Natural rubber is not always suitable for making parts. This is because its natural elasticity is very low, and is highly dependent on external temperature. At temperatures close to 0, rubber becomes hard, or when lowered further it becomes brittle. At a temperature of about + 30 degrees, the rubber begins to soften and with further heating it turns into a melt state. When cooled back, it does not restore its original properties.
To ensure the necessary operational and technical properties of rubber, various substances and materials are added to the rubber - carbon black, chalk, softeners, etc.
In practice, several vulcanization methods are used, but they have one thing in common - processing raw materials with vulcanization sulfur. Some textbooks and regulations state that sulfur compounds can be used as vulcanizing agents, but in fact they can only be considered as such because they contain sulfur. Otherwise, they can affect vulcanization just like other substances that do not contain sulfur compounds.
Some time ago, research was carried out regarding the treatment of rubber with organic compounds and certain substances, for example:
- phosphorus;
- selenium;
- trinitrobenzene and a number of others.
But studies have shown that these substances have no practical value in terms of vulcanization.
Vulcanization process
The rubber vulcanization process can be divided into cold and hot. The first one can be divided into two types. The first involves the use of sulfur semichloride. The mechanism of vulcanization using this substance looks like this. A workpiece made of natural rubber is placed in the vapor of this substance (S2Cl2) or in its solution, made on the basis of some solvent. The solvent must meet two requirements:
- It should not react with sulfur semichloride.
- It should dissolve the rubber.
As a rule, carbon disulfide, gasoline and a number of others can be used as a solvent. The presence of sulfur semichloride in the liquid prevents the rubber from dissolving. The essence of this process is to saturate the rubber with this chemical.
The duration of the vulcanization process with the participation of S2Cl2 ultimately determines the technical characteristics of the finished product, including elasticity and strength.
The vulcanization time in a 2% solution can be several seconds or minutes. If the process takes too long, so-called over-vulcanization may occur, that is, the workpieces lose their plasticity and become very brittle. Experience suggests that with a product thickness of about one millimeter, the vulcanization operation can be carried out in a few seconds.
This vulcanization technology is the optimal solution for processing parts with a thin wall - tubes, gloves, etc. But, in this case, it is necessary to strictly observe the processing modes; otherwise, the top layer of parts can be vulcanized more than the inner layers.
At the end of the vulcanization operation, the resulting parts must be washed with either water or an alkaline solution.
There is a second method of cold vulcanization. Rubber blanks with a thin wall are placed in an atmosphere saturated with SO2. After a certain time, the workpieces are moved into a chamber where H2S (hydrogen sulfide) is pumped. The holding time of workpieces in such chambers is 15 – 25 minutes. This time is sufficient to complete vulcanization. This technology is successfully used for processing glued seams, which gives them high strength.
Special rubbers are processed using synthetic resins; vulcanization using them is no different from that described above.
Hot vulcanization
The technology for such vulcanization is as follows. A certain amount of sulfur and special additives are added to the molded raw rubber. As a rule, the volume of sulfur should be in the range of 5 – 10%; the final figure is determined based on the purpose and hardness of the future part. In addition to sulfur, so-called horn rubber (hard rubber) containing 20–50% sulfur is added. At the next stage, blanks are formed from the resulting material and heated, i.e. curing.
Heating is carried out using various methods. The blanks are placed in metal molds or rolled into fabric. The resulting structures are placed in an oven heated to 130 - 140 degrees Celsius. In order to increase the efficiency of vulcanization, excess pressure can be created in the oven.
The formed blanks can be placed in an autoclave containing superheated water vapor. Or they are placed in a heated press. In fact, this method is the most common in practice.
The properties of vulcanized rubber depend on many conditions. That is why vulcanization is considered one of the most complex operations used in rubber production. In addition, the quality of the raw material and the method of its pre-processing play an important role. We must not forget about the volume of added sulfur, temperature, duration and method of vulcanization. In the end, the properties of the finished product are also affected by the presence of impurities of various origins. Indeed, the presence of many impurities allows for proper vulcanization.
IN last years accelerators began to be used in the rubber industry. These substances added to the rubber mixture speed up the processes, reduce energy costs, in other words, these additives optimize the processing of the workpiece.
When implementing hot vulcanization in air, the presence of lead oxide is necessary; in addition, the presence of lead salts may be required in combination with organic acids or with compounds that contain acid hydroxides.
The following substances are used as accelerators:
- thiuramid sulfide;
- xanthates;
- Mercaptobenzothiazole.
Vulcanization carried out under the influence of water vapor can be significantly reduced if you use chemicals such as alkalis: Ca(OH)2, MgO, NaOH, KOH, or salts Na2CO3, Na2CS3. In addition, potassium salts will help speed up the processes.
There are also organic accelerators, these are amines, and a whole group of compounds that are not included in any group. For example, these are derivatives of substances such as amines, ammonia and a number of others.
Diphenylguanidine, hexamethylenetetramine and many others are most often used in production. It is not uncommon for zinc oxide to be used to enhance the activity of accelerators.
