Effect of crystal modification and superstructure on the static and dynamic creep properties of isotactic polypropylene
Plastics Engineering
Materials in Sintering Processes and Generative Manufacturing Processes
Final Report Abstract
In this project, the influence of nucleating agents on the static and dynamic creep behavior of isotactic polypropylene (iPP) was investigated. The influence of the crystal structure of the samples on the mechanical properties was discussed. The following additives were used as nucleating agents to induce different crystal structures in PP: • Irgaclear XT386 (Irgaclear for short) - α-crystalline, spherulitic. • Millad NX-8000 (Millad for short) - α-crystalline, shish kebab • NJStar NU -100 (in short: NJStar) - β-crystalline, spherulitic. Samples with proportions of 0.05%, 0.1% and 0.2% of these additives were prepared by compounding and injection molding, analyzed for crystallinity and tested in both static and dynamic creep tests. All nucleating agents were found to result in earlier onset of crystallization and produce a finer crystal structure due to an increased number of nuclei. The resulting limited mobility lowers the maximum deformation of the samples compared to pure iPP in all tests performed. With respect to the type of nucleating agent, α-nucleation showed a significant reduction in creep deformation. However, especially in the case of PP -Irgaclear, this was accompanied by earlier tertiary creep or failure, which can be attributed to the embrittlement caused by the shish-kebab structure. Overall, however, both α-nucleated variants showed similar results. Higher deformations were measured with NJStar than with the pure α-modification, since the crystallites can slide past each other more easily. A positive aspect here is that fracture of the specimens occurred only after a longer time or at higher loadings. The influence of the additive concentration was small compared to the influence of the additive type. In the static creep tests, it was found that an elevated temperature of 80 °C resulted in significantly earlier creep failure for all materials, and due to the lower modulus at higher temperature, the maximum loads were lower. Even at low loads, tertiary creep and failure occurred over a 24-hour period. In the comparison between static and dynamic behavior, the hypothesis that dynamic creep is more critical could not be confirmed.
