OXIDATIVE CROSS-LINKING OF UNSATURATED FATTY-ACIDS STUDIED WITH MASS-SPECTROMETRY

The hardening of alkyd paints for do-it-yourself purposes is an autoxi dation reaction with air oxygen. Since the crosslink reactions are har d to follow in an actual coating system, use is being made of model co mpounds. These are (m)ethyl esters of unsaturated fatty acids (linolei c and ricinoic) which contain the reactive substructures responsible f or crosslinking of an alkyd. In this work, the oligomeric mixture resu lting from crosslinking was studied with various mass spectrometric (M S) techniques: direct chemical ionization (DCI), fast atom bombardment (FAB), field desorption (FD), electrospray ionization (ESI) and secon dary ion (SI) MS. Consistent results were obtained with all techniques : oligomers yielded signals consisting of groups of peaks 16 mass unit s apart, pointing to a series of oxygenated homologues. Differences we re encountered, however, in the number of oligomers that could be seen with the various techniques. Using ESI-MS and SIMS, oligomer formatio n up to the tetramer (linoleic) or hexamer (ricinoic) was observed. DC I, FAB and FD generally yielded signals from dimers and trimers only. From the detailed ESI and DCI spectra, a difference in the crosslinkin g mechanism between conjugated and non-conjugated fatty acids becomes apparent (radical addition to the double bond or recombination of radi cals, respectively). Also, using a mixture of the linoleic and ricinoi c esters, mutual reaction between these two types of fatty acids can b e demonstrated.