How Defoamants Work
As air becomes entrained in oil, air bubbles attempt to rise to the surface. While a bubble is rising through the oil, it passes through and picks up any number of defoamant additives that are blended into the lubricant. Defoamants are a little different than most additives in that they are suspended in oil rather than dissolved. This is important because defoamants would lose their ability to minimize foam if they were in a dissolved state.
Once an air bubble traps some of these additives in the bubble wall and finally reaches the oil’s surface, the defoamant works to impair the film strength of the bubble wall. Think of it as providing a weak spot in a chain. The interfacial tension of oil is relatively high, but the interfacial tension between the oil and the defoamant droplet is much lower. At this point, the additive spreads and ruptures, allowing air to spill into the atmosphere as the bubble bursts and the stable foam on the oil’s surface is minimized.
In a perfect world, defoamants would do their job, and foam would never be a problem. However, these additives can lose their effectiveness as a result of a number of issues. Perhaps one of the most widespread is the contamination of the oil. Any contaminant that impairs the oil’s surface tension can diminish the performance of the defoamant. Water is one of the most common contaminants that reduces surface tension and leads to excess foaming. Other contaminants include detergents, solvents, fuels and oxidation byproducts. By keeping out contaminants and maintaining clean oil, you can proactively manage foaming.
Another reason defoamants become ineffective is because of their removal from the oil. This can occur due to filtration, which will be discussed later, or from the oil being stored for an extended period of time. Since these additives are suspended and not dissolved, they are prone to settling out of the oil. Without sufficient agitation, defoamants may not fully suspend when the oil is added to a machine, especially if the oil has been in storage for a considerable length of time. This is why performance testing of stored lubricants is highly recommended. It is also one of the many issues that can arise from lubricants exceeding their shelf life in storage.
Machinery Lubrication (8/2016), Wes Cash, , http://www.machinerylubrication.com/Read/30566/defoamant-additives-filtered.
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