Unexpected core-shell char from polycarbonate/polyborosiloxane composites and its application in improving flame retardancy

A rare charring behaviour was found during the combustion of polycarbonate (PC) composites containing polyborosiloxane (PBS), in which a highly intumescent char layer with core–shell structure was formed. Particularly the core–shell structure is composed of an outer porous structure and an inner lamellar structure, which can effectively isolate external heat radiation and limit the diffusion of degradation products and oxygen. PC/PBS-X composites with 2 wt% PBS-X showed excellent flame retardancy without compromising mechanical properties, including passing the UL-94 V-0 rating and increasing the limiting oxygen index (LOI) to 31.2%. Compared to PC, the peak heat release rate (pHRR) and peak smoke production rate (pSPR) in the case of PC/PBS-1 composites were reduced by 63.2% and 45.5%, respectively. The mechanism for promoting the flame retardancy of PC by PBS-X was studied. The results showed that the PBS decomposed to form linear polysiloxane oligomer with terminal Si-H bond (L-PDMS-H), which reacted with the degraded products of PC to quickly form intumescent char layer with core–shell structure during the combustion. This is the reason why to PBS presented the striking effect on improving flame retardancy of PC.