. In this,rheological properties of wood derived materials, such as mechanical and chemical pulps, must be related to quantitative and structural features of residual wood polymers. Since compression treatments are frequently involved in pulp processing, i t appeared appropriate to examine water and caustic swollen pulp mats subjected to constant compressive strain. It is thought that experimental evidence on relationships between mechanical-rheological properties and chemical-physical features of fibres may be of practical, as well as theoretical value. The study was designed around five main objectives. F i r s t , to extend rheological techniques to wood based cellulosic materials prepared mechanically or by removal and/or reproportioning of wood constituents in industrial and laboratory processes. Second, to determine to what extent such manipulations affect or change wood and pulp rheological properties. The study included materials - 2 -' r epresent ing the whole pulp spectrum (groundwoods, h o l o c e l l u l o s e s , bleached s u l p h i t e , unbleached -and bleached su lpha te , v i s c o s e , a c e t a t e , and a lpha-c e l l u l o s e p u l p s ) . T h i r d , to examine r h e o l o g i c a l e f f e c t s fo l low ing t reatments, such as i n t e r - c r y s t a l l i n e (steeping in d i s t i l l e d , water) and i n t r a - c r y s t a l l i n e swe l l ing (steeping in 18.6% NaOH) and purposefu l degradat ion by gamma-i r r a d i a t i o n . F o u r t h , to e l u c i d a t e the e f f e c t of s t r e s s h i s t o r y on r h e o l o g i c a l p roper t i es by sub jec t ing pulp samples to var ious per iods of swe l l ing s t r e s s e s . F i f t h , to examine s t r e s s r e l a x a t i o n as a phys ica l -mechan ica l procedure f o r c h a r a c t e r i s i n g p u l p s , i . e . , development of r h e o l o g i c a l ind ices f o r p r e d i c t i n g chemical and mechanical p r o p e r t i e s of p u l p s . - 3 -2.0 BACKGROUND TO THE STUDY 2.1 Morphology and Composition of Wood C e l l Walls Woody c e l l s of Coniferales (gymnosperms) and some Dicotyledons (angiosperms) are the p r i n c i p a l sources of commercial pulps. Numerous studies over the l a s t century have centered on morphological and chemical structures of various c e l l types from these sources. Often the purpose has been to increase u t i l i s a t i o n of wood as a f i b r e source, or to improve the properties of fibrous products already being manufactured. In sp i t e of intense research a c t i v i t i e s , which have lead to an ever in c r e a s i n g accumulation of information on the subject, many questions on wood c e l l walls remain unanswered. 2.1.1 Major wood c e l l types Tree cambiums produce four d i s t i n c t types of xyl a r y elements: ( i ) parenchyma c e l l s ; ( i i ) tracheids; ( i i i ) f i b r e s ; and ( i v ) vessel elements. Parenchyma c e l l s are present i n a l l woody species, whereas any of the other s t r u c t u r a l elements may be wanting. In c o n i f e r s l o n g i t u d i n a l tracheids are dominant, with few i f any f i b r e s and no vessels present, but i n angiosperms tracheids are scarce, while f i b r e s and vessels are often dominant c e l l types (103,121,128,182,226,251,301). Longitudinal tracheids i n stem woods of Scotch pine (Pinus  s y l v e s t r i s L. ) and European spruce (Picea abies Karst.) have been found to amount to 95% and 93% of the wood volume and 99% and 98% of the wood weight. In comparison, l i b r i f o r m f i b r e s i n European b i r c h (Betula verrucosa L. ) occupy 65% of the wood volume and account f o r 86% of the wood weight (232,251). For a more complete d e s c r i p t i o n of the various wood elements, the reader i s ref e r r e d to a number of excellent reviews published within recent years (103,226,251). 2.1.2 C e l l wall organisation C h a r a c t e r i s t i c to a l l types of wood c e l l walls i s the f a c t that they are composed of three groups of substances. Wardrop (316) has c l a s s i f i e d - 4 -these as: ( i ) framework substances; ( i i ) matrix substances; and ( i i i ) encrusting substances. Considering the predominant r o l e of tracheids and f i b r e s i n wood struc t u r e , the following review of c e l l wall composition i s limi t e d to these two c e l l types. Since they show very s i m i l a r c e l l wall u l t r a s t r u c t u r e , they may be considered together. The framework substance i s c e l l u l o s e which i s aggregated i n the form of m i c r o f i b r i l s . Hemicelluloses and other n o n - c e l l u l o s i c carbohydrates are constituents of the matrix substances, whereas l i g n i n s are considered to be the main encrusting materials (19,82,316). D i s t r i b u t i o n and o r i e n t a t i o n of these materials within the c e l l wall and between adjacent c e l l s produces one external and three to f i v e i n t e r n a l layers. These are known as middle lamella (M); primary wall (P); outer (S^), main ( S 2 ) and inner (S^) secondary wall layers; and sometimes an inner warty layer (187,301,316). Figure 1 presents three schematic pictures f o r accepted theories on c e l l wall a r c h i t e c t u r e . Since there are a great number of d e t a i l e d reviews on woody c e l l wall layers (12,41,64,79,82,157,158,167,188,314), only a few points pertinent to t h i s study w i l l be discussed. The middle lamella (M) functions as an i n t e r c e l l u l a r adhesive layer between adjoining c e l l s and i s characterised by i t s high l i g n i n content (approximately 70%) and n o n - c e l l u l o s i c carbohydrates (14,156); whereas c e l l u l o s e m i c r o f i b r i l s are possibly absent (12). It (M) i s c l o s e l y associated with the t h i n outermost (P) layer found i n f i b r e s or tracheids. It (P) als o c o n s i s t s mainly of so- c a l l e d amorphous materials, l i s n i n and non-c e l l u l o s i c carbohydrates; but contains only a loose aggregation of c e l l u l o s i c m i c r o f i b r i l s having no d e f i n i t e o r i e n t a t i o n on the outer surface, but a more or less transverse alignment to the f i b r e axis on the inner surface (148,316). According to Bucher (41), the high l i g n i n content, as well as the woven f i b r i l l a r texture, gives the primary wall a very l i m i t e d swelling capacity, which causes i t to burst when treated with swelling agents. It i s generally accepted that the adjoining three layers which comprise the secondary wall (S^, S^ and S^), and al s o lamellae within the S^ and S^, are arranged i n cross-layered constructions (72,101,148,311,316). It has been found i n experiments using a p o l a r i z i n g microscope that m i c r o f i b r i l - 5 -o r i e n t a t i o n i n l i e s between 50° to 70° from the c e l l a x i s ; i n between 10° to 35° depending on species and p o s i t i o n i n the growth zone; and i t ranges i n between 60° to 90° (148). As indicated in F i g . 1, the micro-f i b r i l s form lamellae i n the varions layers. Dunning (60) observed with the electron microscope that the t h i n layer of longleaf pine (Pinus p a l u s t r i s M i l l . ) latewood f i b r e s i s comprised of at least three lamellae, which e x h i b i t a l t e r n a t i n g o r i e n t a t i o n s . When subjected to high swelling stresses t h i s t h i n lamellar system can be r e a d i l y torn from the bulky (251). According to Kollmann and Cote (148), the number of non-alternating lamellae i n the main secondary wall may vary from 30 to 40 f o r earlywood and to 150 and more for latewood, thus c o n t r i b u t i n g most to the c e l l wall bulk, as well as dominating c e i l wall mechanical and physical p r o p e r t i e s . The r o l e of the layer i s also revealed by r e l a t i v e thickness measurements, for example those by Jayme and Fengel (131) on European spruce earlywood tracheids. These showed that the layer occupied 74 to 84% of the c e l l w a l l ; whereas, the primary wall contributed only 7 to 14%; the 5 to 11%; and the only 3 to 4% to the c e l l wall volume. It has been observed (101,102) that m i c r o f i b r i l s i n most S 2 lamellae are oriented nearly p a r a l l e l to the f i b r e a x i s . M i c r o f i b r i l s i n outer lamellae towards and e x h i b i t a gradual change from and o r i e n t a t i o n s to that of the main body. The S^, when present, i s a t h i n layer of f l a t h e l i c a l pattern s i m i l a r to the S^, probably not exceeding f i v e to s i x lamellae in thickness with a l t e r n a t i n g o r i e n t a t i o n s (148,316). Liese (166) reported that the i s somewhat th i c k e r in c o n i f e r s than in pored woods and that i t i s poorly developed or wanting in some genera and in compression wood tracheids. The t h i n layer (W) containing warty structures may cover the or lumen l i n i n g when the i s wanting. It i s generally accepted that W o r i g i n a t e s from deposits of tonoplast residues (167,316). F u n c t i o n a l l y , i t may improve the chemical r e s i s t a n c e . It i s apparent from the above d e s c r i p t i o n s that tracheid or f i b r e c e l l walls are a m u l t i - p l y laminate of c e l l u l o s e m i c r o f i b r i l s which d i f f e r - 6 -c o n s i d e r a b l y i n o r i e n t a t i o n between and w i t h i n r e s p e c t i v e p l i e s . As Mark (182) suggests, such v a r i a t i o n s i n m i c r o f i b r i l o r g a n i s a t i o n can be expected to i n f l u e n c e the p h y s i c a l and mechanical and a l s o the r h e o l o g i c a l p r o p e r t i e s of f i b r o u s c e l l u l o s i c m a t e r i a l s . There are s e v e r a l published works which have i n v e s t i g a t e d m i c r o f i b r i l o r i e n t a t i o n - m e c h a n i c a l property r e l a t i o n s h i p s i n wood and i n i n d i v i d u a l f i b r e s (83,115,136). 2.1.3 C h a r a c t e r i s t i c s of wood chemical c o n s t i t u e n t s Composition s t u d i e s on many c o n i f e r o u s woods have shown that on average c e l l u l o s e amounts to 43%, h e m i c e l l u l o s e s 28% and l i g n i n 29% of the e x t r a c t i v e - and m o i s t u r e - f r e e c e l l w a l l substance ( 4 5 ) . Respective values f o r pored woods are 43%, 35% and 22% (4 5 ) . From t h i s i t i s apparent that c e l l w a l l s i n c o n i f e r o u s woods c o n t a i n c o n s i d e r a b l y l e s s matrix h e m i c e l l u l o s e s and more e n c r u s t i n g l i g n i n substances than those i n pored woods. Chemical compositions f o r a few North American commercially important woods are given i n Table 1 (294). ' 2.1.3.1 C e l l u l o s e C e l l u l o s e i s comprised of jft-D-glucopyranose r e s i d u e s which are bonded together as l i n e a r chains by ( l - * " 4 ) - g l y c o s i d i c l i n k a g e s (119,218). The glucose r e s i d u e s are present i n *C^ c h a i r conformation w i t h i d e n t i c a l bond angles of about 110° and w i t h hydroxyl groups a l l e q u a t o r i a l (119,294). L i g h t s c a t t e r i n g measurements on n i t r a t e d c e l l u l o s e c a r r i e d out by Goring and T i m e l l (90) i n d i c a t e d that at l e a s t 8,000 to 10,000 glucose u n i t s p a r t i c i p a t e i n formation of a wood c e l l u l o s e s t r a n d . A c c o r d i n g to Mutton (203) and Rydholro (251), c e l l u l o s e i s capable of undergoing r e a c t i o n s at i t s h y d r o x y l ( a d d i t i o n , s u b s t i t u t i o n and o x i d a t i o n r e a c t i o n s ) and a c e t a l ( a c i d i c and a l k a l i n e h y d r o l y s i s ) groups. Furthermore, the aldehyde end groups can be o x i d i z e d , reduced or rearranged. I t i s w e l l known that aldehyde end groups are r e s p o n s i b l e f o r degradation r e a c t i o n s i n a l k a l i n e p u l p i n g (251). - 7 -2. 1.3.1.1 Supermolecular arrangement Supermolecular arrangement of the c e l l u l o s e chain molecules within f i b r i l l a r and s u b - f i b r i l l a r c e l l wall structures has been the subject of many theories. Various chemical and physical methods, i n p a r t i c u l a r e l e c t r o n microscopy and X-ray and e l e c t r o n d i f f r a c t i o n measurements, have revealed that c e l l u l o s e - the sole constituent of m i c r o f i b r i l s - i s at least p a r t i a l l y packed into c r y s t a l l i n e regions, known as c r y s t a l l i t e s . Much controversy e x i s t s i n the l i t e r a t u r e on whether or not a l l c e l l u l o s e l i e s within c r y s t a l l i n e regions (109,110,113,141,198,199,238,240,246,249,270). Thus, several systems of f i n e l a t t i c e s t r u c t u r e s , which are assumed to form the m i c r o f i b r i l s , have been proposed as models, such as: ( i ) perfect c r y s t a l ; ( i i ) p a r a c r y s t a l l i n e ; ( i i i ) c r y s t a l defect; ( i v ) fringed m i c e l l e ; (v) f r i n g e d - f i b r i l ; and ( v i ) lamellar. The l a s t three models are presently the most widely discussed and adopted. The fringed m i c e l l a r hypothesis,as f i r s t introduced by Hermann et a_l.(112), states that c e l l u l o s e chain molecules pass through c r y s t a l l i n e regions and then i n the amorphous zone the chains separate. Thereafter they r e a l i g n to form another c r y s t a l l i t e . According to Frey-Wyssling (80), there e x i s t s a gradual t r a n s i t i o n between c r y s t a l l i t e s which are at least 600 A* long, 100 A* wide and 30 A* thick (238,312), and amorphous regions. The c r y s t a l l i t e s are arranged at regular i n t e r v a l s but randomly i n the chain d i r e c t i o n . Marchessault (177) supported t h i s hypothesis and proposed that the c r y s t a l l i t e s are segregated as f i b r i l s which are embedded i n oriented hemicelluloses. More r e c e n t l y , Hearle (108-110) developed a concept, known as the f r i n g e d - f i b r i l model, which considers the almost i n f i n i t e length of micro-f i b r i l s and the phenomenon of polymolecular growth by s p h e r u l i t i c arrangement. In t h i s model m i c r o f i b r i l s are described as long, imperfectly c r y s t a l l i n e , and arranged in s p i r a l - f o r m within the f i b r i l s t ructure and separated by amorphous zones. The c e l l u l o s e molecules may run a l t e r n a t e l y through several c r y s t a l l i n e zones, as well as through amorphous regions. This hypothesis has been found to explain s u f f i c i e n t l y the behavior of cotton c e l l u l o s e (52), but i t f a i l s , as shown by Mark (182) and Cowdrey and Preston (52), to - 8 -'describe s a t i s f a c t o r i l y the mechanical behavior of coniferous tracheids. Lamellar or folded chain theories advocate that c e l l u l o s e chain molecules are folded in-plane to form a f l a t ribbon, which i s wound as a h e l i x (58,59,185,297). By using e l e c t r o n d i f f r a c t i o n techniques, Manley (176) was able to show that c e l l u l o s e d e r i v a t i v e s e x h i b i t t h i s type of s t r u c t u r a l arrangement. Investigations c a r r i e d out by S u l l i v a n (284) on c e l l u l o s e from several coniferous and deciduous species a l s o lend support to the chain f o l d i n g theory. However, as Mark (182) pointed out, t h i s model i s not capable of s a t i s f a c t o r i l y explaining observed phy s i c a l and mechanical properties of the c e l l w a l l . From the above, i t i s apparent that none of the proposed theories seems to represent a l l cases of c e l l u l o s e molecular arrangement i n m i c r o f i b r i l s . Possibly some new or improved p h y s i c a l methods of examination couLd help to e l u c i d a t e m i c r o f i b r i l l a r structure of c e l l u l o s i c s . 2.1.3.1.2 The c r y s t a l structure The d e t a i l e d structure of c e l l u l o s e c r y s t a l l i n e zones has been deduced from X-ray d i f f r a c t i o n measurements. Hartshorne and Stuart (107) and Preston (237) described the smallest grouping of subunits in the c e l l u l o s e c r y s t a l l i t e s , the unit c e l l , as p a r a l l e l e p i p e d and of the mono-c l i n i c system. The c r y s t a l l o g r a p h i c unit c e l l was f i r s t proposed by Meyer and Mark (196) and l a t e r modified by Meyer and Misch (197). As indicated i n F i g . 2, the unit c e l l c o n s i s t s of four c e l l o b i o s e units arranged i n the corners of the p a r a l l e l e p i p e d and a f i f t h c e l l o b i o s e unit turned 180° and passed through the i n t e r s e c t i o n of the diagonals with a d i f f e r e n c e of %-unit spacing. A c h a r a c t e r i s t i c feature of c e l l u l o s e unit c e l l s i s the angle between the axes a and c which has been found to be 84° f o r native c e l l u l o s e . As indicated in F i g . 2, the arrangement of the chains i n the c r y s t a l r e s u l t s i n the;.formation of three planes, known as the 101, 101 and 002 planes. It i s assumed that the 101 planes are p a r a l l e l to the larger surface of the c r y s t a l l i t e , and consequently l i e p a r a l l e l to the c e l l wall surface (237,315). According to Frey-Wyssling (77,78), the 101 plane, which i s p a r t i c u l a r l y - 9 -r i c h i n hydroxyl groups, i s the plane of lamination; that i s , the plane i n which the f i b r i l s tend to aggregate l a t e r a l l y within c e l l wall lamellae (81,315). Forces which keep the glucose residues i n p o s i t i o n are, according to Honeyman and Parsons (120): ( i ) covalent bonds in the b-c plane; ( i i ) f a i r l y strong hydrogen bonds i n the a-b plane; and ( i i i ) much weaker van der Waal's forces i n the a-c plane. A l l of these forces are now considered to be important i n determining c e l l u l o s e properties (212,300). The intermolecular forces i n the u n i t c e l l are s u f f i c i e n t l y strong to r e s i s t the penetration of water molecules into c r y s t a l l i n e zones, in other words, l i m i t the swelling caused by adding water to amorphous regions (206,251). Aqueous polar compounds, such as strongly a l k a l i n e or acid s o l u t i o n s , however, are known to attack and a l t e r the c e l l u l o s e l a t t i c e . According to Rydholm (251) t h i s process involves f i r s t the breaking of hydrogen bonds between two c e l l u l o s e hydroxyIs and thereafter the formation of new hydrogen bonds between reagent and hydroxyl groups. This leads to dimensional changes of the unit c e l l i n transverse d i r e c t i o n , c h i e f l y because of an increase i n the 101 i n t e r p l a n a r distance, whereas l O l and 002 spacings e x h i b i t only minor changes '251). 2.1.3.2 Hemicelluloses Chemical and physical properties of the n o n - c e l l u l o s i c wood carbohydrates, or hemicelluloses, have been c r i t i c a l l y reviewed (2,99,116, 148,169,251,258,293,295). Although r e l a t e d , the hemicelluloses i n the c e l l walls of coniferous and pored woods d i f f e r to some extent. In pored woods, the 0 - a c e t y l - 4 - O-methylglucurono-xylan appears to be the dominant hemicellulose. According to Meier (190), t h i s xylan c o n s i s t s of yS-D-xylopyranose residues, linked together by ( l - > 4 ) - g l y c o s i d i c bonds. It i s p a r t i a l l y sub-s t i t u t e d by 4-0-methyl-c<-D-glucuronic acid and a c e t y l groups. Bouveng et a l . (27) studying glucurono-xylan from b i r c h showed that 1 i n 10 xylan residues has a 4-0-methyl-c^-D-glucuronic acid at the xylan p o s i t i o n and about 7 of 10 carry an a c e t y l group at or C^. The second hemicellulose, a glucomannan, occurs i n only l i m i t e d amounts (294,295). It i s claimed to be l i n e a r and has been found to contain (1-—1*-4) - linked j3-D-glucopyranose and |3-D-mannopyranose residues mostly i n a r a t i o of 1:2 (295). - 10 -In most coniferous woods, according to Abdurahman et a l . (1) and Meier (190), two types of hemicelluloses - glucuronoarabinoxylan and galactoglucomannan - appear to be dominant. The former type c o n s i s t s of |3-D-xylopyranose residues, linked together by (l-*-4 )-g lycos i d i c bonds. Some of the xylose residues carry 4-0-methyl-<X-D-glucuronic a c i d groups at C^. and some are substituted by L-arabinofuranose u n i t s at C^. The glucomannan i s composed of (1—»4) - linked f3-D-glucopyranose and ^-D-mannopyranose residues (1:3.5 r a t i o ) with two or three branches per molecule. Meier (190) and others have reported that a c e t y l and galactose residues are attached to mannose un i t s of the backbone. A c h a r a c t e r i s t i c feature of n o n - c e l l u l o s i c carbohydrates i n wood i s the rather low degree of polymerisation. T i m e l l (294,295) reported that the i s o l a t e d hemicelluloses mentioned above seldom have more than 150 to 200 sugar residues. This may p a r t l y e x plain the higher e x t r a e t a b i l i t y of hemicelluloses i n comparison with c e l l u l o s e (251). Berlyn (19) considers hemicelluloses to be the main constituents of the c e l l wall matrix. Marchessault (178) and Wardrop (316) reported that, f o r instance, native xylan surrounds c e l l u l o s e f i b r i l s by forming a somewhat amorphous matrix, and there i s evidence that i t may be oriented i n wood (70,165).Due to a h i g h l y branched s t r u c t u r e , however, i t i s doubtful that native xylan occurs i n c r y s t a l l i n e form (205). X-ray studies c a r r i e d out by Nelson (205) present evidence that glucomannans may be c l o s e l y associated with c e l l u l o s e m i c r o f i b r i l s and probably are even positioned between c e l l u l o s e chains. It appears from studies of Lindberg and Meier (170) on European spruce h o l o c e l l u l o s e preparations that glucomannans may be un-ordered or only s l i g h t l y ordered. 2.1.3.3 Lig n i n Lignin i s considered as the major encrusting substance between and within wood c e l l w a lls. The knowledge of i t s s t r u c t u r e i s s t i l l f a r from complete due to the f a c t that i t i s a complex, three-dimensional polymer which i s d i f f i c u l t to i s o l a t e from the framework and matrix substances (19). Hypothetical models of i t s chemical s t r u c t u r e , i n p a r t i c u l a r linkages between the main s t r u c t u r a l u n i t s , phenylpropane groups, have been proposed by - 11 -Freudenberg (74,75) and Adler (3) among others. Due to the aromatic nature and f u n c t i o n a l groups on the a l i p h a t i c side-chain and r i n g of the phenylpropane u n i t , the l i g n i n macromolecule, according to Rydholm (251), can undergo several reactions. These include sulphonation, h y d r o l y s i s and condensation i n a c i d i c and a l k a l i n e medium, as well as mercaptation, halogenation, degradative and non-degradative oxidation. These reactions are most important i n many d e l i g n i n i f i c a t i o n processes, such as pulping and bleaching. Excellent reviews on the subject have been published recently (47,57,230,251,309). The physical nature of the l i g n i n polymer system, p o s i t i o n i n g i n the c e l l wall and i t s a s s o c i a t i o n with carbohydrates have been subject of much argument. It "is often assumed that l i g n i n i n wood, due to i t s highly branched nature, i s present i n an amorphous.state. Frey (76) presented evidence that wood l i g n i n does not ex h i b i t a c r y s t a l l i n e structure. Its hydrophobic character, p a r t i c u l a r l y i t s preventing e f f e c t on excessive swelling of hemicelluloses has been mentioned e a r l i e r . Goring (89) reviewed the l i t e r a t u r e on l i g n i n polymer properties and summarized four proposed hypotheses on the physical state of l i g n i n polymeric systems i n wood. These include network theory, small-molecule theory, lignin-carbohydrate bonds and possible "snake cage" theory, and theory on aggregation by secondary bonds. According to the network theory, the l i g n i n s t ructure i s treated as c o n s i s t i n g of short l i n e a r chains c r o s s - l i n k e d by various types of covalent bonds thus forming a three-dimensional structure. Schuerch (257) supported t h i s hypothesis by suggesting that native l i g n i n must be kept i n p o s i t i o n by strong bonds. The small-molecule theory considers native l i g n i n as c o n s i s t i n g of small, r e a c t i v e molecules which tend to polymerize during i s o l a t i o n processes. This theory appears to be questionable, however, since Yean and Goring (324), studying the e f f e c t of sulphonation on l i g n i n molecular weight during e x t r a c t i o n , observed only i n s i g n i f i c a n t changes. The lignin-carbohydrate bond, and possible "snake cage" e f f e c t theory t r i e s to explain the un i v e r s a l p r o t o l i g n i n i n s o l u b i l i t y of low-- 12 -molecular weight. Such bonds might attach the l i g n i n s l i g h t l y to carbohydrate compounds. However, the number of bonds i s probably small and l i m i t e d to "occasional spot welds" (254). Another a l t e r n a t i v e concept was. proposed by Pew and Weyna (234), who stated that l i n e a r carbohydrate molecules may be surrounded by the three-dimensional l i g n i n network. In other words, they may be held in the g e l - l i k e l i g n i n substance by molecular entanglement or "snake cage" structures. Various i n v e s t i g a t i o n s (310,314,317) provide evidence that l i g n i n must i n f i l t r a t e the carbohydrate complex. Based on X-ray measurements, Wardrop (310,314) and Wardrop and Preston (317) c a l c u l a t e d the s i z e of wood and h o l o c e l l u l o s e c r y s t a l l i t e s and found f o r h o l o c e l l u l o s e an increase of 18.8% i n c r y s t a l l i n i t y , which i s i n agreement with values determined from hydrolysed c e l l u l o s e c r y s t a l s . Furthermore, Wardrop (313,314.) observed, that h y d r o l y s i s during d e l i g n i f i c a t i o n treatments and subsequent h y d r o l y s i s of h o l o c e l l u l o s e caused m i c r o f i b r i l s to aggregate i n clumps v i s i b l e under the microscope. Wardrop (314) a t t r i b u t e d t h i s phenomenon to c r y s t a l l i s a t i o n of less ordered regions surrounding the m i c r o f i b r i l s , formerly prevented from c r y s t a l l i s a t i o n by i n f i l t r a t e d l i g n i n . Aggregation by secondary bonds appears to be an important mechanism f o r keeping l i e n i n molecules i n p o s i t i o n within the c e l l w a l l . Benko (17) reported a considerable increase in molecular weight of soluble .kraft l i g n i n s by decreasing the pH from 11.5 to 7.0. Based on these observa-t i o n s Benko suggested that associations of l i g n i n molecules through weak chemical bonds, probably through solvent dependent hydrogen and hydrophobic bonds, influence markedly the actual molecular si z e of lignosulphonates and other l i g n i n materials. Other studies (34,93) on k r a f t l i g n i n a lso support the aggregation hypothesis. 2.2 C h a r a c t e r i s a t i o n of Various Pulp Types Pulping and subsequent bleaching or p u r i f i c a t i o n treatments are known to change d r a s t i c a l l y the chemical and consequently the physical nature of l i g n i n , hemicelluloses and c e l l u l o s e and t h e i r r e l a t i v e composition i n most pulp types. Such changes undoubtedly must exert a profound influence on the mechanical properties of pulp mats. For understanding pulp mat - 13 -responses to mechanical excitations it seems useful to elucidate the chemical, physical and compositional modifications which carbohydrate and lignin fractions undergo in pulping. In the following section the present knowledge and understanding of these modifications will be reviewed in a somewhat summary way, with particular reference to pulp types examined in the present study. 