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Unit 12. HEAT-INSULATING AND ACOUSTIC MATERIALS AND COMPONENTS

 

A. HEAT-INSULATING MATERIALS AND COMPONENTS

Heat-insulating materials are intended for protection against heat and cold; generally they are very porous, with a bulk density below 600 kg/m³ and a coefficient of heat conductivity not more than 0.20 W/m×°C.

Proper use of heat-insulating materials in building practice may gre­atly reduce heat losses to the environmental medium through wall structures and thus reduce fuel consumption, since each ton of rational­ly used heat insulating material can effect a saving of 30 to 200 t of reference fuel per year. Thus, the economic efficiency of thermal insula­tion is very high: investments pay back within 1-1.5 years of service of a thermally insulated conduit or equipment. The economic efficiency of thermal insulation of conduits and surfaces of refrigerating plants is even greater.

The main tasks in the manufacture of heat insulating materials are (along with increased output and improved quality) greater industria­lization of thermal insulation work and organization of manufacture of heat insulating items and components on a large-scale.

1.Structure of Heat-Insulating Materials

Heat-insulating properties of materials are governed not only by their porosity, but also by the nature of pores, their distribution, size and whether they are open or closed. Highest in heat-insulating proper­ties are materials with a greater number of fine, closed and air-filled pores. Air, when immobile, has a very low coefficient of heat conducti­vity (0.02 W/m×°C at 20°C). If the structure of highly porous material with fine and closed pores is studied with a microscope, a host of air cells separated by thin walls becomes readily apparent. The totality of such cells serves as a barrier in the way of heat transfer, and thus makes the material as a whole poorly heat conducting. Heat-insulating properties of material may be improved by increasing the number of air cells and arranging the separating walls in a honeycomb pattern.

Heat-insulating properties of air are greater when it is still, since ' aii motion favours heat transfer. In materials of cavernous structure with large and elongated pores convection air streams arise more ea­sily, thus increasing heat transfer through the material. The finer the pores, or the smaller the volume of air enclosed inside them, the lower is its mobility and the better are its heat-insulating properties.

Heat-insulating properties of material depend upon the ratio of the volume of air enclosed in the air cells, to the volume of solid matter per unit volume of the materials. The thinner the layer of solid matter, surrounding the air cell, the better are the heat insulating properties of material and the lower is its coefficient of heat conductivity. In very porous materials of a very low bulk density, the volume of air enclosed inside the pores is so large and the heat insulating properties are so high that the part of the solid matter engaged in heat transfer becomes negligible. In these materials, the coefficient of heat conducti­vity approximates that of air.

When we compare heat conductivities of materials of a same com­position, but of different porosities, it becomes evident that the coef­ficient of a heat conductivity is almost proportional to the bulk density of material, i.e., to its solid matter content.

Pores and channels in material may be formed by foaming, by intro­ducing gas forming agents, by contact glueing or fritting of separate grains or particles, by random laying of a great number of fibres, etc.

Heat-insulating properties of materia] are greatly affected by its structure, this being particularly evident in fibrous materials. For example, heat conductivity of wood along the fibres is about twice as great as that across the fibres. Heat-insulating properties of powder ma­terials are greatly affected by the size of grains: they improve with the reduction in size of grains even when the bulk density remains constant.

By considering the general nature of heat insulating materials we come to the conclusion that their low heat conductivity is due to air- filled pores; when the surface of these pores is covered with a film of water or the pores are filled with water, the heat insulating proper­ties of material greatly decrease, because water has a much higher (about 25 times) coefficient of heat conductivity than air. Therefore, heat-insulating materials should be protected against moistening.

2. Classification of Heat-Insulating Materials and Components

Heat-insulating materials and components are classed according to structure, shape, kind or origin of basic raw material, volume mass, compressibility (in terms of compression deformation) and heat condu­ctivity.

By structure, heat insulating materials are divided into porous- and-fibrous (mineral wool, glass fibres, etc.), porous-and-granular (pearlite, vermiculite, sovelite, lime-silica and other materials), and cellular varieties (items of cellular concretes, foam glass, foam plastics, etc.).

By shape, heat insulating materials are available in piece (plates, slabs, bricks, cylinders, semi-cylinders, segments), coiled (mats, strips, pads), cord (cords, plaits) and loose kinds.

By origin, heat insulating materials fall into organic and nonorga­nic varieties.

Depending on 'their bulk density, heat insulating materials are sub­divided into three groups: extra-lightweight (ОЛ) with bulk density grades of 15, 25, 35, 50, 75 and 100; lightweight (Jl), grades 125, 150, 175, 200, 225, 250, 300 and 350; and heavy ones (T) available in four grades 400, 450, 500 and 600.

With respect to compressibility (relative compressive deformation), heat insulating materials are known as soft (M), semi-rigid (ПЖ) and rigid (Ж). The compressibility of the soft materials should not exceed 30%, of semi-rigid ones, 6 to 30%, and of rigid ones, 6%.

By heat conductivity (a major characteristic), heat insulating mate­rials are divided into three classes: class A, low heat conductivity; class Б, medium heat conductivity; and class B, high heat conducti­vity.

Materials intended for insulating surfaces of subzero or above-zero temperatures (not higher than 100°C) should be chosen based on their heat conductivity value at 25°C; for insulating surfaces having a tem­perature of 100 to 600°C, on the conductivity value at 125°C; and for insulating surfaces of 600°C and over, on the heat conductivity value at 300°C.

The heat conductivity values as a function of the class of material are given in Table1.

 

Class of material	Name of class of material	Heat conductivity, W/m×°C at temperature 25°C
А	Low heat conductivity	not more than 0,058
Б	Medium heat conductivity	0,058…0,116
В	High heat conductivity	0,116…0,15

 

The classification of the major kinds of construction materials and components is based on four criteria: heat conductivity of the materi­al at a temperature of up to 100°C, at a temperature above 100 and at a temperature of over 600°C; bulk density; compressibility of material (relative compressive deformation); and the suggested field of appli­cation.

According to this classification, heat insulating materials fall into 7 groups:

heat insulating nonorganic friable and loose materials, such as mineral wool, glass wool of continuous fibres, wool of super-thin glass fibres, wool of kaolin composition, expanded pearlite, foamed vermiculite, sovelite powder, asbestos-magnesia powder, asbozurite and diatomite crumbs;

mineral fibre components, such as slabs of mineral wool based on synthetic and bitumen binder, cylinders, semi-cylinders and segments of mineral wool on a synthetic binder, sewn and coiled mineral-wool mats on synthetic binder, flossy cord of mineral' wool, slabs of mineral wool on starch binder, semi-rigid slabs and mats of glass staple fibres, coiled material of staple glass fibres, mats and strips of glass fibres and glass cords;

heat insulating asbestos-bearing components, such as sovelite, lime-silica, vulcanite, asbestos-vermiculite items, asbestos cord, as­bestos flossy cord and asbestos-magnesia cord;

heat-insulating pearlite components, such as ones of pearlite on ceramic and cement bond and on bitumen binder;

miscellaneous heat insulating nonorganic components, such as ones of foamed diatomite, diatomite and tripoli (burned varieties), heat insulating slabs of cellular concrete and foamed plastic slabs;

heat insulating components of organic materials, such as peat heat insulating slabs, insulating wood-fibre slabs, fibrolite slabs on port- land cement, pressed reed slabs, heat insulating natural cork;

heat insulating foamed plastics: heat insulating slabs of foamed-sty- rene foamed plastic, polyurethane foamed plastic, polyvinyl chloride- and mipor (microporous rubber)-hase slab-type foamed plastic.

