Synthesis of novel phosphorus-containing cyanate esters and their curing reaction with epoxy resin

Novel phosphorus-containing cyanate esters with phosphorus contents between 1–4% (badcypx, x=1–4) were synthesized from the addition reaction of 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (dopo) and bisphenol A dicyanate (badcy). The structures of synthesized cyanate ester resins were confirmed by NMR and IR spectra. The modified cyanate esters were either self-curing (badcy and badcypx series) or curing with epoxy (badcye and badcypxe series). Thermal properties of cured cyanate ester were evaluated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermal gravimetric analysis (TGA), dielectric analyzer (DEA) and UL-94 vertical test. It was found that Tg and Td (5%) decrease markedly with the phosphorus content for the self-curing system, while Tg and Td (5%) decrease slightly with the increase of cyanate equivalent weight for the cyanate–epoxy curing system. The flame retardancy increases with the phosphorus content and a UL-94 V-0 rating can be achieved for cured badcyp2-badcyp4 and badcyp3e-badcyp4e. These flame retardant thermosets exhibit low moisture absorption as well as low dielectric constants and dissipation factors. Experimental results indicate that introducing the flame retardant phosphorus element into cyanate ester will not sacrifice its low dielectric characteristic.