1. Mica tape
Mica tape is the refractory insulating material of mica ribbon fire-resistant cable, which determines the fire resistance of mica ribbon fire-resistant cable. The typical structure of mica ribbon fire-resistant cable is shown in figure 1.
Mica tape consists of dielectric material, reinforcing material and adhesive. Mica is widely used in the field of electrical insulation because of its high dielectric strength, small loss, high heat resistance and excellent corona resistance. When fire occurs, mica ensures wire and cable to have higher fire resistance and insulation. There are two kinds of mica in the mica tape: natural mica and synthetic mica. There are many kinds of natural mica, and there are two kinds of muscovite and mica as electrical insulating materials. At present, synthetic mica has only practical value. The electrical properties of Muscovite at room temperature are better than those of mica and fluorine mica, but their insulation properties decrease sharply at high temperature. Therefore, in China, mica or fluorine mica with better fire resistance is often used to make mica tapes for fire-resistant cables.
Reinforcing materials are used to improve the strength of mica tapes and reduce the damage of mica paper. Glass fiber cloth and polymer film are two kinds of reinforcement materials commonly used at present. The glass fiber cloth by alkali free glass silk woven glass fabric reinforced mica tape is divided into single-sided and double-sided reinforcement reinforcement. Because of the backward production process, the domestic mica tape used for fire-resistant cable is mainly made up of double sided glass cloth, so as to reduce mica shedding, but double reinforcement will lead to cable diameter increase and cost increase. In foreign countries, single side reinforcement is the main method to reduce the cost, improve the strength and electrical properties of mica sinter layer, and reduce cable diameter.
At present, the polymer film has polyester film, polyethylene film and so on. Using polymer film as reinforcing material can reduce the cost and improve the surface abrasion resistance and smoothness. However, it is necessary to ensure that the products of combustion do not destroy the insulation of mica tape, and that the elongation of the mica tape should not be too large if the strength of the mica tape is high enough. The function of adhesive is to combine mica paper with reinforcing material, which has decisive influence on heat resistance and high temperature insulation of mica tape. Adhesives need to meet the high bond strength between mica paper and glass cloth, mica tape has a certain flexibility and edge bending, binder inorganic content is high, the three are indispensable.
The general development process of binders for fire resistant mica tapes at home and abroad was discussed. The mica tape prepared with natural adhesive has low insulation, so the polyester adhesive is developed. But the polyester adhesive has poor cohesiveness, shrinkage, and is easy to be cracked and degraded by alkali, chloride, organic solvent and water, so epoxy resin adhesive is developed. Epoxy adhesives have been widely used because of their high insulation and good comprehensive properties, but they are still inadequate in high temperature and insulation. Silicone resin has good resistance to high temperature and insulation, especially after the combustion of silicone binder, SiO2 has good electrical insulation properties, so it is often used in epoxy resin can not be competent.
2. Mineral insulating materials
Mineral insulated refractory cable is a cable consisting of one or several conductors which are insulated by compressed mineral powder in the metal sheath. Its refractory insulation material is made up of insulating filling material and metal sheath, and the two are all inorganic, which makes the mineral insulated fire resistant cable have incomparable advantages in the aspect of fire resistance. The typical structure of mineral insulated refractory cable is shown in figure 2.
Insulating materials for mineral insulated fire resistant cables are usually made of Magnesium Oxide. As an insulating filling material for mineral insulated fire resistant cables, two conditions should be met: high electrical insulation at high temperature and high temperature. Low conductivity material stability and lattice energy of valence cations and there is plenty of space, in order to reduce the ion conduction, so not all refractory oxides are high temperature resistant and good insulating materials.
The high temperature resistance of BeO, MgO and Al2O3 class II and III group oxides can meet the requirements of mineral insulated refractory cables, but almost only Magnesium Oxide can be used as a high-temperature insulation material in the form of powder insulation. The insulating filling material of mineral insulated fire resistant cable is not only one of Magnesium Oxide, but also other inorganic materials with higher heat resistance.
The metal sheath materials of mineral insulated fire resistant cables are copper, copper alloy and stainless steel.
Mineral insulated cable fireproof material composition determines that it has obvious advantages and disadvantages, the advantages are as follows: the fire resistance of refractory insulating material and reliable; all components, from inorganic substances will not burn, not the spread of flame, no toxic substances released; having resistance to mechanical damage, load flow, anti animal bite bite, long life characteristics etc..
