Highly efficient polyethylene composites with flame-retardant, smoke and toxicity suppression properties by incorporating urchin-like ZnO@C-N and intumescent flame retardant

An urchin-like carbon and nitrogen-doped zinc oxide (U-ZnO@C-N) was prepared by calcination of zeolitic imidazolate framework-8, and it was used as synergist. Firstly, the intumescent flame retardant (IFR) containing piperazine pyrophosphate and melamine polyphosphate with a mass ratio of 2:1 was introduced into linear low density polyethylene (LLDPE). The LLDPE/IFR passed UL-94 V-0 rating during the vertical burning tests with a limiting oxygen index (LOI) of 26.0% when 25 wt% IFR was incorporated. The LLDPE composite also reached UL-94 V-0 rating when 23 wt% IFR/U-ZnO@C-N with a mass ratio of 97:3 was introduced, and its LOI value was 26.2%. It was indicated that the addition of U-ZnO@C-N enhanced the flame retardant efficiency of LLDPE. Meanwhile, the total smoke release of LLDPE/IFR/U-ZnO@C-N was reduced by 25.5% and 30.5%, respectively, compared to LLDPE/IFR and LLDPE/commercial zinc oxide (C-ZnO). It was attributed that the U-ZnO@C-N with urchin-like structure served as the skeleton and fulcrum during the formation process of char layer, which effectively improved the strength of char layer. In addition, the free electron-holes in the semiconductor UZnO@C-N oxidized CO to CO2, effectively reducing the release of toxic gas. This work provided a valuable way to prepare LLDPE composites with high flame retardancy, smoke inhibition and low toxicity, which was expected to break the limitations of LLDPE in wires and cables applications.