Inorganic heat-insulating materials

Inorganic (mineral) heat-insulating materials differ from organic by noncombustibility, relatively low hygroscopicity, rot resistance. They can be widely applied in thermal insulation of building structures, industrial equipment and pipelines. Mineral wool materials are most widespread among the heat-insulating materials. Mineral wool mainly consists of glass-like fibers with diameter 1-10 µm and length 2-20 cm (Fig. 17.1), which are manufactured by processing of melts of blast-furnace slag and number of silicate rocks (diabases, basalts, marls, etc.). Silicate melts is converted into a mineral fibers as a result of influence of blowing of steam or gas (blowing method) or centrifugal force (centrifugal method) (Fig. 17.2). Centrifugal - blowing method of fiberizating, which includes application of centrifugal force and blowing is the most widespread. Thermal conductivity of mineral wool at temperature 25°C changes depending on average density in the range of 0.042 – 0.046 W/(m·K), limiting temperature of application is 600°C. Mineral wool is granulated for prevention of compaction during transporting and storage. Glass wool is similar to properties of mineral wool. Glass batch are used for manufacture of glass wool.

Application of mineral and glass wool as ready products is the most rational (Fig. 17.3). Synthetic polymers and bitumens are effective binders for such products. The basic types of products are non-rigid, semi-rigid and rigid board, cylinders and semi-cylinders.

Mats without finishing and with finishing are made of bitumen paper, cardboard, asbestine fabric, glass linen and metal grid. There are mineral wool mats of transversal and longitudinal sewing. More effective is the longitudinal one, because it is based on the longitudinal placing of sewing material is possible to make the products with unlimited length. The mineral wool mats are applied as suspension heaters in light wall structures and also in ceilings. They can be successfully used for insulation of pipelines (Fig. 17.4) and technological equipment. Application of sewed mineral wool mates allows improving conditions of work in the process of heater placing and also decreasing the dust formation.

The semi-rigid boards with average density 100 and 125 kg/m³ are widely used. Also, high rigid mineral wool boards, which have strength 0.04-0.1 MPa, density 75-250 kg/m³and water absorption 15-60%, can be manufactured.

The basic type of the synthetic binder for mineral wool board is phenolic alcohols. It is possible to reduce the content of synthetic binder by additional introduction of bitumen in the mineral wool products.

Mineral wool products are widely used in thermal insulation of pipelines and industrial equipment. Along with mats and boards for this purpose cylinders and semi-cylinders on synthetic binder can be used. High temperature wool and products with the temperature of application up to 1250°C can be manufactured from kaolin and silica initial materials.

Basic physical and mechanical properties of products based on mineral and glass wool are shown in Table 17.1.

Glass-fiber materials occupy about 10 % of general amount of modern heat-insulating materials. Glass-fiber materials economically effective to apply only as light products with an average density 30-50 kg/m³ and thermal-conductivity 0.037-0.04 W/(m·K).

In the last years the technology of basalt fiber materials, which combine high heat-insulating properties and temperature proofness (up to 750°C), longevity, incombustibility and non-toxicity, is developed. Industry produces different types of products based on basalt fibers - linens, cords, rolls, mats, board, etc. The optimum value of density for basalt fiber materials is equal 80-100 kg/m³.

Asbestos can be used for manufacture of heat-insulating products, mastics and mortars in combination with Portland cement or other binders.

Lime-silica and some others heat-insulating products, which contain asbestos, are manufactured. They are used mainly for the insulation of equipment and pipelines which work at temperatures 400-600°C.

Table 17.1

Physical and mechanical properties of products based on

mineral and glass wool

Lime-silica products are made of fine mixture of quicklime and silica materials - diatomite, tripoli, marshalite and others in the conditions of autoclaving. Content of asbestos is in the range of 15-30%. The products of this group produce as boards and semi-cylinders with the minimum density 225 kg/m³ and thermal-conductivity 0.112 W/(m·K) at 300°C.

For thermal insulation large interest is paid to the products based on the expanded perlite and vermiculite. Perlite belongs to the group of volcanic glass, and vermiculite - to hydromica. Both the rocks contain the certain amount of the bound water, which at temperature 800-1000°С is intensively removed as vapour. At firing of perlite the expanding coefficient (ratio between sizes of grains after expanding and initial grains l) is mainly equal 6-15; vermiculite 3-20. After firing, expanded perlite can be divided into sand with bulk density 75-250 kg/m³ and crushed stone with density 300-500 kg/m³. Thermal-conductivity of perlite sand is 0.041-0.07; crushed stone 0.075-0.09. Perlite sand can be used as fine aggregate in concrete and mortars for making heat-insulating products and also fire-proof plasters, which are used at temperature of 200-875°C.

Scaly structure is characteristic feature of expanded vermiculite. It is used as grains, which have coarseness up to 10 mm, bulk density 100-200 kg/m³ and thermal-conductivity 0.064-0.076 W/(m·K). The possible temperature of application of expanded vermiculite is higher, than perlite one and achieves 1100°C.

Production of both non-fired and fired heat-insulation products is possible with application perlite and vermiculite fillers.

Non-fired products can be produced with the application of different binders: bitumen, polymers, liquid glass, gypsum, Portland cement. Their average density is in the range of 150-400 kg/m³. Their range of application depends on the properties of light-weight aggregates and binders. In particular, bitumen perlite can be applied for heat-insulation of pipelines and different coatings; polymer perlite, which has comparatively higher strength, is possible to use as heaters of self-bearing and hangings light panels.

Bitumen perlite and polymer perlite concrete are the varieties of heat-insulating materials, which can be successfully used also for waterproofing arrangement. Heat-insulating, acoustic and decorative properties are combined in vermiculite products.

Perlite and vermiculite in combination with a ceramic bond or liquid glass allow obtaining the fired heat-insulating products with an average density 250-400 kg/m³. Such products can be used for the thermal insulation of technological equipment at operational temperature up to 900-1100°C.

The cellular materials are manufactured by expansion of the different raw material masses in the process of forming or at thermal treatment. Gas- and foam concrete are the most widespread. Cellular concrete products of autoclave and non-autoclave hardening with a density not more than 400 kg/m³ are applied for warming of building structures and thermal insulation of industrial equipment with the temperature of surface, which is insulated, no more 400 °C. They can be used in premises with relative humidity of air no more than 60% without the special protection against moistening.

Heat-insulating porous ceramic materials are manufactured from porous ceramic masses. Porisation of ceramics is executed due to the introduction of combustible additives (sawdusts, lignin, anthracite, etc.) into the batch or by mixing of ceramic mixtures with foam and gasifier. Application of diatomite, tripoli and some other sedimentary rocks containing silica are widespread for the manufacture of light-weight ceramics. Heat-insulating products from porous ceramics are made with a density of 350-500 kg/m³ and compressive strength 1-2 MPa.

Foamglass is manufactured at temperature of 750-850°С by expansion of softened glass mass which contains gasifier. Foamglass is an effective inorganic heat-insulating material. High porosity (80-95%) allows to provide low density (100-700 kg/m³) of foamglass. Also, favourable structure of this material and high content of closed pores provide relatively high strength, water resistance and low water absorption. Temperature stability of ordinary foamglass is 300-400°C and non alkaline foamglass is 800-1000°С. Foamglass is easily processed and can be of different colouring. This material is applied for the thermal insulation of refrigerators, heating systems and also as facing and acoustic material.

Date: 2015-12-18; view: 968