Foaming in Lube

Abstract

Low concentrations of substances having a lower surface tension than oil romote foaming markedly in oils if a single phase is present. If two phases are present and the added phase is finely dispersefin the oil the substance is a strong anti-foamant.Photomicrographs and high-speed motion pictures present a visual confirmation of the mechanism of anti-foamants. IntroductionFoaming and the control of foaming in gasliquid systems represents an area in which few quantitative relations have been developed. Since both static and dynamic properties represent major system variables, precise experimental measurements necessary for theoretical analyses are extremely few. This experimental study, largely qualitative in nature, attempts t o elucidate the mechanisms of foaming and foam stability, and hence provide a sound basis for theoretical attack.Four oils, designated as 555, 702, 528 and 529 and having quite different physical properties and foaming characteristics were chosen as subjects for the study. Dow-Corning 100 C.S. and 1000 C.S. silicones were used as anti-foamants.Foam Studies I n order to evaluate the influence of temperature, physical properties of different oils and concentration of silicone on the foaming properties of the oils, a series of foam tests were carried out in which a measured volume of oil was aerated by nitrogen flowing a t a fixed rate through a porous stone. The temperature was maintained at a desired level by flowing liquid through the jackets of the oil containers. If silicone was to be added, a solution of silicone in n-pentane (0.1%) was added to the oil while stirring in a special high-speed stirrer. The results of these measurements are recorded here.Oil 528.-Samples containing 0, 0.5, 1.0, 2.0, 2.5, 5, 10, and 20 p.p.m. of silicone were aerated over the temperature range of 20 to 120". At 20", none of the samples produced foam; however the bubbles were much smaller than in those with silicones (0.5-3.0 mm., in diameter as compared to 5.0 mm. with silicones). At 60", the sample without silicone began foaming. At 70", the 0.5 p.p.m. sample began t o foam. The number of bubbles floating in the liquid decreased and the size of the bubbles increased with increasing silicone content. At 97" small bubbles began collecting on the surface of the

Mechanisms of Action of Mixed Solid−Liquid Antifoams. 1. Dynamics of Foam Film Rupture

“…The role of the oil (hydrocarbon or poly(dimethylsiloxane)) in liquid or in mixed solid−liquid antifoams is usually explained in the framework of two mechanisms of foam film destruction: (i) spreading-fluid entrainment − and (ii) bridging-dewetting. ,− According to the spreading mechanism, the effective antifoam contains oil that spreads rapidly over the foam film surface. The oil spreading leads to a Marangoni-driven flow of liquid in the foam film (fluid entrainment), resulting in a local film thinning and subsequent rupturesee Figure .…”

Role of Oil Spreading for the Efficiency of Mixed Oil−Solid Antifoams

“…Several exceptions, however, were noticed in the same study. Since that time, there has been an ongoing debate in the literature about the role of oil spreading for the antifoam activity. − Periodically, papers appear in which the relation between the spreading behavior of the oils and their antifoam activity is reiterated in one form or another, − ,,− , whereas this relation is opposed as a general rule in other papers. − ,− ,− …”

Factors That Influence the Coalescence of Bubbles in Oils That Contain Silicone Antifoamants

“…However, silicones are not always effective, and in some instances their use can actually make matters worse (Claxton" 1972;Fowle, 1981). For example, soluble silicones are surface active materials that can stabilize bubbles by reducing interfacial mobility (Shearer and Akers, 1958). Conversely, dispersed drops of silicone can short-circuit the normally slow process of coalescence by surface transport associated with the spreading of small drops (Ewers and Sutherland, 1952).…”