Generally, electric wires and cables are insulated and […]
Generally, electric wires and cables are insulated and sheathed with plastic and rubber. These materials are all conventional materials, have abundant sources, can meet mass production, and the cost is relatively low. But for some special industries such as petrochemical, steel, aerospace, shipbuilding, military industry, pharmaceuticals, food, plastic machinery, boilers and other industries related to heat and high temperature, wires and cables that can withstand a certain higher temperature are required. Ordinary wires Obviously, cables cannot be used, and temperature-resistant wires and cables are required to ensure the safe operation of their power and signals.
With the rapid development of China’s economy, the demand for high-temperature cables in special industries has shown rapid growth. Heat-resistant and high-temperature cables are growing at a rate of 20% each year. As an important part of special cables, high-temperature cables have strong vitality. In short supply, China imports about 2 billion yuan from abroad every year for domestic construction.
The wire and cable insulation and sheath we generally use are ordinary engineering rubber and plastic as the basic resin, but they are required to be insulation grade. Common rubber materials for cables include: styrene-butadiene rubber, ethylene-propylene rubber, natural rubber and chlorosulfonated polyethylene, etc., and the working temperature is 60～75℃; common cable plastic materials include polyvinyl chloride, polyethylene (including cross-linked poly The working temperature is 70～90℃. This shows that these cables are not strictly heat-resistant or high-temperature cables.
Heat-resistant cables generally refer to cables with a temperature of 90-155°C and below, while high-temperature cables are cables with a temperature of 180°C and above. To solve the problem that ordinary cables are not resistant to high temperatures, it is necessary to improve the materials, or use insulating materials that can withstand high temperatures.
Heat-resistant and high-temperature wires and cables are generally determined by two requirements. The first is the high ambient temperature of the wire and cable, and the cable can normally transmit signals or electric energy under high temperature for a long time; the other is the power transmission cable, which is mainly for increasing the interception capacity.
Cable working in high temperature environment. Ordinary cables are prone to insulation aging and scorching at high temperatures, and the use of cables loses performance, is damaged and cannot be used. The high temperature cable can work normally and stably under the rated high temperature, the signal or power transmission performance is not affected, and the cable has a long service life. This type of functional cable is the most common type of high-temperature cable, and its use characteristics are also the easiest to understand.
Load-increasing high-temperature cables are mainly designed to reduce the outer diameter and weight of the cable under the prerequisite of current-carrying, and to develop towards light weight. Generally speaking, the higher the working temperature of the cable, the greater the current carrying capacity of the cable of the same cross-section. For occasions like airplanes and automobiles, weight reduction is of great significance. The use of high-temperature cables greatly reduces the cross-section. When the operating temperature rises from 90°C to 155°C, the current-carrying capacity increases by 50%. Under the same current-carrying capacity, the weight of the cable is reduced by half and the cost is also reduced. Of course, while the current is high, the power loss of most insulating materials will also increase
Heat-resistant wire and cable
Heat-resistant wires and cables are divided into two types: heat-resistant materials and heat-resistant modification of common materials.
Wire and cable made of heat-resistant materials
Heat-resistant wire and cable are insulated and sheathed materials. The body resin has heat-resistant properties. The main varieties are: polyurethane (up to 155°C), polyester (up to 135°C), polyvinylidene fluoride (150°C) ) And nylon (up to 115°C) insulation or sheath material. Commonly used in industries such as communications, automobiles, motors, and construction.
2. Common cable materials are modified in various ways to achieve heat resistance
1 Heat-resistant modification of rubber materials
Because of its poor heat resistance, rubber materials have a small margin for increasing the working temperature. Ordinary rubber can only reach 90°C with more heat stabilizers and cross-linking treatment, so it cannot be called heat-resistant cable, such as: styrene butadiene rubber , Neoprene rubber, chlorosulfonated polyethylene, etc. Mainly used in rubber-insulated mobile flexible wires, rubber-insulated flexible power cables and control cables.
However, EPDM rubber can be modified to increase the temperature resistance level to 135°C. In addition, it has better insulation properties, so it has a better development prospect in rubber.