2.2.1 Mechanical pulps Fibrous materials produced in conventional groundwood processes are obtained by applying mechanical energy to wet wood. The absence of chemical treatments before and during manufacture maintains an almost quantitative yield from wood (95 to 98%). Rydholm (251) suggested that high temperatures developed during the grinding process may have three distinct effects on the lignin-carbohydrate complex: (i) softening of the lignin polymer; ( i i ) weakening of hydrogen bonds; and ( i i i ) probably hydrolysis of carbohydrates. The last effect probably accounts for most of the 2 to 5% yield loss. Brecht and Klemm (30) considered alteration of chemical nature of wood during the grinding process to be below the degree needed to significantly affect physical and chemical responses of cell wal Brightening treatments with oxidative reagents, such as sodium dithionite or hydrogen peroxide, are considered as having only a minor influence on groundwood mechanical properties, in spite of some structural changes imparted to lignin and, under certain circumstances, also to , carbohydrates. The oxidative transformation of lignin chromophores to colourless compounds (251) in peroxide brightening may be accompanied by condensation reactions. This is evidenced by the work of Katuscak e_t a l . (142), who observed that peroxide oxidation of methanol lignins caused an increase in cross-linkage without changing lignin molecular weight; in other words,increased intramolecular covalent bonding. Because these reactions do not seem to lead to enlargements of the lignin network, their effect on the mechanical behavior may be subordinate. The carbohydrates, according to Stobo and Russel (280), are apparently not affected in cold peroxide treatments, but hot treatments cause definite degradation as indicated by decreased viscosity and increased carboxyl content (168). - 14 -'2,2.2 Holo c e l l u l o s e pulps Two widely used procedures f o r h o l o c e l l u l o s e preparation are the peracetic acid method as modified by Leopold (164); and the acid c h l o r i t e method o r i g i n a l l y proposed by Jayme (129), but developed as the present recipe by Wise et al. (323). No proposed method produces h o l o c e l l u l o s e s without some loss of n o n - c e l l u l o s i c carbohydrates at complete l i g n i n removal. Recently, Ahlgren and Goring (4) c a r r i e d out a study on t h i s subject and observed that c h l o r i t e d e l i g n i f i c a t i o n of black spruce (Picea mariana ( M i l l . ) BSP. ) wood produced h o l o c e l l u l o s e in 70% y i e l d containing 40% c e l l u l o s e , 13% glucomannan and galactan, 127. xylan and other carbohydrates and 5% l i g n i n . Further d e l i g n i f i c a t i o n was found to cause considerable carbohydrate l o s s , mainly mannose and galactose. Ahlgren and Goring's observations are i n reasonable agreement with e a r l i e r f i n d i n g s (38,39,44,189,290). Time l l (292) reported that acid c h l o r i t e d e l i g n i f i c a t i o n attacks the carbohydrate portion, as indi c a t e d by decrease i n DP with progressive d e l i g n i f i c a t i o n . This may well explain the loss of polyoses i n l a t e r stages of d e l i g n i f i c a t i o n . Poljak (236), who f i r s t introduced peracetic a c i d f o r wood d e l i g n i f i c a t i o n (235), reported that t h i s method completely removes the l i g n i n without hydrolysing the carbohydrates to simple sugar u n i t s . In l a t e r comprehensive studies on the subject, which lead to a modification of P o l j a k 1 s method, Leopold (164) was able to show that peracid d e l i g n i f i -c a t i o n does not leave the wood carbohydrate portion e n t i r e l y i n t a c t . Some loss of the glucomannan, arabinosalactan and xylan polyoses, and decrease i n c e l l u l o s e DP (164), indicated-carbohydrate chain degradation. Furthermore, the h o l o c e l l u l o s e preparations were found to s t i l l contain between 2 and 3% l i g n i n . Evidence regarding the degrading e f f e c t of peracetic acid on carbohydrates i s also given by work of Shimada and Kondo (264), who observed that increase in chip s i z e lowered y i e l d i n peracid cooking. 2.2.3 Sulphite pulps Dependent on cooking v a r i a b l e s employed, such as temperature, cooking time and liqu o r c h a r a c t e r i s t i c s , sulphite pulping may be used to - 15 -produce a whole s e r i e s of pulps. These range from t y p i c a l heraicellulose-r i c h greaseproof paper pulps at 52% y i e l d (86) through various types of ordinary paper making pulps to high alpha-type d i s s o l v i n g grade pulp at 35% y i e l d (86). it Jorgensen (139) analysed the carbohydrate composition of various pulp types and reported that glucose accounted f o r 90.2%, mannose 5.4% and xylose 4.4% of the carbohydrate portion in an ordinary coniferous s u l p h i t e raw pulp of 47.2% y i e l d with Roe No. of 1.6 (1.3% l i g n i n ) . Other wood carbohydrates were removed during the cooking process. These observations are i n reasonable agreement with data published by Thompson e_t a l . (291) f o r black spruce s u l p h i t e pulps cooked at pH values below 7. In d i s s o l v i n g grade pulps, the hemicellulose portion i s even more reduced through a d d i t i o n a l p u r i f i c a t i o n steps and residues amount to only a few per cent of a l l pulp carbohydrates. For instance, a s u l p h i t e spruce pulp was found by Croon et a_l. (55) to b^e comprised of 95% glucose, 1.7% x y l o s e , 3.3% mannose and 0.1% arabinose. During s u l p h i t e p u l p i n g , l i g n i n undergoes three reactions which are s i g n i f i c a n t to both i t s removal and the physical nature of residues remaining with pulp f i b r e s . These are sulphonation, h y d r o l y s i s and sometimes conden-sation (321). Dependent on how f a r these reactions are allowed to proceed, the l i g n i n content i n unbleached pulps l i e s between 1 and 4% (86). The various f a c t o r s and mechanisms responsible f o r l i g n i n reactions i n s u l p h i t e cooking are discussed in several reviews (86,88,251,321). As i n other t e c h n i c a l pulping processes, d e l i g n i f i c a t i o n i n s u l p h i t e cooking i s very c l o s e l y i n t e r r e l a t e d with degradation of both c e l l u l o s e and hemicelluloses and at least with a p a r t i a l d i s s o l u t i o n of the l a t t e r . Degree of polymerisation (DP) determinations on various tracheid wall layers c a r r i e d out by Luce (171) showed that c e l l u l o s e undergoes serious degradation due to acid h y d r o l y s i s , i n p a r t i c u l a r i n the outer wall layers where DP reaches values as low as 300. This confirms e a r l i e r proposals made by Tayme and von Koppen (134). According to Rydholm (251), cooking below 48% y i e l d causes such serious degradation that part of the c e l l u l o s e goes - 16 -into s o l u t i o n and even f i b r e fragmentation may occur. Hamilton (97) sugeested that under a c i d i c pulping conditions less ordered zones i n m i c r o f i b r i l s are more susceptible to attack and thereby c e l l u l o s e chains are cleaved p r e f e r e n t i a l l y i n amorphous regions. The c e l l u -lose degradation by a c i d i c h y d r o l y s i s seems to be enhanced by mechanical damage of the f i b r e wall during chip preparation,as indicated by lower strength properties of pulps prepared from damaged f i b r e s (106). This may be explained by the f a c t that the p r o t e c t i v e a c t i o n of l i p n i n i n e a r l y cooking stages i s p a r t i a l l y l o s t . The hemicelluloses undergo profound s t r u c t u r a l changes during s u l p h i t e cooking, mainly caused by a c i d i c h y d r o l y s i s of bonds between units of the main chain and between backbone and side-branches. It i s well known that h y d r o l y t i c chain degradation leads to considerable hemicellulose l o s s , e s p e c i a l l y i n low y i e l d pulping which leaves only a few per cent of r e s i d u a l polyoses i n pulp f i b r e s (160). II Studying hemicelluloses in b i r c h pulps, Ohrn and coworkers (174, 217) observed that degradation e f f e c t s on polyoses, as indicated by DP values, are considerably higher f o r s u l p h i t e than f o r k r a f t pulps. DP values.for the former were found to be about 70 and f o r the l a t t e r 130 to 160. Acid s u l p h i t e experiments c a r r i e d out by Hamilton (97), with red alder (Alnus  rubra Bong. ), and western hemlock (Tsuga heterophylla (Raf. ) Sarg. ) showed that xylans are somewhat more susceptible to h y d r o l y t i c cleavage i n a c i d i c medium than the mannan family of carbohydrates. This has been confirmed by Petterson and Rydholm (233) on European b i r c h s u l p h i t e pulps. C h a r a c t e r i s t i c f o r r e s i d u a l xylans i n s u l p h i t e pulps i s a r e l a t i v e abundance of glucuronic acid groups which appear to be f a i r l y stable to acid h y d r o l y s i s (251). S i m i l a r l y , some of the a c e t y l groups tend to remain attached to the xylan backbone. This presence of a c e t y l groups has been proven by II Ohrn and Croon (216) on i s o l a t e d b i r c h s u l p h i t e xylans. However, other linkages with side-branches seem to be a c i d - l a b i l e . As proven by Hamilton (97), arabinofuranose i s r e a d i l y s p l i t from the xylan backbone and therefore s u l p h i t e xylans contain only traces of arabinose (55). - 17 -Likewise, the galactose branches are e n t i r e l y removed i n a c i d i c pulping and the former galactoglucomannan polymer i s reduced to glucomannan. This i s supported by Eriksson and Samuelson (66), who reported considerable amounts of galactose in spent s u l p h i t e l i q u o r . By adding cotton l i n t e r s to a s u l p h i t e cook, Annergren and Rydholm (10) demonstrated that probably a deacetylated glucomannan was absorbed to a c e r t a i n degree on to the f i b r e surface. This indicates that some gluco-mannan i s dissolved in the early stage of cooking and l a t e r adsorbed. They concluded that t h i s rearrangement of glucomannan on the surface and within the f i b r e possibly causes c r y s t a l l i s a t i o n of l i n e a r molecular fragments on to c e l l u l o s e m i c r o f i b r i l s , or at least a better ordered structure on the micro-f i b r i l surface. T i m e l l and Tyminski (296) postulated that some glucomannan adsorption may even occur i n s i d e the m i c r o f i b r i l s . Rydholm (251) pointed out that glucomannan i s p r e f e r e n t i a l l y adsorbed, since l i n e a r xylan chains are l i b e r a t e d l a t e r i n the cook and therefore can not compete f o r the limited m i c r o f i b r i l l a r surface. This may at least p a r t l y explain the r e l a t i v e l y low xylan content i n s u l p h i t e pulps. 2 . 2 . A Sulphate pulps Depending on cooking v a r i a b l e s employed, the k r a f t pulping process provides y i e l d s of AO to 55% based on oven-dry wood (251). Nolan (215) reported that h i g h l y d e l i g n i f i e d bleachable paper grade pulps contain 3 to A% l i g n i n , whereas h i g h - l i g n i n k r a f t pulps may contain as much as 1 0 % l i g n i n . That k r a f t pulps contain considerably more r e s i d u a l l i g n i n than acid sulphite II pulps of s i m i l a r y i e l d has been confirmed by Jorgenson (139). He also analysed the carbohydrate composition of sulphate pulps, i . e . , a pulp of A 7 . 6 % y i e l d was comprised of 8 5 % glucose, 6 % mannose, 8 . 2 % xylose and 0 . 8 % arabinose. Thompson ejt al_. (291) reported that pored wood k r a f t pulps of s i m i l a r y i e l d a lso contained about 0.35% galactose. The hemicellulose portion has been found to be markedly reduced when the k r a f t cook i s preceded by a prehydrolysis stage. For instance, Croon et aJL. (55) found that i n a pre-hydrolysed k r a f t pulp made from southern pine (Pinus spp.), glucose accounted f o r 9 5 . 2 7 o of a l l carbohydrates present, while xylose ( 2 . 6 % ) , mannose ( 2 . 1 % ) , and arabinose ( 0 . 1 % ) totaled only A . 8 % . - 18 -As i n sulphite pulping, d e l i g n i f i c a t i o n i n a l k a l i n e processes i s also accompanied by condensation reactions (251). ^hese seem to be more serious in a l k a l i n e media. As Gi e r t z (86) suggested, the i m p o s s i b i l i t y .of d e l i g n i f y i n g k r a f t pulps to the l e v e l achievable in sulphite pulping has to be e n t i r e l y a t t r i b u t e d to extensive l i g n i n condensation. The d i s t r i b u t i o n II of r e s i d u a l l i g n i n within the f i b r e wall has been studied by von Koppen (149), who found that l i g n i n was almost evenly d i s t r i b u t e d over the e n t i r e wall of k r a f t pulp f i b r e s . In c o n t r a s t , i t appeared to form a considerable accumula-t i o n i n the outer layers of s u l p h i t e pulp f i b r e s . According to Hamilton (97), a l k a l i n e pulping considerably reduces c e l l u l o s e DP by two major re a c t i o n s : ( i ) stepwise end-group degradation; and ( i i ) h y d r o l y t i c chain cleavage. Based on v i s c o s i t y measurements, H a r t l e r (104) and Thompson et a_l. (291) demonstrated that these degradation reactions reduce c e l l u l o s e DP more than corresponding reactions i n acid s u l p h i t e p u l p i n g . But disadvantages of comparatively higher degradation are at least o f f s e t by better uniformity of chain length of a l k a l i degraded c e l l u l o s e s as proved by Luce (171). This phenomenon may be a t t r i b u t e d to f a s t e r and more uniform penetration of a l k a l i n e pulping l i q u o r s . This i s due to i n t e r -and i n t r a - c r y s t a l l i n e a l k a l i n e swelling which, according to Stone (281), enlarges the f i b r e wall c a p i l l a r y system. A l k a l i n e pulping a f f e c t s hemicelluloses p r i n c i p a l l y i n the same way as other commercial chemical pulping processes: ( i ) change of molecular structures by h y d r o l y s i s ; and ( i i ) r e p o s i t i o n i n g by p a r t i a l r e p r e c i p i t a t i o n of dissolved compounds. Residual sulphate pulp hemicelluloses are characterised by high DP, which has been found to be twice that of sulphite hemicelluloses (174,233). This can be a t t r i b u t e d to the tendency of low molecular carbohydrates to d i s s o l v e rather e a s i l y i n a l k a l i n e cooking li q u o r s as has been observed by Axelsson et a_l. (13). It appears from several studies on carbohydrate composition of various pulp types (97,139,174,291) that glucomannans are more s e r i o u s l y degraded and dissolved i n a l k a l i n e pulping than are xylans. This can be at least p a r t l y explained by the greater s u s c e p t i b i l i t y of glucomannans to a l k a l i n e peeling reactions as shown by Richtzenhain and Abrahamson (247). - 19 -Residual glucomannans i n k r a f t pulps occur i n two forms as: ( i ) glucomannan; and ( i i ) a system of galacto-glucomannans (54,97). The former o r i g i n a t e from native glucomannan, but have considerably reduced DP. The l a t t e r a r i s e from O-acetyl-galacto-glucomannan which undergoes deacetylation and p a r t i a l chain degradation during pulping. In prehydrolysed pulp f i b r e s , only l i n e a r glucomannans remain due to removal of the galactopyranose u n i t s i n the prehydrolysis stage '97,160,269). The most c h a r a c t e r i s t i c feature of r e s i d u a l xylans i n sulphate pulps i s the absence (97,291) or infrequency (53,54,191) of 4-0-methyl-c<-D-glucuronic a c i d residues. This i s due to high a l k a l i l a b i l i t y of the (l-*"2) - g l y c o s i d i c linkage. Since ester bonds between the xylan backbone and a c e t y l groups are likewise e a s i l y cleaved (216), r e s i d u a l xylan i n pored wood k r a f t pulps c o n s i s t s only of xylose u n i t s . In coniferous pulps the o r i g i n a l xylan polymer has been found to be reduced to an arabinoxylan (98). Croon and Enstrom (54) have shown that only 10% of the o r i g i n a l arabino-furanose units are s p l i t o f f during the cooking process. But i n prehydrolysed k r a f t pulps a l l xylan molecules are degraded to l i n e a r xylan fragments with an eventual DP of only 22 to 60 (98). Of s p e c i a l importance to chemical and physical c h a r a c t e r i s t i c s of hemicelluloses i s the f a c t that a part of the xylan dissolved i n sulphate l i q u o r s i s readsorbed i r r e v e r s i b l y by the c e l l u l o s e (326). G i e r t z (86) a t t r i b u t e d t h i s phenomenon to cleavage of side-chains a f t e r the branched xylan d i s s o l v e d in the pulping l i q u o r . Thereafter, the unbranched or e s s e n t i a l l y side-chain free xylan molecule i s held no longer in s o l u t i o n , but instead i s adsorbed by or c r y s t a l l i z e d on to c e l l u l o s e surfaces. Meller (193) proposed that xylan may be redeposited i n three d i f f e r e n t forms: ( i ) loosely p r e c i p i t a t e d or adsorbed on to the surface; ( i i ) p r e c i p i t a t e d in c r y s t a l l i n e form on c e l l u l o s e ; or ( i i i ) chemically combined with the c e l l u l o s e by t r a n s g l u c o s i d a t i o n . Clayton and Stone (51) have shown that t h i s redeposition can account f o r as much as 3% of pulp weight, regardless of the amount of hemicelluloses already i n the f i b r e s . Recent studies c a r r i e d out by Simonson (266) provide evidence that even x y l a n - l i g n i n compounds found i n pulping l i q u o r s (265) are redeposited on to c e l l u l o s e . It appears from studies of Meller (193) - 20 -that prehydrolysed xylan from b i r c h and straw shows much less tendency toward redeposition on to cotton c e l l u l o s e when heated i n a l k a l i n e medium. 2.2.5 Bleached pulps An excellent survey on the subject of wood pulp bleaching has been given by Rydholm (251) and f o r t h i s reason only a l i m i t e d number of pub l i c a t i o n s r e l a t e d to the present study w i l l be reviewed here. Depending on f i n a l use, most raw chemical pulps are subjected to furt h e r d e l i g n i f i c a t i o n or l i g n i n m o dification by bleaching treatments. In a d d i t i o n , d i s s o l v i n g grade pulps are subjected to a d d i t i o n a l carbohydrate-removing processes to further reduce hemicellulose portions. Rydholm (251) and Springer ejt a_l. (274) determined o v e r a l l y i e l d loss i n pulp bleaching and reported that approximately 1 to 5% of the f i b r e c e l l wall components, mainly l i g n i n and hemicelluloses are removed i n bleaching of s u l p h i t e and k r a f t pulps cooked to 38 ' to 51% y i e l d . In multi-stage bleaching, which i s generally employed i n bleaching of chemical pulps, r e s i d u a l l i g n i n i s d r a s t i c a l l y reduced to trace amounts, it II II Sjostrom and Enstrom (268), f o r instance, reported that a seven stage bleaching sequence i n v o l v i n g c h l o r i n a t i o n , a l k a l i n e e x t r a c t i o n , hypochlorite and c h l o r i n e dioxide reduced l i g n i n content of a coniferous sulphate pulp from 2.42% to less than 0.05%. In a three stage process c o n s i s t i n g of c h l o r i n a t i o n , a l k a l i n e e x t r a c t i o n and sodium hypochlorite treatment, Kayama and Higgins (143) observed that l i g n i n content of two coniferous k r a f t pulps decreased from 2.13 to 0.51% and from 4.59 to 1.11%. Most l i g n i n a b s tracting stages i n bleaching have been found to a f f e c t also the carbohydrate components. According to Meller (194), both c e l l u l o s e and hemicelluloses undergo hydr o l y s i s and subsequent oxidation reactions in the c h l o r i n a t i o n stage causing some loss i n carbohydrates (143). Subsequent a l k a l i e x t r a c t i o n has been found to reduce the y i e l d (143), probably due to introduced carboxylic groups (194) which render part of the low molecular carbohydrate portion soluble i n a l k a l i n e e x t r a c t i o n treatments. S i m i l a r l y , hypochlorite treatments and subsequent a l k a l i n e - 21 -extraction lead also to a yield loss due to oxidation and hydrolytic cleavage of glycosidic bonds (143,159,194). Viscosity measurements on hypochlorite bleached pulps have indicated that severe chain degradation can occur during this bleaching stage (243,245). At low to medium pH chlorine dioxide does not react with aldehydes or ketones (244) and, consequently, no degradation or noticeable losses of carbohydrates occur in this bleaching stage (251). The work of Springer e_t a_l. (274) indicates that the extraction of lignin in bleaching without alkali purification appears to be the major factor responsible for differences in physical and mechanical properties between bleached and unbleached pulps. In such practice the removal of carbohydrates is comparatively low and these slight losses seem to be insignificant for the development of bleached pulp properties. In the case of cold or hot alkali purification, however, the low molecular carbohydrate portions, mainly as hemicelluloses and to some extent short chain cellulose, . are drastically reduced (251). According to Meller e_t ak (195), cold alkali extractions have considerably higher selective dissolving action on non-alpha cellulose components than do extractions with dilute, hot alkali solutions. The former method produces pulps with up to 98% alpha-cellulose content (195), while the latter gives maximum alpha-cellulose contents at 96 to 97% (251). Croon et ak (55) reported that a prehydrolysed kraft pulp purified in cold alkali s t i l l contained 0.3% mannose, 1.1% xylose and 0.1% arabinose. They attributed the high content of resistant xylan to stabilization during the sulphate cook. The presence of considerable residual mannan and xylan fractions in pulps extracted with hot alkali has been observed. For instance, a bleached standard sulphite pulp with 2.5% xylan and 2.4% mannan was found to s t i l l contain 0.5% xylan and 0.4% mannan after hot alkaline extraction (55). 2.2.6 Alpha-cellulose pulps Alpha-cellulose is generally regarded as pure cellulose, but-several analytical investigations of alpha-cellulose preparations have shown that even exhaustive and repeated alkaline extractions leave non-cellulosic - ll carbohydrate f r a c t i o n s i n the pulp. Uprichard (304) published a n a l y t i c a l data on a l p h a - c e l l u l o s e obtained from Monterey pine (Pinus r a d i a t a D. Don) c h l o r i t e h o l o c e l l u l o s e by t r e a t i n g the pulp with 17.5% NaOH according to the usual a l p h a - c e l l u l o s e procedure. S u r p r i s i n g l y high amounts of hemicelluloses remained i n the pulp; such as 8.8% mannose. 2.3% galactose, 1.3% xylose and 0.3% arabinose. Similar observations have been made by Gillham and T i m e l l (87), who report that an a l p h a - c e l l u l o s e pulp prepared from white b i r c h holo-c e l l u l o s e by exhaustive a l k a l i n e e x t r a c t i o n with aqueous KOH s t i l l contained considerable amounts of mannose and xylose and traces of galactose, arabinose and rhamnose. The work of Croon et a_l. (55) i n d i c a t e d that a l p h a - c e l l u l o s e preparations from viscose pulps likewise contain mannan and xylan residues, but to a much lower extent. 2.3 Time Dependent Mechanical Properties of High Polymers, such as Wood and C e l l u l o s i c s Rheology has an ultimate objective of discovering generalized d e s c r i p t i o n s f o r materials and t h e i r behavior which permit p r e d i c t i o n of time dependent mechanical behavior under s p e c i f i e d e x c i t a t i o n patterns. There are, b a s i c a l l y , two main approaches i n studies of materials v i s c o e l a s t i c behavior. One. the phenomenological approach, i s based on studies l i n k i n g time with stress and s t r a i n (load and deformation), u s u a l l y with macro-samples. An attempt i s made to discover general equations covering v a r i a t i o n s of these parameters, so that behavioral patterns, such as loading rate or rate of elongation, can be deduced. This approach, as employed i n the majority of r h e o l o g i c a l studies, i s e s s e n t i a l l y s u p e r f i c i a l . Such studies deal only with the whole body, or with the body i n i t s macro form i r r e s p e c t i v e of the behavioral i n t e r r e l a t i o n s of s t r u c t u r a l components. In other words, gross and complex changes caused by a p p l i c a t i o n of external forces are reported as averaged values. Another approach, known as molecular rheology, attempts to use rheology as a t o o l f o r e l u c i d a t i n g the fundamental structure of a material and p a r t i c i p a t i o n of i t s components i n r h e o l o g i c a l phenomena. Considering the complex molecular and sometimes also complex anatomical structure of many natural polymers, such as c e l l u l o s i c s and ligno-- 23 -e e l l u l o s i c s , i t i s not s u r p r i s i n g that the majority of studies on these materials have been phenomenological. Presently, the subject of polymer v i s c o e l a s t i c i t y i s s t i l l at the stage of development. The phenomenological theory of l i n e a r v i s c o e l a s t i c i t y i s now e s s e n t i a l l y complete. Much att e n t i o n has also been given to e s t a b l i s h i n g new or improving e x i s t i n g theories of non-linear v i s c o e l a s t i c i t y . Many excellent surveys are now a v a i l a b l e (5,18,22,24,42,49,71,92,117). D i f f i c u l t i e s are experienced within development of a complete molecular theory f o r c e l l u l o s i c s . Even with simple polymers, the molecular o r i g i n of some aspects of v i s c o e l a s t i c behavior i s complicated g r e a t l y by molecular weight, temperature, concentration (e.g. ; moisture content) and other f a c t o r s . This becomes even more complicated f o r wood and wood derived materials due to the complex and incompletely understood chemical structure of the lignin-carbohydrate polymer mixture. Indications are, however, that a molecular approach to polymer rheology, c e l l u l o s i c s i n p a r t i c u l a r , may be f e a s i b l e eventually. 