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UBC Theses and Dissertations

Stress relaxation of paper plastic composites Chen, Chien-pin 1973

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STRESS RELAXATION OF PAPER PLASTIC COMPOSITES by Chien-pin Chen Bi.S. ( For. ) Taiwan P r o v i n c i a l Univ., 1964 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of F o r e s t r y We accept t h i s t h e s i s as conforming to the re q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA May 1973 In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of 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 f o r extensive 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 permission. 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 - i -ABSTRACT 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 ( S(t)/S ( 0|04 ) ) was used to compare time-dependent p r o p e r t i e s of papers, p l a s t i c s and paper p l a s t i c composites ( PPC ). No s i m i l a r observations appear i n the l i t e r a t u r e . Laboratory handsheets were prepared from commercial western hemlock ( Tsuga h e t e r o p h y l l a (Raf.) Sarg. ) unbrightened and brightened groundwoods, as w e l l as unbleached and bleached k r a f t pulps. Adjustments were made to provide e q u i v a l e n t b a s i s weights ( 150 -30 g/m ) f o r m a t e r i a l s of the study. Handsheets were impregnated w i t h methyl methacrylate ( MMA ) and te t r a e t h y l e n e g l y c o l dimethacrylate ( TEGDMA ) (co)monomer systems. Saturated handsheets and p l a s t i c f i l m s were cured by ^ °Co gamma i r r a d i a t i o n ( 1.4 - 0.9 Mrad ). I t was found t h a t the standard log-time equation, S(t)/S ( 0.04) = a + b I n t , ap p l i e d to data c o l l e c t e d between 0.04 and 35 min f o l l o w i n g completion of simulated s t e p - l o a d i n g ( r mostly O.97 or higher ). A second q u a n t i t y , energy d i s s i p a t i o n ( A s ) , A S = 1 - S(35)/S(0.04), - i i -was used to compare between treatments. Some p l a s t i c s gave the highest A S values, while groundwoods gave lowest values and k r a f t papers were intermediate. Pulp d e l i g n i f i c a t i o n l e v e l appeared to r e l a t e d i r e c t l y to & S. Within l i m i t s of the study i t seems th a t PPC s t r e s s r e l a x a t i o n curves were i n f l u e n c e d by both polymer ( matrix ) and f i b r e ( substrate ) employed. The former c o n t r i b u t e d i n minor ways, while the l a t t e r operated i n major ways. * • • - I l l -TABLE OP CONTENTS ABSTRACT i TABLE OF CONTENTS i i i LIST OP TABLES v i LIST OF FIGURES v i i ACKNOWLEDGEMENT v i i i 1.0 INTRODUCTION 1 2.0 LITERATURE REVIEW 3 2.1 Strength P r o p e r t i e s of Paper P l a s t i c Composites 3 2.1.1 G r a f t i n g to pulps 3 2.1.2 G r a f t i n g to papers 6 2.2 E f f e c t of F i b r e Chemical Co n s t i t u e n t s on Grafting» E s p e c i a l l y L i g n i n 10 2.3 E f f e c t of Gamma R a d i a t i o n on C e l l u l o s i c s and Polymers 12 2.4 R h e o l o g i c a l Behaviour of S o l i d s 15 2.4.1 S t r e s s r e l a x a t i o n of s o l i d s ; g eneral survey 16 2.4.2 R e l a x a t i o n of polymers 20 2.4.3 R h e o l o g i c a l p r o p e r t i e s of papers .. 27 2.4.3.1 S t r e s s r e l a x a t i o n of papers 29 2.4.4 Time dependent behaviour of f i b r e p l a s t i c composites 32 - i v -Page 3.0 MATERIALS AND METHODS 34 3.1 Pulps 34 3.2 Monomers and Comonomers 36 3.3 Formation and Treatment of Paper Handsheets. 41 3.4 P r e p a r a t i o n of Polymer Thd& F i l m s 42 3.5 P o l y m e r i z a t i o n 43 3.6 T e s t i n g 44 4.0 RESULTS.. 48 4.1 S t r e s s R e l a x a t i o n of Papers 48 4.2 S t r e s s R e l a x a t i o n of Polymer F i l m s ......... 49 4.3 S t r e s s R e l a x a t i o n of Paper P l a s t i c Composites ( PPC ) 50 5.0 DISCUSSION 53 5.1 Short Term S t r e s s Decay 53 5.2 St r e s s R e l a x a t i o n of Papers 56 5.2.1 E f f e c t of l i g n i n on s t r e s s r e l a x a t i o n of k r a f t papers 57 5.2.2 S t r e s s r e l a x a t i o n of groundwood papers 58 5.2.3 D i f f e r e n c e i n s t r e s s r e l a x a t i o n between k r a f t and groundwood papers . 59 5.3 S t r e s s R e l a x a t i o n of Polymers 61 5.3.1 The c o n t r i b u t i o n of c r o s s l i n k i n g to polymer r h e o l o g i c a l p r o p e r t i e s ... 62 - V -Page 5.4 R h e o l o g i c a l P r o p e r t i e s of Composite M a t e r i a l s 66 5.4.1 Influence of matrix on paper p l a s t i c composite ( PPC ) s t r e s s r e l a x a t i o n . 6? 5.4.2 E f f e c t of su b s t r a t e s on paper p l a s t i c composite ( PPC ) s t r e s s r e l a x a t i o n . 70 5.4.3 E f f e c t of paper cop o l y m e r i z a t i o n with (co)monomers on i n i t i a l m a t e r i a l s t r e s s r e l a x a t i o n 71 5.4.4 E f f e c t of l i g n i n on paper p l a s t i c composite ( PPC ) s t r e s s r e l a x a t i o n . 72 6.0 CONCLUSION 76 7.0 LITERATURE CITED 78 8.0 NOTATIONS 107 - v i -LIST OF TABLES Table Page I . Basis weight ( g/m ) of paper handsheets, paper p l a s t i c composites and polymer f i l m s . 109 I I . P r o p e r t i e s of paper p l a s t i c composites ( PPC ) 110 I I I . S t r e s s r e l a x a t i o n of the d i f f e r e n t treatments studi e d a t constant deformation ( n=5) 112 IV. Regression analyses of s t r e s s r e l a x a t i o n curves according to S(t)/S ( 0.04) = a + b I n t ( n=30 ) 114 V. Test f o r d i f f e r e n c e i n paper s t r e s s r e l a x a t i o n behaviours 116 VI. Test f o r e f f e c t of polymer f i l m c r o s s l i n k i n g on amount of s t r e s s r e l a x a t i o n 117 V i l l i to Test f o r e f f e c t of matrix system on paper V I I s i v . p l a s t i c composite ( PPC ) s t r e s s r e l a x a t i o n 118 V I I I i i to Test f o r e f f e c t of substrate on paper p l a s t i c V I I I i v . composite ( PPC ) s t r e s s r e l a x a t i o n 122 I X i i to Test f o r copolymerization e f f e c t s on paper IX:x. s t r e s s r e l a x a t i o n ( k r a f t papers-polymer systems) 127 X t i to Test f o r copolymerization e f f e c t s on paper Xsx. s t r e s s r e l a x a t i o n ( groundwood papers-polymer systems) 137 - v i i -LIST OF FIGURES Fig u r e Page 1. Polymer s t r e s s r e l a x a t i o n 147 2. Impregnation of papers i n a d e s i c c a t o r w i t h monomers 145 3. Glass frame assembly f o r making polymer t h i n f i l m s 149 4. St r e s s r e l a x a t i o n of papers 150 5. S t r e s s r e l a x a t i o n of polymer f i l m s 151 6. E f f e c t of matrix systems on paper p l a s t i c composite ( PPC ) s t r e s s r e l a x a t i o n 152 7*1 to E f f e c t of sub s t r a t e s on paper p l a s t i c 7- 5. composite ( PPC ) s t r e s s r e l a x a t i o n 15^ 8- 1 to Copolymerization e f f e c t s on paper s t r e s s 8- 4. r e l a x a t i o n ( k r a f t papers-polymer system ) .. 159 9- 1 to Copolymerization e f f e c t s on paper s t r e s s 9-4. r e l a x a t i o n ( groundwood papers-polymer system ) 163 • # * - v i i i -ACKNOWLEDGEMENT The author g r a t e f u l l y acknowledges Dr. J . W. Wilson, P r o f e s s o r , F a c u l t y of F o r e s t r y , U n i v e r s i t y of B r i t i s h Columbia, f o r h i s k i n d guidance i n the planning and experimental phases of the study and pr e p a r a t i o n of the manuscript. A p p r e c i a t i o n i s a l s o due Dr. L. Paszner, Research A s s o c i a t e , f o r h i s c o n s t r u c t i v e c r i t i c i s m of t h i s t h e s i s . S p e c i a l thanks are due the Department of Environement and the Pulp and Paper Research I n s t i t u t e of Canada f o r f i n a n c i a l support. Thanks are a l s o due MacMillan B l o e d e l L t d . f o r m a t e r i a l s s u p p l i e d f o r the study. - 1 -1.0 INTRODUCTION Wood c e l l u l o s e i s an abundant and r e p r o d u c i b l e n a t u r a l polymer wi t h good mechanical p r o p e r t i e s and r e s i s t a n c e to many so l v e n t s and s w e l l i n g agents ( 9 )• I t i s used widely i n numerous ways. I t s weakness f o r some a p p l i c a t i o n s i s due to s e n s i t i v i t y to changing moisture content, a c i d s , bases and oxygen ( 9i 35 )• I n a d d i t i o n , c e l l u l o s i c m a t e r i a l s are f r e q u e n t l y subject to b i o l o g i c a l d e t e r i o r a t i o n . Paper p r o p e r t i e s may be enhanced by treatment of f i b r e s o r papers w i t h c e r t a i n (co)monomers or polymers. I t i s noted t h a t these treatments may improve a v a r i e t y of p r o p e r t i e s i n c l u d i n g t e n s i l e s t r e n g t h , wet str e n g t h , dye r e t e n t i o n and adhesion, as w e l l as r e s i s t a n c e to b i o l o g i c a l degradation, abrasion, a c i d s and bases ( 131 ) . On a weight b a s i s , paper p l a s t i c composites ( PPC ) cost l e s s than plywood and s t e e l , and possess s i g n i f i c a n t l y higher s t r e n g t h . The m a t e r i a l can be used as o v e r l a y f o r plywood to giv e products w i t h smooth p a i n t a b l e surfaces, or as base f o r wood g r a i n p r i n t e d paper. A p p l i c a t i o n s as waterpoof, f i r e p r o o f packaging m a t e r i a l , as w e l l as p a r t s of t r a n s p o r t a t i o n f a c i l i t i e s suehsas r a i l r o a d boxcar s i d e s , c o n t a i n e r s , t r a i l e r t r u c k bodies and automobile bumpers have been proposed ( 14, 123. )• S u i t a b i l i t y f o r such uses depends i n p a r t on m a t e r i a l f l o w p r o p e r t i e s . S t r e s s r e l a x a t i o n i s a technique a p p l i e d to determine f l o w p r o p e r t i e s of substances. When specimens are h e l d a t constant deformation, s t r e s s e s may be d i s s i p a t e d r a p i d l y o r s l o w l y i n accordance w i t h fundamental p r o p e r t i e s of the m a t e r i a l s being examined. Although some PPC product p r o p e r t i e s are documented i n the l i t e r a t u r e , no i n f o r m a t i o n i s a v a i l a b l e on t h e i r s t r e s s r e l a x a t i o n behaviour. The present study was designed about the hypotheses t h a t PPC f l o w p r o p e r t i e s are governed mainly by paper subs t r a t e p r o p e r t i e s . - 3 -2.0 LITERATURE REVIEW 2.1 Strength P r o p e r t i e s of Paper P l a s t i c Composites C e l l u l o s e i s a l i n e a r polymer, composed of re p e a t i n g anhydroglucose u n i t s which are l i n k e d together by 1-4-p-D-glucosidic bonds. Owing to the numerous hydroxyl groups present, c e l l u l o s e i s h i g h l y s e n s i t i v e to change i n moisture content. C e l l u l o s e i s a l s o e a s i l y subjected to d e t e r i o r a t i o n by l i g h t , heat, aqueous a c i d s and bases, oxygen, and microorganisms ( 5. 35 ) . Reduction of s e n s i t i v i t y to water and d e t e r i o r a t i o n by environmental f a c t o r s , can be achieved by copolymer!zation of c e l l u l o s e and l i g n o c e l l u l o s e w i t h c e r t a i n monomers ( 9, 132 ). I n a d d i t i o n , copolymerization techniques have been applied; o f t e n to modify the str e n g t h of c e l l u l o s e and i t s d e r i v a t i v e s . I n paper manufacture, g r a f t i n g of monomers to e i t h e r pulps or papers has been performed and a l t e r a t i o n s of paper p r o p e r t i e s have been achieved. 2.1.1 G r a f t i n g to pulps Copolymerization of woodpulps w i t h monomers - 4 -improves the dry and wet t e n s i l e strengths ( 1, 2, 15i 67, 80, 93. 95. 100, 111, 113, 120 ). B y C g r a f t i n g - a c f y i a i i d e ( AA ) to p a p e r p u l p by an o z o n i z a t i o n technique, Neimo et a l . ( 80 ) r a i s e d wet breaking l e n g t h 150$ above t h a t of the i n i t i a l pulp. Kobayashi et a l . ( 50 ) improved paper stren g t h p r o p e r t i e s by blending a e r y l o n i t r i l e ( AN ) and styrene ( S ) g r a f t e d d i s s o l v i n g pulp and co t t o n l i n t e r s w i t h high y i e l d pulps. Blends c o n t a i n i n g groundwood and g r a f t e d f i b r e s , however, gave poorer p r o p e r t i e s than d i d ungrafted blends. These r e s u l t s i n d i c a t e d t h a t the g r a f t e d polymer was only e f f e c t i v e when i t c o n t r i b u t e d to i n t e r f i b r e bonding. I t ^subtracted from streng?th, however, when i t replaced f i b r e to f i b r e bonds. C h a r a c t e r i s t i c s of p o l y v i n y l a l c o h o l ( PVA ) g r a f t e d pulps were s t u d i e d by Ogiwara e t a l . ( 93. 95 )• They f0und» higher t e n s i l e s t r e n g t h p r o p e r t i e s f o r a i r - d r y papers made from g r a f t e d pulps, but the amount of increase was h i g h l y dependent on pulp grade and beating degree. G r a f t per cent i s a l s o known to a f f e c t paper s t r e n g t h ( 26 ) . I n most cases, g r a f t i n g onto woodpulp f i b r e s p r i o r to sheet formation has been found to be d e t r i m e n t a l - 5 -to paper p r o p e r t i e s ( 10, 41, 61, 79 ). By use of the Ce method, Neimo et a l . ( 7 9 ) g r a f t e d v a r i o u s monomers onto ^ c e l l u l o s e f i b r e s and showed t h a t papermaking p r o p e r t i e s were enhanced only w i t h AA or v i n y l acetate ( VA ), p a r t i c u l a r l y when a p p l i e d to low i n i t i a l s t r e n g t h or damaged pulps. Nevertheless, g r a f t i n g f i b r e s w i t h hydrophobic monomers r e s u l t e d i n a d i s t i n c t lowering of most papermaking p r o p e r t i e s . Lee and F u j i i ( 63 ) declared t h a t b u t y l a c r y l a t e ( BA ) and e t h y l a c r y l a t e ( EA ), when g r a f t e d to pulp,had a d e l e t e r i o u s e f f e c t on subsequent paper p r o p e r t i e s . S i m i l a r r e s u l t s have been obtained when methyl methacrylate ( MMA ), AN, S, or v i n y l c h l o r i d e ( VC ) were used ( 10, 23, 61, 62 ) . Consequently, p r o p e r t i e s of sheets formed from g r a f t i n g depend upon polymers employed. Adherence of a hydrophobic polymer to the f i b r e s e r i o u s l y blocks n a t u r a l bonding s i t e s and i n h i b i t s s w e l l i n g , as w e l l as wet f i b r e f l e x i b i l i t y . These blocked s i t e s i n t e r f e r e with s t r e n g t h development during sheet formation ( 23 ). On the other hand, i f the g r a f t e d polymer i s h y d r o p h i l i c , papermaking p r o p e r t i e s may - 6 -be enhanced due to improved bonding c a p a b i l i t y ( 12, 15, 59, 110, 111, 120), The e f f e c t s ; of g r a f t s with h y d r o p h i l i c and hydrophobic monomers on paper p r o p e r t i e s of g r a f t e d pulps were demonstrated by Lee and F u j i i ( 61- 65 ) . This work showed t h a t h y d r o p h i l i c g r a f t s ( AA ) to pulp gave paper w i t h higher t e n s i l e s t r e n g t h and f o l d i n g endurance, but low t e a r i n g s t r e n g t h . A hydrophobic g r a f t ( MMA ), however, r e s u l t e d i n reduced t e n s i l e s t r e n g t h and t e a r i n g r e s i s t a n c e . 