Common insulating materials and main properties of flame-retardant cables

The performance of flame-retardant cables is determined by the oxygen index of the materials used. The higher the oxygen index, the better the flame-retardant performance of the flame-retardant cable. However, this will also result in the loss of some other properties, increased difficulty of operation, and increased material costs.

The standard C-class test requirements for flame-retardant cables require that the oxygen index of the insulating material reach 30, while for B-class and A-class, both the sheath material and the filler material use flame-retardant materials.

Flame-retardant cables mainly use halogen-free flame-retardant materials and halogen-containing flame-retardant materials.

1. Halogen-free flame-retardant materials

Polyolefins are halogen-free materials, composed of hydrocarbons. When burned, they decompose into carbon dioxide and water, without producing significant smoke or harmful gases. Polyolefins mainly include polyethylene (PE) and ethylene-vinyl acetate copolymer (E-VA). These materials themselves are not flame-retardant and require the addition of inorganic flame retardants and phosphorus-based flame retardants to be processed into practical halogen-free flame-retardant materials. However, due to the lack of polar groups on the molecular chains of non-polar substances, they have hydrophobicity and poor affinity with inorganic flame retardants, making it difficult to firmly bind. To improve the surface activity of polyolefins, surfactants can be added to the formulation; or polymers containing polar groups can be blended into polyolefins to improve the amount of flame-retardant fillers used, improve the mechanical and processing properties of the material, and obtain better flame retardancy.

2. Low smoke low halogen flame-retardant materials

This mainly refers to polyvinyl chloride and chlorosulfonated polyethylene. In the polyvinyl chloride formulation, CaCO3 and A(lOH)3 are added. Zinc borate and MoO3 can reduce the amount of HCL released and the amount of smoke from flame-retardant polyvinyl chloride, thereby improving the flame retardancy of the material and reducing the emission of halogens, acid mist, and smoke, but may slightly reduce the oxygen index. When the amount of additive is large, the mechanical and electrical properties of the material will also decrease.

3. Halogen-containing flame-retardant materials

When heated during combustion, they decompose and release hydrogen halides, which can capture active free radicals HO, thereby slowing down or extinguishing the combustion of the material and achieving the purpose of flame retardancy. Commonly used materials include polyvinyl chloride, chloroprene rubber, chlorosulfonated polyethylene, and ethylene propylene rubber.

(1) Flame-retardant polyvinyl chloride (PVC): Due to its low price, good insulation performance, and good flame retardancy, polyvinyl chloride is widely used in ordinary flame-retardant wires and cables. To improve the flame retardancy of PVC, halogen flame retardants (decabromodiphenyl ether), chlorinated paraffin, and synergistic flame retardants are often added to the formulation to improve the flame retardancy of polyvinyl chloride.

(2) Ethylene propylene rubber (EPDM): It is a non-polar hydrocarbon compound with excellent electrical properties, high insulation resistance, and low dielectric loss. However, ethylene propylene rubber is flammable and it is necessary to reduce the degree of crosslinking of ethylene propylene rubber and reduce the low molecular weight substances produced by molecular chain breakage in order to improve the flame retardancy of the material. As the degree of crosslinking of ethylene propylene rubber increases, the oxygen index increases. For example, adding crosslinking agents DC (P diisopropylbenzene peroxide), crosslinking aids TAIC (triallyl cyanurate), and HVA (2N, N-m-phenylene bis-maleimide) can increase the oxygen index by 10-15.

Another method is to add inorganic flame-retardant fillers to ethylene propylene rubber, which can also increase the oxygen index. Commonly used fillers include A(lOH)3, which can release crystalline water at high temperatures, absorb a large amount of heat, and achieve a flame-retardant effect. However, adding a large amount of filler will reduce the mechanical and electrical properties of the material (such as tensile strength and elongation). Therefore, the filler should not exceed 150 parts. In order to obtain good flame retardancy and maintain high mechanical properties, it is necessary to appropriately reduce the flame-retardant filler and appropriately increase other flame retardants.