Lube-Tech
The polymers swell when the free energy (ΔG) is negative. Therefore, the change in free enthalpy (ΔH) must be smaller than the change in the product of temperature (T) and entropy (ΔS). This approach explains the temperature dependence of the swelling.
Starmer describes the change in enthalpy as a parameter controlling diffusion of the base oil through the liquid / solid interface. This effect reaches a maximum with the same polarity of liquid and rubber. The change in entropy, on the other hand, is responsible for the enclosure of the base oil in the polymer matrix. Smaller molecules of the liquid lead to a greater change in entropy and thus to greater swelling.
Swelling of the rubber is also counteracted by the retractive force of its network structure which is related to the crosslink density. De Broutelles (de Broutelles, et al., 1975) showed that the swelling of a rubber is inversely proportional to the average molecular weight between the crosslink, which stands for a higher density of the network structure.
At the same time, the molar volume between the crosslinks is lower with a higher degree of crosslinking. This results in a smaller change in entropy, which tends towards zero with the same or greater molar volume of the base oil compared to the volume within the network structure. This is shown in figure 6 where the increase in molar mass leads to lower swelling.
Figure 7: Swelling of NBR 1 at 80 °C for 168 h in TMP TO with different hydroxyl values
Figure 6: Volume increase of NBR 1 at 80 °C for 168 h as a function of molecule size
As displayed in figure 7, there is a significantly higher swelling with increasing hydroxyl value. A doubling of the hydroxyl value almost doubles the swelling, both in terms of mass and volume. It can be assumed that the hydroxyl groups have a similar contribution to polarity as the ester number. At the same time, a higher hydroxyl value means there is a higher content of TMP dioleate (TMP DO) in the product. The fact
28 LUBE MAGAZINE NO.167 FEBRUARY 2022
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.138 page 5
For the same type of polymer, these parameters can be changed, for example, by the type of crosslinking or usage of additives, such as fillers.
It also becomes clear that the resistance to lubricants is a specific material property. This explains the fundamental differences between EPDM and FKM materials
The results presented so far were determined on production samples that are characterised by a good degree of esterification. However, there are also products on the market that do not have this characteristic. A lower degree of esterification is reflected in a higher acid value (AV) and hydroxyl value (OHV), i.e. ultimately a higher proportion of the starting materials within the product.
In order to study this influence more in detail, TMP TO with different hydroxyl values were synthesised on lab scale. The acid value of the esters was below 0.6 mg KOH / g.
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