FORMATION AND ELIMINATION OF VOIDS DURING THE PROCESSING OF THERMOPLASTIC MATRIX COMPOSITES

The mechanisms of void formation and elimination during the stamping process of a polypropylene/glass fiber composite have been investigated as a function of temperature, pressure, and time. Experiments were performed in a temperature and pressure controlled chamber, equipped with a rapid cooling facility. Samples of the composite as well as model polypropylene specimens containing calibrated voids were held at 200-degrees-C at different levels of hydrostatic pressure (1, 10, 100, and 300 bars), for a period of either 1 or 10 min. The specimens were subsequently characterized by density measurements and morphological observation. It was shown that: i) the expansion of the composite observed during the heating at 200-degrees-C under atmospheric pressure is largely induced by the gases previously dissolved in the polymeric matrix, ii) the rapid increase of the pressure during the stamping process leads to the closing of voids and, iii) the final holding under high pressure provokes the redissolution of the gases. A simplified thermodynamic calculation of void growth and shrinkage is in agreement with the morphological observations.