Photopolymerization behavior and properties of highly branched poly(thioether-urethane) acrylates used for UV-curing coatings

The highly branched poly(thioether-urethane) acrylate (BPTUA) was synthesized through an “oligomeric A2 + B3” approach. The thiol-endcapped difunctional oligomeric A2 was obtained by the addition reactions of isophorone diisocyanate with 1,4-butanediol, and then further with 1,4-butanediol bis(thioglycolate). Trimethylolpropane triacrylate was used as a B3 monomer. The addition conduct of 1,6-hexanediol diacrylate with 1,4-butanediol bis(thioglycolate) was introduced into the polymeric chain for preparing the modified BPTUA (m-BPTUA). The molecular structures were characterized with FT-IR and 1H NMR spectroscopy, and GPC analysis. The number average molecular weights (Mns) of BPTUA and m-BPTUA were experimentally measured to be 13,500 g mol−1 and 17,900 g mol−1 with the polydispersity indices of 1.86 and 1.94, respectively. A series of UV-curable resins were fabricated based on bisphenol-A epoxy diacrylate (EB600) and tripropylene glycol diacrylate with the addition of either BPTUA or m-BPTUA in different ratios, and exposed to a UV lamp for obtaining the cured films in the presence of 1-hydroxycyclohexylphenyl ketone as a photoinitiator. From the photo-DSC measurements, the polymerization rate at the peak maximum decreased, whereas the final unsaturation conversion in cured film increased, along with the increase of either BPTUA or m-BPTUA. The elastic modulus in the rubbery plateau and the glass transition temperature from DMTA, as well the tensile strength and Young's Modulus of UV-cured film decreased with increasing either BPTUA or m-BPTUA loading, whereas the elongation at break, flexibility and strike performance were greatly enhanced. Moreover, for the m-BPTUA series, the enhancive effects on the properties were much more obviously than the BPTUA series. The water swelling test and thermogravimetric analysis indicated that the water absorption resistance and the oxidative stability of cured films were improved by the incorporation of thioether linkage into the crosslink networks.

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► In this study, we synthesized highly branched poly(thioether-urethane) acrylate (BPTUA), and modified BPTUA (m-BPTUA) with HDDA.
► We find the thioether linkage can improve the thermal stability and water absorption resistance of cured films.
► We find the flexibility and impact performance can be greatly enhanced by the incorporation of the highly branched acrylates.