Comparing bond strength and water resistance of alkali-modified soy protein isolate and wheat gluten adhesives

The tensile strength of beech substrates bonded with dispersions of alkali-denatured soy protein isolate (SPI) and wheat gluten (WG) was measured for comparison of bond strength and resistance to cold water. The proteins were denatured with 0.1 M NaOH (pH 13). Dispersions with different protein concentration and viscosity were investigated. The adhesive properties were studied at different press temperatures (90, 110, and 130 °C) and press times (5, 15, and 25 min). Two types of application methods were used in order to overcome the problem with different viscosity of the dispersions. In addition, SPI was denatured at two different pH levels (approximately 10 and 13) and with two different concentrations of salt (158 μM and 0.1 M), in order to compensate for the different isoelectric points of the proteins. The adhesive properties of WG powder with different particle sizes were also compared. The tensile strengths of the wood substrates were measured according to somewhat simplified versions of the European Standards EN 204 and EN 205. The bond lines were studied with light microscopy. The results indicate that the adhesive properties of SPI are superior, particularly with regard to water resistance. However, the water resistance of WG was to some extent improved when starved adhesive joints could be avoided. Similar tensile strength values were obtained for the dispersions of alkali-denatured SPI regardless of pH or salt concentration. No apparent difference in adhesive strength was observed for the WG dispersions from powder with different particle sizes.