DEVELOPING LIGNIN-BASED EPOXY AND POLYURETHANE RESINS By Saeid Nikafshar A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of Forestry-Doctor of Philosophy 2022 ABSTRACT DEVELOPING LIGNIN-BASED EPOXY AND POLYURETHANE RESINS By Saeid Nikafshar Lignin, the most abundant natural aromatic polymer, is currently produced as by-product during biorefinery and chemical pulping processes. Lignin is rich in hydroxyl functional groups (both phenolic and aliphatic OH), making it an excellent raw material for synthesizing epoxy and polyurethane resins. However, there are several challenges in utilizing unmodified lignins as feedstock for product development, including high polydispersity/heterogeneity, low reactivity, poor accessibility of hydroxyl groups for reaction with co-monomers low solubility in common organic solvents, and dark color. There are significant variations in lignin characteristics, depending on the source of biomass and isolation methods. Therefore, in-depth lignin characterization is needed to provide the basic knowledge of the structural, chemical, and thermal properties to facilitate lignin valorization. This study was focused on lignin characterization and development of lignin-based epoxy and polyurethane resins. First, a wide range of lignin samples was fully characterized by measuring their ash contents, elemental analyses, hydroxyl contents, chemical structures, molar mass distributions, and thermal properties. Next, a novel method was developed to measure the reactivity of thirteen different unmodified lignins toward biobased epichlorohydrin (ECH). A partial least square regression (PLS-R) model (with 92 % fitting accuracy and 90 % prediction ability) was created to study the correlation between lignin properties and epoxy content. The results showed that lignins with higher phenolic hydroxyl contents and lower molecular weights were more suitable for replacing 100 % of toxic bisphenol A (BPA) in the formulation of resin precursors. Additionally, two epoxidized lignin samples (with the highest epoxy contents) were cured using a biobased hardener (Cardolite from cashew nutshell), showed comparable thermomechanical performances and thermal stabilities to a petroleum-based epoxy system. Biobased waterborne polyurethane resins (PUDs) were also developed by entirely replacing the petroleum-based polyol and the internal emulsifier with either alkaline pre-extraction lignins or enzymatic hydrolysis lignins as well as tartaric acid (a biobased diacid). The formulated resins had zero VOC (volatile organic compound), which was achieved by replacing toxic n-methyl-2- pyrrolidone (NMP) with cyrene (a biobased solvent). To further improve the mechanical properties of our biobased PUD resins, 20 wt.% of lignin was substituted with low hydroxyl value soy-polyol, which increased their tensile strength and elongation at break to 87% and 68% of a commercial PUD resin. The results of this study demonstrated that it is imperative to fully characterize lignin and choose the right lignin for each specific application. This approach enabled us to entirely replace petroleum-based raw materials (BPA and polyol) with lignin and formulate biobased epoxy and polyurethane resins. Copyright by SAEID NIKAFSHAR 2022 Dedicated to my beloved wife, Horieh, & My amazing parents, Ali & Mahvash, For your endless encouragement, support, and love v ACKNOWLEDGMENTS Words cannot express the depth of my gratitude to my supervisor Dr. Mojgan Nejad for her suggestions, valuable guidance, endless support, advice, insightful criticism, and patience. She always has gone above and beyond to make sure I was on the right track, and she helped me improve myself in all aspects, such as personality, research, writing, and networking. Moreover, special thanks must go to my committee members, Dr. Eric Hegg (Department of Biochemistry & Molecular Biology), Dr. John Dorgan (Chemical Engineering & Material Science Department), and Dr. Mohammad Rabnawaz (School of Packaging) for their valuable comments, feedback, and suggestions. I gratefully acknowledge Mr. Michael Rich (Composite Materials and Structures Center) and Mr. Aaron Walworth (School of Packaging) for training me to set up lab equipment used in this research study. I would also like to thank my colleague Dr. Nusheng Chen, Dr. Zhen Fang, Dr. Zhaoyang Yuan, Dr. Maryam Arefmanesh, Christian Henry, Mona Alinejad, Mohsen Siahkamari, Kevin Dunne, Jiarun Wang, Akash Gondalyia, and Sasha Bell for their help and support. Of course, I am so grateful to my family. To my kind and always compassionate mother, my strong and supportive father, and my talented brother: Thank you for everything; I truly appreciate it. I could never have done this without you. I am a blessed son to have you by my side. And last, to my beautiful, brilliant, and fantastic wife, Horieh: I love you. Thank you for your patience, inspiration, support, care, and unconditional love for the last few years. You are my favorite. vi TABLE OF CONTENTS LIST OF TABLES .......................................................................................................................... x LIST OF FIGURES ...................................................................................................................... xii LIST OF SCHEMES..................................................................................................................... xv 1 CHAPTER 1 ........................................................................................................................... 1 1.1 Introduction .................................................................................................................. 1 1.2 Lignin Applications ...................................................................................................... 3 1.2.1 Lignin-Based Epoxy ................................................................................................. 4 1.2.2 Lignin-Based Polyurethane ....................................................................................... 6 1.3 Objectives ..................................................................................................................... 8 1.4 Hypothesis .................................................................................................................... 8 2 CHAPTER 2 (Lignin Characterization)................................................................................ 10 2.1 Introduction ................................................................................................................ 10 2.2 Experimental ............................................................................................................... 11 2.2.1 Materials ................................................................................................................. 11 2.3 Characterization .......................................................................................................... 13 2.3.1 Ash Content ............................................................................................................ 13 2.3.2 Elemental Analysis ................................................................................................. 13 31 2.3.3 P NMR (Phosphorus 31-Nuclear Magnetic Resonance Spectroscopy) ............... 15 2.3.4 FTIR (Fourier Transform Infrared Spectroscopy) .................................................. 17 2.3.5 GPC (Gel Permeation Chromatography) ................................................................ 17 2.3.6 DSC (Differential Scanning Calorimetry) .............................................................. 18 2.3.7 TGA (Thermogravimetric Analysis)....................................................................... 19 2.4 Results and Discussion ............................................................................................... 19 2.4.1 Lignin Composition ................................................................................................ 19 2.4.1.1 Ash Content and Elemental Analysis .............................................................. 19 2.4.2 Chemical Structure Analysis................................................................................... 33 31 2.4.2.1 P NMR (Phosphorus 31-Nuclear Magnetic Resonance Spectroscopy) ........ 33 2.4.2.2 FTIR (Fourier Transform Infrared Spectroscopy) ........................................... 37 2.4.3 Molar Mass Distribution ......................................................................................... 39 2.4.4 Thermal Characterization........................................................................................ 40 3 CHAPTER 3 (Lignin-Based Epoxy Resins)143 ..................................................................... 44 3.1 Introduction ................................................................................................................ 44 3.2 Measuring Reactivity of Different Lignins Toward ECH .......................................... 47 3.2.1 Experimental (Reactivity Measurement) ................................................................ 47 3.2.1.1 Materials .......................................................................................................... 47 3.2.1.2 Methods ........................................................................................................... 47 3.2.1.3 Curing of Epoxy Resins ................................................................................... 48 vii 3.2.2 Characterization (Reactivity Measurement) ........................................................... 49 3.2.2.1 Characterization of Epoxidized Lignins .......................................................... 50 3.2.2.1.1 Epoxy Content Measurement (Auto-Titration).......................................... 50 3.2.2.1.2 Epoxy Content Measurements (Proton Nuclear Magnetic Resonance)..... 51 3.2.2.2 Chemometric Modeling ................................................................................... 51 3.2.2.3 Thermomechanical Properties of Cured Epoxy Systems (DMA Analysis) .... 52 3.2.2.4 Thermal Stability of Cured Epoxy Systems (TGA Analysis) ......................... 52 3.2.3 Results and Discussion (Reactivity Measurement)................................................. 52 3.2.3.1 Lignin Characterization ................................................................................... 52 3.2.3.2 Lignin Epoxidation .......................................................................................... 54 3.2.3.3 Modeling .......................................................................................................... 58 3.2.3.4 DMA Analysis of Cured Samples (Thermodynamic Performance) ................ 60 3.2.3.5 TGA Analysis of Cured Samples (Thermal Stability)..................................... 62 3.3 Crosslinking Behavior of Epoxy Resins ..................................................................... 64 3.3.1 Materials and Methodology (Crosslinking Behavior) ............................................ 64 3.3.1.1 Modified Epoxidation Method of Lignin ........................................................ 64 3.3.1.2 Curing of Epoxy Resins ................................................................................... 65 3.3.2 Characterization of Epoxidized Lignin (Crosslinking Behavior) ........................... 66 3.3.2.1 Rheology (Crosslinking Behavior) .................................................................. 66 3.3.2.2 Gel Fraction and Swelling Ratio ..................................................................... 66 3.3.3 Results and Discussion (Crosslinking Behavior) .................................................... 67 3.3.3.1 Characterization of Technical Lignins (Crosslinking Behavior) ..................... 67 3.3.3.2 Characterization of Epoxidized Lignins .......................................................... 71 3.3.3.2.1 FTIR spectroscopy ..................................................................................... 74 3.3.3.2.2 Ethyl Lactate (EL) ..................................................................................... 76 3.3.3.2.3 Viscosity .................................................................................................... 76 3.3.3.2.4 Rheology .................................................................................................... 77 3.3.3.2.5 Gel Fraction and Swelling Ratio ................................................................ 83 4 CHAPTER 4 (Lignin-Based PUD) ....................................................................................... 85 4.1 Introduction ................................................................................................................ 85 4.1.1 Incorporation of Lignin in Polyurethane Formulations .......................................... 85 4.1.2 Waterborne Polyurethane Formulations (PUDs) .................................................... 85 4.2 Experimental ............................................................................................................... 87 4.2.1 Materials ................................................................................................................. 87 4.2.2 Synthesis Lignin-Based PUD Resins ...................................................................... 87 4.3 Characterization of Lignin-Based PUDs .................................................................... 88 4.4 Results and Discussion ............................................................................................... 89 5 CHAPTER 5 (Conclusions and Future Recommendations) ................................................. 92 5.1 Conclusions ................................................................................................................ 92 5.1.1 Insight into the Composition and Structure of Commercial Lignins ...................... 92 5.1.2 Entirely Replacing Bisphenol A with Unmodified Lignin in Epoxy Resin............ 92 5.1.3 Formulating Lignin-Based Waterborne Polyurethane Resins ................................ 93 5.2 Future Recommendation............................................................................................. 94 viii 6 APPENDICES ...................................................................................................................... 95 6.1 APPENDIX A (UV Degradation of Lignin) .............................................................. 96 6.2 APPENDIX B (Epoxy HNT) ................................................................................... 115 REFERENCES ........................................................................................................................... 143 ix LIST OF TABLES Table 1. The list of lignin samples from various isolation processes and sources ....................... 12 Table 2. Mineral concentrations in standard solutions ................................................................. 14 Table 3. Ash contents and elemental results of lignin samples, Mean (s.d.), n=3 ........................ 32 Table 4. Band assignments in mid-infrared region for softwood, hardwood, and herbaceous lignins ....................................................................................................................................................... 38 Table 5. Molecular weight data of lignin samples ........................................................................ 40 Table 6. Thermal characterization of 17 commercial lignin ......................................................... 43 Table 7. Summary of the previously published paper focused on epoxidation of unmodified lignins ....................................................................................................................................................... 46 Table 8. Formulation of different epoxy samples ......................................................................... 48 Table 9. Measured lignin properties (epoxy) ................................................................................ 53 Table 10. Properties of epoxidized lignins, including epoxy content and the epoxy equivalent weight measured by titration and 1H NMR methods, yield (%) based on total hydroxyl content, and average number of epoxy groups in each lignin macromolecule ........................................... 57 Table 11. DMA performance of epoxidized lignins (4-O-CS and 11-K-HW) and DGEBA cured by biobased hardener (GX-3090).................................................................................................. 61 Table 12. Thermal stability of cured epoxidized lignin and DGEBA networks ........................... 64 Table 13. Composition of different epoxy systems ...................................................................... 65 Table 14. Ash content, molecular weight, glass transition temperature, and hydroxyl content of lignin samples ............................................................................................................................... 68 Table 15. Semi-quantification of inter-unit linkages and aromatic units detected in lignin samples ....................................................................................................................................................... 71 Table 16. Properties of epoxidized lignin samples ....................................................................... 74 Table 17. The viscosity of lignin-based and DGEBA-based systems measured at two shear rates (100 and 1000 1/s) ........................................................................................................................ 77 x Table 18. Components of tested PUD formulations ..................................................................... 88 Table 19. Properties of PUD resins............................................................................................... 89 Table 20. Composition of prepared samples............................................................................... 103 Table 21. Relative spectral irradiance of UV-A 340 lamp ......................................................... 104 Table 22. Summary of the major FTIR peaks of the organosolv lignin. .................................... 106 Table 23. Summary of the measured lignin properties before and after 35 days of UV irradiation. ..................................................................................................................................................... 109 Table 24. Morphological characterization of halloysite nanotubes (HNTs) 302.......................... 120 Table 25. Composition of prepared samples............................................................................... 123 Table 26. Amount of UVA/HALS or lignin loaded into HNTs ................................................. 126 Table 27. XPS analyses results of pristine HNTs and encapsulated samples showing successful encapsulation of UVA/HALS mixture and lignin in HNTs........................................................ 127 Table 28. Tg of different samples before and after 35 days of UV irradiation ........................... 141 xi LIST OF FIGURES Figure 1. Hydroxyl contents (mmol/g) of lignins quantified by 31P NMR ................................... 36 Figure 2. Auto-titrator used to measure epoxy content of epoxidized lignin ............................... 50 Figure 3. Hydroxyl contents (mmol g-1) of different lignin samples obtained by 31P NMR ........ 54 Figure 4. 1H NMR spectrum of epoxidized lignin (1-K-SW) ....................................................... 56 Figure 5. Component contribution plot for the response variable (epoxy content) measured both by titration and 1H NMR methods. ............................................................................................... 59 Figure 6. Loadings plot of PLS-R modeling of epoxy content based on lignin properties .......... 60 Figure 7. Storage modulus (a), loss modulus (b), and tan δ (c) of cured epoxidized lignin samples (4-O-CS and 11-K-HW) and DGEBA with GX-3090. ................................................................. 62 Figure 8. TGA profiles of different cured lignin-based and DGEBA epoxy thermosets ............. 64 Figure 9. HSQC spectrum of softwood lignin (SW) .................................................................... 69 Figure 10. HSQC spectrum of hardwood lignin (HW) ................................................................. 70 Figure 11. Lignin interunit presented in original lignins .............................................................. 70 Figure 12. HSQC spectrum of epoxidized hardwood kraft lignin (EHW-K) ............................... 72 Figure 13. HSQC spectrum of epoxidized softwood kraft lignin (ESW-K) ................................. 73 Figure 14. FTIR spectra of original and epoxidized a) HW-K and b) SW-K lignins ................... 75 Figure 15. Oscillation strain on cured sample to determine the linear viscoelastic region .......... 78 Figure 16. a) Multi-wave experiment at 1, 10, 50 Hz, b) Plot of tan δ to identify true gel point . 80 Figure 17. The plot of obtained activation energy of epoxy resins .............................................. 82 Figure 18. Immersed epoxy thermosets in THF ........................................................................... 83 Figure 19. Measured gel fractions and swelling ratios of the cured samples ............................... 84 Figure 20. Wood coated samples .................................................................................................. 90 xii Figure 21. Tensile properties of different PUD resins .................................................................. 91 Figure 22. FTIR spectra of pure lignin sample (control) before and after 35 days of UV-irradiation, 1508 cm-1, vibrations of aromatic rings; 1735 cm-1, vibration of carbonyl groups. ................... 107 Figure 23. Lignin and carbonyl indices of pure lignin at different UV irradiation times. .......... 108 Figure 24. 31P NMR spectra of lignin before(red) and after 35 days of UV irradiation (blue) .. 110 Figure 25. Decrease in lignin index (%) (𝐴 1508𝐴2921) of various samples after 35 days of UV irradiation (lower numbers are better), the bars with the same color are not significantly different (α=0.05)....................................................................................................................................... 111 Figure 26. Increase in carbonyl index (%) (𝐴 1735𝐴2921) of different samples after 35 days of UV irradiation, (lower numbers are better), the bars with the same color are not significantly different (α=0.5). ......................................................................................................................... 113 Figure 27. (a) SEM and (b) TEM images of pristine HNTs with increased magnifications (left to right)............................................................................................................................................ 120 Figure 28. Schematic of HNT-encapsulation process (both UVA/HALS and lignin systems) .. 122 Figure 29. TGA curves of different samples (pristine HNTs, mixture of UVAT-1130/HALS-T282, pure lignin, pristine HNTs, encapsulated UVA/HALS in HNTs, and encapsulated lignin into HNTs) ......................................................................................................................................... 