Innovative strategies to improve the drying properties of castor oil
Because castor oil is a non-drying oil, interesterification and physical blend of castor oil with sacha inchi oil were compared as methods to produce drying oils that could be used for the synthesis of alkyd resins.
First of all, no significant differences were detected between the interesterified product and the physical blend by FTIR, 1H NMR and other chemical analyses (fatty acids content, viscosity and colour).
However, a remarkably different oxidative behavior between the blend and its interesterified product was observed, i.e., the oxidative induction time of the physical blend was 0.08 h, significantly lower than that of the interesterified product (11.20 h). This indicates that, in addition to fatty acid composition, triacylglycerol composition and fatty acid location at glycerol have a strong effect in the oxidative stability. The ethanol solubility test showed a total dissolution of the interesterification product while the physical blend showed turbidity due to the non-polar nature of the sacha inchi oil.
Quite similar properties
The alkyd resins obtained from the physical blend and the interesterified product presented very similar characteristics of chemical composition, acid value, color, non-volatile matter, and viscosity. FTIR and 1H NMR analyses did not show significant differences between both resins. The drying times (set-to-touch, dry-to-touch, dry-hard, and dry-to-handle times) were similar regardless of the resin type. However, the cured film obtained from the interesterified product showed a higher hardness (scratch and gouge) than that of the physical blend. This higher scratch/gouge hardness of the cured films derived from the interesterification product can be explained due to the possibly higher degree of crosslinking of the resin, which is consistent with the higher rate of peroxide radical production observed by FTIR.
The other properties of both cured films were quite similar, i.e., water resistance (good), alkali resistance (poor), acidic resistance (good), non-polar solvent resistance (acceptable), adhesion (strong), and drying times.
The study has been published in Progress in Organic Coatings, Volume 162, January 2022.
