Understanding the degree of estolide enzymatic polymerization and the effects on its lubricant properties

Abstract The aim of this study was to understand estolide enzyme polymerization from free ricinoleic acid (FRA) and its lubricant properties, by controlling the reaction time, during the course of a reaction involving a two-step enzymatic hydroesterification from castor oil. Adding water increased the reaction rate, reducing the total acidity to 30% in 24 h. FRA is the major component of castor oil (up to 90%). A GC analysis of the FRA, expressed as g/100 g of sample, was used to determine the FRA profile during estolide polymerization. These data suggested a relationship between the total acidity of the samples and the FRA concentration. The estolide profile and number (EN) were determined by GPC and NMR analyses. A 14-hour reaction time was needed for total consumption of FRA, with an EN of 5.5 and a degree of polymerization of the major estolide of 6/7. The analysis showed that estolides of several sizes [dimers, trimers, tetramers, and pentamers (+)] were produced during the reaction; between 8 and 14 h the EN grew exponentially until reaching a plateau after about 16 h of reaction. From 14 to 24 h, although the estolide-size profile did not change significantly, EN continued to increase with increasing reaction time, until a polymerization degree of 7/8 at 24 h. The increase in EN corresponds to the increase in the viscosity index and oxidative stability, reaching 164 and 43 min, respectively, after 20 h of reaction; as well as to the decrease in pour point and total acidity number, which reached −48 °C and 36.8 mg KOH/g of sample after 20 h of reaction. This is the first description of a kinetic method to control the degree of estolide polymerization, by controlling the reaction time; and of its relationship between the EN and the lubricant properties of these molecules, aiming toward the production of different biolubricants for several applications and desired uses.