Applied Science, Faculty of
Chemical and Biological Engineering, Department of
DSpace
UBCV
Noki, Gloria
Master of Applied Science - MASc
University of British Columbia
Peroxide brightening of thermomechanical pulp (TMP) in high alkali conditions, has been proven to develop the brightness and tensile properties of the treated pulp. Previous studies have generated a robust understanding of the effects the process has on the optical and strength properties of pulps. The rise in demand for pulps with low bulk and high tensile (better suited to board and packaging grades) has motivated this study on the contributions TMP fines (< 76 μm) make to the development of pulp properties using highly alkaline peroxide treatment (HAPT). When TMP fibres and fines are separately treated with high alkali peroxide, the peroxide reacts more rapidly with the fines, during the initial stages of the treatment. Likewise, weak acid groups are generated more rapidly in fines compared to fibres. Weak acid groups increase the surface charge, promoting fibre swelling and increasing the bonded area between adjacent fibres, which results in sheet densification. Thus, a correlation has emerged between the development of tensile strength and the generation of weak acid groups during HAPT. Due to their smaller size and larger specific surface area, fines are more accessible to the peroxide. Furthermore, fines consume peroxide more rapidly due to their higher lignin content; hence, weak acid groups form more readily on the surfaces of fines. Consequently, it is hypothesized that preferential formation of acid groups on the external surface of fines and fibres makes a significant contribution to tensile strength development. In this investigation, the effect of HAPT on pulp properties is evaluated by varying pulp compositions (fibres and fines % w/w) and treatment time. A better understanding of the correlation between acid group generation and tensile strength development during HAPT will lead to the cost-effective production of customizable pulp grades.
THE ROLE OF FIBRE SIZE IN HIGHLY ALKALINE PEROXIDE TREATMENT OF THERMOMECHANICAL PULP by Gloria Noki BASc., The University of British Columbia, 2017 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Chemical and Biological Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) April 2025 © Gloria Noki, 2025ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled: The Role of Fibre Size in Highly Alkaline Peroxide Treatment of Thermomechanical Pu lp submitted by Gloria Noki in partial fulfilment of the requirements for the degree of Master of Applied Science in Chemical and Biological Engineering Examining Committee: Heather Trajano, Associate Professor, Chemical and Biological Engineering, UBC Supervisor Scott Renneckar, Professor, Forestry, UBC Supervisory Committee Member Peter Englezos, Professor, Chemical and Biological Engineering, UBC Supervisory Committee Member iii Abstract Peroxide brightening of thermomechanical pulp (TMP) in high alkali conditions, has been proven to develop the brightness and tensile properties of the treated pulp. Previous studies have generated a robust understanding of the effects the process has on the optical and strength properties of pulps. The rise in demand for pulps with low bulk and high tensile (better suited to board and packaging grades) has motivated this study on the contributions TMP fines (< 76 μm) make to the development of pulp properties using highly alkaline peroxide treatment (HAPT). When TMP fibres and fines are separately treated with high alkali peroxide, the peroxide reacts more rapidly with the fines, during the initial stages of the treatment. Likewise, weak acid groups are generated more rapidly in fines compared to fibres. Weak acid groups increase the surface charge, promoting fibre swelling and increasing the bonded area between adjacent fibres, which results in sheet densification. Thus, a correlation has emerged between the development of tensile strength and the generation of weak acid groups during HAPT. Due to their smaller size and larger specific surface area, fines are more accessible to the peroxide. Furthermore, fines consume peroxide more rapidly due to their higher lignin content; hence, weak acid groups form more readily on the surfaces of fines. Consequently, it is hypothesized that preferential formation of acid groups on the external surface of fines and fibres makes a significant contribution to tensile strength development. In this investigation, the effect of HAPT on pulp properties is evaluated by varying pulp compositions (fibres and fines % w/w) and treatment time. A better understanding of the correlation between acid group generation and tensile strength development during HAPT will lead to the cost-effective production of customizable pulp grades. iv Lay Summary Paper has evolved over millennia from writing sheets which were made using cellulosic fibres; beginning with papyrus reeds in ancient Egypt, to cotton and hemp rags in 2nd century China, and latterly, to wood chips in Europeâs 1st Industrial Revolution. The mechanized production of paper required high energy inputs and manufactured diversified products. Mechanical pulping separates fibres in softened wood chips through grinding. Mechanical pulpâs pigment is conventionally brightened using hydrogen peroxide. Peroxide brightening in high alkali conditions also improves the strength properties of mechanical pulp. Highly alkaline peroxide treatment (HAPT) can strategically reorient mechanical pulp from the diminishing demand for brighter pulps (printing and writing grades) to the rising demand for stronger durable pulps (board and packaging grades). This research project investigates the ways in which HAPT optimizes the properties of mechanical pulp fibres to suit the current market by varying the fibre size, pulp composition and treatment time. v Preface This thesis is original, unpublished, independent work by the author, Gloria Noki. This research was conducted in Phase 3 of the UBC Energy Reduction in Mechanical Pulping (ERMP) program under the supervision of Dr Heather Trajano and Dr Rodger Beatson. It was conducted by the author in three facilities, namely, the Clean Energy Research Centre in the Department of Chemical and Biological Engineering at UBC, the Paper-Making Lab at UBC Pulp and Paper Centre, and the Pulp and Paper Lab at the British Columbia Institute of Technology (BCIT). The research was focused on the optimization of highly alkaline peroxide treatment in the development of thermomechanical pulp properties. The author designed all pertinent experiments in a joint effort with her supervisors. The high-performance liquid chromatography to determine sugar concentration was performed by Eerik Piirtola in the Analytical Core Facility at UBC. No generative artificial intelligence (AI) was used for any aspect of the work in this thesis. vi Table of Contents Abstract ........................................................................................................................................ iii Lay Summary ............................................................................................................................... iv Preface ........................................................................................................................................... v Table of Contents ......................................................................................................................... vi List of Tables ................................................................................................................................ ix List of Figures ................................................................................................................................ x List of Abbreviations ................................................................................................................. xiv Acknowledgements .................................................................................................................... xvi Dedication ................................................................................................................................. xvii Chapter 1. Introduction ................................................................................................................ 1 1.1 Motivation â¦â¦........................................................................................................................ 1 1.2 Objective ................................................................................................................................... 1 1.3 Overview .................................................................................................................................. 2 Chapter 2. Background ................................................................................................................ 3 2.1 Mechanical Pulping .................................................................................................................. 3 2.2 Fibres and Fines ........................................................................................................................ 3 2.3 Highly Alkaline Peroxide Treatment ......................................................................................... 5 2.4 Acid Group Content .................................................................................................................. 7 2.5 Study Approach ......................................................................................................................... 8 Chapter 3. Materials and Methods ............................................................................................ 10 3.1 Materials ................................................................................................................................. 10 vii 3.2 Pulp Characterization ................................................................................................... ........... 10 3.3 Removal of Fines .................................................................................................................... 10 3.4 Fractionation of Fibres ............................................................................................................ 11 3.5 Chelation .................................................................................................................... ............. 11 3.6 Highly Alkaline Peroxide Treatment (HAPT) ......................................................................... 11 3.7 Final pH and Residual Peroxide .............................................................................................. 12 3.8 Pulp Yield Loss ....................................................................................................................... 13 3.9 Acid Group Content ................................................................................................................ 13 3.10 Standard Handsheets ............................................................................................................. 15 3.11 Chemical Composition Characterization .............................................................................. 16 Chapter 4. Results and Discussion ............................................................................................. 18 4.1 Fines Content .......................................................................................................................... 18 4.1.1 Final pH and Residual Peroxide ........................................................................................... 19 4.1.2 Pulp Yield Loss ................................................................................................................... 21 4.1.3 Optical Properties ................................................................................................................. 22 4.1.4 Strength Properties ........................................................................................... ................... 23 4.1.5 Acid Group Content .................................................................................................... ......... 29 4.1.6 Chemical Composition Characterization ............................................................................. 31 4.2 Fibres Only ............................................................................................................. ................. 33 4.2.1 Final pH and Residual Peroxide ........................................................................................... 33 4.2.2 Pulp Yield Loss ............................................................................................................. ...... 34 4.2.3 Optical Properties ................................................................................................................. 35 viii 4.2.4 Strength Properties .............................................................................................................. 37 4.2.5 Acid Group Content ............................................................................................................. 39 4.3 Fibre Length and Coarseness ............................................................................................. ..... 41 4.3.1 Final pH and Residual Peroxide ........................................................................................... 43 4.3.2 Pulp Yield Loss ................................................................................................................... 44 4.3.3 Optical Properties ........................................................................................................ ......... 