127 Figure 30. XRD diffraction patterns of the Pristine HNTs, HNT-UVA/HALS, and HNT-lignin ..................................................................................................................................................... 128 Figure 31. Variations of ∆E* for different samples after exposure to UV irradiation for 35 days (three replicates for each sample) ............................................................................................... 130 Figure 32. Photos of different epoxy samples before and after UV irradiation .......................... 130 Figure 33. FE-SEM micrographs of pure epoxy, epoxy with addition of 2 wt.% pure UVA/HALS, and 2 % lignin before and after 35 days of UV irradiation ......................................................... 132 Figure 34. FE-SEM micrographs of epoxy resins with addition of (1-3%) of HNT-UVA/HALS loaded samples before and after 35 days of UV irradiation........................................................ 133 Figure 35. FE-SEM micrographs of epoxy samples loaded with different amounts (1-3%) of HNTs encapsulated with lignin, before and after 35 days of UV irradiation ........................................ 135 Figure 36. EPR spectra of pure epoxy (a), 2% HNT-UVA/HALS (b), and 1% HNT-lignin (c), before and after 5 min irradiation. d) The intensity of free radicals before and after 5 min UVC irradiation (EPR data) for different samples ............................................................................... 138 xiii Figure 37. Carbonyl index of various samples obtained from FTIR spectra after 35 days ........ 139 xiv LIST OF SCHEMES Scheme 1. Three main monolignols (precursors) and their structures in lignin ............................. 2 Scheme 2. Chemical structure of diglycidyl ether bisphenol A (DGEBA) .................................... 4 Scheme 3. Urethane synthesis reaction ........................................................................................... 7 Scheme 4. Proposed reaction of lignin with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) ......................................................................................................................................... 16 Scheme 5. Potential reactions occurred during the kraft process 130 ............................................ 35 Scheme 6. Phenol epoxidation mechanisms in lignin ................................................................... 44 Scheme 7. Synthesis and curing reaction of epoxidized lignin .................................................... 49 Scheme 8. Formation of o- and p-quinonoid structures resulting from UV degradation of lignin (Hon 2001) .................................................................................................................................... 99 xv 1 CHAPTER 1 1.1 Introduction Lignin is the second most abundant natural polymer produced as a by-product during chemical pulping and biorefinery processes. It is found in the middle lamella and the primary and secondary cell walls of plants, and it provides rigidity, strength, and protection from microorganism attacks.1- 4 Lignin is defined as a heterogeneous polymer derived from three phenylpropanoid monomers: p- coumaryl, coniferyl, and sinapyl alcohols (the ‘so-called’ H-, G-, and S- monolignols) that radically couple through carbon-carbon or ether linkages (i.e., β-O-4, β-5, and β-β), Scheme 1. The β-O-4 bond is the most common linkage, accounting for almost 50% of the linkages in the lignin structure.5 Monomer composition in lignin varies by different biomass sources. For instance, softwood lignins are primarily composed of G units.1 On the other hand, hardwood lignins are rich in G and S units with low levels of H units,6 while herbaceous plants, like wheat straw, consist of all three monomers with a high amount of H units.7 In addition to the type of monomers, the lignin content significantly depends on plant sources and generally increases in the order of grasses (17 - 24 wt.%), hardwood (18 - 25 wt.%), and softwood (27- 33 wt.%).8 1 Scheme 1. Three main monolignols (precursors) and their structures in lignin A variety of lignin isolation methods have been used 9-12 to extract lignin from biomass by breaking linkages in lignin carbohydrate complexes (LCCs), resulting in smaller polymer chains solubilized in the pulping media.10 During the isolation process, multiple chemical modifications and transformations occur, which significantly change the lignin properties, highlighting the importance of studying the effects of the isolation process on lignin properties. Isolation processes for technical lignins mainly target the maximum valorization of carbohydrates in the biomass 2 without any concern for the fate of lignin. Several commercial isolation methods, including the sulfite (lignosulfonate), kraft, organosolv, and soda processes, are readily available to produce lignin on the commercial-scale, and the properties of these technical lignins significantly differ based on the isolation processes.9, 13-15 In addition, several other methods have also been used for lignin isolation from biomass, including ionic liquid, enzymatic hydrolysis, alkaline wet oxidation, and pyrolysis, but those lignins are mainly available at either the lab or pilot scales.10-12 The global commercial production of technical lignin (excluding bioenergy production) is approximately 1.65 million tons annually, with lignosulfonate accounting for about 80% of the market.16 Approximately 16% of production is kraft lignin,17 and the remainder (4%) is shared between hydrolysis and soda lignins.18 The majority of the lignin produced is burned and used at its lowest value as fuel despite the fact that it has significant potential as a renewable raw material for producing value-added products.19 Therefore, there is a great interest in developing lignin- based products by replacing petroleum-based raw materials using cost-effective and environmentally friendly processes. 1.2 Lignin Applications Lignin has excellent potential to be incorporated into a wide range of products, including as additives or as a raw material for synthesis of polymeric resins. As additives, lignin has been used as an antioxidant,20 an ultraviolet light-stabilizer,21 a flame retardant,22 an antimicrobial,23 an antibacterial,24 and a plasticizer.25 As raw materials in polymeric resins, lignin has been used to partially replace bisphenol A, phenol, and polyol in synthesizing epoxy,26 polyurethane,27 phenolic,28 acrylic,29 and polyester resins.30 In addition, lignin has been used as a sustainable feedstock for the production of low molecular weight aromatic compounds such as vanillin,31 3 vanillic acid,32 syringic aldehyde,33 syringic acid,34 p-hydroxybenzoic acid.35 Lignin is also used as a raw material to produce carbon fibers for advanced composite applications. 36 1.2.1 Lignin-Based Epoxy Epoxy resins refer to low molecular weight prepolymers that contain one or more epoxy groups (oxirane rings).37 They are conventionally prepared by reacting epichlorohydrin (ECH) with the hydroxyl groups of bisphenol A (BPA) under alkaline conditions using sodium hydroxide as a catalyst.38 Epoxy resins are used for the production of adhesives, coatings, and composites due to their excellent chemical, thermal, and mechanical properties.39, 40 One of the most common types of epoxy resin is diglycidyl ether bisphenol A (DGEBA), as shown in Scheme 2.39 This resin forms a crosslinked network by adding different curing agents like polyamines, polyamides, anhydrides, and mercaptans, to cure epoxy resin at different temperatures.38 The final properties of the epoxy system depend on several factors such as curing agents, catalysts, and additives in addition to the curing parameters (i.e., temperature and time).41 Scheme 2. Chemical structure of diglycidyl ether bisphenol A (DGEBA) Due to increased greenhouse gas emissions, and health and environmental concerns, there have been serious efforts to replace fossil fuel-based chemicals with biobased materials.42-44 BPA, which is used as the main raw material in the production of DGEBA epoxy resin, comprises more than 67% of the molar mass of DGEBA.43 BPA has detrimental effects on human health and the environment and has been shown to act as an endocrine disruptor that is highly toxic for living organisms.45 BPA-based products have been banned in food packaging, food-related materials, 4 and baby bottles.46-48 BPA can be leached out by hydrolysis of carbonate linkages in the presence of a base in mild conditions,49 since the ester linkages are hydrolyzed easier and faster compared to ether linkages.50 Therefore, it is of great interest to identify alternative, renewable, and sustainable raw materials that can substitute BPA in the epoxy resin formulation. Several biobased aromatic compounds have been used to synthesize epoxy resin, including itaconic acid,43 eugenol,51 rosins,52 gallic acid,46 vanillin,53 vanillic acid,54 as well as lignin. 26, 55- 62 Renewable compounds like lignin, which are widely available, sustainable, and inedible, are more desirable because they do not compete with food resources like other renewable materials (e.g., vegetable oils). Due to the presence of phenolic hydroxyl groups in lignin’s structure (S, G, and H), lignin is widely recognized as an alternative raw material to substitute BPA in epoxy resin formulations.63 Although many studies have focused on utilizing lignin in epoxy resin,64-67 they mostly used modified lignin (fractionated or lignin monomers).66-68 It was reported that the cured epoxy thermosets made with lignin-derived monomers/dimers had comparable properties to DGEBA thermosets, and properties of the epoxy systems can be tunable by using various curing agents.69- 74 However, the extra cost associated with lignin fractionation and using lignin monomers in epoxy resin formulation has not been viewed favorably by the industry. There have been few studies that 57-62 used unmodified lignin to replace BPA in epoxy resins. Sasaki et al.57 epoxidized lignin isolated from bamboo by the steam-explosion process and reported that the lignin-based epoxy thermosets had lower thermal stability and flexural strength compared with a petroleum-based epoxy system. Other studies used lignin from different isolation methods and sources to epoxidize lignin and replaced up to 42 wt.% DGEBA in epoxy resin. 60-62 They found the epoxy contents of epoxidized lignins were lower than DGEBA. In addition, the thermal stability and mechanical 5 properties of lignin-based epoxy networks were decreased by incorporating a higher amount of DGEBA (more than 42 wt.%) with epoxidized lignin. They could not replace a higher amount of DGEBA in epoxy resin due to lower reactivity and higher molecular weight of epoxidized lignin. In contrast, the factors affecting the suitability of lignin in epoxidation reaction have not been directly evaluated. The structure of lignin is much larger and more complicated than BPA. The high polydispersity index (PDI), low solubility in organic solvents and water, as well as the presence of various functional groups (different types of hydroxyl groups) of lignin are challenges that have limited its application as a BPA replacement.66 These attributes not only result in a non-homogenous network it also impacts lignin reactivity with epichlorohydrin (ECH). 1.2.2 Lignin-Based Polyurethane Polyurethanes (PU) are synthesized based on the chemical reaction between a diisocyanate with a di or polyol, a compound containing more than one hydroxyl group, as shown in Scheme 3.75 Isocyanate compounds are categorized based on their structures (aromatic and aliphatic). MDI (methylene diphenyl diisocyanate) and TDI (toluene diisocyanate) are the aromatic isocyanates most commonly used in PU globally. HDI (hexamethylene diisocyanate) and IPDI (isophorone diisocyanate) are aliphatic isocyanates commonly used for coating formulations because coatings formulated with aromatic isocyanates are sensitive to UV degradation.76 Aromatic isocyanates are more reactive than aliphatic isocyanates due to the delocalization of electron density in the aromatic rings and are usually used in PU adhesive and foam formulations. 6 Scheme 3. Urethane synthesis reaction PU coatings are used for wood, metal, plastic, leather, and textile applications. The automotive and transportation industries are the leading consumer of PU coatings due to their high dielectric strength and high chemical, heat, and weather resistance properties.77 The global market size of PU was $60.5 billion in 2017, with an estimated annual growth rate of 8-10%.78 As the global market of polyols shows strong growth, the demand for biobased polyols is increasing, representing an excellent opportunity for renewable and sustainable feedstocks like lignin. The abundance of hydroxyl groups (aliphatic and phenolic), availability, sustainability, and relatively lower price than most petroleum-based polyols makes lignin an excellent candidate for substituting polyols in polyurethane formulations.79-82 There are several challenges in incorporating lignin into polyurethane. Lignin has a complex structure in which all hydroxyl groups are not accessible to react with isocyanate due to steric hindrance.83 In addition, lignin has three different hydroxyl groups, including aliphatic, aromatic, and carboxylic acid groups, which have different reactivity toward isocyanate in the following order: primary aliphatic > secondary aliphatic >> phenolic >-COOH.84 Therefore, these variations limit the control of the reaction and the performance of final products. Besides, lignin has a relatively high polydispersity index (PDI), which causes the final products to have inconsistent properties due to the low homogeneity of lignin polymeric chains.85, 86 7 1.3 Objectives The main objectives of this thesis are summarized as below, which resulted in multiple side projects for creating value-added products from lignin: 1) Characterizing the structure of a wide range of commercially available lignins isolated from different biomass sources and isolation processes 2) Replacing toxic bisphenol A with unmodified technical lignins in epoxy resin - Investigating the correlation between lignin properties and the epoxy content of epoxidized lignin - Measuring chemical, thermal, and mechanical properties of epoxidized lignin thermosets in comparison with a commercial epoxy network (diglycidyl ether bisphenol A, DGEBA) 3) Formulating lignin-based polyurethane resins - Exploring the possibility of entirely replacing petroleum-based polyol with hardwood lignin isolated through alkaline pre-extraction and enzymatic hydrolysis processes - Increasing the biobased carbon content of resin by substituting other petroleum-based chemicals in polyurethane dispersion formulation (PUD), including internal emulsifier and solvent with biobased compounds - Formulating zero-VOC, high-performance resin 1.4 Hypothesis To successfully develop high-performance lignin-based bioproducts, the first critical step is to start with the right lignin for each specific product. Studying correlations between measured lignin properties and the performance of the developed lignin-based resins can help evaluate the 8 suitability of new lignin for a particular application based on the simple properties of that lignin measured in the lab. The objectives of these proposed projects were as follows: 1) Lignin with higher phenolic hydroxyl content and lower molecular weight, PDI, and ash content are better candidates for replacing BPA in epoxy resin 2) Lignins with a lighter color and higher aliphatic hydroxyl content have great potential to replace petroleum-based polyol in formulating lignin-based PU resins 9 2 CHAPTER 2 (Lignin Characterization) 2.1 Introduction Choosing the right lignin for a specific application is the most crucial step in lignin valorization. To facilitate the utilization of commercial lignin in different products, in-depth characterization of lignins is needed to provide basic knowledge of the structural, chemical, and thermal properties of each sample lignin. Although lignin’s randomness is derived from the radical coupling of H, G, and S monomers, the relative abundance of these monomers and the technique by which lignin is extracted also contribute to variation in lignin properties. Ash content and elemental analysis provide quantitative information about impurities and lignin composition, such as nitrogen, sulfur, carbon, and sodium contents. These compounds can affect physical (like odor caused by sulfur),86 or chemical properties of lignin. For instance, a high amount of sodium and potassium in lignin can act as catalysts to both increase the reaction of lignin with isocyanate,87 and cause trimerization of isocyanate.88 Molar mass distribution of lignin is an important structural property that significantly impacts lignin valorization, and it is commonly determined by gel permeation chromatography (GPC). The molecular weight of lignin can play a crucial role in defining the reactivity of lignin with co- monomers and also its suitability for carbon fiber application.1 Hydroxyl groups play critical roles in determining the physical and chemical properties of lignin.89, 90 Therefore, having a reliable method to quantify and qualify different hydroxyl functional groups of lignin is essential. Phosphorous 31-NMR spectroscopy is a technique where hydroxyl functional groups of lignin are selectively labeled with phosphorus as an active nucleus. This technique is advantageous because of a wide chemical shift, less overlap, and less interference from the homonuclear coupling compared to 1H NMR.91 The 31P NMR technique provides quantitative data 10 for various types of hydroxyl groups (aliphatic, phenolic, and carboxylic acid) that are critical for employing lignin as raw materials in polymeric resin applications. The thermal properties of lignin are usually measured by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). As an amorphous thermoplastic polymer, it is vital to measure the glass transition temperature (Tg) of lignin to evaluate its flexibility/brittleness at certain temperatures. Source and isolation method, moisture content, molecular weight, and dispersity (Ð), as well as thermal history, significantly affect the Tg of lignin.92 In addition to Tg that is usually measured by TGA, this technique can also be used to analyze the thermal stability of lignin and its ash content.93, 94 2.2 Experimental 2.2.1 Materials Seventeen commercially available lignins from different biomass sources and isolation processes were purchased or kindly provided by lignin producers Table 1. 11 Table 1. The list of lignin samples from various isolation processes and sources Lignin No.* Source Isolation Process Supplier 1-SW-K Softwood Kraft Ingevity 2-SW-K Softwood Kraft Arauco 3-SW-K Softwood Kraft UPM Biochemicals 4-SW-K Softwood Kraft West Fraser 5-SW-K Softwood Kraft Domtar 6-SW-K Softwood Kraft Stora Enso 7-SW-K Softwood Kraft Metsa 8-HW-K Hardwood Kraft Suzano 9-HW-K Hardwood Kraft American Science Technology 10-SW-O Softwood Organosolv Fibria (Lignol) 11-HW-O Hardwood Organosolv Fibria (Lignol) 12-HW-O Hardwood Organosolv American Science Technology 13-WS-O Wheat Straw Organosolv CIMV 14-PS-O Peanut Shell Organosolv American Science Technology 15-CS-O Corn Stover Organosolv American Science Technology 16-SW-L Softwood Sulfite (Lignosulfonate) Borregaard (Lignotech) 17-WS-S Wheat Straw Soda Green Value LCC *K: kraft, O: organosolv, L: lignosulfonate, S: soda, SW: softwood, HW: hardwood, WS: wheat straw, PS: peanut shell, and CS: corn stover For 31P NMR, chloroform-d, 99.8% (Cambridge Isotope Laboratories, Inc.), pyridine, anhydrous, 99.8% (Sigma Aldrich), N, N-dimethylformamide, anhydrous, 99.8% (Sigma Aldrich), 12 cyclohexanol, 99% (Sigma Aldrich), chromium (III) acetylacetonate 97% (Sigma Aldrich), and 2- chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphopholane 95%, (Sigma Aldrich) were used. For GPC (THF column), various polystyrene standards ranging from 162-42000 g/mol were purchased from Agilent. All lignin samples were acetylated by acetic anhydride (ACS reagent, 99.7%, Chem-Impex INT’L INC.) and pyridine (99.9%, Sigma Aldrich). Tetrahydrofuran (HPCL grade, J.T. Baker) was used as the mobile phase. 2.3 Characterization 2.3.1 Ash Content Firstly, crucibles were rinsed with DI water and placed in an oven at 105 °C for 2 hours. Then, all crucibles were kept in a desiccator to cool to room temperature before the addition of lignin. Following TAPPI (Technical Association of Pulp and Paper Industry) T 211 om-9395/ASTM (American Society for Testing Materials) D1102,96 2 g of oven-dried lignin was added to each crucible and placed in a muffle furnace and heated with a 5 °C/min ramp to 525 °C with a 4-hour dwell time. At least five replicates were performed for each sample. The percent ash content of lignin samples was determined based on initial lignin and residual ash weights. 2.3.2 Elemental Analysis Samples were dried overnight at 100-105 oC, ground with a Wiley Mill, and passed through a number 10 sieve (2 mm). All samples were prepared following AOAC (Associate of Official Analytical Chemists) Official Methods 922.02 and 980.03.97 Afterward, the lignin samples were digested in an open vessel microwave (CEM, MARS 5) procedure following the SW846-3051A. About 0.2 g of lignin was added to 2 mL of nitric acid (68-70 wt.%), placed in the microwave, and the temperature was ramped up to 90 °C with a 90 second dwell time. The samples were then 13 cooled below 50 oC before adding 1 mL of hydrogen peroxide (30 wt.