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The potential of lignin to increase the hydrophobicity of micro/nanofibrillated cellulose (MNFC) Yeap, Rou Yi 2020

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  THE POTENTIAL OF LIGNIN TO INCREASE THE HYDROPHOBICITY OF MICRO/NANOFIBRILLATED CELLULOSE (MNFC) by  Rou Yi Yeap B.Sc., Illinois Institute of Technology, 2016  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Forestry)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  January 2020  © Rou Yi Yeap, 2020ii  The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, a thesis entitled:  The Potential of Lignin to Increase the Hydrophobicity of Micro/nanofibrillated Cellulose (MNFC)  submitted by Rou Yi Yeap in partial fulfillment of the requirements for the degree of Master of Applied Science in Forestry  Examining Committee: Dr. Jack Saddler (Forestry, UBC) Supervisor  Dr. Scott Renneckar (Forestry, UBC) Supervisory Committee Member  Dr. Richard Chandra (Research Services, Trinity Western University) Supervisory Committee Member Dr. Keith Gourlay (Performance BioFilaments) Additional Examiner   iii   The demand for novel cellulosic material, such as micro/nanofibrillated cellulose (MNFC) is expected to face increasing growth due to its unique properties and evolving, high value applications in packaging, biomedical and nanocomposite materials. However, the hydrophilicity of MNFC has limited some of its applications, particularly in the packaging sector. Common modifications to improve hydrophobicity of MNFC often involves costly and environmentally challenging chemicals. Lignin is naturally hydrophobic, environmentally benign and as described herein, could be an effective agent to improve the hydrophobicity of MNFC.  Softwood Kraft lignin (SWKL) was first considered due to its commercial availability. When SWKL was dissolved in alkaline solution and acid precipitated onto MNFC, substantial amounts of lignin were deposited on the surface, resulting in a two-fold increase in initial water contact angle as compared to the control. However, no significant improvement on MNFC hydrophobicity was observed as the contact angle was unstable over time. It was apparent that the contact angle measurements were strongly influenced by the roughness of the paper, the porous nature of cellulose and the extent of lignin homogeneity on cellulose surface. To enhance the efficacy of the approach, hot pressing lignin-containing papers near lignin’s glass transition temperature was assessed. It was hoped that this would help redistribute the lignin, resulting in better lignin homogeneity on the fiber’s surface. Initial results seemed promising as the contact angle was stable over a period of two minutes after hot pressing. Other attempt to incorporate lignin, such as spray coating with hot pressing, was evaluated to enhance homogenous lignin iv  coverage on the paper. Contact angles as high as 85° and 95° were achieved, for SWKL and organosolv lignin respectively. Lignin coverage as low as 1% was able to impart hydrophobicity using the spray coating method. Although the water vapor transmission rate (WVTR) could be substantially reduced by hot pressing, the incorporation of lignin onto the paper did not reduce the WVTR significantly. This work showed that hot pressing lignin-containing papers resulted in improved hydrophobicity, likely due to the redistribution of lignin on fiber surfaces and the formation of a denser fiber network. v   Currently, most of our daily products are derived from fossil fuels. The unsustainable nature of fossil-derived products has raised grave global concerns. However, some of the impact can be mitigated through the efficient utilization of biomass, notably cellulose. Cellulose is the most ubiquitous biopolymer on earth and it can be used to make innovative, strong cellulosic materials such as micro/nanofibrillated cellulose. However, full commercialization of these products is restricted by their high hydrophilicity, leading to poor performance as packaging products. One challenge is that increasing the hydrophobicity of cellulose often comes at the expense of the environment and cost. The work in this thesis has laid the groundwork of using lignin, a by-product of the pulp and paper industry, to hydrophobize cellulose. This preliminary work showed that lignin could increase hydrophobicity, but further enhancement of lignin’s structure and properties would be needed to substantially increase the hydrophobicity of micro/nanofibrillated cellulose. vi   All of research work reported in this thesis was planned, conducted, and analyzed by Rou Yi Yeap in the Forest Products Biotechnology/Bioenergy laboratory at the University of British Columbia, Vancouver campus, under the supervision of Dr. Jack Saddler. vii   Abstract ................................................................................................................................................ iii Lay Summary ........................................................................................................................................ v Preface .................................................................................................................................................. vi Table of Contents ................................................................................................................................ vii List of Tables ........................................................................................................................................ xi List of Figures ..................................................................................................................................... xiii List of Abbreviations ......................................................................................................................... xvi Acknowledgements .......................................................................................................................... xvii Dedication .......................................................................................................................................... xix  Chapter 1: Introduction ....................................................................................................................... 1 1.1 Cellulose ..................................................................................................................................... 1 1.1.1 Structure and properties ...................................................................................................... 3 1.1.2 Solubility ................................................................................................................................ 4 1.1.3 Reactivity ............................................................................................................................... 6 1.1.4 Biodegradability .................................................................................................................... 8 1.2 Cellulosic material .................................................................................................................... 9 1.2.1 Micro/nano-crystalline cellulose (MCC/NCC) ................................................................ 9 1.2.2 Micro/nano-fibrillated cellulose (MFC/NFC) ................................................................ 11 1.2.3 Micro/nanofibrillated cellulose (MNFC) ........................................................................ 11 1.3 Lignin ........................................................................................................................................ 12 1.3.1 Kraft lignin .......................................................................................................................... 16 1.3.2 Lignosulfonates ................................................................................................................... 17 1.3.3 Organosolv lignin ............................................................................................................... 18 1.3.4 Alkali (soda) lignin ............................................................................................................. 19 1.4 Utilization of lignin ................................................................................................................. 20 viii  1.5 Surface modification of cellulose .......................................................................................... 22 1.5.1 Background ......................................................................................................................... 23 1.5.2 Hydrophobization of nanofibrillated cellulose (NFC)................................................... 26 1.5.3 The potential of lignin to hydrophobize NFC ................................................................ 28 1.6 Wettability characterization of cellulosic material ............................................................. 32 1.6.1 Wettability ........................................................................................................................... 32 1.6.2 Barrier properties ................................................................................................................ 34 1.7 Thesis objectives ...................................................................................................................... 34  Chapter 2: Materials and methods..................................................................................................... 37 2.1 Materials ................................................................................................................................... 37 2.2 Lignin moisture uptake .......................................................................................................... 37 2.3 Fiber quality analyzer (FQA) ................................................................................................. 38 2.4 Handsheet preparation ........................................................................................................... 38 2.4.1 Acid precipitated lignin ..................................................................................................... 38 2.4.2 Spray coating ....................................................................................................................... 39 2.5 Handsheet characterization ................................................................................................... 40 2.5.1 Lignin quantification .......................................................................................................... 40 2.5.2 Thickness ............................................................................................................................. 41 2.5.3 Tensile measurement ......................................................................................................... 41 2.5.4 Contact angle measurement .............................................................................................. 41 2.5.5 Surface roughness ............................................................................................................... 41 2.5.6 Scanning electron microscopy (SEM) imaging .............................................................. 42 2.5.7 X-ray photoelectron spectroscopy (XPS) ........................................................................ 42 2.5.8 Fourier-transform infrared spectroscopy (FTIR) ........................................................... 42 2.5.9 Water vapor transmission rate (WVTR) ......................................................................... 43 2.5.10 Air permeability .............................................................................................................. 43 2.5.11 Lignin leaching measurement....................................................................................... 43  ix  Chapter 3: Results and discussion ..................................................................................................... 45 3.1 Assessment of lignin incorporation via acidification onto cellulosic materials to improve overall hydrophobicity ........................................................................................................................ 45 3.1.1 Background ......................................................................................................................... 45 3.1.2 Optimization of softwood kraft lignin loading on micro/nanofibrillated cellulose (MNFC) ............................................................................................................................................ 46 3.1.3 Softwood kraft lignin deposition via acidification and filtration on different cellulosic substrates .......................................................................................................................................... 50 3.1.4 Conclusions ......................................................................................................................... 59 3.2 To assess the potential of enhancing the hydrophobicity of lignin-containing micro/nanofibrillated (MNFC) paper sheets via hot pressing ....................................................... 60 3.2.1 Background ......................................................................................................................... 60 3.2.2 Influence of hot pressing on lignin-containing MNFC paper’s hydrophobicity ........ 62 3.2.3 Assessment of spray coating with hot pressing as lignin deposition method to enhance hydrophobicity ................................................................................................................................. 73 3.2.4 Conclusions ......................................................................................................................... 84  Chapter 4: Conclusions and future work .......................................................................................... 86 4.1 Conclusions ............................................................................................................................. 86 4.2 Future work ............................................................................................................................. 87 4.2.1 Investigation of lignin structure after hot pressing ........................................................ 87 4.2.2 Lignin modification to enhance moisture barrier performance ................................... 88 4.2.3 Improving the efficiency of producing MNFC papers and the characterization of mechanical properties of resulting papers .................................................................................... 88  Bibliography ........................................................................................................................................ 89 Appendices ........................................................................................................................................ 101 x   Time-dependent water contact angle data of HW organosolv-coated MNFC papers at ten coating applications (A) and twenty coating applications (B) at varying hot pressing temperatures ....................................................................................................................................... 101  Calculated lignin content on HW organosolv lignin-coated MNFC papers at varying coating applications and hot pressing temperatures (On x-axis, “O” denotes organosolv and the following numeric number refers to coating applications) ............................................ 102  xi   Table 1. Demand of dissolving pulp by end-use based on 4.9 million tons total production where 1.1 million tons is fulfilled by cotton linters (Floe 2011) ....................................................................... 2 Table 2. Classification of different cellulose solvents (Heinze and Koschella 2005) ......................... 5 Table 3. Common linkages in lignin and their presence in softwood and hardwood (Henriksson 2009) ........................................................................................................................................................... 15 Table 4. Concentration of cellulose-lignin slurry for each substrate based on 20% lignin loading (UBSWKP: unbleached softwood kraft pulp; UBSWKP-hi: high lignin content unbleached softwood kraft pulp; SWKP: softwood kraft pulp; HWKP: hardwood kraft pulp; MNFC: micro/nanofibrillated cellulose) ............................................................................................................. 39 Table 5. Concentration of MNFC and lignin used for the optimization experiment ..................... 47 Table 6. Conditions employed for three different cellulosic substrates and the final lignin content calculated by Klason on each paper sheet (“C” and “L” denote the control and 20% lignin loaded substrates respectively) ............................................................................................................................ 53 Table 7. Thickness of the resulting paper sheets from different substrates ...................................... 57 Table 8. Density and tensile index for SWKP, HWKP, and MNFC for both control and lignin-loaded samples .......................................................................................................................................... 58 Table 9. XPS analyses showing the O/C ratios of both the air-dried and hot-pressed samples and their theoretical surface lignin coverage (TSLC) .................................................................................. 67 Table 10. Air permeability and water vapor transmission rate data for air-dried and hot-pressed MNFC control and lignin samples ......................................................................................................... 70 Table 11. Summary of previous literature on their water contact angle results of lignin-containing MFC or NFCs ............................................................................................................................................ 75 Table 12. Tensile index, air permeability, and WVTR data from hot-pressed pure and lignin-coated MNFC handsheets .................................................................................................................................... 81 xii  Table 13. Summary of previous WVTR values regarding NFCs and MFCs .................................... 