Long-Term Properties of Butt-Welded Poly(propylene)

It is still not clear why the long‐term properties of plastic weld seams can only be differentiated by the very expensive medium tensile creep tests. One hypothesis for justifying this is based on the change in the structure of the weld seam surroundings, another cites the consumption of antioxidants and the following ageing in the weld seam area to be responsible for this. Butt‐welded weld seams made of poly(propylene) were systematically produced under different process parameters. Corresponding to the particular hypothesis, these weld seams were then analyzed in various ways to find correlations or to prove one of the hypotheses. Regarding their short‐term weld seam quality, the analyzed weld seams could not be differentiated through short‐term tensile or short‐term bend test. However, the medium tensile creep tests showed significant differences in both time until failure and long‐term weld seam quality. Under long‐term loading, the start of the brittle crack could be detected in most weld seams in the fine spherulite‐zone or between this zone and the area of the flow lines. This demonstrated again that only long‐term tests are suitable for examining different weld seam qualities. Depending on the welding parameters, times until failure decline with increasing heated‐tool temperature and heating time. Though these parameters lead to a higher consumption of antioxidants in the weld seam, a degradation was not detected in the breaking area. In fact, increasing heated‐tool temperatures and heating times, as well as higher joining pressures lead to a change in the internal structure of the material. This can be seen in morphological structure analyses in the larger bend of the entire weld seam area. A larger bend, however, correlates with higher residual stresses in the weld seam. In the medium tensile creep tests, these residual stresses as well as the tensile stress in the border region and the compressive stress in the middle are superimposed by the tensile stress resulting from the test stress. Thus a greater bend of the weld seam area and higher residual stresses in the weld seam itself lead to shorter times until failure in medium tensile creep tests. Schematic representation of the formation of residual stresses in a weld seam and residual stresses in the different bended weld seam areas.