%). The samples were microwaved again with a ramp to 105 oC with a 10-minute dwell. After cooling to room temperature, distilled water was added to the solution to a final volume of 25 mL (1:125 dilution). Mixed samples were analyzed with a Thermo iCAP 6500 Duo instrument. A blank along with several multi-element standards obtained from Inorganic Ventures (aluminum, copper, iron, manganese, zinc, boron, sodium, calcium, magnesium, potassium) was run with the lignin samples as external calibration standard solutions according to Around 1.5 mL of standard or sample was injected in an inductively coupled plasma (ICP)-atomic (or optical) emission spectrometer, Thermo Scientific iCAP 6000 series 6500 Duo, in which 1 mL of sample was used to rinse out the sample loop and 0.5 mL of solution was used for analysis. Table 2. Around 1.5 mL of standard or sample was injected in an inductively coupled plasma (ICP)-atomic (or optical) emission spectrometer, Thermo Scientific iCAP 6000 series 6500 Duo, in which 1 mL of sample was used to rinse out the sample loop and 0.5 mL of solution was used for analysis. Table 2. Mineral concentrations in standard solutions Mineral High Medium Low Control Solution Manganese (ppm) 2 1 0.1 0.5 Iron (ppm) 5 2.5 0.25 1.25 Phosphorus (%) 0.01 0.005 0.005 0.0025 Copper (ppm) 0.5 0.25 0.025 0.125 Zinc (ppm) 2 1 0.1 0.5 Boron (ppm) 0.5 0.25 0.025 0.125 Aluminum (ppm) 10 5 0.5 2.5 Calcium (%) 0.05 0.025 0.0025 0.0125 Potassium (%) 0.1 0.05 0.005 0.025 Magnesium (%) 0.01 0.005 0.0005 0.0025 Sodium (%) 0.001 0.0005 0.00005 0.00025 Sulfur (%) 0.01 0.005 0.0005 0.0025 14 31 2.3.3 P NMR (Phosphorus 31-Nuclear Magnetic Resonance Spectroscopy) The hydroxyl contents of the lignin samples were measured using 31P NMR, according to a slightly modified method from the previously published procedure by Asgari et al.98 by adding DMF as a co-solvent to improve the solubility of lignin. About 40 mg (precisely measured using microbalance) of oven-dried lignin sample was dissolved in 325 μL of anhydrous pyridine/ deuterated chloroform mixture (1.6:1, v/v), 8 mL of pyridine and 5 mL of deuterated chloroform, and 300 μL of anhydrous DMF, in which DMF was utilized to fully dissolve the lignin samples. Sequentially, 22 mg of cyclohexanol (precisely measured using a microbalance) was dissolved in 1 mL anhydrous pyridine and deuterated chloroform (1.6:1, v/v). Then 100 μL of the cyclohexanol solution (22 mg/mL) was added as an internal standard, and 50 μL of chromium (III) acetylacetonate solution (5.6 mg/mL) in anhydrous pyridine and deuterated chloroform (1.6:1, v/v) was added as a relaxation reagent. In addition, 100 μL of the phosphitylation reagent (2-chloro- 4,4,5,5-tetramethyl-1,3,2-dioxaphospholane, TMDP) was added to the mixture to react and label hydroxyl and carboxylic acid groups of lignin. Finally, 600 μL of the resulting mixture was transferred to a 5 mm NMR tube, and 31P NMR was conducted using an Agilent DDR2 500 MHz NMR spectrometer equipped with 7600AS, running VnmrJ 3.2A, with a relaxation delay of 5 s, and 128 scans were collected. To prevent decomposition of internal standard, samples were tested within 1 hour of preparation. TMDP is the most common phosphitylation agent for measuring the hydroxyl content of lignin,99 tannins,100, 101 and biobased oils.102-104 When hydroxyl groups in lignin react with TMDP, HCl is released that can decompose phosphitylated compounds. To avoid this, pyridine was used to neutralize HCl by forming pyridine-HCl salt. Deuterated chloroform was used for three purposes: 1) increasing the solubility of lignin, 2) avoiding the precipitation of the pyridine-HCl salt, and 3) 15 locking NMR signals.91 Chromium(III) acetylacetonate was added as a relaxation agent to reduce the spin-lattice relaxation time of the phosphorous nuclei.99, 105 Cyclohexanol was used as an internal standard because it has a sharp and distinguishable chemical shift to avoid overlapping with other chemical shifts of lignin.99 Scheme 4. Proposed reaction of lignin with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) All spectra were analyzed by MestReNova software (version: 14.1.2-25024) in the following steps: each .fid file was opened and focused on the region between 150-132 ppm and “fit to highest intensity” in the View tab was selected. Next, a manual correction was chosen under “Phase Correction”, and “Biggest” was selected. Then two sides of the internal standard were properly aligned. By selecting “Auto Baseline Correction” and selecting “Ablative Point 5-10,” we eliminated variation caused by multiple users performing manual base-line correction, thereby improving the reproducibility of the analyzed data. To use the internal standard as a reference, “Reference” was selected, and 145.1 ppm was entered as the chemical shift. To ensure a sufficient phosphitylation agent was used to react with all hydroxyl groups in the lignin, the peak at 175 ppm must be checked to be present, which corresponds to unreacted phosphitylation agent, ensuring that there was excess phosphitylation reagent than needed for reaction with lignin.106 The hydroxyl content of each lignin sample was calculated based on the ratio of the internal standard peak area (cyclohexanol) to integrated areas over the following spectral regions (using “Manual Integration” in the Analysis tab): aliphatic hydroxyls (149.1-145.4 ppm), cyclohexanol (145.3-144.9 ppm), condensed phenolic units (144.6-143.3; and 142.0-141.2 ppm), syringyl phenolic units (143.3- 16 142.0 ppm), guaiacyl phenolic hydroxyls (140.5-138.6 ppm), p-hydroxyphenyl phenolic units (138.5-137.3 ppm), and carboxylic acids (135.9-134.0 ppm). For softwood lignin, the entire region of (144.6-138.6 ppm) was considered as condensed phenolic since softwoods do not have any syringyl units. The following equation was used to calculate the internal standard peak area: 𝐼𝑆 𝑚𝑚𝑜𝑙 ( )/10 100.158 Internal standard area ( )=[ ] × 1000 𝑔 𝑚 where IS the weight of internal standard (mg), the value 100.158 g/mol is the molecular weight of the internal standard, and m is the weight of the lignin sample used (mg). 2.3.4 FTIR (Fourier Transform Infrared Spectroscopy) The spectrum of each oven-dried lignin sample was recorded on A Perkin Elmer Spectrum II in attenuated total reflectance (ATR) mode with a wavenumber range of 4000-400 cm-1 with a resolution of 4 cm-1 and 32 scans. 2.3.5 GPC (Gel Permeation Chromatography) Gel permeation chromatography technique was used to determine the number average molecular weight, weight average molecular weight, and polydispersity index (PDI) of lignin samples. First, acetylation of lignin was conducted to improve the solubility of lignin in tetrahydrofuran (THF), which was used as the mobile phase.107 To acetylate lignin, 1 g of lignin was dissolved in 40 mL solution comprising pyridine and acetic anhydride (50-50 v/v%) and mixed at room temperature for 24 hours (600 rpm). Then, 150 mL of hydrochloric acid (0.1 N) was used to precipitate acetylated lignin. The precipitates were then vacuum filtered, and the residual solids were washed with 0.05 N hydrochloric acid solution (3 times) and deionized water (3 times). Acetylated lignin samples were then dried in a vacuum oven (Across International) at a temperature of 40 °C for 24 17 hours. Next, the samples were dissolved in THF (HPLC grade) at a concentration of 5 mg/mL and filtered using a syringe filter (PTFE, 0.45 μm); the filtrate samples were used for GPC analysis. A Waters GPC system (Waters e2695 Separation Module) was then used to analyze the filtrate at a flow rate of 1mL/min, using three 300 mm × 7.8 mm Waters columns in series including 1- Styragel HR 4, 5 µm, THF (5k-600kÅ), 2- Styragel HR 3, 5 µm, THF (500-30k Å) and 3- Ultrastyragel THF, 5 µm, (100-10k Å). Polystyrene standards of specific molecular weights (162, 370, 580, 945, 1440, 1920, 3090, 4730, 6320, 9590, 10400, 16700, and 42400 Da) were used as external calibration standards (R2 = 0.99996). The filtrate solution (25 μL) was injected into the system and was detected using a 2414 RI Detector, which was constantly maintained at the same temperature as the columns (35 °C) during the analysis. Data were collected and analyzed using Empower GPC Software. 2.3.6 DSC (Differential Scanning Calorimetry) Glass transition temperatures of different lignins were measured using a Q100 Differential Scanning Calorimeter (TA Instrument). Briefly, 8 mg of oven-dried sample was placed in an aluminum standard pan. One hole was made in the lid by needle with a diameter of 0.45 mm to avoid increasing pressure in the sealed pan caused by evaporation or chaining the volume of the sample upon heating.89, 108 The sample was heated from room temperature to 120 °C at a heating rate of 20 °C/min, then was cooled down to room temperature and kept isothermally for 10 min prior to reheating again to 200 °C under a nitrogen flow of 70 mL/min. The recommended temperature of the first heating cycle of 120 °C was designed for kraft lignins based on the Canadian standard. If there is still noise in the second heating graph, which intends to get rid of the thermal history of polymer, then the sample can be rerun this time using a higher temperature of up to 160 °C, which would be below lignin degradation temperature. The TGA analysis can 18 help with choosing the best temperature for the first cycle. The second heating cycle was used to calculate the Tg of the lignin sample using TRIOS software. 2.3.7 TGA (Thermogravimetric Analysis) Thermal property/stability of lignins were monitored through a thermal gravimetric analyzer (TA Instrument, Q50). Briefly, 8 mg of oven-dried lignin sample was placed in a platinum pan and heated from 25 to 105 °C with a constant heating rate of 10 oC/ min. The temperature was held isothermally at 105 °C for 20 min to evaporate possible hygroscopic water 109 and then continued to ramp up to 800 °C with the same heating rate under a nitrogen flow of 60 mL/min (for thermal stability) and an airflow of 60 mL/min (for oxidation) with a nitrogen flow of 40 mL/min for the balance. 2.4 Results and Discussion 2.4.1 Lignin Composition 2.4.1.1 Ash Content and Elemental Analysis Ash content is usually defined as total inorganic compounds in lignin and categorized as acid- soluble, acid-insoluble ashes, and silicates,110 known to interfere with lignin valorization.86 Ash content of lignin has a broad range (0.5-8%) and strongly depends on the source and isolation process of lignin.86, 111 Table 3 presents the ash contents and elemental analysis results of fifteen commercial lignin samples. Generally, ash and mineral contents of lignin samples are ranked based on their isolation methods are in order of sulfite > kraft > soda > organosolv. Ash contents of all kraft lignins, regardless of their sources, are in a range of 0.5-4.3%. Sulfite lignin (16-SW-L) had extremely high ash content (11.4%) and high sulfur content (5.80%) due to sulfonate groups grafted to lignin 19 phenolic or non-phenolic units. 112 For kraft and soda lignins, the relatively high ash contents are related to the remaining residual of sodium hydroxide and sodium sulfite (only for kraft), which are used during the pulping process, and also sulfuric acid used to precipitate lignin from pulping liquor.113 The variation in ash contents of different kraft lignins could be attributed to lignin isolation methods from black liquor (Lignoboost or Lignoforce) and the washing steps. Organosolv lignins, in contrast, have lower ash content compared to other isolation processes as expected, ranging from 0.1-3.3 %. In the organosolv process, a mixture of solvents (such as ethanol, acetic 114 acid, and formic acid are used). Although the organosolv process is typically sulfur-free, sometimes sulfuric acid is used as catalysts to improve the isolation process;115 therefore, yielding organosolv lignins with a sulfur content ranging from 0.02-1.07%. Organosolv lignins isolated from woody biomass (lignins 15, 13, 14, and 2) had the lowest ash content and inorganic impurities; in contrast, lignins from non-wood species, including wheat straw, corn stover, and peanut shell, had relatively higher ash contents (1.0-3.3%) than woody lignins. This is probably due to fertilizers use and nutrient uptake by plants from soil which is explicitly attributed to high nitrogen and phosphorus content.111, 116, 117 Similarly, fertilizers and usage of potassium hydroxide during the soda cooking process explain relatively high potassium content in lignins 15 ± 1.13% and 14 ± 0.14%.118 20 Table 3. Ash contents and elemental results of lignin samples, Mean (s.d.), n=3 Ash Sample Mg Mn Cu Al content S (%) P (ppm) K (ppm) Ca (ppm) Na (ppm) B (ppm) Zn (ppm) Fe (ppm) ID* (ppm) (ppm) (ppm) (ppm) (%) 153 1-SW-K 4.34 (0.2) 1.97 (0.05) 16 (4) 1160 (200) 142 (7) 430 (17) 6890 (870) 21 (1.2) 17 (0.7) 41 (2.4) 45 (1) 1 (0.4) (11) 165 2-SW-K 1.88 (0.1) 3.00 (0.25) 10 1160 (730) 23 100 8530 (320) 23 (6.8) 130 (70) 23 (5.3) 125 (20) 208 (84) (2) 56 3-SW-K 0.61 (0.05) 1.85 (0.10) 17 (4) 277 (10) 153 (9) 107 (9) 1450 (38) 28 (1.4) 11 (0.8) 39 (1.2) 60 (4) 3 (2.6) (12) 62 4-SW-K 0.54 (0.02) 1.93 (0.35) 10 135 (15) 200 (10) 265 (25) 1100 (80) 9 (4.0) 4 (1.2) 24 (4.5) 30 (7) 3 (0.5) (14) 5-SW-K 0.74 (0.13) 1.51 (0.21) 10 170 (20) 30 (5) 100 1700 (30) 27 (6.2) 8 (1.4) 12 (1.7) 30 (6) 1 39 (5) 6-SW-K 0.76 (0.02) 1.800 (0.03) 10 250 110 106 2030 (86) 19 (1.5) 4 42 (1) 20 (5) 2 (0.43) 22 (1) 7-SW-K 0.93 (0.07) 1.60 (0.09) 150 (20) 610 (15) 150 (10) 155 3800 (300) 17 (1.1) 4.3 (0.17) 29 (1.4) 73 (3) 3 (0.63) 87 (7) 137 8-HW-K 1.05 (0.01) 2.12 (0.06) 200 (5) 570 (10) 100 (7) 630 (3) 1800 (60) 25 (1.2) 4 (0.4) 28 (0.4) 171 (11) 3 (0.5) (11) 9-HW-K 1.45 (0.07) 0.33 (0.01) 10 200 (20) 10 100 4300 (300) 1 1 1 32 (4) 5 (0.9) 17 (7) 10-SW-O 0.34 (0.01) 0.12 (0.07) 10 130 (10) 10 120 (20) 100 7 (4.2) 4 (0.5) 8 (0.5) 240 (15) 2 (0.5) 18 (8) 11-HW-O 0.13 (0.01) 0.02 (0.001) 100 (1) 100 (10) 10 160 (20) 100 3 (1.7) 4 (0.5) 7 (0.5) 25 (3) 1 (0.0) 4 (3) 12-HW-O 0.43 (0.04) 0.25 (0.003) 60 300 (10) 0 100 40 (3) 3 (1.7) 3 (0.5) 7 (0.5) 238 (10) 1 (0.5) 8 (1) 130 13-WS-O 1.40 (0.02) 0.18 (0.004) 210 (5) 300 (10) 10 1370 (10) 100 21 (0.9) 8 (0.5) 4 (1.2) 2380 (300) 65 (0.9) (3) 14-PS-O 0.98 (0.11) 1.07 (0.01) 300 (20) 1430 (200) 10 (0.5) 120 (8) 90 (2) 11 (1.2) 42 (0.9) 21 (0.8) 1950 (180) 21 (1.2) 13 (8) 32 15-CS-O 3.31 (0.04) 0.97 (0.02) 700 (20) 11300 (100) 930 (30) 1950 (70) 20 (9) 7 (1.3) 48 (0.4) 22 (0.8) 540 (7) 95 (3.3) (23) 11.45 45 16-SW-L 5.80 (0.13) 900 (5) 2250 (10) 1640 (50) 39340 (900) 1600 (300) 6 (0.5) 11 (0.5) 138 (7.7) 60 (1) 6 (3.6) (0.03) (16) 17-WS-S 1.76 (0.11) 0.72 (0.01) 20 550 (10) 1100 4830 (100) 1100 15 (1.4) 3 (0.5) 7 (0.5) 80 (4) 10 (0.8) 99 (5) *K: kraft, O: organosolv, L: lignosulfonate, S: soda, SW: softwood, HW: hardwood, WS: wheat straw, PS: peanut shell, and CS: corn stover 32 2.4.2 Chemical Structure Analysis 31 2.4.2.1 P NMR (Phosphorus 31-Nuclear Magnetic Resonance Spectroscopy) 31 P NMR spectroscopy is widely used to quantify different types of hydroxyl groups in lignin, including aliphatic OH, phenolic 5-substituted OH, syringyl OH, guaiacyl OH, and carboxylic acid after phosphitylation.91 Cholesterol,105 bisphenol,119 benzoic acid,120 and cyclohexanol99 have been used as internal standards in 31P NMR analysis of lignin. Some may overlap with the resonance of lignin moieties, thus causing underestimating the OH content of lignin. N-hydroxy-5-norbornene- 121 2,3-dicarboximide (NHND) was introduced as an internal standard to avoid potential overlapping; however, Meng et al. 91 reported that NHND forms a derivative that is not stable, and the prepared sample must be analyzed immediately, which is not always possible for most labs and would result in non-reproducible and overestimated results. Therefore, cyclohexanol was employed as an internal standard in our analyses that has much longer stability than NHND. The amounts of hydroxyl groups of different lignin samples calculated by 31P NMR are presented in Figure 1. For each lignin, three replicates were run, the measurements demonstrated very good reproducibility as evident by the low standard deviation values. The distribution of S/G/H moieties is correlated with the source of biomass. Softwood, hardwood, and herbaceous lignins were rich in guaiacyl, syringyl, and hydroxyphenyl units, respectively, as expected.7, 122 Aliphatic hydroxyl groups in lignin and carbohydrate appear underneath a 31 broadband (145.5-149.5 ppm), and unfortunately, P NMR cannot distinguish them due to overlapping.5 Therefore, compositional analysis of lignin samples that shows high aliphatic OH content should be cautiously considered to track the origin and source of hydroxyl groups (either lignin or carbohydrate). According to Figure 1, the carboxylic acid amount in herbaceous lignins (samples 18, 7, and 9) is higher than softwood and hardwood lignin, possibly due to the presence 33 of hydroxycinnamic acids such as p-coumaric acid and ferulic acid, which are linked to lignin via easter and ether bonds.123, 124 The isolation process has a significant impact on the hydroxyl content of lignin. It is clear that most of the lignin isolated by the kraft process had higher phenolic (average 3.57 mmol/g) and total hydroxyl contents (average 6.07 mmol/g) than lignins isolated from other processes (average 1.99 and 3.82 mmol/g phenolic and total hydroxyl contents, respectively). We surmise that in the kraft process, various types of reactions such as cleavage of lignin-carbohydrate linkages, recondensation, and depolymerization have occurred.111 As illustrated in Scheme 5 quinone methides, the intermediates formed during the isolation process, acted as electrophiles to react with high nucleophilic hydrosulphide ions to form benzylthiol structures. Sequentially, the aryl ether attached to the β carbon was displaced, forming phenolic hydroxyl groups, which eventually increased lignin solubility in an alkali medium.90, 125 Cleavage of phenolic ether linkages and recondensation as a result in kraft cooking can be seen by relatively high phenolic hydroxyl to aliphatic hydroxyl ratio. In addition, a high amount of 5-substituted OH (condensed) confirms recondensation via new-formed and C-C units.126 34 Scheme 5. Potential reactions occurred during the kraft process 111 35 Aliphatic Syringyl Condensed Guaiacyl p-hydroxyphenyl COOH 8.0 7.0 6.0 Hydroxyl Content (mmol/g) 5.0 4.0 3.0 2.0 1.0 0.0 Figure 1. Hydroxyl contents (mmol/g) of lignins quantified by 31P NMR *K: kraft, O: organosolv, and S: soda, SW: softwood, HW: hardwood, WS: wheat straw, PS: peanut shell, and CS: corn stover ** Sample 16-SW-L was not soluble in NMR solution (CDCl3, pyridine, and DMF) 36 Isolated lignins with the organosolv process had lower hydroxyl content than kraft lignin due to less harsh conditions (lower temperature and pressure).127 Soda cooking method is usually used with herbaceous plants, which isolates lignin based on the cleavage of ɑ and β aryl ether bonds, resulting in relatively less modified lignin.111, 117 In this process, depolymerization mainly takes place on non-phenolic β aryl moieties, giving more aliphatic hydroxyl groups.125 2.4.2.2 FTIR (Fourier Transform Infrared Spectroscopy) FTIR spectroscopy can be used to classify different types of lignins based on their sources, namely hardwood, softwood, and herbaceous feedstocks, due to different distributions in aromatic units (G, S, and H). Table 4 presents the band assignments for O-H, C-H, C=O, aromatic skeletal, and C-O groups in lignins based on their lignin sources which are in accordance with previous studies.128-131 Syringyl units showed two characteristic bands at 1326 and 1112 cm-1 related to C- O stretching of syringyl rings and in-plane deformation Syringyl C-H, respectively. In contrast, three peaks were detected for guaiacyl units at 1266, 851, and 812 cm-1, which were correspondingly assigned to C-O stretching of guaiacyl ring, C-H out of plane deformation of guaiacyl. In addition, one characteristic peak at 983 cm-1 was found in herbaceous lignin.128 Although most peaks in FTIR spectra for various lignin sources overlap, specific assignments could be applied to identify the feedstock source of lignin. 37 Table 4. Band assignments in mid-infrared region for softwood, hardwood, and herbaceous lignins Wavenumber (cm-1) No Band Origin Softwood Hardwood Grass 1 O-H stretching 3410 3458 3430 3000 3000 3002 2931 2937 2040 2 C-H stretching in methyl and methylene groups 2880 2880 2889 2833 2833 2835 3 C=O stretching in unconjugated ketones, carbonyls, and esters 1708 1710 1705 4 C=O stretching in conjugated ketones, carbonyls, and esters 1663 1658 - 5 Aromatic skeletal vibration, C=O stretching 1595 1593 1600 6 Aromatic skeletal vibration 1510 1512 1513 7 C-H deformation (asymmetric) in methyl and methylene groups 1455 1458 1460 8 Aromatic skeletal vibrations combined with C-H in-plane deformation 1422 1426 1423 9 Aliphatic C-H stretching in CH3 and phenolic OH 1365 1365 - 10 C-O stretching of syringyl ring - 1326 1318 11 C-O of syringyl ring - 1315 - 12 C-O stretching of guaiacyl ring 1266 1267 1266 13 C-O stretching of syringyl ring and guaiacyl 1213 1213 1212 14 C-O stretching of syringyl ring and guaiacyl 1150 1151 1153 15 C-O of syringyl and guaiacyl ring, C-H bond in guaiacyl ring 1027 1030 1032 16 In-plane deformation syringyl C-H - 1112 1113 17 C-H out of plane deformation of guaiacyl 851, 812 912 983 18 C-H out of plane deformation of syringyl - 830 832 19 C-H out of plane deformation of guaiacyl 812 - - 38 2.4.3 Molar Mass Distribution Gel permeation chromatography (GPC), also known as size exclusion chromatography (SEC), is a common method to determine the molar mass distribution of lignin samples.132, 133 Multiple benefits, including short processing time (0.25-2 hours per sample), ability to analyze sample even in milligram scale and wide range of molecular weight detection, make SEC an attractive technique for lignin characterization. Generally, the experimental setup is inclusive of a column, eluent, standards for calibration, and calibration method significantly affect the molar mass determination. 134-136 In this study, all lignin samples were acetylated prior to eluting in THF using three commercial columns (5k-600 kÅ, 500-30 kÅ, and 100-10 kÅ).110, 137 It should be mentioned that it was difficult to solubilize and acetylate the sulfite lignin (16-SW-Su), making the THF column not suitable to measure its molecular weight. 39 Table 5. Molecular weight data of lignin samples Sample ID* Mn Mw Mz PDI 1-SW-K 1990 6580 15590 3.3 2-SW-K 2080 6920 16910 3.3 3-SW-K 1770 6070 15400 3.4 4-SW-K 2030 8090 19670 4.0 5-SW-K 1740 5510 13410 3.2 6-SW-K 1750 9310 44990 5.3 7-SW-K 1640 7440 44860 4.5 8-HW-K 1270 2830 7360 2.2 9-HW-K 2590 10390 23018 4.0 10-SW-O 1440 4970 13420 3.5 11-HW-O 1290 3440 10700 2.7 12-HW-O 1490 4250 12830 2.9 13-WS-O 1960 15530 26790 7.9 14-PS-O 1640 9080 21400 5.5 15-CS-O 1750 6240 14160 3.6 16-SW-L - - - - 17-WS-S 1480 4330 14210 2.2 *K: kraft, O: organosolv, L: lignosulfonate, S: soda, SW: softwood, HW: hardwood, WS: wheat straw, PS: peanut shell, and CS: corn stover 2.4.4 Thermal Characterization Glass transition temperature (Tg) of lignin, a critical physical property that can be measured by differential scanning calorimetry (DSC),138, 139 generally vary widely between different lignins and 40 range from 64 -176°C. This is due to numerous factors, including the pretreatment strategy, isolation methods, and cross-linking extent during the cooking process. Utilizing DSC, in this study, we found that all the kraft lignin, regardless of their sources, exhibited glass transition temperatures ranging from 119 - 131°C with a concomitant second Tg ranging from 140 - 165°C, illustrating a higher dispersity of these lignins. This is consistent with the fact that harsh conditions, i.e., high temperature or concentrated alkaline treatment, usually yield the resulting lignin in a more heterogeneous form. Organosolv lignins, in contrast, usually exhibit only one Tg (generally lower than that of kraft lignins), associated with the well-preserved inherent backbone of lignin. It is noticeable that peanut shell lignin (14-PS-OS) had the lowest Tg of 72°C, which was significantly lower than other lignins analyzed in this study. Also, organosolv lignin from hardwood (12-HW-OS) showed the low Tg - around 93°C, very likely caused by the high methoxy group content, which are known to increase the heat capacity of lignin, thus lowering its Tg.140, 141 The low Tg of 14-PS-OS and 12-HW-OS lignin makes them good candidates for developing lignin- based polyurethane coatings, flexible foams, and elastomers. Thermostability, one of the most important factors10 that is highly impacted by different process methods and affecting lignin’s application, was also examined in this work. We examined the lignin’s thermo-degradation behavior under both anaerobic (N2 atmosphere) and aerobic (O2 atmosphere) conditions, attempting to reveal the performance of lignin in further application. Lignin pyrolysis often involves multiple stages, comprising various types of reactions over a broad temperature range. As shown in Table 6, kraft lignin’s are more thermally stable than other types of lignin, reflected by the higher decomposition onset temperatures (referred to as T5% and T10%, temperatures corresponding to 5% and 10% of weight loss, respectively) in the range of approximately 190 to 230°C, regardless under N2 or O2 atmosphere. Cross-linking or condensation 41 reactions and formation of 5-5 biphenyls during the kraft process likely contributed to the higher decomposition starting temperature.142 Weight loss at this early stage is usually correlated with dehydration, loss of the side chains, and other fragments.142 Noticeably, organosolv hardwood lignin’s (11-HW-OS and 12-HW-OS) showed similar T5% and T10% as those of kraft lignins. All types of lignin exhibit similar Tmax (temperature corresponding to maximum weight loss) in the range of 350 - 370°C under N2 and 400 - 500°C under O2, respectively, attributed to the decomposition of the aromatic rings. In addition, all lignin samples used in this study had similar solid residue char at the end of thermal analysis, ranging approximately ~ 30 - 40% under N2 atmosphere and an only trace amount of residual (< 5%) left under O2 atmosphere. 