 

3. Nonorganic Heat Insulating Materials and Components

Further advances in industrialized construction are linked to the development of new heat insulating materials, ones of improved pro­perties and quality. Of a greatest interest in this respect are heat in­sulating materials of mineral origin, ones not susceptible to decompo­sition, sufficiently fire-resisting and more durable than materials from plant fibres.

At present, the range of commercially available materials includes over 25 items, of which most important are components and materi­als based on mineral raw materials, such as rock, slags, glass, and as­bestos.

Mineral wool is a heat insulating material obtained from molten rock or metallurgical slags and consisting of glass fibres and miscella­neous nonfibrous inclusions, such as drops of silicate melts and micro­scopic fragments of fibres. The length of mineral wool fibres, depending on the method of manufacture, ranges from 2 to 60 mm, and the bulk of the wool should contain up to 80-90% of thin fibres less than 7 mi­crons in diameter. By the value of bulk density, there are three kinds of mineral wools, or grades 75, 100 and 125. Their heat conductivitie£ at an average temperature of 25±5°C are respectively equal to 0.042, 0.044, and 0.047 W/m-°C, and at the temperature of 100°C to 0.058, 0.059 and 0.060 W/m×°C; the moisture content should not exceed 2%..

Mineral wool is by far in the leading position among heat insulating materials of nonorganic origin. This is due to a ready availability of the raw materials, simplicity of manufacture, high frost resistance, low hygroscopicity, and small cost; it is suitable for the production of heat insulating components and heat insulations intended for service at temperatures of insulated surfaces in the range from -200 to +600°C.

However, it should be noted that the use of a friable mineral wool for heat insulation is hampered by its intrinsic shortcomings. During transportation and storage the wool is compacted and lumped, some of the fibres break and turn to dust; in structures, the friable wool should be protected against mechanical action, and its laying is labo- ur-consuming. The above shortcomings of mineral wool are fully or part­ly obviated when it is transformed into various mineral wool compo­nents.

Mineral raw materials are the basis for manufacturing mineral wool mats, semi-rigid and rigid slabs, shells, segments, cylinders, and si­milar components.

Heat insulating mats based on mineral fibres are intended for heat insulation of building structures, industrial equipment and piping of heat supply lines. The Soviet industry produces several kinds of mineral wool mats.

Sewn mineral wool mats are used for heat insulation of enclosing constructions of buildings and surfaces of industrial equipment and piping at temperatures of up to 400®C.

The mats are manufactured as follows: layers of mineral wool are initially transported by a conveyor from the settling chamber to a cooling chamber, where they are compacted to a specified thickness, cold air being sucked through them in the process. Once cooled, the newly formed mat is transferred to a machine which sews it with threads by means of special needles and slits the mat longitudinally with the aid of disk knives. The mats are then conveyed to a coiler and to packing facilities.

Mineral wool sewn mats are manufactured in length of 2000 mm, widths of 900 to 1300 mm and in thickness of 60 mm. By bulk density in dry state, the mats are manufactured in grade 150 and with a heat conductivity in dry state of not more than 0.046 W/m×°C.

Mineral wool sewn mats on metallic fabric are used for insulation at temperatures of up to 600°C. They are manufactured from spinneret wool, grade ВФ, by sewing a carpet of mineral wool on a metallic fab­ric by cotton threads. Mats are manufactured in sizes of 3000 x 500 X X50 mm and 5000X1000X100 mm, with a bulk density of 100 kg/m³, and a heat conductivity at 100°C of 0.05 W/m×°C.

Mineral wool niats faced, with glass-fibre canvas are used for the in­sulation of surfaces at temperatures of up to 400°C. They consist of a mineral wool impregnated with oil and sewn with glass-fibre cord treated in a soap solution. These mats are available in carpet form measuring 2000x500x40 mm, having a bulk density between 125 and 175 kg/m³ and heat conductivity at 25±5°C of 0.044 W/m×°C.

Mineral wool mats on starch binder and faced with paper are intended for heat insulation of piping laid inside premises and of industrial equipment at temperatures of up to 150°C. These mats are manufactu­red 1000 to 2000 mm in length, 950 to 2000 mm in width, and 45, 50, 60 and 70 mm in thickness, of a bulk density of 100 kg/m³, and a heat conductivity at 25±5°C of 0.044 W/m×°C.

Heat insulating semi-rigid slabs based on mineral fibres are used as an effective heat insulation material for building structures and industrial equipment, piping and cooling installations. Semi-rigid slabs are made on phenol and synthetic binders.

Grade ПП semi-rigid slabs on phenol binder are manufactured from mineral wool by the spraying of a solution of phenol alcohols with subsequent polycondensation and cooling. Slabs are made of a bulk density of up to 100 kg/m⁸, heat conductivity at 25±5°C of 0.046 W/m×°C, in sizes of 1000 x 500 x 30 (40; 60) mm.

Grade ППМ semi-rigid slabs on synthetic binder are made from a carpet of grade ВФ mineral wool impregnated with a synthetic bin­der and thermally processed. They are made of a bulk density of 80 to 100 kg/m³ and heat conductivity (at 0-100°C) of 0.031 and 0.058 W/m×°C, respectively.

 

Heat insulating rigid slabs and components based on mineral fibres

 Rigid mineral components are manufactured as slabs, shells and semi- cylinders based on mineral wool and an organic binder (a synthetic or a bituminous one). Of the synthetic binders use is made of phenol- formaldehyde and carbamide-formaldehyde varieties, while the bitu­minous binders are bitumens of higher grades, whose softening points are not less that 45-50°C.

The manufacture of rigid mineral wool components consists in mixing fibres with binders to form an emulsion or paste, moulding components from said mass at a certain pressure and treating them thermally. Components are moulded in vacuum-presses because of a high content of water in the moulding mass, which does not allow high-pressure moulding. The components are dried at a temperature of 110-120°C, but after the moisture has evaporated the drying temperature is raised to 130-140°C. The items on bitumen binder then acquire improved phy­sical and mechanical properties because bitumen forms a thin molten film which strongly bonds the fibres.

Rigid mineral wool slabs are manufactured in a number of kinds. Type CM250 rigid slabs on bitumen binder are produced with a wet te­chnique by moulding a hydraulic mixture. They are used for heat insulation of building structures, as they possess a low hygroscopicity, are resistant to the action of water and biological agents. Their heat conductivity is 0.042 W/m×°C, and the rated service temperature, as high as 70°C. Slabs measure 1000 x 500 x 60 mm.

Type ПЖ rigid slabs on synthetic binder are used in large-panel en­closing structures for heat insulation of combined roofs in civil and industrial construction. They feature a high rigidity, small bulk den­sity (up to 120 kg/m⁸), and low heat conductivity (0.04 W/m×°C). The slabs measure 1000 x 500 x 600 mm.

Rigid slabs on a bentonite-colloid binder possess a high reflectivity and are thus particularly effective for insulating the installations with high surface temperatures (up to 600°C). The slabs are stable to chemical and biological media, have a bulk density of up to 150 kg/m³, a heat conductivity of 0.04 at a temperature of 25±5°C, and 0.11 W/m ×°C at 270±5 C. The slabs measure 500(1000) X 5000 (50-90) mm.