2.3.1 Molecular approach to v i s c o e l a s t i c i t y V i s c o e l a s t i c behavior of c e l l u l o s i c materials, such as wood and c e l l u l o s e f i b r e networks, i s the product of many fa c t o r s i n t e r a c t i n g i n a complicated and only p a r t l y understood way. This causes great d i f f i c u l t y i n d e r i v i n g q u a n t i t a t i v e and useful expressions f o r d e s c r i b i n g macroscopic e f f e c t s i n terms of molecular parameters. According to Nis san and Sternstein (214,279), the following f a c t o r s are expected to be important i n determining mechanical properties of c e l l u l o s i c s : ( i ) i n t e r - c h a i n covalent bonds; ( i i ) i o n i c bonds between groups which were formed i n oxidation and other degradation processes; ( i i i ) hydrogen bonds between and within c e l l u l o s e and hemicellulose chains; ( i v ) i n t e r - c h a i n van der Waal's bonds; (v) DP; ( v i ) degree of order and disorder; ( v i i ) o r i e n t a t i o n of molecular chains; ( v i i i ) s i z e of p a r a c r y s t a l l i n e or c r y s t a l l i n e regions; ( i x ) morphological structure; and (x) chemical composition and presence of polar l i q u i d s . The representation of these f a c t o r s i s by no means exhaustive but i t shows well the complexity facing attempts aimed at development of formulations which describe accurately the - 24 -mechanical character and behavior of cellulosic materials. During the last two decades some attempt has been made to give molecular interpretations to cellulosic viscoelastic behavior and to deveio a first set of molecular theories based on known properties of hydrogen bonded materials (48,207-214,220,278,279). , It is presently accepted that cellulose subjected to macroscopic deformation will show the following submicroscopic deformations (202,214, 298-300): (i) valence bond length and angle deformation (intra-molecular); ( i i ) secondary bond deformation; and ( i i i ) reorientation of macro-molecules in amorphous regions (inter-molecular). It can be said, therefore, that while ultrastructure is of significance to ultimate fibre property levels, the inter-molecular and intra-molecular bonds are responsible for viscoelastic properties (259). Particularly important in cellulosic viscoelastic processes are inter-molecular forces which arise from the separation of macromolecules that are joined by covalent links, ionic bonds and van der Waal' forces. It is now advanced by one group that the hydrogen bond system dominates viscoelastic processes in cellulosic materials (210,214,220,278). Since each anhydroglucose unit can form three hydrogen bonds, and, as shown by Marrinan and Mann (183), oven-dry cellulose possesses no free hydroxyl groups, the postulation has been made that viscoelastic phenomena are accompanied by breakdown, rearrangement, bending and stretching of these hydrogen bonds (214). The primary importance of the hydrogen bond on mechanical properties is underlined by a number of investigations undertaken by Nissan and Sternstein (214). They observed that dry regenerated cellulose (with high amorphous ratio) and strong papers show more significant hydrogen bond effects than weak papers. In the latter, van der Waal^s forces dominate, while in crystalline or oriented celluloses, such as cotton, flax and ramie, valence bonds appear to be more important. Based on postulated hydrogen bond behavior under stress or strain Nissan and coworkers (208-211,213,278) developed several quantitative - 25 -theories to explain the r h e o l o g i c a l behavior of c e l l u l o s i c materials at the molecular l e v e l . They were able to demonstrate that stress r e l a x a t i o n as a feature of v i s c o e l a s t i c behavior can be studied by following processes of hydrogen bond adjustment:'. According to t h i s theory, stress d i s s i p a t i o n i n a . c e l l u l o s i c material under constant s t r a i n r e s u l t s from an independent change i n the e f f e c t i v e number of hydrogen bonds. This occurs by rupture and formation of new bonds i n a state of lessened s t r e s s . In s p i t e of the f a c t that t h e i r experimental observations agree with t h e i r p o s t u l a t i o n , Nissan and Sternstein (214) consider t h i s theory to be only a f i r s t approximation to the true time dependent behavior of c e l l u l o s i c s . In contemplation of the v i s c o e l a s t i c behavior of c e l l u l o s i c materials other s i g n i f i c a n t f a c t s have to be considered, such as structure of the m a t e r i a l . Page (220) pointed out that r h e o l o g i c a l properties of fibrous networks, such as paper, are also c o n t r o l l e d by structure at the super-molecular l e v e l . He stated that from knowledge of the exceedingly complex structure of paper i t would seem u n l i k e l y that i t s r h e o l o g i c a l behavior can be explained d i r e c t l y i n terms of molecular data. T h i s , of course, p a r t l y refutes Nissan's theory which trea t s c e l l u l o s i c networks only as a molecular assemblage. Page (220) agrees, however, that hydrogen bonds are of considerable and perhaps primary importance i n governing paper sheet p r o p e r t i e s . More r e c e n t l y , Chow (48) reported that r h e o l o g i c a l processes i n coniferous wood tissues involve a two stage molecular motion of a l l three major wood components; c e l l u l o s e , hemicellulose and l i g n i n . Using i n f r a r e d p o l a r i s a t i o n techniques he showed that carbohydrates and l i g n i n move i n opposite d i r e c t i o n s on r e c e i v i n g external e x c i t a t i o n ^ whereby the wood macro-molecular structure maintains an i n t e r n a l e q u i l i b r i u m . In t h i s way, the stress might be transmitted uniformly through the whole lignin-carbohydrate matrix. It may be theorized that c e l l u l o s i c r h e o l o g i c a l processes involve important conformational changes i n a d d i t i o n to s t r e t c h i n g , breaking and re-formation of hydrogen bonds, s t r e t c h i n g of covalent bonds and bond angle deformation (145), provided that the degree of swelling i s s u f f i c i e n t l y high. In dry cellulosic materials hydrogen bonds are abundant and probably prevent glucose units from rotating about glucosidic links as required to change conformation0 However, with increasing moisture content the glucose units should be capable of such rotations and thereby undergo conformational changes in response to excitation. It is known that inter-molecular hydrogen bonds within amorphous regions are widely destroyed by absorption of water (251). According to Lambert (154), six-member ring compounds prefer the more stable chair conformation. Therefore, hydroxyl groups on C^, C^ and the CH OH group on C can be expected to lie in the equatorial position. 1 The arrangement is known as C^ conformation. External energy applied in form of stress could cause these groups to switch to axial positions which changes the ring shape and thereby other conformational isomers are obtained (145')0 These conformers are characterized by higher energy levels and, consequently, lower stability (154). Under suitable conditions removal of external forces or rebalance of stresses internally may result in reversion to the lower energy level ^C^ conformation. In addition to providing an instant response mechanism, the process could well explain the viscoelastic memory behavior of cellulosic materials (145). 2.3.2 Phenomenological study of viscoelasticity In general, phenomenological treatments of viscoelasticity are characterized by two factors: (i) methods employed for describing observed responses; and (i i ) phenomena of viscoelastic behavior. Since both factors are to some extent related to this study they are briefly discussed in the following section. According to Scott-Blair (260) there are two approaches for dealing with phenomenological data: (i) integrative; and (i i ) analytical. The first is the purer phenomenological method, for i t takes the totality of experimenta data as applied to one simple generalized equation with the objective of describing a material behavior under a l l conditions of deformation. An equation employed in the integrative approach is the Nutting equation (242) as: - £ I -where; f = s t r e s s , e = s t r a i n , t = time, and V, fb and k are constants. The constants merely serve to describe behavior of the material as a whole. In other words, constants do not have any s t r u c t u r a l or t h e o r e t i c a l s i g n i f i c a n c e . The a n a l y t i c a l method also attempts to represent phenomenological data by equations, but i t employs an assemblage of c e r t a i n i d e a l elements based on the very fundamental assumption that the material i s s t r u c t u r a l l y comparable to these elements (5,135,147,162,163,231,263,325). The two elements commonly used are id e a l springs which behave according to Hookean e l a s t i c theory (161) and dashpots containing i d e a l viscous l i q u i d s . It has been found that more or less complex combinations of these simple models can be used to produce equations which approximately describe time dependent behavior of many polymeric materials, including dry c e l l u l o s i c s (255). Any m a t e r i a l , which can be characterised by s u i t a b l e combinations of Hookean springs and Newtonian l i q u i d s , i s said to d i s p l a y l i n e a r v i s c o e l a s t i c behavior (18,24,49). This implies that such materials obey the superposition p r i n c i p l e (263). More comprehensive studies on various types of polymers have shown that numerous polymeric materials d i s p l a y non-linear behavior, and f o r t h i s reason non-linear elements have been used also to describe polymer r h e o l o g i c a l behavior (96,228,239,325). The use of models has been widely discussed i n the general f i e l d of rheology. It i s obvious that they have considerable educative value. Whether or not they provide any other advantages i s much debated, since d e s c r i p t i v e equations obtained by i n t e g r a t i v e methods are more simply derived and are just as e f f e c t i v e i n use. Furthermore, as Ranee (241,242) pointed out, the use of models does not account f o r a l l mechanisms responsible f o r the v i s c o e l a s t i c behavior of c e l l u l o s i c s . Ranee maintains that i r r e v e r s i b l e flow, which leads to permanent set, i s not "viscous" i n nature, but i s the e f f e c t of a continuous ser i e s of i n t e r n a l ruptures which u l t i m a t e l y leads to f i n a l break. He also maintains that such models do not deal with t h i s most important aspect of break nor are they linked with the structure of c e l l u l o s e f i b r e mats. Ranee (242) developed a s i m p l i f i e d s t r u c t u r a l theory which described with s u f f i c i e n t accuracy time dependent behavior as the r e s u l t of - -progressive disruption of an elas t i c network. For the reason of studying and understanding the nature of viscoelastic behavior, polymeric materials are subjected to different types of strain and stress patterns. In general, three types of excitation histories may be employed to study rheological responses to mechanical excitations: ( i ) "creep", which i s time dependent strain (AL(t):L) at constant stress (P:A). The constant stress l e v e l .can be reached i n a step, a ramp or after other excitation programs, ( i i ) "stress relaxation", which i s time dependent stress decay at constant level of strain (ALiL). The constant strain level can be reached i n a step, a ramp or after other excitation programs, ^ i i i ) "dynamic damping", which i s normally observation of the mechanical response to sinusodial excitation i n stress . •or strain. The frequencies, can be varied but during each test i t i s normally kept constant. r Stress relaxation phenomena occur when a constant deformation i s imposed on a viscoelastic material and the force required to maintain this continuing deformation i s measured as a function of time (62,263), This can be expressed by the relationship: ^ ( t ) = G(t) € c [2] where: G?(t) «= stress at time t, G(t) = relaxation modulus, and t£ - instantaneous strain, o Figure 3 represents diagrammatically the curve G(t)£ plotted against time, indicating a time dependent decrease of stress in viscoelastic materials. The relaxation modulus (G(t)) as a monotomically decreasing function of time can be expressed also as: G ( t ) = Ge + G d ( t r - 0 where: G e = equilibrium modulus, and G d(t) = relaxation function having i n i t i a l and f i n a l values of d(o) = 1 and d(oo) =0. at time t B 0 the equation reduces to - 29 -G ( t ) = ( O ^ . D - G o [4] where: G = g l a s s modulus, o ° The best way to d e s c r i b e s t r e s s r e l a x a t i o n b e h a v i o r , as e x h i b i t e d by h i g h polymers, i s by a diagram r e l a t i n g r e l a x a t i o n modulus G ( t ) to the logar i t h m of t i m e , t . As i n the case of c r e e p , polymers under constant s t r a i n behave l i k e i d e a l e l a s t i c substances at very short t i m e s . T h e r e a f t e r , they can be desc r i b e d by an operator equation that r e l a t e s s t r e s s , s t r a i n and time. For l i n e a r s o l i d s the v a r i o u s methods of r e p r e s e n t a t i o n are eq u i v a l e n t to each other and one can be obtained from the other (263). 2.3.3 V i s c o e l a s t i c behavior of c e l l u l o s e f i b r e mats Most published s t u d i e s on c e l l u l o s e f i b r e mat v i s c o e l a s t i c p r o p e r t i e s have been based on a n a l y s i s of creep and s t r e s s r e l a x a t i o n curves obtained from moisture c o n d i t i o n e d papers under constant t e n s i l e load or deformation. U n f o r t u n a t e l y , l i t t l e i s known about the v i s c o e l a s t i c behavior of f i b r e networks i n compression and even l e s s i s known about wet systems under any c o n d i t i o n s . Various types of c e l l u l o s e networks such as paper and pulp mats have s e v e r a l f e a t u r e s i n common, important of which i s the f a c t that a l l are made from f i l t e r e d suspensions of c e l l u l o s i c f i b r e s i n water, subsequently pressed and u s u a l l y d r y i e d which g i v e s coherent s h e e t s . Due to these i d e n t i c a l p r e p a r a t i o n procedures, f i b r e networks may a l s o present c e r t a i n common r h e o l o g i c a l features,, For t h i s reason the f o l l o w i n g d i s c u s s i o n of f i b r e network v i s c o e l a s t i c behavior w i l l i n c l u d e both papers and pulp mats. A g a i n , only s t u d i e s r e l e v a n t to t h i s work w i l l be reviewed. 2.3.3.1 Rheology of paper The f i r s t comprehensive s t u d i e s on time dependent s t r e s s - s t r a i n behavior of c e l l u l o s e sheets (papeis) were c a r r i e d out by Steenberg and coworkers (125,276,277) as e a r l y as 1947. They showed that the load-deformation p r o p e r t i e s of paper depend on numerous f a c t o r s , such as manner of sheet p r e p a r a t i o n , e x t e r n a l t e s t c o n d i t i o n s , r a t e of t e s t i n g , and the previous mechanical h i s t o r y of the specimen. Subsequent i n v e s t i g a t i o n s have centered p a r t l y on mechanisms of paper r h e o l o g y , p a r t i c u l a r l y under t e n s i l e e x c i t a t i o n (33,145,186,219,224,225, J V -227,259,260,307,308). It is now generally agreed that both inter- and intra-fibre mechanisms determine the time, dependent behavior of papers. Other studies have attempted to describe the time dependent 11 response of paper mathematically. Thus, Andersson and Sjoberg (7) studied short time relaxation after rapid stretching at constant rates between 1% in 0.01 sec and 1% in 5 sec. They found that rate of stress decay is highly dependent on the i n i t i a l strain rate, i.e., high straining rate is followed by high stress decay rate, and vice versa . By plotting fractional stress relaxationC ' U ) / ^ (O ) against the logarithum of time sigmoid shaped curves were obtained, which indicated a Maxwellian delayed elastic type of relaxation. In his earlier publications on paper relaxation, Kubat (151,152) presented results of stress relaxation measurements following tensile straining at low rates (1% in 100 sec). He was able to demonstrate a linear relationship between residual stress and log t. Nevertheless, he pointed out that this relationship must be regarded only as a temporary approximation. Such curves must, according to this theory, ultimately conform to an ideal Maxwellian trend at long periods of stress dissipation. In other words, the plot o f C a g a i n s t log t becomes asymptotic to a line parallel to the time axis. In paper stress relaxation following constant rates of elongation (1% in 23 sec to 1% i n 10 sec) ya straight line relationship was also found between stress (£>) and log t for the test period of 4 hr (242). The relationship was expressed by the following equation: £ = x - y log t c....[5j where: x and y are constants which depend on preloading history. In a recent study on tensile stress relaxation of paper, Johanson and Kubat (137) determined the effect of strain rate, i n i t i a l stress, moisture content and beating on the viscoelastic behavior of paper under constant strain. They found that the following equation related inflexion slope of stress - log t curves and total dissipated stress Gi^ =^(t 0) — ^ (t^)) applied also to paper: D O - 31 -Furthermore, results of their study indicated that activation energy for the stress relaxation process is stress dependent. The viscoelastic response of air-dry newsprint subjected to constant load and deformation in compression was investigated by Gavelin (85). He observed increased viscoelasticity at higher moisture content and he also found very good correlation between stock freeness and the irreversible compressibility. This showed that beating makes paper less compressible. He also indicated that recovery response (which is the instantaneous and time dependent "spring back" following removal of compressive deformation) is related to the quality of the wood used in making the pulp. Other studies on behavior of air-dry paper under compression were II carried out by Brecht and Schadler (31,32). They observed that increased basis weight reduced relative compression and increased relative expansion. Furthermore, addition of a small percentage of groundwood to a basic Scotch pine sulphite stock caused a substantial increase in relative 2 compression. Creep was found to be unimportant at low loads (20 kp/cm ), 2 but occurred at higher loads (200 kp/cm ). II In a more recent study, Jackson and Ekstrom (127) investigated compressibility characteristics of air-dry sheets prepared from various pulp types. They reported that the recovery response following compression treatments was higher for unbleached than bleached pine sulphate pulp. Experiments with unbleached sulphate and bleached sulphite European birch pulps indicated higher recovery response for the first pulp type. In general, it appeared that sulphate pulps show a higher degree of recovery response than sulphite pulps and unbleached pulps exhibit higher recovery than II bleached pulps. Jackson and Ekstrom observed also a "species effect". Coniferous pulps were found to compress better than hardwood pulps, but the latter group showed higher relaxation responses than the f i r s t . Mardon et a l . (180,181) observed in their investigations on the dynamic and static compressibility of"papers made from various pulp types that compressibility is greatly influenced by moisture content (below the fibre saturation point), caliper, bulk, void volume and.previous stress - 32 -treatments. Degree of deformation was found to increase with increasing values of the first four variables and to decrease towards a constant value with increasing previous peak pressure. Their observation with respect to bulk effects, however, is not in agreement with the findings of Brecht and • II Schadler (31,32), who found that mat response is indirectly proportional to bulk. Similar studies were carried out recently by Bliesner (25), who observed the viscoelastic behavior of two air-dry coniferous kraft pulps and one groundwood pulp under dynamic excitation in compression. It was found 3 that the effect of sheet bulk (cm /g) on compressibility differed widely between these two pulp types. The permanent set (non-recoverable deformation) increased with increase in bulk for kraft pulps but remained constant for the groundwood pulp. Both pulp types showed an increase in relative compression, but responded differently in relative recovery. Increase in bulk improved relative recovery in groundwood, but resulted in lower recovery in kraft pulps. Repeated stressing up to three cycles was found to change the compressive response of pulps, but thereafter no significant changes were noted. 2,3.3.2 Rheology of wet fibre mats in compression According to Brecht and Erfurt (29) and Lyne and Gallay (172), a wet cellulose fibre network consists of fibres arranged in a manner similar to that in dry networks. But considerable differences arise from: (i) distance between the fibres; ( i i ) the degrees of fibre plasticity and elasticity; and ( i i i ) degree of fibre association. Interfibre bonding, a controlling factor for mechanical properties in dry and partly dry fibre webs, does not play a significant role in the mechanical behavior of wet cellulose networks, i.e., below 20% solids. Therefore, rheological behavior of wet pulps is considered only as a function of wood fibre viscoelastic properties and characteristics of the network structure formed as result of fibre-to-fibre contacts. The rheology of wet fibre mats was first studied by Seborg et al_. (262). They compressed cylindrical small pulp sheets in wet condition at 2 a maximum pressure of 8.5 kg/cm » After no further deformation was noted they - 33 -removed the load and observed recovery. It was found that recovery figures were affected by mat caliper, where thicker mats gave less recovery for a given load. Experimental data indicated also a difference in recovery response between unbeaten and beaten pulps, with recovery higher for pulps without beating treatments. In a later publication on the same subject Seborg et a_l„ (261), presented data which indicated that sulphate pulps show higher recovery than comparable sulphite pulps and that groundwood exhibited less recovery than the chemical pulps. A comprehensive study on the effect of moisture content, degree of beating and caliper of pulp mats on their recovery response as derived from compressive deformation was carried out by Ivarsson (124). He observed that absolute compression and permanent set increased with increasing moisture content up to the fibre saturation point, whereas the recovery response decreased with moisture. Further, increasing moisture contents changed insignificantly the mechanical behavior of pulp mats under compression. I^varsson's observations on effects of beating and caliper are in general agreement with those made by Seborg et al_. (262). The relative permanent set was found to decrease slowly with increasing caliper. In addition, I^varsson examined the effect of caliper on relative compression. His data showed that more relative compression is obtained for thinner sheets. Other studies (122,138) reported that beating over wide ranges did not change the nature of wet pulp compressibility for bleached pulps, but they did observe some change for unbleached pulps. Several workers have developed mathematical expressions describing the behavior of wet fibre networks under compressive stress. Campbell (43) was the first to use a mathematical expression which he derived empirically, to describe the relationship between mat consistency (C) and stress (p): C - MpN [7] where: N and M are constants which depend on nature of the pulp. Ingmanson and Whitney (123), studied the filtration resistance of pulp slurries and pointed out the need to include a small but finite constant (C ) in the r o equation to satisfy conditions required for establishing finite consistency - 34 -at zero pressure: C = C + M pN c [8] o Both equations, however, have to be considered as first approximations for describing the stress-pad concentration relationship, since time, known as a highly important variable in mat deformation processes, has been ignored. Wilder (322) studied wet fibre mat creep and creep recovery over long periods and for successive cycles and modified Ingmanson and Whitney's equation by replacing M with the new term (M = A + B log t) which includes time dependent effects. Thereby, the stress - mat consistency relationship became: C = C q + (A + B log t) pN •'••H where: A, B = constants, and t = time. However, as Wilder (322) pointed out, the equation does not apply to extreme-ly short time intervals during which filtration resistance controls the mat response. His experimental data also indicated that the equation constants change for each compression and relaxation phase during the first 5 to 6 cycles. Thereafter, the mat becomes conditioned and the constants remain unchanged, one set for compression and another for recovery. As mentioned earlier, Wilder found his equation insufficient to describe the early phase of mat compression (up to 5 sec). He attributed this deviation to mat permeability and fractional drag of the water being pressed out. Jones (138) carried out compressibility studies on various types of wet fibre mats, and examined effects of fibre length and diameter and elastic modulus on compression recovery response. The purpose of this investigation was to obtain more information on fibre interaction mechanisms, such as fibre bending, fibre slippage and compression of fibres in overall mat deformation during rheological processes. The compression-recovery was found to be highly dependent on fibre length-to-diameter ratio and modulus of elasticity. Fibre diameter alone, however, did not affect significantly the recovery response. Furthermore, fibre repositioning appeared to be very - 35 -important in early compression cycles, but fibre bending was the controlling factor in compression of mechanically conditioned mats. In a comprehensive study on specific permeability and compressi-bility of mats prepared with synthetic and natural pulp fibres, Higgins and De Yong (114) investigated the relationship between mat solids concentration and applied stress. Their experimental data were obtained from two series of coniferous sulphate and pored wood NSSC pulps, with each series covering a wide range of lignin contents. They found that compressibility and consolidation of wet mats was highly influenced by flexibility and lateral conformation of the. wet fibres. These two factors might not only contribute to capacity for elastic deformation but also to the extent to which irreversible processes may occur. Similar investigations were carried out by Kayama and Higgins (143), who examined the'effect of bleaching on wet web compressibility of coniferous kraft pulps. They observed that pulp sedimentation volume decreased as lignin was removed, and as bleaching proceeded the compressibility decreased. This behavior has been attributed also to changes in flexibility and lateral conformability with lignin removal. 2.4 Compressibility of Cellulosic Fibre Mats The behavior of cellulosic fibre mats under compressive deformation can be considered as the combined response of rheological and non-rheological mechanisms. They are probably highly interrelated, particularly in early stages of mat deforming processes. It is the purpose of this section to review information on fibre mat compressibility as related to this work. The behavior of cellulosic networks under compressive stress or strain is. a function of numerous factors interacting in complex and not fully understood ways. In spite of several investigations (100,138) on the subject during the last 15 years, much research is s t i l l required for more complete elucidation of mechanisms and factors determining response of wet cellulosic mats subjected to compressive excitations. It is rather surprising that so l i t t l e emphasis has been placed on the subject, since industrial manufacturing processes frequently involve compression treatments of wet and dry fibre mats. - 36 -Fortunately, the knowledge and theories of air-dry paper response to tensile excitations in particular are rather advanced and, therefore, can offer some help for interpretating observations made on wet fibre mats. The following discussion also includes observations and theories of paper rheology so far as they are relevant to this study. For a comprehensive review it is convenient to subdivide the discussion into two sections; one dealing with the mechanisms which are involved in processes.of compressive deformation, and the other with factors determining the magnitude of instantaneous and time-dependent deformation of cellulosic networks. 2.4.1 Mechanisms in compression of wet cellulosic fibre mats A wet cellulose fibre mat can be defined as a fibre network comprised of elements1 mostly lying loosely over each other and forming many sites of contact and potential contact. The fibres are limited to an approximately planar orientation, where within plane their position and orientation have a degree of randomness arising from the essentially dis-ordered state of suspension from which mats are formed. In the majority of cases, however, the fibres lie in-plane and consequently the regions of contact also lie in the plane of the mat. Complete randomness may be adjusted somewhat by formation of mats on production wires. When a wet mat is subjected to compressive deformation three compressibility mechanisms can be expected to occur during deformation processes (100,138): (i) fibre bending; ( i i ) fibre slippage; and ( i i i ) fibre compression at regions of fibre-to-fibre contact. In the following these potential mechanisms are discussed separately in spite of the fact that they probably interact during compaction. 2.4.1.1 Fibre bending According to Han (100), application of external stresses to a mat will be followed by stress transmittance through fibre-to-fibre contacts and stress distribution along fibres. This is accompanied by bending of fibre segments between two points of support which causes formation of additional contact areas between neighboring fibres and thus increases mat - 37 -density. If cellulosic fibres were entirely elastic, deformation within the fibre would be expected to be entirely recoverable. Due to the visco-elastic nature of cellulosic fibres, however, both recoverable and non-recoverable bending deformations, as well as time dependent bending deforma-tions, take place (138). Since a l l three mechanisms cause permanent deformation in a very complex interaction,no exact information has been obtained as to what extent fibre bending contributes to the total mat response. Elias (63) studied the compression response of mats prepared from various fibre types and was able to show for glass fibres that bending is the dominant mechanism during compaction. Increasing load was found to increase the glass fibre inflection points. A comparison between glass and synthetic fibres indicated that more flexible fibres, such as dacron filaments, give more contacts during bending. Elias (63) also observed that the number of contacts per unit length increased with fibre length for a given load. -Behavior of cellulose fibres as found in wet pulp mats will differ somewhat from Elias' (63) observation on mats formed by non-cellulosic fibres. Bending of pulp fibres in a wet mat is expected to be accompanied by stretching and shearing processes (140), which may be particularly inten-sive for swollen fibres. 