Forestry, Faculty of
DSpace
UBCV
Chen, Siwei
Master of Applied Science - MASc
University of British Columbia
To enhance the sustainability of fused deposition modeling (FDM) 3D printing filaments, wood materials are emerging as eco-friendly fillers in thermoplastics. This study introduced thermomechanical pulp (TMP) fines, a by-product from industrial pulping process, as fillers in polylactic acid (PLA) for 3D printing filaments. To address the limited compatibility between hydrophilic fines and hydrophobic PLA matrices, three strategies were adapted to prevent fine agglomeration and enhance filler-matrix adhesion. Mechanical treatment using ball milling, chemical modification by (2-Dodecen-1-yl)succinic anhydride (DDSA) esterification, and additives of coupling agents (maleic-anhydride grafted polypropylene [MAPP] and maleic-anhydride grafted PLA [MAPLA]) were employed and systematically studied. Tensile, rheology, thermal analysis and printability tests were conducted to evaluate the effect of incorporating fines and the efficacy of above three methods. Further characterization by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) were used to analyze fracture surfaces and understand fine distribution. The PLA/TMP fines composites exhibited higher tensile modulus compared to pure PLA (increased from 792 MPa to 1084 MPa with a 20% w/w loading content), while it lowered the tensile strength, elongation at break and toughness. Compared to untreated TMP fines at the same loading level (20% w/w), the composites with ball-milled (Bm) fines presented a 33% increase in tensile strength, a 115% increase in elongation at break, and a 192% increase in toughness due to smaller fine particles and improved dispersion. DDSA-treated fines showed increased hydrophobicity but limited mechanical improvement. By adding MAPP or MAPLA into the PLA/TMP fines composites, both groups exhibited toughening effects, especially with MAPP. Combining MAPP with ball milling further enhanced the composites (10 % w/w untreated TMP fines in PLA) with an increase of elongation at break by 139% and toughness by 170%, demonstrating the modifications’ synergistic benefits. The PLA/TMP fines composites exhibited pronounced shear thinning behavior and reduced complex viscosity, storage modulus, and loss modulus at higher frequencies compared to PLA. While those incorporating ball-milled fines closely mirrored the rheological characteristics of pure PLA. Particularly, PLA mixed with 10% Bm fines demonstrated excellent printability, offering a sustainable 3D printing material.
THERMOMECHANICAL PULP FINES AS ADDITIVES FOR 3D PRINTING FILAMENTS by Siwei Chen B.Sc., The University of British Columbia, 2021 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 (Forestry) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) April 2024 © Siwei Chen, 2024 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled: Thermomechanical Pulp Fines as Additives for 3D Printing Filaments submitted by Siwei Chen in partial fulfilment of the requirements for the degree of Master of Applied Science in Forestry Examining Committee: Scott Renneckar, Professor, Forestry, UBC Supervisor Heather Trajano, Associate Professor, Chemical and Biological Engineering, UBC Supervisory Committee Member Feng Jiang, Associate Professor, Forestry, UBC Supervisory Committee Member Yasmine Abdin, Assistant Professor, Materials Engineering, UBC Additional Examiner iii Abstract To enhance the sustainability of fused deposition modeling (FDM) 3D printing filaments, wood materials are emerging as eco-friendly fillers in thermoplastics. This study introduced thermomechanical pulp (TMP) fines, a by-product from industrial pulping process, as fillers in polylactic acid (PLA) for 3D printing filaments. To address the limited compatibility between hydrophilic fines and hydrophobic PLA matrices, three strategies were adapted to prevent fine agglomeration and enhance filler-matrix adhesion. Mechanical treatment using ball milling, chemical modification by (2-Dodecen-1-yl)succinic anhydride (DDSA) esterification, and additives of coupling agents (maleic-anhydride grafted polypropylene [MAPP] and maleic-anhydride grafted PLA [MAPLA]) were employed and systematically studied. Tensile, rheology, thermal analysis and printability tests were conducted to evaluate the effect of incorporating fines and the efficacy of above three methods. Further characterization by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) were used to analyze fracture surfaces and understand fine distribution. The PLA/TMP fines composites exhibited higher tensile modulus compared to pure PLA (increased from 792 MPa to 1084 MPa with a 20% w/w loading content), while it lowered the tensile strength, elongation at break and toughness. Compared to untreated TMP fines at the same loading level (20% w/w), the composites with ball-milled (Bm) fines presented a 33% increase in tensile strength, a 115% increase in elongation at break, and a 192% increase in toughness due to smaller fine particles and improved dispersion. DDSA-treated fines showed increased hydrophobicity but limited mechanical improvement. By adding MAPP or MAPLA into the PLA/TMP fines composites, both groups exhibited toughening effects, especially with iv MAPP. Combining MAPP with ball milling further enhanced the composites (10 % w/w untreated TMP fines in PLA) with an increase of elongation at break by 139% and toughness by 170%, demonstrating the modifications’ synergistic benefits. The PLA/TMP fines composites exhibited pronounced shear thinning behavior and reduced complex viscosity, storage modulus, and loss modulus at higher frequencies compared to PLA. While those incorporating ball-milled fines closely mirrored the rheological characteristics of pure PLA. Particularly, PLA mixed with 10% Bm fines demonstrated excellent printability, offering a sustainable 3D printing material. v Lay Summary This thesis investigated the innovative use of thermomechanical pulp (TMP) fines—a byproduct of pulping industry—as sustainable additives for 3D printing filaments. Traditionally, these fines are seen as waste, but this research proposed a method to enhance their value by incorporating them into polylactic acid (PLA), a biodegradable plastic, to create eco-friendly 3D printing filaments. The main objectives were to develop treatment methods for fines to ensure they mix well with PLA without forming clumps, to explore the mechanical and rheological properties of the resulting PLA/fine composites, and to assess their performance in 3D printing applications. Through physical treatment, chemical modifications and adding compatibilizers, the thesis aimed to improve the dispersion, compatibility and interfacial adhesion between fines and PLA. This research not only promoted the utilization of pulp and paper mill waste but also contributed to the development of more sustainable materials for additive manufacturing. vi Preface All of the research work reported in this thesis was planned, conducted, and analyzed by Siwei Chen in the Advanced Renewable Materials Lab in the Wood Science Department of the Faculty of Forestry at the University of British Columbia, Vancouver campus, under the supervision of Dr. Scott Renneckar. In terms of the materials and methods in Chapter 3, the characterization of TMP fines and the ball milling of fines were conducted with the help of Dr. Jie Wu and a co-op student, Nicole Ting. Dr. Jinsheng Gou and a volunteer student, Harold Renneckar, assisted me in synthesizing the MAPLA and producing the masterbatch. Dr. Jinsheng Gou, Harold Renneckar, Nicole Ting, Jason Huang, and Kieran Daniels assisted me with compounding, injection molding, and filament making. In Chapter 4, the SEM images were obtained by Dr. Tao Zou, and the CLSM images by Dr. Gao Guang, with the samples prepared by me. vii Table of Contents Abstract ......................................................................................................................................... iii Lay Summary .................................................................................................................................v Preface ........................................................................................................................................... vi Table of Contents ........................................................................................................................ vii List of Tables ..................................................................................................................................x List of Figures ............................................................................................................................... xi List of Abbreviations ...................................................................................................................xv Acknowledgements .................................................................................................................. xviii Dedication ................................................................................................................................... xix Chapter 1: Introduction ................................................................................................................1 1.1 What are TMP fines and how can they be utilized in printing filaments? ...................... 1 1.2 Thesis objectives ............................................................................................................. 2 Chapter 2: Literature review ........................................................................................................3 2.1 Developments in mechanical pulping ............................................................................. 3 2.1.1 Thermomechanical pulping process ........................................................................... 4 2.1.2 Origins of TMP fines .................................................................................................. 5 2.1.3 Morphology and composition of fines and their effects on papermaking .................. 7 2.2 Introduction of poly(lactic acid) (PLA) ........................................................................ 10 2.2.1 Production of PLA .................................................................................................... 10 2.2.2 High molecular weight PLA ..................................................................................... 12 2.2.3 Properties of PLA ..................................................................................................... 14 viii 2.2.4 Applications and environmental impact of PLA ...................................................... 16 2.3 Literature review of PLA/Wood flour composites ....................................................... 18 Chapter 3: Materials and experimental methods .....................................................................38 3.1 Materials ....................................................................................................................... 38 3.2 Approaches to enhance the mechanical performance of PLA/Fine composites ........... 39 3.2.1 Physical treatment of fines ........................................................................................ 39 3.2.2 Chemical modification of fines ................................................................................. 40 3.2.3 Coupling agent incorporation ................................................................................... 41 3.3 Manufacturing of PLA/fine composites ........................................................................ 42 3.4 Characterization methods.............................................................................................. 44 3.4.1 Fines composition analysis ....................................................................................... 44 3.4.1.1 Moisture content measurement ......................................................................... 44 3.4.1.2 Extractive analysis ............................................................................................ 45 3.4.1.3 Klason analysis ................................................................................................. 45 3.4.2 Fourier transform infrared (FTIR) spectroscopy ...................................................... 46 3.4.3 Water contact angle (WCA) measurement ............................................................... 47 3.4.4 Thermogravimetric analysis (TGA) .......................................................................... 47 3.4.5 1H Nuclear magnetic resonance (NMR) analysis ..................................................... 48 3.4.6 Particle size analysis ................................................................................................. 48 3.4.7 Tensile testing ........................................................................................................... 49 3.4.8 Scanning electron microscopy (SEM) ...................................................................... 50 3.4.9 Confocal laser scanning microscopy (CLSM) .......................................................... 51 3.4.10 Differential scanning calorimetry (DSC) analysis ................................................ 51 ix 3.4.11 Rheometry ............................................................................................................. 52 3.4.12 Printability test ...................................................................................................... 52 Chapter 4: Results and Discussion .............................................................................................54 4.1 Fine treatment (mechanical and chemical) and MAPLA synthesis .............................. 54 4.1.1 The morphology and particle size analysis of thermomechanical pulp fines, before and after ball milling ............................................................................................................. 54 4.1.2 Modification of fines using (2-Dodecen-1-yl)succinic anhydride (DDSA) ............. 56 4.1.3 The synthesis of MAPLA ......................................................................................... 60 4.2 The properties of PLA/fine composites ........................................................................ 62 4.2.1 Visual appearance of PLA/fine composites .............................................................. 62 4.2.2 Tensile properties of PLA/fine composites ............................................................... 64 4.2.3 The fracture surface of PLA/fine composites ........................................................... 75 4.2.4 Fines distribution of PLA/fine composites ............................................................... 80 4.2.5 DSC analysis of PLA/fine composites ...................................................................... 82 4.2.6 Rheological properties of PLA/fine composites ....................................................... 85 4.2.7 3D printing test ......................................................................................................... 92 Chapter 5: Conclusions and future work ..................................................................................95 5.1 Conclusions ................................................................................................................... 95 5.2 Impact of research and future studies ........................................................................... 98 Bibliography .................................................................................................................................99 Appendices ..................................................................................................................................133 x List of Tables Table 2.1 Summary of treatments for PLA/wood flour (WF) biocomposites and their effects ... 22 Table 3.1 The fundamental composition of TMP fines determined by Klason analysis .............. 38 Table 3.2 The composition and treatment of PLA/fine composites (Bm: ball-milled, DDSA: (2-dodecen-1-yl)succinic anhydride, MAPP: maleic anhydride-grafted polypropylene, MAPLA: maleic anhydride-grafted PLA, MTMP: TMP fines masterbatch) ............................................... 42 Table 4.1 The particle size metrics of TMP fines before and after ball milling ........................... 55 Table 4.2 Mechanical properties of the neat PLA and PLA/fine composites from tensile tests with statistical analysis (Bm: ball-milled, DDSA: (2-dodecen-1-yl)succinic anhydride, MAPP: maleic anhydride-grafted polypropylene, MAPLA: maleic anhydride-grafted PLA, MTMP: TMP fines masterbatch. *Values marked with different letters are considerably different at an α level of 0.05.) ......................................................................................................................................... 65 Table 4.3 Thermal characteristics of neat PLA and PLA/fine composites (Bm: ball-milled, DDSA: (2-dodecen-1-yl)succinic anhydride, MAPP: maleic anhydride-grafted polypropylene, MAPLA: maleic anhydride-grafted PLA) .................................................................................... 84 xi List of Figures Figure 2.1 The origins and processing paths of fines (Reprinted from Progress in Materials Science, 83, Odabas, N., Henniges, U., Potthast, A., & Rosenau, T., Cellulosic fines: Properties and effects, 574-594, Copyright (2016), with permission from Elsevier) ...................................... 6 Figure 2.2 Cell wall structure of softwood tracheids and hardwood libriform fibres, and the morphology of the fines derived from these structures (Reprinted from Progress in Materials Science, 83, Odabas, N., Henniges, U., Potthast, A., & Rosenau, T., Cellulosic fines: Properties and effects, 574-594, Copyright (2016), with permission from Elsevier) ...................................... 7 Figure 3.1 Scheme of maleic anhydride grafted PLA (MAPLA) and maleic anhydride grafted polypropylene (MAPP) modification at the interface of PLA and fines .......................... 41 Figure 3.2 Illustration of the various fines’ treatments and PLA/fine composites preparation methods ......................................................................................................................................... 44 Figure 3.3 Tension test specimens for molded plastics ................................................................ 49 Figure 4.1 Microscope images of (a-b) TMP fines and (c-d) ball-milled (Bm) fines; (e) Particle size distribution of fines with water contact angle ........................................................................ 56 Figure 4.2 (a) Reaction scheme of (2-dodecen-1-yl)succinic anhydride (DDSA) esterification of fines; (b-c) characterization of TMP fines and DDSA fines: (b) FT-IR spectrum with water contact angle; (c) TGA analysis .................................................................................................... 59 Figure 4.3 (a) Scheme of reaction between maleic anhydride (MA) and PLA; 1H NMR spectra of (b) PLA and (c) MA-grafted PLA (MAPLA) ............................................................................... 62 Figure 4.4 Imagery of injection molded pure PLA and PLA/fine composites: (a) PLA; (b) 10TMP; (c) 10Bm; (d) 10DDSA; (e) 10TMP_MAPP; (f) 10TMP_MAPLA; (g) 20TMP; (h) 20MTMP; (i) 20Bm; (j) 30Bm; (k) 10Bm_MAPP; (l) 10Bm_MAPLA (Bm: ball-milled, DDSA: xii (2-dodecen-1-yl)succinic anhydride, MAPP: maleic anhydride-grafted polypropylene, MAPLA: maleic anhydride-grafted PLA, MTMP: TMP fines masterbatch) ............................................... 63 Figure 4.5 Three treatments applied to improve the mechanical performance of PLA composites with fines ....................................................................................................................................... 66 Figure 4.6 (a) Representative stress-strain curves; (b) strength and modulus for neat PLA and PLA composites containing 10% TMP fines, ball-milled (Bm) fines, (2-dodecen-1-yl)succinic anhydride (DDSA) fines and TMP fines with coupling agents .................................................... 67 Figure 4.7 Tensile properties for neat PLA and PLA composites with coupling agents .............. 69 Figure 4.8 Two physical methods to optimize the fine-loading content and combining ball milling and coupling agents to further improve the mechanical performance of PLA composites with fines ....................................................................................................................................... 70 Figure 4.9 (a) Representative stress-strain curves; (b) strength and modulus for PLA composites containing TMP fines and ball-milled (Bm) fines at various loading content and 10% Bm fines with coupling agents ..................................................................................................................... 72 Figure 4.10 Elongation at break and toughness for PLA composites containing 10% TMP fines and 10% ball-milled (Bm) fines with coupling agents, TMP fines and Bm fines at various loading content .............................................................................................................................. 74 Figure 4.11 Tensile properties for PLA composites containing 20% ball-milled (Bm) fines and the composites with coupling agents ............................................................................................ 75 Figure 4.12 Low and high magnification SEM images of the fracture surface of (a, d) neat PLA; PLA composites containing (b, e) 10% TMP fines; (c, f) 10% ball-milled (Bm) fines ............... 77 Figure 4.13 Low and high magnification SEM images of the fracture surface of PLA composites containing (a, d) 10% (2-dodecen-1-yl)succinic anhydride (DDSA) fines; (b, e) 10% TMP fines xiii with maleic anhydride-grafted maleic anhydride-grafted polypropylene (MAPP); (c, f) 10% TMP fines with maleic anhydride-grafted PLA (MAPLA) ................................................................... 78 Figure 4.14 Low and high magnification SEM images of the fracture surface of PLA composites containing (a, d) 10% ball-milled (Bm) fines; (b, e) 10% Bm fines with maleic anhydride-grafted polypropylene (MAPP); (c, f) 10% Bm fines with maleic anhydride-grafted PLA (MAPLA) ... 80 Figure 4.15 CLSM images of (a) PLA; (b) 10TMP; (c) 10Bm; (d) 10DDSA; (e) 10TMP_ MAPP; (f) 10TMP_ MAPLA; (g) 20TMP; (h) 10Bm_ MAPP; (i) 10Bm_ MAPLA (Bm: ball-milled, DDSA: (2-dodecen-1-yl)succinic anhydride, MAPP: maleic anhydride-grafted polypropylene, MAPLA: maleic anhydride-grafted PLA) .................................................................................... 82 Figure 4.16 Melting curves of neat PLA and PLA/fine composites (first heating cycle) ............ 84 Figure 4.17 Complex viscosity as a function of frequency at 190 °C for neat PLA and PLA composites containing 10% TMP fines, ball-milled (Bm) fines, (2-dodecen-1-yl)succinic anhydride (DDSA) fines and TMP fines with coupling agents .................................................... 87 Figure 4.18 Complex viscosity as a function of frequency at 190 °C for PLA composites containing TMP fines and ball-milled (Bm) fines at various loading content and 10% Bm fines with coupling agents ..................................................................................................................... 89 Figure 4.19 Storage modulus as a function of frequency at 190 °C for neat PLA and PLA/fine composites..................................................................................................................................... 90 Figure 4.20 Loss modulus as a function of frequency at 190 °C for neat PLA and PLA/fine composites..................................................................................................................................... 90 Figure 4.21 Tan delta as a function of frequency at 190 °C for neat PLA and PLA/fine composites..................................................................................................................................... 92 xiv Figure 4.22 3D printed samples by PLA/10% ball-milled (Bm) fines filaments using different precision (Draft, Speed, Quality) and speed parameters ............................................................... 93 Figure 4.23 SEM images of laminated surface and fracture surface of the 3D-printed samples made of (a, b) pure PLA and (c, d) PLA with 10% ball-milled (Bm) fines ................................. 94 Figure A.1 TGA analysis of PLA ............................................................................................... 133 xv List of Abbreviations AAH Acetic anhydride AM Additive manufacturing ANOVA Analysis of variance Bm Ball-milled CLSM Confocal laser scanning microscopy COOH Carboxylic acid group CTMP Chemi-thermomechanical pulping 3D Three-dimensional DCP Dicumyl peroxide DDSA (2-Dodecen-1-yl)succinic anhydride DSC Differential scanning calorimetry Dv10 The particle size at 10% of the overall particle volume Dv50 The particle size at 50% of the overall particle volume Dv90 The particle size at 90% of the overall particle volume D [4,3] The weighted mean value of particle size by volume FDM Fused deposition modeling FTIR Fourier-transform infrared spectroscopy GMA Glycidyl methacrylate 1H NMR Hydrogen-1 nuclear magnetic resonance HMW High molecular weight MA Maleic anhydride xvi MA-EPDM Maleic anhydride-grafted ethylene propylene diene monomer MAPLA Maleic anhydride-grafted poly(lactic acid) MAPP Maleic anhydride-grafted polypropylene MDI Methylenediphenyl-diisocyanate NMR Nuclear magnetic resonance spectroscopy OH Hydroxyl group PEAA Poly (ethyleneacrylic acid) PET Polyethylene terephthalate PLA Poly(lactic acid) PP Polypropylene PS Polystyrene RMP Refiner mechanical pulping SBS Styrene-butadiene-styrene SEM Scanning electron microscopy TBC Tributyl citrate TGA Thermalgravimetric analysis Tcc Cold crystallization peak temperature Tg Glass transition temperature Tm Melting temperature Td5% Temperature at 5% weight loss Tmax Derivative weight peak temperature TMP Thermo-mechanical pulp/pulping xvii UV-Vis Ultraviolet-visible WCA Water contact angle WF Wood flour WPG Weight percentage gain Wt. Weight Xc Degree of crystallinity ΔHcc Cold crystallization enthalpy ΔHm Melting enthalpy ΔH°m Melting enthalpy for fully crystalline PLA xviii Acknowledgements My heartfelt thanks go to the faculty, staff, and my friends at UBC for inspiring me to pursue my studies in Wood Science. I am deeply grateful to my advisor, Dr. Scott Renneckar, for his invaluable guidance and encouragement throughout my master program. I offer my enduring gratitude to my committee members: Dr. Heather Trajano, for her mentorship during the planning stages of my research and in focusing on specific topics; Dr. Feng Jiang, for posing great ideas and critical questions that enriched my project; and Dr. Yasmine Abdin, for her kind support when I ventured into a new area with my project. I also want to express my sincere thanks to the UBC Energy Reduction in Mechanical Pulping (ERMP) team and our industrial partners for supplying materials, offering suggestions, and providing the financial backing necessary for my research. I am immensely grateful for the invaluable help and insights provided by Dr. Jie Wu, Dr. Muzaffer Karaaslan, Dr. Tao Zou, Dr. Jinsheng Gou, Qi Hua, and Huaiyu Zhang. Their expertise in operating various instruments and their thoughtful advice on interpreting results and analyses have been instrumental in my work. Additionally, I am thankful to my former mentor, Dr. Liyang Liu, for introducing me to the field of biomass materials and equipping me with the foundational lab skills vital for my research endeavors. I must extend my appreciation to the current members of the BiCMat group and the Abdin Lab, as well as to Dr. Gao Guang and Saeid Soltanian for their professional training and prompt responses to my queries, enabling me to acquire reasonable data and achieve meaningful results. Lastly, I thank all my teammates whose assistance was crucial in completing this project. Without your support, it would not have been possible to bring it to fruition. xix Dedication To my beloved parents, thank you for your unwavering love, belief, and support. Your faith in me has been the foundation upon which I’ve built my dreams. To my dear fiancé, thank you for being my rock and my soft place to fall. Your love has been a source of strength and inspiration, pushing me to be the best version of myself. To my cherished friends, thank you for the laughter, the memories, and the comfort of knowing that I am never alone. As I stand on the brink of this new chapter in my life, I carry with me the lessons you’ve taught me, the love you’ve shared, and the strength you’ve instilled. This dedication is but a small token of my gratitude for the roles you’ve played in my story. Here’s to the journey ahead, filled with love, growth, and endless possibilities. With all my heart, thank you. 1 Chapter 1: Introduction 1.1 What are TMP fines and how can they be utilized in printing filaments? Pulping refers to an operation in which wood or other similar fibrous material is transformed into a mass of fibres for the production of paper products. During this process, the bonds holding plant cells together methodically break down, a task that can be achieved through chemical, mechanical, thermal, and/or a combination of approaches such as chemi-thermomechanical pulping (CTMP). This thesis primarily focuses on thermomechanical pulping (TMP) and relevant modifications based on this method. Further, during mechanical pulping small fragments of cell walls along with short ray cells are liberated that are classified as “fines”. As described below, fines are heterogenous materials that can be isolated from waste water streams, reducing the amount of organic effluent from a paper mill. Because of their size, they cannot form paper sheets by themselves. This thesis explores alternative applications for fines. It is expected these materials may be ideal additive to lower the cost and enhance the sustainability of thermoplastics. While many applications require long fibres for reinforcement, newer processing methods such as additive manufacturing (AM), including three-dimensional (3D) printing using a fused deposition modeling (FDM) method, has other requirements including strict viscosity parameters and limitations on additive size to prevent blockage in printing nozzles. Fines would provide an avenue that limits blockage if there is no agglomeration of the particles. Yet fines recovered in the wet-state need to be dried and dispersed. Hence, a challenge is to develop methods to effectively disperse the particles in a polymeric matrix and promote adequate adhesion at the interface of polymer and particles, while avoiding aggregates. 