Semi-cylinders on synthetic binder are used for heat insulation of piping at a temperature from-600 to +400°C. These items are manu­factured by a continuous method on a press-polymerization machine in a manner similar to moulding of items in polymerization chambers. Semi-cylinders on synthetic binders are manufactured of a bulk den­sity of up to 200 kg/m⁸ and heat conductivity of 0.044 to 0.048 W/m•°C.

Glass Wool and Glass Wool-Base Items

Glass wool is a fibrous heat insulating material obtained from molten glass. It has a high chemical resistance, a heat conductivity of 0.05 W/m•°C at 25°C, is incombustible and not susceptible to smoldering, its bulk density in loose state is not more than 130 kg/m⁸. The dia­meter of fibres of glass wool used for heat insulation is not greater than 21 microns. The structure of the wool is required to be friable, and the number of strands composed of parallel fibres should not ex­ceed 20% by mass.

Glass wool is manufactured by spinneret, blast and glass rod moulding methods. Glass wool from continuous glass fibres is used for making heat insulating materials and items and heat insulations at temperatures of insulated surfaces from -200 to +450^eC.

Mats and bands of glass wool are employed for heat insulation of flat surfaces and piping at temperatures of insulated surfaces of -200 to +450°C. They are obtained by sewing glass wool (coated on both sides with layers of glued glass fibres up to 1.5 mm thick) by asbestos or plaited glass fibre threads. The surface of mats is glued with a 2 to 5% solution of dextrin or-some other glue so as to protect the mats and bands against damage. Glass wool mats are available in lengths of 1000-3000 mm, widths of 200-750 mm and thicknesses of 10-50 mm. The bulk density of mats and bands is not higher than 170 kg/m⁹.

Items from glass fibre are used for heat insulation of building stru­ctures of refrigerators and transport vehicles at temperatures between -60 and +180°C.

At present, the Soviet industry manufactures six kinds of items from glass fibre. These are mainly slabs and mats. Mats for building and engineering applications are manufactured of a bulk density of 35 and 50 kg/m⁸, in lengths of 7000-13000 mm, widths of 500-1500 mm and thicknesses of 30-80 mm, and semi-rigid slabs for same engineering applications, of a bulk density of 75 kg/m⁸ in sizes of 1000 x 500 (900, 1000, 1500) X30 (40, 50, 60, 70, 80) mm. The heat conductivity of all the items in dry state should not exceed 0.045 W/m•°C at a tem­perature of 25±5°C.

The manufacture of items from glass fibre consists of the following operations: mixing of fibres with water-soluble synthetic polymer, moulding, thermal processing, cutting and packing.

Wool from extra-thin glass fibre and items from it are good heat- and sound-insulating materials and have been finding in recent years an ever growing application in building practice. Physical and mecha- nical properties of these materials are characterized by the following data: bulk density, 25 kg/m⁸; heat conductivity, 0.03 W/m×°C; li­miting working temperature, from -60 to +450°C; sound-absorption coefficient in the frequency range from 400 to 2000 Hz, 0.65 to 0.95.

Extra-thin basalt glass fibre БСТВ is a high quality material for heat insulation, filtration, manufacture of heat resistant papers, card­boards and mats. This material possesses a very small bulk density (17-25 kg/m⁸), low heat conductivity (0.027-0.037 W/m×°C), and a higft sound-absorption coefficient (0.15 to 0.95) in the frequency range of 100-4000 Hz. Extra-thin basalt glass fibres are rated to serve at temperatures between -200 and +700°C. Heat-insulating mats from extra-thin basalt glass fibres feature similar physical and mechanical properties. Sound-insulating mats of extra-thin basalt glass fibres have a sound-absorption coefficient of 0.70-0.95.

Foamed glass is an excellent heat-insulating highly porous material of cellular structure. Slabs of foamed glass are employed for heat- insulating enclosing structures of buildings (for insulating walls, floors and roofs of industrial and civil buildings), decorative interior finishing of premises, insulating surfaces whose service temperatures range up to 180°C. The porosity of various kinds of foamed glass is 80-95%, and the size of cells, 0.25-0.5 mm. The cells are formed by thin walls and have a microporous structure, which results in the foa­med glass having excellent heat-insulating properties. The heat con­ductivity varies as a function of bulk density (150-250 kg/m³) between 0.058 and0.12 W/m×°C. Foamed glass possesses a number of valuable properties, such as water-resisting properties, incombustibility, frost resistance and a high strength lying between 2 and 6 MPa, depending on the bulk density of the material.

Expanded Pearlite and Items from It

Expanded pearlite is a porous loose material obtained by expansion of natural pearlite in rotary or shaft furnaces at temperatures of 900 to 1200°C. The manufacture of expanded pearlite consists in a rapid heating of the raw materials (igneous pearlite composed of volcanic glass with inclusions of feldspars, quartz and other minerals) to bur­ning temperature. The hydrate water in the rock vaporizes vigorously- and as water vapours leave the softened rock they expand it manyfold (5 to 20) times.

Expanded pearlite sand is composed of white or gray grains with air entrapped inside the closed pores. The grains are 0.1 to 5.0 mm across; the bulk density of pearlite sand is 100 to 250 kg/m⁸; heat conductivity in dry state, 0.046 to 0.071 W/m×°C; true porosity, as high as 85 or 90%; and quantity of open pores, 3 to 20%.

Pearlite sands are used in mortars and concretes for preparing heat insulating items, fire-resisting plasters and heat insulating fillings at temperatures of the insulated surfaces between -200 and +800°C.

Today, expanded pearlite is widely employed for manufacturing heat insulating items. An addition of expanded pearlite to mineral binders makes it possible to obtain incombustible items possessing a high dergee of rigidity and good thermal and physical properties.

Ceramic-pearl ite-phosphate and ceramic-pearlite items have gained wide popularity in construction of electric heating furnaces, chemical reactors and other installations owing to their excellent heat insulating properties.

A production line for the manufacture of pearlite heat insulating burnt items based on various binders (plastic clay, phosphates, liquid glass) includes units for expansion of the pearlite rock, preparation of moulding mass, dry-mass moulding of items and their thermal processing (drying and burning).

 

Ceramic-pearlite-phosphate items on phosphate binders are used for heat insulation of furnaces and equipment at temperatures of up to 1I50°C, inclusive of electric heating furnaces with controlled hydro­carbon-containing atmospheres. The moulding mass is composed of expanded pearlite sand of a bulk density of 60-120 kg/m⁸, of plastic clay of a refractoriness of 1680-1710°C and of a phosphate binder. Items are moulded in presses in metallic moulds. The items feature a bulk density of 250-400 kg/m⁸, refractoriness of 1350-1400°C, heat conductivity of not more than 0.2 W/m×°C at 600°C.

Ceramic-pearlite items, such as slabs, brick, segments, obtained from expanded pearlite sand and clay binder are used for heat insu­lation surfaces of industrial furnaces and equipment at temperatures of up to 900°C. For a bulk density of 250-400 kg/m³, the heat condu­ctivity is 0.07-0.1 W/m×°C.

Cement binder based pearlite items are employed for heat insula­tion of industrial equipment at service temperatures of up to 600°C. The items are manufactured in the form of semi-cylinders,, segments and slabs of a bulk density of 250-350 kg/m⁸ and heat conductivity equal to 0.12-0.13 W/m×°C at S25°C.

