Influence of lignin source and esterification on properties of lignin-polyethylene blends

Lignins from pulp mills and biorefineries will gain increasing commercial importance in the next years. Their utilisation in polyolefin blends is of growing research interest as it offers a value-added usage of lignin as well as substitution of non-renewable resources. However, the low compatibility of lignin and polyolefins restrains a satisfying blend production and leads to poor mechanical properties. This can be overcome by modifying lignin prior to its incorporation into polymers in order to reduce its polarity. In this study, five lignins from different raw material sources and production processes were esterified with acetic, propionic and butyric anhydride and subsequently mixed with polyethylene with a weight ratio of 1:1. The paper provides a comprehensive and systematic evaluation of the influence of esterification as well as chemical composition of lignins on mechanical properties and, for the first time, water absorption of lignin-polyethylene blends. Properties of blends were found to improve upon lignin esterification. Compared to blends with unmodified lignin, a progressive increase of tensile (+45%) and flexural (+30%) strength could be observed with increasing length of the ester carbon chain. Lignin source and chemical composition affect water absorption of blends, but show no significant effect on mechanical properties as similar values were observed for blends produced with the respective lignin esters.