2.4.1.2 Fibre repositioning It is well known that the majority of fibres in a fibre bed obtained from suspensions lie flat in the horizontal plane. There are always a number of fibres, however, which are oriented at appreciable angles with the z-axis (perpendicular to mat direction). Elias (63) observed on non-cellulosic networks that a mat consisting of short fibres contains a higher number of individual fibres out of plane than a mat formed by long-fibre material. During compaction fibres out of plane were found to become nearly horizontal, which implies that those fibres change position relative to their neighbours. This phenomenon is known as slippage or fibre repositioning (63). It might contribute considerably to the non-recoverable deformation of wet cellulose fibre mats, since the forces needed to restore a wet pulp fibre - 38 -to its original position are rather low. Resident restoring forces in the fibre might provide at least partial recovery upon relief of the deformation stress, thereby contributing a component of recoverable fibre slippage. 2.4.1.3 Compression at points of contact Compressive excitations applied to cellulose networks are known to cause stress concentrations at contact areas which induce contacting fibres to conform. The conformation process is essentially characterized by changes in fibre cross-sectional shape, enlargement of contact areas, and increased fibre flattening (100). These phenomena, of course, are accompanied by deswelling processes when the fibre material is saturated with a swelling fluid such as water (100). 2.4.2 Factors determining compression characteristics of cellulose fibre mats The behavior of cellulose fibre mats under compressive stress or strain has to be regarded as the result of numerous factors, most closely interrelated and, therefore, difficult to estimate. It seems reasonable, however, to distinguish between three categories of factors: (i) fibre morphology; ( i i ) fibre properties; and ( i i i ) structure of fibre mats. 2.4.2.1 Fibre morphology There are three fibre characteristics which have been found, or are expected, to influence the response of cellulose fibre mats to compressive excitations; (i) fibre length; (i i ) fibre length/diameter ratio; and ( i i i ) cell wall thickness. The average dimensions of typical pored and coniferous wood fibres differ widely, wherein the latter show the greater dimensions (91).. It is also known that latewood fibres are somewhat longer than those of associated earlywood (40,320). Other differences in fibre dimensions arise from machining processes in pulping. In particular, mechanical pulping produces fibre materials characterised by considerable dimensional variations between fibre fragments. Groundwood pulps are known to contain components as: (i) fibre bundles; ( i i ) separate fibrillated fibres; ( i i i ) broken fibres; and - 39 -(iv) fine wood flour (16). In contrast, chemical pulps consist almost entirely of single fibre skeletons often mechanically damaged in varying amounts during processing. Due to the absence of interfibre hydrogen bonding in wet cellulose fibre mats, fibre length has to be considered important to mat response under compressive stress and strain. It is well known that long fibres cause a high degree of mechanical entanglement (56), which strengthens both air-dry and wet cellulosic networks. Brecht and Erfurt (29) investigated the wet-web strength of mechanical and chemical pulps and pointed out that fibre length, in addition to consideration of flexibility and surface area, has to be considered as the dominant factor in determining wet-web mechanical properties. This concept has been confirmed by experiments of Forgacs e_t al_. (73) with mats formed at low fibre consistencies, such as 0.8%. Further evidence for this can be derived from several studies on paper, which have shown, for instance, that tearing strength is highly influenced by fibre length (28,50). Fibre length can also influence the general fibre mat structure. Watson e_t aJU (3 J. 9) were able to demonstrate this effect on sulphate pulps prepared from Monterey pine fibres of 1.7 mm and 3.0 mm length, but similar in morphological and chemical properties. It was found that the longer fibres gave sheets with much more open structure; in other words, longer fibres produced sheets of higher bulk. Fibre length/diameter ratio was found to have at least some bearing on mat response to mechanical excitation. Hentschel (111), working with synthetic fibres, showed that this ratio influences the strength properties of fibrous networks. Jones (138) also reported a "ratio effect". By studying this effect on synthetic fibre and pulp mats under compression excitation he observed that an increase in length/diameter ratio up to a certain value enhanced the compression recovery response of fibre mats. Cn the other hand, Dadswell and Watson (56) provide evidence that this ratio has very l i t t l e or no influence on mechanical properties of air-dry fibre networks (papers). - 40 -The importance of c e l l wall thickness and degree of f i b r e collapse to paper properties has been recognised f o r many years (65,118,132,150,225, 250,286,318). Most di f f e r e n c e s with coniferous pulps a r i s e from v a r i a t i o n s i n c e l l wall thickness between,earlywood and latewood tracheids. Thin c e l l w a lls, as those i n earlywood, collapse more r e a d i l y and produce a dense and hig h l y packed network; whereas thick-walled c e l l s , such as those i n latewood, co l l a p s e less and give open and bulky networks. The e f f e c t of f i b r e collapse appears to be two-fold: ( i ) increased f i b r e f l a t t e n i n g provides larger contact areas between f i b r e s ; and ( i i ) f i b r e c o l l a p s e e n t a i l s greater f i b r e f l e x i b i l i t y or conformability (248). 2.4.2.2 Fibre properties Variations i n chemical and physical c h a r a c t e r i s t i c s of pulps a r i s i n g during pulping, bleaching and subsequent treatments are expected to influence pulp mat response to compressive e x c i t a t i o n . These v a r i a t i o n s are manifested i n f i b r e c o l l a p s i b i l i t y (influenced by r e s i d u a l l i g n i n ) , f i b r e f l e x i b i l i t y (affected by s t r u c t u r a l c h a r a c t e r i s t i c s of r e s i d u a l hemicelluloses) and f i b r e strength (related to c e l l u l o s e DP). Due to the hydrophobic nature of l i g n i n , as well as i t s probable linkage with hemicelluloses (23,35), the whole l i g n i n - h e m i c e l l u l o s e complex i s made p a r t l y hydrophobic and i s restrained from excessive swelling (86). Moreover, the three-dimensional l i g n i n structure i s known to l i m i t the degree of f i b r e collapse and f l e x i b i l i t y . For instance, high l i g n i n content influences f i b r e properties i n two ways as: ( i ) preventing hemicelluloses from excessive swelling and p l a s t i c i s a t i o n ; and ( i i ) g i v i n g f i b r e s a high degree of s t i f f n e s s which impedes c o l l a p s i b i l i t y (40,86,105,133,184,221,306). Other comprehensive i n v e s t i g a t i o n s on f i b r e properties and co m p r e s s i b i l i t y of f i b r e mats have been published r e c e n t l y (105,221,223). Page and coworkers (221,223) studied paper structure and coll a p s e behavior of pulp f i b r e s , and observed that the percentage of collapsed f i b r e s a f t e r drying increased inversely to the pulp y i e l d and that s u l p h i t e f i b r e s collapsed to a greater extent than sulphate f i b r e s . S i m i l a r observations were made l a t e r by H a r t l e r and Nyren (105), who investigated the transverse - 41 -c o m p r e s s i b i l i t y of pulp f i b r e s . The lower c o l l a p s i b i l i t y of k r a f t f i b r e s was a t t r i b u t e d to more frequent c r o s s l i n k i n g i n those f i b r e parts having high l i g n i n and hemicellulose contents. As a consequence of the hydro-phobic and r i g i d character of l i g n i n , f i b r e s possessing high l i g n i n contents exhibited low conformability and therefore produced bulky f i b r e mats (248). This e f f e c t has been thoroughly investigated by Kayma and Higgins (143), who were able to show that progressive d e l i g n i f i c a t i o n of sulphate pulps produces lower sedimentation volume and reduces mat c o m p r e s s i b i l i t y . The numerous short side chains, c h a r a c t e r i s t i c of many hemicelluloses, are responsible f o r the amorphous state of native h e m i c e l l u l o s i c materials. Therefore, water penetrates more e a s i l y between hemicellulose chains and a f t e r d e l i g n i f i c a t i o n causes considerable swelling. This leads to formation of gels i n s i d e f i b r e walls, such as i n i n t e r - f i b r i l l a r spaces and on f i b r i l surfaces (45). In a d d i t i o n , DP and branching are considerably reduced i n chemical pulping and bleaching treatments (174,217,233). These changes are known to have a pronounced e f f e c t on mechanical properties of papers made from chemical pulps (95,201,233,282). In wet f i b r e s the hemicelluloses enhance g r e a t l y f i b r e l a t e r a l swelling by i n t e r - f i b r i l l a r swelling processes and under stress conditions they are thought to act as a kind of " i n t e r n a l l u b r i c a n t " improving f i b r e f l e x i b i l i t y . S i m i l a r l y , hemicelluloses positioned on f i b r e surfaces are considered to function as an external lubricant promoting f i b r e slippage and r e p o s i t i o n i n g within mats (86). The e f f e c t of f i b r e strength on sheet mechanical properties i s p a r t i c u l a r l y important f o r pulps produced by chemical treatments. In chemical pulping, as well as subsequent d e l i g n i f i c a t i o n and carbohydrate removal processes, a l l c e l l u l o s e molecules are degraded to some extent. This e n t a i l s weakening of the f i b r e u l t r a - s t r u c t u r e and thereby reduction i n whole f i b r e strength. Many studies on paper have shown that sulphate and sulphite pulping produces f i b r e materials which d i f f e r widely i n strength properties (11,84,104,144,233), whereby sulphate pulp f i b r e s u s u a l l y are of superior I I strength. It has been suggested by Jayme and von Koppen (134), f o r instance, that the cause f o r lower strength observed with s u l p h i t e pulp compared with sulphate pulp l i e s i n weaker i n t e r f i b r e bonding i n the s u l p h i t e sheet. This r e s u l t s i n turn from lower carbohydrate DP, i n p a r t i c u l a r of the c e l l u l o s e , - 42 -at the f i b r e surface. Differences in'carbohydrate degradation at the outer parts of f i b r e s may also explain i n part the observation made by Seborg and Simmonds (261) i n compression of wet pulp mats, wherein sulphate pulps showed higher recovery f i g u r e s than sulphite pulps. Another i n v e s t i g a t i o n g i v i n g some i n d i c a t i o n that c e l l u l o s e degradation and consequently weaker f i b r e s e f f e c t f i b r e mat c o m p r e s s i b i l i t y was c a r r i e d out by Petterson and Rydholm (233). They found that overcooked and overbleached pulps form denser papers than would be expected from opacity measurements. 2.4.2.3 Structure of f i b r e mats The structure of f i b r e mats i s determined by d i s t r i b u t i o n and o r i e n t a t i o n of f i b r e s , which i n turn depends on processes of mat formation (8,175). According to f i b r e arrangement the mat structure can be character-ized by: ( i ) the degree of separation of the f i b r e s across the mat thickness; ( i i ) o r i e n t a t i o n ' o f f i b r e s i n the three-dimensional network; ( i i i ) degree of f l o c c u l a t i o n ; ( i v ) extent of c o i l i n g or wrinkling of i n d i v i d u a l f i b r e s ; and (v) d i s t r i b u t i o n of f i n e material (84,175,306). Evidence e x i s t s , i n p a r t i c u l a r from studies on paper, that these mat c h a r a c t e r i s t i c s can have an important bearing on mechanical properties of f i b r e networks (8,15,84). In f i b r e mat compression f a c t o r s ( i ) and ( i i ) are expected to play major r o l e s , as they probably determine the extent to which the three mechanisms are involved during the compaction process. This agrees with Han's statement (100) that resistance of wet f i b r e mats to compression increases with increasing i n i t i a l s o l i d f r a c t i o n . This r e s u l t s i n a lower f i n a l s o l i d f r a c t i o n at a given compression. 3„0 MATERIALS AND METHODS 3<>1 Pulp Samples Pulps were chosen to broadly represent.the wood pulp spectrum, thus providing a wide qu a n t i t i v e range f o r r e s i d u a l carbohydrates and l i g n i n s and hopefully large d i f f e r e n c e i n ph y s i c a l and r h e o l o g i c a l responses. Consequently, 24 commercial and eight laboratory pulps were used i n the study. The sample c o l l e c t i o n included four groundwood pulps, four " h o l o c e l l u l o s e " pulps, four k r a f t pulps, one sulphite pulp, f i f t e e n d i s s o l v i n g grade pulps and four a l p h a - c e l l u l o s e preparations. Other sources of v a r i a t i o n were introduced to the study by in c l u d i n g both bleached and unbleached pulps and by choosing pulps prepared from both coniferous and pored woods. The various pulps employed and some of t h e i r character-i s t i c s are l i s t e d i n Table 2. The eight laboratory pulps included i n the study were holo-c e l l u l o s e and a l p h a - c e l l u l o s e preparations of wood f i b r e skeletons as obtained by standard methods. 3.1.1 H o l o c e l l u l o s e pulps The wood materials used f o r preparing h o l o c e l l u l o s e pulps were taken from one mature stem each of western hemlock (Tsuga heterophil l a (Raf.)Sarg.) and western cottonwood (Populus t r i c h o c a r p a Torr & Gray). Special care was exercised i n d i s s e c t i n g wood blocks into small s i z e pieces of approximately 0.3 x 1.0 x 4.0 cm i n order to minimize mechanical damage to f i b r e s . A f t e r a i r - d r y i n g f o r three days the chips were exhaustive-l y extracted with 2:1 ethanol-benzene. The a i r - d r y extracted chips were divided into two p a r t s , one part was used f o r pulping i n peracetic acid and the other i n a c i d i f i e d sodium c h l o r i t e s o l u t i o n . Peracetic acid cooking accompanied by a l t e r n a t e sodium borohydride reduction stages, was c a r r i e d out according to the method proposed by Leopold (164). Due to rather large chip dimensions, a t o t a l of s i x cycles was required f o r complete d e f i b e r i s a t i o n . Chemical analyses of these pulp preparations indicated a higher loss i n n o n - c e l l u l o s i c carbohydrates (Table - 44 -3g)than reported in the literature (295). This discrepancy can be attributed to overcooking of the outer portion of the chips which obviously caused serious degradation and subsequent dissolution of hemicelluloses. Evidence for this occurred also in work of Shimada and Kondo (264), who observed a decrease in pulp yield with increasing chip thickness. Chlorite holocellulose pulps were obtained by delignifying wood chips in a solution of sodium chlorite and acetic acid at 70° C. Details of the method are reported elsewhere (323). Approximately eight hours chloriting time was required to produce a completely defibrised material. Similar to peratic acid pulps, the chlorite holocellulose pulps contained less hemicelluloses than expected based on values published in the literature. This can be explained by overcooking effects in outer portions of the relatively large chips. An investigation by Eriksson (67) on several factors influencing quality and quantity of chlorite holocelluloses supports this assumption. He observed also that increasing wood particle size and treat-ment time lowered holocellulose yields. In the present study, both factors exceeded considerably the values proposed by Wise et a l . (323). The freshly prepared holocelluloses were air-dried as small flakes for two days at room temperature in order to eliminate or minimize differences in mechanical properties between commercial and laboratory pulps due to moisture history, which has been found to affect pulp mat properties (29). This procedure was necessary as commercial pulps were supplied in air-dry condition following unknown drying histories. 3.1.2 Alpha-celluloses One alpha-cellulose pulp was prepared from each of a commercial viscose and acetate pulp and the two chlorite holocellulose preparations as obtained from western hemlock and western cottonwood. The alkaline extraction with 17.5% NaOH at 20°C was carried out in accordance with TAPPI T203 os-61 (289). Alpha-cellulose residues were teased into small flakes and air-dried before further processing for sample sheets preparation. Since part of a preliminary study on viscose pulps is included in this work and specimens used in the earlier investigation were cut from - 45 -commercial pulp sheets, i t was decided to prepare other fibre mats as close as possible in basis weight and thickness to those supplied commercially. Mats were prepared essentially according to TAPPI Standard T205m-58 (287). Following weighing, disintegration, and sheet formation in the standard sheet machine, 10 wet handsheets of each pulp type were prepared. Prior to pressing these 10 standard wet sheets (each weighing 1.2 - 0.1 g oven-dry basis) were carefully laid over each other. In this way a wet pulp mat was obtained which, after wet-pressing and air-drying, yielded a sample sheet of desired basis weight and thickness. Both pressing stages were performed at 80 psi for time periods according to TAPPI Standard T205m-58. Conditioning of sample sheets was done between drying rings at controlled temperature - humidity conditions (22 C and 50 to 52% R.H.) for at least three days before further processing. The preparation of sample mats by means of assembling handsheets was found by preliminary experiments to produce more uniform structures than when thicker sheets were prepared from higher consistency stock, which gave high flocculation. Thus, serious density variations across mats were avoided with the intention of minimizing variability due to this cause. Test specimens of viscose pulps (1-1 to 4-2) were prepared from commercial sheets, while specimens representing other pulp types were obtained from laboratory made mats as described. Actual test specimens were punched randomly from sample mats by using a carefully honed mechanical cork borer of 1.45 cm diameter. Specimens were either steeped in distilled water or aqueous sodium hydroxide for varying times, or were subjected to gamma-radiation before further treatment. Thereafter, the stress relaxation was performed by methods to be described. The specimens used for studying stress decay on pulps in wet condition were steeped approximately "one minute in distilled water. This was found to be the minimum time required for assuring complete swelling. - 46 -Those specimens exposed to caustic steeping were kept for 25 * 2 sec in 18.6% NaOH, a concentration used in commercial steeping. Both steeping treatments were performed at a temperature of 22°C. Immediately after steeping, the pulp specimens were examined in stress relaxation. The specimens were kept in corresponding solutions during testing. In one experiment, which dealt with the effect of steeping time on stress relaxation, the caustic steeping period was varied from 0.1 to 14400 minutes (more than five time cycles of 10). Sample irradiation treatments were done in a Gamma-Cell 220 at ambient radiation chamber temperature of 34°C. Air-dry pulp samples were exposed in an a i r atmosphere from 0.5 to 6 Mrad dosages at a dose rate of 0.829 Mrad/hr. 3.2 Physical Testing A l l stress relaxation measurements were performed with a Floor Model TT Type C Instron testing machine. The specimens were placed between a 1.0 mm thick microslide glass and a 5.0 mm thick glass plate in order to protect the Instron load heads from corrosion in tests involving caustic steeped specimens. The glass assembly was found not to relax in any measurable way under conditions used in this experiment. A typical test assembly is shown in Fig. 4. Five replications were done for each pulp treatment combination. Since i t is not possible to determine ultimate strength values in pulp compression due to the collapsed state of the fibrous material in the mat, 2 an arbitrary load of 3,5 kg/2.1 cm was applied in a l l tests. It can be assumed that a stress of this magnitude does not cause complete compression of the mat, although i n i t i a l caliper was reduced 60 to. 80%. Strain (&L:L) was not used, since each pulp showed characteristic swelling. The desired stress was applied in a l l tests at the rapid loading speed of 2 cm/min for water treated and 5 cm/min for a l k a l i steeped specimens. These were the highest speeds possible for accurate setting of stress levels with the test engine used. Consequently, the loading process was completed in approximately 1 to 1.5 sec, which is the limit of approximation to step-function excitation of the present study. - 47 -Stress r e l a x a t i o n was observed over a time range of 35 min, except f o r experiments dealing with the e f f e c t of steeping time where r e l a x a t i o n tests were stopped a f t e r 100 min. Two recorders were employed f o r observing stress decay; an X-Y-recorder (Mosely Autograph Model 7000-A) set at chart speed of 2 in/sec was used f o r recording e a r l y stages of stress decay (0 to 15 sec), and the Instron recorder was run at a chart speed of 1 in/min f o r t r a c i n g stress decay from 15 sec onward. Both recorders were fed the same Instron s i g n a l . Data obtained from chart traces were transformed i n t o f r a c t i o n a l s t r e s s r e l a x a t i o n values according to: £(t)/£(o), where: cL, . i s the i n i t i a l stress at time t , i . e . , when the loading process Xo) o was completed, such as 1.0 to 1.5 sec a f t e r e x c i t a t i o n was commenced. This d i f f e r s from much of the e a r l i e r l i t e r a t u r e where, f o r mathematical convenience, t has been taken as 1 min following completion of loading, and t h i s often f o l l o w i n g extensive but seldom described ramp-loading times. 3.3 Determination of Pulp Constituents In order to obtain information on the c o n t r i b u t i o n of various wood and pulp constituents to pulp v i s c o e l a s t i c i t y , several a n a l y t i c a l methods were employed i n c l u d i n g : ( i ) determination of the carbohydrate components; ( i i ) estimation of r e s i d u a l l i g n i n ; and ( i i i ) other measurements. 3.3.1 Carbohydrates Several q u a n t i t a t i v e gas chromatographic methods f o r analysis of wood and pulp carbohydrate compositions have been developed r e c e n t l y . These have proven to be less time consuming and easier to operate than e a r l i e r t r a d i t i o n a l paper chromatographic procedures. According to these methods, monosaccharides in n e u t r a l i s e d hydrolysates o r i g i n a t i n g from c e l l u l o s i c materials can be analysed a f t e r conversion to t r i m e t h y l s i l y l d e r i v a t i v e s . (21,302), acetylated a l d o n i t r i l e s (61) or a l d i t o l acetates (26,94,267,269). T r i m e t h y l s i l y l d e r i v a t i v e s are qu i c k l y prepared but, due to the - 40 -m u l t i p l i c i t y of peaks a r i s i n g from each aldose by anomeric and d i f f e r e n t r i n g - f o r m s , peak i n t e r p r e t a t i o n s are d i f f i c u l t to make. For t h i s r eason, the a l d i t o l a c e t a t e methods which g i v e s o n l y a s i n g l e peak f o r . e a c h monosaccharide seems to be more accurate f o r q u a n t i t a t i v e d e t e r m i n a t i o n of wood and pulp p o l y s a c h a r i d e m i x t u r e s . I t was decided to apply the method developed by Borchardt and P i p e r ( 2 6 ) , which, compared w i t h other a l d i t o l •acetate procedures, i s l e s s time consuming and i s known t o separate a l l known wood su g a r s . A 300 me sample (oven-dry b a s i s ) was taken from each of the 32 pulp mats used f o r r h e o l o g i c a l t e s t i n g . Thus, each pulp type i n v e s t i g a t e d i n the study was represented by one composite sample. A f t e r shredding and weighing, the pulps were hydrolysed f o l l o w i n g the method of Saeman et^ a l . (253). E x a c t l y O.lOO g of m y o - i n o s i t o l as i n t e r n a l standard was added to the aqueous h y d r o l y s a t e before i t was n e u t r a l i s e d and f u r t h e r processed according to the procedure o u t l i n e d by Borchardt and P i p e r ( 2 6 ) . In the f i n a l step of t h i s procedure the a l d i t o l and m y o - i n o s i t o l acetates were d i s s o l v e d i n methylene c h l o r i d e and stored i n t i g h t l y sealed v i a l s , before i n j e c t i o n i n t o the gas chromatograph. Three i n j e c t i o n s of 0.7 p i each were done f o r a n a l y s i s of each sample. Separations were c a r r i e d out w i t h a i d of a MicroTek 150 gas chromatograph equipped w i t h a flame i o n i s a t i o n d e t e c t o r and a Moseley 7100 B s t r i p c h a r t r e c o r d e r . A 6 f t x 1/8-In g l a s s ( a n a l y t i c a l ) column packed w i t h 3% ECNSS - M on Gas Chrom Q 80-100 mesh was i n s t a l l e d f o r on column i n j e c t i o n . Helium was used as c a r r i e r gas w i t h a f l o w r a t e of 33 ml/min. A l l o p e r a t i o n s were is o t h e r m a l w i t h the column oven at 195°C, the i n j e c t i o n port at 210°C, and the d e t e c t o r at 240°C. The a l d i t o l and m y o - i n o s i t o l acetates were e l u t e d i n approximately 40 min w i t h s a t i s f a c t o r y r e s o l u t i o n of each component as can be seen i n F i g s . 5 to 13. The r e l a t i v e amounts of i n d i v i d u a l components were c a l c u l a t e d a c c o r d i n g to ( 2 6 ) : - t y -% Polysaccharide = C x I x F x 100  R x S x H x. k where: C = chromatographic area of the component peak, R = chromatographic area of the myo-inositol peak, I = myo-inositol weight, S = oven-dry sample weight, F = factor converting monosaccharide weight to polysaccharide (0.88 pentose; 0.90 hexose), H = hydrolysis survival factor, and k = calibration factor for the individual component. Peak area measurements were done manually by cutting and weighing chromatograms, since the Disc Chart Integrator was not in working order and the counter did not work satisfactorily for carbohydrate peaks. The hydrol-ysis survival factors of individual sugars used for this equation were determined by Saeman et al_. (253) and k-values were based on slopes of calibration curves which were obtained by plotting chromatographic peak area ratios (monosaccharide area/myo-inositol area) vs. weight ratios (monosaccharide weight/myo-inositol weight) for known samples. Uronic acid residues were determined commercially by Schwarzkoff Microanalytical Laboratory, Woodside, N.J., f o i l owing the method of Anderson et al_. (6). The quantitative analysis of acetyl groups was carried n out by the same company using a modified method developed by Schoniger et a_l. (256). Data on acetyl and uronic acid groups are presented in Table 3. Hydrolysis of wood hemicelluloses according to the procedure of Saeman ejt al.(253) is not without shortcomings. Meier and Wilkie (192) and Zinbo and Timell (327) observed that aldobiouronic acids formed during hydrolysis were rather stable to further hydrolytic degradation. Only one-third of 4-0-methyl-D-glucopyranuronide linkages were found to undergo further hydrolysis under conditions provided by the method of Saeman (192, 327). Since the xylose fraction which is present in non-hydrolysed aldo-biouronic acid residues does not appear in the gas chromatographic analysis, some correction regarding xylose content was undertaken. This was done by multiplying the relative amount of glucuronic acid residues with a factor . [10] of 0.66 (which relates to non-hydrolysed aldobiouronic acid). The value obtained was added to xylose as determined by the gas chromatographic procedure (Table 3). Since the chromatographic analysis gave only information on the total amount of glucose present in pulp, it was useful to estimate those portions of glucose originating from cellulose and glucomannans. This was done according to known ratios between glucose and mannose residues in isolates from coniferous (1:3) and pored (1:2) wood glucomannans (293). The corresponding values are presented in Table 3. 