Thermal and visco-elastic properties of EVA-based hot-melt adhesives: relationship to peel behaviour

Abstract

We have studied the visco-elastic and mechanical behaviour of hot-melt adhesive formulations based on mixtures of an ethylene vinyl acetate (EVA) copolymer, a terpenc phenolic tackifying resin, and increasing amounts of a highly crystalline synthetic Fischer-Tropsch wax. The visco-elastic behaviour has been characterized by oscillatory experiments at a fixed frequency (thermomechanical analysis), and by creep measurements at various temperatures. As a general feature, increasing the wax content leads, at room temperature (i.e. well below the melting point of the wax), to a progressive change from the behaviour of a visco-elastic liquid to the behaviour of a visco-elastic solid (the highly crystalline wax acts like a filler). This feature is clearly evidenced by the variations of the compliance function. The pcel behaviour on aluminium foils is dramatically affected by the wax content; this is closely related to the visco-elastic behaviour described above. One can establish, at a given peel rate, a general relationship between (1) liquid behaviour and cohesive failure at low wax contents, and (2) solid behaviour and interfacial failure at high wax contents. At a given wax content, increasing peel rate results in the generally observed transitions: cohesive→interfacial (rubberlike)→stickslip→interfacial (glasslike); these transitions are shifted towards the right side and the peel energy is decreased as the wax content is increased. The single glass transition temperature (Tg) of the formulations is a function of the polymer/resin ratio, and its variations as a function of resin content confirm the hypothesis of complete polymer/ resin compatibility. Furthermore, the Tg's determined by thermo-mechanical analysis (oscillatory experiments at fixed frequency) arc not affected by the wax content; this suggests that the mechanical transitions cannot be simply understood on the basis of time-temperature equivalence.