Introduction
Wires and cables refer to wire products used to transmit electrical (magnetic) energy, information and realize electromagnetic energy conversion. They are indispensable basic products for realizing electromagnetic energy conversion. They are widely used in various links of power production, transmission and application such as power generation, transmission and distribution, and terminal power consumption. They are closely related to the development of the national economy and people's daily life. They are known as the "blood vessels" and "nerves" of the national economy.
The main structure of wires is "conductor" (conductor) or "conductor + insulation" (cloth wire), and the main structure of cables is "conductor + insulation + sheath". Conductors are generally made of metal materials such as copper, aluminum or aluminum alloy, and insulation and sheaths are generally made of rubber, polyethylene, cross-linked polyethylene and polyvinyl chloride. According to the use, they are mainly divided into five categories: conductors (bare wires), power cables, wires and cables for electrical equipment, communication cables, and winding wires.
At present, in the production of wires and cables, the commonly used fluoroplastics are PTFE, PFA, FEP, PVDF, etc. Of course, generally speaking, if we distinguish them according to market prices, PTFE is at the top of the fluoroplastics, followed by PFA, and then FEP. At present, FEP is the most widely used and has the largest market volume. Of course, some manufacturers choose PVDF/ETFE with a slightly lower temperature grade for related cables. These materials are mainly used to manufacture various heat-resistant and high-temperature insulated wires, well logging (oil) cables, geological detection cables, heating cables, F-class and H-class motor lead wires, and radiation-resistant wires. , electromagnetic wire, RF coaxial cable, etc. Let's put it this way. All plastics containing fluorine atoms in their molecular structure are collectively called fluoroplastics. Fluoroplastics are made from fluorine-containing monomers such as tetrafluoroethylene, hexafluoropropylene, trifluorochloroethylene, vinylidene fluoride and vinyl fluoride through homopolymerization or copolymerization. With the continuous development of polymer technology, the variety of fluoroplastics is gradually increasing, and the scope of application is expanding. Because fluoroplastics contain fluorine atoms in their molecular structure, they have many excellent properties, such as excellent electrical insulation, high heat resistance, outstanding oil resistance, solvent resistance and wear resistance, good moisture resistance and low temperature resistance. Therefore, fluoroplastics occupy an important position in industrial sectors such as national defense, electromechanical, metallurgy, and petrochemicals. The excellent performance of fluoroplastics is achieved by the high binding energy between carbon atoms and fluorine atoms. The main chain skeleton of polytetrafluoroethylene is carbon atoms, and the surrounding is completely surrounded by fluorine atoms, so its various properties are relatively high. However, due to the influence of side groups, the softening point of PFA, FEP, ETFE and other varieties is lower than PTFE, and other properties have also changed.
Main Characteristics of Fluoroplastics
Thermal properties
Fluoroplastics are flame retardant and have excellent heat resistance. The continuous use temperature of PTFE and PFA can reach 260℃, and can be used at 300℃ for a short period of time. The use temperature of FEP is 60℃ lower than them. PCTFE can be used at 120℃. If fluoroplastics are used at high temperatures for a long time, the crystallinity will change, so it is especially important to pay attention to this when manufacturing equipment linings.
Chemical Resistance
Fluoroplastics have excellent chemical resistance and solvent resistance, especially PTFE, PFA, FEP, etc., which are not corroded by acids, alkalis, and solvents. However, molten alkali metals, fluorine, and trifluorochlorocarbons have different degrees of influence on them. PCTFE, ETFE, PVDF, etc. have slightly poor chemical resistance among fluoroplastics, but their corrosion resistance is still much better than other plastics.
Electrical Properties
The electrical properties of fluoroplastics, especially high-frequency electrical properties, are unmatched by other materials. The polarity of PTFE, FEP, and PFA molecules is very low, and changes very little over a wide range of temperature and frequency. The relative dielectric constant is stable, the dielectric loss is very low, and the electrical insulation is excellent. Among them, PVDF also has specific piezoelectricity and pyroelectricity, and can be used to manufacture piezoelectric materials.
Mechanical Properties
The tensile strength of fluoroplastics increases with the increase of hydrogen and chlorine atoms in the molecule. The brittle temperature of PTFE and PCTFE is very low, showing excellent low-temperature performance. PTFE has a low friction coefficient and special self-lubricating properties, but PTFE has its own disadvantages such as large wear and cold flow. Various fillers can be used to improve wear resistance and overcome cold flow.