82 xiii   Figure 1. Molecular chain of cellulose (Habibi, Lucia, and Rojas 2010) ............................................. 3 Figure 2. The hierarchical structure of cellulose (Isogai et al. 2011).................................................... 4 Figure 3. The structure of synthesized cellulose derivatives (Itagaki et al. 1997) ............................... 8 Figure 4. Three types of monolignols (Buranov and Mazza 2008) .................................................... 13 Figure 5. The aromatic constituents of monolignols after polymerization (Whetten, MacKay, and Sederoff 1998) ........................................................................................................................................... 13 Figure 6. Resonance stabilized monolignol radicals formed by laccase and peroxidase (Henriksson 2009) ........................................................................................................................................................... 14 Figure 7. The formation of para-quinone methide and its subsequent reaction mechanisms to delignification (Sixta et al. 2006) ............................................................................................................ 17 Figure 8. The mechanism of sulfonation reaction occurred under acid sulfite process (R represents alkyl group) (Sixta et al. 2006) ................................................................................................................ 18 Figure 9. Delignification mechanism during organosolv pulping process (Berlin and Balakshin 2014) ........................................................................................................................................................... 19 Figure 10. Reaction pathway of lignin hydroxyalkylation using ethylene carbonate and subsequent esterification using oleic acid (Hua et al. 2019) .................................................................................... 22 Figure 11. Modification techniques used to introduce hydrophobicity on NFC (Thomas et al. 2018) .................................................................................................................................................................... 26 Figure 12. Illustration of water contact angle that represent hydrophilic and hydrophobic surfaces (Yuan and Lee 2013) ................................................................................................................................ 33 Figure 13. The intermolecular forces of water molecules at solid-liquid interfaces that result in the formation of liquid droplet ..................................................................................................................... 33 xiv  Figure 14. From left to right: MNFC slurry mixed together with 20% lignin dissolved in alkaline solution; lignin-MNFC slurry after the pH was brought down to 3; resulting MNFC sheets loaded with 20% lignin; pure MNFC sheet ........................................................................................................ 47 Figure 15. Initial water contact angle (A) and tensile index (B) of the control (gold-filled marker) and lignin-containing MNFC sheets based on the amount of lignin content (from left to right: control, L-1%, L-10%, L-5%, L-20%, L-50%) ........................................................................................ 50 Figure 16. SEM images of the fiber morphology of SWKP (left) and MNFC (right) ...................... 51 Figure 17. Lignin content (A) quantified through Klason for different substrates and its resulting initial water contact angle (B) corresponding to its lignin content ................................................... 53 Figure 18. The initial contact angle of lignin-containing SWKP, HWKP, and MNFC sheets ....... 54 Figure 19. Roughness profile (left to right: 2D, 3D) and average roughness value (Ra) obtained from optical profilometer for the lignin-loaded samples (A) SWKP; (B) HWKP; (C) MNFC ............... 56 Figure 20. SEM images showing big and non-uniform patches of lignin on MNFC paper sheets 57 Figure 21. Tensile index for control and lignin-loaded samples among different substrates ......... 58 Figure 22. SEM cross-section images of air-dried (left) and hot-pressed (right) lignin NFC films (scale bar: 30 µm) (Farooq et al. 2019)................................................................................................... 61 Figure 23. Time-dependent contact angle (A) of lignin-containing MNFC sheets after hot pressing and initial contact angle (B) of pure MNFC sheets at varying temperatures ................................... 63 Figure 24. Brownish discoloration for control handsheets pressed at 190°C comparing to air-dried and handsheets hot-pressed at 160°C .................................................................................................... 64 Figure 25. Lignin-containing MNFC handsheets with and without hot pressing ........................... 64 Figure 26. Compact fiber structure resulting from hot pressing pure MNFC handsheets at 160°C .................................................................................................................................................................... 65 Figure 27. SEM images showing hot-pressed lignin-containing MNFC handsheets ...................... 66 Figure 28. FTIR spectra of control and lignin-containing MNFC sheets under air-dried (AD) and hot-pressed (HP) conditions ................................................................................................................... 68 xv  Figure 29. Similarity in tensile index of MNFC with and without hot pressing .............................. 69 Figure 30. Normalized UV absorbance measured at 280 nm for both control and lignin-containing paper sheets under air-dried and hot-pressed conditions (A); water contact angle of hot-pressed lignin-containing MNFC before and after the leaching test (B) ........................................................ 72 Figure 31. Left to right: air-dried and hot-pressed lignin-containing MNFC sheets after stirring for 24 hours in an incubator ......................................................................................................................... 72 Figure 32. Time-dependent water contact angle of SWKL-coated MNFC papers at increasing coating applications (A) and its resulting lignin content (B) (“K” refers to SWKL and the following numeric numbers denotes coating applications) ................................................................................. 76 Figure 33. Time-dependent contact angle of organosolv-coated MNFC papers at five coating applications under increasing temperatures (A); initial contact angle of pure MNFC papers at varying hot pressing temperatures (B) (“O” denotes HW organosolv lignin) ................................. 77 Figure 34. Captured water droplets on organosolv lignin-coated MNFC paper ............................. 77 Figure 35. Left to right: pure MNFC paper hot-pressed at 100°C; organosolv-coated MNFC paper hot-pressed at 100°C; pure MNFC paper hot-pressed at 160°C; SWKL-coated MNFC paper hot-pressed at 160°C........................................................................................................................................ 78 Figure 36. SEM images showing hot-pressed pure MNFC sheet at 160°C (left) and SWKL-coated MNFC sheet hot-pressed at 160°C (right) ............................................................................................ 79 Figure 37. SEM images showing hot-pressed pure MNFC sheet at 100°C (left) and organosolv-coated MNFC sheet hot-pressed at 100°C ............................................................................................ 79 Figure 38. FTIR spectra of hot-pressed pure and lignin-coated MNFC sheets ................................ 80 Figure 39. Measured UV absorbance at 280 nm after leaching test ................................................... 83 Figure 40. Time-dependent contact angle before and after the leaching test for organosolv-coated sample (A) and kraft lignin-coated sample (B) (sheets showed herein were taken after the leaching test) ............................................................................................................................................................. 83 xvi   AGU Anhydroglucose unit ANOVA Analysis of variance CF Carbon fiber DI Deionized DP Degree of polymerization HW Hardwood HWKL Hardwood kraft lignin MCC Microcrystalline cellulose MFC Microfibrillated cellulose MNFC Micro/nanofibrillated cellulose NCC Nanocrystalline cellulose NFC Nanofibrillated cellulose Sln Solution SW Softwood SWKL Softwood kraft lignin TMP Thermo-mechanical pulp TSLC Theoretical surface lignin coverage WCA Water contact angle Wt. Weight WVTR Water vapor transmission rate  xvii   First, I would like to express my gratitude towards my supervisor, Dr. Jack Saddler, who has taught me how to better my research story, and for his guidance and financial support throughout my MASc program. I owe particular thanks to my committee members, Dr. Richard Chandra for his mentorship and introducing me to new ideas; and Dr. Scott Renneckar, for his selfless help and advice when I had doubts regarding my project.  A special thanks to Performance BioFilaments, for providing me with the material for my work. I am very fortunate and thankful to have received invaluable feedbacks and advice from Dr. Sai Swaroop Dalli, Dr. Fredrik Nielsen, and Dr. Keith Gourlay. A special mention goes to Kevin Aïssa, for being such a coherent person, and whose illuminating questions have taught me to develop my own. Additional thanks to my past mentors, Dr. Jacob Kruger and Dr. Rui Katahira, who first introduced me to the field of bioenergy and taught me all the basic laboratory skills that will be useful for my career.    I would like to extend my thanks to the past and current members of the Forest Products Biotechnology/Bioenergy group, the Advanced Renewable Materials Lab, the Sustainable Functional Biomaterials Lab and the Evans Lab, for your help, suggestions, and friendships, which have certainly made my time here so much better and enjoyable. I have also benefited from many great friendships that have kept me company during my very unpredictable ups and downs, for which I am earnestly grateful.  xviii   Last but not at all least, to the most amazing family of mine, thank you for everything. I am eternally grateful. xix   To my parents, for your abiding love and support. To my siblings, for always being wonderful. 1   In the early 1900s, the most well-known synthetic polymers, plastics, were created, which not only helped disengage human dependence on natural resources, but also successfully revolutionized society and increased the number of products derived from the petroleum industry. However, the unfettered optimisms about plastics did not last, primarily due to the escalated concerns about the environment and “plastic-pollution” in particular. Since 1950s, it was estimated that 6.3 billion tonnes of plastic waste have been generated and about 80% of it was left in the landfills or the environment (Anon 2018b). It is apparent that global society would now like to shift from an unsustainable petroleum-based economy to a more sustainable and renewable economy. Fortunately, Nature has provided us with one of the products to accomplish this – cellulose.    Cellulose can be found throughout nature, particularly in plants, animals, fungi, bacteria, algae and minerals. Cellulose makes up approximately 40% of the carbon found in plants and usually coexists with hemicellulose and lignin in the cell wall to provide mechanical support to the plants.  Cellulose is the most abundant renewable biopolymer on earth and is considered to be an inexhaustible source of environmentally friendly and biocompatible products. Historically, cellulose has been used in industries such as lumber, paper and textiles. Wood pulp is one of the most common raw materials used to produce cellulose. Dissolving pulp (i.e. high cellulose content pulp) has been used as a chemical feedstock to produce cellulose derivatives such as cellulose 2  xanthate (viscose), cellulose acetate and cellulose ethers. These derivatives have found applications in rayon, textile filament, cellophane and cigarette filter tow (Bajpai 2012; Floe 2011). For example, viscose (rayon) is a type of regenerated cellulose that has been chemically and structurally altered, making it one of the most versatile synthetic fibers (Wilkes 2001). Previous work has reported the demand for dissolving pulp based on their end-use (Table 1) (Floe 2011). As well as established high value products such as dissolving pulp that have uses beyond paper products, a new range of cellulosic fibers have also emerged that emphasize cellulosic structure at the nanoscale to impart novel properties. These products include micro/nano-fibrillated and micro/nano-crystalline cellulose. These intriguing new cellulose products have tremendous potential for products such as coating additives, absorbent products and packaging products (e.g. paper bags or cardboards). More importantly, the development of these substrates helps diversify the current pulp and paper industry and strengthens the bio-based economy.  Table 1. Demand of dissolving pulp by end-use based on 4.9 million tons total production where 1.1 million tons is fulfilled by cotton linters (Floe 2011) Derivatives End-use products (%) Xanthate (Viscose) 69 Acetate 15 Ethers 12 Nitrates 3 Others 1  3   Cellulose is a linear homogeneous polysaccharide chain consisting of β-D-glucopyranose as the basic unit. Two adjacent units are connected by a β-1,4-glycosidic bond through the elimination of water to form cellobiose. The formation of glycosidic bonds requires a 180° rotation of one of the two glucose units around its C1-C4 axis, as shown in Figure 1. Therefore, cellobiose is the smallest repeating unit of cellulose with a length of 1.03 nm instead of glucose. These glucan chains make up cellulose microfibrils (3-4 nm in diameter), and are further bundled into macrofibrils (>15nm in diameter), subsequently into cellulose fibers (20-30 µm in diameter) (Figure 2) (Isogai, Saito, and Fukuzumi 2011). The hierarchical structure of cellulose enables researchers to employ both its nano- and macro-properties for high-end applications.    Figure 1. Molecular chain of cellulose (Habibi, Lucia, and Rojas 2010)  The structure of cellobiose gives rise to the reducing and non-reducing end properties of a cellulose chain. One end (C1) contains a cyclic hemiacetal that is in equilibrium with the aldehyde (reducing end). The other end (C4) consists of an alcoholic hydroxyl group, hence non-reducing. The degree of polymerization (DP) is used to describe the chain length of cellulose polymer, which is heavily dependent on the origin and pretreatment of the feedstock. For instance, wood pulp and cotton have a DP of 300-1,700 and 800-10,000 respectively (Klemm et al. 2005). 4   Figure 2. The hierarchical structure of cellulose (Isogai et al. 2011)  Cellulose can form two types of hydrogen bonds: intra- and intermolecular hydrogen bonds. Intramolecular hydrogen bonds are formed between the hydroxyl groups of adjacent glucose units within a cellulose chain whereas intermolecular hydrogen bonds are formed between the hydroxyl groups of different cellulose chains. The hydrogen bonds, along with the β-1,4-glycosidic bond give rise to the stiffness and rigidity of a cellulose polymer.    The intra- and intermolecular hydrogen bonds formed within the cellulose make the polymer very hygroscopic. Therefore, finding an effective way to dissolve cellulose is essential for characterization and homogeneous chemistry to take place. Cellulose can be dissolved in derivatizing solvents (formation of cellulose derivatives to dissolve the polymer) or non-5  derivatizing solvents (dissolving cellulose through physical interaction with the polymer). An example of non-derivatizing solvents is sodium hydroxide solution. It is commonly used to swell the cellulose, allowing subsequent reactions to happen. The well-known viscose process employs this method to “activate” the cellulose, followed by a reaction with carbon disulfide (CS2) to form viscose. Derivatizing solvents are usually less preferred as it promotes side reactions and formation of side products. One of the derivatizing solvent systems utilizes a combination of dimethyl formamide (DMF) and N2O4 (Heinze and Koschella 2005). It can dissolve and derivatize the cellulose into cellulose nitrite. Other solvent system such as hydrazine and dimethyl sulfoxide (DMSO) under certain pressure and temperature have also been proven to successfully dissolve cellulose (Fengel and Wegener 1989). A summary of different derivatizing and non-derivatizing solvents are shown in Table 2. Even though homogenous reactions are favored, those solvents that are able to dissolve cellulose (e.g. aprotic dipolar media and salt components) pose technical challenges (Klemm et al. 2005).  Table 2. Classification of different cellulose solvents (Heinze and Koschella 2005) Non-derivatizing solvents  Derivatizing solvents Aqueous media Non-aqueous media   Inorganic complexes Bases (e.g. NaOH) Mineral acids Melts of inorganic salt hydrates Organic liquid/ inorganic salt Organic liquid/ amine/ SO2 NH3/ ammonium salt  CF3COOH HCOOH DMF/ N2O4  6  Due to the poor solubility and compatibility with common solvents, the modification of cellulose is often carefully controlled to achieve the desired degree of substitution and targeted properties at the laboratory and production scale. Cellulose ether is first produced by “activating” the cellulose with sodium hydroxide solution where the base solution also catalyzes the etherification reaction (Majewicz and Podlas 2000; Thielking and Schmidt 2012). However, cellulose often exists in the presence of water as drying is known to disrupt the internal fibers irreversibly, generally termed as hornification (Fernandes Diniz, Gil, and Castro 2004). When cellulose is dried, the internal fiber volume shrinks irreversibly due to the modification in fiber structure, hence resuspension of pulp in water will not regain the original water-swollen state (Fernandes Diniz et al. 2004). Therefore, in cases such as cellulose esterification, water is often solvent exchanged prior to the modification to prevent water participation in subsequent reactions such as esterification (Sehaqui, Zimmermann, and Tingaut 2014; Vuoti et al. 2013).    