(139d) Foaming Using a Polystyrene / Poly(Methyl Methacrylate) Blend and Nanocomposites

Physical blowing agents for thermoplastic foams include refrigerants, alcohols, hydrocarbons, nitrogen, carbon dioxide, and water. The foaming industry is currently transitioning those systems that use ozone-depleting refrigerants into either hydrofluorocarbons or carbon dioxide (CO2). CO2 is an inexpensive, effective, and friendly blowing agent for thermoplastic foams. To expedite the introduction of CO2 to industrial foaming applications, the fundamentals governing the foaming process need to be understood. Some of the most important fundamentals involve nucleation rate, location of (or time to) onset of nucleation, bubble growth rate/duration, viscosity, surface tension for a given pressure drop profile and temperature drop profile. Specifically, this study analyzes batch foaming of the following materials: pure polystyrene (PS), pure poly(methyl methacrylate) (PMMA), a PS/PMMA blend, and nanocomposites of these polymers. Batch foaming involved soaking the samples with supercritical carbon dioxide at the foaming temperature until equilibrium was reached, then quickly releasing the pressure. Cell size and density were obtained using image analysis of cryo-fractured SEM micrographs. The polymer blends were prepared to obtain varying systems. One technique was to foam these materials using different levels of mixing from a twin screw extruder. The other technique involved joining two films to form a bilayer. Since it is not possible to measure the true nucleation or bubble growth of these systems, it is not possible to fully understand the foaming of these blends from their final (aged) bubble morphologies. Thus, a dynamic method is developed to monitor the macroscopic foam expansion rate. This relates to both bubble growth rate and nucleation rate. The initial results indicate that each polymer, if dispersed to a particular domain size, can serve as a CO2 reservoir to increase the number of foam bubbles. As expected, nanoparticles speed up the rate at which the foam expands, though its exact impact on nucleation rate, onset of nucleation and bubble growth rate is not quantified.