Cooling rate-induced glass-adhesion variations using crystallizing hot-melt adhesives

Polychlorotrifluoroethylene (PCTFE) and PCTFE copolymeric films are bei ng used in the electronic packaging industry as insulating dielectric layers between microwave circuits. Because these films are semicrystalline and, in this application, are being used as hot-melt adhesives, the cooling rate is an important processing variable, affecting the crystallinity of the PCTFE films which, in turn, affect many properties including dielectric characteristics and mechanical properties. In this study, the crystallinity of PCTFE and PCTFE copolymeric films as a function of cooling rate was characterized by wide-angle X-ray scattering. As expected, the degree of crystallinity decreased as the cooling rate increased. Analysis of mechanical properties as a function of cooling rate by tensile testing showed that the mechanical behaviour of the films became more ductile with faster cooling rates. Because the cooling rate has also been shown to significantly affect adhesion in previous studies, the effect of cooling rate on the bond strength between PCTFE and a glass substrate was analysed. Peel testing was performed on PCTFE/glass laminates revealing that the adhesive bond strength increased as the cooling rate was increased. Thus, optimum adhesion is achieved with faster cooling rates. This was attributed to the higher fracture energy and greater ductility of the adhesive.