Flame retardant, strong and tough, low dielectric epoxy thermoset enabled by a novel bi-DOPO type epoxy with aggregation-induced emission

Multifunctional epoxy resins with excellent flame retardancy, high strength, high toughness, low dielectric loss and strong fluorescence show great potentials in electrical/electronic applications; however, simultaneously achieving those goals are difficult. To address those challenges, an epoxy resin with 9,10-dihydro-9-oxa-10-phos-phaphenanthrene-10-oxide (DOPO) pendant group and flexible aliphatic spacer (DOPOEP) was developed. V-0 rating in UL-94 test and LOI of 33.1% are achieved at only 0.45 wt% phosphorus loading, benefitting from the flame retardant mechanism in gas and condensed phase. Compared with neat DGEBA, the cured epoxy with 10 wt% DOPOEP shows 13.4% and 37.6% improvements in tensile strength and impact strength, respectively; in addition, the tensile strength of the cured epoxy with 20 wt% DOPOEP is as high as 102.2 MPa, a 35.3% increase, without compromising toughness. The strengthening effects of DOPOEP derive from pi-pi stacking and hydrogen bonding, and the improved toughness is due to the flexible aliphatic spacer and reduced crosslinking density. Meanwhile, free volume reduction was observed after introducing DOPOEP, which is responsible for the improved modulus. DOPOEP addition also lowers the hydroxyl group concentration, leading to lower dielectric constant and dielectric loss tangent. Moreover, the aggregation-induced emission behavior of DOPOEP was re-ported, and its application in anti-counterfeiting field was explored. In air, the fluorescence of DOPOEP-containing thermoset steadily diminishes with increasing temperature, however the fluorescence changing trend is complex in vacuum. Besides, the fluorescence of DOPOEP is effective in the visual detection of defects and flaws in cured products. This work provides a new direction for developing high-performance epoxy with multifunction.