Low smoke and halogen-free flame-retardant cable materials not only have flame retardancy, but also have characteristics such as low smoke, halogen-free, low corrosion, and low toxicity compared to halogen-containing flame-retardant cable materials.

（1） Oxygen index of low smoke and halogen-free cable materials

At present, the GB/T 2406 method is mainly used in China to test the oxygen index of flame retardant cable materials at room temperature to assess their flame retardancy. Although the oxygen index at high temperature is also important for flame retardant wires and cables, it is generally rarely tested due to testing conditions and other factors. Another parameter closely related to material flame retardancy - temperature index - is also rarely tested. The testing method is specified in NES 715, which indicates how high the material can ignite in air at ambient temperature. Low smoke halogen-free cable material can meet the general flame retardant requirements of cables with an oxygen index of 33-35 at room temperature, and its temperature index is around 300.

The oxygen index, as the most commonly used combustion test worldwide, cannot be the only indicator to determine the flame retardancy of materials. The assessment of material self extinguishing performance seems to be a more appropriate measure of the flame retardancy of materials. For example, if the oxygen index of polyvinyl chloride or halogen-containing flame-retardant polyolefin reaches around 30, it can be used as a wire with an insulation cross-section of 0.5mm to pass the VW-1 combustion test in UL standards. However, even if the oxygen index of low smoke halogen-free flame-retardant polyolefin materials reaches 34 or above, they may not meet the VW-1 combustion requirements. In addition, for low smoke halogen-free flame retardant polyolefin cable materials, products with high oxygen index may not necessarily have good self extinguishing properties. For example, a halogen-free material using calcium carbonate and polysiloxane as flame retardants can achieve a high oxygen index, even reaching 36-38, through appropriate substrate coordination. However, it is likely that its self extinguishing performance is not as good as that of a polyolefin/mineral hydroxide system with an oxygen index of 32-34. Some foreign users have started using the V-0 level in UL 94 to assess the self extinguishing performance of halogen-free materials, as a supplement to the oxygen index assessment indicators. In terms of flame retardant indicators, a cone calorimeter method that is closer to the actual combustion performance is increasingly receiving attention. This method can quantitatively evaluate combustion parameters such as ignition time, heating rate, and total heat generated during material combustion. Experiments have shown that halogen-free materials with higher heating rates are more likely to spread heat to the surrounding area and expand the combustion range, which means they are prone to extended combustion and have poor self extinguishing properties. However, halogen-free materials with higher oxygen index may not necessarily have lower heating rates than those with lower oxygen index. Although using the oxygen index alone to assess the flame retardancy of materials has certain limitations, due to its simple and feasible testing method, and the fact that in most cases, the difference in flame retardancy of the same type of material can still be relatively indicated, it is generally acceptable to use the oxygen index as an assessment indicator of material flame retardancy.

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（2） Formula technology

Low smoke halogen-free flame retardant polyolefin cable materials are usually composed of polyolefin blend resin, flame retardant fillers aluminum hydroxide, magnesium hydroxide, and some appropriate antioxidants added to improve heat resistance life. Sometimes, in order to reduce the amount of smoke produced during combustion, some smoke suppressants such as vanadium, nickel, molybdenum, iron, silicon, and nitrogen compounds are added. The flame retardant mechanism is that during combustion, the flame retardant fillers aluminum hydroxide and magnesium hydroxide will release crystalline water and absorb a large amount of heat; At the same time, the dehydration reaction produces a large amount of water vapor, which can dilute flammable gases and prevent combustion. In addition, a layer of non melting and non combustible oxide hard shell will form on the surface of the material, blocking the channel for the reaction between high polymers and external thermal oxygen, ultimately leading to flame retardancy and self extinguishing of the material.

Low smoke and halogen-free flame-retardant polyolefin cable materials must have good flame retardancy. The formula must have a large filling amount of aluminum hydroxide and magnesium hydroxide, usually reaching 150 parts or more. However, a large amount of inorganic flame retardant filling will inevitably lead to significant degradation of the material's physical and mechanical properties, electrical properties, and extrusion process performance. In order to balance its flame retardancy and physical and mechanical properties, and ensure that the material can fully meet the technical requirements of final use, commonly used methods include: on the one hand, modifying and grafting polyolefin materials to improve their polarity; On the other hand, the surface of inorganic flame retardants is chemically modified, usually treated with coupling agents.

（3） Performance of low smoke and halogen-free cable materials

The use of a large amount of inorganic flame retardants endows low smoke halogen-free flame retardant polyolefin cable materials with flame retardant, low smoke, halogen-free, low toxicity and other characteristics, while also making them different from other non flame retardant and halogen-containing flame retardant materials in terms of physical and mechanical properties, electrical properties, and process performance. Due to the different usage scenarios and production processes of low smoke and halogen-free wires and cables, as well as their supporting products, the performance requirements are also different. For example, in terms of tensile strength, elongation at break, aging temperature and indicators, volume resistance coefficient, oil resistance, scratch resistance, flexibility, flame retardant requirements, etc., different cables often have a bias. In halogen-free material technology, some of the above indicators are mutually restrictive, and it is impossible for a versatile product to meet the requirements of all types of wires and cables mentioned above. The most prominent contradiction is the contradiction between elongation at break and flame retardancy, as well as the contradiction between flexibility and thermal deformation and aging performance. The only thing that material manufacturers can do is to balance the advantages and disadvantages of certain properties while meeting the basic performance requirements, and adapt different grades of products to different requirements of wires and cables.