Equilibrium coexistence between polyol, CO2, and a physical blowing agent at elevated pressures*

Rigid polyurethane (PU) foams are widely used as thermal and acoustic insulation materials. The insulation properties depend critically on the size and distribution of cells from nucleation and growth of small-molecule bubbles during foaming. To achieve desired cell structures, the foaming recipe needs to be formulated. Besides reactants(polyols, isocyanates and water), the recipe typically contains physical blowing agents (PBAs) to increase the nucleation rate of bubbles. However, due to complex chemical and physical reactions, how PBAs affect the bubble formation are not understood. As a fist step to approach the issue, we develop a liquid-state theory of a ternary foaming system (PBA, CO₂ and polyols) and study the phase behaviors of the system at manufacture-relevant conditions. Our results show a transition in the system from a liquid-vapor into a liquid-liquid coexistence via an intermediate triple-phase coexistence with increasing amount of PBA. When the polyol-rich phase in the coexistence is destabilized (e.g. upon rapid decompression), we find the incipient phase that metastablly coexists with the polyol-rich phase changes its nature from a vapor-like to a liquid-like PBA/CO₂-rich phase with increasing weight fraction PAB in the polyol-rich phase.