Oxidative Crosslinking of Unsaturated Fatty Acids Studied with Mass Spectrometry

Abstract

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.

Autoxidation of Conjugated Linoleic Acid Methyl Ester in the Presence of α‐Tocopherol: The Hydroperoxide Pathway

“…The mechanisms for the formation of the oligomers and the identified secondary products remain more or less a matter of speculation, due to the lack of knowledge of the early autoxidation steps and of the preceding autoxidation products. Furthermore, analyses of oligomeric product mixtures can be anticipated to be challenging due to heterogeneity of the oligomers (Muizebelt & Nielen 1996;Luna et al 2007) and therefore characterization of the primary products may prove helpful in elucidation not only of the formation mechanisms of the oligomers but also of their structures. Moreover, if sufficient quantities of pure primary products could be produced, their further reactions may be investigated and their biological activity evaluated in a quantitative manner.…”

“…For example, some important mechanistic conclusions have been drawn based on MS analysis of the oxidative crosslinking products of ethyl linoleate and methyl 9-trans,11-trans-octadecadienoate (Me 9t,11t-CLA) formed in the presence of a Co/Ca/Zr drier (Muizebelt & Nielen 1996). While the oxidative crosslinking of ethyl linoleate yielded dimers to tetramers, that of Me 9t,11t-CLA yielded dimers to hexamers.…”

“…19). However, this does not explain the observed heterogeneity of the oligomeric fraction (Muizebelt & Nielen 1996). In effect, the autoxidative polymerization can be envisioned to involve several basic chemical reactions in the propagation and termination phases of the autoxidation, as discussed in Pajunen & Kamal-Eldin 2008, pp.…”

A study of new manganese complexes as potential driers for alkyd paints

“…Since, in alkyd paint drying, most of the chemistry takes place at the unsaturated fatty acid tails of the binder compound, the entire binder resin can be modeled by using compounds that structurally or chemically resemble this fatty acid chain. Dienes such as 4,7-heptadiene, 9 3,6-nonadiene, 10 and 2,5-undecadiene, 11 as well as methyl and ethyl esters of the unsaturated fatty acids linoleic, linolenic, and ricinoleic acid, [12][13][14][15] have been used as model compounds for alkyd resin precursors. An advantage of this approach, besides having a much simpler system to study, is that when model compound mixtures become 'dried', they remain liquid and consequently are more easily studied using standard analytical techniques.…”

“…These techniques include 13 C-NMR, 10-13 GC, 13 HPLC, [9][10][11] FT-IR, 14,[16][17][18] SEC, 12,13 and SIMS. 15 To screen different metal complexes quickly for their ability to enhance the 'drying' of EL, the two more useful parameters to monitor are the rate of the autoxidation reaction and the extent of the crosslinking that takes place. Time-resolved Fourier transform infrared spectroscopy (FT-IR) has been used to study the kinetics of EL oxidation.…”