2 1.2 Thesis objectives At the time of this study, no mechanical pulp fine filled polylactic acid (PLA) filaments for FDM techniques had been reported in the literature. Therefore, the thesis focused on investigating the effective treatment methods for dispersion and surface modification, and relevant properties to highlight both the optimum performance and scalable methods of integrating renewable additives into filaments. The overall goal of this project was to create a printing material using TMP fines as 3D printing filament fillers by blending them with PLA and to determine the mechanical properties and rheological properties of the PLA/fine composites compared to the neat PLA. The central research objective was to compare and identify physical and chemical modification methods of the wood-based fines by evaluating the mechanical performance of the thermoplastic material. Three different methods were proposed to achieve the above goal that included: (i) ball milling fines to physically eliminate fine agglomerates and facilitate uniform dispersion within PLA; (ii) esterifying the fines with (2-Dodecen-1-yl)succinic anhydride (DDSA) to increase their hydrophobicity, thus improving the compatibility between fines and PLA; (iii) introducing coupling agents to link fines and PLA during the mixing process, enhancing the interfacial adhesion of the composites. 3 Chapter 2: Literature review 2.1 Developments in mechanical pulping The origin of mechanical pulping lies in the groundwood process, through which fibres are isolated from the wood log using a grinding stone (Smook, 2016d). This method later evolved into refiner mechanical pulping (RMP). RMP employs a refiner device with rotating discs to shred and defibrillate wood chips, and sometimes sawdust. Typically, the process involves two stages of refining: primary and secondary refining (Smook, 2016b). The refining mechanism shares some similarities with grinding, yet there are fundamental distinctions. The initial separation in grinding involves softening the lignin through the generation of heat from the friction between the stone and the wood, or between the wood pieces themselves. However, in refining, the wood chips are gradually untangled into smaller pieces, eventually forming individual fibres. The resulting fibres are stiff and twisted due to the cooling and locking of lignin within the fibres after refining, a phenomenon known as the latency effect (Pettersson et al., 2021). This not only decreases the fibre length but also limits the bonding between fibres, affecting the strength of the paper sheet. To mitigate these effects and facilitate subsequent processing of RMP, the fibres are treated in hot water for a certain period to remove the “latency” (Smook, 2016d). With advancements in mechanical pulping, contemporary RMP setups incorporate thermal or chemical treatments, such as TMP and CTMP. These treatments pre-soften the wood chips and have changed both the characteristics of the resulting fibres and the energy requirements of the process. 4 2.1.1 Thermomechanical pulping process Thermomechanical pulping (TMP), which emerged as the first significant alteration of RMP in the mid-1960s (Vena, 2005), continues to be widely used for producing high-tear pulps suitable for newsprint and board production. This process combines thermal and mechanical energies, starting with the pre-steaming of wood chips under high pressure to plasticize the lignin, followed by mechanical shear grinding against a rotating refiner plate to liberate the fibres. Initially, TMP involved pre-heating and primary refining under pressure, succeeded by secondary refining at atmospheric conditions. Modern advancements have incorporated pressurized secondary refining to improve steam recycling and heat recovery (Smook, 2016d). The application of pressurized steam softens the fibres, facilitating their release during refining, leading to fewer shives (incompletely refined fibre bundles), minimal screen rejects, and higher pulp yield. A greater percentage of long fibres of TMP leads to stronger paper compared to outputs from traditional stone ground pulping and RMP methods. Previously, to produce high-quality newsprint paper, the industry was required to blend chemical pulp with traditional mechanical pulp for reinforcement. However, the strength enhancements in TMP now allow for the production of 100% TMP newsprint, without the need for long-fibred chemical pulp (Smook, 2016d). Nevertheless, TMP still contains a small fraction of fibre fragments, unlike in chemical pulping. After pre-treatment and twice refinement, a typical TMP process involves a series of steps similar to other mechanical pulping methods. These mainly include screening, cleaning, and thickening (Smook, 2016d). Screening is a critical part of this process as not all wood converts 5 into individual fibres during pulping. This step uses perforated barriers to filter out contaminants—such as dirt, knots, shives, and oversized particles—from the high-quality fibres, ensuring uniform paper quality. Material failing to pass initially undergoes further reject refinement and screening (Smook, 2016c). The last barrier sets the minimum debris size that can be eliminated, introducing the concept of fines—particles small enough to pass through holes, typically separated with a 76 µm diameter metal plate or a #200 mesh screen (Kangas & Kleen, 2004). Despite their potential as paper material, fines are a common by-product often removed from the pulp stream due to their mixed impacts on paper quality which will be further explained in the next section. Cleaning is crucial to prevent screening equipment blockages, facilitating smooth and continuous processing. Subsequently, fibres proceed to a thickener and can undergo additional treatments like brightening before entering the paper machine. 2.1.2 Origins of TMP fines In TMP, intense mechanical forces and harsh conditions produce various lengths of pulp fibres, wood tissue elements, small fragments, and a fines fraction from wood chips. Fibres intended for papermaking must exhibit specific properties, such as conformability for even sheet formation and the ability to form durable bonds (Smook, 2016a). Fibre length and cell wall thickness are key structural characteristics influencing these properties. For example, a minimum fibre length is necessary for inter-fibre bonding, correlating with tear strength, while thick-walled fibres hinder bonding (Smook, 2016b). Although fines may resemble fibres with smaller diameters and lengths, this thesis distinguishes them from pulp fibres meant for paper products. Generated at various stages (Figure 2.1), the focus here is on pulp fines rather than chip fines, which are removed before pulping and do not mix with pulp fines (Odabas et al., 2016). 6 Figure 2.1 The origins and processing paths of fines (Reprinted from Progress in Materials Science, 83, Odabas, N., Henniges, U., Potthast, A., & Rosenau, T., Cellulosic fines: Properties and effects, 574-594, Copyright (2016), with permission from Elsevier) Pulp fines, making up 20-40% of the mechanical pulp by weight, vary in quantity based on the pulping process, affected by factors like energy consumption, wood species, and process. For instance, in a TMP process, fines make up about 30% of the pulp and increase with specific energy consumption, while the amount of TMP fines is nearly independent of the wood species (Odabas et al., 2016). Besides their quantity, the pulping conditions significantly impact fines’ morphology and chemical composition, dependent on their cell wall origin based on cell type (Krogerus & Tiikkaja, 2002; Mosbye et al., 2002). 7 2.1.3 Morphology and composition of fines and their effects on papermaking The morphology and composition of fines in the TMP process are intricate and vary significantly, reflecting the fines’ origins from different parts of the wood structure including the middle lamella, cell wall, and other cell components (e.g. ray parenchyma cells) as Figure 2.2 shows (Odabas et al., 2016). The fines can be broadly categorized into two types: fibrillar fines and non-fibrillar fines, each with distinct characteristics and implications for papermaking (Odabas et al., 2016). Figure 2.2 Cell wall structure of softwood tracheids and hardwood libriform fibres, and the morphology of the fines derived from these structures (Reprinted from Progress in Materials Science, 83, Odabas, N., Henniges, U., Potthast, A., & Rosenau, T., Cellulosic fines: Properties and effects, 574-594, Copyright (2016), with permission from Elsevier) In the production of TMP, the refining begins with the isolation of fibres from wood matrix, the loosening of middle lamella and shedding of surface material from the cell wall, where most non-fibrillar fines are generated (Kangas & Kleen, 2004). Thus, non-fibrillar fines include flake-like particles, broken cell pieces, ray cells and fragments of the middle lamella (Hafrén et al., 8 2014; Kangas, H., Pöhler, T., Heikkurinen, A. and Kleen, 2004). Compared to fibrillar fines, these types of fines are richer in lignin and other extractives, such as terpenes, fatty acids, and their derivatives, contributing to less hydrophilic nature (Mosbye et al., 2003; Obst, 1990). Among all types of fines, ray cells have the highest lignin content (Kleen et al., 2003; Westermark & Capretti, 1988) and consume more bleaching agents (Karlsson & Agnemo, 2010). Due to their larger size (Kangas & Kleen, 2004) and lower aspect ratio (Odabas et al., 2016), non-fibrillar fines tend to be less effective in enhancing paper strength and cause linting problems in paper printing processes due to their poor bonding ability (Westermark & Capretti, 1988; Wood et al., 2000). They can also impact the optical properties of paper, such as increasing opacity which is helpful for printing papers (Luukko & Paulapuro, 1998). Fines produced by peeling off the inner layers of the cell walls in the later stage of refining, particularly in rejects refining, contain a high proportion of ribbon-like and fibrillar fines (Smook, 2016d). Fibrillar fines are characterized by their thin, thread-like appearance, consisting of microfibrils and fibril bundles. The fibrillar fines originated from primary cell walls, contain pectin and small amount of protein (Kleen et al., 2003; Westermark et al., 1986). Predominantly derived from the cellulosic components of the wood fibre, particularly the secondary cell walls, fibrillar fines tend to have a higher concentration of cellulose than non-fibrillar fines (Kangas & Kleen, 2004). Their slender shape and large surface area make them a key indicator of bonding potential. Fibrillar fines can act as binding agents, filling the spaces between longer fibres and increasing the paper’s strength by improving fibre-to-fibre bond formation (Motamedian et al., 2019). The presence of fibrillar fines in the pulp is crucial for achieving a higher degree of paper 9 smoothness and density, which are desirable qualities in many paper grades (Odabas et al., 2016). The ratio of fibrillar to flake-like fines is influenced by the pulping process. For instance, higher energy input leads to a higher fibril content and thinner particles in pulps during low-consistency refining (Hafrén et al., 2014). In the following screening step, research (Luukko & Paulapuro, 1999) also indicates that screening increases fine yield but reduces fibrils among them. This could be because fine fibrils entangle in the fibre sheet retained on the mesh plate, allowing smaller flakes to accumulate readily. In most circumstances, fines in papermaking furnish are inevitable and difficult to separate. The impacts of the whole fines fraction on paper sheets were summarized by Retulainen et al. (1993) and revised by Odabas et al. (2016). Based on their summary, the influence of fines on paper’s mechanical properties is multifaceted. The presence of fines affects paper’s mechanical resistance, density, wet strength, and tensile strength, with TMP fines showing benefits in inter-fibre bonding over conventional dry-strength additives (Belle et al., 2014; Honkasalo, 2004; Vainio & Paulapuro, 2007). However, fines can also impair drainage, air permeability, and bleachability, negatively impacting paper machine productivity and increasing chemical additive consumption (Ferreira et al., 1999; Hubbe, 2002; Odabas et al., 2016; Richter, 1941; Rundlöf, 2002). The general effects of fines in papermaking are variably defined, but it is challenging to quantify their effects on paper properties and process performance, sometimes necessitating their removal (Fischer et al., 2017). 10 2.2 Introduction of poly(lactic acid) (PLA) Due to human-induced climate change and the harmful effects of global plastic pollution, developing a large-scale bio-based materials economy is essential. To replace plastics made from fossil carbon sources, bioplastics with low carbon footprints and biodegradability are needed. These can be achieved by specifically using or altering plant-based materials, a process necessary for creating new types of polymers to meet society needs (Cywar et al., 2022). According to European Bioplastics, bioplastic is classified as either biobased, biodegradable, or possessing both properties. “Biobased” indicates that the material or product is (at least partially) derived from biomass, such as corn, sugarcane, or cellulose (European Bioplastics, 2023). “Biodegradation” refers to a chemical process in which microorganisms in the environment break down materials into natural substances like water, carbon dioxide, and compost. Poly(lactic acid) (PLA) is an example of a plastic that is considered both biobased and biodegradable (Garlotta, 2019). This section of the thesis will explore PLA, covering its production, properties, and applications. 2.2.1 Production of PLA PLA uses plant-based sugar sources and combines this with fermentation and industrial chemical engineering processes to form the polymer in reactors. The production process of PLA mainly consists of three stages: extracting sugar from carbohydrate sources, fermenting the sugar to form lactic acid, and then polymerizing the lactic acids. The resulting polymer is then shaped into various forms, such as pellets, fibres, or films, by extrusion or injection molding. These 11 forms are used in various applications including medicine, agriculture, textiles, and packaging (Auras et al., 2004). The synthesis of PLA is based on lactic acid units, which can be produced either through the fermentation of carbohydrates or by chemical synthesis. Initially, chemical synthesis produced racemic lactic acid but was deemed economically unviable (Kricheldorf et al., 1996). As a result, the predominate commercially produced lactic acid worldwide is obtained through bacterial fermentation of carbohydrates, using optimized or modified strains of the genus Lactobacilli (Hartmann, 1998). A wide range of carbohydrates, such as simple sugars derived from agricultural byproducts, can be used in this fermentation process. Lactic acid is typically produced from fermented dextrose (glucose), a sugar commonly obtained from hydrolyzed corn starch (Baran & Yildirim Erbil, 2019). Alternatively, this foundational component can also be sourced from other plant sources such as potato starch, sucrose from sugarcane, and lactose from cheese whey. To extract sugar from corn, the corn is first subjected to wet milling, a mechanical process in water that separates the starch from other plant materials. This starch is then treated with acid and/or enzymes and heated, a step that hydrolyzes the starch and allows for the isolation of dextrose either through crystallization or as a liquid concentrate. Subsequently, this dextrose undergoes fermentation with proteins and other nutrients in the presence of bacteria, resulting in the production of lactic acid monomers. Finally, the lactic acid is purified, especially for use in pharmaceuticals, food derivatives, and then polymerized to form PLA polymer (Datta et al., 1995). The various polymerization methods will be introduced in detail in the following section. PLA contains a stereocenter that can have either an L or D configuration. 12 Microorganisms primarily produce L-lactic acid, making it the dominant form in PLA (Auras et al., 2004). 2.2.2 High molecular weight PLA High molecular weight (HMW) PLA, typically around 100,000 g/mol, can be obtained through various methods. Three main approaches are: direct condensation polymerization, azeotropic dehydrative condensation and ring-opening polymerization of lactide, a cyclic dimer of lactic acid (Auras et al., 2004). Direct condensation polymerization is cost-effectiveness, but it requires the use of solvents to attain high molecular weights, a requirement that renders it impractical and infeasible for large-scale industrial applications. The initial step in this method involves the self-condensation of lactic acid, resulting in a product with a low molecular weight characterized by equal concentrations of carboxyl and hydroxyl end-groups. In the pursuit of elevating the polymer’s molecular weight, the introduction of chain coupling agents and adjuvants becomes necessary. While these additives have the potential to enhance molecular weight, they also escalate the complexity and cost into the production process (Aharoni et al., 1986; Cotarca Pietro; Nardelli, Alfonso; unji, Vitomir, 1996; Inata & Matsumura, 1985). Azeotropic condensation polymerization offers a method in which lactic acid and catalysts are azeotropically dehydrated under reduced pressures in an aprotic solvent that is resistant to boiling and capable of refluxing. By forming an azeotropic mixture with a solvent, water can be effectively removed from the reaction mixture. This process drives the reaction forward, 13 enabling the production of high average molecular weight PLA, exceeding 300,000 g/mol, without the need for chain extenders (Lunt, 1998). While it does yield high-molecular-weight polymers, drawbacks of this technique include the use of solvents and the presence of catalysts to ensure efficient reactions, resulting in notable catalyst impurities. These impurities can lead to various issues in subsequent processing, including undesired degradation, unpredictable or inconsistent hydrolysis rates, and, especially in medical applications, potential concerns related to catalyst toxicity and variations in slow-release characteristics (Garlotta, 2019). Carothers et al. initially discovered ring-opening polymerization of lactide in 1932 (Carothers et al., 1932), and in 1954, DuPont further advanced this process (Lowe & Du Pont de Nemours, 1954). The progress was made possible by refining lactide purification techniques, ultimately leading to the creation of HMW PLA. Currently, the commercial method used by Cargill Dow LLC involves the pre-polymerization of low molecular mass PLA derived from dextrose fermentation. This is followed by catalytic conversion of the intermediate PLA into a mixture of lactide stereoisomers (L-lactide (two L-lactic acid molecules), D-lactide (two D-lactic acid molecules) and meso-lactide (a L-lactic acid and a D-lactic acid molecule)) under lower pressure, and subsequent vacuum distillation to yield HMW PLA. This method offers precise control over optical purity and chemical composition (Drumright et al., 2000; Hartmann, 1998; Kricheldorf et al., 1996). The initiation method of the polymerization through lactide formation has been extensively studied and can be initiated either cationically or anionically (Albertsson & Varma, 2003; Auras et al., 2004; Hartmann, 1998; Jacobsen et al., 2000; Kricheldorf et al., 1990, 1996; H. Tsuji & Ikada, 1999). However, these initiations are typically carried out in solvent-based systems, which are susceptible to issues such as racemization, transesterification, and elevated 14 impurity levels due to their high reactivity. These issues are not conducive to industrial-scale production. Hence, substantial research efforts were undertaken to address these challenges by investigating the application of metal carboxylates, oxides, and alkoxides that exhibit lower reactivity. Studies have confirmed that polymerization can be effectively accomplished using heavy metal catalysts, notably tin(II) and zinc, resulting in polymers of outstanding purity (Dahlmann et al., 1990; Kricheldorf & Kreiser-Saunders, 1990; Stevels et al., 1995). 2.2.3 Properties of PLA PLA exhibits a range of properties intricately linked to its polymer architecture and molecular weight (Garlotta, 2019). Control over the stereochemical makeup of the backbone can be achieved through polymerization with L-lactide, D-lactide, or meso-lactide, allowing for the formation of precise stereo-copolymers. The molecular weight can be controlled through the addition of hydroxylic compounds such as lactic acid, water, and alcohols (Hartmann, 1998). This control influences mechanical properties, crystallization behavior, thermal properties, rheological characteristics, and processing temperatures of the resulting PLA. Thus, PLA is a thermoplastic material with tunable properties for broad applications (Liber-Kneć & Łagan, 2014; Norazlina & Kamal, 2015; Zenkiewicz et al., 2009). The mechanical properties of PLA vary depending on its structure, composition, and molar mass. It can be made as soft and pliable to stiff and with high strength (Hossein Ramezani Dana & Ebrahimi, 2023). To be more specific, PLA polymers exhibit their highest modulus and tensile strength when their viscosity-average molecular weight exceeds 55,000 g/mol. As molecular weight and crystallinity increase, both impact strength and softening temperature also rise. 15 Notably, molecular orientation enhances the mechanical strength and properties such as piezoelectricity of PLLA (Poly-L-lactic acid) (Ikada et al., 1987; Ikada & Tsuji, 2000). When this alignment is achieved at lower temperatures, it results in PLLA with improved strength, without a substantial increase in crystallinity. In terms of crystallinity, PLA can be produced in two main forms: completely amorphous or with up to 40% crystallinity at room temperature. One of the key factors influencing crystallinity is the regular structure found in homopolymers such as PLLA or PDLA (Poly-D-lactic acid), and high L- or D-lactide copolymers, which tend to result in crystalline phases. For instance, when the PLA contains over 93% L-lactic acid, it leads to semicrystalline PLA, while a L-lactic acid content ranging from 50% to 93% results in strict amorphousness (Hideto Tsuji, 1996). In contrast, the presence of meso- and D-lactide introduces twists in the PLLA molecular architecture, reducing its crystallinity due to structural imperfections (Auras et al., 2004). The physical characteristics of PLA, such as density, heat capacity, and rheological properties, can be adjusted by controlling its glass transition temperature (Tg) (Farah et al., 2016). Around the Tg, PLA exhibits a transition from brittle to ductile fracture, and a change from rubbery to viscous state, marks the temperature at which significant changes in polymer chain mobility occur. In semicrystalline PLA, Tg is also essential for understanding its thermal behavior and both Tg and melting temperature (Tm) are influenced by lactide molecular weight, processing conditions and stereochemistry (Auras et al., 2004). For example, PLLA exhibits a higher Tg compared to PDLA, primarily attributed to disparities in crystallinity between these two polymers (Celli & Scandola, 1992). Additionally, the commercial viability of PLA is closely 16 linked to its Tg and Tm, dictating the conditions under which PLA can be processed or stored. While the PLLA is sensitive to degrade, its Tm can be reduced through the addition of D-lactide, widening the processing window (Celli & Scandola, 1992; Garlotta, 2019). PLA’s processing temperature depends on melt viscosity, which is affected by several factors, including the L/D ratio, molecular weight, processing conditions, and additives (Jamshidi et al., 1988). PLA melt exhibits viscoelastic behavior and does not conform to Newtonian fluid dynamics. With increasing temperature, PLA exhibits a decrease in shear viscosity (Fang & Hanna, 1999). Additionally, elevating the shear rate can reduce the viscosity of PLA melts by unwinding the entangled PLA chains. In conclusion, by changing its chemical composition and varying its molecular characteristics, it is possible to manipulate the physical, mechanical, and rheological properties of PLA. Therefore, understanding these relationships is crucial for tailoring PLA for specific applications. 