Physical properties of saturated estolides and their 2-ethylhexyl esters

Abstract

Biodegradable, vegetable oil-based lubricants must have better low temperature properties before they can become widely acceptable in the marketplace. These low temperature properties are usually measured as the material's pour point, the minimum temperature at which a material will still pour. Viscosity and viscosity index also provide information about a fluid's properties where a high viscosity index denotes that a fluid has little viscosity change over a wide temperature range. Oleic acid and a series of saturated fatty acids, butyric through stearic, were treated with 0.4 equivalents of perchloric acid at either 45 or 55 °C to produce complex estolides, dimers and tetramers of fatty acids linked through the double bond and carbonyl group. Yields varied between 45 and 65% after Kugelrohr distillation. The estolide number (EN), the average number of fatty acid units added to a base fatty acid, varied with reaction temperature as well as with the change in saturated fatty acids. The saturate-capped, oleic estolides were esterified with 2-ethylhexanol to obtain high yields of the corresponding ester. As the chain length of saturate capping material increased from C-4 to C-10, the low temperature performance of the estolide 2-ethylhexyl esters, namely pour point, decreased to −39 °C. The other mid-chain, saturated estolide 2-ethylhexyl esters C-6 through C-14 also had superior low temperature properties compared with their competitors; i.e. soy-based, synthetic-based and petroleum-based oils. The amount of oligomerization (EN) had an important role with the viscosities. Viscosity increased with higher oligomerization and the free acid estolides were generally several hundred centistokes (cSt) more viscous than the corresponding esters. The viscosity index ranged from 122 to 155 for the free acids estolides while the estolide 2-ethylhexyl esters had slightly higher indices which ranged from 172 to 196. These new estolide esters displayed far superior low temperature properties, and were more suitable as a base stock for biodegradable lubricants and functional fluids than current commercial materials.