In addition to additives and accelerators, an important role is played by environment. For example, the presence of atmospheric air creates unfavorable conditions for vulcanization at standard pressure. In addition to air, carbonic anhydride and nitrogen have a negative effect. Meanwhile, ammonia or hydrogen sulfide have a positive effect on the vulcanization process.
The vulcanization procedure gives rubber new properties and modifies existing ones. In particular, its elasticity improves, etc. The vulcanization process can be controlled by constantly measuring the changing properties. As a rule, the determination of tensile strength and tensile strength is used for this purpose. But these control methods are not accurate and are not used.
Rubber as a product of rubber vulcanization
Technical rubber is a composite material containing up to 20 components that provide various properties of this material. Rubber is produced by vulcanizing rubber. As noted above, during the vulcanization process, macromolecules are formed that ensure the performance properties of rubber, thus ensuring high rubber strength.
The main difference between rubber and many other materials is that it has the ability to undergo elastic deformations, which can occur at different temperatures, ranging from room temperature to much lower ones. Rubber significantly exceeds rubber in a number of characteristics, for example, it is distinguished by elasticity and strength, resistance to temperature changes, exposure to aggressive environments, and much more.
Cement for vulcanization
Cement for vulcanization is used for self-vulcanization operation, it can start from 18 degrees and for hot vulcanization up to 150 degrees. This cement does not contain hydrocarbons. There is also OTR type cement used for application to rough surfaces inside tires, as well as Type Top RAD and PN OTR series adhesives with extended drying time. The use of such cement makes it possible to achieve long service life for retreaded tires used on special construction equipment with high mileage.
Do-it-yourself hot vulcanization technology for tires
To perform hot vulcanization of a tire or tube, you will need a press. The welding reaction between the rubber and the part occurs over a certain period of time. This time depends on the size of the area being repaired. Experience shows that it will take 4 minutes to repair damage 1 mm deep, subject to the specified temperature. That is, to repair a defect 3 mm deep, you will have to spend 12 minutes of pure time. We do not take preparation time into account. Meanwhile, putting the vulcanization device into operation, depending on the model, can take about 1 hour.
The temperature required for hot vulcanization ranges from 140 to 150 degrees Celsius. To achieve this temperature there is no need to use industrial equipment. To repair tires yourself, it is quite acceptable to use household electrical appliances, for example, an iron.
Eliminating defects in a car tire or tube using a vulcanization device is a rather labor-intensive operation. It has many subtleties and details, and therefore we will consider the main stages of repair.
- To provide access to the damage site, the tire must be removed from the wheel.
- Clean the rubber near the damaged area. Its surface should become rough.
- Blow off the treated area using compressed air. The cord that appears outside must be removed; it can be bitten off with wire cutters. Rubber must be treated with a special degreasing compound. Processing must be carried out on both sides, outside and inside.
- On the inside, a pre-prepared patch of size should be placed on the damaged area. Laying begins from the side of the tire bead towards the center.
- From the outside, pieces of raw rubber, cut into pieces of 10–15 mm, must be placed on the damaged site; they must first be heated on the stove.
- The laid rubber must be pressed and leveled over the surface of the tire. In this case, it is necessary to ensure that the layer of raw rubber is 3–5 mm higher than the working surface of the chamber.
- After a few minutes, using an angle grinder (angle grinder), it is necessary to remove the layer of applied raw rubber. If the bare surface is loose, that is, there is air in it, all applied rubber must be removed and the operation of applying rubber must be repeated. If there is no air in the repair layer, that is, the surface is smooth and does not contain pores, the part being repaired can be sent under preheated to the temperature indicated above.
- To accurately position the tire on the press, it makes sense to mark the center of the defective area with chalk. To prevent the heated plates from sticking to the rubber, thick paper must be placed between them.
DIY vulcanizer
Any hot vulcanizing device must contain two components:
- a heating element;
- press.
For self-made vulcanizer may be required:
- iron;
- electric stove;
- piston from internal combustion engine.
A do-it-yourself vulcanizer must be equipped with a regulator that can turn it off when it reaches operating temperature (140-150 degrees Celsius). For effective clamping, you can use an ordinary clamp.
Basic methods of rubber vulcanization. To carry out the main chemical process of rubber technology - vulcanization - vulcanizing agents are used. The chemistry of the vulcanization process consists in the formation of a spatial network, including linear or branched rubber macromolecules and cross-links. Technologically, vulcanization consists of processing the rubber mixture at temperatures from normal to 220˚C under pressure and less often without it.
In most cases, industrial vulcanization is carried out using vulcanizing systems that include a vulcanizing agent, accelerators and vulcanization activators and contribute to a more efficient process of formation of a spatial network.
The chemical interaction between the rubber and the vulcanizing agent is determined by the chemical activity of the rubber, i.e. the degree of unsaturation of its chains, the presence of functional groups.
The chemical activity of unsaturated rubbers is due to the presence of double bonds in the main chain and the increased mobility of hydrogen atoms in α-methylene groups adjacent to the double bond. Therefore, unsaturated rubbers can be vulcanized with all compounds that react with the double bond and its neighboring groups.
