The Role of Gas Dissolution and Induced Crystallization During Microcellular Polymer Processing: A Study of Poly (Ethylene Terephthalate) and Carbon Dioxide Systems

One of the critical steps in the production of microcellular polymers is the dissolution of gas into a polymer matrix. In this paper, the formation of a gas and semi-crystalline thermoplastic solution is studied in the presence of a crystallizing matrix with particular emphasis on the ultimate effects of crystallinity on microcellular polymer processing. In this particular study, carbon dioxide was selected as the gas and poly (ethylene terephthalate) (PET) as the polymer. Polymer/gas solution formation is the precursor to the microvoid nucleation and growth during microcellular polymer processing. In batch processing, the solution formation is typically accomplished by placing a polymer sample in a high pressure gas environment resulting in the diffusion of gas into the polymer matrix. For gas and semi-crystalline thermoplastic systems, the solution formation process is notably more complex. In particular, PET crystallizes in the presence of high Co₂ solution concentrations. The crystallization results in a solution that is relatively difficult to microcellular process, requiring relatively high temperatures as compared to amorphous polymer/gas solutions. However, the resulting crystalline foam has a superior microcellular morphology. In addition, the crystallization of the solution results in a lower solubility, an increased matrix stiffness, and a lower diffusivity. Our analysis includes (1) an experimental characterization of the carbon dioxide-induced crystallization occurring during microcellular polymer processing, indicating a critical gas concentration is required for crystallization, (2) an experimental estimation of the vis-coelastic behavior of amorphous and semi-crystalline PET/CO₂ solutions, and (3) an experimental investigation of the effects of crystallinity on microcellular processing and the resulting cell morphology. Crystallinity was found to play a major role in microcellular processing through its effects on (a) cell nucleation mechanisms resulting in larger cell densities due to heterogeneous nucleation at the amorphous/crystalline boundaries and (b) cell growth mechanisms resulting in smaller cell sizes due to the increased matrix stiffness of the semi-crystalline matrix.