The three hydroxyl groups in each anhydroglucose unit (AGU) at C2, C3, and C6 atoms (Figure 1) are responsible for the reactivity of cellulose. The primary hydroxyl group at the C6 atom is generally more accessible and reactive due to less steric hindrance, while the secondary hydroxyl group at the C2 atom has higher acidity. Depending on the type of reactions desired, AGUs can be “activated” selectively along the chain to favor particular reaction, termed regioselectivity of cellulose. The two classic functionalization of cellulose are etherification and esterification. A few excellent reviews have been published in the past that delved into the details 7  of regioselectivity of cellulose in both reactions (Fox et al. 2011; Klemm et al. 2003; Koschella et al. 2006). Etherification of cellulose is generally more efficient in selective modification than esterification and usually takes place at the O-6 position. Tritylation is a prominent example to synthesize regioselective cellulose derivatives and is frequently employed by chemists as protective techniques in organic syntheses (Klemm et al. 2005). The attached trityl group can be further removed and reverted to hydroxyl group through acid hydrolysis once the other hydroxyl groups (O-2 and O-3) have been reacted with stable functional groups (Fox et al. 2011). In previous work, five different cellulose derivatives were synthesized through tritylation and benzylation or methylation, resulting in three of the –OH groups either being substituted by benzyl or methyl or hydrogen (Figure 3). When the cellulose derivatives were cooled down, only 23B6O was found to form gels and the others did not. It was discovered that the cellulose gel formation was largely influenced by the hydrogen bonding at O-6 position, while the O-2 and O-3 did not contribute significantly (Itagaki, Tokai, and Kondo 1997). Understanding the reactivity and regioselectivity of cellulose can enable a more efficient downstream functionalization of cellulose in many applications.    8   Figure 3. The structure of synthesized cellulose derivatives (Itagaki et al. 1997)   Cellulose is known to be biodegradable, which could be an attractive property in applications such as packaging. The microorganisms (i.e. fungal and bacteria) responsible for the hydrolytic and oxidative cleavage of cellulose are present in our daily life and degradation of cellulose can be observed through changes in cellulose characteristics such as a decrease in molecular weight or an increase in solubility. For example, cotton fibers are known to degrade over time, which is reflected in its discoloration and loss in strength. However, modification of cellulose has been shown to compromise its biodegradability depending on the modification pathway used. Previous work has looked at the effect of imidazolidinone modified cellulose on biodegradability through soil burial test and enzymatic hydrolysis (Tomšič et al. 2007). These researchers found that the biodegradability of modified cellulose greatly decreased when compared to pure cellulose. Sample Abbreviation R1 R2 R3 Cellulose  H H H 6-O-benzylcellulose 23O6B H H Benzyl 2,3-di-O-methyl-6-O-benzylcellulose 23M6B Methyl Methyl Benzyl 2,3-di-O- benzylcellulose 23B6O Benzyl Benzyl H 2,3-di-O-benzyl-6-O-methylcellulose 23B6M Benzyl Benzyl Methyl 2,3,6-tri-O-benzylcellulose 236B Benzyl Benzyl Benzyl 9  Therefore, it was apparent that it was important to control the degree of substitution and the choice of side chains when modifying cellulose (Simon et al. 1998).    Over the past several decades, cellulose-based products have continued to be of great interest to researchers due to their low environmental impact and their renewable characteristics. In this thesis, nanocellulose is used as an “umbrella” term to describe all micro- or nano- dimensions cellulose particles. Nanocellulose specifically has many applications in the pharmaceutical, drug delivery, separation membrane, transparent film, fiber and textile, electronic components and many other areas. Nanocellulose is known to have high aspect ratio which is defined as the ratio of fibre length (L) to diameter (D). Most nanocellulose also have low density and a number of reactive sites (hydroxyl groups) which are capable of attaching varying functional groups to create different surface properties. In addition to that, the exceptional mechanical properties of nanocellulose makes it an appealing candidate as a reinforcing agent. The elastic modulus of nanocellulose can reach as high as 200 GPa, which is comparable to steel and current commercial products such as Kevlar (Moon et al. 2011).    Microcrystalline cellulose (MCC) is a widely used fibrous material that has vast applications in the food, cosmetic and medical sectors as a water-retainer, reinforcing agent and suspension stabilizer. Microcrystalline cellulose is commercially available and is usually obtained via acid hydrolysis such as hydrochloric acid, which attacks the amorphous regions of cellulose. 10  This treatment is then followed by back-neutralization with alkali and spray-drying prior to recovery. The resulting material is mainly composed of crystalline cellulose with diameters ranging from 10 to 50 μm (Moon et al. 2011). One commercially available MCC, Avicel® PH-101, has an average particle size of 50 μm (Levis and Deasy 2001). Nanocrystalline cellulose (NCC) also called cellulose nanocrystals or cellulose whiskers, is composed of rod-like crystals (about 3-5 nm in diameter, 50-500 nm in length) with high crystallinity. Nanocrystalline cellulose has a broad range of applications in the food, chemical, and pharmaceutical sectors due to their nanoscale dimensions, strong mechanical properties and high surface area that enhances chemical reactivity. Like MCCs, NCCs are also produced through acid hydrolysis, often using sulfuric acid, consequently introducing surface charges on NCCs. The surface charges on NCCs are known to aid dispersion in water. NCCs are known to show the typical rheological behavior of liquid crystals in solutions (Lima and Borsali 2004; Moon et al. 2011). In “dilute” applications, cellulose whiskers are shear thinning and concentration dependant at low rates. However, at higher concentrations, cellulose whiskers show atypical behavior, likely due to the tendency of the rod-like nanocrystals to align with each other at higher shear rate (George and Sabapathi 2015). NCC has tremendous potential in biomedical, electronic and membrane applications, largely due to its large surface area, surface charge and biocompatibility (Dong et al. 2014; Jorfi and Foster 2015). Previous literature showed that NCCs were able to bind with ionizable drugs, resulting in the slow release of drugs over time (Jackson et al. 2011). However, despite its high potential to be incorporated into different applications, the production cost remained relatively high, due to the use of acid and the cost of starting material.  11   Unlike MCC or NCC, microfibrillated cellulose is produced via mechanical treatment of wood or plant fibers via microfluidization, homogenizing and grinding. Microfibrillated cellulose fibers have a diameter of 10-100 nm and is 0.5-10’s μm long, which results in its high aspect ratio (Moon et al. 2011). Microfibrillated cellulose contains both crystalline and amorphous regions of cellulose as well as hemicellulose resulting in the formation of  a fibrillated network in contrast to the rod-like shape of nanocrystals (Moon et al. 2011). The first patented process to produce MFC implemented a high-pressure homogenizer (Turbak, Snyder, and Sandberg 1983). In this process, a fibrous material is passed through a high pressure homogenizer with a small diameter opening several times until a stable suspension is achieved. MFCs are widely used as a thickening agent in the food and cosmetic industries (Rebouillat and Pla 2013). Nanofibrillated cellulose fibers generally have a lateral dimension of 4-20 nm and are about 500-2000 nm in length (Moon et al. 2011). The difference between MFC and NFC is largely between their particle diameters with NFC having a large surface area and excellent mechanical properties. These properties have led to various applications in the food packaging, paper making, and nanocomposites industries. For example, NFC is used as an additive during the paper-making process to decrease basis weight while maintaining good physical properties (Wernersson Brodin, Weiby Gregersen, and Syverud 2014).    Micro/nanofibrillated cellulose (MNFC) was the main cellulosic substrate used in the thesis work. Micro/nanofibrillated cellulose is composed of long, thin and flexible filamentous cellulose, 12  commonly extracted from wood pulp fibers. Micro/nanofibrillated cellulose retains the length of the fiber after the extraction process therefore, it has a higher aspect ratio (about 300-3000) than other nanofibers. Micro/nanofibrillated cellulose also has a large surface area and high degree of polymerization (Hua, Laleg, and Owston 2015). It is widely used as a reinforcing agent, rheology modifier and many other applications in manufacturing, construction, and consumer products (Anon 2014). FPInnovations has developed a process where MNFC can be obtained from wood fibers using mechanical process without the addition of chemicals and enzymes (Anon 2013). However, transporting micro- and nano-scale cellulose could become a major problem due to the hornification of fibers, as mentioned in Section 1.1.2.   Lignin is one of the three components in plant’s cell wall that provides rigidity to the plants, aids internal transport of water and nutrients and protects the plants against microorganisms. It is a heterogeneous polymer with phenylpropane acting as the basic unit. The three precursors, also called monolignols that makes up lignin are p-coumaryl (1), coniferyl (2), and sinapyl alcohols (3) (Figure 4). When it undergoes polymerization, the aromatic constituents of these monolignols are called p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) unit respectively (Figure 5) (Lewis and Yamamoto 1990).   13   Figure 4. Three types of monolignols (Buranov and Mazza 2008)   Figure 5. The aromatic constituents of monolignols after polymerization (Whetten, MacKay, and Sederoff 1998)  Nowadays, better understanding lignin biosynthesis remains an active research pursuit due to its economic relevance such as lignin valorization to higher value-added products (Ragauskas et al. 2014). Lignin is synthesized through dehydrogenation and subsequent polymerization from monolignols in plants (Boerjan, Ralph, and Baucher 2003; Henriksson 2009). Several proteins such 14  as peroxidases, laccases, and polyphenol oxidases are responsible for the dehydrogenation of monolignols to monolignol radicals. However, the exact mechanism is still unclear. Peroxidases and laccases utilize hydrogen peroxide (H2O2) and oxygen as the oxidant, respectively to oxidize monolignols to form resonance stabilized monolignol radicals (Figure 6) (Boerjan et al. 2003; Henriksson 2009). The unpaired electron picks up an electron on another monomer radicals, thereby creating new linkages on the lignin polymer, which is referred to as end-wise polymerization (Boerjan et al. 2003).   Figure 6. Resonance stabilized monolignol radicals formed by laccase and peroxidase (Henriksson 2009)  Lignin content and types vary in different terrestrial plants. Softwood generally has more lignin (15-35%) compared to hardwood (~20%) (Henriksson 2009). Softwood is composed of mainly coniferyl alcohol and some p-coumaryl alcohol, resulting in mostly G lignin and small 15  amounts of H lignin (Henriksson 2009). Hardwood consists of both coniferyl and sinapyl alcohol, and small amount of p-coumaryl alcohol, thus both S and G lignin dominate in hardwood (Henriksson 2009). In grasses, all three types of monolignols are present with a clear dominance of p-coumaryl alcohol. Therefore, all types of lignin can be found with higher amount of H lignin (Henriksson 2009). Due to the variety of lignin types in softwood and hardwood, the linkages can also vary. The most common linkage observed in lignin is the alkyl-aryl-ether linkage, called ꞵ-O-4’, which is the most susceptible to fragmentation or rearrangement during chemical pulping (discussed in detail in a later section). Condensed C-C bonds require a lot of energy input to break, thus they often survive chemical pulping. Moreover, the extra methoxyl group on S unit makes it less likely to form branching and crosslinked C-C structures due to steric hindrance. Therefore, hardwoods usually have less C-C linkages compared to softwoods. As indicated in Table 3, the proportion of common inter-linkages of lignin in both softwood and hardwoods can vary.  Table 3. Common linkages in lignin and their presence in softwood and hardwood (Henriksson 2009) Name Bonds Frequency in softwood (%) Frequency in hardwood (%) Ether bonds    ꞵ-aryl-ether ꞵ-O-4’ 35-60 50-70 Diaryl ether 4-O-5’ <4 ~7 Carbon-carbon bonds    Dihydroxy biphenyl 5-5’ 10 ~5 Phenyl coumarane ꞵ-5’ 11-12 4-9 Pinoresinol ꞵꞵ’ 2-3 3-4  16   Lignin is a by-product of the pulping/paper-making and the presence of lignin can result in the discoloration and strength impairment of the final products. Therefore, delignification is a major focus during pulping. Currently, the most prominent paper making process is kraft pulping which involves the digestion of wood chips in “white liquor” made up of caustic sodium hydroxide and sodium sulfide at elevated temperature and pressure under alkaline conditions (Sixta, Potthast, and Krotschek 2006). The cooking chemicals promote delignification and degradation of hemicellulose to produce cellulose fibers. However, hemicellulose is retained as much as possible to boost overall pulp yield. After cooking, most of the lignin is removed from the wood chips in a solution called black liquor. The degradation of lignin under alkaline pulping conditions involves the formation of an intermediate called para-quinone methide which resembles a β-aryl ether structure (Sixta et al. 2006). The intermediate is formed through the ionization of phenolic group and the elimination of aryl group at the α-position, resulting from a vinylogous β-elimination (Sixta et al. 2006). The resulting intermediate is electron-deficient and can go through three different reaction mechanisms, namely addition of nucleophiles (nucleophilic attack by hydrosulfide anion), elimination reactions, and electron transfer reactions to promote delignification (Figure 7) (Sixta et al. 2006). Black liquor is then concentrated and incinerated to provide energy to the pulp mills.  Kraft lignin could be recovered by precipitating black liquor using acid, and the lignin is typically recovered in a powder form. The recovered lignin can be soluble in alkaline solution through deprotonation of the phenolic groups. Kraft lignin is known to have high polydispersity index (PDI), a high content of phenolic hydroxyl groups and some sulfur (Henriksson 2009). 17  There are currently two well-known processes that precipitates kraft lignin by acidification, LignoForce and LignoBoost. Both processes utilize carbon dioxide to bring down the pH close to neutral to promote the coagulation of lignin (Anon n.d.; Kouisni et al. 2012). The pH is further brought down using sulfuric acid and subsequently dried to produce lignin powder. The difference between the two processes is that LignoForce utilizes oxygen to first oxidize the black liquor prior to acidification, which is reported to aid downstream filtration rate by resulting in larger lignin particles (Kouisni et al. 2012).    Figure 7. The formation of para-quinone methide and its subsequent reaction mechanisms to delignification (Sixta et al. 2006)   Lignosulfonates are typically produced via the acid sulfite process. The cooking liquor is usually made by burning sulfur with oxygen to produce sulfur dioxide which is then subsequently dissolved in water to generate sulfurous acid and further mixed with a base chemical (e.g. magnesium) (Linero 1977). Delignification occurs through sulfonation, resulting in lignin 18  solubilisation and subsequent removal. In brief, the oxygen of α-ether or α-hydroxyl group is first protonated under acidic condition to produce a resonance-stabilized benzylium cation (Sixta et al. 2006). The benzylium cation then undergoes nucleophilic addition by the bisulfite ions from the cooking liquor (Sixta et al. 2006) (Figure 8). Due to the sulfate ester formed during delignification, lignosulfonates are highly soluble in water and usually contain 3-5% sulfur.    Figure 8. The mechanism of sulfonation reaction occurred under acid sulfite process (R represents alkyl group) (Sixta et al. 2006)   Organosolv lignin is produced through solvent pulping which involves mixing water and organic solvents (e.g. ethanol or methanol) with an acid catalyst at 140-200°C (Doherty, Mousavioun, and Fellows 2011; Johansson, Aaltonen, and Ylinen 1987). During the pulping process, the more labile β-O-4 linkages are usually broken via the acidic hydrolysis pathway, resulting in the formation of ketones products, as shown in Figure 9 (Berlin and Balakshin 2014). It is often regarded as a more environmentally benign route compared to kraft and acid sulfite processes. Organosolv lignin resembles native lignin, contains no sulfur and low ash content and 19  tends to be more hydrophobic (Lora and Glasser 2002). It also has a narrow polydispersity, lower molecular weight and a lower glass transition temperature (Lora and Glasser 2002).   Figure 9. Delignification mechanism during organosolv pulping process (Berlin and Balakshin 2014)   Alkali lignin is the by-product of soda pulping, which is predominantly used with non-woody biomass such as wheat straw and corn stover (Doherty et al. 2011). Alkali lignin involves heating the substrate to 140-170°C with alkali (e.g. sodium hydroxide) and the recovered lignin contains a higher carboxylic acid content but no sulfur.  These properties are more conducive to developing higher value products (Doherty et al. 2011).  