Polyborosiloxane(PB)was synthesized through two-step reaction by using alkoxysilanes and boric acid as raw materials and synthesis optimization was studied through adjusting boric acid /silane molar ratio

condensation temperature and hydrolysis time.The effect of PB on flame retardancy and physical properties of polycarbonate(PC)was investigated.The results showed that strict control of boric acid/silane molar ratio and condensation temperature was the key to preparation of high efficiency flame retardant PB.When boric acid /silane molar ratio was 1:1

and condensation temperature was 100℃

PB had the best flame retardancy on PC.After adding 5%(mass)of PB

LOI of PC could increase from 26% to 39.4%.Addition of PB could decrease thermal degradation onset temperature of PC.However PB decreased mass loss rate of PC and increased degradation residue of PC at 800℃.PB could promote the formation of char residue which could serve as a good insulating barrier to reduce the radiant heat of flame and lower decomposition rate of PC.Addition of PB increased tensile strength and flexural strength of PC

but decreased impact strength of PC.In addition

PB had little effect on optical properties of PC

and the PC/PB composites were still transparent.