An intramolecular hybrid of metal polyhedral oligomeric silsesquioxanes with special titanium-embedded cage structure and flame retardant functionality

Abstract

A novel metal–organic-inorganic ternary intramolecular hybrid of N,N′-bis(methylene)-bis(dibenz[C,E][1,2]oxaphosphorin6-oxide)-propylisobutyl-amino-titanium-polyhedral oligomeric silsesquioxanes (Ti-POSS-bisDOPO) was synthesized via a feasible route including the corner-opening, corner-capping and Kabachnik-Fields reactions. The uniqueness of this compound lies in its structural design and has the function of flame retardant. Firstly, one Ti atom has been embedded into the polyhedral oligomeric silsesquioxanes (POSS) cage framework and two dibenz[C,E][1,2]oxaphosphorin6-oxide (DOPO) units with anti-burning functionality have been linked to the aminopropyl group on the corner of siliceous cage. For each molecule, the affinities of Ti-embedded POSS part and DOPO unit are different, which make Ti-POSS-bisDOPO to show the amphiphilicity. Therefore, Ti-POSS-bisDOPO could self-assemble into nanoparticle and achieve good dispersion in epoxy matrix with a particle size varying from 150 nm to 1.5 μm. Secondly, when Ti-POSS-bisDOPO was incorporated into EP matrix, the prepared EP/Ti-POSS-bisDOPO composites exhibited good comprehensive performances. Compared with pure EP, both limited oxygen index and char yield of EP/Ti-POSS-bisDOPO composites were increased greatly at a loading range of 1.5 wt%–6.0 wt%. Meanwhile, the corresponding heat release and smoke production of EP/Ti-POSS-bisDOPO were all reduced. It was observed that the interior layers of char residues were honeycomb-like multilayer structures and the exterior layers were compact, which could effectively hinder the heat transfer and gas diffusion by a barrier effect. Besides, when Ti was doped into the vertex of siliceous cage, it worked by catalyzing char formation, producing thermodynamically stable TiO2, and enhancing the flame retardancy. It was deduced that both condensed-phase and gas-phase flame-retardancy made contributions to the flame retardant mechanism. Overall, the metal-embedded and phosphorus-modified Ti-POSS-bisDOPO was a new kind of promising fire retardants in the field of non-halogen flame-retardant polymers.