Rubber production and properties. Rubbers for special purposes. Durability and fatigue endurance of rubbers

Rubber is a well-known material that is used in almost all spheres of human life. Medicine, agriculture, industry cannot do without this polymer. In many production processes rubber is also used. What this material is made of and what are its features are described in the article.

What is rubber

Rubber is a polymer with high elasticity. Its structure is represented by randomly arranged carbon chains held together by sulfur atoms.

In the normal state, carbon chains have a twisted appearance. If the rubber is stretched, the carbon chains will unwind. The ability to stretch and quickly return to its original shape has made such material as rubber indispensable in many areas.

What is it made from? Typically, rubber is made by mixing rubber with a vulcanizing agent. After heating to the desired temperature, the mixture thickens.

The difference between rubber and rubber

Rubber and rubber are high molecular weight polymers obtained naturally or synthetically. These materials are different physical and chemical properties and production methods. Natural rubber is a substance made from the sap of tropical trees - latex. It flows out of the cortex when it is damaged. Synthetic rubber is obtained by polymerization of styrene, neoprene, butadiene, isobutylene, chloroprene, nitrile. During the vulcanization of artificial rubber, rubber is formed.

What are they made of different types rubbers? For certain types of synthetic materials, organic substances are used, which make it possible to obtain a material identical to natural rubber.

Rubber properties

Rubber is a versatile material that has the following properties:

1. High elasticity - the ability to large reverse deformations in a wide temperature range.
2. Elasticity and stability of forms at small deformations.
3. Amorphous - easily deformed with slight pressure.
4. relative softness.
5. Does not absorb water well.
6. Strength and wear resistance.
7. Depending on the type of rubber, rubber can be characterized by water, oil, gasoline, heat resistance and resistance to action. chemical substances, ionizing and light radiation.

Rubber eventually loses its properties and loses its shape, which is manifested by destruction and a decrease in strength. The service life of rubber products depends on the conditions of use and can range from several days to several years. Even with prolonged storage, rubber ages and becomes unusable.

Rubber production

Rubber is made by vulcanizing rubber with the addition of mixtures. Typically, 20-60% of the processed mass is rubber. Other components of the rubber compound - fillers, vulcanizing agents, accelerators, plasticizers, antioxidants. Dyes, fragrances, modifiers, fire retardants and other components can also be added to the composition of the mass. The set of components is determined by the required properties, operating conditions, the technology of using the finished rubber product and economic calculations. In this way, high-quality rubber is created.

What are rubber semi-finished products made of? For this purpose, the production uses the technology of mixing rubber with other components in special mixers or rollers designed for the manufacture of semi-finished products, followed by cutting and cutting. The production cycle uses presses, autoclaves, drum and tunnel vulcanizers. The rubber compound is given high plasticity, due to which the future product acquires the necessary shape.

Rubber products

Today, rubber is used in sports, medicine, construction, agriculture, in production. The total number of products made from rubber exceeds more than 60 thousand varieties. The most popular of them are seals, shock absorbers, tubes, oil seals, sealants, rubberized coatings, and facing materials.

Rubber products are widely used in manufacturing processes. This material is also indispensable in the production of gloves, shoes, belts, waterproof fabrics, transport tapes.

Most of the rubber produced is used to make tires.

Rubber in the tire industry

Rubber is the main material in the production of car tires. This process begins with the preparation of a rubber compound from natural and synthetic rubber. Then silica, soot and others are added to the rubber mass. chemical components. After thorough mixing, the mixture is sent to the oven. The output is rubber bands of a certain length.

On next step cord rubberizing occurs. Textile and metal cord is filled with hot rubber mass. In this way, the inner, textile and belt layer of the tire is made.

What is tire rubber made of? All tire manufacturers use different formulations and rubber manufacturing technologies. To give the finished product strength and reliability, various plasticizers and reinforcing fillers can be added.

Tires are made from natural rubber. Its addition to the rubber compound reduces tire heating. Most of the rubber compound is synthetic rubber. This component gives the tires elasticity and the ability to withstand heavy loads.

Rubber(from lat. resina - resin) (vulcanizate), an elastic material resulting from natural and synthetic rubbers. Represents a mesh elastomer - a product of cross-linking of rubbers by chemical bonds.

Getting rubber

Rubber obtained mainly by vulcanization of compositions (rubber compounds), the basis of which (usually 20-60% by weight) are rubbers. Other components of rubber compounds are vulcanizing agents, accelerators and vulcanization activators (see), anti-aging agents, (softeners). The composition of mixtures may also include regenerate (plastic product of regeneration rubber, capable of revulcanization), retarders, modifiers, fragrances and other ingredients, the total number of which can reach 20 or more. The choice of rubber and composition is determined by the purpose, operating conditions and technical requirements to the product, production technology, economic and other considerations (see , ).

