Chemical processes accompanying the autoxidation of paint films

Abstract

Film forming systems were prepared from allyloxy compounds, in conjunction with polyacrylates, together with a redox promoter and solvent. The allyloxy compounds used were based upon two broad types namely (1) tris-(N-allyloxymethyl-N-methoxymethyl) melamine and (2) tri (2-(3' propenoxymethyl)ethylene) glycol 1-ethyl ether. The structure of the crosslinked matrix appeared to consist mainly of polymerised allyloxy compound. If the polyacrylate, (derived from butyl acrylate, methyl methacrylate and methacrylic acid), used in the film forming system consisted of non-derivatised carboxyl groups along the chain, then no chemical interaction seemed to occur between the drying system and polymer. The polymer merely became entrapped in the crosslinking matrix of the allyloxy compound. However, if some of the free carboxyl groups were reacted with allyl glycidyl ether, so that some pendant allyloxy groups were present, then the rate of drying was marginally increased and more polymer was incorporated in the dried film than with the unmodified polyacrylate. These conclusions were based upon a combination of results using G.P.C., FTIR and radiolabelling. The 'yellowing' of alkyd films, particularly in the dark and at elevated temperatures was investigated by attempting the characterisation of the yellow productsderived from trans-2-hexenal. This compound has been identified as one of the autoxidative degradation products of both the methyl esters of linoleic and linolenic acids and alkyds derived therefrom. Analysis by gc-ms, with El and NH3-CI and accurate mass measurement of the derivatised alkali soluble products derived from trans-2-hexenal, which had yellowed in the dark at 70°C, gave structures which showed epoxide formation with possible rearrangement to carbonyl compounds. 2-Propenal was identified as a by-product of the autoxidative degradation of the allyloxy drying systems. Similar analysis by gc-ms of 'yellowed' 2-propenal gave structures of the form H(CH=CH-CHOH)- CH=CH-CHO. It is postulated that with sufficiently large n and oxidation of the secondary alcohol function that compounds would have a chromophore in the yellow region of the spectrum.