2.2.4 Applications and environmental impact of PLA PLA is one of the most significant polymers to gain the most market share in the 21st century. This has to do with both its biobased origins and also its potential for biodegradation. In terms of its end-of-life options, PLA can undergo degradation and recycling. PLA, like most synthetic polymers, has the potential to be biodegradable, meaning it can be broken down by microorganisms in the environment, typically in soil or compost (Kale et al., 2007). The biodegradation process of PLA involves two stages: hydrolytic degradation, in which water molecules reduce the polymer’s molar mass, and microbial assimilation, during which microorganisms break down the low molecular weight polymer chains for energy, resulting in 17 the production of compounds like CO2 and H2O (N. K. Kalita et al., 2019; Sikorska et al., 2015; Valapa et al., 2016). The rate of hydrolytic degradation of the polymer matrix depends strongly on the crystallinity of the samples. Amorphous samples degrade in weeks while highly crystalline PLA may take months or even longer to completely hydrolyze to lactic acid because of the impermeability of the crystalline regions (Kricheldorf et al., 1996). Temperature also plays a role, with composting environments being optimal due to higher temperatures, especially around the glass transition temperature of PLA (approximately 60 °C), which accelerates biodegradation (Delacuvellerie et al., 2021). PLA typically decomposes in more than a year under natural soil conditions (N. K. Kalita et al., 2021), but it can decompose up to 90% into CO2 at 70 °C after 180 days under anaerobic conditions. In industrial-grade composting environments, it is able to fully biodegrade in 180 days (Al Hosni et al., 2019; N. K. Kalita et al., 2019, 2020). Considering the stringent conditions required for composting PLA, an attractive alternative approach is recycling (Åkesson et al., 2016; F R Beltrán et al., 2018; Freddys R. Beltrán et al., 2019; Fazelinejad et al., 2017; Goto et al., 2020; Zenkiewicz et al., 2009). This addresses the challenges associated with composting and has the potential to further reduce its environmental impact by utilizing previously produced biomass as PLA feedstock. Life cycle assessments have demonstrated that PLA can achieve reductions of up to 40% in greenhouse-gas emissions and up to 25% in fossil energy consumption (Baran & Yildirim Erbil, 2019) compared to traditional petrochemical-based polymers like polystyrene (PS), polyethylene terephthalate (PET), or polypropylene (PP) (Gironi & Piemonte, 2011; Hossein Ramezani Dana & Ebrahimi, 2023; Vink et al., 2010). Additionally, PLA is considered an environmentally friendly alternative as it can be derived from microbial fermentation of agricultural by-products, 18 primarily carbohydrate-rich resources, through lactide production and direct polymerization (John et al., 2006). The significance of PLA extends beyond its green credentials. With desirable thermoplasticity, PLA is well-suited for various manufacturing processes, including sheet extrusion, film blowing, and fibre spinning (Drumright et al., 2000). Its versatility allow it to mimic the mechanical and physical characteristics of some petroleum-derived thermoplastics, making it a promising candidate for a wide spectrum of applications, ranging from packaging and consumer goods to medical and pharmaceutical applications (Jubinville et al., 2022; Ncube et al., 2020; Sinclair, 1996; Taib et al., 2023; Trivedi et al., 2023). Moreover, due to its ease of processing, PLA and PLA-based biocomposites have undergone extensive research and are being in 3D printing (Ilyas et al., 2021). In 2019, PLA dominated the plastics segment of the 3D printing filament market, accounting for more than 39% of the revenue share (Research, 2019). AM is a revolutionary technology used in industries like aerospace, automotive, healthcare, construction, and energy, where three-dimensional objects are created from a virtual model (Kristombu Baduge et al., 2021; H. Ramezani Dana et al., 2022; Hossein Ramezani Dana & Ebrahimi, 2023; Roque et al., 2021; Shapiro et al., 2016; C. Sun et al., 2021; Wiese et al., 2021). 2.3 Literature review of PLA/Wood flour composites Additive manufacturing using FDM has emerged as a material-efficient approach for fabricating thermoplastic materials to complex structures (Drummer et al., 2012). Executed through an extruder head, this layer-by-layer manufacturing process is notable for its flexibility, contrasting with traditional methods like subtractive techniques, molding, and casting. Its applications vary, 19 ranging from creating simple prototypes to industrial-grade products, proving useful in both professional and home-based settings. Over the past decade, the adoption of this technology has significantly driven the growth of the global market for 3D printing materials (Ligon et al., 2017). Given concerns about non-renewable resources and plastic pollution, there is a growing interest in creating renewable, sustainable, and cost-effective 3D printing filaments (Suárez & Domínguez, 2020). This includes incorporating natural materials into standard thermoplastic 3D printing materials, which can contribute to a more environmentally friendly FDM process (Bhagia et al., 2021). Numerous studies have focused on reinforcing PLA composites with lignocellulosic materials, such as natural fibres, wood flour (WF), nanoparticles, and other additives (Balla et al., 2019; Satyanarayana et al., 2009), which not only maintains the biodegradability of the materials but also enhances their properties and reduces the costs of the composites (Baran & Yildirim Erbil, 2019; Bhagia et al., 2021; X. Zhao et al., 2020). Meanwhile, the challenge of incompatibility between hydrophilic natural materials and hydrophobic polymer matrices has spurred studies into modification methods to improve interfacial adhesion and composite properties. In the thermomechanical pulping (TMP) process, fines are a fraction of fibres isolated from waste streams that can pass through a mesh screen with round holes 76 microns wide. These fines, comprising up to 30% of the weight of TMP in some cases (Odabas et al., 2016), also have the potential to serve as 3D printing material additives. These particulate materials, smaller than the bulk fibres, have a distinct composition; for example, they have a higher concentration of lignin (35%-40%) and more extractives on their surfaces. Due to their heterogeneous character and small size, these fines unpredictably impact the mechanical properties of the final paper 20 sheets as their concentration increases (Page, 1969). Consequently, the pulping and papermaking processes may lose these fines in the waste stream instead of capturing them as valuable co-products. Given the desire to valorize this by-product in the TMP industry and to create a more sustainable 3D printing material, these naturally abundant fines may be of interest for creating value-added products by blending with PLA. According to the composition and size of fines (less than 76 µm), they can be categorized as wood flour (WF) among various natural fillers, which refers to wood particles that are small enough to pass through an 850 µm screen size (US standard mesh size of 20) (Clemons & Caufield, 2005; Das et al., 2021). With the burgeoning demand for diverse materials in FDM and its expanding applications, the incorporation of WF as a filler into the PLA polymer matrix offers the prospect of developing environmentally friendly and cost-effective biocomposite materials. For example, Ayrilmis et al. (2019) investigated the impact of WF loading content on the surface properties of PLA/wood composite filaments. This research found that incorporating more WF into PLA significantly increased the surface roughness of specimens, attributed to the presence of micropores in the printed parts. Additionally, the water contact angle of specimens with 30% w/w WF was less than 90 degrees, indicating they remained hydrophilic. The results highlight one of the challenges faced by wood-plastic materials: the hygroscopic nature of wood (Dalu et al., 2019). During the heating process, moisture within wood materials can evaporate, leading to the formation of an interface cavity layer. This results in issues such as low dimensional stability, carbonization, and filament blockage in PLA/WF composites (Tao et al., 2017). Therefore, it is essential to consider adjusting the hydrophobicity of wood fillers to mitigate these problems (Arminger et al., 2019; Kumar, 1994; Rowell, 1983). 21 Another significant challenge in fabricating PLA composites reinforced with wood materials is achieving even dispersion of biomass fillers within the polymer matrix. Due to the abundance of polar groups such as hydroxyl groups (Elrhayam & Elharfi, 2019; J. Zhang et al., 2021), wood powders tend to agglomerate after drying or during melt processing within the polymer matrix due to their inter- and intra-molecular interactions amongst fibre (Chinga-Carrasco et al., 2011). Furthermore, the hydrophilic nature of wood materials and the hydrophobic characteristics of polymers containing nonpolar ester groups (R. Liu et al., 2016) result in poor interfacial compatibility (Haryńska et al., 2021). This incompatibility leads to filler aggregation, weak interfacial adhesion, phase separation, and initiation of failure, consequently causing a significant reduction in the composite’s mechanical properties. To enhance the mechanical properties of the composites, extensive research has focused on ensuring a uniform distribution of WF within PLA matrix and improving the interfacial adhesion between wood fillers and PLA. This literature review examines a range of strategies proposed to enhance the compatibility and mechanical properties of PLA/WF biocomposites, with some work focus FDM applications (Table 2.1). 22 Table 2.1 Summary of treatments for PLA/wood flour (WF) biocomposites and their effects PLA WF Content Method type Method Compounding and sample type Effects Reference 4032D Poplar wood (100 mesh) 10% Coupling agent 2%, 4%, 6% glycidyl methacrylate (GMA) grafted PLA high-speed mixer and FDM printed specimens Adding 2 wt.% GMA grafted PLA improved the interfacial adhesion, impact strength and tensile strength. (R. Guo et al., 2018) 4032D Norway spruce wood (70-150 µm) 0 to 40% Coupling agent 5 phr MA-grafted ethylene propylene diene monomer (MA-EPDM) twin screw extruder and FDM printed specimens Adding MA-EPDM improved elongation and impact strength. (Bae et al., 2021) 23 PLA poplar WF (140-160 mesh) 30% Plasticizer 2%, 4% glycerol or tributyl citrate (TBC) twin screw extruder and FDM printed specimens Adding 4% TBC showed better performance in compatibility, mechanical properties, water absorption, and thermal stability. (Xie et al., 2017) 4043D Teak WF (74 µm, 125 µm) 1%, 3%, 5% Coupling agent 5% γ-aminopropyl trimethoxy silane twin screw extruder and extruded filaments Printed only for the composites with 74 µm WF. Adding silane lowed water uptake, improved interfacial bonding and mechanical strength. (Petchwattana et al., 2019) 3052D WF (60 mesh) 10% Coupling agent, plasticizer KH550, glycerol high-speed mixer and FDM printed specimens Modification improved interfacial bonding, hydrophilicity, tensile strengths and anti-hydrothermal aging properties. (Yu et al., 2023) 4032D Pulp, wood or lignin 0, 4-20% Chemical modification soaked in 3% KH550 solution at RT for 24h torque rheometer and FDM printed specimens The modified PLA/lignin composites had the best overall performance. (L. Liu et al., 2019) 24 powder (300 mesh) 2003D Nova wood (100 µm) 5%, 10% Physical and chemical treatment thermal treated WF/ soaked in silver nitrate solution twin screw extruder and FDM printed specimens Treated WF improved interfacial adhesion and mechanical properties. Adding silver nanoparticles positively affected mechanical properties and compatibility with treated WF. (Yurttas & Ayrilmis, 2023) 4043D Poplar WF 20% Physical treatment hammer-mill and sieve to: 850–2360, 425–850, 180–425, and <180 µm brabender and plunger-type batch extrude specimens Decrease in fibre size promoted fewer voids, better dispersion into PLA, increased tensile strength, and demonstrated a suitable viscosity range for 3D printing. (X. Zhao et al., 2019) 25 4043D Poplar (7 and 153 µm) 20% Physical treatment fibrillation and ball milling single screw extruder and FDM printed specimens Biomass size reduction through ball milling was preferred for FDM processes, tensile properties, particle dispersion, and appearance of printed parts than fibrillation. (Bhagia et al., 2020) PLA Poplar (100-200, 200-300, <300 mesh) 20% All high-temperature hot air/ silane/ glacial acetic acid, acetic anhydride (AAH) high-speed mixer and FDM printed specimens The heat-treated AAH esterified WF improved hydrophobicity, tensile and flexural strength, decreased the cold crystallization temperature, and thermal stability. (Chen et al., 2022) 2100D WF (100-120 mesh) 0 to 40% Coupling agent 1%, 3% silane was integral-blended twin screw extruder and injection molded specimens Adding silane improved interfacial bonding, tensile modulus, tensile strength (1 wt.%) decreased in the composite crystallinity. (Lee et al., 2008) 26 3001D Pine (40 mesh/ 425 µm) 20%, 40% Coupling agent 0.5% silane kinetic-mixer and injection molded specimens Adding silane showed higher storage modulus, higher crystallinity. (Pilla et al., 2008) 7000D WF (250 µm) 0 to 40% Coupling agent methylenediphenyl-diisocyanate (MDI), poly (ethyleneacrylic acid) (PEAA) twin screw extruder and injection molded specimens Adding MDI increased interfacial adhesion, tensile strength, and modulus. Adding PEAA reduced tensile strength and slightly increased impact strength. (Petinakis et al., 2009) PLA Pine 0 to 30% Toughening agent 5% poly(styrene-butadiene-styrene) (SBS) high-speed blender and injection molded specimens Adding SBS increased impact strength and elongation but decreased tensile strength. (Qiang et al., 2012) 27 2002D Pine (120 mesh) 0 to 30% Coupling agent 1.3 to 6% rheology modifier kitchen mixer and single screw extruded specimen Adding rheology modifier can lower and match the melt flow index (or viscosity) of the composite melt to that of the neat PLA. (Matuana & Diaz, 2013) 2002D Poplar (325 mesh) 0 to 50% Coupling agent 10 phr MA, 3 phr DCP brabender and compression molded specimens Grafting of MA to PLA improved tensile strength, compatibility and biodegradability. (Tsou et al., 2015) 4032D WF (210 mesh) 0 to 60 vol% Coupling agent 0 to 20% MA grafted PLA (MAPLA) brabender and compression molded specimens Increased strength, reinforcement and prevented debonding of fibres. Efficiency increased with increasing number of functionality and MAPLA amount. (Csikós et al., 2015) 28 2002D European spruce and silver fir 40% Fibre reinforced Cordenka cellulose fibres/ E-glass fibres twin screw extruder and injection molded specimens Adding cellulose fibres increased specific tensile and flexural strength, specific notch impact strength. (Bledzki et al., 2015) L-PLA, 306 D WF (40–60 mesh) 30% Coupling agent 0-3% maleic anhydride (MA), 0.4% dicumyl peroxide (DCP) high-speed mixer and single-screw extrude specimen Adding MA improved the compatibility, water resistance and mechanical strength; decreased storage modulus and complex viscosity. (Lv et al., 2016) 2003D WF (100 mesh) 30% Coupling agent 0-4% styrene-assisted MA-grafted-PLA high-speed blender and compression molded specimens Increased mechanical properties (max. at 3 wt.%), compatibility, storage modulus, complex viscosity, equilibrium torque, and shear heat. (Du et al., 2017) 29 7000D Norway spruce (120 µm) 40% Physical and chemical treatment hydrothermal/ MA/ silane/ stearic acid treatment solvent casting and compression molded specimens Modified WF increased storage modulus, interfacial bonding and decreased dynamic fragility. Adding silane increased tensile strength and Young’s modulus. (Gregorova et al., 2011) 6202D Pine (20 mesh) 0, 30-50% Chemical modification alkaline treatment, pre-impregnation with PLA solution twin screw extruder and injection molded specimens Both treatments further increased the mechanical properties including tensile strength, modulus and impact strength. Pre-impregnation reduced the water uptake. (Altun et al., 2013) Luminy L105 Maple (0.3 to 1.0 mm, 20 to 60 mesh) 30%, 50% Chemical and coupling agent dodecenyl succinic anhydride (DDSA); 5 phr MA, 0.1 phr DCP batch mixer and injection molded specimens DDSA increased hydrophobicity, and dimensional stability. DDSA and maleation of PLA both aided in compatibility, without affecting tensile strength and modulus. (Jubinville et al., 2022) 30 Based on the table, it can be concluded that there are generally three methods to enhance the properties of PLA/WF composites: 1) physical treatment to optimize processing conditions or particle size distribution, ensuring a uniform dispersion of WF; 2) chemical modification of wood materials to improve their plasticity, thermal stability, and hydrophobicity (Chen et al., 2022); and 3) the addition of compatibilizers to the melt to enable intimate bonding between the fillers and the PLA matrix (Zarna et al., 2021). Some studies have combined these methods to achieve a synergistic effect. 2.3.1 Physical treatment Physical treatments of WF encompass a wide array of strategies aimed at enhancing its distribution, compatibility and performance within polymer matrices, particularly in applications like 3D printing. These strategies range from modifying the morphology of WF to adjusting processing techniques. Techniques such as heat treatment, stretching, calendaring, low-temperature plasma, radiation, and various discharge technologies effectively alter the WF’s properties, making it more suitable for composite applications (Chen et al., 2022; Gregorova et al., 2009; Yurttas & Ayrilmis, 2023). The choice of process methods (W. Liu et al., 2007; Migneault et al., 2009; K. Zhang et al., 2013) and processing parameters, such as extrusion speed (Yilmazer & Cansever, 2002)—plays a crucial role in determining the WF’s distribution within the PLA matrix, impacting the final composite’s mechanical properties. One notable approach to enhancing WF performance is through particle size reduction for improved dispersion, achieved through various milling and compounding techniques. For example, Zhao et al. (2019) demonstrated that employing a hammer mill to prepare poplar fibres 31 of different mesh sizes notably improved the tensile strength of PLA composites—from 34 MPa to 54 MPa as the fibre size decreased from 850−2360 µm to less than 425 µm. This surprising improvement is attributed to the larger exposed surface area of smaller fibres (<180 µm), which facilitates PLA penetration and leads to fewer voids at the composites interface. Thanks to their porous and hollow microstructures exposed at fibre surface, facilitated better PLA penetration and mechanical interlock. This result indicates the potential of smaller particle sizes for creating bio-derived material feedstocks optimized for 3D printing applications. Another issue impedes the performance of WF-based composites is the naturally occurred agglomeration of fibres and the inherent heterogeneity of untreated lignocellulosic materials, leading to defect agglomerated structures in natural material-reinforced polymer composites. Such defects can detrimentally affect the mechanical properties, mixing quality, and overall appearance of the composites (Joffre et al., 2014; Zykova et al., 2018). Furthermore, while exceptionally refined wood fibre is required for wood-based 3D printing materials used in FDM, nozzle blockage issues remain a significant concern due to the potential aggregation of wood particles (Huang et al., 2021; Petchwattana et al., 2019; Wimmer et al., 2015). To mitigate this issue, mechanical pre-treatments like ball milling could be an effective solution for distributing fibres evenly within the PLA matrix (Berthet et al., 2015; Qu et al., 2017; X. Zhao et al., 2020). Bhagia et al. (2020) highlighted ball milling’s efficacy in preparing ball-milled poplar for 3D printing, finding it superior to high-speed friction grinding due to its enhanced particle dispersion, improved tensile properties, and better appearance of printed samples. Moreover, Bai et al. (2019) showed that ball milling facilitated the penetration of bonding additives into 32 hardwood structures, improving biocomposite processing and resulting in increased flexural strength and modulus. Additionally, the treatment’s impact extends beyond mechanical enhancements. For instance, Isa et al. (2016) found that dry-milled particles could increase compound flowability, beneficial for applications like injection molding. In the realm of natural fibres, Ma et al. (2020) discovered that ball milling significantly reduced the size of silver grass particles, ensuring their homogeneous dispersion in a PLA matrix and improved printability with 3D printer nozzles. Combined with acid treatment to the particles, these PLA/silver grass composites exhibited superior thermal stability and mechanical performance compared to pure PLA, highlighting their suitability for 3D printing applications. Despite the promising advancements in physical treatments for WF, it’s crucial to consider the intensive energy consumption and cost implications of these techniques. Future research should aim to balance performance enhancements with sustainability and economic viability, paving the way for more environmentally friendly and cost-effective solutions in the 3D printing industry. 2.3.2 Chemical modification The concept of chemical modification involves altering the chemical composition on the surface of wood materials to enhance their properties for improved moisture resistance, better compatibility with PLA, and increased mechanical strength. Typical methods encompass esterification, etherification, oxidation, benzoylation, grafting, crosslinking, isocyanate treatment (Ying Guo et al., 2018), as well as alkali treatment (Bhagia et al., 2021). These techniques 33 primarily target the hydroxyl groups present in wood flour (WF) to diminish water affinity and mitigate fibre-fibre hydrogen bonding, while the alkali treatment method focuses on the removal of lignin, hemicellulose, and extractives, thereby improving mechanical interlocking with polymers (Altun et al., 2013). Among the various chemical modification strategies, esterification methods, such as acetylation, maleation, and silylation, are particularly common for converting hydroxyl groups into more hydrophobic entities, which decreases surface polarity and hydrophilicity of wood fillers, thereby enhancing their compatibility with PLA and the overall durability of the composite material (Chen et al., 2022; Gregorova et al., 2009; Hou et al., 2021; Hubbe & Grigsby, 2020; Jamaluddin et al., 2021; L. Liu et al., 2019; Stamm & Tarkow, 1947; Tserki et al., 2005; Wang et al., 2017). Recent research into lignocellulosic biomass, such as the innovative one-pot, two-step homogeneous transesterification method introduced by Suzuki et al. (2021), has demonstrated promising results in converting biomass into thermoplastic viable materials. This method involved mixing decanoyl with acetyl groups in ionic liquid, successfully converting sugarcane bagasse into injectable thermoplastics while simultaneously separating lignin from the parent materials. Furthermore, studies combining heat treatment with esterification have investigated the impact of various chemical modifications on the mechanical and thermal properties of PLA biocomposites. For example, Chen et al. (2022) employed high-temperature moist air treatment and acetic anhydride esterification. This multifaceted approach significantly enhances the composites’ mechanical properties, and hydrophobicity. Specifically, heat-treated and esterified WF added to PLA resulted in a 40% increase in tensile strength and a 13% increase in flexural strength compared to untreated composites. These modifications also lead to a lower cold 34 crystallization temperature and a significant reduction in the composites’ crystallinity, suggesting improved movement of chain segments, enhanced interaction and greater compatibility between phases. Another study found that the similar combined treatments enhanced the composite’s thermal, mechanical, and viscoelastic properties by increasing the rigid amorphous phase content, associated with improved thermal stability, stiffness, and reduced damping factors. The surface modification employed in this study, especially with silane, decreases the dynamic fragility of PLA/WF composites, indicating better interfacial compatibility and performance (Gregorova et al., 2011). However, many reports on WF modification dealing with esterification, have highlighted the extensive use of organic solvents, which are difficult to remove fully (Ying Guo et al., 2018; Wang et al., 2017). For instance, Gregorova et al. (2009) employed stearic acid to treat wood products, which were then blended with degradable aliphatic polyesters. Although the resulting materials exhibited promising thermoplastic and mechanical properties, the use of toluene as a solvent conflicted with green chemistry principles. This modification step not only raises the cost and complexity of post-treatment but also poses environmental concerns. Thus, the choice of greener solvents and more efficient reaction conditions is imperative, underscoring the importance of ongoing research in this field. In conclusion, selecting appropriate nucleophilic compounds (such as carboxylic acid or anhydride) and optimizing reaction conditions can effectively reduce the hydroxyl content of esterified WF, enhancing its thermoplasticity. The chemical modification of WF is important in tailoring PLA biocomposites’ properties based on the specific needs of targeting applications. 35 Future research should aim to develop novel bio-based modifiers and apply green chemistry techniques to balance performance enhancements with environmental sustainability. 2.3.3 Coupling agent The introduction of a coupling agent as a compatibilizer into the melting process has been employed to modify the fibre-matrix interface, enhancing compatibility between wood materials and the PLA matrix (Hamdan et al., 2019; Ihemouchen et al., 2013). Serving as a bridge, the coupling agent establishes chemical and physical bonds with both the polymer and fillers, thereby improving the interfacial adhesion between the fillers and the matrix. This interfacial adhesion is a critical factor that significantly influences the mechanical properties of the biocomposite (Chauhan et al., 2021; Huda et al., 2006). Since both the addition of a coupling agent and surface modification induce chemical reactions, this thesis distinguishes between them by identifying the stage at which each process is incorporated. Surface modification involves treating the biomass prior to its blending with the polymer, enabling the modified wood materials to better interact with the polymer. In contrast, the addition of a coupling agent during the thermal compounding process allows for direct grafting onto the polymer or simultaneous reaction with both the polymer and wood fillers. Various compatibilizers and coupling agents, such as silane, maleic anhydride (MA), MA-grafted polymers, and functionalized nanoparticles, have been explored to enhance the interfacial adhesion between WF and PLA, thereby improving the composite’s mechanical properties. Recent studies have shown that treatments like grafting MA onto the PLA matrix can improve the compatibility between WF and PLA by introducing anhydride groups, which can react with 36 the hydroxyl groups on the surface of WF and the end groups of PLA, forming ester linkages (covalent bonds) and carboxylic acids (hydrogen bonds) (Chauhan et al., 2021). This reaction improves the moisture resistance of the WF and creates strong bonding between fillers and PLA, significantly enhancing mechanical locking and stress transfer efficiency between the filler and the polymer matrix (Tsou et al., 2015). Lv et al. (2016) revealed through a one-step reactive extrusion process that optimal concentrations of MA (1% w/w) lead to the best mechanical properties, including increased tensile and flexural strength. However, higher concentrations of MA result in decreased thermal stability and rheological properties of the composites. MA grafted polymers have also proven to be effective coupling agents (Bae et al., 2021). Currently, the regularly used coupling agents include maleic anhydride grafted propylene (MAPP) (Etaati et al., 2014; Huda et al., 2006; Huner, 2017; R. Liu et al., 2013; Mohamad et al., 2004) and maleic anhydride grafted polylactic acid (MAPLA) (Dimzoski et al., 2008; Dzul-Cervantes et al., 2017; Rahem et al., 2019; L. Zhang et al., 2017). These agents intertwine with the polymer, initiating chemical reactions with fillers to increase interfacial bonding. For instance, Huner (2017) demonstrated that incorporating MAPP in rice husk-reinforced composites can enhance tensile strength by 41.7% and tensile modulus by 13% compared to neat PP. Similarly, adding 5% MAPLA to PLA composites with 30% rice hulls significantly increased tensile and flexural strength by 33% and 40%, respectively, due to improved interfacial adhesion (Dimzoski et al., 2008). Furthermore, employing a low molecular weight MAPP as a rheology modifier has enabled the production of microcellular foamed PLA/WF composites with uniform and homogeneous cellular structures through a continuous extrusion process, effectively matching the melt flow index and viscosity to those of neat PLA (Matuana & Diaz, 2013). While 37 MAPP has been commercialized and is readily available in the market, MAPLA has not yet reached commercial availability and can be synthesized via reactive extrusion (Nyambo et al., 2011). In conclusion, the use of coupling agents, particularly MA-grafted polymers, play a pivotal role in enhancing the compatibility and mechanical properties of WF/PLA composites. These advancements not only improve the material’s physical properties but also contribute to its environmental sustainability, offering promising directions for future research in the development of biodegradable polymer composites. 