Purpose - The purpose of this paper is to develop an environmentally safe aqueous polymer-isocyanate (API) wood adhesive for structural uses with whey protein isolate (WPI) that is a by-product of cheese making. Design/methodology/approach - The API formulations with whey proteins denatured at different heating temperatures and times, WPI/polyvinyl alcohol (PVA) denaturing processes, PVA contents and nano-CaCO3 (as filler) contents were investigated and optimised according to the JIS K6806-2003 standard. Findings - A whey-protein based API adhesive was developed which had 28 h boiling-dry-boiling wet compression shear strength 6.81 MPa and dry compression shear strength 13.38 MPa beyond the required values (5.88 and 9.81 MPa, respectively) for structural use of commercial standards. The study also indicated that the thermal denaturation of 40 per cent WPI solution at 60-63°C could unfold the globular structure of whey protein to some extent and therefore improve the bond strength and bond durability of whey-protein based API adhesive; the additions of PVA and nano-CaCO3 as filler had a significant effect on the bond strength and bond durability of whey-protein based API adhesive. Research limitations/implications - The thermally denatured WPI solutions (40 wt%) incline towards being decayed by moulds if not properly formulated. Practical implications - Owing to the good bond strength and durability and environmental safety, the optimised whey-protein based API adhesives have greater potential for commercial applications, especially for the structural wood bonds. Originality/value - A novel API wood adhesive for structural use was developed using whey proteins that are often regarded as a waste due to their relatively small molecules and compact globular structures.
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