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 in Amorphous Thermoplastic Polymeric Materials: I 341 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.

Thermal degradation of polyetherimide joined by friction riveting (FricRiveting). Part I: Influence of rotation speed

“…Unfortunately, in the case of a process where high temperature regimes are present, such as hot-tool welding, resistance welding, induction welding and FricRiveting, volumetric defects will be inherently present [47], so one can only reduce their density but not fully eliminate them. It has been reported that PEI has a significant affinity for atmosphere moisture [48].…”

“…While for the majority of the polymers it would negatively affect short-term mechanical properties, moisture does not considerably affect PEI strength [49]. Nevertheless, under high temperatures (T > T g ), high heating rates, and increasing heating time, desorption of moisture and other volatiles in the polymer structure can lead to nucleation and growth of voids [47], in addition to flaws associated with chain scissioning and entrapped air. The separation of these types of flaws is a very complex matter and is out of the scope of this work.…”

Deconsolidation of C/PEEK blanks: on the role of prepreg, blank manufacturing method and conditioning

“…The effect is less pronounced for unidirectional pregreg, as this undulation is not present and hence less elastic energy is stored in the fiber bed [24]. Secondly, a well known source of deconsolidation is the thermal expansion of dissolved gas [25] and moisture [15,21,26,27] in the matrix, especially at high heating rates. Deconsolidation can easily be prevented by applying external pressure during heating [15,20,28].…”

Analysis of void formation in thermoplastic composites during resistance welding

“…As void nucleation in the polymer can be assumed to occur instantaneously 43 and the rate of nucleation is proportional to the equilibrium number of critical size voids, 42 the probability of void nucleation can be calculated using the nucleation rate for any given spatial location and heating history.…”