Expandable graphite encapsulated by magnesium hydroxide nanosheets as an intumescent flame retardant for rigid polyurethane foams

Encapsulation of expandable graphite (EG) particles by organic or inorganic shells has been proved to efficiently enhance the expandability of EG, and thus to improve the flame-retardant efficiency of EG. In this study, magnesium hydroxide (MH) nanosheets were utilized to fabricate core-shell EG@MH flame-retardant particles through a heterocoagulation method. It was observed that after the encapsulation by MH nanosheets, the edges of the char residue of the EG layer were sealed after combustion, which contributed to the enhancement of expandability. The expansion volume of EG@MH increased dramatically to 456 mL/g, in contrast to 338 mL/g for pure EG. By incorporating 11.5 wt % of flame-retardant particles, polyurethane foam containing EG@MH (here PU-EG@MH) displayed excellent flame retardancy. Compared with the physically mixed sample, PU-EG+MH, the limiting oxygen index value for the PU-EG@MH sample increased from 29.8% to 32.6%. Furthermore, the shell of MH nanosheets was beneficial for improving the interfacial adherence between EG and the rigid polyurethane foam (RPUF) matrix, due to the reaction between isocyanate functional groups and MH. The cell structure and storage modulus of PU-EG@MH were improved. In other words, the shell of MH nanosheets successfully improved the flame-retardant efficiency and enhanced the interface adhesion between EG and the matrix. (c) 2018 Wiley Periodicals, Inc.