Role of Silicone Surfactant in Flexible Polyurethane Foam X. D. Zhang,* C. W. Macosko,* ,1 H. T. Davis,* A. D. Nikolov,† and D. T. Wasan† *Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave. SE, Minneapolis, Minnesota 55455; and †Department of Chemical Engineering, Illinois Institute of Technology, Chicago, Illinois 60616 Received August 11, 1998; accepted March 26, 1999 Grafted copolymers which consist of a polydimethylsiloxane backbone and polyethylene oxide-co-propylene oxide pendant groups are used as surfactants to stabilize the foam cells in the flexible polyurethane foaming process. The mechanical properties of the cured polyurethane foam such as airpermeability and foam cell size are affected significantly by the structure of the silicone surfactant used in the formulation. It is shown that silicone sur- factant has an important impact on both the bubble generation and the cell window stabilization stage. A series of silicone surfac- tants with different structures was tested. Surfactants with higher silicone content will provide lower surface tension and thus help increase the number of air bubbles introduced during mixing. These air bubbles serve as the starting point for foam cell growth. As a result, the cured polyurethane foam made with higher sili- cone content surfactant has a smallerbubble size. It is also shown that silicone surfactant can reduce the cell window drainage rate due to the surface tension gradient along the cell window. The Gibbs film elasticity, the dynamic film elasticity, and the film drainage rate were measured for the first time versus surfactant composition. Surfactants with longer siloxane backbones are shown to give higher film elasticity. Using the vertical film drain- age and foam column tests, it is shown that surfactants with higher film elasticity will yield slower drainage rate and better foam cell stability. © 1999 Academic Press Key Words: polyurethane foam; silicone surfactant; film tension; film elasticity; nucleation; foam stability; film drainage. INTRODUCTION The addition polymerization reaction of diisocyanates with alcohols was discovered by Bayer and co-workers in 1937 (1). The discovery provided the fundamental chemistry for the polyurethane industry. Of all the polyurethane products, flex- ible polyurethane foam has the largest production and is most widely used. During flexible polyurethane foam formation, water reacts with the isocyanate group and produces carbon dioxide gas. The carbon dioxide gas will provide volume for bubble expansion and occupy over 95% of the final volume of the product. Figure 1 shows the four stages of the foaming process: (1) bubble generation and growth, (2) packing of the bubble network and cell window stabilization, (3) polymer stiffening and cell opening, and (4) final curing (2, 3). Silicone surfactants, which consists of a polydimethylsilox- ane (PDMS) backbone and polyethylene oxide-co-propylene oxide (PEO-PPO) random copolymer grafts, are used as sur- factants in flexible polyurethane foaming systems. It was shown that these surfactants do not alter the reaction kinetics in the flexible polyurethane foaming process (4). In the absence of these surfactants, a foaming system will experience cata- strophic coalescence and eventually cause foam collapse. These surfactants are efficiently adsorbed at the air–polyol interface and thus may have a significant effect on both the bubble generation and the cell window stabilization steps in the polyurethane foaming process. The silicone surfactant struc- ture is shown to have a great impact on the foam cell size and air permeability of the final foam product which is directly related to the open cell window percentage of a foam (4). Many physical properties of flexible polyurethane foam are highly affected by its porosity and cell size (5). However, porosity as well as foam cell size are not perfectly regulated in industry because the effect of the surfactant is not well understood. Traditionally, it is believed that the silicone surfactant can lower surface tension, emulsify incompatible formulation in- gredients, promote generation of bubbles during mixing, and stabilize cell windows (3). Among these functions, cell win- dow stabilization is the most important. During foaming, the bubbles initially introduced by mechanical mixing will grow. As the volume fraction of the gas bubbles exceeds 74%, the spherical bubbles will distort into multisided polyhedrals and cell windows with Gibbs Plateau Borders are formed. Due to capillary pressure, pressure inside the plateau borders is lower than that in the cell windows. This pressure difference will cause liquid in the cell window to drain into the struts. Without adding silicone surfactant, this drainage rate will be very fast so that film rupture and bubble coalescence occur rapidly. It is shown in this work that due to the surface tension gradient generated by the silicone surfactant the cell window drainage rate is slower. Different surfactant structures will have differ- ent effects on cell window drainage, resulting in a distribution of different cell window thicknesses at the time of cell opening (4). Thus the porosity or the air permeability of the solid 1 To whom correspondence should be addressed. Journal of Colloid and Interface Science 215, 270 –279 (1999) Article ID jcis.1999.6233, available online at http://www.idealibrary.com on 270 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.