20   To fully realize the potential of a biorefinery, it is essential to effectively utilize lignin to convert into higher value products (Ragauskas et al. 2014). During the past few decades, many studies have been published that made use of the functionality and reactivity of lignin to convert them into materials, fuels, and chemicals (Kai et al. 2016; Lora and Glasser 2002). However, traditionally, lignin can be utilized without modification, such as when technical lignin is directly added to a matrix for reinforcing purpose. Technical lignin such as lignosulfonates and sulfonated kraft lignin are often used as dispersants, dust control, food additives, and resin and binder compositions (Berlin and Balakshin 2014). Phenol formaldehyde (PF) is an important type of adhesives commonly used in wood-based composites and is usually synthesized using phenol and formaldehyde (Ghorbani et al. 2016). The use of lignin as a bio-substitute to phenol has been widely explored by researchers due to its renewability and considerable lower price point (Ghorbani et al. 2016; Xu and Ferdosian 2017). The potential of raw lignin as an additive in polypropylene or polylactic acid (PLA) matrices has been explored, to improve the antioxidant properties of the resulting material (Domenek et al. 2013; Pouteau et al. 2003). Lignin can also be utilized as a flame retardant to increase the thermal degradation temperature of the final product (De Chirico et al. 2003).  Alternatively, lignin can be chemically modified and tailored to specific needs. Lignin chemical modification is usually classified into three groups, namely lignin depolymerisation to yield lignin monomers; modification to create new functional sites; and functionalization of hydroxyl groups (Laurichesse and Avérous 2014). However, it is important to understand the market and economic relevance of the modified lignin products before further pursuance. One of 21  the prime examples of lignin valorization for emerging applications is producing lignin-based carbon fibers (CF). Carbon fibers are high performance and lightweight reinforcing material that are used in the aviation, automotive and marine industries. Nowadays, poly-acrylamide-co-methylacrylate-co-itaconic acid (PAN) is the most common carbon fibre precursor, with an anticipated annual growth of 11-18% in the market share of carbon fibre (Grasselli and Trifirò 2016). However, the high production cost and unsustainable production scheme have fueled the research in producing lignin-based carbon fibre. Lignin was first investigated as a potential precursor for carbon fibre production due to its high carbon content and low cost in 1960s (Otani et al. 1969). Since then, there has been continuous technological development to improve the thermal stability and heterogeneity of lignin based carbon fibers (Baker and Rials 2013). In early 2000s, carbon fibers derived from commercially available lignin (i.e. Alcell organosolv lignin and hardwood kraft lignin) were first investigated and tensile strength around 0.5 GPa was achieved with the addition of poly(ethylene oxide) (PEO) (Kadla et al. 2002). Compared to conventionally PAN-derived carbon fibers with a measured tensile strength up to 7 GPa, the mechanical properties of lignin based carbon fibers are still lacking due to its heterogeneity and complex structure (Frank et al. 2014; Ragauskas et al. 2014). The abundancy of functional groups in addition to the aromatic ring present in lignin provides ample opportunity to transform lignin into valuable fuels and chemicals (Ragauskas et al. 2014). Moreover, lignin can be utilized as a coating due to its hydrophobic characteristics. For instance, lignin was esterified with tall oil fatty acid (TOFA) and subsequently applied as a coating layer on paperboard to enhance its barrier properties (Hult, Koivu, et al. 2013; Hult, Ropponen, et al. 2013). The coated paperboard exhibited stable contact angle up to 80° and the water vapor 22  transmission rate was reduced by four-fold (Hult, Koivu, et al. 2013; Hult, Ropponen, et al. 2013). When organosolv lignin was esterified with long aliphatic chain and applied on wood, water contact angles as high as 140° were achieved (Gordobil et al. 2017). More recently, researchers developed a pathway to hydroxyalkylate carboxylic acid and phenolic groups on softwood kraft lignin using ethylene carbonate and sodium carbonate as the catalyst (Liu et al. 2018). Consequently, greener esterification using carboxylic acids (i.e. oleic acid) without organic solvent were achieved (Figure 10). The resulting lignin esters were applied as a hydrophobic coating, where water contact angles as high as 150°C were achieved on solid wood (Hua et al. 2019; Liu et al. 2018).   Figure 10. Reaction pathway of lignin hydroxyalkylation using ethylene carbonate and subsequent esterification using oleic acid (Hua et al. 2019)   The unique hierarchical structure of cellulose, along with its outstanding mechanical properties and its renewability character has sparked interests among researchers, notably in the 23  field of nanocellulose. However, in order to broaden its utilization in different industries, surface modification of cellulose will be key in improving its compatibility with certain matrices and if we hope to add new functionality. The following section of the introduction deals mainly with the modification of micro- or nano-fibrillated cellulose, as this substrate is the main focus of the thesis. For clarification purposes, nanofibrillated cellulose implies both MFC and NFC in this thesis as films produced from MFC and NFC exhibited similar “fibrillated” network, but at the micro- and nano-scale respectively (Toivakka et al. 2014). NFC films also possessed superior mechanical properties and higher transparency as compared to MFC films. However, MFC films displayed higher oxygen and water vapor barrier ability, have been shown to be influenced by the choice of raw material and the process of producing them (Toivakka et al. 2014).   Surface modification of NFC can be categorized into two groups, namely physical interactions or adsorption and molecule or polymer grafting (Missoum, Belgacem, and Bras 2013). As an example, modified amphiphilic hemicellulose (i.e. modified galactoglucomannans) has been shown to be adsorbed onto NFC film due to its natural affinity with cellulose (Lozhechnikova et al. 2014). However, the resulting films did not show promising wetting properties and oxygen permeability when compared to the control (Lozhechnikova et al. 2014). Typically, adsorption takes advantage of the charged cellulose surface (e.g. carboxymethylated or TEMPO oxidized NFC) to attach either surfactants or polyelectrolytes to introduce new properties (Missoum et al. 2013). For example, negatively-charged NFC was prepared through 2, 2, 6, 6-Tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation and subsequently reacted with 24  different chain length of cationic surfactants (e.g. cetyltrimethylammonium bromide (CTAB)) to render NFCs hydrophobic (Xhanari et al. 2011). It was apparent that the CTAB was only adsorbed onto TEMPO-oxidized NFCs and this was attributed to the electrostatic interaction between the negatively-charged cellulose surface and positively-charged CTAB. The hydrophobicity was slightly enhanced with the water contact angle increased from 42° to 60°. Enhanced adsorption can also be achieved in polyelectrolyte solutions using carboxymethylated NFC to obtain polyelectrolyte multilayers (PEMs) (Wågberg et al. 2008). The carboxymethylation was first performed on never-dried fibers to convert the hydroxyl to carboxyl groups, prior to homogenization to produce nanoscale cellulose. The resulting charge was found to be 515 µeq/g. Silicon wafers were chosen as the substrates and anchored with a layer of polyethyleneimine (PEI) prior to the formation of multilayers using either dipping or spraying. Alternating layers of NFC suspensions and different polyelectrolyte solutions such as PEI, poly(diallyldimethylammonium chloride) (PDADMAC), and poly(allylamine hydrochloride) (PAH) were attached to the substrates and the build-up of layers was monitored using ellipsometry. It was shown that uniform and well-defined PEMs could be synthesized via this treatment and well-defined layers were observed, with potential applications such as sensor materials (Missoum et al. 2013).  An alternative approach involves chemical grafting such as molecule or polymer grafting, to create covalent bonds between the grafting agent and the hydroxyl groups on nanocellulose (Missoum et al. 2013). Some common molecule grafting reactions include esterification, etherification, and silylation, with the common intention to increase the hydrophobicity of the resulting material (Abdul Khalil et al. 2014; Thomas et al. 2018). These reactions are described in 25  the following section. A recent study has taken advantage of the Diels-Alder reaction to functionalize TEMPO-oxidized NFC, resulting in thermally reversible NFC hydrogels (Kramer et al. 2019). The carboxylic acid groups induced by the TEMPO on NFC was covalently grafted with furfurylamine to create an amine functional group. This could subsequently be reacted with a synthesized imide. The advantage of this approach is carrying out the reaction in water, and when heated above 100°C, the product can be reversibly converted back to the precursors. However, the drawback lies in the “unfriendly” chemicals used during the synthesis of modified furan-NFC and imide. Functionalizing cellulose surface with polymers has applications in increasing elasticity, stimuli-responsive materials, ion exchangers and self-cleaning devices (Thomas et al. 2018). Polymer grafting can be divided into grafting-to and grafting-from approaches (Missoum et al. 2013; Thomas et al. 2018). Grafting-to implies existing and well-characterized polymers being covalently attached to NFC with the aid of a coupling agent (Missoum et al. 2013). However, the grafting efficiency is usually limited due to steric hindrance caused by the polymeric chains (Missoum et al. 2013). The latter approach, grafting-from, involves first functionalizing the cellulose with an initiator, followed by the grafting of monomers to induce direct polymerization on the surface (Thomas et al. 2018). Previous work used cerium ammonium nitrate as an initiator on the NFC, followed by the polymerization of various acrylic monomers (Littunen et al. 2011). When these researchers assessed the grafting densities of different monomers, up to 80 wt% of the polymer was found in the resulting material. This work also revealed that the “architecture” of NFC was preserved, using atomic force microscopy (AFM), resulting in promising modification method to improve compatibility with synthetic polymer matrices. 26   The main drawback of nanofibrillated cellulose lies in the high content of hydroxyl groups which results in strong hydrogen bond interactions that limits its applications in packaging and composites due to the tendency to agglomerate. However, as shown in Figure 11, hydrophobicity on cellulose can be achieved by coating or functionalization. For example, previous work attempted to incorporate polyethylene wax on handsheets and a high water contact angle was achieved (Xu et al. 2015). Others have also looked at using naturally occurring waxes from plants to induce hydrophobicity on filter paper (Yadav, Datta, and Gour 2014).   Figure 11. Modification techniques used to introduce hydrophobicity on NFC (Thomas et al. 2018)  27  Esterification of cellulose typically involves the use of acyl chloride or anhydrides, which can be harmful to the environment. Past work has attempted to impart hydrophobicity through solvent exchange of NFC, followed by esterification using anhydrides with different carbon chain length (Sehaqui et al. 2014). This work confirmed the wetting properties of nanopaper using water contact angle, where a WCA as high as 118° was achieved. Moreover, the modified hydrophobic nanopaper had improved wet strength properties when compared to the unmodified NFC. Subsequent work attempted to introduce superhydrophobicity onto filter paper through esterification using pentadecafluorooctanoyl chloride (Nyström et al. 2006). Analysis by FT-IR confirmed the adherence of pentadecafluorooctanoyl chloride onto the surface of cellulose. However, the water contact angle of the filter paper was greatly reduced from around 150º to below 90º after 50 minutes. However, this might have been due to the inconsistency of surface coverage. A more typical example of etherification of cellulose involves carboxymethylation where an alkoxide ion is first generated on the cellulose using base, followed by nucleophilic substitution using chloroacetic acid. The carboxymethylated cellulose can be further modified via adsorption to introduce hydrophobic groups, as mentioned previously. Silylation is another technique that has been used to hydrophobize NFC. As described earlier, silylated NFC can be produced using hydrogen chloride (HCl) and isopropyldimethyl chlorosilane (IPDMSiCl) (Goussé et al. 2004). The resulting material retained the same morphology as untreated NFC and was able to disperse uniformly in solvents such as methyl oleate due to the ensuing hydrophobicity. Nevertheless, these techniques often suffer drawbacks such as tedious set-up and the use of organic solvent. More recently a novel method, using methyltrimethoxysilane (MTMS) was used to silylate NFC in water via fast freezing and 28  subsequent sublimation under low pressure (Zhang et al. 2014, 2015). This group investigated two different conditions, namely at pH 0.4 and 4 during silylation (Zhang et al. 2015). The former protocol hoped to promote the condensation of silanes and potential adsorption through hydrogen bonding with cellulose. The resulting suspension was washed with water to remove the unbounded MTMS. The latter protocol hoped to foster the adsorption or grafting of hydrolyzed MTMS to cellulose surface during the displacement of water. These workers found that NFC was coated with a firm layer of polysiloxane under the condition at pH 4, and when mixed in a polydimethylsiloxane (PDMS) matrices, the composites showed uniform dispersion and improved mechanical properties (Zhang et al. 2015). Despite being a “greener” process, this set-up is relatively difficult to scale up due to the need for fast freezing and subsequent sublimation.   Motivated by the challenges faced during the hydrophobization of NFC, this research work aimed to employ lignin to develop an economically viable, greener, and facile method to hydrophobize nanocellulose. Current literature have looked at the impact (e.g. mechanical properties, hydrophobicity) of residual lignin on nanocellulose (Chen et al. 2018; Ferrer et al. 2012; Rojo et al. 2015). In previous work, NFCs containing 0, 2, 4, and 14 wt% residual lignin were produced from SO2-ethanol-water (SEW) treated Norway spruce by microfluidization (Rojo et al. 2015). The NFC suspensions were filtered, followed by hot-pressing at 100°C and 220 bar for 2 hours to form respective films. The highest lignin-containing NFC film exhibited highest water contact angle (78°) compared to lignin-free NFC film (35°). Contrary to what was expected, the lignin content had only a negligible effect on the mechanical properties of all the resulting films, 29  which was presumed to be counteracted by the uniform distribution of lignin (Rojo et al. 2015). The oxygen permeability was found to be lower for lignin-containing films than for lignin-free films, which could be beneficial for food packaging. In related work, alkali treated bark fibers were mechanically grinded to produce NFC with residual lignin as high as 23 wt% (Nair et al. 2017). The contact angle for these lignin-containing films was around 55°. When added into epoxy matrices, the lignin-containing NFC displayed excellent dispersion and reinforcing properties, with the composite showing a two-fold increase in tensile strength compared to pure epoxy. When high lignin-content NFC (~22 wt% lignin) was produced after grinding of poplar wood (Chen et al. 2018), no significant water contact angle improvement was observed.  Recently, the American Process Inc. company (currently acquired by GranBio) patented a process, called American Value-Added Pulping (AVAP) to produce lignin-coated nanocellulose (i.e. hydrophobic nanocellulose). Thin films obtained from lignin-coated NFC resulted in a water contact angle as high as 93⁰, which was substantially higher as compared to pure NFC (~40°) (Nelson et al. 2016). It could also be freeze-dried and readily dispersed in hydrophobic polymers such as silicone. Gupta et al. (2017) reported that the addition of lignin-coated nanocrystals into PLA enhanced its interfacial interaction with the PLA matrix. This resulted in major improvements to its rheological as well as mechanical properties. The observed variation of these studies in hydrophobicity (e.g. water contact angle values) indicated that the choice of raw material, pretreatment procedure and mechanical treatment will all impact the extent of hydrophobicity of the final materials. In an alternative approach, lignin particles can be added to NFC to introduce hydrophobicity. Previous work has shown that the addition of colloidal lignin particles (CLPs) or 30  cationic lignin particles (c-CLPs) to NFC suspension could render the NFC strong and water-resistant (Farooq et al. 2019). In this work, lignin particles were stirred with the NFC suspension for two hours prior to filtration and drying. These workers found that the addition of 10 wt% of CLPs or c-CLPs on NFC resulted in the production of a NFC-lignin composite film that is nearly double the toughness of pure NFC films. Additionally, subsequent water permeability tests on the nanocomposite films showed that nearly no water was passed through for two hours.  It is recognised that exploiting the inherent properties of lignin (e.g. fluidization during heat exposure) can be an effective technique to enhance the hydrophobicity of NFC. Lignin is known to change its structure and molecular weight around and above its glass transition temperature (Tg), often causing fluidization (Cui et al. 2013). Organosolv lignin has been reported to exhibit flow-like properties when heated (Lora and Glasser 2002). Several research groups have investigated the changes in lignin structure using milled wood lignin as it was reported to most resemble native lignin (Brosse et al. 2010; Kim et al. 2014). In one study, beech boards were heat-treated to 230°C for 7 hours and the lignin was extracted before and after heat treatment (Brosse et al. 2010). 13C and 31P NMR showed that ꞵ-O-4 linkages in the lignin were cleaved during thermal treatment, followed by lignin recondensation. Recent work has looked at preparing wood-lignin composites by mixing poplar particles and alkaline lignin, followed by hot pressing (Zhang et al. 2017). These researchers observed, using 2D-NMR that, at temperature above 130°C, there was a decrease in ꞵ-O-4 bonds with a concomitant increase in ꞵ-ꞵ bonds (i.e. “hydrophobic” bonds). Thus it appeared that the changes in lignin structure during heat treatment might be beneficial in improving the hydrophobicity of this material. In other work, where pine blocks were heat-treated at 240°C for 8 hours, the water contact angle increased from 55° to 81° (Gérardin et al. 2007).  