Anisotropy of expandable graphite to explain its behavior as a flame-retardant

Abstract

Expandable graphite is used as a flame-retardant in polymers. Its expansion on heating leads to a network of graphite worms which acts as a thermal barrier. However, mechanisms of action of worms are not yet well known. An original experimental approach is performed to study the heat dissipation in the network of worms. The network is made by the burning of polypropylene with 10 wt% expandable graphite during cone calorimeter experiment. After the burning, a hot spot is applied on the char. Temperature is monitored at different locations of sample during the combustion and after the application of the hot spot. During cone calorimetry, the char develops homogeneously over the whole sample. The hot spot test evidences the anisotropy of the entangled network of graphite worms. This anisotropy of heat conductivity allows the dissipation of heat in-plane and poorly out of plane, which explains the thermal barrier effect made by entangled worms.

Ternary intumescent flame retardant composition containing ionic liquid towards efficiently flame‐retardant polystyrene

“…Once exposed to fire, the acidic substances (H 2 SO 4 , HNO 3 ) intercalating graphite layer produce blowing gas by chemical reaction to promote irreversible expansion and further evolve into vermiform carbonization, followed by constructing network as protective shield 3 . Although graphite is considered a good thermal conduction, anisotropy of heat diffusion of network formed by expanded graphite would dramatically inhibit heat transfer to interface between expansion layer and matrix 4 . However, the expansion behavior also probably results in insufficient flame retardancy, because graphite sheets grow disorderly into vermiform carbon branch (worms) and form loose charring bulk with fluffy layer under thermal flow disturbing, that is “popcorn effect”.…”

“…3 Although graphite is considered a good thermal conduction, anisotropy of heat diffusion of network formed by expanded graphite would dramatically inhibit heat transfer to interface between expansion layer and matrix. 4 However, the expansion behavior also probably results in insufficient flame retardancy, because graphite sheets grow disorderly into vermiform carbon branch (worms) and form loose charring bulk with fluffy layer under thermal flow disturbing, that is "popcorn effect". Accordingly, some previous researches reported utilization of phosphates, 5 carbon nanotube, 6 ferrocenebased polymer, 7 etc.…”