ASPECTS OF THE INHIBITION OF POSTCURE PHOTOYELLOWING OF NOVEL AMINE-DIACRYLATE-TERMINATED ULTRAVIOLET-BEAM-CURED AND ELECTRON-BEAM-CURED TMPTA-BASED COATINGS

The post-ultraviolet (UV) and -electron-beam (EB) cure photo-yellowing of four novel amine cosynergist terminated diacrylate monomers has be en compared with that of a standard commercial diethylamine diacrylate monomer based on trimethylol propane triacrylate (TMPTA) using second -order derivative UV spectroscopy. Inhibition of photo-yellowing was a lso studied using four acrylated hindered piperidine stabilisers as we ll as an acrylated hindered phenol. Of the amine co-synergists the die thylamine diacrylate system gave the greatest degree of photo-yellowin g in both UV- and EB-cured coatings whilst those based on piperazine g ave the least yellowing. The presence of readily abstractable hydrogen atoms alpha to the nitrogen atom and subsequent ease of amine oxidati on appears to be a key feature in the yellowing mechanism. The incorpo ration of acrylated and methacrylated hindered piperidine stabilisers as well as that of a hindered phenol are effective in impairing the ph oto-yellowing of both the UV- and EB-cured diethylamine diacrylate sys tem. Of the stabilisers studied the acrylated hindered phenol was the most effective in both UV- and EB-cured coatings. Inhibition of hydrop eroxidation of the tertiary amine functionality in the amine diacrylat e coating is considered to be of primary importance in the overall mec hanism. Significantly lower levels of photo-yellowing are observed in the non-amine TMPTA. Here UV curing of the stabilised coatings gave ri se to an initial yellowing which was photodecomposed on subsequent irr adiation, the rate depending upon the stabiliser structure. The EB-cur ed coatings however, exhibited no initial yellowing followed by only a minor growth in photoyellowing on subsequent irradiation, again with a rate dependent upon the stabiliser structure.