31  When lignin containing NFC films were treated by heat and pressing (i.e. hot pressing) this resulted in smoother and more hydrophobic NFC films. For instance, hot-pressing lignin-containing NFC films allowed lignin to soften, thus filling the voids of the fibrillated network (Rojo et al. 2015). Other workers have tried to improve the compatibility between cellulose fibers and hydrophobic polymers by producing NFC using chemi-thermomechanical pulp (Abe, Nakatsubo, and Yano 2009). These workers reported the plasticization of lignin when the film was hot-pressed at 180°C, resulting in a glossy surface. Another recent study attempted to produce lignin-containing NFC from unbleached softwood kraft pulp via TEMPO oxidation and subsequent microfluidization (Imani et al. 2019). The film was hot-pressed at 85°C at 220 bar for 40 minutes prior for further characterizations. They hypothesized that hot pressing helped to “melt” the amorphous lignin, successfully filling the voids between fibrils, creating a smooth surface with increased hydrophobicity when compared to the control (~60°). More recently, inspired by the nature of lignin to hold plants together, a research group successfully produced a cellulose-based paper by incorporating lignin into cellulose, achieving enhanced tensile strength (~200 MPa) when compared to conventional cellulose paper (~40 MPa) and a hydrostable cellulosic material (Jiang et al. 2019). In this work, cellulose paper was first swelled in a sodium hydroxide solution, followed by successive infiltration in acetone-soluble lignin. This was followed by hot pressing, where the optimum temperature was found to be around 100-130°C (Jiang et al. 2019). These workers proposed that, when hot-pressed at around the glass transition temperature of the lignin, crosslinking reactions occurred which subsequently dispersed into the cellulose matrix, acting as a reinforcing agent (Jiang et al. 2019). The resulting lignin-32  cellulose paper was stable in water up to 7 days, reaching a stable contact angle of 60° for 45 minutes.   This section focuses on the common characterization techniques used to study hydrophobicity of cellulosic material, specifically for packaging applications. It is estimated that the paper-based packaging industry will grow from USD 64 billion to 82.4 billion by 2023 as a result of the proliferation of e-commerce (Anon 2018a). Currently, the packaging industry utilizes materials such as non-renewable petroleum-derived plastics, glass, aluminium and tin. Thus, there is a tremendous pressure to develop alternative, sustainable packaging. With this goal in mind, this section outlines wettability techniques, followed by a description of the methods used to examine the barrier properties of the final products.  Wettability specifies the extent of wetting between a solid and liquid surface and is most often quantified through the water contact angle measurement (Yuan and Lee 2013). Contact angles that are higher than 90°, indicate a low wettability and usually referred to materials that are hydrophobic (Mirvakili 2018). The definition of contact angle refers to the resulting angle between the tangent lines of the liquid-vapor interfaces and solid-liquid interfaces, as shown in Figure 12 (Mirvakili 2018).  33   Figure 12. Illustration of water contact angle that represent hydrophilic and hydrophobic surfaces (Yuan and Lee 2013)  The surface tension of a liquid and external forces (i.e. gravity) are responsible for the resulting shape of the liquid droplet on the solid surface (Yuan and Lee 2013). In an ideal bulk liquid, each molecule interacts with their neighboring molecules, where no preferential affinity takes place, resulting in a net force of zero. However, the net force is not zero when the liquid is dropped on a solid surface because part of the molecules is exposed to the solid’s surface, resulting in disparate distribution of intermolecular forces. Therefore, the liquid molecules are contracted inward (shown in Figure 13) to minimize its surface area to achieve lower surface free energy (Yuan and Lee 2013). Theoretically, the contact angle is represented by the equation below, first described by Thomas Young in 1805, where θY refers to Young’s contact angle; γsv, γsl, and γlv represent the surface tension exerted by the solid-vapor, solid-liquid, and liquid-vapor interfaces respectively (Young 1805). 𝛾𝑙𝑣 cos 𝜃𝑌 = 𝛾𝑠𝑣 − 𝛾𝑠𝑙   Figure 13. The intermolecular forces of water molecules at solid-liquid interfaces that result in the formation of liquid droplet 34   Good packaging materials usually possess sufficient barrier against water vapor and oxygen. NFC films can be employed in packaging applications as they have shown relatively decent barrier properties (Ferrer, Pal, and Hubbe 2017). The ability of water vapor to penetrate through a barrier is often quantified by using the water vapor transmission rate for cellulosic material. In brief, the water vapor transmission rate is defined as the mass of water vapor transmitted per unit time per unit area from one face of the material to the other, under specified steady conditions (as described by TAPPI 448 om-97). A higher transmission rate implies a lower barrier ability, which is not favored if the intended use is as packaging material. The oxygen barrier performance can be also evaluated from the oxygen transmission rate. NFC films generally display low oxygen permeability due to the disorganized entanglements exhibited by NFC, resulting in a tortuous diffusion path (Ferrer et al. 2017).   The primary aim of the thesis work was to develop an inexpensive and more environmentally friendly way of using industrial lignin to hydrophobize MNFC. The thesis is organised into two chapters with the first chapter focusing on precipitating softwood kraft lignin onto different cellulosic substrates. We hoped to assess the preference of lignin deposition on the “fibrillated” MNFC network as compared to regular pulps, hopefully resulting in improved hydrophobicity. The second chapter assessed hot-pressing as a potential method to enhance lignin distribution on the MNFC’s surface to try to increase the hydrophobicity of the resulting paper sheets. Each chapter is summarised below. 35  Chapter 1: Can lignin can be used to impart hydrophobicity on cellulose surfaces and does MNFC have the potential to deposit more lignin due to its highly fibrillated network, resulting in hydrophobic paper sheets? Softwood kraft lignin was chosen due to its commercial availability. As demonstrated at an industrial scale, kraft lignin is soluble in alkaline solutions and can be further precipitated to form agglomerates during acidification. We first optimised the lignin dosage added to MNFC by determining the resulting water contact angles. Once the optimum lignin amount was determined, the second phase involved loading the same amount of lignin onto different cellulosic substrates that included MNFC, softwood and hardwood kraft pulps (coarser fibers). The water contact angles for all substrates were assessed. Lignin retention was quantified using the Klason method and the morphology of the resulting paper sheets was assessed using SEM.  Chapter 2: Can hot pressing lignin-containing MFC sheets further enhance the hydrophobicity of these materials? We initially hot-pressed the lignin-containing handsheets (and the controls) produced from Chapter 1 at varying temperature for 20 minutes. As kraft lignin is known to change its structure at around 120°C, an initial temperature of 130°C was chosen, followed by incremental 30°C increases until 190°C. The water contact angle was measured every 30 seconds for a total of 150 seconds and the best condition was chosen. The preliminary experiments with an additional hot pressing showed promising results based on the water contact angle data. However, with the precipitation technique, the drainage time increased when lignin was present in the slurry and it was difficult to control lignin distribution on fiber’s surfaces. Therefore, we assessed if spray 36  coating lignin with subsequent hot pressing could be a potential method to ensure homogenous lignin distribution. Two types of lignin, softwood kraft lignin and hardwood organosolv lignin, were used and dissolve in 60/40 wt% ethanol/ water. The solutions were sprayed onto MNFC paper sheets and dried at room temperature before hot-pressing. Different pressing temperatures were assessed, based on lignin types, and the resulting water contact angle data recorded. Further characterization such as water vapor transmission rate, FTIR, SEM and tensile properties were also determined. 37    Northern bleached softwood kraft pulp (SWKP), hardwood kraft pulp (HWKP), and unbleached softwood kraft pulp (UBSWKP) were received as sheets from an industrial source in British Columbia. The sheets were disintegrated at a 2% consistency in a pulp disintegrator at 3000 rpm for 15 minutes. The higher lignin content softwood kraft pulp (UBSWKP-hi) was produced using a mixture of softwood chips at 1:4 chips: liquid ratio, 30% sulfidity and 18.5% active alkali, with H factor=500 at 160°C in a pulp digester. The pulps were thoroughly washed and disintegrated before refining. After pulp refining, the total pulp yield was 43.3%, where 41.8% and 1.5% were refines and rejects respectively. Micro/nanofibrillated cellulose (MNFC) with a 98% moisture content was provided courtesy from Performance BioFilaments. The production scheme for MNFC followed the previous work of  (Gourlay et al. 2018). In brief, NBSK was passed through a refiner at high consistency (~30 wt% solids) multiple times, resulting in final material that consists of ~50 wt% larger fibers with diameter of 20-30 µm, and 50 wt% of micro/nanofibrillated material with diameter below 1 µm. All reactions described later were run at 1 wt% of pulp consistency. Industrial softwood kraft lignin (Indulin-AT) and hardwood organosolv lignin were supplied by WestRock Company and Fibria Innovations respectively.   Lignins were dried in a 100°C oven and the dry weight was measured. Once dried, lignins were taken out and left in the fume hood at room temperature. The weight was measured for each lignin after being left overnight to assess the tendency of kraft and organosolv lignin to absorb 38  moisture. The following equation was employed to calculate the moisture uptake for each type of lignin, where Wwet and Wdry represented the wet weight and dry weight of lignin, respectively. 𝑀𝑜𝑖𝑠𝑡𝑢𝑟𝑒 𝑢𝑝𝑡𝑎𝑘𝑒 =𝑊𝑤𝑒𝑡 − 𝑊𝑑𝑟𝑦𝑊𝑑𝑟𝑦× 100   Fiber length was calculated using a high-resolution fiber quality analyzer (FQA). The minimum fiber count per sample was set at 10,000 and the range of mean length was between 0.05 to 10 mm. The numbers reported here were length-weighted (Lw).    This method was applied to the SWKP, HWKP, and MNFC used in Chapter 1. 20 g of softwood kraft lignin (SWKL) was first dissolved in 0.1M sodium hydroxide solution (lignin concentration of 20 g/L) and stirred overnight. For the SWKP and HWKP, 1.2 g dry weight of pulps was mixed with 12 g of lignin solution (20% of lignin based on dry weight of pulps) for an hour at a 1 wt% pulp consistency. For the MNFC, 0.4 g dry weight of pulp was mixed with 4 g of lignin solution for an hour at a 1 wt% pulp consistency. After an hour, the lignin slurry was precipitated dropwise using 64 wt% sulfuric acid solution until the pH reached 3. The slurries were left overnight prior to forming the handsheets. Controls without lignin, including unbleached controls, followed the same procedure as described above. Handsheets were prepared according to TAPPI 205 sp-02. In brief, the slurry was poured into a handsheet maker that had around 1 L water 39  inside and subsequently filled up with 7 L of water. The slurry was agitated to ensure perfect mixing using a propeller in the cylinder and the suspension was drained to form a handsheet. After draining, the sheet was collected by putting a few blotter papers on top of the sheet and pressing it with a couch roll to absorb the water. The handsheet was pressed at 345 kPa twice for 330 seconds and 150 seconds, and further dried in a controlled room (50% RH, 23°C). For some experiments, the dry handsheets were further hot-pressed using a Carver bench top hot presser at 345 kPa for 20 minutes at different temperature as specified in the thesis later on.  Table 4. Concentration of cellulose-lignin slurry for each substrate based on 20% lignin loading (UBSWKP: unbleached softwood kraft pulp; UBSWKP-hi: high lignin content unbleached softwood kraft pulp; SWKP: softwood kraft pulp; HWKP: hardwood kraft pulp; MNFC: micro/nanofibrillated cellulose) Samples Control  Lignin Dry pulp (g) Total sln (g)  Dry pulp (g) Lignin sln (g) Total sln (g) UBSWKP 1.2 120  - - - UBSWKP-hi 1.2 120  - - - SWKP 1.2 120  1.2 12 120 HWKP 1.2 120  1.2 12 120 MNFC 0.4 40  0.4 4 40   In Chapter 2, spray coating was applied to the MNFC sheets. As shown in previous work, softwood kraft lignin and hardwood organosolv lignin were dissolved in 60/40 wt% ethanol/ water to achieve highest solubility (Goldmann et al. 2019). The solutions were centrifuged at 5000 rpm 40  for 20 minutes, after which the supernatant was stored for future use. The final concentration for both lignin solutions was 14 g/L. The lignin solution was sprayed on the surface of the sheets using a spray gun at a distance of 15 cm (Hua et al. 2019). The coating procedure was repeated 5, 10, and 20 times for different experiments. The sheets were dried under room temperature prior to hot pressing. Hot pressing was performed as described above. After hot pressing, the sheets were dried in a controlled room (50% RH, 23°C).    For the acid precipitated lignin experiments, the amount of lignin on the handsheets was quantified via the Klason method (Sluiter et al. 2010) with slight modification. The handsheets were cut into pieces before adding sulfuric acid. The substrates were mixed with 72 wt% sulfuric acid with rigorous mixing for two hours. All of the handsheet pieces were observed to be fully dissolved prior to autoclaving. Insoluble lignin was quantified through mass balance and acid soluble lignin were quantified through UV-Vis spectroscopy. The sugar content was assessed using HPLC.  For the spray coating experiments, the amount of lignin was quantified using mass balance. The coated lignin was measured by taking the weight difference of the sheets before and after the treatment, as denoted by the equation below. 𝑊𝑐𝑜𝑎𝑡𝑒𝑑 𝑙𝑖𝑔𝑛𝑖𝑛 =𝑊𝑓𝑖𝑛𝑎𝑙 − 𝑊𝑖𝑛𝑖𝑡𝑖𝑎𝑙𝑊𝑓𝑖𝑛𝑎𝑙 × 100  41   The thickness of handsheets were calculated using the L&W micrometer according to TAPPI 411 om-97. Each sheet was measured five times to obtain the average thickness value.  The tensile indexes for handsheets were measured using the L&W tensile strength tester according to TAPPI 494 om-01 with a load cell of 200 N.   The water contact angles of handsheets were determined using the sessile drop method with an optical contact angle meter on the wire-side of the handsheet (CAM 101, KSV Instruments Ltd.) (van der Zwan 2018). 5µL of distilled water was added onto the surface using a syringe and the image of the first stabilized sessile drop was taken for contact angle analysis using the KSV CAM software (v 3.99). Five measurements were taken for each sheet. For the time-dependent contact angle experiments, the image was taken every 30 seconds for a period of 150 seconds.   A Wyko NT1100 Optical Profilometer was used to measure the roughness of handsheets. The VSI scan was chosen as the measurement option and the modulation threshold was set at 0.001%. The values were analyzed using Vision software (Veeco Instruments Inc.) and the average roughness value (Ra) was obtained in triplicate (Mirvakili 2018).  42   To study the lignin morphology on the handsheets, a Hitachi S2600 Variable Pressure Scanning Electron Microscopy (SEM) was used. Samples were freeze dried and placed on a SEM stub prior to being gold coated using a Cressington 208HR High Resolution Sputter Coater. Observations were performed at 10 kV accelerating voltages and 15 mm working distance. In some experiments, field emission scanning electron microscopy (FESEM) (Carl Zeiss NTS Ltd.) was used. In this case, observations were performed between 5-15 kV and ~4 mm working distance.    The XPS measurements were carried out using a Leybold Max 200 X-ray photoelectron spectrometer (Cologne, Germany) with a monochromated Al Ka X-ray source (van der Zwan 2018). Handsheets were vacuum dried prior to the measurements. The theoretical surface lignin coverage (TSLC) was calculated based on the oxygen/ carbon ratio, as described in previous work (Laine et al. 1994). The O/Csample represented the O/C ratio of the analyzed sample, while O/Ccellulose and O/Clignin were 0.83 and 0.33 respectively. ∅𝑙𝑖𝑔𝑛𝑖𝑛 =(𝑂/𝐶𝑠𝑎𝑚𝑝𝑙𝑒 − 𝑂/𝐶𝑐𝑒𝑙𝑙𝑢𝑙𝑜𝑠𝑒)(𝑂/𝐶𝑙𝑖𝑔𝑛𝑖𝑛 − 𝑂/𝐶𝑐𝑒𝑙𝑙𝑢𝑙𝑜𝑠𝑒)   The presence of lignin on handsheets were analyzed by FTIR transmission. Each sample was dried in a 40°C vacuum oven prior to IR measurements. The spectra were recorded using Spectrum One FTIR Spectrometer (PerkinElmer instrument), in which 32 scans with 4 cm-1 43  spectral resolution were collected. The spectra were normalized at 1060 cm-1 since the ether group was chemically inactive during the reactions. (Hu et al. 2017).   The WVTR of handsheets were measured at 23°C and 50% RH according to TAPPI 448 om-97 with slight modification. In brief, 7 cm diameter of mason jars were used and filled with calcium chloride desiccant that has been dried in 100°C oven at least overnight. The cut handsheets were sandwiched between the jar and jar band. To ensure perfect sealing, the outer band was sealed with extra parafilm sealing film. The weight of the entire set up was weighed every 24 hours until the gained weight was constant.   The air permeability of handsheets was measured according to TAPPI 460 om-02, where the time required for 100 mL of air to pass through the sheets at a pressure of 1.22 kPa was measured. Handsheets were pre-conditioned at 23°C and 50% RH prior to all measurements. Triplicate was performed for all handsheets.   The stability of the control and lignin-incorporated handsheets in water were evaluated according to previous work, with slight modification (Farooq et al. 2019). 