Novel phosphorus-containing silazanes as flame retardants in epoxy resins

Molecules containing different elements and moieties like P-N and P-O-Si have high impact on flame retardant properties. In this study, a novel group of flame retardants based on phosphasilazanes containing a P-N-Si structure is established. Due to introduction of the silazane moiety to various phosphorus environments, further properties, like mechanical or thermal, besides flame retardancy can be altered. The influence of phosphorus or silicon moieties and their mode of action on flame retardant properties is examined. In formulations with a high-performance epoxy resin, thermal properties (glass transition temperature and moisture absorption) and flame retardant performance (UL94 vertical burning test, cone calorimetry) are investigated. Alteration of chemical environment at the phosphorus atom determines various implications on burning behavior. For example, phosphorus-containing moieties promoting char generation and intumescence enhance char yields and other parameters describing flame retardant properties of phosphasilazanes in thermo-analysis as well as in cone calorimetry. On the other hand, higher concentration of silazane groups in the structure leads to higher flammability. Phosphorus moieties acting in gaseous phase cause poor flame retardant properties because they antagonize the char generation of silazanes.