42 Table 6. Thermal characterization of 17 commercial lignin Tg (°C) TGA (under N2) TGA (under O2) Sample ID T5% T10% Tmax Residue T5% T10% Tmax Residue Tg-1 Tg-2 (oC) (oC) (oC) (%) (oC) (oC) (oC) (%) 1-SW-K 127 148 189 275 352 38 211 257 425 1.2 2-SW-K 131 153 190 260 373 41 207 262 418 2.9 3-SW-K 131 145 222 270 364 42 221 269 459 5.7 4-SW-K 119) 162 223 267 350 40 223 268 487 5.4 5-SW-K 120 153 233 279 372 37 233 280 470 5.0 6-SW-K 125 145 227 268 381 36 235 271 456 5.2 7-SW-K 121 155 231 271 383 38 234 269 469 5.4 8-HW-K 120 140 228 271 364 30 242 288 477 0 9-HW-K 119 165 217 267 349 34 227 262 432 0.8 10-SW-O 64 130 184 229 376 39 185 229 449 0 11-HW-O 100 - 231 267 351 37 239 281 525 0 12-HW-O 93 - 213 247 372 35 210 247 471 0 13-WS-O 119 176 194 236 356 35 206 241 398 6.5 14-PS-O 72 - 174 203 367 35 169 199 508 0 15-CS-O 65 120 165 207 217 32 185 209 523 2.5 16-SW-L 138 175 191 255 363 35 208 258 518 0 17-WS-S 120 150 182 213 293 19 170 206 509 6.9 *K: kraft, O: organosolv, L: lignosulfonate, S: soda, SW: softwood, HW: hardwood, WS: wheat straw, PS: peanut shell, and CS: corn stover 43 3 CHAPTER 3 (Lignin-Based Epoxy Resins)143 3.1 Introduction Hydroxyl groups of lignin (mainly phenolic hydroxyl groups) can undergo epoxidation via two possible mechanisms: SN2 and ring-opening (two steps epoxidation). In the SN2 reaction, an epoxidized product is obtained (also called direct epoxidation). In the ring-opening reaction, first a chlorinated intermediate is formed, then an intermolecular SN2 reaction with an aqueous solution of NaOH will create the epoxide ring.38 The general reaction mechanisms are illustrated in Scheme 6. Scheme 6. Phenol epoxidation mechanisms in lignin 44 The epoxidation reaction of unmodified lignin depends on several factors, including the type of catalyst, amount of ECH, co-solvent, and type of lignin. Two types of catalysts are commonly used for lignin epoxidation, alkaline and phase transfer catalysts. An alkaline catalyst like NaOH and KOH deprotonates the hydroxyl groups of lignin and forms a phenolate ion (the conjugate base of phenol obtained by deprotonation of the OH group) to react with ECH.58 While, phase transfer catalysts such as tetrabutylammonium bromide (TBAB) and triethyl benzyl ammonium chloride (TEBAC) allows phenolate ion to transfer in organic solution and react with ECH by facilitating the migration of phenolate ion to the organic phase.75, 76 Usually, the alkaline catalyst is added in a significantly higher amount (0.5-2.5 eq of total hydroxyl groups of lignin). The equivalent amount is calculated based on the total hydroxyl content of lignin obtained from phosphorus-31 nuclear magnetic resonance (31P NMR) analysis result according to the following equation: Total OH content (mmol/g) × number of equivalent × molar mass of catalyst) compared to quaternary ammonium salts as catalyst (0.05-0.26 eq of total hydroxyl groups in lignin) because the alkaline catalyst is consumed during the reaction (by neutralizing released hydrochloric acid) and formed NaCl salt. Most lignin samples from different sources and isolation methods are soluble in ECH at elevated temperatures. However, co-solvent is necessary to keep the epoxidized lignin soluble since the solubility of epoxidized-lignin drastically decreases during epoxidation reaction.143 Dimethyl sulfoxide (DMSO) was used as co-solvents for lignin epoxidation. It was shown that not only using co-solvents decrease the side reactions, it also increases the epoxy content of epoxidized lignin.58 In addition, a higher amount of ECH is always used (10-20 eq of total hydroxyl groups) to increase the epoxy content of epoxidized lignin and reduce the side reactions.38 45 Table 7 summarizes the published papers which used unmodified lignins from different isolation processes (kraft, soda, organosolv, steam explosion) and various biomass sources (softwood, hardwood, herbaceous plants) for epoxy resin applications.57-60, 144 Although several studies used lignins from different isolation processes and sources for epoxidation reaction, the factors affecting the suitability of lignin in epoxidation reaction have not been evaluated. On the other hand, only two studies fully replace BPA with unmodified, non-technical lignins (bamboo steam exploded and enzymatic hydrolysis corncob). Therefore, it is critical to developing a new method to epoxidize unmodified technical lignins and find the most important lignin properties affecting their suitability for epoxidation. Table 7. Summary of the previously published paper focused on epoxidation of unmodified lignins Epoxy equivalent BPA replacement NO Lignin Type* Catalyst Co-solvent Ref weight (EEW) (wt.%) Bamboo steam 57 1 TBAB - 333 0-100 exploded 58 2 Kraft softwood DMSO 309-1075 2-10 Enzymatic 59 3 TBAB - 294 0-100 hydrolysis corncob Organosolv 60 4 TBAB/KOH - 312 0-42 hardwood 61 5 Alkali Sal leaves NaOH - - 5-20 62 6 Alkaline lignin NaOH - - 0-20 In this chapter, first, the reactivity of thirteen unmodified lignins toward epichlorohydrin was measured to identify the most important factors affecting lignin epoxidation. Next, two technical 46 hardwood and softwood lignins from kraft processes were epoxidized and their crosslinking behaviors and thermomechanical properties were investigated. 3.2 Measuring Reactivity of Different Lignins Toward ECH 3.2.1 Experimental (Reactivity Measurement) 3.2.1.1 Materials Thirteen commercially available lignin samples from different plant sources and isolation processes were provided by Advanced Biochemical (Thailand) Co., Ltd. Other chemicals used in this study were: N, N Dimethylformamide (DMF; 99.8 %, extra dried, Acroseal, Acros Organics); tetrabutylammonium bromide (TBAB; Tokyo Chemical Industry Co., LTD, Purity >98 %); biobased ECH (Advanced Biochemical Thailand Co., Ltd, 99.9 %). GX-3090 and DGEBA (EPON 828) were obtained from Cardolite and E. V. Roberts, respectively. Additional reagents were purchased from Fisher Scientific, Alfa Aesar, Sigma-Aldrich, and Acros Organics and were used as received. 3.2.1.2 Methods First, 4 g of each lignin sample was dissolved in 20 g dimethylformamide (DMF) and stirred for 10 min at room temperature (DMF was used as co-solvent since all lignin samples were completely soluble in DMF). Then 0.4 g tetrabutylammonium bromide (TBAB) and 40 g biobased ECH were added to the lignin/DMF solution and stirred for 3 h at 60 °C under reflux conditions (Scheme 1). The mixture was then cooled down to room temperature, and 50 mL of 2% w/w NaOH solution containing 1.2% w/w TBAB was gradually added to the mixture dropwise (one drop every 5 s). Then, the reaction was continued at room temperature for 8 h while stirring at 500 rpm using a magnetic stirrer. After that, 1000 mL deionized (DI) water was added to the solution to precipitate 47 epoxidized lignin. The epoxidized lignin was collected by vacuum filtration and washed several times with DI water to remove formed salt and unreacted ECH. Finally, a vacuum oven was used to dry the epoxidized lignin samples at 40 °C, 76 kPa for 48 h.145 3.2.1.3 Curing of Epoxy Resins The two epoxidized lignin samples (4-O-CS and 11-K-HW) with the highest epoxy contents and a commercial DGEBA epoxy resin were cured with a biobased diamine (GX-3090) (Scheme 7). The epoxy equivalent weight (EEW) of epoxidized lignin was calculated by the following equation 146 : 4300 EEW= % 𝐸𝑝𝑜𝑥𝑦 𝐶𝑜𝑛𝑡𝑒𝑛𝑡 Each hydrogen of the amine group could react with one epoxy group based on active hydrogen equivalent weight (AHEW), then the stoichiometric ratio between the hardener epoxy resins can be calculated as AHEW/EEW. First, epoxidized lignin samples were dissolved in acetonitrile, then a specific amount of GX-3090 was added and mixed according to Table 8. To evaporate the solvent, epoxidized lignin systems were heated at 50 °C for 1 h. All samples were cured at 130 °C for 2 hrs and post-cured at 150 °C for 1 h (as recommended by the supplier of hardener). Table 8. Formulation of different epoxy samples Sample ID EEW Mass Ratio (epoxy resin/hardener) 4-O-CS/GX-3090 346.8 1/0.21 11-K-HW/GX-3090 354.2 1/0.20 DGEBA/GX-3090 185 1/0.37 48 Scheme 7. Synthesis and curing reaction of epoxidized lignin 3.2.2 Characterization (Reactivity Measurement) All lignin properties were measured according to characterization methods explained in Chapter 2. 49 3.2.2.1 Characterization of Epoxidized Lignins 3.2.2.1.1 Epoxy Content Measurement (Auto-Titration) The epoxy content of epoxidized lignin was measured according to a modified version of ASTM D1652-11 using an auto-titrator (Metrohm, 916 Ti-touch Swiss Mode). Due to the dark color of lignin, it was impossible to use a color-changing indicator; instead, the electric potential was measured to determine the endpoint of the titration. Briefly, 0.2-0.3 g epoxidized lignin was dissolved in 30 mL dichloromethane and 15 mL of a prepared tetraethylammonium bromide reagent (100 g of tetraethylammonium bromide in 400 mL of glacial acetic acid). The resulting solution was stirred for 5 min to ensure the epoxidized lignin was entirely dissolved in the solution. The titration is based on the in-situ formation of hydrobromic acid by the reaction of perchloric acid with excess tetraethylammonium bromide. Initially, the produced hydrobromic acid (HBr) immediately reacts with epoxy rings; thus, there is no change in potential. After all epoxy rings are consumed, the formed HBr drops the pH and increases the potential of the solution, which is used as the endpoint. The auto-titrator was used to titrate the solution with 0.1 N perchloric acid reagent until the endpoint (Figure 2). Figure 2. Auto-titrator used to measure epoxy content of epoxidized lignin 50 3.2.2.1.2 Epoxy Content Measurements (Proton Nuclear Magnetic Resonance-1H NMR) Epoxy contents of epoxidized lignin samples were measured by 1H NMR according to a previously published report.147 Around 50 mg of each epoxidized lignin sample was dissolved in 700 µl of deuterated dimethyl sulfoxide (d-DMSO). Then approximately 20 mg internal standard (1,1,2,2 tetrachloroethane) was added. NMR analysis was performed using an Agilent DDR2 500 MHz NMR spectrometer equipped with 7600AS, running VnmrJ 3.2A, with a 10 s relaxation delay, and 64 scans. The epoxy content of each epoxidized lignin was calculated based on the ratio of the following peaks δ [ppm, DMSO-d6]: 2.77 (m, 1H); 2.92 (m, 2H); 3.41 (m, 1H), 4.32 (dd, 1H), and 4.64 (m, 1H); these peaks are assigned to the epoxy ring chemical shifts and peaks of internal standard (6.89 ppm, S, 1H). In addition, the average number of epoxy groups in each macromolecule was calculated according to the following equation: 𝑛̅ = epoxy group (mol/g) × Mn 3.2.2.2 Chemometric Modeling UMetric Simca 16.0.2 software was used to model the correlation between lignin properties and their reactivity toward ECH. The partial least square regression (PLS-R) modeling method was used, as it can tolerate highly correlated variables (lignin properties).20 To build the model, the properties of lignin such as ash content, elemental analysis, hydroxyl content, and molecular weight of lignin samples were considered as inputs (X-variables). The epoxy content (measured reactivity of lignins with ECH, using both titration and 1H NMR techniques) were considered as responses or Y-variables. 51 3.2.2.3 Thermomechanical Properties of Cured Epoxy Systems (DMA Analysis) The thermomechanical properties of two cured epoxidized lignin samples and a cured DGEBA- based sample were analyzed using a dynamic mechanical analyzer (DMA). A TA Instrument Q800 with a single cantilever under airflow, and a heating rate of 3.0 °C/min from room temperature to 250 °C, with a constant deformation frequency of 1 Hz was used to analyze the properties of cured epoxy systems. All samples were cured at 130 °C for 2 hours and post-cured at 150 °C for 1 hour in Teflon molds measuring 35 mm (length) by 12 mm (width) by 3 mm (depth). Samples were polished (by different sandpaper grits 1500, 2000, 2500, 3000, 5000, and 7000) to have smooth surfaces before analysis. 3.2.2.4 Thermal Stability of Cured Epoxy Systems (TGA Analysis) Thermogravimetric analysis (TGA, TA Analysis, Q100) was carried out to compare the thermal stability of cured lignin-based epoxy samples with a DGEBA-based sample as a reference. Briefly, 5-10 mg of each sample was placed on a platinum pan and heated from 30 to 700 °C, with a constant heating rate of 20 °C, under an airflow of 25 mL/min for the sample and 10 mL/min for balance. 3.2.3 Results and Discussion (Reactivity Measurement) 3.2.3.1 Lignin Characterization Table 9 shows the physicochemical properties of thirteen lignin used. 52 Table 9. Measured lignin properties (epoxy) Sample Ash Content C H N S Mn Mw Tg PDI ID* (%) (%) (%) (%) (%) (Da) (Da) (°C) 1-K-SW 0.52 (0.10) 62.9 5.9 0.1 1.7 1800 7000 3.9 144 2-K-HW 1.39 (0.14) 60. 5.8 0.2 0.3 2700 12400 4.6 164 3-S-HW 4.84 (0.11) 58.5 5.8 0.8 1.9 1900 6400 3.4 158 4-O-CS 0.50 (0.20) 63.7 5.7 0.5 0.1 1900 5380 2.8 174 5-O-Ba 3.37 (0.04) 61.1 5.5 0.7 0.1 2300 11500 5.0 130 6-O-PS 0.88 (0.02) 63.9 6.6 1.8 1.1 1750 9300 5.3 83 7-O-HW 0.47 (0.02) 62.9 6.0 0.2 0.2 1800 8200 4.6 79 8-K-SW 0.54 (0.02) 62.7 6.0 0.1 1.4 2000 8700 4.4 159 9-K-SW 0.65 (0.01) 62.9 6.0 0.1 1.3 1900 7200 3.8 150 10-K-HW 5.19 (0.01) 58.7 5.7 0.1 1.9 1600 4000 2.5 167 11-K-HW 1.62 (0.01) 60.9 5.8 0.1 2.3 1400 3200 2.3 146 12-O-WS 1.73 (0.06) 58.1 5.8 2.1 0.2 3100 15300 4.9 123 13-K-SW 0.75 (0.02) 63.7 6.0 0.1 1.8 2000 9300 4.7 143 *K: kraft, S: soda, O: organosolv, SW: softwood, HW: hardwood, CS: corn stover, Ba: bagasse, PS: peanut shell, and WS: wheat straw, PDI= Polydispersity index 53 Figure 3. Hydroxyl contents (mmol g-1) of different lignin samples obtained by 31P NMR 3.2.3.2 Lignin Epoxidation The epoxy contents of different epoxidized lignin were measured by titration and 1H NMR methods. Figure 4 shows the 1H NMR spectrum of epoxidized lignin (1-K-SW). The results of these tests on all lignin samples, based on epoxy content and epoxy equivalent weight (EEW), are reported in Table 10. Epoxy contents measured by titration are based on three replicates, while epoxy contents based on 1H NMR are based on one replicate. As shown, there were no significant differences (p-value: 0.671) between the results of the two methods, which confirms both are 54 reliable methods for measuring the epoxy content of epoxidized lignin samples. Samples 4-O-CS and 10-K-SW had the highest yield (89.9% and 66.9%, respectively). In addition, the average number of epoxy groups (n ̅) in each macromolecule [epoxy group (mol/g) × Mn] are presented in Table 10. The results showed that the reactivities of hydroxyl (OH) functional groups in lignin toward ECH, in decreasing order, are phenolic-OH > carboxylic acid > aliphatic-OH.148 It has been reported that the phenol epoxidation mechanism has three steps.149 During the epoxidation reaction, a phase transfer catalyst (TBAB) transfers the phenolate ion into the organic solution. In the second step, deprotonated lignin (phenolate ion) reacts with ECH via two mechanisms: 1) SN2, and 2) ring- opening reactions. In the third step, the chlorinated intermediate is closed in the presence of NaOH to form the epoxy ring. It was found that the hydroxyl groups of lignin could only partially react with ECH. The reaction was also incompletely quenched due to side reactions between lignin’s OH groups, ECH, and epoxidized lignin.149 This may lead to the formation of ether bonds between epoxidized lignin functional groups and ECH. In addition, unreacted hydroxyl groups could potentially react with epoxy groups and form crosslinked products.150 The formation of crosslinked epoxidized lignin reduces its solubility in organic solvents, negatively affecting the curing reaction of epoxidized lignin with a hardener. 55 Figure 4. 1H NMR spectrum of epoxidized lignin (1-K-SW) 56 Table 10. Properties of epoxidized lignins, including epoxy content and the epoxy equivalent weight measured by titration and 1H NMR methods, yield (%) based on total hydroxyl content, and average number of epoxy groups in each lignin macromolecule % Epoxy % Epoxy Content EEW EEW Sample ID* Content (1H Yield % ̅ 𝒏 (Titration) (Titration) (1H NMR) NMR) 1-K-SW 9.56 ± 0.26 450 9.72 442 39 4.0 2-K-HW 6.79 ± 0.12 633 7.00 614 26 4.4 3-S-HW 8.59 ± 0.35 501 8.21 524 39 3.5 4-O-CS 12.40 ± 0.31 347 12.53 343 90 5.6 5-O-Ba 5.93 ± 0.13 725 5.87 732 25 3.1 6-O-PS 5.18 ± 0.12 830 4.93 872 35 2.0 7-O-HW 8.75 ± 0.19 491 8.93 481 42 3.7 8-K-SW 7.97 ± 0.15 539 7.88 546 31 3.8 9-K-SW 10.01 ± 0.24 430 9.81 438 44 4.2 10-K-HW 11.27 ± 0.28 381 11.50 374 67 4.1 11-K-HW 12.14 ± 0.15 354 11.98 359 46 3.7 12-O-WS 4.35 ± 0.08 988 3.81 1129 19 2.5 13-K-SW 8.63 ± 0.18 498 8.98 479 32 4.1 *K: kraft, S: soda, O: organosolv, SW: softwood, HW: hardwood, CS: corn stover, Ba: bagasse, PS: peanut shell, and WS: Wheat straw Samples 2-K-HW, 4-O-CS, 9-K-SW, 10-K-HW, and 13-K-SW had higher 𝑛̅ compared to other lignin samples. The higher 𝑛̅ indicates that the crosslinking density of the cured sample is higher. The weight of epoxidized lignin after the reaction was precisely measured for 11-K-HW to be 4.8 g. Although lignins 4-O-CS, 10-K-HW, and 11-K-HW all have high epoxy contents, based on the 57 overall data, the organosolv corn stover lignin (4-O-CS) seems to be a better lignin for epoxy resin applications due to its low ash content, low molecular weight, and low polydispersity index. In addition, this lignin has low carboxylic acid content, which will reduce potential hydrolysis and increase the service life of epoxy systems after crosslinking with a hardener.151 3.2.3.3 Modeling Partial least-square regression modeling was used to find correlations between different lignin properties and their epoxy contents after epoxidation (reaction with ECH). The PLS model was developed with two PLS components, which had 92% fitting accuracy (R2Y=the explained variation) and 90% prediction ability (Q2Y=the predicted variation) based on a cross-validation method. Two PLS components were shown to be optimal for the model. Figure 5 plots the components’ contributions; light blue-colored bars represent R2 as an indicator of how well the model fits the measured data for each performance criterion, and the dark blue-colored bars show Q2 as an indicator of how well the model can predict the epoxy content of a new lignin sample based on its measured properties. 58 Figure 5. Component contribution plot for the response variable (epoxy content) measured both by titration and 1H NMR methods. According to the loading plot (Figure 6), Mn, Mw, PDI, and nitrogen content have strong negative (opposite side) correlations with epoxy content. In contrast, phenolic hydroxyl content has strong positive correlation (same side) with the epoxy content of lignin. In other words, lignins with lower molecular weights, molecular numbers, PDI, nitrogen, and higher phenolic hydroxy contents are more suitable for replacing BPA in epoxy resin formulation. We believe the reason that higher nitrogen content seems to have a very high negative effect on epoxy content is because there is a strong negative correlation (r=-73%) between the nitrogen content of the lignin samples used in this study with their total phenolic hydroxyl groups. Rather than higher nitrogen content being a fact that contribute to the reaction of lignin and ECH. It simply indicates that among the lignins used in this study, most of them that had nitrogen content (mainly from annual crops) had relatively lower phenolic hydroxyl content, which resulted in lower epoxy content. 59 Figure 6. Loadings plot of PLS-R modeling of epoxy content based on lignin properties 3.2.3.4 DMA Analysis of Cured Samples (Thermodynamic Performance) Thermodynamic performances of lignin-based epoxy and DGEBA-based thermosets cured by GX- 3090 (biobased hardener) were studied by DMA using a single cantilever mode. Figure 7 shows storage modulus, tan δ, and loss modulus as a function of temperature; these parameters are summarized in Table 11. The storage modulus (E´) and loss modulus (E´´) represent the elastic and viscoelastic response of a material, respectively. The ratio of loss modulus to storage modulus is tan δ. The peak temperatures of tan δ and loss modulus are usually reported as glass transition temperature, where a network transits from a glassy state to a rubbery state. The storage moduli (E´) of all cured samples ranged between 1.3 to 1.6 GPa at 25 °C. The storage moduli of lignin-based epoxy networks (1.3-1.4 GPa) were lower than the DGEBA system (1.6 GPa), which could be related to the lower epoxy content of the epoxidized lignins compared to DGEBA resin. This shows that the lignin-based epoxy system had a lower crosslinking density than the petroleum-based epoxy system (DGEBA) prepared using bisphenol A. The organosolv corn stover lignin (4-O-CS) had a much higher storage modulus than kraft hardwood (11-K-HW). This could be due to the higher average number of epoxy groups (𝑛̅) and lower molecular weight of 4-O-CS compared to 11-K-HW. At the higher temperature (100 °C), the storage moduli of 4- O-CS and 11-K-HW samples were higher than that of DGEBA, possibly due to the higher glass transition temperature of cured lignin-based epoxy systems. The loss moduli (E´´) of 4-O-CS and 11-K-HW thermosets were also higher than that of the DGEBA sample at higher temperatures (120-200 °C), which shows they can better dissipate deformation energy at higher temperatures.152 60 The tan δ peak gives valuable information regarding cured epoxy networks; generally, higher tan δ peaks correspond to better fracture toughness and higher Tg.94 The width of tan δ represents sample homogeneity, with broader peaks indicating less homogeneous samples.94 Both lignin- based epoxy thermosets showed significantly broader tan δ peaks, meaning that they are less homogeneous than the DGEBA system, as expected due to the high polydispersity index of lignin compared to BPA. Side reactions at different temperatures 152 as well as multiple functionalities 94 in the system could also result in observing broader tan δ peaks. Also, the glass transition temperatures (Tg) (recorded from tan δ profile) of epoxidized lignin samples (181 °C and 173 °C) were significantly higher than the Tg of the DGEBA system (106 °C), which indicates that lignin- based epoxy systems have higher toughness.56 Table 11. DMA performance of epoxidized lignins (4-O-CS and 11-K-HW) and DGEBA cured by biobased hardener (GX-3090) Sample ID E´ (GPa, 25 °C) E´ (GPa, 100 °C) Tan δ 4-O-CS/GX-3090 1396 701 181 11-K-HW/GX-3090 1275 613 173 DGEBA/GX-3090 1557 331 106 61 Figure 7. Storage modulus (a), loss modulus (b), and tan δ (c) of cured epoxidized lignin samples (4-O-CS and 11-K-HW) and DGEBA with GX-3090. 