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.