Technology for the production of products from rubber includes rubber with ingredients in mixers or on rollers, production of semi-finished products (cast profiles, calendered sheets, rubberized fabrics, cord, etc.), cutting and cutting of semi-finished products, assembly of workpieces of complex design or configuration using special assembly equipment and vulcanization of products in devices of periodic (presses, boilers, autoclaves, shapers-vulcanizers, etc.) or continuous action (tunnel, drum, and other vulcanizers). At the same time, high rubber compounds are used, thanks to which they are given the shape of the future product, fixed as a result of vulcanization. Molding in a vulcanizing press and is widely used, in which the molding and vulcanization of products are combined in one operation. The use of powdered rubbers and compositions and the production of molded rubbers by liquid molding methods from compositions based on are promising. With mixtures containing 30-50% by weight S based on rubber, get .

Rubber properties

Rubber can be considered as cross-linked, in which the rubber is the dispersion medium, and the dispersed phase. The most important property rubber- high elasticity, i.e. the ability to large reversible over a wide temperature range (see).

Rubber combines the properties (elasticity, shape stability), (amorphousness, high deformability at low volumetric compression) and (increase in elasticity of vulcanization nets with increasing temperature, entropy nature of elasticity).

Rubber- relatively soft, almost incompressible material. The complex of its properties is determined primarily by the type of rubber (see Table 1); properties can change significantly when rubbers of different types are combined or modified.

Elastic modulus rubber of various types at small deformations is 1-10 MPa, which is 4-5 orders of magnitude lower than for steel; the Pausson coefficient is close to 0.5. Elastic properties rubber are nonlinear and have a pronounced relaxation character: they depend on the loading mode, magnitude, time, speed (or frequency), repetition of deformations, and temperature. Reversible stretch strain rubber can reach 500-1000%.

Lower limit of high elasticity temperature range rubber is mainly due to the glass transition temperature of rubbers, and for crystallizing rubbers it also depends on temperature and speed. Upper temperature limit of operation rubber associated with the thermal stability of rubbers and cross chemical bonds formed during vulcanization. Unfilled rubber based on non-crystallizing rubbers have a low . The use of active fillers (highly dispersed, SiO 2, etc.) makes it possible to increase the strength characteristics by an order of magnitude rubber and reach the level of indicators rubber from crystallizing rubbers. rubber is determined by the content of fillers and plasticizers in it, as well as the degree of vulcanization. Density rubber calculated as the volume-weighted average of the densities of the individual components. In a similar way, they can be approximately calculated (for bulk filling less than 30%) thermophysical characteristics rubber: thermal expansion coefficient, specific volumetric heat capacity, thermal conductivity coefficient. Cyclic deformation rubber accompanied by an elastic hysteresis, which determines their good damping properties. rubber are also characterized by high frictional properties, wear resistance, tear and fatigue resistance, heat and sound insulation properties. They are diamagnetic and good dielectrics, although conductive and magnetic ones can be obtained. rubber.

rubber slightly absorb water and swell to a limited extent in organic solvents. The degree of swelling is determined by the difference in the solubility parameters of rubber and solvent (the smaller, the higher this difference) and the degree of cross-linking (the equilibrium swelling value is usually used to determine the degree of cross-linking). known rubber, characterized by oil, benzo, water, steam and heat resistance, resistance to chemically aggressive environments, ozone, light, ionizing radiation. During long-term storage and operation rubber are subject to aging and fatigue, leading to a deterioration in their mechanical properties, a decrease in strength and destruction. Life time rubber depending on operating conditions from several days to several tens of years.

Rubber classification

By appointment, the following main groups are distinguished rubber: general purpose, heat-resistant, frost-resistant, oil and petrol resistant, resistant to chemically aggressive environments, dielectric, electrically conductive, magnetic, fire-resistant, radiation-resistant, vacuum, friction, food and medical purpose, for tropical climate conditions, etc. (Table 2); they also get porous, or spongy (see), colored and transparent rubber.

Application of rubber

rubber widely used in engineering, agriculture, everyday life, medicine, construction, sports. The range of rubber products includes more than 60 thousand items. Among them: tires, conveyor belts, drive belts, sleeves, shock absorbers, seals, seals, cuffs, rings, etc., cable products, shoes, carpets, tubes, coatings and facing materials, rubberized fabrics, sealants, etc. More than half of the volume generated rubber used in the manufacture of tires.

Rubber is a product of special treatment (vulcanization) of a mixture of rubber and sulfur with various additives.

Rubber differs from other materials in its high elastic properties, which are inherent in rubber, the main initial component of rubber. It is capable of very large deformations (relative elongation reaches 1000%), which are almost completely reversible.

The peculiarity of rubber is its low compressibility, relaxation nature of deformation, at room temperature the relaxation time can be -10 ~ 4 s or more, high abrasion resistance, gas and water resistance, chemical resistance, electrical insulating properties and low density.

Composition and classification of rubbers. The basis of rubber is natural rubber (NC) or synthetic rubber (SC), which determines the basic properties of the rubber material. To improve the physical and mechanical properties of rubbers, various additives (sulfur and selenium) are introduced.

Antioxidants(antioxidants) slow down the aging process of rubber, which leads to deterioration of its performance properties. There are chemical antioxidants (aldolneozone is used) and physical action (paraffin, wax) that form surface protective films.

