The nucleation of microcellular thermoplastic foam with additives: Part I: Theoretical considerations

TLDR

Microcellular foam consists of bubbles ≤10 µm formed by saturating a polymer with gas and then heating while reducing pressure to trigger thermodynamic instabilities that nucleate cells. The study develops a model for nucleation of microcellular foam in amorphous polymers containing additives. The model links nucleation to additive solubility, concentration, and interfacial energy, predicting homogeneous nucleation at low additive levels, heterogeneous nucleation above the solubility limit, and a competition near the limit, and is applied to a polystyrene‑zinc stearate system.

Abstract

Abstract Microcellular foam is a polymeric foam with bubble sizes of 10 microns or less that is produced by saturating a polymer with gas and then utilizing the thermodynamic instabilities that result when the polymer is heated and the pressure is reduced to nucleate the cells. A model for the nucleation of microcellular foam in amorphous polymers with additives has been developed. The nucleation process depends on the solubility, concentration, and interfacial energy of any additives present. At very low levels, additives in solution act to increase the free volume of the polymer, resulting in homogeneous nucleation within the free volume Well above the solubility limit, heterogeneous nucleation dominates, as it lowers the activation energy for nucleation to levels below that for homogeneous nucleation. In the vicinity of the solubility limit of the additive, these two nucleation mechanisms compete. The polystyrene‐zinc stearate system has been chosen for experimental evaluation.