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Foaming Polystyrene with Mixtures of Carbon Dioxide and HFC-134a

Mixtures of blowing agents are becoming widely used in the industry either for economical reasons or for achieving better control of processing conditions. Despite the fact that they are commonly used for foaming, the literature is fairly scarce on that particular subject and the fundamentals are not very well understood. This work studies the effect of blending carbon dioxide and 1,1,1,2-tetrafluoroethane (HFC-134a) in polystyrene. Ultrasonic monitoring and online rheology provided information on the solubility and plasticizing effect of the gases. Results show that, on an equivalent molar basis, HFC-134a is slightly more soluble than CO2 and is a more effective plasticizer. Moreover, HFC-134a generates foam samples with a higher nucleation density than CO2 using similar processing conditions. Blending the two gases generates nucleate cell densities, which are intermediate to the pure gases but do not follow a log-additivity rule. It is hypothesized that blending gases affect their mutual diffusion coefficients, which in turn, largely dictates the final foam morphology.