Softeners(plasticizers) facilitate the processing of the rubber mixture, increase the elastic properties of rubber, increase the frost resistance of rubber. Paraffin, vaseline, stearic acid, bitumen, dibutyl phthalate, and vegetable oils are used as softeners. The amount of softeners 8 - 30% by weight of rubber.

Fillers according to the effect on rubber, they are divided into active (reinforcing) and inactive (inert). Reinforcing fillers (carbon black and white soot - silicic acid, zinc oxide, etc.) increase the mechanical properties of rubber: strength, abrasion resistance, hardness. Inactive fillers (chalk, talc, barite) are introduced to reduce the cost of rubber.

Often, regenerate is introduced into the composition of the rubber mixture - a product of the processing of old rubber products and rubber production waste. In addition to reducing the cost, the reclaim improves the quality of rubber, reducing its tendency to ageing.

By purpose, the following main groups of rubber are distinguished:

general purpose and special purpose, including:

heat resistant,

frost-resistant,

oil and petrol resistant,

Resistant to the action of chemically aggressive media, including resistant to hydraulic fluids,

dielectric,

conductive,

magnetic,

fire resistant,

radiation resistant,

vacuum,

Friction (wear-resistant*),

Food and medical purposes,

for tropical and other climate conditions

By types:

Porous or spongy

Colored and transparent rubbers.

Physical and mechanical properties of rubbers.

Wear resistance - the main indicator of wear resistance is abrasion and abrasion resistance. Abrasion (defined as the ratio of the decrease in sample volume during abrasion to the work expended on abrasion. Abrasion resistance (defined as the ratio of work expended on abrasion to a decrease in sample volume during abrasion.

The fatigue-strength properties of rubbers are determined by their fatigue when, under the action of mechanical stresses, destruction occurs. Fatigue is also promoted by exposure to light, heat, aggressive environments, which cause aging. The number of loading cycles that a sample can withstand without breaking is called fatigue endurance. Almost not subject to ozone cracking rubber based on butyl rubber and chloroprene rubber. To achieve high fatigue strength, high strength, low internal friction and high chemical resistance of rubber are required. At elevated temperatures(150°C) organic rubbers lose their strength after 1-10 hours of heating, rubbers on SKT can work for a long time at this temperature. The strength of siloxane rubber at room temperature is less than that of organic rubbers, however, at 200°C, the strengths are the same, and at a temperature of 250–300°C, the strength is even higher. The impact on rubber of negative temperatures causes a decrease and even complete loss of highly elastic properties, a transition to a glassy state and an increase in its rigidity by thousands and tens of thousands of times.

Rubber aging is observed during storage and operation of rubber products under the influence of non-mechanical factors. The aging test is carried out both in natural and in artificial conditions. The aging process affects rubber differently.

Types of rubber compounds

Natural (NK) and synthetic isoprene (SKI). The density of rubbers is 910-920kg/m 3 , tensile strength is 24-34MPa, relative elongation is 600-800%. Isoprene rubbers are used in the production of conveyor belts, molded products, sponge medical and other products.

Butadiene (SKD). Rubber density 900-920kg/m 3 , tensile strength 13-16MPa, relative elongation 500-600%. SKD has high frost resistance and abrasion resistance. Rubber compounds based on SKD are poorly processed by extrusion and calendering. Mixtures based on SKD are characterized by low stickiness. SKD is inferior to NK in terms of the strength of vulcanizates.

Butyl rubber (BR) resistant to oxygen, ozone and other chemicals. Rubber has high abrasion resistance and high dielectric performance. In terms of temperature resistance, it is inferior to other rubbers. Main physical property BC is an unusually high gas and moisture resistance. The inner tube of this material retains air 10 times longer than a natural rubber tube. Butyl rubber is widely used as a rubber for general and special purposes. In the production of rubber goods from BR, steam pipeline hoses, conveyor belts and rubber technical parts are produced, which require increased heat, steam, ozone and chemical resistance. BC is used for the manufacture of electrical insulating rubbers, various rubberized fabrics and linings of chemical equipment. BR rubbers are used in parts of milking machines and in the food industry.

Styrene-butadiene (SBS) and methylstyrene-butadiene (SMS) rubbers. Rubber density 919-920kg/m 3 , tensile strength 19-32MPa, relative elongation 500-800% Rubbers based on styrene-butadiene and methylstyrene-butadiene rubbers have high abrasion resistance. Rubbers from these rubbers are widely used in the production of conveyor belts for covering rubbers, various rubber goods.

Urethane (SKU) / Polyurethane have high strength, elasticity, abrasion resistance, oil and petrol resistance. Resistant to oxygen and ozone, its gas impermeability is 10 - 20 times higher than that of NK. Urethane rubbers are resistant to radiation. Rubbers based on SKU are used for car tires, conveyor belts, lining of pipes and gutters for transporting abrasive materials, shoes, etc.

