Volatile-induced void formation in amorphous thermoplastic polymeric materials: I. Modeling and parametric studies

Abstract

Void formation in polymeric materials is of critical importance in a wide range of process technologies such as composites manufacturing, polymer devolatilization and foam molding. A predictive model for volatile-induced void formation and growth during the processing of thermoplastic polymeric materials is presented. This model is comprehensive, incorporating the effect of material properties, such as surface tension and moisture solubility and diffusivity in these materials, and processing conditions and part thickness on the spatial nucleation and growth of voids. Fickian diffusion of volatiles during polymer processing provides concentration profiles that are coupled with classical nucleation theory to predict nucleation density through the part thickness as a function of process temperature, heating rate and applied pressure. Void growth is predicted at each material point based on local vapor pressure, material properties and process conditions. Parametric studies to elucidate the effect of various processing parameters and material properties on moisture-driven void formation are conducted for amorphous thermoplastic engineering polymers such as polyetherimide (PEI). Further, the role of material and processing parameters in the generation of a spatial distribution of void content within the part being processed is also demonstrated.