The main vulcanizing agent for unsaturated rubbers is sulfur, which is usually used as a vulcanizing system in conjunction with accelerators and their activators. In addition to sulfur, you can use organic and inorganic peroxides, alkylphenol-formaldehyde resins (APFR), diazo compounds, and polyhalide compounds.
The chemical activity of saturated rubbers is significantly lower than the activity of unsaturated rubbers, so for vulcanization it is necessary to use substances with high reactivity, for example various peroxides.
Vulcanization of unsaturated and saturated rubbers can be carried out not only in the presence of chemical vulcanizing agents, but also under the influence of physical influences that initiate chemical transformations. These are high-energy radiation (radiation vulcanization), ultraviolet radiation (photovulcanization), prolonged exposure to high temperatures (thermovulcanization), the action of shock waves and some other sources.
Rubbers that have functional groups can be vulcanized across these groups using substances that react with the functional groups to form a cross-link.
Basic principles of the vulcanization process. Regardless of the type of rubber and the vulcanizing system used, some characteristic changes in the properties of the material occur during the vulcanization process:
The plasticity of the rubber mixture sharply decreases, and the strength and elasticity of vulcanizates appears. Thus, the strength of a raw rubber mixture based on NC does not exceed 1.5 MPa, and the strength of a vulcanized material is not less than 25 MPa.
The chemical activity of rubber is significantly reduced: in unsaturated rubbers the number of double bonds decreases, in saturated rubbers and rubbers with functional groups the number of active centers decreases. Due to this, the resistance of the vulcanizate to oxidative and other aggressive influences increases.
The resistance of the vulcanized material to low and high temperatures increases. Thus, NK hardens at 0ºС and becomes sticky at +100ºС, and vulcanizate retains strength and elasticity in the temperature range from –20 to +100ºС.
This nature of the change in the properties of the material during vulcanization clearly indicates the occurrence of structuring processes, ending in the formation of a three-dimensional spatial network. In order for the vulcanizate to retain its elasticity, the cross-links must be sufficiently rare. Thus, in the case of NC, the thermodynamic flexibility of the chain is preserved if there is one cross-link per 600 carbon atoms of the main chain.
The vulcanization process is also characterized by some general patterns of changes in properties depending on the vulcanization time at a constant temperature.
Since the viscosity properties of mixtures change most significantly, shear rotational viscometers, in particular Monsanto rheometers, are used to study the kinetics of vulcanization. These devices allow you to study the vulcanization process at temperatures from 100 to 200ºС for 12 - 360 minutes with various shear forces. The recorder of the device writes out the dependence of the torque on the vulcanization time at a constant temperature, i.e. kinetic vulcanization curve, which has an S-shape and several sections corresponding to the stages of the process (Fig. 3).
The first stage of vulcanization is called the induction period, scorch stage or pre-vulcanization stage. At this stage, the rubber mixture must remain fluid and fill the entire mold well, therefore its properties are characterized by the minimum shear moment M min (minimum viscosity) and the time t s during which the shear moment increases by 2 units compared to the minimum.
The duration of the induction period depends on the activity of the vulcanization system. The choice of a vulcanizing system with a particular t s value is determined by the weight of the product. During vulcanization, the material is first heated to the vulcanization temperature, and due to the low thermal conductivity of rubber, the heating time is proportional to the mass of the product. For this reason, for the vulcanization of large-weight products, vulcanizing systems should be selected that provide a sufficiently long induction period, and vice versa for low-weight products.The second stage is called the main vulcanization period. At the end of the induction period, active particles accumulate in the mass of the rubber mixture, causing rapid structuring and, accordingly, an increase in torque to a certain maximum value M max. However, the completion of the second stage is not considered the time of reaching M max, but the time t 90 corresponding to M 90. This moment is determined by the formula
M 90 =0.9 M + M min,
where M is the difference in torque (M = M max – M min).
Time t 90 is the optimum of vulcanization, the value of which depends on the activity of the vulcanizing system. The slope of the curve in the main period characterizes the vulcanization rate.
The third stage of the process is called the re-vulcanization stage, which in most cases corresponds to a horizontal section with constant properties on the kinetic curve. This zone is called the vulcanization plateau. The wider the plateau, the more resistant the mixture is to over-vulcanization.
The width of the plateau and the further course of the curve mainly depend on the chemical nature of the rubber. In the case of unsaturated linear rubbers, such as NK and SKI-3, the plateau is not wide and then the properties deteriorate, i.e. decline in the curve (Fig. 3, curve A). The process of deterioration of properties at the stage of re-vulcanization is called reversion. The reason for the reversion is the destruction of not only the main chains, but also the formed cross-links under the influence of high temperature.
In the case of saturated rubbers and unsaturated rubbers with a branched structure (a significant number of double bonds in the side 1,2-units) in the re-vulcanization zone, the properties change slightly, and in some cases even improve (Fig. 3, curves b And V), since the thermal oxidation of double bonds of side units is accompanied by additional structuring.
