Hollow glass microspheres (sodium borosilicate, 30-120 µm diameter) were tested as synergist with the intumescent PIN flame retardant, ammonium polyphosphate (APP) and pentaerithyritol (PER), in epoxy resin. 3 mm thickness epoxy resin sheets were tested with 20% total loading of flame retardant (of which 0 – 4% glass microspheres).
Peak heat release of the epoxy sheets with flame retardants was around 1/3 of without, and was further reduced by around a quarter with 2% or 4% glass. UL94-V1 (3mm) was achieved with APP only, and V0 with 2% or 4% glass. Smoke production rate of epoxy with the PIN FR was less than half that of untreated epoxy, and the lowest smoke production rate was with 2% glass. The PIN FR also increased the epoxy’s thermal decomposition temperature (TG thermogravimetric analysis), further increased with 2% glass. The authors conclude that glass microspheres act as an effective synergist to intumescent PIN flame retardant (APP), apparently by increasing density and compactness and reducing cracking of char.
“The synergistic effect of hollow glass microsphere in intumescent flame-retardant epoxy resin” J. Zhou et al., J Therm Anal Calorim, http://dx.doi.org/10.1007/s10973-017-6142-6