Optimizing impact properties of PP composites by control of spherulitic morphology

The objective of this work was to investigate methods for increasing the im pact resistance of composite sheets made from glassfiber mat-reinforced pol ypropylene prepeg by varying its thermal history. A 60:40 (wt %) mixture of woven glass fiber-PP was crystallized at various temperatures and times to examine the effect of the thermal history (during cooling fi om the melt) on the degree of crystallinity and spherulitic morphology and to study the relationship between these factors and mechanical properties. The composite laminates were manufactured within a flat mold using a compression molding press and then crystallized from the melt in the range 106-156 degreesC fo r 10, 30, 60, and 240 min in an air oven. The degree of crystallinity that developed in the matrix polymer was determined using differential scanning calorimetry (DSC), and the matrix morphology was examined by permanganic et ching followed by scanning electron microscopy. The highest peak and failur e energies during impact were achieved when maximum crystallinity was produ ced in the specimen crystallized at 134C for 4 h. Electron microscopy of et ched interior sections of impacted specimens has enabled a more detailed un derstanding of the impact behavior of these materials. The greatest improve ment appears to result from an. increase in the propensity of cracks to pro pagate along spherulitic boundaries by virtue of mechanisms facilitated by the results of the differential contraction of the crystalline and amorphou s phases within the polypropylene. Differential contraction of the glass an d polypropylene appears to be a less important factor, although voids creat ed by such processes have to be taken into account. (C) 2000 John Wiley & S ons, Inc.