Chemical modification of starch and the application of expanded starch and its esters in hot melt adhesive

Abstract

Adhesives are of pivotal importance in modern society as over 6 billion pounds of adhesives are used globally per annum. Among these, hot melt adhesives (HMAs) represent the most dynamically developing area, reaching 15–21% of the global volume of production and usage of adhesives. The application of expanded high amylose corn starch (HACS) and its propionate derivatives with differing degrees of substitution (DS) in a formulation comprising polyvinyl alcohol (PVOH) and glycerol to afford 100% biodegradable HMAs is reported. Esterification of expanded starch was conducted to increase the stability and hydrophobicity of starch. The effects of amounts of esterifying reagent and reaction time on DS of starch propionates were investigated. Native starch was expanded (BET surface area, 176 m² g⁻¹; DS = 0) and derived propionate esters were studied by ATR-IR, TGA, ¹³C CPMAS NMR, ¹H NMR and titrimetric methods. The HMAs, irrespective of DS, displayed a T_g at approximately 0 °C, melting (T_peak) at approximately 160 °C and crystallisation (on cooling) at approximately 115 °C. The adhesive properties (tensile strength) with respect to DS of expanded high amylose corn starch and its propionate esters show a distinct structure–property relationship. Expanded high amylose corn starch (DS = 0) gives the strongest adhesion, outperforming native (non-expanded) starch. The expansion process is beneficial in promoting adhesion which may be linked to the increased availability of hydroxyl moieties promoting better non-covalent interactions and mixing with PVOH and glycerol. Adhesion strengths decrease with increasing DS of base polymer and those of starch propionates with DS in the range 1.46–1.82 are comparable to that of non-expanded HACS based HMA. This is the first reported occurrence of the use of expanded starch and its propionate esters as HMAs.