Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
Estolides Estolides are naturally occurring branched lipid structures formed by esterifying secondary fatty acids are esterified to hydroxyl groups of a primary fatty acid [5]. This provides estolides with a unique set of characteristics: the derived hydroxyl ester linkages lead to an increased polarity and metal surface adhesion, while the branched structure disrupts ‘molecular packing’ to maintain fluidity at lower temperatures [5]. The estolides identified in Orychophragmus violaceus seed oil truly shows this molecular design, consisting of TAG molecules where a single glycerol bound fatty acid carries additional fatty acids esterified to hydroxyl groups at the C7 and C18 places [5, 6].
Tribological testing of Orychophragmus violaceus oil exemplified a significant tribological improvement: the coefficient of friction demonstrated a value about three times lower than castor oil at 100°C, alongside reduced wear rates and better resistance to metal surface oxidation [5]. The biosynthesis of estolides needs enzymatic activities such as the hydroxylation of fatty acids (typically catalysed with FAD2 enzymes like OvFAD2-2) and the esterification of additional fatty acids to these hydroxyl groups [6]. In the elongation pathway, OvFAE1-1 extends three hydroxy intermediates without finishing the steps of normal dehydration and reduction steps, leading to fatty acids with hydroxyl groups that can serve as sites for the formation of estolides [6]. Further, when the genes were expressed together with Arabidopsis seeds, the produced oils showed presence of dihydroxy acids, or molecules that act as sites for estolides. Evidently, engineering this pathway into crop plants is possible [6]. Also, the ester linkages are less susceptible to autoxidation compared to the isolated double bonds in polyunsaturated fatty acids, while the branching structure limits oxidative reactions [5]. All these characteristics make estolides a very promising candidate for the applications of biolubricants, specifically the aspects of biodegradability and durability under heat stress.
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Wax esters Wax esters, made of long chain fatty alcohols esterified to long chain fatty acids, represent the third primary molecular target. Unlike the TAG structure of most plant seed oils, wax esters lack a ‘glycerol backbone,’ leading to higher molecular weight that ranges between 600 and 900 Da, alongside higher viscosities [8]. Jojoba oil, which naturally consists of about 98% of wax esters, has been renowned to have great lubricant properties, particularly the highly sought properties of oxidative stability and low temperature fluidity [7]. The main wax ester species in jojoba oil contains C20 and C22 monounsaturated fatty acids and fatty alcohols, creating molecular structures like C40:2 and C4:2 [7].
Recent efforts in genetic engineering have aimed to transfer wax ester biosynthesis similar to that found in jojoba oil into viable oilseed crops [8]. This requires the coordination of genes responsible for encoding fatty acyl-CoA reductases, which convert fatty acyl-CoA to fatty alcohols, and wax synthases, which drives the esterification of fatty alcohols with fatty acyl-CoA substrates [8]. Wax ester-containing oils from genetically modified oilseed crops have shown promising tribological performance, even exceeding those of conventional mineral-based lubricants under the same extreme-pressure conditions [8].
Stemming from earlier, the basis for this performance lies in the combination of high molecular weight, which enhances film strength and viscosity, and the absence of polyunsaturated acids, which effectively eliminates the main site of oxidative degradation [7, 8]. Further, the linear structure of wax esters leads to more ordered molecular packing that contributes to the formation of robust boundary films on metal surfaces [8]. Although current wax ester production levels in transgenic oilseeds remain lower than the almost complete wax ester content of jojoba oil, improvements through metabolic engineering and optimal gene expression continue to forge the path
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