2.1.2 G r a f t i n g to papers Some disadvantages of g r a f t i n g to pulps can be reduced by d i r e c t impregnation and copolymerization of paper sheets with monomers. Lynch ( 68 ) g r a f t e d AA and MMA onto k r a f t , groundwood and f i l t e r papers by a f r e e r a d i c a l c a t a l y s i s method. The t e n s i l e s t r e n g t h of these papers increased w i t h i n c r e a s i n g polymer content. Using the e e r i e i o n technique to g r a f t AN onto paper made from a l p h a - c e l l u l o s e pulps, a l a r g e improvement of wet and dry t e n s i l e strengths w i t h increase i n polymer content was noted ( 26 ). At about 40% - 7 -r e t e n t i o n , the wet strengt h was equal to the c o n t r o l dry s t r e n g t h . Using the same method when g r a f t i n g d i f f e r e n t papers w i t h AN, AA, MMA,EA-AN, a c r y l i c a c i d and other monomers r e s u l t e d i n great improvement i n dry and wet strengths ( 26, 43, 68, 105, 109, 120, 121 ) . Increased bonding has been c i t e d to e x p l a i n increased s t r e n g t h p r o p e r t i e s of g r a f t e d papers. D a n i e l et a l . ( 26 ) assumed t h a t the enhanced st r e n g t h from AN g r a f t e d to f i b r e s u b s t r a t e s was due to p o l a r bonds between neighbouring poly( AN ) chains. Putnam ( 105 ) concluded t h a t s t r e n g t h reinforcement on AN p o l y m e r i z a t i o n r e s u l t s from a d d i t i o n of hydrogen bonds between the polymer and the c e l l u l o s e f i b r e matrix. AN and AA appear to be p a r t i c u l a r l y b e n e f i c i a l w i t h r e s p e c t to dry paper stren g t h p r o p e r t i e s ( 10, 69» 78, 80, 105, 112, 121, 130 ). Such in c r e a s e s were a t t r i b u t e d to g r e a t l y increased number of i n t e r f i b r e bonds ( 26 ) . Some f i n d i n g s of Neimo and S i h t o l a ( 78 ) i n d i c a t e d t h a t f i r m hydrogen bonds probably e x i s t between amide groups of g r a f t e d p o l y ( AA ) chains and the c e l l u l o s e - 8 -h y d r o x y l s . This may g i v e r i s e to a s h e l l type s t r u c t u r e with c e l l u l o s e . " e n c a p s u l a t e d " i n p o l y ( AA ) c o i l s . C o n t r a s t i n g w i t h the above r e s u l t s , Ogiwara et a l . ( 9 5 ) i n d i c a t e d t h a t g r a f t i n g commercial p r i n t i n g papers with AN i n a i r , w i t h e e r i e s a l t as i n i t i a t o r , l e d to decreased breaking l e n g t h and dimensional s t a b i l i t y . MMA and methyl a c r y l a t e ( MA ) have been found to lower the strength of papers ( 79» 96, 121 ). Oraby et a l . ( 97 )» as w e l l as Crook et a l . ( 25 ), s t u d i e d the r a d i a t i o n - i n d u c e d g r a f t i n g of S to paper sheets. They found t h a t breaking l e n g t h , burst and f o l d i n g endurance a l l decreased a f t e r g r a f t i n g , w i t h strength d e t e r i o r a t i o n p r o p o r t i o n a l to the amount of polymer g r a f t e d . Reinforcement of paper s t r e n g t h was a l s o attempted by c o p o l y m e r i z a t i o n w i t h formed polymers and p a r t i a l polymers. Berger and G e l b e r t ( 11 ) i n d i c a t e d t h a t treatment w i t h urethane emulsions increased paper dry and wet strengths, p a r t i c u l a r l y a t low l e v e l s of s a t u r a t i o n . A small amount of urethane produced a l a r g e increase i n s t r e n g t h . Paszner ( 100 ) polymerized a hydrocarbon d r y i n g o i l w i t h i n handsheets made from chemical pulps, groundwood and groundwood-chemical pulp combinations. I n t e r f i b r e bonding was strengthened without - 9 -n o t i c e a b l e e f f e c t on i n t r i n s i c f i b r e s t r e n g t h . The a d d i t i o n of butadiene-styrene copolymers ( Gr-S rubber ) to paper sheets improved wet and dry t e n s i l e strengths ( 3 9 ). S t r e s s - s t r a i n curves f o r l a t e x - t r e a t e d papers i n d i c a t e d t h a t , w i t h few exceptions, a d d i t i o n of the 60% 3 and 40$ \" a c r y l a t e elastomer to paper mainly improved the paper " p o s t - y i e l d e x t e n s i b i l i t y ". L i t t l e v a r i a t i o n i n modulus of e l a s t i c i t y , i . e . , e a r l y s t r e s s - s t r a i n behaviour, was observed. S t r e s s - s t r a i n curves a t a number of Gr-S rubber a d d i t i o n l e v e l s showed t h a t e l o n g a t i o n a t f i n a l y i e l d increased w i t h i n c r e a s i n g l a t e x content, but t h a t t e n s i l e s t r e n g t h a t f i n a l y i e l d remained f a i r l y constant. Heyse et a l . ( 39 ) concluded t h a t impregnation by an elastomer introduced two important f a c t o r s which could g r e a t l y a f f e c t paper s t r e s s - s t r a i n p r o p e r t i e s « i . the s t r e s s - s t r a i n and adhesive p r o p e r t i e s of the Gr-S f i l m ; and i i . the f i l m forming temperature of the l a t e x , i . e . , the Gr-S f i l m would not be formed i f the temperature i s below a c r i t i c a l p o i n t . Thommen and Stannett ( 139 ) found t h a t the i n i t i a l polymer - 10 -a d d i t i o n brought about a sharp increase i n s t r e n g t h , f o l l o w e d by g r a d u a l l y diminished i n c r e a s e s of b e n e f i c i a l e f f e c t s w i t h f u r t h e r a d d i t i o n s , u n t i l the curve became n e a r l y f l a t . This was a t t r i b u t e d to the q u a n t i t y of s i t e s a v a i l a b l e f o r reinforcement, making f u r t h e r a d d i t i o n s l e s s e f f e c t i v e . I t can be concluded t h a t a l t e r a t i o n of paper sheet p r o p e r t i e s by c o p o l y m e r i z a t i o n methods r e s u l t s from choice of monomer, amount of g r a f t i n g , technique empolyed and the o r i g i n a l paper m a t e r i a l used. I n a d d i t i o n , f i b r e chemical composition and environmental c o n d i t i o n s d u r i n g processing have s i g n i f i c a n t e f f e c t s . 2.2 E f f e c t of F i b r e Chemical Constituents on G r a f t i n g j E s p e c i a l l y L i g n i n S u i t a b i l i t y of woodpulps f o r g r a f t i n g depends on f i b r e chemical c o n s t i t u e n t s and s w e l l i n g c h a r a c t e r i s t i c s . Wood i s a complex substance w h i c h i i s composed of c e l l u l o s e , h e m i c e l l u l o s e s , l i g n i n s and extraneous compounds. The term " l i g n i n s " r e f e r s to a mixture - 11 -of substances t h a t have s i m i l a r chemical composition but may have s t r u c t u r a l d i f f e r e n c e s . The aromatic s t r u c t u r e of l i g n i n has been proven by i t s c h a r a c t e r i s t i c u l t r a v i o l e t a b s o r p t i o n spectrum, which i s very s i m i l a r to th a t of some guaiacylpropane compounds ( 16, 82 ). L i g n i n s appear to be amorphous substances. I t should be noted t h a t coniferous wood l i g n i n s c o n t a i n e x c l u s i v e l y g u a i a c y l p r o p y l u n i t s , whereas deciduous wood l i g n i n s c o n t a i n both g u a i a c y l -and s y r i n g y l p r o p y l m o i e t i e s ( 16 ). The nature of the bond between l i g n i n s and carbohydrates i n not c l e a r . I t seems, however, t h a t some p a r t of l i g n i n i s chem i c a l l y bound to carbohydrates ( 82 ). The presence of l i g n i n i n f i b r e s may a c t as a r e t a r d e r to g r a f t i n g r e a c t i o n s ( 79. 83, 103 ) . The d e l e t e r i o u s e f f e c t of l i g n i n on g r a f t i n g by the e e r i e i o n technique has been reported ( 83 ). wherein g r a f t i n g y i e l d s v a r i e d according to r e s i d u a l l i g n i n content of the pulps ( 10 ) . Increased r e s i d u a l l i g n i n decreased g r a f t i n g r a t e c o n s i d e r a b l y ( 10 ). By using the e e r i e i o n - 12 -technique, Kubota and Ogiwara ( 56, 94 ) s t u d i e d the e f f e c t of l i g n i n i n a s e r i e s of coniferous pulps on MMA grafting..They showed t h a t g r a f t i n g occurred o n l y a f t e r an i n d u c t i o n p e r i o d . Length of the p e r i o d increased w i t h i n c r e a s i n g pulp l i g n i n content. I t was concluded t h a t the e e r i e i o n r e a c t e d a t a much f a s t e r r a t e w i t h l i g n i n than w i t h c e l l u l o s e i n wood pulp. Coniferous l i g n i n was not as severe a r e t a r d e r as pored wood l i g n i n ( 103 ) . The s u i t a b i l i t y of l i g n i n - c o n t a i n i n g pulps f o r g r a f t i n g i s dependent i n par-t on the p a r t i c u l a r monomer used. P h i l l i p s et a l . ( 103 ) revealed t h a t the g r a f t y i e l d of S to wood pulps increased w i t h l i g n i n content, w h i l e the g r a f t i n g of AA was s t r o n g l y i n h i b i t e d by only a few per cent l i g n i n . 2.3 E f f e c t of Gamma R a d i a t i o n on C e l l u l o s i c s and Polymers Gamma rays are high energy photons and as such c a r r y no charge o r r e s t mass. These rays r e s u l t from nuclear processes, such as r e l e a s e i n f i s s i o n and decay of r a d i o a c t i v e isotopes ( 48 ). - 13 -I t has been observed t h a t r a d i a t i o n a f f e c t s polymers by causing e i t h e r c r o s s l i n k i n g or degradation. e v e n t u a l l y to formation of i n s o l u b l e three-dimensional networks ( 20, 21 ). Degradation i s re v e a l e d by f r a c t u r e of polymer molecules, l e a d i n g to decreased average molecular weight ( 20, 21 ). Even i n systems showing i n i t i a l c r o s s l i n k i n g , degradation begins to predominate at s u f f i c i e n t l y high r a d i a t i o n doses. Both r e a c t i o n s are found to be p r o p o r t i o n a l to dose and r a t h e r independent of r a d i a t i o n i n t e n s i t y ( 48 ). I f only chain s c i s s i o n takes place and the r a t e of breaking l i n k s i s a l i n e a r f u n c t i o n of r a d i a t i o n dose, an equation i s obtained as i C r o s s l i n k i n g leads t o . i n c r e a s e d molecular weight and ( 1/ M n ) - ( 1/ ^ ) = r / E d N where j f i n a l number average molecular weight, iio i n i t i a l number average molecular weight, r r a d i a t i o n dose, E average energy absorbed by the system f o r each main s c i s s i o n , and - 14 -N = Avogadro's number. Conversely, i f o n l y random c r o s s l i n k i n g takes place and the r a t e of c r o s s l i n k i n g i s a l i n e a r f u n c t i o n of the r a d i a t i o n dose, them q = q 0 r { 2 J where i q = r a t e of c r o s s l i n k i n g , q = the c r o s s l i n k i n g d e n s i t y produced per u n i t r a d i a t i o n dose, and r = r a d i a t i o n dose. The t o t a l dose absorbed to i n i t i a t e damage i n p l a s t i c s and elastomers i s i n the range of 2.1 x 10^ to 7.3 x 1 0 1 0 ergs g ~ 1 0 ( 48 ). Depolymerization of c e l l u l o s e chains as i n i t i a t e d by gamma r a d i a t i o n , occurs mainly by breaking of the 1-4- p - D - g l u c o s i d i c l i n k a g e . Carbonyl and c a r b o x y l groups are formed i n both amorphous and c r y s t a l l i n e r e g i o n s , but n e g l i g i b l e c r y s t a l l i n i t y i s l o s t ( 7, 8, 101 ). Paszner ( 101 ) i n d i c a t e d t h a t c e l l u l o s e degradation under Ng i s a f u n c t i o n of t o t a l i r r a d i a t i o n dose, whereby the l o g DP - l o g dose r e l a t i o n s h i p i s almost l i n e a r . No changes of paper mechanical p r o p e r t i e s 6 or b r i g h t n e s s were noted w i t h doses up to 10 r a d . - 15 -F u r t h e r i r r a d i a t i o n l e d to a marked d e t e r i o r a t i o n i n both stren g t h and brig h t n e s s (55). Loos(66) found t h a t i r r a d i a t i o n l e v e l s of lo'' r ad or greater d e f i n i t e l y degraded wood and decreased i t s toughness. I t has been found a l s o t h a t 6 dosages below 10 rad s l i g h t l y increase some st r e n g t h p r o p e r t i e s (18). Wood i s more r e s i s t a n t to r a d i a t i o n degradation than c e l l u l o s e , probably due to the presence of l i g n i n and e x t r a c t i v e s a s s o c i a t e d w i t h i t s l a t t i c e s t r u c t u r e (124). The fragmentation of c e l l u l o s e chains under i r r a d i a t i o n e f f e c t s takes place randomly, thus reducing p o l y d i s p e r s i t y . This process, however, can a l s o be accompanied'by c r o s s l i n k i n g and e n d l i n k i n g , p r o v i d i n g i r r a d i a t i o n occurs under s u i t a b l e c o n d i t i o n s (99). 2.4 R h e o l o g i c a l Behaviour of S o l i d s Rheology i s the science concerned w i t h study of deformation and f l o w of m a t e r i a l s , e s p e c i a l l y the i n t e r r e l a t i o n s h i p of s t r e s s , s t r a i n and time. I n i t s broadest sense the subject ranges from f l o w of gases and l i q u i d s to e l a s t i c deformation of c r y s t a l l i n e s o l i d s . I n p r a c t i c e the term g e n e r a l l y r e f e r s to the study of m a t e r i a l s having e l a s t i c and f l o w p r o p e r t i e s intermediate between those of i d e a l s o l i d s and o r d i n a r y l i q u i d s . Wood, wood products and p l a s t i c s are of t h i s type. - 16 -Two approaches have been developed to describe v i s c o e l a s t i c p r o p e r t i e s of m a t e r i a l s . One i s c h a r a c t e r i z a t i o n of the m a t e r i a l by a complete d e s c r i p t i o n of i t s behaviour, while the other i s e l u c i d a t i o n of the atomic or molecular s t r u c t u r e of the m a t e r i a l and assignment of mechanisms r e s p o n s i b l e f o r r h e o l o g i c a l phenomena. The f i r s t i s g e n e r a l l y r e f e r r e d to as phenomenological rheology, while the second may be c a l l e d molecular rheology C 102 ). Nevertheless, both approaches are concerned w i t h a n a l y s i s of s t r e s s -s t r a i n - t i m e r e l a t i o n s h i p s as responses of m a t e r i a l s to v a r i o u s e x c i t a t i o n s . 2.4.1 S t r e s s r e l a x a t i o n of s o l i d s j general survey S t r e s s r e l a x a t i o n , as an aspect of v i s c o e l a s t i c behaviour of m a t e r i a l s , i s observed when a m a t e r i a l i s subjected to s t r a i n t h a t i s h e l d constant. The s t r e s s i s found to decrease w i t h time. According to l i n e a r v i s c o e l a s t i c theory, r e l a x a t i o n curves of s o l i d s are expressed by a s i n g l e Maxwellian model which i n v o l v e s an i d e a l s p r i n g and dashpot i n s e r i e s ( 3, - 17 -30, 71. 72, 81, 129, 141, 143 ). The model, however, i s described by E(t) = E exp ( - t / x ) as d e r i v e d i n the f o l l o w i n g steps ( 71. 142 )« i . the s t r e s s - s t r a i n r e l a t i o n s h i p f o r the..spring i s S = E&» where j S = s t r e s s measured across the e n t i r e model, E = Young's modulus, and £, = deformation of the i i . the s t r e s s - s t r a i n r e l a t i o n s h i p f o r the dashpot i s where ; n = Newtonian v i s c o s i t y - of the l i q u i d , £ t = deformation of l i q u i d i n 5 Ithe dashpo t , and r^T>= r a t e of 'defamation i sp r i n g j S = IJ ( dfci/ d t ) - 18 -i i i . s i n ce t o t a l deformation ( % ) of the model i s t = e, + £ * / 5 / the s t r e s s - s t r a i n r e l a t i o n s h i p f o r the e n t i r e s e r i e s can be obtained by s u b s t i t u t i n g { 4 J and i n t e g r a t i n g [ 3 / i n t o [ 5 J* thus« d f c / d t = (1/E) dS/dt + S/f)../"6 / where ; d £ / dt = s t r a i n r a t e ; i v . d uring r e l a x a t i o n , macroscopic deformation of the specimen i s h e l d constant so t h a t d f c / dt = 0, which a l t e r s { 6 J to 0 = (1/E ) ( dS/dt ) + S/ I J . . . / 7 a / or ; dS/S = ( - d t / (Q /E) ) / 7b / v. a f t e r i n t e g r a t i o n under the boundary c o n d i t i o n t h a t t =0, S = S Q « E ( S Q = i n i t i a l s t r e s s ), - 19 -f 7 J becomes S(t) = E t exp ( - t / t ) { 8a / or; E ( t ) = E exp ( - t / r ) /"8b J where \ S(t ) = time dependent s t r e s s , E ( t ) = time dependent r e l a x a t i o n modulus, and ^ = IJ /E = r e l a x a t i o n time. I n the m a j o r i t y of cases, s t r e s s r e l a x a t i o n occurs more s l o w l y than p r e d i c t e d by the simple Maxwellian model ( 44, 51, 54, 116, 132-134, 142, 145 ). A f e a t u r e of s t r e s s p l o t t e d a g a i n s t the l o g a r i t h m of time i s the comparatively broad i n f l e c t i o n range over which the curves are l i n e a r ( the l i n e a r i t y i n f l e c t i o n r e g i o n ). The r e g i o n u s u a l l y extends over about two decades of time. This l i n e a r i t y has been c a l l e d the l o g a r i t h m i c time law ( 146 ), which may be formulated a s i S = S Q - F l o g t {9 / where % S = s t r e s s a t time t , S„ = i n i t i a l s t r e s s , and o - 20 -F = i n f l e c t i o n slope. Experimental r e s u l t s of r e l a x a t i o n may be f i t t e d by the Maxwell-Weiehert model ( 3t 30, 81, 142 ), which contains a s u f f i c i e n t number of Maxwellian elements i n p a r a l l e l arrangement, such t h a t « E ( t ) = E1 exp ( - t / r , ) + E 2 exp ( - t / t x ) . + + E n exp ( - t / r n ) n =£ E. exp ( - t / r . ) or? ^ E ( t ) = )0 E(C ) exp ( - t / r ) d t . . . / 1 0 a / o r j E ( t ) = H ( t l n v r ) exp ( . - t / r ) d(lnC) / " 1 0 b / where j H( I n C ) = H ( r ) = t E ( T ) ( H( l n t ) ) = d i s t r i b u t i o n of r e l a x a t i o n times. 2.4.2 R e l a x a t i o n of polymers The r e l a x a t i o n modulus ( E ( t ) , { 8b J ) can be used to describe s t r e s s r e l a x a t i o n behaviour of high polymers by p l o t t i n g l o g E ( t ) a g a i n s t l o g time. I n - 21 -amorphous polymers f o u r s t a t e s , such as g l a s s y , t r a n s i t i o n , rubbery, and Newtonian f l o w are r e a d i l y observed i n such diagrams as shown i n F i g . 