Structure evaluation of polyethylene and polypropylene hot plate welds

TWI Industrial Member Report Summary 466/1996

By S M Stevens

Background

Little is known about the molecular changes induced in plastics by welding, although they are expected to influence mechanical properties.

Polarised Fourier transform infrared (FTIR)-microspectrometry, differential scanning calorimetry (DSC), and light and scanning electron microspectrometry (SEM) have been used to study the structure and morphology of polyethylene (PE) and polypropylene (PP) hot plate welds. Results were related to tensile properties.

Welds made using standard and non-standard conditions could be distinguished in terms of morphological or structural behaviour, but not necessarily by tensile strength. Welding did not affect PE morphology, but melting-recrystallisation-annealing ocurred in PP welds. Scanning electron microscopy on etched samples revealed areas not shown by transmitted light microscopy, such as the extent of the heat affected zone (HAZ) in PP welds.