Polysulfide (PSK) Thiokol. Resistant to fuels and oils, to the action of oxygen, ozone, sunlight. It has high gas impermeability - good sealing material, good performance aging, high tear resistance. Aqueous dispersions of thiokols are used for sealing reinforced concrete tanks. The mechanical properties of rubber based on thiokol are low.

Acrylate (AK) / Polyacrylate. The advantage of acrylate rubber is resistance to the action of sulfur-containing oils at high temperatures; they are widely used in the automotive industry. They are resistant to oxygen, quite heat resistant, have adhesion to polymers and metals. Distinctive properties of acrylic rubbers are their high heat and oil resistance. Acrylate rubbers are used for various heat and oil resistant sealing products (for example, oil seals, rings, gaskets), hoses, diaphragms, protective coatings, gumming equipment, sticky tapes; for the manufacture of products operating in abrasion conditions: various molded products, printing rollers, pipeline linings and chutes through which abrasive materials are transported, etc.

The disadvantages are low frost resistance, low resistance to hot water and steam.

Siloxane-Silicone (SKT). Rubber density 1700-2000kg/m 3 , tensile strength 35-80MPa, relative elongation 360%. SKT - heat-resistant synthetic rubber. They are used as elastic materials for special purposes in various industries, many fields of technology. Siloxane rubbers are used for the manufacture of seals, membranes, profile parts for sealing doors and windows, aircraft cabins, as well as flexible joints that can withstand very low temperatures in the high layers of the atmosphere, significant concentrations of ozone and solar radiation. Their aging resistance and dielectric performance are also very high.

The high heat resistance of rubbers made from silicone rubber allows them to be used also for the manufacture of rubber-metal vibration isolators (shock absorbers), air duct antivibrators, spark plug shells, searchlight seals, etc.

Rubber is a product of special treatment (vulcanization) of a mixture of rubber and sulfur with various additives.

Rubber as a technical material differs from other materials in its high elastic properties, which are inherent in rubber, the main initial component of rubber. It is capable of very large deformations (relative elongation reaches 100%), which are almost completely reversible. At normal temperatures, rubber is in a highly elastic state, and its elastic properties are maintained over a wide temperature range.

The temperature modulus lies within 1-10 MPa, i.e. it is thousands and tens of thousands of times less than for other materials. A feature of rubber is its low compressibility (for engineering calculations, rubber is considered incompressible); Poisson's ratio is 0.4-0.5, while for metal this value is 0.25-030. Another feature of rubber as a technical material is the relaxation nature of the deformation. At normal temperature, the relaxation time can be 10 4 s or more. During the operation of rubber under conditions of repeated mechanical stresses, part of the energy perceived by the product is lost to internal friction; this friction is converted into heat and is the cause of the hysteresis loss. During the operation of thick-walled parts, due to the low thermal conductivity of the material, the increase in temperature in the mass of rubber reduces its performance.

In addition to the noted features, rubber materials are characterized by high abrasion resistance, gas and water resistance, chemical resistance, electrical insulating properties, and low density.

15.1. Composition and classification of rubbers.

The basis of any rubber is natural rubber (NC) or synthetic rubber (SC), which determines the basic properties of the rubber material. To improve the physical and mechanical properties of rubbers, various additives (ingredients) are introduced. Thus, rubber is made up of rubber and the ingredients discussed below.

1. Vulcanizing substances (agents) are involved in the formation of the spatial-grid structure of the vulcanizate. Usually, sulfur or selenium is used as such substances, for some rubbers, peroxides. For rubber for electrical purposes, instead of elemental sulfur (which interacts with copper), organic sulfur compounds, thiuram, are used.

Accelerators of the vulcanization process: polysulfides, oxides of lead, magnesium and others affect both the mode of vulcanization and the physical and mechanical properties of vulcanizates. Accelerators show their greatest activity in the presence of oxides of certain metals (zinc, etc.), which are therefore called activators in the composition of the rubber compound.

2. Antioxidants (antioxidants) slow down the aging process of rubber, which leads to a deterioration in its performance properties. There are chemical and physical antioxidants. The action of the first is that they delay the oxidation of rubber. Physical antioxidants (paraffin, wax) form surface protective films, they are used less frequently.

H. Softeners (plasticizers) facilitate the processing of the rubber mixture, increase the elastic properties of rubber, and increase the frost resistance of rubber. Paraffin, vaseline, stearic acid, bitumen, dibutyl phthalates, vegetable oils are introduced as softeners. The amount of softeners is 8-30% of the mass of rubber.

4. Fillers according to their effect on rubber are divided into active (reinforcing) and inactive (inert). Active fillers (carbon black and white carbon black) increase the mechanical properties of rubber: strength, abrasion resistance, hardness. Inactive fillers (chalk, talc, barite) are introduced to reduce the cost of rubber.

Often, reclaimed rubber is introduced into the composition of rubber - a product of processing old rubber products and rubber production waste. In addition to reducing the cost, the reclaim improves the quality of rubber, reducing its tendency to aging.

5. Mineral or organic dyes are used to color rubbers. Some dyes (white, yellow, green) absorb the short-wave part of the solar spectrum and thus protect rubber from light aging.

