Contribution of nanoclays to the flame retardancy of polyethylene-based cable insulation materials with aluminum hydroxide and zinc borate

The main aim of this study was to investigate contribution of nanoclays to the flame retardancy of two cable insulation materials: low-density polyethylene and its blend with ethylene vinyl acetate. For this purpose, nanoclays were first incorporated alone, then together with traditional flame-retardant aluminum hydroxide, and then together with aluminum hydroxide–zinc borate system. Compounds and nanocomposites were prepared by melt mixing method with a twin-screw extruder, while specimens were shaped by compression and injection molding. X-ray diffraction analysis and transmission electron microscopy revealed that nanoclay silicate layers were mainly intercalated with certain level of exfoliation in both matrices. Limiting oxygen index, UL-94 vertical burning, and mass loss cone calorimeter analyses indicated that for both matrix materials even use of nanoclays alone could improve many flammability parameters including peak heat release rate, time to ignition, and fire growth index. Contributions of nanoclays were much more significant when they were incorporated together with traditional aluminum hydroxide or together with aluminum hydroxide–zinc borate system. Residue analysis revealed that contribution of nanoclays to the flame retardancy mechanisms of aluminum hydroxide and zinc borate was mainly by formation of strong and tough char structure via well-dispersed and intercalated/exfoliated silicate layers shielding the underlying polymer matrices from heat and mass transfer.