Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


esterification at 170°C, using an ion exchange resin catalyst (Purolite CT269 and CT169) to reach quantitative reaction yields. After synthesis, the esters were distilled and purified.


The purity standard of the lubricant industry for ester base oils is typically higher than 98%. After thorough purification, the sample of PE-C5 could be purified to 98.2% and PE-C6 and PE-C7 reached 97.9% purity, as shown in Table 1. The remaining percentage consists of incomplete pentaerythritol esters (tri-esters instead of the desired tetra-esters). It is also hypothesised that some esters of di- and tri-pentaerythritol are present. Acid values and water content met the requirements reported in the literature and the market standard.


No.157 page 3 Kinematic Viscosity (ISO 3104)


Figure 4: Kinematic viscosity of PE-C5, PE-C6 and PE-C7


Viscosity index (ISO 2909) (ISO 3104)


Table 1: Purity, acid value and water content of the three synthesised POEs.


All three batches of POEs had a light odour and were clear with a slight yellow colour. Due to the possibility of slight degradation of the catalyst during the synthesis, the final products were tested for residual sulphur; all samples were below 500 parts per million. This catalyst residue might have affected the base oil properties; however, adjustments in the synthesis process can easily avoid this issue.


To evaluate the potential of PE-C6 as a base oil, its general physicochemical properties were analysed and compared with those of PE-C5 and PE-C7.


The kinematic viscosities of PE-C6 at 100°C and 40°C fall between those of PE-C5 and PE-C7 (Figure 4). The same is observed for the viscosity index (VI) (Figure 5). As expected, the viscosities and VIs increase with the length of the fatty acid chains.


Figure 5: Viscosity index of PE-C5, PE-C6 and PE-C7


Base oils used in internal combustion engine (ICE) oils, electric vehicle (EV) fluids, and aviation lubricants can benefit from lower viscosities, achievable through the use of low molecular weight POEs. These applications also benefit from low volatility for safety and durability reasons. The volatility of all three molecules was found to be lower than 1.5% weight loss at 150°C for 22 hours, as shown in Figure 6.


Compared to PE-C5, PE-C6 has a lower volatility, offering a better compromise between viscosity and volatility for many applications. Volatility is influenced by factors such as purity and molecular weight, with the general trend being a decrease in volatility as the fatty acid chain length increases. However, the results showed a deviation: PE-C7 exhibited higher volatility than PE-C6, which could likely be due to the


LUBE MAGAZINE NO.186 APRIL 2025 35


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