Oxidation reactions and spontaneous ignition of linseed oil

The drying properties of linseed oil have been known since the 15th century, and exploited in artistic and industrial paints. Unfortunately, in the presence of metal salts, linseed oil applied to cotton fabric may induce self-heating and spontaneous ignition. The present study elucidates the chemical mechanisms which trigger both phenomena. Low-temperature oxidation of linseed oil and its active components, linoleic, linolenic and oleic acids in equimolar O₂/N₂ mixture has been investigated in a plug flow reactor housed inside an oven at an initial temperature between 60 and 100℃, with the liquids impregnated onto the glass wool support. In particular, we have studied the effect of transition metal salts on the oxidation of linseed oil. We identified the gaseous species produced in the oxidation by means of Fourier transform infrared (FTIR) spectroscopy and gas chromatography–mass spectrometry (GC–MS), and quantified them by micro gas chromatography (µGC). FTIR spectroscopy indicated the presence of aldehydes and carboxylic acids, with identification of these species confirmed by GC–MS. We propose that in the presence of a metal catalyst, the oxidation process involves the formation of metal-dioxygen (superoxide) adducts. The catalytic effect of a metal cation depends on its ability to form superoxide. Cobalt(II) is the most effective catalyst among other transition metal salts, followed by manganese(II) and iron(II). In the absence of a catalyst, we found the oil samples to undergo a slower autoxidation process, probably associated with cross-linking or polymerisation together with partial fragmentation to form the observed low molecular weight products