In the previous chapter, we focused on the molecular structure, molecular chain structure and phase characteristics of polytetrafluoroethylene (PTFE). Through the analysis of the structure and characteristics of PTFE, we have an in-depth understanding of PTFE as a stable phase cable dielectric layer. Reasons for a great choice of material. In this chapter, we will specifically introduce the common forms of PTFE in production and application and its processing technology in stable phase cable applications.

Common forms of PTFE membrane in production and application

PTFE membrane is a microporous membrane made of polytetrafluoroethylene as raw material by special processes such as calendaring, extrusion and biaxial stretching. In terms of form, the three common forms of PTFE membrane in production and application are flat membrane, hollow fiber membrane and tubular membrane. 

The surface of the PTFE flat membrane has many microporous structures, the pore size ranges from 0.02 μm to 15 μm, and the surface microporous structure is similar to spider net. DuPont first introduced the stretching process into the production of PTFE membranes in 1960 and successfully produced PTFE microporous membranes. Due to the limitations of the process conditions at that time, the micropores produced still have many defects. After that, the United States, Europe, and Japan successively used the biaxial stretching method to successfully prepare PTFE flat membranes with uniform pores and stable dimensions. PTFE hollow fiber membrane has strong hydrophobicity, uniform and controllable pores, and due to the existence of hollow fiber structure, it has good self-supporting property, which can significantly improve the filtration efficiency, so it is often used in membrane distillation. The diameter of PTFE tubular membrane is larger than that of hollow fiber membrane, which is also a self-supporting structure and can be used for air evolution and water filtration.

In the stable phase cable application, the PTFE film is cut to form a microporous tape, and the insulating layer of the cable is formed after multi-layer wrapping.

Application of PTFE in stable phase cable

PTFE is a crystalline polymer material. In terms of molecular structure, PTFE is a thermoplastic material with a melting point around 327°C. Due to the strong electronegativity fluorine atoms on both sides of the macromolecular carbon main chain, the repulsion between the fluorine atoms is very large, which makes it difficult to rotate within the macromolecular chain and the molecular chain segment is rigid. In addition, its linear structure also increases the force between molecular chains, which makes the melt viscosity of PTFE extremely high. Even if it is heated to above the melting point, PTFE will only show a gel state instead of a molten state, and the viscosity can reach 1010~1011Pa·S. When the environment reaches the decomposition temperature and decomposes, the PTFE still cannot flow and cannot be injected or extruded. the PTFE powder is easy to be fibrous under the action of shearing force. Therefore PTFE cannot be processed using conventional thermoplastic melt processing methods.

The PTFE insulation of cables is usually prepared by pressing and extrusion in the industry. PTFE insulated cable is prepared by adding lubricant to the dispersed powder of PTFE to make it become a sticky paste, extruding the paste to obtain a uniform blank, then putting it into extruder to press and extrude, and then removing the lubricant through degreasing process and sintering at high temperature, so that the originally dispersed molecular chains are entangled to form a block material.

Wrapping PTFE microporous tape as cable insulation medium is another important processing form of stable phase cable insulation.

The front-end production process of PTFE microporous film is very similar to that of PTFE extrusion, but the extruded semi-finished products need to be turned or rolled to form a film, and stretched under different speed rollers, and finally a PTFE microporous film with a unidirectional or biaxial stretching state is formed. Reducing the density of PTFE and preparing it into a microporous film can effectively reduce the dielectric constant of PTFE insulation, which is one of the most common and important ways to reduce cable attenuation and weight.

Figure 7.  Production flow chart of PTFE microporous membrane

Figure 8. Microstructure of unidirectional and biaxial stretched PTFE microporous membrane

There are many specifications of PTFE wrapping tapes on the market. The choice of PTFE wrapping tapes by cable manufacturers mainly depends on their own production experience and product properties. Generally, wrapping tapes with different widths and thicknesses are selected according to the inner conductor and medium radius of the cable. The difference in the thickness of the wrapping tape and the width of the overlap will affect the number of layers of dielectric wrapping, which in turn affects the diameter of the cable.

The performance of the cable dielectric layer is inseparable from the wrapping equipment. After setting the parameters such as the outer diameter and pitch of the wrapping equipment, it is necessary to adjust the speed and tension of the equipment according to the actual wire diameter. Focusimple adopts the most stable and precise wrapping equipment. Experienced designers debug the equipment according to the requirements of different types of cables, and choose the optimal production speed.

Figure 9. Schematic diagram of production process of PTFE microporous tape insulated phase stable cable

In addition, due to the special phase structure and characteristics of PTFE, PTFE insulating materials must be stored and operated in an environment where the temperature is below 19 °C and the humidity is below 50%, so as to avoid changes in its structure and properties before processing, which may affect product processing and performance. The temperature and humidity are strictly controlled in Focusimple's material warehouse and component production workshops, which ensure the performance of PTFE does not change.

Density and transmission rate of PTFE insulating medium

The transmission rate v of electromagnetic waves in stable coaxial cables is mainly related to the dielectric constant εr of the medium. The dielectric constant of the stable phase cable dielectric produced by pushing and sintering is generally 1.7~2.3, so the transmission rate of the pushing dielectric is generally 70~77% of the transmission rate of electromagnetic waves in vacuum.

In addition, the transmission attenuation a is greatly affected by the dielectric constant εr and the loss tangent tan θ. Therefore, Focusimple and other companies in the industry generally choose to optimize dielectric materials or increase the proportion of air in the material to reduce the density of the dielectric layer and achieve the purpose of reducing the dielectric materials’ dielectric constant εr and loss tangent tan θ.

The low-density PTFE microporous tape forms the dielectric of the stable phase cable through the multi-layer wrapping process. Since the membrane itself has a low density, it can effectively reduce the εr and tan θ of the cable dielectric, so as to improve the transmission speed and reduce transmission attenuation of the stable phase cable. The transmission rate of the PTFE microporous tape-wrapped insulated cable of the same specification is about 15% higher than that of the PTFE solid push cable, and the transmission attenuation is optimized by more than 15%.

Conclusion

With its superior dielectric properties, chemical corrosion resistance, high temperature resistance, low friction coefficient, and high electrical breakdown strength, PTFE is widely used in telecommunications, testing and measurement, national defense and aerospace. It has become an indispensable important insulating medium in modern industry. As an excellent choice of coaxial cable insulation medium, PTFE plays an important role in reducing attenuation and maintaining stable signal transmission.

After in-depth study of the material properties, relying on its own research and development capabilities and complete hardware facilities, Focusimple R&D team has designed and produced a series of cables with high performance, low loss and stable phase, such as the FSB series and FSD series, and the FABtest series for test cables. The in-depth research and application of materials and structures has laid a solid foundation for the design and production of cable assemblies suitable for different application scenarios.

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