In this article, a new function of siloxane polyether surfactants is described. It turns out that these compounds are able to stabilize the rising flexible polyurethane foam by adsorption on the surface of precipitating urea particles. In this way, foam collapse is prevented. The influence of siloxane surfactants on the properties of suspended urea particles was investigated by TEM and AFM. Basic research in the field of polyurethane foam induced the synthesis of amphiphilic particles with designed anisotropic distributions of hydrophilic and hydrophobic domains. Particles derived from hollow glass spheres showed some principal properties of this class of amphiphiles: size and curvature of these particles allowed the stabilization of emulsified droplets of unusual shape and size (cauliflower, inverted cauliflower, and highly distorted structures). Several synthetic approaches for Janus-like particles are given. A very simple and efficient way is the restructuring of clusters of coated silica particles with different degrees of hydrophobicity at a water/oil interface. These amphiphilic particles are an excellent example for "physically coupled surfactants".Key words Polyurethane foam siloxane surfactant -Janus beadamphiphilic particle physically coupled surfactants
From polyurethane foam stabilization to amphiphilic particlesIn flexible polyurethane foam, which is often used for typical slabstock formulations, polysiloxane polyether copolymers are an essential compound for increasing foam stabilization. Originally, this phenomenon was just tracedback to the rather unique capability of the siloxane derivatives to reduce the surface tension of the main component of the foam, a polyether-triol. Optimization was thought to be realized by adjusting the dynamical parameters of the system.In former investigations we noticed [1] that the colloidal aspects of stabilizing flexible polyurethane foams are, indeed, more complicated. The surface tension measured by addition of polysiloxane polyether copolymers gives no information on the quality of suitable surfactants even if one includes the time dependence of the surface tension reduction. By close inspection of the mechanisms of the reactions of toluene diisocyanate with water and polyol, it becomes clear that the main function of the surfactant is related to the precipitation of polyurea particles in the foam mixture. (In this approach we have neglected the important but not critical assistance of the surface active compound in the nucleation of gas bubbles.) Precipitation of polyurea particles takes place in a late phase of the foam rise. Without a suitable surfactant, urea precipitation is immediately followed by a foam collapse. The quality of the surfactant can easily be demonstrated
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