Formation of integral skin polyurethane foams

Abstract

Polyurethane integral skin foams comprise a low density foamed core surrounded by a high density skin of the same material, and are made in a single molding operation. Polyurethane integral skin foams find wide application as structural materials, and the mechanical properties of such foams are significantly affected by the foam structure, which in turn is determined by the foaming process. An experimental and theoretical study of the foam formation process is presented. Experimental results show that increase in the mold wall temperature results in an increase in the upper skin thickness, a decrease in the lower skin thickness, and a decrease in the density of both skins. The core density increases with increasing wall temperature. Increase in the blowing agent concentration produces a small increase in the skin thickness. Computations based on the model of Marciano et al. [Polym. Eng. Sci., 26, 717 (1986)] are presented using an efficient numerical technique. Predictions of the model for the skin thickness, core density and pressure and temperature variation with time are in reasonable agreement with experimental data. The model, however, predicts higher skin densities than the experimental measurements. A modification of the initial condition of the model to account for the air bubbles dispersed during mixing gives better agreement between theory and experiment.