Improving adhesion performance of cottonseed protein by the synergy of phosphoric acid and water soluble calcium salts

Abstract Cottonseed protein has great potential to replace formaldehyde-based adhesives due to the nature of non-toxicity, harmlessness, and non-pollution. Like soy protein-based adhesives, cottonseed protein-based adhesives also face a huge challenge in water resistance. In this work, the adhesion performance of cottonseed protein isolate (CSPI) modified by combination of H3PO4 and CaCl2 with different mole ratios was investigated and compared with those modified by 40 mM H3PO4, CaCO3, CaO, or CaHPO4. Results showed that CSPI with 40 mM CaHPO4 did not show better adhesion performance when compared with 40 mM H3PO4, CaCO3, or CaO, indicating the little/no synergy between Ca and P due to the spatial structure restriction of water insoluble CaHPO4. However, the combination of H3PO4 and CaCl2 enhanced the adhesion performance of CSPI in comparison with other modifiers, due to free Ca2+ released from water soluble CaCl2 presenting a better synergy with PO43-. At the optimal loadings of 40 mM H3PO4 and 20 mM CaCl2, 71, 86, and 56% increase in dry, wet, and soaked strengths was achieved. In addition, comparison of CSPIs with H3PO4/CaCl2 and H3PO4/Ca(NO3)2 indicates that the anions of water soluble calcium salts (CaCl2 and Ca(NO3)2) had insignificant effects on adhesion performance of CSPI. Those findings indicated that mixing of phosphoric acid and some water soluble calcium salts could synergistically enhance the adhesion performance of CSPI, thus improving the industrial acceptability of CSPI-based adhesives.