Long-term behavior of oil-based varnishes and paints.: Fate of hydroperoxides in drying oils

The thermal stability of linseed oil and poppy-seed oil hydroperoxides in a temperature range from ambient to 120 degrees C has been investigated on the basis of iodometric titration. The peroxide value (PV) vs. oxidation time curves show similar developments at the six temperatures chosen for the experiments. These curves are characterized by a fast increase in the PV up to a maximal value, followed by a decrease in the PV at a lower rate. The maximal PV is higher when the curing temperature is lowered. This result indicates thermal decomposition of the hydroperoxides. The peroxy crosslink concentration in the dried oil film varies similarly to the hydroperoxide concentration. This indicates that, for dried films, the network is almost totally constituted of ether and C-C crosslinks. A comparison of the rates of peroxide decomposition under thermolytic and thermooxidative conditions has evidenced that the only homolytic scission of the O-O bonds cannot justify the decrease of the PV in thermooxidation. Another mechanism accounting for hydroperoxide decomposition, based on an induced decomposition of the hydroperoxides, has been proposed. These re suits have permitted completing the description of the curing mechanisms of drying oils.