1 ( 36, 60, 118, 140, 142 ). At very short times amorphous polymers behave l i k e an e l a s t i c s o l i d . The high modulus observed a t short times i s c h a r a c t e r i s t i c of g l a s s y , b r i t t l e s o l i d s and i s a s s o c i a t e d with r i g i d i t y of the molecular chain backbone. The modulus, E ( t ) , i s e s s e n t i a l l y independent of time and temperature ( 40, 118 ). The motion of molecular segments i s r e s t r i c t e d p r i m a r i l y to v i b r a t i o n s , and l a r g e s t r e s s e s are r e q u i r e d to cause deformation of s t i f f molecules. As time progresses, the t r a n s i t i o n range i s entered, wherein polymers a c t l e s s g l a s s y and the modulus decreases s h a r p l y . A d d i t i o n a l modes of motion, which might a l l o w increased r e l a x a t i o n , are r o t a t i o n of segments about t h e i r main chain backbone and other segmental adjustments w i t h respect to neighbouring segments ( 40, 118 ). At s t i l l longer times, the rubbery p l a t e a u r e g i o n i s reached. In the r e g i o n E ( t ) approaches a constant value and the m a t e r i a l d i s p l a y s l a r g e e l a s t i c - 22 -deformation, which i s mostly r e c o v e r a b l e . R e l a x a t i o n e f f e c t s are absent. Since E ( t ) i s c h a r a c t e r i s t i c of the value encountered i n rubber e l a s t i c i t y , and i s r e l a t i v e l y independent of temperature or time, t h i s rubbery pl a t e a u has been a s s o c i a t e d w i t h entanglements between and among long chain molecules. I n the Newtonian r e g i o n , the modulus f a l l s o f f r a p i d l y , i n d i c a t i n g t h a t a s t a t e of l i q u i d f l o w has been reached. Plow behaviour i s c h a r a c t e r i z e d by even g r e a t e r loosening of the molecular s t r u c t u r e and g r e a t e r molecular m o b i l i t y . Among c r o s s l i n k e d polymer m a t e r i a l s a r e s t r i c t i o n i s placed on looseness of the s t r u c t u r e , and the modulus l e v e l s o f f to a r a t h e r constant value i n the f i n a l Newtonian r e g i o n ( 81 ). A s u i t a b l e mode re p r e s e n t i n g E ( t ) f o r these m a t e r i a l s i s i E ( t ) = j** E( T ) exp (-t/ c ) d t + Eoc - . . . ./• or; = L e H< l n t ) e x P ( - * / r ) d ( l n t ) + Eoc / ' l i b J D i f f e r e n c e s appear between p o l y e r y s t a l l i n e and amorphous polymers. The ehange i n E ( t ) f o r - 23 -p o l y c r y s t a l l i n e types i s much smal l e r , and the t r a n s i t i o n r e g i o n extends over a much wider temperature range than f o r amorphous polymers ( 4 0 , 142 ), s i n c e o n l y disordered regions take p a r t i n the t r a n s i t i o n . I n the rubbery p l a t e a u r e g i o n , the disordered regions are rubbery i n nature. The modulus i s higher than w i t h amorphous polymers s i n c e c r y s t a l l i n e regions are r i g i d and a c t as c r o s s l i n k s f o r the disordered r e g i o n s . Molecules pass through both ordered and disordered regions and o n l y short molecular lengths are rubbery. I n t h i s r e g i o n the m a t e r i a l may be compared to a h i g h l y c r o s s l i n k e d rubber which a l s o contains r i g i d f i l l e r p a r t i c l e s . Where the Newtonian r e g i o n occurs the c r y s t a l s melt, the modulus f a l l s d r a s t i c a l l y and the polymer shows vi s c o u s l i q u i d f l o w . The t r a n s i t i o n r e g i o n of p l a s t i c m a t e r i a l i s u s u a l l y c h a r a c t e r i z e d by a r e l a x a t i o n time. C» which gi v e s r i s e to a f a m i l y of r e l a x a t i o n curves having d i f f e r e n t t v a l u e s . S t r e s s r e l a x a t i o n curves f o r m a t e r i a l s with low Z values show r e l a t i v e l y r a p i d decay and are i n d i c a t i v e of l i q u i d - l i k e behaviour,whereas those f o r - 24 -m a t e r i a l s w i t h higher r values cmaintain r e l a t i v e l y -h igh s t r e s s values f o r l a r g e r time periods and are i n d i c a t i v e of s o l i d - l i k e behaviour. Within l i m i t s , as X approaches^zerosto c approaches ©c, completely v i s c o u s to completely e l a s t i c behaviour i s expected ( 72, 81, 142 ). and fj i s temperature s e n s i t i v e , the e f f e c t of temperature on v i s c o e l a s t i c p r o p e r t i e s of p l a s t i c m a t e r i a l s i s nota b l e . Increase i n temperature decreases both TJ and x pr o v i d e s ^ a f l u i d - l i k e s t a t e w i t h i n c r e a s i n g temperature, i n c o n t r a s t to the s o l i d - l i k e s t a t e with decreasing temperature. ( 69» 84 ) have proposed t h a t the e f f e c t s of time and temperature on v i s c o e l a s t i c p r o p e r t i e s of n o n - c r y s t a l l i n e polymers can be i n t e r c o n v e r t e d above the g l a s s t r a n s i t i o n temperature and w i t h i n l i n e a r v i s c o e l a s t i c l i m i t s . W i l l i a m s et a l . ( 150 ) converted the moduli obtained from d i f f e r e n t temperatures by us i n g a s h i f t f a c t o r ( A ( t ) ) to make a master curve ( WLF equation ) d e s c r i b i n g the moduli over s e v e r a l time decades ass Since r depends on v i s c o s i t y , fj , ( T Smith ( 125-127; ), Tobolsky ( 140 ), and others - 25 -l o g A ( t ) = l o g t / t Q = CJ( T - T g ) / ( C 2 + T - T g ) / " 1 2 / where j C l * C2 = constants, and T = the g l a s s t r a n s i t i o n temperature. Time- temperature equivalence becomes apparent when an amorphous polymer i s subjected to heat above i t s T . aThe^molecular "chains- tend to rearrange and take on the most probable c o n f i g u r a t i o n s commensurate wi t h the s t a t e of s t r e s s or s t r a i n being endured. The r a t e of rearrangement depends on l o c a l r e s i s t a n c e encountered by any chain. This r e s i s t a n c e can be expressed by a v i s c o u s f r i c t i o n c o e f f i c i e n t which i s d e r i v e d as the f o r c e r e q u i r e d to move a chain through the surrounding medium at u n i t v e l o c i t y ( 127 ). Thus, the f a s t e r the chain i s r e q u i r e d to move, the g r e a t e r i s the f o r c e which must be a p p l i e d . L i k e w i s e , the r e q u i r e d f o r c e becomes g r e a t e r as temperautre i s decreased. Consequently, i t appears l o g i c a l t h a t some r e l a t i o n s h i p should e x i s t between time- and temperature-dependence of amorphous polymer v i s c o e l a s t i c p r o p e r t i e s ( 128 ) - 26 -The time-temperature s u p e r p o s i t i o n theory, however,is not v a l i d f o r c r y s t a l l i n e polymers ( 6, 81, 141, 142, 145 )• This i s due to changes i n m i c r o c r y s t a l l i n e s t r u c t u r e and i t s s t r e s s - b e a r i n g mechanisms a t v a r i o u s temperatures ( 127 ). I t was found 127 ) w i t h t e n s i l e s t r e s s r e l a x a t i o n of PVA d e r i v a t i v e s t h a t no s i n g l e time-temperature s u p e r p o s i t i o n was v a l i d over a l l s t a t e s , i n c l u d i n g the T and rubbery r e g i o n s . Changes from s e m i c r y s t a l l i n e to amorphous s t r u c t u r e were observed with such m a t e r i a l s a t v a r i o u s temperatures above T^ . These anomalies were a t t r i b u t e d to existence of an i n t e r - m o l e c u l a r r e l a x a t i o n mechanism, c h a r a c t e r i z e d by loosening of the c r y s t a l s t r u c t u r e through breakage of strong secondary valence bonds ( 32 ) . The s u p e r p o s i t i o n p r i n c i p l e has been examined with copolymer systems. F u j i n o et a l . ( 31 ) examined s t r e s s r e l a x a t i o n of MMA-MA copolymers, as w e l l as poly(MMA) and p o l y (MA), at v a r i o u s temperatures and found t h a t these could be s h i f t e d to g i v e master curves covering numerous time decades. In summary, polymer s t r e s s r e l a x a t i o n depends upon the c h a r a c t e r i s t i c s of m a t e r i a l s and the - 27 -t e s t environment, i n c l u d i n g temperature and time. E f f e c t s of temperature and time may be superimposed i n some cases, such as with amorphous polymer systems. 2.4.3 R h e o l o g i c a l p r o p e r t i e s of papers E a r l y understanding of paper v i s c o e l a s t i c behaviour was drawn from simple i n v e s t i g a t i o n s of l o a d -deformation. I t i s noted here t h a t the i n i t i a l response i n a load-deformation t e s t i s l a r g e l y e l a s t i c , and at higher loads paper does deform i n a manner i n d i c a t i n g f l o w . This deformation caused by m a t e r i a l f l o w i s l a r g e l y non-recoverable. When specimens are subjected to l o a d i n g -unloading c y c l e s , deformation a f t e r the f i r s t c y c l e s i s almost e n t i r e l y r e c o v e r a b l e . A marked h y s t e r e s i s i n the l o a d i n g ^ unloading curves has been observed. Studies by Steenberg and a s s o c i a t e ( 132-134 ) on paper load-deformation p r o p e r t i e s , i n d i c a t e d t h a t these r e s u l t s depend on numerous f a c t o r s r e l a t i n g to the manner of sheet pr 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 previous mechanical h i s t o r y of the specimen. S u p e r f i c i a l i n v i s i b l e creep of f i b r e s , macroscopic - 28 -unkinking and f i b r e u n c u r l i n g i n the sheet were considered to be r e s p o n s i b l e f o r non-recoverable deformation ( 71 ) . Attempts have been made to d e s c r i b e v i s c o e l a s t i c p r o p e r t i e s of papers by mechanical s p r i n g and dashpot combinations. Anders son ( 4 ) caKdQSt.©enberg,iand coworker ( 132- 134 ) employed Eyring's theory of r a t e processes ( 34 ) and the s p r i n g and dashpot combination developed f o r t e x t i l e s by Halsey and coworkers ( 3 7 . 38 ) to d escribe paper s t r e s s - s t r a i n curves according to a threes element model. The model elements c o n s i s t e d of a s p r i n g p a r a l l e l to a Haxwellian body. Mason ( 71 ) suggested a f o u r element model w i t h a Maxwellian body i n s e r i e s w i t h a body c o n s i s t i n g of a s p r i n g and dashpot i n p a r a l l e l , to e x p l a i n paper s t r e s s - s t r a i n phenomena. In the molecular approach, i t i s b e l i e v e d t h a t H-bonds pl a y an important r o l e i n v i s c o e l a s t i c behaviour of c e l l u l o s i c m a t e r i a l s ( 85-92, 135, 136 ) . Ranee ( 106, 107 ) proposed t h a t delayed e l a s t i c and p l a s t i c cmoponents of paper e l o n g a t i o n are due to the breakage of f i b r e to f i b r e bonds. Forces a c t i n g between - 29 -two f i b r e s were a t t r i b u t e d to H-bonds or van der Waal's f o r c e s , which c o n t r i b u t e to the fl o w of m a t e r i a l s . F u r t h e r , Nissan ( 85-90 ) and coworker. ( 92; 135, \ 136 ) p o s t u l a t e d t h a t paper v i s c o e l a s t i c p r o p e r t i e s are s t r o n g l y a f f e c t e d , o r even governed, by H-bonds. The formation of i n t e r - f i b r e bonds a t d i f f e r e n t stages during sheet d r y i n g was found to be important to paper r e l a x a t i o n behaviour ( 29 ). I n a d d i t i o n , Page ( 98 ) argued t h a t 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 networks, such as paper, were a l s o dependent upon s t r u c t u r e a t the supermolecular l e v e l . I n c o n c l u s i o n, i t seems t h a t paper r h e o l o g i c a l p r o p e r t i e s have been a t t r i b u t e d to valence bond le n g t h , bond deformation angle, secondary bond deformation, r e o r i e n t a t i o n of macromolecules i n amorphous re g i o n s , r e o r i e n t a t i o n of c r y s t a l l i n e regions and c o n f i g u r a t i o n a l entropy e f f e c t s , as w e l l as d r y i n g c o n d i t i o n s and paper m a c r o - s t r u c t u r a l e f f e c t s ( 24, 92 ) . 2.4.3.1 Str e s s r e l a x a t i o n of papers Studies on paper s t r e s s r e l a x a t i o n have been - 30 -l i m i t e d mostly to phenomenological d e s c r i p t i o n of s t r e s s versus time r e l a t i o n s h i p s and e f f e c t s of t e s t i n g environment on these measurements. P l o t t i n g of s t r e s s decay agai n s t time s c a l e s "between 0.01 and 10 sec by Anderson and Sjoberg ( 5 ) provided sigmoid shaped curves implying Maxwellian r e l a x a t i o n . Mason ( 71 ) showed, with exception of very e a r l y p a r t s , l i n e a r r e l a t i o n s h i p s f o r l o g s t r e s s decay aga i n s t time. Kubat ( 52, 53 ) p o s t u l a t e d a l i n e a r r e l a t i o n s h i p f o r s t r e s s ( S ) against l o g dS/dt according to» . dS/dt = b ( exp a S - 1 ) / 13 / where ? S = s t r e s s at time t , dS/dt = s t r e s s decay, and a, b = constants. Ranee . ~ ( 107 ) ' " r e p o r t e d l ^ o n c i paper s t r e s s r e l a x a t i o n f o l l o w i n g constant e l o n g a t i o n r a t e s between 1% i n 23 sec and 1% i n 10 sec. L i k e Kubat, he has recorded t h a t the p l o t of S against l o g t f o l l o w e d a s t r a i g h t l i n e , such t h a t t - 31 -S = x - y l o g t / 14 J where ? x,y = constants, dependent on amount of p r e l o a d i n g . Recently, Kubat ( 54 ) described r e l a x a t i o n k i n e t i c s i n s o l i d m a t e r i a l s , i n c l u d i n g paper, as not complying w i t h the simple Maxwellian model. The curves extend, however, over a p p r e c i a b l y longer i n t e r v a l s than would be c o n s i s t e n t with exponential decay. I t has been noted ( 44 ) t h a t P, i n f l e x i o n slope of s t r e s s , v e r s u s In t f o r paper and t o t a l s t r e s s d i s s i p a t i o n are r e l a t e d byi P = 0.1 A s f 15 / where j A S = s t r e s s d i s s i p a t i o n . I n t h i s respect, paper conforms to the general behaviour of other types of s o l i d s as i n t e r p r e t e d by { 10 / . Shape of the r e l a x a t i o n curve i s a f f e c t e d by the t e s t i n g environment and t e s t c o n d i t i o n s . I t has been noted t h a t an i n c r e a s e s i n moisture content r e s u l t s i n s u b s t a n t i a l increase i n r e l a x a t i o n r a t e ( 71 )• Increase i n i n i t i a l s t r e s s , i n analogy w i t h promotion - 32 -by temperature, a l t e r e d curves toward s h o r t e r r e l a x a t i o n time ( 44 ). By i n c r e a s i n g s t r a i n i n g time a t constant i n i t i a l s t r e s s , e f f e c t i v e width of the <£^distribution was decreased ( 5^ )• I n a d d i t i o n , Craven ( 24 ) reported t h a t i n f l u e n c e of d r i e d - i n s t r e s s e s waso s i g n i f i c a n t on the r e l a x a t i o n - c u r v e . The e f f e c t of beating on shape and p o s i t i o n of the curves was r a t h e r l i m i t e d ( 44 ). 2.4.4 Time dependent behaviour of f i b r e p l a s t i c composites Composite m a t e r i a l s are important to development of high s t r e n g t h s t r u c t u r e s . P r o p e r t i e s of composites vary w i t h d i f f e r e n t substrate and ma t r i x ( binder ) systems used . I t has been found ( 108 ) t h a t f i b r o u s composite c h a r a c t e r i s t i c s are s t r o n g l y a f f e c t e d by f i b r e l e n g t h, f i b r e c o n c e n t r a t i o n , f i b r e o r i e n t a t i o n , f i b r e diameter, f i b r e shape and component behaviours, as w e l l as composition of substrate and ma t r i x . Rauch et a l . ( 108 ) expressed the b e l i e f t h a t f i b r e composite stren g t h depends on« - 33 -i . f i b r e s strength} and i i . degree to which a l l f i b r e s w i t h i n the matrix are s t r e s s e d . Studies on wood p l a s t i c composites; ( WPC ) i n d i c a t e t h a t u l t i m a t e p r o p e r t i e s seem to be more strongly-i n f l u e n c e d by wood species than by chemical nature of the impregnating monomer ( 131 ). P h y s i c a l p r o p e r t i e s of g r a f t e d papers are decided by the nature of f i b r e s , c h a r a c t e r i s t i c s of monomers ( or polymers ) and g r a f t i n g techniques used. L i t t l e work has been done on time dependent p r o p e r t i e s of wood f i b r e composites. Heyse et a l . ( 39 ) observed t h a t when paper s u b s t r a t e s were formed w i t h s o f t , e x t e n s i b l e polymer matrices, i n c r e a s e s occurred* i n t e n s i l e s t r e n g t h and e l o n g a t i o n with i n c r e a s i n g r a t f i of s t r a i n . However, the product behaved l i k e untreated paper i f s t i f f e r copolymers and PVA were a p p l i e d . I n summary, there seems to be a s c a r c i t y of observations on time-dependent behaviour of composite m a t e r i a l s . Among these needs, i t seems e s p e c i a l l y c h a l l e n g i n g and u s e f u l to describe s t r e s s responses of substrate and " m a t r i x complexes as found i n paper p l a s t i c composites ( PPC ). - 3^ -3.0 MATERIALS AND METHODS M a t e r i a l s with v e r y d i f f e r e n t p r o p e r t i e s were s e l e c t e d f o r study. These in c l u d e d bleached and unbleached k r a f t , as w e l l as brightened and unbrightened groundwood pulps, w i t h and without monomer and comonomer l o a d i n g . In a d d i t i o n , a s s o c i a t e d polymer and copolymer f i l m s were prepared and t e s t e d . 