Effects of Nucleating Agents’ Shapes and Interfacial Properties on Cell Nucleation

Abstract

The classical nucleation theory (CNT) provides the fundamentals to understand the cell nucleation phenomena theoretically during polymeric foaming processing. Since a variety of additives and impurities are ubiquitous in commercial plastic resins and crevices are omnipresent on the internal walls of processing equipment, heterogeneous nucleation plays a significant role in plastic foaming. According to the CNT, interfacial tensions at different interfaces and surface geometries at various heterogeneous nucleating sites are important factors that govern the heterogeneous nucleation rate. Therefore, the elucidation of their roles in cell nucleation will be beneficial to the foaming industry, which can use the information to optimize its foaming technologies and develop effective nucleating agents. In this study, a series of sensitivity analyses were performed; the results indicated that the contact angle (θ_c ) at the gas—liquid—solid junction and the semi-conical angle (β) at the nucleating site are critical parameters that govern the free energy barrier to initiate heterogeneous nucleation (W_het ). Moreover, regardless of the surface geometries of the nucleating agents, W_het decreases as θ_c increases. For moderate values of θ_c (e.g., θ_c = 90°), heterogeneous nucleation is promoted in conical cavities with a smaller β. However, a smaller β is unfavorable for cell nucleation when θ_c is too small.