45 4.3.4 Strength Properties .............................................................................................................. 47 4.3.5 Acid Group Content ............................................................................................................. 53 Chapter 5. Conclusions and Recommendations ....................................................................... 55 5.1 Fines Content .......................................................................................................................... 55 5.2 Fibres Only ............................................................................................................................. 56 5.3 Fibre Length and Coarseness ......................................................................................... ......... 57 5.4 Limitations and Recommendations ........................................................................................ 58 5.5 Future Work ............................................................................................................. ............... 61 Bibliography ................................................................................................................................ 62 Appendix ..................................................................................................................................... 69 ix List of Tables Table 4.1 Percentage of fines content present in pulp samples ..................................................... 18 Table 4.2 Bauer McNett fibre classification of pulp by size ......................................................... 41 Table 4.3 Fibre size distribution .................................................................................................... 42 x List of Figures Figure 2.1 Schematic cross-section of a plant cell wall with its representative distribution of cellulose, hemicellulose and lignin; the cell boundary is at the top and the interface of the cell wall with lumen is at the bottom (adapted from Lui, 2015) ................................................................... 4 Figure 2.2 Scale breakdown of the cellulose structure from wood to its molecular unit (adapted from Peneder, 2021) ........................................................................................................................ 4 Figure 3.1 Schematic of the conductometric titration apparatus setup for the analysis of acid group content .......................................................................................................................................... 14 Figure 3.2 Titration curve indicating the equivalence points used in determining the weak acid content .......................................................................................................................................... 15 Figure 4.1 Final pH after 60 minutes of HAPT remains constant as fines content increases ....... 19 Figure 4.2 Percentage residual peroxide after 60 minutes of HAPT decreases as fines content increases. The error bars are based on standard deviations of triplicates. ..................................... 20 Figure 4.3 Pulp yield loss after 60 minutes of HAPT increases as fines content increases .......... 20 Figure 4.4 Optical properties after 60 minutes of HAPT: (a) Brightness generally decreases as fines content increases; (b) Opacity increases as fines content increases; (c) Scattering coefficient increases as fines content increases; (d) Absorption coefficient generally increases as fines content increases ....................................................................................................................................... 22 Figure 4.5 Strength properties after 60 minutes of HAPT: (a) Tear decreases as fines content increases; (b) Burst decreases as fines content increases; (c) Tensile strength decreases as fines content increases; (d) Characteristic stress-strain curves for varied fines content. The error bars are based on standard deviations, with up to 4 replicates of tear index and up to 10 replicates for the burst index and tensile strength. .................................................................................................... 24 xi Figure 4.6 (a) Stress-strain curve focus on 0% fines; (b) Optical microscopy image of the break interface for 0% fines; (c) Stress-strain curve focus on 30% fines; (d) Optical microscopy image of the break interface for 30% fines; (e) Stress-strain curve focus on 50% fines; (f) Optical microscopy image of the break interface for 50% fines; (g) Stress-strain curve focus on 70% fines; (h) Optical microscopy image of the break interface for 70% fines; (i) Stress-strain curve focus on 100% fines; (j) Optical microscopy image of the break interface for 100% fines ........................ 28 Figure 4.7 After 60 minutes of HAPT, tensile strength decreases as scattering coefficient increases, due to the decrease in bonded area. The error bars are based on standard deviations of up to 10 replicates. ...................................................................................................................................... 29 Figure 4.8 After 60 minutes of HAPT: (a) Acid group content increases as fines content increases; (b) Tensile strength declines with increase in acid group content corresponding to the increase in fines content; (c) Bulk generally increases as fines content increases. The error bars for tensile strength are based on standard deviations of up to 10 replicates. .................................................. 30 Figure 4.9 Chemical compositional analysis for fibres only (0% fines), whole pulp (30% fines), and fines only (100% fines) for untreated (control) and 60-minute treated (HAPT) pulps .......... 32 Figure 4.10 Final pH increases more rapidly for the whole pulp during the first 30 minutes of HAPT ............................................................................................................................................ 34 Figure 4.11 Percentage residual peroxide decreases over treatment time, dropping sharply during the first 30 minutes of HAPT. The error bars for are based on standard deviations of triplicates. .................................................................................................................................................. ..... 34 Figure 4.12 Pulp yield loss increases over treatment time ............................................................ 35 xii Figure 4.13 (a) Brightness increases rapidly during the first 30 minutes of HAPT; (b) Opacity is generally higher for the whole pulp; (c) Scattering coefficient is higher for the whole pulp; (d) Absorption coefficient drops rapidly during the first 30 minutes of HAPT .................................. 36 Figure 4.14 (a) Tear index fluctuates over treatment time; (b) Burst index is considerably higher for the whole pulp; (c) Tensile strength increases rapidly during the first 30 minutes of HAPT; (d) Characteristic stress-strain curves contrasting control (incubation for 60 minutes) and HAPT (treatment for 60 minutes) pulp. The error bars are based on standard deviations, with up to 4 replicates of tear index and up to 10 replicates for the burst index and tensile strength. .............. 38 Figure 4.15 Tensile strength decreases as scattering coefficient increases. The error bars are based on standard deviations for up to 10 replicates. .............................................................................. 39 Figure 4.16 (a) Acid group generation is more rapid for the whole pulp during the first 30 minutes of HAPT; (b) Acid group content and tensile strength area linearly correlated; (c) Tensile strength decreases as bulk increases. The error bars for the tensile strength are based on standard deviations for up to 10 replicates. ................................................................................................................... 40 Figure 4.17 Fibre length and coarseness of the fractionated fibres are linearly correlated. The error bars are based on standard deviations of triplicates. ...................................................................... 43 Figure 4.18 The variation in final pH with respect to fibre length is insignificant after 60 minutes .................................................................................................................................... ................... 43 Figure 4.19 Percentage residual peroxide increases as coarseness increases after 60 minutes of HAPT ............................................................................................................................................ 44 Figure 4.20 Pulp yield loss decreases as fibre length increases after 60 minutes of HAPT .......... 45 xiii Figure 4.21 Optical properties after 60 minutes: (a) Brightness as fibre length increases; (b) Opacity decreases as fibre length increases; (c) HAPT has little influence on the scattering coefficient; (d) Absorption coefficient decreases as the fibre length increases ............................. 46 Figure 4.22 S Strength properties after 60 minutes: (a) Tear increases as fibre length increases; (b) Burst is higher the shorter less-coarse fibres; (c) Tensile strength development is higher in the shorter less-coarse fibres; (d) Elastic modulus versus scattering coefficient across fibre length. The error bars are based on standard deviations, with up to 4 replicates of tear index and up to 10 replicates for the burst index, tensile strength and elastic modulus. ............................................. 48 Figure 4.23 Characteristic stress-strain curves for control pulp by the Lw mean fibre length, after incubation for 60 minutes .............................................................................................................. 51 Figure 4.24 Characteristic stress-strain curves for HAPT pulp by the Lw mean fibre length, after HAPT for 60 minutes .................................................................................................................... 51 Figure 4.25 (a) Optical microscopy image of the fracture line for the Lw = 2.555 mm (control) showing fibres withdrawn from the matrix; (b) Optical microscopy image of the fracture line for the Lw = 0.625 mm (control) showing cleaved/broken fibres ....................................................... 52 Figure 4.26 After 60 minutes: (a) Acid group generation resulting from HAPT is greater in the shorter less-coarse fibres; (b) Acid group generation leads to tensile strength development; (c) Increase in bulk of HAPT fibres is due to fibre swelling; (d) Tensile strength increases as bulk decreases for both pulps. The error bars for tensile strength are based on standard deviations for up to 10 replicates. ........................................................................................................................ 54 Figure A.1 Representative refractive index signal (control whole pulp sample) showing the coelution of galactose/mannose/xylose ......................................................................................... 69 xiv List of Abbreviations λ Mean fibre length Ï Fibre density A Titrant volume for sample (Equation 1) A 1st equivalence point (Equation 2) A Fibre cross-sectional area (Equation 5) AIL Acid insoluble lignin AIR Acid insoluble residue ASE Accelerated solvent extraction ASL Acid soluble lignin B Titrant volume for blank (Equation 1) B 2nd equivalence point (Equation 2) b Shear bond strength per unit area CD Cross direction COD Chemical Oxygen Demand DTPA Diethylenetriamine pentaacetate ERMP Energy Reduction in Mechanical Pulping FQA FTIR Fibre Quality Analyzer Fourier Transform Infrared HAPT Highly alkaline peroxide treatment LC Low consistency Lw Mean fibre length MD Machine direction xv N Normality of sodium thiosulfate solution (Equation 1) OD Oven dried P PLS Fibre perimeter Partial Least Squares RBA Relative bonded area T Long-span tensile index TAPPI Technical Association of the Pulp and Paper Industry TMP Thermomechanical pulp UTS Ultimate tensile strength UV Ultraviolet Z Zero-span tensile index xvi Acknowledgements My deepest gratitude to my supervisors, Dr Heather Trajano and Dr Rodger Beatson, whose invaluable mentorship has helped me balance curiosity and pragmatism in this research. The financial support provided by Dr Heather Trajano and the ERMP consortium facilitated my graduate studies, research project and conversance with industry, for which I am immensely grateful. Special thanks Dr Christer Sandberg and Jonas Jonsson at Holmen Paper Braviken (Sweden) for generously providing all the pulp I used to conduct my research. The considered questions and expert insights of the ERMP industry partners and interdisciplinary research team have refined and fortified my research. In addition to their logistical and technical support, Daniela Alejandra Vargas Figueroa, George Soong and Norman Roberts, effectively troubleshot problems as they arose, which enabled me to safely and successfully conduct experiments. I am eternally grateful to my family and friends, who make life vibrant. xvii Dedication To my mother, my first and most dedicated teacher. Soli Deo Gloria 1 Chapter 1. INTRODUCTION 1.1 Motivation Despite its ubiquity, the economy of paper is often misunderstood. In 2019, a Canadian consumer survey conducted by Toluna and Two Sides North America reported common consumer misperceptions concerning the sustainability of forestry reliant industries (Pulp and Paper Canada, 2020). Nonetheless, the Canadian pulp and paper industry exemplifies a circular economy, with its natural resource management, versatile biodegradable products, subsidiary energy generation, integrated recycling and waste management. In the industry, mechanical pulp is distinguished by its comparatively low cost and high yield (90 â 98%). However, the steady decline in demand of printing and writing grade mechanical pulp, has necessitated a shift in the favored pulp properties, to better suit board and packaging grades. The brightening treatment of mechanical pulp using alkaline peroxide was detected to simultaneously improve tensile strength, depending on the alkaline and peroxide charges applied (Moldenius, 1984). The motive behind this research project is to adapt alkaline peroxide treatment to develop customizable pulp grades, cost effectively. 