3.1 Pulps K r a f t and groundwood western hemlock ( Tsuga  h e t e r o p h y l l a ( Raf. ) Sarg. ) pulps were obtained from the Powell R i v e r m i l l of MacMillan B l o e d e l , L t d . The pulps were at about 300?S moisture content ( based on oven-dry „ weight. )1 as c o l l e c t e d . These were stored i n p l a s t i c bags at 2°C i n a c o l d room p r i o r to handsheet p r e p a r a t i o n . Groundwood pulp i s formed by mechanical s e p a r a t i o n of f i b r e s from wood. The product represents almost the t o t a l wood f r a c t i o n subjected to the s e p a r a t i o n a c t i o n and i s obtained a t high y i e l d ( almost 98$ ) i n conventional operations ( 28 ). The pulp i s composed mainly of f i b r e bundles and f i b r e fragments w i t h some whole i n d i v i d u a l f i b r e s . Groundwood - 35 -pulps are r i g i d due to t h e i r high l i g n i n content, whereby they do not c o l l a p s e on l o s s of water and do not conform w e l l during d r y i n g or p r e s s i n g of f i b r e webs. Therefore, webs prepared from groundwoods are bulky and show good o p a c i t y but low s t r e n g t h . Groundwood b r i g h t e n i n g , such as done with one of the present samples, is:a" n o n - l i g n i n a b s t r a c t i o n techniques. The mechanisms of groundwood b r i g h t e n i n g may be c l a s s i f i e d e i t h e r as o x i d a t i o n or r e d u c t i o n ( 49 ). The b r i g h t e n i n g a c t i o n of reducing agents i s most probably r e l a t e d to a d d i t i o n of d i s s o c i a t i o n products to the c o l o r i n g m a t e r i a l s i n the wood, or i t may be due to changes of these m a t e r i a l s . On the other hand, o x i d i z i n g type b r i g h t e n i n g agents d i s s o c i a t e i n water by forming H0~ i o n s . I t i s b e l i e v e d t h a t t h i s i o n s e l e c t i v e l y o x i d i z e s and/or hydrolyses coloured organic compounds i n wood. Some condensation may occur, but t h i s does tiSt a f f e c t a p p r e c i a b l y the b a s i c p h y s i c a l s t a t e of the l i g n i n and c e l l u l o s e . K r a f t pulps, such as used i n t h i s study d i f f e r from groundwood i n processing f i b r e s e p a r a t i o n . This i s accomplished through expenditure of chemical energy - 36 -preceding a g i t a t i o n , which maintains i n d i v i d u a l s i b r e s k e l e t o n s . The va r i o u s pulping steps i n v o l v e d i s s o l v i n g l i g n i n and at the same time h e m i c e l l u l o s e s are degraded and p a r t l y removed. High hemiee l l u l o s e s o l u b i l i t y i n the a l k a l i cooking medium u s u a l l y leads to lower hem i e e l l u l o s e r e t e n t i o n ( 73 )» whereby s w e l l i n g c a p a c i t y of the f i b r e s i s f u r t h e r reduced. K r a f t pulps show good strength p r o p e r t i e s , as the name i m p l i e s . Unbeaten k r a f t pulp i s known to provide mats wit h open s t r u c t u r e and poor l y conforming f i b r e s which do not c o l l a p s e on removal of water from the sheet, as w e l l as v i r t u a l absence of f i b r i l l a t i o n ( 17 ). F a i l u r e of papers made from unbeaten k r a f t f i b r e s i s due mainly to f i b r e p u l l - o u t over a deep f r a c t u r e zone ( 17 ) . 3.2 Monomers and Comonomers Methyl methaerylate ( MMA ) and t e t r a e t h y l e n e g l y c o l dimethacrylate ( TEGDMA ) were s e l e c t e d as monomers f o r the study. These were s u p p l i e d , r e s p e c t i v e l y , by Lor t e c h S e r v i c e s , L t d . , Burnaby, B r i t i s h Columbia and Monomer- Polymer Lab, Borden Chemicals Div., Borden, Inc., P h i l a d e l p h i a . - 37 -The i n h i b i t o r was removed from MMA by shaking approximately 250 ml of monomer i n a stoppered separatory f u n n e l with 25 ml of an aqueous s o l u t i o n c o n t a i n i n g 2g NaOH and lOg NaCl per 100 ml. The aqueous l a y e r was allowed , to se p a r a t e ^ and run o f f . The procedure was repeated u n t i l the aqueous l a y e r remained c o l o u r l e s s . The monomer was then washed wi t h successive 10 ml p o r t i o n s of water u n t i l t h i s gave no a l k a l i n e r e a c t i o n . The organic l a y e r was then d i s t i l l e d to remove water. Since TEGDMA i s cured by gamma r a d i a t i o n w i t h dose as low as 0.2 Mrad, no i n h i b i t o r removal was considered necessary. Some in f o r m a t i o n on these two monomers and t h e i r uses followsrr;cc* „ MMA i s u s u a l l y prepared from acetone, hydrocyanide, and o x i d a t i o n of isobutylene with n i t r i c a c i d ( 45, 114 ). The monomer i s a c o l o u r l e s s l i q u i d w i t h molecular weight 100.11, b o i l i n g p o i n t 100.6 to 101.1 °C, and w i t h i n f i n i t e s o l u b i l i t y i n a l c o h o l and ether ( 14, 114 ). MMA p o l y m e r i z a t i o n occurs e a s i l y , and i s q u i t e s e n s i t i v e to a v a r i e t y of c a t a l y s t s and i n h i b i t o r s ( 46 ). Molecular oxygen i n h i b t s the p o l y m e r i z a t i o n . P o l y m e r i z a t i o n of MMA i s exothermic to the extent t h a t 13,000 c a l o r i e s - 38 -per gram-monomer u n i t are r e l e a s e d . The energy r e l e a s e d during p o l y m e r i z a t i o n r a i s e s the system temperature, a c c e l e r a t i n g the r e a c t i o n somewhat during e a r l y p o l y m e r i z a t i o n and f u r t h e r i n c r e a s i n g r a t e of energy r e l e a s e ( 45 ). Poly(MMA) i s s o l u b l e i n i t s own monomer, i n aromatic and most c h l o r i n a t e d hydrocarbons, e s t e r s , ketones, and te t r a h y d r o f u r a n s o l v e n t s ( 114 ). The s p e c i f i c g r a v i t y of poly(MMA) i s 1.18 to 1.19. The polymer i s temperature s e n s i t i v e , undergoing a second order t r a n s i t i o n when warmednto 60 °C t tha t i s , there i s a t r a n s f o r m a t i o n at t h a t temperature without an accompanying l a t e n t heat e f f e c t . Above t h i s t r a n s i t i o n temperature, the polymer becomes r u b b e r - l i k e . On f u r t h e r warming to 138 to 156 °C poly(MMA) melts to give a visc o u s l i q u i d . I t i s a b r i t t l e , transparent polymer w i t h good st r e n g t h p r o p e r t i e s , such as t e n s i l e 2 strength of 2.1 kg / mm and compression strengths of 2.4 to 4.0 kg / mm2 ( 45 ). Poly(MMA) shows a pronounced delayed p s e u d o - e l a s t i c or memory e f f e c t . This i s u s u a l l y something of a disadvantage because i t may le a d to dimensional - 39 -i n s t a b i l i t y . I t al s o i n d i c a t e s the presence of l o c k e d - i n s t r e s s e s which may lead to surface c r a c k i n g . A network of cracks causes l o s s i n transparency and st r e n g t h . The chemical behaviour of poly(MMA) i s t h a t of a s t a b l e m a t e r i a l . Thus, i t has no c h l o r i n e , which might make i t s e n s i t i v e to u l t r a v i o l e t exposure, and no uns a t u r a t i o n which might make i t s e n s i t i v e otherwise to atmospheric oxygen ( 45 ) . TEGDMA i s a h y d r o p h i l i c , d i v i n y l monomer wit h b o i l i n g p o i n t g r e a t e r than 160 °C and v i s c o s i t y of 12 c.p.. I t i s i n s o l u b l e i n water, but has good s o l u b i l i t y i n MMA, S, VA, p o l y e s t e r s , a c r y l i c a c i d , d i a l l y l maleate and aromatics, but l i m i t e d s o l u b i l i t y i n a l i p h a t i c hydrocarbons. TEGDMA hardens i n t o a g l a s s y s o l i d a t -55 to -60 °C, which i s al s o the lowest temperature at which p o l y m e r i z a t i o n can be e f f e c t e d ( 117 )« The TEGDMA monomer polymerizes much more r e a d i l y than monomers with o n l y one double bond. I t was found to produce 18 5.4 x 10 f r e e r a d i c a l s per Mrep. I t s peroxide c a t a l i z e d p o l y m e r i z a t i o n i s i n h i b i t e d by both benzoquinone and oxygen ( 119 ). P o l y m e r i z a t i o n by high energy r a d i a t i o n - 40 -was found to be dim e n s i o n a l l y s p e c i f i c and t h i s can be used to form o b j e c t s of predetermined shape ( 119 )• Eoly(TEGDMA) i s a transparent c r o s s l i n k e d polymer. According to Micleo ( 76 ), y i e l d s t r e s s i n compression of the polymer i s 5.7 kg / mm with rupture s t r e s s of 19 2 kg / mm 7 The shear modulus of poly(TEGDMA) i s d i f f e r e n t from t h a t of poly (MMA)'>r^s„ since i t l e v e l s o f f with a high modulus i n the rubbery p l a t e a u ( 144 ). Pol y m e r i z a t i o n of two or mareLcomanomersccan provide a wider v a r i e t y of p h y s i c a l and chemical p r o p e r t i e s than when s i n g l e monomers are polymerized,!. As example, i n t r o d u c t i o n of TEGDMA i n t o MMA suppresses the regions of rubbery f l o w and l i q u i d f l ow, and increases the"rubbery p l a t e a u modulus ( 144 ). Comonomer mixtures f o r impregnating paper handsheets and forming t h i n f i l m s were prepared by mining volumes of MMA and TEGDMA to giu>e three mixtures to be desc r i b e d . These mixtures were chosen to represen1?Gpartial;lyj to f u l l y r : c r o s s l i n k e d polymer systems. - 41 -3.3 Formation and Treatment of Paper Handsheets Handsheets were prepared on a B r i t i s h Sheet Machine f o l l o w i n g the recommended pulp d i s i n t e g r a t i o n and d i l u t i o n steps of TAPPI Standard T-205-M58 ( 138 ). o The handsheets were then pressed twice a t 0.325 kg/ mm with standard p r e s s - p l a t e and dry b l o t t e r s i n p i l e s of seven handsheets. Press d w e l l times were 5 and 2 min, r e s p e c t i v e l y . The handsheets were then conditioned i n press r i n g s f o r a minimum of 2 days at $0%'JR®M'L aftd 21?;C. The handsheets were made up i n two c a t e g o r i e s . Sheets with the standard nominal 1.2 g m o i s t u r e f f r e e weight were employed to prepare paper p l a s t i c composites ( PPC ). In order to maintain constant b a s i s weights between paper handsheets and PPC, heavier handsheets were prepared by d r a i n i n g e x t r a amounts of the prepared stock ( Table I ). Paper p l a s t i c composites ( PPC ) were prepared by p o l y m e r i z a t i o n of impregnated (co)monomer systems i n standard paper handsheets. Bleached and unbleached k r a f t , as w e l l as brightened and unbrightened groundwood western hemlock pulps were t r e a t e d with MMA » TEGDMA - 42 -(co)monomer mixtures as 100 j 0, 95 « 5. 85 « 15» 60 : 40; and 0 : 100. I n p r a c t i c e , l a b o r a t o r y handsheets were cut i n t o r e c t a n g l e s 15.2 x 12.7 cm which were evacuated f o r 24 hr i n a d e s i c c a t o r at 10 t o r r . The monomer mixture was then introduced through the d e s i c c a t o r l i d v i a a separatory f u n n e l ( F i g . 2 ). Vacuum was maintained continuously u n t i l no more bubbles were apparent on the paper surface, i . e . , approximately 48 hr . Saturated papers were wrapped i n smooth aluminum f o i l ( shiny side inward ) and pressed between g l a s s p l a t e s before c u r i n g i n the Gammacell. 3.4. P r e p a r a t i o n of Thin Polymer F i l m s F i l m s were prepared i n the l a b o r a t o r y from the f i v e (co)monomer systems described above. A f t e r some p r a c t i c e , f i l m t h i c k n e s s was c o n t r o l l e d a t 10 - 1 m i l . Thin f i l m s were made by f l o w i n g monomer from a m i c r o - p i p e t t e between two spaced g l a s s p l a t e s sealed along t h r e e edges ( F i g . 3 ). To e l i m i n a t e s t i c k i n g of cured f i l m s to the g l a s s p l a t e s , a l a y e r of s i l i c o n e mold r e l e a s e agent was a p p l i e d to both i n t e r n a l s u r f a c e s . - 43 -The whole assembly was h e l d together by s e v e r a l pinch clamps. Upon s e a l i n g of the f o u r t h edge, the samples were cured i n the Gammacell. 3.5 P o l y m e r i z a t i o n P o l y m e r i z a t i o n of saturated handsheets and pure (co)monomer systems was done i n a Gammacell 220 a t dose r a t e of 0.762 Mrad/hr. Appropriate dose requirement f o r each i n d i v i d u a l treatment was determined from exothermic temperature-time records run simultaneously on bulk (co)monomer samples placed i n the Gammacell during i r r a d i a t i o n . Sample i n i t i a l temperatures were around 3^ °C. Dose f o r each treatment i s g i v e n i n Table I I . Moisture contents ( M.C. ), i n c l u d i n g paper handsheets and PPC, were measured by d r y i n g samples i n an oven at 95 to 105 °C f o r 24 hr and c a l c u l a t i n g according t o i M.C.* = 1 Sample a i r - d r y weight ± Sample oven-dry weight x 10© { 16 J - 44 -Polymer loadings i n PPC were c a l c u l a t e d according to f 17 / based on oven-dry weight ( o.d. wt. ) of m a t e r i a l s , which were obtained by f 18 J\ PPG o l d . wt. - Paper o.d. wt. Loading, % = Paper o.d. wt. x 100 / 17 / where ; o.d. wt.t = a i r - d r y weight x ( 1 + M.C. ) \ f^J 3.6 Te s t i n g S t r e s s r e l a x a t i o n t e s t s i n t e n s i o n were performed on PPC and corresponding paper handsheets and polymer f i l m s of comparative b a s i s weight. Ten specimens f o r each treatment were prepared ; h a l f of these were used f o r u l t i m a t e strength determination and h a l f f o r s t r e s s r e l a x a t i o n t e s t s . The 15.2 x 12.7 cm PPC sheets were cut i n t o 1.5 x 1.5 cm s t r i p s . These were used to determine u l t i m a t e s t r e s s and r e l a x a t i o n behaviour of the - 45 -m a t e r i a l s . Paper and polymer s t r i p s were comparable i n b a s i s weight and dimensions to the corresponding PPC ( Table I ). A l l specimens were conditioned i n a CTH room at 50% R.H. and 21 °C f o r two weeks p r i o r to t e s t i n g . Since s t r e s s r e l a x a t i o n curves may be a f f e c t e d by r a t e of s t r a i n and the s t r e s s l e v e l s e t , u l t i m a t e t e n s i l e s t r e n g t h f o r each sample was determined p r i o r to r e l a x a t i o n t e s t s . Tests, i n c l u d i n g u l t i m a t e t e n s i l e s t r e n g t h and s t r e s s r e l a x a t i o n , were done on a f l o o r model I n s t r o n housed i n the CTH^ room. Headspeed was 5 cm/min, the maximum machine s e t t i n g . This provided simulated step^loadingLof LspecimeiTs 'InCo£o§ to 0.06 min. Ultimate strengths f o r various samples t e s t e d under these conditions. 'axe .ilitfte'd ' i n UtiS&Mm !'SfI. S t r e s s r e l a x a t i o n t e s t s were c a r r i e d out by deforming samples to a f i x e d s t r a i n and measuring s t r e s s r e q u i r e d to maintain t h i s s t r a i n as a f u n c t i o n of time. An e x c i t a t i o n energy of 50% of the u l t i m a t e t e n s i l e s t r e n g t h was used. The s t r e s s - t i m e r e l a t i o n s h i p f o r a t e s t e d specimen was read from the I n s t r o n s t r i p chart recorder. Data i n Table I I I represent i n t e r n a l - 46 -energy d i s s i p a t i o n ( s t r e s s decay, or s t r e s s r e t e n t i o n ) as the average of f i v e r e p l i c a t i o n s f o r each sample over observations periods up to 35 min. The s t r e s s a f t e r e x c i t a t i o n was read between 0.04 and 35 min and subjected to r e g r e s s i o n a n a l y s i s according to l i n e a r v i s c o e l a s t i c ! L behaviour; S / S0.04 where j a + b In t s s a b t 0.04 s t r e s s a t time t , s t r e s s a t t Q Q ^ min f o l l o w i n g l o a d i n g , i n t e r c e p t , slope, and time. Re s u l t s of these r e g r e s s i o n analyses are g i ven i n Table IV.. Energy d i s s i p a t i o n ( 33 ) f o r each m a t e r i a l was c a l c u l a t e d according to £ 20 J and subjected to Duncan's New M u l t i p l e Range Test i n order to compare energy f l o w behaviours between m a t e r i a l s ( TablesvV to X ) as» - 47 -A S = 1 - S( 3 5 ) / S(0.04>... { 20 J where ; A S = amount of energy d i s s i p a t i o n i f s t r e s s a t t Q Q l, i s regarded as 1. * ^(35) = s * r e s s a t 35 m^ n» and s ( 0 04) = 3 ^ r e s s a - t °»°4 mi n» - 4 8 -4.0 RESULTS As determined by r e g r e s s i o n a n a l y s i s , r e l a x a t i o n curves f o r a l l three types of m a t e r i a l s were found to f o l l o w s t r a i g h t l i n e s according to 7 * Regression c o e f f i c i e n t s ranged from 0.957 to 0.997 f o r papers, 0.946 to 0 . 