High‐strength, tough, and hygrothermal aging‐resistant UV cationic‐cured epoxy resin improved by castor oil fatty acid

Abstract This work investigated the effect of toughening agents on the hygrothermal aging resistance of ultraviolet (UV) cationic‐cured epoxy resin 3,4‐epoxycyclohexymethyl‐3′,4′‐epoxycyclohexyl carboxylate (CE). After being toughened by polycaprolactone diol (PCL), the resulting epoxy sample (PCL‐CE) exhibited excellent toughness but poor hygrothermal aging resistance, with a tensile strength of 39.8 MPa and elongation at break of 16.9%. After being toughened by poly(tetramethylene ether glycol) (PTMG), the resulting epoxy sample (PTMG‐CE) showed good hygrothermal aging resistance but limited toughness, with a tensile strength of 33.9 MPa and elongation at break of 8.2%. Another epoxy sample (COFA‐CE) was prepared by introducing hydrophobic castor oil fatty acid (COFA) into CE, which combined the high toughness of PCL‐CE and the hygrothermal aging resistance of PTMG‐CE, reaching a tensile strength of 47.8 MPa and elongation at break of 12.9%. After 72 h of hygrothermal aging at 50°C and 100% RH, COFA‐CE retained 46.9% and 88.8% of its original tensile strength and storage modulus, respectively. These were higher than the values retained by PCL‐CE (16.6% and 49.9%) and PTMG‐CE (44.5% and 77.7%). Finally, the hygrothermal aging study revealed that the performance of the epoxy samples degraded due to plasticization caused by moisture absorption, but the properties could be recovered after drying. Highlights Castor oil fatty acid (COFA) was employed to modify UV‐cured epoxy resin (CE). The pre‐reaction between COFA and CE increased their compatibility. Epoxy sample COFA‐CE showed a high toughness and hygrothermal aging resistance.