The vast majority of rubbers are unsaturated, high-polymer compounds with a double chemical bond between carbon atoms in the elementary units of the macromolecule. The molecular weight of rubbers is estimated at 400,000 - 450,000. The structure of macromolecules is linear or slightly branched and consists of individual links that tend to curl up into a ball, occupy a minimum volume, but this is prevented by the forces of intermolecular interaction, because rubber molecules are winding (zigzag). This form of molecules is the reason for the exceptionally high elasticity of rubber (under a small load, the molecules straighten). The properties of rubber are similar to thermoplastic polymers. The presence of unsaturated bonds in rubber molecules allows certain conditions transfer it to a thermally stable state. To do this, divalent sulfur is added at the place of the double bond, which forms in the transverse direction, as it were, “bridges” between the filamentous rubber molecules, resulting in a spatial-network structure inherent in rubber (vulcanisate). The process of chemical interaction of rubbers with sulfur in technology is called vulcanization.

Depending on the amount of sulfur introduced, a different frequency of the polymer network is obtained. With the introduction of 1-5% S, a sparse mesh is formed and the rubber is highly elastic and soft. With an increase in the percentage of sulfur, the network structure becomes more frequent, the rubber becomes harder, and with the maximum possible saturation (30%) of the rubber with sulfur, a solid material called ebonite is formed.

During vulcanization, the molecular structure of the polymer is used, which entails a change in its physical and mechanical properties: the tensile strength and elasticity of rubber increase sharply, and plasticity almost completely disappears (for example, natural rubber has σ in=1.0-1.5MPa, and after vulcanization σ in=35MPa); increase in hardness and wear resistance. Many rubbers are soluble in solvents, rubbers only swell in them and are more resistant to chemicals. Rubbers have a higher heat resistance (NK softens at a temperature of 90°C, rubber works at temperatures above 100°C).

The change in the properties of rubber is affected by the interaction of rubber with oxygen, therefore, during vulcanization, two processes occur simultaneously: structuring under the action of a vulcanizing agent and destruction under the influence of oxidation and temperature. The predominance of one or another process affects the properties of the vulcanizate. This is especially true for NK rubbers. For synthetic rubbers (SC), the vulcanization process is supplemented by polymerization: under the action of oxygen and temperature, intermolecular carbon bonds are formed that strengthen the thermostable structure, which gives an increase in strength.

The thermal stability of the vulcanizate depends on the nature of the bonds formed during the vulcanization process. The strongest, and therefore, heat-resistant bonds -C-C-, the lowest strength of the polysulfide bond -C-S-C.

In terms of world consumption, NK is 30%, the rest is SC, of ​​which 250 species are known.

Rubbers are divided into general-purpose rubbers and special-purpose rubbers.

General purpose rubbers. The group of general-purpose rubbers includes vulcanizates of non-polar rubbers - NK, SKB, SKS, SKI.

NK - natural rubber is a polymer of isoprene (C 5 H 8) P. It dissolves in fatty and aromatic solvents (gasoline, benzene, chloroform, carbon disulfide, etc.), forming viscous solutions used as adhesives. When heated above 80-100 ° C, rubber becomes plastic and at 200 ° C begins to decompose. At a temperature of -70°C, NC becomes brittle. Usually NC is amorphous. However, during long-term storage, crystallization is possible. The crystalline phase also occurs when rubber is stretched, which significantly increases its strength. To obtain rubber, NK is vulcanized with sulfur. Rubbers based on NK are distinguished by high elasticity, strength, water and gas impermeability, high electrical insulating properties: ρ V\u003d 3 * * 10 14 - 23 * 10 18 Ohm * cm; ε =2,5.

SKB - butadiene synthetic rubber(divinyl) is obtained according to the CB method. Lebedev. Polybutadiene formula (C 4 H 6) P. It is a non-crystallizing rubber and has a low tensile strength, so it is necessary to add reinforcing fillers to the rubber based on it. Frost resistance of butadiene rubber is low (from - 40 to - 45 °). It swells in the same solvents as NK. Stereoregular divinyl rubber SKD is close to NK in terms of its main technical properties. Divinyl rubbers are vulcanized with sulfur in a manner similar to natural rubber.

SCS - styrene-butadiene rubber obtained by joint polymerization of butadiene (C 4 H 6) and styrene (CH \u003d CH - C 6 H 5). It is the most common general purpose rubber.

Depending on the percentage of styrene, rubber is produced in several grades: SKS-10, SKS-30, SKS-50. The properties of rubber depend on the content of styrene units. So, the more styrene, the higher the strength, but the lower the frost resistance. From the most common rubber SKS-30, rubbers are obtained with good aging resistance and good performance under repeated deformations. In terms of gas impermeability and dielectric properties, they are equivalent to rubbers based on NK. Rubber SKS-10 can be used at low temperatures (from -74 to -77°C). With the selection of appropriate fillers, it is possible to obtain rubbers with high mechanical strength.