The disadvantages of mineral insulated refractory cables are shown as follows:
(1) the production process is complex. The production process of mineral insulated fire-resistant cable with a prefabricated Magnesium Oxide porcelain column method, magnesium oxide powder automatic filling method and continuous argon welding method, each process is very complex.
Installation and construction technology is complex. Materials for mineral insulated fire resistant cables
Its composition and structure make it more complex in installation and construction than in other cables, mainly because it is not easy to bend and requires a large number of cable heads, and the manufacture of cable head is more complicated.
Magnesium Oxide is easy to damp, so it has strict requirements for the place and cable connection of mineral insulated fire resistant cable, so as to avoid the short-circuit caused by Magnesium Oxide damp.
Fourth, the price is higher. Because of its special material composition, manufacturing process, connection processing and other aspects of the special, making its price higher than other cables. The production of mineral insulated cables began in France and England in 1930s, and developed rapidly. In our country, the development of mineral insulated cable started late, although the Shanghai Cable Research Institute can produce, but the varieties, specifications and other foreign countries have a big gap. The basic development of mineral insulated refractory cables in China is described. At present, its radiation protection performance is more under the condition of nuclear radiation.
3. Rapid ceramic refractory insulating layer
The refractory insulating material of rapid ceramic refractory cable is a polymer matrix composite. It can be made into porcelain quickly at low temperature. Therefore, it is called low temperature rapid ceramic refractory cable material. Good processability, insulation, flexibility in the polymer composites are environment, similar to ordinary cable insulation insulation layer; in high temperature flame, polymers were burned, inorganic filler at certain temperature fast porcelain, the conductor surface to form a layer of refractory insulating shell. To ensure the integrity of the line. It can be fire-resistant and insulated in one, and can only be used as refractory layer. Moreover, the ceramic fire-resistant cable can use the existing ordinary cable production equipment and process, without adding new equipment and adjusting the process. The typical structure of the ceramic fire-resistant cable is similar to that of the common cable structure, as shown in Figure 3
The refractory insulating layer is composed of polymer and inorganic filler. Polymer as matrix material provides basic properties for cable material; inorganic filler as porcelain component, provides the material basis for residual porcelain. Theoretically, the rubber and plastic materials used for ordinary wires and cables can be used as matrix materials, such as polyethylene, polypropylene, polyvinyl chloride, rubber or elastomer, such as ethylene propylene rubber, EVA elastomer, POE elastomer, silicone rubber, etc..
Polyethylene has excellent physicochemical properties, non-toxic itself, emit carbon dioxide and water during combustion, produce less smoke, but its poor performance of flame retardant resin, pure oxygen index is only about 17%, burning drip serious, for refractory cable material to higher requirements for the development of flame retardant polyethylene brought no small difficulty. The amount of total filler is too much, which will cause bad effects on the processing and product performance. Polypropylene is similar to that of polypropylene.
PVC has excellent flame retardant properties, making it widely used in wire and cable. But from the present work, the volume expansion is very serious in combustion, and as the refractory insulating layer of fast ceramic refractory cable, the size of the refractory layer should not be changed too much during combustion. Ethylene propylene rubber elastomer is a kind of excellent performance, the combustion performance of polyethylene and polypropylene and other similar, but has better filling property, can maintain a good comprehensive performance in the packing amount is larger, but from the current perspective, it has no polyvinyl chloride combustion expansion defects.
Silicone rubber has excellent resistance to high temperature and low temperature, slow combustion and less heat release. It can be used as cable material for a long time under high temperature. Silicone rubber as a refractory insulating layer material is another advantage: when it is decomposed, will produce a large number of SiO2 for refractory insulation ceramic layer to provide the material basis, therefore, using silicone rubber as the base material, can reduce the amount of inorganic filler, to ensure the performance of composite system. Inorganic fillers in refractory insulation include flame retardant fillers and non flame retardant fillers. The commonly used flame retardant fillers are magnesium hydroxide and aluminum hydroxide, and the commonly used non flame retardant fillers are glass powder, ceramic powder, talc powder, clay, glass fiber, etc.. In order to ensure the processing performance and mechanical properties of the composites, elected is suitably shaped filler and filler surface treatment agent. In order to ensure the porcelain and compactness of the residue, attention should be paid to the coordination of inorganic fillers with high and low softening point. In order to ensure the high temperature insulation of porcelain paste, it is necessary to pay attention to the composition of the selected fillers, if the filler with higher alkali metal content can not be selected, so as to avoid the high-temperature insulation of the porcelain paste.