Photoinduced chemical crosslinking activity and photo-oxidative stability of amine acrylates: photochemical and spectroscopic study

A number of dialkyl and heterocyclic amine-terminated diacrylate resins were prepared by a Michael addition reaction of the appropriate secondary amine with a triacrylate diluent monomer. Photocuring rates are found to be dependent upon the extent of UV absorption of the photoinitiator used with ITX in this case exhibiting the most rapid cure. There is no consistency in cure rate with amine structure. Transient absorption spectra on conventional microsecond flash photolysis are assigned to the formation of free radical intermediates by electron transfer. Photoinduced polymerisation activities of the resins measured by real-time infrared spectroscopy appear to be closely related to the formation of such transient species. The initiator structure is the more determining factor. Their photo-oxidative stability and photoyellowing (by UV derivative absorption) are studied after UV and electron-beam curing through Fourier transform infra-red (FTIR) and second derivative UV spectroscopic methods and hydroperoxide analysis. On photo-oxidation W cured diluent monomer undergoes oxidation at a higher rate than EB cured systems. Initial photo-oxidation of the coatings as measured by hydroperoxide analysis and hydroxyl index via FTIR analysis exhibited similar changes. Again the oxidation profiles are dependent upon the terminal amine structure with I-methylpiperazine exhibiting the strongest oxidation. Hydroxyl index showed generally an initial rise followed by a sharp decline and then a slow increase over much longer irradiation periods. The presence of the amine functionality is found to be an effective scavenger of oxygen and hydroperoxide formation in EB cured coatings. In the UV cured coatings the hydroperoxide levels are found generally to be significantly higher than in EB systems due to the photosensitising effect of the residual photoinitiator. Diluent monomer terminated with dialkylamine groups are found to be more prone to oxidation and UV absorption increase than alkanolamines, cycloaliphatic amines and heterocyclic amines. W cured resins exhibit a more facile transient photoyellowing than the same electron-beam cured systems, and this is associated with hydrogen-atom abstraction and oxidation of the alkylamine group by the residual photoinitiator enhancing the rate of hydro-peroxidation of the amine group. (C) 2001 Elsevier Science Ltd. All rights reserved.