1.2 Objective Energy Reduction in Mechanical Pulping (ERMP) is a consortium comprised of institutional researchers and industry partners, collaborating to reduce electrical energy consumption in mechanical pulping and to valorize low energy mechanical pulping products. One of the goals in the third phase of the ERMP program was to investigate the effects alkaline peroxide treatment has on second stage thermomechanical pulp (TMP) properties succeeding low consistency (LC) refining. It was previously found that the enhanced optical and strength properties of pulp treated with highly alkaline peroxide permitted energy reduction in second-stage refining. Furthermore, 2 the treatment increased pulp flexibility and resistance to cutting during the ensuing LC refining. These improvements are attributed to the generation of weak acid groups on the surfaces of fibres and fines (Chang et al., 2011). This thesis will examine the correlation between acid group generation and tensile strength development of highly alkaline peroxide treated pulp. 1.3 Overview There are five chapters in this thesis. Chapter one introduces the research with a synopsis of its motivation and objective, along with an outline of the scope. The motivation is to understand the impact highly alkaline peroxide treatment (HAPT) has on the development of TMP fibre and finesâ properties; with the objective of developing economically viable processes that combine HAPT with LC refining to produce printing and writing, board and packaging grades. Chapter two surveys the literature reviewed in this research. The methods used to conduct this research are enumerated in chapter three. HAPT conditions are constrained to charges of 6% sodium hydroxide and 4% hydrogen peroxide with 3% sodium silicate (based on OD weight), at a consistency and temperature of 10% and 65°C, respectively. Chapter four discusses the impacts of varying the pulp composition (fibre and fines % w/w) and treatment time on the development of the TMP optical and strength properties relative to the generation of weak acid groups. In conclusion, chapter five summarizes the research findings, reviews the associated limitations, and proposes prospective research projects. 3 Chapter 2. BACKGROUND 2.1 Mechanical Pulping In mechanical pulping, refiner plates (metal discs) concurrently apply pressure and friction on wood chips, shredding and defibrillating the wood into its constituent fibres (Smook, 2016). In high consistency refining (30%), a narrow refiner plate gap produces fibres which are twisted and curled, at suitably high temperatures (Pettersson, 2021). Low consistency refining (3 â 5%), is a less energy-intensive process in which discrete fibres are fibrillated with an optimized refiner gap with the aim to improve pulp strength and smoothness while avoiding excessive fibre shortening. In thermomechanical pulping, wood chips are pre-steamed at a high pressure to plasticize the lignin prior to refining, which results in higher pulp quality and fewer shives. 2.2 Fibres and Fines In mechanical pulping, fibres (elongated plant cells) in the wood matrix are liberated by breaking the middle lamella (outer edge of plant cell walls). Figure 2.1 illustrates the layers of a plant cell wall, which contain lignocellulose (a nanocomposite of cellulose fibrils embedded in a hydrated matrix of lignin) and hemicellulose (Herrera, 2018). Cellulose is comprised of cellobiose units which are two glucose molecules linked with β-(1-4)-glycosidic bonds, whose crystalline and amorphous regions cluster into fibrils (Figure 2.2). Hemicellulose is a branched amorphous polysaccharide made up of short lateral chains of different monosaccharides (pentoses, arabinose, xylose; and hexoses: galactose, glucose, mannose) and functional groups (Scheller et al., 2010). 4 Figure 2.1 Schematic cross-section of a plant cell wall with its representative distribution of cellulose, hemicellulose and lignin; the cell boundary is at the top and the interface of the cell wall with lumen is at the bottom (adapted from Lui, 2015) Lignin is an amorphous polyphenol with aromatic compounds (Smook, 1982), whose formation of chromophores give mechanical pulp its pigmentation (Ajao et al., 2018). The pigment in mechanical pulp is also attributed to extractives, which are mainly comprised of water-soluble colorless/pale-yellow low-molecular-weight materials (tropolones and lignans) and a poorly characterized âbrownish materialâ (Johansson et al., 2000). Figure 2.2 Scale breakdown of the cellulose structure from wood to its molecular unit (adapted from Peneder, 2021) 5 The shredding effect of mechanical pulping results in significant size variations of pulp fibres, with intact longer fibres ranging from 1 mm to 5 mm (Odabas, 2016), and short fibre fragments. Fibre fragments smaller than 76 µm are considered as fines. In contrast to fibres, fines are typified by their larger specific surface area, higher lignin content and lower cellulose content. Fibrillar fines are generated in thermomechanical pulping when the surface of the loosened middle lamella is shredded (Kangas & Kleen, 2004). Fibrillar fines enhance fibre â fibre bonding in sheet formation by occupying the spaces between the longer fibres, improving tensile strength (Motamedian et al., 2019); however, negatively affecting the dewatering process in sheet formation (Fischer et al. 2017). Conversely, non-fibrillar fines are flake like fragments of the middle lamella (Hafrén et al., 2014; Kangas & Kleen, 2004), and contain yet higher lignin and extractives content (Mosbye et al., 2003). Due to their lower aspect ratio, non-fibrillar fines exhibit inferior bonding ability, causing linting during the paper printing process (Westermark et al.,1988, Wood et al., 2000). Non-fibrillar fines are also known to influence the optical properties of paper, for instance increasing the opacity of printing grade pulps (Luukko et al.,1998). A disproportionately large amount of bleaching reagent is consumed by ray cells (Karlsson & Agnemo, 2010, Fischer et al. 2017), which are fines with the highest lignin content (Kleen et al., 2003). 2.3 Highly Alkaline Peroxide Treatment (HAPT) In industry, the brightening of mechanical pulp using HAPT is considered a lignin-preserving âbleachingâ treatment (Smook, 1997). In keeping with unstandardized industry terminology, this section will use âbrighteningâ and âbleachingâ interchangeably. Peroxide bleaching reasonably retains the high yield of mechanical pulp by oxidizing the chromophores in lignin; however, sustained exposure to UV light causes brightness reversion or photoyellowing (Heitner 1993). 6 Notably, peroxide bleaching of mechanical pulps can cause lignin degradation to occur under certain conditions (Ramos, 2008). In peroxide bleaching under alkaline conditions, hydrogen peroxide (H2O2) is activated by the hydroxide ion (OH-) to yield the perhydroxyl anion (HOO-) (Göran et al. 2009). Perhydroxyl anion is the active bleaching species which modifies the chromophoric groups into colorless structures by oxidizing carbonyl groups and reducing quinone structures (Yang et al., 2022). According to Tchepel et al. (2006), peroxide bleaching of TMP in high alkali conditions decreases the fibre cross-sectional dimensions and increases single fibre flexibility due to the reduced moment of inertia. In addition, bleached fibresâ increased bonding capacity and collapsibility produce sheets with high density and tensile strength. In contrast to conventional peroxide bleaching which uses sodium hydroxide, Savoye et al. (2011) observed that pulps bleached using magnesium hydroxide and magnesium carbonate had decreased tensile strength. Even so, pulps treated with magnesium additives demonstrated greater resilience when UV-irradiated for 72 hours, in comparison to those treated with sodium additives, whose brightness reversion occurred sooner. Moreover, Savoye et al. (2018) noticed a reduction in recalcitrant chemical oxygen demand associated with magnesium hydroxide, magnesium carbonate and calcium hydroxide, contrary to sodium hydroxide. Pulps brightened in the alternative alkali sources (such as magnesium hydroxide) were bulkier and opaquer than those conventionally brightened, and resulted in lowered tensile strength due to the lower pH. 7 The presence of transition metal ions (such as copper, iron and manganese) in the pulp during highly alkaline peroxide bleaching catalyze peroxide decomposition through the formation of radicals (Lapierre et al. 2003), which diminishes the efficiency of the bleaching process (Sun et al., 1999). Chelating the pulp with diethylenetriamine pentaacetate (DTPA) prior to HAPT, significantly decelerates the rate of peroxide decomposition by forming stable water-soluble complexes with the alkali metal ions, which are then rinsed out of the pulp. Pulp chelation is supplemented by incorporating sodium silicate in the HAPT process, which stabilizes hydrogen peroxide (Li et al., 2015) by reacting with multivalent metal ions to produce metal silicates which prevent peroxide decomposition, and provide a buffering capacity (Göran et al., 2009). The typical industrial conditions for peroxide bleaching of mechanical pulps are: a peroxide charge of 1 â 5% w/w, a pH range of 10.5 â 11.2, at a pulp consistency of 4 â 35%, a temperature of 45 â 85°C, and a retention time of 30 â 120 minutes (Presley & Hill, 1996). Comparably, the lab scale conditions for the HAPT experiments performed in this research are: a peroxide charge of 4% w/w, a pH range of 11 â 13 (achieved with 6% w/w sodium hydroxide), at a medium pulp consistency of 10%, a temperature of 65°C, and retention times of up to 200 minutes. 2.4 Acid Group Content During peroxide bleaching under alkaline conditions, the peroxide reactions with chromophores also affect fibre properties by generating carboxylic acid groups and phenolic groups (Xu, 2001). These are some of the anionic functional groups that determine the chemical properties of pulp (Li et al., 2020). The generation of new anionic groups in peroxide bleaching result from the hydrolysis of uronic acid residues in wood (Sundberg et al., 2007). 8 A compelling linear relationship exists between the cationic demand measurement and the carboxylic acid content, and by extension, fibre bonding, pulp density and tensile strength (Pan, 2004). Carboxylic acid groups play a crucial role in the papermaking process, by facilitating the swelling of pulp fibres in suspensions and improving paper strength (Sundberg et al., 2007). Similarly, Huang et al. (2017) observed that the increased carboxylic acid content promotes fibre swelling, which impacts fibre flexibility and increases the bonded area between adjacent fibres. Therefore, acid group generation in fibres resulted in sheet densification and greater bonded area, which are determinants of tensile strength. 2.5 Study Approach Previous unpublished studies in the ERMP program have investigated the effects of varying the reagent charges, pH, pulp consistency and temperature in HAPT, on pulp property development. In 2022, Maulit, Po and Lai, varied the H2O2 charge (2% and 6%) and maintained all other conditions at 6% NaOH and 3% Na2SiO3 at 20% consistency and 65°C for up to 120 minutes. The key result of the study was that after 120 minutes of HAPT, the difference in strength gains between the 2% and 6% peroxide charges was insignificant. In another unpublished 2022 study, Maulit, Po and Lai varied the treatment time (0, 5 and 15 minutes) and ratio of fines content (0, 30, 50 and 70% w/w) to untreated fibres in HAPT and maintained all other conditions at 4% H2O2, 6% NaOH and 3% Na2SiO3 at 20% consistency and 75°C. The key result of the study was that acid group generation occurred most rapidly in the fines, and tensile strength of the pulp improved with addition of HAPT fines. 9 By extension, this study investigates the effects of varying the pulp composition (fibres and fines % w/w) and treatment time, on pulp property development during HAPT; with a focus on the correlation between tensile strength development and acid group generation. The HAPT conditions for this study are 4% H2O2, 6% NaOH and 3% Na2SiO3 at 10% consistency and 65°C. 10 Chapter 3. MATERIALS AND METHODS 3.1 Materials Holmen Paper Braviken, Sweden generously provided the secondary finer thermomechanical pulp (TMP) and fines of 100% spruce that were used throughout all the experiments. The chips were sourced from approximately 70% roundwood and 30% sawmill. After determining the initial moisture content of the pulp and fines as 36% and 48.9%, respectively, the pulp and fines were treated with biocide. All chemical reagents were purchased from the Chemistry (Chem) Store and Chemical & Biological Engineering (CHBE) Store at the University of British Columbia (UBC). 