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(also heat insulation, thermal insulation, or heat insulator), a material or article used for heat insulation of buildings, structures, industrial equipment, and transportation vehicles. Insulating materials have low thermal conductivity (less than 0.2 watt per [m·°K]), high porosity (70–98 percent), and low density and strength (their compressive strength is 0.05–2.5 meganewtons per sq m).
The main indicator of the quality of an insulating material is the thermal conductivity. However, determination of this indicator requires considerable labor and special equipment; therefore, the indicator used in practice—the grade of insulating material—is the dry density expressed in kilograms per cubic meter (kg/m3). This gives an adequate approximation of the thermal conductivity of the material. Nineteen grades of heat insulators are distinguished, with values ranging from 15 to 700. Insulating materials must be protected against moisture during use; upon saturation with water, their thermal conductivity increases severalfold.
Insulating materials are used primarily to insulate enclosing members and such equipment as industrial furnaces, heating units, refrigeration chambers, and pipelines. They may be rigid (boards, blocks, bricks, casings, and segments), flexible (blankets, bats, and rope), loose-fill (granular and powdered), or fibrous. Insulating materials are subdivided into organic, inorganic, and mixed, depending on the raw material used.
Predominant among the organic insulating materials are those obtained by processing noncommercial wood and by-products from woodworking (fiberboard and chipboard sheets), agricultural waste products (strawboard, reed board), peat (peat board), and other local organic raw materials. These materials usually have low water resistance and are readily biodegradable. The gas-filled plastics (expanded, porous, cellular, and other types) do not have these shortcomings; they are highly effective organic insulators, with densities of 10–100 kg/m3. Low refractoriness is characteristic of most organic insulating materials, and consequently they are not usually used at temperatures above 150°C. Materials of mixed composition, such as fibrolit and arbolit, which are produced from a mixture of a mineral cement and an organic filler such as wood chips or shavings, are more refractory.
The inorganic insulating materials include mineral wool and articles made from it (among the latter the solid and highly rigid mineral wool sheets are very promising), lightweight and cellular concretes (primarily gas and foamed concrete), foam glass, fiberglass, and articles made of expanded perlite. Articles made of mineral wool are obtained by processing melts of rock or metallurgical slags, mainly blast-furnace slags, into a vitreous fiber. The density of articles made of mineral wool is 75–350 kg/m3.
Inorganic insulating materials used as installation materials are made with asbestos (asbestos millboard, paper, and felting), mixtures of asbestos and mineral cements (asbestos-diatomaceous, asbestos-tripoli, asbestos-lime-alumina, and asbestos-cement articles), or expanded rock (vermiculite and perlite). For insulating industrial equipment and installations that operate at temperatures above 1000°C (for example, metallurgical, heating, and other types of furnaces and boilers), lightweight refractories are made in discrete units such as bricks or blocks of various shapes from refractory clays or highly refractory oxides. The manufacture of fibrous insulating materials from refractory fibers and mineral cements is also promising; their thermal conductivity at high temperatures is 25–50 percent lower than that of traditional materials with cellular structure.
REFERENCESSpravochnik po proizvodstvu teploizoliatsionnykh i akusticheskikh materialov. Moscow, 1964.
Kitaitsev, V. A. Tekhnologiia teploizoliatsionnykh materialov, 3rd ed. Moscow, 1973.
Sukharev, M. F. Proizvodstvo teploizoliatsionnykh materialov i izdelii. Moscow, 1973.
IU. P. GORLOV and K. N. POPOV
(also electrical insulating material, insulation, electrical insulation, or insulant), a material used in electrical or radio equipment to separate current-carrying parts at differing potentials or to increase the capacitance of capacitors. Insulating materials may also be used to provide a heat-conducting medium in electric machinery and electrical equipment.
Dielectrics are used as insulating materials. As compared with conducting materials, dielectrics have a high volume resistivity ρv = 109–1020 ohm·cm. In contrast, conductors have a volume resistivity of 10–6–10–4 ohm·cm.
The main characteristics of insulating materials are as follows: volume resistivity ρv; surface resistivity ρs; the dielectric constant, or specific inductive capacity ε; the temperature coefficient of specific inductive capacity, 1/ε × dε/dT°C–1; dielectric loss angle 8; and dielectric strength Ebr, which is the electric field intensity at which breakdown occurs (seeBREAKDOWN, DIELECTRIC). When insulating materials are evaluated, the dependence of the characteristics on the frequency of a current and on voltage is also taken into account.
