Characteristics of the basic properties of raw materials, materials...

7.2. Characteristics of the basic properties of raw materials, materials and products

The chemical properties of materials depend on the chemical composition, are manifested when the materials are exposed to various substances in the process of production, during the operation or consumption, storage, disposal of finished products.

chemical properties include water resistance, acid resistance, alkali resistance, the ratio to the action of oxidants, reducing agents and organic solvents, etc. These properties show the reaction of the material of manufacture to the action of various chemicals and the environment.

Water resistance - is the ratio of the material to the effect of water under different temperature conditions and the duration of the exposure; acid resistance and alkali resistance is the resistance of the material to the action of organic and inorganic acids, alkaline media; the relation to the action of light-duty is a complex influence of several factors, for example solar insolation and atmospheric precipitation.

For example, the chemical properties of glass are characterized by resistance to short-term exposure at normal temperatures of various chemical media (moisture, salts, gases, etc.), with the exception of HF. With prolonged exposure to reagents and elevated temperature, the glass is gradually degraded - corrosion, which is accompanied by the appearance on the surface of a white coating or an irrigation film. Under the influence of hydrofluoric acid, glass is easily destroyed, which is used to apply to the products of the figure.

The stability of glass to acid solutions is higher the higher the content of SiO2, A12O3, ZrO2, and to alkalis - SiO2, CaO, BeO2. With the increase in BaO, MgO, and TiO2, the alkali resistance decreases. Glass is less resistant to NaOH than KOH.

Physical properties of goods are divided into size-mass (density, bulk weight, specific gravity, weight), mechanical (hardness, toughness, elasticity, ductility, brittleness) , thermal (thermal resistance, frost resistance, thermal expansion, heat capacity, thermal conductivity), optical (refraction, transmission, reflection), acoustic, electric (electrical resistance, electrification), sorption properties (absorption, adsorption, chemisorption), permeability (water, gas -, dust, air permeability). As well as chemical, physical properties affect the behavior of goods during operation, transportation, storage, disposal.

The physical properties of the materials are largely due to their structure. In their study, the structure of atoms and molecules of matter, the location and connections of molecules (internal structure of matter), microstructure and macrostructure are considered.

The microstructure of a material depends on the shape, size and relative positioning of the complexes of atoms and molecules. It is visible under a microscope. The nature of the microstructure (size, shape and mutual arrangement of crystals) has a great influence on the properties of materials.

Study of the microstructure of the skin, fibrous materials allows to establish mechanical properties, porosity, water permeability, thermal conductivity, etc .; data on the microstructure of metals constitute the most important section of the theory of the physico-mechanical properties of metals and their alloys.

The macrostructure of the material is characterized by the arrangement, shape and size of large groups of molecules or material components that can be observed with the naked eye or with a magnifying glass. The macrostructure is determined by the structure of solids, which is also visible to the naked eye or under a magnifying glass.

The macrostructure of the materials differs. Thus, the macrostructure of metals is characterized by the apparent location of the crystallites of the substance, their size, shape and other indices determining the surface pattern and the nature of the fracture, and the macrostructure of the tissues is the structure of the constituent filaments of the fibers, threads, their interweaving and some other factors determining the visible structure of the material. The macrostructure determines a significant part of the properties, and its features are always considered by commodity experts, in addition, the macrostructure is investigated in the identification of materials.

Mesostructure is characterized by the structure and arrangement of elementary particles - subnuclear particles, i.e. the smallest particles of matter (for example, electrons), which are not molecules, atoms, ions, etc.

One of the important properties researched by commodity experts in the study of micro- and macrostructure is the porosity of the structure of materials (wood, leather, ceramic products, etc.). Porosity is an indicator that characterizes the filling of the material with pores. Pores - small cells in a material filled with air or gases; Large cells are also called voids.

Pores are divided:

- through, passing through the thickness of the material;

- closed, which do not communicate with the external environment;

- semi-closed - going deep into and ending in the thickness of the material;

- surface, open - causing material surface irregularities (semiclosed pores that do not go into the depth of the material).

Many properties of materials depend on the degree and nature of the porosity of the material - bulk mass, water absorption, thermal conductivity, mechanical properties, etc.

Semi-closed and through pores can have three forms: cylindrical, funnel-shaped and bottle-shaped (with constrictions). The presence of some form of pores (especially bottle-like) has a significant effect on the nature of adsorption and desorption of liquids by a porous material.

Dimensional properties of materials and products, such as density, bulk mass, volume, weight affect the quality. Dimensional-mass properties are used to characterize materials. For example, for fabrics and building roll materials, paper an important indicator is the mass of 1 m2.

The weight is important for sports goods, hunting weapons, fishing goods, clothing, footwear. Comparison of weights before and after wetting the samples allows us to determine the moisture content and porosity of the materials.

Mass of products determines the choice of packaging, technology of transportation and storage of goods. Mass affects the ease and convenience of using consumer goods (shoes, clothes, goods for active sports and recreation, tools, etc.).

Size-mass properties and indicators are taken into account when assessing the hygienic properties of materials and garments (weight of materials for footwear and clothing, weight of sewing, fur products, etc.).

The main indicators of weight properties are as follows.

Density (p) is the mass of an absolutely dense substance per unit volume (kg/m3, g/cm3).

In commodity science, density is determined by pycnometers (for liquids and solids) and hydrometers (for liquids). The density of porous materials is established after their grinding.

The density of some materials: vulcanized rubber - 1.1 g/cm3; keratin of wool 1,3-1,33 g/cm3; celluloid - 1.4 g/cm3; cellulose 1.5-1.55 g/cm3; glass - 2,5 g/cm3; aluminum - 2.7 g/cm3; chromium - 7.1 g/cm3; tin - 7.3 g/cm3; steel - 7.9 g/cm3; copper - 8.9 g/cm3; lead - 11.35 g/cm3.

Mechanical properties are manifested when external forces are applied to materials.

These properties are characterized by deformations of materials and products under the action of compressive, tensile, bending loads.

The mechanical properties of materials include hardness, toughness, elasticity, ductility, brittleness.

Mechanical properties are manifested in deformation-strength characteristics: the ultimate strength, elongation, narrowing, fatigue strength, hardness.

Mechanical properties determine the strength of goods.

All bodies consist of atoms and molecules, between which there are forces of interaction that are in balance and do not manifest themselves in a visible way. When an external force (load) is applied to the material, the elementary particles of matter are moved, as a result of which the shape of the material changes. The change in the shape of the material or the distances between any points of matter under the action of a load on it is called deformation.

If, after the removal of the load, the particles of matter under the action of internal (elastic) forces return to their original position and the material completely restores its shape, the deformation is called reversible.

If after the removal of the load the material receives a constant change in shape determined by the new arrangement of elementary particles (atoms, molecules or their complexes) and a new equilibrium state, this modification of the shape is called irreversible (plastic) deformation. strong>

Deformation depends on the structure and properties of the material, the magnitude and speed of application of the load, the time of its action, the state of the material (temperature, humidity, etc.).