The behavior of rubber mixtures at the stage of over-vulcanization is important in the production of massive products, especially car tires, since due to reversion, over-vulcanization of the outer layers can occur while the inner layers are under-vulcanized. In this case, vulcanizing systems are required that would provide a long induction period for uniform heating of the tire, high speed in the main period and a wide vulcanization plateau at the re-vulcanization stage.
3.2. Sulfur vulcanizing systems for unsaturated rubbers
Properties of sulfur as a vulcanizing agent. The process of vulcanization of natural rubber with sulfur was discovered in 1839 by C. Goodyear and independently in 1843 by G. Gencock.
Natural ground sulfur is used for vulcanization. Elemental sulfur has several crystalline modifications, of which only the modification is partially soluble in rubber. It is this modification, which has a melting point of 112.7 ºC, that is used for vulcanization. Molecules of the -form are an eight-membered ring S 8 with an average activation energy of ring rupture E act = 247 kJ/mol.
This is a fairly high energy, and the splitting of the sulfur ring occurs only at temperatures of 143ºC and above. At temperatures below 150ºC, heterolytic or ionic decomposition of the sulfur ring occurs with the formation of the corresponding sulfur biion, and at 150ºC and above, homolytic (radical) decomposition of the S ring occurs with the formation of sulfur biradicals:
t150ºС S 8 →S + – S 6 – S – → S 8 +–
t150ºС S 8 →Sֹ–S 6 –Sֹ→S 8 ֹֹ.
Biradicals S 8 ·· easily break down into smaller fragments: S 8 ֹֹ→S x ֹֹ + S 8 ֹֹ.
The resulting sulfur biions and biradicals then interact with rubber macromolecules either at the double bond or at the site of the α-methylene carbon atom.
The sulfur ring can also disintegrate at temperatures below 143ºС if there are some active particles in the system (cations, anions, free radicals). Activation occurs according to the following scheme:
S 8 + A + →A – S – S 6 – S +
S 8 + B – → B – S – S 6 –
S 8 + Rֹ→R – S – S 6 – Sֹ.
Such active particles are present in the rubber mixture when vulcanizing systems with vulcanization accelerators and their activators are used.
To transform soft plastic rubber into hard elastic rubber, a small amount of sulfur is sufficient - 0.10.15% wt. However, actual dosages of sulfur range from 12.5 to 35 parts by weight. per 100 parts by weight rubber.
Sulfur has limited solubility in rubber, so the dosage of sulfur determines the form in which it is distributed in the rubber mixture. At actual dosages, sulfur is in the form of molten droplets, from the surface of which sulfur molecules diffuse into the rubber mass.
The preparation of the rubber mixture is carried out at elevated temperatures (100-140ºС), which increases the solubility of sulfur in rubber. Therefore, when the mixture is cooled, especially in cases of high dosages, free sulfur begins to diffuse onto the surface of the rubber mixture with the formation of a thin film or deposit of sulfur. This process is called fading or sweating in technology. Fade rarely reduces the stickiness of the workpieces, and therefore, to freshen the surface of the workpieces, they are treated with gasoline before assembly. This worsens the working conditions of assemblers and increases the fire and explosion hazard of production.
The problem of fading is especially acute in the production of steel cord tires. In this case, to increase the strength of the bond between metal and rubber, the dosage of S is increased to 5 parts by weight. To avoid fading in such formulations, a special modification should be used - the so-called polymer sulfur. This is the -form, which is formed when the -form is heated to 170ºC. At this temperature, a sharp jump in the viscosity of the melt occurs and polymer sulfur Sn is formed, where n is over 1000. In world practice, various modifications of polymer sulfur are used, known under the brand name “Cristex”.
Theories of sulfur vulcanization. Chemical and physical theories have been put forward to explain the process of sulfur vulcanization. In 1902, Weber put forward the first chemical theory of vulcanization, elements of which have survived to this day. By extracting the product of the interaction of NC with sulfur, Weber found that part of the introduced sulfur was not extracted. He called this part bound, and the released part - free sulfur. The sum of the amount of bound and free sulfur was equal to the total amount of sulfur introduced into the rubber: S total = S free + S bound. Weber also introduced the concept of vulcanization coefficient as the ratio of bound sulfur to the amount of rubber in the rubber mixture (A): K vulc = S bond / A.
Weber managed to isolate polysulfide (C 5 H 8 S) n as a product of the intramolecular addition of sulfur at the double bonds of isoprene units. Therefore, Weber's theory could not explain the increase in strength as a result of vulcanization.
In 1910, Oswald put forward a physical theory of vulcanization, which explained the effect of vulcanization by the physical adsorption interaction between rubber and sulfur. According to this theory, rubber-sulfur complexes are formed in the rubber mixture, which interact with each other also due to adsorption forces, which leads to an increase in the strength of the material. However, the adsorbed sulfur should be completely extracted from the vulcanizate, which was not observed under real conditions, and the chemical theory of vulcanization began to prevail in all further studies.
The main evidence of the chemical theory (bridge theory) is the following:
Only unsaturated rubbers are vulcanized with sulfur;
Sulfur interacts with molecules of unsaturated rubbers to form covalent cross-links (bridges) of various types, i.e. with the formation of bound sulfur, the amount of which is proportional to the unsaturation of the rubber;
The vulcanization process is accompanied by a thermal effect proportional to the amount of added sulfur;
Vulcanization has a temperature coefficient of approximately 2, i.e. close to the temperature coefficient of a chemical reaction in general.
