Bubble nucleation in nonpressurized polymer foaming systems

Abstract

The mechanism of bubble nucleation in the foaming process under atmospheric pressure is investigated in the present study. The experimental observations using a plastic‐foaming visualization setup revealed two stages of nucleation, primary nucleation in interstitial regions and secondary nucleation in the polymer melt, which followed the sintering and densification of the polymer matrix. Statistical analysis of the evolving cellular structure during the nucleation stage was used to study the significance of rheology on the behavior of polymer materials during the nucleation. The role of viscosity on the nucleation rate was also investigated theoretically by using a modified form of the classical nucleation theory and it was verified with the experimentally observed data. POLYM. ENG. SCI., 54:1201–1210, 2014. © 2013 Society of Plastics Engineers

Experimental and numerical studies on bubble dynamics in nonpressurized foaming systems

“…It was observed that low viscosity and melt elasticity for a gas‐laden matrix allowed a larger number of bubbles to survive and grow larger than the critical nucleus. A detailed explanation of the effect of rheology on the nucleation behavior of polyethylene materials has been discussed in our previous work . The trend of decline in bubble density after nucleation was notably similar for all resins; however, resins with higher melt viscosity/elasticity resulted in lower rate of bubble density reduction.…”

“…The difference between numerical and experimental results can be explained by the effect of viscosity during the nucleation stage. Our previous experimental and theoretical studies showed that higher viscosity could impede the number of nuclei survived during the nucleation stage in the foaming system . Because of lower nucleated bubble density and higher availability of foaming gas in the polymer matrix, foams produced by resins with higher viscosity showed in larger average bubble size during all stages of the foaming process.…”

“…Considering the effect of melt viscosity/elasticity during the bubble nucleation and contribution of melt strength to the stabilization of the cellular structure in the bubble growth stage, nucleation, growth and bubble coalescence must be considered simultaneously to arrive at a satisfactory conclusion. Results indicate that low shear viscosity accelerates the rate of removing the interstitial voids during polymer melt sintering and allows a greater number of bubbles to survive and grow at the nucleation stage , while high elasticity hinders bubble formation and strain‐hardening reduces the bubble coalescence and stabilizes the growing bubble. On the other hand, very low shear viscosity may not be desirable for the foaming process because of the associated reduction in melt strength.…”

Polymer foaming with chemical blowing agents: Experiment and modeling

“…In our case, the diffusion process is in the opposite direction from the bubble, which is supposed to be nucleated around the CBA particle, towards the polymer matrix. Recently Emami et al analyzed the bubble nucleation in non‐pressurized foaming CBA systems starting from solid materials, composed of PP and CBA powders as in our experiments. It was observed that the nucleation process proceeded in two distinct stages, namely primary and secondary nucleation.…”

Thermal Characterization of Microwave Assisted Foaming of Expandable Polystyrene

“…Commercial interest in polymeric foams has been increased due to new emerging applications and the ability to foam a variety of polymeric materials or composites. Despite their success, the continuous growth in research of foamed polymers into new markets depends on the ability to enhance control over its mechanical structure and performance (Emami et al, 2014).…”