A reversible deformation that lasts a long time is called elastic (for metals), or elastic deformation (for high molecular weight organic compounds).

The total deformation is the sum of the elastic, elastic and plastic deformations:

(6)

The time of determination of elastic, elastic and plastic deformations can take a long time (for example, elastic deformation is determined 3 minutes after removal of the load, plastic - after 24 hours). These deformations are conventionally elastic, conditionally plastic and conditionally elastic.

Materials whose elastic deformations are insignificant or practically absent under the action of a load are called plastic deformations (clay, tin, lead, etc.).

The elastic properties of elastically plastic materials are characterized by the elasticity index (E).

Elasticity is expressed by the ratio of the conditionally elastic deformation to the total (D) and is calculated by the formula

(7)

When the material is subjected to an external load, elastic forces arise in it, which tend to return it to its original position, as a result of which internal stresses arise in the material.

The load balancing the effect of elastic forces per unit area of ​​the material characterizes the stress (σ, kgf/cm2).

Internal voltage is calculated by the formula

σ = P/S, (8)

where P - magnitude of the load, kgf; S - the cross-sectional area of ​​the material, cm2 or mm2.

With a large amount of external forces applied to the material, the particles of matter move, lose mutual connection, and there is a separation - the destruction of the material. The load at which this occurs is called destructive (discontinuous) (Razr). The stress at which the material collapses is called ultimate strength ( endurance) of the material and is denoted by σv.

Loads (stresses) and deformations at which destruction occurs are important for goods. The parameters of deformations are the changes in shape observed during stretching, compression, shear, torsion and bending.

Deformations with stretching are shown in Fig. 7.1.

When applied to a sample of length l and the cross section F of the tensile force P directed along the beam, under the action P , a deformation occurs in the form of a length increment - an elongation (Δ l ) with a simultaneous decrease in transverse dimensions.

The tests determine absolute l ) and sample elongation (ε):

(9)

Compression set calculated as a tensile strain with the opposite sign when the action pattern is shortened while increasing transverse dimensions of the compressive load. Warp shear occurs when the resultant forces are in two nearby cross-sections and act oppositely. Deformation torsional arise when rotating rod, the end of which is fixed.

Fig. 7.1. Strain strain on a sample

In the production and operation of goods simple deformations are in most cases found in various combinations (stretching with simultaneous twisting, compression and bending, etc.), such deformations are called complex.

Loads , depending on the area of ​​the application to the body, are divided into distributed and focused: to permanent and temporary. For example, a cord with a chandelier suspended on it is under constant load, time (at the moment of pressing). A repeated temporary load is called multiple. This load is subject to clothing, shoes, floor coverings, furniture for rest and sleep, tools.

By the nature of the action of external forces applied to the body, distinguish their static and dynamic effects. With static action , the external forces are applied statically very slowly, the accelerations of the material particles created by them are so small that they can be neglected. With dynamic action the load is applied to the material quickly, dynamically, the material particles receive appreciable accelerations and the material is removed from equilibrium. After the damping of the elastic oscillations, the effect of the load becomes static. For example, when walking slowly, shoes experience a static load, while running and jumping - a dynamic; metal pressing is the action of static loads, forging is dynamic loading.

Dynamic loads destroy the material more than static loads. The greatest force of destruction has multiple variable dynamic loads.

If the tensile (compression) load does not reach a certain limit (different for each material), the material remains elastic and returns to its original length after removing the applied load. According to Hooke's law, the resulting elongation (compression) is proportional to the voltage, ie:

σ = Eε, (10)

where E - is a proportionality factor called the modulus of elasticity (when stretching or compressing).

For a relative elongation of one, σ = E, , the elastic modulus indicates the design stress at which the elastic elongation of the material is equal to the length of the original sample.

The reciprocal of the modulus of elasticity is called the stretch factor (compression), which determines the amount of strain corresponding to the stress.

The modulus of elasticity is expressed in kg/cm2 or kg/mm2.

Hooke's law in expanded form is written as follows:

(11)

The absolute deformation (elongation, compression) obtained by the material under tension (compression) is proportional to the tensile (compressive) force and length, inversely proportional to the modulus of elasticity and the cross-sectional area of ​​the product.

Not all materials are strictly subject to Hooke's law, to the greatest extent he will apply to characterize the mechanical properties of metals within the linear relationship between stress and elongation. Mechanical properties of wood, fabrics, leather and some other materials do not obey this law, and the presence of remaining elongations is noted even under the action of small loads.

Tensile strength is an important property of metal, leather, fabric and other materials. The stretch curves can be different in form-they are shallow or steeply rising, and their bulges-face down or up. The shape of the curve makes it possible to judge the relationship between the magnitude of the load and the elongation throughout the whole stretch of the sample until its destruction.

In Table. 7.3 shows the tensile strength values ​​of some materials.

For leather, fabrics, twisted products, plastic masses - i.e. Materials that do not obey Hooke's law even at low loads, the proportionality limit can not be established. The results of the tests are limited by the values ​​of the load and the percentage elongation observed at the time of sample failure or the construction of a tension diagram from intermediate loads and their corresponding elongations.

Table 7.3

Ultimate tensile strength of some materials

Material name

Ultimate strength, kg/mm 2

Material name

Ultimate strength, kg/mm 2

Steel

40-120

Natural silk

45-50

Copper

15-43

Cotton (fiber)

30-50

Cast iron

17-26

Wool

10-20

Aluminum

10-18

Skin

1-7

Tree in the direction of the fibers

8-15

Cotton fabrics

1.5-2.5

Glass

6-8

Rubber

0.6-1

Porcelain

4-8

Woolen fabrics

0,2-1

In addition to the strength index, the resistance of the material to failure can additionally be characterized by the work of the fracture, which is determined by the amount of energy absorbed by the material under mechanical influences. The latter is characterized by the work spent on the deformation and rupture of the sample.

Deformations of plastic materials are associated with the movement of material particles. Particles can move slowly at a constant load. The property of the material is slowly and continuously deformed under the action of a constant load called creep (flow). When moving material particles during plastic deformation, they experience internal friction, resulting in the release of thermal energy. The increase in internal stresses during deformation depends on the rate of increase in the load. At a high rate of increase in the load, the particle movement "lags behind" from the growth of loads and the material breaks down at a higher voltage.

If, after stretching the plastic material, to stop further loading of the sample, the particles of matter will for some time tend to move into a state of equilibrium, as a result of which the internal stress in the material will fall. The voltage drop in the presence of this deformation is called relaxation (relaxation).

The plastic properties of materials are affected by temperature. For example, for metals with increasing temperature, the elastic moduli decrease, the plastic deformation resistance also decreases, the effect of time (speed) of deformation increases strongly, the relaxation properties manifest themselves in a sharper form.

For hygroscopic materials (leather, fabric, wood, etc.), the moisture content of the material is also an important factor, with an increase in which the material is fully elongated.

