Functional lignin for coating and 3D printing

Abstract

The abundance of lignin, aromatic structure, and multifunctional groups uniquely appeal for use in advanced sustainable functional materials. Currently, the available lignin is not derived from a lignin-first strategy; instead, it is a byproduct of the paper and pulp industry, where the primary focus is on cellulose, resulting in structural alterations. Consequently, lignin exhibits variable structural characteristics and includes impurities that restrict its valorization for diverse applications. This thesis employed functionalization and fractionation combined with functionalization to leverage lignin in advanced applications, including functional coatings, composite films, and 3D printing. The initial portion of this thesis focuses on the eco-friendly grafting of a silsesquioxane chain onto kraft lignin to produce a homogeneous superhydrophobic and flame-retardant lignin. A detailed study was conducted on the chemical interaction between softwood kraft lignin and aminopropyl/methyl silsesquioxane. Nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS) measurements validated the transformation of hydroxyl groups in kraft lignin to Si-O-C by polycondensation. The resulting lignin was employed with an aluminum phosphate binder to dip-coat a stain-grade pine wood species. The second study concentrated on integrating lignin-silsesquioxane copolymer at an elevated concentration with water-based polyurethane (PU) polymer. Sulfoethylated lignin was employed as a dispersant in the lignin-PU composite formulation to enhance the dispersion of the lignin-silsesquioxane polymer in PU. In both studies, the functionalized lignin surpassed unfunctional lignin in hydrophobic and thermal properties, endowing coated wood with superhydrophobic characteristics and flame-retardant features. The functionalized lignin incorporation in PU at a higher concentration (>50) was possible while improving the hydrophobic, thermal, and flame-retardant characteristics of PU. A reduced concentration of functionalized lignin (10%) enhanced the surface, mechanical, and thermal characteristics of pure PU compared to unfunctionalized lignin at an equivalent concentration. This is related to the improved properties of lignin due to functionalization. The optimization conditions derived from the initial investigation were employed in the third study, whereby lignin was functionalized using a fluoro silsesquioxane agent, effectively incorporating lignin into natural rubber latex. [...]