Synergistic effect of expanded graphite and lignin-based intumescent flame retardant on the flame retardancy and thermal stability of high density polyethylene

High-Density Polyethylene (HDPE) is a widely used material whose flame retardancy is an important issue. To enhance its flame retardancy, synergists are often needed. In recent years, as a biomass material with high carbon content, lignin has attracted more attention used for environment-friendly bio-flame retardant. In this paper, the effects of Intumescent Flame Retardant (IFR) composed of alkali lignin (A-lig) and Ammonium Polyphosphate (APP) to improve the flame retardant properties of HDPE were studied by Limiting Oxygen Index (LOI) and vertical burning test (UL-94). The results showed that when the ratio of APP to A-lig was 3:1 and the addition amount of them was 30wt%, the LOI value of HDPE/IFR reached 23.8%, with no rating in UL-94 test. Then, the effect of Expanded Graphite (EG) on improving the above HDPE/IFR systems was studied. When the EG content was 8.3wt%, the LOI value reached 28.5%, with V-0 grade in UL-94 test. The results of Thermogravimetric Analysis (TGA) showed that the flame retardant mechanism between EG and IFR was mainly based on physical interaction, which improved the density and barrier properties of the carbon layer.