38 Chapter 3: Materials and experimental methods 3.1 Materials Thermomechanical pulp fines (as never-dried unbleached softwood extracted from primary refiners), kindly provided by Holmen Paper AB (Sweden), were used to produce modified fines and later as PLA composite fillers. The initial moisture content of the TMP fines as received was 79.6%. The contents of cellulose, hemicellulose, and lignin in the TMP fines, determined by Klason analysis, are shown in the table below (Table 3.1). The data reveals a significant lignin concentration of approximately 41.3% and an extractive content of 6.0% within the TMP fines. Table 3.1 The fundamental composition of TMP fines determined by Klason analysis Sample Cellulose Arabinan Galactan Xylan Mannan Acid insoluble lignin Acid soluble lignin Extractives TMP fines 26.6 (0.9) 1.9 (0.3) 3.5 (0.2) 5.2 (1) 8.9 (3.1) 41.3 (0.4) 0.6 (0.1) 6.0 (1.1) Ingeo 3D850 biopolymer (PLA pellets) developed by NatureWorks was used in this study. The density of this PLA was 1.24 g/cc and the glass transition temperature was between 55-60 °C. Acetone was purchased from UBC Chemistry Store (MilliporeSigma). All of the other chemical reagents were purchased from MilliporeSigma (Canada) at reagent grade and were used as received, including (2-dodecen-1-yl)succinic anhydride (DDSA), imidazole, maleic anhydride (MA), dicumyl peroxide (DCP), and maleic anhydride-grafted polypropylene (MAPP). 39 3.2 Approaches to enhance the mechanical performance of PLA/Fine composites To determine the optimum performance of PLA/fine composites, a number of methods including physical treatment, chemical modification of fines, and coupling agent incorporation were adopted in this thesis to improve the interfacial bonding and compatibility between TMP fines and PLA. The procedures for these methods are described in the following sections. 3.2.1 Physical treatment of fines Ball milling was employed as a potential way to reduce fines agglomeration and achieve a more uniform fines dispersion in the PLA matrix. In this approach, each batch of freeze-dried 4-gram TMP fines was equally loaded into two ceramic grinding jars with nominal volumes of 125 ml. Then, TMP fines were mechanically milled through a planetary ball mill (Retsch PM200) using ceramic balls as the grinding medium. The rotation speed was set to 600 rpm, and the total grinding time was 15 min with a 5 min interval break after 5 min. After being taken out from the container, ball-milled (Bm) fines were stored in a 40 °C Isotemp 280A vacuum oven and were ready to be used in the subsequent compounding process. Another physical method to improve the separation of fine clusters and the dispersion of fines within the PLA matrix involved a masterbatch procedure. The masterbatch was prepared by mixing never-dried fines (60.0 g wet mass, equivalent to 12.0 g dry mass) with PLA (18 g) in a Brabender Prep-Center® Mixer (Brabender, Duisburg, Germany) at 190 ℃ and a rotor speed of 50 rpm for 10 minutes. The resulting masterbatch, containing 40% fines by dry weight, was then combined with additional fresh PLA and fed back into the extruder to produce dumbbell-shaped PLA/20% TMP fines specimens. The tensile properties of the PLA/20% TMP fines specimens 40 produced using the masterbatch step were compared with those obtained by directly extruding 20% TMP fines with PLA. 3.2.2 Chemical modification of fines Surface modification was used to tune the hygroscopic nature of TMP fines into more hydrophobic, potentially improving their compatibility to a hydrophobic matrix (PLA). Esterification was applied to modify the hydroxyl rich fibre surface and tune their hydrophobicity. Based on Beaumont’s work (2021), DDSA was chosen to achieve the surface esterification of fines in this study and the method shown below was adjusted accordingly. Never-dried fines (50.0 g wet mass, 10.0 g dry mass) were first weighed and transferred into centrifuge tubes, followed by acetone solvent exchange three times via centrifugation. In a separate container, 61.7 mL of a 3 M solution of imidazole (12.6 g, 68.1 g/mol) in acetone was mixed with 123.4 mL of a 1 M succinic anhydride solution (32.9 g, 266.4 g/mol) in acetone and stirred for 10 min (Beaumont et al., 2021). Later, the solvent exchanged fines were added to this mixture and went for closed reacting in a shaker at 40 °C for 6.25 h. Eventually, the reaction was stopped through addition of extra acetone and filtration. To separate the unreacted DDSA and imidazole from the modified fines, the slurry was washed by Soxhlet extraction with acetone. The washed fines were then rinsed by deionized water and freeze dried before further application. Note, the modified fines were resistant to dispersion in water showing qualitative surface changes. Weight percentage gain (WPG) was calculated as % WPG = ((W2-W1)/W1), where, W1 is the dry weight of the fines prior to the modification and W2 is the weight after DDSA esterification and lyophilization. 41 3.2.3 Coupling agent incorporation The third method to improve the interfacial adhesion between PLA matrix and fine fillers was the addition of coupling agents during extrusion. Specifically, the two coupling agents applied in this work were MAPP (maleic anhydride 8-10 wt.%) and maleic anhydride-grafted PLA (MAPLA). Since MAPLA was not commercially available, it was fabricated through a melt grafting process in the literature (Jang et al., 2022). At 190 °C with a rotor speed of 60 rpm, the PLA was first added to the Brabender Prep-Center® Mixer (Brabender, Duisburg, Germany), and melted for 2 min. Next, the free radical initiator, DCP (1 wt.%) was introduced and blended for 3 min, followed by the MA (8 wt.%) reaction for another 5 min. Eventually, batches of MAPLA were taken out from the Brabender mixer. Coupling agents were incorporated during the composites processing (melt mixing) in this study. The reaction scheme of adding coupling agents to the melt of PLA and fines is illustrated in Figure 3.1, which indicates esterification between succinyl anhydride groups and the hydroxyl groups of fines as well as the bonding between coupling agents and the polymer matrix (Oliver-Ortega et al., 2022). Figure 3.1 Scheme of maleic anhydride grafted PLA (MAPLA) and maleic anhydride grafted polypropylene (MAPP) modification at the interface of PLA and fines 42 3.3 Manufacturing of PLA/fine composites The extrusion of composite materials was conducted using a twin-screw micro compounder (Xplore MC l5 HT). The extrusion was set at a melting temperature of 190 °C over a processing time of 10 minutes, and a screw speed of 100 rpm with a nozzle diameter of 3 mm. The mixing procedure varied dependent on the composition. For untreated fines and treated fines, including DDSA fines and ball-milled fines, the extrusion process began with the melting of PLA, followed by the inclusion of fines with concentrations of 10% and 20%. For composites with coupling agents, the process was modified by pre-mixing PLA with a coupling agent (4%) for 3 minutes, and then adding the fines (10%) for an additional 7 minutes. Pure PLA was also extruded using the same procedure to serve as the control material for this research project. Before melt compounding, all materials were pre-dried in a 40 °C Isotemp 280A vacuum oven for at least 4 hours to eliminate moisture. An overview of the composites’ composition and treatments is given in Table 3.2. Table 3.2 The composition and treatment of PLA/fine composites (Bm: ball-milled, DDSA: (2-dodecen-1-yl)succinic anhydride, MAPP: maleic anhydride-grafted polypropylene, MAPLA: maleic anhydride-grafted PLA, MTMP: TMP fines masterbatch) Sample name PLA (wt.%) Coupling agent (wt.%) Fine (wt.%) Fine treatment a. PLA 100 0 0 No b. 10TMP 90 0 10 No c. 10Bm 90 0 10 Ball milling d. 10DDSA 90 0 10 DDSA 43 e. 10TMP_MAPP 86 4 10 No f. 10TMP_MAPLA 86 4 10 No g. 20TMP 80 0 20 No h. 20MTMP 80 0 20 Masterbatch i. 20Bm 80 0 20 Ball milling j. 30Bm 70 0 30 Ball milling k. 10Bm_MAPP 86 4 10 Ball milling l. 10Bm_MAPLA 86 4 10 Ball milling The extruded materials were then drawn into filaments (selected formulation) and test specimens, respectively. Filaments were produced using a controlled filament collector with a diameter of approximately 30 centimeters at a low speed that matched the extrusion speed, maintaining a stable filament diameter. With this approach, an average filament diameter of around 1.75 mm was achieved. For composite property testing, standard dog-bone shaped specimens (63.5 mm × 9.53 mm × 3.2 mm, ASTM D 638 Type V) and rheometer test disc-shaped specimens (25 mm diameter, 1 mm thickness) were produced using a micro injection molder (Xplore IM 5.5). Through a nozzle unit that fitted seamlessly with the compounder, the injection molder allowed for direct transfer of the composite melt into standard specimen molds under a controlled three-step pressure. The injection unit was pre-heated to 190 °C to ensure a consistent melt temperature and quality of the specimens for mechanical and rheological 44 behavior analysis. The process for modifying fines in various ways and preparing composite samples is illustrated in Figure 3.2 below. Figure 3.2 Illustration of the various fines’ treatments and PLA/fine composites preparation methods 3.4 Characterization methods 3.4.1 Fines composition analysis 3.4.1.1 Moisture content measurement TMP fines (as received) were weighed to approximately 2 g to the nearest 0.001 g in a tared dish. Next the samples were dried for 2 h in an oven at 105 °C, cooled in a desiccator and weighed. Samples were weighed repeated for successive hourly periods until the mass did not change by more than 0.002 g. The moisture content can be calculated by determining the difference between the initial weight of the fines (2 g) and their dry weight. 45 3.4.1.2 Extractive analysis Following Tappi T 204 cm-97, the freeze-dried fines were first ground to pass a 40 mesh on a Wiley mill. Then, extractives were removed by Soxhlet extraction using a mixture of toluene/ethanol (2:1), for at least 6 hours. After extraction, the solvent mixture was evaporated partially to collect the extractives. The extract was then transferred to a tared weighing dish and air-dried in a chemical fume hood. To weigh the content of the extractives, the dish was dried in an oven for 1 hour at 105 °C and then cooled in a desiccator. Meanwhile, the extracted fines were transferred to a funnel and the excess solvent was removed by suction. After washing the fines and extraction thimble with ethanol to remove the toluene, the fines were returned to Soxhlet extraction again with 95% w/w ethanol for at least 4 h, until the alcohol became colorless. The remaining solid residue in the thimble were air dried in a chemical fume hood and further dried in an oven at 105 °C until the weight remained constant. By subtracting the weight of the original freeze-dried fines by the weight of the extractive-free fines, the extractive contents of fines could be calculated. The extractive analysis was measured in duplicate. 3.4.1.3 Klason analysis 200mg oven dried extractive-free fines were weighed and mixed with 3 ml 72% w/w sulfuric acid to pre-treat at room temperature for 2 hours, ensuring that the cellulose and hemicellulose components were hydrolyzed, leaving behind a solid residue primarily composed of lignin and other acid-insoluble materials. Three parallel samples were prepared. After the pre-treatment, the mixture was diluted with 112 ml water to reduce the sulfuric acid concentration. This diluted mixture was then autoclaved for 1 hour at 121°C to complete the hydrolysis of the cellulose and 46 hemicellulose into soluble sugars. After heating, the mixture was cooled down to ambient temperature. The solid residue was filtered and washed to remove any soluble components and acids thoroughly, and the residue was later dried in an oven at 105 °C till a constant weight to determine the acid-insoluble Klason lignin content. The percentage of Klason lignin in the fines sample was calculated by dividing the weight of the dried residue by the weight of the original dry fines (200 mg). The acid-soluble lignin content could be measured from the filtrate using UV-Vis Spectroscopy (Agilent, Cary 60) at 205 nm using 0.00909 g/L as the molar absorptivity constant. With the preparation of sugar standards, including arabinose, galactose, glucose, xylose, and mannose, the sugar analysis of the sample filtrates was completed using a high-performance ion chromatography system (Dionex ICS-6000 HPIC). This was done according to the Laboratory Analytical Procedure from the National Renewable Energy Laboratory for the general chemical composition analysis of lignocellulosic biomass (Sluiter et al., 2008). 3.4.2 Fourier transform infrared (FTIR) spectroscopy The chemical structure of the untreated and DDSA treated fines was investigated through Fourier transform infrared (FTIR) spectroscopy to confirm the introduction of the new functional groups induced by the modification. The FTIR spectra were carried out at room temperature on an INVENIO S (Bruker Corp, United States) and collected in transmission for the wavenumber range from 4000 to 400 cm−1 using 64 scans, and a resolution of 4 cm−1. The sample was made by grinding fines and KBr with a wight ratio of 1:80 using a press and dried in a 40 °C Isotemp 280A vacuum oven prior to the measurements. The background noise for the moisture and air was collected and subtracted from the sample measurement. Automatic baseline corrections were made by OPUS software after each measurement, and the spectra of all fines were normalized at 47 1060 cm-1 as the represented ether group was chemically inactive during reactions (Hu et al., 2017). 3.4.3 Water contact angle (WCA) measurement The hydrophobicity of the DDSA modified fines was evaluated by measuring the water contact angle (WCA). Both the untreated TMP fines (as control), ball-milled fines and DDSA modified fines were pressed to a flat mat and the contact angle was determined using a Theta Flex optical tensiometer (Biolin Scientific). Contact angle measurements were carried out in triplicate and for a period of 90 seconds. 3.4.4 Thermogravimetric analysis (TGA) Thermal stability of the fines and DDSA modified fines under steady heat exposure were examined using thermogravimetric analysis (TGA). This study utilized a TA Instruments TGAQ500, applying a heating rate of 10 °C/min across a span from 30 °C to 600 °C. The procedures included a 10-minute isothermal phase at 105 °C to eliminate any remaining moisture, and a nitrogen flow of 60 ml/min throughout the experiment. Around 10 mg of the sample, previously dried in an Isotemp 280A vacuum oven, was loaded on a platinum plate to perform the test. Due to the inherent moisture content of fines, 100% was offset at 100 °C and the temperature at which 5% weight loss (Td5%) was then determined using TA Instruments Universal Analysis 2000 software along with Microsoft Excel. To ensure repeatability, the experiment was conducted three times. 48 3.4.5 1H Nuclear magnetic resonance (NMR) analysis The grafting reaction of maleic anhydride on PLA was investigated through 1H nuclear magnetic resonance (NMR) analysis carried out on a Bruker Advance 300 MHz machine (Bruker Corp, United States). 20 mg of PLA-g-MAH was dissolved into 400 mL of chloroform-D. After thoroughly stirring via vortex mixer, the NMR spectra were then obtained using parameters: 256 scan number, 10 s relaxation delay, and acquisition time of 5.3 s. As a comparison, 20 mg of pure PLA was also examined by NMR under the same parameters and conditions. 3.4.6 Particle size analysis The particle-size distribution of the untreated fines and ball-milled fines was determined by laser light scattering via a Mastersizer 3000 particle size analyzer (Malvern Instruments Ltd, UK). The fine suspension samples were prepared by diluting with water to 0.1 % and sonicating in a sonication bath for 1 hour before test to avoid multiple scattering effects. The mean particle size was determined by volume moment mean diameter, D [4,3], which better indicates the presence of large particles and particle aggregation (Han et al., 2021; Mun et al., 2011). The particle size at 10% (Dv10), 50% ((Dv50), median diameter), 90% (Dv90) of the overall particle volume were also determined (Ahmed et al., 2016). The measurement was carried out in triplicate. The percentage change for each metric was calculated by dividing the difference between its values before and after ball milling by its initial value. Each dispersed fine suspension was also placed in a glass microscope slide via a transfer pipette and examined by a Reichert-Jung Polyvar light microscope to compare the particle shape and size of TMP fines and ball-milled fines qualitatively. 49 3.4.7 Tensile testing The tensile testing method and analysis for this study were rigorously conducted in alignment with the ASTM D638 standard, which is akin to the ISO 527-1 standard. This comprehensive approach encompassed sample preparation, testing, and data analysis to ensure consistency across all samples (ASTM, 2014). Due to the variances in the mold geometries and the thermal expansion coefficient of the material being injected, the actual size of the sample might exceed the tolerance of the standard. The specimen prepared from injection molder was type V and the actual size was taken from each sample by a micro caliper. As shown in Figure 3.3 (ASTM, 2014), the averaged thickness (T) of 3.29 mm and width (Wc) of 3.49 mm of the narrow section were used for calculating the tensile strength. The gauge length (G) was set at 7.73 mm, essential for normalizing strain in subsequent calculation. Figure 3.3 Tension test specimens for molded plastics Tensile tests were conducted by a universal testing machine (Instron 5969 Dual Column Load Frame) at room temperature. Tensile measurement was performed using a 2 kN load cell combined with the Bluehill software at a displacement rate of 5 mm/min, which leads to rupture 50 within the standard testing time range of 0.5 to 5 minutes. A minimum of five specimens were tested for each determination to ensure statistical reliability. According to the requirement of the calculation section of the standard, cross-sectional area (T*Wc = 11.5 mm2) and gauge length (G = 7.73 mm) were used to calculate engineering stress (σ) and strain (ε). The modulus of elasticity was determined by extending the initial linear part of the load-extension curve and computing the ratio of the change in stress for any section of this straight line to the corresponding change in strain (ASTM, 2014). The data (Table 4.2) was shown as an average from five samples that broke within the narrow section. Adhering to the guide, the results reported mechanical properties as averages with three significant figures and standard deviations to two significant figures. The results of mechanical tests were evaluated using a One-Way Analysis of Variance (ANOVA), with probability values of less than 0.05 (i.e., 95% confidence interval) considered as statistically significant. 3.4.8 Scanning electron microscopy (SEM) The morphological examinations of untreated fine and treated fines as well as the fracture surfaces of the fine-based PLA composites after tensile testing were conducted using a Hitachi SU3500 scanning electron microscope (SEM). Prior to measurement, the fractured samples were affixed vertically to SEM stubs and secured onto sample holders with carbon tape. Subsequently, the specimens were coated with a thin iridium layer (5-10 nm). The images were taken at an accelerating voltage of 1.5 kV. 51 3.4.9 Confocal laser scanning microscopy (CLSM) The flat surface of the narrow section of the PLA/fine dumbbell specimen was also observed by a Leica Stellaris laser scanning confocal microscope (Leica Microsystems, Mannheim, Germany) on a DNi8 stand equipped with a Diode 405 nm laser and a HyD S detector, operated with Leica Application Suite X and imaged with an HC FLUOTAR L VISIR 25x/0.95 water objective. 3.4.10 Differential scanning calorimetry (DSC) analysis The evaluation of the physical characteristics and crystallinity of PLA/fine composites was conducted using differential scanning calorimetry (DSC) on a TA Q1000 instrument (TA Corporation, USA). For this analysis, samples were cut from the fracture surface of injection molded dumbbell specimens. The sliced samples were then prepared by accurately weighing approximately 10 mg and encapsulating them in aluminum pans. The measurements were carried out in triplicate, with the temperature increasing to 200 °C at a heating rate of 10 °C/min and cooled from 200 °C to 30 °C at a rate of 2 °C/min, under a protective atmosphere of nitrogen gas flow at 60 ml/min. The thermal transitions, including the glass transition temperature (Tg), cold crystallization temperature (Tcc), melting temperature (Tm), and the enthalpy of melting (ΔHm), were identified based on the endothermic peaks through linear peak integration. The crystallinity level (Xc) of the samples was assessed using the formula: Xc (%) = (ΔHm – ΔHcc) / (ΔH°m * w) * 100 where ΔHcc is the cold crystallization enthalpy, and ΔH°m denotes is the melting enthalpy for fully crystalline PLA (93.6 J/g), and w is the PLA content in the composite by weight percentage (Song et al., 2021). 52 3.4.11 Rheometry Rheological measurements of PLA and PLA/fine composites were conducted using a rotational rheometer AR-2000 from TA Instruments, USA, equipped with a parallel plate geometry (plate diameter: 25 mm). The disc-shaped specimens (25 mm diameter, 1 mm thickness) were injection molded. A fixed gap of 1 mm between the plates and a nitrogen atmosphere were maintained throughout all experiments. The temperature was set at 190 °C as this is above the melting temperature (Tm 165-180 °C) of the studied PLA. At this temperature, the PLA is completely melted, ensuring that the viscosity is relatively low. It also remains below the onset of degradation for PLA (Td5% 332 °C) as shown in the appendix, and fines (Td5% 216-250 °C) as indicated in Figure 4.2 (c). To assess the linear and non-linear viscoelastic characteristics of the samples, strain sweep tests were conducted in the strain range of 0.01%-100% at a fixed frequency of 0.1 Hz in oscillatory mode. The storage modulus (G’) and loss modulus (G”) were recorded by conducting frequency sweep tests on each sample. The complex viscosity (η*) was determined as a function of frequency (ω) based on the formula: η* = √(𝐺’ω)2+ (𝐺"ω)2 These tests ranged from low to high frequencies (0.1 to 100 rad/s) at a fixed strain of 0.5%, within the linear viscoelastic region. 3.4.12 Printability test PLA/fine filaments were printed using an Original Prusa MK4 FDM 3D printer with a nozzle diameter of 0.4 mm to obtain the final 3D printed products. The filaments were fed into the 3D printer after the nozzle temperature reaching 210 °C and build plate temperature reaching 60 °C. 53 The layer height of the printed part was set to 0.2 mm, and the filling density was established at 15% using a grid pattern. Printing speeds for solid infill and top infill were set at 90 mm/s and 40 mm/s, respectively. These relevant parameters could be adjusted via slicer software to tune the quality of the printed objects. Samples that did not print completely due to clogging were discarded and reprinted. 54 Chapter 4: Results and Discussion 4.1 Fine treatment (mechanical and chemical) and MAPLA synthesis 4.1.1 The morphology and particle size analysis of thermomechanical pulp fines, before and after ball milling The morphology of TMP fines was observed through microscopy (Figure 4.1 (a-b)), which provided a clear visualization of the heterogeneous shape of the fines. To minimize agglomeration, TMP fines were freeze-dried before analysis. The presence of both flake and fibrillar fines highlighted the diverse shapes (and potential compositions) inherent in TMP fines. The observed agglomerates, represented by large dark areas in the optical microscopic image (Figure 4.1 (a)), indicated the natural tendency of fine fibres to adhere and entangle into clusters. The laser diffraction determined the particle size distribution TMP fines (Figure 4.1 (e)). During the measurement, unmodified TMP fines exhibited multi-modal distributions (Laouchedi et al., 2017). The presence of larger size distributions potentially caused by particle agglomeration before milling underscores the challenge in achieving uniform dispersion of untreated and dried TMP fines in PLA matrix. The effects of ball milling on the fines were also investigated through optical microscopy, SEM and laser diffraction, which revealed a significant impact of mechanical processing on the morphology of fines, especially on their size distribution. After 10 minutes of ball milling, Figure 4.1 (c-d) revealed a partial breakdown of the original TMP fines into smaller pieces and fragments. This visual observation was pivotal as it qualitatively confirmed the effectiveness of ball milling in mitigating fines’ agglomeration and reducing particle size (Figure 4.1 (c-d)). The results of laser diffraction quantified the impact of 10 mins of ball milling on the particle size 55 distribution of fines and offered a more detailed analysis, suggesting the same trend as the microscopy results (Figure 4.1 (e)). Significantly, after ball milling, the fines exhibited more particles with size below 10 µm and agglomerates above 100 µm. This shift was indicative of the ball milling’s efficiency in breaking down larger aggregates into finer particles. The detailed particle size metrics and the percentage change of each metric after ball milling are listed in Table 4.1. The 10% (Dv10), 50% (Dv50), and 90% (Dv90) volume distribution of TMP fines, were 12.63 µm, 39.17 µm, 129.60 µm, respectively. The corresponding values for ball-milled fines were 4.09 µm, 13.93 µm, 102.17 µm, showing the particle size decrease after ball milling. Moreover, the decrease in the volume mean diameter (D [4,3]) from 77.56 µm to 45.2 µm confirmed a reduced average particle size of ball-milled fines. These metrics provided a comprehensive view of the overall effect of ball milling, emphasizing the process’s efficacy in achieving a smaller particle size distribution for the fines. Note the volume mean diameter of the untreated TMP fines is near that of the fine classification size (less than 76 µm). Table 4.1 The particle size metrics of TMP fines before and after ball milling Particle size metrics TMP fines (µm) Bm (ball-milled) fines (µm) Change (%) Dv10 12.63 4.09 67.62 Dv50 39.17 13.93 64.44 Dv90 129.60 102.17 21.17 D [4,3] 77.56 45.20 41.72 Figure 4.1 (e) also presents the images of a water droplet captured on the surface of TMP fines and ball-milled fines at 0 seconds. The measured WCAs for both were less than 90°, classifying 56 them as hydrophilic (Samyn, 2013; T. Zhao & Jiang, 2018). It was reported that the WCA of pulp fibres typically ranges between 30-50° (Jin et al., 2022). Therefore, the initial contact angles of 72° for TMP fines and 70° for ball-milled fines likely result from a high percentage of lignin and extractives within the fines, as suggested by previous literature (Koljonen & Stenius, 2005; Spence et al., 2010). Figure 4.1 Microscope images of (a-b) TMP fines and (c-d) ball-milled (Bm) fines; (e) Particle size distribution of fines with water contact angle 4.1.2 Modification of fines using (2-Dodecen-1-yl)succinic anhydride (DDSA) DDSA was chosen to esterify fines to create a more hydrophobic filler. During the esterification process, DDSA and imidazole (as a catalyst) form an acyl transfer agent, introduced alkynyl chains onto the available aliphatic primary OH groups on the surface of the fines, as shown in Figure 4.2 (a) (Beaumont et al., 2021; Jubinville et al., 2022). This modification can increase the hydrophobicity of fines, which may weaken their intermolecular hydrogen bonding, thus reduce agglomeration and promote interaction with PLA through secondary interactions. The degree of 57 this esterification yielded a weight gain of 51.77% and was examined through FTIR analysis and WCA tests. These methods provide a qualitative analysis of the alteration in functional groups and hydrophobicity, offering a comprehensive understanding of the chemical treatment’s effects on fine materials. FTIR analysis serves as a powerful tool in identifying the functional groups present in molecules especially for ones that cannot be dissolved and analyzed with other methods. Figure 4.2 (b) displays the FTIR spectra of untreated TMP fines and fines after DDSA modification. The broad band observed at 3000-2800 cm-1 in the spectrum of the unmodified fines (Figure 4.2 (b)) is indicative of aliphatic C-H stretching and bending vibrations (Gnanasambandam & Proctor, 2000). The differentiation of this broad band into several distinct signals in the spectrum of DDSA fines (Figure 4.2 (b)) suggests a variety of aliphatic C-H groups in the DDSA chain with their asymmetrical and symmetrical stretching (Yingting Guo & Bustin, 1998). Among these, strong intensities observed around 2925 cm-1 and 2850 cm-1 correspond to aliphatic CH2 groups (Igisu et al., 2018; Jandura et al., 2000). The alkene C-H stretching band was found around 3150 cm-1, and moderate absorption near 1600 cm-1 may represent the vinyl groups, which also mark the incorporation of the long carbon chains onto the surface of fines from DDSA (Silverstein & Webster, 1998). Furthermore, the presence of additional ester groups in the modified fines is demonstrated by strong bands around 1739 cm-1, attributed to the ester carbonyl (C=O) groups (Yingting Guo & Bustin, 1998; Jandura et al., 2000; Tao et al., 2021) and 1163 cm-1, associated with C-O stretching vibrations in esters (Gnanasambandam & Proctor, 2000; Niu et al., 2017). These findings indicate that the hydroxyl groups on the surface of the fines were effectively esterified, introducing new ester functionalities. Despite these modifications, the bands within 58 3600-3100 cm-1 maintain strong intensities in both untreated and DDSA fines, suggesting the preservation of hydroxyl groups (Maréchal & Chanzy, 2000). This observation could be attributed to the fact that DDSA modification primarily occurs on the surface of the fines, while inner hydroxyl groups can also be detected by FTIR if they vibrate and absorb the radiation. This result is highlighted in previous research around the oxidation of cellulose pulp (Li & Renneckar, 2011). Another possible explanation for the -OH absorption could be the formation of free carboxylic acids through anhydride esterification. The WCA analysis complements the FTIR findings by providing insights into the changes in surface hydrophobicity of fines. As mentioned in the previous section, the measured WCA of the TMP fines was around 72° (Figure 4.2 (b)) and the water droplets were rapidly absorbed within 10 seconds, indicating efficient wicking into the TMP fines mat. Conversely, DDSA modification significantly increased the hydrophobicity of fines, resulting in a measured WCA of around 126° (Figure 4.2 (b)). This significant change in wettability remained stable even after an absorption test time of 90 seconds. The enhancement in the hydrophobic properties underscores the successful substitution of hydroxyl groups with hydrophobic long aliphatic chains of DDSA on the surface of fines, corroborating findings from a previous study (Jubinville et al., 2022). Moreover, the thermal gravimetric curves (Figure 4.2 (c)) revealed decreased thermal stability in the fines after DDSA modification. The Td5% for the DDSA-modified fines was around 216 °C, which was lower than the 250 °C for unmodified TMP fines. Previous studies also observed less thermally stable esterified starch, sander dust, cellulose, and hemicellulose (Hansen & Plackett, 2011; Y. Sun et al., 2018; Vaidya et al., 2016). 