Originally Posted By: MolaKule
The answer is E and B in that order.
Seals can be damaged by mechanical abrasion, heat, and chemical attack. Sludge formation is the most prevalent form of seal degradation. But why sludge? Sludge contains high polarity and very small molecules such as amine molecules from various additives, most notably, the dispersants. (Microfine abrasion particles such as metals and silicon can damage them as well, but modern filtration usually removes most abrasion particles).
While esters can cause seal swelling by altering the plasticizer a bit, Dispersants not only can extract the plasticizer but also alter other material properties of the seal.
Dispersants containing high nitrogen content (such as amine-type or succinimide-type dispersants) have been implicated in causing seal damage to fluoroelastomer seals such as Viton. The low molecular weight and high polarity molecules in some dispersants can "diffuse" into the seal material and extract the plasticizer. [The plasticizer is that material which makes the seal pliable.]
To mediate ("counteract") dispersant attack on seals, one can "post-treat" the dispersant with various reagents such as boric acid or epoxides.
So for any additive, one must fully test that additive to determine seal compatibility, and to determine how it behaves with other additives.
Since PAO molecules do not diffuse into seals, they do not swell or condition seals. In piston engines, seal swell volume needs to be 18-30% and can be provided by using various esters or aliphatic alcohols. One of those esters (not used as a base lubricating oil) is triphosphite esters in combination with hydrocarbonyl-substituted phenols.
If the base oil is a PAO/ester combination which contains certain percentages of lubricating esters, then seal swell additives are not needed.
[ March 24, 2004, 11:28 PM: Message edited by: MolaKule ]
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