Conclusions
Experiments with four model molecules (stearic acid, trans-7-tetradecene, methyl oleate and oleic acid) under high frying thermal oxidation (210°C, 3h, in air) demonstrated that oxidative polymerization predominates only when there are olefin and carboxylic acid functions on the same molecule. Oleic acid is thus an ideal model molecule for investigating the initial polymerization events in vegetable oil.
NMR spectroscopy experiments identified the first major (non-volatile) thermal oxidation products generated. Under our oxidation conditions, with model molecules, we evidenced: stearic acid forms a bit of internal ester, trans-7-tetradecene forms (30%) epoxide, methyl oleate largely forms (90%) epoxide and oxidized monomers while oleic acid forms (40%) much larger species and fragments. Changes in NMR signal intensities reveal oxidation occurs at the double bond. Long-range13C-1H correlation experiments confirm there are no species with oxygen near a double bond.
Monomeric epoxide species (M2), formed first during oxidization of oleic acid at 210°C, undergo rapid di-acylation through inter-molecular ester cross-links (T1 in Figure 11 ). Fragmentation of oleic acid during oxidation was revealed by formation of methylene groups bonded to oxygen (CH2-O). Observation of these unprecedented fragments (CH2-O rather than CH2-C(O)O) along with the di-ester trimer demonstrate experimentally the first steps in vegetable oil polymerization.
Ester cross-links are seldom contemplated in vegetable oil polymers. Drying and poly-unsaturated oils are thought to increase molecular size by ether cross-links. Here we show, for a mono-unsaturated oil, the reactive olefinic groups form epoxides which then cross-link through ester groups. Surprisingly, this knowledge, from experimental NMR observations and model chemical reactions, is being realized only now . The first trimer formed for oleic acid is still reactive since it possesses both oleic and carboxylic acid functions. This allows propagation of deleterious polyesters at longer reaction times.
Facile formation of polyesters provides a new perspective on oxidation of lipids and vegetable oils. This report dispels traditional, widely accepted, reaction schemes involving ether cross-links and hydroxy-olefin species.