3.3.2 Lignin Lignin determinations were carried out according to the Klason lignin method as described by Browning (37) on groundwoods (10-1 to 10-4), holocelluloses (9-1 to 9-4) and paper grade, pulps (6-1 to 8-2). Data are presented as Klason lignin in Table 3. Only groundwood pulps were subjected to ethanol-benzene extraction treatments prior to acidification, since chemical pulps are known to contain no or only traces of extraneous compounds. No lignin determinations were undertaken on viscose, acetate and alpha-cellulose pulps,due to their extremely low lignin contents which are not measurable by the Klason lignin method. Their lignin contents were estimated to be ^ 0. l7o. 3.3.3 Other measurements Viscosity measurements were done for characterising pulps and for determining degree of cellulose degradation by irradiation treatments. They were carried out in a modified FeTNa (EWNN, alkaline iron tartaric acid sodium complex)solution prepared according to Valtasaari (305), but with an additional 10 g/1 sodium tartrate used as stabilizer. Oven-dry sample weights of 0.015 to 0.25 g pulp per 100 ml solution were used according to the method of Paszner (229), with amount adjusted to the degradation level. First, specific viscosity Ctfsp) was determined with an Ubbelohde viscosimeter No. + o r- -1 la at 20 - 0.02 C. Intrinsic viscosity (\_K>j) w a s then calculated by the equation (305): - 51 -M - [nJ l"C J C (140.339 ^sp) L J where: C = amount of moisture-free pulp, g/100 ml. TAPPI Standard method T293 os-6l (289) was followed in determining alpha-cellulose contents. This particular method was scaled to accommodate micro-amounts of material. The solubility determinations were performed according to TAPPI Standard T 235 m - 60 (288) with 107o NaOH solution at a constant temperature of 20°C. Special care was exercised to keep the temperature at 20 - 0.02°C for a l l tests, since it has been found that even slight changes in temperature influence results of alkali solubility measurements (126). - 52 -4.0 RESULTS AND DISCUSSION 4.1 Fractional Stress Relaxation Tests As described earlier, five replicates were used in a l l pulp treat-ment combinations. Table 4 presents fractional stress relaxation data (1.000 - ^ (35 min)) as obtained from individual measurements after 35 min i o (o) relaxation time, mean values and standard deviations for each of the pulps tested for both water and caustic steeped conditions. Standard deviations show relatively l i t t l e variation between measurements at 35 min relaxation time as taken from individual pulps given the same steeping treatment. This means that the results are consistent, which is a strong point of the experiments. Degree of variation was approximately the same for all pulp types with comparable steeping treatments, but differed considerably between water and caustic steeped specimens of the same pulp. On average, caustic treated samples exhibited less variability in rate of stress decay, as compared with water saturated specimens.after 35 min relaxation. As will be shown later, the reproducibility of stress relaxation data, particularly when obtained from caustic treated samples, appears to be sufficiently high to employ relaxation measurements for characterising pulps. Characteristic of a l l viscoelastic materials is their ability to dissipate stress when subjected to deformation. It is well known that stress relaxation of viscoelastic solids occurs at the highest rate immediate-ly after a constant deformation is applied, and thereafter decreases gradually with time. A similar type of response can be expected also from pulp mats placed under constant compressive strain. Two-dimensional £(t )/€T(o) vs. log time plots of typical fractional stress relaxation curves, as obtained from pulps steeped either in distilled water or 18.6% NaOH prior to relaxation tests, are shown in Figs. 14 to 18. The curves represent average values of five measurements. It appears from each set of plots that rate of stress dissipation in pulp compression follows patterns similar to those observed in woods or other cellulosics examined under constant compressive or tensile strains (137,145,146). Wet or caustic - 53 -steeped pulp mats exhibited the highest rate of stress decay immediately a f t e r s t r a i n a p p l i c a t i o n , which i s c h a r a c t e r i s t i c f o r r e l a x a t i o n processes of l i g n o - c e l l u l o s i c materials and often expressed as log time r e l a t i o n s h i p s . t A more d e t a i l e d analysis of the ^ ( t )/Sto) vs. log time p l o t s reveals that s t r e s s r e l a x a t i o n processes take place as two d i s t i n c t phases. These are in d i c a t e d as two slopes within each f r a c t i o n a l stress r e l a x a t i o n t r a c e . The w r i t e r emphasizes that responses have been observed only over the short time range of 35 min following almost instantaneous s t r a i n a p p l i -c a t i o n . In p a r t i c u l a r , rate of s t r a i n has been found to exert a profound influence on the magnitude of i n i t i a l s t r e s s decay. Kirbach (145) studied the s t r e s s decay i n wood microspecimens and observed that a high s t r a i n i n g r a t e was followed by a high i n i t i a l s tress decay, but that t h i s d i d not a f f e c t s i g n i f i c a n t l y the l a t e r process of s t r e s s d i s s i p a t i o n . Similar observations were reported a l s o by Anderson and Sjoberg (7) who explain t h i s phenomenon as an immediate consequence of the Boltzmann superposition p r i n c i p l e or as dependent on previous h i s t o r y . Such observation show that s p e c i a l care i s needed f o r i n t e r p r e t i n g i n i t i a l s tress r e l a x a t i o n responses. The f i r s t and non-linear period of t h e ^ ( t ) / ^ t o ) vs. log t plot w i l l be c a l l e d Phase I r e l a x a t i o n . It v a r i e s i n duration ( F i g s . 14-18) and has been found to involve rather short times f o r viscose and high alpha-pulps (0.00 to 0.03 min); often intermediate times f o r h o l o c e l l u l o s e and chemical paper pulps (0.00 to approximately 0.10 rain); and comparatively longer times f o r groundwoods, p a r t i c u l a r l y when steeped i n c a u s t i c (to approximately 1.00 min). The common c h a r a c t e r i s t i c s i n Phase I f o r a l l pulp-water or pulp-caustic treatments i s the sigmoid type r e l a t i o n s h i p between ^ ( t )/g£(o) vs. log t . Such r e l a t i o n s h i p s have been found a l s o by n Anderson and Sjoberg (7), and to some extent by Johanson and Kubat (137), f o r paper t e n s i l e stress r e l a x a t i o n . They a l s o used extremely short periods II f o r load or s t r a i n a p p l i c a t i o n . Anderson and Sjoberg (7) strained t h e i r 100 mm long specimens at constant rates between 1% i n 0.01 sec and 17, i n 5 sec and Johanson and Kubat (137) employed a s t r a i n rate of 1.25 x 10 sec * i n the majority of experiments-on 10 mm long s t r i p s . As mentioned e a r l i e r , 1,0 to 1.5 sec was the elapsed time f o r completion of loading i n t h i s study. - 54 -The "linear" period, which will be termed Phase II relaxation, characterises the.remaining part of the 35 min response of pulp mats steeped in either water or 18.6% NaOH. A straight line relationship between ^(t)/£>to) vs. log t has been reported also for other cellulosic materials, such as wood at micro- and macro-levels (145,146) and papers (137,242), while several equations have been developed to describe this linear inflection region. For instance, Johanson and Kubat (137) used the following equation to describe rate of stress decay (Phase II) mathematically as: • < 0^-F log t [ l i ] where: F = slope of the main inflection of C(log t ) . A similar equation was derived empirically by Kitazawa (146) earlier and was used to describe wood stress relaxation: ^(t)/eto) - 1 - m log t [l4J where: m = constant relaxation coefficient which takes into account factors such as species variations. Kitazawa reported that his equation covered the total stress relaxation process as observed after 1 min. In contrast to other studies in stress relaxation (7,137,145), Kitazawa's work ignored stress relaxation in the 1 min period and therefore does not report existence of a non-linear Phase I relationship. Since the constants F and m in the two equations above may be replaced by any number representing a certain slope, both equations can be employed to describe pulp mat stress relaxation following water or caustic steeping. Such application, of course, is limited to the linear (Phase II) inflection region. The steep slopes observed in Phase I, compared with Phase II, for other cellulosics suggests profound differences in mechanisms responsible for i n i t i a l stress decay, especially in the water and caustic swollen state of pulp mats and other cellulosic materials. This includes observations by Johanson and Kubat (137) for paper 3tress relaxation in tension and Xirbach - 55 -(H5) f o r wood microspecimens i n compression and tension. The p o s s i b i l i t y that rate of s t r a i n i n g e f f e c t s may have induced these d i f f e r e n c e s can be excluded, since f o r a l l experiments i n v e s t i g a t i n g short term or i n i t i a l s t r e s s d i s s i p a t i o n (Phase I) s t r a i n was applied at high r a t e s . D i s s i m i l a r i t i e s i n i n i t i a l s tress d i s s i p a t i o n (Phase I) between various c e l l u l o s i c s are a t t r i b u t e d mainly to f a c t o r s which determine i n t e r -r e l a t i o n s between i n d i v i d u a l s t r u c t u r a l units i n wood and f i b r o u s c e l l u l o s i c s , such as type and i n t e n s i t y of f i b r e a s s o c i a t i o n and order. In water and c a u s t i c saturated pulp mats the randomly arranged f i b r e s can exert only rather l i m i t e d i n t e r a c t i o n . Hydrogen bonds, c h a r a c t e r i s t i c f o r i n t e r f i b r e bonding of a i r - d r y fibrous m aterials, are lacking and the only i n t e r a c t i n g forces i n saturated pulp mats are due to mechanical entanglement or f r i c t i o n at f i b r e surfaces. Consequently, external mechanical e x c i t a t i o n s are expected to r e p o s i t i o n e a s i l y i n d i v i d u a l f i b r e s . In woods, where the strong middle lamella holds various s t r u c t u r a l elements i n s p a c i a l arrangement and i n dry papers which obtain strength from i n t e r - f i b r e hydrogen bonding along contact areas, mechanical e x c i t a t i o n s encounter considerably higher r e s i s t a n c e before s t r u c t u r a l units are separated from each other or repositioned. During, and immediately following load a p p l i c a t i o n , saturated pulp mat response i s expected to include both i n i t i a l f i b r e rearrangements and, of course, i n t r a - f i b r e processes causing molecular rearrangements. Consequently, the e a r l y stages of s t r e s s decay (Phase I) would seem to be c o n t r o l l e d by two fundamentally d i f f e r e n t mechanisms. One of these, which may be c a l l e d M^ , can be a t t r i b u t e d to i n t e r - f i b r e processes. These processes might l a r g e l y c o n t r o l rate of Phase I st r e s s decay. The other mechanism, which may be c a l l e d M^, i s associated with i n t r a - f i b r e processes taking place at the molecular l e v e l . As w i l l be discussed below, these may be of i n t e r - and intramolecular nature. Based on slopes of curves i n F i g s . 14 to 18, i t can be concluded that M2 i s the less important mechanism i n Phase I, but seems to c o n t r o l almost e n t i r e l y Phase I I , i . e . , a f t e r i n t e r f i b r e processes are at a minimum. The i n t e r f i b r e processes which are thought to play the major r o l e i n Phase I st r e s s decay involve e s s e n t i a l l y f i b r e movements, such as bending - 56 -slippage, deswelling effects at points of contact and, to a lesser extent, movement of fluids in the saturated mat. Certainly, most fibre slippage occurs during the loading process, but some fibre repositioning can be expected in early stages of the stress relaxation process. This might contribute significantly to Phase 1 stress decay. The rather long periods of Phase I observed for groundwood pulps support this assumption. The low length/diameter ratio of groundwood fibres, together with presence of a considerable amount of fines, provides a mat of low coherence. Therefore, some fibre movement in particular toward the sample edge can be expected. The consequence is decreasing stress. o Changes in fibre bending, which according to Jones (138) are time dependent, may be also of great importance to stress relaxation in Phase I. Deswelling processes at points of contact are, according to Han (100), a significant feature of water saturated mat compression. Removal of internal fluids from areas under compression requires time, therefore, these processes will continue beyond conclusion of strain application (t )» adding to the high rate of Phase I stress dissipation. It is not believed that filtration resistance plays more than a subordinate role in init i a l stress relaxation processes. Slopes of the loading curves indicate that only low stresses (less than 5% of^at t ) develop during early periods of the straining process. This holds even when a high degree of straining (approximately 90% of total mat deformation) is employed. It appears from this that almost a l l free liquid is removed from the mat before the desired init i a l stress level is attained. The conclusion can be drawn that init i a l stress relaxation (Phase I) in saturated pulp mats involves mechanisms at least partly different from those responsible for stress decay in dry fibrous webs or in dry or saturated woods. This is due mainly to the fact that associations between structural units of saturated fibre mats are profoundly different from those, existing in wood and papers. 4.2 Effect of Steeping Media (Water vs. Caustic) It is known from studies on wood and other cellulosic materials - 57 -that steeping treatments with strong a l k a l i n e solutions cause both i n t e r -and i n t r a - c r y s t a l l i n e swelling, whereas steeping i n water e n t a i l s only extension of i n t e r - c r y s t a l l i n e regions (251,275). Former i n v e s t i g a t i o n s on c a u s t i c steeping f o r adjustment of c e l l u l o s e f i b r e mechanical properties (46) and creep behavior of wood (204) provide evidence that d i f f e r e n c e s i n swelling exert profound influences on the response of c e l l u l o s i c s to mechanical e x c i t a t i o n s . Based on t h i s information, and because a major obje c t i v e of the present study was to el u c i d a t e the r o l e of r e s i d u a l wood pulp polymers i n rh e o l o g i c a l processes, i t appeared useful to undertake r e l a x a t i o n studies on both ca u s t i c and water steeped samples. F r a c t i o n a l stress r e l a x a t i o n traces as averages of f i v e t r i a l s obtained i n 35 min tests of various pulp types steeped i n water, and in 18.6% NaOH, are presented i n Fi g s . 14 to 18; whereas Table 4 contains i n d i v i d u a l s t r e s s r e l a x a t i o n data f o r a l l pulps as observed at 35 min. Each plot and the Table 4 data show considerably higher rates of stress d i s s i p a t i o n i n pulp mats treated with c a u s t i c solutions than in water saturated mats. In other words, ca u s t i c steeping i n comparison with steeping i n water resulted i n reduction of the e l a s t i c and enhancement of the p l a s t i c behavior of the lignin-carbohydrate or carbohydrate complex. As can be seen, rate of stress d i s s i p a t i o n increased between 10 and 54% at 35 min a f t e r conclusion of loading. Slopes of the quickly descending portion (Phase I) of r e l a x a t i o n curves i n F i g s . 14 to 18 i n d i c a t e c l e a r l y that d i f f e r e n c e s i n r e l a t i v e amount of str e s s decay between water or cau s t i c treated pulps are set e s s e n t i a l l y during the f i r s t 0.01 to 0.1 min of r e l a x a t i o n . It appears from t h i s observation that e f f e c t s of cau s t i c swelling within the pulp f i b r e s t r u c t u r a l polymer complex i s only a c t i v e during e a r l y stages of the st r e s s d i s s i p a t i o n processes. It can be concluded that c a u s t i c swelling must a c t i v a t e some mechanisms which, a f t e r d i s s i p a t i n g considerable amounts of s t r e s s , exhaust q u i c k l y . The co n t r i b u t i o n to r h e o l o g i c a l response i s q u a n t i t a t i v e l y large but li m i t e d i n duration. - 58 -As noted above, differences in stress decay over 35 min, as caused by various steeping treatments, ranged between 10 and 54%. It can be seen in Figs. 14 to 18 and Table 4 that the "steeping" effect is essentially independent of pulp type tested, in spite of wide variations in lignin and hemicellulose contents (see Table 3). This proves that the higher rate of stress decay observed following caustic steeping is mainly caused by (M ) processes at inter- and intra-molecular levels of cellulose organisation, in particular within highly oriented regions. It is generally accepted that strong alkaline solutions, of at least 8 to 9% NaOH, penetrate cellulose crystalline regions by first breaking H-bonds and thereafter forming new H-bonds between the reagent and cellulose hydroxyl groups (173,251,275). This causes dimensional changes in the unit c e l l , in particular an increase in the 101 inter-planar distance (179). The former strong inter-molecular bonds in crystallites, H-bonds in the a-b plane and van der Waal's forces in the a-c plane (120, see Fig. 3), are replaced by weak H-bonds which have been found to be associated with decreased crystallinity (153). The higher stress decay in intra-crystalline swollen pulps under compressive strain may involve any of several processes. These may include: (i) conformational changes of cellulose; ( i i ) sliding effects within the cellulose crystallites; ( i i i ) enhanced molecular motion of cellulose chains on the cellulose crystallite surface, and (iv), in the case of chemical pulps, possibly enhanced mobility of formerly crystalline hemicelluloses. Since the swelling power of water is limited to amorphous zones of the lignin-carbohydrate complex, no short term viscoelastic processes probably take place within crystalline portions of cellulose or redeposited crystalline xylan and glucomannan in water treated samples. These components may undergo elastic deformation when subjected to stresses, which would mean storage but not dissipation of energy. That elastic deformations of the cellulose lattice can occur has been shown by X-ray measurements on Hinoki (Cupressus obtusa, Koch) wood specimens subjected to tensile strains (285). It has been proposed (145) that the *"C. conformation in unstressed - 59 -cellulosic material can be expected to switch by mechanical excitations to the less stable (higher energy level) conformation, thus providing an abrupt means for energy storage. The reduction of inter- and intra-crystalline forces by caustic solutions, which converts the cellulose from a three-dimensional body to a more or less two-dimensional form, may allow glucose units to switch conformations more easily by mechanical excitation. The fact that capacity of glucose units to absorb energy is limited, and that glucose units in alkaline swollen celluloses are rather flexible, can be used to explain the high but limited rate of stress decay observed immediately after application of straining. At the same time, converting transition zones between crystalline and amorphous regions, and part of cellulose crystallites (153) into amorphous zones, may also contribute to ini t i a l stress relaxation. Such changes can be expected to increase molecular movement around the crystalline cores. The changes in cellulose crystalline lattice, particularly increase in 101 interplanar distance (179) provides a further system for redistributing or dissipating stress through less tight bonding. In addition, the deposition of water and hydroxyl ions with counter ions between 101 planes (153) can be expected to act as lubricants promoting sliding along 101 planes when the lattice system is stressed above a critical value. This, of course, is accompanied by breaking and reforming the rather weak H-bonds between reagents and cellulose hydroxyl groups. That sliding phenomena are involved only in i n i t i a l stress relaxation can be explained by a critical stress level below which no further intra-crystalline sliding occurs. At beginning of the stress relaxation process, the strain level used in this work may have exceeded such a critical level. With progressive relaxation, however, the residual stress would soon f a l l below that level. Thereby, intra-crystalline processes would be excluded from further involvement in the rheological behavior of pulp mats maintained at constant strain. Stress relaxation differences between caustic and water steeped chemical pulp samples (with the exception of holocellulose preparations) can be attributed also to some extent to crystalline (redeposited) hemi-- 60 -c e l l u l o s e s . Observations on wood glucomannans i n su l p h i t e pulping (10), and xylans i n sulphate pulping (51,326), show that these hemicelluloses can be redeposited or adsorbed during l a t e r cooking stages. The redeposition process occurs p r e f e r e n t i a l l y on the f i b r e surface or, more acc u r a t e l y , on the surface of m i c r o f i b r i l s on the outer part of the f i b r e s . The r e s u l t i s formation of c r y s t a l l i n e l a y e r s . The abundant H-bonds between i n d i v i d u a l xylan or glucomannan chains i n these layers are s u f f i c i e n t l y strong to r e s i s t the penetration of water, but y i e l d to attack by strong a l k a l i n e s o l u t i o n s . Consequently, the short chain and i n t e r - and i n t r a - c r y s t a l l i n e swollen hemicelluloses may provide a l u b r i c a n t layer between f i b r i l s of the same f i b r e or ad}acent f i b r e s . This might promote r e p o s i t i o n i n g of f i b r e s within the mat which, as described e a r l i e r , i s thought to be p a r t l y responsible f o r high r a t e of i n i t i a l (Phase I ) s t r e s s r e l a x a t i o n . Such l u b r i c a n t e f f e c t s may be expected also to enhance slippage of m i c r o f i b r i l s within f i b r e s . Figure 19, which shows c o r r e l a t i o n between 1-^(35 min)/Ca(o) and hemicellulose content f o r 14 viscose pulps steeped e i t h e r i n water or 18,6% NaOH, lends evidence to t h i s assumption. The inverse r e l a t i o n s h i p f o r water and ca u s t i c treatments i n d i c a t e s that an increase i n e s s e n t i a l l y c r y s t a l l hemicelluloses reduces the v i s c o e l a s t i c response of water swollen pulp mats, but enhances the time dependent response i n c a u s t i c swollen pulps. That t h i s e f f e c t i s rather short, i . e . , l i m i t e d to Phase I, i s evidenced by F i g . 18 which shows t y p i c a l stress r e l a x a t i o n traces of three viscose pulps. The hemicellulose content of viscose pulps examined i n the study ranged from 2.2 to 6.1%. As the work of Clayton and Stone (51) showed, xylan adsorption i n raw paper pulps can reach values up to 3% of the pulp weight and 10.2% of t h i s redeposited xylan was found to r e s i s t a 90 min e x t r a c t i o n with 10% NaOH at room temperature. Therefore, i t can be expected that, i n s p i t e of severe p u r i f i c a t i o n treatments, a c e r t a i n portion of the adsorbed hemicelluloses are retaine d . Further, F i g . 19 also i n d i c a t e s that the hemicelluloses, a f t e r removal of side branches and redep o s i t i o n , have l o s t most of t h e i r o t i g i n a l s t r e s s d i s s i p a t i o n function i n the lignin-carbohydrate complex of wet pulps. - bl This function seems to be partly regained, however, when caustic solutions "soften" the layer of highly oriented hemicellulose on the f i b r i l surface, A more detailed discussion on this particular subject appears in a following paragraph. 4.3 Effect of Residual Chemical Constituents Stress relaxation data obtained from a l l water and caustic steeped pulps at 35 min relaxation time (Table 4) indicate wide variations in stress decay between various pulps given the same steeping treatment. These variations, however, were less pronounced between pulps of the same type (for instance within groundwood pulps, holocellulose preparations, bleached sulphate and viscose pulps) than between different pulp types. It is also apparent that stress decay variations seem to be somewhat higher for pulps steeped in caustic. It is further evident from Table 4 that the highest rates in stress relaxation following caustic steeping occurred in groundwoods, wherein the cottonwood pulps exhibited slightly higher rates. In this respect, ground-wood pulps were followed in order by holocellulose preparations, sulphite, bleached sulphate, unbleached sulphate, acetate and viscose pulps and, finally, by alpha-cellulose preparations (with the exception of 0-1 and 0-2). In water saturated pulp mats differences were less distinct and the sequence for stress dissipation is not identical with results from caustic treated samples. It appears that water saturated groundwood pulps and holocellulose preparations exhibited approximately the same rate of stress decay, followed by unbleached sulphate pulp, bleached sulphate pulps, sulphite pulp; then viscose, acetate and alpha-cellulose pulps which showed the lowest rates of stress relaxation. Experimental variables such as sample preparation and testing conditions were carefully controlled throughout the experiments. Therefore, the above differences must be attributed to inherent variations in chemistry of the pulp samples. The analytical data presented in Table 3 exhibit large differences in chemical composition for the various materials. Lignin contents were estimated to vary between ^ 0.1 and 30.5%, total hemicelluloses - 62 -.between 0.8 to 31.9%, while the c e l l u l o s e portion ranged from 42.2 to 99.3%. Some evidence that such broad v a r i a t i o n s can be expected to influence pulp mat r h e o l o g i c a l properties appears i n several works showing that q u a n t i t a t i v e changes of l i g n i n and hemicelluloses r e s u l t i n profound v a r i a t i o n s of mechanical-rheological properties of l i g n o - c e l l u l o s i c s (200,201,283). The following d i s c u s s i o n r e l a t e s these v a r i a t i o n s i n pulp chemistry with r h e o l o g i c a l responses observed i n r e l a x a t i o n t e s t s on water and c a u s t i c swollen samples. For convenience, i n d i v i d u a l e f f e c t s of the three components, l i g n i n , hemicelluloses and c e l l u l o s e , on time dependent behavior of various pulp types, w i l l be discussed separately. 4.3.1 Lignin The most c h a r a c t e r i s t i c features of l i g n i n are i t s three-dimensional supposedly amorphous st r u c t u r e , formed by phenylpropanoid u n i t s , and i t s hydrophobic nature which r e s t r a i n s associated carbohydrates from excessive swelling (3,74-76,86). In the native state l i g n i n i s thought to be deposited between c e l l s and within amorphous parts of c e l l walls as encrusting sub-stances (316). It i s apparent from the l i g n i n s t r u c t u r e , physical nature and p o s i t i o n , as well as a s s o c i a t i o n with hemicelluloses, that i t may play an outstanding r o l e i n governing the response of wood and wood derived materials, e s p e c i a l l y i n the water saturated s t a t e , to mechanical e x c i t a t i o n . Lignin can be expected to r e s i s t mechanical e x c i t a t i o n both d i r e c t l y and i n d i r e c t l y . The work of Murakami and Yamada (200) showed that l i g n i n r e s t r i c t s absorption of water i n plant c e l l w a l l s . This proves l i g n i n to be a c o n t r o l l i n g f a c t o r i n swelling processes and leads to the conclusion that l i g n i n must exert also an i n d i r e c t influence on mechanical prop e r t i e s . Eriksson (68,69) studied the e f f e c t of d e l i g n i f i c a t i o n on wood creep and explained h i s data according to i n d i r e c t e f f e c t s of l i g n i n on the r h e o l o g i c a l response. It i s unquestioned that the aromatic character and three dimensional structure of the l i g n i n polymeric system (3) influences d i r e c t l y mechanical and r h e o l o g i c a l behavior. This has been evidenced f o r strength p r o p e r t i e s , f o r instance by an i n v e s t i g a t i o n c a r r i e d out by Stone and Kallmes (283). - 63 -They showed that progressive d e l i g n i f i c a t i o n of wood reduced d r a s t i c a l l y i t s shear strength. The a b i l i t y of aromatic systems to store energy, and the high r i g i d i t y provided by the three-dimensional polymeric s t r u c t u r e , i s expected to exert a s i m i l a r d i s t i n c t e f f e c t on r h e o l o g i c a l properties of l i g n o - c e l l u l o s i c s . It can be assumed that the l i g n i n component w i l l retard v i s c o e l a s t i c processes i n wood. As shown i n F i g s . 