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[Display omitted] • Imidazole-based liquid compounds containing different P oxidation states were synthesized for curing EP. • EP-DCPPI and EP-PPOAI show the combined flame-retardant mechanism of gas-phase and condensed-phase flame retardancy. • EP-DCPPI and EP-PPOAI show excellent flame-retardant, mechanical properties and heat resistance. In this work, two phosphorus-containing liquid imidazole compounds with different phosphorus oxidation states (1,1′-(phenylphosphanediyl)bis(1 H -imidazole) (DCPPI) with + 1 valence phosphorus, and imidazole-phenylphosphate (PPOAI) with + 3 valence phosphorus) were designed and synthesized. Both of DCPPI and PPOAI were used as curing agents to obtain intrinsic flame-retardant epoxy resin (EP-DCPPI and EP-PPOAI), in which the imidazole group was used as the curing group of EP, and the phosphorus-containing group was directly connected to the EP crosslinking network. On the one hand, the efficient flame-retardant groups containing + 1 or + 3 valence phosphorus were incorporated into the EP system to obtain excellent flame-retardant properties; on the other hand, the introduction of phenylphosphine or phenoxyphosphine groups in the molecular chain of EP greatly enhanced the interaction between molecular chains, adjusted crosslinking density and endowed EP with excellent tensile strength and toughness. Compared with imidazole cured EP (EP-MZ), the peak heat release rate (PHRR) of EP-15DCPPI and EP-15PPOAI decreased by 70 % and 68 %, and the total heat release rate (THR) decreased by 37 % and 36 %, respectively. Meanwhile, EP-15DCPPI passed UL-94 V-0 rating, and the limiting oxygen index (LOI) reached 36.1 %. EP-15PPOAI passed UL-94 V-1 rating and the LOI reached 29.1 %. The flame-retardant mechanism was analyzed and discussed. Compared with EP-MZ, the dynamic and static mechanical properties of EP-15DCPPI and EP-15PPOAI were enhanced simultaneously, and the glass transition temperature (T g) was obviously increased. [ABSTRACT FROM AUTHOR]

[Display omitted] • Highly intumescent char layer with core–shell structure was formed during combustion of PC/PBS composites. • The core–shell char layer is composed of an outer porous structure and an inner lamellar structure. • Linear PDMS with terminal Si-H is the key intermediate to join reaction for forming core–shell char. • The addition of 2 wt% PBS reduce the pSPR of PC by 63.2%, increase the LOI to 31.2% and pass the UL94 V-0 rating. 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. [ABSTRACT FROM AUTHOR]