Low temperature oxidation reactions of linseed oil and its active components

Drying oils, especially linseed oil, have been used since the 15th century as a painting medium. Hardening of linseed oil is a complex and little understood process involving autoxidation that has frequently led to the spontaneous ignition of rags doused with linseed oils. In the present study, low temperature oxidation of linseed oil and its active components, linoleic, linolenic and oleic acids in 50 mol% 0₂ has been investigated in a plug flow reactor, with and without an added cobalt catalyst. Glass wool, silanised glass wool and cotton wool have been impregnated with the drying oils and held in the flow reactor at an initial temperature between 60 and 100 °C. Online analysis of products has been performed by micro gas chromatography. Sampling gas analysis involved μGC, FTIR spectroscopy and GC-MS. Reaction has been monitored over a period of several hours. An induction period which depends on the substrate and the initial temperature has been observed prior to rapid increase in the evolution of products of oxidation including CO, C0₂ and H₂0. The ratio of carbon monoxide to carbon dioxide becomes larger as the initial temperature increases suggesting distinct pathways for the evolution of CO and C0₂. FTIR spectroscopy indicated the presence of aldehydes and carboxylic acids, with identification of these species confirmed by GC-MS. Light hydrocarbons, especially ethane and ethylene were quantified by the micro gas chromatography. Cobalt(II) nitrate catalysed the autoxidation reactions. In the absence of a catalyst, the oil samples were found to undergo a slower hardening process, probably associated with cross-linking or polymerisation together with partial fragmentation to form the observed low molecular weight products.