Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE DSC
One of the most efficient methods for investigating the thermal stability of esters is the Differential Scanning Calorimetry (in short: DSC). The principle of the method provides a heat flow differential measurement between the sample and an inert reference. Both samples are placed in a furnace which has a selectable heating rate and brought to specified temperature. If an endothermic or exothermic reaction takes place in the sample, this reduces or increases the heat flow between the furnace and the sample. This leads to a temperature difference to the comparison sample. A HP-DSC, which worked isobarically with 100 bar nitrogen, was used for investigations as part of this work. In this way oxidation, but also faults due to evaporation of low molecular components, are avoided.
If there is thermal decomposition of ester oils there are endothermic reactions. The evaluation of the onset and the peaks of the response curve allow statements about the thermal stability.
Test results Various model substances were tested in order to investigate the stability of saturated and unsaturated esters. Various polyols were used, as well as substances networked with dimeric acid. From the trimethylol propane a triester of a commercial oleic fatty acid and a triester of caprylic-caproic acid (50:50) were used. Because there are significant differences in viscosity in both variants, a partly networked ester was manufactured with adipic acid and C8-C10. This, as well as the trimethylol propane trioleate, has a viscosity of 46 m² s-1.
Oxidation stability Figure 8 below shows the results of the RPVOT test graphically. If one looks at the results of the ISO VG 46 esters, the saturated esters, as was to be expected, have a significantly better oxidation stability. The reason for this is that the unsaturated fatty acids promote the bonding of ROO. radicals.
32 LUBE MAGAZINE NO.150 APRIL 2019
No.121 page 5
Figure 8. Results of the RPVOT measurements.
On the other hand, the effect is significantly lower in the complex esters of the ISO VG 320. The unsaturated complex esters reach values, which also lie in the area of saturated esters. This difference can be attributed to the steric hindrance, as well as to the low iodine value contribution of the mono-carboxylic acids.
The pure base liquids - mineral oils, polyalphaolefins (PAO) and polyalkylene glycols (PAG) included - generally have values, which are considerably lower than the requirements of the formulated lubricants. In the RPVOT, for example, usually over 200 hours are required for ester based hydraulic oils and for turbine oils more than 600 hours. The additives are crucial, since there is no base oil as such that fulfils these conditions. If unsaturated fatty acids are used special attention must be given to the additive package.
Hydrolytic stability The results of the beverage bottle test are shown in Figure 9. These show that the unsaturated esters - contrary to popular opinion - generally prove to be hydrolytically more stable. The reason for this is primarily the steric hindrance due to the larger scale structure of the oleic acid. This reasoning is supported by the fact that the penta esters within the polyol test series had the highest stability.
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