Photopolymerization behavior and properties of highly branched polyester acrylate containing thioether linkage used for UV curing coatings

The highly branched polyester acrylate containing thioether linkage (HBAT) was synthesized using an “oligomeric A2 + B3” approach. The dithiol, 1,6-hexamethylene bis(3-mercaptopropionate) (HMBM), was prepared through the esterification between 1,6-hexanediol and 3-mercaptopropionic acid, and then underwent an amine-catalyzed thiol-Michael addition to the acrylic double bond of 1,6-hexamethylene diacrylate to obtain an oligomeric dithiol (oligomeric A2), then further with trimethylolpropane triacrylate as a B3 monomer. The molecular structure was characterized with FT-IR and 1H NMR spectroscopy. A broader molecular weight distribution from 1.74 to 2.75 was obtained for different oligomeric dithiol chains by GPC measurements. The photopolymerization kinetics study by photo-DSC analysis showed that the HBAT greatly reduced the oxygen inhibition in radical photopolymerization of acrylates in air. The maximum polymerization rate decreased along with the addition of HBAT into the UV curable resin (EB605) containing bisphenol A epoxy diacrylate and tripropylene glycol diacrylate due to the reduced double bond density and increased viscosity, whereas the final unsaturation conversion in cured film increased because of the longer spacer chain in HBAT compared to that in EB605 and the reduced crosslinking density by HBAT addition. The dynamic mechanical thermal analysis results showed that both the elastic modulus in the rubbery plateau and glass transition temperature of cured film decreased along with the incorporation of HBAT into EB605. The water swelling test and thermogravimetric analysis indicated that the thioether linkage in HBAT can greatly improve the water absorption resistance and oxidative stability of cured film.