20 and 21, such an e f f e c t was not observed f o r water and NaOH steeped pulp mats subjected to compressive stress r e l a x a t i o n t e s t i n g . Both f i g u r e s i n d i c a t e , independent of steeping treatment, that pulps with the highest l i g n i n content (ground-woods) exhibited higher rates of s t r e s s r e l a x a t i o n than most materials of low l i g n i n content. There were s u r p r i s i n g l y large d i f f e r e n c e s between groundwoods and h i g h l y d e l i g n i f i e d pulps such as a l p h a - c e l l u l o s e s , acetate and viscose pulps. Altogether, only two of the h o l o c e l l u l o s e preparations d i s s i p a t e d stress at s l i g h t l y higher rate than the. groundwoods when steeped i n d i s t i l l e d water. Two a l t e r n a t i v e explanations may account f o r t h i s unexpected observation. F i r s t l y , perhaps no r e t a r d i n g e f f e c t of l i g n i n occurs i n compressive stress r e l a x a t i o n of pulp. Secondly, the more l i k e l y case that l i g n i n e x h i b i t s only a modest retarding e f f e c t which i s much subordinate to a more dominant r o l e of hemicelluloses. Thereby, the l i g n i n e f f e c t i s not recognized i n the complex t o t a l response. Unfortunately, the pulps used i n t h i s experiment varied in both hemicellulose and l i g n i n contents so that no d i r e c t information on c o n t r i b u t i o n of l i g n i n to compressive pulp mat r e l a x a t i o n was obtained. However, i t can be concluded that p a r t i c i p a t i o n of native or r e s i d u a l l i g n i n s i n f i b r e mat r h e o l o g i c a l properties does not reach a c o n t r o l l i n g l e v e l . These unexpected r e s u l t s may be r e l a t e d also to other f a c t o r s . Thus, the r e s i s t a n t behavior of l i g n i n i n the case of groundwood could be p a r t l y l o s t due to the high mechanical damage imparted during the grinding process. Mechanical damage could cause a loosening of the i n t e r - f i b r i l l a r s t r ucture i n such ways that p a r t i a l weakening of the close lignin-carbohydrate a s s o c i a t i o n may occur. T h i s , i n turn, may reduce the retarding e f f e c t of l i g n i n on swelling and consequently on the time dependent response of carbohydrates to s t r a i n i n g . Support to t h i s assumption i s lent by the - 64 -observation that unbeaten wet pulps ex h i b i t a higher degree of recovery from compressive creep, i n other words,showed less v i s c o e l a s t i c i t y , than beaten wet pulps (261). Beating i s known to cause f i b r i l l a t i o n and delamination of the f i b r e wall (222), which has been found to f a c i l i t a t e swelling of f i b r e s and i n h i b i t i o n of l i q u i d s (251),. The^se e f f e c t s are expected to be s i m i l a r to those occurring i n grinding of wood. Another f a c t o r possibly responsible f o r the subordinate r o l e of l i g n i n in comnressive pulp mat r e l a x a t i o n might be an o r i e n t a t i o n and/or lamellation e f f e c t . There e x i s t s evidence from several studies (130,155, 200,252) i n v e s t i g a t i n g the status of l i g n i n i n the f i b r e r w a l l that l i g n i n i s at least p a r t l y oriented and layered. An o r i e n t a t i o n might be r e l a t e d to the function of l i g n i n which i s to r e s i s t i n p a r t i c u l a r high stresses i n the l i v i n g tree along the f i b r e a x i s , and less to counteract stresses perpendicular to t h i s a x i s . In t h i s experiment the f i b r e s were loaded p r e f e r e n t i a l l y perpendicular to the f i b r e a x i s . The e x c i t a t i o n , t h e r e f o r e , was applied to a kind of layered system at two l e v e l s . One of the layered systemsoccurs at the m i c r o l e v e l . It i s formed by a l i g n i n - r i c h (20) layer (M and P) and the layer predominately occupied by carbohydrates. The other layer type of structure which i s found at the u l t r a - s t r u c t u r a l l e v e l , i s caused by the deposition of l i g n i n and hemicelluloses between m i c r o f i b r i l s . In such layered systems the carbohydrates are forced to carry the same loads as the l i e n i n complex when compressed i n transverse d i r e c t i o n . This seems to be d i f f e r e n t f o r compressive stresses applied p a r a l l e l to the grain where the r e s i s t a n c e to deformation w i l l be predominantly c o n t r o l l e d by the more r i g i d l i g n i n s t r u c t u r e . Consequently, i t can be believed "that compressive s t r a i n i n g of f i b r e s perpendicular to t h e i r axis lessens the influence of l i g n i n on rate of stress decay, but enhances v i s c o e l a s t i c response of the carbohydrates. The only i n d i c a t i o n that l i g n i n may exert some i n d i r e c t influence on rate of stress decay of chemical paper pulps can be deduced from the f a c t that the unbleached sulphate pulp (7-1)^ when steeped in water, d i s s i p a t e d stress at a somewhat higher rate than the bleached sulphate paper pulps (7-2,8-1,3-2) (Fig.20). When the same pulps were steeped in c a u s t i c , however, - 65 -the response level was reversed (Fig. 21). Since hemicellulose content did not vary greatly between these pulps (Table 3) the observation has to be attributed to increased cellulose crystallinity by removal of lignin through purification treatments. That cellulose crystallinity increases as deligni-fication progresses has been proven by Wardrop (310,314), Wardrop and Preston (317) and Murakami and Yamada (200) with holocellulose preparations. In water steeping, the relatively larger crystalline portion in bleached pulp cellulose caused' a somewhat lower degree of swelling and, consequently, less viscoelasticity than in unbleached pulps. However, by steeping in 18.6% NaOH this crystallinity effect seemed to be lost or of subordinate importance in rheological processes, due to intra-crystalline swelling phenomena in strong alkaline media. 4,3.2 Hemicelluloses It is well known that hemicelluloses contribute to mechanical properties of cellulosics, such as strength of individual fibres and papers and pulp beating behavior (95,173,201,272,273). In pulping, most chemical treatments have been found to cause profound changes, in particular in the molecular structure of non-cellulosic carbohydrates. Such changes certainly must affect pulp mat viscoelasticity. In this study an attempt was made to relate pulp viscoelastic behavior, under constant compressive strain, to residual hemicelluloses in the pulp material. Emphasis is placed on'establishing an interrelationship between quantitative features and rheological response. The discussion is also concerned with structural characteristics of residual hemicelluloses as they may relate to viscoelastic behavior. Experimental data obtained from stress relaxation measurements at 35 min relaxation time (Table 4), and from carbohydrate analyses (Table 3), are presented in Fig. 22 as plots showing 1 -(^ >(35 min)/<o(o) as a function of hemicellulose content. The points show average values of five relaxation measurements. It is apparent from the two regression lines, and evidenced by statistical calculations, that rate of stress decay in pulp mats under - 0 0 -constant strain is highly correlated with the amount of hemicelluloses present in the pulp materials. As indicated by slope changes, the relationships are characterized by two distinct stages. In the case of water steeped pulps, the slope descends from 0 to approximately 6% hemi-celluloses, which shows that pulp viscoelasticity was lowered by increasing contents up to the 6% level. Further increases in hemicelluloses reversed this relationship and lead to a considerable enhancement of stress relaxation. The positive slope from approximately 5 to 32% hemicelluloses confirms this. In respect to caustic steeped pulps, slope variation was less distinct. The slope covering the range from 0 to 6% hemicelluloses exhibits a somewhat steeper ascent than that for the range above 6%. Consequently, changes in hemicellulose content below 6% appear to have a more pronounced effect on rate of stress decay than variations above this level. In the range from 6 to 32% hemicellulose similar slopes for both water and caustic treatment groups are evident. This indicates that effect of hemicellulose variation above 6% becomes independent of the steeping medium. As mentioned above, distance between the two regression lines originates primarily from phenomena which are inherent in alkaline swelling effects of cellulose. Somewhat unexpected is the reverse relationship for water and 18.6% NaOH steeping in the range below 6%. It is obvious that hemicelluloses at such low contents resist stress relaxation when the .pulp is soaked in distilled water, but contribute to stress dissipation when alkaline steeping is applied. This behavior, can be attributed particularly to qualitative features of residual hemicelluloses, in other words, to structural changes of hemicellulose molecules. It seems reasonable to base the following discussion about the stress relaxation - hemicellulose relationship on a detailed analysis of characteristic features of pulp types or individual pulps, such as amount and molecular structure of hemicelluloses. This facilitates a better under-standing of the hemicellulose contribution to rheological processes in the fibre wall and, consequently, to the pulp mat. The discussion on this particular subject is subdivided according to pulp types. 4.3.2,1 Mechanical pulps In the caustic swollen state groundwood pulps, which were found to contain the largest hemicellulose quantities (Table 3), exhibited the highest stress dissipation rates (Fig.22). This differs to some extent from observations made on water steeped samples. Here, the Cottonwood holocelluloses appeared to dissipate energy at the highest rate, slightly more than groundwood pulps and hemlock holocelluloses. The superiority of groundwoods in stress dissipation, compared with low hemicellulose pulps, can be considered as strong evidence for involvement of hemicelluloses in rheological processes. In groundwood, the wood polymers are more or less present in the native state (30). It means that the hemicelluloses - as in the undamaged wood fibres - are fully branched, amorphous and deposited on the f i b r i l surface as matrix substance which surrounds the microfibrils (19.178.316). These polyoses are believed to form connecting layers between microfibrils and the encrusting lignin polymer system. It is well known that lignin and cellulose vary widely in physical and mechanical properties. Consequently, in the stressed state, hemicelluloses acting as connecting links have to function as a stress adjusting system. Low DP, high degree of branching and hygroscopic nature make the hemicelluloses well suited to dissipate or distribute stresses uniformly in the cell wall. In wet condition, their strong tendency to swell, provides a gel system which, as a lubricant-like layer, allows at least some gliding along the f i b r i l surface. An excessive gliding, however, may be hindered particularly by covalent bonds connecting them to the rigid three-dimensional lignin polymer system. The existence of such bonds has been proposed by several workers (36,272). The low DP is expected to contribute to gliding or flow processes. Due to the gel-like state and numerous side branches, the strength and density of H-bonds is fairly low, This possibly facilitates flow between - 68 -adjacent hemicellulose or hemicellulose and c e l l u l o s e molecules and. therefore, aids in d i s s i p a t i o n of s t r e s s . The numerous side branches may not only contribute to the flow response^but may also be involved i n s t r e s s d i s t r i b u t i o n processes. On the other hand, possible l i g n i h - h e m i c e l l u l o s e bonds and mechanical entanglement within hemicelluloses and between hemicelluloses and l i g n i n may hinder the flow processes but c e r t a i n l y r e s u l t i n microscopic deformations of the lignin-carbohydrate complex. The complex, i n turn, may be adapted better to provide uniform d i s t r i b u t i o n and storage of energy. Both microscopic deformation i n hemicellulose molecules and g l i d i n g processes may be part of the v i s c o e l a s t i c memory behavior. In the case of compressive or t e n s i l e s t r a i n s along the f i b r e a x i s , the m i c r o f i b r i l angle, which i s known to change between c e l l wall layers (148), may exert a considerable influence on the r h e o l o g i c a l processes between m i c r o f i b r i l s i n the hemicellulose matrix. The m i c r o f i b r i l angle has been found to a f f e c t mechanical properties of wood p a r a l l e l to the grain (83). I n t e r e s t i n g l y , the c o n t r i b u t i o n of hemicelluloses to the r e l a x a t i o n response does not seem to depend on any s i n g l e h e m i c e l l u l o s i c component. The function of various hemicellulose types i n t h i s regard appears to be very much the same. This i s evidenced by the f a c t that, although profound d i f f e r e n c e s in hemicellulose composition e x i s t between angiospermous and coniferous groundwoods (Table 3), s i m i l a r rates of stress r e l a x a t i o n were observed for a l l groundwood pulps. The s l i g h t d i f f e r e n c e s in s t r e s s decay are caused by v a r i a t i o n s i n t o t a l amount of hemicelluloses i n the pulp as shown i n F i g . 22. The conclusion can be drawn that wood hemicelluloses i n the native s t a t e , and/or even a f t e r chemical adjustment, function i n stress adjusting systems. M nreover, they may c o n t r o l the r h e o l o g i c a l properties of wood f i b r e s . It can be assumed that a minimum portion of hemicelluloses must be present i n the c e l l wall to withstand sudden e x c i t a t i o n s in the l i v i n g t ree. The v a l i d i t y of t h i s assumption i s supported by the f a c t that wood - 69 -hemicellulose content increased considerably during evolution from coniferous to pored woods. These changes may have occurred to enhance the energy transf system i n woody c e l l w a l l s . Previously, the function of hemicelluloses i n wood of the l i v i n g tree has been rather obscure. Kollmann and. Cote (148), f o r instance, consider hemicelluloses i n wood c e l l walls as possible r e l i c t s of c e l l wall formation. According to them, these serve as a temporary matrix preceeding l i g n i f i c a t i o n . Hopefully, the findings of the present study w i l l cause reconsideration of the r o l e of wood hemicelluloses, which appear to have been underrated in importance or misinterpreted i n the past. 4.3.2.2 Ho l o c e l l u l o s e pulps The h o l o c e l l u l o s e preparations d i s s i p a t e d stress at somewhat lower rates than groundwoods when steeped in 18.6% NaOH, but exhibited more or equal stress decay i n the water saturated s t a t e . This observation i s not quite i n accordance with expected r e s u l t s . Normally, the removal of l i g n i n would be expected to increase the r e l a t i v e amount of hemicellulose i n the pulp so that a higher rate of stress decay would seem l i k e l y . The somewhat unexpected r e s u l t s must be p r i m a r i l y a t t r i b u t e d to "overcooking" e f f e c t s . Shimada and Kondo (264) reported that large chips, s i m i l a r to those used i n t h i s study, cause f i b r e overcooking in outer portions of the chips. T h i s , i n turn, can be expected to r e s u l t in consider-able hemicellulose losses and, p o s s i b l y , i n severe degradation of the re-maining carbohydrate portion. The rather low hemicellulose contents f o r these preparations, compared with those obtained f o r groundwoods of the same species, provide evidence f o r t h i s assumption. It i s apparent from Table 3 that s u b s t a n t i a l portions of xylan and glucomannan were l o s t during pulp preparation. The losses were so high that, despite d e l i g n i f i c a t i o n which normally would have increased the r e l a t i v e amount of n o n - c e l l u l o s i c carbohydrates, the hemi-c e l l u l o s e portion was considerably below values obtained f o r groundwoods. On the other hand, overcooking must have had a rather l i m i t e d e f f e c t on sidebranches. As Table 3 suggests, degree of branching was s t i l l - 70 -f a i r l y h i g h . I t i s c o n c l u s i v e from a n a l y t i c a l d a t a i n T a b l e s 3 and 4 t h a t the lower r a t e o f s t r e s s r e l a x a t i o n e x h i b i t e d by c a u s t i c s w o l l e n h o l o c e l l u l o s e s , compared w i t h t h a t o b s e r v e d on groundwoods o f t h e same t r e a t m e n t , was p r i m a r i l y due to h e m i c e l l u l o s e l o s s e s . T h i s o b s e r v a t i o n can be c o n s i d e r e d as a f u r t h e r p r o o f f o r the i m p o r t a n t r o l e o f h e m i c e l l u l o s e s i n wood and p u l p r h e o l o g y . The r e s i d u a l h e m i c e l l u l o s e s i n c a u s t i c s w o l l e n h o l o c e l l u l o s e s may f u n c t i o n e s s e n t i a l l y i n t h e same way as d e s c r i b e d e a r l i e r f o r ground-wood p u l p s . I t i s u n l i k e l y t h a t the r a t h e r l i m i t e d d e c r e a s e i n b r a n c h i n g e x e r t e d a p a r t i c u l a r change. But i t can be e x p e c t e d t h a t t h e e f f e c t o f br a n c h e s might have changed t o some e x t e n t due to t h e removal o f l i g n i n . U n f o r t u n a t e l y , any c o n c l u s i o n on how d e l i g n i f i c a t i o n a f f e c t s the r h e o l o g i c a l b e h a v i o r o f the h e m i c e l l u l o s e complex i s d i f f i c u l t t o draw. The l o s s i n h e m i c e l l u l o s e d u r i n g p u l p i n g makes i t e x t r e m e l y d i f f i c u l t t o e v a l u a t e e f f e c t o f l i g n i n - h e m i c e l l u l o s e a s s o c i a t i o n on the s t r e s s d i s s i p a t i o n mechanism i n b o t h c a u s t i c and water s t e e p e d p u l p s . I t can be assumed t h a t the d e c r e a s e i n s t r e s s r e l a x a t i o n caused by l o s s o f h e m i c e l l u l o s e s i s t o some e x t e n t r e d u c e d by d e g r a d a t i o n e f f e c t s on b o t h c e l l u l o s e and r e s i d u a l h e m i c e l l u l o s e s . P u l p i n g i n both a c i d c h l o r i t e and p e r a c e t i c a c i d media has been r e p o r t e d to cause d e c r e a s e d DP f o r a l l c a r b o h y d r a t e s (4,164.292). As i r r a d i a t i o n e x p e r i m e n t s , which w i l l be d i s c u s s e d below,show, a d e c r e a s e i n DP enhances the v i s c o e l a s t i c r e s p o n s e . C o n s e q u e n t l y , the d e g r a d a t i o n r e a c t i o n s can be e x p e c t e d t o i n c r e a s e to some e x t e n t the c o n t r i b u t i o n o f r e m a i n i n g h e m i c e l l u l o s e s i n both water and c a u s t i c s w o l l e n p u l p s . With r e s p e c t to the water s w o l l e n s t a t e , cottonwood h o l o c e l l u l o s e was found t o e x h i b i t t h e h i g h e s t r a t e o f s t r e s s decay ( F i g . 22) i n s p i t e o f the lower h e m i c e l l u l o s e c o n t e n t as compared t o groundwood p u l p s . T h i s phenomenon seems to be r a t h e r complex. P o s s i b l y , an " a c t i v a t i n g e f f e c t " on the branched x y l a n m o l e c u l e s may have taken p l a c e d u r i n g the d e l i e n i f i -c a t i o n p r o c e s s . - 71 -As Table 3 indicates, no severe loss of glucuronic acid side chains occurred during pulping treatments. It can be assumed that these side chains were closely associated with lignin and consequently exerted a,retarding effect on swelling or viscoelastic behavior. By removing lignin this effect was eliminated and flexibility of the xylan molecule was considerably enhanced, possibly by the swelling tendency of the now un-hindered glucuronic acid groups. This assumption is supported by Pew and Weyna (234) who proposed that carbohydrate molecules are surrounded by lignin. In other words, they may be held in the gel-like lignin substance by molecular entanglement ("snake cage" structures). The considerably lower relaxation of water steeped hemlock holo-celluloses, compared with cottonwood pulps, can be attributed to the lower content of acidic xylans in coniferous wood. Due to this quantitative difference the "activating effect" of acidic xylans, as caused by delignifica-tion . may contribute less to the total viscoelastic response of hemlock pulps. In 'addition, some removal of galactose side branches from glucomannans may have reduced to some extent the viscoelastic contribution of hemi-celluloses. 4.3.2.3 Sulphite and sulphate paper pulps As indicated by Table 4 and Fig. 22 the group of paper pulps ranked third with respect to hemicellulose content, which explains the lower rate of stress decay compared with that exhibited by groundwoods and holocelluloses. Again, the amount of residual hemicelluloses appeared to dominate the viscoelastic behavior of these pulps under compressive straining. As indicated by Fig. 22, however, the corresponding data are rather scattered around the two regression lines showing that pulping method exerts an influence on paper pulp viscoelasticity. The variations between individual pulps within this group seem to be less due to quantitative, but more due to structural differences of the remaining carbohydrate complex. The rather similar hemicellulose content of these pulps (Table 3) confirms this. In addition, relative amount of lignin seems to be of some - 72 -importance f o r unbleached pulp stress r e l a x a t i o n . The r e l a x a t i o n response of sulphite pulps i n water and c a u s t i c swollen state i s characterised by a number of f a c t o r s inherent to t h i s p a r t i c u l a r pulp type. It i s apparent, from F i g . 22 that the s u b s t a n t i a l loss of hemicelluloses during s u l p h i t e pulping was the major f a c t o r responsible f o r the generally lower rates of stress decay i n comparison with groundwoods and h o l o c e l l u l o s e s . Other f a c t o r s which may have enhanced or reduced sulphite pulp s t r e s s r e l a x a t i o n are: ( i ) removal of l i g n i n to 0,7%; ( i i ) high degree of c e l l u l o s e degradation within amorphous regions, p a r t i c u l a r l y i n the outer part of the f i b r e ; ( i i i ) low average DP of r e s i d u a l hemicelluloses; ( i v ) glucomannan redeposition; and (v) r e l a t i v e abundance of glucuronic a c i d groups. That severe degradation of c e l l u l o s e i n sulphite pulping, as proven by Luce (171), may influence sulphite pulp v i s c o e l a s t i c i t y w i l l be discussed i n a l a t e r s e c t i o n . S i m i l a r l y , hemicellulose degradation to a rather low average DP (174,217) may also contribute to the r e l a x a t i o n response. The considerably shorter chains might improve the " l u b r i c a t i o n e f f e c t " of hemicelluloses and so that rate of stress decay i s enhanced i n both water and caustic swollen st a t e s . The highly ordered structure of redeposited glucomannan (10) i s believed to have played a subordinate r o l e . It probably regarded stress decay to some extent in water saturated pulp, but exerted no e f f e c t on r e l a x a t i o n i n caustic swollen samples. The r e l a t i v e l y high amount of glucuronic a c i d (Table 3), i n d i c a t i n g a high degree of r e s i d u a l xylan branching, may have influenced the stress decay process i n a s i m i l a r way to that described f o r h o l o c e l l u l o s e s . It may have enhanced r e l a x a t i o n p a r t i c u l a r l y i n the caustic swollen s t a t e . In the case of c a u s t i c steeping, these physical and chemical changes of sulphite pulp hemicelluloses (factors i i i to v) appear to be overshadowed by c e l l u l o s e degradation e f f e c t s caused by sulphite pulping. Similar rates of stress decay for c a u s t i c swollen s u l p h i t e pulp and holo-c e l l u l o s e s ( F i g . 22) provide t h i s evidence. It seems that a reduction i n - 73 -relaxation due to hemicellulose losses is at least partly compensated for by the cellulose degradation.phenomena. In the case of sulphate pulps, the following pulp characteristics, in addition to quantitative changes in hemicelluloses, are believed to have influenced stress decay: (i) removal of lignin; ( i i ) hemicellulose degradation; and ( i i i ) xylan redeposition. This is confirmed by the scattered distribution of sulphate pulps around the regression lines which indicates considerable variation between individual pulps. Figure 22 shows that pulps 7-1 and 8-2 in the caustic swollen state dissipated less stress than predicted from residual total hemicellulose contents. It is difficult to attribute this deviation to any particular factor.- It seems likely, however, that in the case of pulp 7-1 the relatively high lignin content (9.1%) and probably lignin condensation are responsible for the comparatively low rate of stress decay. Hemicellulose degradation effects may account most for the compar-atively low relaxation rate observed for angiospermous pulp 8-2. It is known that the susceptibility of glucuronic acid and acetyl groups to loss in alkaline pulping media leaves the pored wood xylan without or with only few residues of side chains (53,54,97,191,216,291). This essentially linear xylan, which is the dominant hemicellulose in this hardwood pulp (Table 3), was substantially reduced in its capability to assist in stress dissipation. In the water swollen state, the xylan degradation effect seems to play only a subordinate role in viscoelasticity of the same pulp. Further, the close location of this pulp to the lower regression line in Fig. 22 indicates also that effect of xylan redeposition, which could be expected to be particularly high for this pulp species, may not reach a significant level. The considerably lower relaxation rate of bleached pulps in comparison with unbleached sulphate pulp 7-1 when steeped in water has to be considered as an indirect effect of lignin. It can be expected that removal of lignin and subsequent drying of pulp increased considerably the number of H-bonds in the remaining carbohydrate portion of the cell wall. - 74 -A c e r t a i n portion of newly formed H-bonds i n the close v i c i n i t y of c r y s t a l -l i t e s may have been s u f f i c i e n t l y strong to r e s i s t water s w e l l i n g . This i n turn may have reduced the v i s c o e l a s t i c response of the bleached sulphate pulps. 4.3.2.4 Viscose and acetate pulps In viscose and acetate pulps the r e s i d u a l hemicellulose portion i s reduced to l e s s than 7%. In the c a u s t i c swollen s t a t e , t h i s leads, as evidenced by F i g . 22, to less stress r e l a x a t i o n . The steeper slope of the regression l i n e i n the range of viscose and acetate pulps, i n comparison with that of the remaining range, i n d i c a t e s that hemicellulose v a r i a t i o n s below 6% cause considerably greater changes i n rat e of s t r e s s r e l a x a t i o n . This phenomenon i s considered to be the r e s u l t of two f a c t o r s : ( i ) increased v i s c o e l a s t i c c o n t r i b u t i o n of hemicelluloses- at lower contents; and ( i i ) c e l l u l o s e degradation. The comparatively high stress r e l a x a t i o n values (groups of pulps l y i n g above the regression l i n e ) obtained from s u l p h i t e pulps confirm the l a t t e r suggestion. In the water swollen s t a t e , rate of viscose and acetate pulp stress r e l a x a t i o n i s i n v e r s e l y r e l a t e d to r e s i d u a l hemicelluloses ( F i e . 