Low-Density polyethylene (LDPE), as a hydrocarbon thermoplastic polymer, has combustion performance that directly affects the safety assessment of the material during processing, use, and waste disposal.LDPE molecules are composed of a long-chain carbon skeleton and hydrogen atoms, and do not contain halogens, phosphorus, or other flame-retardant elements. Therefore, it exhibits typical polymeric flammability under heating or fire conditions. In-depth research on its combustion mechanism and control measures is crucial for the safe application of this material.
From the perspective of the combustion mechanism, LDPE first undergoes thermal decomposition under the influence of an external heat source, breaking down molecular chains to generate low-molecular-weight hydrocarbon gases such as methane, ethylene, propylene, and various olefins. When the ambient temperature reaches its pyrolysis range (approximately 300-400℃) and oxygen is sufficient, these volatile flammable gases mix with air to form a combustible mixture. Upon encountering an ignition source, gas-phase combustion occurs, with a light blue or yellow flame accompanied by molten dripping. Because hydrocarbons burn to produce carbon dioxide and water, the combustion process releases a high amount of heat, approximately 46 MJ/kg. The flame propagation rate varies with the sample shape, thickness, and environmental conditions.
LDPE's oxygen index (LOI) is generally around 17%–18%, lower than the 26% threshold required by most flame-retardant materials, indicating that it is highly ignitable and burns continuously in air. In vertical burning tests, LDPE's molten dripping can ignite combustibles below, resulting in a rapid burning rate. This necessitates additional protective measures in applications such as electrical insulation, building interiors, and vehicle interiors. The gases produced during combustion are mainly carbon dioxide and water vapor, but under incomplete combustion conditions, carbon monoxide and a small amount of black smoke are generated. The latter originates from the formation of carbon particles, which may affect visibility and the respiratory system.
To improve combustion safety, flame retardants are often added to LDPE or it is blended for modification in industry. While halogenated flame retardants can significantly reduce combustion rate and smoke, they may produce toxic hydrogen halide gases. Halogen-free flame retardant systems, such as aluminum hydroxide, magnesium hydroxide, or phosphorus-nitrogen-based flame retardants, delay combustion and reduce harmful smoke through endothermic decomposition and gas-phase dilution, meeting both environmental and health requirements. Furthermore, increasing the char layer barrier through structural design can inhibit the diffusion of combustible gases and improve flame retardant durability.
In practical use, LDPE products should avoid direct contact with high-temperature surfaces or open flame sources. Storage and processing areas should be equipped with fire extinguishers and ventilation facilities to prevent gas accumulation and subsequent fires. For LDPE insulation layers in wires and cables, flame retardant ratings must be assessed according to relevant standards to ensure self-extinguishing or low-smoke halogen-free characteristics under abnormal operating conditions.
In summary, the combustion performance of low-density polyethylene is dominated by its hydrocarbon structure, exhibiting characteristics of flammability, high calorific value, and molten dripping. Through scientific flame-retardant modification and standardized application protection, fire risks can be effectively reduced, ensuring personal and property safety and opening up possibilities for its expansion into more safety-sensitive areas.