Starch ester based hot-melt adhesives obtained by co-esterification with fatty acids and propionic anhydride

As an alternative to current commercially available fossil-based raw materials used in hot-melt adhesives, starch is attractive due to its broad availability. In addition, it is a bio-based and renewable raw material. However, starch could not be used in hot-melt adhesives without the ability to melt at a dedicated temperature without thermal decomposition. To overcome the physical interactions between the starches OH-groups, starch esters were synthesized with fatty acids having chain lengths from C5 to C18, together with propionic anhydride. They were synthesized to achieve thermoplastic behavior with suitable cohesion, thermal stability and crystallinity to be applied in hot-melt adhesives. FTIR and NMR confirmed formation of co-esters with propionic and fatty acid hydrocarbon chains. The degree of substitution ranged from 2.26 to 2.99 from a theoretical maximum of 3. By fatty acid chain length variation glass transition temperature, melting behavior, viscosity and modulus could be varied. For the preparation of hot-melt adhesives tackifier resins and wax, which are also bio-based, were added, similar to formulation of synthetic hot-melt adhesives. Lap-shear testing of the obtained hot-melt adhesive formulations on beech wood confirms that with these bio-based formulations an adhesion performance close to certain synthetic reference adhesives can be obtained. The resulting performance depends on the specific fatty acid combination used. It was obtained that short-chain esters contribute to higher tensile and lap-shear strength, while starch esters with long alkyl chains (e.g., myristate or oleate) lead to ductile materials with higher elongation at break.