Non-Stick
Fluoroplastic has a special non-stick property. Especially PTPE, FEP, PFA and other molecules have a high fluorine content, and the surface contact angle is very large, making the liquid on the surface of fluoroplastic products spherical. It is not easy to bond with resin, so it is often used to make non-stick coating on the surface of cookware.
Weather Resistance
All types of fluoroplastics have excellent weather resistance, and their properties remain unchanged even after long-term exposure to harsh temperatures.
Hydrophobicity
Fluoroplastic has low water absorption, especially PTFE. Its hydrophobicity can be used to manufacture breathable and water-tight composite fabrics and other equipment.
What is the manufacturing process of fluoroplastic wires?
The main insulating materials for fluoroplastic insulated wires are PTFE, FEP and other fluoroplastics. According to the processing characteristics of different fluoroplastics, the following three processing techniques are generally used.
Hot Extrusion Process
When the temperature of the barrel inside the extruder reaches about 350°C~390°C, add F46 fluoroplastic into the hopper, and use the thrust of the rotating screw to evenly and continuously coat the conductive wire core through the forming mold, and then cool it down to shape. This method uses Φ30, Φ60, Φ90 and other high-temperature plastic extruders, and often produces F46, F40 and other fluoroplastic insulation products.
Pushing Process
The powdered polytetrafluoroethylene plastic is pre-pressed into a cylinder shape, placed in the barrel, and evenly and continuously coated on the conductive wire core through the forming mold using the piston thrust, and then sintered at 380°C and shaped after cooling. This method uses the F4 push press to produce F4 (PTFE) type products.
Wrapping Process
The polytetrafluoroethylene film tape cut into a certain width is wrapped around the wire core and then sintered to shape it. This method uses a wrapping machine and a sintering furnace, and usually produces AFR type, FSFB type and other wires.
Several points to note in the extrusion process of fluoroplastic cables
1. Choice of Extrusion Method
The melt viscosity of fluorine-containing plastic melt is extremely high. In the processing of thermoplastic fluoroplastics, pressure extrusion is not suitable. Since the melt has sufficient strength and allows stretching, sleeve-type die extrusion can be used to produce fluoroplastic wires and cables.
2. Extrusion Mold
When the sleeve-type die is extruded, the die is the most critical factor in the thermoplastic fluoroplastic extrusion process. An inappropriate die may cause loose sleeves, melt fracture, surface corrugations, large outer diameter fluctuations, and other problems. In severe cases, extrusion molding may not be possible. In actual production, it is particularly important to control a reasonable stretch coefficient to ensure the bonding force between insulation and conductor.
The stretch balance DRB value should not be less than 1.0, otherwise the insulation will be loose, especially when the single wire is extruded, the value should be adjusted to between 1.05 and 1.15, but the DRB value should not be greater than 1.2, otherwise the melt cone will rupture when the extrusion speed is fast, affecting the product quality.
In general, it is more appropriate for the wire length of the thermoplastic fluoroplastic extrusion die not to exceed 15mm. For processing wires with larger diameters and a stretch ratio DDR between 5 and 10, the wire length of the extrusion die needs to be increased to about 12mm to increase the pressure of the molten resin in the die and reduce the fluctuation of the extrusion outer diameter, so as to achieve the purpose of stable extrusion.
3. Extrusion Temperature
When extruding thermoplastic fluoroplastics, the temperature setting is proportional to the extruded wire diameter and speed. The larger the wire diameter, the higher the extruder screw speed, and the higher the set temperature. During the extrusion process, the temperature is judged to be appropriate by observing the state of the molten cone of the fluoroplastic. If the temperature is too low, the insulation will crack due to the existence of internal stress; excessive heating will cause local decomposition of the fluoroplastic, resulting in bubbles on the surface of the cable insulation or sheath, seriously affecting the product quality. At the same time, the wire core should be preheated before entering the die, and a heat preservation heating cover should be added outside the mold and the molten cone to avoid temperature changes causing changes in the viscosity of the molten cone, which will affect the extrusion speed and product quality.
4. Cooling Control
Fluoroplastic is a crystalline organic polymer material, so it is not suitable to use the shaping cooling and quenching method. Since the extrusion temperature of fluoroplastic is high, the temperature of the cooling medium should reach the optimal ratio of the material's crystal nucleus growth rate and grain growth rate. The insulation or sheath after extrusion should be cooled by segmented cooling, that is, the water temperature of the water tank should be set from high to low to avoid rapid cooling causing loose sleeves or forming internal stress in the extrudate.
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