9 9 7 f o r polymer f i l m s , and 0.976 to 0.997 f o r PPC ( Table IV ). This means t h a t l i n e a r v i s c o e l a s t i c theory can be a p p l i e d to e x p l a i n the r e l a x a t i o n obehaviour of the m a t e r i a l s i n v e s t i g a t e d over o b s e r v a t i o n times up to 3 5 min. 4 . 1 S t r e s s R e l a x a t i o n of Papers As described, v i s c o e l a s t i c behaviour i s u s u a l l y c h a r a c t e r i z e d by a r e l a x a t i o n time ( :t'j ) which g i v e s r i s e to a f a m i l y of curves r e l a t i n g r e l a x a t i o n behaviour f o r d i f f e r e n t m a t e r i a l s . Curves w i t h low t values describe a r e l a t i v e l y r a p i d decay and are i n d i c a t i v e of f l u i d - l i k e behaviour. Samples g i v i n g higher x values m a i n t a i n r e l a t i v e l y high s t r e s s l e v e l s f o r longer p e r i o d s , which i n d i c a t e s s o l i d - l i k e behaviour. Among papers of the present study, i t was found t h a t those prepared from the bleached k r a f t pulp r e l a x e d s t r e s s f a s t e r than those - 49 -from the unbleached counterpart. I n co n t r a s t , no d i f f e r e n c e s were observed i n s t r e s s r e l a x a t i o n between paperscmade from brighteded and unbrightened groundwood pulps. R e l a x a t i o n between papjers from k r a f t and groundwood were s i g n i f i c a n t l y d i f f e r e n t . Even unbleached k r a f t papers re l a x e d f a s t e r than those from e i t h e r groundwood pulp ( Table V, and F i g . 4 ). 4.2 St r e s s R e l a x a t i o n of Polymer F i l m s The s e r i e s demonstrated the e f f e c t of c r o s s l i n k i n g on r e l a x a t i o n behaviour. Since TEGDMA served as the c r o s s l i n k i n g agent, i t s gradual increase i n MMA l e d to reducing the r e l a x a t i o n amounts. Some unexpected r e s u l t s were obtained. The polymer f i l m s arranged i n order of decreasing s t r e s s decay were M95, TEGDMA, M«5, M60, MMA ( F i g . 5 )• Again data were subjected to Duncan's New M u l t i p l e Test, which revealed that there were no s i g n i f i c a n e e : i n energy d i s s i p a t i o n among the group M85, M60, and MMA ( Table VI ). - 50 -4.3 S t r e s s R e l a x a t i o n of Paper P l a s t i c Composites ( PPC) The p h y s i c a l p r o p e r t i e s of composite m a t e r i a l s depended on t h e i r r e s p e c t i v e components. Studies on r e l a x a t i o n behaviour of composites showed t h a t handsheets t r e a t e d with comonomers i n r a t i o s of 85 » 15. 60 i 40, and MMA were more e l a s t i c than those from 95 « 5 and TEGDMA, despite o b v i o u s l y d i f f e r e n t p r o p e r t i e s of the v a r i o u s pulps ( Tables VII «i to V I I t i x , and P i g . 6 ). This meant t h a t d i s t r i b u t i o n of the r e l a x a t i o n curves was more or l e s s a f f e c t e d by p r o p e r t i e s of the matrices used ( compare P i g . 6 with F i g . 5 ) . The e f f e c t of p u l p i n g process on s t r e s s r e l a x a t i o n was not l o s t a f t e r formation of composites. Although d i s t r i b u t i o n of r e l a x a t i o n curves was a f f e c t e d by the matrices used, energy^ d i s s i p a t i o n s . wjer&; more dependent on c h a r a c t e r i s t i c s of s u b s t r a t e s . I t was found t h a t papers from k r a f t pulps r e l a x e d f a s t e r than those from groundwood pulps ( Tables V I I I l i to V I I I i v , and F i g s . 7-1 to 7-5 ) . Comparing r e l a x a t i o n curves of c e r t a i n - 51 -composites with r e spect to t h e i r corresponding components, the s t r e s s decays f o r kraft-polymer systems were commonly l e s s than those f o r e i t h e r paper handsheets or p l a s t i c f i l m s ( Tables I X i i To IX:x, F i g s . 8-1 to 8- 4 ) . The amount of r e l a x a t i o n f o r groundwood-polymer systems, however, was s i m i l a r to t h a t f o r those papers. Both papers and composites r e l a x e d r e l a t i v e l y more sl o w l y than observed f o r the corresponding pure (copolymer f i l m s ( Tables X : i to X i x , and F i g s . 9- 1 to 9-4 ) . S t r e s s r e t e n t i o n between unbleached- and bleach e d - k r a f t papers t r e a t e d with e i t h e r MMA ( l i n e a r polymer ), or TEGDMA ( c r o s s l i n k e d polymer ), o r the 95 » 5 comonomer mixture ( s l i g h t l y c r o s s l i n k e d polymer ) were not s i g n i f i c a n t l y d i f f e r e n t ( Tables V i l l i i , V l l l i i i , and V I I I i v ). on the other hand, bleached k r a f t papers r e l a x e d f a s t e r than unbleached papers when they were impregnated w i t h e i t h e r the 85 : 15 or 60 : 40 comonomer mixtures ( Table VIII»iii, V l l l i i v , and F i g s . 7-3. 7-4 )• Nevertheless, amount of r e l a x a t i o n a f t e r treatment was not s i g n i f i c a n t l y - 52 -d i f f e r e n t between brightened and unbrightened groundwoods ( Tables V I I I : i to VIII»iii and V I I I : v ), except f o r substrates t r e a t e d w i t h the 95 » 5 comonomer ( Table V I I I l i i ). - 53 -5.0 DISCUSSION Stre s s r e l a x a t i o n i s concerned with the i n v e s t i g a t i o n of deformation and f l o w behaviour of r m a t e r i a l s . I n t h i s regard, polymeric molecules, i n c l u d i n g p l a s t i c s and c e l l u l o s i c s , are s i m i l a r to simple monomer molecules. The molecules are held together by covalent, i o n i c and hydrogen bonds, as w e l l as d i p o l e i n t e r a c t i o n s . A p p l i c a t i o n of s t r e s s to these m a t e r i a l s may cause deformation, s l i p p a g e , breakage of molecular bonds and p o s s i b l y changes i n molecular conformation. Thereby, the r e l a x a t i o n behaviour of m a t e r i a l s can be e l u c i d a t e d i n terms of molecular s t r u c t u r e s which p a r t i c i p a t e i n mechanisms r e s p o n s i b l e f o r the r e l a x a t i o n phenomenon. 3.1 Short Term S t r e s s Decay At constant specimen deformation, s t r e s s decay w i t h i n the f i r s t m i l l i s e c o n d s a f t e r e x c i t a t i o n has been s t u d i e d by s e v e r a l workers ( 46, 47, 74, 115, 149 ). Watson et a l . ( 149 ) measured s t r e s s r e l a x a t i o n on d i f f e r e n t polymers over the approximate time range of 0.01 to 2.5 sec and reported t h a t hard, s t i f f , and b r i t t l e m a t e r i a l s were c h a r a c t e r i z e d by l i t t l e r e l a x a t i o n w i t h i n such time range. S t r a i n s which these m a t e r i a l s - 54 -could s u s t a i n without f r a c t u r e , however, were s m a l l . S o f t e r , more d u c t i l e p l a s t i c s , on the other hand, showed f a s t e r r e l a x a t i o n r a t e s and were considerably more e x t e n s i b l e than b r i t t l e p l a s t i c s . Working with c e l l u l o s e f i b r e s , Meredith ( 74 ) demonstrated th a t the f a s t e r the extension, the f a s t e r the subsequent decay of s t r e s s w i t h i n a f r a c t i o n of a second a f t e r reaching a gi v e n e x c i t a t i o n . This was i n f e r r e d as the f a s t e r the i n i t i a l extension, the l e s s time there was f o r i n t e r n a l r e o r g a n i z a t i o n of the s t r u c t u r e , w i t h the r e s u l t t h a t more damage was done. P o s s i b l e damage of the s t r u c t u r e s was al s o observed with polymer substances (.115 )• I n v e s t i g a t i o n of some wood t i s s u e s and vi s c o s e pulps by Kirbach ( 46, 47 ) l e d to a p o s t u l a t i o n t h a t conformational changes of h e m i c e l l u l o s e s and glucose may c o n t r i b u t e to high s t r e s s decay i n e a r l y stages of wood product:rheolog"ical processes. As regards mechanism, e x t e r n a l energy a p p l i e d as s t r e s s might cause the anhydroglucose " ' " C ^ conformation to 4 switch to C-L conformation, which i s c h a r a c t e r i z e d - 55 -by higher energy content and lower s t a b i l i t y . Removal of e x t e r n a l f o r c e s or processes of i n t e r n a l accomodation might r e s u l t i n r e v e r s i o n to the "^ C^  conformation w i t h r e l e a s e of stored energy. In polymer t e s t i n g , Schmitz and Brown ( l i b ) h i g h l y recommend t h a t data should be taken from one time decade a f t e r the i n i t i a l s t r a i n was achieved f o r meaningful measurements. Unfortunately, no e x p l a n a t i o n was suggested. A sharp drop i n i n i t i a l s t r e s s was observed w i t h i n the f i r s t 0.04 min of the present study ( Table I I I There i s ho strong t h e o r e t i c a l evidence supporting such extensive s t r e s s l o s s over short time. At t h i s stage i t i s a d v i s i b l e to f o l l o w Bergen's advice, i n which a n a l y s i s of s t r e s s - l o g time curves i s s t a r t e d o n l y a f t e r a c e r t a i n p e r i o d of r e l a x a t i o n ( l i b ). Consequently, e a r l y p a r t s of r e l a x a t i o n curves are l e f t alone u n t i l f u r t h e r evidence i s obtained as to t h e i r s i g n i f i c a n c e . - 56 -5.2 S t r e s s R e l a x a t i o n of Papers Paper r h e o l o g i c a l p r o p e r t i e s r e l a t e d i r e c t l y to p r e s s i n g , machining and p r i n t i n g p r o p e r t i e s . I t i s noted t h a t paper r h e o l o g i c a l behaviour has been the subject of i n v e s t i g a t i o n s s i n c e the 1920's. In early, days, the mechanism of s t r e s s / e l o n g a t i o n was explained by simple models of spring-dashpot combinations (171, 132-134 ), and l a t e r by hydrogen bonding theory ( 85-92, 106, 107, 135. 136?). The most acceptable i n t e r p r e t a t i o n s of paper mechanical p r o p e r t i e s considers i . sheet s t r u c t u r e parameters, i n c l u d i n g such f a c t o r s as f i b r e length d i s t r b u t i o n , nature and extent of f i b r e to f i b r e bonding, and other d i s t r i b u t i o n f u n c t i o n s ; i i . i n s t r i n s i c s trength of f i b r e to f i b r e bonds; and i i i . t e n s i l e strength of i n d i v i d u a l f i b r e s ( 147 ). Besides these s t r u c t u r a l parameters, such other f e a t u r e s as d r y i n g c o n d i t i o n s , t e s t environment - 57 -( temperature, humidity, s t r e s s or s t r a i n l e v e l s ), i n f l u e n c e of l i g n i n and pulping process have been s t u d i e d . 5.2.1 E f f e c t of l i g n i n on s t r e s s r e l a x a t i o n of k r a f t papers L i g n i n i s c h a r a c t e r i s t i c a l l y a three-dimensional substance. The p a r t i c i p a t i o n of l i g n i n i n r e l a x a t i o n of wood substances d i f f e r s from c e l l u l o s e . The h i g h l y c r o s s l i n k e d l i g n i n s t r u c t u r e could r e s t r i c t c e l l u l o s e chain movement, thereby decreasing energy d i s s i p a t i o n . Studies on d e l i g n i f i e d wood t i s s u e s ( 29 ) revealed t h a t the f u n c t i o n of l i g n i n i n r e l a x a t i o n was to reduce carbohydrate m o b i l i t y . L i k e w i s e , magnitude and r a t e of s t r e s s decrement increased r a p i d l y as specimen l i g n i n content decreased ( 22, 33 ). Since l i ' g n i n a f f e c t s r a t e of wood s t r e s s r e l a x a t i o n , the d i f f e r e n t r e l a x a t i o n behaviours between unbleached and bleached k r a f t papers observed i n the present study may be a t t r i b u t e d l i k e w i s e to v a r i o u s amounts of l i g n i n i n the m a t e r i a l s . - 58 -The l a r g e r q u a n t i t y of l i g n i n i n unbleached k r a f t papers may have l e d to energy storage, delayed energy t r a n s m i s s i o n and, thereby, the higher s t r e s s r e t e n t i o n observed ( Table V, F i g . 4 ) . 5.2.2 S t r e s s r e l a x a t i o n of groundwood papers R e l a x a t i o n of groundwood papers was found to d i f f e r from t h a t of k r a f t papers. Not only were values much lower, but 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 energy d i s s i p a t i o n were found between brightened and unbrightened groundwoods. The r e s u l t s can be a t t r i b u t e d to t h e i r s i m i l a r chemical s t r u c t u r e s . Since there i s no r e a l l o s s of substances d u r i n g the t y p i c a l groundwood b r i g h t e n i n g process ( 49 ), the chemical components between brightened and unbrightened pulps are almost i d e n t i c a l . Therefore, they are expected to a f f e c t the amount of energy d i s s i p a t i o n i n a s i m i l a r manner. The present r e s u l t s are c o n s i s t e n t with these s p e c u l a t i o n s ( Table V, F i g . 4 ) . - 59 -5.2.3 D i f f e r e n c e s i n s t r e s s r e l a x a t i o n between k r a f t and groundwood papers The r h e o l o g i c a l behviour of d i f f e r e n t papers i n response to s t r e s s e x c i t a t i o n i s a t t r i b u t e d to v a r i a t i o n i n f i b r e nature and paper s t r u c t u r e . R e s u l t s from the present study r e v e a l r e l a t i v e l y l e s s amount of energy d i s s i p a t i o n f o r groundwoods as opposed to the k r a f t papers. This c o n f l i c t s with p r e v i o u s l y e s t a b l i s h e d patterns by Jackson and Ekstrom ( 42 ), as w e l l as Seborg and Simmonds ( 122 ). They have shown t h a t groundwood pulp mats have. lower «.recovery% value's f r o m compressive deformation than sulphate and s u l p h i t e pulps. This was a t t r i b u t e d to a l a r g e r percentage of f i n e s i n groundwood pulps ( 122 ). The r e l a t i v e expansion recovery a f t e r pressure removal was used to c h a r a c e r i z e these m a t e r i a l s . For a p e r f e c t l y e l a s t i c substance the expansion would be equal to 100%, and f o r a p e r f e c t l y p l a s t i c m a t e r i a l i t would-be equal to u%. Paper c o m p r e s s i b i l i t y was v a r i e d by adding unbleached s u l p h i t e pulp to a b a s i c groundwood stock. Sheet expansion increased as chemical pulp content was - 60 -increased. Up to 60% a d d i t i o n of chemical pulp, however, had l i t t l e e f f e c t on r e l a t i v e expansion. The present r e s u l t s , on the other hand, seem to agree w i t h work done by Drummond and Parsons ( 2b ). They reported t h a t b u l k i n e s s gave a cushioning e f f e c t i n groundwood sheets, since the f i b r e s tended to r e g a i n t h e i r shape f o l l o w i n g r e l e a s e a f t e r compression. This cushion e f f e c t may c o n t r i b u t e to e l a s t i c p r o p e r t i e s of paper. Besides, f i b r e s t i f f n e s s may a l s o be used to e x p l a i n the r e s u l t s . S t i f f n e s s i s defined as the product of the modulus of e l a s t i c i t y of the f i b r e m a t e r i a l and the moment of i n e r t i a of the f i b r e cross s e c t i o n ( 116 ). S t i f f n e s s i s , thereby, i n f l u e n c e d by f i b r e w a l l m a t e r i a l s and the dimension of the f i b r e cross s e c t i o n . Paper s t i f f n e s s i s r e l a t e d to these i n d i v i d u a l f i b r e p r o p e r t i e s . Papers made from pulps high! i n hemieel l u l o s e content are s t i f f e r than papers made from pulps low i n hemieel l u l o s e content. Papers made from s h o r t - f i b r e d pulps, e.g., straw, chestnut and groundwood are g e n e r a l l y s t i f f e r than papers made - 61 -from l o n g e r - f i b r e d pulps, e.g., fiouglas-fir. Groundwood produces very high s t i f f n e s s papers, but r e s u l t s i n t h i c k e r sheets a t the same b a s i s weight than sheets prepared from other pulps*( 19 ) The s t i f f n e s s of f i b r e s a f f e c t s mechanical p r o p e r t i e s of the f i b r e network. Shear modulus of f i b r e networks i s d i r e c t l y r e l a t e d to f i b r e s t i f f n e s s ( 75 ) . Since groundwood f i b r e i s more s t i f f than k r a f t pulp f i b r e , the r e l a t i v e l y l e s s amount of s t r e s s d i s s i -pation!