Soy Protein Isolate Non-Isocyanates Polyurethanes (NIPU) Wood Adhesives

Abstract

Soy-protein isolate (SPI) was used to prepare non-isocyanate polyurethane (NIPU) thermosetting adhesives for wood panels by reacting it with dimethyl carbonate (DMC) and hexamethylene diamine. Both linear as well as branched oligomers were obtained and identified, indicating how such oligomer structures could further cross-link to form a hardened network. Unusual structures were observed, namely carbamic acid-derived urethane linkages coupled with lactam structures. The curing of the adhesive was followed by thermomechanical analysis (TMA). It appeared to follow a two stages process: First, at a lower temperature (maximum 130°C), the growth of linear oligomers occurred, finally forming a physically entangled network. This appeared to collapse and disentangle, causing a decrease of MOE, as the temperature increases. This appears to be due to the ever more marked Brownian movements of the linear oligomer chains with the increase of the temperature. Second, chemical cross-linking of the chains appeared to ensue, forming a hardened network. This was shown by the thermomechanical analysis (TMA) showing two distinct MOE maxima peaks, one around 130°C and the other around 220°C, with a very marked MOE decrease between the two. Plywood panels were prepared and bonded with the SPI-NIPU wood adhesive and the results obtained are presented. The adhesive appeared to pass comfortably the requirements for dry strength of relevant standards, showing to be suitable for interior grade plywood panels. It did not pass the requirements for wet tests. However, addition of 15% of glycerol diglycidyl ether improved the wet tests results but still not enough to satisfy the standards requirements.