3.2 Pulp Characterization A pulp freeness of 197.2 mL was measured using the Technical Association of the Pulp and Paper Industry (TAPPI) Method T 227 (Canadian Standard Method). A standard procedure was used to hot disintegrate 10 g oven-dried (OD) pulp. Using the Bauer McNett fibre classifier, the fibre size distribution of the pulp was determined as 71% fibres and 29% fines. A Fibre Quality Analyzer (FQA) was used to determine the length-weighted mean fibre length, mean fibre width and coarseness corresponding to the fibre fractions from the Bauer McNett. 3.3 Removal of Fines To obtain only fibres, batches of 20 g OD pulp were hot disintegrated using a standard procedure. 0.5% consistency was achieved by adding deionized water to the pulp batches. Each suspension was subsequently transferred to a 200 mesh basket and continuously agitated using a Sommerville shive analyzer, while frequently rinsed with 16 L of deionized water. Only fibres were retained in 200 mesh basket whereas the fines contained in the filtrate were discarded. 11 3.4 Fractionation of Fibres Following the removal of fines, batches of 10 g OD fibres were hot disintegrated using a standard procedure. 0.5% consistency was achieved by adding distilled water. The Bauer McNett fibre classifier was used to separate fibres into fractions with narrow fibre length distributions. This process was repeated until each fibre fraction collected was sufficient for the subsequent tests. 3.5 Chelation The interference of metal ions during the treatment process was minimized by chelating all samples in advance. 30 g OD pulp was chelated with 0.2% charge diethylene triamine pentaacetic acid (DTPA) at a consistency of 4% consistency in a water bath of 60°C for 30 minutes. The suspension was thoroughly mixed every 5 minutes for the first 10 minutes and every 10 minutes for the next 20 minutes. Following chelation, the suspension was rinsed thrice with 1 L of distilled water. For any sample containing fines, the rinsed filtrate was recirculated in the rinse step to restore the drained fines. The rinsed pulp was then dewatered to attain as high a consistency as possible and homogenized in a mixer. 3.6 Highly Alkaline Peroxide Treatment (HAPT) In a resealable plastic bag, the consistency of 25 g OD pulp was adjusted to 10% by adding a suitable amount of distilled water. The bag contents were squeezed and released repeatedly to ensure adequate mixing. The bag was then placed in a hot water bath set to 65°C. Once preheated, the pulp was treated with 4% H2O2, 6% NaOH and 3% Na2SiO3 at 65°C for the required treatment time. In the treatment duration, the pulp was mixed every 5 minutes for the first 10 minutes, every 10 minutes for the next 20 minutes, and every 30 minutes for the remainder. To stop the reaction, 12 the bag was transferred to an ice water bath and chilled until it was cool to the touch. The treated pulp was filtered immediately to extract approximately 50 mL of reagent filtrate for subsequent testing. After which, the pulp was rinsed thrice with 1 L of distilled water. For any sample containing fines, the rinsed filtrate was recirculated in the rinse step to restore the drained fines. The rinsed pulp was then dewatered to attain as high a consistency as possible and homogenized in a mixer. 3.7 Final pH and Residual Peroxide The final pH of the reagent filtrate was tested. The residual peroxide of the filtrate was determined using iodometric titration with 0.1N sodium thiosulfate solution. Prior to titration, 200 mL of distilled water, 20 mL potassium iodide solution and 25 mL of acid mixture were added to 3 g of the filtrate, thoroughly mixed, capped and allowed to stand for 5 minutes. After which, titrant was added to the filtrate solution until its brown triiodide color was reduced to a light straw color. A few drops of 10 g/L starch solution were added to the filtrate solution, and titration resumed until its color changed suddenly from blue to colorless. The initial (A) and final (B) dispensed volumes of the titrant were used to determine the residual peroxide per Equation (1) (Solvay Chemicals 2004). Hydrogen Peroxide [% w/w] = (A - B) N (1.7007)Sample Weight (1) where: A = Titrant volume for sample B = Titrant volume for blank N = Normality of sodium thiosulfate solution 13 3.8 Pulp Yield Loss Equations (2) & (3) which model the correlation of pulp yield loss to chemical oxygen demand (COD) was adopted to determine the loss attributed to HAPT (Möbius, 2006 and Pan, 2018). COD of reagent filtrate (effluent) was tested using the USEPA Reactor Digestion Method, to determine the amount of organic material present. COD [kgton of OD pulp]= 0.34 Ã TOC [kgton of OD pulp] (2) TOC [kgton of OD pulp] = 4.4173 Ã δ [%] + 9.3454 (3) where: TOC = Total organic carbon ð¿ = Pulp yield loss 3.9 Acid Group Content The acid group content of the treated pulp was determined using conductometric titration. To convert the acid groups present into their protonated form, preceding titration, 3 g OD pulp was soaked twice in 0.1 M hydrochloric acid for 45 minutes intervened with vacuum drying. The pulp was then rinsed repeatedly until constant conductance of the rinse filtrate was attained in consecutive conductivity measurements. The pulp was subsequently vacuum dried to attain as high a consistency as possible. It was then dispersed in a wide mouth Erlenmeyer flask containing 450 mL of 0.001 M sodium chloride. To account for the lack of strong acid groups in secondary refiner 14 pulp, 5 mL of hydrochloric acid was added to the mixture. 0.1 M sodium hydroxide was titrated into the mixture and once equilibrated (constant conductance), the conductance was measured using a dip-type electrode and recorded to generate a titration curve. Figure 3.1 Schematic of the conductometric titration apparatus setup for the analysis of acid group content The emergent equivalence points of the titration curve represent the strong acid groups attributed to the hydrochloric acid and total acid content present in the pulp. A nitrogen gas atmosphere was maintained over the pulp suspension throughout the titration duration to prevent absorbance of carbon dioxide from the air. Figure 3.1 shows a schematic of the conductometric titration apparatus setup. Equation (4) was used to convert the conductometric titration curve (Figure 3.2) to the weak acid content. 15 Figure 3.2 Titration curve indicating the equivalence points used in determining the acid group content Weak acid content [mmol/kg] = (B - A)Sample Weight x 0.001 (4) where: A = 1st equivalence point B = 2nd equivalence point 3.10 Standard Handsheets Handsheets averaging a grammage of 60 cm2/g were prepared from treated and untreated pulp samples using the TAPPI T 205 standard method. The handsheets were air-dried in a temperature and moisture-controlled room for a minimum of 24 hours. 16 Prior to testing their properties, each handsheetâs weight and thickness was determined using an analytical balance and L&W Micrometer, respectively. The Technidyne ColorTouch PC was used to test the optical properties (brightness, opacity, absorption coefficient and scattering coefficient) according to the ISO 2470-1 and TAPPI T 525 protocols. The tensile strength, tear index and burst index tests were performed using the 84-56 Horizontal Tensile Tester according to TAPPI Method T 494, the Elmendorf Tearing Tester according to TAPPI Method T 414, and the Mullen Tester according to TAPPI Method T 403, respectively. 3.11 Chemical Composition Characterization National Renewable Energy Laboratory (NREL) Laboratory Analytical Procedures (LAP) for standard biomass analysis were used to characterize the chemical composition of pre- and post-treatment pulp samples. A two-step accelerated solvent extraction (ASE) method was applied to remove water and ethanol (95% w/w) soluble extractives from the samples according to the NREL/TP â 510 â 42619 (Sluiter et al, 2008) method. Firstly, the extractives-free samples were air-dried for 24 hours in a fume hood, then oven-dried at 40°C for 24 hours. The reduced weight of the initial 3 g OD samples was attributed to the extractives removed. Subsequently, a two-step acid hydrolysis was conducted on 300 mg of the dried extractives-free samples according to the NREL/TP â 510 â 42618 (Sluiter et al, 2012) method to initiate the determination of structural carbohydrates and lignin content. The samples were first hydrolyzed with 3 mL of 72% w/w sulfuric acid at 30°C for 1 hour, then diluted with 84 mL of ultrapure water to attain 4% w/w sulfuric acid and hydrolyzed in an autoclave at 121°C for another hour. Once cooled to room temperature, the mixtures were vacuum filtered through glass crucibles, retaining 17 both acid insoluble residue (AIR) and the hydrolysis liquor aliquots. To determine the acid insoluble lignin (AIL) on an extractives-free basis, the crucibles containing the AIR were oven-dried at 105°C for 24 hours. The reduced weight of the initial 300 mg samples was attributed to AIL. The oven-dried AIR was then incinerated at 575°C in a furnace for 24 hours. The residual weight of the samples was attributed to the ash content according to the NREL/TP â 510 â 42622 (Sluiter et al, 2005) method. The acid soluble lignin (ASL) was measured using UV-Vis Spectroscopy (Shimadzu UV-1800 spectrophotometer) at 240 nm wavelength and 12 L/g cm absorptivity. 1 â 4 mL of hydrolysis liquor aliquots were diluted with ultrapure water to attain absorbance values within the required range (0.7 â 1.0). Calibration sugar standards were prepared for D(+)galactose, D(+)glucose, D(+)mannose, D(+)xylose and L(+)arabinose in a linear range of concentrations to derive calibration curves used to identify and quantify the carbohydrates present in the pulp samples with high-performance liquid chromatography refractive index detection (HPLC-RID). A set of sugar recovery standard (SRS) were subjected to the second hydrolysis step to correct for the destruction of sugars during dilute acid hydrolysis. In preparation for chromatography, 20 mL aliquot of each liquor was neutralized with calcium carbonate to the required pH range (5 â 6). 2 mL supernatant of each neutralized liquor was first filtered through a 1 μm and then a 0.22 μm filter, into a chromatography autosampler vail. The UBC Analytical Core Facility performed the sugar analysis and provided a report summarizing the carbohydrate chromatograms and concentrations for each pulp sample. 18 Chapter 4: RESULTS AND DISCUSSION 4.1 Fines Content Page (1969) observed that due to their heterogeneous nature, fines unpredictably affect the mechanical properties of paper products, as their concentration increases. The effect of varying fines content in pulp is investigated to better understand its impact on pulp properties developed with HAPT. Table 4.1 lists the variations in fines content of pulp combinations used. Except for the 30% fines content, which represents the unaltered whole pulp, all pulp combinations were made up of TMP fines (provided by Holmen Braviken) and isolated fibres (whose fines were previously eliminated). After chelation, the pulp was treated with 4% H2O2, 6% NaOH, 3% Na2SiO3 at 65°C and 10% consistency for 60 minutes. Following each treatment, the slurry was filtered and washed as previously described. The final pH of the filtrate was promptly measured. The residual peroxide post-HAPT was determined using iodometric titration. The consequential pulp yield loss was determined using the COD of the filtrate. The weak acid content was determined using conductometric titration. Handsheets were made and tested according to the TAPPI standards. Table 4.1 Percentage of fines content present pulp samples Fines [%] Fibres [%] 0 100 30 70 50 50 70 30 100 0 19 4.1.1 Final pH and Residual Peroxide After treatment for 60 minutes, the variation in the final pH with respect to fines content was insignificant (Figure 4.1) since pH is mainly controlled by the stable strong base (NaOH). Figure 4.1 Final pH after 60 minutes of HAPT remains constant as fines content increases Due to their small size and large specific surface area, fines are more easily accessed by the peroxide ions. Thus, the percentage residual peroxide predictably decreases as the fines content increases (Figure 4.2), owing to the quicker consumption of peroxide by fines (which contain high residual metals) since decomposition of hydrogen peroxide by transition metal ions increases its consumption (Li et al. 2015). Ray cells consumed more peroxide since they contain the most lignin (Karlsson & Agnemo, 2010, Fischer et al. 2017). 7891011120 20 40 60 80 100Final pHFines Content [% w/w]20 Figure 4.2 Percentage residual peroxide after 60 minutes of HAPT decreases as fines content increases. The error bars are based on standard deviations of triplicates. 4.1.2 Pulp Yield Loss In Figure 4.3, pulp yield loss increases as fines content increases, similarly indicating an increase in the rate of peroxide consumption. Yield loss is attributed to the removal of extractives (lower molecular weight acidic components). Figure 4.3 Pulp yield loss after 60 minutes of HAPT increases as fines content increases 0123450 20 40 60 80 100Residual Peroxide [%]Fines Content [% w/w]024680 20 40 60 80 100Yield Loss [%]Fines Content [% w/w]21 4.1.3 Optical Properties Figures 4.4 (a) to (d) demonstrate the dependence of brightness and opacity on the absorption and light scattering coefficients of pulp (Odabus et al, 2016). Fines content of 50% and above saw a decline in brightness due to the increase in light absorption coefficient (Karlsson & Agnemo, 2010). Scattering coefficient is a measure of fibre surface area which is exposed to incident light; that is, the optically unbound area. In fact, the fines content is linearly correlated with the scattering coefficient. The brightness decreases as the fines content increases, which is driven by increase in the absorption coefficient. Harrison (2018) emphasized the importance of optimizing the scattering coefficient to achieve high opacity of printing and writing paper, corroborated by the increase in opacity with scattering coefficient. 