The drying of alkyds was studied using NMR and mass spectrometry employing model compounds. Crosslinking occurs via oxidation of unsaturated fatty (linoleic) acids in the resin. Hydroperoxides formed as intermediates are degraded to alkoxy and peroxy radicals by cobalt catalyst. These radicals then recombine to form mainly ether and peroxy crosslinks. However, with conjugated fatty acids another mechanism prevails in which addition of radicals to the double bonds occurs. Despite the differences in mechanism the rates of both reactions were found to be similar. High-solids alkyd coatings may employ reactive diluents replacing traditional solvents (white spirit). The rate of incorporation of these reactive diluents into the actual paint film during drying can be followed with NMR. Allyl ether groups appear to react fastest whereas allyl esters show generally little reactivity. Using mass spectrometry it was found that incorporation involves recombination of radicals as well. However, reactive diluents with conjugated double bonds will be incorporated by radical addition.
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