Weld line morphology of injection molded polypropylene.

One of the main goals of this research was to develop an understanding of the specific cause(s) of mechanical weakness at weld lines in injection molded plastic parts. In this study, a variety of techniques have been used to evaluate polypropylene weld lines: optical microscopy, electron microscopy, x-ray photoelectron microscopy, Fourier transform infrared spectroscopy and mechanical property measurements. Optical microscopy results showed that the weld line penetrates about 10 microns into the sample, and that the crystalline morphology near the weld line was very different than in the polymer further removed. Transmission electron microscopy was used to determine that the material at the weld line was of slightly different density and stained differently than the rest of the polypropylene material. X-ray photoelectron spectroscopy (XPS) determined that the material at the flow front was enriched in elemental sulfur and oxygen, which helped identify it as an antioxidant additive. Finally, FTIR was used to confirm that the flow front tip was enriched in the antioxidant material by comparing spectra of the neat antioxidant. The data cumulatively demonstrate that a low concentration ($<$0.5% by weight) heat stabilizer additive was accumulating at the flow front surface during mold filling, and was subsequently trapped at the weld line as it was formed. This additive accumulation is responsible in a large part for the weakness observed at the weld line in the polypropylene system studied. Other low concentration additives were also found to accumulate at polypropylene weld lines, also making the interface weak. Even an incompatible, higher surface free energy polymer, polystyrene, when added at low concentration to polypropylene, was found to accumulate at the weld line. Therefore, surface free energy was found not to play a role in these accumulations. Homogeneous elongation was found not to reproduce the enrichments observed. The mechanism by which low concentration additives accumulate at flow fronts is speculated to involve incompatible droplets experiencing a stress gradient due to the elongation gradient at the flow front during fountain flow which pushes them out toward the free surface. In addition, large concentrations of the heat stabilizer additive was also observed on the injection molded part surface, probably deposited there by fountain flow of the flow front. This observation could have practical implications for using low concentration additives to achieve surface modification of injection molded parts. Attempts were made to modify part surfaces to improve conductivity and part release from the mold. Unfortunately, the base polypropylene polymer used in these experiments contained low level heat stabilizer additives which masked any surface accumulation measurements of other added materials using XPS. In one instance, a mixture of polypropylene with a low concentration of low molecular weight polystyrene, the intended surface modifier was observed by TEM below a thin layer that was determined by XPS to be heat stabilizer. Obviously, a more controlled study of the surface deposition is necessary before it can be exploited.Ph.D.Applied SciencesMaterials sciencePlasticsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131051/2/9825304.pd