Non-reacting compounds known as “physical blowing agents” (PBAs) are often added to polymer foams to increase bubble nucleation and produce higher quality foams. One such foam is rigid polyurethane foam (RPUF), which is used as thermal insulation in buildings and refrigeration. Often, the exothermic temperature rise during RPUF manufacturing is treated as the critical driver of increased bubble nucleation. Here, at constant temperature, we demonstrate in a high-pressure (&gt; 10 MPa) microfluidic flow cell that the addition of a common PBA (cyclopentane) to a non-reacting model system for polyurethane foam (polyol and CO2) dramatically increases the rate of bubble nucleation.<br/>Using the string method based on density functional theory, we showed that a possible explanation for the increased bubble nucleation upon addition of cyclopentane is the temporary condensation of a liquid-like film along the interface of the nucleating and mother phases of intermediate composition, which would reduce the surface energy required to nucleate a bubble. As further support for this explanation, we theoretically (PC-SAFT) and experimentally (high-pressure gas and liquid sampling) demonstrated a three-phase, liquid–liquid–vapor equilibrium (LLVE) under similar conditions, which has been shown in the literature to support the condensation of such films. By understanding why bubble nucleation increases with the addition of cyclopentane, we may better understand how to develop more environmentally friendly and safe alternatives.

Addition of cyclopentane to supersaturated polyol + CO2 drives increased bubble nucleation

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