59 In the derivative thermogravimetric curve, the thermal degradation of DDSA occurred within the temperature range of approximately 150-250 °C, as shown in Figure 4.2 (c). Consequently, the first degradation stage of DDSA fines, happening around 200-300 °C, might be attributed to the breakdown of ester groups and unsaturated bonds in the side chains introduced by DDSA. These changes in the fines’ composition could reduce their stability at high temperatures (Hou et al., 2021). Figure 4.2 (a) Reaction scheme of (2-dodecen-1-yl)succinic anhydride (DDSA) esterification of fines; (b-c) characterization of TMP fines and DDSA fines: (b) FT-IR spectrum with water contact angle; (c) TGA analysis 60 4.1.3 The synthesis of MAPLA Another method for enhancing the adhesion between fibre and matrix is by the use of a coupling agent that reacts with fibre surface and interacts with the polymer. Figure 4.3 (a) illustrates the chemical grafting of maleic anhydride onto the PLA backbone using DCP as an initiator. This reaction, achieved through free radical mechanisms via reactive extrusion, has already been detailed and experimented with various polyolefins in the literature (M. E. González-López et al., 2018; Rigotti et al., 2022). The recognized grafting process involves the generation of primary radicals via the decomposition of DCP, which subsequently initiates the formation of PLA macroradicals by extracting hydrogen atoms from the PLA framework. These macroradicals provide reaction sites for bonding MA to the PLA through the unsaturated bond of the maleic anhydride. The NMR spectra of the neat PLA and MAPLA are presented in Figure 4.3, along with their chemical structures corresponding to each spectrum. The assignment of the 1H NMR signals was supported by the literature (Hwang et al., 2012; Orozco et al., 2013; Quero et al., 2012). In the spectrum of pure PLA (Figure 4.3 (b)), the highest intensity peaks, located at 1.8 ppm, indicated the internal –CH3 groups within the polymeric chain, corresponding to the protons labeled as “3” in the chemical structure of PLA. The low-intensity signals near these peaks are associated with the terminal protons of the –CH3 groups, labeled “1” and “5”. Meanwhile, the signals for the internal –CH methine protons of PLA, labeled “4”, were observed as peaks around 5.1 ppm. The weak signals at each side of the peaks are linked to the protons of –CH methine groups, labeled “2” and “6”, next to the terminal alcohol and acid groups (Orozco et al., 2013; Quero et al., 2012). 61 After the grafting reaction, the protons of the MA moiety and the succinyl anhydride groups (product of MA after loss of unsaturated bond) are generally found in the region of 2-3 ppm (Hwang et al., 2012). A peak at 2.1 ppm in Figure 4.3 (b) was evident for the MAPLA samples but not for PLA samples, indicating the presence of protons labeled as “7” and “8” and confirming the grafting of MA. Compared to the strong signals of the PLA’s internal groups shown in the spectrum, the signals of the protons in MA were relatively weak due to the small amount of grafted MA. Moreover, the signal of the “7” proton can overlap with the peaks of the inner −CH3 groups of PLA, making the signal more difficult to be identified (Orozco et al., 2013). Although the NMR spectra indicated successful grafting of MA groups onto the PLA chains, two peaks positioned at 6.5 and 7.0 ppm (representing protons labeled as “9”), which appeared only in the MAPLA spectra, demonstrated that the MAPLA synthesized in the lab still contained some unreacted MA. The NMR trace for MAPLA synthesized by Quero et al (2012) also showed these peaks, suggesting the presence of unreacted MA and ring-opened MA ester derivatives. They suggested that the MA not bonded to PLA could be removed through purification. Another study specified that the unreacted MA can be eliminated by dissolving it in chloroform and precipitating it into excess methanol (Gardella et al., 2014). The effectiveness of purification remains a topic for future research of this study. 62 Figure 4.3 (a) Scheme of reaction between maleic anhydride (MA) and PLA; 1H NMR spectra of (b) PLA and (c) MA-grafted PLA (MAPLA) 4.2 The properties of PLA/fine composites 4.2.1 Visual appearance of PLA/fine composites The visual appearances of dogbone-shaped specimens, crafted from neat PLA and PLA mixed with fines subjected to various treatments or at different loadings, were depicted in Figure 4.4. The neat PLA composite exhibited a transparent and consistent appearance (Figure 4.4 (a)), which was noticeably altered upon adding fines. Specifically, composites with 10% TMP fines, regardless of coupling agent use, and those with 20% TMP fines (Figures 4.4 (b, e-g)) clearly displayed the fine particles at the surface of the samples. In contrast, composites containing 10% ball-milled fines, with or without coupling agents, had a minimal amount of visible fine particles at the surface, presenting more uniform coloration and suggesting a more even distribution of 63 fines within the PLA matrix (Figures 4.4 (c, k, l)). However, increasing ball-milled fines content (Figures 4.4 (c, i, j)) resulted in visible fine particles on the sample surfaces. While both 20% TMP and balled milled fines composites revealed fine particles, the surface of the sample with 20% TMP-masterbatch fines processed through the masterbatch step was smooth and flawless (Figure 4.4 (h)). In general, the colour of the composites turned brown after the addition of fines and the colour varied based on the inclusion of both coupling agents and fine contents. Generally, adding fines led to a brown coloration in the composites, with slight variations depending on the coupling agents and fines content. Previous studies have shown that small-sized wood materials, like wood flour or rice hulls less than 106 µm, act as pigments, reflecting the color of the wood fillers (Petchwattana et al., 2019; Petchwattana & Covavisaruch, 2013). Guen et al. (2019) attributed the brown colour to carbohydrate oxidation and the Maillard reaction at similar processing temperatures. Notably, the composite with DDSA fines turned exceptionally dark brown (Figure 4.4 (d)), likely due to the lower thermal stability of DDSA fines, as suggested by TGA results (Figure 4.2), indicating potential degradation. Figure 4.4 Imagery of injection molded pure PLA and PLA/fine composites: (a) PLA; (b) 10TMP; (c) 10Bm; (d) 10DDSA; (e) 10TMP_MAPP; (f) 10TMP_MAPLA; (g) 20TMP; (h) 20MTMP; (i) 20Bm; (j) 30Bm; (k) 10Bm_MAPP; (l) 10Bm_MAPLA (Bm: ball-milled, DDSA: (2-dodecen-1-yl)succinic anhydride, MAPP: maleic anhydride-grafted polypropylene, MAPLA: maleic anhydride-grafted PLA, MTMP: TMP fines masterbatch) 64 4.2.2 Tensile properties of PLA/fine composites The relevant mechanical data derived from tensile tests on pure PLA and PLA/fine composites and their statistical analysis were arranged by treatment and fibre loading (Table 4.2). The data show that incorporating original TMP fines into the composites resulted in an enhanced tensile modulus, which is verified by statistical analysis (Table 4.2). This enhancement is attributed to the inherent rigidity and stiffness of wood fillers, which generally surpass those of most thermoplastics (Lendvai et al., 2023). In terms of other mechanical properties, pure PLA demonstrated superior performance over all composites containing fines in tensile strength, elongation at break, and toughness. This discrepancy underscores the challenges of integrating wood materials into PLA, mainly due to their poor compatibility. To improve the mechanical performance of PLA composites with fines, the study successfully applied various techniques, including ball milling, surface modification and the use of coupling agents (as highlighted in Figure 4.5), achieving enhancements in several areas. 65 Table 4.2 Mechanical properties of the neat PLA and PLA/fine composites from tensile tests with statistical analysis (Bm: ball-milled, DDSA: (2-dodecen-1-yl)succinic anhydride, MAPP: maleic anhydride-grafted polypropylene, MAPLA: maleic anhydride-grafted PLA, MTMP: TMP fines masterbatch. *Values marked with different letters are considerably different at an α level of 0.05.) Sample (w/w) Tensile modulus (MPa) Tensile strength (MPa) Elongation at break (%) Toughness (J/m3) Avg Std Class* Avg Std Class* Avg Std Class* Avg Std Class* PLA 796 36 ae 63.0 1.6 a 22.0 2.5 a 1028 133 a 10% TMP fines 925 18 bcd 55.7 2.7 bc 8.41 0.3 bd 276 23 bd 10% Bm fines 874 21 acd 59.0 1.5 abc 9.70 1.3 bc 342 62 bc 10% DDSA fines 848 30 ade 54.4 2.1 cd 8.82 0.9 bd 288 47 bd 10% TMP fines_MAPP 779 26 e 46.1 1.3 de 13.3 1.3 e 428 61 bc 10% TMP fines_MAPLA 889 17 cd 55.9 1.6 bc 10.7 0.2 bce 362 10 bc 20% TMP fines 1084 44 f 42.1 6.7 e 5.87 1.3 df 141 40 de 20% MTMP fines 930 24 bc 54.7 1.0 c 10.8 1.1 bce 358 42 bc 20% Bm fines 977 34 b 56.4 1.2 bc 12.6 0.6 ce 412 23 bc 30% Bm fines 892 61 cd 29.2 2.5 f 3.92 0.5 f 62.5 11 e 10% Bm fines_MAPP 787 38 e 48.4 1.4 d 20.1 2.9 a 744 126 f 10% Bm fines_MAPLA 803 12 ae 61.0 1.3 ab 13.0 1.0 ce 486 50 c 66 Figure 4.5 Three treatments applied to improve the mechanical performance of PLA composites with fines To facilitate a detailed comparison of the impacts of different treatments, Figure 4.6 (a) outlines the representative stress-strain curves for pure PLA and its composites with TMP fines, ball-milled fines, surface-treated DDSA fines at a concentration of 10%, and TMP fines (10%) integrated with coupling agents. Please note that in the subsequent results and discussion, when referring to composites containing fines, the 10% loading content by weight may not be explicitly mentioned. However, for loading contents of 20% or 30%, these percentages will be specified. Figure 4.6 (b) demonstrates the corresponding values of tensile strength and modulus for pure PLA and the PLA/fine (10%) composites. Following the classification of polymeric materials, different types of behaviour can be observed in Figure 4.6 (a) (Singh et al., 2020). Firstly, pure PLA showed ductile behaviour and plastic 67 deformation. Upon achieving the peak yield value, a reduction in stress ensued, potentially indicative of localized necking followed by rupture (Singh et al., 2020). Because these tests were performed at room temperature, maximum elongation at break occurred under 25% strain. The inclusion of fines changed the behaviour pattern, with ductile failure turning to brittle failure without showing any necking. The transition to brittle behaviour reduced tensile strength and significantly diminished the elongation at break and the toughness of the composites incorporating 10% TMP fines. The suboptimal stress transfer from the PLA matrix (see below for fracture analysis) to the fines and the weak interfacial adhesion between them were identified as possible causes for this brittle failure. Figure 4.6 (a) Representative stress-strain curves; (b) strength and modulus for neat PLA and PLA composites containing 10% TMP fines, ball-milled (Bm) fines, (2-dodecen-1-yl)succinic anhydride (DDSA) fines and TMP fines with coupling agents Nonetheless, it is apparent from Figure 4.6 (a) that ball-milled fines composites possessed better tensile strength, elongation at break, and toughness relative to TMP fines composites. This enhanced performance could be attributed to the smaller average particle size and fewer 68 aggregates of mechanically treated fines, which ensure better homogeneity and dispersion within the PLA matrix. The mechanical processing also eliminated fine agglomerates, thereby preventing failure and crack propagation initiated by these agglomerated defects. While the reduced particle size of the ball-milled fines caused a slightly lower tensile modulus than composites with TMP fines, but still higher than neat PLA (Figure 4.6 (b)). The improvements in tensile strength and modulus for DDSA fines composites and TMP fines composites with coupling agents were not substantial (Figure 4.6). This limited enhancement might be due to the decreased thermal stability and potential degradation during compounding of DDSA fines and the plasticizing effect of the coupling agents (Martín E. González-López et al., 2019; Hong & Kim, 2013; Khalid et al., 2008). Moreover, the incorporation of DDSA fines induced chemical reactions during the fabrication process, forming ester groups on the surface of the fines. The chemical modification may facilitate the inter and intra chain mobility of PLA, consequently reducing the tensile modulus of the composites (Altun et al., 2013). On the other hand, the inclusion of coupling agents notably increased elongation at break and toughness of TMP fines composites. As shown in Figure 4.6 (a), the composites with MAPP exhibited modest increase in ductility in tension relative to the other composites due to the plasticization of the MAPP (Xu et al., 2023). However, this plasticization effect is less pronounced with MAPLA coupling agents. In this study, PLA composites were produced using only coupling agents and no fines, serving as a control to isolate the effect of the coupling agents on PLA. Figure 4.7 illustrates that incorporating coupling agents did not result in a notable plasticization effect or significant enhancements. This limited impact may be due to the coupling agents disrupting the continuity of the original PLA matrix. 69 Figure 4.7 Tensile properties for neat PLA and PLA composites with coupling agents Given the efficiency of ball milling in enhancing the dispersion of fines within the PLA matrix and the toughening effects of adding coupling agents, the study aimed to explore the potential for further enhancing the overall performance of the composites by incorporating coupling agents into the PLA composite containing ball-milled fines. In an effort to optimize the fine-loading content, the first method involved increasing the content of ball-milled fines to 30%. Another method entailed using a masterbatch to produce PLA composites containing 20% fines, as highlighted in Figure 4.8. 70 Figure 4.8 Two physical methods to optimize the fine-loading content and combining ball milling and coupling agents to further improve the mechanical performance of PLA composites with fines The representative stress-strain traces, tensile strength and modulus for PLA composites containing TMP fines and ball-milled fines at various loadings, as well as PLA/10% ball-milled fines composites with coupling agents were compared in Figure 4.9. Increasing the filler content (TMP fines and ball-milled fines) from 10% to 20% resulted in a higher tensile modulus for the composites (Figure 4.9 (b)). Notably, the modulus value of the composite with 20% TMP content, marked as “f” (Table 4.2), was significantly higher than all other values. This increase aligns with the principle of mixture that fillers with greater stiffness than the polymer matrix can improve the composite’s tensile modulus, a trend that continues with increasing filler content, as demonstrated in previous studies (Altun et al., 2013; Pilla et al., 2008). Fibre geometry plays a role as well; the reduced tensile modulus in composites with ball-milled fines relative to the 71 TMP fines could likely be attributed to their smaller particle sizes and shorter fibre lengths, which result in decreased stiffness. As shown in Figure 4.9 (a), incorporating 20% TMP fines into the composites led to reductions in tensile strength, elongation at break, and toughness. This outcome suggested that the composites exhibit more brittle behavior, with an increased concentration of fines likely resulting in a higher number of stress concentrators throughout the cross-section. These concentrators interfere with the continuity of the PLA matrix, thereby facilitating failure. However, composites containing 20% TMP fines, when produced using a masterbatch technique, exhibited tensile properties comparable to those made by directly blending 10% TMP fines with PLA via an extruder. This finding demonstrates the efficacy of the masterbatch process in achieving a more uniform distribution of fines within the PLA matrix. Interestingly, the tensile strength did not significantly decrease when the content of ball-milled fines was increased from 10% to 20% (Figure 4.9). Additionally, an improvement in elongation at break and toughness was observed with a 20% ball-milled fines loading (Figure 4.9 (a) and Table 4.2). Although further analysis is required to fully explain this phenomenon, it may suggest that ball milling aids the fines in mitigating their negative impact on the PLA matrix. However, the beneficial effect of adding ball-milled fines to a composite has its limits. For instance, introducing 30% ball-milled fines resulted in a significant decrease in the tensile properties of the composite, indicating that there is an optimal filler content threshold for maximizing mechanical performance. 72 Figure 4.9 (a) Representative stress-strain curves; (b) strength and modulus for PLA composites containing TMP fines and ball-milled (Bm) fines at various loading content and 10% Bm fines with coupling agents The addition of coupling agents led to a decrease in the tensile modulus while notably improving the elongation at break and toughness of the PLA/10% Bm fines composites, although the effects on tensile strength varied, as shown in Figure 4.9. Importantly, the inclusion of MAPLA did not compromise the tensile strength of the composites, which could be attributed to MAPLA’s compatibilization effect. This effect might be further enhanced by increasing the grafting degree and the purity of MAPLA. Zhu et al. (2012) explored the compatibilizing influences of MAPLA on PLA/soy protein composites, revealing that the tensile strength incrementally increased with the grafting degree of 4 phr MAPLA. Conversely, the addition of MAPP reduced tensile strength, likely because of its plasticizing effect. The use of coupling agents further demonstrated the benefit of ball milling in counteracting the adverse effects of fines on the PLA matrix. For instance, in Figure 4.9, significant rubber toughening mechanisms, such as plastic deformation before fracture (referred to as “shear yielding”), were observed in the stress-strain traces of PLA/10% Bm fines with MAPP (Kaynak & Meyva, 2014). However, adding MAPP to PLA 73 composites containing TMP fines only showed modest ductile behavior (Figure 4.6). According to the data in Table 4.2, adding MAPP resulted in a 107% increase in elongation at break and a 117% increase in toughness for 10% ball-milled fines composites, whereas the improvements for 10% TMP fines composites were only 59% and 55%, respectively. This indicates a more favorable interaction between coupling agents and ball-milled fines compared to TMP fines. The preferable interaction could be attributed to the smaller particle size and fewer aggregates of ball-milled fines, which increase the surface area of the fines, leading to stronger compatibilization with the coupling agents (Kaynak & Meyva, 2014). Additionally, a more even distribution of ball-milled fines in the PLA matrix further enhances the compatibilization effect. Thus, through the application of ball milling or its combination with the addition of MAPLA, the tensile strength of the resulting composites was found to be on par with that of pure PLA, exhibiting no significant difference (Table 4.2). Regarding elongation at break, the PLA and ball-milled fines composites with MAPP attained a value that was not significantly different from that of pure PLA (Table 4.2). The column chart (Figure 4.10) more effectively illustrates the treading impacts of the content of fines on the elongation at break and toughness, as well as enhanced toughening effects achieved by combining ball milling with coupling agent treatment. 74 Figure 4.10 Elongation at break and toughness for PLA composites containing 10% TMP fines and 10% ball-milled (Bm) fines with coupling agents, TMP fines and Bm fines at various loading content However, the synergistic toughening effects anticipated from the combination of ball milling and coupling agent treatment were not observed when the ball-milled fine loading content reached 20%, as illustrated in Figure 4.11. This outcome suggests that the functionality of the coupling agents may be hindered by the elevated content of ball-milled fines. Alternatively, the toughening effects might be reinstated by increasing the quantity of coupling agents, a prospect that warrants further investigation in future studies. 75 Figure 4.11 Tensile properties for PLA composites containing 20% ball-milled (Bm) fines and the composites with coupling agents 4.2.3 The fracture surface of PLA/fine composites To gain insights on the failure nature of the composites and investigate the interface interactions between fines and PLA, the fracture surfaces of the injection-molded specimen after tensile testing experiments was investigated by SEM. The SEM images of fracture surfaces of pure PLA and PLA composite specimens containing 10 % fines with different treatments are shown in Figure 4.12 and Figure 4.13. The fracture surface of pure PLA appeared clean and smooth with some distinct furrow marks at low magnification (Figure 4.12 (a)), while closer observation in Figure 4.12 (d) revealed drawing fibrils suggesting higher ductility (Todo & Takayama, 2011). In contrast, adding TMP fines significantly increased the degree of inhomogeneity due to the aggregation of fines, which caused higher roughness of fracture surfaces with visible phase separation (Figure 4.12 (b)). Parallel to the result of the tensile tests, TMP fines disrupted the PLA matrix’s continuity and 76 acted as stress concentration points as suggested by previous work, lowering the tensile strength, elongation at break and toughness of the composites (Moradi & Yeganeh, 2020). Upon closer examination (Figure 4.12 (e)), layers of TMP fine aggregates were visibly present at the PLA/TMP fines fracture interface, surrounded by large and deep cracks. These cracks indicate a brittle structure and weak bonding caused by stress concentrators arising from poor compatibility between the surface of the fines and the hydrophobic surface of PLA (Tao et al., 2017). The inhomogeneity at the fracture surface reduced with addition of ball-milled fines, as reflected in the higher tensile strength of PLA/ball-milled fines composites. Besides demonstrating a smoother fracture surface, ball-milled fines showed uniform dispersion without fine aggregates (Figure 4.12 (c)). In the high magnification image, fine breakage could be observed, as highlighted by the yellow arrows (Figure 4.12 (f)). This result may be from damage during milling or compounding or from adequate interfacial bonding, as the fines were not stripped from the PLA matrix. If the latter, the ruptured ball-milled fines suggest that stress was effectively transferred from the PLA matrix to the fillers, further explaining their high tensile strength (Ning et al., 2015). However, there were still gaps at the PLA- ball-milled fines interface with no evidence of polymer or particle deformation. These gaps indicate that the compatibility between ball-milled fines and PLA was limited despite the enhancements from ball milling. 