Rheological Characterization of a Wood Adhesive Based on a Hydrolyzed Soy Protein Suspension

Soy protein (SP) is a promising raw material for the development of renewable and environmentally friendly adhesives. This work aims to study the effect of protein denaturation and/or hydrolysis on the rheology of a soy protein isolate (SPI) suspension. Four stages of chemical modification of SPI were evaluated: unmodified (U), denatured (D), partially hydrolyzed (PH) and fully hydrolyzed (H). Fourier-transform infrared spectroscopy (FTIR) and sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) confirmed that polypeptides with high molecular weight were fully fragmented into smaller ones. Rheological analysis showed that U behaves as a gel-like material while PH and H act as viscous-like materials. Hydrolysis caused a significant decrease in the viscosity, even for suspensions with high solid content. Furthermore, comparing U and H, the viscosity at high shear strength dropped 65% with an increase of 219% in the solid content. The shear strength of the adhesives can be increased through two strategies: crosslinking with epoxy resin and the addition of different reinforcements. Considering suspension H as reference, the addition of 20% epoxy resin and 5% montmorillonite clay caused an increase in the shear strength of almost 39%. These results could contribute to the development of high solid content SP-based adhesive with desirable rheological properties and zero formaldehyde emission.