Pearlite-gel items find applications as heat insulators of surfaces of power equipment and piping at service temperatures of up to 650°C. Pearlite-gel slabs, shells, and segments are obtained from expanded sulfuric acid-treated pearlite sand and finely ground silicate with addi­tion of sodium silicofluoride. The items have a bulk density of 200- 250 kg/m⁸, heat conductivity of 0.06 and 0.11 W/m×°C at 25 and 325°C, respectively.

Bitumen-pearlite items are used (at service temperatures of -60 to +50°C) for heat insulation of structural elements of buildings, for monolithic insulation of heating and refrigerating installations and surface-laid piping. Grade БН-IV bitumen heated to a tempera­ture of 170-180®C is fed to a mortar mixer and stirred with pearlite sand, the ratio of components being 1 to 6 and 1 to 9 (by volume). The hot bitumen-pearlitemass is suitable for the manufacture of shells, segments or monolithic heat insulation of piping.

The bulk density of bitumen-pearl ite items is 300-450 kg/m⁸, and heat conductivity, 0.08-0.10 W/m×°C at 20°C.

Expanded Vermiculite and Items from It

Expanded vermiculite is obtained by rapid burning of vermiculite rock (of the hydromica group) until it expands. When heated to 1000- 1100°C, vermiculite gives off crystallization water and expands rapid­ly. Water vapours act at right angles to the cleavage planes and move apart the plates of mica, thereby increasing the original volume by 20 times and over.

The manufacture of expanded vermiculite consists of the following principal operations: crushing and sizing of natural vermiculite, dry­ing, burning in shaft or rotary furnaces, and cooling. Expanded ver­miculite is a porous material composed of goldish yellow scaly par­ticles, 5 to 15 mm across, of a bulk density of 80-150 kg/m⁸; for the finer grains, the bulk density is 200-400 kg/m⁸. The heat conduc­tivity at 100^oC is 0.048-0.10 W/m×^oC. In the temperature range up to 400°C, the heat conductivity increases to 0.14-0.18 W/m×°C. When heated to 1100^oC, vermiculite decomposes, and at 1300°C it melts. Its water absorption may be as high as 300% by mass.

Expanded vermiculite is used as a heat insulating filling at tempera­tures of insulated surfaces of up to 900^oC, as a component of lightweight concretes and also for the manufacture of heat insulating items, fire-resisting, heat-insulating and sound-absorbing plasters.

Grade 300 cement-vermiculite slabs are used for heat insulation of the enclosing constructions of civil and industrial buildings and instal­lations. They are manufactured from expanded vermiculite on port- land cement binder. Cement-vermiculite slabs have a small heat con­ductivity, 0.08 W/m×^oC, a bulk density of up to 300 kg/m⁸, a compres­sive strength of 0.5 MPa, and measure 500 x 500x100 mm. The slab manufacturing procedure consists of mixing expanded vermiculite with a laitance, moulding slabs and thermally processing the latter.

Grade 350 ceramic-vermiculite slabs are employed for heat insula­tion of enclosing structures of buildings, hot surfaces of furnaces and other thermal units and equipment. Slabs are of a bulk density of 350 kg/m^s, a heat conductivity of up to 0.08 W/m×^oC, and measure 500 x 500x125 mm. They are rated to serve at temperatures of up to 1200°C. The manufacture of slabs involves mixing expanded ver­miculite with a liquor prepared from refractory clay and water. The moulding mass is fed to an auger machine, and the moulded items are conveyed to drying and burning units.

 

Asbestos-Containing Heat Insulating Materials

The main raw material for the manufacture of asbestos-containing heat insulating materials and items is chrysotile asbestos. Asbestos is the basis for producing loose (powder like) materials, coiled and piece items and such products as cardboard, slabs, shells and segments.

Depending on their composition, asbestos materials are subdivided into asbestos items, composed of asbestos fibres only, and into as- bestos-containing items.

Asbestos-cardboard is a fire-resisting heat insulating material ob­tained from chrysotile asbestos. The starting materials for manufa­cturing the cardboard are grades 4 and 5 asbestos (65%), kaolin (30%) and starch (5%). When manufacturing cardboard, asbestos is fluffed in a crusher-roll mill and a hoi lander (beating machine), kaolin and starch being added in the process. From the hollander, the resultant mass is fed to a mixer, and therefrom, after it is cleaned of rock particles and pelletized asbestos fibres, transferred to a wire drum of the cardboard machine. Asbestos fibres settle on the drum to form sheets of loose asbestos cardboard, the latter then being compa­cted in a hydraulic press under a pressure of up to 5 MPa. Compacted cardboard sheets are dried and then cut to standard dimensions. As­bestos cardboard is also manufactured in sheet-forming machines as sheets of equal length and width of 900 to 1000 mm and a thickness of 2 to 10 mm. The heat conductivity of cardboard in dry state is 0.157 W/m×^oC, bulk density, 1000 to 1400 kg/m³, tensile strength, not less than 0.6 MPa, and moisture content, not more than 3% by mass.

Asbestos cord is employed for heat insulation of surfaces of indust­rial equipment and piping at temperatures: when cord incorporates an organic fibre, of up to 200°C, and when no organic fibres are used, of up to 500°C. Cord is obtained from a number of twisted threads or coarse linen which are laid together to form a core and wound or braided on the outside by the asbestos thread or yarn. Asbestos cord may also be manufactured without braiding. The diameter of asbestos cords may range, from 3 to 25 mm, heat conductivity, not more than 0.12 W/m×^oC, and the moisture content, not more than 4% by mass. By its bulk density in dry state, asbestos cord is divided into grades 100 to 380.

Asbestos-magnesia powder finds application as a heat insulator of industrial equipment at temperatures of up to 350°C. It is obtained by mixing ground asbestos with a water solution of a magnesium salt. To obtain newvel, magnesia is ground and mixed with 15% of asbestos. This heat insulating material is available in the form of powder which is usable not only as a heat-insulating filling, but also for preparing mastics and manufacturing slabs, shells and segments.

Moulded and dried items have a bulk density of up to 350 kg/m^a, heat conductivity of 0.08 W/m×°C at 50°C and a bending strength of not less than 0.15 MPa.

Sovelite materials and items are employed for heat insulation of surfaces of industrial equipment and piping at temperatures of up to 500°C. They consist of lightweight magnesium and calcium carbona­tes resulting from the processing of caustic dolomite and asbestos. Sovelite is generally manufactured from grade 5 and 6 asbestos, the procedure involving fluffing of asbestos, mixing it with caustic mag- nesite and curing the mass in tanks. The resifltant mass is filtered to give a sort of paste with a moisture content of about 70-75%, which is usable for heat-insulating constructions or moulding slabs, shells and segments.

Items are moulded in vacuum-filters or hydraulic presses under a pressure of 0.16-0.18 MPa. Pressed slabs have a moisture content of 66-70%. In the vacuum-filtering method, from the curing tanks the mass passes into vacuum-filters adapted for moulding blocks which, once dried, are sawn into slabs. The slabs so obtained feature a high porosity, but also a high moisture content (up to 75%).

The "green" items are then transferred to driers kept at a tempera­ture of 200°C. After drying the blocks are sawn into slabs measuring 500x170 (30-60) mm. By their bulk density in dry state, slabs are divided into grades 350 and 400, their bending strength being respec­tively not more than 0.17 and 0.2 MPa, heat conductivity not more than 0.075-0.86 W/m×°C, and moisture content not more than 15% by mass.