5×5 cm pieces were cut from the handsheets and placed in a beaker with 15 mL of water. After 24 hours of continuous mixing at 150 rpm in the incubator, intact handsheets were collected and the leftover solutions 44  were saved for UV measurements. The same amount of sodium hydroxide solution was added to all solutions to ensure all the lignin that had leached out had been dissolved. To evaluate the leaching of lignin, absorbance readings at 280 nm were recorded using a UV-Vis spectroscopy. 45     Producing the hydrophobic MNFC will help increase the potential of packaging products as well as improving this materials compatibility with hydrophobic matrices. As described earlier, common hydrophobization techniques for cellulose often involve expensive chemicals, complex chemistry and tend to be environmentally unfriendly. Therefore,  using lignin to make hydrophobic MNFC sheets has potential, especially if we can use kraft lignin, as about 70 million tons of kraft lignin are available worldwide annually (Ragauskas et al. 2014). However, the literature regarding incorporation of lignin onto NFC is scarce. One of the few papers, described in the introduction, assessed a variety of colloidal lignin particles that were subsequently mixed with NFCs. This resulted in strong and ductile nanocomposite films (Farooq et al. 2019). However, the potential utilization of raw and commercially available lignin (i.e. kraft lignin) to make hydrophobic NFC sheets has not yet been studied. During the kraft pulping process the black liquor, which mainly composed of lignin, is precipitated by acidification to aid in lignin removal. Lignin is known to exhibit colloidal behavior in alkaline solution (Lindström 1980). The phenolic and carboxyl groups on lignin dissociates when the pH is higher than their pKa, resulting in repulsive electrostatic forces that stabilize lignin colloids in alkaline solution (Lindström 1980; Zhu 2013). Aggregation of kraft lignin macromolecules is known to occur when the pH drops below the pKa of phenolic or carboxyl 46  groups. Other parameters include the ionic strength of the solution and temperature (Norgren et al. 2001; Zhu 2013). The aggregation is attributed to the domination of attractive forces, such as van der Waals force and hydrophobic interactions, over repulsive electrostatic force. Lignin has been reported to precipitate on fiber surfaces. For example, when other workers investigated if a decrease in pH aided lignin precipitation from softwood kraft pulp slurry from the second oxygen delignification stage (Koljonen et al. 2004), they observed a slight increase in the amount of residual lignin in both the pulp and the resulting handsheets when pH was decreased from 5 to 3. The lignin which had precipitated on the fibers showed a granular structure, in accordance with other studies (Gilli et al. 2012; Maximova et al. 2001; Schmied et al. 2012). The addition of a cationic polyelectrolyte (e.g. PDADMAC) onto cellulosic fibers was also reported to promote lignin adsorption onto the fiber due to the opposite surface charge between the anionic lignin and cationic fibers (Maximova et al. 2001). In the work reported here, softwood kraft lignin was deposited onto surfaces of different cellulosic substrates.  We initially wanted to determine the optimum lignin loading onto MNFC and once the optimum lignin loading was determined, the second phase investigated how well the lignin was retained on different cellulosic substrates. Last but not least, the paper sheets were evaluated and characterised.   Optimization was based on the resulting water contact angle when varying amounts of lignin were loaded, based on the dry weight of MNFC (0.4 g), namely 1%, 5%, 10%, 20%, and 50%. 47  An amount of 0.4 g dry weight of MNFC was used throughout the experiments, and was chosen due to the smaller fiber sizes of MNFC, resulting in longer drainage time and the possibility of clogging if more than 0.4 g was loaded. As indicated below (Table 5) the concentration of MNFC and lignin used and the amount of lignin retained on MNFC sheets varied. As indicated in Figure 14, the lignin-MNFC slurry before and after acid precipitation and the resulting lignin-containing MNFC sheet varied in colour.  Table 5. Concentration of MNFC and lignin used for the optimization experiment Samples MNFC dry wt. (g) Lignin added (g) Total sln (g)  Final lignin content (%) Control 0.4 - 40  0.8 L-1%  0.4 0.004 40  1.8 L-5% 0.4 0.02 40  6.0 L-10% 0.4 0.04 40  3.8 L-20% 0.4 0.08 40  8.4 L-50% 0.4 0.2 40  12.1   Figure 14. From left to right: MNFC slurry mixed together with 20% lignin dissolved in alkaline solution; lignin-MNFC slurry after the pH was brought down to 3; resulting MNFC sheets loaded with 20% lignin; pure MNFC sheet  48  As the cellulose and lignin slurry were filtered together at the same time, the amount of lignin retained in the final paper sheets could vary, due to the differing affinity of physical retention of precipitated lignin particles and cellulose. Lignin retention on the resulting paper sheets was likely due to imbibition of lignin into fiber pores, as demonstrated in earlier work where it was precipitated because of filtration and agglomerated lignin was trapped within the paper sheets (Maximova 2004). In addition, as more lignin was loaded, the drainage time increased, was likely due to the physical blocking caused by lignin particles and cellulose to the handsheet mesh screen. When a determination of the mass balance was used to calculate the amount of lignin adsorbed, based on the amount of lignin added, about 35-50% of lignin was retained based on the amount of lignin added, for the 10%, 20%, and 50% conditions. The L-5% component had a higher lignin content when compared to L-10% despite having a higher lignin loading. This experimental error was likely due to undissolved paper pieces in the substrate or it could be due to the detection limit for the Klason assay as the same could be observed for the control and the 1% sample, where the final lignin content tended to appear higher. When doing a Klason assessment of the MNFC sheets, it was important to ensure that the cut pieces were fully dissolved prior to autoclaving The initial water contact angle was found to increase with the increasing lignin content of the resulting MNFC sheets, resulting in improved hydrophobicity (Figure 15 (a)). However, the water contact angle (~70°) plateaued when the final lignin amount exceeded 4%, except for L-5% (55°). This seemed to be an outlier, as explained earlier. A lignin loading of 20% was chosen based on these results as L-20% produced a more homogenous paper sheet when compared to L-10%. Subsequent experiments were run using a 20% lignin loading. After the water contact angle was 49  determined, we wanted to investigate if the tensile properties were impaired due to the presence of lignin.  The tensile properties of a film or paper sheet can be interpreted in different units, such as tensile strength (often has units in MPa or GPa) and tensile index (kNm/kg). The tensile strength is an indication of the maximum force that could be withstood by a test material before it ruptures and is often used in the field of material sciences, where other mechanical measurements such as Young’s modulus is also carried out. The tensile index is a more common method used in the pulp and paper sector, as it is essentially tensile strength that is based on a grammage basis (weight ÷ cross sectional area of paper). The tensile strength can be obtained by multiplying tensile index by the density of the material. In the work reported here, the tensile properties were described using the tensile index as the thickness (ranging from 26 to 32 µm) of the paper sheets were of similar values. It was shown in Figure 15 (b) that the tensile index of the MNFC paper sheets did not differ greatly with values at about 75 kNm/kg. When ANOVA analysis was tested in Excel, it resulted in p-value > 0.05, implying no significant difference across different samples. The tensile indexes obtained were slightly lower when compared to literature values, which usually ranged between 80-150 kNm/kg (Ferrer et al. 2012; Syverud and Stenius 2009). This could be due to some of the finer fibrils being lost during the filtration process. It was also apparent (Figure 15 (b)) that the amount of lignin retained on the paper sheets had a negligible effect on its tensile index. This was unexpected as it is often reported that residual lignin often negatively interferes with the mechanical strength of paper, especially during the papermaking process. However, this confirmed that lignin deposited on MNFC was weakly bound, and consequently not able to disrupt the hydrogen bonding between cellulose. Previous work on unbleached NFC films that contained 3% 50  residual lignin also concluded that no significant effect was observed on the tensile properties of lignin-containing NFC films compared to pure NFC films (Ferrer et al. 2012). These workers attributed this result to the plasticizing effect caused by lignin, resulting in better adhesion capacity between the fibers and lignin. However, how the lignin was incorporated into fiber pores or bound to fibers was not comprehensively studied. Additionally, spatial localization of lignin on fibers was poorly controlled, making it more challenging to draw any firm conclusions (Farooq et al. 2019).  Figure 15. Initial water contact angle (A) and tensile index (B) of the control (gold-filled marker) and lignin-containing MNFC sheets based on the amount of lignin content (from left to right: control, L-1%, L-10%, L-5%, L-20%, L-50%)   As mentioned earlier, a 20% lignin loading was chosen as the best case and was thus used in all subsequent cases. As supported by the FQA results, the softwood kraft pulp (SWKP) had 51  longer fibers when compared to the hardwood kraft pulp (HWKP) and the MNFC, with the weighted length being 2.2 mm, 0.63 mm, and 1.24 mm respectively. It is important to understand the potential limitation of using FQA to measure MNFC fibers, as some finer fibrils (i.e. nanoscale fibers) are beyond the detectable limits for FQA, consequently resulting in a greater length for MNFC. The fiber width in the SWKP had a relatively uniform distribution, resulting in an average width of about 25 µm. However, this was not observed with the MNFC substrate as the original fibers were often irregularly split into smaller fibrils, making it difficult to quantify the width of the fibrillated cellulose. The SEM images (Figure 16) indicated the difference between SWKP and MNFC fiber morphology with the MNFC showing a much higher surface area. Thus, it was likely that, due to the greater surface area of the MNFC, more lignin was deposited on the surfaces due to its fibrillated network, conferring greater hydrophobicity. To try to test this hypothesis, we first compared a variety of cellulosic substrates, including SWKP, HWKP and MNFC, to assess how the coarser fibers (SWKP and HWKP) performed when compared to the finer and more fibrillated fibers (MNFC).    Figure 16. SEM images of the fiber morphology of SWKP (left) and MNFC (right) 52   The fiber consistency was slightly modified for the SWKP and HWKP after processing by the standard procedure published by TAPPI T 205 sp-02. Therefore, the dry weight of SWKP and HWKP fibers loaded was 1.2 g while the amount of lignin loaded was 20% based on the dry weight of the fibers. The other conditions for the MNFC remained the same as described in the previous section. As summarised in Table 6, the SWKP, HWKP and MNFC controls contained 0.4%, 0.9%, and 0.8% lignin, likely due to the residual lignin remaining after the pulping process. As expected, when additional lignin was added, the more fibrillated network of the MNFC allowed more lignin to be deposited on its surface (~8%), when compared to the SWKP and HWKP substrates, which showed minimal lignin coverage (~1%). As indicated in Figure 17 (a), the amount of lignin that was trapped on the paper sheet showed that most of the lignin was washed off the SWKP and HWKP substrates during the papermaking process, resulting in less than 5% of lignin retention for both cases, comparing to MNFC that achieved roughly 40% of lignin retention. When the initial water contact angle was determined (Figure 17 (b)), the lignin-loaded substrates did show an increase in the water contact angle while the water contact angle for SWKP, HWKP, and MNFC controls were 28°, 30°, and 32° respectively. Surprisingly, the SWKP and HWKP substrates showed drastically higher contact angles (between 90° and 100°) when compared to the MNFC (around 60°), implying greater hydrophobicity despite having a lower lignin content when compared to the MNFC (Figure 17 (b)). As described in Section 1.6.1., Young’s equation was used to measure static water contact angle and it gives approximate values on solid surfaces that are relatively smooth, flat, and homogenous (Chau et al. 2009). However, different equations, such as the Wenzel and Cassie equation, can also be used to measure superhydrophobicity on solid surfaces that are rougher and more heterogeneous (Chau et al. 2009). As described in this earlier work, the water 53  contact angle values can be affected by surface roughness, heterogeneity, particle shape and size (Chau et al. 2009). Consequently, the observed changes in the contact angle were also likely influenced by the observed variation in surface roughness of the different cellulosic substrates, the heterogeneity of surfaces as well as the porous nature of the cellulosic materials.  Table 6. Conditions employed for three different cellulosic substrates and the final lignin content calculated by Klason on each paper sheet (“C” and “L” denote the control and 20% lignin loaded substrates respectively) Samples Dry wt. of fibers (g) Lignin added (g) Total sln (g)  Final lignin content (%) SWKP-C 1.2 0.24 120  0.4 SWKP-L  1.2 0.24 120  1.0 HWKP-C 1.2 0.24 120  0.9 HWKP-L 1.2 0.24 120  1.2 MNFC-C 0.4 0.08 40  0.8 MNFC-L 0.4 0.08 40  8.4   Figure 17. Lignin content (A) quantified through Klason for different substrates and its resulting initial water contact angle (B) corresponding to its lignin content 54    Figure 18. The initial contact angle of lignin-containing SWKP, HWKP, and MNFC sheets   The paper sheets were held together by a random and unaligned network of fibers, which likely contributed to variations in surface roughness when different type of fibers were used. Generally, rougher surfaces resulted in higher water contact angle. In order to quantify the roughness of the different cellulosic substrates, an optical profilometer was used, which gave calculated average roughness value for each paper sheet and its roughness profile. The roughness profile in Figure 19 was illustrated in the variation of colors, as shown in the scale bar on the right. As seen below, it was easier to spot individual fibers on SWKP and HWKP, whereas MNFC showed a relatively smooth surfaces. The average roughness values for the SWKP and HWKP were also higher when compared to the MNFC. P-value < 0.05 was obtained when ANOVA was performed, indicating significant difference on the roughness values of SWKP, HWKP, and MNFC. The lower roughness value and the abundancy of surface area in the MNFC likely resulted in the decrease in water contact angle. This was in agreement with previous work when kraft pulps were refined to produce finer and smaller fibers ranging from ~150 µm to ~900 µm in fiber sizes (Mirvakili 2018). All of the unrefined and refined fibers were hydrophobically treated by vapor phase silanization using dimethyldichlorosilane at 75°C for 15 minutes. It was found that the contact angle of unrefined samples were much higher (~140°) compared to the highly refined sample (~95°). The 55  observed higher contact angle was attributed to the consolidated effects of greater surface roughness and surface chemistry (Mirvakili 2018). In addition to the higher surface roughness observed on the SWKP and HWKP substrates, the more uniform lignin coverage on the regular pulps could also be advantageous to the contact angle measurements. As lignin appeared to act as a hydrophobic agent, the better lignin coverage on the surface resulted in less exposure of the hydroxyl groups on cellulose, consequently leading to a high contact angle. As illustrated on the SEM images (Figure 20), big patches of lignin precipitation could be observed on the MNFC substrates, but they were not observed on the SWKP material. The production of thinner MNFC paper sheets, combined with the higher drainage time during the papermaking process might also be responsible for the relatively heterogeneous lignin deposition or trapping on fibers. As noted earlier, it was more difficult to spatially control lignin precipitation, leading to a lower contact angle. For comparison the thickness of the SWKP, HWKP, and MNFC summarised in Table 7 where the thickness of SWKP and HWKP is shown to be about four times more than the MNFC sheets. As mentioned earlier it was likely that the porous nature and large surface area of the pulps, even more evident with MNFC, was another contributing factor to the observed low hydrophobicity. It was also likely that the uniform distribution of the lignin on the cellulose surface played a more important role than the amount of lignin to confer greater hydrophobicity. It is also worth noting that it was beneficial to add lignin onto cellulose in conferring hydrophobicity. When high lignin content (10.1 ± 0.2%) unbleached softwood fibers were produced, the measured water contact angle was 69°, which was roughly 20° lower than the lignin-loaded SWKP fibers. 56   Figure 19. Roughness profile (left to right: 2D, 3D) and average roughness value (Ra) obtained from optical profilometer for the lignin-loaded samples (A) SWKP; (B) HWKP; (C) MNFC  57     Figure 20. SEM images showing big and non-uniform patches of lignin on MNFC paper sheets   Table 7. Thickness of the resulting paper sheets from different substrates Substrates Thickness (µm) Control Lignin SWKP 130.4 125 HWKP 131.2 128.1 MNFC 28.6 31.8  To compare the mechanical properties of the different cellulosic substrates, we measured the tensile index, on a grammage basis, as explained in Section 3.1.2. It was apparent that the SWKP control and its respective lignin-loaded samples showed some differences, as supported by ANOVA test with p-value < 0.05. This could be due to the higher density (Table 8) of lignin-loaded samples. However, between the different substrates, we could clearly see that the tensile properties of the MNFC was about 2.5 times higher when compared to the SWKP and HWKP material. This was due to the denser network of the MNFC (Figure 21, Table 8). The superior tensile strength of MNFC, in addition to its lightweight properties have been shown to be beneficial for packaging 58  applications (Li, Mascheroni, and Piergiovanni 2010). Therefore, one of our subsequent goals was to try and improve the hydrophobic properties of the MNFC.  