Biobased Epoxidized Starch Wood Adhesives: Effect of Amylopectin and Amylose Content on Adhesion Properties

Formaldehyde-free, water-resistant, and environmentally friendly wood adhesives were prepared from starches via a simple one-step epoxidation reaction followed by cross-linking using diethylenetriamine. This study focused on the effect of amylose/amylopectin ratio of starch (high amylose, low amylose, and amylopectin) on the epoxidation reaction and on the bonding performance of the prepared epoxidized starches. The epoxidation of starches altered the fluid behavior of the resins, from Newtonian for pristine epoxy to shear thinning, that corroborated the successful integration of starch chains as part of the epoxy resin. Epoxidized amylopectin had the highest viscosity, 114 Pa.s, and a high degree of substitution, 2.33 ± 0.10, among the starches that indicated that amylopectin is more favorable for the grafting of epoxy groups and for the integration of starch chains in epoxy resins compared to the starches with high amylose content. Bonding strengths were determined by lap shear tests on yellow birch specimens in dry, wet, and boiled conditions. High amylopectin epoxidized starch showed the highest lap shear bonding performance under wet conditions (5.50 ± 0.451 MPa). Under boiled conditions, all three epoxidized starches showed equally better performance than the unreacted starch-filled epoxy blends. This study provides fundamental insights into the effect of starch molecular structure on epoxidation reaction and adhesion properties and suggests a promising approach for developing strong formaldehyde-free sustainable biobased wood adhesives.