In airports, hospitals, shopping centres, subways, high-rise buildings and other important places, in order to ensure the safety of people in the event of a fire and the normal operation of emergency systems, it is necessary to use fire-resistant wire and cable with excellent fire resistance. Due to the increasing attention to personal safety, the market demand for fire-resistant cables is also increasing, and the application areas are becoming more and more extensive, the quality of fire-resistant wire and cable requirements are also increasingly high.
Fire-resistant wire and cable refers to wire and cable with the ability to operate continuously in a specified state when burning under a specified flame and time, i.e. the ability to maintain line integrity. Fire-resistant wire and cable is usually between the conductor and insulation layer plus a layer of refractory layer, the refractory layer is usually multi-layer refractory mica tape directly wrapped around the conductor. It can be sintered into a hard, dense insulator material attached to the surface of the conductor when exposed to fire, and can ensure normal operation of the line even if the polymer at the applied flame is burnt. The choice of fire-resistant mica tape therefore plays a crucial role in the quality of fire-resistant wires and cables.
1 The composition of refractory mica tapes and the characteristics of each composition
In the refractory mica tape, the mica paper is the real electrical insulation and refractory material, but the mica paper itself has almost no strength and must be reinforced with reinforcing material to enhance it, and to make the mica paper and reinforcing material become one must use the adhesive. The raw material for refractory mica tape is therefore made up of mica paper, reinforcing material (glass cloth or film) and a resin adhesive.
1. 1 Mica paper
Mica paper is divided into three types according to the properties of the mica minerals used.
( 1) Mica paper made from white mica;
( 2) Mica paper made from gold mica;
( 3) Mica paper made of synthetic mica as raw material.
These three types of mica paper all have their inherent characteristics
In the three kinds of mica paper, room temperature electrical properties of white mica paper is the best, synthetic mica paper is the second, gold mica paper is poor. The electrical properties at high temperatures, synthetic mica paper is the best, gold mica paper is the second best, white mica paper is poor. Synthetic mica does not contain crystalline water and has a melting point of 1,370°C, so it has the best resistance to high temperatures; gold mica starts releasing crystalline water at 800°C and has the second best resistance to high temperatures; white mica releases crystalline water at 600°C and has poor resistance to high temperatures. Gold mica and synthetic mica are usually used to produce refractory mica tapes with better refractory properties.
1. 2 Reinforcing materials
Reinforcing materials are usually glass cloth and plastic film. Glass cloth is a continuous filament of glass fibre made from alkali-free glass, which should be woven. The film can use different types of plastic film, the use of plastic film can reduce costs and improve the abrasion resistance of the surface, but the products generated during combustion should not destroy the insulation of the mica paper, and should have sufficient strength, currently commonly used is polyester film, polyethylene film, etc. It is worth mentioning that the tensile strength of mica tape is related to the type of reinforcing material, and the tensile performance of mica tape with glass cloth reinforcement is generally higher than that of mica tape with film reinforcement. In addition, although the IDF strength of mica tapes at room temperature is related to the type of mica paper, it is also closely related to the reinforcement material, and usually the IDF strength of mica tapes with film reinforcement at room temperature is higher than that of mica tapes without film reinforcement.
1. 3 Resin adhesives
The resin adhesive combines the mica paper and the reinforcement material into one. The adhesive must be selected to meet the high bond strength of the mica paper and the reinforcement material, the mica tape has a certain flexibility and does not char after burning. It is essential that the mica tape does not char after burning, as it directly affects the insulation resistance of the mica tape after burning. As the adhesive, when bonding the mica paper and reinforcing material, penetrates into the pores and micropores of both, it becomes a conduit for electrical conductivity if it burns and char. Currently, the commonly used adhesive for refractory mica tape is a silicone resin adhesive, which produces a white silica powder after combustion and has good electrical insulation properties.
( 1) Refractory mica tapes are usually manufactured using gold mica and synthetic mica, which have better electrical properties at high temperatures.
( 2) The tensile strength of mica tapes is related to the type of reinforcement material, and the tensile properties of mica tapes with glass cloth reinforcement are generally higher than those of mica tapes with film reinforcement.
( 3) The IDF strength of mica tapes at room temperature is related to the type of mica paper, but also to the reinforcement material, and is usually higher for mica tapes with film reinforcement than for those without.
( 4) Adhesives for fire-resistant mica tapes are often silicone adhesives.