In-situ measurement of internal mold pressure during one-shot chemical foaming process

Abstract

One-shot chemical foaming of metallocene linear low-density polyethylene（LLDPE）cross-linked by dicumyl peroxide（DCP）was investigated using azodicarbomide（ADCA）as the foaming agent. A novel technique for the measurement of the instantaneous internal mold pressure was developed. The s-shaped time-dependence of the mold pressure suggested that, after an initial induction phase during which the pressure remained low, the ADCA decomposed quickly leading to a rapid increase in the pressure. The rapid pressure increase eventually saturated indicating reaction completion. The amount of gas（taken to be proportional to the mold pressure pressure）increased linearly with increasing ADCA content. ZnO content did not affect the amount of gas released but accelerated the rate of thermal decomposition of ADCA. The prepared foams displayed a closed cell structure. The average cell diameter was found to be essentially independent of both the ADCA and the ZnO contents. For all of the investigated ADCA and ZnO contents, the expansion ratio of the foams was uniquely determined only by the mold pressure at saturation. The cell number density calculated for the unfoamed specimen increased linearly with ADCA content. The behavior of the expansion ratio and the cell number density suggested that, during foaming, the ADCA particles served as sites for cell nucleation.