Optimizing Film Formation by Oil-in-Water Emulsions

Abstract

Oil-in-water (O/W) emulsions are widely used in metal rolling and cutting and also as fire-resistant hydraulic fluids. For both of these applications they require a reasonable degree of lubricating ability—in metal rolling to protect the surfaces against wear and seizure and to provide a controlled value of friction, and in hydraulics to limit the extent of wear, scuffing and fatigue of rubbing pump components. The lubricating properties of O/W emulsions are significantly poorer than those of most water-free oils and this deficiency can create service problems, such as excessive wear or rolling contact fatigue of hydraulic pumps or the uneven rolling or heat streaking of metal strip. Considerable work has gone into the design of O/W emulsions which have acceptable lubricating properties combined with satisfactory emulsion stability and corrosion resistance. Much of this work has been empirical, although there have also been a number of more fundamental studies of the film-forming and friction properties of emulsions. The latter have shown that the ability of O/W emulsions to form films in rubbing contacts is very dependent on the rolling speed of the contact and also the composition of the emulsion. This paper describes a study of the relationship between the composition and properties of O/W emulsions and the extent to which they form elastohydrodynamic films in contacts over a range of speeds. By applying surface chemical principles to maximize the tendency of the oil in O/W emulsions to wet the surfaces, it is shown how the composition of an O/W emulsion can be optimized to yield satisfactory film formation and, thus, low friction and wear over a wide range of speed conditions.