22). The unexpected behavior i s believed to o r i g i n a t e p r i m a r i l y from degradation and redeposition phenomena of hemicelluloses. The tendency of glucomannan and xylan to r e p r e c i p i t a t e as h i g h l y ordered deposits on c e l l u l o s e f i b r i l s has been noted above. It i s l i k e l y that a portion of redeposited hemi-c e l l u l o s e s survived p u r i f i c a t i o n treatments, p a r t i c u l a r l y i n pulps of lower p u r i t y f o r which less severe p u r i f i c a t i o n treatments were employed. Further, i t i s a l s o l i k e l y that part of the hemicelluloses remained i n t h e i r o r i g i n a l p o s i t i o n i n the f i b r e w a l l , but were subject to degradation (removal of side chains). Durine pulp drying the e s s e n t i a l l y l i n e a r chains formed H-bonds between themselves and with adjacent c e l l u l o s e chains. These bonds can be expected to be s u f f i c i e n t l y strong to r e s i s t water penetration. Consequently, a large portion of the r e l a t i v e l y small amount of hemicelluloses i n viscose and acetate pulps i s not a c c e s s i b l e to water and, therefore, does not a s s i s t , or only a s s i s t s to rather l i m i t e d extent, i n s t r e s s d i s s i p a t i o n . Since pulps with r e l a t i v e l y large portions of hemicellulose residues 75 -exhibited the lowest r e l a x a t i o n response ( F i g . 22), the i n a c c e s s i b l e hemi-c e l l u l o s e s seem to be d i r e c t l y proportional to t o t a l amount of r e s i d u a l hemicelluloses. In caustic steeping, however, the newly formed H-bonds are r e a d i l y broken and the t o t a l remaining hemicellulose portion e x i s t s i n a g e l - l i k e state which f a c i l i t a t e s stress d i s s i p a t i o n . This explains why these chemical and physical changes of polyoses do not a f f e c t the v i s c o e l a s t i c response of caustic swollen pulps. It i s obvious that the o r i g i n a l function of polyoses i n green, or swollen wood f i b r e s was almost e n t i r e l y l o s t during pulping and p u r i f i c a t i o n treatments. This observation underlines again the importance of side branches i n native hemicelluloses f o r maintaining the h i g h l y e f f i c i e n t energy d i s s i p a t i o n and storage systems of wood f i b r e s . 4.3.2.5 Alpha-cellulose pulps No a l p h a - c e l l u l o s e preparation was found to be free of residual, polyoses. As indicated in Table 4 and supported by other work (55,87,304) alp h a - c e l l u l o s e s from wood pulps s t i l l contain various amounts of hemicellu-loses. The r e s i d u a l quantity seems to depend on the o r i g i n a l hemicellulose content. The hemicellulose portions i n the two alpha-pulps 0-3 and 0-4, which were prepared from viscose pulp 1-3 and acetate pulp 5-1, were found to account f o r 0.8 and 1.5% of pulp composition. It i s apparent from F i g . 22 that such low contents as the above contribute very l i t t l e to stress decay i n both water and c a u s t i c steeped mats. This i s evidenced by the close l o c a t i o n of the corresponding points to the i n t e r s e c t i o n s of the two regression l i n e s with the Y-axis. The i n t e r s e c t i o n points in d i c a t e stress decay following complete removal of hemicelluloses, i . e . , the response contributed by pure c e l l u l o s e as obtained by pulping and a l k a l i n e e x t r a c t i o n treatments. As i n the case of viscose and acetate pulps the r e s i d u a l hemi-c e l l u l o s e s are e s s e n t i a l l y l i n e a r (degraded) chains c l o s e l y associated with c e l l u l o s e . It i s assumed that t h i s r e s i d u a l portion o r i g i n a t e s p r i m a r i l y from degraded but non-redeposited hemicelluloses t i g h t l y embedded between - 76 -c e l l u l o s e m i c r o - f i b r i l s . Even strongly a l k a l i n e extractions are not s u f f i c i e n t to remove t h i s r e s i d u a l portion from the f i b r e w a l l . The c o n t r i b u t i o n of these hemicelluloses to pulp v i s c o e l a s t i c i t y i s believed i d e n t i c a l to that observed f o r viscose and acetate pulps. They reduced str e s s decay i n water steeped a l p h a - c e l l u l o s e pulps, but enhanced the d i s s i p a t i o n of stress i n the c a u s t i c swollen s t a t e . The alpha-pulps prepared from cottonwood and hemlock h o l o c e l l u l o s e s s t i l l contained considerable amounts of r e s i d u a l hemicelluloses (Table 3). This i s r e f l e c t e d i n t h e i r s u b s t a n t i a l l y d i f f e r e n t v i s c o e l a s t i c responses compared with those observed on the other two alpha-pulps. In the water saturated state they d i s s i p a t e d less stress,but exhibited a higher r e l a x a t i o n r a t e when steeped i n c a u s t i c . As can be seen i n F i g . 22, t h e i r response i s e s s e n t i a l l y i d e n t i c a l with that observed on bleached paper and viscose pulps of low p u r i t y . It appears that r e s i d u a l hemicelluloses a f f e c t the r e l a x a t i o n response of these two alpha-pulps i n a way s i m i l a r to that proposed i n e a r l i e r discussions dealing with paper and viscose pulps. 4.3.3 C e l l u l o s e The i n t e r s e c t i o n of the two regression l i n e s with the Y-axis i n F i g . 22 provides evidence that c e l l u l o s e plays the dominant r o l e i n v i s c o -e l a s t i c behavior of both water and c a u s t i c treated pulps. It i s apparent that c e l l u l o s e accounts f o r at least more than 50% of t o t a l s t r e s s d i s s i p a t i o n observed f o r the various pulp types. Its independent v i s c o e l a s t i c behavior and i t s c o n t r i b u t i o n to r e l a x a t i o n in h i g h l y p u r i f i e d pulps i s obtained from the i n t e r s e c t i o n point. In the case of paper grade, h o l o c e l l u l o s e and groundwood pulps the v i s c o e l a s t i c c o n t r i b u t i o n of c e l l u l o s e i s more d i f f i c u l t to estimate. But i t can be assumed that, i n s p i t e of complex i n t e r a c t i o n s between the three wood s t r u c t u r a l polymers i n r h e o l o g i c a l processes, the c e l l u l o s e c o n t r i b u t i o n i s b a s i c a l l y the same f o r a l l pulp types. This conclusion, however, i s made on the assumption that the c e l l u l o s e has not been subjected to serious degradation or does not contain degraded short chain f r a c t i o n s . - 77 -Variations in DP above 1000 are known to not influence s i g n i f i c a n t l y the pulp mechanical properties, which are d r a s t i c a l l y lowered as DP values f a l l below 1000 (251). Consequently, c e l l u l o s e r h e o l o g i c a l properties can be expected not to be changed s i g n i f i c a n t l y by pulp processes which degrade c e l l u l o s e uniformly to a DP l e v e l not lower than 1000. This implies that c e l l u l o s e s of groundwoods, h o l o c e l l u l o s e s and also of unbleached and bleached paper grade k r a f t pulps show e s s e n t i a l l y the same v i s c o e l a s t i c response. In sulphate pulping the c e l l u l o s e degradation i s uniform and u s u a l l y r e s u l t s i n DP values above 1000 (171). This i s r e f l e c t e d i n the high strength properties of unbleached k r a f t pulps i n p a r t i c u l a r (104). Based on t h i s observation, i t seems u n l i k e l y that c e l l u l o s e v i s c o e l a s t i c i t y i n un-bleached and bleached k r a f t pulps d i f f e r s s i g n i f i c a n t l y from that i n groundwoods and h o l o c e l l u l o s e s . Consequently, not c e l l u l o s e but instead the s u b s t a n t i a l q u a n t i t a t i v e and s t r u c t u r a l changes of hemicelluloses and possibly l i g n i n must be responsible f o r the profound v i s c o e l a s t i c d i f f e r e n c e s observed between groundwoods, h o l o c e l l u l o s e s and unbleached and bleached k r a f t pulps. This leads to the conclusion that the high c o r r e l a t i o n between c e l l u l o s e and rate of stress r e l a x a t i o n ( F i g s . 23 and 24) has to be considered as a secondary phenomenon. Thus,the decreasing order of hemicellulose content i n groundwood to alpha-pulp preparations allows f o r a r e l a t i v e increase i n c e l l u l o s e content. The loss of hemicelluloses i s accompanied by a proportional decrease in v i s c o e l a s t i c i t y (except f o r c a u s t i c treated v i s c o s e , acetate and two alpha-pulps) which i s r e f l e c t e d i n the negative r e l a t i o n s h i p between f r a c t i o n a l stress r e l a x a t i o n and c e l l u l o s e content (Figs. 23 and 24). As noted above, the c o n t r i b u t i o n of c e l l u l o s e to v i s c o e l a s t i c behavior i n pulps of no or s l i g h t c e l l u l o s e degradation i s e s s e n t i a l l y the same. It appears that the c e l l u l o s e response i s based on molecular processes or rearrangements of c e l l u l o s e chains or parts thereof found in the amorphous regions. In the c a u s t i c swollen s t a t e , molecular displacement in parts of the swollen c e l l u l o s e c r y s t a l structure may be involved i n v i s c o e l a s t i c processes. - 78 -In d r a s t i c pulping treatments, such as sulphite pulping or c e r t a i n bleaching procedures which cause oxidative degradation, e.g. hypochlorite bleaching, c e l l u l o s e i s subject to considerable degradation (171,243,245). This i s known to reduce pulp strength properties extensively (104). In s u l p h i t e pulping, the cleavage of the (1—>4) g l u c o s i d i c linkaee between |*J-D-glucopyranose residues occurs p r e f e r e n t i a l l y i n the amorphous regions thus c r e a t i n g weak areas i n m i c r o f i b r i l s . Since a great number, of c e l l u l o s e chains i n amorphous regions, acting as connecting l i n k s between c r y s t a l l i t e s i n the m i c r o f i b r i l s , are cleaved, the former high resistance of the amorphous-c r y s t a l l i n e system to s t r a i n i n g i s s u b s t a n t i a l l y reduced. Thus the c r y s t a l l i t e s , which can be considered as centres of energy storage and r e s i s t a n c e to s t r a i n i n g , become more or less i n e f f e c t i v e without a s u f f i c i e n t * number of connecting c e l l u l o s e chains. They y i e l d r e a d i l y to mechanical e x c i t a t i o n when chain degradation exceeds the c r i t i c a l l e v e l . It appears from Figs.22 and 24 that some c e l l u l o s e degradation e f f e c t s may have contributed to rate of stress r e l a x a t i o n . A l l s u l p h i t e pulps (viscose, acetate and paper pulps) steeped i n caustic d i s s i p a t e d stress at higher rates than prehydrolized sulphate pulps of s i m i l a r r e s i d u a l hemicellulose content. A l l viscose and acetate pulps located above the regression l i n e , representing the data obtained from c a u s t i c treated pulps (Figs.22 and 24) and pulp 6-1, are sulphite pulps. Since c e l l u l o s e degradation appears to r e l a t e to stress r e l a x a t i o n of low y i e l d pulps, i t was decided to provide more experimental evidence demonstrating relevance of c e l l u l o s e DP to pulp v i s c o e l a s t i c i t y . For t h i s reason, gamma-irradiation treatments at various dose l e v e l s ( F i g . 25) were employed on two viscose pulps (3-2 and 3-4). These pulps were found to d i f f e r i n both hemicellulose content and rate of stress r e l a x a t i o n . Radiation degradation e f f e c t s on c e l l u l o s e s of these two pulps as followed by changes i n a l p h a - c e l l u l o s e , i n t r i n s i c v i s c o s i t y and 107. NaOH s o l u b i l i t y are shown i n Table 5. It i s evident from F i g . 25, which presents the l-££t )/,€>(o) - Mrad dose r e l a t i o n s h i p at 6 sec and 35 min r e l a x a t i o n times, that gamma-irradiation changed r e l a x a t i o n responses of the c e l l u l o s e . As the p o s i t i v e slopes show, • - 79 -the time dependent resistance of pulps to s t r a i n i n g decreased s t e a d i l y with increasing dose l e v e l . Since t h i s experiment was c a r r i e d out without removing or reportioning i n i t i a l carbohydrates the ascending slopes prove unequivocally that progressive degradation or shortening of c e l l u l o s e chains leads to increased p l a s t i c i s a t i o n o f . m i c r o f i b r i l s and i n turn to increased stress r e l a x a t i o n . As discussed above, the actual mechanism responsible f o r enhanced v i s c o e l a s t i c i t y of c e l l u l o s e i s increased m o b i l i t y of c r y s t a l l i t e s caused by chain degradation i n amorphous regions. It i s apparent that i n pulps undergoing progressive degradation of carbohydrates the v i s c o e l a s t i c response changes according to both the hemicellulose portion and amount of low DP c e l l u l o s e induced. Since hemi-c e l l u l o s e molecules i n low y i e l d pulps are e s s e n t i a l l y l i n e a r , hemicelluloses and c e l l u l o s e can be assumed to function s i m i l a r l y with respect to stress d i s s i p a t i o n p a r t i c u l a r l y i n the c a u s t i c swollen state. This means that changes i n v i s c o e l a s t i c responses of pulps, such as vi s c o s e , acetate and s u l p h i t e paper pulps, have to be considered as the r e s u l t of v a r i a t i o n s i n t o t a l short chain material i n the f i b r e w a l l . The v a l i d i t y of t h i s suggestion i s supported by F i g . 26 i l l u s t r a t i n g the regression of 1 - G£(35')/£"(o) on 10% NaOH s o l u b i l i t y of i r r a d i a t e d and untreated viscose pulps,and the corresponding s t a t i s t i c a l c a l c u l a t i o n s given i n Table 6. As F i g . 27 i n d i c a t e s , the c a u s t i c s o l u b i l i t y of non-irradiated pulps was p r i m a r i l y dependent on hemicellulose content. Consequently, the high c o r r e l a t i o n between 1 -€J35' )/e""(o) and c a u s t i c s o l u b i l i t y of non-i r r a d i a t e d pulps ( F i g . 26) i s believed to be e s s e n t i a l l y caused by r e s i d u a l hemicelluloses. Testing of p a r a l l e l i s m and coincidence f o r multiple c u r v i l i n e a r regression (Table 6) showed no s i g n i f i c a n t d i f f e r e n c e s i n slopes and l e v e l s of the three regression l i n e s in F i g . 26. This provides evidence that s t r e s s r e l a x a t i o n i n h i g h l y degraded pulps seems to be s o l e l y a function of t o t a l amount of low DP carbohydrates arid not of a s p e c i f i c carbohydrate component. This observation i s useful i n explaining the r e l a t i v e l y small chain length of native hemicelluloses. As proposed e a r l i e r , l i m i t e d chain length and high degree of branching appear useful to v i s c o e l a s t i c processes in the undamaged or slightly degraded fibre wall. In the water swollen state, severe chain degradation, particularly in low yield pulping, changes profoundly the original mechanical function of carbohydrates in the fibre wall. Highly degraded cellulose appeared to behave similarly to native hemicelluloses,whereas that part of the degradated hemicelluloses, which is redeposited in highly ordered form or remains after degradation (removal of side chains) in close association with cellulose, responded like crystalline cellulose. 4.4. Interchangeability of Stress Systems When dry cellulosic materials are steeped in water or alkaline solutions, swelling takes place. This phenomenon is known to cause enormous swelling stresses which, in turn, can be assumed to induce rheological processes in the lignin-carbohydrate complex. Weakening, breaking and reformation of hydrogen bonds, bond angle changes, stretching of primary bonds, or even conformational changes can be expected to occur in cellulosics under swelling stresses. Therefore, i t was hypothesized that time of steeping in swelling reagents, i.e, the time period over which the sample is subjected to swelling stress, may significantly affeet'rheological behavior of ligno--cellulosics under subsequent mechanical excitation. Two experiments were carried out to study the effect of steeping (swelling) period on viscose pulp relaxation. In the first experiment this effect was investigated on samples steeped in water and 18.6% NaOH for short (1 min and 25 sec) and long periods (48 hr). For a more complete study of this interaction a second experiment with a sequence of tests following steeping periods in 18.6% NaOH ranging from 0.1 to 14400 min was undertaken. The data are shown in Figs.28 and 29. Figure 28^ in which fractional stress relaxation at 35 min is plotted against steeping time, compares stress dissipation in viscose pulps following short(one minute or less) and long (48 hr) term steeping in water or caustic. Figure 29 represents the data obtained from pulps steeped between 0.1 and 14400 min in 18.6% NaOH. It appears from both figures that steeping time influenced residual stress dissipation. Extension of steeping time unequivocally resulted in reduced ability of. the cellulosic material to absorb or dissipate energy. - VI -This i s true f o r both water and caustic steeping treatments. As exhibited i n F i g . 28, approximately 10% less stress r e l a x a t i o n was observed with specimens steeped 48 hr in water or i n 18.6% NaOH, compared to specimens steeped one minute i n water or 25 sec in 18.6% NaOH. Since the e f f e c t occurred with water steeping, although to a lesser extent than with c a u s t i c , i t i s assumed that possible extractions of soluble f r a c t i o n s during steeping could not e n t i r e l y cause the change. The r e s i d u a l solubles f r a c t i o n following prolonged steeping was not examined i n the present study. In F i g , 29 a log-log plot i s used f o r presenting data obtained with specimens treated over very short to long steeping periods. The two l i n e s show the r e l a t i o n s h i p s between <^ ( t )/(^(o) observed at 6 sec and 100 min a f t e r t and steeping time. Again, i t can be seen that the c a p a b i l i t y of pulp to d i s s i p a t e energy decreased considerably with increased treatment time. The dependence of str e s s r e l a x a t i o n on steeping period can be best expressed by a c u r v i l i n e a r r e l a t i o n s h i p as indicated i n F i g . 29. The slope reveals that short steeping periods (below one min) do not show large d i f f e r e n c e s in str e s s decay. This implies that swelling stresses did not occur over a short time period or were less than the s e n s i t i v i t y of the test method. It appears to take at least f i v e minutes steeping to observe the steeping e f f e c t a t the temperature employed. The slopes of the curves i n F i g . 29 show, furthermore, that increased steeping periods above approximately 5 minutes reduced both short (below 6 sec) and long time (above 6 sec) rates of stress dissipation-. Both curves approach each other, i n d i c a t i n g that a f t e r i n f i n i t e l y long steeping periods <^(6 sec) w i l l f a l l c lose to ^ (100 min). It may be proposed that a f t e r such a long steeping period the pulp i s "conditioned" or "s e t " i n a way that l i m i t s f u r t h e r chemical adjustment under temperature-concentration conditions of the system. Such s e t t i n g or pre s t r e s s i n g simultaneously reduces the time dependent physical-mechanical p r o p e r t i e s , at least as measured over short r e l a x a t i o n periods. It i s evident from the data presented i n F i g . 29_, that a b i l i t y of c e l l u l o s i c s to d i s s i p a t e stress can be diminished by two types of e x c i t a t i o n : - 82 -( i ) by an i n f i n i t e l y long steeping time; ( i i ) by an i n f i n i t e l y long period of mechanical e x c i t a t i o n ; and that exhaustion should occur through a combination of both. The inverse r e l a t i o n s h i p between stress d i s s i p a t i o n and steeping time i n swelling solutions may be explained i n the following way: The assumption can be made that c e l l u l o s i c materials possess a maximum capacity f o r absorption or d i s s i p a t i o n of stress under set conditions and according to t h e i r h i s t o r y . Steeping time adds a new component to that h i s t o r y . When, for instance, pulps are steeped i n water or other swelling s o l u t i o n s , the enormous swelling forces induce r h e o l o g i c a l processes within the f i b r e w a l l s . Under s u i t a b l e conditions these kinds of s t r a i n s may produce " i n t e r n a l chemical stress r e l a x a t i o n " . The longer the pulp i s kept i n the swelling environment, the larger i s t h i s component. When the material i s subsequently subjected to external physical-mechanical s t r a i n only a f r a c t i o n of the i n i t i a l capacity f o r r e l a x a t i o n can be observed. This assumes that adjustments i n chemical components, such as a l k a l i solubles, have not occurred during the chemical treatment, i . e . s c a u s t i c steeping. This phenomenon, as observed on viscose pulps, has to be a t t r i b u t e d p r i m a r i l y to molecular processes i n the amorphous regions of c e l l u l o s e . The high swelling stresses may cause molecular motion of c e l l u l o s e , probably i n v o l v i n g the mechanisms mentioned at the beginning of the sect i o n . However, i t can be assumed that these molecular rearrangements of amorphous c e l l u l o s e are not of i n f i n i t e extent due to the f a c t that c e l l u l o s e chains leave and enter c r y s t a l l i n e zones. Such dichotomy l i m i t s the l i n e a r c e l l u l o s e chains from u n r e s t r i c t e d motion. This means, that energy d i s s i p a t i o n by r e o r i e n t a t i o n of amorphous c e l l u l o s e i s of l i m i t e d extent. In the swollen state the swelling stresses w i l l e n t a i l molecular rearrange-ments which, i n turn, reduce the c a p a b i l i t y of c e l l u l o s e to d i s s i p a t e s t r e s s . Under subsequent external s t r a i n i n g the magnitude of c e l l u l o s e reponse w i l l be s o l e l y dependent on " r e s i d u a l " molecular rearrangements of amorphous e e l l u l o s e . These observations on i n t e r c h a n g e a b i l i t y of chemical and physical s t r e s s systems carry the profound suggestion of a u n i f y i n g concept for - 83 -adjudicating r e s u l t s of mixed systems. Both, as has been discussed, t r a n s l a t e i n to the same basic f u n c t i o n : r e - o r i e n t a t i o n of c e l l u l o s e i n amorphous zones. Difference l i e s only in the ways e x c i t a t i o n i s applied, i . e . , " i n t e r n a l " or " e x t e r n a l " . 4.5 A p p l i c a t i o n of Stress Relaxation Measurements f o r Characterising Pulps I n t e r r e l a t i o n s h i p between many chemical, physical and mechanical properties of c e l l u l o s i c materials i s frequently used f o r p r e d i c t i n g material behavior by estimating r e l a t e d c h a r a c t e r i s t i c s . Many cases are known where such i n d i r e c t methods of material c h a r a c t e r i s a t i o n s are employed with great success. In the f o l l o w i n g , an attempt i s made to describe a few p o t e n t i a l uses of stress r e l a x a t i o n as a tool f o r determining chemical and mechanical properties of various wood pulps and papers. 4.5.1 Estimation of viscose pulp a l k a l i s o l u b i l i t y The highly s i g n i f i c a n t c o r r e l a t i o n between f r a c t i o n a l s t r e s s r e l a x a t i o n and a l k a l i s o l u b i l i t y of viscose pulps ( F i g . 30) may be of p r a c t i c a l importance. Determination of s t r e s s r e l a x a t i o n on pulp samples i s done r a p i d l y , e a s i l y and reproducibly, whereas customary multi-step s o l u b i l i t y tests f o r determining viscose pulp q u a l i t i e s are time consuming. Consequently, r e l a x a t i o n tests may be s u i t a b l e f o r r e p l a c i n g chemical methods with less c o s t l y procedures. S o l u b i l i t y values could be estimated by using the s t r a i g h t l i n e r e l a t i o n s h i p , such as between l-£(t)/^(o) and s o l u b i l i t y i n 10% NaOH as shown i n F i g . 30, or other c a u s t i c concentrations. The inverse r e l a t i o n s h i p of f r a c t i o n a l s t r e s s r e l a x a t i o n between water and a l k a l i steeped materials ( F i g . 30) suggests that measurements done on water saturated pulps might be used also to predict a l k a l i s o l u b i l i t y . The only uncertain f a c t o r remaining f o r a p p l i c a t i o n of the technique i s the s i z e of error i n p r e d i c t i n g s o l u b i l i t y by using s t r e s s r e l a x a t i o n and r e l a t i o n of t h i s to a c c e p t i b i l i t y c r i t e r i a . From t h i s study, including numerous viscose pulp types, the accuracy of t h i s new method was found to be reasonably good, as i n d i c a t e ^ by the standard er r o r values i n F i g . 30. V a r i a t i o n within any s i n g l e pulp type under c o n t r o l l e d manufacturing conditions could be much lower. The importance here could be the u t i l i z a t i o n • - 84 -of rheology dynamics f o r "on machine" t e s t i n g , thereby introducing a point of pulp process control not presently exercised. Viscose pulp a l k a l i s o l u b i l i t y has meaning to viscose manufacturers as regards y i e l d and possibly other viscose processing f a c t o r s . It i s l i k e l y that other useful parameters, as well as t h i s , are associated with f r a c t i o n a l stress r e l a x a t i o n . 4.5.2 P r e d i c t i n g viscose pulp pressing c h a r a c t e r i s t i c s The response of fibrous structures to c o m p r e s s i b i l i t y has an important bearing on a number of f i b r e processing operations. Thus, the rate of water or a l k a l i n e s o l u t i o n flow from a fibrous mat should depend to a large extent upon i t s rheology i n compression. Compressibility of f i b r e mats or shreddings i s of p a r t i c u l a r importance to the pressing of steeped viscose pulps, where i t has been observed that time or pressure required to obtain the desired "press r a t i o " v a r i e s with i n d i v i d u a l pulps. The present work suggests that these v a r i a t i o n s are r e l a t e d d i r e c t l y to some i n t r i n s i c a b i l i t y of the pulp to absorb and d i s s i p a t e s t r e s s e s , since i t i s found that viscose pulps e x h i b i t d i f f e r e n c e s i n rate of stress decay corresponding to reported pressing behavior. Thus, high i n i t i a l rate of stress decay shows rapid energy d i s s i p a t i o n , which may ind i c a t e ease i n pressing. A fur t h e r study needs to be done f o r examining t h i s r e l a t i o n s h i p as regards i t s p r a c t i c a l i m p l i c a t i o n s . . 4.5.3 P r e d i c t i n g f i b r e response to machining In pulp beating, the deformation of f i b r e s can be considered as being of the r e l a x a t i o n type (271). Pulp f i b r e s of high p l a s t i c i t y are said to deform r e a d i l y and, therefore, respond e a s i l y to the beating treatment (272). It appears that a c e r t a i n minimum low molecular f r a c t i o n content, p a r t i c u l a r l y of hemicelluloses, i s necessax-y f o r good beating performance. It i s believed that new surfaces are formed in pulp machining only when some energy l e v e l c r i t i c a l to a given pulp i s exceeded. This may be - 85 -accomplished by further loading of the molecular system already strained by swelling processes i n the lignin-carbohydrate complex which, of course, reduces the energy a b s o r p t i o n / d i s s i p a t i o n r a t i o . Based on this work, the suggestion i s made that a high c o r r e l a t i o n should e x i s t between beating response and rate of stress decay. This i s demonstrated by the high c o r r e l a t i o n found between r e s i d u a l low-molecular f r a c t i o n s , i . e . hemicelluloses and low DP c e l l u l o s e , and r e l a x a t i o n response. It would be of p a r t i c u l a r i n t e r e s t to examine the contents of t h i s r e l a t i o n s h i p , since the stress r e l a x a t i o n test could be applied as a dynamic system f o r "on machine" p r e d i c t i o n of pulp beating response. 