Synthesis of Unsaturated Drying Oils from Saturated Fatty Oils Derived from Renewable Feedstocks

Abstract

Surface coating industry uses drying and semi-drying oils containing varying amounts of oleic, linoleic, and linolenic fatty acids or dehydrated ricinoleic acid (castor oil) fractions (with iodine values in the range of 125–160 g/100 g) for manufacturing alkyd resins. These are used in large volumes in air drying paints and varnishes. Different methods are reported in the literature for increasing the unsaturation in the available fatty oils/acids. Most of these methods use feedstocks that already contain unsaturation. In this research, we studied functionalization of renewable saturated oils (iodine value ≤ 5 g/100 g) as potential replacements for costly unsaturated vegetable oils such as soybean oil, linseed oil, castor oil, and so forth. This process includes radical bromination of saturated fatty acids/oils followed by dehydrobromination using an alcoholic base to obtain the desired unsaturation. Computational studies were performed to find out stability of bromo compounds. Dehydrobromination was found to be quantitative when fully saturated feedstocks were employed. On the other hand, feedstocks bearing unsaturation could not be fully dehydrobrominated, demonstrating thereby that the approach adopted herein is ideally suited to completely saturated oil as a replacement. The maximum achieved iodine value was 125–130 g/100 g for 1:3 ratio of [substrate]/[Br]. Performance tests were carried out on the product, and the results were found to be along expected lines with commercial products. Further increase of the iodine value to 132 g/100 g was achieved by separating out the unreacted saturates by crystallization. In this way, product efficacy could be enhanced while recycling the saturated fraction in a subsequent batch. Separation of saturated components from unsaturated components would also be beneficial for successful use of inexpensive feedstocks such as tallow. It is further shown that the brominating reagent and catalyst can be recycled and reused, making the whole process industrially attractive.