Lube-Tech
the same. Nevertheless, the differences in the mass absorption shown in figure 3 result primarily from the contribution of the double bonds to the overall polarity of the molecules.
Therefore, the proportion of double bonds was also considered in a newly calculated npi which is called npi 2. Npi and npi 2 are both displayed in figure 3. This approach enables a better explanation for the swelling behaviour. However, as expected the double bonds have less influence on the swelling action than the carboxyl groups.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.138 page 4
Figure 4: Volume increase of NBR 1 at 80 °C as a function of the polyol type
Figure 5 shows the swelling of TMP TO for different elastomers. The data for NBR 1 and HNBR 1 are not surprising. The hydrogenation of the double bonds in the HNBR reduces the polarity. Therefore, HNBR is more stable for polar base oils like esters and there is less swelling. Furthermore, the results show that EPDM 1 is incompatible and FKM 2 is resistant. This swelling cannot be explained by the npi. Due to its more non-polar nature, EPDM, for example, should be significantly more resistant.
Figure 3: Mass increase of NBR 1 at 80 °C as a function of the iodine value (IV)
Figure 4 shows the swelling behaviour of four different polyol esters based on oleic acid. It becomes clear that the different swelling actions cannot only be explained by the npi. The gap between NPG and glycerol esters is somewhat more pronounced than anticipated from the npi. The TMP ester does not behave as expected and shows significantly lower values. The size of the molecules will certainly play a role here, but also the difference in steric arrangement of the acid group. However, all these factors cannot explain the swelling of the PE ester. Less swelling would be expected due to the molecular size, the npi and the steric arrangement of the fatty acid groups. Obviously, in the area of low swelling other factors also need to be considered. Fundamental aspects of thermodynamics, as discussed in the next section, probably play an essential role here.
Figure 5: Swelling of different polymers at 80 °C for 168 h in TMP TO
In order to explain the unexpected behaviour of EPDM 1 and FKM 2 the following aspect could be considered: According to Starmer (Starmer, 1993), the swelling behavior of elastomers can be described by the Gibbs-Helmholtz equation:
LUBE MAGAZINE NO.167 FEBRUARY 2022
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