Low smoke halogen-free cable material selection substrate is polyolefin halogen-free materials, such as polyethylene, crosslinked polyethylene, irradiation crosslinked polyethylene, polypropylene, ethylene - vinyl acetate polymer, etc. These materials themselves are not flame retardant, and halogen-free flame retardants need to be added to make them low smoke halogen-free materials. At present, ethylene - vinyl acetate copolymer (EVA) and PE are used as substrates, which also lays a foundation for the development of low-smoke halogen-free cable materials.

Low smoke halogen-free flame retardants

The flame retardants of low smoke halogen-free cable materials are mainly aluminum hydroxide and magnesium hydroxide flame retardants, and their flame retardancy mechanism is similar. The biggest advantage of aluminum hydroxide and magnesium hydroxide as flame retardants is that they do not produce harmful gases in the combustion process compared with other flame retardants, and have a very obvious cost advantage.

Aluminum hydroxide alone can have an obvious flame retardant effect only when the general dosage reaches 50% (mass fraction) or more. Such a large amount of addition will increase the viscosity of the plastic, reduce the toughness, and decrease the fracture growth rate. Magnesium hydroxide flame retardants are similar to aluminum hydroxide and are also highly added flame retardants, and a large number of additions make the mechanical properties and processing properties of the material significantly decreased, and the surface of the material is rough. This is due to the difference in surface energy between the inorganic flame retardant and the organic polymer matrix, and the highly filled inorganic powder makes it easy to form a network structure and an aggregate structure, resulting in the decline of the strength of the cable material and the deterioration of the fluidity. Therefore, it is necessary to conduct in-depth research on this kind of flame retardant, and the treatment methods of flame retardants mainly include surface treatment, micro-refinement treatment, and the study of synergistic effects.

Application of compatibilizers

In addition to the above treatment of flame retardants, in order to improve the compatibility of inorganic components and resins, low-smoke halogen-free compatibents have also been successfully developed. EVA grafting material is more widely used, mainly EVA grafting maleic anhydride. The principle is to use a large number of polar hydrophilic groups (hydroxyl groups) on the surface of metal hydroxide; maleic anhydride graft EVA polarity is stronger, can participate in the reaction of maleic anhydride and hydroxide, and form a firm bond so that the hydroxide in the basic resin achieves uniform distribution, which is beneficial to greatly improve the mechanical properties, while the oxygen index can also be improved. In order to expand the application range of polyolefin and develop more valuable new materials, functional polyolefin as a compatibilizer has been an important field in scientific research and industrial production. So far, due to cheapness, high activity, and good processability, maleic anhydride-grafted polyolefin (PO-g-MAH) is the most important functional polyolefin, in addition to EVA-g-MAH and PE-g-MAH as compatibiliers, in improving the tensile strength of the product, and elongation at tensile break has a relatively large contribution.

Application of irradiation crosslinking technology

With the introduction of irradiation crosslinking technology, the temperature resistance of low-smoke halogen-free materials has been significantly improved. The principle of irradiation cross-linking is to use high-energy rays (β ray acceleration of radioactive elements such as cobalt-60) or high-speed electrons as energy sources to form polymer free radicals, and then polymer free radicals recombine to become cross-linked bonds, so that the original linear molecular structure into a three-dimensional network molecular structure, so that the polymer chain is directly connected from the linear structure to the body structure. Due to the high energy of the ray, it does not need a crosslinker, nor does it need high temperature and high pressure conditions to cause the fracture and combination of chemical bonds, directly connecting the C-C bond, and thus has a high heat resistance grade. According to the formula of the material and the different processing technology, it can be made into 90, 105, and 125 °C insulation material. The general use temperature of chemical crosslinking can only reach 90 °C, and irradiation crosslinking makes up for the shortcomings of insufficient temperature resistance of chemical crosslinking products. In the United States, Japan, Europe, and other industrially developed countries, the amount of cross-linked insulated cable products is much more than the warm water cross-linked insulated cable and has become the main cable coating material.

Compared with chemical crosslinking products, irradiation crosslinking also has the characteristics of large volume resistivity and small medium loss, which greatly improves the current carrying capacity of products of the same specification, and the performance of chemical crosslinking can also be met. Irradiation crosslinked low-smoke halogen-free flame retardant and wire and cable for 10 kV and below power transmission and distribution systems, control lines, and a variety of requirements of flame retardant, fire-resistant, smokeless, non-toxic, and high temperature-resistant important places can be widely used in nuclear power plants, power plants, steel mills, oil wells, computer rooms, schools, entertainment venues, densely populated places, and high-rise buildings.