Mola is on the money as usual. There are many types of defoamers with the most common being silicone or methacrylate based. It is probably easier to make some statements that are general with respect to foam. Defoamers have a window of which they work in. More is not necessarily better. In fact if you treat a product with too much defoamer it will actually cause foam that would be much worse than you were originally dealing with. High treat rates of defoamers while reducing foam can actually have a negative effect on air release. Defoamers are very surface active and will cling to surfaces that will deplete their concentration in the oil. Defoamers will be depleted by things like changing filters but not the oil, or transferring from drums to other containers for example. As mentioned above the API allows 50 mils of foam and additionally allows 10 minutes for the foam to break. This is a very loose specification. Our oils will go from a trace of foam to zero in seconds.
We do not treat products any differently based on intended use. We simply ship products that don't foam. Below are some pictures. First of the equipment we use to test for foam. We have 4 testing stations and one dedicated to measuring the cracking pressure and flow rate of the stones used to distribute air into the oil during the foam test. The test is open loop feeding a given amount of air to the stones so the stones must function within a window to be valid. When the stones function outside of this window we will spend $300 on a new one that does. We have invested nearly $100,000 in foam testing equipment. Each foam station has a camera which optically measures the foam and additionally video records every foam test we perform. Foaming is the most common compatibility issue when changing oils. Defoamers from manufacturer A can actually conflict with manufacturer B and cause foam. It is not a common thing overall but does occasionally happen.
Next would be an example of what 50 mils of foam looks like during a sequence II foam test. Sequence II is run at 94 degrees C.
Lastly an image of what a trace looks like in a sequence II test. A trace means that there are bubbles that form at the interface with the oil and glass at the perimeter and do not form a layer of foam on top of the oil.
Hopefully this will give some insight into the topic of foam and further what it takes to legitimately understand what you are shipping as a company.
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Our foam testing stations
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50 mils of foam on sequence II
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This is a trace, actually a very slight trace of foam in a sequence II test
David