• expandable graphite (EG);
  • melamine − formaldehyde resin (MF);
  • core–shell structure;
  • flame retardant;
  • rigid polyurethane foam (RPUF)


Pulverized expandable graphite (pEG) and melamine − formaldehyde (MF) resin core − shell structure particles (pEG@MF) as specific flame retardants for rigid polyurethane foam (RPUF) were synthesized by encapsulating pEG particles with a layer of MF resin via in situ polycondensation. The initial feed weight ratio of pEG and MF prepolymer was found to be the key factor affecting the shell forming process, and the shell growth can be regarded as a combination of ‘raspberry-like’ and conventional ‘core–shell’ formation mechanisms. With the encapsulation of a well formed MF shell, the expandability of pEG particles was significantly enhanced from 42 mL g–1 to 76 mL g–1 and thus the pEG@MF particles showed good flame-retardant performance in RPUF. The RPUF/pEG@MF composites passed the V-0 rate and the limiting oxygen index was remarkably increased from 21 to 28 vol% by adding only 10 wt% pEG@MF particles; both the expandability and available expandable graphite content played an important role in controlling the flame-retardant performance of pEG@MF particles. With a loading of fine sized pEG@MF particles, desirable mechanical and thermal insulation properties of RPUF/pEG@MF composites were achieved by preserving the complete cell structure of RPUF and screening the high thermal conductivity of the pEG particles with the thermally inert MF resin shell. The exciting application of the novel pEG@MF particles indicates that the core–shell structure design of expandable graphite can serve as promising solution for fabricating halogen-free flame-retardant RPUF composites with high performance. © 2013 Society of Chemical Industry