Deformations of high molecular substances (rubber, synthetic resins, fibers from cellulose, protein) are characterized by the presence, along with elastic, plastic - elastic deformation, which is usually many times greater than elastic deformation. The mechanism of elastic deformation consists in increasing the average distances between atoms in tension, in connection with which the volume of the deformed object increases. Elastic deformation of stretching is caused by straightening of long molecules, i.e. the amount of stretching is determined, on the one hand, by the length of the molecules of the substance, by the location in the sample (the value of their bending, by the dimensions in the folded form) and by the magnitude of their stretching during stretching.

The straightening of molecules under tension is accompanied by the release of heat; after removing the external stress, thermal motion tends again to disrupt the orientation of the straightened chains of molecules, they again take a curved shape, absorbing heat. When heated, the rate of elastic deformation increases significantly, and the relaxation time, depending on external conditions, can vary within wide limits (from fractions of seconds to several years). At sufficiently large elongations, a denser "pack" is also observed; oriented molecules, which is accompanied by a certain decrease in the volume of the sample.

Discontinuous length - is the minimum length of the sample at which it undergoes destruction by its own weight, or the length of the sample at which the stress under its own weight becomes equal to the ultimate strength .

The discontinuous length (L) is calculated on the basis of the tensile strength values ​​(Rasr), the bulk density of the material ( d) and the sample cross-sectional area (F):

(12)

The sample collapses at a time when, as a result of the increase in the length of the specimen, its total weight becomes equal to the tensile strength of the specimen, i.e. when g = P then L = L Image

Material fatigue is a property that manifests itself when a multiple load is applied to a material. When the load is repeatedly applied, the material loses its mechanical properties, and then it can completely collapse. This property is characterized by the endurance (fatigue) limit of the material (σw).

For materials whose fatigue curve is inclined and does not end with a horizontal section, the endurance limit can be arbitrarily chosen as the value of the stress (load) that does not destroy the material with the set number of cycles ( n ).

High endurance is an important property of goods subjected to multiple loads (springs, springs, crankshafts, etc.). In particular, when testing metals, tensile and bending loads are applied, changing their directions. These properties are important for yarn, fabrics, rubber, shoe leathers, plastics.

Hardness is the ability of a material to resist the penetration of other (more solid) material into it. If the compressive force is transferred to a relatively small area of ​​an article, it causes a local compression deformation that does not propagate to a greater depth, thereby causing a crumpling. When pressing one object on the surface of another, there may be an imprint, which is the remaining deformation of the crumple. Hardness is an important property characteristic of metals, plastics and other materials, and also very important for knife, tool products, materials used to make frames, load-bearing structures, building blocks, building materials.

The durability of materials and products is an important set of properties that are taken into account when developing products.

Wear is a change in the appearance, design, or properties of the product in which it requires repair (partial wear) or becomes unusable (wear and tear). Wear of the product is a complex phenomenon that occurs during its operation. It is caused by mechanical and physicochemical effects on the goods.

Depending on the conditions of use, the nature of the material and the design of the product, during the wear process, certain effects predominate, and in each individual case, various prevailing causes of wear can be established. So, when using automobile tires, wear is mainly determined by mechanical influences - tire abrasion when driving on road surfaces; wear of clothes depends on the intensity of physicochemical effects of direct sunlight, air, humidity, washing, and also wear friction.

The external mechanical wear, at which the material surfaces are abraded, is the simplest kind of destruction, it is accompanied by the loss of the surface of the particles of the material and, consequently, the thickness, weight and strength of the material on wear areas, as in cases of abrasion of the lower part of the footwear, coins with long use, etc. The intensity of external wear depends on the natural properties of the material, the structure of the surface exposed to external influences, and the intensity of these impacts.

Internal mechanical wear is caused by repeated mechanical influences on the material during operation, accompanied by a change in the internal structure and, in connection with this - the physical and mechanical properties of the material.

The signs of such wear are the loss of elastic-elastic and plastic properties of the material, an increase in rigidity, brittleness, a drop in mechanical strength, although the appearance and design of the product remain unchanged. With further wear, the product becomes unusable due to a sharp deterioration in consumer properties, partial or complete destruction. This kind of wear is observed in products or their parts, which are subjected to repeated mechanical stresses such as stretching, compression, bending, impact forces, causing fatigue of the material. This kind of wear is characteristic, in particular, for many machine parts (for example, axes of machines, pistons, etc.).

A significant factor in the deterioration of goods are physical and chemical effects - light, heat, moisture, oxidants, alkaline and acid solutions, other reagents. Their influence, as a rule, causes more wear and tear on materials and products than mechanical stresses: changes in the chemical composition and surface of the material (color fading, formation of oxide films, etc.), loss of mass, deterioration of physical and mechanical properties, partial or complete destruction of the product . The physicochemical effect on the material causes external and internal mechanical wear, which in this case proceed more intensively.

Thermal properties are manifested when acting on materials or finished products of thermal energy. The most important of them are heat capacity, coefficient of thermal expansion, thermal conductivity, heat radiation and heat absorption, heat-insulating properties, thermal stability, parameters of the change in the aggregate state.

Fire resistance, frost resistance and thermal insulation properties are most important for building materials; thermal resistance - for dishes; heat radiation and heat absorption - for household appliances; changes in the aggregate state of substances - to determine the storage conditions of fuels and lubricants, household chemicals, food.

The heat capacity (C, cal/hail) is the amount of heat (Q), needed to increase temperature (T) of the body at 1 ° C:

(13)

Specific heat (cal/g · deg or kcal/kg · deg) is the ratio of heat capacity to mass of a substance. The heat capacity at constant pressure is denoted as C p a at constant volume - C v . The heat capacity is significant for metals, insulating materials and other materials (Table 7.4).

Table 7.4

Specific heat of some substances and materials

Name

Material

Specific heat, cal/g · deg

Name

Material

Specific heat, cal/g · deg

Ice (0 °)

0.49

Copper

.091

Air

0.24

Brass

.092

Asbestos

0.195

Carbon steel

0.12

Concrete

0.21

Aluminum

0.20

Brick

0.2

Rubber

0.5

Glass

0.16

Cellulose

0.36

Porcelain

0.26

Wool

0.41

The coefficient of thermal expansion - is an important indicator for products in which it is associated with the judgment of strength, the correctness of their design (for metals, glasses, glassware); for products, the size of the parts of which affect the main function of the thing (details of particularly precise mechanisms, measuring tools).

Thermal conductivity is the ability of a material to conduct heat in the presence of a temperature difference between individual parts of the product. The amount of heat passing through the material layer is directly proportional to its area, the temperature difference between the two surfaces of the layer, the time and inversely proportional to the thickness of the material layer. In addition, the thermal conductivity depends on the properties of the material:

(14)

where F - the area of ​​the material; T1-T2 - temperature difference; t is the time; h - the thickness of the material; λ - is a coefficient that depends on the properties of the material, called the coefficient of thermal conductivity. The thermal conductivity of a substance depends on its state, i.e. temperature and pressure.

