Viscoelastic properties and plasticization potential of native, residual, and technical lignin

Licentiate thesis, 2023

Lignin in biomass, pulp, and isolated during pulping all have great potential as renewable and inexpensive thermoplastic components and materials, but widespread utilization is hindered by high glass transition temperatures (Tg), brittleness, and poor flow properties. The aim of this thesis is to elucidate what molecular structures govern lignin thermoplasticity and how these can be efficiently manipulated. To accomplish this, we first isolated lignins of high yield and purity from wheat straw, Norway spruce, and softwood kraft pulp, and fractionated softwood kraft lignin. The Tg of these lignins, as well as their relative dynamic moduli, were determined using a novel dynamic mechanical analysis (DMA) approach for non-self-supporting materials. Application of the Flory-Fox equation for modelling the Tg revealed that residual lignin in pulp had an apparent free volume more similar to that of kraft lignin than isolated native lignin. However, the chain length had a larger influence on the flow properties than degree of condensation. To elucidate the role of lignin structure in plasticization, we blended four different lignins with three distinct plasticizers (protic, aprotic, and aromatic) selected based on the hypothesis that various lignin structures would be susceptible to different types of plasticizing mechanisms. The viscoelastic properties of the resulting blends were studied using the DMA setup. Contrary to the hypothesis, the plasticizers exhibited a remarkably consistent Tg-depression and mechanical damping profiles across all four lignins. This would imply that the similarities in lignin structure (aromatic backbone, OH groups) were more important than the differences (alcohol/phenol content, lignol unit and linkages between lignols) for achieving efficient plasticization. However, one notable difference was observed upon plasticization of the diverse lignins: the condensed lignins exhibited a more pronounced reduction in Tg compared to the native lignins. This phenomenon could be attributed to the greater benefit of increasing the intermolecular distances within the rigid, condensed structures present in these lignins. Additionally, the impact of external plasticization was not limited to reducing Tg; it also led to a narrower transition, suggesting a more homogeneous ordering of the material.