Lube-Tech
high temperatures and chemicals. The structure of the elastomers themselves as well as the used additives play an important role on how much they are affected by the aforementioned parameters.
When elastomers have direct contact with an ester, various processes can take place. The elastomer can absorb the lubricant. This leads to swelling of the elastomer as it gains volume and weight. It is also possible that substances from the elastomer are dissolved in the ester. This would cause shrinkage of the elastomer as volume and thickness decrease. In general, chemical reactions of the elastomer and a liquid medium are also possible but not very likely when it comes to esters.
Besides the elastomer itself, the used additives are also involved in these processes. Plasticisers or antioxidants have a high probability of being dissolved in the lubricants. As a result, elasticity, wear resistance and durability are affected. Fillers, on the contrary, tend to lead to a better resistance of the elastomers.
Both processes, adsorption and elution, can overlap. A discoloration of the medium indicates the latter.
Materials and Method Elastomers
For the swelling tests, mainly elastomers with a good compatibility with lubricants were selected. All test specimens were obtained from Bundesanstalt für Materialforschung und - prüfung (BAM) in Berlin and comply with ISO 6072.
NBR – Nitrile Butadiene Rubber NBR generally provides good oil resistance. It is widely used for gaskets, seals, diaphragms, gasoline hoses, liners, tubing, O-rings and cable jacketing.
Table 1: Ester base oils used without additivation
The elastomer is produced by radical polymerisation of acylonitrile and butadiene. Higher proportions of acrylonitrile lead to better stability and mechanical
LUBE MAGAZINE NO.167 FEBRUARY 2022 25
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.138 page 2
strength. A higher butadiene content results in better low-temperature flexibility.
HNBR – Hydrogenated Nitrile Butadiene Rubber HNBR is the hydrogenated variant of NBR. The elimination of the double bonds in the butadiene part increases the resistance to oxidation, chemicals and oils. Simultaneously, the resistance to aromatics or polar substances is reduced.
EPDM – Ethylene Propylene Diene Rubber EPDM elastomers are insensitive to physical influences and have a high resistance to aging. The polymer shows good chemical compatibility, even with more polar solvents, while it is less resistant to low molecular weight hydrocarbons.
EPDM is produced by copolymerising ethylene, propylene and a non-conjugated diene, mostly 1,4 hexadiene or dicyclopentadiene.
FKM – Based on Vinyldenfuorid- Hexafluorpropylen-Copolymer Fluoro-Elastomer (FKM) rubber is a product group of elastomers based on fluorocarbon polymers. They are characterised by good high temperature properties and excellent chemical resistance.
Base Oils To assess the seal compatibility, the following ester base oils were used without additivation:
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