Novel low phosphorus-content bismaleimide resin system with outstanding flame retardancy and low dielectric loss

A novel modified bismaleimide resin system (BDP) with significantly improved flame retardancy and decreased dielectric loss was successfully prepared by copolymerizing 4,4′-bismaleimidodiphenyl methane (BDM) with 2,2′-diallyl bisphenol A (DBA) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). Compared with BDM/DBA resin, BDP resin has obviously different crosslinked structure, and thus simultaneously improved dielectric properties and flame retardancy. Most attractively, with a very low content of phosphorus element, the BDP resins show significantly improved flame retardancy. For example, when the content of phosphorus is as low as 0.5 wt%, the flame retardancy of BDP resin is evaluated to be UL94 V-0 level, while that of BD resin is classified as UL94 V-1 level; in addition, the total heat release (THR) of BDP resin reduces to about 61% of that of BD resin, and similar phenomenon is also observed on the weight ratio of CO/CO2. This outstanding flame retardancy of BDP resins is attributed to the attractive phosphorus-nitrogen synergistic effect. The main flame retardancy mechanism of BDP resins is evaluated to be condensed phase mechanisms. On the other hand, BDP resins exhibit improved dielectric properties; specifically, the dielectric constant and loss at 1 GHz of BDP resin with 19.7 wt% of phosphorus are 2.90 and 0.0058, only about 92% and 51% of that of neat BD resin, respectively. These interesting results demonstrate that the co-reaction of DOPO with BD resin is important, and the method proposed herein is a new approach to develop high performance resins with attractive flame retardancy and dielectric properties.