Depending on the accepted dimensions, the physical coefficient of thermal conductivity (λphysis) and technical (λ tech) are distinguished:

; (15)

; (16)

(17)

The metals have the highest thermal conductivity, so they are not used as thermal insulators. Materials with low thermal conductivity (up to 0.2 kcal/m · deg/hr) are called heat insulators (asbestos, felt, wood, leather, etc.). Air has the lowest thermal conductivity, so its presence in porous bodies sharply reduces the thermal conductivity of the latter. The thermal conductivity of hygroscopic materials increases sharply with increasing moisture content of the material, the thermal conductivity of the wet material being higher than the thermal conductivity of both the material and water. Thermal conductivity is an important property of fabrics, clothing, footwear, fur products, metal and glassware, building materials, etc.

Heat Radiation and Heat Absorption are related to the following phenomena. Thermal radiation is the result of the vibrational motion of atoms and molecules. When the material is heated, some of the thermal energy becomes radiant, the amount of which increases with the temperature of the product. To the greatest extent, the properties of thermal radiation have light and infrared rays with wavelengths from 0.4 to 400 ц (Table 7.5).

Table 7.5

Comparative wavelength

Radiation

Wavelength

Radiation

Wavelength

Radio waves

from 30 km to 0.4 mm = 400 m

Ultraviolet

Rays

0,4-0,01 ц

Infrared heat rays

400-0.76 c

X-ray beams

0.01-0.0001 cps

Light rays

0,76-0,4 ц

Gamma rays

(y-rays)

Radiation is inherent in all bodies, and each of them radiates energy continuously. Getting to the material, the radiant energy Q0 is partially reflected (Q1), partly absorbed (Q2) and partly passes through the body (Q3).

Thus:

(18)

(19)

Relationships , and characterize the degrees (reflection coefficients) of the reflection ( R ), absorbing (A) and missing ( D ) of the thermal rays. Most radiant energy is absorbed by absolutely black bodies, for them A = 1, R = 0 and D = 0. The luminous capacity of a material is the greater, the greater their absorbance. Accordingly, at any temperature, the emission of an absolutely black body is maximum.

The thermal properties of various materials (fabrics, leather, etc.) are determined not only by the heat conductivity of this material, but also by some of its other properties, isolated from the external environment by a heat insulator, occurs in three ways - heat conduction, radiation (thermal radiation) and convection.

The thermal insulation properties of materials are characterized by the heat transfer coefficient or the total heat loss through the material under investigation, which characterizes the total heat losses (by thermal conductivity, radiation and convection).

Thermal resistance characterizes the ability of a material or product to withstand, within certain limits, temperature changes without deterioration or a marked deterioration in their properties. It is an important property for glass and ceramic dishes, plastics. For example, some types of artificial fibers (acetate silk, etc.) do not withstand temperatures above 80-100 ° C; some materials under the influence of low temperatures sharply increase fragility, reduce strength. The thermal resistance of glassware is characterized by the temperature limit, which can be heated and cooled rapidly, by the number of heat exchangers held by the products. For glass products, the thermal resistance depends on the heat capacity, the coefficient of thermal expansion, thermal conductivity, the strength of the glass, the shape and shape of the product.

An important property of structural, combustible and lubricating materials used in construction is fire resistance, characterized by their ability to ignite and burn with a greater or lesser intensity. The least fire-resistant wood, paper, fabrics from vegetable fibers, many plastics, varnishes. Metallic and silicate products are the most fire resistant. The fire resistance of some materials is enhanced by special impregnation or by applying flame retardant coatings.

For flammable substances (gasoline, kerosene, lubricating oils, etc.), a significant characteristic is the flash point (the temperature at which the fumes of the combustible material flare with subsequent damping) and the ignition temperature (flash with subsequent combustion).

If the temperature changes, change the aggregate state of the material. This property is considered in the technological process of production, storage and use of finished products and is characterized by softening and melting temperatures for solids (metals, silicates, plastics) and boiling and solidifying temperatures for liquids (combustible and lubricating substances, solvents for paint and varnish goods, etc.). . The characteristics of the change in the aggregate state are also used in assessing the purity of the material, determining the presence of impurities in it.

Sorption properties (absorption, adsorption, chemisorption, capillary condensation) are physicochemical processes in which the material absorbs gases, vapors or substances from the environment. They affect the retentivity, strength, change in weight, volume, elongation, stiffness, thermal conductivity, plasticity, moisture of the material.

Sorption properties are of great importance for trading and manufacturing enterprises, since it is necessary to know the loss of moisture under shrinkage under certain conditions until the onset of "constant mass" (absolutely dry material), i.e. mass, which during the subsequent drying and weighing of the sample will not change. This is especially important for products made of leather, textiles, wood.

Adsorption - is the absorption of gases, vapors and solutes on the surface of solids. A solid body capable of absorbing gases, vapors or solute is called an adsorbent, called an adsorbate. The phenomenon, the reverse of the adsorption associated with the decrease in the amount of absorbed substance by the adsorbent is called desorption.

Absorption - is the absorption of matter due to its diffusion into the mass of the body and the introduction between the atoms and molecules of the sorbent. Capillary condensation occurs as a result of the fusion of liquid layers formed on the walls of the capillaries (pores) due to the adsorption of vapor.

Chemisorption - the absorption as a result of the chemical interaction of gas or vapor with a solid, occurring either on the surface of the solid phase, or propagating to the entire mass of the solid object.

When studying the properties of solid materials and the production processes of their processing, the greatest practical interest is the adsorption of gases, vapors or particles of dissolved substances. Hygroscopic properties of materials, individual processes of coloring, cleaning of oils and other processes are explained in whole or in part by adsorption phenomena.

The mechanism of adsorption on the surface of solids is very complex, and so far there is not a clear enough idea about it. This is explained, on the one hand, by the inhomogeneous activity of individual sections of the surface of a solid body, and on the other hand, by the fact that adsorption can be accompanied by chemisorption and other sorption phenomena. An important property is the sorption ability of substances, the amount of adsorbed substances (gas, vapor, dissolved substances) under various environmental conditions and the effect of adsorbed substances on the properties of the adsorbent. The main factors affecting the amount of adsorption are the nature of the components (adsorbent and adsorbable substance), the concentration of the adsorbed substance and the temperature. Adsorption is the higher the specific surface area of ​​the adsorbent. When adsorption from solutions, its value depends not only on the nature of the adsorbent, but also on the nature of the solvent and the adsorbed material.

The moisture contained in the materials can be in a different state: in the form of adsorbed moisture, condensed moisture in the capillaries (small pores) of the substance and in a chemically bound form (crystallization, constitutional water). In addition, the materials can contain drip-free, free water that mechanically fills the pores of the material.

The moisture content of the material is characterized by the ratio of the total moisture loss during drying (gb - g c) to the weight of the material after drying, (%):

(20)

where g in - the mass of the wet material; g c - the mass of the material after drying under appropriate conditions.

The moisture content (Wc) is determined by the ratio of the mass of moisture to the weight of the wet material and is calculated by the formula

(21)

Many materials have the ability to adsorb moisture

As already mentioned, moisture (due to adsorption, condensed, chemically bound moisture) of wood, leather, textile fibers, plastics is accompanied by a change in volume, mass, strength, elongation, softness, heat conductivity and other properties, but, in addition, it affects the biological stability and the material's antipollution properties.

