Long-term behavior of oil-based varnishes and paints I. Spectroscopic analysis of curing drying oils

The curing of drying oils at 60°C has been investigated by Fourier transform infrared spectroscopy and Fourier transform Raman analysis of linseed oil and poppyseed oil. In the first step, hydroperoxides are formed (broad vibration band centered around 3425 cm⁻¹) with concomitant conjugation and cis-trans isomerization of the double bonds (disappearance of cis bands at 3011 and 716 cm⁻¹, appearance of trans conjugated and trans nonconjugated bands at 987 and 970 cm⁻¹). The subsequent decomposition of hydroperoxides in the presence of oxygen leads to the formation of alcohols (nitrite band at 779 cm⁻¹ after nitrogen monoxide treatment), aldehydes (bands at 2810 and 2717 cm⁻¹ in gas phase), ketones (saturated and unsaturated at 1720 and 1698 cm⁻¹, respectively), carboxylic acids (saturated and unsaturated acid fluorides identified at 1843 and 1810 cm⁻¹ after SF₄ treatment), and peresters or γ-lactones (near 1770 cm⁻¹). A rapid decrease in the double-bond concentration is recorded when curing continues, and the formation of epoxides, characterized by a vibration band at 885 cm⁻¹, is observed. Thermolysis experiments have suggested the proposal of a reaction of addition of peroxyl radicals on the conjugated double bonds as a probable mechanism. This mechanism explains both the rapid disappearance of conjugated double bonds and the formation of epoxides as intermediate products observed in the initial step of curing.