The increase in strength as a result of sulfur vulcanization occurs due to the structuring of the system, as a result of which a three-dimensional spatial network is formed. Existing sulfur vulcanization systems make it possible to specifically synthesize almost any type of cross-link, change the vulcanization rate, and the final structure of the vulcanizate. Therefore, sulfur is still the most popular crosslinking agent for unsaturated rubbers.
Technologically, the vulcanization process is the transformation of “raw” rubber into rubber. As a chemical reaction, it involves the combination of linear rubber macromolecules, which easily lose stability when exposed to external influences, into a single vulcanization network. It is created in three-dimensional space due to cross-sectional chemical bonds.
This seemingly “cross-linked” structure gives the rubber additional strength properties. Its hardness and elasticity, frost and heat resistance are improved while solubility in organic substances and swelling are reduced.
The resulting mesh has a complex structure. It includes not only nodes connecting pairs of macromolecules, but also those that combine several molecules at the same time, as well as transverse chemical bonds, which are like “bridges” between linear fragments.
Their formation occurs under the influence of special agents, the molecules of which partially act as building materials, chemically reacting with each other and rubber macromolecules at high temperatures.
Material properties
The performance properties of the resulting vulcanized rubber and products made from it largely depend on the type of reagent used. Such characteristics include resistance to exposure to aggressive environments, rate of deformation during compression or increased temperature, and resistance to thermal-oxidative reactions.
The resulting bonds irreversibly limit the mobility of molecules under mechanical action, while simultaneously maintaining the high elasticity of the material with the ability to undergo plastic deformation. The structure and number of these bonds is determined by the rubber vulcanization method and the chemical agents used for it.
The process does not proceed monotonously, and individual indicators of the vulcanized mixture in their changes reach their minimum and maximum at different times. The most suitable ratio of the physical and mechanical characteristics of the resulting elastomer is called the optimum.
The vulcanizing composition, in addition to rubber and chemical agents, includes a number of additional substances that contribute to the production of rubber with specified performance properties. According to their purpose, they are divided into accelerators (activators), fillers, softeners (plasticizers) and antioxidants (antioxidants). Accelerators (most often zinc oxide) facilitate the chemical interaction of all ingredients of the rubber mixture, help reduce the consumption of raw materials and time for its processing, and improve the properties of vulcanizers.
Fillers such as chalk, kaolin, carbon black increase the mechanical strength, wear resistance, abrasion resistance and other physical characteristics of the elastomer. By replenishing the volume of feedstock, they thereby reduce rubber consumption and reduce the cost of the resulting product. Softeners are added to improve the processability of rubber compounds, reduce their viscosity and increase the volume of fillers.
Plasticizers can also increase the dynamic endurance of elastomers and abrasion resistance. Antioxidants that stabilize the process are introduced into the mixture to prevent “aging” of the rubber. Various combinations of these substances are used in the development of special raw rubber formulations to predict and adjust the vulcanization process.
Types of vulcanization
Most often, commonly used rubbers (styrene-butadiene, butadiene and natural) are vulcanized in combination with sulfur, heating the mixture to 140-160°C. This process is called sulfur vulcanization. Sulfur atoms participate in the formation of intermolecular cross-links. When up to 5% sulfur is added to a mixture with rubber, a soft vulcanizate is produced, used for the manufacture of automobile tubes, tires, rubber tubes, balls, etc.
When more than 30% of sulfur is added, a rather hard, low-elastic ebonite is obtained. Thiuram, captax, etc. are used as accelerators in this process, the completeness of which is ensured by the addition of activators consisting of metal oxides, usually zinc.
Radiation vulcanization is also possible. It is carried out through ionizing radiation, using streams of electrons emitted by radioactive cobalt. This sulfur-free process produces elastomers that are particularly resistant to chemical and thermal attack. To produce special types of rubber, organic peroxides, synthetic resins and other compounds are added under the same process parameters as in the case of adding sulfur.
On an industrial scale, the vulcanizable composition, placed in a mold, is heated at elevated pressure. To do this, the molds are placed between heated plates of a hydraulic press. When producing non-molded products, the mixture is poured into autoclaves, boilers or individual vulcanizers. Heating of rubber for vulcanization in this equipment is carried out using air, steam, heated water or high-frequency electric current.
For many years, the largest consumers of rubber products have been automotive and agricultural engineering enterprises. The degree of saturation of their products with rubber products serves as an indicator of high reliability and comfort. In addition, parts made from elastomers are often used in the production of plumbing installations, footwear, stationery and children's products.
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VulcanizAtion-- a technological process of interaction of rubbers with a vulcanizing agent, during which rubber molecules are cross-linked into a single spatial network. Vulcanizing agents can be: sulfur, peroxides, metal oxides, amine-type compounds, etc. To increase the rate of vulcanization, various accelerator catalysts are used.
