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The hardening of alkyd paints for do-it-yourself purposes is an autoxidation reaction with air oxygen. Since the crosslink reactions are hard to follow in an actual coating system, use is being made of model compounds. These are (m)ethyl esters of unsaturated fatty acids (linoleic and ricinoic) which contain the reactive substructures responsible for crosslinking of an alkyd. In this work, the oligomeric mixture resulting from crosslinking was studied with various mass spectrometric (MS) techniques: direct chemical ionization (DCI), fast atom bombardment (FAB), field desorption (FD), electrospray ionization (ESI) and secondary ion (SI) MS. Consistent results were obtained with all techniques: oligomers yielded signals consisting of groups of peaks 16 mass units apart, pointing to a series of oxygenated homologues. Differences were encountered, however, in the number of oligomers that could be seen with the various techniques. Using ESI-MS and SIMS, oligomer formation up to the tetramer (linoleic) or hexamer (ricinoic) was observed. DCI, FAB and FD generally yielded signals from dimers and trimers only. From the detailed ESI and DCI spectra, a difference in the crosslinking mechanism between conjugated and non-conjugated fatty acids becomes apparent (radical addition to the double bond or recombination of radicals, respectively). Also, using a mixture of the linoleic and ricinoic esters, mutual reaction between these two types of fatty acids can be demonstrated.
Muizebelt et al. (Wed,) studied this question.