LAUSR

Abstract Cottonseed protein is a potential alternative to formaldehyde-based adhesives as a renewable resource without negative environmental impact in use. Like other protein-based adhesives, unmodified cottonseed protein isolate needs improved water resistance. Thus, in this work, the effects of phosphoric acid (H3PO4), dipotassium hydrogen phosphate (K2HPO4), calcium oxide (CaO), and calcium hydrogen phosphate (CaHPO4) at four concentrations (i.e., 20, 40, 60, and 80 mM) on the adhesive strengths of cottonseed protein isolate were evaluated. Compared with control (protein without modifiers), H3PO4 (20 and 40 mM) increased dry, wet, and soaked strength by 20–30, 72–88, and 24–30%, K2HPO4 (40 and 60 mM) by 8–22, 2–10, and 1–8%, CaO (20 and 40 mM) by 8–16, 11, and 6%, and CaHPO4 (40, 60, and 80 mM) by 12–26, 6–13, and 7–14%, respectively. Cottonseed protein with 40 mM H3PO4 had the best adhesive performance, especially water resistance (88% higher in the wet strength than control), due to the increased crosslinking at its isoelectric point caused by acidity of H3PO4 (pH, 4.13). Solid state 13C nuclear magnetic resonance spectroscopic analysis showed that the amino groups interacted with these P compounds in the blending adhesives. Finally, an optimized protein adhesive preparation was applied to bond 3-ply pine plywood. The water resistance of these 3-ply strips passed the industrial soak test as Type II plywood used for hardwood and decorative plywood, indicating that the adhesive under optimal conditions can be used as a green and environment-friendly adhesive resource in wood industry.