77 Figure 4.12 Low and high magnification SEM images of the fracture surface of (a, d) neat PLA; PLA composites containing (b, e) 10% TMP fines; (c, f) 10% ball-milled (Bm) fines Microscopy analysis of the samples shows the impact of chemical treatment and coupling agents on the composites. Although the tensile properties were not improved by the DDSA modification (Figure 4.13 (a, d)) and the addition of MAPLA (Figure 4.13 (c, f)), the fractured samples seemed to show an enhanced distribution of fines within the PLA. The apparent phase separation was mitigated, and the formation of cavities of various sizes encapsulating the fines was observed, indicated by the yellow arrows (Figure 4.13). The fracture surfaces revealed irregular holes and the rough edges of the voids being the result of fines protruding. However, even with improved distribution, adhesion between the modified fines and PLA was poor. DDSA fines pulled out from the PLA matrix, clearly shown in the high magnification image (Figure 4.13 (d)). This phenomenon is attributed to the weaker interfacial strength between the fines and PLA compared to the cohesive strength of the fines themselves (Pilla et al., 2008). The fracture interface of PLA/TMP fines with MAPP (Figure 4.13 (a, d)) also exhibited rough surfaces and a 78 heterogeneous distribution of fines, mirroring the interface observed in PLA/TMP fines without MAPP. The image in the high magnification also presented deep gaps caused by fines agglomeration, as shown in Figure 4.13 (e), which would have facilitated crack initiation and propagation. This observation seems to suggest that MAPP failed to improve the compatibility and interfacial bonding between the fines and PLA, ultimately compromising the mechanical properties of the composites. Figure 4.13 Low and high magnification SEM images of the fracture surface of PLA composites containing (a, d) 10% (2-dodecen-1-yl)succinic anhydride (DDSA) fines; (b, e) 10% TMP fines with maleic anhydride-grafted maleic anhydride-grafted polypropylene (MAPP); (c, f) 10% TMP fines with maleic anhydride-grafted PLA (MAPLA) In comparing 10% ball-milled fines with and without a coupling agent, the fracture surfaces of PLA/Bm fines (Figure 4.14 (a, d)), PLA/Bm fines with MAPP (Figure 4.14 (b, e)), and PLA/Bm fines with MAPLA (Figure 4.14 (c, f)) are illustrated below. The PLA/Bm fines that included coupling agents exhibited rough surfaces following tensile tests (Figure 4.14 (b, c)). Under 79 higher magnification (Figure 4.14 (e, f)), these rough surfaces indicated plastic deformation that could absorb the energy necessary for initiating and propagating cracks, which explains the observed increase in elongation at break and toughness (Kaynak & Meyva, 2014). Specifically, the sample containing MAPP showed a stretched fracture surface, indicative of higher ductility, as evidenced by the tensile stress-strain curve. Although it failed in a ductile manner, the fracture surface of the PLA/Bm fines with MAPP revealed gaps at the interface between the PLA and the fines, as well as the formation of voids due to fines pullout, highlighted by yellow arrows in Figure 4.14 (e). Based on these observations and the results presented in Table 4.2, the proposed explanation for the reduction in tensile strength associated with the presence of MAPP is a significant weakening of the interfacial adhesion between the PLA matrix and ball-milled fines or the polypropylene segments of MAPP. On the contrary, the introduction of MAPLA did not diminish the tensile strength of the PLA/Bm fines. As shown in Figure 4.14 (f), the fines are more tightly encased in the PLA matrix, exhibiting narrower gaps at the interface with the occurrence of MAPLA. This may suggest improved compatibility, albeit without a significant increase in interfacial adhesion, as evidenced by the fines’ pullout indicated by the yellow arrows. 80 Figure 4.14 Low and high magnification SEM images of the fracture surface of PLA composites containing (a, d) 10% ball-milled (Bm) fines; (b, e) 10% Bm fines with maleic anhydride-grafted polypropylene (MAPP); (c, f) 10% Bm fines with maleic anhydride-grafted PLA (MAPLA) 4.2.4 Fines distribution of PLA/fine composites To better represent the distribution of fines, laser scanning fluorescent confocal microscopy was used to image the injection-molded PLA/fine composites (Figure 4.15). The black and white regions in the images represent the PLA matrix and the fines, respectively. Generally, the autofluorescence of lignocellulosic materials is attributed to the presence of lignin fractions, due to the intrinsic property of lignin chromophores that naturally fluoresce (Chimenez et al., 2014). Although autofluorescence in cellulose is recognized, the mechanism behind it remains partially unclear. The most accepted explanation is the clustering-triggering emission hypothesis. This hypothesis suggests that the autofluorescence in cellulose arises from the interaction of electron-rich molecular components such as carbonyls, esters, hydroxyls, and imides, which reduce 81 energy barriers and enhance electron mobility—a process that is further facilitated by strong hydrogen bonds in carbohydrate compounds (Teodoro et al., 2024). In the control experiment, the PLA matrix (Figure 4.15 (a)) was not visible in the fluorescence micrograph and appeared as a black region. Meanwhile, Figure 4.15 (b) shows the fines distributed throughout the matrix, as indicated by the white regions. It is apparent that the ball milling of fines altered their shape and size. The reduction in the areal size of the black regions and the increase in the white region in Figure 4.15 (c) indicate a more uniform dispersion of ball-milled fines as suggested in the SEM analysis of the fractured surface. Moreover, as suggested in the literature, mechanical milling could convert the phenylcumaran compounds present in fines into phenylcumarones, thus inducing autofluorescence (E. Kalita et al., 2015). Interestingly, the fluorescence intensity was weakened by the chemical modification of the fines (Figure 4.15(d)). This reduction is likely due to the introduced carboxyl groups acting as electron acceptors, which disrupt the flow of π-electrons and consequently reduce the fluorescence capability, as indicated by the literature (Ding et al., 2020). However, when comparing Figure 4.15 (b) with (e, f) and Figure 4.15 (c) with (h, i), the effects of the coupling agents on the distribution of fines were not readily apparent. For higher loading contents, denser regions of fines were observed, evidenced by the presence of whiter regions and a reduced number of black regions (Figure 4.15 (g)). Overall, it appears that all samples had good distribution throughout the cross section of the materials. 82 Figure 4.15 CLSM images of (a) PLA; (b) 10TMP; (c) 10Bm; (d) 10DDSA; (e) 10TMP_ MAPP; (f) 10TMP_ MAPLA; (g) 20TMP; (h) 10Bm_ MAPP; (i) 10Bm_ MAPLA (Bm: ball-milled, DDSA: (2-dodecen-1-yl)succinic anhydride, MAPP: maleic anhydride-grafted polypropylene, MAPLA: maleic anhydride-grafted PLA) 4.2.5 DSC analysis of PLA/fine composites To further elucidate the impact of fines modification and content to the stiffness of PLA, differential scanning calorimetry (DSC) was employed to study the crystallization behavior of both PLA and PLA/fine composites as crystallinity has been considered highly relevant to the stiffness of semicrystalline polymeric materials. The obtained thermograms are presented in 83 Figure 4.16, while Table 4.3 provides the corresponding numerical values during the first heating cycle. The thermograms revealed that all tested composites exhibited peaks related to glass transition (Tg), cold crystallization, recrystallization, and melting. The incorporation of 10% fines and treated fines had a minimal impact on the thermal properties of PLA including total crystallinity (Table 4.3). With the introduction of MAPP, the Tg experienced a slight decrease, implying that PLA may be softened by this coupling agent at a molecular scale (Fico et al., 2022). Additionally, the inclusion of DDSA fines resulted in a minor reduction in the temperature of cold crystallization (Tcc) and an increase in the enthalpy of cold crystallization compared to neat PLA. This effect can be attributed to DDSA fines acting as nucleating agents, perhaps rising from altered interactions, thereby promoting the initial cold crystallization of the PLA matrix (Kuciel et al., 2020). In terms of increasing the content of fines, the presence of more fines throughout the composites had minimal impact on crystallite growth. Surprisingly, there was slight decrease in crystallinity with 20% ball-milled fines and even more for 30% ball-milled fines. The presence of the fines clearly impacted the mobility of polymer chains, as the Tg was supressed (Fazita et al., 2015). The result of decreased crystallinity with increased fine content contrasts with that of most semicrystalline polymers, as fillers typically serve as a nucleating zone. A possible reason could be high concentrations of fines adversely affects the motility of PLA molecular chains, thereby limiting the crystalline matrix formation (Shazleen et al., 2021). Despite no significant difference in crystallinity between the PLA/fine composites and virgin PLA, the tensile modulus of the composites increased due to the presence of stiff wood fibres, rather than an increase in crystallinity. 84 Figure 4.16 Melting curves of neat PLA and PLA/fine composites (first heating cycle) Table 4.3 Thermal characteristics of neat PLA and PLA/fine composites (Bm: ball-milled, DDSA: (2-dodecen-1-yl)succinic anhydride, MAPP: maleic anhydride-grafted polypropylene, MAPLA: maleic anhydride-grafted PLA) Sample (w/w) Tg (°C) Tcc (°C) Tm (°C) ΔHcc (J/g) ΔHm (J/g) Xc (%) PLA 65.56 161.41 176.16 22.58 50.46 26.17 10% TMP fines 63.49 161.45 175.53 23.05 50.67 28.95 10% Bm fines 65.20 161.80 176.59 23.45 49.18 26.12 10% DDSA fines 63.25 158.30 174.26 25.54 53.95 29.65 10% TMP fines_MAPP 63.07 160.56 175.43 20.56 47.30 28.17 10% TMP fines_MAPLA 65.89 160.76 175.21 24.47 51.85 29.96 20% TMP fines 62.13 161.15 174.18 20.95 43.79 27.30 85 20% Bm fines 62.30 160.94 175.02 21.01 43.08 25.23 30% Bm fines 58.41 161.23 172.73 20.45 36.70 23.96 4.2.6 Rheological properties of PLA/fine composites In an investigation into the viscoelastic characteristics of fine-based PLA composites, oscillatory rheological tests were conducted on injection molded disc samples to assess their complex viscosity and dynamic moduli. This study was used to further understand the effects of fine modification and loading on these composites. The rheological data were also used to predict the printability and the quality of the final product, as these properties across a range of shear rates significantly impact the material’s behaviour during the 3D printing process. For instance, viscosity reflects the flowability and extrudability of the melt through the nozzle of the 3D printer, as well as the shape stability after extrusion. Elasticity determines the material’s response to changing stress and strain during printing (Herrada-Manchón et al., 2023). Polymer viscosity typically results from the permanent deformation of molecular chains when they slide past neighboring chains (Jubinville et al., 2022). As illustrated in Figure 4.17, pure PLA exhibited a Newtonian plateau at low frequencies, characterized by mild shear thinning behavior, indicating lower viscosity at higher frequencies/shear rates when tested at 190 C. The introduction of TMP fines altered this behavior to non-Newtonian, introducing increased yield stress in the low-frequency region alongside moderate shear thinning behavior, which is a typical characteristic of the branched or entangled polymer blends (Lv et al., 2016). A pronounced shear thinning property was revealed to enhance the processability of polymers in the printing process 86 in previous studies, because it indicates that the viscosity of the material is low enough to allow extrusion from the nozzle, yet high enough upon exiting to prevent dripping (Bertolino et al., 2021; Compton & Lewis, 2014). This change is likely due to the fines’ tendency to partially agglomerate, potentially disrupting the interactions within PLA matrix. At low frequencies, there was enough time for PLA chains to rebuild disentanglements, while at higher frequencies, the polymer chains were unable to reform the entangled structure (Lv et al., 2016). The inclusion of ball-milled fines resulted in mild shear thinning behavior with a Newtonian plateau at intermediate frequencies, similar to pure PLA, but further reduced the composite’s viscosity. This mild shear thinning behavior was also observed in PLA/TMP fines with MAPLA. For ball-milled fines, the reduction in size and potential spherical shape allows for easier slip and even dispersion, consequently lowering the viscosity. The spherical ball-milled fines might also act as lubricants and pack efficiently within the PLA matrix, facilitating easier flow. A similar effect was observed by incorporating ball-milled rice husk particles into the PLA, which halved the complex viscosity compared to neat PLA (Guen et al., 2019). In other researches, Xianhui et al. (2019) and Vo et al. (2017) suggested that using smaller poplar fibres (less than 180 µm) and shorter stem fragments can reduce viscosity of the composites due to fewer topological contacts between inter-filler particles during flow. The observed decrease in viscosity upon the addition of MAPLA to the PLA/TMP fines composite could be attributed to its lubricating effect or decreased molar mass of the matrix (Lv et al., 2016). Similarly, Du et al. noted a decrease in both the storage modulus (G’) and complex viscosity upon adding more than 3% w/w of PLA-g-St/MAH, which they attributed to the lubricating effect (Du et al., 2017). 87 PLA/DDSA fines and PLA/TMP fines with MAPP exhibited a higher viscosity in the low-frequency region and a more pronounced shear thinning behavior than other composites (Fig, 4.13). Compared to neat PLA, the higher initial viscosity might indicate an improved interaction between PLA and fines, attributed to the vinyl groups of DDSA fines and a network-like structure formed by MAPP, respectively. The improved interaction restricted polymer chain movement and increases resistance to flow. Additionally, at low frequencies, the composites had more time to respond to the applied stress, leading to a greater resistance to flow and higher measured viscosities. As the frequency increases, the fine fillers and additives were expected to align with the flow direction due to higher shear rates, resulting in a further shear-thinning behavior (Zhao et al., 2019). Figure 4.17 Complex viscosity as a function of frequency at 190 °C for neat PLA and PLA composites containing 10% TMP fines, ball-milled (Bm) fines, (2-dodecen-1-yl)succinic anhydride (DDSA) fines and TMP fines with coupling agents Regarding fines loading content, Figure 4.18 demonstrates that a 20% loading of TMP fines in PLA composites exhibited a higher initial complex viscosity compared to 10% and neat PLA. 88 This increase is attributed to greater restriction of polymer mobility, a phenomenon also observed by Jubinville et al. (2022). As the frequency increases, the polymer chains gradually separate from particle surfaces, resulting in lower viscosity. Increasing the loading content of ball-milled fines from 10% to 30% had an altered impact relative to the 20%-30% enhanced the lubricating effect due to the small, spherical particles, as evidenced by the further reduction in viscosity. Introducing a coupling agent to PLA/Bm fine composites did not significantly alter their behavior, highlighting the dominant effect of balled milled fines. Interestingly, at high frequencies, all fine-based composites showed lower complex viscosity than pure PLA. This suggests that rapid oscillations cause the fines to orient in the flow direction and disrupt PLA chain interactions, allowing the PLA polymer chains to disentangle and flow more freely (Bertolino et al., 2021). Although low viscosities at high frequencies or shear rates are favored for ensuring a smooth and energy-efficient extrusion of feedstock materials, if the complex viscosity is too low, the material becomes too fluid to maintain the printed shape. Thus, composites containing more than 20% ball-milled fines might not be ideal candidates for 3D printing. 89 Figure 4.18 Complex viscosity as a function of frequency at 190 °C for PLA composites containing TMP fines and ball-milled (Bm) fines at various loading content and 10% Bm fines with coupling agents The storage modulus (G’) and loss modulus (G”) displayed similar trends with the result of complex viscosity for the different samples when tested at a constant temperature of 190 °C. According to Figure 4.19, the addition of TMP fines (at both 10% and 20% concentrations), DDSA fines, and TMP fines combined with MAPP significantly enhanced the composites’ elastic characteristics at low and intermediate frequencies. This enhancement is attributed to the introduction of rigid particles. Furthermore, the presence of additives with large sizes and irregular shapes might impede molecular motion, leading to an increase in the storage modulus (Guo et al., 2018). Consequently, PLA composites containing ball-milled fines did not significantly contribute to elasticity. As can be seen in Figure 4.20, compared to pure PLA, an increase in the loss modulus was noted for composites containing PLA/DDSA fines, PLA/TMP fines with MAPP, and PLA with 20% TMP fines at low frequencies. This increase is due to improved interaction between fines and PLA for PLA/DDSA fines and PLA/TMP fines with 90 MAPP. The reason for 20% TMP fines could be the formation of physical aggregate barriers. The observed lower dynamic moduli at high frequencies correspond with the fine-based PLA composites’ reduced viscosity in the terminal region when compared to pure PLA. This indicates less entanglement/adhesion between the fines, coupling agents, and PLA. Figure 4.19 Storage modulus as a function of frequency at 190 °C for neat PLA and PLA/fine composites Figure 4.20 Loss modulus as a function of frequency at 190 °C for neat PLA and PLA/fine composites 91 For effective bonding of the printed part, it is preferable that the storage modulus be lower than the loss modulus (G’ < G”) during extrusion. This condition suggests a complete relaxation and diffusion of polymer chains, which facilitates the welding of consecutively deposited layers by the 3D printer (Yang et al., 2021). The tan δ value, which represents the ratio of the loss modulus (G”) to the storage modulus (G’), effectively illustrates the dynamic relationship between these moduli. Figure 4.21 shows tan δ curves as a function of angular frequency for PLA and fine-based PLA composites. A tan δ value greater than 1 indicates that G” exceeds G’, suggesting a more liquid-like nature within the tested frequency range, as observed with pure PLA (Figure 4.21). With the exception of PLA composites containing DDSA fines, TMP fines combined with MAPP, and 20% TMP fines, which exhibited solid-like behavior (G’ > G”) at low and intermediate frequencies, all other samples exhibited a contrasting behavior. Specifically, the tan δ value of pure PLA and the remaining PLA/fine composite samples was above 1 throughout the entire testing range (Figure 4.21), indicating a more liquid-like behavior (G” > G’). This behavior is considered ideal for the printing process. Notably, in Figure 4.21, the tan δ value for PLA gradually decreased with the increase of the frequency, indicating that its storage modulus (G’) was approaching the loss modulus (G”), which is typical of linear polymer melts (Shin & Han, 2017). This trend was predominantly observed in PLA composites containing 10% ball-milled fines and those combined with 10% ball-milled fines and MAPLA, suggesting these materials are more suitable for 3D printing applications due to their favorable viscoelastic properties. Note the rate of polymer material cooling would impact the behaviour as well, and so it is a fine balance of temperature and shear rate for polymer diffusion and welding of deposited layers. 92 Figure 4.21 Tan delta as a function of frequency at 190 °C for neat PLA and PLA/fine composites 4.2.7 3D printing test After a series of experiments, PLA/10% Bm fines filaments were selected for testing the printability of the composite, owing to their overall acceptable mechanical properties and feasible rheological properties. Before printing, a model was created and sliced using software. During this slicing process, the appearance of the model can be adjusted through parameter settings such as printing temperature, printing speed, infill pattern, and fill density. As illustrated in Figure 4.22, the surface roughness of the 3D-printed part, a tree, varied when using different precision parameters and speeds. If the printing speed is too high, there will be less material, resulting in an unfilled pattern; conversely, if the printing speed is too low, the material may accumulate and stack together. Thus, to better control the printability of different fine-based PLA materials, specific parameter settings may be required for each formulation. 93 Figure 4.22 3D printed samples by PLA/10% ball-milled (Bm) fines filaments using different precision (Draft, Speed, Quality) and speed parameters The final products printed with PLA/10% Bm fines filaments exhibited a wooden color and texture. During a continuous printing duration of 45 minutes, the filaments showed no breakage or nozzle obstructions, meeting the requirements for FDM 3D printing. The comparative analysis of the laminated surface and fracture surface of the 3D-printed samples made of pure PLA and PLA with 10% ball-milled fines further confirms its compatibility with FDM 3D printing, as illustrated in Figure 4.23. Figure 4.23 (a, b) shows that the pure PLA samples had notably smooth printed lines and clear boundaries, allowing for precise determination of line width and layer thickness. In contrast, the inclusion of ball-milled fines within the PLA matrix led to a discernible enhancement of surface roughness and variability in the dimensions of the extruded lines, as seen in Figure 4.23 (c). This again indicated that the ball-milled fines were evenly dispersed. Although there was a slight reduction in the clarity of the stacking pattern (Figure 4.23 (d)), the composite demonstrated commendable layer-to-layer cohesion. This investigation substantiates the potential of PLA composites enhanced with 10% ball-milled fines for additive manufacturing, showing satisfactory adhesion between layers and maintaining an acceptable standard of print quality. 94 Figure 4.23 SEM images of laminated surface and fracture surface of the 3D-printed samples made of (a, b) pure PLA and (c, d) PLA with 10% ball-milled (Bm) fines 95 Chapter 5: Conclusions and future work 5.1 Conclusions This thesis aimed to create a more environmentally friendly 3D printing material by incorporating TMP fines into biodegradable PLA. The objective was to compare the interactions of modified fines and coupling agents with PLA, in contrast to unmodified fines. Therefore, this research examined the potential of TMP fines as fillers for PLA and investigated modifications to enhance the performance of fine-based PLA composites for 3D printing applications. Through extensive mechanical, thermal, and rheological testing, the effects of physical and chemical modification methods, as well as coupling agents, on PLA/fine composites were assessed. The morphology of tensile test specimen fracture surfaces and filler dispersion in the PLA matrix were analyzed using scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM), respectively. Additionally, the composite’s printability for 3D printing applications was explored. Injection molded test samples were made to create wood fine filled composites for mechanical and rheological testing. Incorporating TMP fines changed the composites’ color from translucent to brown and increased their tensile modulus, with the highest increase observed at 20% TMP fines. However, the addition led to decreases in tensile strength, elongation at break, and toughness of the composites due to the introduction of stress concentrators. These properties would make the 3D filament brittle. A more pronounced shear thinning behavior was observed in the rheological behavior of the PLA/TMP fines composites, which could potentially facilitate 3D printing. However, achieving smooth, stable, and continuous 3D printing filament from the PLA/TMP fines blend via the extruder and controlled filament collector was challenging due to 96 the heterogeneous dispersion of fines. Moreover, the addition of fines did not cause significant changes to the crystallinity, glass transition, and melting temperature of the composites. It was hypothesized that by employing various treatment methods, such as ball milling, surface modification with DDSA esterification, and the use of coupling agents (MAPP and MAPLA), the dispersion, compatibility, and interfacial adhesion of fines within the PLA matrix could be optimized. The efficacy of these treatments was evaluated by analyzing the mechanical properties of the composites containing 10% w/w fines. • Incorporating ball-milled fines into PLA composites, as opposed to TMP fines, resulted in more consistent coloration, uniform dispersion, and improvements in tensile strength, elongation at break, and toughness. These improvements were attributed to the smaller particle size and better dispersion of ball-milled fines. While the thermal behavior exhibited only minor changes, ball mill fines significantly reduced the PLA composites’ complex viscosity, likely enhancing their flowability through the 3D printer nozzle. The more spherical shape of the ball-milled fines, led to lower flow resistance, explaining this improvement. • The esterification of fines with DDSA introduced alkynyl chains with the substitution of hydroxyl groups, increasing their hydrophobicity. This fine modification step darkened the colour of the composites, suggesting reduced thermal stability. Consequently, no improvements in tensile properties were observed, along with minor changes in Tg, Tm, and crystallinity. As for the rheological behaviour, the PLA composites treated with DDSA fines showed a yield stress and increased storage modulus at low frequencies, indicating an initial strong interaction between the DDSA fines and PLA chains. 97 • Adding both MAPP and MAPLA to the fine-based PLA composites demonstrated a toughening effect on mechanical properties without significantly affecting thermal behaviour in the DSC analysis. MAPP notably enhanced both elongation at break and toughness, but reduced tensile strength of the composites. Although MAPLA’s toughening effect was more subdued, it maintained the composites’ tensile strength. The impact of these coupling agents on rheological behavior of the PLA/TMP fines composites differed: MAPP increased the complex viscosity, storage modulus, and loss modulus, indicating a higher yield stress. In contrast, the addition of MAPLA decreased these values, suggesting a potential lubricating effect with the oxidized PLA chains. The comparison of PLA/fine (10% w/w) composites subjected to various treatments revealed that ball milling is an efficient method for the improved dispersion of fines within the PLA matrix, demonstrating optimal performance among all composites. This method enhanced mechanical properties and reflected the thermal and rheological behaviors of the neat PLA. The impacts of the two coupling agents varied across tests; however, both enhanced the composites’ elongation at break and toughness. To optimize the fine loading content and overall performance further, the study increased the ball-milled fines content to 30% and combined ball milling with coupling agent incorporation. It was observed that ball milling could mitigate the adverse effects associated with the addition of fines, maintaining acceptable performance of the PLA composites even with a 20% concentration of fines. However, when the content of fines was elevated to 30%, the tensile properties of the composites significantly deteriorated. Incorporating MAPP in the 10% ball-milled fines composites yielded the highest elongation at break and toughness among all composites, while adding MAPLA achieved the highest tensile strength. Moreover, 98 the printability of PLA/10% Bm fines filaments was assessed, showing satisfactory printing quality. In conclusion, the objectives outlined in the introduction—to incorporate fines into thermoplastics and explore different modification methods of renewable materials—were successfully met with promising outcomes. The research demonstrated that ball-milled fines could be effectively dispersed in PLA, reducing the viscosity of the material and preventing nozzle blockage during printing without significantly compromising the mechanical strength of the printing materials. 5.2 Impact of research and future studies Strengths of this research include its novel approach to using a by-product from the pulp industry, and its contribution to sustainable manufacturing practices by blending TMP fines with PLA. The potential applications of this research are vast, extending to industries seeking environmentally friendly alternatives to traditional plastic fillers. Limitations are primarily related to the scale of experiments. Thus, future research could focus on several key areas: • Scaling up the production process for industrial applications, including more tests on the masterbatch method, which presents a promising approach to keep fines separated in the PLA matrix and to utilize PLA/fine composite as an industrial product. • Determining the grafting degree of MAPLA and the optimal percentage of coupling agents that can be added to the fine-based PLA composites to enhance their properties. • Creating filaments from other possible formulations and evaluating the mechanical properties of 3D printed specimens to assess their suitability for various applications. • Lastly, examining the environmental impact and biodegradability of TMP fine-filled PLA composites to provide deeper insights into their sustainability benefits. 