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Lube-Tech Introduction


Synthetic esters have been used as base fluids or additives for lubricants for decades. The initial driving force behind this evolution were worries about the finite nature of crude oil reserves. But thoughts about toxicity and biodegradability also began to play a significant role. Sustainability is an aspect of ever greater importance. Following nature’s example, circular material cycles using renewable materials are to be implemented as far as possible. A zero balance in CO2


emissions is one of the important factors here.


The form of these sustainability factors is, to a large extent, determined by the base fluids of the lubricants. Mineral oils are still predominantly in use today but increasingly being replaced by synthetic esters.


Esters stand out when compared to mineral oils due to a number of advantages. On account of the ester group’s polarity, they exhibit a high affinity to metallic surfaces. This results in good lubricating properties. In addition, they have lower volatility than mineral oils along with a high viscosity index, which means the viscosity is less dependent on the temperature. By choosing different polyols, using various fatty acids and esterification with dicarboxylic acids, it is possible to vary properties such as viscosity and viscous behaviour, stability and cold-temperature behaviour over a wide range. However, not all properties can be optimised simultaneously since many are diametrically opposed in their behaviour. For example, improved resistance to hydrolysis inevitably results in reduced biodegradability (Stolz, et al., 2019).


26 LUBE MAGAZINE NO.160 DECEMBER 2020


PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


No.131 page 1


Estolides - The innovative alternative of ester base oils


Dr. Hermann-Josef Stolz, Dr. Wilhelm Huber, Mrs Nadine Flügel, Peter Greven GmbH & Co.KG


For specific applications it would be desirable to have esters with more degrees-of-freedom of modification, since this enables further optimisation of the products. The estolides provide a highly promising approach for this matter.


The substance class of estolides Estolides are oligomeric esters based on a special fatty acid with an additional functional group. They cover a wide range of viscosity and, by virtue of their adaptable molecule, can have product properties that vary within a spectrum of above-average width.


Figure 1: Basic structure of an estolide


Figure 1 shows the basic structure of an estolide. Here, a hydroxy-fatty acid forms the basis of the estolide. This can form oligomers at first by esterification with itself. The extent of oligomerisation is expressed as estolide number (EN). The hydroxy acids used may come from natural sources such as ricinoleic acid, for example. This is available as such or in its hydrogenated form. Alternatively, a hydroxy acid may be obtained from unsaturated fatty acids using mineral acids as a catalyst. The oligomerisation reaction is interrupted by a capping fatty acid which, in turn, by skilful selection, offers the ability to influence the product properties.


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