STRENGTH DEVELOPMENT AT INCOMPATIBLE SEMICRYSTALLINE POLYMER INTERFACES

The structure of an interface that develops from laminating polypropylene/polyethylene (PP/PE) by hot compression‐molding was analyzed by optical and electron microscopy. The interface strength was measured by means of a butt‐joint specimen. For the laminate formed by isothermal crystallization above the PE melting temperature, it was found that the interface strength increased with time due to the formation of PE influxes between the PP spherulites. The inter‐spherulitic influxes broke up the sharp PP/PE interface plane and provided considerable strength by mechanical interlocking. Intra‐spherulitic entanglements were also noted for PP spherulites nucleated near the interface. Fast crystallization prevents the formal/on of influxes and results in a weak interface. For the hon‐iscthennal crystallization case during cooling at 10°C/min, the interface morphology of PP/HDPE was found to be different from that of PP/LLDPE. The difference in crystallization temperatures and growth rates between PP and HDPE during rapid cooling permitted the HDPE to crystallize first which prevented the formation of influxes. With PP/LLDPE, the PP crystallized first allowing some influxes to develop. Normally the interface between PE and PP is very weak. However, to obtain good mechanical properties for a given PP/PE couple, one can control the processing thermal history to optimize the development of influxes and mechanical strength at the interface. Copyright © 1990 Society of Plastics Engineers