Y. Leterrier et C. Gsell, FORMATION AND ELIMINATION OF VOIDS DURING THE PROCESSING OF THERMOPLASTIC MATRIX COMPOSITES, Polymer composites, 15(2), 1994, pp. 101-105
The mechanisms of void formation and elimination during the stamping p
rocess of a polypropylene/glass fiber composite have been investigated
as a function of temperature, pressure, and time. Experiments were pe
rformed in a temperature and pressure controlled chamber, equipped wit
h a rapid cooling facility. Samples of the composite as well as model
polypropylene specimens containing calibrated voids were held at 200-d
egrees-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 subs
equently characterized by density measurements and morphological obser
vation. It was shown that: i) the expansion of the composite observed
during the heating at 200-degrees-C under atmospheric pressure is larg
ely induced by the gases previously dissolved in the polymeric matrix,
ii) the rapid increase of the pressure during the stamping process le
ads to the closing of voids and, iii) the final holding under high pre
ssure provokes the redissolution of the gases. A simplified thermodyna
mic calculation of void growth and shrinkage is in agreement with the
morphological observations.