Chemical Tailoring of Lignin for Functional and Sustainable Material Solutions
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Date
2023-09-01
Authors
Shorey, Rohan
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
Lignin, being the second most abundant natural biopolymer after cellulose, constitutes between 18 wt.% and 35 wt.% of wood. Due to its abundance, sustainability, renewability, and the amenability to undergo numerous modifications through chemical reactions, there is a significant interest in lignin valorization for material applications. However, its hydrophilicity, lack of melt processability, and poor dispersibility have hindered its wide-scale application. Structural modifications are expected to counter its detrimental properties effectively. Thus, the first part of this work employed a silylation reaction to successfully modify kraft lignin and enhance its hydrophobicity for eventual incorporation in natural rubber matrix. Lignin being the hard moiety imparted mechanical strength to the composites, while NR being the soft moiety, provided it with elastomeric characteristics. The enhanced hydrophobicity enabled the specimens to better disperse in natural rubber composites, with the sample consisting of 5 wt.% modified lignin in the hydrophobic NR matrix recording a 44.4% increase in the tensile strength.
Furthermore, the incorporation of lignin in a compostable bioplastic for sustainable and biodegradable packaging materials was studied. Although petrochemical-derived and non-biodegradable plastics dominate the packaging material market, an increasing interest in sustainable alternatives is being witnessed. However, their inferior barrier properties and high cost inhibit their widespread applications. Therefore, the second part of this study investigated the use of lignin as a functional filler of a compostable bioplastic, poly (butylene adipate-co-terephthalate) (PBAT), for paper coating applications. The palmitoyl chloride esterified lignin exhibited excellent compatibility with the PBAT polymer matrix and a comprehensive characterization recorded its enhanced dispersion in the PBAT matrix, resulting in excellent wet tensile strength and barrier properties.
In another effort to effectively valorize lignin for packaging applications, cellulose acetate and lignin blends were formulated. The inherently excellent mechanical properties, optical clarity, and compostability of pristine cellulose acetate makes it a great alternative to the single-use, petrochemical-sourced plastic materials. However, due to the lack of dimensional stability and hygroscopic behavior exhibited by cellulose acetate specimens, its applicability in the food packaging industry is limited. Therefore, the third study of this work investigated the effect of blending different concentrations of lignin (oleic acid modified and pristine) with cellulose acetate on the resulting film’s morphological, barrier, mechanical, and viscoelastic properties. Moreover, the thermomechanical processed lignin-cellulose acetate sheets exhibited enhanced barrier properties (UV and moisture), better tensile properties, melt processability, and a reduction in overall moisture absorption.
Finally, the fourth part of this thesis focused on investigating the stability of lignin incorporating oil-in-water emulsions as a renewable and non-toxic alternative to organic and inorganic emulsifying agents. Three acid chlorides with varying aliphatic carbon chain lengths were employed as modifying agents. Moreover, the effect of varying pH and concentration of lignin on the stability of Pickering emulsions was studied. Post a comprehensive characterization, it was witnessed that the esterified lignin loaded emulsions outperformed the unmodified lignin stabilized Pickering emulsions. Overall, the rational and controlled modification of lignin can make it a suitable functional and sustainable additive to various materials to achieve enhanced properties.
Description
Keywords
Lignin, Lignin silylation, Hydrophobicity, Rubber composites, Lignin-rubber composites, Esterified lignin, PBAT, Paper coatings, Oil barrier, Grease barrier, Kraft lignin, Pickering emulsions, Creaming index, Zeta potential, cellulose acetate, food grade films, UV barrier, Sustainability