©;;: f o r groundwoods seems reasonable ( Table V, F i g . 4 ). 5.3 S t r e s s R e l a x a t i o n of Polymers Polymer s t r e s s r e l a x a t i o n processes can be thought of i n terms of thermal motion a f f e c t i n g polymer molecule o r i e n t a t i o n . I n t h i s , mechanical s t r e s s a p p l i e d to a polymer increases f r e e energy of the system. I f the sample i s kept i n deformed s t a t e , - 6 2 -s t r e s s r e l a x a t i o n takes place as r e s u l t of thermal motion of the chains, molecular deformations are o b l i t e r a t e d , and excess f r e e energy i s d i s s i p a t e d as heat. D e t a i l s of such a s t r e s s r e l a x a t i o n process u s u a l l y depend upon the m u l t i p l i c i t y of ways i n which polymer molecules can r e g a i n t h e i r most s t a b l e conformations through thermal motion ( 74 ) . 5.3.1 The c o n t r i b u t i o n of c r o s s l i n k i n g to polymer r h e o l o g i c a l p r o p e r t i e s Polymer molecules c o n s i s t of many u n i t s c h e m i c a l l y l i n k e d together. Some atoms i n these u n i t s form the polymer chain backbone while others are chemical s u b s t i t u e n t s attached to the backbone. I f backbone atoms form a l i n e a r sequence, the polymer i s c a l l e d l i n e a r . I n some cases the chain atoms which are separated by macroscopic d i s t a n c e s may be l i n k e d through backbone atoms i n the mode of a three-dimensional network. These are c a l l e d c r o s s l i n k e d polymers. As r e s u l t of c r o s s l i n k i n g many chains become, i n f a c t , one chain. Although chain fragmental motion i s p o s s i b l e , the - 63 -s l i d i n g of one chain past another and loosening of the s t r u c t u r e i s no longer p o s s i b l e . Polymer s t r e s s r e l a x a t i o n t e s t s i n the present study i n v o l v e d the l i n e a r poly(MMA), as w e l l as a s e r i e s of polymer products prepared to in c l u d e v a r y i n g degrees of c r o s s l i n k i n g . I t was a n t i c i p a t e d t h a t i n t r o d u c t i o n of primary-valence c r o s s l i n k s would i n v a r i a b l y increase creep r e s i s t a n c e and decrease r e l a x a t i o n r a t e ( 3, 141, 14-2, 145 ). I n t r o d u c t i o n of samil amounts of TEGDMA should suppress rubbery and l i q u i d f l o w regions and enhance the gl a s s y , t r a n s i t i o n and rubber plateau r e g i o n s . I n c r e a s i n g the amounts of TEGDMA i n poly(MMA) should increase the time at onset of the t r a n s i t i o n r e g i o n and als o increase the rubbery p l a t e a u modulus ( 144 ). However, somewhat opposite r e s u l t s were obtained !C Table VI, F i g . 5 ). I t was found th a t the amounts of s t r e s s r e t e n t i o n "did not s y s t e m a t i c a l l y decrease w i t h the planned increase i n c r o s s l i n k i n g agent ( TEGDMA ) c{ p o s s i b l y these r e s u l t s may be i n t e r p r e t e d as f o l l o w s . Observation time was not long enough. At o room temperature poly(MMA) i s f a r below i t s T (110 C). - 64 -As a matter of f a c t , c r o s s l i n k i n g of MMA with TEGDMA incr e a s e s T of the b a s i c polymer. Low degree of c r o s s l i n k i n g i s expected to cause a small s h i f t i n T , hut at high c r o s s l i n k i n g degrees the s h i f t . i s very l a r g e . I t i s noted t h a t "below T these polymers are c h a r a c t e r i z e d as hard, b r i t t l e s o l i d s , w i t h s t r u c t u r e s s i m i l a r to supercooled l i q u i d s i n which thermal motion i s exceedingly slow. Since c r o s s l i n k i n g has no major e f f e c t on time dependent behaviour of polymers a t temperatures; w e l l below t h e i r T r e g i o n or over very short o b s e r v a t i o n periods ( 80 ), the motion of molecular segments below T i s r e s t r i c t e d p r i m a r i l y to V i b r a t i o n s without f l o w phenomena. Thus, time dependent behaviours among these polymers any not be c l e a r l y separated. The polymer s t r u c t u r e s may have been damaged during preparatory step l o a d i n g . Rogovian and S l o n i m s k i i ( 115 ) conducted s t r e s s r e l a x a t i o n experiments w i t h polyethlene t e r e p h t h a l a t e ( PETP ) and reported t h a t s t r e c h i n g i n the g l a s s y r e g i o n was accompanied by formation of transverse f i s s u r e s . These f i s s u r e s arose i n specimens subjected to e l a s t i c deformation, and - 65 -began at low extension l e v e l s of 0.5$. The growth of f i s s u r e s l e d to greater s t r e s s l o s s . When r a t e of extension was low the s t r e s s produced was a l s o s m a l l ; consequently, f i s s u r e s were found to be sm a l l e r and t h e i r e f f e c t was l e s s s i g n i f i c a n t . P o s s i b l e damage to polymer s t r u c t u r e s might be suspected i n t h i s study, s i n c e specimens were deformed a t cates as high as 5 cm/ min while below t h e i r T temperatures. Degradation might have been induced by gamma r a d i a t i o n . P r e c i s e chemical r e a c t i o n s which l e a d to degradation under i r r a d i a t i o n have been subject to much d i s c u s s i o n . Indeed, i t could be argued t h a t r a d i a t i o n -induced i o n i z a t i o n and e x c i t a t i o n produce r a d i c a l s which are r e l a t i v e l y s t a b l e and o n l y cause main chain s c i s s i o n . I t i s noted, however, t h a t the change i n mechanical p r o p e r t i e s of an i r r a d i a t e d poly(MMA) sample might w e l l depend on the time lapse between i r r a d i a t i o n and t e s t i n g ( 20, 21 ). I n a d d i t i o n to main chain s c i s s i o n , i r r a d i a t e d poiy(MMA) may al s o show changes i n mechanical p r o p e r t i e s due to trapped gases. I t i s known tha t the side group C00CH., decomposes and, - 66 -together w i t h hydrogen from;, another: parte of the?^hain t may giv e r i s e to G, CO, C0 2, CH^ or H 2 ( 20, 21 ). When i r r a d i a t e d poly(MMA) i s allowed tot stand f o r long periods of time f i s s u r e s can be observed w i t h i n the bulk of the specimen. The r h e o l o g i c a l p r o p e r t i e s of p l a s t i c f i l m d i f f e r e d from those of papers. The former showed more f l u i d - l i k e behaviour. The 95 J 5 comonomer and TEGDMA re l a x e d almost twice as much as groundwood ( Tables V, VI, F i g s . 4,5 ). These may r e s u l t from d i f f e r e n t molecular s t r u c t u r e , cidegisees of c r y s t a l l i n i t y and s t r u c t u r a l v a r i a t i o n s between the twocmaterials. 5.4 R h e o l o g i c a l P r o p e r t i e s of Composite M a t e r i a l s Composites are combinationsnof m a t e r i a l s d i f f e r i n g i n form on a macro-scale. Consituents more or l e s s r e t a i n i n d i v i d u a l i d e n t i t i e s i n composites. The c o n s i t u e n t s of a composite may assume va r i o u s forms. In some, d i s c r e t e u n i t s of the substrate are embeded i n and bonded together by a continuous m a t r i x , while i n o t h e r s , the bonding phase may be discontinuous - 67 -or i t may be absent a l t o g e t h e r i f the d i s c r e t u n i t s are bonded or i n t e r l o c k e d d i r e c t l y . Composite p r o p e r t i e s are s t r o n g l y i n f l u e n c e d by m a t e r i a l s of which they are composed, d i s t r i b u t i o n of these c o n s t i t u e n t s and i n t e r a c t i o n s among them. P r o p e r t i e s may be the sumcof c o n s t i t u e n t p r o p e r t i e s , or the c o n s t i t u e n t s may i n t e r a c t i n such ways as to provide e s s e n t i a l l y new p r o p e r t i e s i n the composite which were not present i n i n d i v i d u a l c o n s t i t u e n t s . The v i s c o e l a s t i c behaviour of composites i s al s o a f f e c t e d by the c o n s t i t u e n t s . Time-temperature s u p e r p o s i t i o n p r i n c i p l e s , and thus the WLF equation, have been a p p l i e d to i n v e s t i g a t e some time-dependent p r o p e r t i e s of composites ( 27, 57, 5», 70, 104 ). 5.4.1 Influence of m a t r i x on paper p l a s t i c composite ( PPC ) s t r e s s r e l a x a t i o n S t r e s s r e l a x a t i o n moduli of composites are c h a r a c t e r i z e d by t h e i r c o n s t i t u e n t s . I n p r a c t i c e the moduli are de r i v e d by e x t r a p o l a t i o n s i n which the s h i f t f a c t o r A ( t ) i s obtained f o r d i f f e r e n t temperatures. - 6b -This modulus can he superimposed "by a h o r i z o n t a l t r a n s l a t i o n of temperature sequence r e l a t i v e to a base reference temperature. Results i n the s o - c a l l e d master curve, a t a given temperature, cover a s e r i e s of time decades. I n almost every system i n v e s t i g a t e d the binder ( matrix ) has been found to c o n t r o l shape of the A(t) versus temperature curve. As examples, an i n v e s t i g a t i o n by P i s t e r and Monsmith ( 104 ) showed e x c e l l e n t agreement between A ( t ) f o r pure a s p h a l t and tha t f o r bituminous concrete, when normalized constant r a t e s of compression data were s h i f t e d . Very l i t t l e d i f f e r e n c e was found ' ( 27 ) i n the constant values f o r the WLF equation a p p l i e d to u n f i l l e d and f i l l e d p o l y - i s o b u t y l e n e systems, when dynamic shear storage and l o s s compliances were s h i f t e d . S i m i l a r r e s u l t s were a l s o reported f o r p r o p e l l a n t systems by Lander, and CSmith ( 58 ) "and Mar t i n (' 70 f. In the present study, composite s t r e s s r e l a x a t i o n curves were a f f e c t e d by the v a r i o u s m a t r i x systems employed. A gradual increase of c r o s s l i n k i n g d e n s i t y d i d not s y s t e m a t i c a l l y decrease the siopesoof r e l a x a t i o n curves f o r t r e a t e d papers. Slo/pes were, howeverr,i s i m i l a r to - 69 -those described f o r pure polymers, i . e . , pure TEGDMA and 95 » 5 coraonomer f i l m s , as w e l l as corresponding composites showing smaller s t r e s s r e t e n t i o n than other samples ( Tables VI, V I I s i to V I I s i x , F i g s . 5. 6 ) . These observations support r e s u l t s from Heyse et a l . ( 39 ), who proclaimed that matrix systems a f f e c t e d time dependent behaviour of lajfcex t r e a t e d papers. Papers t r e a t e d w i t h s o f t e x t e n s i b l e polymers were found to give increased t e n s i l e s t r e n g t h and el o n g a t i o n w i t h i n c r e a s i n g r a t e s of s t r a i n , ©n the other hand, papers t r e a t e d w i t h s t i f f e r polymers d i s p l a y e d a behaviour s i m i l a r to untreated papers. This meant that t e n s i l e s t r e n g t h and modulus f o r s t i f f e r polymer t r e a t e d paper were somewhat higher and t h a t e l o n g a t i o n was about the same f o r high r a t e s of s t r a i n i n g . The p o s s i b l e e f f e c t of polymer on composite s t r e s s r e l a x a t i o n may be s i m i l a r to that employed to e x p l a i n l t h e t e n s i l e s t r e n g t h increment f o r g r a f t e d c e l l u l o s i c s u b s t r a t e s . When f i b r e ( paper ) i s modified by g r a f t p o l y m e r i z a t i o n the observed increase i n stre n g t h r e s u l t s from pendant g r a f t chain aggregations i n domains t h a t act as secondary valence c r o s s l i n k s ( 131 ) . - 70 -A d d i t i o n a l bonds produced by p o l y m e r i z a t i o n r e s t r i c t f i b r e motion i n the composite. Besides, the increased f a c i l i t y f o r energy t r a n s f e r from f i b r e to f i b r e by v a r i o u s polymer matrices may a f f e c t ; t h e composite energy d i s s i p a t i o n . 5.4.2 E f f e c t of substrate on paper p l a s t i c composite (PPC) s t r e s s r e l a x a t i o n The e f f e c t of substrate on composite v i s c o e l a s t i c p r o p e r t i e s depends on percentage, shape, s i z e and d i s t r i b u t i o n of substrate p a r t i c l e s , and the nature of bonding between the substrate and matrix. The e f f e c t of substrate content has been discussed i n s e v e r a l p u b l i c a t i o n s . Landel and coworker ( 57. 58 ) reported t h a t increase i n substrate content l e d to decreased dynamic shear compliances f o r poly-isobutylene-binder-glass-bead systems. Increased shear moduli were found by Davis and Krokosky ( 27 ) f o r asphalt-aggregate systems with i n c r e a s i n g substrate contents. Despite matrix systems used i n the present - 71 -study, groundwood paper-polymer-composites always d i s p l a y e d lower r e l a x a t i o n than found f o r k r a f t paper-polymer- composites. This confirms t h a t s t r e n g t h p r o p e r t i e s of composites depend mainly upon c h a r a c t e r i s t i c s of the substrate ( 108 ). This a l s o provides evidence supporting the p o i n t t h a t WPC st r e n g t h p r o p e r t i e s are governed by c h a r a c t e r i s t i c s of the wood used ( 131 ). Fu r t h e r , the present data also support Heyse et a l . ( 39 ) i n th a t the time dependent behaviour of papers t r e a t e d w i t h s t i f f e r polymers i s s i m i l a r to th a t of untreated paper. I n summary, PPC r e l a x a t i o n behaviour was a f f e c t e d is'ubgrdi'nat(glyGcc' by the polymer ( matrix ) system employed, but governed,-mainiyy by f i b r e ( substrate ) c h a r a c t e r i s t i c s . 5.4.3 E f f e c t of paper copolymerization w i t h (co)monomers on i n i t i a l m a t e r i a l s t r e s s r e l a x a t i o n Copolymerization o f paper w i t h (co)monomers may have increased secondary valence c r o s s l i n k s among - 72 -the f i b r e s ( 131 ). As a r e s u l t of c r o s s l i n k i n g , the s l i d i n g of f i b r e s past one another and loosening of entanglements among f i b r e s may be r e s t r i c t e d . This r e s u l t s i n a m a t e r i a l with d i f f e r e n t r a t e of s t r e s s decay. Copolymerization of k r a f t papers w i t h (co)monomers may have given r i s e to strong bonds between f i b r e and polymer chains, l e a d i n g to the ob s e r v a t i o n t h a t composites r e l a x i n t e r n a l s t r e s s slower than e i t h e r c o n s t i t u e n t ( 51 ). On the other hand, some t r e a t e d groundwood papers r e l a x e d more r a p i d l y than the untreated papers. The tendency f o r c r o s s l i n k i n g between f i b r e and polymer chains may be governed byffitoxe p r o p e r t i e s , e s p e c i a l l y l i g n i n content. 