The amount of adsorption moisture contained in the material depends mainly on the type of material (adsorbent properties), relative humidity and air temperature. As the relative humidity of the air increases (at a constant temperature), the amount of adsorbed moisture increases, and adsorption equilibrium occurs at higher moisture values ​​of the material.

The maximum hygroscopicity of the material is characterized by the moisture content of the material held to adsorption equilibrium under conditions of air humidity of 100% at a temperature of 20 ° C.

water absorption. The water absorption of a material depends on the hydrophilicity, the porosity of the material, the type and size of its pores, the adsorption capacity, etc. The water absorption is judged usually by weight gain after full saturation; it should be borne in mind that some part of the water can be absorbed in the form of chemically bound or adsorbed, part of the water will fill through or semi-closed pores, some of the water will only wet the surface of the material and the inner surface of large pores.

Total water absorption by weight (%) is calculated using the formula

(22)

where g c - the mass of absolutely dry material; g n is the mass of the material after one hundred saturation with water.

With the help of this indicator, both adsorbed (condensed, chemically bound) and droplet liquid moisture are taken into account.

Subtracting the value of the maximum hygroscopicity from the value of the total water absorption index, we get the water absorption of the drop-water by the material with the maximum adsorption moisture.

In the commodity practice for dry the material receives a sample with normal adsorption moisture, sometimes called "air-dry", and the weight of this sample is the weight gain of the sample after its saturation with water.

Water absorption by volume characterizes the degree of water filling of the total volume of the material and is expressed as the amount of absorbed water referred to the volume of the material:

(23)

because, where y is the bulk weight of the material.

Volumetric the water absorption can be represented as Gob = Gweight · γ. However, the volume water absorption can be less, since some of the pores are not completely filled with water.

The water absorption of various materials varies widely; so, weight water absorption of dense stone materials 0,2-0,7%, ordinary brick 8-20, a tree 20-200, fabrics 60-150% and more.

Permeability - This property of the materials passes through itself gas particles (gas permeability), air (air permeability), vapor (vapor permeability), dust (dust permeability), water (water permeability) .

These properties have textile, shoe, construction materials and products. The level of these properties affects the quality and safety of products. For example, the vapor permeability of clothing determines the normal hygienic state of the susceptible environment, normal heat exchange and heat regulation of the human body. The low water permeability of natural leather determines their use for the top of everyday shoes, the waterproofness of rubber and polymeric materials (polyethylene, polyurethane) allows them to be used in the manufacture of clothing and footwear that protect people from rain and moisture.

Let us consider in more detail certain types of permeability.

Breathability depends on the pressure, is experimentally determined, as a rule, at a pressure of 5 or 10 mm of water column by passing air from chamber 1 to chamber 2 through the air-permeable material being examined.

If the air pressure in the chambers is different (P1> P2, P 1 - P 2 = h), then air permeability Bh (ml/cm2 · s) is calculated using the formula

(24)

where F - the area of ​​the partition, cm2; V - volume of air flowing through the material under study, cm3; t - the time of passage of air through the partition, pp.

Bh is called the air permeability of the material at the pressure difference h.

Water vapor permeability - is the ability of a material to carry water vapor. In the case of static evaporation from an aqueous or humidified surface, water vapor is diffused in air by diffusion due to the created pressure drop of water vapor in the atmosphere and at the water surface. If there is a barrier in the way of diffusion in the form of a sample of the material, the diffusion slows down to a greater or lesser extent depending on the porosity of the material; simultaneously a part of the water vapor is adsorbed and is transferred to the external medium by desorption.

The rate of static evaporation in cylindrical vessels (the weight of water vapor evaporating in 1 cm2 of area per hour) depends on the pressure, temperature and relative humidity of the air, and also on the depth of the water level in the vessel. For the conditions t = 20 °, φ = 60%, P = 760 mm Hg. Art. at a distance of the water surface from the edges of the glass of 3 cm and a diameter of the glass of 8 cm, the amount of evaporated water is ~ 3 mg/cm2 · h.

With dynamic evaporation, when air flows are created at the water surface, passing through the sample under some pressure h, the water vapor passes through the pores of the sample along with the airflow.

In this case, with a steady parallel sorption process, the amount of water vapor passing along with the airflow will be proportional to the air permeability and relative humidity of air.

If V passes through a sample of F cm 2 in time t ml of air at the pressure h mm of the water column, then the amount of water vapor passing through the sample, together with the air stream having the temperature m,

(25)

where G S is the amount of mg vapor in 1 ml of air at full saturation for temperature τ; φ is the relative humidity of air in fractions of a unit.

Relative vapor permeability is the ratio of the amount of water vapor that has passed through the sample to the amount of water evaporated from an open cup of the same size over the same time.

The relative vapor permeability (P0,%) is calculated by the formula

(26)

where g1 is the weight of water vapor passing through the sample during the time t g2 is the weight loss in an open glass with water for the same time.

For tissues, the relative permeability varies between 20-50%.

Water permeability - is the ability of a material to flow water at a certain pressure. The resistance of the material to the penetration of water on the opposite side of the material is called water resistance.

Water permeability (V) is measured by the amount of water (ml) that has passed per hour through 1 cm2 of material and is calculated by the formula

where v is the amount of water in ml that passed through the sample during t; F - area of ​​the sample, cm2.

The water resistance of the material is characterized by the height of the column of water that the test specimen withstands without the water passing to the opposite side in the form of freely falling drops.

Dust permeability is a property important for filter materials, clothing fabrics, shoe materials, etc. In general, it depends on the same factors as air permeability, as well as on the characteristics of dust (composition, particle size, air dust, g/m3). Dust penetration is determined by increasing the weight of the sample through which dust is passed through the air for a certain period of time at a set differential pressure.

Optical properties (light refraction, light transmission, light reflection) are perceived by a person in visual perception. They determine primarily the aesthetic properties of goods. Optical properties of goods include their color, brightness, lightness, saturation.

These characteristics determine the appearance, perception of color, gloss, texture of the surface of products. Optical properties are important for jewelry products, glassware and ceramics, they are based on the work of electronic goods and video and photographic equipment.

Color along with form is the main element of visual perception when radiant energy acts on our eyes. The visible part of the spectrum lies within the wavelength range from 400 to 760 nm, and waves of different lengths cause different color sensations. In the spectrum, up to 130 colors can be distinguished, which can be connected in groups that are close in color tone.

Table 7.6

Colors and their respective wavelengths

Colors

Wavelength, im

Colors

Wavelength, nm

Reds

760-620

Green

530-500

Orange

620-590

Blue

500-470

Yellow

590-560

Blue

470-430

Yellow-green

560-530

Purple

430-380

If the material reflects the rays of all wavelengths of the spectrum in the same ratio, such colors are called achromatic - from white through gray to black - depending on the number of reflected beams. Full reflection gives an ideally white color, full absorption - ideally black. The most white color is barium sulfate, magnesium oxide. Plates pressed from these substances reflect 94-98% of the light incident on them; The surface of black velvet, which will give almost the most black color, reflects about 0.2% of the incident light. Achromatic colors differ from each other by the degree of reflection of light rays, or by lightness. Achromatic scale from white to black can be divided into a maximum of 300 degrees of light, which is determined by the sensitivity limit of our eyes.