Figure 21. Tensile index for control and lignin-loaded samples among different substrates  Table 8. Density and tensile index for SWKP, HWKP, and MNFC for both control and lignin-loaded samples Substrates Density (kg/m3)  Tensile index (kNm/kg) Control Lignin  Control Lignin SWKP 499.5 530  28.6 36.5 HWKP 511.5 518.4  19.1 21.1 MNFC 761.4 738  75.2 76.1  However, the major bottleneck that was encountered was that the water sank into the fiber pores after a few seconds, despite it showing an initial, high contact angle. To try to enhance the hydrophobic properties of MNFC, without neglecting its excellent mechanical properties, we 59  wanted to improve the lignin coverage on the MNFC surface. We hoped that this would result in less exposure of the porous surface, providing better stability for the water contact angle measurements.  This work showed we could physically trap lignin on pulp fibers via acidification to enhance hydrophobicity. The optimum lignin loading was found to be 20% based on the weight of dry fibers. As anticipated the “fibrillated” and more entangled network of micro/nanofibrillated cellulose (MNFC) was able to achieve greater deposition of the lignin when compared to regular pulps (e.g. softwood and hardwood kraft pulp). However, the contact angle for the MNFC paper sheets was lower when compared to the SWKP and HWKP paper sheets. It appeared that the observed changes in the water contact angle was the combined effect of surface roughness, the heterogeneity of surface lignin coverage, and the intrinsic, porous nature of the cellulosic material. However, the tensile properties of the MNFC were roughly 2.5 times higher when compared to the regular pulps, thus showing considerable potential for products such as packaging.         60    It was apparent that incorporating lignin onto the MNFC handsheets increased their hydrophobicity to a certain extent. However, not as much as we would like. The rationale behind assessing hot pressing of lignin-containing MNFC sheets was to better distribute the deposited lignin. This would, hopefully, result in improved lignin homogeneity on the surface and reduced porous network, consequently enhancing hydrophobicity. Previous work has looked at the difference in water permeability and mechanical properties between air-dried and hot-pressed lignin NFC films (Farooq et al. 2019). In this work the workers produced air-dried and hot-pressed (1.8 kPa, 100°C for 90 mins) lignin containing NFC films by mixing 10 wt% of kraft lignin suspension with NFC suspension under room temperature for two hours, followed by filtration (Farooq et al. 2019). As shown in Figure 22, the fibril structure was decreased after hot pressing, which these workers attributed to the applied compression force pushing the kraft lignin matrix into the porous network of the NFC (Farooq et al. 2019). They also observed improved water permeability due to the decreased porosity resulting from drying under higher temperature and pressure. Therefore, we wanted to assess if hot pressing lignin containing MNFC paper sheets would enhance the hydrophobicity of the handsheets. We also wanted to assess if hot pressing would result in a stable water contact angle for more than two minutes as this was not achieved in our earlier work. To compare the lignin and cellulose ratio on the surface between air-dried and hot-pressed samples, X-ray photoelectron spectroscopy (XPS) was used to assess the assumption that some lignin particles might have been “pushed” into the fiber pores. Physical properties such 61  as tensile strength, water vapor transmission rate, and air permeability were also measured to try to better elucidate the consequences of hot pressing.   Figure 22. SEM cross-section images of air-dried (left) and hot-pressed (right) lignin NFC films (scale bar: 30 µm) (Farooq et al. 2019)  The work outlined in the previous section had shown that it was important to achieve a homogeneous lignin distribution on the surface to impart hydrophobicity. Thus, we hoped that by coating lignin onto the paper sheets with subsequent hot pressing would result in an efficient and scalable technique to confer hydrophobicity. Employing coatings to improve hydrophobicity has been previously explored in several cases (Gordobil et al. 2017; Hua et al. 2019; Hult, Ropponen, et al. 2013). As reported earlier, organosolv lignin was esterified and coated on wood veneer by press moulding at varied temperature (90°C and 100°C) and pressure (100 bar and 200 bar), where the temperature was thought to provide additional energy to aid homogeneous treatment (Gordobil et al. 2017). In this work the hydrophobicity of the treated wood veneer improved drastically, as confirmed by the water contact angle, which reached as high as 120°C and was stable over two minutes. In a related study, a low molecular weight lignin fraction from black liquor was 62  isolated and esterified with vegetable oils and used as hydrophobic coating on paper substrates. This work resulted in a contact angle as high as 120°C and was stable over ten minutes (Antonsson et al. 2008). Esterified lignin coating have also been used to improve water vapor and oxygen barrier for paper-based packaging (Hult, Koivu, et al. 2013). However, there have been few studies that assessed the use of raw or technical lignin, concurrently with hot pressing (Zhang et al. 2017). Thus, in the work described below we assessed the hydrophobicity potential of using technical lignin to coat MNFC paper sheets. Initially we looked at how the water contact angle responded to different types of lignin, temperature and coating applications. Once the best case was chosen, we performed several physical characterizations pertinent to paper-based packaging including water vapor transmission rate, oxygen permeability, and tensile strength to investigate the consequences of hot pressing. We also confirmed the presence of lignin using FTIR. Lastly, we did a straightforward leaching test to assess the impact of leaching on the paper’s hydrophobicity.   As mentioned earlier, 20% of lignin based on the dry weight of MNFC fibers was incorporated onto the MNFC via the acidification approach and subsequently pressed and dried in a humidity controlled room prior to any measurements. Hot pressing was performed after the paper sheets were formed, pressed, and dried in the controlled room. The only parameter to be altered was the temperature, which was carried out in increments of 30°C from 130°C to 190°C. This temperature range was based on the reported glass transition temperature of softwood kraft lignin, which is assumed to start at around 120°C (Cui et al. 2013). The pressure (345 kPa) was chosen to match the standard handsheet pressing pressure. As summarised in Figure 23 (a), the 63  water contact angle of MNFC sheets after hot pressing were measured every 30 seconds, up to 150 seconds. However, it should be noted that the contact angle for the controls (i.e. pure MNFC sheets at different temperature) were undetectable after 30 seconds, hence were present separately in the right figure.   Figure 23. Time-dependent contact angle (A) of lignin-containing MNFC sheets after hot pressing and initial contact angle (B) of pure MNFC sheets at varying temperatures   As illustrated in Figure 23 (a), when heated to 130°C, the contact angle decreased over time resulting in a similar value to its control after 150 seconds. At higher temperature, the contact angle of the hot-pressed lignin-containing MNFC handsheets showed substantial improvement when compared to their respective controls, with the contact angle staying stable for 150 seconds. Hot pressing at 190°C showed a slightly lower contact angle compared to 160°C, likely due to thermal decomposition of the matrix substances at elevated temperature (Abe et al. 2009). The 190°C 64  control handsheets seemed slightly “burned”, showing a brownish discoloration, when compared to handsheets that were pressed at 160°C (Figure 24).    Figure 24. Brownish discoloration for control handsheets pressed at 190°C comparing to air-dried and handsheets hot-pressed at 160°C   Figure 25. Lignin-containing MNFC handsheets with and without hot pressing  Hot pressing at 160°C yielded the most promising contact angle results (~75°) as they were stable after 2 minutes, even though no significant visual difference was observed between the air-dried and hot-pressed lignin containing MNFC sheets (Figure 25). Thus we next primarily focused on the MNFC handsheets dried under ambient atmosphere and hot-pressed at 160°C. Compared to the initial contact angle obtained without hot pressing (Section 3.1.3), there was about a 15° improvement in the water contact angle. However, more importantly, the contact angle was stable 65  over time after hot pressing, which was not achieved previously. As described in previous work, the combined effects of elevated temperature and pressure likely helped redistribute the lignin on the MNFC, pushing some of the lignin between fiber pores and creating a more compact fiber network. To try to support this assumption, we took some SEM images for control and lignin-containing MNFC handsheets after hot pressing at 160°C. From the control SEM images (Figure 26), it appeared that the fibers were more compact when compared to Figure 16. When the SEM images of hot-pressed lignin-containing MNFC sheets were assessed the lignin boundaries were not as clearly defined as before (Figure 20). We also observed lignin droplets when the sheets were hot-pressed under elevated temperatures (Figure 27). This was likely due to the hydrophobic lignin particles tending to “rearrange” to minimize their surface area (Selig et al. 2007).    Figure 26. Compact fiber structure resulting from hot pressing pure MNFC handsheets at 160°C  66   Figure 27. SEM images showing hot-pressed lignin-containing MNFC handsheets  In earlier work, when X-ray photoelectron spectroscopy (XPS) was used to approximate the oxygen/ carbon (O/C) ratios of the lignocellulosic material the lignin-rich substrates were shown to have values closer to the O/C ratio of pure lignin (0.33) while the carbohydrate-rich substrates had values closer to the O/C ratio of pure cellulose (0.83) (Laine et al. 1994). As indicated by the XPS results in Table 9, the O/C ratios of pure MNFC (carbohydrate-rich substrate) were higher than their lignin-containing counterparts under both air-dried and hot-pressed conditions. Under both air-dried and hot-pressed conditions, lignin-containing MNFC showed higher theoretical surface lignin coverage (TSLC) compared to their respective controls. The hot-pressed lignin containing MNFC showed a lower lignin coverage compared to the air-dried lignin containing MNFC. Therefore, additional samples for both conditions were produced and re-analyzed to determine if the difference between the two was significant. ANOVA test with p-value < 0.05 was obtained, implying the difference was significant. The values in bold (Table 9) were the average of quadruplicate samples and their standard deviations were at 3% and 1.1% for air-dried and hot-pressed lignin-containing samples, respectively. The XPS was able to detect surface 67  composition at the level of 5 nm, thus, it was possible that, under hot pressing conditions, the lignin particles were fluidized into the fiber pores due to the applied pressure and heat, resulting in a decrease in surface lignin coverage. The observed higher standard deviation for the air-dried samples could also be attributed to the less homogeneous lignin distribution on the MNFC surface.  Table 9. XPS analyses showing the O/C ratios of both the air-dried and hot-pressed samples and their theoretical surface lignin coverage (TSLC)  Air-dried MNFC  Hot-pressed MNFC Control Lignin  Control Lignin O/Csample 0.49 0.41  0.50 0.43 Φlignin 0.67 0.84 (± 3%)  0.66 0.79 (± 1.1%)  As indicated in Figure 28, the presence of lignin on the paper sheets was detected by FTIR with the air-dried and hot-pressed pure MNFC handsheets showing similar spectra, where the main peaks for cellulose were observed. The broad region of 3600-3200 cm-1 was assigned to O-H vibrations observed in cellulose, while the 2900 cm-1 and 1000 cm-1 corresponded to C-H and C-O bonds, respectively (Garside and Wyeth 2004; Kondo 1997; Li and Renneckar 2011). The presence of lignin was confirmed by a small peak detected at the region 1600-1500 cm-1, which was responsible for C=C aromatic vibrations (Garside and Wyeth 2004).  68   Figure 28. FTIR spectra of control and lignin-containing MNFC sheets under air-dried (AD) and hot-pressed (HP) conditions  As indicated in Figure 29, the tensile index of the MNFC paper sheets decreased after hot pressing, as supported by ANOVA test with p-value < 0.05, suggesting notable difference. This result agreed with previous work where hot pressing lignin-containing nanofibrillated cellulose film resulted in a reduction in tensile strength, which was suggested to be due to the formation of new chemical bonds from hot pressing, concurrently restricting interfibrillar sliding (Farooq et al. 2019). The air permeability and water vapor transmission rate data were next assessed (Table 10). 69  It is important to note that the working principle of air permeability and water vapor transmission rate (WVTR) tests are different. The air permeability test measures the time it takes for a fixed volume of air to pass through the connected pores in a porous material under a constant pressure gradient, which is sometimes referred to as penetration test (Wang et al. 2018; Zeigler 2008). Thus, the results derived from the air permeability test are often an indication of material defects such as cracks and pores (Wang et al. 2018). In contrast, the WVTR is measured under static state flux, where the partial pressure drives molecules from one side of the material to the other side via diffusion (Wang et al. 2018).   Figure 29. Similarity in tensile index of MNFC with and without hot pressing   70  Table 10. Air permeability and water vapor transmission rate data for air-dried and hot-pressed MNFC control and lignin samples  Air-dried MNFC  Hot-pressed MNFC Control  Lignin  Control Lignin Air permeability (s) 1217 ± 14.6 Imp  Imp Imp WVTR (g/ m2·day) 351.4 ± 7.1 497 ± 11.0  142.2 ± 5.5 164.6 ± 0.5 Thickness (µm) 39.6 38.2  46.8 38.0 WVTR × 1000 (g/ m2·day)*m 13.9 ± 0.3 19.0 ± 0.4  6.7 ± 0.3 6.3 ± 0.02   From the air permeability results (Table 10), the air-dried MNFC control sheets took about 20 minutes for 100 mL of air to pass through, while the rest of the handsheets were practically impermeable. The Gurley test is usually used for materials that are relatively permeable. As stated in the TAPPI protocol 460 om-02, the Gurley equipment is not suitable for materials that need longer than 30 minutes to drain 100 mL of air due to the possibility of air leaking from the equipment. The water vapor transmission rate (WVTR) is often interpreted by the unit of g/m2× day if homogeneous sheets are produced with similar thicknesses. In our case, the “WVTR × 1000” was normalized according to their thicknesses to prevent bias. This approach has also been adopted by other researchers (Nair et al. 2017; Spence, Richard A. Venditti, et al. 2010). It was apparent (Table 10) that hot pressing could significantly reduce the WVTR by about 2-3 times when compared to their counterparts under air-dried condition. This resulted in improved moisture barrier performance. It was also apparent that hot pressing produced a more compact fiber network (Figure 26), consequently making it more difficult for the water vapor to diffuse through. The presence of lignin seemed to increase the WVTR under air-dried condition (Table 10) which is in agreement with the previous literature. Some of this earlier work examined the moisture 71  barrier performance among MFCs with different lignin contents. These workers found that the WVTR increased with a higher lignin content, which they suggested was due to the large hydrophobic pores present in the film (Hu et al. 2001; Spence, Richard A. Venditti, et al. 2010). In addition, another group produced unbleached nanopaper and reported the same observation, that the contact angle was higher compared to bleached nanopaper, but that unbleached nanopaper resulted in higher water sorption capacity (Ferrer et al. 2012). These workers attributed the effects to the higher charge density and hemicellulose content of unbleached nanopaper (Ferrer et al. 2012).  We next assessed the stability of lignin-containing MNFC sheets in water with the absorbance taken at 280 nm for aromatic detection (Figure 30 (a)) with the resulting UV absorbance normalized based on the sheet’s weight. It was apparent that the controls showed minimal absorbance at 280 nm due to the lack of aromatic ring compounds. However, the lignin-containing MNFC sheets displayed higher absorbance, with the air-dried lignin-containing MNFC sheets about 5 times higher than the hot-pressed lignin-containing MNFC sheets. This indicated that hot pressing improved the adhesion ability between lignin and fibers as the air-dried paper sheets were relatively easy to disintegrate after 24 hours of continuous stirring in the incubator (Figure 31). This resulted in a higher amount of leached lignin in the solution, when compared to hot-pressed sheets (Figure 30 (a)). The intact MNFC sheet (i.e. hot-pressed lignin-containing MNFC) was collected and dried and further hot-pressed at 160°C for 20 minutes to examine the change in contact angle (Figure 30 (b)). The contact angle experienced a steady decrease over time, suggesting that the lignin that had leached out to the media was enough to reduce its hydrophobicity.  