Colloquia

Summary

Glass fiber reinforced isotactic polypropylene is an interesting material for use in engineering applications such as automotive. The material has a decent mechanical performance while it is also lightweight and affordable. However, like most composite materials, it has a complex failure behavior which involves the formation of transverse matrix cracks. These cracks may lead to delamination, which ultimately results in failure. In previous research was found that an increase in molecular weight and the addition of a nucleating agent results in better mechanical performance for neat iPP. The focus of this research is to translate those results to glass fiber reinforced iPP.

As a first step, tensile tests were performed to find the transverse strength of glass fiber reinforced iPP as a function of temperature and strain rate. Several grades were selected such that both the influence of molecular weight and nucleating agent were determined. It was found that a higher molecular weight iPP gives a higher tensile strength which is in line with previous found results for neat iPP. However, the tensile strength remained the same when nucleating agent was added which is an unexpected finding based on the results for neat iPP.

As a next step, the transverse crack density growth rate is determined for different grades by performing tensile tests on specimens with a cross-ply layup. The number of cracks occurring during the test were counted with a microscope. It was found that the crack densities remained the same with increasing molecular weight, while the crack growth rate was reduced when nucleating agent was added.

DSC measurements were done to study the crystallization behavior. It is hypothesized that nucleation of crystals primary occurs around the glass fibers for the non-nucleated grades, while nucleation also takes place in the bulk when a nucleation agent is added. It is believed that nucleation in the bulk causes a more homogeneous microstructure and a reduction of the residual stresses, although this required further research. The believed reduction of residual stresses results in a lower crack growth rate from which can be concluded that addition of nucleation agent has a positive effect on the mechanical performance of glass fiber reinforced iPP.