Oxidative crosslinking of alkyd resins studied with mass spectrometry and NMR using model compounds

The crosslinking of alkyd resins has been studied using ethyl linoleate and methyl ricinoate as model compounds. With quantitative C-13 NMR is was established that ether- and peroxy-crosslinks were formed in roughly equal amounts. Double bonds reacted to give epoxides, endoperoxides, and beta-scission into aldehydes. Using SIMS it was established that dimers through pentamers were formed having included several oxygen atoms. After reduction of peroxide (crosslinks) with stannous chloride, the higher oxygen homologues decreased. In the NMR spectrum of the reduced material, peroxide and epoxide signals completely disappeared. High resolution electrospray ionization mass spectra (ESI-MS) yielded evidence regarding the crosslink mechanism. The non-conjugated linoleic acid was found to crosslink by combination of radicals: dimers were having masses 2(M-1). The conjugated linoleic acid crosslinked by addition of radicals to the double bond and disproportionation, yielding masses 2M as well. (M = mass of ethyl linoleate or methyl ricinoate, respectively, including several oxygen atoms.) EIS-MS of oligomers after isotopic exchange enable estimation of OH and OOH groups.