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 t he basis of iodometric titration. The peroxide value (PV) vs. oxidation tim e curves show similar developments at the six temperatures chosen for the e xperiments. 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 m aximal PV is higher when the curing temperature is lowered. This result ind icates thermal decomposition of the hydroperoxides. The peroxy crosslink co ncentration in the dried oil film varies similarly to the hydroperoxide con centration. This indicates that, for dried films, the network is almost tot ally 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 fo r hydroperoxide decomposition, based on an induced decomposition of the hyd roperoxides, has been proposed. These re suits have permitted completing th e description of the curing mechanisms of drying oils.