99 Bibliography Aharoni, S. M., Forbes, C. E., Hammond, W. B., Hindenlang, D. M., Mares, F., O’Brien, K., & Sedgwick, R. D. (1986). High-temperature reactions of hydroxyl and carboxyl PET chain end groups in the presence of aromatic phosphite. Journal of Polymer Science Part A: Polymer Chemistry, 24(6), 1281–1296. https://doi.org/https://doi.org/10.1002/pola.1986.080240614 Ahmed, J., Al-Attar, H., & Arfat, Y. A. (2016). Effect of particle size on compositional, functional, pasting and rheological properties of commercial water chestnut flour. Food Hydrocolloids, 52, 888–895. https://doi.org/10.1016/j.foodhyd.2015.08.028 Åkesson, D., Fazelinejad, S., Skrifvars, V.-V., & Skrifvars, M. (2016). Mechanical recycling of polylactic acid composites reinforced with wood fibres by multiple extrusion and hydrothermal ageing. Journal of Reinforced Plastics and Composites, 35(16), 1248–1259. https://doi.org/10.1177/0731684416647507 Al Hosni, A. S., Pittman, J. K., & Robson, G. D. (2019). Microbial degradation of four biodegradable polymers in soil and compost demonstrating polycaprolactone as an ideal compostable plastic. Waste Management, 97, 105–114. https://doi.org/10.1016/j.wasman.2019.07.042 Albertsson, A. C., & Varma, I. K. (2003). Recent developments in ring opening polymerization of lactones for biomedical applications. Biomacromolecules, 4(6), 1466–1486. https://doi.org/10.1021/bm034247a Altun, Y., Doğan, M., & Bayramlı, E. (2013). Effect of Alkaline Treatment and Pre-impregnation on Mechanical and Water Absorbtion Properties of Pine Wood Flour Containing Poly (Lactic Acid) Based Green-Composites. Journal of Polymers and the 100 Environment, 21(3), 850–856. https://doi.org/10.1007/s10924-012-0563-x Arminger, B., Gindl-Altmutter, W., Keckes, J., & Hansmann, C. (2019). Facile preparation of superhydrophobic wood surfaces: Via spraying of aqueous alkyl ketene dimer dispersions. RSC Advances, 9(42), 24357–24367. https://doi.org/10.1039/c9ra03700d ASTM, I. (2014). ASTM D638-14,Standard Test Method for Tensile Properties of Plastics. ASTM International, 1–15. https://doi.org/10.1520/D0638-14.1 Auras, R., Harte, B., & Selke, S. (2004). An overview of polylactides as packaging materials. Macromolecular Bioscience, 4(9), 835–864. https://doi.org/10.1002/mabi.200400043 Ayrilmis, N., Kariž, M., & Kitek Kuzman, M. (2019). Effect of wood flour content on surface properties of 3D printed materials produced from wood flour/PLA filament. International Journal of Polymer Analysis and Characterization, 24(7), 659–666. https://doi.org/10.1080/1023666X.2019.1651547 Bae, S. U., Seo, Y. R., Kim, B. J., & Lee, M. (2021). Effects of Wood Flour and MA-EPDM on the Properties of Fused Deposition Modeling 3D-printed Poly Lactic Acid Composites. BioResources, 16(4), 7122–7138. https://doi.org/10.15376/biores.16.4.7122-7138 Bai, Y., Zhang, X., & Xia, K. (2019). Biocomposites produced from hardwood particles by equal channel angular pressing without additives. Journal of Composites Science, 3(2), 1–11. https://doi.org/10.3390/jcs3020036 Balla, V. K., Kate, K. H., Satyavolu, J., Singh, P., & Tadimeti, J. G. D. (2019). Additive manufacturing of natural fiber reinforced polymer composites: Processing and prospects. Composites Part B: Engineering, 174(May), 106956. https://doi.org/10.1016/j.compositesb.2019.106956 Baran, E. H., & Yildirim Erbil, H. (2019). Surface modification of 3d printed pla objects by 101 fused deposition modeling: A review. Colloids and Interfaces, 3(2). https://doi.org/10.3390/colloids3020043 Beaumont, M., Tardy, B. L., Reyes, G., Koso, T. V., Schaubmayr, E., Jusner, P., King, A. W. T., Dagastine, R. R., Potthast, A., Rojas, O. J., & Rosenau, T. (2021). Assembling Native Elementary Cellulose Nanofibrils via a Reversible and Regioselective Surface Functionalization. Journal of the American Chemical Society, 143(41), 17040–17046. https://doi.org/10.1021/jacs.1c06502 Belle, J., Kleemann, S., & Odermatt, J. (2014). Weighing of different impact factors on wet web strength by full-factorial design of experiments. BioResources, 9(2), 1830–1844. https://doi.org/10.15376/biores.9.2.1830-1844 Beltrán, F R, Lorenzo, V., Acosta, J., de la Orden, M. U., & Martínez Urreaga, J. (2018). Effect of simulated mechanical recycling processes on the structure and properties of poly(lactic acid). Journal of Environmental Management, 216, 25–31. https://doi.org/https://doi.org/10.1016/j.jenvman.2017.05.020 Beltrán, Freddys R., Infante, C., de la Orden, M. U., & Martínez Urreaga, J. (2019). Mechanical recycling of poly(lactic acid): Evaluation of a chain extender and a peroxide as additives for upgrading the recycled plastic. Journal of Cleaner Production, 219, 46–56. https://doi.org/10.1016/j.jclepro.2019.01.206 Berthet, M. A., Angellier-Coussy, H., Chea, V., Guillard, V., Gastaldi, E., & Gontard, N. (2015). Sustainable food packaging: Valorising wheat straw fibres for tuning PHBV-based composites properties. Composites Part A: Applied Science and Manufacturing, 72, 139–147. https://doi.org/10.1016/j.compositesa.2015.02.006 Bertolino, M., Battegazzore, D., Arrigo, R., & Frache, A. (2021). Designing 3D printable 102 polypropylene: Material and process optimisation through rheology. Additive Manufacturing, 40(October 2020). https://doi.org/10.1016/j.addma.2021.101944 Bhagia, S., Bornani, K., Agarwal, R., Satlewal, A., Ďurkovič, J., Lagaňa, R., Bhagia, M., Yoo, C. G., Zhao, X., Kunc, V., Pu, Y., Ozcan, S., & Ragauskas, A. J. (2021). Critical review of FDM 3D printing of PLA biocomposites filled with biomass resources, characterization, biodegradability, upcycling and opportunities for biorefineries. Applied Materials Today, 24. https://doi.org/10.1016/j.apmt.2021.101078 Bhagia, S., Lowden, R. R., Erdman, D., Rodriguez, M., Haga, B. A., Solano, I. R. M., Gallego, N. C., Pu, Y., Muchero, W., Kunc, V., & Ragauskas, A. J. (2020). Tensile properties of 3D-printed wood-filled PLA materials using poplar trees. Applied Materials Today, 21, 100832. https://doi.org/10.1016/j.apmt.2020.100832 Bledzki, A. K., Franciszczak, P., & Meljon, A. (2015). High performance hybrid PP and PLA biocomposites reinforced with short man-made cellulose fibres and softwood flour. Composites Part A: Applied Science and Manufacturing, 74, 132–139. https://doi.org/10.1016/j.compositesa.2015.03.029 Carothers, W. H., Dorough, G. L., & Natta, F. J. van. (1932). Studies of Polymerization and Ring Formation. X. the Reversible Pplymerization of Six-membered Cyclic Esters. Journal of the American Chemical Society, 54(2), 761–772. https://doi.org/10.1021/ja01341a046 Celli, A., & Scandola, M. (1992). Thermal properties and physical ageing of poly (l-lactic acid). Polymer, 33(13), 2699–2703. https://doi.org/10.1016/0032-3861(92)90440-8 Chauhan, S., Raghu, N., & Raj, A. (2021). Effect of maleic anhydride grafted polylactic acid concentration on mechanical and thermal properties of thermoplasticized starch filled polylactic acid blends. Polymers and Polymer Composites, 29(9_suppl), S400–S410. 103 https://doi.org/10.1177/09673911211004194 Chen, F., Ni, X., Liu, Y., Xia, X., & Gao, X. (2022). Preparation and properties of heat-treated esterified wood flour/polylactic acid composites for FDM 3D printing. Journal of Materials Science, 57(31), 14819–14834. https://doi.org/10.1007/s10853-022-07419-x Chimenez, T. A., Gehlen, M. H., Marabezi, K., & Curvelo, A. A. S. (2014). Characterization of sugarcane bagasse by autofluorescence microscopy. Cellulose, 21(1), 653–664. https://doi.org/10.1007/s10570-013-0135-9 Chinga-Carrasco, G., Miettinen, A., Luengo Hendriks, C. L., Kristofer, E., & Kataj, M. (2011). Structural Characterisation of Kraft Pulp Fibres and Their Nanofibrillated Materials for Biodegradable Composite Applications. Nanocomposites and Polymers with Analytical Methods. https://doi.org/10.5772/21580 Clemons, C. M., & Caufield, D. F. (2005). Wood Flour. In X. M (Ed.), Functional Fillers for Plastics (pp. 269–290). USA: WILEY-VCH Verlag GmbH & Co KGaA. https://doi.org/10.1002/9783527629848.ch15 Compton, B. G., & Lewis, J. A. (2014). 3D-printing of lightweight cellular composites. Advanced Materials, 26(34), 5930–5935. https://doi.org/10.1002/adma.201401804 Cotarca Pietro; Nardelli, Alfonso; unji, Vitomir, L. D. (1996). Bis(trichloromethyl) Carbonate in Organic Synthesis. Synthesis, 1996(05), 553–576. https://doi.org/10.1055/s-1996-4273 Csikós, Á., Faludi, G., Domján, A., Renner, K., Móczó, J., & Pukánszky, B. (2015). Modification of interfacial adhesion with a functionalized polymer in PLA/wood composites. European Polymer Journal, 68, 592–600. https://doi.org/10.1016/j.eurpolymj.2015.03.032 Cywar, R. M., Rorrer, N. A., Hoyt, C. B., Beckham, G. T., & Chen, E. Y. X. (2022). Bio-based 104 polymers with performance-advantaged properties. Nature Reviews Materials, 7(2), 83–103. https://doi.org/10.1038/s41578-021-00363-3 Dahlmann, J., Rafler, G., Fechner, K., & Mehlis, B. (1990). Synthesis and properties of biodegradable aliphatic polyesters. British Polymer Journal, 23(3), 235–240. https://doi.org/https://doi.org/10.1002/pi.4980230310 Dalu, M., Temiz, A., Altuntaş, E., Demirel, G. K., & Aslan, M. (2019). Characterization of tanalith E treated wood flour filled polylactic acid composites. Polymer Testing, 76(December 2018), 376–384. https://doi.org/10.1016/j.polymertesting.2019.03.037 Das, A. K., Agar, D. A., Rudolfsson, M., & Larsson, S. H. (2021). A review on wood powders in 3D printing: processes, properties and potential applications. Journal of Materials Research and Technology, 15, 241–255. https://doi.org/10.1016/j.jmrt.2021.07.110 Datta, R., Tsai, S., Bonsignore, P., Moon, S., & Frank, J. R. (1995). Technological and economic potential of poly(lactic acid) and lactic acid derivatives. FEMS Microbiology Reviews, 16, 221–231. Delacuvellerie, A., Benali, S., Cyriaque, V., Moins, S., Raquez, J. M., Gobert, S., & Wattiez, R. (2021). Microbial biofilm composition and polymer degradation of compostable and non-compostable plastics immersed in the marine environment. Journal of Hazardous Materials, 419. https://doi.org/10.1016/j.jhazmat.2021.126526 Dimzoski, B., Bogoeva-Gaceva, G., Gentile, G., Avella, M., Errico, M. E., & Srebrenkoska, V. (2008). Preparation and characterization of poly(lactic acid)/rice hulls based biodegradable composites. Journal of Polymer Engineering, 28(6–7), 369–384. https://doi.org/10.1515/polyeng-2008-286-704 Ding, Q., Han, W., Li, X., Jiang, Y., & Zhao, C. (2020). New insights into the autofluorescence 105 properties of cellulose/nanocellulose. Scientific Reports, 10(1), 21387. https://doi.org/10.1038/s41598-020-78480-2 Drummer, D., Cifuentes-Cuéllar, S., & Rietzel, D. (2012). Suitability of PLA/TCP for fused deposition modeling. Rapid Prototyping Journal, 18(6), 500–507. https://doi.org/10.1108/13552541211272045 Drumright, R. E., Gruber, P. R., & Henton, D. E. (2000). Polylactic acid technology. Advanced Materials, 12(23), 1841–1846. https://doi.org/10.1002/1521-4095(200012)12:23<1841::aid-adma1841>3.3.co;2-5 Du, J., Wang, Y., Xie, X., Xu, M., & Song, Y. (2017). Styrene-assisted maleic anhydride grafted poly(lactic acid) as an effective compatibilizer for wood flour/poly(lactic acid) bio-composites. Polymers, 9(11). https://doi.org/10.3390/polym9110623 Dzul-Cervantes, M., Herrera-Franco, P. J., Tábi, T., & Valadez-Gonzalez, A. (2017). Using Factorial Design Methodology to Assess PLA-g-Ma and Henequen Microfibrillated Cellulose Content on the Mechanical Properties of Poly(lactic acid) Composites. International Journal of Polymer Science, 2017. https://doi.org/10.1155/2017/4046862 Elrhayam, Y., & Elharfi, A. (2019). 3D-QSAR studies of the chemical modification of hydroxyl groups of biomass (cellulose, hemicelluloses and lignin) using quantum chemical descriptors. Heliyon, 5(8), e02173. https://doi.org/10.1016/j.heliyon.2019.e02173 Etaati, A., Pather, S., Fang, Z., & Wang, H. (2014). The study of fibre/matrix bond strength in short hemp polypropylene composites from dynamic mechanical analysis. Composites Part B: Engineering, 62, 19–28. https://doi.org/10.1016/j.compositesb.2014.02.011 European Bioplastics. (2023). What are bioplastics? European Bioplastics. https://www.european-bioplastics.org/bioplastics/ 106 Fang, Q., & Hanna, M. A. (1999). Rheological properties of amorphous and semicrystalline polylactic acid polymers. Industrial Crops and Products, 10(1), 47–53. https://doi.org/10.1016/S0926-6690(99)00009-6 Farah, S., Anderson, D. G., & Langer, R. (2016). Physical and mechanical properties of PLA, and their functions in widespread applications — A comprehensive review. Advanced Drug Delivery Reviews, 107, 367–392. https://doi.org/10.1016/j.addr.2016.06.012 Fazelinejad, S., Åkesson, D., & Skrifvars, M. (2017). Repeated Mechanical Recycling of Polylactic Acid Filled with Chalk. Progress in Rubber, Plastics and Recycling Technology, 33(1), 1–16. https://doi.org/10.1177/147776061703300101 Fazita, M. R. N., Jayaraman, K., Bhattacharyya, D., Hossain, M. S., Haafiz, M. K. M., & Abdul Khalil, H. P. S. (2015). Disposal options of bamboo fabric-reinforced poly(lactic) acid composites for sustainable packaging: Biodegradability and recyclability. Polymers, 7(8), 1476–1496. https://doi.org/10.3390/polym7081465 Ferreira, P. J., Matos, S., & Figueiredo, M. M. (1999). Size characterization of fibres and fines in hardwood kraft pulps. Particle and Particle Systems Characterization, 16(1), 20–24. https://doi.org/10.1002/(SICI)1521-4117(199905)16:1<20::AID-PPSC20>3.0.CO;2-M Fico, D., Rizzo, D., De Carolis, V., Montagna, F., Palumbo, E., & Corcione, C. E. (2022). Development and characterization of sustainable PLA/Olive wood waste composites for rehabilitation applications using Fused Filament Fabrication (FFF). Journal of Building Engineering, 56(March), 104673. https://doi.org/10.1016/j.jobe.2022.104673 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). 107 https://doi.org/10.3390/polym9080366 Gardella, L., Calabrese, M., & Monticelli, O. (2014). PLA maleation: An easy and effective method to modify the properties of PLA/PCL immiscible blends. Colloid and Polymer Science, 292(9), 2391–2398. https://doi.org/10.1007/s00396-014-3328-3 Garlotta, D. (2019). A Literature Review of Poly ( Lactic Acid ) A Literature Review of Poly ( Lactic Acid ). Journal of Polymers and the Environment, 9(2), 63–84. Gironi, F., & Piemonte, V. (2011). Life Cycle Assessment of Polylactic Acid and Polyethylene Terephthalate Bottles for Drinking Water. Environmental Progress & Sustainable Energy, 33(3), 459–468. https://doi.org/10.1002/ep Gnanasambandam, R., & Proctor, A. (2000). Determination of pectin degree of esterification by diffuse reflectance. Food Chemistry, 68, 327–332. González-López, M. E., Robledo-Ortíz, J. R., Manríquez-González, R., Silva-Guzmán, J. A., & Pérez-Fonseca, A. A. (2018). Polylactic acid functionalization with maleic anhydride and its use as coupling agent in natural fiber biocomposites: a review. Composite Interfaces, 25(5–7), 515–538. https://doi.org/10.1080/09276440.2018.1439622 González-López, Martín E., Pérez-Fonseca, A. A., Cisneros-López, E. O., Manríquez-González, R., Ramírez-Arreola, D. E., Rodrigue, D., & Robledo-Ortíz, J. R. (2019). Effect of Maleated PLA on the Properties of Rotomolded PLA-Agave Fiber Biocomposites. Journal of Polymers and the Environment, 27(1), 61–73. https://doi.org/10.1007/s10924-018-1308-2 Goto, T., Kishita, M., Sun, Y., Sako, T., & Okajima, I. (2020). Degradation of polylactic acid using sub-critical water for compost. Polymers, 12(11), 1–14. https://doi.org/10.3390/polym12112434 Gregorova, A., Hrabalova, M., Kovalcik, R., & Wimmer, R. (2011). Surface Modification of 108 Spruce Wood Flour and Effects on the Dynamic Fragility of PLA/Wood Composites. Polymer Engineering and Science, 51, 143–150. https://doi.org/10.1002/pen.21799 Gregorova, A., Wimmer, R., Hrabalova, M., Koller, M., Ters, T., & Mundigler, N. (2009). Effect of surface modification of beech wood flour on mechanical and thermal properties of poly (3-hydroxybutyrate)/wood flour composites. Holzforschung, 63(5), 565–570. https://doi.org/10.1515/HF.2009.098 Guen, M.-J. Le, Hill, S., Smith, D., Theobald, B., Gaugler, E., Barakat, A., & Mayer-Laigle, C. (2019). Influence of Rice Husk and Wood Biomass Properties on the Manufacture of Filaments for Fused Deposition Modeling. Frontiers in Chemistry, 7(October), 15–20. https://doi.org/10.3389/fchem.2019.00735 Guo, R., Ren, Z., Bi, H., Song, Y., & Xu, M. (2018). Effect of toughening agents on the properties of poplar wood flour/poly (lactic acid) composites fabricated with Fused Deposition Modeling. European Polymer Journal, 107(June), 34–45. https://doi.org/10.1016/j.eurpolymj.2018.07.035 Guo, Ying, Chen, J. Q., Su, M., & Hong, J. G. (2018). Bio-based Plastics with Highly Efficient Esterification of Lignocellulosic Biomass in 1-methylimidazole under Mild Conditions. Journal of Wood Chemistry and Technology, 38(4), 338–349. https://doi.org/10.1080/02773813.2018.1488876 Guo, Yingting, & Bustin, R. M. (1998). Micro-FTIR spectroscopy of liptinite macerals in coal. International Journal of Coal Geology, 36(3–4), 259–275. https://doi.org/10.1016/S0166-5162(97)00044-X 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), 737–753. https://doi.org/10.1007/s00226-014-0636-1 Hamdan, M. H. M., Siregar, J. P., Rejab, M. R. M., Bachtiar, D., Jamiluddin, J., & Tezara, C. (2019). Effect of Maleated Anhydride on Mechanical Properties of Rice Husk Filler Reinforced PLA Matrix Polymer Composite. International Journal of Precision Engineering and Manufacturing - Green Technology, 6(1), 113–124. https://doi.org/10.1007/s40684-019-00017-4 Han, J., Fitzpatrick, J., Cronin, K., Maidannyk, V., & Miao, S. (2021). Particle size, powder properties and the breakage behaviour of infant milk formula. Journal of Food Engineering, 292(June 2020), 110367. https://doi.org/10.1016/j.jfoodeng.2020.110367 Hansen, N. M. L., & Plackett, D. (2011). Synthesis and characterization of birch wood xylan succinoylated in 1-n-butyl-3-methylimidazolium chloride. Polymer Chemistry, 2(9), 2010–2020. https://doi.org/10.1039/c1py00086a Hartmann, M. H. (1998). High Molecular Weight Polylactic Acid Polymers BT - Biopolymers from Renewable Resources (D. L. Kaplan (ed.); pp. 367–411). Springer Berlin Heidelberg. https://doi.org/10.1007/978-3-662-03680-8_15 Haryńska, A., Janik, H., Sienkiewicz, M., Mikolaszek, B., & Kucińska-Lipka, J. (2021). PLA-Potato Thermoplastic Starch Filament as a Sustainable Alternative to the Conventional PLA Filament: Processing, Characterization, and FFF 3D Printing. ACS Sustainable Chemistry and Engineering, 9(20), 6923–6938. https://doi.org/10.1021/acssuschemeng.0c09413 Herrada-Manchón, H., Fernández, M. A., & Aguilar, E. (2023). Essential Guide to Hydrogel Rheology in Extrusion 3D Printing: How to Measure It and Why It Matters? Gels, 9(7). https://doi.org/10.3390/gels9070517 Hong, J., & Kim, D. S. (2013). Preparation and physical properties of polylactide/cellulose 110 nanowhisker/nanoclay composites. Polymer Composites, 34(2), 293–298. https://doi.org/https://doi.org/10.1002/pc.22413 Honkasalo, J. (2004). Behaviour of Different Furnish Mixtures in Mechanical Printing Papers. In Helsinki University of Technology. Helsinki University of Technology Department. Hou, D. F., Li, M. L., Yan, C., Zhou, L., Liu, Z. Y., Yang, W., & Yang, M. B. (2021). Mechanochemical preparation of thermoplastic cellulose oleate by ball milling. Green Chemistry, 23(5), 2069–2078. https://doi.org/10.1039/d0gc03853a Hu, Z., Berry, R. M., Pelton, R., & Cranston, E. D. (2017). One-Pot Water-Based Hydrophobic Surface Modification of Cellulose Nanocrystals Using Plant Polyphenols. ACS Sustainable Chemistry & Engineering, 5(6), 5018–5026. https://doi.org/10.1021/acssuschemeng.7b00415 Huang, Y., Löschke, S., & Proust, G. (2021). In the mix: The effect of wood composition on the 3D printability and mechanical performance of wood-plastic composites. Composites Part C: Open Access, 5(March), 100140. https://doi.org/10.1016/j.jcomc.2021.100140 Hubbe, M. A. (2002). Fines Management for Increased Paper Machine Productivity. Proc Sci Tech Advan Wet End Chemistry, 1–22. https://repository.lib.ncsu.edu/server/api/core/bitstreams/b233a504-1933-4e82-9a89-33586648339f/content Hubbe, M. A., & Grigsby, W. (2020). Reinforcing particle size. BioResources, 15(1), 2030–2081. Huda, M. S., Drzal, L. T., Misra, M., & Mohanty, A. K. (2006). Wood-fiber-reinforced poly(lactic acid) composites: Evaluation of the physicomechanical and morphological properties. Journal of Applied Polymer Science, 102(5), 4856–4869. 111 https://doi.org/10.1002/app.24829 Huner, U. (2017). Effect of chemical treatment and maleic anhydride grafted polypropylene coupling agent on rice husk and rice husk reinforced composite. Materials Express, 7(2), 134–144. https://doi.org/10.1166/mex.2017.1359 Hwang, S. W., Lee, S. B., Lee, C. K., Lee, J. Y., Shim, J. K., Selke, S. E. M., Soto-Valdez, H., Matuana, L., Rubino, M., & Auras, R. (2012). Grafting of maleic anhydride on poly(L-lactic acid). Effects on physical and mechanical properties. Polymer Testing, 31(2), 333–344. https://doi.org/10.1016/j.polymertesting.2011.12.005 Igisu, M., Yokoyama, T., Ueno, Y., Nakashima, S., Shimojima, M., Ohta, H., & Maruyama, S. (2018). Changes of aliphatic C–H bonds in cyanobacteria during experimental thermal maturation in the presence or absence of silica as evaluated by FTIR microspectroscopy. Geobiology, 16(4), 412–428. https://doi.org/10.1111/gbi.12294 Ihemouchen, C., Djidjelli, H., Boukerrou, A., Fenouillot, F., & Barres, C. (2013). Effect of compatibilizing agents on the mechanical properties of high-density polyethylene/olive husk flour composites. Journal of Applied Polymer Science, 128(3), 2224–2229. https://doi.org/10.1002/app.38434 Ikada, Y., Jamshidi, K., Tsuji, H., & Hyon, S. H. (1987). Stereocomplex Formation between Enantiomeric Poly(lactides). Macromolecules, 20(4), 904–906. https://doi.org/10.1021/ma00170a034 Ikada, Y., & Tsuji, H. (2000). Biodegradable polyesters for medical applications. Macromol. Rapid Commun, 21(3), 117–132. https://doi.org/1022-1336/2000/0302–0117$17.50+.50/0 Ilyas, R. A., Sapuan, S. M., Harussani, M. M., Hakimi, M. Y. A. Y., Haziq, M. Z. M., Atikah, M. S. N., Asyraf, M. R. M., Ishak, M. R., Razman, M. R., Nurazzi, N. M., Norrrahim, M. N. F., 112 Abral, H., & Asrofi, M. (2021). Polylactic Acid (PLA) Biocomposite: Processing, Additive Manufacturing and Advanced Applications. Polymers, 13(8). https://doi.org/10.3390/polym13081326 Inata, H., & Matsumura, S. (1985). Chain extenders for polyesters. I. Addition-type chain extenders reactive with carboxyl end groups of polyesters. Journal of Applied Polymer Science, 30(8), 3325–3337. https://doi.org/https://doi.org/10.1002/app.1985.070300815 Isa, A., Minamino, J., Kojima, Y., Suzuki, S., Ito, H., Makise, R., Okamoto, M., & Endo, T. (2016). The influence of dry-milled wood flour on the physical properties of wood flour/polypropylene composites. Journal of Wood Chemistry and Technology, 36(2), 105–113. https://doi.org/10.1080/02773813.2015.1083583 Jacobsen, S., Fritz, H. G., Degée, P., Dubois, P., & Jérôme, R. (2000). New developments on the ring opening polymerisation of polylactide. Industrial Crops and Products, 11(2–3), 265–275. https://doi.org/10.1016/S0926-6690(99)00053-9 Jamaluddin, N., Hsu, Y. I., Asoh, T. A., & Uyama, H. (2021). Effects of acid-anhydride-modified cellulose nanofiber on poly(Lactic acid) composite films. Nanomaterials, 11(3), 1–15. https://doi.org/10.3390/nano11030753 Jamshidi, K., Hyon, S.-H., & Ikada, Y. (1988). Thermal characterization of polylactides. Polymer, 29(12), 2229–2234. https://doi.org/https://doi.org/10.1016/0032-3861(88)90116-4 Jandura, P., Kokta, B. V., & Riedl, B. (2000). Fibrous long-chain organic acid cellulose esters and their characterization by diffuse reflectance FTIR spectroscopy, solid-state CP/MAS 13C-NMR, and X-ray diffraction. Journal of Applied Polymer Science, 78(7), 1354–1365. https://doi.org/10.1002/1097-4628(20001114)78:7<1354::AID-APP60>3.0.CO;2-V Jang, H., Kwon, S., Kim, S. J., & Park, S. Il. (2022). Maleic Anhydride-Grafted PLA Preparation 113 and Characteristics of Compatibilized PLA/PBSeT Blend Films. International Journal of Molecular Sciences, 23(13). https://doi.org/10.3390/ijms23137166 Jin, H., Kose, R., Akada, N., & Okayama, T. (2022). Relationship between wettability of pulp fibers and tensile strength of paper during recycling. Scientific Reports, 12(1), 1560. https://doi.org/10.1038/s41598-022-05514-2 Joffre, T., Miettinen, A., Wernersson, E. L. G., Isaksson, P., & Gamstedt, E. K. (2014). Effects of defects on the tensile strength of short-fibre composite materials. Mechanics of Materials, 75, 125–134. https://doi.org/10.1016/j.mechmat.2014.04.003 John, R. P., Nampoothiri, K. M., & Pandey, A. (2006). Solid-state fermentation for L-lactic acid production from agro wastes using Lactobacillus delbrueckii. Process Biochemistry, 41(4), 759–763. https://doi.org/10.1016/j.procbio.2005.09.013 Jubinville, D., Tzoganakis, C., & Mekonnen, T. H. (2022). Recycled PLA – Wood flour based biocomposites: Effect of wood flour surface modification, PLA recycling, and maleation. Construction and Building Materials, 352(August), 129026. https://doi.org/10.1016/j.conbuildmat.2022.129026 Kale, G., Kijchavengkul, T., Auras, R., Rubino, M., Selke, S. E., & Singh, S. P. (2007). Compostability of bioplastic packaging materials: An overview. Macromolecular Bioscience, 7(3), 255–277. https://doi.org/10.1002/mabi.200600168 Kalita, E., Nath, B. K., Agan, F., More, V., & Deb, P. (2015). Isolation and characterization of crystalline, autofluorescent, cellulose nanocrystals from saw dust wastes. Industrial Crops and Products, 65, 550–555. https://doi.org/https://doi.org/10.1016/j.indcrop.2014.10.004 Kalita, N. K., Bhasney, S. M., Kalamdhad, A., & Katiyar, V. (2020). Biodegradable kinetics and behavior of bio-based polyblends under simulated aerobic composting conditions. Journal 114 of Environmental Management, 261(October 2019), 110211. https://doi.org/10.1016/j.jenvman.2020.110211 Kalita, N. K., Hazarika, D., Kalamdhad, A., & Katiyar, V. (2021). Biodegradation of biopolymeric composites and blends under different environmental conditions: Approach towards end-of-life panacea for crop sustainability. Bioresource Technology Reports, 15(March), 100705. https://doi.org/10.1016/j.biteb.2021.100705 Kalita, N. K., Nagar, M. K., Mudenur, C., Kalamdhad, A., & Katiyar, V. (2019). Biodegradation of modified Poly(lactic acid) based biocomposite films under thermophilic composting conditions. Polymer Testing, 76(September 2018), 522–536. https://doi.org/10.1016/j.polymertesting.2019.02.021 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), 298–306. Kangas, H., & Kleen, M. (2004). Surface chemical and morphological properties of mechanical pulp fines. Nordic Pulp and Paper Research Journal, 19(2), 191–199. https://doi.org/10.3183/npprj-2004-19-02-p191-199 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), 256–268. https://doi.org/10.3183/npprj-2010-25-03-p256-268 Kaynak, C., & Meyva, Y. (2014). Use of maleic anhydride compatibilization to improve toughness and other properties of polylactide blended with thermoplastic elastomers. Polymers for Advanced Technologies, 25(12), 1622–1632. https://doi.org/10.1002/pat.3415 Khalid, M., Salmiaton, A., Chuah, T. G., Ratnam, C. T., & Choong, S. Y. T. (2008). Effect of 115 MAPP and TMPTA as compatibilizer on the mechanical properties of cellulose and oil palm fiber empty fruit bunch-polypropylene biocomposites. Composite Interfaces, 15(2–3), 251–262. https://doi.org/10.1163/156855408783810876 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), 361–368. https://doi.org/10.3183/npprj-2003-18-04-p361-368 Koljonen, K., & Stenius, P. (2005). Surface characterisation of single fibres from mechanical pulps by contact angle measurements. Nordic Pulp & Paper Research Journal, 20(1), 107–114. https://doi.org/doi:10.3183/npprj-2005-20-01-p107-114 