(451a) Debonding at Elevated Temperatures in Polypropylene Composites

Most studies on debonding of fillers from the matrix polymer have been carried out at room temperature and not near the melting point of the polymers. The test temperature is significant because it is known that when unfilled isotactic polypropylene is held at elevated temperatures, the degree of crystallinity increases. The objectives of this study were to investigate the effects of temperature on the onset of debonding in (20% vol.) glass bead filled polypropylene and (20% vol.) glass flake filled polypropylene at elevated temperatures ranging from 130°C to 150°C, below the melting temperature of the polymer.

The onset of debonding in the composites was identified from kinks in the stress-strain curves of tensile tests. The kink in the stress-strain curve was also investigated with scanning electron microscopy (SEM) on samples that were strained and frozen in the tensile frame. The degree of crystallinity of the polymer matrix in the composite specimens was investigated by differential scanning calorimetry (DSC) as a function of holding temperature with a fixed holding time of 30 min.

The degree of crystallinity in the matrix at the start of the test was found to increase with the test temperature for the bead filled composites but it was unchanged for the flake filled composites. This difference in initial crystallinity trends for the two composites influenced the variation of debonding stress with temperature for the two composites. Debonding stress decreased with increasing temperature for both composites but the slopes were different. The increase in crystallinity with increasing temperature for the glass bead composite counteracts the decrease in matrix modulus due to increasing the temperature. It must be noted that the degree of polymer matrix crystallinity is different in the two composites even before conducting the high temperature tests because of greater nucleation in the glass flake filled polypropylene. Hence the in-situ degree of matrix crystallinity in the composite is an important parameter that must be accounted for in models of debonding.

The applied tensile stress σi at the onset of debonding may be related to the local debonding stress σd by means of a stress amplification factor as after accounting for the thermal residual stress σT in the sample as follows. For example, the stress amplification factor as is 2 for a single rigid sphere in an elastic matrix.

σi = [-σT + σd]/as

The relevant factors including particle geometry and their effects on the local debonding stress may be seen more clearly by inspecting an energy balance that holds for a linear elastic matrix at the onset of debonding. These equations have been used along with debonded particle sizes from micrographs to interpret observations of debonding stress in the two composites at various temperatures.