2 Modification of polyvinyl chloride cable
The working temperature of ordinary PVC cable is 70℃. The high miscibility of PVC cable material makes it possible to modify it. The use of a large amount of heat stabilizer can facilitate the heat resistance of PVC to rise from 70℃ to 90℃. ℃ or 105℃, which greatly expands the applicability of PVC, an old-fashioned material. Perhaps this is one of the reasons why PVC cables will last forever? 90℃ PVC cable materials are often used in cross-linked polyethylene cable sheaths. For power, control and electrical equipment cables, due to the modification of PVC, the use of PVC cable materials that can be eliminated will last a long time in the use of sheaths. The main component of polyvinyl chloride butyronitrile compound is PVC, so it has the same modification performance as polyvinyl chloride butyronitrile compound cable and PVC insulated cable.
3 Modification of polyethylene cable
The plasticity of polyethylene material is better, but the fillability is poor, so it is impossible to fill the heat stabilizer method to increase the heat resistance temperature. Polyethylene cables can be used for DCP dry chemical crosslinking and silane warm water crosslinking to increase the working temperature to 90°C. The former is used for medium and high voltage power cables and the latter is used for low voltage cables. But another cross-linking method—irradiation cross-linking modification can greatly increase the working temperature of polyolefin (mainly polyethylene). The irradiated insulating material can vary according to the conditions, and the temperature resistance can reach 105 ℃, 125℃, 135℃, 150℃, foreign countries can increase to 180℃. It is mainly through the conversion of high-energy electrons into stable bond energy to strengthen the thermal stability of its molecular structure, and at the same time, it is equipped with an appropriate thermal stabilizer. According to the energy level and the performance of the thermal stabilizer, it is divided into different heat resistance levels.
The commonly used processing equipment in the radiation cross-linking industry is an electron accelerator, which increases the energy of the electron beam at a high voltage to achieve the purpose of cross-linking polyolefin materials. The energy level of the accelerator commonly used in electricity processing is 1.0-3MeV. Radiation crosslinking can also crosslink materials such as rubber, PVC and fluoroplastics. Irradiation cross-linked polyolefin wires and cables are mainly used for heat-resistant building wires, automotive wires, aviation wires, locomotive wires and electrical lead wires.
A heat-resistant cable is a medium-temperature cable that has a certain degree of heat resistance and can adapt to a certain temperature environment. The most widely used is that in power transmission cables, it is of great significance to increase the current-carrying capacity of the cable and reduce the weight and cross-section of the cable while ensuring the insulation performance.
High temperature and ultra-high temperature wire and cable
High temperature cables are divided into high temperature cables of organic polymer materials and ultra-high temperature cables of inorganic materials.
Organic polymer material high temperature cables are mainly fluorine plastic and silicone rubber cables
1Fluoroplastic insulated wire and cable
Fluoroplastics are an important category of plastics, and people usually recognize fluoroplastics from contact with the king of plastics-polytetrafluoroethylene (PTFE). In fact, polytetrafluoroethylene is only the fluoroplastic with the largest output and consumption. The commonly used fluoroplastics for wires and cables are: polyperfluoroethylene propylene (FEP, commonly known as F46), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene copolymer (ETFE, commonly known as F40), polyvinylidene fluoride (PVDF), etc. Fluoroplastic cables mainly include: high temperature communication cables, high temperature resistant lead wires and installation wires, high temperature compensation wires and industrial high temperature resistant power and control signal cables, radiation resistant cables for nuclear reactors, and wires for rolling stock.
2 Silicone rubber insulated wires and cables
Silicone rubber is mainly a polymer material formed by replacing carbon element with silicon element. Silicone rubber has good heat resistance. The commonly used silicone rubber for wires and cables is methyl vinyl silicone rubber, and the operating temperature range is -60°C to 180°C.
Silicone rubber has good bending performance and low-temperature performance, and is not easy to be damaged and cracked. These properties are not available in general high-temperature cables. Therefore, silicone rubber cables have a wide range of applications and are already a bright spot of high-temperature cables.