Heat insulating cellular concretes (of a bulk density 500 kg/m^s and less) are used to insulate enclosing constructions of buildings and surfaces of industrial equipment and piping at temperatures of up to 400°C. Heat-insulating items from cellular concretes are made in the form of slabs measuring 1000 x 500 x (80-200) mm. By their bulk den­sity in dry state, slabs fall into grades 300-500 with a compressive strength of not less than 0.9 to 12 MPa, heat conductivity in dry state of 0.08-0.11 W/m•°C, and a moisture content of not more than 15% by mass.

We have considered the main and most characteristic inorganic heat insulating materials. Along with them, there are many others, such as composite mixtures (volcanic, lime-silica), burnt materials, as pearlite-ceramic, diatomite or tripoli, foamed-diatomite, cellular ceramic materials, and compositions obtained from expanded natu­ral rocks (pearlite, vermiculite) and binders (lime, cement, synthetic compounds).

4. Organic Heat Insulating Materials and Components

Organic heat insulating materials and items are manufactured from various raw materials of vegetal origin, such as wood waste (shavings, sawdust, outside planks, etc.), reed, peat, flax tow, hemp tow, com­bings of animal wool, and from polymers.

Many organic heat insulating materials are susceptible to rapid de­composition, damage from various insects and liable to spontaneous ignition, and so they require preventive treatment. Since organic materials are ineffective as fillings due to the unavoidable settling and tendency to decompose, they are used for manufacturing slabs (boards). In the latter, the basic material is almost fully protected aga­inst moistening and thus against rotting. In addition, it is treated by antiseptics and fire retardants which enhance its service life.

Heat insulating materials and items from organic raw materials. Among the great variety of heat insulating organic items, of parti­cular interest are wood-fibre, reed, fibrolite, and peat slabs (boards) and also natural heat insulating cork and heat insulating foamed plas­tics.

Wood-fibre boards are used for heat and sound insulation of enclo­sing constructions (walls). They are manufactured from fluffed wood or other plant fibres of substandard wood, waste from the woodworking industry, boon, straw, reed, and cotton plants. Most popular are wood- fibre boards manufactured from wood waste. The production method of boards includes the following operations: crushing and breaking the wood, glueing the fibrous mass, moulding and thermal processing. To minimize combustibility, the wood-fibre boards are impregnated with fire retardants, while the resistance to the action of water is en­hanced by adding paraffin, resin, oil and other emulsions to the fibrous mass.

Insulating wood-fibre boards have a bulk density of 250 kg/m³, a bending strength of 1.2 MPa, and a heat conductivity of not more than 0.07 W/m×°C. They are made 1200-3000 in length, 1200-1600 in width and 8-25 mm thick.

Along with the insulating varieties, there are also insulating-and- finishing boards having a painted face surface or one prepared for painting.

Reed pressboard is used to insulate enclosing constructions of class III buildings, dwelling houses of a few storeys, small industrial pre­mises and also find use in agricultural construction. This heat insula­ting material is composed of stems of reed pressed to form boards which are then bound with steel galvanized wire. These pressboards are manu­factured from mature one-year stems of common reed. The best are stems 7 to 15 mm in diameter as they are most amenable to pressing. In addition to the common reed, use can also be made of cane, cattail and other plants. Stems of these plants should be harvested during the autumn-winter period. Boards are compacted in special presses. By arrangement of stems, there are boards with transversal (i.e., along the shorter side of the board) and longitudinal positioning of stems.

By their bulk density pressboards fall into three grades, 175, 200 and 250, of a bending strength not lower than 0.18-0.5 MPa, heat conductivity 0.06-0.09 W/m×°C and a moisture content not more than 18% by mass. Reed pressboards are available in lengths of 2400-2800, widths of 550-1500 and thicknesses of 30-100 mm.

Peat heat insulating items are available as boards, shells and seg­ments and used for heat insulation of enclosing structures of class III buildings and surfaces of industrial equipment and piping at temperatu­res from -60 to -100°C. The raw materials for them are slightly dec­omposed high-moor peat having a fibrous structure which facilitates the manufacture from it of quality pressed items.

Boards of 1000 x 500 x 30 mm are obtained by pressing a peat mass in metallic moulds with or without additives and subsequently drying the boards at a temperature between 120 and 150°C.

Depending on the initial moisture content of the peat mass, there are two methods for manufacturing the pressboards: a wet (a moisture of 90-95%) and a dry (a moisture of 35%) method. In the wet method, excess moisture is squeezed out through the fine metal wire mesh du­ring pressing. In the dry method, no such wire fabrics are placed in the moulds.

Insulating peat pressboards are divided by their bulk density into grades 170 and 220 kg/m⁸, of a bending strength of 0.3 MPa, a heat con­ductivity in dry state of 0.06 W/m×°C and a moisture content of not more than 15%.

Cement-fibrolite boards consist of heat-insulating or heat-insula- ting-and-structural material obtained from a hardened mixture of port- land cement, water and wood wool, the latter serving as a reinforcing carcass. Wood wool consists of thin wood shavings up to 500 mm long, 4 to 7 mm wide and 0.25 to 0.5 mm thick prepared from unworkable wood of conifer species in special machines. The wool is dried, imp­regnated with mineralizing agents (calcium chloride, soluble glass) and mixed with cement paste according to the wet method, or with cement according to the dry method (the wool is powdered or sprayed with cement) in mixing machines of various types. Care is taken to coat the wood wool uniformly with cement. Pressboards are either moulded by pressing or prepared on conveyors, where the fibrolite is moulded as a continuously moving band which is then cut to boards (similar to vibro-rolled items of reinforced concrete). The specific pressure applied in moulding heat-insulating fibrolite is up to 0.1 MPa, and in preparing structural fibrolite, up to 0.4 MPa. Once moulded, the pressboards are steamed for 24 hours at a temperature of 30-35°C. By their bulk density, cement-fibrolite pressboards are divided into grades 300, 350, 400 and 500 with a bending strength of not less than 0.4, 0.5, 0.7 and 1.2 MPa, respectively, heat conductivity of 0.09-0.15 W/m×°C, and a water absorption of not more than 20%. The length of pressboards is 2000-2400, width 500-550, and thickness 50, 75, and 100 mm.

Fibrolite pressboards based on portland cement are used as heat- insulating, heat-insulating-and-structural material, and acoustic ma­terial for walls, partitions, floors, and roofs of buildings. Fibrolite pressboards are also obtained by moulding and thermal processing (or without it) of organic short-fibre raw materials. Suitable as raw mate­rials are crushed machine-shavings or chips, cut straw or cane, saw­dust, boon and the like. The second component for the manufacture of fibrolite pressboards is portland cement. The^bulk density in dry state is 500 kg/m⁸, the bending strength, not less than 0.7 MPa; the heat conductivity in dry state, not more than 0.12 W/m•°C; the mois­ture content, not more than 20% by mass. Pressboards are available in lengths and widths of 500, 600 and 700 mm and thicknesses of 50, 60 and 70 mm.

Heat-insulating cork materials and items (pressboards, shells and segments) are used for heat insulation of enclosing constructions of buildings, refrigerators, surfaces of refrigerating equipment and piping (at temperatures of the insulated surfaces between -150 and +70°C), and for insulation of ship hulls. They are manufactured by pressing crumbs waste of bottle cork obtained from bark of the cork-oak or the so-called cork which grows in the Soviet Far East, in the Amur region and on the Sakhalin island. The cork, due to its high porosity and pre­sence of resinous matter, is one of the best heat insulating materials. It is used for manufacture of pressboards, shells and segments.

