Foaming polystyrene with a mixture of CO2 and ethanol

Use of mixtures of blowing agents in thermoplastic foam extrusion has been an industrial practice for a long time. However, it has gained renewed interest in the past few years due to the introduction of difficult-to-process alternative gases, targeted as potential replacement for the banned ozone-depleting blowing agents. Reasons for blending physical foaming agents (PFA) are numerous. The incentives may be economical, environmental, or technical. With respect to that latter factor, blending suitable PFAs is often regarded as providing a better control of processing conditions. For example, a specific PFA could be selected for its inflation performance and blended with other co-blowing agents chosen for their stabilizing role. Although a considerable amount of work has been done in that area, very little information has been disclosed in open literature. Carbon dioxide (CO2) has been reported as an interesting candidate for low-density polystyrene (PS) foaming, although the required concentrations are associated with high processing pressures due to the low solubility of the gas. Thus, stable processing conditions are difficult to achieve. This work studies the effect of blending CO2 with ethanol (EtOH) as a co-blowing agent for PS foaming. Extrusion foaming performance of this mixture is discussed, with respect to its solubility (i.e., degassing conditions) and rheological behavior. The function of each blowing agent during the process is analyzed with respect to the plasticization, nucleation, expansion, and stabilization phases. Attention is also paid to the interaction involving the two PFA components.