Vulcanization increases the strength characteristics of rubber, its hardness, elasticity, heat and frost resistance, and reduces the degree of swelling and solubility in organic solvents. The essence of vulcanization is the combination of linear macromolecules of rubber into a single “cross-linked” system, the so-called vulcanization network. As a result of vulcanization, cross-links are formed between macromolecules, the number and structure of which depend on method B. During vulcanization, some properties of the vulcanized mixture do not change monotonically over time, but pass through a maximum or minimum. The degree of vulcanization at which the best combination of various physical and mechanical properties of rubber is achieved is called the vulcanization optimum.
Vulcanization is usually carried out on a mixture of rubber with various substances that provide the necessary performance properties of rubber (fillers, for example, soot, chalk, kaolin, as well as softeners, antioxidants, etc.).
In most cases, general purpose rubbers (natural, butadiene, styrene butadiene) are vulcanized by heating them with elemental sulfur at 140-160°C (sulfuric acid). The resulting intermolecular cross-links occur through one or more sulfur atoms. If 0.5-5% sulfur is added to rubber, a soft vulcanizate is obtained (car tubes and tires, balls, tubes, etc.); the addition of 30-50% sulfur leads to the formation of a hard, inelastic material - ebonite. Sulfur vulcanization can be accelerated by adding small amounts of organic compounds, so-called vulcanization accelerators - captax, thiuram, etc. The effect of these substances is fully manifested only in the presence of activators - metal oxides (most often zinc oxide).
In industry, sulfur vulcanization is carried out by heating the vulcanized product in molds under high pressure or in the form of unmolded products (in “free” form) in boilers, autoclaves, individual vulcanizers, and devices for continuous vulcanization. etc. In these devices, heating is carried out with steam, air, superheated water, electricity, and high-frequency currents. The molds are usually placed between heated platens of a hydraulic press. Vulcanization with sulfur was discovered by Charles Goodyear (USA, 1839) and T. Hancock (Great Britain, 1843). For the vulcanization of special-purpose rubbers, organic peroxides (for example, benzoyl peroxide), synthetic resins (for example, phenol-formaldehyde), nitro- and diazo compounds and others are used; The process conditions are the same as for sulfur vulcanization.
Vulcanization is also possible under the influence of ionizing radiation - g-radiation from radioactive cobalt, a flow of fast electrons (radiation vulcanization). Sulfur-free and radiation rubber methods make it possible to obtain rubbers that have high thermal and chemical resistance.
In the polymer industry, vulcanization is used in the extrusion production of rubber.
Vulcanization at prepairetires
The technological process of tire repair consists of preparing damaged areas for applying repair materials, applying repair materials to damaged areas and vulcanizing the areas being repaired.
Vulcanization of the repaired areas is one of the most important operations when repairing tires.
The essence of vulcanization is that when heated to a certain temperature, a physicochemical process occurs in unvulcanized rubber, as a result of which the rubber acquires elasticity, strength, resilience and other necessary qualities.
When two pieces of rubber glued together with rubber glue are vulcanized, they turn into a monolithic structure and the strength of their connection does not differ from the adhesion strength of the base material inside each piece. At the same time, to ensure the necessary strength, the pieces of rubber must be pressed - pressed under a pressure of 5 kg/cm 2.
In order for the vulcanization process to take place, it is not enough to only heat it to the required temperature, i.e., to 143+2°; The vulcanization process does not occur instantly, so heated tires must be kept for a certain time at the vulcanization temperature.
Vulcanization can occur at lower temperatures than 143°, but it takes longer. So, for example, if the temperature decreases from the specified one by only 10°, the vulcanization time should be doubled. In order to reduce the time for preheating during vulcanization, electric cuffs are used, which allow heating simultaneously on both sides of the tire, thereby reducing the vulcanization time and improving the quality of repairs. When one-sided heating of thick tires occurs, over-vulcanization of the rubber sections in contact with the vulcanization equipment occurs, and under-vulcanization of the rubber on the opposite side. Vulcanization time, depending on the type of damage and tire size, ranges from 30 to 180 minutes for tires and from 15 to 20 minutes for tubes
For vulcanization in motor vehicles, a stationary vulcanization apparatus model 601, produced by the GARO trust, is used.
The working set of the vulcanization apparatus includes corsets for sectors, corset tightening, tread and side profile linings, clamps, pressure pads, sand bags, mattresses.
With a steam pressure in the boiler of 4 kg/cm2, the required surface temperature of the vulcanization equipment is 143"+2°. At a pressure of 4.0--4.1 kg/cm2, the safety valve must open.
Vulcanizing devices must be inspected by a boiler inspector before being put into operation.
Internal damage to tires is vulcanized on sectors, external damage is cured on slabs using profile linings. Through damage (in the presence of electric cuffs, they are vulcanized on a plate with a profile lining, in the absence of electric cuffs, separately: first from the inside on the sector, then from the outside on a plate with a profile lining.
