Effect of the particle diameter of the chemical foaming agent on the foaming process and the cellular structure of one-shot compression molded polyethylene foams

Abstract

Abstract We investigated the effect of varying the particle diameter and the particle content of a chemical foaming agent on the foaming process and the cellular structure of one-shot compression molded polyethylene foams. We foamed metallocene linear low-density polyethylene (LLDPE) crosslinked by dicumyl peroxide using azodicarbonamide (ADCA) as the foaming agent. We used a purpose-built metal mold equipped with a built-in pressure sensor to monitor the time evolution of the cavity pressure. The time dependence of the cavity pressure, which is indicative of the thermal decomposition of the ADCA, was s-shaped. The s-shaped time dependence of the cavity pressure indicates that, after an initial induction phase during which there is no thermal decomposition of the ADCA, the ADCA rapidly decomposes until the reaction is essentially complete. The cavity pressure at saturation, which corresponds to the mass of the generated gas, was linearly proportional to the ADCA content but only weakly dependent on the particle diameter of the ADCA. Upon decreasing the particle diameter of ADCA, rise in the cavity pressure at intermediate times, which indicates the thermal decomposition of the ADCA, began earlier and was faster. The cell size in the final foam decreased with a decrease in the particle diameter of the ADCA. The cell number density in the unfoamed samples (NF ) was proportional to the ADCA particle number density in the unfoamed samples (NC ), with a proportionality constant less than 1. A model for the foaming process where the ADCA particles nucleate cells and the nucleated cells then coalesce, thereby reducing the number of remaining cells, can offer an explanation for the observed proportionality between NF and NC and the proportionality constant being less than 1. Furthermore, it is proposed that cell coalescence occurs if the increase in the moduli of the LLDPE due to crosslinking is not sufficient to prevent the coalescence of the cells triggered by the rapid increase in the cavity pressure due to the rapid decomposition of ADCA.