Chromatic colors differ from each other in lightness and color tone (red, yellow, blue, etc.). The color of opaque bodies is determined as a result of selective absorption of certain rays of the spectrum or absorption of some wavelengths to a greater extent, others - to less. The color of the material is summed from the light waves that remain unabsorbed and reflected.

When light rays pass through a transparent material (glass, crystal, etc.), some of them are reflected from the surface of the body, a part is absorbed by the substance of the body and most of it is passed by the body.

The reflection coefficient (p) is the ratio of the reflected light flux S O to the total light flux S %:

(28)

The ratio of the light absorbed by the body S α to the total S, is the percentage of the absorption coefficient α).

(29)

where S α is the amount of light absorbed when passing through a 1 mm thick body.

The number of light rays transmitted through the body, referred to the total light flux and expressed in percent, characterizes the transparency of the body (τ):

where S τ - the number of light rays transmitted through a 1 mm thick body.

When light passes through transparent bodies that have selective absorption (for example, through colored glass), light rays of different wavelengths will be characterized by unequal absorption and transmission coefficients.

Red, orange, yellow and yellow-green colors are called warm & quot ;; they are considered more vivid, "catchy". Green-blue, blue, blue and blue-violet colors are called "cold", more calm, less prominent. As the intensity of illumination decreases, the chromatic colors gradually cease to differ; Previously, others disappear red, yellow and longer than others are kept blue and blue. In very low light, yellow and red colors darken, blue and blue, on the contrary, lighten. With a large intensity of light (in direct sunlight), all the colors become whitish and yellowish, with violet color changing more than others, and red colors less than others.

The color perception affects the background, which perceives the color of the object. For example, on a light background, gray and black colors appear darker; on the contrary, gray and white on a dark background appear lighter than, for example, on a gray background (light contrast). The same effect is obtained if light colors are perceived after viewing dark colors.

The brightness of the color tone increases even if it is perceived by the eye after viewing the dark tone; if you translate the eye from the white background to black, then black is perceived as a deeper. Chromatic colors, located on a colored background, change their color top depending on the background color. So, on a blue background red gets a yellow-orange tint, on a red background, yellow and blue appear greener, on a green background red changes towards purple, yellow - towards orange, orange - toward reddish.

Under the influence of light stimuli, vision "wears out", which affects the lowering of the sensitivity of the retina to this irritant, and therefore the ability to perceive a given color is reduced. Working with colored objects, it is necessary to rest the eye, because fatigue can lead to incorrect perception of color. For example, after a long work with the green color, at first the objects appear pinkish.

The color of the materials stained with the same dye will be different depending on the structure of the painted surfaces.

The visible structure of the surface of the material is called the texture of the material. The texture of the surface can be smooth and rough. This distinction is conditional, since surfaces with a low degree of roughness refer to smooth ones.

Reflection of light from a smooth surface occurs in a directed manner, without scattering of light rays, due to which such surfaces have glare ( glare ) and are called glossy, shiny, mirror. When reflected from a rough surface, the light is dissipated, such a surface does not give light reflections and appears matte. Colors on glossy, shiny surfaces are characterized by greater brightness, greater lightness; matte surface, painted in the same color, is more dark, painted in darker colors.

Different colors in different degrees emphasize or obscure the structure of the surface. For example, white color, as well as warm colors more than others reveal the surface of the material; cold & quot ;, more saturated colors, hide the texture. If the fabric is made from coarse yarn, it has external defects, then surface defects will be less noticeable when the fabric is dyed in dark colors, which to some extent hide the texture of the fabric.

The perception of the color of the material depends on the light source, i.e. from the composition of the light flux incident on the painted surface. For example, in the light of an electric lamp with more yellow and less blue and blue rays in the composition of the light than in the solar spectrum, the colors change: the yellow colors become more saturated, the red colors become lighter, they become orange, the orange becomes yellowish, the blue slightly darken or acquire a green tint, dark blue darkens strongly, dark blue becomes difficult to distinguish to black, lilacs get pinkish tinge, and violet - red.

Changes in colors depending on lighting should be considered when evaluating the color of materials.

Ultraviolet radiation is a kind of optical radiation characterized by wavelengths in the range of 10-400 nm. The shortwave region of ultraviolet radiation (10-180 nm) is strongly absorbed by all known materials and media (solids, liquids, air). The far field of ultraviolet radiation (180-275 nm) has a bactericidal effect, is used in special light sources to sterilize air and water, improve the storage of food. In addition, this radiation can ozonize the air. Ultraviolet lamps, giving a wavelength of 254 nm, have bactericidal efficacy, have the greatest effect on DNA, cause dimerization of thymine in DNA molecules. The accumulation of such changes in the DNA of microorganisms leads to a slowdown in the rate of their multiplication and extinction.

The ultraviolet region of the spectrum (290-400 nm) increases the tone of the sympathetic-adrenaline system, activates protective mechanisms, raises the level of nonspecific immunity, and also increases the secretion of a number of hormones. Under the influence of ultraviolet radiation, histamine and the like are formed, which have a vasodilating effect, increase the permeability of the skin vessels. The carbohydrate and protein metabolism in the body changes.

The effect of optical radiation changes pulmonary ventilation - the frequency and rhythm of breathing; increased gas exchange, oxygen consumption, the activity of the endocrine system is activated.

The average area of ​​ultraviolet radiation (275-320 nm) is characterized by an anhydrite effect on organisms, the ability to form vitamin D in the subcutaneous cells, a beneficial effect on the growth of poultry and animals , and also erythemal effect, i.e. ability to cause reddening and sunburn of human skin.

Near field of ultraviolet radiation (320-400 nm) contains radiation widely used for luminescent analysis, and also for excitation of luminous substances in signal, decorative and other devices.

The division of the spectrum into the listed areas is not accurate, since the properties of ultraviolet radiation attributed to one region are inherent in often and neighboring regions, albeit to a lesser extent.

In practical activity, ultraviolet radiation is conventionally divided into the regions indicated in Table. 7.7.

Table 7.7

Types of ultraviolet radiation

Name

Wavelength, nm

Ultraviolet A (long-wave range of UVA)

400-315

Ultraviolet B (medium range UVB)

315-280

Ultraviolet C (short-wave range UVC)

280-100

Long-term lack of ultraviolet radiation causes light fasting. The most frequent manifestation of this disease is a violation of mineral metabolism, reduced immunity, fast fatigue, etc.

Ultraviolet radiation, when the natural protective ability of the skin is exceeded, can have a negative effect on the skin (tanning) and cause burns, prolonged action of the ultraviolet promotes the development of melanoma, various types of skin cancer, causes a typically radiation damage to the eyes (a retinal burn).

Acoustic properties represent the ability of materials and products to emit, absorb and conduct sound. These qualities are the basis for using household audio and video equipment, wire and wireless communications, musical instruments, building sound-proof materials, electrical appliances and other products.

