Effect of soft segment molecular weight and NCO:OH ratio on thermomechanical properties of lignin-based thermoplastic polyurethane adhesive

Abstract

During the last decades lignin has been largely applied on development of reactive polyurethanes. More recently, significant efforts were dedicated to synthesizing lignin-based thermoplastic polyurethanes (LTPUs). However, the relationship between molecular structure, properties and feasible LTPU applications still lacks in the literature. Aiming to overcome this absence, kraft lignin was chemically modified by partial acetylation in order to reduce the reactive sites and avoid the formation of a highly cross-linked structure. The extent of the functionalization reaction was properly investigated by Fourier transformed infra-red spectroscopy (FTIR-ATR) and quantitative 31P nuclear magnetic resonance analysis and the results revealed that the hydroxyl content from the parental lignin could be reduced by half in a controlled manner. In the second stage of this research, lignin-based thermoplastic polyurethanes (LTPUs) were successfully synthesized using the partially acetylated Kraft lignin macromonomer as the polyol to form a stable and thermo-formable polymer. The effect of lignin content, soft segment molecular weight, and the NCO:OH ratio on chemical, thermal, and mechanical properties were investigated. At last, LTPUs were applied as hot-melt adhesive for wood and their mechanical behavior evaluated by single lap shear tests. The results elucidated how those parameters influence LTPU properties and might be used as a design tool to tune LTPUs properties, supporting the development of industrial LTPU application.