22 Figure 4.4 Optical properties after 60 minutes of HAPT: (a) Brightness generally decreases as fines content increases; (b) Opacity increases as fines content increases; (c) Scattering coefficient increases as fines content increases; (d) Absorption coefficient generally increases as fines content increases 60646872760 20 40 60 80 100Brightness [%ISO]Fines Content [% w/w](a)848892961000 20 40 60 80 100Opacity [%]Fines Content [% w/w](b)R² = 0.95684060801001200 20 40 60 80 100Scattering Coefficient [m2/kg]Fines Content [% w/w](c)00.511.522.530 20 40 60 80 100Absorption Coefficient [m2/kg]Fines Content [% w/w](d)23 4.1.4 Strength Properties Figures 4.5 (a), (b) and (c) show that the tear index, burst index and tensile strength all decrease as the fines content increases, owing to the reduced mean fibre length and its consequent effect on the bonded area. That is, the increase in fines content is balanced by a decrease in fibres, and thus a reduction in fibre â fibre bond frequency and strength. Figure 4.5 (d) shows that the maximum stress the handsheets can withstand before fracture (ultimate tensile strength, UTS) decreases with increase in fines content. Similarly, the maximum failure strain of the handsheets decreases with increase in fines content. This is likely due to the reduction in mean fibre length, and consequent fibre â fibre bonding. The greatest UTS is exhibited by whole pulp (30% fines content) handsheets, which have the most favorable consolidation of fibres and fines; whereby, fibrillar fines occupy the spaces between the longer fibres, thus increasing fibre â fibre bonding and improving tensile strength (Motamedian et al., 2019). 24 Figure 4.5 Strength properties after 60 minutes of HAPT: (a) Tear decreases as fines content increases; (b) Burst decreases as fines content increases; (c) Tensile strength decreases as fines content increases; (d) Characteristic stress-strain curves for varied fines content. The error bars are based on standard deviations, with up to 4 replicates of tear index and up to 10 replicates for the burst index and tensile strength. 02468100 20 40 60 80 100Tear Index [mNm2/g]Fines Content [% w/w](a)00.511.522.533.50 20 40 60 80 100Burst Index [kPam2/g]Fines Content [% w/w](b)0123450 20 40 60 80 100Tensile Strength [km]Fines Content [% w/w](c)05101520250 0.5 1 1.5 2 2.5Stress [MPa]Strain [%]0% Fines 30% Fines50% Fines 70% Fines100% Fines(d) 25 When fines are absent (Figure 4.6 (a)), the fibres in the handsheet showed the greatest elongation under tension, prior to failure. The fibre â fibre bonds withstand high load before breaking due to the load dispersion within the fibre matrix. The break interface for 0% fines (Figure 4.6 (b)) is markedly bristly, with fibres on the edge overlaying each other. The fibre matrix for 30% fines (whole pulp) appears to have the most favorable consolidation of fibres and fines as evidenced by the high UTS (Figure 4.6 (c)). This is because the fibrillar fines occupy the spaces between the longer fibres, which increases fibre â fibre bonding and boosts tensile strength. The break interface for 30% fines (Figure 4.6 (d)) is less bristly with the addition of fines. Figures 4.6 (e) and (g) show that the decrease in elongation of the fibre â fines matrices for the 50% and 70% fines, respectively, is due to the reduced bond area. The handsheets withstands less tensile stress prior to failure as the fines content increases since sheet consolidation decreases. For both 50% and 70% fines, the break interfaces (Figure 4.6 (f) and (h), respectively) are less bristly and opaquer than that of the 30% fines, reflecting the reduction in bonded area. The handsheets made of 100% fines withstand the least tension prior to failure due to the low bonded area (Figure 4.6 (i)). The fibrillar and non-fibrillar fines stretch significantly less than fibres (which are absent). The break interface for 100% fines (Figure 4.6 (j)) is the least bristly, indicating the brittleness of the fines only matrix. 26 05101520250 0.5 1 1.5 2 2.5Stress [MPa]Strain [%]0% Fines 30% Fines50% Fines 70% Fines100% Fines(a)05101520250 0.5 1 1.5 2 2.5Stress [MPa]Strain [%]0% Fines 30% Fines50% Fines 70% Fines100% Fines(c)(b) (d) 27 05101520250 0.5 1 1.5 2 2.5Stress [MPa]Strain [%]0% Fines 30% Fines50% Fines 70% Fines100% Fines(e)05101520250 0.5 1 1.5 2 2.5Stress [MPa]Strain [%]0% Fines 30% Fines50% Fines 70% Fines100% Fines(g)(f) (h) 28 Figure 4.6 (a) Stress-strain curve focus on 0% fines; (b) Optical microscopy image of the break interface for 0% fines; (c) Stress-strain curve focus on 30% fines; (d) Optical microscopy image of the break interface for 30% fines; (e) Stress-strain curve focus on 50% fines; (f) Optical microscopy image of the break interface for 50% fines; (g) Stress-strain curve focus on 70% fines; (h) Optical microscopy image of the break interface for 70% fines; (i) Stress-strain curve focus on 100% fines; (j) Optical microscopy image of the break interface for 100% fines Figure 4.7 shows that the tensile strength decreases as the scattering coefficient increases, due to the decrease in bonded area, which corroborates the stress-strain curves discussed above. The decrease in bonded area per fibre leads to less bonds that transmit stress from fibre to fibre, thereby concentrating more load on individual fibres, augmenting fracture. 05101520250 0.5 1 1.5 2 2.5Stress [MPa]Strain [%]0% Fines 30% Fines50% Fines 70% Fines100% Fines(i) (j) 29 Figure 4.7 After 60 minutes of HAPT, tensile strength decreases as scattering coefficient increases, due to the decrease in bonded area. The error bars are based on standard deviations of up to 10 replicates. 4.1.5 Acid Group Content Acid group content increases as fines content increases (Figure 4.8 (a)). Like the decrease in residual peroxide and increase in yield loss seen earlier, this suggests that fines react more rapidly with peroxide than fibres. The difference in reaction rate between fibres and fines may be attributed to the larger specific surface area and higher lignin content of the fines. An increase in weak acid content typically results in increased bonded area between adjacent fibres. However, Figure 4.8 (b) shows that the tensile indices of the handsheets decrease as the acid group content increases. This discrepancy suggests that the boost of fibre â fibre bonding resulting from acid group generation is countered by the reduced mean fibre length resulting from increased fines content. A balance between the combined effects of increased acid content and reduced mean fibre length, is achieved by the whole pulp (30% fines content); which is the least bulky (Figure 4.8 (c)). 01234540 50 60 70 80 90 100 110Tensile Strength [km]Scattering Coefficient [m2/kg]30 Figure 4.8 After 60 minutes of HAPT: (a) Acid group content increases as fines content increases; (b) Tensile strength declines with increase in acid group content corresponding to the increase in fines content; (c) Bulk generally increases as fines content increases. The error bars for tensile strength are based on standard deviations of up to 10 replicates. 2102402703003303600 20 40 60 80 100Acid Group Content [mmol/kg]Fines Content [% w/w](a)0% 30%50%70%100%012345210 240 270 300 330 360Tensile Strength [km]Acid Group Content [mmol/kg](b)1.81.922.12.22.30 20 40 60 80 100Bulk [cm3/g]Fines Content [% w/w](c)31 4.1.6 Chemical Composition Characterization Figure 4.9 shows the chemical compositional analysis for fibres only (0% fines), whole pulp (30% fines), and fines only (100% fines) before and after HAPT. Carbohydrates, which constitute most of the pulp, are not categorized as cellulose and hemicellulose due to the coelution of the galactose, mannose and xylose peaks in the chromatograms. The hemicellulose (mannans and glucomannans) backbone, β-1,4-linked mannose, would have been used to determine the quantity of glucose attributed to hemicellulose, and its balance to cellulose. The coelution may have occurred because the HPLC column used was not well suited to this application. During the accelerated solvent extraction (ASE) process, some fines strained through ASE filter with ethanol-soluble extractive liquor for the control (untreated) fines samples. Likewise, fines content appeared to form colloidal dispersions in both water and ethanol extractive liquors for the HAPT fines samples. These incurred losses skewed the extractives content and undermined the overall material balances for the finesâ samples. Furthermore, the chemical distribution for fines does not distinguish between fibrillar and non-fibrillar, ray cells or flaky fines, which are varied. The sugars that dissolved in the water-soluble extractive liquor during the removal of extractives were not identified or quantified. The carbohydrates content decreased with increase in fines content while the lignin and extractives content increased, which is consistent with literature (Benouadah et al., 2019). The lignin content represents the total of ASL and AIL on an extractives-free basis. The decrease in extractives content after HAPT is reflected in the pulp yield loss; that is, the removal of lower molecular weight acidic components resulting from HAPT. 32 Figure 4.9 Chemical compositional analysis for fibres only (0% fines), whole pulp (30% fines), and fines only (100% fines) for untreated (control) and 60-minute treated (HAPT) pulps 53.0 49.737.457.349.735.219.618.622.920.824.520.323.426.828.121.021.722.13.8 5.94.81.92.22.61.3 1.71.60.30.31.9Fibres Whole Pulp Fines Fibres Whole Pulp FinesCONTROL _ HAPTGlucan Ara/Gal/Man/Xyl Lignin Extractives Ash/Minerals33 4.2 Fibres Only Chelated samples of whole pulp and fibres were each treated with 4% H2O2, 6% NaOH, 3% Na2SiO3 at 10% consistency and 65°C for up to 200 minutes. For the 0-minute treatment time, the HAPT reagents were substituted with distilled water to maintain 10% consistency. Following each treatment, the slurry was filtered and washed as previously described. The final pH of the filtrate was promptly measured. The residual peroxide post-HAPT was determined using iodometric titration. The consequential pulp yield loss was determined using the COD of the filtrate. The weak acid content was determined using conductometric titration. Handsheets were made and tested according to the TAPPI standards. 4.2.1 Final pH and Residual Peroxide During the initial stages of the reaction, the pH increased as the percentage residual peroxide drops (Figures 4.10 and 4.11). These changes occurred more rapidly for the fines-containing whole pulp. The drop in pH after 120 minutes could be attributed to the consumption of alkali by the acid group content generated within the fibre walls. The acid group content is discussed in section 4.2.5. The rise in pH at lower residual peroxide content occurs because hydrogen peroxide forms a hydroperoxide ion with the base, owing to its acidity. The decrease in residual peroxide is attributed to its consumption and decomposition throughout the reaction with pulp. 34 Figure 4.10 Final pH increases more rapidly for the whole pulp during the first 30 minutes of HAPT Figure 4.11 Percentage residual peroxide decreases over treatment time, dropping sharply during the first 30 minutes of HAPT. The error bars for are based on standard deviations of triplicates. 4.2.2 Pulp Yield Loss During the initial stages of the reaction, the pulp yield loss increases rapidly then plateaus at around 3.5% for the fibres and 4% for the whole pulp (Figure 4.12). In an unpublished ERMP study where 6789101112130 30 60 90 120 150 180 210Final pHTreatment Time [Minutes]Whole PulpFibres Only0481216200 30 60 90 120 150 180 210Residual Peroxide [%]Treatment Time [Minutes]Whole PulpFibres Only35 the caustic charge was varied in the treatments conducted, the pulp yield loss appeared to be correlated with the initial caustic charge. Yield loss is attributed to the removal of extractives. Figure 4.12 Pulp yield loss increases over treatment time 4.2.3 Optical Properties Figure 4.13 (a) demonstrates the brightening effect of HAPT on pulp. Brightness developed rapidly during the initial stages of the reaction, was sustained for both fibres (~ 69% ISO) and whole pulp (~ 71% ISO) between 60 and 120 minutes. The decline in brightness of the whole pulp handsheets at 150 and 180 minutes could be attributed to the lowered residual peroxide (Figure 4.11). High alkalinity in the absence of peroxide leads to alkali darkening. Conversely, at 200 minutes, the handsheets of the fibres only gained brightness. The opacity decreased rapidly during the initial stages of the reaction for both fibres and whole pulp (4.13 (b)). The scattering coefficient for whole pulp handsheets is higher (4.13(c)), due to the higher specific surface area of the fines present. The decrease in absorption coefficient over time is due to the modification/removal of chromophores during peroxide brightening (Figure 4.13(d)). 