SKI - synthetic rubber isoprene- polymerization product of isoprene (C 5 H 8). Obtaining SKI became possible in connection with the use of new types of catalysts. In terms of structure, chemical and physical-mechanical properties, SKI is close to natural rubber. The industry produces rubbers SKI-3 and SKI-3P, which are closest in properties to NK; rubber SKI-3P, intended for the production of electrical insulating rubbers, SKI-ZV - for vacuum technology.

General purpose rubbers can work in water, air, weak solutions of acids and alkalis. The operating temperature range is from -35 to 130°C. Tires, belts, sleeves, conveyor belts, cable insulation, and various technical products are made from these rubbers.

Special rubbers are divided into several types: oil and petrol resistant, heat resistant, light and ozone resistant, wear resistant, electrical, resistant to hydraulic fluids.

Oil and petrol resistant rubbers are produced on the basis of chloroprene (nairit), SKN and thiolok rubbers.

Nairit is a domestic chloroprene rubber.

Vulcanization can be carried out by heat treatment even without sulfur, since under the influence of temperature the rubber passes into a thermostable state. Rubbers based on nairite have high elasticity, vibration resistance, ozone resistance, resistance to fuel and oils, and good resistance to thermal aging. In terms of temperature resistance and frost resistance (from -35 to -40 ° C), they are inferior to both NK and other SC. The electrical insulating properties of rubber based on polar nairite are lower than those of rubber based on non-polar rubbers.

Depending on the composition, rubber is produced in the following grades: SKN-18, SKN-26, SKN-40. The presence of the CN group in rubber molecules gives it polar properties. The higher the polarity of rubber, the higher its mechanical and chemical properties and the lower the frost resistance. Vulcanize SKN with sulfur. Rubbers based on SKN have high strength (σ in\u003d 35 MPA), resist abrasion well, but are inferior in elasticity to rubbers based on NK, surpass them in resistance to aging and the action of dilute acids and alkalis. Rubbers can work in the environment of gasoline, fuel, oils in the temperature range -30 to 130°C. Rubbers based on SKN are used for the production of belts, conveyor belts, sleeves, oil and petrol resistant rubber parts.

Heat-resistant rubbers are produced on the basis of SKT rubber.

SKT - heat resistant synthetic rubber is an organosilicon compound with the chemical formula:

…-Si(CH 3) 2 O-Si(CH 3) 2 -…

Rubber is vulcanized by peroxides and requires the introduction of reinforcing fillers. The presence of a strong siloxane bond in the main molecular chain gives rubber high heat resistance. Because SQT is weakly polar and has good dielectric properties. The operating temperature range of the SKT is from -60 to 250°C. The low adhesion inherent in organosilicon compounds makes SKT water-resistant and hydrophobic. It swells in solvents and oils, has low mechanical strength, high gas impermeability, and poorly resists abrasion. When the methyl group is replaced by other radicals, other types of siloxane rubbers are obtained. Vinyl group rubber (VCR) is resistant to heat aging and has lower compression fluidity, operating temperature from -55 to 300°C.

Frost-resistant rubbers are based on rubbers with low glass transition temperatures. Rubbers based on SKS-10 and SKD can operate at temperatures up to -60°C; NK, SKV, SKS-30, SKN-up to -50°С, SKT-below -75°С.

Wear-resistant rubbers are produced on the basis of SKU polyurethane rubbers.

Polyurethane rubbers have high strength, elasticity, abrasion resistance, oil and petrol resistance. There are no unsaturated bonds in the structure of rubber, so it is resistant to oxygen and ozone. Operating temperatures of rubbers based on it range from -30 to 130°C. Based on complex polymers, rubbers SKU-7, SKU-8, SKU-50 are produced; based on polyethers - SKU-PF, SKU-PFL. The latter are characterized by high frost resistance and hydrolytic resistance. Urethane rubbers are resistant to radiation. Rubbers based on SKU are used for car tires, conveyor belts, lining of pipes and chutes for transporting abrasive materials, shoes, etc.

Electrical rubbers include electrically insulating and electrically conductive rubbers. electrical insulating rubber, used to insulate the conductive core of wires and cables, for special gloves and shoes, are made only on the basis of non-polar rubbers NK, SKB, SKS, SKT and butyl rubber.

Conductive rubbers for shielded cables are obtained from NK, SKN rubbers, especially for polar rubber SKN-26 with the introduction of carbon black and graphite into their composition

Rubber resistant to hydraulic fluids is used to seal mobile and stationary hydraulic systems, hoses, diaphragms, pumps; for operation in oil, rubber based on SKN rubber is used, the swelling of which in a liquid is no more than 1-4%. For organosilicon liquids, non-polar rubbers based on rubbers NK, SKMS-10, etc. are used.

15.2. Influence of operating factors on the properties of rubber.

During operation, rubber products are subjected to various types aging (light, ozone, thermal, radiation, vacuum) which reduces their performance; property changes can be irreversible. The resistance of rubber during aging depends on the degree of unsaturation of rubber, the flexibility of macromolecules, the strength of chemical bonds in the chain, the ability to orient and crystallize. The change in properties is evaluated by the change in strength and elastic characteristics; on the recoverability of rubber, tear resistance.