Heat-insulating cork materials and items may be manufactured with and without additions of organic binders (organic glue, gelatin, bitumen, resin and others). In the former case, the cork crumbs coated with a thin layer of organic glueing matter are pressed to boards ha­ving a length of 500 to 1000 mm, a width of 500 mm and a thickness of 20 to 80 mm. These boards are termed "impregnated". In the latter case boards are manufactured in the same sizes, as above with the cork crumbs being pressed at a pressure of 0.7 MPa, but without the binder, and then thermally processed at a temperature of 250 to 300°C. In this process, the resinous matter of the cork volatilize and the cork crumbs bake into a monolithic mass. Pressboards obtained according to the second method are known as "expansite". Once the hot-pressed boards are cooled, they are sawn to a required size.

By their bulk density in dry state, cork heat insulating materials and items are divided into grades 150 to 350 with a bending strength of 0.15 to 0.25 MPa, a heat conductivity in dry state of 0.05 to 0.09 W/m×°C at a temperature of 25°C.

Among the advantages the pressboards offer is that they are incom­bustible, smolder but with great difficulty, are resistant to wood fungi and are not attacked by rodents.

Cork materials are packed in cages of 0.25 to 0.5 m⁸, stored in dry closed premises and shipped by rail in boxcars.

Heat insulating foamed plastics. Heat insulating materials based on polymers in the form of gas-expanded plastics and items or mineral and glass wool products are manufactured with the use of polymer binders.

By their physical structure, gas-expanded plastics may be divided into three groups: cellular or foamed (foamed or gas-expanded plas­tics), porous, and honeycomb plastics. Foamed and honeycomb plas­tics based on polymers are not only heat-insulating, but also structu­ral materials. Heat-insulating materials from plas­tics fall (by the kind of polymers used for their manufacture) into the following varieties: polystyrene materials, porous plastics based on suspension-type (beaded) or emulsion-type polystyrene; polyvinyl- chloride materials, porous plastics based on polyvinylchloride; phe­nol materials, porous plastics based on formaldehyde.

Pore formation in polymers is based on the use of substances that intensively evolve gases and expand a polymer softened by heating.

These porogene substances may be solid, liquid or gaseous. Solid po- rogenes having the greatest practical significance are carbonates, bi- carbonates of sodium and ammonium, which decompose with the evolution of CO₂ and NH₃, azodinitriles, azodicarbonic acid ethers, which produce a mixture of abietic acid with calcium carbonate that generates CO,. Liquid porogenes include benzene, light fractions of benzene, alcohol, etc. Gaseous porogenes are air, nitrogen, carbon dio­xide, and ammonia.

Plasticizers, such as phosphates, phthalates and others are added to polymers to make porous plastics adequately elastic.

Porous and cellular plastics may be manufactured by two met­hods-with and without the use of presses. When manufactured by the press method, a finely ground polymer powder mixed with gas- generating agents and other additives is pressed under a pressure of 15-16 MPa, after which the batch (generally 2 to 2.5 kg) is expanded to produce a material of a cellular structure.

The manufacture of. porous plastics without pressing involves the heating of a polymer containing a gas-generating agent, curing and other components to a required temperature. The polymer melts, the gas-generating agent decomposes, and the gas that evolves expands the polymer to produce a material of a cellular structure with uniformly distributed fine pores.

Slabs, shells and segments of porous plastics are used to insulate en­closing constructions of buildings and surfaces of industrial equipment and piping at a temperature of up to 70°C.

Items from porous plastics based on suspension polystyrene are di­vided, on the basis of bulk density in dry state, into grades 25 and35 of a bending strength of not less than 0.1-0.2 MPa, a heat conductivity of 0.04 W/m×°C, and a moisture content of not more than 2% by mass. Similar emulsion polystyrene-based components have a grade, in terms of bulk bensity, of 50 to 200 with a bending strength of not less than 1.0 to 7.5 MPa, a heat conductivity of not more than 0.04-0.05 W/m×°C, and a moisture content of not more than 1 % by mass. Slabs of porous plastics are manufactured in lengths of 500-1000 mm, widths of 400- 700 mm and thicknesses of 25-80 mm.

The heat insulating plastics widely used in practice are polystyrene expanded plastic, mipor and others. Polystyrene expanded plastic is an excellent heat insulator in multi-layer panels, and a good match to aluminium, asbestos cement and glass-fibre plastic. It is much used as an insulating material in the refrigerating, shipbuilding and the railway car industry to insulate walls, ceilings and roofs in constru­ction. Polystyrene porous plastic manufactured from beaded (suspen­sion) polystyrene is a material consisting of fine spherical particles fritted to one another. There are voids of various sizes between the particles. Tne most valuable properties of polystyrene porous plastic are its low bulk density and a small heat conductivity. Polystyrene porous plastic is available as slabs and variously shaped items. It is produced of a bulk density of up to 60 kg/m³, of a strength, in terms of 10% compression, of up to 0.25 MPa and a heat conductivity of 0.03-0.04 W/m×°C. The standard size of slabs is 900 X 650x100 mm.

Polyurethane expanded plastic is used for heat insulation of enclo­sing constructions of buildings and surfaces of industrial equipment and piping at temperatures of up to 100°C. It is obtained from polyes­ter polymers with addition of porogenes and other admixtures.

Polyester polymers form a large group of artificial polymers obtai­ned by condensation of multi-atomic alcohols (glycol, glycerine, pen- taerythritols and others) and chiefly dibasic acids, such as maleic, phthalic and other acids. To improve the elasticity of items, fat acids or vegetable oils are added to the multi-atomic alcohols and dibasic acids during their condensation. By their bulk density in dry state, mats from porous polyurethane are divided into grades 35 and 50, of a heat conductivity in dry state of 0.04 W/m×°C, and a moisture con­tent of not more than 1 % by mass. Porous polyurethane is also used as the basic material for manufacturing hard and soft slabs of a bulk density of 30-150 kg/m³ and a heat conductivity of 0.022-0.03 W/m×°C. Mats of porous polyurethane are made in slabs 2000 mm long, 1000 mm wide and 30 to 60" mm thick.

Mipor is a porous material obtained on the basis of an urea-formal­dehyde polymer. The raw materials for the manufacture of mipor are an urea-formaldehyde polymer, a 10% solution of sulfonaphthene acids (Petrov's contact) and fire-resistant additions (20% solution of ammonium phosphate).

Mipor is used for heat insulation of building structures, industrial equipment and piping at temperatures of up to 70°C.

To obtain mipor, a water solution of an urea-formaldehyde polymer and a porogene are charged into a reactor fitted with a stirrer, the mix­ture then being vigorously stirred. The resultant foam is cast into me­tallic moulds which are then transferred to curing chambers where the mass hardens at a temperature of 18-22®C over a period of 3-4 h. The obtained slabs are conditioned for 60-80 h in driers at a temperature of 30-50°C.

Mipor is available in slabs of a volume of not less than 0.005 m³, a compressive strength of 0.5-0.7 MPa, a specific impact strength of 0.04 kgf×cm/cm², a water absorption of 0.11% in 24 h, and a heat conductivity of 0.03 W/m×°C.

