Serena Frasca: Lignocellulosic Biomass Components for Materials Applications

Abstract

This thesis presents a comprehensive study of the use of lignocellulosic biomass components in materials, targeting two distinct applications: conductive materials and additive manufacturing. The lignocellulosic biomass components -lignin, cellulose, and hemicellulose- have been identified as promising renewable feedstocks to replace fossil resources and contribute to the green transition. The first work presented focuses on the synthesis of conductive polymers, specifically polypyrroles. The synthesis of the monomers, N-functionalized pyrroles, was achieved by a modified Clauson-Kaas protocol, both performed in continuous flow and in batch. The substrates used, 2,5-dimethoxytetrahydrofuran and anilines, are promising candidates for biomass-based chemical production. The produced N-functionalised pyrroles were then deposited onto electrodes via electropolymerization to obtain thin films and their electrical properties were characterized. Next, the thesis delves into the isolation and valorisation of lignin, specifically into softwood lignin, modified and isolated via a phenol-assisted fractionation. This approach supresses the formation of condensed lignin while simultaneously introducing new functional groups that could be beneficial for a number of applications. Phenolated lignin was obtained with a high degree of functionalization, a well-defined structure and relatively low molecular weight. Detailed analysis of the fractionation conditions and of the corresponding lignin structures gave insights on how to tailor lignin on demand. The potential of the one-step phenolated lignin was investigated for materials applications.

Filaments of lignin and polylactic acid (PLA) were produced to be used in additive manufacturing. The study focused on high lignin incorporation to PLA at three different concentrations (30, 50, and 70 wt%). The lignin-PLA filaments were used for 3D printing of dog bone shaped specimens to examine their mechanical properties. Additionally, detailed thermal and chemical analysis were performed to get an in-depth understanding of the materials. The results were compared to the performance of technical lignins that were also included in the study. Importantly, recycling studies of the filaments indicated good printing performance up to three recycling cycles.

Another application explored was the production of conductive carbon materials, starting from the modified lignin. The carbonization was performed using a CO2 laser engraver and lignin was the main component in the wet film formulation. The optimized carbonization parameters afforded carbonized films with low sheet resistance (< 7 Ω sq-1). The structural analysis of the carbonized materials revealed the formation of few-layers graphene-like carbon structures. Further applications of these materials are under investigation.

These innovative applications showcase the potential for sustainable, biomass-based materials. The lignin fractionation method reported herein can contribute to further advancements in lignin research. A phenol modified lignin with defined structure offers more opportunities in comparison with bulk lignins, with the advantage of tailoring lignin properties to its end use in the same number steps. Biomass-based organic electronics will help paving the way for environmentally friendly advancements in the energy sector.