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

Void formation in polymeric materials is of critical importance in a wide r ange of process technologies such as composites manufacturing, polymer devo latilization and foam molding. A predictive model fur volatile-induced void formation and growth during the processing of thermoplastic polymeric mate rials is presented. This model is comprehensive, incorporating the effect o f material properties, such as surface tension and moisture solubility and diffusivity in these materials, and processing conditions and part thicknes s on the spatial nucleation and growth of voids. Fickian diffusion of volat iles during polymer processing provides concentration profiles that are cou pled 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. Parametri c studies to elucidate the effect of various processing parameters and mate rial properties on moisture-driven void formation are conducted for amorpho us thermoplastic engineering polymers such as polyetherimide (PEI). Further , the role of material and processing parameters in the generation of a spa tial distribution of void content within the part being processed is also d emonstrated.