0123450 30 60 90 120 150 180 210Yield Loss [%]Treatment Time [Minutes]Fibres OnlyWhole Pulp36 Figure 4.13 (a) Brightness increases rapidly during the first 30 minutes of HAPT; (b) Opacity is generally higher for the whole pulp; (c) Scattering coefficient is higher for the whole pulp; (d) Absorption coefficient drops rapidly during the first 30 minutes of HAPT 5055606570750 30 60 90 120 150 180 210Brightness [% ISO]Treatment Time [Minutes]Whole PulpFibres Only(a)607080901000 30 60 90 120 150 180 210Opacity [%]Treatment Time [Minutes]Whole PulpFibres Only(b)30405060700 30 60 90 120 150 180 210Scattering Coefficient [m2/kg]Treatment Time [Minutes]Whole PulpFibres Only(c)00.511.522.533.50 30 60 90 120 150 180 210Absorption Coefficient [m2/kg]Treatment Time [Minutes]Whole PulpFibres Only(d)37 4.2.4 Strength Properties The tear indices for both fibres and whole pulp fluctuate around 8 mNm2/g over treatment time (Figure 4.14 (a)). Burst is roughly three times greater for whole pulp than for fibres, pointing to the role fines play in paper strength development (Figure 4.14 (b)). During the first 30 minutes, tensile strength development is faster for the whole pulp due to the rapid reaction of peroxide with the fines. The tensile strength for whole pulp appears to plateau around 4.5 km after 60 minutes, indicating that further treatment does not achieve additional strength gains. However, tensile strength for fibres continues to develop. This may be due to continued reactions within the fibre structure as a result of continued diffusion. Although the tensile strength (Figure 4.14 (c)) for fibres and whole pulp at 0 minutes (control) and 60 minutes (HAPT) are proximate (around 2.8 km and 4.4 km, respectively), the control stress-strain curves (Figure 4.14 (d)) vary by an order of magnitude. In particular, whole pulp â control (incubated for 60 minutes) withstands greater stress before yielding and ultimately fracturing, pointing to the role untreated fines play in paper strength development. The impact of HAPT on stress-strain relationship is discussed in greater detail in section 4.3.5. Seth et al. (1981) propose that the stress-strain curve lengthens as the fibres straighten during the final stages of straining preceding failure. The mean fibre length (Lw) of the fibre â control and fibre â HAPT samples is higher due to the absence of fines, thus their interfibrillar matrices endure more strain (extension) before failure. 38 Figure 4.14 (a) Tear index fluctuates over treatment time; (b) Burst index is considerably higher for the whole pulp; (c) Tensile strength increases rapidly during the first 30 minutes of HAPT; (d) Characteristic stress-strain curves contrasting control (incubation for 60 minutes) and HAPT (treatment for 60 minutes) pulp. The error bars are based on standard deviations, with up to 4 replicates of tear index and up to 10 replicates for the burst index and tensile strength. 02468100 30 60 90 120 150 180 210Tear Index [mNm2/g]Treatment Time [Minutes]Whole PulpFibres Only(a)02468100 30 60 90 120 150 180 210Burst Index [kPa.m2/g]Treatment Time [Minutes]Whole PulpFibres Only(b)22.533.544.555.50 30 60 90 120 150 180 210Tensile Strength [km]Treatment Time [Minutes]Whole PulpFibres Only(c)01101000 0.5 1 1.5 2 2.5Stress [MPa]Strain [%]Fibres - HAPT Whole Pulp - HAPTFibres - Control Whole Pulp - Control(d) 39 The tensile strength of both the whole pulp and fibres only handsheets decrease as the scattering coefficient increases due to the reduction in bonded area (Figure 4.15). The scattering coefficient for the whole pulp is higher than the fibres only on account of the fines present, which result in greater unbound area. HAPT is known to increase fibre swelling and flexibility which in turn increases the bonded area between adjacent fibres, due to acid group generation (discussed next). Figure 4.15 Tensile strength decreases as scattering coefficient increases. The error bars are based on standard deviations for up to 10 replicates. 4.2.5 Acid Group Content Weak acid groups are generated more rapidly for the whole pulp during the first 30 minutes of the reaction (Figure 4.16(a)), possibly due to the higher specific surface area of fines present. A linear correlation exists between acid group generation and tensile strength development (Figure 4.16 (b)). Over longer treatment times, acid group generation is thought to continue within the fibre structure, due to mass transfer. Acid group content within fibre structure may not impact the tensile strength development as much as acid group content on the external surfaces of fibres and fines. Tensile strength decreases as bulk increases, owing to the decreased bonded area (Figure 4.16 (c)).22.533.544.555.540 45 50 55 60 65 70Tensile Strength [km]Scattering Coefficient [m2/kg]Whole PulpFibres Only40 Figure 4.16 (a) Acid group generation is more rapid for the whole pulp during the first 30 minutes of HAPT; (b) Acid group content and tensile strength area linearly correlated; (c) Tensile strength decreases as bulk increases. The error bars for the tensile strength are based on standard deviations for up to 10 replicates. 0501001502002503000 30 60 90 120 150 180 210Acid Group Content [mmol/kg]Treatment Time [Minutes]Whole PulpFibres Only(a)2.533.544.555.50 50 100 150 200 250 300Tensile Strength [km]Acid Group Content [mmol/kg]Whole PulpFibres Only(b)22.533.544.555.51.75 2 2.25 2.5 2.75 3 3.25Tensile Strength [km]Bulk [cm3/g]Whole PulpFibres Only(c)41 4.3 Fibre Length and Coarseness This section investigates the effect of varying fibre length and coarseness on HAPT in the development of handsheet properties. A Bauer McNett was used to classify and fractionate whole pulp by size as outlined in Table 4.2. Each fibre fraction was characterized using an FQA to determine its fibre length, width and coarseness. A summary of fractionated fibre morphology is recorded in Table 4.3. Figure 4.17 shows the correlation between fibre length and coarseness, which is considered proportional to fibre wall thickness. Table 4.2 Bauer McNett fibre classification of pulp by size Mesh Sieve [#] Fraction [% w/w] 14 2.6 28 27.9 48 21.8 100 11.7 200 7.1 Fibres 71.0 Fines 29.0 42 Table 4.3 Fibre size distribution Mesh Mean Fibre Length Mean Fibre Width Coarseness Sieve # Lw ÏLw W ÏW C ÏC mm mg/m μm μm mg/m mg/m 14 2.555 0.004 29.95 0.45 0.231 0.011 28 2.231 0.003 31.85 0.36 0.210 0.007 48 1.293 0.003 27.85 0.24 0.206 0.010 100 0.625 0.004 21.80 0.33 0.090 0.002 200 0.295 0.005 18.35 0.83 0.062 0.001 Whole Pulp 1.140 0.005 26.05 0.37 0.168 0.005 The fibre fractions were chelated and treated with 4% H2O2, 6% NaOH, 3% Na2SiO3 at 10% consistency and 65°C for 60 minutes. The HAPT reagents for the control samples were substituted with distilled water to maintain 10% consistency, and incubated at 65°C for 60 minutes. Following each treatment, the slurry was filtered and washed as previously described. The final pH of the filtrate was promptly measured. The residual peroxide post-HAPT was determined using iodometric titration. The consequential pulp yield loss was determined using the COD of the filtrate. The weak acid content was determined using conductometric titration. Handsheets were made and tested according to the TAPPI standards. On average, the handsheets for the shortest least-coarse fibres (200 mesh sieve) had 5 â 12% higher grammage than the other fibre fractions (approximately 60 cm2/g). This discrepancy had an effect on the optical and strength properties in the upcoming discussions. The formation of handsheets was made further difficult by the limited supply of short fibres, which restricted the adjustments made the pulp-suspension consistency. 43 Figure 4.17 Fibre length and coarseness of the fractionated fibres are linearly correlated. The error bars are based on standard deviations of triplicates. 4.3.1 Final pH and Residual Peroxide After treatment for 60 minutes, the variation in the final pH with respect to fibre length was insignificant for both HAPT and control as shown in Figure 4.18. The basic pH of HAPT is consistent with its high alkali charge while the controlâs neutrality signifies the absence of alkali. Figure 4.18 The variation in final pH with respect to fibre length is insignificant after 60 minutes R² = 0.850500.050.10.150.20.250.30 0.5 1 1.5 2 2.5 3Coarseness [mg/m]Lw Mean Fibre Length [mm]6789101112130.0 0.5 1.0 1.5 2.0 2.5 3.0Final pHLw Mean Fibre Length [mm]HAPTControl44 In Figure 4.19, the percentage residual peroxide increases as the coarseness increases, indicating that peroxide consumption is faster in the shorter less-coarse fibres, likely due to their higher specific surface area and thinner cell walls. Figure 4.19 Percentage residual peroxide increases as coarseness increases after 60 minutes of HAPT 4.3.2 Pulp Yield Loss Figure 4.20 shows that pulp yield loss decreases as fibre length increases. This may be attributed to the removal of extractives, which likely have a higher content in the finer material (Kangas & Kleen, 2004). The influence fibre morphology has on yield loss associated with HAPT requires a focused study. 024681012140.05 0.1 0.15 0.2 0.25Residual Peroxide [%]Coarseness [mg/m]HAPT45 Figure 4.20 Pulp yield loss decreases as fibre length increases after 60 minutes of HAPT 4.3.3 Optical Properties Increase in brightness (Figure 4.21 (a)) results from removal of chromophores by the hydrogen peroxide, as shown by the decrease in absorption coefficient for HAPT pulps (Figure 4.21(d)). The absorption coefficient decreases as the fibre length increases, especially for the control, likely due to a higher lignin content of the shorter less-coarse fibres. Figure 4.21 (b) shows that the shortest fibres (0.3 mm) produce remarkably opaque paper (98.58% opacity for the control), corresponding to the high absorption and scattering coefficients. The opacity decreases as the absorption coefficient decreases for both HAPT and control pulps, revealing the dependence of opacity on absorption coefficient. Brightness decreases as the fibre length increases for both the control and HAPT pulps due to the decreasing specific surface area. HAPT has little influence on the scattering coefficient since the behavior of both the control and HAPT pulps is similar (Figure 4.21 (c)). This indicates that the bonded areas are similar; therefore, the strength grains are attributed to the increase in bond strength resulting from HAPT.00.511.522.530 0.5 1 1.5 2 2.5 3Yield Loss [%]Lw Mean Fibre Length [mm]HAPT46 Figure 4.21 Optical properties after 60 minutes: (a) Brightness as fibre length increases; (b) Opacity decreases as fibre length increases; (c) HAPT has little influence on the scattering coefficient; (d) Absorption coefficient decreases as the fibre length increases 40455055606570750 0.5 1 1.5 2 2.5 3Brightness [%ISO]Lw Mean Fibre Length [mm]HAPTControl(a)7075808590951000 0.5 1 1.5 2 2.5 3Opacity [%]Lw Mean Fibre Length [mm]HAPTControl(b)203040506070800 0.5 1 1.5 2 2.5 3Scattering Coefficient [m2/kg]Lw Mean Fibre Length [mm]HAPTControl(c)0123450 0.5 1 1.5 2 2.5 3Absorption Coefficient [m2/kg]Lw Mean Fibre Length [mm]HAPTControl(d)47 4.3.4 Strength Properties Figure 4.22 (a) shows that tear increases as fibre length increases, reflecting its high dependence on fibre length. The tear resistance for HAPT pulp is greater than the control pulp on account of the enhanced fibre bonding. The burst index mirrors the tensile strength since the sheet is isotropic, as evidenced by their comparable graphs (Figures 4.22 (b) and (c)). Tensile strength for the control pulp is independent of coarseness, except for shortest least coarse fibres (0.30 mm). The difference in tensile between the HAPT and control pulps, or tensile strength development, is highest for the shorter less-coarse fibres. This is likely due to the greater extent of reaction with peroxide on account of their higher lignin content and higher specific surface area. Figure 4.22 (d) shows that the elastic modulus for HAPT pulp increases as scattering coefficient (unbound area) increases and mean fibre length decreases. This counter intuitive correlation will be examined more closely. 48 Figure 4.22 Strength properties after 60 minutes: (a) Tear increases as fibre length increases; (b) Burst is higher the shorter less-coarse fibres; (c) Tensile strength development is higher in the shorter less-coarse fibres; (d) Elastic modulus versus scattering coefficient across fibre length. The error bars are based on standard deviations, with up to 4 replicates of tear index and up to 10 replicates for the burst index, tensile strength and elastic modulus. 