Fluoroplastics: Halogen-Containing Insulation Materials Part II

Fluoroplastic insulation materials have extensive applications in the cable industry, with notable examples including PTFE, ETFE, and PVDF. Among them, PTFE can operate long-term at temperatures up to 200°C. Its lightweight nature, excellent corrosion resistance, mechanical properties, outstanding dielectric performance, and resistance to breakdown make it widely used in the aerospace industry.

Most fluoroplastics, especially PTFE, are considered radiation-degradable materials. PTFE can undergo degradation and generate PTFE microparticles under different conditions. Irradiation at temperatures above the melting point of PTFE, which is approximately 330°C to 340°C, in a vacuum or inert atmosphere can induce cross-linking in PTFE. Cross-linking significantly improves the radiation resistance and abrasion resistance of PTFE, compensating for the shortcomings of non-cross-linked PTFE. However, the application of cross-linked PTFE in cables is limited because PTFE can only be cross-linked in its molten state.

Among other types of fluoroplastics, ETFE and PVDF exhibit good radiation resistance but have lower operating temperatures compared to PTFE. After irradiation cross-linking, their temperature ratings can be increased. For example, ETFE wires, when cross-linked using electron beam irradiation, can have their temperature ratings increased from 150°C to 200°C while maintaining other excellent characteristics. XL-ETFE insulated wires are one of the two most commonly used types of wires in the aviation industry today.

XL-ETFE insulated wires use special cross-linkable ETFE insulation material that is extruded into wires and then cross-linked through electron beam irradiation. ETFE molecules contain ethylene structural units, which make them prone to cross-linking under irradiation. However, additional cross-linking sensitizers are required to promote cross-linking. Furthermore, the cross-linking stability of ETFE during irradiation is affected by the presence of oxygen. Irradiation in an inert gas atmosphere at higher temperatures improves the stability of wire cross-linking.

Relative to common PE and PVC cables, fluoroplastic cables have several outstanding advantages:

1. High Temperature Resistance:

Fluoroplastics exhibit exceptional heat stability, allowing fluoroplastic cables to withstand temperatures ranging from 150°C to 250°C. In other words, under the same conductor cross-sectional area, fluoroplastic cables can transmit larger allowable currents, significantly expanding their range of applications. Due to this unique property, fluoroplastic cables are used in aircraft, ships, high-temperature ovens, and for internal wiring and jumper cables in electronic equipment.

2. Excellent Flame Resistance:

Fluoroplastics have a high oxygen index and are generally difficult to ignite, with small flame spread when burning. Wires made from fluoroplastics are suitable for tools and environments with strict flame resistance requirements. Examples include computer networks, subways, vehicles, airplanes, and other public spaces. In the event of a fire, people have some time for evacuation, achieving safety and emergency response.

3. Excellent Electrical Properties:

Fluoroplastics have a lower dielectric constant compared to PE. As a result, fluoroplastic cables have lower attenuation compared to coaxial cables with similar structures, making them suitable for high-frequency signal transmission. Fluoroplastic cables are commonly used for internal wiring of communication equipment, jumpers between wireless transmitters and transceivers, and video and audio cables. Additionally, fluoroplastic cables have excellent dielectric strength and insulation resistance, making them suitable for control cables in critical instruments and equipment.

4. Excellent Mechanical and Chemical Properties:

Fluoroplastics have strong chemical bonds, high stability, and are nearly unaffected by temperature changes. They exhibit excellent resistance to weather aging and mechanical strength. Moreover, they are resistant to various acids, alkalis, and organic solvents. Therefore, they are suitable for use in environments with large climate variations and corrosive conditions, such as petrochemicals, oil refining, and wellhead instrument control.

5. Suitable for Welded Connections:

In electronic instruments, many connections are made through welding. Due to their low melting temperature, common plastics can melt at high temperatures, requiring skilled welding techniques. Some solder joints must have a certain welding time. This is another reason why fluoroplastic cables are popular, especially for internal wiring in communication equipment and electronic instrument control.

Fluoroplastics also have some limitations that restrict their use:

1. High Raw Material Costs:

Fluoroplastic raw materials are expensive, and domestic production in China still heavily relies on imports, mainly from companies like Japan’s Daikin and the U.S.’s DuPont. Although the domestic fluoroplastic production industry has developed rapidly in recent years, the product range is relatively limited, and there is still some gap in thermal stability and other comprehensive properties compared to imported materials.

2. Challenging Production Processes:

Compared to other insulation materials, fluoroplastic production processes are more challenging, with lower production efficiency and issues like printability and high wear and tear. These factors contribute to higher production costs.

3. Poor Radiation Resistance of PTFE:

For example, PTFE fluoroplastics exhibit poor radiation resistance. In normal temperature or in the presence of air, when the radiation dose reaches several Mrad, accelerator electron beam irradiation can cause carbon main chain breakage in PTFE molecules, leading to rapid decomposition of PTFE.