Application of polytetrafluoroethylene in wires and cables

Polytetrafluoroethylene (PTFE) has the characteristics of high temperature resistance, corrosion resistance, high insulation, and excellent electrical properties, which make it widely applicable. Therefore, wires insulated with PTFE are a very good choice and are widely used in various types of wiring in space and aviation. Most of the wires in the US military standard MIL-W-22759 use this type of wire. Its outstanding advantage is that its temperature resistance level can reach up to 250 ℃, and its mechanical strength and electrical performance will not be affected by long-term use at high and low temperatures. Secondly, due to the wide frequency range of polytetrafluoroethylene, it is commonly used as insulation for coaxial cables; There are also insulation for thermocouple wires and lead wires for H-class and F-class motors.

The raw material for making corrugated pipes is polytetrafluoroethylene (PTFE), which is a highly crystalline polymer. Its crystals undergo volume changes at the turning points of 19 ℃, 29 ℃, and 327 ℃. Therefore, the transition points of 19 ℃ and 327 ℃ are crucial for the processing technology of polytetrafluoroethylene.
The crystal transition temperature of 19 ℃ is crucial for processing raw materials. When using polytetrafluoroethylene to make thin films or push wire insulation layers, there is a process of molding polytetrafluoroethylene powder resin. If the temperature control is not good, the lattice distance will increase, causing voids in the pre formed product and ultimately leading to internal cracking.
327 ℃ is the melting point of polytetrafluoroethylene. Strictly speaking, above this temperature, the crystal structure disappears and changes to a transparent amorphous gel state, accompanied by a 25% increase in specific volume. This gel block has high melt viscosity and still cannot flow at 360 ℃. This characteristic determines that polytetrafluoroethylene cannot be processed by the same method as typical thermoplastic resins (such as melt extrusion), but rather by a similar powder metallurgy method that combines pressure and sintering. Due to the low thermal conductivity of polytetrafluoroethylene, there is a significant volume change at temperatures above and below the melting point. Therefore, during the sintering process, the heating rate near the melting point must be slow to ensure uniform temperature inside and outside the product. Otherwise, it may cause stress inside the product, and in severe cases, even cracking.
The crystallinity of polytetrafluoroethylene has a certain impact on the physical and mechanical properties of wires. Usually, with high crystallinity, the density of polytetrafluoroethylene is also high, and its physical and mechanical properties are improved, while the opposite is small.
The crystallinity of tetrafluoroethylene is related to the size of its molecular weight and the cooling rate after sintering. At the same cooling rate, the smaller the molecular weight, the easier it is to crystallize, and the higher the crystallization rate; Under the same molecular weight, an extremely slow cooling rate facilitates the recrystallization of large molecules, as the crystallinity of the product is higher. The maximum is about 73%. Rapid cooling can prevent the recrystallization of amorphous gel. The crystallinity is small, but even at the fastest cooling rate, the crystallinity is about 50%. So the cooling rate is different, and the crystallinity of sintered polytetrafluoroethylene is usually between 50% and 70%, with the maximum crystallization rate in the temperature range of 310 ℃ -315 ℃.

01 Physical Performance

PTFE is a tough, soft, non elastic material with moderate tensile strength. It has good low-temperature performance and can still be stretched under pressure even at temperatures as low as -269 ℃.
02 Electrical characteristics of PTFE insulated wires
Polytetrafluoroethylene has excellent electrical insulation properties over a wide range of temperatures and frequencies. Due to the symmetry and uniform distribution of fluorine atoms in the polytetrafluoroethylene molecular chain, there is no inherent dipole distance, resulting in minimal variation in the dielectric loss tangent tg δ and relative dielectric constant ε r within the power frequency range of 109Hz. Between room temperature and 300 ℃, the actual change in TG δ value of polytetrafluoroethylene is very small, while ε r decreases with increasing temperature.
03 Heat resistance
Polytetrafluoroethylene has high heat resistance and low temperature resistance, with a long-term use temperature range of -200-+250 ℃.

The heat resistance of polytetrafluoroethylene is very high among existing engineering plastics. Although trace amounts of decomposition products begin to appear at 200 ℃, the decomposition rate remains very slow from 200'C to temperatures above the melting point of 327 ° C, almost negligible; When the temperature rises to 400 ℃, there is significant decomposition but only a loss of about 0 0.1% by weight. After thermal decomposition, the average molecular weight of polytetrafluoroethylene decreases, the crystallinity increases, and the tensile strength decreases. When heated at 300 ℃ for one month, its tensile strength decreases by about 10% to 20%; After long-term heating at 260 ℃, its tensile strength remains basically unchanged. Therefore, from the perspective of thermal decomposition, polytetrafluoroethylene can be used for a short period of time at 300 ° C and for a long period of continuous use at 260 ° C. From the perspective of thermal deformation, polytetrafluoroethylene can be used continuously for a long time below 260 ° C under low load conditions; When the load is high, thermal deformation is significant, and its operating temperature is relatively reduced.
04 Electrical Insulation
Polytetrafluoroethylene has extremely stable arc resistance, usually exceeding 300 seconds. This is because during high-voltage surface discharge, it does not cause short circuits due to carbonization, but only decomposes into gas. Even under long-term outdoor exposure and pollution from dust, rain, and dew, its insulation performance is not affected. However, due to the high electronegativity of fluorine atoms in polytetrafluoroethylene, electrons of 1~2Ev will cause it to dissociate and decompose, resulting in poor corona resistance.
05 Other Performance
Although polytetrafluoroethylene has many advantages such as weather resistance, water resistance, and chemical stability, it is difficult to process, has poor processability, cannot be continuously extruded, and has low production efficiency; Under continuous load, there is cold flow phenomenon and poor cutting resistance; Due to poor resistance to electrical ionization and radiation, polytetrafluoroethylene as an insulation material for wires and cables still has some drawbacks.

Types and Applications of Polytetrafluoroethylene
Tetrafluoroethylene can be divided into two categories based on different polymerization methods: suspended polytetrafluoroethylene and dispersed polytetrafluoroethylene. Suspended polytetrafluoroethylene resin is a white powder with larger particles. After appropriate post-treatment, powders of different particle sizes can be obtained. This powdered resin is used for molding and rolling processing, rather than directly for the production of wires and cables. When used for insulation of wires and cables, suspended polytetrafluoroethylene should be molded and sintered into cylindrical blanks, and then machined into polytetrafluoroethylene films on a lathe. This type of film, also known as clinker tape, is used for insulation of power lines and cables. Dispersed polytetrafluoroethylene is divided into two forms: powder and concentrated dispersion. Among them, the powdered dispersed resin is mixed with a certain amount of additives (such as petroleum ether) and fillers (such as quartz powder), and is specially used for compression molding, suitable for thin materials such as wires and cables. The pressing processing of wall products is widely used in current wire production: powdered dispersed resin can also be pressed into shape, and then rolled into a thin film (also known as raw material tape) for insulation or sheath wrapping of thin wire diameters. The concentrated dispersion of polytetrafluoroethylene is mainly used for impregnating porous materials (such as asbestos, glass, fiber weaving) and for surface coating of metal bearings made by powder metallurgy. The glass fiber braided layer of polytetrafluoroethylene insulated electromagnetic wires and high-temperature resistant wires is used for coating with concentrated polytetrafluoroethylene solution.