03691215180 0.5 1 1.5 2 2.5 3Tear [mNm2/g]Lw Mean Fibre Length [mm]HAPTControl(a)00.511.522.533.50.05 0.1 0.15 0.2 0.25Burst [kPam2/g]Coarseness [mg/m]HAPTControl(b)024680.05 0.1 0.15 0.2 0.25Tensile [km]Coarseness [mg/m]HAPTControl(c)2.562.231.29 0.630.302.562.23 1.29 0.630.300100020003000400020 30 40 50 60 70 80Elastic Modulus [MPa]Scattering Coefficient [m2/kg]HAPTControl(d)49 The Page (1969) equation, a fundamental expression for pulp tensile strength, was modeled to describe the tensile failure of chemical pulps: 1T = 98 Z+ 12 A Ïb P λ RBA (5) where: T = Long-span tensile index [Nm/kg] Z = Zero-span tensile index [Nm/kg] A = Fibre cross-sectional area [m2] Ï = Fibre density [kg/m2] b = Shear bond strength per unit area [N/m2] P = Fibre perimeter [m] λ = Mean fibre length [m] RBA = Relative bonded area The first and second terms on the right-hand-side of Equation (5) represent the contribution of fibre strength and bond strength, respectively, to the tensile strength. The second term in particular associates the bond strength with bonded area, fibre morphology and bulk. Page proposed that the fibre shear strength may be the product of the relative bonded area in the sheet and the shear strength of the fibreâs bonded areas. According to Shallhorn and Karnis (1979), the Page equation does not adequately describe the tensile failure of mechanical pulps, which fail in shear. For mechanical pulps, the intrinsic fibre strength can be considered constant for a given wood species and yield. Hence, at least two 50 parameters are needed to adequately characterize mechanical pulp, namely fibre length and bonding potential (shear strength) based on the mean fibre length and specific surface area, respectively. The theory presented by Shallhorn and Karnis considers paper (a handsheet) as a continuum of a composite structure to which the following assumptions apply: (1st) the fibres enmesh to form a fibre â fibre matrix that shares a load uniformly; (2nd) the consistent fibre morphology justifies the uniform distribution of fibre strength properties (tensile strength and bond shear strength); (3rd) the orientation of all the fibres is perpendicular to the tear and tensile fracture line, and (4th) both the fibre tensile and bonding strengths contribute to the sheet tensile strength, and the energy required to induce fibre fracture has negligible effect on the tear strength. If the assumptions above are considered applicable for all fibre fractions, then the tensile failure can be summarily explained using the mean fibre length (Lw) and the specific surface area. Figures 4.23 and 4.24 show that capacity of the handsheets to withstand tensile stress before failure (UTS) increases as the mean fibre length decreases and the specific surface area increases. The shear strength due to fibre bonding in the matrix likely increases as the fibre length decreases, resulting in strongly embedded fibres which preferentially break rather than being withdrawn from the fibre matrix. That is, the applied stress cleaves the more securely bonded fibres at the fracture line, instead of extracting the them from the matrix, as shown in Figure 4.25. The strength gains from HAPT narrows the margin between the stress-strain curve for all longer coarser fibres and the shortest least-coarse fibres (0.30 mm), on account of the increased bonding capacity/potential of the treated fibres evidenced by the generation on weak acid groups discussed next. 51 Figure 4.23 Characteristic stress-strain curves for control pulps by the Lw mean fibre length, after incubation for 60 minutes Figure 4.24 Characteristic stress-strain curves for HAPT pulps by the Lw mean fibre length, after HAPT for 60 minutes 0.010.11101000.02 0.2 2Stress [MPa]Strain [%]2.555 mm2.231 mm1.293 mm0.625 mm0.295 mm01101000.02 0.2 2Stress [MPa]Strain [%]2.555 mm2.231 mm1.293 mm0.625 mm0.295 mm52 Figure 4.25 (a) Optical microscopy image of the fracture line for the Lw = 2.555 mm (control) showing fibres withdrawn from the matrix; (b) Optical microscopy image of the fracture line for the Lw = 0.625 mm (control) showing cleaved/broken fibres (a) (b) 53 4.3.5 Acid Group Content In Figure 4.26 (a), the initial weak acid content present in each control pulp fraction is constant. These are the carboxylic acid groups attributed to the hemicelluloses. Acid group generation after HAPT is greater in the shorter less-coarse fibres, likely due to their higher lignin content resulting in higher formation of muconic acid groups. Figure 4.26 (b) shows that acid group generation in the fibres following HAPT causes an increase in the tensile strength. The increase in acid content promotes fibre swelling (Figure 4.26 (c)) and flexibility, and increases the bonded area between adjacent fibres, thereby improving the tensile strength of handsheets. Figure 4.26 (d) shows that the tensile strength increases as the bulk decreases for both pulps, due to the increase in bonded strength of the fibres. 54 Figure 4.26 After 60 minutes: (a) Acid group generation resulting from HAPT is greater in the shorter less-coarse fibres; (b) Acid group generation leads to tensile strength development; (c) Increase in bulk of HAPT fibres is due to fibre swelling; (d) Tensile strength increases as bulk decreases for both pulps. The error bars for tensile strength are based on standard deviations for up to 10 replicates. 60901201501802100.05 0.1 0.15 0.2 0.25Acid Group Content [mmol/kg]Coarseness [mg/m]HAPTControl(a)2.562.231.290.630.302.562.231.290.630.3060901201501802100 1 2 3 4 5 6 7 8Acid Group Content [mmol/kg]Tensile Strength [km]HAPTControl00.511.522.530.05 0.1 0.15 0.2 0.25Bulk [cm3/g]Coarseness [mg/m]HAPTControl00.511.522.530 2 4 6 8Bulk [cm3/g]Tensile Strength [km]HAPTControl(d)(c) 55 Chapter 5. CONCLUSIONS AND RECOMMENDATIONS This research project set out to understand the impact HAPT has on the development of TMP properties, with aim of developing economically viable processes that combine HAPT with LC refining to produce diversified pulp grades. To that end, this thesis examines the correlation between tensile strength development and acid generation resulting from the treatment of TMP with highly alkaline peroxide. All treatments were conducted with 6% sodium hydroxide, 4% hydrogen peroxide and 3% sodium silicate charges (based on OD weight), at 10% consistency and 65°C. The effects of varying the pulp compositions (fibre and fines % w/w) and treatment times, on pulp properties (optical, strength and acid group content), were evaluated. 5.1 Fines Content The pulp composition was varied by adjusting the fines content as follows: 0%, 30%, 50%, 70% and 100%; counterbalancing with fibres. Each pulp mixture was treated for 60 minutes. The residual peroxide decreases as the fines content increases indicating that the fines consume more peroxide on account of their larger specific surface area and higher lignin content. The decrease in brightness with increased fines content, is driven by the increase in absorption coefficient, attributed to the increased lignin content. Similarly, the increase in opacity with increased fines content, is driven by the increase in scattering coefficient and absorption coefficient, attributed to the larger specific surface area of the fines. The tear index, burst index and tensile strength all decrease as the fines content increases, on account of the reduced fibre content and fibre â fibre bonding leading to localized stress; which is corroborated by the decrease in maximum failure strain with increased fines content. The acid 56 group content increases as the fines content increases, due to the finesâ larger specific surface area and higher lignin content. A 16% increase in acid group content from 0% fines (fibres only) to 30% fines (whole pulp) causes a 2% increase in tensile strength, suggesting that the influence of mean fibre length and strength on tensile is more significant than that of acid group content. 5.2 Fibres Only The pulp composition was also varied by removing the fines from the whole pulp (identified as fibres only). The fibres only and parallel whole pulp samples were treated for up to 200 minutes. During the first 30 minutes of HAPT, the increase in final pH with decreasing residual peroxide is attributed to peroxide consumption in the reaction. These changes occurred more rapidly in the whole pulp due to the presence of fines, which have larger specific surface area and higher lignin content. After 120 minutes, the decrease in final pH with constant residual peroxide is attributed to the consumption of alkali by the acid content generated within the fibre walls. The increase in pulp yield loss during the first 30 minutes is attributed to the extraction of lower molecular weight acidic components, which is higher for the whole pulp due to the finesâ faster consumption of alkali. Brightness develops within the first 30 minutes of the reaction, and is higher for the whole pulp because its absorption coefficient decreases more rapidly, due to the fines present. The scattering coefficient and consequent opacity, are higher for the whole pulp due to the presence of fines, whose large specific surface area increase the unbonded area. The tear indices for both the fibres and whole pulps fluctuate over treatment time, on account of the interplay between the bonded area, fibre â fibre bonding, fibre length and strength properties. The burst indices for the whole 57 pulps are more than 3 times higher than those for the fibres, due to the presence of fines, which enhance fibre â fibre bonding and sheet densification. Tensile strength develops more rapidly for the whole pulp within the first 30 minutes of HAPT, which corresponds to the acid group generation; indicating that the strength gains and acid group generation occur concurrently for consistent pulp compositions. 5.3 Fibre Length and Coarseness Finally, the pulp composition was varied by fractionating the whole pulp fibres using the Bauer McNett, and discarding the fines. Each fibre fraction was identified by its mean fibre length (Lw) and coarseness. The fibre fractions were treated for 60 minutes (HAPT), and contrasted with incubated untreated fibre fractions (control). The increase in residual peroxide with increased coarseness indicates that peroxide consumption is faster in the shorter less-coarse fibres, on account of their larger specific surface area and thinner cell walls. The increase in pulp yield loss with increased fibre length is attributed to the removal of extractives (low molecular weight polar compounds), which are more prevalent in the shorter less-coarse fibres. The decrease in brightness with increased fibre length for both the HAPT and control pulps is driven by the decrease in absorption coefficient, likely attributable to decrease in lignin content with increased fibre length. The scattering coefficient behaves similarly for both the control and HAPT pulps, indicating that the bonded areas are similar; therefore, the strength grains are attributed to the increase in bond strength resulting from HAPT. 58 The tear indices increase with increasing fibre length for both the HAPT and control pulps, because the tear resistance depends on the fibre length [cubed]. The tensile strength development is highest for the shortest least-coarse fibres, likely due to the greater extent of reaction with peroxide on account of their higher lignin content. This is mirrored by the increase in elastic modulus for the HAPT pulp, as the fibre length decreases. The generation of acid groups increases as the coarseness decreases, likewise due to the greater extent of reaction with peroxide on account of the increasing specific surface area and lignin content. 5.4 Limitations and Recommendations Repeated rinsing with vacuum filtration likely strained away some the finest materials for the pulp compositions containing fines, despite filtrate recirculation. This material loss mostly impacted the pulp mixtures with the highest fines content. In addition, maintaining uniformity in the handsheets became more difficult as the fines content increased. Handsheets containing 70% and 100% fines proved to be the most challenging to separate from the blotters and pressing discs, after pressing and drying, respectively. The subsequent surface unevenness likely affected the thickness accuracy and handsheet properties. The probable presence of fines in the HAPT filtrate (effluent) used to measure the COD and later determine pulp yield loss, misrepresented the amount of organic material present. This was most evident for the 90-minute and 120-minute treatment times of whole pulp. Immediately after terminating the reaction, the final pH and residual peroxide of the HAPT filtrate is measured as to minimize the pH variation caused by alkali consumption and peroxide decomposition. Recycling 59 the remaining HAPT filtrate in order to recover the strained fines, would result in a more accurate estimation of the yield loss. Obtaining sufficient amounts of the shortest (200 mesh sieve) and longest (14 mesh sieve) fibre fractions using the Bauer McNett required multiple runs since these fibre fractions were the scarcest (7.1 and 2.6% w/w, respectively). Consequently, the experiments conducted for these outlying fibre fractions began with less than the standard 25 g OD pulp. This reduced stock material eventually produced less handsheets; therefore, the evaluated pulp properties were derived from fewer tests. The material and time constraints which brought about these limitations could be readily addressed in further studies. In general, this research project assessed the absence, presence and quantity of fines in TMP treatment with highly alkaline peroxide. As such, the characterization of modified/treated and unmodified/untreated fines has been critical in this research. 0.07 mm is the lower bound of any given sample size that can be analyzed using the fibre quality analyzer (FQA), which excludes the majority of fines (Lw â¤ 0.076 mm). Moreover, the FQA is not designed to detect and/or quantify lignin, which constitutes 20 â 25% w/w of TMP fines. For these reasons, the FQA was unsuitable for determining the morphology of fines and in turn, pulp compositions containing fines. These restrictions do not apply to the L&W Fibre Tester, which can be used to categorize fibre and fines (Lw â¤ 0.004 mm) properties into mean weighted values and statistical distributions of length, width, shape factor, fibrillar and non-fibrillar fines, and macro fibrillations. 60 Throughout the duration of this research, the 30% w/w hydrogen peroxide (lab grade) solution from which peroxide was sourced, was stored in a refrigerator to slow its decomposition. However, the inevitable decomposition of the reagent to water and oxygen diluted its original concentration over time. This discrepancy in concentration affected the peroxide charge achieved in HAPT experiments which were performed near the end of this research, such as experiments to bridge the gaps between data points and repetitions. For future studies that use the same source of hydrogen peroxide over a prolonged period, it is recommended that the concentration of hydrogen peroxide be routinely determined prior to use, by titrating with a standard potassium permanganate solution. This precautionary measure will improve accuracy in achieving the target peroxide charge through adjustments made beforehand; thereby minimizing potential inconsistencies in pulp treatments and subsequent fluctuations in paper properties. The Klason Lignin Method, used in this research, gravimetrically determines the content of lignin in pulp by hydrolyzing the carbohydrates in sulfuric acid, leaving acid insoluble residue. This method is susceptible to error in a several ways, namely, the incomplete removal of extractives from pulp, the loss of the fines during extractivesâ removal, unaccounted protein, lignin solubilization, and human error. The lignin analysis in prospective studies can be supplemented with a reliable and less labor-intensive method such as Fourier Transform Infrared Spectroscopy (FTIR), which identifies the characteristic absorption bands in the infrared spectrum corresponding to the chemical structure of lignin, together with Partial Least Squares (PLS) regression (chemometric method which relates the FTIR spectra to lignin content). 