Choline chloride deep eutectic solvents as plasticizers for thermoplastic starch adhesives for wood bonding: thermal...

Regulatory and environmental pressure on petroleum-derived polymers, including those used as thermoplastic and hot-melt adhesives, is driving demand for renewable alternatives across multiple applications, such as non-structural wood adhesives. Thermoplastic starch (TPS) is a promising candidate, derived from an abundant, biodegradable polysaccharide, but how plasticizer chemistry, particularly chloride-based deep eutectic solvents (DESs), governs its adhesive performance on wood remains poorly understood. Eight TPS adhesive films were prepared from native corn starch by twin-screw extrusion and compression molding with glycerol, D-sorbitol, ChCl:Glycerol (1:2), or ChCl:Urea (1:2) at total low-molecular-weight phase content of 40 and 50 wt% (31.8 and 41.2 wt% plasticizer, dry basis). Films were bonded to birch wood by thermocompression at 140 °C, and thermal, viscoelastic, structural, and moisture-sorption analyses were related to single lap-shear strength and SEM fractography. Sorbitol-plasticized TPS gave the strongest joints at both loading levels (Sor-L40, 4.71 ± 0.45 MPa; Sor-L50, 4.14 ± 0.52 MPa), together with the widest thermal processing window and the lowest equilibrium moisture uptake. Two-factor analysis of variance confirmed a significant plasticizer × loading interaction. All DES-plasticized formulations exceeded 1 MPa, in the same order of magnitude as ethylene-vinyl acetate adhesive and poly(butylene succinate) reference materials bonded identically. Lap-shear strength correlated inversely with equilibrium moisture uptake (r = -0.930, p < 0.001) and positively with free-film tensile storage modulus (r = +0.954, p < 0.001), rather than with melt fluidity, identifying absorbed water as a plasticizer-dependent co-plasticizer and placing TPS adhesive design within a ternary starch-plasticizer-water framework.