Under the influence of atmospheric conditions, ozone cracks stressed rubbers from unsaturated rubbers; resistant to ozone aging saturated rubbers. Light causes photooxidation of rubbers, which depends on the presence of double bonds in them. In descending order of the rate of photooxidation, rubbers can be arranged in a row: NK, SKB>SK>BK. Light-resistant rubbers from SKF and SKT. The photoprocess accelerates with increasing temperature. The rate of aging of rubber in a stressed state is higher than in a free state. An increase in ozone resistance is achieved by introducing the appropriate ingredients and applying a protective film.

The service life or storage of rubber products is determined by the change in residual compression strain ε ost, which is allowed up to 80% for sealing materials, and stress relaxation 0.2.

Questions to repeat the section.

1. Name the composition and classify rubber.

2. When are special rubbers used?

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Rubbers are elastic multicomponent materials based on rubber. Elasticity of rubbers, i.e. the ability to very large (500 - 800%) reversible deformations is their most valuable property. Rubbers have a very low modulus of elasticity (E = 1...10 MPa) and are easily deformed under the action of relatively small stresses; their Poisson's ratio is close to 0.5. Rubbers are characterized by σ Β = 10 ... 60 MPa and a very large relative elongation at the moment of specimen rupture - up to 900 - 1000%. Cracks in rubber propagate slowly; tear resistance (GOST 262-93) varies from 20 to 150 kN/m.

After rupture, the specimen had a residual relative elongation of 20–30%, i.e. less than 5% of the maximum elongation before breaking. This permanent elongation is mainly an irreversible deformation due to cross-link breaks and slippage of macromolecules, the smaller the permanent elongation, the higher the quality of the rubber.

Rubber products during operation experience stresses that are much less than the temporary resistance. Under load, part of the mechanical energy supplied to the product is converted into heat. Hysteresis losses occur during a single loading cycle. They acquire special significance under repeated cyclic loading (Fig. 20.1).

Rice. 20.1. Dependence of stress in rubber on tensile strain

In massive products, the temperature of rubber rises by 100 °C or more. Hysteresis heating of rubber is accompanied by a decrease in its strength and an increase in oxidative aging. As a result, the service life of products is reduced, and in some cases their destruction is possible. Hysteresis losses ensured the use of rubber as the main material for shock absorbers. The energy of vibrations, shocks or impacts is absorbed by the deformation of the rubber elements of shock-absorbing devices.

Rubbers are made on the basis of natural and synthetic rubbers with glass transition temperatures below 0°C. The main operation for the transformation of rubber into rubber is vulcanization, when the linear molecules of thermoplastic rubber are connected by cross chemical bonds. The molecular structure of rubber is a three-dimensional network capable of highly elastic deformation due to the low density of cross-links. Compared to rubber, rubber is stronger, not prone to irreversible deformation under load and does not dissolve, but only swells in those solvents in which rubber is soluble.

For general purpose rubbers, the operating temperature range is -50 ... + 150 °С. When heated above 150 ° C, the rubber quickly collapses, and when cooled below -50 ° C, it loses elasticity. For lower and higher temperatures, special rubbers have been developed - frost-resistant and heat-resistant, respectively.

Depending on the location of the nearest groups of atoms in relation to double bonds, various configurations of rubber molecules are possible (Fig. 20.2).

Rice. 20.2. Configurations of rubber molecules:

a- 1,4-trans; b- 1,4-cis

With the same chemical composition, rubber isomers differ in flexibility, i.e. in terms of the number of possible conformations, some isomers significantly outnumber others. For example, natural rubber (1,4-cis) differs from gutta-percha (1,4-trans) in increased elasticity.

Preservation of the main set of double bonds in the bulk molecular network of rubber is the reason for its rapid aging. Ozone is especially destructive, aging is accelerated by heating and the simultaneous action of oxidizing agents and mechanical stresses. As a result of aging, rubber from the surface is covered with a network of cracks. In particular, under an alternating loading cycle, rubber simultaneously undergoes oxidation and mechanical failure. Breaking bonds in rubber molecules and recombination of fragments of molecules reduce the elasticity of rubber and are accompanied by constant cracking of its surface layers.

Depending on the resistance to aging, rubbers are divided into three groups: persistent (does not contain double bonds); moderately resistant and unstable. Resistant are rubbers based on ethylene propylene, organosilicon and fluoroelastomers, as well as horsulfated polyethylene. They are insensitive to ozone, neither at its equilibrium concentration in the air, equal to (2 - 4) · 10 -6%, nor with an increase in this concentration to 0.1 - 1.0%. The effect of aging becomes noticeable in them only after years.

Moderately resistant rubbers are based on chloroprene and butyl rubbers and thiokols. In these materials, cracks begin to develop after several months of exposure.

Unstable rubbers are general purpose, which are produced in mass quantities. These are natural rubber and rubbers based on isoprene, butadiene-styrene, butadiene-nitrile and a number of other rubbers. Cracks occur in them after a short stretching, bending or torsion. Increasing the concentration of ozone in the air to 10–2–10–4% leads to cracking of the surface of these materials at 20–25 °C already after 1 hour of exposure. Surface cracks contribute to further destruction and reduce the wear resistance of rubber.

