Introduction
Fluoroplastics are well known in the cable industry as near-perfect resin materials with a unique combination of excellent properties that provide outstanding performance in many demanding applications and are therefore widely used in wire and cable for high-speed data transmission, military and aerospace applications. Foamed fluoroplastics have excellent electrical properties while maintaining the inherent flame retardancy, temperature resistance, chemical resistance and weather resistance of fluoroplastics, making foamed fluoroplastic cables a great advantage for applications. This paper elaborates on the technical development of fluoroplastic foaming cable technology and equipment.
1. Advantages of fluoroplastic foaming cable applications
1.1 Foamed fluoroplastics performance characteristics
Fluoroplastics unique atomic basic properties and the formation of molecular bonds is the key to its excellent performance combination. PTFE based on its own characteristics can not melt extrusion. 1960 FEP as a real melt processing of fluoropolymers was first developed.
ETFE allows the polymer to be cross-linked to further improve properties such as cut-through resistance and is used primarily in aerospace, nuclear power wire and cable applications. Fluoropolymers are used in high frequency applications due to their low dielectric constant and very small dielectric loss tangent.
They are the best insulation material for high frequency transmission lines due to their low dielectric constant and very small dielectric loss angle tangent. In recent years, the excellent physical and electrical properties of fluoroplastics far exceed those of other materials, making them widely used in high-end communication transmission lines and high-temperature resistant wires and cables. However, the high price of fluoroplastics has limited its further application.
Therefore, based on the successful application of foaming technologies such as polyethylene (PE), foamed fluoroplastics were also developed.
Compared to fluoroplastics and other cable insulation materials, foamed fluoroplastics have the following advantages
a. Better electrical properties, with a significantly lower dielectric constant ε and a lower tangential value of the dielectric depletion angle tanδ (as shown in Figure 2). For example, the relative dielectric constant εr for solid FEP is 2.1 and tanδ is 5 x 10-4 at 1 MHz, while at 60% FEP foaming, εr is reduced to 1.4 and tanδ is reduced to 2.4 x 10-4 at 1 MHz. and a smaller cable outer diameter (impedance unchanged), resulting in a more compact product. For example, by using 60% foamed FEP for coaxial cable insulation, the cable attenuation can be reduced by 20% at 1 MHz, while the cable outer diameter can be reduced by about 12% (with no change in resistance).
b. Savings in high material costs. Due to the foaming of the insulation material, the bubble part is gas, which directly saves a large amount of insulation material, if the foaming degree is 60%, then 80% of the insulation material can be saved.
c. It does not affect the other good properties of fluoroplastics. Foamed fluoroplastics maintain the inherent flame retardancy, temperature resistance, chemical resistance and weather resistance of fluoroplastics, and basically do not affect the mechanical properties of fluoroplastics.
1.2 Application characteristics of foamed fluoroplastic cables
The main application characteristics of foamed fluoroplastic cables are: a. To meet the needs of data network cables for higher transmission rates and fire retardant (especially the US legislation). Although the market for CAT6 and CAT6A cables is growing, it is difficult to combine the increase in effective transmission distance and transmission speed (>10Gb/s) and bandwidth (>500MHz) of traditional 100m cables. As a result, fluoroplastic cables with lower dielectric constant foams are the obvious choice for higher frequency, low latency cables. In addition, CAT6 and CAT6A cables and assemblies with FEP, PFA/MFA insulation are available with fire ratings up to CMP. b. Power over Ethernet (PoE) cables meet the need to provide both power and communication at the same time. Foamed fluoroplastic PoE cables can provide power for equipment that implements the "Internet of Things" and new generation enterprise technologies.
The PoE cable provides power and communication for devices that implement the 'Internet of Things' and new generation enterprise technologies. From smart lighting to wireless access points (WAPs), PoE cables are transforming the future of wiring infrastructure by combining the functions of power and communication cables for devices in the home, office buildings and the future of autonomous vehicles.
c. Meeting the demand for higher frequency data transmission capability in consumer electronics cables. Foamed fluoroplastic coaxial cable can be used as a smaller, lighter, ultra-fine coaxial cable in the mobile phone and medical cable industries.
d. The demand for higher data transmission capacity for ultra-high frequency transmission cables in data centres can be met. Foamed fluoroplastic cables can be used as more miniaturised, lightweight and highly temperature resistant fire retardant cables.
2 Fluoroplastic foamed cable technology
2.1 Fluoroplastic foaming technology
As early as 1995, the Massachusetts Institute of Technology (MIT) conducted pioneering research into fluoroplastic foaming technology and reported the results in detail in the article "Microporous processes for fluoropolymers and the design of microporous foam extrusion systems for wire cladding".
The relevant research results are reported in detail in
a. It was pointed out that supercritical fluids can influence fluoroplastic foaming under certain conditions. The density of supercritical gas
The density of a supercritical gas is essentially the same as that of a liquid, and its viscosity is only 2 to 3 times that of a normal gas (about 1/10 of that of a liquid), with a dispersion coefficient about 10 times that of a liquid. In addition to increasing the bubble density of fluoroplastics foaming, supercritical fluids can also reduce the saturation time. For example, supercritical CO2 (critical temperature of 31 °C and critical pressure of 7.38 MPa) is used for fluoroplastic foaming, and the test results show that fluoroplastics have the best absorption of gas at the melting temperature.
The results show that fluoroplastics have an optimal absorption of gas at the melting temperature and undergo a rapid thermodynamic state change, forming small, uniformly distributed bubbles.
b. Evaluate the bulk foaming characteristics of FEP4100 and PFA440HP fluoroplastics developed by Dupont. PFA is a more crystalline polymer than FEP and therefore diffusion of gases in its matrix is more difficult.
c. The characteristics of microcellular foaming are summarised, including the fact that microcellular foaming is induced by the thermodynamic instability of the homogeneous supercritical gas/polymer system, that the number of nucleation in microcellular foaming is much greater than that of typical chemical foaming, and that the pore size of microcellular foaming is smaller than that of typical chemical foaming.
Many studies have shown that CO2 and nitrogen are suitable gases for fluoroplastic foaming, with a critical temperature of -147 °C and a critical pressure of 34 bar (3.4 MPa) for nitrogen.