Insulating materials may be classified according to several criteria, such as state of aggregation, chemical composition, and production method. Depending on its state of aggregation, an insulating material may be a solid, a liquid, or a gas.
Solids constitute the most extensive group of insulating materials. According to their physical and chemical properties, structure, and production characteristics, solid insulating materials are divided into a number of subgroups, for example, laminated plastics, papers, fabrics, varnished cloths, micas, materials based on micas, and insulation ceramics. Varnishes, poured insulating compounds, and impregnants may be arbitrarily classified as solid insulating materials; although such substances occur in the liquid state, they are used as insulating materials in the solid state. At a temperature of 20°C and a current frequency of 50 hertz (Hz), the dielectric strength of solid insulation ranges from 1 megavolt per meter (MV/m)—for example, for certain resin-based materials—to 120 MV/m—for example, for polyethylene terephthalate.
The uses and production of solid insulating materials are discussed in INSULATION, ELECTRICAL; INSULATOR, ELECTRICAL; VARNISHES; MICA; FIBER GLASS REINFORCED PLASTIC; POLYMER COMPOUNDS; and RESIN, SYNTHETIC.
Many insulating liquids are insulating oils, which include petroleum, vegetable, and synthetic oils. Specific types of insulating liquids differ from one another in viscosity and in the values of their electrical properties. The oils used to insulate capacitors and oil-filled cables have the best electrical properties. At 20°C and a frequency of 50 Hz, the dielectric strength of insulating liquids ranges from 12–25 MV/m—for example, for transformer oils—to 15–20 MV/m (see alsoLIQUID DIELECTRICS). Insulating greases, called petrolatums, are also used.
Insulating gases include air, elegaz (sulfur hexafluoride), and Freon-21 (dichlorofluoromethane). Air, a natural insulator used for air gaps in electric machinery and electrical equipment, has a dielectric strength of about 3 MV/m. Elegaz and Freon-21 have a dielectric strength of approximately 7.5 MV/m; they are used mainly to insulate cables and various types of electrical equipment.
According to chemical composition, a distinction is made between organic and inorganic insulating materials. Inorganic substances, such as mica and ceramics, are the most widely used insulating materials. Both natural and synthetic materials are used for electrical insulation. Synthetic insulating materials may be produced with a desired set of required electrical, physical, and chemical properties. Therefore, such materials find the widest applications in electrical engineering and radio engineering.
According to the electrical properties of the substance’s molecules, an insulating material may be polar or nonpolar. Polar insulating materials include thermosetting resins, polychlorinated biphenyl, chlorinated naphthalene, polyvinyl chloride, and certain organosilicon materials. Nonpolar insulating materials include hydrogen, benzene, carbon tetrachloride, polystyrene, and paraffin. Polar insulating materials are distinguished by a high dielectric constant, a relatively high conductivity, and a relatively high hygroscopicity.
For solid insulating materials, mechanical and thermal properties are very important, as are moisture resistance, hygroscopici-ty, and spark resistance. The mechanical properties include tensile strength, compressive strength, static flexural strength, dynamic flexural strength, hardness, and workability. The thermal properties are heat resistance and thermal stability. Heat resistance characterizes the upper limit of the temperature region in which an insulating material can retain its mechanical and service properties. Thermal stability is the ability of an insulating material to withstand the effects of high temperatures (90° to 250°C) without pronounced changes in its electrical properties. In electric-machine building, a division of insulating materials into seven classes is used. Inorganic materials exhibit the highest thermal stability; such materials include mica, porcelain, ordinary glass, and glass with a hetero-organic binder. For brittle materials, such as glass and porcelain, heat-shock resistance is also important.
Insulating materials separate conductors in an electric circuit. At the same time, they should not impede the elimination of heat from windings, cores, and other components of electric machinery and electrical installations. Therefore, thermal conductivity is an important property of such materials. To increase the thermal conductivity of insulating liquids, mineral fillers are added to the liquids.
Most insulating materials absorb moisture to some extent; that is, they are hygroscopic. To increase the moisture resistance of porous insulating materials, the materials are impregnated with oils, synthetic liquids, or various compounds. Only glazed porcelain, glass, and a few other materials may be considered absolutely moisture-proof.
REFERENCEElektrotekhnicheskii spravochnik, 5th ed., vol. 1. Moscow, 1974.
A. I. KHOMENKO