Kricheldorf, H. R., & Kreiser-Saunders, I. (1990). Polylactones, 19. Anionic polymerization of L-lactide in solution. Die Makromolekulare Chemie, 191(5), 1057–1066. https://doi.org/https://doi.org/10.1002/macp.1990.021910508 Kricheldorf, H. R., Kreiser-Saunders, I., Jürgens, C., & Wolter, D. (1996). Polylactides - synthesis, characterization and medical application. Macromolecular Symposia, 103(1), 85–102. https://doi.org/https://doi.org/10.1002/masy.19961030110 Kricheldorf, H. R., Kreiser-Saunders, I., & Scharnagl, N. (1990). Anionic and pseudoanionic polymerization of lactones - a comparison. Makromolekulare Chemie. Macromolecular Symposia, 32(1), 285–298. https://doi.org/https://doi.org/10.1002/masy.19900320122 Kristombu Baduge, S., Navaratnam, S., Abu-Zidan, Y., McCormack, T., Nguyen, K., Mendis, P., Zhang, G., & Aye, L. (2021). Improving performance of additive manufactured (3D printed) concrete: A review on material mix design, processing, interlayer bonding, and reinforcing methods. Structures, 29(December 2020), 1597–1609. https://doi.org/10.1016/j.istruc.2020.12.061 116 Krogerus, B., & Tiikkaja, E. (2002). Fines from different pulps compared by image analysis. Nordic Pulp and Paper Research Journal, 17(4), 440–444. https://doi.org/10.3183/npprj-2002-17-04-p440-444 Kuciel, S., Mazur, K., & Hebda, M. (2020). The Influence of Wood and Basalt Fibres on Mechanical, Thermal and Hydrothermal Properties of PLA Composites. Journal of Polymers and the Environment, 28(4), 1204–1215. https://doi.org/10.1007/s10924-020-01677-z Kumar, S. (1994). Chemical Modification of Wood. Society of Wood Science and Technology, 26(2), 270–280. Laouchedi, D., Bezzazi, B., & Aribi, C. (2017). Elaboration and characterization of composite material based on epoxy resin and clay fillers. Journal of Applied Research and Technology, 15(2), 190–204. https://doi.org/10.1016/j.jart.2017.01.005 Lee, S. Y., Kang, I. A., Doh, G. H., Yoon, H. G., Park, B. D., & Wu, Q. (2008). Thermal and mechanical properties of wood flour/talc-filled polylactic acid composites: Effect of filler content and coupling treatment. Journal of Thermoplastic Composite Materials, 21(3), 209–223. https://doi.org/10.1177/0892705708089473 Lendvai, L., Omastova, M., Patnaik, A., Dogossy, G., & Singh, T. (2023). Valorization of Waste Wood Flour and Rice Husk in Poly(Lactic Acid)-Based Hybrid Biocomposites. Journal of Polymers and the Environment, 31(2), 541–551. https://doi.org/10.1007/s10924-022-02633-9 Li, Q., & Renneckar, S. (2011). Supramolecular Structure Characterization of Molecularly Thin Cellulose I Nanoparticles. Biomacromolecules, 12(3), 650–659. https://doi.org/10.1021/bm101315y 117 Liber-Kneć, A., & Łagan, S. (2014). The use of contact angle and the surface free energy as the surface characteristics of the polymers used in medicine. Polimery w medycynie, 44(1), 29–37. Ligon, S. C., Liska, R., Stampfl, J., Gurr, M., & Mülhaupt, R. (2017). Polymers for 3D Printing and Customized Additive Manufacturing. Chemical Reviews, 117(15), 10212–10290. https://doi.org/10.1021/acs.chemrev.7b00074 Liu, L., Lin, M., Xu, Z., & Lin, M. (2019). Polylactic acid-based wood-plastic 3D printing composite and its properties. BioResources, 14(4), 8484–8498. https://doi.org/10.15376/biores.14.4.8484-8498 Liu, R., Cao*, J., & Ou-Yang, L. (2013). Degradation of Wood Flour / Poly (lactic acid) Composites Reinforced by Coupling Agents and Organo-Montmorillonite in a Compost Test. Wood and Fiber Science, 45((1)), 105–118. Liu, R., Luo, S., Cao, J., & Chen, Y. (2016). Mechanical properties of wood flour/poly (lactic acid) composites coupled with waterborne silane-polyacrylate copolymer emulsion. Holzforschung, 70(5), 439–447. https://doi.org/10.1515/hf-2015-0064 Liu, W., Drzal, L. T., Mohanty, A. K., & Misra, M. (2007). Influence of processing methods and fiber length on physical properties of kenaf fiber reinforced soy based biocomposites. Composites Part B: Engineering, 38(3), 352–359. https://doi.org/10.1016/j.compositesb.2006.05.003 Lowe, C. E., & Du Pont de Nemours, E. I. (1954). Preparation of high molecular weight polyhydroxyacetic ester. United States Patent Office, 2–4. https://patents.google.com/patent/US2668162A/en%0Ahttps://patents.google.com/patent/US2668162 118 Lunt, J. (1998). Large-scale production, properties and commercial applications of poly lactic acid polymers. Polymer Degradation and Stability, 59(1–3), 145–152. https://doi.org/10.1016/s0141-3910(97)00148-1 Luukko, K., & Paulapuro, H. (1998). Mechanical pulp fines: Effect of particle size and shape. Tappi Journal, 82(2), 95–101. Luukko, K., & Paulapuro, H. (1999). Development of fines quality in the TMP process. Journal of Pulp and Paper Science, 25(8), 273–277. Lv, S., Gu, J., Tan, H., & Zhang, Y. (2016). Modification of wood flour/PLA composites by reactive extrusion with maleic anhydride. Journal of Applied Polymer Science, 133(15), 1–9. https://doi.org/10.1002/app.43295 Ma, S., Kou, L., Zhang, X., & Tan, T. (2020). Energy grass/polylactic acid composites and pretreatments for additive manufacturing. Cellulose, 27(5), 2669–2683. https://doi.org/10.1007/s10570-019-02927-7 Maréchal, Y., & Chanzy, H. (2000). The hydrogen bond network in I(β) cellulose as observed by infrared spectrometry. Journal of Molecular Structure, 523(1–3), 183–196. https://doi.org/10.1016/S0022-2860(99)00389-0 Matuana, L. M., & Diaz, C. A. (2013). Strategy to produce microcellular foamed poly(lactic acid)/wood-flour composites in a continuous extrusion process. Industrial and Engineering Chemistry Research, 52(34), 12032–12040. https://doi.org/10.1021/ie4019462 Migneault, S., Koubaa, A., Erchiqui, F., Chaala, A., Englund, K., & Wolcott, M. P. (2009). Effects of processing method and fiber size on the structure and properties of wood-plastic composites. Composites Part A: Applied Science and Manufacturing, 40(1), 80–85. https://doi.org/10.1016/j.compositesa.2008.10.004 119 Mohamad, Z., Ismail, H., & Bakar, A. A. (2004). Rice husk powder filled polystyrene/styrene butadiene rubber blends: chemical treatment. JIRCAS Working Report, No.39, 194–199. Moradi, S., & Yeganeh, J. K. (2020). Highly toughened poly(lactic acid) (PLA) prepared through melt blending with ethylene-co-vinyl acetate (EVA) copolymer and simultaneous addition of hydrophilic silica nanoparticles and block copolymer compatibilizer. Polymer Testing, 91, 106735. https://doi.org/https://doi.org/10.1016/j.polymertesting.2020.106735 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), 63–68. https://doi.org/10.3183/npprj-2003-18-01-p063-068 Mosbye, J., Moe, S., & Laine, J. (2002). The charge and chemical composition of fines in mechanical pulp. Nordic Pulp and Paper Research Journal, 17(3), 352–356. https://doi.org/10.3183/npprj-2002-17-03-p352-356 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), 4099–4124. https://doi.org/10.1007/s10570-019-02349-5 Mun, S., McClements, D. J., & Surh, J. (2011). Influence of maltodextrin addition on the freeze-dry stability of β-lactoglobulin-based emulsions with controlled electrostatic and/or steric interactions. Food Science and Biotechnology, 20(4), 1143–1150. https://doi.org/10.1007/s10068-011-0156-9 Ncube, L. K., Ude, A. U., Ogunmuyiwa, E. N., Zulkifli, R., & Beas, I. N. (2020). Environmental impact of food packaging materials: A review of contemporary development from conventional plastics to polylactic acid based materials. Materials, 13(21), 1–24. https://doi.org/10.3390/ma13214994 120 Ning, F., Cong, W., Qiu, J., Wei, J., & Wang, S. (2015). Additive manufacturing of carbon fiber reinforced thermoplastic composites using fused deposition modeling. Composites Part B: Engineering, 80, 369–378. https://doi.org/https://doi.org/10.1016/j.compositesb.2015.06.013 Niu, S., Zhou, Y., Yu, H., Lu, C., & Han, K. (2017). Investigation on thermal degradation properties of oleic acid and its methyl and ethyl esters through TG-FTIR. Energy Conversion and Management, 149(17923), 495–504. https://doi.org/10.1016/j.enconman.2017.07.053 Norazlina, H., & Kamal, Y. (2015). Graphene modifications in polylactic acid nanocomposites: a review. In Polymer Bulletin (Vol. 72, Issue 4). https://doi.org/10.1007/s00289-015-1308-5 Nyambo, C., Mohanty, A. K., & Misra, M. (2011). Effect of maleated compatibilizer on performance of PLA/wheat straw-based green composites. Macromolecular Materials and Engineering, 296(8), 710–718. https://doi.org/10.1002/mame.201000403 Obst, J. R. (1990). Lignins: Structure and Distribution in Wood and Pulp. MRS Proceedings, 197, 11–20. https://doi.org/10.1557/proc-197-11 Odabas, N., Henniges, U., Potthast, A., & Rosenau, T. (2016). Cellulosic fines: Properties and effects. Progress in Materials Science, 83, 574–594. https://doi.org/10.1016/j.pmatsci.2016.07.006 Oliver-Ortega, H., Reixach, R., Espinach, F. X., & Méndez, J. A. (2022). Maleic Anhydride Polylactic Acid Coupling Agent Prepared from Solvent Reaction: Synthesis, Characterization and Composite Performance. Materials, 15(3). https://doi.org/10.3390/ma15031161 Orozco, V. H., Palacio, J., Sierra, J., & López, B. L. (2013). Increased covalent conjugation of a 121 model antigen to poly(lactic acid)-g-maleic anhydride nanoparticles compared to bare poly(lactic acid) nanoparticles. Colloid and Polymer Science, 291(12), 2775–2781. https://doi.org/10.1007/s00396-013-3023-9 Page, D. H. (1969). A theory for the tensile strngth of paper. Tappi Journal, 52((4)), 674–681. Petchwattana, N., Channuan, W., Naknaen, P., & Narupai, B. (2019). 3D printing filaments prepared from modified poly(lactic acid)/teak wood flour composites: An investigation on the particle size effects and silane coupling agent compatibilisation. Journal of Physical Science, 30(2), 169–188. https://doi.org/10.21315/jps2019.30.2.10 Petchwattana, N., & Covavisaruch, S. (2013). Effects of Rice Hull Particle Size and Content on the Mechanical Properties and Visual Appearance of Wood Plastic Composites Prepared from Poly(vinyl chloride). Journal of Bionic Engineering, 10(1), 110–117. https://doi.org/https://doi.org/10.1016/S1672-6529(13)60205-X Petinakis, E., Yu, L., Edward, G., Dean, K., Liu, H., & Scully, A. D. (2009). Effect of matrix-particle interfacial adhesion on the mechanical properties of poly(lactic acid)/wood-flour micro-composites. Journal of Polymers and the Environment, 17(2), 83–94. https://doi.org/10.1007/s10924-009-0124-0 Pettersson, G., Norgren, S., Engstrand, P., Rundlöf, M., & Höglund, H. (2021). Aspects on bond strength in sheet structures from TMP and CTMP – A review. Nordic Pulp and Paper Research Journal, 36(2), 177–213. https://doi.org/10.1515/npprj-2021-0009 Pilla, S., Gong, S., O’Neill, E., Rowell, R. M., & Krzysik, A. M. (2008). Polylactide-Pine Wood Flour Composites. Polymer Engineering and Science, 48(3), 578–587. https://doi.org/10.1002/pen.20971 Qiang, T., Yu, D., Gao, H., & Wang, Y. (2012). Polylactide-Based Wood Plastic Composites 122 Toughened with SBS. Polymer - Plastics Technology and Engineering, 51(2), 193–198. https://doi.org/10.1080/03602559.2011.618518 Qu, T., Zhang, X., Gu, X., Han, L., Ji, G., Chen, X., & Xiao, W. (2017). Ball Milling for Biomass Fractionation and Pretreatment with Aqueous Hydroxide Solutions. ACS Sustainable Chemistry and Engineering, 5(9), 7733–7742. https://doi.org/10.1021/acssuschemeng.7b01186 Quero, F., Eichhorn, S., Nogi, M., Yano, H., Lee, K.-Y., & Bismarck, A. (2012). Interfaces in Cross-Linked and Grafted Bacterial Cellulose/Poly(Lactic Acid) Resin Composites. Journal of Polymers and the Environment, 20, 916–925. https://doi.org/10.1007/s10924-012-0487-5 Rahem, Z., Mayouf, I., Guessoum, M., Delaite, C., Douibi, A., & Lallam, A. (2019). Compatibilization of biocomposites based on sponge-gourd natural fiber reinforced poly(lactic acid). Polymer Composites, 40(12), 4489–4499. https://doi.org/10.1002/pc.25338 Ramezani Dana, H., El Mansori, M., Barrat, M., & Seck, C. A. (2022). Tensile behavior of additively manufactured carbon fiber reinforced polyamide-6 composites. Polymer-Plastics Technology and Materials, 61(6), 624–641. https://doi.org/10.1080/25740881.2021.2005094 Ramezani Dana, Hossein, & Ebrahimi, F. (2023). Synthesis, properties, and applications of polylactic acid-based polymers. Polymer Engineering and Science, 63(1), 22–43. https://doi.org/10.1002/pen.26193 Research, G. V. (2019). Global 3D Printing Filament Market Share Report, 2020-2027. https://doi.org/GVR-4-68039-304-2 Retulainen, E., Moss, P., & Nieminen, K. (1993). Effect of fines on the properties of fibre 123 networks. Products of Papermaking, Trans. of the Xth Fund. Res. Symp. Oxford, 727–769. https://doi.org/10.15376/frc.1993.2.727.Effect Richter, G. A. (1941). Cellulose from Hardwoods. Industrial & Engineering Chemistry, 33(1), 75–83. https://doi.org/10.1021/ie50373a016 Rigotti, D., Fambri, L., & Pegoretti, A. (2022). Bio-composites for fused filament fabrication: effects of maleic anhydride grafting on poly(lactic acid) and microcellulose. Progress in Additive Manufacturing, 7(4), 765–783. https://doi.org/10.1007/s40964-022-00264-z Roque, R., Barbosa, G. F., & Guastaldi, A. C. (2021). Design and 3D bioprinting of interconnected porous scaffolds for bone regeneration. An additive manufacturing approach. Journal of Manufacturing Processes, 64(February), 655–663. https://doi.org/10.1016/j.jmapro.2021.01.057 Rowell, R. M. (1983). Chemical Modification of Wood. Forest Products Abstracts, 6(12), 363–382. https://doi.org/10.3139/9783446442504.022 Rundlöf, M. (2002). Interaction of dissolved and colloidal substances with fines of mechanical pulp - Influence on sheet properties and basic aspects of adhesion [Royal Institute of Technology, Stockholm, Sweden]. https://www.diva-portal.org/smash/get/diva2:9082/FULLTEXT01.pdf Samyn, P. (2013). Wetting and hydrophobic modification of cellulose surfaces for paper applications. Journal of Materials Science, 48(19), 6455–6498. https://doi.org/10.1007/s10853-013-7519-y Satyanarayana, K. G., Arizaga, G. G. C., & Wypych, F. (2009). Biodegradable composites based on lignocellulosic fibers-An overview. Progress in Polymer Science (Oxford), 34(9), 982–1021. https://doi.org/10.1016/j.progpolymsci.2008.12.002 124 Shapiro, A. A., Borgonia, J. P., Chen, Q. N., Dillon, R. P., McEnerney, B., Polit-Casillas, R., & Soloway, L. (2016). Additive Manufacturing for Aerospace Flight Applications. Journal of Spacecraft and Rockets, 53(5), 952–959. https://doi.org/10.2514/1.A33544 Shazleen, S. S., Yasim-Anuar, T. A. T., Ibrahim, N. A., Hassan, M. A., & Ariffin, H. (2021). Functionality of cellulose nanofiber as bio-based nucleating agent and nano-reinforcement material to enhance crystallization and mechanical properties of polylactic acid nanocomposite. Polymers, 13(3), 1–19. https://doi.org/10.3390/polym13030389 Shin, B. Y., & Han, D. H. (2017). Viscoelastic properties of PLA/PCL blends compatibilized with different methods. Korea Australia Rheology Journal, 29(4), 295–302. https://doi.org/10.1007/s13367-017-0029-8 Sikorska, W., Musiol, M., Nowak, B., Pajak, J., Labuzek, S., Kowalczuk, M., & Adamus, G. (2015). Degradability of polylactide and its blend with poly[(R,S)-3-hydroxybutyrate] in industrial composting and compost extract. International Biodeterioration and Biodegradation, 101, 32–41. https://doi.org/10.1016/j.ibiod.2015.03.021 Silverstein, R. M., & Webster, F. X. (1998). Infrared Spectrometry. In Spectrometric Identification of Organic Compounds (Sixth Edit, pp. 84–85). Wiley. Sinclair, R. G. (1996). The case for polylactic acid as a commodity packaging plastic. Journal of Macromolecular Science - Pure and Applied Chemistry, 33(5), 585–597. https://doi.org/10.1080/10601329608010880 Singh, S., Santana-Pérez, O., Rodriguez, C., Oksman, K., & Maspoch, M. L. (2020). Mechanical behaviour of poly(Lactic acid)/cellulose nanocrystal nanocomposites: A comparative study between conventional tensile test and small punch test. Express Polymer Letters, 14(12), 1127–1136. https://doi.org/10.3144/expresspolymlett.2020.92 125 Sluiter, A., Hames, B., Ruiz, R., Scarlata, C., Sluiter, J., Templeton, D., & Crocker, D. (2008). Determination of structural carbohydrates and lignin in Biomass. National Renewable Energy Laboratory, April 2008, 17. http://www.nrel.gov/docs/gen/fy13/42618.pdf Smook, G. A. (2016a). 1.5 Requirements and Sources for papermaking fibers. In Handbook for Pulp & Paper Technologists (4th Edition) (p. 5). Smook, G. A. (2016b). 2.3 Effect of Fiber Structure (Morphology) on Fiber and Paper Properties. In Handbook for Pulp & Paper Technologists (4th Edition) (pp. 17–18). Tappi. Retrieved from app.knovel.com Smook, G. A. (2016c). 9.4 Screening. In Handbook for Pulp & Paper Technologists (4th Edition) (p. 115). Smook, G. A. (2016d). Mechanical Pulping. In Handbook for Pulp & Paper Technologists (4th Edition) (pp. 47–60). Song, L., Li, Y., Meng, X., Wang, T., Shi, Y., Wang, Y., Shi, S., & Liu, L.-Z. (2021). Crystallization, Structure and Significantly Improved Mechanical Properties of PLA/PPC Blends Compatibilized with PLA-PPC Copolymers Produced by Reactions Initiated with TBT or TDI. Polymers, 13(19). https://doi.org/10.3390/polym13193245 Spence, K. L., Venditti, R. A., Rojas, O. J., Habibi, Y., & Pawlak, J. J. (2010). The effect of chemical composition on microfibrillar cellulose films from wood pulps: Water interactions and physical properties for packaging applications. Cellulose, 17(4), 835–848. https://doi.org/10.1007/s10570-010-9424-8 Stamm, A. J., & Tarkow, H. (1947). Dimensional Stabilization of Wood. The Journal of Physical and Colloid Chemistry, 51(2), 493–505. https://doi.org/10.1021/j150452a016 Stevels, W. M., Ankone, M. J. K., Dijkstra, P. J., & Feijén, J. (1995). Stereocomplex formation 126 in ABA triblock copolymers of poly(lactide) (A) and poly(ethylene glycol) (B). Macromolecular Chemistry and Physics, 196(11), 3687–3694. https://doi.org/https://doi.org/10.1002/macp.1995.021961121 Suárez, L., & Domínguez, M. (2020). Sustainability and environmental impact of fused deposition modelling (FDM) technologies. International Journal of Advanced Manufacturing Technology, 106(3–4), 1267–1279. https://doi.org/10.1007/s00170-019-04676-0 Sun, C., Wang, Y., McMurtrey, M. D., Jerred, N. D., Liou, F., & Li, J. (2021). Additive manufacturing for energy: A review. Applied Energy, 282(November 2020). https://doi.org/10.1016/j.apenergy.2020.116041 Sun, Y., Gu, J., Tan, H., Zhang, Y., & Huo, P. (2018). Physicochemical properties of starch adhesives enhanced by esterification modification with dodecenyl succinic anhydride. International Journal of Biological Macromolecules, 112, 1257–1263. https://doi.org/https://doi.org/10.1016/j.ijbiomac.2018.01.222 Suzuki, S., Hikita, H., Hernandez, S. C., Wada, N., & Takahashi, K. (2021). Direct Conversion of Sugarcane Bagasse into an Injection-Moldable Cellulose-Based Thermoplastic via Homogeneous Esterification with Mixed Acyl Groups. ACS Sustainable Chemistry and Engineering, 9(17), 5933–5941. https://doi.org/10.1021/acssuschemeng.1c00306 Taib, N. A. A. B., Rahman, M. R., Huda, D., Kuok, K. K., Hamdan, S., Bakri, M. K. Bin, Julaihi, M. R. M. Bin, & Khan, A. (2023). A review on poly lactic acid (PLA) as a biodegradable polymer. In Polymer Bulletin (Vol. 80, Issue 2). Springer Berlin Heidelberg. https://doi.org/10.1007/s00289-022-04160-y Tao, Y., Liu, M., Han, W., & Li, P. (2021). Waste office paper filled polylactic acid composite 127 filaments for 3D printing. Composites Part B: Engineering, 221(July 2020), 108998. https://doi.org/10.1016/j.compositesb.2021.108998 Tao, Y., Wang, H., Li, Z., Li, P., & Shi, S. Q. (2017). Development and application ofwood flour-filled polylactic acid composite filament for 3d printing. Materials, 10(4), 1–6. https://doi.org/10.3390/ma10040339 Teodoro, K. B. R., Silva, M. J., Andre, R. S., Schneider, R., Martins, M. A., Mattoso, L. H. C., & Correa, D. S. (2024). Exploring the potential of cellulose autofluorescence for optical detection of tannin in red wines. Carbohydrate Polymers, 324, 121494. https://doi.org/https://doi.org/10.1016/j.carbpol.2023.121494 Todo, M., & Takayama, T. (2011). Fracture Mechanisms of Biodegradable PLA and PLA/PCL Blends. In R. Pignatello (Ed.), Biomaterials - Physics and Chemistry (p. Ch. 19). IntechOpen. https://doi.org/10.5772/24199 Trivedi, A. K., Gupta, M. K., & Singh, H. (2023). PLA based biocomposites for sustainable products: A review. Advanced Industrial and Engineering Polymer Research, 6(4), 382–395. https://doi.org/10.1016/j.aiepr.2023.02.002 Tserki, V., Matzinos, P., Kokkou, S., & Panayiotou, C. (2005). Novel biodegradable composites based on treated lignocellulosic waste flour as filler. Part I. Surface chemical modification and characterization of waste flour. Composites Part A: Applied Science and Manufacturing, 36(7), 965–974. https://doi.org/10.1016/j.compositesa.2004.11.010 Tsou, C. Y., Wu, C. L., Tsou, C. H., Chiu, S. H., Suen, M. C., & Hung, W. S. (2015). Biodegradable composition of poly(lactic acid) from renewable wood flour. Polymer Science - Series B, 57(5), 473–480. https://doi.org/10.1134/S1560090415050164 Tsuji, H., & Ikada, Y. (1999). Stereocomplex formation between enantiomeric poly(lactic acid)s. 128 XI. Mechanical properties and morphology of solution-cast films. Polymer, 40(24), 6699–6708. https://doi.org/10.1016/S0032-3861(99)00004-X Tsuji, Hideto. (1996). Crystallization from the melt of poly(1actide)s with different optical purities and their blends. 197, 3483–3499. Vaidya, A. A., Gaugler, M., & Smith, D. A. (2016). Green route to modification of wood waste, cellulose and hemicellulose using reactive extrusion. Carbohydrate Polymers, 136, 1238–1250. https://doi.org/https://doi.org/10.1016/j.carbpol.2015.10.033 Vainio, A. K., & Paulapuro, H. (2007). Interfiber bonding and fiber segment activation in paper. BioResources, 2(3), 442–458. https://doi.org/10.15376/biores.2.3.442-458 Valapa, R. babu, Pugazhenthi, G., & Katiyar, V. (2016). Hydrolytic degradation behaviour of sucrose palmitate reinforced poly(lactic acid) nanocomposites. International Journal of Biological Macromolecules, 89, 70–80. https://doi.org/10.1016/j.ijbiomac.2016.04.040 Vena, P. F. (2005). Thermomechanical pulping (TMP), chemithermomechanical pulpinfg (CTMP) and biothermomechanical pulping (BTMP) of bugweed (solanum mauritianum) and pinus patula (Issue December) [University of Stellenbosch]. https://core.ac.uk/download/37322847.pdf Vink, E. T. H., Davies, S., & Kolstad, J. J. (2010). ORIGINAL RESEARCH: The eco-profile for current Ingeo® polylactide production. Industrial Biotechnology, 6(4), 212–224. https://doi.org/10.1089/ind.2010.6.212 Vo, L. T. T., Girones, J., Beloli, C., Chupin, L., Di Giuseppe, E., Vidal, A. C., Soutiras, A., Pot, D., Bastianelli, D., Bonnal, L., & Navard, P. (2017). Processing and properties of sorghum stem fragment-polyethylene composites. Industrial Crops and Products, 107, 386–398. https://doi.org/https://doi.org/10.1016/j.indcrop.2017.05.047 129 Wang, H., Chen, W., Zhang, X., Liu, C., & Sun, R. (2017). Esterification mechanism of bagasse modified with glutaric anhydride in 1-allyl-3-methylimidazolium chloride. Materials, 10(8), 1–15. https://doi.org/10.3390/ma10080966 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), 95–99. https://doi.org/10.3183/npprj-1988-03-02-p095-099 Westermark, U., Hardell, H.-L., & Iversen, T. (1986). The Content of Protein and Pectin in the Lignified Middle Lamella/Primary Wall from Spruce Fibers. Holzforschung, 40(2), 65–68. https://doi.org/10.1515/hfsg.1986.40.2.65 Wiese, M., Kwauka, A., Thiede, S., & Herrmann, C. (2021). Economic assessment for additive manufacturing of automotive end-use parts through digital light processing (DLP). CIRP Journal of Manufacturing Science and Technology, 35, 268–280. https://doi.org/10.1016/j.cirpj.2021.06.020 Wimmer, R., Steyrer, B., Koddenbeerg, T., & Mundigler, N. (2015). 3D Printing and Wood. Pro-Ligno, 11(4), 144–149. 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), 83–87. Xie, G., Zhang, Y., & Lin, W. (2017). Plasticizer combinations and performance of wood flour-poly(lactic acid) 3D printing filaments. BioResources, 12(3), 6736–6748. https://doi.org/10.15376/biores.12.3.6736-6748 Xu, X., Yu, J., Yang, F., Li, Y., Ou, R., Liu, Z., Liu, T., & Wang, Q. (2023). Preparation of degradable chemically cross-linked polylactic acid films and its application on disposable 130 straws. International Journal of Biological Macromolecules, 251(June), 126394. https://doi.org/10.1016/j.ijbiomac.2023.126394 Yang, Z., Feng, X., Xu, M., & Rodrigue, D. (2021). Printability and Properties of 3D-printed Poplar Fiber/Polylactic Acid Biocomposite. BioResources, 16(2), 2774–2788. https://doi.org/10.15376/biores.16.2.2774-2788 Yilmazer, U., & Cansever, M. (2002). Effects of Processing Conditions on the Fiber Length Distribution and Mechanical Properties of Glass Fiber Reinforced Nylon-6. Polymer Composites, 23(1). https://doi.org/10.1002/pc.10412 Yu, W., Sun, L., Li, M., Peng, Y., Wei, C., Lei, W., Qiu, R., & Ge, Y. (2023). Effect of Modification and Hydrothermal Ageing on Properties of 3D-Printed Wood Flour–Poly(butylene succinate)–Poly(lactic acid) Biocomposites. Polymers, 15(18). https://doi.org/10.3390/polym15183697 Yurttas, E., & Ayrilmis, N. (2023). Mechanical and Thermal Properties of 3D-Printed Biocomposites of Polylactic Acid and Thermally Modified Wood Flour with Silver Nanoparticles. Macromolecular Materials and Engineering, 308(12), 1–9. https://doi.org/10.1002/mame.202300180 Zarna, C., Opedal, M. T., Echtermeyer, A. T., & Chinga-Carrasco, G. (2021). Reinforcement ability of lignocellulosic components in biocomposites and their 3D printed applications – A review. Composites Part C: Open Access, 6. https://doi.org/10.1016/j.jcomc.2021.100171 Zenkiewicz, M., Richert, J., Rytlewski, P., Moraczewski, K., Stepczyńska, M., & Karasiewicz, T. (2009). Characterisation of multi-extruded poly(lactic acid). Polymer Testing, 28(4), 412–418. https://doi.org/10.1016/j.polymertesting.2009.01.012 Zhang, J., Yang, L., & Liu, H. (2021). Green and efficient processing of wood with supercritical 131 CO2: A review. Applied Sciences (Switzerland), 11(9). https://doi.org/10.3390/app11093929 Zhang, K., Gong, Y., Niu, P., Wang, X., & Yang, J. (2013). Properties of polypropylene/hemp fibers flame-retardant composites: Effects of different processing methods. Journal of Reinforced Plastics and Composites, 32(9), 644–653. https://doi.org/10.1177/0731684412472247 Zhang, L., Lv, S., Sun, C., Wan, L., Tan, H., & Zhang, Y. (2017). Effect of MAH-g-PLA on the Properties of Wood Fiber/Polylactic Acid Composites. Polymers, 9(11), 5–8. https://doi.org/10.3390/polym9110591 Zhao, T., & Jiang, L. (2018). Contact angle measurement of natural materials. Colloids and Surfaces B: Biointerfaces, 161, 324–330. https://doi.org/10.1016/j.colsurfb.2017.10.056 Zhao, X., Li, K., Wang, Y., Tekinalp, H., Larsen, G., Rasmussen, D., Ginder, R. S., Wang, L., Gardner, D. J., Tajvidi, M., Webb, E., & Ozcan, S. (2020). High-Strength Polylactic Acid (PLA) Biocomposites Reinforced by Epoxy-Modified Pine Fibers. ACS Sustainable Chemistry and Engineering, 8(35), 13236–13247. https://doi.org/10.1021/acssuschemeng.0c03463 Zhao, X., Tekinalp, H., Meng, X., Ker, D., Benson, B., Pu, Y., Ragauskas, A. J., Wang, Y., Li, K., Webb, E., Gardner, D. J., Anderson, J., & Ozcan, S. (2019). Poplar as Biofiber Reinforcement in Composites for Large-Scale 3D Printing. ACS Applied Bio Materials, 2(10), 4557–4570. https://doi.org/10.1021/acsabm.9b00675 Zhu, R., Liu, H., & Zhang, J. (2012). Compatibilizing Effects of Maleated Poly(lactic acid) (PLA) on Properties of PLA/Soy Protein Composites. Industrial & Engineering Chemistry Research, 51(22), 7786–7792. https://doi.org/10.1021/ie300118x 132 Zykova, A. K., Pantyukhov, P. V., Kolesnikova, N. N., Monakhova, T. V., & Popov, A. A. (2018). Influence of Filler Particle Size on Physical Properties and Biodegradation of Biocomposites Based on Low-Density Polyethylene and Lignocellulosic Fillers. Journal of Polymers and the Environment, 26(4), 1343–1354. https://doi.org/10.1007/s10924-017-1039-9 133 Appendices The thermal stability of PLA is critical for determining its thermal processing window. As shown in the thermal gravimetric curves below (Figure A.1), the Td5% of PLA is around 332°C. Figure A.1 TGA analysis of PLA
Thesis/Dissertation
eng
Forestry
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Renneckar, Scott
Thermomechanical pulp fines as additives for 3D printing filaments
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