68 Refining Used Lubricating Oils
1. Surface foam formation—this can be counteracted with antifoaming agents
2. Internal foam in which the bubbles of gas are dispersed throughout the
oil—this cannot be treated with the antifoaming agents, which tend to sta-
bilize the dispersion
3. Dissolved air in the oil—which can cause problems with cavitation caused by
pressure variations as the oil ows through the engine or mechanical equipment
The most efcient defoamers are those containing silicon as liquid silicones, such
as the cyclic poly-dimethylsiloxane derivatives, and these are dissolved in aromatic
solvents and have a strong attraction to polar metal surfaces. Increasing use is being
made of defoamer additives that are free of silicones and consist ...