Lube-Tech
Figure 1: Classical primary ester of a neo-polyol ester (NPE).
There is a vast array of polyols available beyond those used for the manufacture of NPEs. By carefully selecting the choice of polyol, it is possible to design a wide range of secondary polyol esters. A simplistic structure for an SPE™ bond is shown (Figure 2).
Figure 2: Secondary ester of an SPE.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.144 page 3
The choice of polyol and acid, and indeed their mixtures, can lead to a very wide range of SPE products which have low viscosities or very high viscosities, and these can be saturated or unsaturated SPEs. Their functional and tribological characteristics can be controlled to a degree, by manipulating the polymer architecture. It is therefore possible to tailor this chemistry to design base oils for a specific application. This paper reports on SPEs exhibiting viscosities in the range ISO VG 32 to 100.
Properties and Functional Aspects The lightly branched SPE structures offer a significant increase in the hydrolytic stability of the ester
bond and improves low temperature properties. Because of this structural feature, SPE base oils can utilise saturated fatty acids in their design without sacrificing cold temperature performance in contrast to, for example, natural esters in which a high saturated fatty acid content results in high pour points or solidification at room temperature. Furthermore, for the saturated SPEs this leads to more thermo-oxidatively stable products due to the absence of olefinic moieties in their structures. Although unsaturated esters are known to be more prone to oxidation, the oxygen-rich structure of unsaturated SPEs results in minimal or no deposit formation when they thermally degrade. This is also a well-known feature of PAGs. The high polarity of SPEs and PAGs facilitates the solubilisation of dangerous oxidation by-products that can deposit on equipment surfaces. This functionality is an important benefit over conventional mineral oils which can degrade to form insoluble polar deposits in the non-polar parent base oil that may lead to varnish formation on surfaces and result in equipment failure. This issue can be mitigated by including SPEs as additives or co-base oils in formulations where the primary base oil is a hydrocarbon.
The viscosity indices of SPEs are also high, in part due to the choice of polyol in the backbone such that their viscosity indices are similar to the high values associated with PAGs. Typical physical properties for structures in the ISO VG 32-100 range are shown in Table 2.
Table 2: SPEs derived from saturated (S) and unsaturated (U) acids.
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LUBE MAGAZINE NO.173 FEBRUARY 2023
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