Free fatty acids are usually generated spontaneously in oils either by action of enzymes inadvertently entrained during storage or mineral acids inherited from the refining process. The study investigated the effect of the presence of excess fatty acids on the drying/curing behaviour of drying oil admixed with drying catalysts. The oxygen absorption behaviours of Linseed oil (LSO) and rubber seed oil (RSO) on one hand, and that of their corresponding free fatty acids, namely linseed fatty acids (LSA) and rubber seed fatty acids (RSA) on the other hand were first compared followed by drying tests of same samples using the set-to-touch method. Results obtained showed the following order of oxygen absorption behaviour: LSA>LSO>RSA>RSO. Although oxygen absorption is a stage subsequent to drying process, the drying behaviour, however, did not follow the same trend with the oxygen absorption pattern. Whereas the oils/drier mixtures underwent drying between 3 and 4 h after they were casted as thin films, the fatty acids did not at all even in presence of varying concentrations, up to maximum of 50 wt, % of the curing agent. Therefore, the presence of free fatty acids in drying oils is assumed to be capable of hampering the curing of the latter. Hence, it is recommended that drying oils intended as oleoresinous binders for surface coatings be refined so as to remove or reduce to the barest minimum of their free fatty acids content in order to obtain products with optimum drying rate at reasonable time period.
Key words: drying oil, free fatty acids (FFAs), oxygen absorption, drying catalyst, surface coating.
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