When working with musical instruments, audio equipment and other goods, the acoustic field influences the hearing of a person. It is characterized by the frequency of elastic vibrations, the spectrum and speed of sound, the amplitude, the wave and the specific electrical resistance of the medium. The parameters of acoustic properties are sound pressure, sound power, tone and other characteristics.

Depending on the type of goods, various acoustic indicators are used to characterize its performance characteristics by commodity experts: height, strength, frequency, timbre of sound, sound absorption, sound permeability, etc.

Sound is an oscillation that propagates wavy in an elastic medium and is perceived by the ear. Sound vibrations that lie beyond the threshold of hearing are called infrasonic (in the low frequency region, less than 16 Hz) and ultrasonic (in the high frequency range, more than 16-20 000 Hz). In gases and liquids sound propagates in the form of longitudinal waves - alternating condensations and rarefactions of the elastic medium; In solid bodies, shear waves, shear waves, etc. can also appear.

The length of the sound wave, i.e. The distance between two neighboring points of space that are currently in the same sound mode (for example, between two rarefactions or two condensations) is related to the frequency and speed of sound by a simple relationship:

(31)

where λ is the wavelength, m; C is the speed of sound, m/s; f is the oscillation frequency, Hz (number of vibrations per second).

The speed of propagation of sound vibrations depends on the properties and the state of the environment in which sound propagates - from its density, elasticity and temperature (Table 7.8).

Table 7.8

Audio propagation speeds in various environments

Environment

Speed ​​of sound propagation, m/s

Environment

Sound Propagation Rate, m/s

Air, t = 0 °

331.7

Steel

5000

Air, t = 22.5 °

344.7

Lead

1320

Distilled water, t = 13 °

1441

Brick

3650

Tree (of various kinds)

3360-5300

Cork

430-530

Glass

5950

Rubber

54-69

Sound sensations are usually divided into two groups - tops and noises. When the sound pressure varies in time according to the sinusoidal law, the correct periodicity of oscillations takes place; the corresponding sounds are perceived by the hearing as elementary simple sounds, they refer to pure tones. The collection of pure gons forming a complex sound is called the sound spectrum. Noise is a term in technical acoustics that designates a complex sound with a large number of constituent frequencies that does not have a periodicity. For noise sounds, the distribution of sound energy in a wide range of frequencies and amplitudes of oscillations is characteristic, the presence of shock sounds in a number of cases (machine noise, street noise, etc.).

The top is characterized by height, strength and timbre or shade (frequency, amplitude, and shape). The pitch of a musical sound is determined by the frequency of the oscillations - their number per unit of time. The area of ​​musical tones lies approximately in the range of 16-16 000 vibrations per second, accessible to human hearing.

Strength, or intensity, sound refers to the average sound power passing through a unit of surface perpendicular to the direction of propagation sound. The unit of sound strength is erg per second per square centimeter (erg/s · cm2) or watt per square centimeter (W/cm2), which is equal to 107 erg/s · cm2. The level of sound intensity, or noise, (3 is expressed in conventional units - decibels (dB), which show how much the sound (noise) I exceeds the sound power unit I0, t the sound strength at the threshold of audibility (I0 is assumed to be 10-16 W/cm2) (Table 7.9).

Table 7.9

Characteristics of sound levels

Sound level, dB

Sound Strength, W/cm 2

Note

0

10-16

Audibility threshold

10

10-13

The rustling of leaves with a weak wind

30

10-13

Whispering at a distance of 1 m

50

10-11

Talking in a low voice

70

10-9

Electric vacuum cleaner

90

10-7

Tram in a narrow street

130

10-3

Painful sensation

The sound intensity level is calculated as a tenfold logarithm of the ratio :

(dB). (32)

The increase in the sound level by 1 dB corresponds to a 26% increase in the sound power. This is approximately the smallest change in the sound force that is picked up by the ear.

On the strength of sound expressed by certain physical quantities, it is necessary to distinguish loudness of sound - its subjective quality, determined by auditory sensations.

A 10 dB change in sound level is subjectively perceived as approximately a two-fold change in the sound volume regardless of the original level. Thus, the sound volume at 70 dB will be 4 times greater than at 50 dB.

The timbre of sound characterizes the color of the sound at the same height. The musical sound has along with the fundamental frequency a number of so-called overtones with frequencies corresponding to different harmonic components of the given sound. Depending on the ratio in the height of the amplitudes of its harmonic components, the tone, or color, of the sound is determined.

Reflection of sound, soundproofing also represent the most important properties of materials. Sound waves are reflected, refracted according to the same basic laws as reflected and refracted by light rays (waves).

If the sound wave I falls on the plate A , a portion of the sound energy I otr is reflected, part of the energy passes into the material, with some of its quantity I absorbed, and part I pr passes through the plate.

The ratios of the reflected, absorbed and transmitted energy to the incident are called the corresponding coefficients:

reflection coefficient:

(33)

absorption coefficient:

(34)

sound transmission coefficient, or sound conductivity:

(35)

These indicators are used in commodity science when characterizing materials for musical instruments, soundproof materials, etc.

High values ​​of reflection are characterized by metals, wood, silicate materials; good sound absorbers are various porous and fibrous materials (felt, cotton wool, fabrics, especially pile). Absorption of sound waves by the material occurs to different degrees for waves of different frequencies (Table 7.10).

Table 7.10

Sound Absorption Coefficients for Some Materials

The name of the material and its thickness, mm

Sound Absorption Rate

Sheet metal h = 1

0.002

Ordinary brick wall

0.032

Tree

0.06-0.1

Rubber carpet h = 5

0.1

Technical Felt h = 10

0.3

Technical Felt h = 25

0.5

Vata loosely h = 50

0.7

The absorption coefficient is defined as the ratio of the power absorbed to the incident power.

The ability to penetrate sound from one part of space to another is characterized by the coefficient of sound conductivity t. Usually use the reverse logarithmic value, called the sound insulation factor (Table 7.11), expressed in decibels, i.e.

(36)

Table 7.11

Sound insulation factors for some materials

The name of the material and its thickness, mm

Sound insulation factor, db

Aluminum h = 0.6

16

Steel h = 2

33

Three-ply plywood h = 3.2

19

Oak door h = 45

20-25

Wooden room partition

30-50

Windows, double frame with a gap of 24 cm

46

Electrical properties are basic for materials used in electrical engineering, for electrical and electronic products. The main ones are electrical resistance and electrical conductivity, electrical permeability and dielectric properties (Table 7.12). Depending on these properties, the materials are divided into conductors, semiconductors and insulators.

Table 7.12

Electrical resistance and electrical conductivity of metals and alloys

Metal or alloy

p · 10 4

γ · 10 4

Aluminum

.027

37

Bronze (Cu 88, Sn 12)

0.02

50

Iron

0.1

10

Gold

.022

45

Constantan

0.5

2.0

Brass (Si 66, Zn 34)

0.064

15.6

Manganine

0.43

2.3

Copper

0.017

59

Nickeline

0.4

2.5

Nickel

0.07

14

Nichrome

1.0

1.0

Platinum

0.11

9.0

Lead

0.21

4.8

Silver

0.016

62.0

Fechral

1.2

0.83

Cast iron

0.5

2.0

Conductors have a low resistivity, of the order of 10-6-10-4 Ohm · cm, and, correspondingly, high electrical conductivity . Conductors include metals and alloys (silver, copper, aluminum, tin, bronze), which are used for the manufacture of microcircuits, wiring accessories, wires and cords.