5.4.4 E f f e c t of l i g n i n on paper p l a s t i c composite ( PPC ) s t r e s s r e l a x a t i o n As mentioned e a r l i e r , PPC s t r e s s r e l a x a t i o n depended on the c o n s t i t u e n t s used. This property was als o a f f e c t e d by bonding of substrate to ma t r i x . Substrates which develop good adhesion w i t h the - 73 -binder ( matrix ) could r a i s e s h e a r - a n d - s t r e s s - r e l a x a t i o n moduli and lower creep and dynamic compliance. However, i n i t i a t i o n of dewetting between components may reverse t h i s behaviour ( 51 ).The PPC s t r e s s r e l a x a t i o n behaviour may r e l a t e to e f f e c t i v e n e s s of bonding between the v a r i o u s pulps and polymer chains and r e f e r to g r a f t copolymerization of (co)monomers to papers. As noted, g r a f t i n g e f f i c i e n c y depends on c h a r a c t e r i s t i c s of m a t e r i a l s used, as w e l l as g r a f t i n g technique. I t i s shown i n e a r l i e r r e p o r t s t h a t presence of l i g n i n i n wood pulps d e l e t e r i o u s l y a f f e c t e d g r a f t i n g of most monomers ( 103 ). By using Ce - as an i n i t i a t o r , Bergem et a l . ( 10 ) i n d i c a t e d t h a t g r a f t i n g y i e l d s v a r i e d according to pulp r e s i d u a l l i g n i n content. I t was-established^ -.1 t h a t w i t h increased^oresiduali§ a i'gnin t h e t r a t e of g r a f t i n g decreased co n s i d e r a b l y ( 83). R e s i d u a l l i g n i n i n wood pulp ac t s as an i n h i b i t o r and r e t a r d s r during the g r a f t i n g r e a c t i o n . I t was found r e c e n t l y t h a t i n h i b i t i o n and r e t a r d a t i o n r e s u l t e d from t r a n s f e r r e a c t i o n s of polymer r a d i c a l s with l i g n i n ( 137 ). This prevents fo r m a t i o n of macroradicals necessary f o r the g r a f t i n g r e a c t i o n to get under way ( 83 ) . - 74 -Kobayashi e t a l . ( j>® ) Jb.lended g r a f t e d c o t t o n l i n t e r s and d i s s o l v i n g pulps i n v a r i o u s p r o p o r t i o n s w i t h unbleached k r a f t and groundwood pulps. They showed tha t when even small amounts of g r a f t e d f i b r e s were present, a s i g n i f i c a n t increase i n paper dry - s t r e n g t h could be achieved i f the pulp contained s m a l l amounts of l i g n i n . The advantage of g r a f t i n g became l e s s as l i g n i n content of the blended pulp increased. When groundwood pulp was blended, s t r e n g t h p r o p e r t i e s were worse than those f o r the corresponding ungrafted blend. Because groundwood pulps c o n t a i n a great d e a l of l i g n i n and h e m i c e l l u l o s e s , tooth of which d e i e t e r i o u s l y a f f e c t g r a f t i n g ( 103 )t i t i s reasonable to assume th a t bonding may not occur between groundwoods and polymer chains. Such composites are simply p h y s i c a l mixtures w i t h c h a r a c t e r i s t i c s s i m i l a r to paper p r o p e r t i e s . Conversely, most l i g n i n and p a r t of the h e m i c e l l u l o s e s had been removed by k r a f t p u l p i n g . Bondiformation between k r a f t f i b r e s and polymer chains seems more l i k e l y . Thereby, c r o s s l i n k s might e x i s t , g i v i n g papers - 75 -with more compacted s t r u c t u r e . More energy, t h e r f o r e , would he r e q u i r e d to cause m a t e r i a l f l o w . The s t r e s s r e l a x a t i o n i n t r e a t e d k r a f t papers was r e l a t i v e l y slower than that i n untreated papers ( Tables IX »i to IX i x , F i g s . 6-1 to ). The e f f e c t of d e i i g n i f c i a t i o n upon paper s t r e s s r e l a x a t i o n was discussed e a r l i e r . As noted, l i g n i n a v a i l a b l e i n unbleached k r a f t papers l e d to lower s t r e s s r e l a x t i o n than f o r bleached pulp c o n t e r p a r t s . This r e l a t i o n s h i p d i d not continue with PPC prepared w i t h MMA, 95 » 5 comonomer, or TEGDMA. This r e s u l t demonstrates g r e a t e r i n t e r a c t i o n between f i b r e and polymer, than caused by small d i f f e r e n c e s i n l i g n i n content. - ?6 -6.0 CONCLUSION Stress r e l a x a t i o n t e s t s i n t e n s i o n were conducted on groundwood and k r a f t papers, p l a s t i c s and t h e i r composites ( PPC ) over time between 0.04 and 3 5 min. Res u l t s were as f o l l o w s i 1. The l i n e a r v i s c o e l a s t i c i t y equation could be employed to decribe r e l a x a t i o n behaviour of a l l m a t e r i a l s i n v e s t i g a t e d . 2. Papers prepared from groundwood pulps r e l a x e d i n t e r n a l s t r e s s much slower than those made from k r a f t pulps. 3 . L i g n i n a f f e c t e d r e l a x a t i o n of k r a f t papers. Paper made from unbleached pulp seemed to p a r t i c i p a t e d i f f e r e n t l y i n energy storage and t r a n s m i s s i o n , r e s u l t i n g i n l a r g e r s t r e s s r e t e n t i o n . Conversely, brightened and unbrightened groundwood gave almost i d e n t i c a l r e s u l t s . 4. Energy d i s s i p a t i o n i n p l a s t i c s d i d not s y s t e m a t i c a l l y decrease according to the amount of c r o s s l i n k i n g o intended by choice of pre p a r a t i o n s . 5 . PPC s t r e s s r e l a x a t i o n curves were i n f l u e n c e d by both polymer ( matrix ) and f i b r e ( substrate ) employed. The former c o n t r i b u t e d i n minor ways, while the l a t t e r , operated'in"major ways. - 77 -6. PPC and untreated paper handsheets d i s s i p a t e d i n t e r n a l s t r e s s e s i n r e l a t i v e l y s m a ll amount compared to some polymer t h i n f i l m s . PPC made from k r a f t papers d i s p l a y e d r e l a t i v e l y l a r g e r s t r e s s d i s s i p a t i o n than untreated k r a f t papers, hut no such d i f f e r e n c e s were found between t r e a t e d and untreated groundwoods. - 70 -7.0 LITERATURE CITED 1 . 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John Wiley & Sons, Inc., New York. 374 pp. 149. Watson, M.T., Kennedy, W.D, and G.M, Armstrong. 1 9 5 5 . Short time s t r e s s r e l a x a t i o n behaviour of p l a s t i c s . J . Appl . Phys. 2 6 * 7 0 1 - 7 0 5 . 1 5 0 . W i l l i a m s , M.L., Landel, R.F. and J.D, P e r r y . 1 9 5 5 . Temperature dependence of r e l a x a t i o n mechanisms i n amorphous polymers and other glass-forming l i q u i d s . J . Am. Chem. Soc. 7 7 » 3 7 0 1 - 3 7 0 7 . - 107 -b.O NOTATIONS 1. BK, UK = Bleached k r a f t , unbleached k r a f t hemlock pulps. 2. BG, UG = Brightened groundwood, unbrightened groundwood hemlock pulps. 3. MMA = Methyl methacrylate. 4. (TEGDMA = Tetraethylene g l y c o l dimethacrylate. 5 . M95 = MMA » TEGDMA i n volume proportions of 95 « 5» 6. Mb 5 = MMA < TEGDMA i n volume prop o r t i o n s of b5 « 1 5 . 7. M60 = MMA i TEGDMA i n volume proportions of 60 j 40. b. MBK, MUK, MBG, MUG = BK, UK, BG, UG t r e a t e d w i t h MMA . 9 . 9BK, 9UK, 9BG, 9UG = BK, UK, BG, UG t r e a t e d w i t h M 9 5 . 10. bBk, bUK, bBG, bUG, = BK, UK, BG, UG t r e a t e d w i t h Mb5. 11. 6Bk, 6UK, 6BG, f;6UG = BK, UK, BG, UG t r e a t e d w i t h M60. - lob -1 2 . PBK, PUK, PBG, PUG, = BK, UK, BG, UG, t r e a t e d w i t h TEGDMA, 1 3 . r = C o r r e l a t i o n c o e f f i c i e n t . 14. r = Square of r = r e g r e s s i o n SS d i v i d e d by t o t a l SS. 15. S W 1 ? = Standard e r r o r of estimate. 2 Table I . Basis weight ( g/m ) of paper handsheets, paper p l a s t i c composites and polymer f i l m s . Samples (a) Standard handsheets Treated papers (b) Matched papers (a) P o l y m e r ^ f i l m s BK UK BG UG MMA M95 M85 M60 TEGDMA 5 6 . 8 7 5 « . 63 5 9 . 0 0 5 8 . 9 0 1 2 2 . 2 0 W . 3 7 171 .20 1 7 3 . 0 1 (a) = Average reading of 5 sheets (b) = Average reading from Table I I 141 . 2 5 164.47 171 . 13 185 . 5 1 120.06 142.60 161.37 132.74 177.99 o c Table I I . P r o p e r t i e s of paper p l a s t i c composites ( PPC ) . Monomer System MMA M95 M»5 Paper sample Dose Loading B.W. Dose Loading B.W. Dose Loading • B.W. Mrad. fo g/m2 Mrad. fo g/m2 Mrad. * g/m2 BK 2.3 87 111.06 2.1 85 110.00 1 . 5 199 116 . 5 9 UK 2 . 3 136 148.62 2.1 221 1 3 2 . 5 9 1 . 5 127 136.11 BG 2 . 3 160 167 .37 2.1 261 1 3 5 . 5 2 1 . 5 274 169.19 UG 2 . 3 159 163.57 2.1 270 1 5 5 . 6 7 1 . 5 273 164.64 Table I I . Continued. Monomer System M60 TEGDMA Paper sample Dose Loading B.W. Dose Loading B.W, Mrad. % g/m* Mrad. % g/m* BK 1.1 105 1 1 7 . 3 5 0 . 5 161 155.89 UK 1.1 133 137.76 0 . 5 204 1 9 1 . 7 7 BG 1.1 188 1 7 5 . 3 2 0 . 5 227 £208.61 UG 1.1 279 174..88 0 . 5 222 2 0 6 . 2 7 Table I I I . S t r e s s , r e l a x a t i o n of" the d i f f e r e n t treatments s t u d i e d at constant deformation ( n=5). Ultimate (a) Treatment strength, 0 .004 .01 kg BK 16.70 8.35 7.76 7.56 Paper UK 14.54 7.27 6.74 6.54 BG 10.00 5.00 4.46 4.27 UG 10.41 5.21 4.64 4.43 BK 13.00 6.50 6.02 5.85 PPC UK 16.50 8.25 8.00 7.85 (MMA) BG 12.91 6.46 8.00 7.85 UG 12.94 6.47 6.15 5.81 BK 13.16 6.58 6.35 6.17 PPC UK 16.42 8.21 7.87 7.71 (M95) BG 12.34 6.17 5.72 5.55 UG 11.45 5.73 5.25 5.09 Time, min .02 .04 1 10 20 30 35 7.47 7.37 6.65 6.04 5.88 5.77 5.74 6.48 6.42 5.85 5.39 5.24 5.15 5.12 4.20 4.17 3.88 3.64 3.57 3.53 3.50 4.37 4.32 4.03 3.80 3.72 3.67 3.65 5.75 5.69 5.27 4.90 4.77 4.71 4.67 7.77 7.69 7.12 6.65 6.48 6.39 6.33 7.77 7.69 7.12 6.65 6.48 6.39 6.33 5.65 5.58 5.20 4.92 4.81 4.75 4.73 6.09 6.02 5.51 5.17 5.00 4.91 4.88 7.63 7.54 6.91 6.43 6.21 6.11 6.07 5.48 5.42 5.01 4.70 4.59 4.51 4.48 5.02 4.96 4.62 4.35 4.25 4.19 4.17 Table I I I . ............ C o n t i n u e d . -Ultimate (a) Treatment strength kg 0 .004 .01 BK 14 . 0 3 7.01 6.63 6.47 PPC UK 16.20 8.10 7.78 7 . 6 5 (M85) BG 11.84 5 . 9 2 5 . 5 1 5.36 UG 12.68 6.34 5 . 9 5 5.80 BK 13.01 6 . 5 1 6.07 5.91 PPC UK 1 5 . 0 5 7 . 5 3 7.18 7.04 (M60) BG 11 . 8 9 5 . 9 5 5.54 5.40 UG 10.62 5.81 5.42 5 . 2 5 BK 12.97 6 . 4 9 6.00 5.84 PPC UK 10.16 5.08 4 . 5 0 4.23 (TEGDMA) BG 11 .77 5.89 5.46 5 . 3 0 UG 12.49 6.49 5.83 5.67 MMA 10 . 2 3 5.12 4 . 6 3 4 . 3 4 Polymer M95 8.40 4 . 7 0 4.28 4 . 0 5 M85 9.98 4 . 9 9 4.47 4.22 M60 7.69 3.84 3 . 7 3 3.65 TEGDMA 5 . 1 9 2 . 5 9 2 . 3 4 2.24 (a) t ( o ) i s defined as time ( 0.05 to 0.06 min ).. Time, min . 0 2 .04 1 10 20 30 35 6.40 6 . 3 2 5 . 7 9 5 . 3 6 5 . 2 3 5.14 5 . 1 1 7 . 5 9 7 . 4 9 6 . 9 1 6 . 4 3 6 . 3 1 6 . 2 0 6 . 1 5 5 . 2 9 5.24 4 . 8 8 4.61 4 . 5 1 4 . 4 7 4 . 4 4 5 . 7 3 5 . 6 6 5.28 4 . 9 7 4 . 8 8 4.82 4 . 8 0 5 . 8 5 5 . 7 8 5 . 3 3 4 . 9 7 4 . 8 3 4 . 7 5 4 . 7 1 6 . 9 7 6 . 9 0 6 . 3 9 5 . 9 8 5 . 8 5 5 . 7 7 5 . 7 4 5 . 3 2 5 . 2 7 4 . 9 2 4 . 6 6 4 . 5 8 4 . 5 4 4 . 5 2 1 5 . 1 9 5 . 1 3 4.80 4 . 5 4 4 . 4 4 4 . 3 b 4 . 3 6 H> 1—1 5 . 7 6 5 . 6 8 5 . 2 0 4.80 4 . 6 8 4.61 4 . 5 7 1 4 . 0 9 4 . 0 3 3 . 6 7 3 . 3 8 3 . 3 0 3 . 2 5 3 . 2 3 5 . 2 3 5 .17 4 . 7 8 4 . 4 7 4 . 3 7 4 . 3 2 4 . 2 9 5.60 5 . 5 4 5.14 4 . 8 3 4 . 7 2 4 . 6 3 4 . 6 0 4 . 2 2 4.14 3 . 7 6 3 . 4 9 3.40 3 . 3 4 3 . 3 2 3 . 9 3 3 . 8 3 3 . 2 9 2 . 7 8 2 . 5 7 2 . 4 4 2.40 4 . 1 1 4 . 0 2 3.61 3 . 3 0 3 . 2 0 3.14 3 . 1 1 3.61 3 . 5 0 3 . 2 3 2 . 9 8 2 . o 9 2 . 8 3 2 . 0 1 2.18 2.14 1 .87 1 . 6 5 1 . 5 9 1 . 5 5 1 . 5 3 f o l l o w i n g i n i t i a l s t r e s s e x c i t a t i o n - 114 -Table IV. Regression analyses of s t r e s s r e l a x a t i o n curves according to S ( t ) / S (0.04) = a + b I n t ( n= 3 0 ) t Sample a b r 2 r ( a ) SEE BK .896201 - . 0 3 3 0 2 2 3 .993485 - . 9 9 6 7 3 7 .00670831 UK . 9 0 6 5 5 3 -.0300249 .990802 - . 9 9 5 3 9 0 . 0 0 7 2 5 7 0 6 BG .927076 - . 0 2 3 4 8 0 6 .990690 -.995334 .00571015 UG .932837 - . 0 2 3 6 8 8 0 .915262 - . 9 5 6 6 9 3 .0180804 MBK .919441 - . 0 2 6 3 2 9 9 . 9 7 9 3 2 3 - . 9 8 9 6 0 8 . 0 0 9 5 9 7 3 1 MUK . 9 2 0 7 3 5 -.0258323 .980188 -.990045 .00921264 MBG .925464 - . 0 2 4 0 1 6 3 . 9 9 0 4 7 9 -.995228 . 0 0 5 9 0 6 5 9 MUG .929332 -.0224682 .980830 -.990369 .00787951 9BK .913846 -.0276479 .967385 -.983557 .0127347 9UK .911670 -.0286777 .977678 -.988776 .0108699 9 B G .921399 -.0253904 . 9 8 9 7 5 2 -.994863 .00648123 9UG .926548 -.0236242 .983108 -.991518 .00776823 8BK .911352 -.0282873 .994659 -.997326 .00519992 8 UK .917915 -.0263141 .992718 -.996352 .00565358 8BG .929525 -.0222995 .979172 -.989531 .00815838 8UG .929622 -.0224843 .988718 -.994343 .00602506 6BK . 9 1 6 7 3 9 -.0271276 .979745 -.989821 .00978446 6UK .922244 -.0248270 .993607 -.996799 .00499573 6BG . 9 3 2 8 2 1 -.0210792 .987797 -.993880 .00587733 - 115 -Table IV Continued. Sample a b r 2 r ( a ) SEE 6UG .931970 -.0220568 ^951641 -.975521 .0124727 PBK .910471 -.0287440 .985148 -.992546 .00885337 PUK .906519 -.0295265 .989351 -.994661 .00768447 PBG .921407 -.0251466 .987505 -.993733 .00709584 PUG .92377? -.0247468 .983903 -.991919 .00794060 MMA .907496 -.0290840 .993107 -.996548 .006078553 M95 .835979 -.0553927 .985690 -.992819 .0167423 M85 .895049 -.0553927 .985690 -.992819 .0167423 M60 .905316 -.0304165 .987857 -.993910 .00845967 TEGDMA .868065 -.0420076 .894819 -.945949 .0361277 (a) i m p l i e s t h a t a l l r values are s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l .with degree of freedom of 1 and 28. - 116 -Table V. Test f o r d i f f e r e n c e i n paper s t r e s s r e l a x a t i o n behaviours. „ a. A n a l y s i s of Variance Source DP SS MS P Treatment 3 .016241 .0054138 62.14 E r r o r 16 .0013940 .000087126 T o t a l 19 .017635 ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . b. Duncan 1s New M u l t i p l e Range Test Treatment UG BG UK BK Amount of energy d i s s i p a t i o n .1551 .1591 .2024 .2221 Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y a t the 0.05 p r o b a b i l i t y l e v e l . - 117 -Table VI. Test f o r e f f e c t of polymer f i l m c r o s s l i n k i n g on amount of s t r e s s r e l a x a t i o n . a. A n a l y s i s of Variance Source DP SS MS P Treatment 4 .10743 .02685b 51.»3 E r r o r 20 .010364 .0005181b* T o t a l 24 .11779 ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment Amountsof energy d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y a t the 0 . 0 5 p r o b a b i l i t y l e v e l . MMA M60 Mb5 TEGDMA M95 .19b6 .2069 .2272 .2b67 .3745 - 118 -Table V I I t i Test f o r e f f e c t of matrix system on paper p l a s t i c composite (PPC) s t r e s s r e l a x a t i o n (bleached k r a f t paper-polymer system) . a. A n a l y s i s of Variance Source DF SS MS F „ „ n. s. Treatment 4 .00082781 .00020695 2.15 E r r o r 20 .0019273 .000096365 T o t a l 24 .0027551 n.s. not s i g n i f i c a n t . (a) b. Duncan's New M u l t i p l e Range Test ' Treatment MBK 6BK 9BK 8BK PBK Amount of energy d i s s i p a t i o n .1786 .1848 .1897 .1917 .1951 Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y a t the 0.05 p r o b a b i l i t y l e v e l . (a) the t a b l e i s s e t f o r reference only. - 119 -Table V I I t i i Test f o r e f f e c t of matrix system on paper p l a s t i c composite (PPC) s t r e s s r e l a x a t i o n (unbleached k r a f t paper-polymer system). a. A n a l y s i s of Variance Source DP SS MS P Treatment 4 .0034212 .00085530 9.50 E r r o r 20 .0018010 .000090051 T o t a l 24 .0052222 ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment 6 UK MUK 8 UK 9 UK PUK Amount of energy d i s s i p a t i o n .1681 .1758 .1791 .1941 .1995 Means underlined by the same l i n e do not d i f f e r s i g n i n f i c a n t l y a t the 0.05 p r o b a b i l i t y l e v e l . - 120 -Table V l l i i i i . Test f o r e f f e c t of m a t r i x system on paper p l a s t i c composite ( PPC ) s t r e s s r e l a x a t i o n ( brightened groundwood paper-polymer system ). a. A n a l y s i s of Variance Source DP SS Treatment 4 .0033822 E r r o r 20 .0014805 T o t a l 24 .0048672 MS P .00084555 11.42' .000074025 ** s i g n i f i c a n t at the 0.01 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment 6BG 8BG MBG PBG 9BG Amount of energy .1425 .1513 .1631 .1700 .1736 d i s s i p a t i o n ' Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y at. the 0.05 p r o b a b i l i t y l e v e l . - 121 -Table V I I l i v . Test f o r e f f e c t of matrix system on paper p l a s t i c composite ( PPC ) s t r e s s r e l a x a t i o n ( unbrightened groundwood paper- polymer system ). a. A n a l y s i s of Variance Source DP SS MS P Treatment 4 .0013068 .00032670 2.62 n , s* E r r o r 20 .0024901 .00012451 n.s. not s i g n i f i c a n t . (a.) b. Duncan's New M u l t i p l e Range T e s t x ' Treatment 6UG 8UG MUG 9UG PUG Amount of energy .1496 .1518 .1534 .1605 .1694 d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y at the O.05 p r o b a b i l i t y l e v e l , (a) the t a b l e i s set f o r reference onl y . - 122 -Table VIII«i Test f o r e f f e c t of substrate on paper p l a s t i c composite ( PPC ) s t r e s s r e l a x a t i o n ( MMA-substrate system ) , 0 a. A n a l y s i s of Variance Source DF SS MS Treatment 3 .0020508 .00068260 Error' 16 .0019314 .00012071 T o t a l 19 .0039822 5.66 ** s i g n i f i c a n t a t the 0.01 p r o b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment MUG MBG MUX MBK Amount of energy . 1534 . 1631 .1758 . 1786 d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y at the 0 . 0 5 p r o b a b i l i t y l e v e l . - 123 -Table V I I I s i i . Test f o r e f f e c t of substrate on paper p l a s t i c composite ( PPC ) s t r e s s r e l a x a t i o n ( M 9 5 -substrate system ). a. A n a l y s i s of Variance Source Treatment E r r o r T o t a l DF 3 16 19 SS .0035642 .001057b* .0046220 MS I .0011881 17.97 .00006610.33 ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment 9UG 9BG 9BK 9UK Amount of energy .1605 .1736 .1897 .1941 d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y a t the 0 . 