Influence of processing conditions on the structure and surface microhardness of injection-moulded polyethylene

Abstract

Linear polyethylene (PE) was injection-moulded into Standard tensile bars using a range of melt,Ts, and mould,Tw, temperatures. Microhardness testing and X-ray small- and wide-angle diffraction techniques were used to investigate the changes in mechanical properties, microstructure and crystalline orientation, occurring throughout the range of mouldings. A correlation was shown to exist between microstructure and processing variables. Thus, a clear increase in hardness anisotropy, ΔMH, (from 15 up to about 30%), corresponding to a well-developed molecular and lamellar orientation, and hardening (for high molecular grade PE), especially when decreasing the melt temperature below 200° C, has been detected. This increase in ΔMH favours the view of an increase of a substantial fraction of tie molecules contributing to the local instant elastic recovery beneath the indenter along the injection direction. ΔMH is, however, nearly independent ofTs for conventional moulding-grade PE. Here the absence of a unit-cell orientation is evident in theTs range investigated while a lamellar orientation only prevails forTs<200° C. In this latter caseMH⊥ is a linear function ofTw. This result is consistent with the fact that microhardness increases with the fraction of crystallized material. The obtained results suggest that the three-dimensional molecular network existing in the material plays a relevant role in steering the mechanical behaviour of the final lamellar moulded material.