Study of modification of hyperbranched polyester and properties of UV curable coatings

The UV curable hyperbranched polyester with vinyl end-groups (VHBP) was prepared by solution method. During the preparation, the hyperbranched polyester (HBP) was modified by using acrylic acid as end-group modifier. The structure, glass transition temperature and intrinsic viscosity of the HBP and VHBP prepared were characterized by FT-IR, 1H-NMR, DSC and Ubbelohde viscometer, respectively. The prepared VHBP was used as cross-linking agent of polyurethane acrylate (PUA). The pendulum hardness, flexibility, impact resistance and adhesion of the UV-curable coating of PUA/VHBP system were studied in comparison with those of the traditional UV-curable coating of polyurethane acrylate/epoxy acrylate (PUA/EA) system. The FT-IR and 1H-NMR spectra indicate that, after modification by acrylic acid, the hydroxyl end groups of HBP are modified into vinyl end gropes; the modification degree of VHBP is 88.7%, and its glass transition temperature and intrinsic viscosity decrease after modification. The pendulum hardness of the UV curable coating increases greatly after adding VHBP to PUA system, and with the increase of VHBP content, the pendulum hardness of PUA/VHBP coating increases gradually and its flexibility remains almost unchanged. When the VHBP content is higher, the impact resistance and adhesion decrease slightly, and the PUA/VHBP coating has a good performance when the VHBP content in it is 10% and 20%. For the PUA/EA system, although after adding EA to PUA system, the pendulum hardness of the coating also increases, but it is lower than that of PUA/VHBP coating with the same composition ratio. When the EA content is higher, the flexibility, impact resistance and adhesion of the coating decrease. It indicates that, using VHBP as cross-linking agent to modify PUA, the performance of UV curable coating formed by PUA/VHBP is superior to that formed by conventional PUA/EA.