Due to their pliability under mechanical stress, rubbers are resistant to many types of abrasive wear. At the same time, they wear out by "rolling". During friction, the microroughnesses of the rubber are deformed, rolled up and detached from the surface. The wear rate of rubber increases sharply when heated above 150 °C. Wear develops under the action of tangential stresses tearing the surface layer and the more intense, the greater the friction coefficient. Micro cracks due to aging increase wear.

As a polymeric material, rubber is characterized by gas and water resistance, chemical resistance (with the exception of strong oxidizing agents). Rubber slightly absorb water (natural rubber - up to 2% H 2 O). Of exceptional importance is the resistance of rubber to oil and motor fuel. General purpose rubbers, including natural rubber, are not resistant to these substances, swell in them and quickly lose strength. Special rubbers - nitrile butadiene, polyurethane, polysulfide, chloroprene, as well as rubbers based on fluororubbers are oil resistant. Rubber based on butyl rubber is superior to others in terms of gas impermeability, its main application is the tubes of car tires.

In the operating temperature range, the mechanical properties of rubbers change: the elasticity of rubbers decreases when approaching t CT and at temperatures above 100 °C due to thermal destruction and aging. With short-term heating to 120 ° C (to exclude aging), the strength of all rubbers, without exception, is halved. Heat-resistant rubbers are based on ethylene-propylene, organosilicon and fluoroelastomers (up to 300 - 400 ° C instead of 150 ° C for conventional rubbers). Rubber is a dielectric.

The properties of rubber vary depending on the choice of components, the ratio between them and the vulcanization condition. The composition of rubber includes: rubber, 8-30% plasticizer to prepare raw rubber for molding, filler in the form of a fine powder, vulcanizer to connect rubber molecules with cross-links, antioxidant

Fillers are divided into active (soot, silicon oxide) and inert (chalk, talc, etc.). Active fillers in the form of a specially prepared finely dispersed powder interact with rubber molecules and increase the strength of rubber. Inert fillers reduce the cost of rubber without increasing its strength. Regenerate (8 - 30%) is introduced into raw rubber - finely ground waste and old rubber products, which also reduces its cost. The higher the content of active filler and vulcanizer, the higher the strength, modulus of elasticity and hysteresis loss. The higher the content of the plasticizer, the weaker the intermolecular interaction, the lower the strength and the lower the hysteresis loss.

The manufacturing technology of rubber products includes plasticizing rubber together with plasticizers, mixing components and obtaining raw rubber, forming, assembling and vulcanizing products. Plasticizing is the repeated deformation of raw rubber. As a result of plasticization, the mixture is heated, the average molecular weight is halved due to mechanical destruction, and a pliable viscous material is obtained that is easy to mix with other components, and then form a product from raw rubber. Vulcanization is carried out at 140 - 180 ° C in molds or autoclaves. Exposure is made as short as possible to reduce the thermal destruction of rubber (vulcanization accelerators are used for this purpose). The vulcanizing agent is usually sulfur, it is added in an amount of 5 - 6%, while maintaining the elasticity of the rubber. At a sulfur concentration of 30 - 50%, the frequency of cross-links is so great that elasticity is completely eliminated; the hard material obtained after vulcanization is called ebonite.

Rubbers are easily combined with other substances - styrene, acrylonitrile, isobutylene. In general purpose rubbers, the elastic component is usually a copolymer, such as styrene butadiene rubber. With an increase in the content of styrene or nitrole units in rubber molecules, the frost resistance of rubber decreases. Reinforcing elements made of fibers and fabrics are often used in rubber products to increase strength.

The range of rubber products is exceptionally wide. For mechanical engineering, tires for vehicles, shock absorbers, drive belts, sleeves, various gaskets and cuffs, and plain bearings are of primary importance. Rubber is used as a die material for sheet metal stamping. For the production of rubber products, general-purpose and special rubbers are used. The former include natural rubber and isoprene rubber, which is almost identical in properties to it, butadiene-styrene rubber, which is superior in wear resistance to natural rubber.

Special rubbers include: a) frost-resistant rubbers that retain elasticity up to -70 ... -100 ° C; these are organosilicon rubbers and rubbers with special plasticizers, for example, nitrile butadiene, plasticized with sebacates; b) heat-resistant rubbers - ethylene-propylene (up to 200 - 300 ° C), fluoro-rubber (up to 300 ° C), organosilicon (up to 250 ° C); c) oil-resistant rubbers based on chloroprene, nitrile butadiene, fluoroelastomers, as well as polysulfide (thiokols) and polyurethane rubbers; d) radiation-resistant rubbers filled with lead or barium compounds to absorb γ-radiation.

The exceptional value of the unique properties of rubber made it necessary to comprehensively test its properties using standard methods. In addition to tensile and tear tests, frost resistance, resistance to thermal aging, resistance to liquid media and other properties of rubber are evaluated.