Unravelling the effect of carbon black in the autoxidation mechanism of polyunsaturated oils

Carbon black-based particles are widely employed as pigment and they are known to slow down the drying time of oil paints. In this work, the effect of amorphous carbon black on the mechanism and speed of autoxidation of a polyunsaturated oil was investigated. Moreover, the effect of the addition of aluminium stearate and zinc stearate was studied. These are two common additives used in the artists’ paint industry to facilitate pigment dispersion. The curing of the oil paints with and without the addition of the two stearates at 80 °C under airflow was followed by isothermal Thermogravimetry. The oxygen uptake profiles were fitted by a semi-empiric equation to comparatively study the kinetics of the oil oxidation and estimate oxidative degradation. Moreover, model paintings were left to cure at ambient conditions and Differential Scanning Calorimetry was then used to monitor their curing progress over time and to evaluate the stability of peroxides formed in the paint layers. Gas Chromatography–Mass Spectrometry was performed at 7 and 12 months of natural ageing of the model paintings, to investigate the non-covalently cross-linked fractions. Analytical Pyrolysis coupled with Gas Chromatography–Mass Spectrometry was used to characterise the whole organic fraction of the model paintings, including the cross-linked network. Amorphous carbon has an antioxidant effect and inhibits the radical chain propagation. The presence of aluminium and zinc stearates in the black paint affects the autoxidation process, by leading, in the first months, to a faster consumption of unsaturated moieties, and, accordingly, to accelerate and increase peroxides formation. After a few months though, the whole curing slows down, and active peroxides and radicals are still present even after 12 months.