The broadening of the carbonyl absorption at 1744 cm−1[31] is associated with the formation of aldehydes, ketones, carboxylic acids and esters as the result of the oxidation of linseed oil. In the discussion that follows, we will employ the band at 1463 cm−1 representing CH2 bending [5] as a reference peak for normalising other peaks owing to the invariance of 1463 cm−1 peak with respect to the oxidation reaction of linseed oil.
Fig. 4(a) shows the trends of peaks 970 cm−1 and 987 cm−1 during the oxidation process indicating that a trans conjugated double bond is formed as the reaction progresses. This confirms the above mechanism in which non-conjugated double bonded acids progressively react to form conjugated compounds. This transition occurs after hydrogen abstraction and addition of oxygen to the unsaturated compounds. The broad band in the range of 3100–3600 cm−1 shown in Fig. 4(a), which progressively increases during the oxidation process, suggests the formation of peroxides, alcohols and acids during the oxidation of linseed oil. The relative absorbance of the peak at 1744 cm−1 increases with oxidation time as shown in this figure. The secondary oxidation products containing carbonyl groups as the result of peroxide decomposition and scission reaction lead to the increase in absorbance of this band. Fig. 4(a) also illustrates the decreasing of the peak at 722 cm−1 associated with cis-double bond compounds and also from CH2 bending vibrations. Some of the cis-double bond compounds undergo isomerisation to trans double bonds resulting in appearance of a peak at 987 cm−1. In addition to this geometric isomerisation, some cis-double bond compounds are involved in saturation and scission reactions.