3.2.3.5 TGA Analysis of Cured Samples (Thermal Stability) Thermal stabilities of cured lignin-based and DGEBA thermosets were evaluated (Figure 8) using thermal gravimetric analysis (TGA). The temperatures at 5% (Td5%), 30% (Td30%), weight loss, maximum weight loss (Tmax), and the statistic heat-resistance index temperature (Ts) for different samples were summarized in Table 12. Ts was calculated from Td5% and Td30% according to equation 1, which represents the thermal stability of the crosslinked polymers.68, 153 62 Ts = 0.49 [Td5% + 0.6 (Td30% - Td5%)] All epoxy thermosets showed two-step degradation profiles, including a considerable weight-loss stage around 400 °C and another weight-loss stage at above 500 °C. The first stage of degradation is primarily due to the breaking of aliphatic chains and releasing small molecules like CO, CO2, and water.154 The second stage of degradation is most likely associated with the degradation of aromatic rings and oxidation of C-C linkages and different functional groups such as methoxy, phenol, and carbonyl.155 Although the degradation of lignin-based thermosets was started at lower temperatures (241-245 °C) compared to the DGEBA thermoset (350 °C), the difference was smaller at higher temperatures. According to TGA analysis results (Table 12), the Td30% of lignin- based thermosets were 40-50 °C lower than the DEGBA thermoset. Also, the degradation temperatures of both lignin-based thermosets are remarkably higher than their Tg, indicating that they can be used for applications that do not require high-temperature stability.156 The statistical heat resistant-indices (Ts) of cured lignin-based samples were about 33-36 °C lower than Ts of cured DGEBA, showing their lower heat tolerance.155 The epoxy system made with organosolv lignin (4-O-CS) had higher thermal stability than the epoxy made with kraft hardwood (11-K- HW). This can be explained by higher 𝑛̅ of 4-O-CS thermoset, which resulted in an epoxy system with higher crosslinking density. The lower thermal stability of lignin-based thermosets is probably due to the lower crosslinking density of epoxidized lignin and lignin's lower thermal stability.157 63 Table 12. Thermal stability of cured epoxidized lignin and DGEBA networks Sample ID Td5% (°C) Td30% (°C) Ts (°C) 4-O-CS/GX-3090 241 345 149 11-K-HW/GX-3090 245 334 146 DGEBA/GX-3090 350 385 182 Figure 8. TGA profiles of different cured lignin-based and DGEBA epoxy thermosets 3.3 Crosslinking Behavior of Epoxy Resins 3.3.1 Materials and Methodology (Crosslinking Behavior) Two commercially available lignins (hardwood kraft (HW-K) and softwood kraft (SW-K) were provided by Suzano, and West Fraser. Ethyl lactate was purchased from Scientific Fisher Co. 3.3.1.1 Modified Epoxidation Method of Lignin Some parameters of the previous procedure of lignin epoxidation were changed, resulting shorter reaction time (3 hrs total), and replacing DMF with a biobased solvent (ethyl lactate). First, 4 g of 64 lignin was dissolved in 20 g ethyl lactate and mixed for 10 min at room temperature. Then, biobased ECH (20 eq) and TBAB (0.1 eq), based on the total hydroxyl content of lignin, were added to the mixture and stirred for 2 h at 80 °C under reflux conditions. Next, the mixture was cooled down to 10-15 °C, and 20 wt.% NaOH solution (2 eq of total hydroxyl OH) containing 20 wt.% TBAB was slowly added to the mixture. The mixture was stirred for 1 h. After that, the lignin was precipitated by adding 1000 mL deionized (DI) water. Epoxidized lignin was separated using vacuum filtration and washed multiple times to remove salt, unreacted ECH, and ethyl lactate. Lastly, the epoxidized lignin was freeze-dried (Labconco, FreeZone 4.5) at -52 °C for 6 h. 3.3.1.2 Curing of Epoxy Resins Table 13 shows the compositions of different epoxy systems. Epoxidized lignin was dissolved in ethyl lactate (40 wt.%). The stoichiometrically determined amount of curing agent was then added to the mixture, followed by mixing for 2-3 min. To slowly evaporate ethyl lactate, prepared samples were kept at a regular oven at 40 °C for 8 h, then heated at 80 °C for 1 h, cured at 130 °C for 2 h, followed post-cured at 150 °C for 1 h. The same method was used for Epon (EP) systems (with and without solvent). Table 13. Composition of different epoxy systems Sample ID EEW Mass ratio (epoxy resin/curing agent) Ethyl lactate (wt.%) HW-K/GX-3090 320.9 1: 0.22 40 SW-K/GX-3090 364.4 1: 0.19 40 EP-EL/GX3090 185 1: 0.37 40 EP/GX3090 185 1: 0.37 0 65 The physiochemical properties of two lignins were measured according to explained methods in Chapter 1. Also, the epoxy contents of epoxidized lignins were measured according to methods discussed in sections 3.2.2.2.1 and 3.2.2.2.2. 3.3.2 Characterization of Epoxidized Lignin (Crosslinking Behavior) The epoxy contents of epoxidized lignins were measured according to methods discussed in sections 3.2.2.2.1 and 3.2.2.2.2. 3.3.2.1 Rheology (Crosslinking Behavior) Viscosity measurements and curing studies were carried out on a TA Instrument DHR-1. The viscosity of uncured epoxy systems was measured in scanning mode (0.02-1000 1/s). Isothermal curing studies were measured using a parallel plate geometry (25 mm diameter, 1 mm gap) at 40, 45, 50, 55, 60, 65, and 70 °C. Before running the sample, the linear viscoelastic region of cured epoxy systems was determined by running a strain sweep experiment. 3.3.2.2 Gel Fraction and Swelling Ratio The gel fraction and swelling ratio of cured thermosets were measured according to a previously published method by Tellers et al.158 All samples were vacuum dried, and 200 mg of each sample was placed in a 20 mL vial, and 5 mL THF was added. Vials were kept at room temperature for 7 days while the lids were closed. After that, samples were removed from the vials and immediately dried with a paper towel before weighing the swollen samples. Then, samples were placed in a vacuum oven until their weight stabilized. The gel fraction and swelling ratio were obtained according to the following equations: 𝑚 Gel fraction = ( 𝑚𝑑 ) × 100 𝑖 66 𝑚𝑠 −𝑚𝑖 Swelling ratio = ( ) × 100 𝑚𝑖 Where mi, md, and ms are initial mass, dry mass, and swollen mass, respectively. 3.3.3 Results and Discussion (Crosslinking Behavior) 3.3.3.1 Characterization of Technical Lignins (Crosslinking Behavior) Two kraft lignins derived from different sources (hardwood and softwood) were chosen since kraft lignin is widely available (265,000 tons produced annually),15 and the effect of their different monolignol compositions can be investigated on the epoxidation reaction. The measured properties of the two kraft lignin samples are presented in Table 14. HW had higher ash content than SW, which might be related to the different methods (Lignoboost Vs. Lignoforce) or various parameters (pH, temperature, time, and acid concentration) used to isolate these lignins from black liquor or severity of the final washing steps. GPC data showed that SW had a higher molecular weight than HW, as expected. This could be due to the high amount of sinapyl alcohol in HW, which has two methoxy groups and limits the formation of 5–5 and dibenzodioxins linkages in the hardwood lignin.159 Therefore, HW has a more linear structure with a lower molecular weight compared to SW. In addition, SW had a significantly higher PDI than HW, which could be related to the isolation process conditions (such as temperature and time), which resulted in breaking the intermolecular linkages and potential repolymerization of lignin chains.94 The hydroxyl contents 31 of lignin samples were measured by P NMR (Figures S1 and S2). The HW lignin had significantly higher phenolic hydroxyl content (4.88 mmol/g) than SW (2.38 mmol/g) used in this study. Generally, high phenolic and total hydroxyl contents of kraft lignins is due to cleavage of lignin-carbohydrate linkages, recondensation, and depolymerization.111 67 Table 14. Ash content, molecular weight, glass transition temperature, and hydroxyl content of lignin samples Properties HW-K* SW-K* Ash % 0.34 (0.01) 1.62 (0.01) Mn (Da) 1370 2250 Mw (Da) 3160 12100 PDI 2.3 5.4 Aliphatic OH (mmol/g) 1.37 1.65 Condensed phenolic OH (mmol/g) 0.77 0.57 Syringyl (mmol/g) 2.78 - Guaiacyl (mmol/g) 1.14 1.62 Hydroxyphenyl (mmol/g) 0.19 0.19 Carboxylic acid (mmol/g) 0.34 0.61 Total phenolic (mmol/g) 4.88 2.38 Total OH (mmol/g) 6.59 4.64 *HW: hardwood, SW: softwood, and K: kraft 13 C 1H HSQC was used to further study lignin intermolecular linkages and skeletal structure. The HSQC spectra of SW and HW are shown in Figure 9 and Figure 10. Several linkages including β- O-4, β-5, and β-β are present in the structure of native lignin. Still, due to the harsh conditions during the kraft isolation process, the amount of these linkages are significantly reduced in technical lignin.160, 161 For example, phenolic ether linkages are cleaved and recondensed, resulting in a high amount of phenolic hydroxyl groups in kraft lignins.90, 125 Table 15 presents the corresponding abundances of inter-unit linkages for the hardwood and softwood kraft lignin samples. Based on HSQC spectra, the methoxy groups are dominant in both lignin samples. The residual carbohydrates were observed in both lignins (black spots). The S/G ratio of lignin samples are correlated with the sources of lignin. Also, the abundance of β-5 and β-O-4 linkages was higher 68 in SW than HW, while HW had higher β-β linkages than SW. Lignin interunit are illustrated in Figure 11. Figure 9. HSQC spectrum of softwood lignin (SW) 69 Figure 10. HSQC spectrum of hardwood lignin (HW) Figure 11. Lignin interunit presented in original lignins 70 Table 15. Semi-quantification of inter-unit linkages and aromatic units detected in lignin samples Sample ID* S/G B5% BO4 % BB% MeO/Aro HW-K 3.1 6.2 53.5 40.2 1.7 SW-K 0 15.9 59.2 23.5 1.4 *HW: hardwood, SW: softwood, and K: kraft 3.3.3.2 Characterization of Epoxidized Lignins Epoxy functional groups were selectively introduced on two lignin samples by reacting ECH in ethyl lactate solvent under mild conditions for a relatively short time (only 3 h reaction time). HW 31 and SW kraft lignins were modified by epoxidation (EHW and ESW). P NMR analysis confirmed that only phenolic hydroxyl groups and carboxylic acid groups in lignin had undergone epoxidation, while aliphatic hydroxyl groups were left unreacted, which confirms the optimized conditions of the reaction. HSQC analysis of epoxidized lignin (Figure 12 and Figure 13) identified several peaks (70/4.4, 7- /3.8, 50/3.3, and 45/2.4 ppm), assigned to introduced epoxy rings in lignin.162, 163 Besides epoxidation, no other major structural changes were observed, indicating that the epoxidation reaction was run with a mild condition that maintained the structural integrity of the lignin skeleton. 71 Figure 12. HSQC spectrum of epoxidized hardwood kraft lignin (EHW-K) 72 Figure 13. HSQC spectrum of epoxidized softwood kraft lignin (ESW-K) The epoxy contents of two epoxidized lignins measured by titration and 1H NMR methods are presented in Table 16. The epoxy content for HW-K lignin was higher than SW-K (13.4% and 11.8%, respectively). In a previous study, we showed that lignin with lower molecular weight and higher phenolic hydroxyl content is more reactive toward ECH and resulted in higher epoxy content after epoxidation reaction.143 In addition, the average number of epoxy groups in each lignin macromolecule (𝒏 ̅ ) was calculated according to the following equation: 𝑛̅= epoxy content (mmol/g) × Mn, 73 The 𝑛̅ of EHW-K and ESW-K were 4.3 and 5.7, respectively, while DGEBA resin has only 2 epoxy groups per molecule. In this case, ESW-K provides a higher number of reacting sides compared with EHW due to the higher number average molecular weight (Mn) of SW lignin. Table 16. Properties of epoxidized lignin samples Epoxy content (%) by Epoxy content (%) Average number of Sample ID titration by 1H NMR epoxy groups 𝒏̅ a EHW-K 13.4 (0.1) 13.2 4.3 ESW-K 10.8 (0.2) 10.5 5.7 a) 𝑛̅= epoxy content (mmol/g) × Mn 3.3.3.2.1 FTIR spectroscopy The FTIR spectra of epoxidized lignin samples (Figure 14) confirmed the epoxidation by forming new peaks of oxirane ring at 760, 840, 908, and 3000 cm-1.58 Complete epoxidations was confirmed by the disappearance of phenolic OH peaks at 1365 cm-1. The intensity of non-phenolic OH groups at 3500 cm-1 was not changed after epoxidation, which means those hydroxyl groups were not converted during the epoxidation reaction.56, 144 74 Figure 14. FTIR spectra of original and epoxidized a) HW-K and b) SW-K lignins 75 3.3.3.2.2 Ethyl Lactate (EL) Ethyl lactate was used as a solvent to dissolve epoxidized lignins to cure with a curing agent. It is an environmentally friendly compound (in accordance with at least eight of twelve principles of Green Chemistry). Ethyl lactate can be generated from renewable raw materials (synthesized by the esterification reaction between lactic acid and ethanol)164 and due to its high polarity and boiling point (151-155 °C),165 it is a good solvent to replace high boiling point solvents such as dimethyl sulfoxide (DMSO) and dimethylformamide (DMF). It was shown that ethyl lactate is fully biodegradable, recyclable, non-corrosive, non-carcinogenic, and non-ozone depleting.166 Hence, the United States Food and Drug Administration (FDA) approved its use in food products.166 3.3.3.2.3 Viscosity The viscosity of lignin-based epoxy sample dissolved in 40 wt.% ethyl lactate as well as Epon system without and with 40 wt.% ethyl lactate before curing was measured at a constant shear rate (Table 17). It was observed that the viscosity of lignin-based epoxy systems (including ethyl lactate and curing agent) are significantly higher than the viscosity of the Epon system containing 40 wt.% solvents, while their viscosities are much lower than the Epon system without solvent. Between the two lignin-based epoxy systems, the EHW-K system had a lower viscosity than ESW-K. It is due to the higher amount of liquid curing agent used in the EHW-K system (more liquid curing agent was used because EHW-K has a higher epoxy content than ESW-K). In addition, the higher molecular weight of SW-K lignin may increase viscosity. 76 Table 17. The viscosity of lignin-based and DGEBA-based systems measured at two shear rates (100 and 1000 1/s) Sample ID Viscosity cP (100 1/s) Viscosity cP (1000 1/s) EP/GX-3090 (no solvent) 4810 3485 EP-EL/GX-3090 286 264 EHW-K/GX-3090 545 520 ESW-K)/GX-3090 602 548 In addition, all epoxy systems displayed shear-thinning flow behavior (reducing viscosity with increasing shear rate). 3.3.3.2.4 Rheology Chemical composition, curing time, and curing temperatures are among the processing parameters that greatly influence the performance of epoxy systems. Different thermal and mechanical techniques such as differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), rheological analysis, and thermogravimetric analysis (TGA) were employed to investigate the curing process of epoxy systems.167-170 Although DSC is widely used to study the epoxy curing process and provides valuable information about the degree of conversion and heat capacity changes, it does not reveal the rate of the curing process (gelation).171 This information can be obtained from rheological analysis by measuring viscoelastic properties such as storage modulus (G’), loss modulus (G’’), and damping factor (tan δ) throughout the curing process. Curing studies of lignin-based epoxy and Epon systems were carried out under isothermal conditions at different temperatures (40, 45, 50, 55, and 60 °C). Higher temperatures were not tested to avoid evaporation of ethyl lactate. To obtain a good signal-to-noise ratio close to the gel 77 point, a 25 mm diameter geometry was used to stay within the linear viscoelastic region of the cured sample and avoid overloading the instrument torque while a small strain was applied. The applied strains for all experiments were determined to be in the viscoelastic region of each sample. This was achieved by using a strain sweep on a fully cured sample and selecting the lowest strain that either resulted in a non-linear stress-strain relationship or reduced the storage modulus by 5% (Figure 15). A non-iterative sampling feature was activated on the rheometer to automatically adjust the strain through the experiment. This mode is important and applicable for thermoset resins where rapid measurements over accurate strain control is required. In addition, the axial force was controlled within 0 ± 0.1N to compensate generated forces due to sample shrinkage during the curing process as well as to monitor possible solvent evaporation. Figure 15. Oscillation strain on cured sample to determine the linear viscoelastic region One of the most important characteristics of thermoset curing systems is the gel point, which is the boundary between the polymerization and crosslinking step; in simpler terms, it losses 78 flowability (dramatic increase in the viscosity of the curing resin) where the crosslinking density reaches infinity.172 It is defined as the time or temperature point where G’ and G’’ intercept and the damping factor (tan δ) is equal to 1. The gel point is frequency-dependent. It was shown that using the G'/G" crossover to measure gel point is accurate if the stress relaxation in gel point follows a power law of G(t)=t -1/2. To overcome this issue and design a more accurate method, Winter et al. 172 proposed to use the intersection of tan (δ) cures obtained at multiple frequencies to determine the gel point. The rheological multi-wave test was used to avoid several sample preparations and save time by applying multiple frequencies in sequence (1, 10, and 50 Hz). All three curves of tan (δ) were overlapped, and the true gel point which is independent of frequency was obtained (Figure 16). 79 a) Storage Modulus (G') Loss Modulus (G'') 80000 70000 60000 Modulus (Pa) 50000 40000 30000 20000 10000 0 3300 3500 3700 3900 4100 Time (s) Figure 16. a) Multi-wave experiment at 1, 10, 50 Hz, b) Plot of tan δ to identify true gel point 80 The gel point (tα gel) always occurs at the same conversion for the same material, regardless of temperature. Therefore, the activation energy (EA) of gelation based on this iso-conversional phenomenon can be measured using the following equation.173-175 𝐸𝐴 𝑙𝑛(𝑡α,𝑔𝑒𝑙 ) = 𝐶 + 𝑅𝑇𝑖 Where C is a polymer constant, R is the gas constant, and T is the temperature. The measured activation energy for each system obtained at different temperatures is shown in Figure 17. Interestingly, both lignin-based systems showed significantly lower activation energies compared to EP systems (with and without solvent). Two reasons could explain this behavior. According to statistical approaches by Flory 176 and Stockmeyer 177, monomer functionality is inversely related to the critical extent of reaction; this means the system will reach the gel point at a lower extent of reaction. On the other hand, the system is gelled faster. The functionalities of lignin-based epoxy systems were 2-3 times higher than EP system; therefore, the gel point is achieved at a lower conversion, according to the following equation. 1 𝑃𝑐 = [𝑟 + 𝑟𝜌(𝑓 − 2)]1/2 Where Pc is the gel point, r is the ratio of two different monomers (ratio of epoxy groups and amine groups in this case), ρ is the fraction of multifunctional groups to all functional groups in the mixture, and f is the functionality of a multifunctional monomer. In addition, it was shown that hydroxyl functional groups accelerate the curing reaction of epoxy/amine systems, but they only serve as a catalyst and do not compete with the reaction between amines and epoxy groups.178, 179 81 When solvent was added to the epoxy resin (EP-EL), the gel point and the activation energy increased. Wu et al.180 reported that the presence of solvent in epoxy resin results in a lower reaction rate and reaction order. Thus, adding solvent could decrease the curing rate and increase the activation energy, possibly due to a decrease in the interaction of functional groups. 5.5 ESW EHW EP (no solvent) EP with 40% Ethyl lactate 5 EA= 7.7 kJ/mol 4.5 EA = 7.2 kJ/mol 4 EA = 6.9 kJ/mol ln(tα, gel) 3.5 EA= 6.6 kJ/mol 3 2.5 2 2.98 3.03 3.08 3.13 3.18 3.23 1000/T (K-1) Figure 17. The plot of obtained activation energy of epoxy resins E-SW: epoxidized softwood E-HW: epoxidized softwood Ep: Epon 828 (DGEBA) 82 3.3.3.2.5 Gel Fraction and Swelling Ratio The gel fraction and swelling ratio of cured samples were measured to obtain further information regarding their network density. Sample swelling and dry states are presented in Figure 19, following immersion of samples in THF for 24 h. The EP sample without EL showed the highest gel fraction and lowest swelling ratios. In the EP-EL system, the gel fraction decreased, which indicates the solvent (EL) was partially washed out because it was not incorporated into the network. For lignin-based epoxy systems, both cured samples (EHW and ESW) showed higher gel fractions, which might be related to a 3D crosslink network that retained more EL solvent than the EP-EL system. Sample EHW-K had a higher gel fraction than ESW-K, probably due to its higher epoxy content, which provides a higher crosslinking density. Figure 18. Immersed epoxy thermosets in THF 83 Gel fraction Swelling ratio 99 18 16 98 14 97 Gel fraction (%) Swelling ratio (%) 12 96 10 95 8 6 94 4 93 2 92 0 EP EP-EL EHW-K ESW-K Figure 19. Measured gel fractions and swelling ratios of the cured samples 84 4 CHAPTER 4 (Lignin-Based PUD) 4.1 Introduction 4.1.1 Incorporation of Lignin in Polyurethane Formulations Lignins derived from different sources and isolation methods, including wheat straw,181 organosolv,182 and kraft lignin 183 have been used to partially (40- 80 wt.%) replace polyol in PU resin formulations used for coating applications. Different modification approaches such as solvent fractionation,184 depolymerizations,185 and functionalization186 have been used to improve the suitability of lignin as polyol replacement in polyurethane formulations. Although several studies successfully replaced petroleum-based polyol with unmodified lignin, all formulated resins were designed for solvent-borne systems, which require a high amount of organic solvents (2-3 times the weight ratio of lignin) to dissolve lignin. Previous studies dissolved lignin first in tetrahydrofuran (THF) or DMF before PU formulation, which caused the final resin to contain about 40-60 wt.% volatile organic compound (VOC).184, 187 Reducing VOC emissions is the goal of most industries, resulting in most resins and coating producers shifting from solvent- borne to waterborne systems.188 Thus, it is crucial to develop lignin-based polyurethane systems that meet the requirement of green chemistry in formulating zero or low VOC waterborne resins. 