Silicone rubber cables are used in high-temperature mobile cables, flexible power cables, motor lead wires, and high-temperature operating places in low-temperature environments.
2. Inorganic material ultra-high temperature cable
Inorganic materials do not have excellent processing properties such as extrudability, so it is more difficult to process and form cable insulation, with poor adhesion and uniformity, and the insulation performance is lower than that of polymer materials, but the excellent high temperature performance can meet the special high temperature industry Demand.
1. Mineral insulated fireproof cable
The product structure is copper core copper sheathed magnesium oxide insulated cables, mainly power cables, control cables, heating cables and cloth wires. The normal working temperature is 250℃, but the actual working temperature can be higher, because the melting point of magnesium oxide is 2852℃, which is much higher than the melting point of copper (1083.4℃), and the cable can be used repeatedly and safely after fire.
The production of this cable is that the magnesium oxide shaped ceramic pillar is passed through the copper conductor, and the copper conductor and the outer copper tube are drawn and annealed at the same time. The magnesium oxide pillar is evenly crushed to form cable insulation. The production length of mineral insulated fire-resistant cables is subject to certain restrictions. Larger specifications are shorter.
The production process of mineral insulated fireproof cables is as follows:
Mineral insulated fireproof cables have the characteristics of high temperature resistance, fire resistance, explosion resistance, water resistance, radiation resistance, strong corrosion resistance, high mechanical strength, good grounding performance, small size, long life, large current carrying capacity, and strong overload capacity. Magnesium oxide is used for the insulation of mineral insulated cables. The advantage is that it has a high melting point (2852°C), but the disadvantage is that the dielectric constant is large (9.8), easy to absorb moisture, and the resistance at high temperatures is 1-2 orders of magnitude lower than that of silica. Therefore, it is not suitable as an insulating medium for signal cables. Moreover, this production method cannot achieve precise control of electrical performance indicators. Therefore, in demanding signal transmission occasions, especially microwave transmission, silica cables are the only available inorganic insulated cables. Magnesium oxide insulated cables are generally used in the transmission of electrical energy. The silica ultra-high temperature cable uses a high-temperature resistant stainless steel sheath, and the inner conductor uses 27% nickel-clad copper alloy, and even uses pure nickel wire as the conductor (melting point is 1455℃), special high-temperature silica (melting point is 1455℃) 1723℃), so the use temperature can reach 1000℃, or even 1300℃, and the short-term temperature resistance can be higher.
2. Inorganic coated insulated ultra-high temperature cable
Inorganic insulation coated insulated cables are generally made of inorganic tapes and wires, which are processed by cable technology. Refractory tape, as a high temperature resistant material, can still maintain the normal insulation performance at 800℃, so it is one of the main materials for high temperature cables; the alkali-free glass filament is an inorganic silicon material with a certain degree of insulation, and the melting temperature is above 600℃. It is processed by weaving technology and also used as reinforcement; the above two materials cannot form a tightness and tight insulation, so the insulator can be formed by applying inorganic curing paint, silicon powder and borax. Among the inorganic materials, there are many high temperature resistant materials, but the workability and formability are poor. The cable insulation formed by the above method is also easy to be damaged. Depending on the material selected, it can be made into a working temperature of 500℃, 800℃, or even 1000 ℃ cable, in addition to the conductor, nickel-plated copper conductor or matching high temperature alloy conductor should be used. For signal cables, using the above winding method is an unreliable insulating layer, especially when there are various additives, so there is currently no inorganic insulated signal cable with this production method.
Ultra-high temperature cables are often used in industries such as aerospace, military industry, heating furnaces, chemical reactors, nuclear islands in nuclear power plants, and steel.
Conclusion Among the varieties of special cables, high-temperature cables and ultra-high-temperature cables have become an important trend. Although the power loss of high-temperature cables is slightly larger, in some high-temperature situations, ordinary cables are easily damaged. High-temperature cables can operate safely. An indispensable variety in modern industry and military industry.