by Robert Sanborn on Tue, Jun 19, 2007, who was a member of the STLE Society of Tribologists and Lubrication Engineers for 15 years.
Generally, additives to be blended in lubricants are mostly polar compounds, which are surface active. Therefore, when they are added to lubricant base oils, foaming readily occurs. Furthermore, when lubricants are oxidized and deteriorated during use, or as additives decompose, highly polar oxides may be formed. The increase in polarity makes the lubricants more surface active, increasing the tendency of foaming.
When lubricants are foaming, the following drawbacks may occur problematically;
hydraulic operation is deteriorated because of the increase in compaction of lubricants
the efficiency of hydraulic pumps decreases
oil supply into a frictional part is insufficient, causing wear, seizing, and the like
oxidation is facilitated because of the increase in the contact area between lubricants and air, and the like.
Thus, generally, dimethylsilicones (dimethylsiloxanes) have been most commonly used as an anti-foaming agent for lubricants. One or more dimethylsilicones with a viscosity of 100 mm2 /s to 100,000 mm2 /s at 40° C. may be used depending on the base composition of lubricants and the temperature at which lubricants are used.