Lube-Tech
Determination of the change in weight and volume of the elastomers
The seal compatibility was determined according to ISO 6072:2011. The elastomer test specimens were all cut to the same size and weighed. Testing glasses of a proper size were filled with the test liquid. The test specimens were attached to a bracket and then completely immersed in the test liquid. After that, the glasses were closed.
In order to achieve a possibly high comparability of the results, every tested combination of elastomer and ester was stored at 80 °C for 168 hours. When the test period was over, weight and dimensions of the test specimens were determined again.
The changes in weight and volume were calculated based on the measurements before and after the test. Both values are given as a change in percent.
Results and Discussion
The following explanations focus on the swelling behaviour of the elastomers once they get in touch with an ester base oil. Of course, this does not allow any conclusions on the suitability of seals under operating conditions. But it enables an assessment of the resistance of the elastomers to different base oils.
In terms of chemical structure, the fundamental difference between mineral oils or polyalphaolefins (PAO) and ester base oils is their polarity. As hydrocarbons, mineral oils and PAOs are non-polar, highly lipophilic substances. With at least one ester group, esters have polar molecular components. This difference in polarity constitutes the different effects on elastomers. Therefore, esters are more likely to dissolve in more polar polymers than in typically non-polar ones.
EPDM has a saturated, non-polar structure. Double bonds may originate from non-crosslinked diene components. As a result of the hydrogenation, HNBR has a lower polarity than NBR, which represents a
26 LUBE MAGAZINE NO.167 FEBRUARY 2022
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.138 page 3
polymer of medium polarity. FKM polymers also have a low polarity, but are in general very resistant to chemical influences.
Different types of esters also show considerable differences in seal compatibility within the same group of polymers. Therefore, G. van der Waal (Van der Waal, 1985) suggested the use of a non-polarity index (npi). The npi should help to estimate the influence of an ester on the resistance of seals. It is calculated according to the following equation:
total number of carbon atoms X molecular weight of the ester number of carboxyl groups * 100
The test results of 4 different polyol esters based on trimethylolpropane (TMP) are given in figure 2. They show that there is a strong correlation between the npi and the swelling behaviour for TMP TCC, TMP TCo and TMP TO. This applies to both, the weight and volume increase of the test specimens. Based on this information a moderate decrease could also be expected for TMP TiS. However, the data show that this assumption is not correct.
Figure 2: Swelling of NBR 1 at 80 °C as a function of the fatty acid
Other molecular components, such as double bonds or free OH valences, also have a considerable influence. The npi of a TMP tri-ester of the pure stearic, oleic or linoleic fatty acid is approximately
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