4.1.2 Waterborne Polyurethane Formulations (PUDs) Waterborne polyurethane dispersions (PUD) are widely used as coatings for wood, plastic, metallic, and mineral substrate. PUD is a polyurethane polymer resin dispersed in water rather than solvent due to its hydrophilic functional groups.189 PUD has attracted great interest in coating applications due to its excellent flexibility, impact and abrasion resistance, and low or even zero VOC content.189 85 Based on the synthesis method, PUDs are classified into two groups: 1. PUDs are first synthesized in a water-miscible organic solution, then water is added to the resin 2. PUDs synthesized in the presence of water In both groups, a medium molecular weight isocyanate-terminated prepolymer (polymer chains containing active isocyanate functional groups in the structure) is formed by a reacting di- or poly- ols and an excess of di- or poly-isocyanates. In this way, the formed prepolymer is chemically active for further reactions. The main differences between the two methods in PUD systems are how the prepolymer chain is extended. In the first method, the water-solubilizing/dispersing groups from the internal emulsifier are functionalized (neutralized) in the prepolymer before chain extension. Then, the water is added to the polymer, and stable small particle dispersions are obtained without using an external surfactant. Later on, the solvent is removed to obtain the desired PUD system. This PUD system is not chemically active and can be directly applied on substrate.189 In the second method, the PUD is synthesized by adding di or polyamines in the aqueous phase to the isocyanate-terminated PU prepolymer for chain extension. Chain extension with amine is possible because amine groups in polyamine are more reactive towards isocyanates than water. It is essential to keep the water temperature low enough to maintain this reactivity preference.189 Several biobased PUD systems have been developed using castor oil,190 linseed oil,191 canola oil,192 tung oil,193 and, cardanol 194 , which all are edible feedstock and not desirable by the industry as they compete with food sources. In one study,195 unmodified alkali lignin dissolved in DMF to replace 25-33 wt.% of petroleum-based polyol to synthesize PUD resin. It was found that 86 introducing alkaline lignin in PUD formulation improved the thermal stability and tensile strength of PUD films. 4.2 Experimental In the present study, lignin-based PUD formulations were prepared. Two approaches were utilized to modify and improve the PUD formulations. Tartaric acid (TA) was used as an emulsifier to replace DMPA and increase the percentage of biobased reagent in PUD formulation. In addition, a soy-based polyol with a low OH value was added to the formulation to increase the flexibility and elongation of lignin-based PUD. 4.2.1 Materials Hardwood lignin isolated from alkaline pre-extraction pretreatment was used. Isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), dimethylol propionic acid (DMPA), tartaric acid (TA), methyl ethyl ketone (MEK), and triethylene amine (TEA) were purchased from Fisher Scientific. Hydrazine monohydrate (HZ), and cyrene were supplied from Sigma Aldrich. Cargill kindly provided the soy-based polyol with OH value of 40 mg KOH/g. 4.2.2 Synthesis Lignin-Based PUD Resins First, 5g dried alkaline pre-extraction lignin was dissolved in 25 g cyrene and 80 g MEK. Then, different formulations were prepared according to Table 18. The mixture of lignin, emulsifier, and isocyanate was mixed under reflux and a nitrogen atmosphere at 70 °C for 4 hours. Then, it was cooled to 50 °C, and TEA was added as a neutralizing agent and mixed for 1 hour. Next, an equivalent amount of HZ was gradually added at room temperature to react with the remaining NCO groups to complete the polymerization step. Finally, 40 g DI water was added to the mixture at a high shear rate (2000 rpm) to disperse formed PUD particles. A rotary evaporator at 50 °C 87 was employed to remove MEK from the solution. Commercial PUD resin from WIL was used as a reference for comparison with lignin-based formulations. Table 18. Components of tested PUD formulations Soy-polyol Tartaric acid Triethylene amine Isocyanate Sample ID Lignin (g) (g) (g) (g) (g) Pre-ex IPID-PUD 5 - 1 1.3 IPDI*- 4.4 Pre-ext HDI-PUD 5 - 1 1.3 HDI**- 3.3 Pre-ext 10% soy HDI-PUD 4.5 0.5 1 1.3 HDI**- 3.1 Pre-ext 20% soy HDI-PUD 4 1 1 1.3 HDI**- 2.6 Pre-ext 30% soy HDI-PUD 3.5 1.5 1 1.3 HDI**- 2.4 *Isophorone diisocyanate **Hexamethylene diisocyanate 4.3 Characterization of Lignin-Based PUDs Solid contents of all resins were measured according to the ASTM D4426-01. First, aluminum pans were placed in a furnace at 270 °C for 1-2 min to burn off any contaminations. Then they were cooled to room temperature in a desiccator and weighted. Then, around 1 g of resin was added to a pan and placed in an oven for 105 min at 125 °C. Later, samples were moved to a desiccator to reach the room temperature and then weighted. The solid content of resins was calculated according to the following equation: Weight of dried resin (g) Solid Content (%) = × 100 Weight of initial resin (g) 88 The pH of all resins was measured using a calibrated Mettler Toledo S220 pH meter at room temperature. The viscosity measurement was performed using a Discovery HR-1 hybrid rheometer (TA instrument) at room temperature and a constant shear rate (100 1/s). PUD resins were added to a silicon mold and dried at room temperature for 72 hrs. Then, the formed films were taken out and subjected to a micro-tensile test, using an Instron universal testing machine, to measure the tensile strength and elongation at break. The sample dimensions were 25×5×1 mm, and the elongation rate was 500 mm/min. 4.4 Results and Discussion Cyrene, also called dihydrolevoglucosenone, is a heterocyclic cycloalkanone that is made either by hydrogenation,196 or enzymatic reduction of levoglucosenone.197 It is a biobased solvent and is considered a green alternative for toxic aprotic solvents such as dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP); it has recently been commercially available.198 It is widely used in different applications such as synthesis chemistry, pharmaceutical products, and polymers.199- 202 DMF is known as a good solvent for lignin due to their relatively low Hansen relative energy difference (RED), which is 0.77. Because of similar aprotic dipolar properties to DMF, the RED of lignin and cyrene is 0.89, indicating that cyrene can be a great solvent for lignin. 203 Therefore, cyrene was used to dissolve lignin for further reaction in PUD formulations. Table 19 shows the properties of prepared PUD resins. Table 19. Properties of PUD resins Sample ID Solid Content (%)a pH Viscosity (cP)b Pre-extraction PUD 33.7 (0.5) 8.0 228 Cu-AHP (1.2% H2O2+O2) PUD 31.8 (0.4) 8.2 211 89 Cu-AHP (4% H2O2+O2) PUD 34.2 (0.3) 7.9 234 Commercial PUD 34.9 (0.2) 8.1 240 a) Dried at 125 °C, according to ASTM D4426-01 b) 25 °C, constant shear rate 100.0 (1/s) Figure 20. Wood coated samples with PUD resins The mechanical testing results of the formulations are illustrated in Figure 21. The sample prepared with IPDI showed low tensile strength and significantly lower elongation at break than the commercial PUD. HDI was used as an isocyanate reagent to improve the elongation at the break of PUD films since it has a linear structure. It was found that replacing IPDI with HDI did not substantially improve the tensile properties of PUD film. Therefore, soy-polyol with a low hydroxyl value partially replaced the lignin (10, 20, and 30 wt.%) to provide flexibility. By adding 10 wt.% soy-polyol, the elongation at break and tensile strength increased by 160% and 116%, respectively, compared with lignin-based PUD formulated with HDI. Both tensile strength and elongation at break continued to increase with higher incorporation of soy-polyol, but the increase 90 rate plateaued from 20wt.% to 30 wt.% soy-polyol. The lignin-based PUD formulation with 20% soy-polyol displayed a tensile strength that was 88% of the commercial PUD’s tensile strength; similarly, its elongation to break was 68% of the commercial PUD’s corresponding value. Tensile Strength (Mpa) Elongation at break % 20 400 Tensile Strength (MPa) Elongation at Break (%) 16 320 12 240 8 160 4 80 0 0 Pre-extraction Pre-extraction Pre-ext 10% soy Pre-ext 20% soy Pre-ext 30% soy Commercial PUD (IPDI) PUD (HDI) PUD PUD PUD PUD Figure 21. Tensile properties of different PUD resins 91 5 CHAPTER 5 (Conclusions and Future Recommendations) 5.1 Conclusions Lignin, the most abundant renewable aromatic polymer, is recovered as a by-product of pulp and paper and bioethanol production. It is currently burned to provide energy for pulp and paper plants, while lignin has excellent potential to be used for synthesis of different value-added products. The structural complexity of lignin varies among biomass sources and isolation processes. Comprehensive characterization is required to find the most suitable applications for a particular lignin. To achieve this, seventeen commercially available lignins covering softwood, hardwood, and herbaceous sources, isolated by four common methods including kraft, organosolv, sulfite, and soda, were thoroughly characterized. 5.1.1 Insight into the Composition and Structure of Commercial Lignins According to results obtained by multi-techniques characterization, hardwood lignins generally had more linear structures and lower molecular weights. Furthermore, kraft lignins on average had higher aliphatic and phenolic hydroxyl contents because of the cleavage of phenolic ether linkages (β-O-4, α-O-4, and 4-O-5), which provide more hydroxyl groups for further reactions. Also, lignin isolated through the organosolv process had lower hydroxyl content due to milder conditions (lower temperatures and concentrated alkaline pretreatment) used in that process. In summary, the molecular weight, impurity, and chemical structure of lignin should be all taken into consideration when choosing a specific lignin for valorization. 5.1.2 Entirely Replacing Bisphenol A with Unmodified Lignin in Epoxy Resin The efficacy of a wide range of commercially available unmodified lignins in replacing 100 % of bisphenol A in epoxy resin formulation was investigated. According to our developed method, 92 molecular weight and phenolic hydroxyl content had the most significant impact on lignin suitability in replacing BPA in epoxy resin. It was found that lignins which had lower molecular weights (Mw) and higher phenolic hydroxy contents, had higher reactivities towards biobased epichlorohydrin. Dynamic mechanical analysis (DMA) proved that the fully biobased cured systems containing epoxidized lignin and biobased diamine (from cashew nutshell) as a hardener had comparable thermomechanical properties to the petroleum-based epoxy system (diglycidyl ether bisphenol A) but had lower thermal stability based on TGA results. This is an important step forward in establishing the possibility of formulating a fully biobased epoxy resin using unmodified technical lignins, biobased ECH, and biobased hardener with comparable performance to petroleum-based resins for coating, adhesive and composite applications. 5.1.3 Formulating Lignin-Based Waterborne Polyurethane Resins The high aliphatic hydroxyl contents of lignins (3-5 mmol/g) isolated through an MSU patented technology (pre-extraction and enzymatic hydrolysis) indicated that these lignins are excellent candidates to formulate polyurethane waterborne resins due to the high reactivity of aliphatic hydroxyl groups toward isocyanates. In this study, several lignin-based waterborne polyurethane dispersion (PUD) resins with zero volatile organic compounds were developed for the first time by entirely replacing petroleum-based polyols with unmodified lignins. Additionally, the petroleum-based emulsifier in PUD formulations was successfully substituted with a biobased compound (tartaric acid). Then to improve the flexibility of developed lignin-based PUD resin, 20 wt.% of lignin was replaced with soy-polyol, which enhanced tensile strength and elongation of the resin. The promising results of this study can potentially expand the application of biorefinery lignin and soy-polyol in formulating low VOC, high biobased content PUD resins. 93 5.2 Future Recommendation This study was focused on developing lignin-based epoxy and PUD resins using unmodified lignins from different isolation processes and sources. For considering these resins for coating applications (mainly wood coatings), it is recommended to evaluate the performance of formulated coatings made with the developed lignin-based PUD resins when applied on different substrates. In addition, to consider the feasibility of producing lignin-based epoxy and PUD resins on commercial scales, it is helpful to conduct a comprehensive techno-economic analysis (TEA) for both processes. 94 6 APPENDICES 95 6.1 APPENDIX A (UV Degradation of Lignin) Evaluating Efficacy of Different UV-Stabilizers/Absorbers in Reducing UV-Degradation of Lignin Abstract Susceptibility of wood to UV degradation decreases the service life of wood products used outdoors. Organic UV absorbers and hindered amine light stabilizers (HALS), as well as inorganic UV absorbers, are added to coatings to improve the UV stability of coated wood products. Although about 85% of UV radiation is absorbed by lignin in the wood, it is unclear which UV stabilizers can minimize lignin degradation. In this study, the photodegradation of softwood organosolv lignin was monitored over 35 days of UV exposure. Changes in lignin properties were assessed using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), gel permeation chromatography (GPC), and phosphorus nuclear magnetic spectroscopy (31P NMR). It was found that the aromatic rings of lignin underwent significant degradation, resulting in increased glass transition temperature and molecular weight of lignin. Subsequently, eighteen different additives were mixed with lignin and exposed to UV irradiation. The analysis of samples before and after UV exposure with FTIR revealed that inorganic UV absorbers (cerium oxide and zinc oxide) and a mixture of organic UV absorbers and HALSs (T-479/T-292, T-5248, and T-5333) were the most effective additives in reducing lignin degradation. This study can help coating scientists to formulate more durable transparent exterior wood coatings. Keywords: Lignin; Photodegradation; UV Absorber; UV Stabilizer 96 Introduction Wood products used outdoors are susceptible to photodegradation,204 which usually causes yellowing, discoloration, loss of gloss, increased roughness, and diminished mechanical and physical properties.205-207 Wood is mainly composed of three compounds: cellulose, hemicellulose, and lignin; these compounds have different sensitivities to UV light during photodegradation. Lignin is a macromolecule in wood that acts as a binder in holding cellulosic fibers;208 it is composed of various arrangements of three monolignols: guaiacyl, syringyl, and p-hydroxyphenyl units.209 Cellulose and hemicellulose only absorb 5-20% of UV light, while lignin absorbs about 80-95% of the UV light due to the presence of chromophores and aromatic rings, making it more prone to decomposition by photooxidation reactions.208, 210 During UV irradiation, three chemical reactions occur in lignin, 1) dehydrogenation, 2) dehydroxymethylation, and 3) demethoxylation.211 The formation of free radicals triggers the UV degradation process, followed by the oxidation of phenolic hydroxyl groups in lignin.212, 213 Free phenolic radicals are generated immediately under UV irradiation. This way, the radicals delocalization favors the formation of o- and p-quinonoid structures after demethylation and cleavage of the side chain.214 As shown in Scheme 8, the newly formed carbonyl groups in o- and p-quinonoid are considered chromophoric groups that cause significant color changes on the wood surface.214 In addition to lignin, extractives are also susceptible to UV degradation. Extractives types and contents significantly affect the color, odor, and biological durability of the wood.215 Similar to lignin, extractives will also undergo structural changes after UV exposure contributing to wood discoloration.216-219 97 UV radiation (295-400 nm) provides sufficient energy to dissociate lignin moieties that have carbonyl, biphenyl, or ring-conjugated structures. It was shown that violet light (380- 430 nm) extends photodegradation into the wood beyond the area affected by UV light since larger wavelengths penetrate deeper into the wood than UV light.220 In addition to UV light, water also plays a vital role during the photodegradation, of wood, including carrying the radicals formed on the wood surface to deeper layers in the wood, forming hydroperoxide that can initiate chain scission reactions in polymeric wood compounds. Kalnins 221 reported that oxygen is necessary for free radical initiation, and phenoxy radicals are formed during lignin photodegradation can react with oxygen to form O-quinoid structures after demethylation. Hon also stated that the reaction of oxygen to form hydroperoxide is an essential part of the photodegradation process.222 98 Scheme 8. Formation of o- and p-quinonoid structures resulting from UV degradation of lignin 223 Researchers have explored many ways to improve weathering performance of wood. Treating wood surfaces with chromic acid is a well-known method that stabilizes wood by oxidizing phenolic sub-units at the surface, making them more resistant to photodegradation.224, 225 However, concern about chromium compounds’ potential adverse health effects limited the commercialization of chromic acid treatment for wood.226 Chemical modification has also been widely used to enhance the weathering stability of wood. Chemically treated wood with butylenes, butylene oxide, and methyl isocyanate, benzoyl chloride, as well as acetylation showed better weathering performance.227-230 Additionally, 1,3-dimethylol-4,5-dihydroxyethyleneurea (mDMDHEU) was used to treat scots pine wood and reported to significantly improved the 99 weathering stability of the wood.231 One of the most common ways to increase the photostability of wood while maintaining its aesthetic appeal is to use transparent coatings such as epoxy, polyurethane, alkyd, and acrylic, containing UV stabilizers and/or nano-pigments.232-236 Organic UV stabilizers are categorized into two groups: 1) UV absorbers (UVAs) and 2) hindered amine light stabilizers (HALSs). UVAs like 2-(2-hydroxyphenyl)-benzotriazoles (BTZ) filter out the vulnerable wavelengths of the light before they reach the wood surface; therefore, decreasing the rate of radical formation due to their high absorbance profile in the UV region (typically 300- 350 nm).237 The most common UVAs have primary photophysical properties, including a high absorbance profile in the UV range and high photochemical stability.238 HALS (a common derivative of 2,2,6,6-tetramethylpiperidine), also known as a radical scavenger, inhibiting the photo-oxidative degradation of polymers.239 It was shown that a combination of organic UV absorber and HALS improved the transparent wood coating performance.240 On the other hand, inorganic UV absorbers are based on metal oxide particles like ZnO or TiO2, which are applied to scatter or absorb light. Using nanosized pigments can protect both coating and the substrate (wood) while preserving transparency in the visible spectrum.241, 242 Pretreatment of wood for dimensional stability and applying the flexible and photostable coating is another way to achieve durable exterior coated wood products.243, 244 Also, it has been shown that phenol- formaldehyde resins can improve weathering stability of plywood samples.213, 245 Although several studies have evaluated the photostability performance of different wood coatings, they were mainly focused on assessing color change as a visual indicator,246 to the best of our knowledge, there is no published work on examining the interaction between lignin, as the main UV susceptible component of the wood, with a wide range of UV stabilizers. The aim of the present research was to find the most effective additives that can reduce lignin degradation. The 100 results of this study can help coating formulators or preservative producers choose additives proven to minimize lignin degradation, thus improving the UV performance of wood products in exterior applications. Experimental Materials and Methods Since most exterior wood products (fences and decks) are made of softwood in North America, a high purity softwood organosolv lignin provided by Lignol (Current Suzano) was used. An organosolv lignin was chosen because, among technical (commercially available) lignins, organosolv lignin has the closest structure to the structure of native lignin in the wood due to the mild ethanol biorefinery isolation process.247 Eighteen different additives (Table 1), including organic UV absorbers (UVA), organic hindered amine light stabilizers (HALS), and inorganic UV absorbers, were kindly supplied by chemical industries. All other chemicals were purchased from Fisher Scientific Co and Sigma Aldrich and used as received. Sample Preparation First 4 g of organosolv softwood lignin was dissolved in 15 g of tetrahydrofuran (THF). Next 2 wt.% of individual additives were added to the mixture (lignin and THF). The solution was mixed for 15 min at room temperature (200 rpm) and then was poured into aluminum pans (40 mm diameter, 10 mm height). For some samples, the mixture of both organic UV absorber and HALS was used, as recommended by the additives manufacturers. The mixtures of inorganic UV absorbers in lignin-THF were placed in a sonication bath for 10 min to ensure their homogenous dispersions. Minex, a functional filler made from nepheline syenite, that is used as a performance enhancer for brightness, weathering stability, abrasion, and burnish resistance in coating 101 formulations, was also added to the lignin as an additive. The control sample (lignin, without any additives) was also dissolved in THF to obtain a uniform and smooth surface similar to other samples. Later, all samples were placed under a fume hood (with lights off for 24 hrs) to let the solvent evaporate (THF). Then the samples were kept in the dark place for one week at room temperature to ensure complete removal of solvent while avoiding any potential exposure to light that might trigger undesirable reactions of photoactive compounds. The names and compositions of all samples are shown in Table 20. 