Study On Sythesis And Application Of Photolatent-amidine Photoinitiators

Photolatent bases are compounds that can generate basic compounds upon UVirradiation. Early photolatent bases release primary or secondary amines as theyexposed to UV irradiation. The photo-generated amines have mainly been used forpattern imaging as cross-linkers in electronics. They were rarely used as basiccatalysts for cross-linking, due to their low-basicity. More and more attentions havebeen focusing on the photolatent bases that generate tertiary amines because tertiaryamines have strong basicity and great ability to catalyze cross-linking reactions.Several photolatent bases have been developed, such as quaternary ammonium salts,-aminoketon. As exposure to UV irradiation, these novel photolatent bases releasetertiary amines, amidines or guandidines that are able to catalyze the curing reactionof oligomers continually after UV exposure. The novel photolatent basicphotoinitiators may broaden the applications of the UV curing technique.In this study, DBN and DBU were reduced with LiAlH4and variouschromophore groups were attached to the secondary nitrogen in the reduced DBN andDBU by replaceing reaction. A series of photolatent-bicycloamidines have beensynthesized and their molecular structures have been identified by NMR. Their UVabsorption spectra, fluorescence emission spectra and thermal stabilities have beenanalyzed. The applications of the photolatent-bicycloamidines as novel photoinitiatorsfor epoxy acrylate resin, epoxy resin and acrylate/epoxy hybrid resin have beenstudied. The effects of the molecular structures and content of the novelphotoinitiators on UV curing speed have been investigated by means of FTIR and gelmeasurement, and the effects on the gel contents have been analyzed. The catalyzingabilities of the photolatent-bicycloamidines to the reaction of epoxy groups have beenexamined on DSC before and after UV exposure. The effect of UV-thermal curingconditions on the gel content and tensile strength of the hybrid resin have beendetermined.From DBN and DBU, photolatent-bicycloamidines with different chromophore groups can be synthesized through reducing and replacing reactions. As theconjunction of the attached chromophore groups increases, the molar extinctioncoefficients of the photolatent–bicycloamidines increase and UV absorption spectrashift towards long wavelength. For acetophenonic chromophore, the molar extinctioncoefficient is1.78×10⁴mol^-1·L·cm^-1, the wavelength at max absorption locates at256nm.The synthesized photolatent-bicycloamidines can trigger UVcuring of epoxyacrylate and the curing rate o increase with the increasing conjunction in the chromo-phore groups. The gel content of the epoxy acrylate resin at2phr synthesized photo-latent base initiator can be over95%.The synthesized photolatent-bicycloamidines exhibit the character of photolatentbases. Their photolyzed fragments are able to catalyze the reaction of epoxy groupand make the initial temperature of exothermic heat of epoxy resin decreased by about30℃. After UV-exposure and thermal curing at temperature above80oC, the gelcontent of epoxy resin at3phr photolatent-bicycloamidines can be more than90%.After UV curing for3min and thermal cuing at100℃for2hrs, the acrylate/epoxyhybrid resin exhibit gel content above90%and tensile strength above70MPa.