Physical Properties of Flexible Polyurethane 'Liquid CO_2' Slabstock Foaming Mixtures

摘要

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℃ 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 ℃ 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.