Plastic wood is a type of environmentally friendly composite material made by melt blending wood fibers and thermoplastic through extrusion hot pressing or injection molding. It not only inherits the advantages of natural affinity, light weight, high strength, and easy processing of wood, but also overcomes the defects of poor dimensional stability, corrosion resistance, anisotropy, etc. of wood. It can be used as a high economic composite material and widely applied in many fields such as landscaping, packaging logistics, home decoration, and construction.
However, wood plastic composites mainly composed of wood fibers and polyethylene/polypropylene are highly flammable and require flame retardant treatment in practical use. The expansion flame retardant (IFR) system is usually composed of acid source, carbon source, and gas source, and has been proven to significantly improve the flame retardant performance of plastic wood. When IFR is heated or subjected to high-temperature combustion, the carbon source is dehydrated into carbon under the catalytic action of the acid source. The carbides are decomposed by the gas source to form a thick porous expanded carbon layer wrapped around the material surface, which is used for insulation and diffusion of flammable gases, thereby suppressing the combustion of plastic wood. Expanded graphite (EG), as a representative of IFR, is inexpensive, non-toxic, low smoke, and has a high expansion ratio, demonstrating excellent flame retardant properties. A study has prepared flax fiber/polypropylene composite materials using EG as a flame retardant, and found that when the EG addition amount is 25%, plastic wood passes UL94V-1 grade [13]. Wang Di et al. used EG/ammonium polyphosphate (APP) synergistic flame retardant EPS based wood plastic composite materials. The results showed that when the ratio of EG to APP was 1:1, the flame retardant performance of the composite material was the best. When the amount of synergistic flame retardant system added was more than 10 parts, the wood plastic could reach UL94V-0 level. From this, it can be seen that EG can be used as an ideal additive type IFR for flame retardant treatment of plastic wood.
However, like most additive flame retardants, the introduction of high levels of EG typically leads to a significant decrease in the mechanical properties of plastic wood. Therefore, researchers have attempted to improve the mechanical properties of composite materials through various means, minimizing the impact of flame retardants as much as possible. Huang et al. designed a branched cross-linked network polyelectrolyte composite (PEC) by self-assembly of polyethyleneimine (PEI)/cellulose nanocrystals (CNC)/PP, and confirmed that PEC provides hydrogen bond interactions that can effectively enhance the interfacial compatibility between plastic matrix, wood fibers, and flame retardants, thereby improving the overall mechanical properties of the composite material [18]. Shen Hui used sodium stearate to modify the surface of magnesium hydroxide and combined it with EG to prepare flame-retardant polyethylene material.
The results showed that compared to the unmodified material system, the compatibility between modified magnesium hydroxide and polyethylene particles was improved, enhancing the bonding performance between flame retardants and matrix materials during processing. Researchers have investigated the effect of SiO2 on the mechanical properties of plastic wood, and the results show that under the premise of uniform particle dispersion, 0.5-9wt% SiO2 can improve the mechanical properties of composite materials by 15% -30%; Some people have also explored the effect of different contents of nano silica (n-SiO2) on the mechanical properties of HDPE based wood plastic composites. The results show that the best reinforcement effect is achieved when the amount of n-SiO2 is 5wt%; A comparative study was conducted on the effects of dispersed and layered distribution of EG/APP in plastic wood on the flame retardancy and mechanical properties of the material. The results showed that layered distribution of flame retardant plastic wood can better improve the flame retardancy and bending strength of the material. Based on the above research conclusions, a multi-layer sandwich structure wood plastic composite material with surface flame retardancy and core reinforcement was prepared using EG as a flame retardant and n-SiO2 as a reinforcing agent. The influence of different structural designs on the flame retardancy and mechanical properties of the composite material was explored. The research results have important practical significance for the development of high-performance and functional wood plastic composites and the expansion of the application field of wood plastic composites.