UV Curing Technology Of Polyurethane Acrylate

Polyurethane acrylate (PUA) is an UV-curable coating material, having the characteristics of fast solidifying, low energy cost, and limiting the discharge of the volatilization organic compounds. It has been used widely in the fields of wood, leather, metallic trimming, printing, optical fiber and so on. PUA curing in the UV ray is affected by many factors, including the compositions of the curable system and the solidifying conditions.In this thesis, PUA was synthesized by bulk polymerization, using isophorone diisocyanate (IPDI), polyether glycol (PEG) and hydroxyl propyl acryl ate (HPA) as the raw materials. The effects of different photoinitiators, reactive diluents and tertiary amine co-initiators were studied. This could be a theory guide for getting a good UV curable system. Gas Phase Chromatography-Mass Spectra was used to analysis the photoinitiators' photolysis products, which were benzoin di-methylether (651), 2-hydroxy-2-methyl-1-phenyl-1 -acetone (1173), 1-hydroxy-cyclohexyl-phenyl-ketone (184), diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO),4-benzoyl-4'-methyl-dibenzolsulfurether (BMS) and isopropyl thioxane authrone (ITX). The effects of photoinitiators on the surface dry time and gel fraction of the UV-cured film were studied. It was concluded that 651 photoinitiator had the best capability to make the film surface dry, while TPO photoinitiator had the lowest gel fraction as cracking free radical photoinitiators. Hydrogen transfer photoinitiators always had a good capability to make the film surface dry quickly and get a higher gel fraction. TPO, 1173 and 184 photoinitiators were better choices for anti-yellowed changing purpose as photoinitiators. It was found that the UV curing would behave well when the mass concentration of the photoinitiators at about 4% of the prepolymer. Two kinds of reactive diluents, tripropylene glycol diacrylate (TPGDA) and trimethylolpropane triacrylate (TMPTA), were studied. Itwas found that TPGDA had the contribution to the extensibility of the solidified coatings and TMPTA had the contribution to the tensile-strength of the solidified coatings. The UV curing would behave well when the mass concentration of TPGDA at about 30% of the prepolymer, while TMPTA at 15%. When the mixture of TPGDA and TMPTA was used, both of the solidified coatings' extensibility and tensile-strength could be increased. Four kinds of tertiary amine co-initiators, triethylamine (TEA), triethanolamine (TEOA), 4-dimetolamino isocapryl benzoate (EHA) and 4-dimetol ethyl benzoate (EDAB), were studied. EDAB and EHA were much more active than TEOA and TEA. It was found that adding tertiary amine to the system could increase the solidified coatings' tensile performance, while to the adherence capability, adding EHA and EDAB is much better than TEA and TEOA.Because PUA cured by the UV ray always took place in the air, the inhibition of oxygen in the air to the polymerization would play an important role to the UV-curing speed and the final degree of the UV-curing. To avoid the inhibition of oxygen, several physical methods were involved in the present thesis, including covering by the polyester film, protecting by the nitrogen, prolonging the curing time and increasing the radiation intensity. All these methods could decrease the oxygen inhibition, but because of the difficulty in operations and increasing the cost, they could not used in the industry directly. Then some chemical methods were studied, using ethyl di-thiol to modify the PUA. There were three different methods, including using the ethyl di-thiol directly, ethyl di-thiol substituting part of the PEG to synthesis PUA, and mixing two different kinds of PUA. Using the ethyl di-thiol directly could shorten the surface dry time and enhance the anti-yellow changing, but the gel fraction would be decreased. The tensile performance and the adherence capability of the solidified coatings would not be affected. Ethyl di-thiol substituting part of the PEG to synthesis PUA would shorten the surface dry time greatly, 5% of PEG substituted by the ethyl di-thiol, the surface dry time cut down to nearly fourteenth, enhance the anti-yellow changing, increase the gel fraction, and make sure the solidified coatings have good tensile performance and the adherence capability. Mixing PUA synthesized by ethyl di-thiol and no modified PUA, the surface dry time would be shorten, the gel fraction would be decreased and the anti-yellow changing would decrease, but thetensile performance and the adherence capability of the solidified coatings would not be affected.