Wj. Muizebelt et Mwf. Nielen, OXIDATIVE CROSS-LINKING OF UNSATURATED FATTY-ACIDS STUDIED WITH MASS-SPECTROMETRY, Journal of mass spectrometry., 31(5), 1996, pp. 545-554
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.