Expandable graphite systems for halogen-free flame-retarding of polyolefins. I. Flammability characterization and synergistic effect

The use of some types of expandable graphite (EG) as an intumescent flame-retardant additive in polyolefins was studied using the cone calorimeter test (CCT), thermogravimetric analysis (TGA), the limiting oxygen index (LOI), and the-UL 94 test and through measurement of EG's mechanical and electrical properties. The present study has shown that some suitable EG systems combined with other organic and inorganic halogen-free flame-retardant (HFFR) additives apparently can improve the flame-retardant capacity with good mechanical properties of polyolefin blends. For linear low-density polyethylene and/or ethylene vinyl acetate/EG/HFFR blends the limiting oxygen index can reach a rating above 29, and the UL-94 test can produce a value of V-0. The CCT and TGA data show that the EG and EG/HFFR additives not only promoted the formation of carbonaceous char but also greatly decreased the heat release rate and the effective heat of combustion and increased the residues after burning. The synergistic effect of EG with other HFFR additives, such as zinc berate, the phosphorus-nitrogen compound NP28, and microcapsulated red phosphorus is examined and discussed in detail in this article. (C) 2001 John Wiley & Sons, Inc.