EFFECT OF MATERIAL CHARACTERISTICS OF POLYOLEFINS ON WELD LINE MORPHOLOGY AND ITS CORRELATION TO MECHANICAL-PROPERTIES

The effect of a weld line on the tensile, tensile impact and environme ntal stress cracking properties of a number of polyethylene (PE) and p olypropylene (PP) samples has been investigated. The observed mechanic al behaviour has been correlated to material properties, and also to t he weld line morphology. PE samples differed in branching, molar mass (M-w) and molar mass distribution (MWD), whereas the PP samples differ ed in nucleation. The morphology of the weld line formed in injection moulding was analysed by optical microscopy and transmission electron microscopy. The mechanical strength was studied by tensile, tensile im pact and constant tensile stress methods. In polyethylene samples with a high M-w, the weld line area was seen through the skin layer to the shear layer, and even down to the beginning of the core layer. The ef fect of a high M-w on morphological changes was diminished by a broad MWD. Short chain branching limited the morphological change solely to the skin layer. Both PP samples were morphologically rather homogeneou s. The weld line created a V-notch on the surface that acted as a crac k initiator in mechanical tests and thus reduced the mechanical streng th of the weld line samples. The V-notch mainly hid the effect of the morphology on the mechanical properties. (C) 1998 Chapman & Hall.