Effect of network structure on lignin-derived polyurethane film properties

Abstract

Thermosetting polyurethane films were prepared from solution by crosslinking hydroxypropyl lignin derivatives with diisocyanates. The network structure, characterized by the crosslink density, was controlled by two variables: the hydroxy content of the lignin polyol and the molar ratio of NCO:OH. In addition, the effect of network structure on thermal and viscoelastic properties was determined.

Both the thermal and viscoelastic properties of the polyurethane films followed the trends found for crosslink density. For those films prepared from the various polyols with reduced hydroxy functionality, T_g was found to vary linearly with the extent of crosslinking as well as with the T_g of the polyol from which it was prepared. As the NCO:OH ratio was increased, the major difference in T_g occurred within the range from 1:1 to 3:1. Similar trends were noted for the viscoelastic properties in both instances. However, it was apparent that the sol fraction contributed significantly to the overall properties of the network, particularly at the high weight fractions.

Fracture surface analysis of model substrates (cellulose acetate and cellulose tri-acetate) bonded with a lignin-polyisocyanate resin by Electron Spectroscopy for Chemical Analysis (ESCA) revealed the formation of interfacial bonds on the surface of the cellulose acetate adherend. Furthermore, it was indicated that failure occurred well within the bulk of the cellulose acetate while interfacial failure was apparent for the cellulose tri-acetate model. While mechanical attachment is unquestionably a contributing mechanism, the formation of interfacial bonds should significantly contribute to the performance of bonded joints for this particular adhesive system.