Physical Properties of Flexible Polyurethane “Liquid CO2” Slabstock Foaming Mixtures

Abstract

In commercial slabstock foaming, liquid CO2 technology is increasingly applied to replace previously used liquid auxiliary blowing agents like CFCs and methylene chloride. Knowledge of physical properties of mixtures which are used in liquidCO2 foaming may contribute to the further development of the liquid CO2 technology. Equations have been determined which do predict, for specific operational conditions and formulations, the pressures which are required in commercial foam operation to keep theCO2 dissolved both in the polyol/CO2 and in the foaming mixture/CO2 solutions. The evident dependence on temperature ofCO2 solubility, established for both polyol and foaming mixtures, strongly supports tight temperature control of the feedstocks in slabstock liquid CO2 foaming. The solubility of CO2 in polyol was found to increase with the EO content of the polyol where the effect of molecular weight was found to be negligible. It was established that small amounts of water and surfactant lower the CO2 solubility, and it is predicted that the commonly used levels of TDI do the same. Based on this information, it is advisable to conduct the mixing of TDI and auxiliary components at pressures a fraction higher than the saturation pressure of the liquid CO2 polyol mixture. A significant decrease of both viscosity and surface tension is quantified for polyol and model foaming systems at increasing liquidCO2 concentration. These decreases may explain why very small cell sizes can be obtained with liquid CO2 foaming, as the nucleation process takes place at high liquid CO2 concentration. The rise in viscosity upon evaporation ofCO2 into the froth may explain the relative stability of the unreacted froth. A cooling capacity of about 1°C per part of CO2 per hundred parts of polyol is determined from two independent experiments. However, for safety reasons, it is advisable to keep the current assumption of 0°C per part of CO2 per hundred parts of polyol in commercial foaming until a further study on the temperature evolution of liquidCO2 blown foaming under practical conditions is conducted.