Photodecarboxylation Of α-Carboxyl Thioether And Its Application In The UV-curable Water-borne Coatings

In this dissertation, two aliphaticα-carboxyl thioethers, methyl 3-(carboxymethylthio) propionate (MCP) and trimethylolpropane mono- 3-(carboxymethylthio)propyl ether (TMCPE), were prepared by the additions of mercapto acetic acid and double bond of trimethylolpropane monoallyl ether.The photodecarboxylation of MCP were investigated by 1H NMR and it was found that the type II photoinitiators such as benzophenone (BP) and isopropyl thioxanthone (ITX) were able to induce the photodecarboxylation of MCP rather than the type I photoinitiator (2-hydroxy-1-[4-(2-hydroxy-ethoxy)phenyl]-2-methyl-propan - 1-one (D2959). It was suggested that photoinduced electron transfer fromα-carboxyl thioethers to the triplet state of type II photoinitiator was a primary mechanism of the photodecarboxylation.The photodecarboxylation products of the MCP/BP system which was analyzed with 1H NMR and GC-MS included four different compounds, though the primary product was methyl 3-methylthio propionate (CH3SCH2CH2COOCH3). The kinetics research indicated that the photodecarboxylation rate of MCP increased with the ratio of BP to MCP and reached the ultimate at the equal ratio. The photodecarboxylation of MCP was found to decrease in the presence of triethylamine, pyridine and trichloroacetic acid. TMCPE could decarboxylate in the presence of ITX with UV irradiation and transform from heterogeneous phase into homogeneous phase with toluene for the hydrogen bond decreased after photodecarboxylation.Two kinds of water-borne UV-curable resin with potential of photodecarboxylation were prepared by means of introducing the carboxymethylthio group (-SCH2COOH) into the side chain of resin. Both of the resins, dispersed in water after neutralized with alkaline and added ITX and D2959, respectively, were exposed to UV irradiation after film drying. The influences of photodecarboxylation upon water resistance and alkaline resistance of the cured films were investigated. The experiment result indicated: 1) ITX can induce the photodecarboxylation of carboxymethylthio groups in the film during the curing process and post irradiation in certain extent. 2) the photodecarboxylation is favorable for improving water-resistance of the UV-curing coating; 3) Tetrabutyl ammonium hydroxide (TBAH), applied as neutralizer for the mercaptoacetic acid modified resin, could effectively accelerate the photodecarboxylation. However, triethylamine (TEA) would impair the photodecarboxylation seriously. The combined neutralization with both of TBAH and TEA could avoid the disadvantage of TEA. 4) The photodecarboxylation is also favorable for improving the base-resistance of the UV-curing coating.