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

In commercial slabstock foaming, liquid CO 2 technology is increasingly applied to replace previously used liquid auxiliary blowing agents like CFC’s and methylene chloride. Knowledge of physical properties of mixtures which are used in liquid CO 2 foaming may contribute to the further development of the liquid CO 2 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 the CO 2 dissolved both in the polyol/CO 2 and in the foaming mixture/CO 2 solutions. The evident dependence on temperature of CO 2 solubility, established for both polyol and foaming mixtures, strongly support tight temperature control of the feedstocks in slabstock liquid CO 2 foaming. The solubility of CO 2 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 CO 2 solubility and it is predicted that the commonly used levels of TDI do the same. Based on this information it is advised to conduct the mixing of TDI and auxiliary components at pressures a fraction higher than the saturation pressure of the liquid CO 2 polyol mixture. A significant decrease of both viscosity and surface tension is quantified for polyol and model foaming systems at increasing liquid CO 2 concentration. These decreases may explain why very small cell sizes can be obtained with liquid CO 2 foaming, as the nucleation process takes place at high liquid CO 2 concentration. The rise in viscosity upon evaporation of CO 2 into the froth may explain the relative stability of the unreacted froth. A cooling capacity of about 1°C per part of CO 2 per hundred parts of polyol is determined from two independent experiments. However, for safety reasons, it is advised to keep the current assumption of zero °C per part of CO 2 per hundred parts of polyol in commercial foaming until a further study on the temperature evolution of liquid CO 2 blown foaming under practical conditions is conducted.