Building felt is used as a lining and a heat insulating material for separate parts of constructions (ends of beams in stone walls, window and door frames in exterior walls, joints of panels in residential pre­fabricated buildings) and surfaces of industrial equipment and piping at temperatures of up to 100°C. Felt is employed for lining ceilings prior to plastering. Felt is manufactured as piece items of rectangular shape by watting wool and waste from wool, fur and other processing

1. Basic Information on Sound Waves and Noises

Sound is a mechanical disturbance in an elastic medium (air, water, solid body). The frequency range of sounds perceptible to the human ear lies between 15 and 20000 Hz (1 Hz is one oscillation per second). Sound waves of lower or greater frequencies (pitch) cannot be perceived by man. Sounds may be divided into musical sounds, noises and sound pulses.

The quantity of energy carried by a sound wave per 1 s through an area of 1 cm² normal to the direction of its travel is known as sound intensity; it is proportional to the square of the amplitude of oscilla­tions of the medium particles and to the square of pressure fluctuations in the sound wave. Sound intensity is measured in decibels, and its physiological characteristic (loudness), in phons. Velocity of sound in the air at 15^oC is equal to 340 m/s.

Noise is a combination of various sounds rapidly changing in pitch and intensity. In everyday life, any sound that brings discomfort is called noise. Noises in buildings are most various in nature. Noises may be audible and inaudible, they may originate in the air or by direct impact. A prolonged action of audible sound, in particular, that of a high pitch sound is detrimental to human health. Not less injurious is a prolonged action of an intensive inaudible ultrasound (of a pitch above 20000 Hz). Sources of high-intensity audible and inaudible noi­ses are generally various engines and mechanisms. Noise can be consi­derably lowered, if buildings are designed and finished with due re­gard for architectural acoustics. Architectural acoustics deal with sound phenomena inside premises. Architectural acoustics are comple­mented by constructional acoustics which are concerned with sound insulation of exterior walls of buildings against air-borne and solid- borne noises. These noises are controlled by various techniques.

Sound energy that falls upon an exterior structure (wall, floor, cei­ling), is partly reflected, partly absorbed and partly passes through the structure with the effect that sound is transmitted to its other side. The ability of material to pass sound is called its sound transmission, and its reciprocal, soundproofing. Soundproofing of a wall material is evaluated by the difference in sound levels on both sides of the wall and is expressed in decibels. Materials which, preferably absorb sound energy are called sound-absorbing materials, and those capable of insulating premises against the ingress of sound, soundproofing ones. All together they are called acoustic materials.

Acoustic materials are subdivided, similarly to heat-insulating ones, by structure, kind of basic raw material, bulk density, and should meet the same overall requirements. By application, acoustic materi­als are subdivided into two main groups: soundproofing-and-lining and sound-absorbing ones.

2. Soundproofing-and-Lining Materials and Components

Soundproofing materials and items are used chiefly in the form of liners and interlayers in floors, interior and exterior walls and other parts of buildings in order to damp impact noises transmitted through floors (walking), vibration (machine operation), etc.

Soundproofing materials may be incorporated in building structu­res in free (non compressed) or even in suspended state (e.g., slabs fas­tened to ceiling in a manner to leave an air space) or in compressed state (e.g., between the load-bearing panels of ceiling and floor).

Soundproofing materials, in free or loose state, are used for insula­tion against air-borne noises, whereas compressed ones, against solid- borne noise. Thickness of soundproofing materials used in free state (in walls, floors) should not exceed 5 cm, and those compressed (e.g., in floors underneath the flooring), should not be less than 1.2 cm.

Sound-insulation materials are of a porous-fibrous structure (based on mineral or glass wool, asbestos or any other kind of fibres), of a po­rous-jagged structure (based on plastics and various kinds of rubber), and of a loose constitution (as natural or artificial sands, slags, etc.). The first are shaped as slabs, rolls, mats, strips and piece liners. By the relative compression under load, a distinction is made between rigid, semi-rigid and soft sound-insulation materials. The main de­sign parameter which predetermines their use in structures is the dyna­mic modulus of elasticity. In terms of this parameter, sound-insulation materials fall into three groups: 1st group, materials with a dynamic modulus of elasticity less than 1 MPa; 2nd group, from 1 to 5 MPa; 3rd group, from 5 to 15 MPa.

Materials of the 1st group are used in the form of slabs, rolls and mats laid in solid layers in "floating" and multi-layer floors, walls and partitions as insulation against air-transmitted and impact noise. Sound-insulation materials of the 2nd group are employed as strip and piece liners in "floating" and multi-layer floors as insulation against impact noise. The liners may be 400-1000 mm wide and 500-1000 mm long, depending on the design load, and not more than 30 mm thick. Sound-insulation materials of the 3rd group find application as fillings in multi-layer structures of floors to improve insulation against im­pact and air-transmitted noise.

Vibrations may be damped and localized by the use of vibration- absorbing materials, such as rigid and soft polyvinylchloride and po­lyethylene sheets, sheet rubber, bitumen and polymer mastics, inc­lusive of rubber, polyvinylacetate, epoxy and other kinds. Sound-insu­lation materials are mainly of the elastic variety, such as semi-rigid mats and slabs from mineral wool on synthetic binder; slabs, mats, and coils of staple glass fibres; wood-fibre insulating boards; slabs from plasticized polystyrene foamed plastic; fibrolite slabs on portland ce­ment; river sand, metallurgical slag and fuel cinder, cork crumbs.

Glass and mineral wool mats and slabs on synthetic binder have a bulk density of 50-225 kg/m³, a relative compression factor of 15-40% under a load of 0.02 MPa, and a dynamic modulus of elasticity of 0.3- 0.7 MPa.

Wood-fibre and fibrolite boards on portland cement are used in con­structions of subfloors as insulation against impact noise; they feature a relative compression under the same load of up to 1.5%, and dyna­mic modulus of elasticity of 1.0 to 1.8 MPa.

Slabs from plasticized polystyrene foamed plastic, grade ПСБ-Э, have a bulk density of 20 to 35 kg/m³, and dynamic modulus of elasti­city, 0.8 to 1.0 MPa.

Above items provide sound damping of reinforced concrete floors by 35 to 40 dB, which answers the design norms.

The chief material for the production of new acoustic slabs are mi­neral wool, glass staple, starch, lithopone and polyvinyl acetate emul­sion.

Slab manufacture involves loosening and granulation of mineral wool, mixing of resultant granules with starch binder, slab moulding, drying and working, painting and packing.

Soundproofing-and-lining materials are used as solid interlayers underneath floors (mats and slabs of mineral or glass wool, wood fib­re insulation slabs), as strip linings in common floors (wood fibre and asbestos-cement slabs), as strip linings in multi-layer floor structures (sandwich formed of asbestos cardboard).

3. Sound-Absorbing Materials and Items

Sound-absorbing materials are used for interior finishing of pre­mises with the aim of improving their acoustic properties. These ma­terials lower audible noise (a combination of numerous sounds which rapidly vary in frequency and amplitude) in industrial and public buildings.

Sound-absorbing materials are capable of ensuring a required dura­tion of reverberation in premises for various applications, their coef­ficient of sound-absorption, measured in the diffusion field (in a re­verberation chamber with the material or item placed directly upon a rigid foundation) in frequency bands of 125-500, 500-2000 and 2000- 8000 Hz, should not be lower than 0.2, 0.4 and 0.6, respectively. Rever­beration is a gradual damping of an emitted sound in closed premises due to repeated reflections. Depending on the kind of premises and the sound frequency ranges, the reverberation time is 0.2 to 2 s.

Sound-absorbing materials are employed to obtain a correct distri­bution of useful signal levels over the floorspace of premises and to pre­vent the propagation of sound along premises of considerable extent.

