Comparison of adhesive properties of water- and phosphate buffer-washed cottonseed meals with cottonseed protein isolate on maple and poplar veneers

Water- and phosphate buffer (35 mM Na2HPO4/NaH2PO4, pH 7.5)-washed cottonseed meals (abbreviated as WCM and BCM, respectively) could be low-cost and environmentally friendly protein-based adhesives as their preparation does not involve corrosive alkali and acid solutions that are needed for cottonseed protein isolate (CSPI). In this work, we comparatively tested the adhesive shear strength and water resistance properties of WCM, BCM and CSPI on two hard wood veneers, maple and poplar, at press temperatures of 80, 100, 110 and 130 °C and a press pressure of 2.8 MPa. The adhesive behaviors of the three adhesives on maple and poplar veneers were similar, although not always identical. Both the adhesive shear strength and water resistance properties of the three adhesives improved with the increase of press temperature from 80 °C to over 100 °C. However, raising the press temperature improved the water resistance properties of WCM and BCM more than that of CSPI. With a press temperature of 110 °C, the adhesive shear strength and water resistance properties of WCM on both maple and poplar veneers were comparable to those of CSPI. Thus, the low-cost preparation of WCM, in comparison to the CSPI preparation, is a strong candidate for substituting synthetic wood adhesives. FT-IR spectra showed higher contents of carbohydrate in WCM and BCM than in CSPI. Cross-linking reactions between carbohydrate and protein at higher press temperatures (≥100 °C) may have contributed to the improvement of the water resistance properties of WCM and BCM.