The electric cuff consists of several layers of rubber and an outer layer of rubberized chafer, in the middle of which there is a spiral of nichrome wire for heating and a thermostat to maintain a constant temperature (150°).
vulcanization industry repair tire
Rice. 4. Stationary vulcanizing apparatus GARO model 601: 1 - sector; 2 -- side plate; 3 -- boiler-steam generator; 4 -- small clamps for cameras; 5 -- bracket for cameras; 6 -- pressure gauge; 7-clamp for tires; 8 - firebox; 9 -- water meter glass; 10 -- manual plunger pump; 11 -- suction tube
Before vulcanization, the boundaries of the tire area to be repaired are marked. To eliminate sticking, powder it with talcum powder, as well as a sand bag, an electric cuff and vulcanization equipment (sectors, profile linings, etc.) in contact with the tire.
When vulcanizing on a sector, crimping is achieved by tightening a corset, and when vulcanizing on a slab, using a bag of sand and a clamp.
Profile linings (tread and bead) are selected in accordance with the location of the tire being repaired and its size.
During vulcanization, the electric cuff is located between the tire and the sand bag.
The start and end times of vulcanization are marked with chalk on a special board installed near the vulcanization equipment.
Repaired tires must meet the following requirements:
1) tires should not have unrepaired areas;
2) on the inside of the tire there should be no swelling and traces of patch delamination, under-vulcanization, folds and thickenings that impair the performance of the tube;
3) the rubber sections applied along the tread or sidewall must be completely vulcanized to a Shore hardness of 55-65;
4) tread areas larger than 200 mm restored during the repair process must have a pattern identical to the entire tread of the tire; an “All-terrain vehicle” pattern must be applied regardless of the size of the restored tread area;
5) the shape of the tire beads should not be distorted;
6) thickenings and depressions that distort the external dimensions and surface of the tire are not allowed;
7) repaired areas should not have any backlogs; the presence of shells or pores up to 20 mm 2 in area and up to 2 mm in depth is allowed in an amount of no more than two per square decimeter;
8) the quality of tire repair must ensure their guaranteed mileage after repair.
Vulcanization at prepairecameras
Similar to the tire repair process, the tube repair process consists of preparing damaged areas for patching, patching, and curing.
The scope of work to prepare damaged areas for patching includes: identifying hidden and visible damage, removing old unvulcanized patches, rounding edges with sharp corners, roughening rubber around the damage, cleaning chambers from roughening dust.
Rice. 5. Sector for vulcanization of tires: 1 -- sector; 2 -- tire; 2 -- corset; 4 -- tighten
Rice. 6. Vulcanization of bead damage to the tire on the bead plate: 1 - tire; 2 -- side plate: 3 -- side lining; 4 -- sandbag; 5 -- metal plate; 6 -- clamp
Visible damage is revealed by external inspection in good lighting and outlined with a chemical pencil.
To identify hidden damage, i.e. small punctures that are invisible to the eye, the camera, in an inflated state, is immersed in a bath of water, and the puncture site is determined by the escaping air bubbles, which is also outlined with a chemical pencil. The damaged surface of the chamber is roughened with a carborundum stone or a wire brush at a width of 25-35 mm from the boundaries of the damage, preventing roughening dust from getting inside the chamber. Rough areas are cleaned with a brush.
Repair materials for repairing inner tubes are: unvulcanized inner tube rubber 2 mm thick, rubber for inner tubes unsuitable for repair, and rubberized chafer. All punctures and tears up to 30 mm in size are sealed with raw, unvulcanized rubber. Damage greater than 30 mm is repaired using rubber for cameras. This rubber must be elastic, without cracks or mechanical damage. Raw rubber is refreshed with gasoline, coated with glue with a concentration of 1:8 and dried for 40-45 minutes. The chambers are roughened with a wire brush or carborundum stone on a roughening machine, after which they are cleaned of dust, refreshed with gasoline and dried for 25 minutes, then coated twice with glue with a concentration of 1: 8 and dried after each application for 30-40 minutes at a temperature 20--30°. The chafer is coated once with glue with a concentration of 1:8, then dried.
The patch is cut out in such a way that it covers the hole on all sides by 20-30 mm and is 2-3 mm smaller than the boundaries of the rough surface. It is applied to the repaired area of the chamber with one side and gradually rolled with a roller over the entire surface, so that there are no air bubbles left between it and the chamber. When gluing patches, you must ensure that the surfaces to be glued are completely clean, free from moisture, dust and greasy stains.
In cases where the chamber has a tear of more than 500 mm, it can be repaired by cutting out the damaged piece and inserting in its place the same piece from another chamber of the same size. This repair method is called chamber joining. The width of the joint must be at least 50 mm.
Damaged external threads of valve bodies are restored using dies, and internal threads are restored using taps.
If it is necessary to replace the valve, it is cut out together with the flange and another valve is vulcanized in the new location. The location of the old valve is repaired as normal damage.
Vulcanization of damaged areas is carried out using a model 601 vulcanization apparatus or a GARO vulcanization apparatus for vulcanizing chambers. Vulcanization time for patches is 15 minutes and flanges are 20 minutes at a temperature of 143+2°.
During vulcanization, the chamber is pressed with a clamp through a wooden plate to the surface of the plate. The overlay should be 10-15 mm larger than the patch.
