Bernard Kanner

Mechanism of antifoaming action

Abstract Short-range interfacial forces are known to play an important role in the performance of antifoams. To be effective in a given foaming medium, an antifoam agent should have a positive spreading co-efficient and form an unstable interfacial film incapable of foaming; i.e., it should be highly surface active and of limited solubility. Although these conditions are sufficient to describe the essentials of defoaming activity, they are inadequate to explain the variable effectiveness of different antifoams. In the present investigation, using silicone antifoams, we have shown that antifoam effectiveness can be strongly influenced by dispersibility and long-range interfacial electrical forces. The origin of such electrical forces is ionic surfactant adsorption which can impart identical surface charges to a foam bubble and the antifoam droplet, resulting in a repulsive interaction between them. Using the Gouy electrical double-layer model, this repulsive force has been estimated and correlated with antifoam effectiveness. It is also shown that in foaming systems containing anionic or cationic surfactants, electrical repulsive forces change significantly near the critical micelle concentrations of the foaming surfactants.

New Aspects of the Stabilization of Flexible Polyether Urethane Foam by Silicone Surfactants

The earliest urethane foam systems were based on prepolymers or quasi prepolymers of relatively high viscosity, were slow draining and of good inherent stability. For these systems, stabilization requirements are quite modest and were readily met by polydimethylsiloxane oils which provided surface tension lowering of the reacting components, but little else. With the introduction of the one-shot system, viscosity of the foaming component mixture was considerably lowered, film drainage accelerated and stabilization demands considerably increased. These were satisfied by the hydrolyzable and nonhydrolyzable families of polymethylsiloxanepolyoxyalkylene block copolymers which are formally analogous to conventional nonionic surfactants (1). These copolymers not only lower surface tension of the urethane reactant mixture, but also confer foam stability by means that are still the subject of investigation (2, 3, 4, 5, 6, 7, 8, 9). More recently, the development of frothed polyvinyl chloride and urethane foam systems have added yet another dimension to the requirements of foam stabilization for surfactants. In these systems as in the earlier SBR latex foamed rubber, foaming of the system and polymer gelation or fusion are separated appreciably in time by from minutes to as long as several hours. Foam stabilization demands are much greater than for the &dquo;one-shot&dquo; system and the performance demands of the latter (10). For these frothed systems, new families of silicone surfactants have been