Study on the Structure and Properties of PTFE and the Application of Coaxial Cables Ⅰ
RF coaxial cable is an indispensable component in various radio communication systems and electronic equipment. It’s widely used in wireless communication and broadcasting, base stations, television, radar, navigation, computers and instruments.
According to the functional attributes of each structure of the RF cable, the cable structure can be divided into five parts: the inner conductor, the dielectric, the wrapping shielding outer conductor, the braiding outer conductor and the jacket. The inner and outer conductors of the RF coaxial cable are in a concentric position. Due to the high-frequency skin effect of the RF signal, the propagation of electromagnetic energy is confined within the medium between the outer surface of the inner conductor, the inner surface of the outer conductor and the inner and outer conductors. As the core structure of the RF cable, the choice of the material of the dielectric greatly affects the performance of the RF cable. The dielectric occupies the largest proportion in the functional layer of the RF cable, so it must have the characteristics of wide frequency band, low attenuation, high stability and good reliability.
Coaxial cable dielectric material
Commonly used dielectric materials for coaxial cables are polyethylene (PE), polypropylene (PP), fluorinated ethylene propylene (FEP), and soluble polytetrafluoroethylene (PFA), polytetrafluoroethylene (PTFE), silica, etc. PE is a good electrical insulator, but PE has high hardness, poor mechanical strength, low heat resistance, and poor resistance to environmental stress cracking, the phase stability is also poor. PE has higher requirements on the ambient temperature when it is used as the dielectric layer. The maximum service temperature of PTFE, PFA and FEP can reach 200 ℃, among them, PTFE has the lowest dielectric constant and loss factor. Its dielectric properties and electrical insulation properties are basically not affected by temperature, humidity and frequency. After combining the conditions of temperature, attenuation and phase stability, PTFE becomes the best choice for the dielectric material of stable phase cable. Focusimple and other first-class coaxial cable manufacturers at home and abroad usually choose PTFE as the dielectric material.
Molecular structure of PTFE
PTFE, commonly known as "Plastic King", has a molecular structure of , it was discovered by Dr. Roy J. Plunkett of DuPont in 1938. In 1945, DuPont applied for the registration of the Teflon trademark for polytetrafluoroethylene, realizing the industrialization of PTFE.
PTFE is a high molecular polymer formed by polymerization of tetrafluoroethylene monomer. Its polymerization preparation method is the same as that of conventional polymers. There are four polymerization methods including bulk polymerization, solution polymerization, suspension polymerization and lotion polymerization (dispersion polymerization). Suspension polymerization and dispersion polymerization are generally used in industry. Suspension polymerization can obtain PTFE resin particles with a particle size of 35-500 μm, while dispersion polymerization can obtain PTFE resin particles with a smaller particle size.
Figure 1. Schematic diagram of SP3 hybridization of carbon atoms in the main chain of PTFE
The fluorine atom has the highest electronegativity of all elements. The greater the electronegativity, the smaller the atomic radius and the greater the bond energy formed with other atoms. Therefore, the C-F bond energy is very large and the esterification reaction is not easy to occur. It ensures that PTFE has high thermal stability, chemical stability and anti-aging ability, and can work for a long time at temperature of -190℃~250℃. The molecular structure of PTFE only contains two elements—carbon and fluorine. The valence electrons in the outer layer of carbon atoms adopt SP3 hybridization to form a regular tetrahedron with a bond angle of 109°28'. The main chain of C-C bonds is in a zigzag distribution, and the symmetrical distribution of F atoms makes the positive and negative charge centers of the entire molecular chain overlap. Therefore, PTFE is a linear non-polar polymer that is completely symmetrical and has no branches and side groups. Its dielectric constant and dissipation factor are the smallest among existing dielectric materials. PTFE can be used in electronic devices such as wires and cables, connectors, printed circuit boards, etc. The C-F bond length in the molecular structure is short. In the crystalline state, PTFE has a helical structure, the fluorine atoms can just cover the main chain of PTFE, and the shell of fluorine atoms protects the main chain of carbon atoms from attack and erosion by external molecules. As a result, PTFE has low surface energy and friction coefficient, high acid and alkali corrosion resistance, further, it is resistant to almost all other chemicals except molten alkali metal and liquid fluorine.
Figure 2. Schematic diagram of the basic helical structure of the molecular chain of PTFE
PTFE with unbranched linear molecular chain structure
The molecular chain of PTFE is an unbranched linear structure with excellent regularity and symmetry. It’s easy to form an ordered arrangement, the simpler the structure of the molecular chain, the better the symmetry and regularity, and the stronger the crystallization ability. As a crystalline polymer, PTFE generally has a crystallinity of 55% to 75%, with a maximum of 93% to 97%. The higher the crystallinity, the more regular the molecular chain arrangement is, the higher the temperature is required to destroy, the higher the melting point. The higher the crystallinity of PTFE, the whiter the color. The modified PTFE has low crystallinity and trends to be transparent and translucent.
Figure 3. Schematic representation of the condensed matter structure of PTFE
Figure 4. Phase diagram of PTFE
Figure 5. Schematic illustration of the three-dimensional superstructure of the molecular chain of PTFE
Special phase properties of PTFE
PTFE has different condensed state structures at different temperatures and pressures. It is currently known that PTFE crystals have four two-dimensional structures and phase structures. As shown in the figure, when the temperature is lower than 19°C, PTFE exhibits the second phase state, the molecular chain is helical, the pitch is 1.69nm, the helix angle is 13.8°, including 13 carbon atoms, 6 monomer units. The crystal structure is triclinic, the lateral diameter of the segment is 0.27nm and the lattice constant is 0.559nm. When the temperature rises above 19°C, the PTFE crystal undergoes a phase transition and becomes the fourth phase hexagonal system, the helix chain begins to unwind, the helix pitch increases to 1.95nm, the helix angle is 12°, it contains 15 carbon atoms, 7 monomer units, the lateral diameter of the segment is 0.28 nm and the lattice constant is 0.566 nm. When the temperature continues to rise to 30 °C until the melting point of 327 °C, the PTFE crystal transforms and remains in the first phase, which is a pseudo-hexagonal system. The helical chain was further unwinded into a random helical shape, the lateral diameter of the segment increased to 0.29 nm, the lattice constant was 0.567-0.574 nm. The crystalline state transition around 19°C-30°C will cause a volume change of about 1% in the polymer, resulting in many sudden changes in properties. When the ambient temperature is 75°C, if the pressure is increased to 500MPa or above, the PTFE crystal will transform into the third phase with monoclinic and orthorhombic crystals, and the helical chain molecular structure will unwind into a zigzag straight chain structure. The segment transverse diameter is reduced to 0.24 nm.
Figure 6. Focusimple temperature phase change curve of stable phase cable
From the molecular structure, phase characteristics and phase transition of PTFE, it can be known that when the temperature rises, the thermal motion of the polymer molecules intensifies, the molecular polarization increases, the dielectric constant of the material changes in direct proportion to the temperature. The dielectric properties will be improved when the temperature increases. But at the same time, the volume expansion caused by the temperature rise will reduce the dielectric constant of the whole material, so the change of the dielectric constant due to the temperature change is comprehensive. Different densities of dielectric materials have different rates of change. When PTFE is in the phase transition region of 19°C-30°C, the change in dielectric constant caused by the phase transition is far greater than molecular motion and volume expansion. In the fourth phase state and boundary, the temperature phase of the stable phase cable has an inflection point, which usually changes inversely proportional to the temperature.
To be continued
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