102 Table 20. Composition of prepared samples ID Sample Name Role Supplier 1 Tinuvin-1130 Organic UV absorber BASF 2 Tinuvin-400 Organic UV absorber BASF 3 Tinuvin-479 Organic UV absorber BASF 4 Tinuvin-384 Organic UV absorber BASF 5 Chiguard 5330 Organic UV Absorber Chitec Technology 6 Tinuvin-292 Organic HALS1 BASF 7 Tinuvin-123 Organic HALS BASF 8 Chiguard 101 Organic HALS Chitec Technology 9 Tinuvin-5333 Organic UV absorber & HALS BASF 10 Tinuvin-5248 Organic UV absorber & HALS BASF 11 Tinuvin-479-123 Organic UV absorber & HALS BASF 12 Tinuvin-292-1130 Organic UV absorber & HALS BASF 13 Tinuvin-384-292 Organic UV absorber & HALS BASF 14 Tinuvin-400-123 Organic UV absorber & HALS BASF 15 Tinuvin-479-292 Organic UV absorber & HALS BASF 16 Fe3O42 Inorganic UV absorber US Research Nanomaterials 17 Cerium oxide2 Inorganic UV absorber Strem Chemical, Inc. 18 Zirconium oxide2 Inorganic UV absorber US Research Nanomaterials 19 Zinc Oxide2, 3 Inorganic UV absorber Zochem 20 Titanium oxide2, 3 Inorganic UV absorber NYACOL Nano Technologies Inc. 21 Minex3 UV and binder stabilizer Covia Canada Ltd. 22 Control (pure lignin) ---- Lignol (Current Suzano) 1 HALS= Hindered amine light stabilizer 2 Nanosized pigments 3 Provided by the third-party supplier, and it is assumed were obtained from listed producers 103 UV Exposure Photodegradation of the lignin mixtures was studied following the EN 927-6 Standard procedure.248 The UVA-340 lamp (recommended by EN 927-6 Standard) manufactured by Q-Lab Corporation (Canada) was chosen for this study due to its similar spectrum to sunlight (Table 21). All samples were placed under UV light irradiation at a 5 cm distance from the lamp for 35 days. UV exposure was run in ambient conditions (24 °C, 70-80 RH%, and air atmosphere). There were not any dark periods during the UV exposure, except for brief sample collections. Because powdered lignin was used during the test, the samples were not exposed to water during the photodegradation test. Table 21. Relative spectral irradiance of UV-A 340 lamp Wavelength (nm) Relative Spectra Irradiance % 290 < λ ≤ 400 100 λ ≤ 290 0.0 290 < λ ≤ 300 0.2 300 < λ ≤ 320 6.2 to 8.6 320 < λ ≤ 340 27.1 to 30.7 340 < λ ≤ 360 34.2 to 35.4 360 < λ ≤ 380 19.5 to 23.7 380 < λ ≤ 400 6.6 to 7.8 Lignin Characterization The pure lignin and mixture of additives and lignin samples were analyzed with Fourier-transform infrared spectroscopy (FTIR) before UV irradiation and then every week during 35 days of UV irradiation. A Jasco FTIR-ATR-6600 equipped with an attenuated total reflection accessory (ATR) was used to monitor potential chemical changes of samples because of the photodegradation 104 reaction. Spectra were recorded in a wavenumber range between 500 and 4000 cm-1 at a resolution of 4 cm-1 with 64 scans. All spectra were normalized and baseline-corrected before quantitative analysis using OriginPro 2015 software (version 2.214). The spectra were normalized by selecting the C-H deformation band at 2921 cm-1 as a reference since its intensity remained the same during UV exposure. Linear background correction in the absorbance mode was applied to remove any potent background compound. The same procedure was applied for selected bands (aromatic skeleton or carbonyl), which are expected to change during the UV exposure. The lignin and carbonyl indexes were calculated by dividing the absorbance of a specific band (after background correction) by the absorbance of the C-H band (the unchanged band used as reference). The glass transition temperature, molar mass distribution, and hydroxyl content of lignin before and after 35 days UV irradiation were measured according to the procedures discussed in Chapter 2. Results and Discussion Lignin Degradation To confirm that the lignin structure was not changed after dissolving it in THF and solvent was 31 entirely removed, the sample was analyzed with P NMR. The analysis showed that the drying method was effective with no residual THF in lignin (based on quantitative data). No change in lignin structure was observed compared with spectra of original lignin before dissolving it in THF. FTIR spectroscopy was used to monitor chemical changes of lignin (with and without additives) before UV irradiation and at weekly intervals over 35 days of UV irradiation. Table 22 shows different assignments of FTIR bands in lignin. The C=C stretching vibrations of aromatic rings, which have an absorption band at 1508 cm-1, were considered as the lignin bands 249. The carbonyl groups showed an absorption band at 1714 cm-1, which moved to 1735 cm-1 after UV exposure that was assigned to formed quinone groups 250. The wide absorption band around 3400 cm-1 was 105 assigned to hydroxyl groups.251 The FTIR spectra of pure lignin before and after each week of UV irradiation are shown in Figure 22. Table 22. Summary of the major FTIR peaks of the organosolv lignin. Wavenumber (cm-1) Band Assignment 3414 O-H stretching 2925 C-H stretching 2842 C-H stretching 1714 C=O stretching 1508 Aromatic skeletal vibration 1182 Guaiacyl C-H 106 Figure 22. FTIR spectra of pure lignin sample (control) before and after 35 days of UV- irradiation, 1508 cm-1, vibrations of aromatic rings; 1735 cm-1, vibration of carbonyl groups All spectra were baseline corrected and normalized using the band at 2921 cm-1 as reference band, which was not affected by photodegradation.252 Two parameters were used to monitor 1508 1735 potential photodegradation of lignin: I 2921 and I , corresponding to lignin and carbonyl 2921 indices, respectively, where “I” represents the measured intensity from the top band to the baseline.252 𝐴 1508 Lignin index = 𝐴2921 𝐴 1735 Carbonyl index = 𝐴2921 107 Different indices of pure lignin at different UV irradiation times are displayed in Figure 23. The lignin content decreased rapidly as a result of photodegradation, which was accompanied by the formation of carbonyl groups.253 It can be seen that with the increase in exposure time, lignin index (I 1508/2921) decreased as a result of the decreased amount of aromatic rings during the photodegradation phenomenon, as also confirmed by previous studies.250, 254 On the contrary, carbonyl index (I 1735/2921) increased due to ortho and para-quinoid formation forming as free radicals react with oxygen forming carbonyl and carboxylic groups.214, 255, 256 The results showed that the rate of carbonyl formation significantly increased after 21 days of UV irradiation. 10 9 Lignin Carbonyl 8 7 6 Index 5 4 3 2 1 0 0 5 10 15 20 25 30 35 UV Irradiation Time (days) Figure 23. Lignin and carbonyl indices of pure lignin at different UV irradiation times. Table 23 shows the properties of pure lignin before and after 35 days of UV irradiation. The molecular weight of lignin increased after UV irradiation. This enhancement may be related to radical coupling reactions, causing the formation of higher molecular weight compounds such as condensed phenyl propane structures like 5-5, 4-O-5, and β-5.257, 258 Also, the 108 polydispersity index (PDI) of lignin was increased significantly, which is probably due to depolymerization and repolymerization of lignin polymeric chains under UV light.259, 260 The Tgs of lignin increased from 93 °C to 122 °C after UV exposure. The increase in Tg is reported to be due to the formation of polar groups, such as carbonyl during UV irradiation, confirmed by FTIR data.261, 262 Table 23. Summary of the measured lignin properties before and after 35 days of UV irradiation. Lignin Properties Before After 35 days of UV Irradiation Mn (Da) 1190 1290 Mw (Da) 5250 7050 PDI 4.4 5.5 Tg (°C) 93 ± 2 122 ± 3 Aliphatic Hydroxyl (mmol/g) 1.31 1.04 Phenolic Hydroxyl (mmol/g) 2.39 1.99 Carboxylic Acid (mmol/g) 0.42 0.68 31 Figure 24 illustrates the P NMR spectra of lignin before and after UV irradiation. The aliphatic and phenolic hydroxyl functional groups of lignin decreased by 21% and 17%, respectively, after UV exposure, while the carboxylic groups increased by 62%. The decrease in aliphatic and phenolic hydroxyl groups could be related to the formation of quinone structures, whereas an increase in carboxylic acid functional groups could be due to the ring- opening of quinone compounds.263 109 Figure 24. 31P NMR spectra of lignin before(red) and after 35 days of UV irradiation (blue) Efficacy of Different Additives on Improving UV Stability of Lignin 𝐴 1508 Figure 25 shows the percent decrease of the lignin index ( ) as a measure of lignin loss. Pure 𝐴2921 lignin (control) showed the highest decrease (85%), and the sample containing 2 wt.% cerium oxide had the lowest lignin loss (28%), possibly due to its excellent UV absorption capacity, as also indicated by Dao et al. 264 Additionally, samples containing a mixture of organic UV absorber and HALS (T-479/T-292, T-5248, and T-5333) illustrated lower lignin loss than other additives. The average lignin loss of organic UV absorbers (65%) was significantly lower than the average lignin loss of HALSs (74%) and other additives (78%). The combination of organic UV absorbers and HALSs showed 58% lignin loss, which is remarkably lower than using them individually. Overall, inorganic UV absorbers (56%) were more effective in reducing lignin loss than a mixture 110 of organic UV absorbers and HALSs (UV absorbers 65% and HALSs 74%) and other additives (78%). Also, using a mixture of both organic UV absorbers and HALSs were more effective in reducing the photodegradation of lignin (58% lignin loss) than using individual components. When they are used together, UV absorber helps to absorb UV light, and HALS captures any free radicals that form during exposure resulting in reducing the UV degradation of lignin.265 Control 85.1 Minex 74.0 Titanium oxide 61.6 Zinc Oxide 69.7 Zirconium oxide 60.3 Cerium oxide 28.0 Fe3O4 59.9 Chiguard 5330 59.2 Chiguard 101 69.5 T-479/ T-292 46.9 T-400/ T-123 52.9 T-384/ T-292 77.6 T-292/ T-1130 69.8 T-479/ T-123 65.1 T-5248 47.7 T-384 71.4 T-479 68.7 T-400 67.0 T-5333 49.2 T-123 77.2 T-292 74.4 T-1130 57.2 0 10 20 30 40 50 60 70 80 90 % Decrease in Lignin Index 𝐴 1508 Figure 25. Decrease in lignin index (%) ( 𝐴2921 ) of various samples after 35 days of UV irradiation (lower numbers are better), the bars with the same color are not significantly different (α=0.05). 111 When lignin undergoes photodegradation, quinone groups, which have two carbonyl groups, are formed. Therefore, the photodegradation of lignin will increase the number of carbonyl groups in the lignin. As such, the carbonyl index of all samples increased after 35 days of UV irradiation (Figure 26). Among tested additives, the organic UV absorber (T-479), zinc oxide, and to some extent, cerium oxide proved to be the most effective additives in protecting lignin, which resulted in lower carbonyl group formation. It was shown that zinc oxide could quench free radicals and acts as a radical scavenger.266, 267 UV absorbers filter the high-energy UV spectrum while radical scavengers neutralize high-energy and destructive free radicals. Therefore, when organic UV absorbers and HALSs were used together, the UV protection was significantly improved compared to using each additive individually. 112 Control 94.5 Minex 85.2 Titanium oxide 81.3 Zinc Oxide 63.0 Zirconium oxide 79.9 Cerium oxide 77.3 Fe3O4 81.2 Chiguard 5330 75.6 Chiguard 101 78.8 T-479/ T-292 84.1 T-400/ T-123 76.1 T-384/ T-292 80.6 T-292/ T-1130 88.1 T-479/ T-123 71.1 T-5248 76.8 T-384 82.7 T-479 60.9 T-400 80.7 T-5333 84.0 T-123 86.2 T-292 78.1 T-1130 82.6 0 10 20 30 40 50 60 70 80 90 100 % Increase in Carbonyl Index 𝐴 1735 Figure 26. Increase in carbonyl index (%) ( 𝐴2921 ) of different samples after 35 days of UV irradiation, (lower numbers are better), the bars with the same color are not significantly different (α=0.5). In this study, lignin was used as a model compound to evaluate the efficacy of a wide range of light stabilizer additives used in coatings. It was observed that zinc oxide and a mixture of organic UV absorber/HALS were the most effective additives for increasing the photostability of lignin. It was assumed that these additives can potentially increase the UV stability of wood when added to 113 coating formulations designed for exterior applications. Although other wood components, especially extractives, can also play a role in photodegradation of wood, reducing structural degradation of lignin which absorbs 80-95% of UV-light,214 is a good starting point in choosing the right additives for formulating clear wood coatings. It is important to point out that the organosolv lignin used in this study has a different structure than native lignin in the wood. It is indisputable that any isolation process will change the structure of lignin; therefore, the additives that have worked in this study might not have the same interaction effect with the lignin on the surface of the wood. That is why the effectiveness of these best-performing additives are being studied (by authors) when added into different resins (alkyd, acrylic, and polyurethane). The performance of coated-wood samples will be monitored during exposure to a combination UV and rain (accelerated weathering). Some of these additives can still protect the wood surface without chemically bonding with lignin. They can reduce the UV degradation of wood by shielding (dispersion or absorption) the UV light 268 or acting as radical scavengers to capture the formed radicals. Panel et al. reported that the combination of UV absorber and HALS was the most effective treatment for color stabilization of wood in exterior applications. Since similar results were found, studying lignin degradation as a simpler structure than wood seems to be a reliable method to evaluate the performance of newly developed light stabilizers. 114 6.2 APPENDIX B (Epoxy HNT) Improving UV-Stability of Epoxy Coating Using Encapsulated Halloysite Nanotubes with Organic UV-Stabilizers and Lignin Published in Progress in Organic Coatings 269 Abstract Epoxy coatings are used in a wide variety of applications due to their excellent chemical, thermal, and mechanical properties. However, their susceptibility to UV degradation has limited their use in exterior applications. Usually organic UV absorbers and stabilizers are added to epoxy systems to improve theirs UV stability, but their performance decreases over time due to the degradation and loss of organic UV-stabilizers. In this study, a novel method was developed to encapsulate organic UV stabilizers and lignin (as a natural UV absorber) into halloysite nanotubes (HNTs). To ensure successful encapsulation, the pristine and filled halloysite nanotubes were characterized quantitatively using thermogravimetric analyzer (TGA), and qualitatively with X-ray photoelectron spectroscopy (XPS). Then, encapsulated nanotubes (HNTs) were added to an epoxy system (1-3 wt%) and their efficacy was evaluated before and after 840 hours of accelerated weathering. Changes in physical, chemical, and thermal properties of coatings were measured using a spectrophotometer, field emission scanning electron microscopy (FE-SEM), electron paramagnetic resonance spectroscopy (EPR), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). The results showed that epoxy samples containing 2 wt% HNT-encapsulated with organic UV-stabilizers, and samples containing 1 wt% HNTs encapsulated with lignin had significantly higher UV stability than epoxy resin prepared with the same concentration of these individual components (UV-stabilizers or lignin). This study 115 demonstrates the efficacy of encapsulated HNTs, with UV stabilizers or lignin, in improving UV stability and extending service life of epoxy coatings. Keywords: UV stability, halloysite nanotubes, encapsulation, lignin, epoxy Introduction Epoxy coatings are widely used in various applications due to their excellent chemical and thermal resistance and their outstanding adhesion to different substrates (metals, woods, and plastics).270- 272 However, in outdoor conditions, these coatings degrade as a result of exposure to UV light. The UV degradation causes chalking, delamination, and discoloration while also negatively affecting the gloss, hardness, and surface roughness of coatings.273, 274 Although only 5% of the total solar UV radiation reaches the earth’s surface (280-400 nm), its high energy can easily induce the formation of free radicals, causing chain scission and secondary oxidative reactions of polymeric chains.275, 276 Aromatic and ether groups in epoxy resin are photo-initiating sites that are more susceptible to UV degradation.277 Therefore, one of the main challenges in the coatings industry is to formulate transparent epoxy coatings with exceptional UV stability for exterior applications. To increase the UV stability of epoxy coatings, organic UV stabilizers (including UV absorbers and hindered UV stabilizers), as well as inorganic UV absorbers are added to epoxy formulations. UV-absorbers (UVAs) filter out the short and harmful wavelengths of light before they reach the polymeric chains. UVAs can be categorized into two main groups; 1) benzotriazoles (2-(2- hydroxyphenyl)-benzotriazole) with two absorption peaks at 300 and 350 nm, and 2) hydroxyphenyl triazines (2-hydroxyphenyl-s-triazine), which are newer and have two strong absorption peaks at 300 and 340 nm.237, 278 Currently, 2-(2-hydroxyphenyl)-benzotriazoles are commonly used as organic UV-absorbers in transparent coating formulations.232 Most transparent coating formulations contain hindered amine light stabilizers (HALSs), which interact with 116 radicals and decrease the photooxidative degradation of polymers.239, 279 Due to high vapor pressure, leaching, and chemical loss by photochemical reactions, the concentration of organic UV stabilizers (both UVAs and HALSs) decreases over time, which reduces their efficacy in service.237, 238 One way to avoid leaching is to chemically bond UV-absorbers to the resins,280, 281, which will hinder their migration towards the surface. However, this approach limits the performance of UV stabilizers and requires specific absorbers for each individual resin formulation. Metal oxides are also used as inorganic UV absorbers that can scatter or absorb light. Nanosized metal oxides (<100nm) like zinc and titanium oxide, as well as nano-silica, are commonly used in clear coating formulations.241, 282-286 Although inorganic UV absorbers are more permanent, they are less efficient than organic UV stabilizers.287 Lignin, the most abundant natural aromatic polymer in the world, constitutes 15-35 wt.% of the wood and other plant biomass.288 Lignin is a natural UV absorber with excellent radical scavenging and antimicrobial properties.289 However, difficulty in the synthesis and homogeneous dispersion of lignin nanoparticles into coatings,290, 291 has limited its application at the industrial level. Encapsulation of UV stabilizers and lignin could be used to potentially solve their weaknesses and in increases their efficiency. To increase the photostability performance of acrylic coatings, Quenant et al.292 found that the addition of encapsulated poly (methyl methacrylate) microcapsules with UV stabilizers (both UV absorber and HALS) improved the mechanical properties and reduced the discoloration of coated-wood samples after exposure to artificial weathering. Halloysite nanotubes (HNTs) can be used as effective nanocarriers for the encapsulation, and controlled release of chemical and biological compounds, owing to their biocompatible nature as well as their tubular shape.293 HNTs, natural clay nanoparticles, are nontoxic and have high surface areas.294, 295 HNTs have been frequently used as additives in epoxy, polyamide, and styrene- 117 butadiene rubber resins, due to their ability to improve the chemical, thermal, and mechanical properties of composite materials.296 One study showed that the addition of pristine HNTs to the polylactide resin resulted in a significant decrease in the chemical and physical properties of the coating system after 300 hours of artificial weathering.297 In another study, the addition of modified HNTs with a surfactant into a polystyrene formulation resulted in improving the UV stability of the resins.298 Hu et al.299 showed that immobilization of zinc oxide nano-protrusions on the HNTs’ surface, improves the UV shielding performance of nanocellulose films. Another study investigated how zinc oxide immobilization on HNTs affects the photo stability of poly (lactic acid) after 60 days of artificial weathering.300 It was found that HNT-ZnO nanocomposites showed less discoloration and higher photostability compared to pristine HNT nanocomposites.300 To the best of our knowledge, there is no study on the encapsulation of organic UV-stabilizers or lignin into HNTs to improve the UV stability of polymers. The main objective of this study was to examine the UV stability of epoxy resins after the addition of halloysite nanotubes loaded with UVA/HALS or lignin. It was supposed that encapsulation of organic UV stabilizers in HNTs could be an effective method to preserve them in the system. Moreover, it was hypothesized that lignin could potentially serve as an excellent UV absorber by encapsulation. The goal was to improve the efficacy of these organic UV stabilizers and lignin by encapsulating them into a natural nanotube that can easily be added to any transparent coating formulations to extend their exterior service life. 118 Experimental Materials Halloysite nanotubes with the trade name “Hallo Pure” were supplied by I-Minerals Inc. Liquid diglycidyl ether of bisphenol A (Epon Resin 828) with epoxy equivalent weight of 185-192 g/eq was supplied by E.V. Roberts. Isophorone diamine (IPDA) and acetone were purchased from Fisher Scientific Co. UVA T1130, and HALS T292 were provided by BASF. Hardwood organosolv lignin was kindly supplied by Fibria (Lignol), Vancouver, Canada. Figure 27 shows SEM and TEM images of pristine HNTs at different magnifications. Matauri Bay (MP) HNTs generally include short and stubby tubes with lengths ranging from 100 nm to 3 µm; some long and thin tubes were also observed. As shown in Figure 27, HNTs clump together. The morphological characteristics of HNTs are presented in Table 24. They have tubular shapes, with inner diameters of 15-70 nm and outer diameters of 50-200 nm. The chemical composition is based on Al2Si2O5(OH)4. The surface area of MB HNTs is around 22.10 m2/g, which is less than other types of HNTs such as patch (PT), Dragonite (DG), and Camel Lake (CLA)).301 119 a) SEM Images of HNTs at different magnifications b) TEM images of HNTs at different magnifications Figure 27. (a) SEM and (b) TEM images of pristine HNTs with increased magnifications (left to right) Table 24. Morphological characterization of halloysite nanotubes (HNTs) 301 Dominant Inner Outer Aspect Length Empirical HNT-Type Particle Diameter Diameter Ratio (nm) Formula Shapes (nm) (nm) (L/D) Matauri Bay Tubular 100−3000 15−70 50−200 12 Al2Si2O5(OH)4 (MB) 120 Methods Encapsulation of HNTs with Organic UV Stabilizers and Lignin A 400 mg/mL solution of UVA T1130/ HALS T292 (1:1 weight ratio) or lignin was prepared in acetone. T1130 is an organic UV absorber based on hydroxyphenyl benzotriazole, and T292 is a liquid HALS that is widely used for coatings and contains two active compounds, including bis (1, 2, 2, 6, 6-pentamethyl-4-piperidyl) sebacate and methyl 1, 2, 2, 6, 6-pentamethyl-4-piperidyl sebacate. The mixture of UVA/HALS was used based on the manufacturer’s recommendation 302 (according to a filed provisional patent) . Acetone was selected as a non-VOC solvent, and it has a low surface tension (25.2 mN/m at 20 °C) 303 can easily solubilize organosolv lignin. Then, 0.6 g HNTs were added to the solution of UVA/HALS, or lignin and acetone and mixed for 48 h at room temperature. Next, the suspension was subjected to a vacuum for 5 min at 76 kPa, until there were no air bubbles coming off from the surface of the solution. This indicated the highest loading (complete encapsulation), which occurs when all the air is removed from inside the nanotubes, and the voids are filled with a solution of UVA/HALS or lignin and acetone. Then, the suspension was centrifugated (5000 rpm for 5min), and the loaded HNTs were washed three times with acetone and with excess DI water to remove any potential residual UVA/HALS or lignin from the surfaces of the HNTs. Figure 28 shows different steps involved in the encapsulation process. 121 Figure 28. Schematic of HNT-encapsulation process (both UVA/HALS and lignin systems) Preparation of HNT-Epoxy Nanocomposites Twelve different sets of samples were prepared as presented in Table 25. To prepare samples containing HNTs, specific amounts of HNTs (1 wt.