By character of sound absorption, sound-absorbing materials fall into porous materials with a solid skeleton, in which the sound is ab­sorbed as a result of viscous friction inside the pores, so that the so­und energy transforms into heat (foamed glass, gas concrete and other porous materials with a solid skeleton); porous materials with a fle­xible skeleton, in which, in addition to the viscous friction in the po­res, there appear relaxation losses due to the deformation of the non- rigid skeleton (mineral, glass, basalt and cotton wool; wood-fibre bo­ards and other materials of similar characteristics); panel materials and constructions whose sound absorption is due to an active resistance of the system which effects forced vibrations under the action of an in­cident wave (thin veneer panels, rigid wood-fibre boards, soundproof cloths and others). Sound absorption of porous materials may be impro­ved by providing an air interlayer between the enclosing construction and the material.

Sound-absorbing materials may have a porous-granular, a porous- fibrous and a porous-spongy structure. By the hardness of their skele­ton, they are divided into soft, semi-rigid and rigid varieties. Sound- absorbing materials are available as boards, coiled and loose materi­als; they are also used in the form of plaster boards having a smooth- porous, perforated and grooved structure.

In the enclosing structures, the sound-absorbing materials are in­corporated as single-layer homogeneous textured-face components, multi-layer porous-fibrous ones with a rigid perforated coat, and as piece items of various sizes and shapes, both single- and multi-layer ones.

At present, in wide use are the following kinds of sound-absorbing materials employed in structures that carry no protective sheathing: mineral wool acoustic boards on synthetic binder, types ПА/С, ПА/'О and ПА/Д; boards from granulated mineral wool on starch binder; boards from staple glass fibre, types ПС and ПЖС; basalt sound-ab­sorbing mats, grade БЗМ; wood-fibre boards with perforations; gyp­sum slabs reinforced with glass fibre and with through perforations; slabs of cellular concrete, type "Silakpor", of porous structure and with perforations of the face layer; slabs from gas (foamed) lime-sand con­crete, etc.

The following types of sound-absorbing materials are used in enclo­sing structures with protective sheathing: grade ПП and ППМ semi­rigid mineral wool slabs on synthetic binders; mineral-wool sewn mats on metal wire fabric; mats from staple glassfibres on synthetic binders; mats from extra-thin glassfibres and canvases and mats of intertwi­ned extra-thin basalt fibres.

A successful use in the construction of public buildings is being made of protective sheathing and baffles from glass or capron (nylon) fib­res and of gypsum perforated boards backed with glued-on commercial cotton cloth.

In some instances, wood-fibre boards, acoustic plaster with a filler of burnt kaolin crumbs or pearlite sand have been used as sound-absor­bing materials.

Type ПА/С, ПА/О and ПА/Д mineral wool acoustic (soundproofing) boards are manufactured from mineral fibres by impregnating them with a synthetic binder and subsequently submitting them to thermal and moisture curing in special chambers. The blanks so obtained are mechanically worked, then given an ornamental face finish. Said bo­ards measure 500 x 500 x 20 mm and feature a bulk density of 130- 140 kg/m³, a tensile strength of up to 0.4 MPa and a sound-absorption coefficient in the 500-2000 Hz frequency range of 0.4 to 0.87. Excel­lent ornamental properties of the mineral wool acoustic boards un­derlie their wide use for facing ceilings, entrance halls, theaters, con­cert halls, radio studios and noisy premises.

Akminite and Akmigran boards are new acoustic materials ba­sed on granulated mineral wool and compositions of a starch binder with additions. The boards feature a bulk density of 350-400 kg/m³, a bending strength of 0.7 to 1.0 MPa, a sound absorption coefficient of up to 0.8, and measure 300 x 300 x 20 mm. The boards are intended for sound-absorbing finishing of ceilings and top parts of walls of public and administration buildings where the relative air humidity is not more than 70%.

The face of boards is textured as directional cracks (caverns) simi­lar to the texture of weathered limestone.

The boards are secured to the floors by a system of metal sections. They may also be glued with the aid of special mastics directly to any rigid surface.

Specific textures and a wide range of colours of the ornamental "silakpor" and foamed lime-sand boards provide the necessary va­riety of interior finishes even when they are used on a large scale.

Silakpor slabs are manufactured of lightweight gas concrete of spe­cial structure and a bulk density of 300-350 kg/m⁸. The faces of slabs can be provided with longitudinal slot perforations which not only improve their appearance; but also enhance their ability to absorb noi­se. The sound absorption coefficient of the silakpor slabs in the 200- 4000 Hz frequency range is 0.3-0.8.

Slabs of gas lime-sand concretes possess good service and architec- tural-and-construction properties. They form a particular group of sound-absorbing materials, including the ones with a macroporous structure. Gas lime-sand concrete is used to make slabs of a bulk den­sity of 500-600 kg/m⁸, a compressive strength of 1.5-2.0 MPa, and a sound-absorption coefficient in the 500-4000 Hz frequency range for microporous slabs of 0.2-0.3, and for macroporous slabs, of 0.6-0.9; the slabs are 750x350x25 mm in size.

The slab manufacturing procedure consists of mixing the raw mate­rials (lime, sand and dye), pouring the prepared mix into moulds and autoclaving the latter, after which the product is milled and sized.

Acoustic perforated plasterboards and gypsum perforated slabs with a mineral wool sound-absorbing component feature good appea­rance, adequate fire-resisting and high soundsbsorbing properties. They are widely used for interior finishing of walls and ceilings in entertainment, service and public buildings.

Acoustic baffles of plasterboard are obtained by stamping. Gypsum plasterboards, cut to slabs measuring 1000 x 500 x 8 mm, are worked in a die press to form holes 6 and 10 mm in diameter. Once stamped, the baffles are transferred to grinders for removing rough spots, and then onto a conveyor line for glueing an underlying cloth with si­multaneous drying of the glue. The baffles are used together with mi­neral wool and glass-fibre sound-absorbing materials for facing walls and ceilings of premises where the relative air humidity is not higher than 70%.

Acoustic perforated gypsum slabs with a mineral wool sound- absorbing material consist of a gypsum shell reinforced with glass- fibre cord and a steel wire 0.8 to 1.2 mm in diameter, type ПП-80 mineral wool inserted in the free sections of the gypsum slab and of aluminium foil which protects the wool against humidification. The slabs have a sound absorption coefficient of up to 0.7 in the 400-1500 Hz sound frequency range.

Asbestos-cement acoustic baffles are of a particular interest as they are durable and hygienic, feature a high mechanical strength (up to 10.0 MPa), fire-resisting properties, good ornamental properties and an excellent sound absorption factor valued at 0.6 to 0.9. Asbestos- cement acoustic items are manufactured as perforated slabs with ro­und or slot-type through holes and as perforated asbestos-cement baffles with mineral wool sound-absorbing filler. Slabs and baffles are emplo­yed for facing suspended ceilings or walls to dampen noise.

Pearlite sound-absorbing slabs'are used for minimizing noise and providing good acoustic conditions inside premises. Slabs are prepared from expanded pearlite on a binder of soluble glass or synthetic re­sins with addition of pigments to obtain required colouring. Pearli­te slabs are manufactured of a bulk density of 250-500 kg/m^s, a ben­ding strength of 0.4-1.2 MPa, a sound absorption coefficient in the 500-2000 Hz frequency range of up to 0.7, and measure 300 x 300 x 300 mm.