Synthesis and physical properties of estolide-based functional fluids

Abstract

Biodegradable, vegetable oil-based lubricants must have better low temperature properties as well as comparable cost to petroleum oil before they can become widely acceptable in the marketplace. These include low pour point temperature, low viscosity and viscosity indices that do not change over a wide temperature range. Our objective was to synthesize estolides from various sources with improved lubricating physical properties. Oleic acid and lauric acid were treated with varying equivalents of perchloric acid at 60 °C to produce complex estolides. Yields ranged between 45 and 75% after purification by Kugelrohr distillation. The estolide number (EN), the average number of fatty acid units added to a base fatty acid, varied with reaction conditions. The saturate-capped, oleic estolides were esterified with 2-ethylhexanol to obtain high yields of the corresponding ester. Coconut–oleic 2-ethylhexyl estolide esters were produced by varying the ratio of oleic and coconut fatty acids with 0.05 equivalents of HClO 4 to give estolides with excellent cold temperature properties. The amount of oligomerization (EN) played an important role on viscosity; viscosity increased with higher oligomerization. The free acid estolides were generally several hundred centistokes (cSt) more viscous than their corresponding esters. The viscosity index ranged from 141 to 170 for the free acid estolides, whereas the complex estolide 2-ethylhexyl esters had slightly higher viscosity indices, which ranged from 159 to 232. These new coco–oleic estolide esters displayed superior low temperature properties (−36 °C), were of reasonable cost, and were more suitable as a base stock for biodegradable lubricants and functional fluids than current vegetable oil-based commercial materials.