%, 2 wt.%, or 3 wt.%) were added to the epoxy resin. Then, benzyl alcohol was added as a viscosity reducer (1:10 mass ratio) to improve the dispersion of HNTs into the epoxy resin formulations. Afterward, the resin was placed in a sonication probe at 50% amplitude for 5 min, followed by an ultrasonication bath for 30 min at room temperature (to homogenously disperse HNTs into the epoxy matrix). Further, a curing agent was added (0.22 g IPDA to 1 g epoxy resin) to the solution, and mechanically mixed for 1 min, and later placed in an ultrasonication probe for 2 min. For control samples (without HNTs), the specified amount of epoxy resin, curing agent, benzyl alcohol, and additives (either a pure mixture of UVA/HALS, or lignin) were mixed using a digital overhead stirrer (Caframo) for 2 min at 500 122 rpm. The mixture was poured into an aluminum pan (10 cm diameter and 0.5 cm height) and heated in an oven at 60 °C for 2 hours (as recommended by the supplier of the epoxy resin). Based on the manufacturer’s recommendation, all samples were kept in the dark environment for 7 days at room temperature before further analysis, reaching maximum crosslinking density and preventing potential unwanted reactions of photoactive compounds in the epoxy system. Table 25. Composition of prepared samples Sample ID Type of Additive Amount of Additive (%) Epoxy None - UVA/HALS UVA T1130 and HALS T292 (1:1) 2 Lignin Hardwood organosolv lignin 2 1% Pristine HNTs Pristine HNTs 1 2% Pristine HNTs Pristine HNTs 2 3% Pristine HNTs Pristine HNTs 3 1% HNT- UVA/HALS UVA/HALS loaded into HNTs 1 2% HNT-UVA/HALS UVA/HALS loaded into HNTs 2 3% HNT-UVA/HALS UVA/HALS loaded into HNTs 3 1% HNT-lignin Lignin loaded into HNTs 1 2% HNT-lignin Lignin loaded into HNTs 2 3% HNT-lignin Lignin loaded into HNTs 3 Characterization The photostability of the prepared coating samples were evaluated before and after accelerated weathering in a QUV machine (source: UVA 340 nm) at an irradiance of 0.68 W/m2 with a chamber temperature of 60 °C. The samples were exposed to UV irradiation for a period of 35 123 days (840 hours) without condensation or a water spray cycle. The samples were evaluated before and after exposure to UV irradiation. Field emission scanning electron microscopy (JEOL JSM 7500 F) was used to evaluate the morphology of the HNT samples, as well as to study the effect of HNT samples on the UV stability of epoxy coatings. HNTs and epoxy samples were respectively coated with iridium and gold to increase their conductivity for SEM. Also, a small amount of HNTs were dispersed in a methanol solution, and one drop of the mixture was placed into a TEM grid (carbon film 200 mesh, copper) and heated in a vacuum oven at 50 °C prior to TEM (JEOL, JEM-2200FS) analysis. To quantify the loading of organic UV stabilizers and lignin, in the HNT samples, thermogravimetric analysis (TGA) was used. The samples were run from 30°C to 800°C with a heating rate of 10 °C/min, under a nitrogen flow of 25 mL/min for the sample, and 10 mL/min for the balance (TGA TA, Q50). X-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD) were used to ensure that the loading of UVA/HALS or lignin into HNTs did not change the structure of the HNTs. The color of the epoxy films, before and after UV exposure was monitored using a spectrophotometer (CM-2300d- Konica Minolta) in SCE mode. The L*a*b* color space before and after 35 days of UV irradiation was quantified to study the effects of UV light exposure on the color change of epoxy coatings.304 Electron paramagnetic resonance (EPR) spectroscopy was carried out on small slices of epoxy samples, with a 0.9 mm in thickness, 1 mm wide, and 30 mm long at X-band, on a Bruker E-680X spectrometer equipped with an SHQE–W1 resonator. The resonator was flushed with nitrogen gas during the measurements to maintain the ambient temperature and remove oxygen. Continuous- wave (CW) EPR spectra were detected under non-saturating conditions with 0.5 mW incident microwave power, a microwave frequency of 9.87 GHz, magnetic field modulation with 0.4 mT 124 amplitude, and 100 kHz frequency, and a data conversion time of 163 ms. For in situ EPR experiments, all samples were irradiated by a UVC lamp (25 W) for 5 min. The optical fiber of the lamp was adjusted in front of an EPR resonator 2 cm from the sample probe. The number of radical centers in each sample was determined by double integration of the first derivative cw-EPR spectra preceded by a first-order polynomial baseline correction. Samples were also analyzed with a Spectrum II PerkinElmer Fourier transform infrared spectrophotometer in attenuated total reflectance mode (FTIR-ATR) to monitor possible chemical changes in epoxy resins, before and after UV exposure. The absorbance mode was used, with wavenumbers ranging from 400-4000 cm-1 with 4 cm-1 resolution and 32 scans. Glass transition temperatures (Tg) of epoxy samples were measured at different exposure times using a differential scanning calorimeter (DSC 6000, PerkinElmer). 7-10 mg of each epoxy sample was placed in an aluminum pan, and the samples were subjected to a heat/cool/heat cycle with a temperature range of 10-180°C, under a nitrogen flow of 40 mL/min, and a heating rate of 10 °C/min. The second heating curve was used to calculate the Tg. Results and Discussion The results of TGA and XPS analysis showed that the UVA/HALS or lignin were successfully loaded into halloysite nanotubes (HNTs). Thermogravimetric analyzer (TGA) was previously used by other researchers to determine the quantity of encapsulation of benzotriazole (as a corrosion inhibitor) into the HNTs as anti-corrosion coatings and salicylic acid (as a biocidal agent) as an antibacterial for food packaging.305, 306 Five different samples, including (1) pristine HNTs; (2) UVA T1130 and HALS T292 (1:1 mixture); (3) hardwood organosolv lignin; (4) UVA/HALS loaded into HNTs; and (5) lignin loaded into HNTs, were analyzed using TGA. Since the thermal stability of UVA/HALS and lignin are much lower than that of pristine HNTs (due to their mineral 125 structures), the residual amount of loaded HNT samples were lower than that of pristine HNTs, at levels proportional to the amounts in the encapsulated samples.306 The results of the TGA analyses are summarized in Table 26 (three replicates were run for each sample). The residual amount of pristine HNTs at 800 °C was around 80 wt.%, while the residual amounts of HNTs loaded with UVA/HALS and lignin were around 68 wt.% and 67 wt.%, respectively. This indicates that about 11 wt.% UVA/HALS and about 13 wt.% lignin were loaded into HNTs. TGA curves of pristine HNTs, a mixture of UVA/HALS, lignin, and loaded HNT samples are shown in Figure 29. Table 26. Amount of UVA/HALS or lignin loaded into HNTs Loading Amount into Sample Code % Residual at 800 °C HNT (%) Pristine HNTs 80.5 ± 0.5 - Mixture of UVA/HALS (T1130-T292) 1.6 ± 0.05 - Lignin 0.2 ± 0.02 - Encapsulated UVA/HALS in HNTs 68.1 ± 0.3 10.8 ± 0.2 Encapsulated lignin in HNTs 67.0 ± 0.3 13.3 ± 0.4 100 90 80 Pristine HNTs 70 60 UVA/HALS Mass (%) 50 Lignin 40 30 Encapsulated UVA/HALS into HNT 20 10 Encapsulated lignin into HNT 0 0 100 200 300 400 500 600 700 800 Temperature (°C) 126 Figure 29. TGA curves of different samples (pristine HNTs, mixture of UVAT-1130/HALS-T282, pure lignin, pristine HNTs, encapsulated UVA/HALS in HNTs, and encapsulated lignin into HNTs) The XPS analysis was conducted to qualitatively confirm the loading of UVA/HALS and lignin into HNTs.305 The results of XPS analyses are presented in Table 27. Pristine HNTs are composed of oxygen, aluminum, and silicon. Also, carbon was observed in pristine HNTs, probably due to contamination of the surface.307 The mixture of UVA/HALS contain C, O, H and N. Additionally, loading lignin into HNTs should significantly increase the carbon and oxygen content in encapsulated samples, because lignin contains around 60% carbon and 35% oxygen.308 The presence of nitrogen, observed in the loaded UVA/HALS of HNTs, confirms encapsulation of UVA/HALS into HNTs, since only UVA/HALS contain nitrogen in their structures. In addition, for HNTs loaded with lignin, the composition of carbon relative to oxygen increased, which confirms the loading of lignin, as lignin structure contains mainly CHO. Table 27. XPS analyses results of pristine HNTs and encapsulated samples showing successful encapsulation of UVA/HALS mixture and lignin in HNTs Sample Code C% O% Al% Si% N% Pristine HNTs 31.11 43.02 6.91 18.96 - Loaded UVA/HALS in HNTs 55.95 30.06 3.64 8.15 2.19 Loaded lignin in HNTs 61.17 28.88 1.57 8.39 - Figure 30 illustrates the XRD spectra of three different samples (pristine HNTs, HNT- UVA/HALS, and HNT-lignin). There are two diffraction peaks, at 2θ= 12.2° and 2θ= 20.1°, that are related to the 001 and 101 planes in HNTs, respectively. These peaks are attributed to HNTs’ tubular morphology, high degree of disorder, small crystalline structures, and interstratifications 309, 310 of the layers with various hydration state . The structure of HNTs did not change after the 127 loading process since the main peaks of the HNTs are still present. After loading, the intensity and sharpness of some peaks decreased, which is possibly related to the loading of UVA/HALS or lignin into HNTs. Since they are both amorphous compounds, loading of UVA/HALS or lignin decreased the crystallinity of the HNTs. Figure 30. XRD diffraction patterns of the Pristine HNTs, HNT-UVA/HALS, and HNT-lignin The color changes of the epoxy coatings are due to the increasing numbers of chromophores produced during UV irradiation. Color difference values (∆E) for different samples after 35 days of UV irradiation are shown in Figure 31. The pure epoxy resin had the highest color change (∆E=46), while the color change of epoxy resin with the inclusion of 2% pure UVA/HALS (mixture of UVA-T1130/HALS-T292) was 22. Even though the color change of epoxy resin with the addition of 2% lignin was very low (16), the sample was not necessarily more photostable. Rather, the low color change was due to the initial dark color of the lignin itself. By adding 1 wt.% HNT-UVA/HALS, the ∆E of the epoxy resin samples decreased from 46 to 22, which indicated 128 that the coatings became more photostable. Also, ∆Es of epoxy coatings filled with 1% HNT- lignin (2) and 2% HNT-lignin (6) (indicated using different shades of green) were significantly lower than epoxy resins with pure lignin (46). Hence, these two samples had the highest color stability among the different prepared coatings, even substantially greater than the samples containing the commercial organic UV stabilizers (UVA/HALS). Interestingly, ∆E of samples containing different ratios of pristine HNTs (1, 2 and 3%) (indicated using different shades of grey) are lower than color change of pure epoxy sample (Epoxy). This behavior may be related to the photocatalytic effects of HNTs interaction with the epoxy matrix after UV radiation exposure 304. The images of some samples before and after 35 days of UV exposure are shown in Figure 32 which has same pattern to the color change data (∆Es). It was seen that both pure epoxy and UVA/HALS samples underwent huge color changes. On the other side, both samples 2% HNT- UVA/HALS and 2% HNT-lignin showed less color changes. ∆E (ColorChange) 0 5 10 15 20 25 30 35 40 45 50 55 Epoxy 46 UVA/HALS 22 Lignin 16 1% HNT-UVA/HALS 22 2% HNT-UVA/HALS 29 3% HNT-UVA/HALS 35 1% HNT-lignin 2 2% HNT-lignin 6 3% HNT-lignin 20 1% Pristine HNT 23 2% Pristine HNT 26 3% Pristine HNT 27 129 Figure 31. Variations of ∆E* for different samples after exposure to UV irradiation for 35 days (three replicates for each sample) Figure 32. Photos of different epoxy samples before and after UV irradiation Figure 33 shows FE-SEM micrographs of the pure, UVA/HALS, and lignin epoxy samples before and after 35 days of UV irradiation. After UV exposure, the pure epoxy sample degraded drastically. Several cracks appeared on the sample, and the surface became extremely uneven. This result may be related to the chemical decomposition of epoxy and the release of gases from the epoxy surface, as reported by a previous study 276. Samples containing pure UVA/HALS (mixture of UVA-T1130/HALS-T292) or lignin showed some visible cracks on their surfaces after UV exposure. The number of cracks on the surface UVA/HALS sample was smaller than that in the 130 samples containing lignin, indicating that the UVA/HALS samples underwent lower degrees of photodegradation. Pure epoxy-before Pure epoxy-after 35 days Epoxy with mixture of UVA/HALS-before Epoxy with mixture of UVA/HALS-after 35 days Epoxy with 2% Lignin-before Epoxy with 2% Lignin-after 35 days 131 Figure 33. FE-SEM micrographs of pure epoxy, epoxy with addition of 2 wt.% pure UVA/HALS, and 2 % lignin before and after 35 days of UV irradiation Figure 34 shows samples that were filled with different amounts of HNT-encapsulated UVA/HALS. The samples prepared with addition of 2% HNT-UVA/HALS appear more resistant to UV light, since the crack density after UV irradiation is significantly lower than that of other samples. 132 Epoxy with 1% HNT- UVA/HALS-before Epoxy with 1% HNT- UVA/HALS-after 35 days Epoxy with 2% HNT- UVA/HALS-before Epoxy with 2% HNT- UVA/HALS-after 35 days Epoxy with 3% HNT- UVA/HALS-before Epoxy with 3% HNT- UVA/HALS-after 35 days Figure 34. FE-SEM micrographs of epoxy resins with addition of (1-3%) of HNT-UVA/HALS loaded samples before and after 35 days of UV irradiation 133 Figure 35 displays FE-SEM micrographs of the epoxy resins filled with different amounts (1-3%) of HNT-encapsulated lignin. The results show that the epoxy resin that contained 1% HNTs encapsulated lignin had fewer visible cracks after exposure to UV irradiation, compared with other samples. Increasing the amount of HNTs encapsulated lignin into the epoxy resins increased the frequency of cracks indicating that addition of higher amounts of HNTs encapsulated lignin into the epoxy resins negatively impacts their UV stability. 134 Epoxy with 1% HNT- lignin-before Epoxy with 1% HNT- lignin-after 35 days Epoxy with 2% HNT- lignin-before Epoxy with 2% HNT- lignin-after 35 days Epoxy with 3% HNT- lignin-before Epoxy 3% HNT- lignin-after 35 days Figure 35. FE-SEM micrographs of epoxy samples loaded with different amounts (1-3%) of HNTs encapsulated with lignin, before and after 35 days of UV irradiation 135 According to the SEM images of all samples, epoxy coatings filled with 2% HNT-UVA/HALS and 1% HNT-lignin had higher photostability than other samples. Note that the sample with 1% HNT-lignin was more photostable (having less visible cracks) than the sample filled with 2% HNT-UVA/HALS. Additionally, the samples containing HNT-UVA/HALS cracked less than the sample containing pure UVA/HALS. It can be suggested that the higher efficiency of the encapsulated systems is due to the fact that encapsulation of UV stabilizers into halloysite nanotubes (HNTs) reduced their migration to the surface of the coatings although, the migration of UV stabilizers to the surface of the coatings was not measured in this study. Lignin has previously been reported to be a good UV absorber and radical scavenger.311, 312 This study found that photo stability substantially improved when lignin was encapsulated into HNTs versus when pure lignin was directly added to the epoxy coatings. Electron paramagnetic resonance spectroscopy (EPR) was used to monitor the formation of free radicals in each sample before and after UV exposure. The intensities of various EPR signals, which are directly related to free radical concentrations, were measured in different samples. Figure 36 shows the EPR spectra of pure epoxy collected before irradiation (dark blue trace). The spectrum shows a nonhomogeneous-broadened resonance, typical for an organic radical in the solid-state, centered at approximately g = 2.011.313, 314 After 5 min of UV irradiation, the EPR signal intensity of pure epoxy increased significantly (Figure 36, red trace) and a new EPR signal characterized by a narrower linewidth was resolved at g = 2.003. The number of paramagnetic centers present in these two samples was determined by double integration of the spectra over their full 30 mT width. Integration values of 104 and 140 were obtained for the “Epoxy-dark” (blue- gray) and “Epoxy-5min” (orange) spectra, respectively, as shown in Figure 36. Parallel sets of EPR spectra are shown in Figures 10b and 10c for the 2% HNT-UVA/HALS and 1% HNT-lignin 136 samples, respectively. For both samples, radical centers at g = 2.011 and 2.003 were observed prior to illumination. After 5 min of UV irradiation, the EPR signal intensities increased by 20% for 2% HNT-UVA/HALS, and 32% for 1% HNT-lignin (Figure 36). For 2% HNT-UVA/HALS, the observed increase was significantly smaller than the 35% increase observed for the pure-epoxy sample. EPR signal intensities of all samples before and after UV irradiation are shown in Figure 36. Sample with 2% HNT-UVA/HALS had the lowest change in free radical concentration after 5 min UV irradiation. The samples containing lignin, 1% HNT-lignin, 2% HNT-lignin, and 3% HNT-lignin, showed a nearly uniform increase in radical intensity with illumination ranging from 33–42%, essentially independent of doping level. In contrast, the change in radical concentrations upon illumination recorded for the 1% pristine HNT through 3% pristine HNT series showed a steady increase, from 31% at the lower doping level to 92% for the 3% pristine HNT sample. 137 a) b) c) 3 3 4 cw EPR Int/a.u 2 cw EPR Int/a.u cw EPR Int/a.u 2 1 1 0 0 -1 -1 -2 -2 -3 -3 -4 -4 -5 3350 3450 3550 3650 3350 3450 3550 3650 3350 3450 3550 3650 Magnetifc Field/mT Magnetifc Field/mT Magnetifc Field/mT Epoxy (dark) 2% HNT-UVA/HALS (dark) 1% HNT-lignin (dark) Epoxy (5min Irradiation) 2% HNT-UVA/HALS (5 min irradiation) 1% HNT-lignin (5min irradiation) d) Max Int/a.u 0 50 100 150 104 Epoxy 140 70 UVA/HALS 92 75 Lignin- dark 101 49 1% HNT-UVA/HALS 87 Dark 54 2% HNT-UVA/HALS 65 50 5 min irradiation 3% HNT-UVA/HALS 81 83 1% HNT-lignin 110 73 2% HNT-lignin 101 78 3% HNT-lignin 111 78 1% Pristine HNTs 102 68 2% Pristine HNTs 103 67 3% Pristine HNTs 129 Figure 36. EPR spectra of pure epoxy (a), 2% HNT-UVA/HALS (b), and 1% HNT-lignin (c), before and after 5 min irradiation. d) The intensity of free radicals before and after 5 min UVC irradiation (EPR data) for different samples 138 After 35 days of UV irradiation, FTIR spectra of all samples were normalized, and carbonyl indexes were calculated (Figure 37) and used to evaluate chemical changes in the samples after UV exposure. The epoxy resin with 2 wt.% pure lignin (“Lignin”), had a carbonyl index of 0.81, which is very high possibly due to the degradation of lignin and formation of quionone.212 The carbonyl index of pure epoxy and sample with UVA/HALS were 0.74 and 0.59, respectively. The lowest carbonyl indexes were related to the samples containing 1% HNT-UVA/HALS and 1% HNT-lignin which were 0.37 and 0.36, respectively. These results indicate that samples with 1% HNT-UVA/HALS and 1% HNT-lignin underwent less chemical degradation than other samples, complimenting the SEM and EPR results. FTIR- Carbonyl Index (1730/1180) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Epoxy 0.74 UVA/HALS 0.59 Lignin 0.81 1% HNT-UVA/HALS 0.37 2% HNT-UVA/HALS 0.55 3% HNT-UVA/HALS 0.61 1% HNT-lignin 0.36 2% HNT-lignin 0.51 3% HNT-lignin 0.59 1% Pristine HNT 0.74 2% Pristine HNT 0.75 3% Pristine HNT 0.84 Figure 37. Carbonyl index of various samples obtained from FTIR spectra after 35 days 139 When polymeric materials are exposed to UV light, their glass transition temperatures (Tg ) increase due to the formation of more polar groups (like carbonyls) which hinder chain movements, and breaks up some polymeric chains.261, 262 The Tg of all samples before and after 35 days of UV irradiation are tabulated in Table 28. Pure epoxy had the highest Tg changes (∆Tg), as expected, due to its high susceptibility to UV degradation. The addition of organic UV stabilizers to epoxy resin decreased the Tg, while the addition of 2 wt.% lignin did not change the Tg of the epoxy system. In all samples, addition of HNTs increased Tg, due to the inorganic nature of HNTs.315 Samples that contained HNTs encapsulated lignin, regardless of the loading amount, displayed the lowest change in Tg (smaller ∆Tg). This could well be an indication of the higher photo stability of these samples (especially samples with 1% HNT-lignin) as confirmed by other techniques. In addition, ∆Tg (5.3) of epoxy resin with 1%HNT-UVA/HALS is slightly lower than ∆Tg (6.9) of epoxy resin with pure UVA/HALS, indicating also that encapsulation of UVA/HALS into HNTs reduced the photo degradation of epoxy polymer chains. 140 Table 28. Tg of different samples before and after 35 days of UV irradiation Tg °C Tg °C Sample ID (After 35 days ∆Tg (Before Irradiation) Irradiation) Epoxy 53.1 60.5 7.4 UVA/HALS 51.3 58.2 6.9 Lignin 53.5 60.3 6.8 1% HNT- UVA/HALS 53.6 58.9 5.3 2% HNT-UVA/HALS 56.1 62.6 6.5 3% HNT-UVA/HALS 59.8 63.5 3.7 1% HNT-lignin 57.6 60.4 2.8 2% HNT-lignin 58.2 60.8 2.3 3% HNT-lignin 59.7 60.9 1.2 1% Pristine HNTs 57.1 61.0 3.9 2% Pristine HNTs 57.3 62.8 5.5 3% Pristine HNTs 59.6 62.7 3.1 By monitoring the physical, chemical, and thermal properties of different samples, it was observed that the encapsulation of UVA and HALS into HNTs were an effective method for increasing the photostability of epoxy systems. The highest photostability was achieved when 2 wt.% HNTs encapsulated with UVA/HALS and 1 wt.% HNTs encapsulated with lignin were added to the epoxy system. Our results showed that 1 wt.% HNTs loaded with lignin, a natural UV absorber can effectively be used as a biobased photo stabilizer for epoxy systems. While the higher additions (> 2%) of these encapsulated compounds into epoxy systems resulted in a decrease in photostability, they still performed better than pure epoxy systems. This trend may be related to the photoactivity of HNTs. When the amount of added HNTs increased, the photostability performance decreased, due to an increase in free radical formations caused by the higher UV 141 absorption (as observed for pristine HNT samples).300 The addition of pristine HNTs (specifically more than 2%) into epoxy systems negatively impacted the photostability performance of resins (color change, EPR, and FTIR spectroscopy clearly exhibited this trend). The present research shows that using pristine HNTs in epoxy formulations did not reduce the UV degradation of epoxy resins, as also indicated by another study using HNTs in polylactic acids (PLA).297 Although pristine HNTs may increase the UV absorption of the system, it does not make it is more photostable. Other published studies used ZnO-HNT systems and found that the addition of these systems improved photostability of natural rubbers and composites.310, 316 Another study also showed that the encapsulation of UV stabilizers into poly (methyl methacrylate) microcapsules could increase the photostability of water-based acrylic systems,293 confirming the findings of this study. 142 REFERENCES 143 REFERENCES 1. Laurichesse, S.; Avérous, L., Chemical modification of lignins: Towards biobased polymers. Progress in polymer science 2014, 39 (7), 1266-1290. 2. Stewart, D., Lignin as a base material for materials applications: Chemistry, application and economics. 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