The role of joint viscoelastic function in the adhesion of low-density polyethylene to thermoplastic starch

Abstract

Measured adhesion energies of low-density polyethylene (LDPE) to thermoplastic starch (TPS) joint and of joints in presence of poly(ethylene-co-vinyl acetate) (EVA), polyethylene grafted with maleic anhydride (PE-g-MAH) and styrene-ethylene-butadiene-styrene grafted with maleic anhydride (SEBS-g-MAH) compatibilizers were investigated. The compatibilizers were introduced to the interface via their pre-mixing with the adherend (PE) or adhesive (TPS). The results showed adhesion energy improvement from 41 J/m2 for PE/TPS to 118, 151 and 272 J/m2 by adding 3.3 wt% of EVA, SEBS-g-MAH and PE-g-MAH to the PE adherend, respectively. On the other hand, by raising the compatibilizers to 5.75 wt%, similar joint adhesion energies of about 250 J/m2 were found for all studied systems. The measured adhesion energy increments were attributed to the migration of the compatibilizers to the interface during high temperature joint preparation. In addition, the observed efficacies of various compatibilizers were ascribed to their interfacial stress transfer capabilities. Joint viscoelastic function, i.e., joint adhesion energy divided by its thermodynamic work of adhesion, showed similar dependence on adherend tan δ divided by the adhesive tan δ (A) divided by the compatibilizer tan δ as the measured adhesion energy. This interesting finding supports the hypothesis that the main viscoelastic loss effects in joints with stiff adherends are localized in the interphase adjacent to the crack tip.