From polyurethane foam stabilization to amphiphilic particles — An example of industrial R&D in colloid chemistry

Abstract

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

Adsorption of Disk-Shaped Janus Beads at Liquid−Liquid Interfaces

“…For traditional surfactant molecules, effects of the molecular shape on the interfacial properties have been extensively studied by using the models proposed by Israelachvili or Helfrich. , However, there is little precedent literature as to the shape of the surface-active particle. In our previous experimental studies, we showed that the shape of the particles indeed affects their self-assembled structures. , Rossmy also reported that the morphology of the emulsions stabilized by J-beads depends on the geometrical property of the beads . Recently, Brown et al have shown that amphiphilic curved disks distort an air−water interface, giving rise to anisotoropic forces between the disks .…”

Particles Adsorbed at the Oil−Water Interface: A Theoretical Comparison between Spheres of Uniform Wettability and “Janus” Particles

“…The varnish is subsequently dissolved yielding Janus particles. Rossmy describes their synthesis either from hollow glass spheres, with the ability to coat the inner and outer surfaces differently, or in situ at an oil−water interface. The use of particle monolayers spread at air−water surfaces or adsorbed at solid−liquid interfaces appears to be a successful way of preparing microspheres with nonuniform coatings .…”

Magnetic Anisotropic Particles: Synthesis and Applications

“…The confinement of different physical and chemical properties on either sides of the JPs can generate asymmetric interfaces and open up possibilities for unique ways of interfacial meso-structuration. Some of the very early accounts on the amphiphilic nature of JPs were reported by both Casagrande [103] and Rossmy [104]. Casagrande et al described the use of 60 μ m glass spheres with one hemisphere protected with a varnish and the other hemisphere chemically treated with a silane reagent.…”