Comparison of the wood bonding performance of water- and alkali-soluble cottonseed protein fractions

Abstract

The interests in biobased wood adhesives have been steadily increasing in recent years. Cottonseed protein isolate has shown potential as a renewable biobased adhesives. For a better understanding of the adhesive performance and mechanisms of cottonseed protein, we sequentially separated the isolated material into water-soluble (CSPw) and alkali-soluble (CSPa) fractions. The adhesive properties of both fractions on bonding maple wood veneers were tested at their natural pH (4.0 for CSPw and 7.0 for CSPa) and a common pH 11.0. The dry, wet, and soaked (i.e. wet and then re-dried) strength at break of the wood pairs bonded by CSPa were always higher than the values found for CSPw under the same conditions. Per the solubility and rheological analysis, the observed differences in bonding performance could be attributed to the fact that CSPw is more hydrophilic and lower in pI. ATR FTIR analysis was directly applied to the adhesive-coated surface area of maple strips cohesively broken by shear tests. Qualitative FTIR spectral features and quantitative contact angle measurement of the adhesive-coated maple surface evidenced the better wettability of cured CSPa than CSPw adhesives. Lesscarbohydrates leached out of CSPa during water soaking relative to CSPw. These observations increased the mechanistic knowledge of seed protein products functioning as wood adhesives, and would be helpful for the design and tuning of their future functional performance in green applications.