Adhesive properties of urea-formaldehyde resins blended with soy protein concentrate

Commercial urea-formaldehyde (UF) resin was blended with a renewable and environmentally friendly soy protein concentrate (SP) adhesive in order to reduce the emission of volatile organic compounds. The UF resin was replaced by SP in different proportions (10 wt%, 20 wt%, and 30 wt%). Rheology showed pseudoplastic behavior of UF/SP slurries, even at low SP content, caused by the entanglements between polymer chains. Nevertheless, viscosity of UF/SP slurries dropped to 100 mPa·s at high shear rate, reaching the viscosity value of the UF resin alone. Fourier transform infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy confirmed a chemical reaction between the two components. Shear strength in wet conditions of UF/SP blends was 29% higher than SP alone. Water extraction showed an increase of 55% in the insoluble fraction caused by the formation of methylene bridges between UF and SP. Finally, the incorporation of soy protein slurries to a urea-formaldehyde resin led to the development of adhesives with high solid content and appropriate viscosity to be used by the spray method, and environmentally friendlier than the resins currently used by the wood industry. Graphical abstract Diagram of UF/SP adhesive preparation followed by rheological characterization, the study of UF-SP interaction, and the evaluation of thermal and mechanical properties