61 5.5 Future Work The findings of this research trace the effect TMP fines have on the development of pulp properties as a result of HAPT. Specifically, the absence, presence and quantity of fines in the pulp compositions, have been shown to influence the development of tensile strength in dynamic ways. The mechanical pulp properties developed using HAPT, can be modified by adjusting the TMP fines content to achieve low bulk and high tensile pulps, which are better suited for board and packaging grades. Further studies in which the HAPT conditions (charges, pH, temperature) are varied with respect to fines content, can contribute greatly towards the optimization of HAPT for the development of diversified pulp grades. The effects of 2-stage peroxide brightening at lower charges, in developing tensile strength while curbing brightening, could be investigated to eliminate the need for dying the treated pulp. This research concludes the third phase of the ERMP program. During the third phase of the program, HAPT was successfully trialed at Holmen Hallstaâs paper mill (Sweden); that is, this technology has demonstrated readiness (TRL 6). In anticipation of the programâs next phase, preliminary experimental results for the combined treatment of TMP with HAPT and enzymatic hydrolysis coupled with refining, have demonstrated feasibility (TRL 1). Future research on the synergistic effects of these combined treatments, could investigate the sequence, reaction conditions and enzyme dosages that cost-effectively improve pulp properties. 62 Bibliography Ajao, O., Jeaidi, J., Benali, M., Restrepo, A. M., El Mehdi, N., & Boumghar, Y. (2018). Quantification and Variability Analysis of Lignin Optical Properties for Colour-Dependent Industrial Applications. Molecules (Basel, Switzerland), 23(2), 377. https://doi.org/10.3390/molecules23020377 Bajpai, P. (2018). Biermann's handbook of pulp and paper.: (raw material and pulp making) (Third ed.). Elsevier. Benouadah, N., Aliouche, D., Pranovich, A., & Willför, S. (2019). Chemical characterization of pinus halepensis sapwood and heartwood. Wood Material Science and Engineering, 14(3), 157-164. https://doi.org/10.1080/17480272.2018.1448436 Chang, X. F., Bridges, C., Vu, D., Kuan, D., Kuang, L., Olson, J. A., Luukonen, A., & Beatson, R. P., (2011). Saving electrical energy by alkaline peroxide pretreatment of TMP prior to low consistency refining. Journal of Pulp and Paper Science. Chang, X. F., Luukkonen, A., Olson, J. A., & Beatson, R. P. (2016). Pilot-scale investigation into the effects of alkaline peroxide pre-treatments on low-consistency refining of primary refined softwood TMP. Bioresources, 11(1), 2030-2042. https://doi.org/10.15376/biores.11.1.2030-2042 Fischer, W. J., Mayr, M., Spirk, S., Reishofer, D., Jagiello, L. A., Schmiedt, R., Colson, J., Zankel, A., & Bauer, W. (2017). Pulp fines-characterization, sheet formation, and comparison to microfibrillated cellulose. Polymers, 9(8). Göran, G., Monica, E., & Gunnar, H. (2009). Production of bleaching chemicals at the mill. In M. Ek, G. Gellerstedt & G. Henriksson (Eds.), Pulp and paper chemistry and technology - pulping chemistry and technology, volume 2 (pp. 1-1). De Gruyter. https://doi.org/10.1515/9783110213423 63 Guo, T. X., Zhao, Y., Ma, S. C., & Liu, S. T. (2012). Decomposition Characteristics of Hydrogen Peroxide in Sodium Hydroxide Solution. Advanced Materials Research, 610â613, 359â362. https://doi.org/10.4028/www.scientific.net/amr.610-613.359 Hafrén, J., Fernando, D., Gorski, D., Daniel, G., & Salomons, F. A. (2014). Fiber- and fine fractions-derived effects on pulp quality as a result of mechanical pulp refining consistency. 109 Wood Science and Technology, 48(4), 737â753. https://doi.org/10.1007/s00226-014-0636-1 Harrison, V. G. W. (2018). Optical properties of paper. In The Formation and Structure of Paper, Trans. of the IInd Fund. Res. Symp. Oxford, 1961, (F. Bolam, ed.), pp 467â485, FRC, Manchester, DOI: 10.15376/frc.1961.1.467. Heitner, H. (1993). Light-induced yellowing of wood- containing papers. In: Heitner, C. and Scaiano, J.C. Eds., Photochemical of Lignocelluosic Materials, ASC Symposium Series 531, 1, 2-25. Herrera, M., Thitiwutthisakul, K., Yang, X., Rujitanaroj, P.-on, Rojas, R., & Berglund, L. (2018). Preparation and evaluation of high-lignin content cellulose nanofibrils from eucalyptus pulp. SpringerLink. https://link.springer.com/article/10.1007/s10570-018-1764-9 Huang, C., Chen, Y., Zhao, Y., Xue, D., Li, C., & Zhu, M. (2017). Physical strength improvement of eucalyptus alkaline hydrogen peroxide mechanical pulp by low-temperature plasma treatment. Bioresources, 12(4), 9075-9085. https://doi.org/10.15376/biores.12.4.9075-9085 Johansson, C. I., Beatson, R. P., & Saddler, J. N. (2000). Fate and influence of western red cedar extractives in mechanical pulping. Wood Science and Technology, 34(5), 389-401. https://doi.org/10.1007/s002260000051 Kangas, H., & Kleen, M. (2004). Surface chemical and morphological properties of mechanical pulp fines. Nordic Pulp and Paper Research Journal, 19(2), 191â199. https://doi.org/10.3183/npprj-2004-19-02-p191-199 64 Kangas, H., Pöhler, T., Heikkurinen, A. and Kleen, M. (2004). Development of the mechanical pulp fibre surface as a function of refining energy. Journal of Pulp and Paper Science, 30(11), 298â306. Karlsson, A., & Agnemo, R. (2010). High consistency hydrogen peroxide bleaching of mechanical pulps with varying amounts of fines. Nordic Pulp and Paper Research Journal, 25(3), 256â268. https://doi.org/10.3183/npprj-2010-25-03-p256-268 Kleen, M., Kangas, H., & Laine, C. (2003). Chemical characterization of mechanical pulp fines and fiber surface layers. Nordic Pulp and Paper Research Journal, 18(4), 361â368. https://doi.org/10.3183/npprj-2003-18-04-p361-368 Lapierre, L., Berry, R., & Bouchard, J. (2003). The effect of magnesium ions and chelants on peroxide bleaching. Holzforschung, 57(6), 627-633. https://doi.org/10.1515/HF.2003.094 Li, M., Yin, J., Hu, L., Chen, S., Min, D., Wang, S., & Luo, L. (2020). Effect of hydrogen peroxide bleaching on anionic groups and structures of sulfonated chemo-mechanical pulp fibers. Colloids and Surfaces. A, Physicochemical and Engineering Aspects, 585, 124068. https://doi.org/10.1016/j.colsurfa.2019.124068 Li, Z., Dou, H., Fu, Y., & Qin, M. (2015). Improving the hydrogen peroxide bleaching efficiency of aspen chemithermomechanical pulp by using chitosan. Carbohydrate Polymers, 132, 430-436. https://doi.org/10.1016/j.carbpol.2015.06.062 Liu, S. (2015). A synergetic pretreatment technology for woody biomass conversion. Applied Energy, 144:114â128. Luukko, K., & Paulapuro, H. (1998). Mechanical pulp fines: Effect of particle size and shape. Tappi Journal, 82(2), 95â101. 65 Möbius, C. H. (2006). Water use and wastewater treatment in papermills. Augsburg, Germany. Herstellung und Verlag. Moldenius, S. (1984). The effects of peroxide bleaching on the strength and surface properties of mechanical pulping. Journal of Pulp and Paper Science., vol. 10, pp. 172-177. Mosbye, J., Laine, U., & Moe, S. (2003). The effect of dissolved substances on the adsorption of colloidal extractives to fines in mechanical pulp. Nordic Pulp and Paper Research Journal, 18(1), 63â68. https://doi.org/10.3183/npprj-2003-18-01-p063-068 Motamedian, H. R., Halilovic, A. E., & Kulachenko, A. (2019). Mechanisms of strength and stiffness improvement of paper after PFI refining with a focus on the effect of fines. Cellulose, 26(6), 4099â4124. https://doi.org/10.1007/s10570-019-02349-5 Odabas, N., Henniges, U., Potthast, A., & Rosenau, T. (2016). Cellulosic fines: Properties and effects. Progress in Materials Science, 83, 574â594. https://doi.org/10.1016/j.pmatsci.2016.07.006 Page, D. H. (1969). A theory for the tensile strngth of paper. Tappi Journal, 52((4)), 674â681. Pan, G. X. (2004). Relationship between dissolution of fibre materials and development of pulp strength in alkaline peroxide bleaching of mechanical pulp. Holzforschung, 58(4), 369-375. https://doi.org/10.1515/HF.2004.056 Pan, H. Y. G. (2018). Measurement of dissolved organic substances as a predictive tool to monitor pulp qualities in the alkaline peroxide bleaching of aspen mechanical pulp. Pulp and Paper Canada. http://www.pulpandpapercanada.com/paptac/PDFs/Jun04/PulpProperties2.pdf Peneder, N. (2021). What is microfibrillated cellulose (MFC)? Weidmann Fiber Technology. https://weidmannfibertechnology.com/what-is-mfc/ 66 Pettersson, G., Norgren, S., Engstrand, P., Rundlöf, M., & Höglund, H. (2021). Aspects on bond strength in sheet structures from TMP and CTMP â A review. Nordic Pulp and Paper Research Journal, 36(2), 177â213. https://doi.org/10.1515/npprj-2021-0009 Presley, J.R. & Hill, R.T. (1996). Peroxide bleaching of (chemi) mechanical pulps, In: Dence, C.W. and Reeve, D.W. (eds.), Pulp Bleaching: Principles and Practice, TAPPI Press, Atlanta, GA, USA, pp. 459-489 Pulp and Paper Canada, S. (2020). Two sides releases facts on Canadian Paper Industryâs Environmental Impact. https://www.pulpandpapercanada.com/two-sides-releases-facts-on canadian-paper-industrys-environmental-impact/ Ramos, E., Calatrava, S. F., & Jiménez, L. (2008). Bleaching with hydrogen peroxide. A review. Afinidad, 65(537) Savoye, L., Petit-Conil, M., & Meyer, V. (2011). Mechanisms of TMP peroxide bleaching using mg-based alkalis. Holzforschung, 65(6), 797-803. https://doi.org/10.1515/HF.2011.109 Scheller, H. V., & Ulvskov, P. (2010). Hemicelluloses. Annual review of plant biology, 61. Seth R. S., & Page. D. H. (2018). The stress strain curve of paper. In The role of fundamental research in paper-making, Trans. of the VIIth Fund. Res. Symp. Cambridge, 1981, (Fundamental Research Committee, ed.), pp 421â452, FRC, Manchester. DOI: 10.15376/frc.1981.1.421. Sluiter, A., Hames, B., Ruiz, R., Scarlata, C., Sluiter, J. and Templeton, D. (2006). Determination of Sugars, Byproducts, and Degradation Products in Liquid Fraction Process Samples. Golden, CO: National Renewable Energy Laboratory. NREL/TP-510-42623. Sluiter, A., Hames, B., Ruiz, R., Scarlata, C., Sluiter, J., Templeton, D. and Crocker, D. (2012). Determination of Structural Carbohydrates and Lignin in Biomass. Golden, CO: National Renewable Energy Laboratory. NREL/TP-510-42618 67 Sluiter, A., Ruiz, R., Scarlata, C., Sluiter, J. and Templeton, D. (2005). Determination of Extractives in Biomass. Golden, CO: National Renewable Energy Laboratory. NREL/TP-510 42619. Smook, G. A. (1997). Handbook for Pulp & Paper Technologists. 2nd Edition, Angus Wilde Publications Inc., Bellingham. Smook, G. A. (2016). Mechanical Pulping. In Handbook for Pulp & Paper Technologists (4th Edition) (pp. 47â60). Smook, G. A., & Kocurek, M. J. (1982). Handbook for pulp & paper technologists. Canadian Pulp and Paper Association. Sun, Y., Fenster, M., Yu, A., Berry, R. M., & Argyropoulos, D. S. (1999). The effect of metal ions on the reaction of hydrogen peroxide with kraft lignin model compounds. Canadian Journal of Chemistry, 77(5-6), 667-675. https://doi.org/10.1139/v99-036 Sundberg, K. E., Holmbom, B. R., & Pranovich, A. V. (2007). Chemical changes in thermomechanical pulp at alkaline conditions. Journal of Wood Chemistry and Technology, 23(1), 89â112. https://doi.org/10.1081/WCT-120018617 Tchepel, M. V., McDonald, J., D., & Dixon, T. (2006). The effect of peroxide bleaching on the mechanical properties of black spruce fibres. Journal of Pulp and Paper Science, 32(2), 193-200. Westermark, U., & Capretti, G. (1988). Influence of ray cells on the bleachability and properties of CTMP and kraft pulps. Nordic Pulp & Paper Research Journal, 3(2), 95â99. https://doi.org/10.3183/npprj-1988-03-02-p095-099 Wood, J., Zhang, X., Chagaev, O., Stationwala, M., Goel, K., Shallhorn, P. M., & Beaulieu, S. (2000). Effect of various mechanical and chemical treatment of ray cells on sheet properties and linting. Pulp and Paper Canada, 101(10), 83â87. 68 Xu, E. C. (2001). Chemical treatment in mechanical pulping, part 4 : H2O2 bleaching and pulp property development. Tappi Journal, 84(1) Yang, Q., Chen, Y., Yu, S., Hou, Q., Wu, M., Jiang, T., Wang, K., & Liu, W. (2022). Changes of lignin structure of poplar wood chips in autohydrolysis pretreatment and bleachability of chemi-thermomechanical pulp. Industrial Crops and Products, 176, 114420. https://doi.org/10.1016/j.indcrop.2021.114420 Zhao, Y., & Deng, Y. (2006). Improvement of peroxide bleaching yield and efficiency of TMP using glyoxal crosslink agents. Industrial & Engineering Chemistry Research, 45(17), 5813 5818. https://doi.org/10.1021/ie060234c 69 Appendix Figure A.1 Representative refractive index signal (control whole pulp sample) showing the coelution of galactose/mannose/xylose
Thesis/Dissertation
eng
Chemical and Biological Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Trajano, Heather L.
The role of fibre size in highly alkaline peroxide treatment of thermomechanical pulp
Text