Toughness modification of cationic UV-cured cycloaliphatic epoxy resin by hydroxyl polymers with different structures

In this work, cationic UV-curable epoxy-expansive matrix composites based on cycloaliphatic epoxy resin (CE) were prepared using trarysulfonium hexafluorophosphate salt as photoinitiator, and hyperbranched polyester terminated with 6 hydroxy groups (H2004) or linear hydroxy-ended polytetramethylene ether glycol (PTMG) as toughness modifiers. Effects of H2004 and PTMG on the UV-curing kinetics, mechanical and thermal properties were systematically compared. An acceleration phenomenon was observed by real-time FTIR characterisation during the UV-curing of CE/H2004, which may be attributed to the hydroxy group participation of H2004 based on an activated monomer mechanism. However, the epoxy group conversion in the CE/PTMG composite decreased during UV irradiation owing to the random coil morphology of PTMG. Thermal-mechanical properties of the CE/H2004 composite were investigated by dynamic mechanical thermal analysis, which revealed that the glass transition temperature (Tg) decreased slightly as the H2004 dosage increased, in contrast to the Tg of the CE/PTMG composite, which increased as the PTMG content rose. Compared with CE, the thermal stability of CE/H2004 and CE/PTMG was less, but toughness was improved dramatically by increasing the content of H2004 or PTMG. At concentrations of H2004 and PTMG of 20% and 30%, respectively, the impact strength was improved from 1.14 J/cm2 for CE to 4.62 J/cm2 for CE/H2004 and 10.44 J/cm2 for CE/PTMG. This was further elucidated by fracture surface analysis with a scanning electron microscope.