Fabrication and origin of new flame retarding bismaleimide resin system with low dielectric constant and loss based on microencapsulated hexaphenoxycyclotriphosphazene in low phosphorus content

To maintain sustainable development, developing new and effective phosphorus flame retardants that can simultaneously endow thermally resistant thermosetting resins with outstanding flame retardancy and dielectric properties is a great but worthy challenge. A new flame retardant coded as MH, polyamide microencapsulated hexaphenoxycyclotriphosphazene (HPCTP), was synthesized through interfacial polymerization. The influences of main synthesis parameters on the morphology and thermal stability were discussed. Furthermore, a new kind of modified bismaleimide resins (MH/BD) were prepared by incorporating MH into 4, 4' - bismaleimidodiphenylmethane/o, o'-diallylbisphenol A (BD) resin, and the structure and performances of MH/BD resins were studied and compared with those of HPCTP/BD resins. Results show that the MH/BD resin, of which the content of phosphorus is as small as only 0.33 wt%, has outstanding flame retardancy, mainly reflected by high limiting oxygen index (LOI), low total heat evolved and total smoke production; while the content of phosphorus in the HPCTP/BD resins should be higher than 1.30 wt% to get similar flame retardancy. Moreover, when the contents of MH and HPCTP are the same, the MH/BD resins exhibit lower dielectric constant and loss over the wide frequency from 102 to 104 Hz than HPCTP/BD resins, suggesting that the former is more attractive to be used in applications related with electric and electronic fields. The mechanisms behind these attractive performances were intensively discussed. The interaction between core and shell of MH and the effect of MH on the crosslinked structure of cured resins are responsible for these attractive properties. The research reported herein provides a new method and origin to prepare effective phosphorus flame retardant and related high performance flame retardant resins.