0 5 p r o b a b i l i t y l e v e l . - 124 -Table V I I I l i i i . Test f o r e f f e c t of substrate on paper p l a s t i c composite ( PPC ) s t r e s s r e l a x a t i o n ( M 8 5 - s u b s t r a t e system ). a. A n a l y s i s of Variance Source Treatment E r r o r T o t a l ** s i g n i f i c a n t at the 0 . 0 1 p r o b a b i l i t y l e v e l . DF SS MS F 3 . 0 0 6 1 2 8 3 .002042b 3 3 . 1 9 16 . 0 0 0 9 8 4 8 9 . 0 0 0 0 6 1 5 5 6 19 . 0 0 7 1 1 3 2 b. Duncan's New M u l t i p l e Range Test Treatment 8BG 8UG 8 UK 8BK Amount of energy . 1 5 1 3 . 1 5 1 8 . 1 7 9 1 . 1 9 1 7 d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y a t the 0 . 0 5 p r o b a b i l i t y l e v e l . - 1 2 5 -Table V l l l s i v . Test f o r e f f e c t of substrate on paper p l a s t i c composite ( PPC ) s t r e s s r e l a x a t i o n ( M60-substrate system ). a. A n a l y s i s of Variance Source Treatment E r r o r T o t a l ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . DP SS MS F 3 . 0 0 5 4 4 1 4 .0018138 13.17 16 . 0 0 2 2 0 3 9 . 0 0 0 1 3 7 7 4 19 . 0 0 7 6 4 5 3 , b. Duncan's New M u l t i p l e Range Test Treatment 6BG 6UG 6UK 6BK Amount of energy . 1 4 2 5 .1496 .1681 .1848 d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y at the 0 . 0 5 p r o b a b i l i t y l e v e l . - 126 -Table V I I I : v . Test f o r e f f e c t of substrate on paper p l a s t i c composite ( PPC ) s t r e s s r e l a x a t i o n ( TEGDMA- substrate system ). a. A n a l y s i s of Variance Source DF SS MS Treatment 3 .0038471 .0012824 13.49 E r r o r 16 .0015208 .000095058 T o t a l 19 .0053680 ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment PUG PBG PBK PUK Amount of energy . 1 6 9 4 . 1 7 0 0 . 1 9 5 1 . 1 9 9 5 d i s s i p a t i o n Means un d e r l i n e d by the same l i n e do not d i f f e r s i g n i f i c a n t l y a t the 0 . 0 5 p r o b a b i l i t y l e v e l . - 127 -Table IX»i Test f o r copo l y m e r i z a t i o n e f f e c t s on paper s t r e s s r e l a x a t i o n ( bleached k r a f t paper-MMA system ). a. A n a l y s i s of Variance Source Treatment E r r o r T o t a l ** s i g n i f i c a n t at the 0 . 0 1 p r o b a b i l i t y l e v e l . DF SS MS F 2 . 0 0 4 7 5 3 3 .0023767 28 . 7 4 12 . 0 0 0 9 9 2 4 5 . 0 0 0 0 8 2 7 0 4 14 .0057458 b. Duncan's New M u l t i p l e Range Test Treatment Amount of energy d i s s i p a t i o n Any two means d i f f e r s i g n i f i c a n t l y a t the 0 . 0 5 p r o b a b i l i t y l e v e l . MBK MMA BK .1786 .1986 .2221 - 128 -Table I X i i i . Test f o r copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( bleached k r a f t paper-M95 system ). a. A n a l y s i s of Variance Source Treatment E r r o r T o t a l ** s i g n i f i c a n t at the 0X01 p r o b a b i l i t y l e v e l . DF SS MS F 2 . 0 9 7 4 0 7 . 0 4 8 7 0 3 7 0 8 . 3 6 12 . 0 0 0 8 2 5 0 6 . 0 0 0 0 6 8 7 5 5 14 . 0 9 8 2 3 b. Duncan's New M u l t i p l e Range Test Treatment 9BK BK M95 Amount of energy .1897 .2221 . 3 7 4 5 d i s s i p a t i o n Any two means d i f f e r s i g n i f i c a n t l y at the 0 . 0 5 p r o b a b i l i t y l e v e l . - 129 -Table I X j i i i . Test f o r copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( bleached k r a f t paper-M85 system ). a. A n a l y s i s of Variance Source Treatment E r r o r T o t a l ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . DF SS MS F 2 .0036972 .0018486 21.08 12 .0010524 .000067702 14 .0047497 b. Duncan's New M u l t i p l e Range Test Treatment 8BK BK M85 Amount of energy .1917 .2221 .2272 d i s s i p a t i o n Me'ansB underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y at the 0.05 p r o b a b i l i t y l e v e l . - 130 -Table I X t i v . Test f o r cop o l y m e r i z a t i o n e f f e c t s on paper s t r e s s r e l a x a t i o n ( bleached k r a f t paper-M60 system ). a. A n a l y s i s of Variance Source DF SS MS Treatment 2 .0035165 .0017583 E r r o r 12 . O O I 3 0 5 0 .00010875 T o t a l 14 .0048216 16.17 ** s i g n i f i c a n t at the 0.01 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment 6BK .M60 BK Amount of energy .1847 .2069 .2221 d i s s i p a t i o n Any two means d i f f e r s i g n i f i c a n t l y a t the O.05 p r o b a b i l i t y l e v e l . - 131 -Table I X J V . Test f o r cop o l y m e r i z a t i o n e f f e c t s on paper s t r e s s r e l a x a t i o n ( bleached k r a f t paper-TEGDMA system ). a. A n a l y s i s of Variance Source Treatment E r r o r T o t a l ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . DF SS MS F 2 .022181 .011090 13.71 Ik .0097106 .00080922 16 .031891 b. Duncan's New M u l t i p l e Range Test Treatment PBK BK TEGDMA Amount of energy .1951 .2221 .2867 d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y at the 0.05 p r o b a b i l i t y l e v e l . - 132 -Table I X t v i . Test f o r copolymerization e f f e c t s on paper r e l a x a t i o n ( unbleached k r a f t paper-MMA system ). A n a l y s i s of Variance Source DP SS MS Treatment 2 .0020723 .0010361 E r r o r 12 .0013506 .00011255 T o t a l 14 .0034228 9.21 ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment MUK MMA UK Amount of energy .1758 .1986 .2024 d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y a t the 0.05 p r o b a b i l i t y l e v e l . - 133 -Table I X i v i i . Test f o r copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( unbleached k r a f t paper-M95 system ). a. A n a l y s i s of Variance Source DP SS MS F ** Treatment 2 . 1 0 3 7 4 . 0 5 1 8 7 1 5 6 7 . 3 5 E r r o r 12 .0010971 . 0 0 0 0 9 1 4 2 7 T o t a l 14 .10484 ** s i g n i f i c a n t at the 0 . 0 1 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment 9UK UK M95 Amount of energy . 1 9 4 1 .2024 . 3 7 4 5 d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y at the 0 . 0 5 p r o b a b i l i t y l e v e l . - 134 -Table I X i v i i i . Test f o r copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( unbleached k r a f t paper-M85 system ). a. A n a l y s i s of Variance Source Treatment E r r o r T o t a l ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . DF SS MS F 2 .0057812 .0028906 34.06 12 .0010185 .000084877 14 .0067998 b. Duncan's New M u l t i p l e Range Test Treatment 6UK UK M85 Amount of energy .1791 .2024 .2272 d i s s i p a t i o n Any two means d i f f e r s i g n i f i c a n t l y a t the 0 . 0 5 p r o b a b i l i t y l e v e l . - 135 -Table I X t i x . Test f o r copo l y m e r i z a t i o n e f f e c t s on paper s t r e s s r e l a x a t i o n ( unbleached k r a f t paper-M60 system ). a. A n a l y s i s of Variance Source DP SS MS F Treatment 2 .0045076 ,002253b 3 1 .bl E r r o r 12 .00085024 .000070854 T o t a l 14 .0053579 ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment 6UK UK M60 Amount of energy .1681 .2024 .2026 d i s s i p a t i o n Means und e r l i n e d by the same l i n e do not d i f f e r s i g n i f i c a n t l y a t the 0.05 p r o b a b i l i t y . - 136 -Table I X t x . Test f o r copo l y m e r i z a t i o n e f f e c t s on paper s t r e s s r e l a x a t i o n ( bleached k r a f t paper-TEGDMA system ). a. A n a l y s i s of Variance Source Treatment E r r o r T o t a l ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . DP SS MS F 2 .023560 .012280 15.38 12 .0095791 .00079826 14 .034139 b. Duncan's New M u l t i p l e Range Test Treatment PUK UK TEGDMA Amount of energy .1995 .2024 .2867 d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y at the 0.05 p r o b a b i l i t y l e v e l . - 137 -Table X $ i . Test f o r copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( brightened groundwood paper-MMA system ). a. A n a l y s i s of Variance Source DP SS MS I Treatment 2 .0047412 .0023706 43.81 E r r o r 12 .00064929 .000054108 T o t a l 14 .0053905 ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment Amount of energy d i s s i p a t i o n BG MBG MMA .1591 .1631 .1986 Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y a t the 0.05 p r o b a b i l i t y l e v e l . - 138 -Table X i i i . Test f o r copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( brightened groundwood paper- M95 system ). a. A n a l y s i s of Variance Source DF SS MS Treatment 2 . 1 4 5 0 2 . 0 7 2 5 0 9 1 1 5 9 . 5 7 E r r o r 1 2 . 0 0 0 7 5 0 3 7 . 0 0 0 0 6 2 5 3 1 T o t a l 14 . 1 4 5 7 7 ** s i g n i f i c a n t a t the 0 . 0 1 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment BG 9BG M 9 5 Amount of energy . 1 5 9 1 . 1 7 3 6 . 3 7 4 5 d i s s i p a t i o n Any two means d i f f e r s i g n i f i c a n t l y a t the 0 . 0 5 p r o b a b i l i t y l e v e l . - 139 -Table X t i i i . Test f o r copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( brightened groundwood paper-M85 system ). a. A n a l y s i s of Variance Source DP SS MS P Treatment 2 .17450 .0087249 79.46 E r r o r 12 .0013176 .00010980 T o t a l 14 .018767 ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment 8BG BG M85 Amount of energy .1513 .1591 .2272 d i s s i p a t i o n Means und e r l i n e d by the same l i n e do not d i f f e r s i g n i f i c a n t l y a t the 0.05 p r o b a b i l i t y l e v e l . - 140 -Table X i i v . Test f o r copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( brightened groundwood paper-M60 system ). a. A n a l y s i s of Variance Source DF SS MS F Treatment 2 .011169 .0055844 74.82 E r r o r 12 .00089562 .000074635 T o t a l 14 .012064 ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . b. Duncan!s New M u l t i p l e Range Test Treatment 6BG BG M60 Amount of energy .1425 .1591 .2069 d i s s i p a t i o n Any two means d i f f e r s i g n i f i c a n t l y a t the 0.05 p r o b a b i l i t y l e v e l . - 141 -Table X J V . Test f o r copo l y m e r i z a t i o n e f f e c t s on paper s t r e s s r e l a x a t i o n ( brightened groundwood paper-TEGDMA system ). a. A n a l y s i s of Variance Source Treatment E r r o r T o t a l DF SS 2 .050076 12 .0094236 14 .059500 MS .025038 .00078530 31.88 ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment BG PBG TEGDMA Amount of energy .1591 .1700 .2867 d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y a t the 0.05 p r o b a b i l i t y l e v e l . - 142 -Table X : v i . Test f o r c o p o l y m e r i z a t i o n e f f e c t s on paper s t r e s s r e l a x a t i o n ( unbrightened groundwood paper-MMA system ), a. A n a l y s i s of Variance Source Treatment E r r o r T o t a l DP 2 12 14 SS .0065845 .0012556 .0078401 MS .0032923 .00010463 31.47 ** s i g n i f i c a n t a t the 0..XJ1 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment MUG UG MMA Amount of energy .1534 .1551 .1986 d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y a t the 0.05 p r o b a b i l i t y l e v e l . - 143 -Table X»vii. Test f o r copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( unbrightened groundwood paper-M95 system ). a. A n a l y s i s o f Variance Source DP SS MS F Treatment 2 .15665 .078323 985.92 E r r o r 12 .00095330 .000079441 T o t a l 14 .15760 ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment 9UG UG M95 Amount of energy .1551 . 1 6 0 5 . 3 7 4 5 d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y a t the 0 . 0 5 p r o b a b i l i t y l e v e l . - 144 -Table X i v l i i . Test f o r copo l y m e r i z a t i o n e f f e c t s on paper s t r e s s r e l a x a t i o n ( unbrightened groundwood paper-M85 system ). a. A n a l y s i s of Variance Source Treatment E r r o r T o t a l DP 2 12 14 SS .018182 .0012963 .019479 MS I .009012 84.16 .00010802 ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Range Test Treatment Amount of energy d i s s i p a t i o n 8UG UG M85 .1518 .1551 .2272 Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y at the 0.05 p r o b a b i l i t y l e v e l . - 145 -Table X i i x . Test f o r cop o l y m e r i z a t i o n e f f e c t s on paper s t r e s s r e l a x a t i o n ( unbrightened groundwood paper-M6G system ). a* A n a l y s i s of Variance Source Treatment E r r o r T o t a l ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . DP SS MS F *# 2 .0099748 .0049874 27.08 12 .0022100 .00018416 14 .012185 b. Duncan's New M u l t i p l e Range Test Treatment 6UG UG M60 Amount of energy .1496 .1551 .2069 d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y at the 0.05 p r o b a b i l i t y l e v e l . - 146 -Table X i x . Test f o r copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( unbrightened groundwood paper-TEGDMA system ). a. A n a l y s i s of Variance Source DP SS MS Treatment 2 .52173 .026087 E r r o r 12 .0095905 .00079920 T o t a l 14 .061764 32.64 ** s i g n i f i c a n t a t the 0.01 p r o b a b i l i t y l e v e l . b. Duncan's New M u l t i p l e Test Treatment UG PUG TEGDMA Amount of energy .1551 .1694 .2867 d i s s i p a t i o n Means underlined by the same l i n e do not d i f f e r s i g n i f i c a n t l y a t the 0.05 p r o b a b i l i t y l e v e l . l o g a u ) / ^ i l o g t F i g u r e 1. Polymer s t r e s s r e l a x a t i o n ( 148 ). - 148 -- 149 -F i g u r e 3. Glass frame assembly f o r making polymer t h i n f i l m s . - 150 -- 151 --1.2 -1.0 0 1.0 1.3 1.5 I n t , min F i g u r e 5. S t r e s s r e l a x a t i o n of polymer f j i l m s . - 152 -I n t , min I n t , min F i g u r e 6. E f f e c t of m a t r i x systems on paper p l a s t i c composite (PPC) s t r e s s r e l a x a t i o n . - 153 -I n t, min - 154 -l . o ^ w 0 . 9 MUG MBG MUK MBK 03 03 2 0.8 -p w 1 - j . 2 J l . o n ' 6——Tto—i.b 1.^ I n t , min F i g u r e 7 - 1 . E f f e c t of substrates on paper p l a s t i c composite (PPC) s t r e s s r e l a x a t i o n 1 MMA-substrate system ). - 155 -1.0 9UG 9BG 9BK 9UK s -1.2 -1 .0 0 1.0 1.3 1.5 P i g u r e 7-2. I n t , min E f f e c t of substrates on paper p l a s t i c composite(PPC) s t r e s s r e l a x a t i o n ( M95-Substrate system ). - 1 5 6 -I n t , min F i g u r e 7-3. E f f e c t of s u b s t r a t e s on paper p l a s t i c composite (PPC) s t r e s s r e l a x a t i o n ( Mb5-substrate system ). - 157 -6BG 6UG 6UK 6BK -1.2 -1.0 1.0 F i g u r e 7-4. 1.3 1.5 I n t , min E f f e c t of s u b s t r a t e s on paper p l a s t i c composite (PPC) s t r e s s r e l a x a t i o n ( M60-substrate system ). - 158 -F i g u r e 7-5. 0 ~ 1 . 0 1 . 3 1.5 In t , min. E f f e c t of s u b s t r a t e s on paper p l a s t i c composite (PPC) s t r e s s r e l a x a t i o n ( TEGDMA-substrate system ). - 159 -I I I <s» » RF>U I I L 1 -1.2 -1.0 0 ~ 1 . 0 1 . 3 1.5 I n t , min Figu r e b-1. Copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( bleached k r a f t paper-polymer system ). - 160 -I S I FTU I t\f I I I I -1.2 -1 .0 0 1.0 1.3 1.5 I n t , min 1 I ' ,1. ' AI J '• ~u - 1 - 2 - l . o o l . o 1.3 1.5 I n t , min e a-2. Copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( bleached k r a f t paper-polymer system ). - 1 6 1 -1 * • *u -i • . . . - 1 . 2 - l . O O 1 7 0 1 . 3 1.5 I n t , min Figu r e b - 3 . Copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( unbleached k r a f t paper-polymer system ). - 162 -H ° S to co to \ CO CO o H IS! u 3 03 CO CO \ © +» +> —. to CO o • H >j3 ° co co CO CO PUK UK -1.2 - l i O TEGDMA 1.3 1.5 In t , min 1.0 0.9 0.85 0.75 6UK UK M60 -1.2 -1.0 TTo 1.3 1.5 I n t , min 1.0 0.9 X 0.85 0.75 8 UK UK M85 -1.2 -1.0 0 T7G 1.3 1.5 I n t , min F i g u r e 8-4. Copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( unbleached k r a f t paper-polymer system ). - 163 -I I I KM I *m I I I. -1.2 -L.O O ~ l . O 1 7 3 175 lrig t , min F i g u r e 9-1. Copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( brightened groundwood paper-polymer system ). - 164 --1.2 -L.O 0 L.O 1.3 1.5 I n t , min -1.2 -1.0 1.0 1.3 1.5 In t , min 8BG BG M85 -1.2 -1.0 0 1.0 1.3 1.5 I n t , min F i g u r e 9-2. Copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( brightened groundwood paper-polymer system ). - 165 --1.2 - l . O O l . O 1.3 1.5 I n t , min Figu r e 9 - 3 . Copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( unbrightened groundwood paper-polymer system ). - 166 -n ° 1.0 •ri o •}U) o OB \ o / • H / «! \r+ 0); O T4/ o a> -w w 0.9 0.8 PUG TEGDMA -1.2 -1.0 T7o 1.3 1.5 In t , min 1.0 0.9 0.8 6UG UG M60 /IS (a 3-O CQ W » \ -P -1.2 -1.0 0 1.0 1.3 1.5 I n t , min 1.0 0.9 0.8 8UG UG M85 F i g u r e 9-4. -i.2 - l . O S O H — — i t n t s I n t , min Copolymerization e f f e c t s on paper s t r e s s r e l a x a t i o n ( unbrightened groundwood paper-polymer system ). 

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