Functionalized lignin derivatives for flame retardant, thermal insulation, and coating applications

Abstract

Lignin, a highly branched and heterogeneous biopolymer composed of phenolic units, is a key structural component of plant cell walls, providing mechanical strength, water resistance, and protection against microbial degradation. Due to its complex chemical structure and functional versatility, lignin has gained significant attention for its potential applications in biomaterials, emulsifiers, and sustainable industrial processes. Chitosan, a biopolymer derived from chitin, is widely recognized for its biocompatibility, biodegradability, and antimicrobial properties, making it a valuable material for various industrial and biomedical applications. When combined with lignin, the resulting composite benefits from enhanced structural stability, improved functional properties, and synergistic effects, offering promising potential for sustainable biomaterials, emulsifiers, and advanced functional coatings. In this thesis, functionalized lignin was produced following different pathways and used as an emulsifier for oil-water emulsions and advanced coating applications. It was also combined with chitosan to develop multifunctional materials with enhanced thermal insulation properties. The first experimental part of this thesis presents a method for producing a phosphorylated lignin-derived (PK) flame retardant following a solvent-free polycondensation reaction of kraft lignin (KL) and phytic acid (PHA). The reaction was optimized for low temperature and high decomposition temperature. Advanced techniques confirmed the covalent bonding between PHA and KL oxygen, resulting in high decomposition temperature and char formation. The study provided a new approach for preparing a fully bio-based flame retardant with limited smoke density and higher limiting oxygen index, following a green chemistry concept. The second experimental chapter utilized optimized reaction conditions to produce lightweight, thermally insulated, and flame-retardant aerogels (APK) from chitosan (CH) and phosphorylated kraft lignin (PK). The production process improved porosity reduced thermal conductivity, and enhanced compression strength, making APK an excellent thermal insulator for construction. The third experimental part of this thesis explores the production of sustainable aerogels from carboxymethylated lignin (CM), a biodegradable material with a lower environmental footprint. The research found that increasing the charge density of CM intensified the crosslinking bond between CM and chitosan (CH), reducing porosity and compression strength, and increasing thermal conductivity. The least charged CM (CM1) aerogel had the least thermal conductivity and the highest compression strength. The results of this chapter suggest a promising strategy for creating eco-friendly, sustainable aerogels. The fourth experimental section of this thesis investigated the interaction between lignin derivatives and oil and water in emulsion systems. The reaction condition for the charge density of -1.5 mmol/g was optimized with Taguchi for both sulfoethylation (SL) and carboxyethylation (CL) of kraft lignin (KL). These modifications were found to generate functional emulsifiers for soybean emulsions at different pH levels, such as 3, 7, and 11. The study found that SL and CL produced Pickering emulsions with oil droplet sizes of 436 and 452 nm at acidic pH but had shorter lifespans under acidic conditions. The study also found that SL had higher elasticity and interaction at pH 11, highlighting the importance of lignin upgrading techniques in generating functional emulsifiers. Overall, this thesis advances the development of multifunctional materials by combining functionalized lignin with chitosan, resulting in enhanced thermal insulation and flame retardancy. The research introduces innovative methods for producing bio-based flame retardants, lightweight aerogels, and sustainable emulsifiers, highlighting the potential of lignin in various industrial applications. The findings contribute to the creation of environmentally friendly, high-performance materials with improved structural stability and functional properties.