High-resistance conductors with low oxidizability (fekhrali (iron, chromium and aluminum alloy), nichrome (nickel, chromium, iron alloy)) are important. They are used as heating elements of electrical household goods.

Semiconductors have high conductivity - 101-1010 Ohm · cm, but also a rather high electrical resistivity. Free electrons in a semiconductor appear as a result of thermal motion, which distinguishes semiconductors from metals in which the ability to change their motion under the influence of electrical forces and, consequently, to carry current is inherent in electrons regardless of the presence of thermal energy of the atoms of the body.

Semiconductors have Si, C, S, Se, As, alloys AlSb, Mg2Sn, oxides A12O3, ZnS, Cu2O, sulfides ZnS and Cu2S, etc.

Semiconductor materials are used in the production of electronic equipment, wiring and electrical goods. In household appliances, semiconductors are used to convert AC to DC (in power supplies and chargers), amplify high-frequency oscillations and generate radio waves (in telephones, radio stations), regulate current and voltage (in electrical household goods), protect against overvoltages and lightning discharges lines of high-voltage transmissions, the creation by means of electric current of heat or cold, the concentration of electric and magnetic energy, the transformation of sound energy into electrical energy, ƃ and others.

Insulators - are materials that have a high resistivity (1011-1018 Ohm-cm), therefore they are used as electrical insulating materials. These include glass, porcelain, rubber, plastics, mica and so on.

The electromagnetic radiation created by the goods (wireless communication means) is conventionally divided into two zones - the induction zone (next to the transmitter or emitter) and the wave (far) zone, which lies outside the antenna field. Electromagnetic radiation is divided by frequency ranges (Table 7.13), between which there are no abrupt transitions, they sometimes overlap and have only conditional boundaries.

Radio waves are divided into super long, long, medium, short and ultrashort. In turn, ultrashort radio waves are usually divided into meter, decimeter, centimeter, millimeter and submillimeter or micrometer. Waves of length λ & lt; 1 m ( v & gt; 300 MHz) is also referred to as microwaves, or ultrahigh-frequency (UHF) waves.

Table 7.13

Frequency bands of electromagnetic radiation

Range name

Wavelengths, λ

Frequencies, v

Sources

Radio

waves

Extra long

more than 10 km

<30 kHz

Atmospheric phenomena, alternating currents in conductors and electronic flows

Long

10-1 km

30-300 kHz

Avg

1 km - 100 m

300 kHz - 3 MHz

Short

100-10 m

3-30 MHz

Ultra-short

10 m - 2 mm

30 150 MHz

Optical radiation

Infrared radiation

2 mm - 760 nm

150 MHz -

42.9 THz

Radiation of molecules and atoms under thermal and electrical influences

Visible Radiation

760-400 im

42.9-75 THz

Ultraviolet

400-10 nm

7.5x1013Hz -

3x1016Hz

Radiation of atoms under the action of accelerated electrons

Ionizing electromagnetic radiation

X-ray

10-5 x 10: t nm

3x1016 -

6x1019Hz

Atomic processes under the action of accelerated charged particles

Gamma

less than 5 x 10-3 nm

more

6x1019Hz

Nuclear and cosmic processes, radioactive decay

Radio waves arise when AC current flows through the conductors of the corresponding frequency. Conversely, an electromagnetic wave passing through space excites an alternating current corresponding to it in the conductor. This property is used in radio engineering when designing antennas.

Ionizing radiation - different types of microparticles and physical fields that can ionize a substance. Radiation (from the Latin radius) is any radiation. In practice, radiation is called ionizing radiation. Ionizing electromagnetic radiation includes X-ray and gamma radiation, but ultraviolet radiation and visible light can ionize atoms.

The boundaries of the X-ray and gamma-ray regions can be determined conditionally - the energy of X-ray quanta lies within the limits of 20 eV - 0.1 MeV, and the energy of gamma quanta is more than 0.1 MeV. The gamma radiation is emitted by the nucleus, and the X-ray is emitted by the atomic electron shell when the electron is knocked out from low lying orbits; rigid radiation generated without the participation of atoms and nuclei (for example, synchrotron or bremsstrahlung) does not fit into these definitions. The radiation hazard of consumer goods will be discussed below.

In the International System of Units (SI), the unit of absorbed dose is gray (Gy), numerically equal to a ratio of 1 J to 1 kg. Previously, the exposure dose of radiation was also widely used - a quantity that shows what charge produces a photon (gamma or X-ray) radiation per unit volume of air. The most commonly used unit of exposure dose was X-ray (P), numerically equal to 1 CGS-unit charge per 1 cm3 of air.

Biological properties of food and non-food products are manifested in resistance to negative effects of microorganisms, fungi, algae, insects, rodents.

Goods consisting of organic substances of natural origin (food, cosmetics, wood materials, textiles, leather and fur products, etc.) are subject to substantial damage and destruction by microorganisms, fungi, insects and rodents.

Goods from synthetic and artificial organic materials - plastic masses, fibrous synthetic materials - are more resistant to the action of biological pests. However, materials known to be subject to significant effects are known, for example articles made from polyvinylchloride are subjected to destruction by rodents.

Products consisting of inorganic materials, biological pests are practically not destroyed, or are destroyed to an insignificant extent.

The degree of negative impact on goods depends on the state of the environment that affects the vital activity of microorganisms (temperature, humidity, pH). The effect of microorganisms and fungi on products is manifested in the deterioration of aesthetic properties, changes in taste, molding, rotting and subsequent destruction of products. With an increase in humidity, the activity of the activity of mold fungi increases. For most microorganisms, the optimal temperature is 25-40 ° C. Microorganisms, fungi, rodents can have a negative effect on the goods at all stages of the product's life cycle - during production, during transportation, storage and use or consumption.

The persistence of goods to the action of living organisms is enhanced by special methods of treatment, for example impregnation with antiseptic compounds (tissues, wood, paper, leather), preservatives (cosmetics and food products), impregnation with silver (fabrics, cosmetics, household appliances ), placing the goods in an impermeable packaging with evacuation (food).

For example, the resistance of wood materials to fungi, mold and insects depends on the content of resinous and tannic substances. Wood species, depending on biological resistance are divided into resistant (yew, chestnut, oak, karagach, larch), medium-hardy (pine, cedar) and low-resistant (birch, beech, aspen , linden, maple, etc.) of the rock.

To increase rottenness, wood is impregnated, coated with water soluble (sodium fluoride, sodium fluoride, zinc chloride, ammonium silicofluoride and other preparations) and water insoluble antiseptics (dry distillation products of coal, peat and wood: creosote, anthracene and shale oil).

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