72    Figure 30. Normalized UV absorbance measured at 280 nm for both control and lignin-containing paper sheets under air-dried and hot-pressed conditions (A); water contact angle of hot-pressed lignin-containing MNFC before and after the leaching test (B)   Figure 31. Left to right: air-dried and hot-pressed lignin-containing MNFC sheets after stirring for 24 hours in an incubator  73   Although it was apparent that hot pressing could enhance hydrophobicity and improved moisture barrier performance, with the acidification approach, it was time consuming to drain the handsheet. Therefore, we next attempted a relatively scalable and straightforward technique – which involved spray coating a lignin solution onto the MNFC handsheets, followed by hot pressing (Hua et al. 2019). The advantages of coating lignin onto already-made MNFC papers should be twofold. Not only can we retain the mechanical properties of MNFC, but we can also efficiently utilize the lignin to hydrophobize the surface. Earlier work has shown that lignin-containing nano- or microfibers can be produced and how they contributed to the improvement in water contact angle (Rojo et al. 2015; Spence, Richard A. Venditti, et al. 2010). In these past studies, lignin was used as the raw material, therefore resulting in moderate hydrophobicity due to the exposed porous network. Table 11 summarises some of the past work on this topic area.  Two types of lignin were assessed, namely SWKL and hardwood organosolv lignin. Past work had suggested that organosolv lignin was of higher purity and more hydrophobic (Lora and Glasser 2002). The moisture uptake of SWKL and organosolv lignin was calculated and found out to be 0.7% and 0.4% respectively, implying the greater tendency for SWKL to absorb moisture, (based on the method described in Section 2.2). For spray coating, we needed to take into account varying factors, such as the hot pressing temperature for the different types of lignin, lignin concentration and coating applications. To eliminate the complication of having too many variables, both lignin types were dissolved in 60/40 wt% ethanol/water and diluted to the same concentration of 14 g/L. For SWKL, the optimum hot pressing temperature had been determined 74  in the previous section (160°C). Its coating applications was selected to be 5, 10, and 20, where 5 denoted five repeated layers. The best case scenario was determined based on water contact angle results. For organosolv lignin, its lower glass transition temperature (reportedly to be at around 90-110°C) prompted us to investigate the hot pressing temperature at increments of 30°C, starting from 100°C to 160°C (Lora and Glasser 2002). The coating applications was also selected to be 5, 10, and 20 layers, and the optimum condition was determined.  We first varied the coatings of SWKL on MNFC papers and hot-pressed at 160°C for 20 minutes. The time-dependent water contact angles were determined (Figure 32 (a)). A water contact angle as high as 90° was achieved for all handsheets and was quite stable over time. A mere five coating applications achieved the best results. Looking at the amount of lignin that was actually “stuck” on the papers, 5, 10, and 20 coating applications led to 0.7%, 2.9%, and 5.1% of lignin content, respectively. The highest contact angle observed was after five coating applications, despite having lower lignin content. However, this could be due to the roughness effect conferred by the MNFC. As the coating applications increased, the water droplet became heavily dependent on the surface created by just the lignin instead of the combined effects of lignin and fibers. From these results, we concluded that five coating applications on SWKL-coated MNFC paper, further hot-pressed at 160°C for 20 minutes gave the optimum results.   75  Table 11. Summary of previous literature on their water contact angle results of lignin-containing MFC or NFCs Raw material Lignin (%) Process WCA (°) References Unbleached SW 8.8 – 13.5  - Mechanically refined in Valley beater, followed by homogenization at 55 MPa - MFC was poured into petri dish to dry and conditioned at 50% RH, 23°C (diameter: 85 – 265 nm)  48 - 74 Spence et al. 2010 Unbleached HW 2.7 - Mechanically refined in a PFI mill, followed by microfluidization at 55 MPa - NFC suspension was filtered using an over-pressurized device with subsequent cold and hot-pressed at 100°C for 2 hrs, conditioned at 50% RH, 23°C   60 Ferrer et al. 2012 TMP 31.2 - Mechanically refined in Valley beater, followed by homogenization at 55 MPa  - MFC suspension was poured into petri dish to dry and conditioned at 50% RH, 23°C (diameter: 1 µm)  88 Spence et al. 2010 Poplar powder 2 – 22.1 - Mechanically ground, followed by high shear disintegration using a grinder - NFC was poured into a mold for film casting and conditioned at 50% RH, 23°C (diameter: 15 nm)  17 – 45 Chen et al. 2018 Spruce fibers 1.7 – 13.5 - Mechanically refined, followed by high shear disintegration in a microfluidizer - NFC was filtered using an over-pressurized device with subsequent cold and hot-pressed at 100°C for 2 hrs, conditioned at 50% RH, 23°C (diameter: 16 – 44 nm)  49 – 78 Rojo et al. 2015   76   Figure 32. Time-dependent water contact angle of SWKL-coated MNFC papers at increasing coating applications (A) and its resulting lignin content (B) (“K” refers to SWKL and the following numeric numbers denotes coating applications)  Following the same path, we next assessed the ideal conditions for using HW organosolv lignin as a coating agent to introduce hydrophobicity on MNFC papers. Varying coating applications (i.e. 5, 10, and 20 layers) and hot pressing temperatures (e.g. 100°C, 130°C, and 160°C) were performed and the water contact angle results of five coating applications at varying hot pressing temperatures are reported in Figure 33. The results of increasing coating applications and hot pressing temperatures are summarised in Appendix A. It was apparent that the best case scenario for organosolv-coated MNFC papers was at five coating applications with hot pressing temperature of 100°C, resulting in a contact angle as high as 100°. The amount of organosolv lignin coated onto the MNFC papers after five coating applications and hot-pressed at 100°C was determined to be 0.9%. This was in agreement with the previous value reported for SWKL. As the 77  coating applications increased, the lignin content onto the MNFC papers also increased (details shown in Appendix B). However, it was apparent that lignin distribution onto the MNFC surface had more influence on hydrophobicity as a minimal amount of lignin was able to improve the overall water contact angle.    Figure 33. Time-dependent contact angle of organosolv-coated MNFC papers at five coating applications under increasing temperatures (A); initial contact angle of pure MNFC papers at varying hot pressing temperatures (B) (“O” denotes HW organosolv lignin)   Figure 34. Captured water droplets on organosolv lignin-coated MNFC paper 78    Figure 35. Left to right: pure MNFC paper hot-pressed at 100°C; organosolv-coated MNFC paper hot-pressed at 100°C; pure MNFC paper hot-pressed at 160°C; SWKL-coated MNFC paper hot-pressed at 160°C   When the lignin-coated MNFC sheets were observed under SEM, both the lignin-coated MNFC sheets showed conspicuous hydrophobic lignin droplets on the sheet surface, (Figure 36) with the kraft lignin uniformly coated on the MNFC with some slight inhomogeneities and some fiber pores were still visible on the images. Subsequent SEM images (Figure 37) showed patches of organosolv lignin on the MNFC paper, indicating the tendency of organosolv lignin to interact with itself. It was likely that the inherent hydrophobicity of the organosolv lignin and the roughness created between the lignin and fibers led to the higher contact angle than was reported previously.   The presence of lignin on MNFC papers was confirmed by FTIR with the broad region of 3600-3200 cm-1 assigned to the O-H vibrations observed in cellulose and the 2900 cm-1 and 1000 cm-1 bands corresponding to the C-H and C-O bonds, respectively (Figure 38) (Garside and Wyeth 2004; Kondo 1997; Li and Renneckar 2011). The presence of lignin was confirmed by a small peak 79  detected at the region 1600-1500 cm-1, which was responsible for the C=C aromatic vibrations (Garside and Wyeth 2004).  Figure 36. SEM images showing hot-pressed pure MNFC sheet at 160°C (left) and SWKL-coated MNFC sheet hot-pressed at 160°C (right)   Figure 37. SEM images showing hot-pressed pure MNFC sheet at 100°C (left) and organosolv-coated MNFC sheet hot-pressed at 100°C    80   Figure 38. FTIR spectra of hot-pressed pure and lignin-coated MNFC sheets    When the tensile index of the handsheets was assessed (Table 12) no significant variation was observed between the 100°C control and its respective lignin-coated handsheets, as supported by a p-value > 0.05 (no significant difference). The lower tensile index observed for the 160°C hot-pressed pure MNFC sheet was likely due to its lower density. The tensile strength of the lignin-coated MNFC was calculated to fall within 44- 54 MPa, which possessed similar or better performance when compared to the common plastics used today. For example, high density 81  polypropylene (HDPE) is reported to have a tensile strength between 15-31 MPa (Spence, Richard A Venditti, and Habibi 2010). For all of the handsheets, the Gurley equipment was not able to detect any air flow, indicating we had made impermeable handsheets. It should be noted that the literature values for the WVTR varied based on different starting feedstock, instrument used, and the standard or protocol followed. Previous WVTR values for micro- and nano-fibrillated films and the instrument and standard employed by researchers is summarised in Table 13. The values reported here fall in the range of the reported values. When comparing the HP 100°C pure and organosolv lignin-coated MNFC, the WVTR values showed a slight improvement after lignin coating. However, looking at the HP 160°C pure and lignin-coated MNFC sheets, the WVTR values did not seem to vary. This suggested that hot pressing was a beneficial step needed to reduce the WVTR. Despite the observed improvement in WVTR values after hot pressing, the values were still lacking when compared to low density polyethylene film (LDPE) which has reported values close to 0.0004 (g/ m2·day)*m (Mehta 2018).  Table 12. Tensile index, air permeability, and WVTR data from hot-pressed pure and lignin-coated MNFC handsheets  HP 100°C  HP 160°C Control  O5-coated  Control K5-coated Tensile index (kNm/kg) 82.7 ± 9.0 80.9 ± 4.5  68.6 ± 4.1 80.0 ± 4.6 Air permeability (s) Imp Imp  Imp Imp WVTR (g/ m2·day) 197.6 ± 18.5 164.7 ± 2.8  142.2 ± 5.5 123.2 ± 6.4 Thickness (µm) 46.2 45.4  46.8 53.8 WVTR × 1000 (g/ m2·day)*m 9.1 ± 0.9 7.5 ± 0.1  6.7 ± 0.3 6.6 ± 0.3  82  Table 13. Summary of previous WVTR values regarding NFCs and MFCs Material Thickness (µm) Instrument and standard Condition WVTR × 1000 (g/ m2·day)*m References NFC 5.7 Mocon Permatran 3/33 apparatus ASTM D 3985-06 23°C, 50% RH 0.4 Aulin, Salazar-Alvarez, and Lindström 2012  NFC 25 ASTM E96/E96M-05 cup method 23°C, 50% RH 4 - 5.5 Toivakka et al. 2014  MFC 25 ASTM E96/E96M-05 cup method 23°C, 50% RH 1.2 – 1.3 Toivakka et al. 2014  MFC 30 Wet cup method 23°C, 50% RH 20 - 24 Spence et al. 2010    When the amount of leached lignin was next examined using UV spectroscopy (Figure 39) both the MNFC controls exhibited minimal absorbance at 280 nm. However, higher aromatic compounds were detected for the HP 100°C organosolv-coated sample, when compared to the HP 160°C kraft lignin-coated sample. This suggested that hot pressing at higher temperature helped better retain the lignin on the handsheets. When the time-dependent water contact angle was measured for both lignin-coated samples and after lignin leaching (Figure 40), the hydrophobicity was impaired after the leaching test for organosolv-treated sample as the contact angle dropped to about 40° after 2 minutes. However, the kraft lignin-treated sample showed superior stability even 83  after overnight mixing with water, showing almost identical water contact angle as what was obtained before the test was carried out.  Figure 39. Measured UV absorbance at 280 nm after leaching test   Figure 40. Time-dependent contact angle before and after the leaching test for organosolv-coated sample (A) and kraft lignin-coated sample (B) (sheets showed herein were taken after the leaching test) 84   This work indicated the benefits of hot pressing lignin-containing MNFC papers, in terms of improved hydrophobicity, moisture barrier performance and stability in water. Using the lignin-containing MNFC papers produced in the last section, the additional hot pressing step was able to produce a relatively more homogeneous lignin distribution. These results were supported by SEM images and XPS results. The XPS results in particular showed a decrease in lignin surface coverage after hot pressing, suggesting that the lignin particles were being “forced” into fiber pores and dispersed into cellulose matrix. The water contact angle drastically improved when compared to the earlier work reported in the previous section, as the water droplets were able to stay on the surface after 2 minutes.  Nevertheless, depositing lignin via acidification on cellulose took longer to drain during paper formation and it was also relatively difficult to localize lignin deposition. Therefore, we combined spray coating and hot pressing as a potential technique to enhance hydrophobicity of MNFC. We employed two types of lignin, namely HW organosolv and SW kraft lignin and both were spray-coated on already-made MNFC papers followed by hot pressing at 100°C and 160°C, respectively. Five coating applications showed better contact angle data for both types of lignin – numerically resulting in about ~1% lignin coverage. This was likely due to the combined effects of uniform lignin distribution and the roughness effect conferred by MNFC. The organosolv lignin-coated MNFC paper showed slightly higher water contact angle when compared to the SWKL-coated MNFC papers, likely due to the inherent hydrophobicity of organosolv lignin. However, the SWKL-coated MNFC papers displayed lower WVTR, likley due to the higher hot pressing temperature. As improved hydrophobicity and moisture barrier performance are critical 85  requirements when it comes to packaging-related applications, the resulting lignin-containing MNFC papers showed potential to be incorporated into these types of applications. 86    The overall goal of the thesis was to develop an inexpensive and more environmentally friendly method to hydrophobize micro/nanofibrillated cellulose (MNFC). Previous literature mainly focused on modifications employing chemicals and involved a time-consuming set up. Despite promising results, these modifications mostly would not be adopted in a larger scale. At the same time, various studies have looked at producing lignin-containing micro/nano-fibrillated cellulose to improve the hydrophobicity of the material, in which the material could be well-suited as a reinforcing material in hydrophobic matrices, if succeed. Nevertheless, research involving utilizing lignin as a hydrophobic agent on micro/nano-fibrillated cellulose is scarce. Thus, this thesis work examined the potential of using industrial lignin (i.e. softwood kraft lignin and hardwood organosolv lignin) to improve the hydrophobicity of MNFC, which could be beneficial to packaging-related applications. In Section 3.1, softwood kraft lignin (SWKL) was deposited on MNFC via acidification, where SWKL was first dissolved in alkaline solution and further acidified on cellulose to make MNFC papers. The optimum lignin loading was found out to be 20% based on the dry weight of cellulose, in order to achieve highest water contact angle. It was hypothesized that, the “fibrillated” network of MNFC would help to retain more lignin, thus increasing the hydrophobicity of the final material. Comparing among different substrates, namely SWKP and HWKP, MNFC was showed to deposit more lignin, but resulting in a lower contact angle (~67°). It was apparent that water contact angle, as a surface property, was highly dependent on surface roughness, heterogeneity of 87  surface lignin coverage, and the porous nature of cellulose. These were corroborated by roughness measurement of resulting papers and SEM images taken, showing deposited lignin as big patches. To better control lignin distribution on cellulose surface and reduce the porous network of cellulose, spray coating followed by hot pressing was investigated as a viable route to hydrophobize MNFC. SWKL and organosolv lignin were spray coated on MNFC papers, followed by hot pressing at their optimum temperature for 20 minutes, 160°C and 100°C respectively. Contact angle as high as 85° and 100° were achieved and stable over 2 minutes, for SWKL and organosolv respectively. The contact angle results were corresponding to five coating applications on the papers, which was roughly 1% of lignin coverage, if stated in numbers. The improvement in contact angle was likely due to the combined effects of surface roughness conferred by MNFC and homogeneous lignin distribution. Besides, hot pressing showed to improve moisture barrier ability because of the formation of denser and more compact fiber network. Tensile measurements revealed that the lignin-containing papers were unaffected by the addition of lignin. In a nutshell, this thesis work suggested that incorporation of lignin on MNFC is a potential method to increase hydrophobicity of the final material. When combined with heat treatment, hydrophobicity and moisture barrier performance could be further enhanced.    It appeared that hot pressing lignin-containing MNFC papers helped to increase the hydrophobicity of the papers, suggesting chemical changes of lignin structure. Previous studies have shown that heat treatment of lignin could cause a decrease in the more susceptible ꞵ-O-4 88  bonds with a concomitant increase of C-C bonds. Consequently, further assessment on lignin structural changes would be an interesting focus to better understand the outcome of hot pressing.   Using technical lignin, as surveyed in this thesis, showed potential to make hydrophobic papers. However, to further improve moisture and oxygen barrier performance, lignin modification prior to spray coating and hot pressing could be advantageous to open up more applications.   As the methods described herein, a handsheet maker was used to produce MNFC papers, which unavoidably led to a considerable amount of lost fines. Other methods to produce MNFC films should be assessed to avoid the loss of fines. To better control lignin distribution on surface, a commercial coater should be used. Different hot pressing pressure should be explored as greater compression could further reduce fiber pores, consequently leading to lower WVTR. 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