4.5.4 P r e d i c t i n g paper p r i n t a b i l i t y Among other f i b r e machining operations, the p o s s i b i l i t y of em-ploying stress r e l a x a t i o n f o r p r e d i c t i n g paper p r i n t a b i l i t y i s a t t r a c t i v e . According to Ivarsson (124) and Steenberg (277), softness/hardness of a paper product c o r r e l a t e s with i t s compression behavior and, together with other f a c t o r s , the compression properties of a paper or board a f f e c t p r i n t i n g r e s u l t s . This implies that i f paper i s s o f t and capable of deforming r a d i l y under pressure, required intimate contact w i l l be assured between the f i b r e s and the p r i n t i n g p l a t e , even at some v a r i a t i o n s i n surface smoothness. It can be supposed that high rate of stress d i s s i p a t i o n i n steploaded operations would i n d i c a t e a high degree of softness and, therefore, good p r i n t a b i l i t y . E x i s t i n g methods f o r assessing p r i n t i n g q u a l i t y of various paper products are not always s a t i s f a c t o r y (9,302). According to Ullman (303), current measurements can d i s t i n g u i s h r e l i a b l y only between papers of considerable d i f f e r e n c e s . It i s obvious from the ereat number of v a r i a b l e s , which have been found to influence paper p r i n t a b i l i t y , that one t e s t i n g method alone i s not s u f f i c i e n t to supply accurate information. For instance, current smoothness measurements of paper surface, widely practised for evaluating h e a v i l y coated paper surfaces (127), give some good i n d i c a t i o n regarding p r i n t i n g q u a l i t y . However, due to the f a c t that t h i s evaluation considers only surface c h a r a c t e r i s t i c s and neglects - 86 -the compressibility behavior of the paper, the information i s of limited value. It appears feasible to obtain accurate information by combining physical and mechanical testing methods, such as smoothness and stress relaxation measurements. Such a pair of measurements could be carried out simultaneously on the paper machine. The same stress relaxation data may also furnish information about runnability of paper, a property extremely important in high speed printing. It is well known that the modulus of e l a s t i c i t y (E) is closely related to frequency of breaks and bagginess (303). Low E indicates an extensible paper which adapts smoothly to the printing r o l l s , thus limiting the number of breaks. Since viscoelastic and elastic properties are highly interrelated, stress relaxation measurements would be expected to be useful in predicting the runnability of various p*per grades. 0 / -5.0 CONCLUSIONS 1. Rate of stress relaxation varied widely between individual pulps or pulp types. However, the relaxation measurements obtained from individual pulps, in particular when steeped in caustic, appeared to be highly consistent indicating good reproducibility. 2. Stress dissipation in water or caustic steeped pulp mats followed a similar pattern observed on wood and other ligno-cellulosics under constant compressive or tensile strain. 3. Two different mechanisms (M^  and appeared to be involved in pulp mat stress relaxation following quasi steploading (1.0 to 1.5 sec). One dominated the ini t i a l short period (0.0 to approximately 1.0 min), while the other featured longer time response 'from approximately 1.0 min to 35 min). It is thought that is mainly caused by inter-fibre processes, whereas M comprises intra-fibre, essentially molecular processes. 4. Pulps dissipate stress at substantially higher rates in the caustic than in the water swollen state. This underlines the importances of inter-(in water and 18.6% NaOH) and intra-(in 18.6% NaOH) crystalline swelling phenomena which determine the molecular forces between individual molecules of the pulp polymeric system. These forces, in turn, control the relaxation response. 5. Pulp chemistry exerts a profound influence on stress relaxation of both water and caustic (18.6% NaOH) steeped pulp mats. 6. Lignin appeared to make no or only an insignificant contribution to pulp mat stress relaxation in compression. This is probably due to a "layer-effect" causing discontinued stress communication in the lienin-(carbohydrate complex and due to the dominant role of hemicelluloses which seem to obscure a moderate contribution of lignin. 7. The hemicelluloses were found to be of extreme importance for stress dissipation or redistribution processes in water and caustic swollen pulp mats. Both quantitative and structural features of degraded or non-degraded residual hemicelluloses appeared to play an outstanding role in the - 88 -r e l a x a t i o n response. High hemicellulose contents in a s s o c i a t i o n with no or i n s i g n i f i c a n t degradation provide a considerably higher degree of visco-e l a s t i c capacity than extremely low contents of h i g h l y degraded and redeposited hemicellulose residues. In water swollen low y i e l d pulps,the h i g h l y degraded and redeposited residues were found to exert even a reverse e f f e c t , i . e . they retarded stress decay. 8. It i s apparent from t h e i r r h e o l o g i c a l behavior i n pulp mats that hemicelluloses must function as an extraordinary part i n the stress d i s s i p a t i n g system i n l i v i n g wood. This function i s f a c i l i t a t e d by the high degree of branching and the low DP. 9. It i s evident that c e l l u l o s e accounts f o r most of the d i s s i p a t e d stress i n pulp mats steeped i n water or c a u s t i c . In the native or only s l i g b t l y degraded state i t s c o n t r i b u t i o n to- pulp mat rheology seems to vary only i n s i g n i f i c a n t l y between pulps of various hemicellulose contents. However, progressive chain cleavage leading to DP values below 1000 d r a s t i c a l l y changes the r h e o l o g i c a l behavior. This i s i n d i c a t e d by considerably increased rates of stress r e l a x a t i o n of gamma-irradiation degraded pulps. 10. High degree of c e l l u l o s e degradation and degradation and redeposition phenomena of hemicelluloses reverse the o r i g i n a l f u nction of these carbohydrates in r h e o l o g i c a l processes i n the. water swollen pulp mats. Degraded c e l l u l o s e d i s s i p a t e d stress at increased r a t e s , whereas degraded and redeposited hemicelluloses retard r h e o l o g i c a l processes. 11. In the c a u s t i c steeped s t a t e , pulp mat rheology appeared.-to be c o n t r o l l e d s o l e l y by the short chain material present i n the pulp. This implies that i t does not depend on any s p e c i f i c carbohydrate component. 12. The profound e f f e c t of steeping time, observed on water and c a u s t i c (18.6% NaOH) steeped viscose pulps, on r e s i d u a l s t r e s s r e l a x a t i o n suggests the i n t e r c h a n g e a b i l i t y of chemical and physical stress systems. Therefore, the c a p a b i l i t y of pulps to d i s s i p a t e or r e d i s t r i b u t e stress can be diminished by two types of e x c i t a t i o n : ( i ) an i n f i n i t e l y long steeping time; ( i i ) an i n f i n i t e l y long period of mechanical e x c i t a t i o n ; and a combination of both. ov 13. F r a c t i o n a l stress relaxation measurements on pulps and papers are suggested as useful tools f o r determining chemical and mechanical pulp properties, such as pressing c h a r a c t e r i s t i c s and a l k a l i s o l u b i l i t y of viscose pulps, estimation of pulp f i b r e response to machining (beating) and evaluation of r u n n a b i l i t y and p r i n t a b i l i t y of paper. 14, Further studies need to be done f o r examining the r e l a t i o n s h i p between stress r e l a x a t i o n and pulp mat c h a r a c t e r i s t i c s as regards i t s p r a c t i c a l i m p l i c a t i o n s . - 90 -6.0 LITERATURE CITED Abdurahman, N. , Dutton, G.G.S., McLardy, D.M. , P i e r r e , K.E. and B.I. Stephenson. 1964. Hemicelluloses of black spruce, Si t k a spruce, ponderosa pine and Douglas f i r , Tappi 47: 812. Adams, G.A. 1964. Wood carbohydrates. Pulp Paper Mag. Can. 65: T13-T24. Adler, E. 1957. Newer views of l i g n i n formation. Tappi 40: 294-301. 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Cellulose 45 42 48 Lignin 24 19 21 Glucuronpxy Ian 25 35 24 Glucomannan 4 3 3 Pectin, starch 2 1 4 Balsam fir White spruce (Abies bolsameo (Piceo g louco Eastern white pine Component [L.] Mill) tMoen chl Voss) (Pinus strobus L.) Cellulose 42 41 41 Li gnin 29 27 29 Arabinoglucuronoxylan 9 13 9 Galoctoglucomannan 18 18 18 Pectin, starch 2 1 3 Table 1. The chemical composition of woods from three angiosperms and three c o n i f e r s ( a f t e r T i m e l l (294)). - 1 2 5 -Table 2/a.Description of pulps used i n the study. Product Wood Pulp Pulping process A d d i t i o n a l Code No. source ) type or method of pulping preparation (and treatments s t a r t i n g experi-. mental material )) 0 - 1 ( 9 - 1 )° ) C (W) — 0-2(9-2) A Alpha- TAPPI CW) — 0-3(5-1) C c e l l u l o s e standard (s) T.203 os— 61 (S) 0-4(1-3) C 7 + -1-1 C L Sulphate (S) -1-2 ' C Sulphate (S) -1-3 c Sulphate (S) -1-4 c Sulphate (S) -2-1 c Sulphite (S) _ Viscose (S) 2-2 c Sulphite pulps 2-3 A Sulphate Cs) -2-4 C+A Sulphate (S) -3-1 C Sulphite (S) -3-2 C Sulphite (s) -3-3 C Sulphate (s) -3-4 C Sulphate (s) -4-1 C Sulphite (S) -4-2 1 i f Sulphite (S) -a ) Coniferous (C), Angiospermous (A). k) S t a r t i n g experimental material as wood (W), pulp f i b r e s l u r r y (P) or pulp sheet (S). c ) Numbers i n brackets r e f e r to code No. of pulps used f o r a l p h a - c e l l u l o s e preparations. - 126 -Table 2/b. Product Wood Pulp Pulping process Additional Code No. source ) type or method of (and pulping preparation treatments starting experi-. mental material )) 5-1 C Acetate Sulphite (S) — 6-1 c Sulphite Cs) Bleached. 7-1 c Sulphate (P) Unbleached 7-2 c Paper Sulphate (P) Bleached grade (s) 8-1 c Sulphate Bleached 8-2 10% c+ Sulphate (s) Bleached 90% A r (w) 9-1 C Chlorite — 9-2 A Holo- Chlorite (W) — 9-3 C cellulose Peracetic acid (W) 9-4 A Peracetic — r acid (W) . 10-1 C Mechanical (P) Unbleached 10-2 C Grc >und- Mechanical (P) Bleached 10-3 A wood Mechanical (P) Unbleached 10-4 A r Mechanical (P) Bleached - 127 -Table 3/a. Summative data ( i n per cent of e x t r a c t i v e - f r e e wood) f o r chemical composition of.pulps tested i n the study. Values f o r c e l l u l o s e and t o t a l amount of hemicelluloses have been adjusted according to ratios: of glucose-mannose i n i s o l a t e d wood.gluco-mannans ( i n c o n i f e r s 1:3, and i n pored wood 1:2(295)). Component Alpha - c e l l u l o s e preparations 0-1 0-2 0-3 0-4 Ac e t y l — - — Uronic anhydride - - - -Residues of: Galactose - - - -Glucose 92.9 97.1 99.5 99.2 Mannose 6.2 2.0 0.5 0.8 Arabinose 40.1 0.1 - — Xylose 0.6 1.3 0.2 0.5 Rhamnose . - - - -; C e l l u l o s e 90.8 96.1 99.3 99.0 ! Hemicelluloses 8.9 4.4 0.9 1.5 ! L i g n i n i — -| T o t a l 99.7 100.5 100.2 100.5 I - 128 -Table 3/b. Component Viscose pulps 1-1 ; 1-2 1-3 1-4 A c e t y l -; - -Uronic anhydride -: - — Residues of: Galactose - - - -Glucose 95.1 97.0' 96.6 97.5 Mannose 2.4 2.0 1.8 2.0 Arabinose ^0.1 0.1 /.0.1 <0.1 Xylose 1.9 2.4 0.8 0.7 Rhamnose - — -C e l l u l o s e 94.3 96.3 96.0 96.8 Hemicelluloses 5.1 5.2 3.2 3.4 L i g n i n i . - — — -T o t a l 99.4 j 101.5 j i 99.2 100.2 - 129 -liable 3/c. Component Viscose pulps 2-1 2-2 2-3 2-4 A c e t y l - - - -Uronic anhydride - - mm* Residues of: Galactose - - - -Glucose 92.5 97.3 94.2 96.3 Mannose 2.3 1.5 1.0 0.7 Arabinose <0.1 ^0.1 0.1 ^0.1 Xylose 1.8 1.9 2.6 1.2 Rhamnose ' — - — C e l l u l o s e 91.7 96.8 93.7 96.1 Hemicelluloses 4.9 3.9 4.2 2.2 L i g n i n — — • — — • T o t a l 96.6 100.7 97.9 98.3 | - 130 -Table 3/d. Component Viscose pulps 3-1 3-2 3-3 3-4 A c e t y l - - - -Tronic anhydride - - - -Residues of: Galactose - - - mm-Glucose 93.1 92.4 96.1 96.5 Mannose 1.8 3.5 1.6 1.6 Arabinose ^0.1 -:0.1 40.1 40.1. Xylose 1.7 1.4 1.9 0.6 Rhamnose . - - - -C e l l u l o s e 92.5 91.2 95.6 96.0 Hemicelluloses 4.1 6.1 4.0 2.7 L i g n i n — - -T o t a l 96.6 97.3 | 99.6 98.7 - 131 -Table 3/e. • 1 Component Viscose pulps Acetate pulp Bleached s u l p h i t e pulp 4-1 4-2 5-1 6-1 A c e t y l - - - 3.1 Uironic anhydride - — - 0.9 Residues of:. Galactose - - - 0.2 Glucose 97.0 93.9 97.0 86.5 Mannose 2.3 2.2 2.2 7.2 Arabinose ZO.l *0.1 ^0.1 0.3 Xylose 1.5 1.5 1.2 Rhamnose Ce l l u l o s e 96.2 93.2 96.3 84.1 Hemicelluloses 4.6 4.4 4.1 17.9 L i g n i n — — - 0.7 T o t a l 100.8 97.6 100.4 102.7 *) values i n d i c a t e t o t a l xylose residues as obtained from gas- l i q u i d chromatography measurements ( i n brackets) and corrected f o r xylose residues i n aldobiouronic a c i d groups which r e s i s t e d a c i d hydro-l y s i s (according to Meier and Wilkie (192} and Zinbo . and Timell (327)' ), - 1 3 2 -T a b l e 3/f. Component Unbleached-s u l p h a t e pulpsi Bleached; sulphate pulps \ 7-1 / 7-2 8-1 8-2 Acetyl - - -Uronic anhydride 1.1 0.7 0.7 1.6 Residues ofr •Galactose 0.3 0.3. 0.2 0.2 Glucose 74.6 83.4 83.1 83.0 Mannose 7.5 8.4 7.2 1.5 Arabinose 0.5 0.5 0.3 0.5 Xylose 6.0 (5.3)*) 6.0 (5.5)*) 7.4 (6.9)*) 14.7 (13.6)*) Rhamnose - - - -C e l l u l o s e 72.1 80.6 80.7 82.2 Hemicelluloses 17.9 18.7 18.2 19.3 Li g n i n 9.1 1.0 0.3 0.5 Tot a l 99.1 100.3 99.2 • " ! 102.0 j - 133 -Table 3/g. Component Holocellulose pulps 9-1 9-2 9-3 9-4 Acetyl 5.1 6.0 3.8 5.6 "(Ironic anhydride 3.0 3.0 2.6 2.2 Residues of: Galactose 0.9 0.6"' 1.1 0.7 Glucose 72.4 75.1 71.7 73.8 Mannose 11.9 4.3 12.0 4.6 Arabinose 0.4 0.3 0.5 0.3 Xylose 5.4 (3.4)*] 13.3 (11.3)*) 5.7 (4.0)*) 12.3 (10.8)*) Rhamnose - 0.1 - 0.1 Cellulose 68.5 73.0 67.7 71.5 Hemicelluloses 30.6 29.7 29.7 28.1 Lignin 0.3 0.4 0.3 0.4 Total 99.4 103.1 j 97.7 t 100.0 j Table 3/h. Component Groundwoods 10-1 10-2 10-3 10-4 Acetyl 3.7 3.8 4.7 4.7 llronic anhydride 2.8 2.8 4.0 4.0 Residues oft. Galactose 1.2 1.1 1.0 1.2 Glucose 47.0 46.5' 50.5 50.8 Mannose 12.4 12.7 2.9 2.9 Arabinose 0.5 0.6 0.7 0.6 Xylose 5.4 5.7 16.4 16.8 (3.6)*) (3.9)*) (13.8)*) (14.2)*) Rhamnose 0.1 0.1 0.3 0.2 C e l l u l o s e 42.9 42.2 49.0 49.3 Hemicelluloses 30.1 31.0 31.5 31.9 L i g n i n r f 30.5 30.3 21.7 21.6 ! T o t a l | i J 103.5 103.5 102.2 ; 102.8 j i - 135 -Table 4/a. Summary of f r a c t i o n a l stress r e l a x a t i o n r e s u l t s on pulps tested i n the study as read a f t e r 35 min r e l a x a t i o n time (1 - £ ( 3 5 m i n ) / £ ( 0 ) ) f o l l o w i n g steeping i n water or ca u s t i c . 1 -£(35 min) Product £ ( o ) Code H20 , distilled 18.6% NaOH No. Single >iean Std. ' Single Mean Std. Measur. Value 3?ev. i-ieasur. Value Dev. r-.cellulose pulps 0-1 .495 .667 .480 .655 .479 .652 .592 .661 .487 .497 .026 .648 .657 .008 0-2 .510 .702 .498 .705 .492 .693 .494 .699 .464 .492 .017 .702 .701 .003 0-3 .534 .625 .543 .618 .550 .628 .537 .638 .523 .537 .010 .639 .630 .009 0-4 .527 .582 .546 .576 .515 .579 .541 .580 .513 .528 .015 .577 .579 .002 Viscose Pulps 1-1 .436 .648 .443 .634 .477 .642 .470 .673 .477 .461 .020 .668 .653 .017 1-2 .481 .654 .456 .615 .470 .659 .435 .669 .461 .461 .017 .649 .649 .021 - 136 -Table 4/b. 1 - <^ >(35 min) Product £ (o ) Code d i s t i l l e d 18.6% I-;aOH No. Single Mean Std. Single Mean Std. lieasur. Value 3>ev. Measur. Value Bev. Viscose Pulps 1-3 .485 .610 .485 .616 .506 .622 .500 .618 .486 .492 .010 .621 .617 .005 1-4 .509 .576 .482 .611 .512 .619 .497 .618 .496 .499 .012 .614 .608 .018 2-1 .468 .720 .487 .730 .400 .714 .467 .725 .443 .453 .034 .734 .725 .008 2-2 .503 .683 .448 .702 .487 .702 .422 .716 .514 .475 .039 .759 .712 .029 2-3 .484 .677 • .482 .682 .530 .679 .520 .676 .500 .503 .021 .690 .681 .006 2-4. .532 .599 .554 .569 .534 .594 .549 .593 .562 .546 .013 .601 .591 .013 3-1 .533 .642 .506 .644 .496 .647 .470 .643 .497 .500 .023 .628 .641 .007 - IM -Table 4/c. 1 - £(35 rain) Product (o) Code , d i s t i l l e d J 18.( 5% NaOH No. Single Mean Std. Single Lean Std. Heasur. Value Uev. Measur. Value J>ev. Viscose Pulps 3-2 .474 .695 .461 .690 .428 .716 .461 .699 .465 .458 .018 .706 .701 .010 3-3 .508 .617 .527 .613 .511 .615 .505 .622 .496 .509 .011 .622 .618 .004 3-4 .517 .600 .520 .590 .524 .608 .537 .617 .529 .525 .008 .610 .605 .010 4-1 .466 • .704 .497 .709 .512 .727 .498 .725 .491 .493 .017 .746 .722 .017 4-2 .486 .707 .456 .741 .416 .713 .458 .724 .485 .460 .029 .751 .727 .019 Acetate Pulp 5-1 • 527 .682 .529 .633 .543 .673 .525 .701 .547 .535 .010 .701 .678 .028 - 138 -Table 4/d. Product 1 - <o(35 £» (o rain) ) Code fl2°» distilled 18.6% NaOH-No. Single Measur. Mean Value Std. Dev. Single Measur. Mean - Value Std. Dev. Sulphite Pulp 6-1 .506 .559 .519 .518 .513 .523 .021 .795 .807 .797 .809 .798 .801 .006 Unbleached Sulphate Pulp 7-1 .574 .572 .533 .570 .536 .577 .007 .735 .723 .731 .724 .734 .729 .006 Bleached Sulphate Pulps 7-2 .533 .533 .548 .532 .526 .534 .782 .769 .781 .775 .785 .778 .006 8-1 .510 .530 .545 .506 .524 .523 .012 .811 .804 .810 .802 .808 .807 .004 8-2 .539 .538 .506 .496 .538 .524 .021 .741 .744 .735 .748 .735 .741 .006 - 139 -Table 4/e. Product 1 - £(35 min) <Z (o) Code H20, distilled 13.6% NaOH. No. Single Measur. Mean Value Std. Vev. Single Measur. Mean Value Std. 3>ev. Holocellul-ose Prepara-tions 9-1 .669 .695 .642 .670 .641 .643 .022 .823 .793 .810 .311 .829 .813 .014 9-2 .703 .725 .730 .706 .712 .715 .012 .872 .858 .869 .821 .872 .858 • .022 9-3 .652 .658 .596 .647 .663 .643 .027 .820 .771 .773 .783 .808 .792 .021 9-4 .695 .686 .702 .672 .709 .693 .014 .817 .820 .826 .820 .814 .819 .005 Groundwood Pulps ' 10-1 .661 .671 .633 .651 .653 .654 .014 .865 .874 .870 .855 .875 .868 .008 10-2 .643 .673 ' .685 .639 .657 .659 .020 .845 .845 ,847 .868 .867 .855 .012 - 1 4 0 -T a b l e 4/f. Product Code No. •1 - £(35 min) <£(o) •HO, distilled 18.6% NaOH Single Measur. Mean Value Std. J>ev. Single Measur. Mean Value Std. 3»ev. Groundwood Pulp& 10-3 10-4 .672 .662 .664 ,653 .663 .663 .007 .648 .644 .649 .685 .677 .661 -.019 .888 .890 .890 .893 .891 .890 .002 .903 .900 .899 .902 .874 .896 .012 Table 5. Properties of viscose pulps treated with various doses of gamma-radiation. Product Treatment, Alpha- I n t r i n s i c 10% NaOH Code No. Mrd c e l l u l o s e , V i s c o s i t y , S o l u b i l i t y , % [?3 % 3-2 0.0 90.3 12.8 9.5 0.5 90.0 6.0 9.9 1.0 8S.4 4.5 12.6 2.0 82.4 3.0 15.5 4.0 63.2 2.1 25.2 6.0 47.5 1.3 43.1 3-4 0.0 97.5 13.2 1.6 0.5 97.5 6.2 1.8 1.0 96.0 4.7 3.3 2.0 91.1 3.1 5.1 4.0 75.9 2.1 14.1 6.0 66.5 1.3 32.9 - 142 -Table 6. Mul t i p l e c u r v i l i n e a r covariance a n a l y s i s f o r the " r e l a t i o n s h i p between 10% NaOH s o l u b i l i t y and f r a c t i o n a l s t r e s s r e l a x a t i o n of i r r a d i a t e d and untreated viscose pulps. Groups DP P Set 1 (untreated pulps) 11 Set 2 ( i r r a d i a t e d pulp No. 3--2) 3 Set 3 ( i r r a d i a t e d pulp No. 3--4) 3, To t a l 17 Difference f o r t e s t i n g slopes 4 1.343 N.S. Sums 21 Difference f o r t e s t i n g l e v e l s 2 3.074 N.S. Combined regres s i o n 23 FIGURES - 1 4 4 -CA) Warty layer F i b r i l l a r layer Secondary wall Transition layer Primary wall Middle lamella } Tertiary layer Angiospermous Coniferous wood (birch) wood (spruce) (B) Outer surface (PJ Inner surface lPldl Primary wall \ (0-B.Lamellae) Innsr layer of secondary wall Outer layer of secondary wall IS,) [Several lamellae intermedial! between of the Ss a layers) Middle layer of secondary nil (S?) (Ca. 30 - ISO lamellae) (Several lamellae of orientation intermediate between that of the 5t and layers) (1-S Lamellae) (C) Warty layer (W) mm Inner (S.)] /fiJjK •—-Main (SX)?- Secondary wall Outer (SJ)J Primary wall (P) Middle lamella (M) Figure 1. Schematic representation of the c e l l wall organisation i n a coniferous tracheid and/or angiospermous wood fibre , with respective m i c r o f i b r i l orientation: (A) after Meier (187); (B) after Wardrop and Harada as i n Wardrop (316). after Tsoumis (301). * Figure 2. The unit c e l l as proposed by Meyer and Misch (197). - 146 -S t r e s s <£(t) \ G° o s /£6l) s t r a i n /applied at / f i n i t e rate 1 ***'-t ,i Time F i g u r e 3. S t r e s s r e l a x a t i o n under cons tan t s t r a i n ( a f t e r Leaderman ( 1 6 1 ) ) . - 147 -Figure 5. Gas chromatogram of the ac e t y l a t e d hydrolysates of western hemlock groundwood pulp No. 10-2 (brightened). Components: 1. Rhamnose 5. Galactose Time, min Figure 6", Gas chromatogram of the acetylated hydrolysates of western cottonwood groundwood No. 10-4 (brightened). Components: Figure 7. Gas chromatogram of the acetylated hydrolysates of western cottonwood peracetic a c i d h o l o c e l l u l o s e No. 9-4. 5 Recorder Response T- 1 I 1 , 2G 30 AO Time, min Figure 8. Gas chromatogram of the acetylated hydrolysates of unbleached ' coniferous sulphate pulp No. 7-1. Figure 9. Gas chromatogram of the acetylated hydrolysates of bleached coniferous sulphate pulp No. 7-2. Components: Figure 10. Gas chromatogram of the acetylated hydrolysates of bleached predominantly angiospermous sulphate pulp No. 8-2. Figure 11. Gas chromatogram of the acetylated hydrolysates of coniferous viscose pulp No. 1-2. Figure 12. Gas chromatogram of the acetylated hydrolysates of alpha-c e l l u l o s e pulp No. 0-3. as Time, wir. Figure 13» Gas chromatogram of the acetylated hydrolysates of bleached coniferous sulphite pulp No. 6-1. 1.00 0.00 0.00125 0.0.1 0.10 1.00 10.00 ' 35.00 Relaxation time (min) 14. F r a c t i o n a l stress r e l a x a t i o n curves f o r groundwood — pulps following water or ca u s t i c steeping (n = 5). 3.-Q0-1 Residual;' Stress v :<5(t) .0.80 0.60 0.40-0.20 -0.00 J^y Key: 1. Coniferous h o l o c e l l u l o s e pulp No. 9-3. 2. Angiospermous h o l o c e l l u l o s e pulp No. 9-4. Steeped i n d i s t i l l e d Ho0 (1 rain, 2 2° C ) . Steeped i n 18.6$ NaOH (25 sec, 2 2° C ) . ' < 2. 1 2' 0.00 0.00125. 0.01 0.10- 1.00 1 ; 1 10.00 35.0Q. Relaxation time (rain) Figure 15. T y p i c a l f r a c t i o n a l stress r e l a x a t i o n curves f o r h o l o c e l l u l o s e pulps f o l l o w i n g water or caustic steeping (n = 5). VO COO 0.00]25 0.01 0.10 1.00 10.00 35.00 Relaxation time (min) Figure 16. F r a c t i o n a l stress r e l a x a t i o n curves f o r paper pulps, following water or ca u s t i c steeping (n = 5 ) . 1.00 -, Residual Stress (£<t)/€#o>) 0.30 • 0.60* 0.40 • 0.20 Key: 1. 2. Coniferous s u l p h i t e acetate pulp No. 5-1. ©<-cellulose pulp No. 0-3 prepared from coniferous sulnhite acetate pulp No. 5-1. steeped steeped i n l8.6?o NaOH i n d i s t i l l e d H o0 (1 (25 m m , 22°C). 0.00 0.00125 — I 0.01 — I — 0.10 1.00 10.00" 35.00 Relaxation time (min) Figure 17. F r a c t i o n a l stress r e l a x a t i o n curves f o r acetate and alpha-cellulose pulps following water or caustic steeping (n = 5 ) . 1.00 Residual stress ( £ ( t > / £ ( o ) ) 0.80 0.60 « 0.40 Key: 1. Coniferous s u l p h i t e pulp No. 2-2 2. Coniferous sulphate pulp No. 3-4 3. Angiospermous sulphate pulp No. 2-3 steeped i n d i s t i l l e d water (1 min,22°C) steeped i n 18.6% NaOH (25. sec,22°C) 0.20 0.00 0.00125" 0.01 — i — 0.10 1.00 1 0 . 0 0 3 5 . 0 0 Relaxation time ( n i n ) ' Figure 18. T y p i c a l f r a c t i o n a l stress r e l a x a t i o n curves f o r three viscose pulps f o l l o w i n g water or ca u s t i c steeping (n = 5). . -162 -0.30 0.75 0.70 . 0.65 . 0.60 Dissipated Stress 0.55 1 - ^ (35 rr.in) <5 (o) 0.50 0.45 0.40 . y = 0.4782 + 0,3011 l o - x tfhere y = 1'- €1* (35 run) of NaOH steeped snecir.ens x = pulp henicellulor.e content r => 0.71**; s E E = 0.037; DF = 12 °o o ">eeped i n 18.6% N,;.0H (25 sec, 22°C) Stepped i n d i s t i l l e d vatei (1 min, 22°C) y = 0.6147 - 0.2083 log where y = 1 - (35 r.in) of water steeped speci-ens <S ( n ) x - pulp hemicellulose content r = 0 . 8 7 * * ; Ser » 0 .014 ; DF = 12 T T TT 5 Hemicellulose content, % Figure 19. Correlation between fractional stress relaxation of 14 viscose pulps read after 35 min relaxation time following steeping in water or caustic and pulp hemicellulose content. 0.80 0,70. Dissipated Stress 1 - (35 min) (o) O.60 0.50 0.40 .9-2 • 9-4 9-3 ®9-l 10-4 10-3 10-2 10-1 7-1? 19 pulps (0-1—5-1) ON u> —T— 10 20 30 Lignin content, % Figure 20. Relationship between f r a c t i o n a l stress r e l a x a t i o n (n = 5) and l i g n i n contents as observed on water steeped samples of various pulp types a f t e r 35 min r e l a x a t i o n time. Code numbers r e f e r to pulps l i s t e d i n Table 2. 0.90% 0.30. .ipnted Stress g£(35 min) 0.60 0.50 10-4 10-3-•-9-2 ^9-4 - 8-1 -9-1 \>6-l •>9-3 N7-2 8-2 10-1-« 1 0 - 2 - « 0 7-1 • •19 pulps (0-1 — 5-1) 10 20 30 Lignin content, % Figure 21, Relationship between f r a c t i o n a l s t r e s s r e l a x a t i o n (n = 5) and l i g n i n contents as observed on c a u s t i c steeped (18.6% NaOH) samples of various pulp types a f t e r 35 min re l a x a t i o n time. Code numbers r e f e r to pulps l i s t e d i n Table 2. 0.90 Dis s i p a t e d Stress 1 - (2(35 min) ~STol 0.B0 y = 0.595 + 0.005x + 0.076 log x where y = 1 - d(35 min) x = hemicellulose content r = 0.92**; S E £ = 0.039: DP => 29 6-1 CO Caustic steeped specimens 0.70 0.60 0.50 • 10-4 10-3 \ oT 9-4 n^°°-V '.Vater steeped specimens 0.40 '8-1 y » 0.549 + 0.013 x - 0.189 l o g x where y = 1 - £(35 min) <£(o) x = hemicellulose content r = 0.94 **; = 0.026; DP = 29 i • 1 . • 10 20 30 Hemicellulose content,% Figure 22. Relationship between f r a c t i o n a l s t r e s s r e l a x a t i o n (n=5) and hemicellulose contents as observed on water and c a u s t i c (18.6% NaOH) steeped samples of various pulp types a f t e r 35 min r e l a x a t i o n time. 0.80. •• Dissipatcd Stress 1 -d<3*> mU: <&<•-•) 0. 70. 10-2 10-1 — • 9-2 10-3 • 10-4 — 9-3 ..9-1 O.60. 7-1 • • > 0.50j y r c "£E 7-2 X . S"1 I • 9 6^V. - -0.7004 - 0.0106x + 1.1155 log x , 8 - 2 = 0.842*"* - 0.043; DF » 29 0-1*0,5-1 • • • • e 9 9 9 ^ 0.40 1 40 50 60 70 80 90 100 Cellulose content, Z. F i g u r e 23* R e l a t i o n s h i p between f r a c t i o n a l s t r e s s r e l a x a t i o n ( n = 5) and c e l l u l o s e c o n t e n t s as o b s e r v e d on w a t e r s t e e p e d samples o f v a r i o u s p u l p t y p e s a f t e r 35 min r e l a x a t i o n t i m e . Code numbers r e f e r t o p u l p s l i s t e d i n T a b l e 2. 0.90 Dissipated S t r e s s (35 run) <S(c) 0.80 0.70 0.60 0.50 10-4 ©• 10-3 10-1 10-2 40 y r 1.5841 - 0.0168x - 1.9435 log x 0.905** 0.041; DF = 29 50 60 70 0-1-U5-1 80 90 100 C e l l u l o s e content, % Figure 24. Relationship between f r a c t i o n a l s t r e s s r e l a x a t i o n (n = 5) and c e l l u l o s e contents as observed a f t e r 35 min re l a x a t i o n time on ca u s t i c steeped (18.6% NaOH) samples of various pulp types. Code numbers r e f e r to pulps l i s t e d i n Table 2. - 1 6 8 -0.90 • 0.80 • 0.70 • 0.60 0.50 • 0.40 Dissipated Stress 1 ^ ( t ) + -+- + • O i l T " 2 T " 3 .+ 3-2 ( 6 sec) •+ 3-4 ( 6 sec) 4 5 Mr ad T -6 Figure 25. Short and long time response i n fractional stress relaxation (n = 5) of two viscose pulps exoosed (air-dry) to different gamma-radiation levels and then steeped i n 18.6% NaOH (30 sec, 22°C). ' ' ' I I 1 I 0 10 20 30 40 10% NaOH solubility, % Figure 26. Correlation between fractional stress relaxation (n = 5) of irradiated and untreated viscose pulps read after 35 min relaxation time following steeping in 18.6% NaOH and pulp caustic solubility. - I /u -Figure 27. Relationship between caustic s o l u b i l i t y and relative amount of hemicelluloses of 14 viscose pulps. 0.80 0.70 D i s s i p a t e d S t r e s s 1 * ~ <^  (35 min) S'(o) 0.60 0.50 a AO Steeped i n 18.6% NaOH (25 s e c , 22°C) 2-1' o I I I I I 2-2' o I-I 2-3' o I I I I I 3-2' o I I I 2-4' o 3-3' o I 3-4' o zzzzzzzzzzzz i i i 6 ' i ! ! 2-4 ! I I o I 2-3 o 3-3 I o 3-4 o I 2-2 o 2-1 o 3-2 Steeped i n d i s t i l l e d water (I min, 22°C) Steeped i n 18.6° (48 h r , 22°C) 2-1' o I 2-2' I o I I T 2-3 I ZZZZ2? ' 3-2' o I I o NaOH 3-3' O 3-4 i ?3 / / / / / / . I 2-1 O a 2-2 Steeped i n d i s t i l l e d (43 h r , 22°C) I I I o 3-3 *i.t;er o 3-4 Figure 28". Fractional stress relaxation results (n = 5) on seven viscose pulps as read after 35 min relaxation time following short and long time steeping i n water or caustic. l . O O l Residual Stres ( t ) G"( o ) 8. 80 <X60. O.A0. 0.25 0.1 (£(t)/G£(o) as read after 6 sec relaxation time O- • ° > <s(t)/^(o) as read aftea* 100 min relaxation time 10 100 I 1000 10000 100000 Steeping time, min Figure 29. E f f e c t of steeping time i n 18.6% NaOH ( 2 2°C ) on r e s i d u a l stress r e l a x a t i o n (n = 5) of viscose pulp No. 3-2. - 1 / J -DVGOT 0.75 0.70 0.65. 0.60 D i s s i p a t e d S t r e s s 1 - (35 ntn) (o) 0.55 0.50 0.45 0.4o. y = 0.58*911 + 0.0105S5x where y = 1 - g£ (35 - i n ) of XaOH steeped specimens x = s o l u b i l i t y i n 10% I^aOI! r - 0.S3**; S F E = 0.030; . 0 ° DF = 12 Steeped i n 18.6% NaOH (25 s e c , 22o c) Steeped i n d i s t i l l e d water (1 n»in, 22°C) v = x r 0.5181.39 - 0.004345 x where y = 1 - c& (35 r-irQ er (c) o f water s t e e p e d specimens s o l u b i l i t y i n 10% :;aOH. 0.60*; S E E = 0.023; DF = 12 10 —T" 12 —r .14 10% NaOH s o l u b i l i t y . % Figure 30. C o r r e l a t i o n between f r a c t i o n a l s t r e s s r e l a x a t i o n (n=5) of 14 viscose pulps read a f t e r 35 min r e l a x a t i o n time f o l l o w i n g steeping i n water or caustic (18.6% NaOH) and ca u s t i c s o l u b i l i t y . 

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