Phosphorus-containing active esters modified dicyclopentadiene epoxy resins with simultaneously improved flame retardancy, thermal stability, and dielectric properties

[Display omitted] •Synthesis of DPO(DOPO)-based active esters and their application in DCPD epoxy resin.•Achievement of UL-94 V-0 rating for thermosetting epoxy resins with 0.9 wt% P.•Significant reduction in Dk and Df of thermosetting epoxy resins.•Low water absorption (0.32 % to 0.45 %) of active ester cured epoxy resins. In the pursuit of epoxy thermosets with low dielectric properties and excellent flame retardancy, the 2- (diphenylphosphino)-1,4-benzenediol active ester (DPO-HQ-AE). and 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide active ester (DOPO-HQ-AE) were synthesized as bifunctional phosphorus-containing active ester flame retardants. The molecular structures of DPO-HQ-AE and DOPO-HQ-AE were characterized by MS, 1H NMR, 13C NMR, 31P NMR and FTIR. The flame retardancy, thermal stability, crosslink density, water absorption and dielectric properties of active ester cured dicyclopentadiene (DCPD) epoxy resins were investigated. DCPD/DPO-HQ-AE with a phosphorus content of 0.9 wt% passed the UL-94 V-0 test. Meanwhile, DCPD/DOPO-HQ-AE containing 1.2 wt% phosphorus achieved a V-0 rating. DPO-HQ-AE and DOPO-HQ-AE showed noticeable gas-phase flame-retardant mechanism with active phosphorus species in the gas phase of pyrolysis. At the frequency of 1 GHz, the dielectric constant and dielectric loss of DCPD/DPO-HQ-AE-0.9 were 2.67 and 0.0042, respectively. Compared with the unmodified epoxy resin, the dielectric loss was reduced by approximately 5.5 times. The as-prepared epoxy thermosets with high thermal stability, excellent flame retardancy, low dielectric properties and low water absorption satisfied the requirements of high-speed and high-frequency encapsulation materials.