If the area to be repaired does not fit on the slab, then it is vulcanized in two or three successive installations (rates).
After vulcanization, the beads on the unroughened surface are cut off with scissors, and the edges of the patches and burrs are removed on the stone of a roughening machine.
Repaired cameras must meet the following requirements:
1) the chamber filled with air must be sealed both along the body of the chamber and at the place where the valve is attached;
2) the patches must be tightly vulcanized, free from bubbles and porosity, their hardness must be the same as the rubber of the camera;
3) the edges of patches and flanges should not have thickenings or peeling;
4) the valve thread must be in good condition.
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The control method relates to the production of rubber products, namely to methods of controlling the vulcanization process. The method is carried out by adjusting the vulcanization time depending on the time of obtaining the maximum shear modulus of the rubber mixture during vulcanization of samples on a rheometer and the deviation of the tensile modulus of rubber in finished products from a given value. This makes it possible to work out disturbing influences on the vulcanization process based on the characteristics of the initial components and the operating parameters of the processes for obtaining the rubber mixture and vulcanization. The technical result is to increase the stability of the mechanical characteristics of rubber products. 5 ill.
The present invention relates to the production of rubber products, namely, to methods for controlling the vulcanization process.
The production process of rubber products includes the stages of obtaining rubber mixtures and their vulcanization. Vulcanization is one of the the most important processes in rubber production technology. Vulcanization is carried out by keeping the rubber mixture in presses, special boilers or vulcanizers at a temperature of 130-160°C for a given time. In this case, rubber macromolecules are connected by transverse chemical bonds into a spatial vulcanization network, as a result of which the plastic rubber mixture turns into highly elastic rubber. The spatial grid is formed as a result of heat-activated chemical reactions between rubber molecules and vulcanizing components (vulcanizers, accelerators, activators).
The main factors influencing the vulcanization process and the quality of finished products are the nature of the vulcanization medium, vulcanization temperature, vulcanization duration, pressure on the surface of the vulcanized product, and heating conditions.
With existing technology, the vulcanization mode is usually developed in advance by calculation and experimental methods and a program is set for the vulcanization process during the production of products. To ensure punctual implementation of the prescribed regime, the process is equipped with control and automation tools that most accurately implement the prescribed strict program for carrying out the vulcanization regime. The disadvantages of this method are the instability of the characteristics of the manufactured products due to the impossibility of ensuring complete reproducibility of the process, due to limitations in the accuracy of automation systems and the possibility of shifting modes, as well as changes in the characteristics of the rubber mixture over time.
There is a known method of vulcanization with temperature control in steam boilers, plates or mold jackets by changing the flow rate of coolants. The disadvantages of this method are the wide variation in the characteristics of the resulting products due to shifts in operating conditions, as well as changes in the reactivity of the rubber mixture.
There is a known method for controlling the vulcanization process by continuously monitoring those process parameters that determine its progress: the temperature of the coolants, the temperature of the surfaces of the vulcanized product. The disadvantage of this method is the instability of the characteristics of the resulting products due to the instability of the reactivity supplied to the molding of the rubber mixture, and the obtaining of different characteristics of the product during vulcanization under the same temperature conditions.
There is a known method for adjusting the vulcanization mode, including determining the temperature field in the vulcanized product using controlled external temperature conditions on the vulcanizing surfaces of products, determining the kinetics of non-isothermal vulcanization of thin laboratory plates using the dynamic modulus of harmonic shift in the found non-isothermal conditions, determining the duration of the vulcanization process at which an optimal set of the most important properties of rubber, determination of the temperature field for multilayer standard samples simulating a tire element in composition and geometry, obtaining the kinetics of non-isothermal vulcanization of multilayer plates and determining the equivalent vulcanization time based on the previously selected optimal level of properties, vulcanization of multilayer samples on a laboratory press at a constant temperature in the course of the equivalent vulcanization time and analysis of the resulting characteristics. This method is significantly more accurate than the methods used in industry for calculating effects and equivalent vulcanization times, but it is more cumbersome and does not take into account the change in the instability of the reactivity of the rubber mixture supplied for vulcanization.
There is a known method for regulating the vulcanization process, in which the temperature is measured in the areas of the product that limit the vulcanization process, the degrees of vulcanization are calculated from these data, and when the specified and calculated degrees of vulcanization are equal, the vulcanization cycle stops. The advantage of the system is the adjustment of the vulcanization time when the temperature fluctuations of the vulcanization process change. The disadvantage of this method is the large scatter in the characteristics of the resulting products due to the heterogeneity of the rubber mixture in terms of reactivity to vulcanization and the deviation of the vulcanization kinetics constants used in the calculation from the actual kinetics constants of the rubber mixture being processed.
There is a known method for controlling the vulcanization process, which consists in calculating the temperature in the controlled shoulder zone on the R-C grid using boundary conditions based on measurements of the surface temperature of the molds and the temperature of the diaphragm cavity, calculating equivalent vulcanization times that determine the degree of vulcanization in the controlled area, when implementing the equivalent time vulcanization in the real process the process stops. The disadvantages of this method are its complexity and the wide variation in the characteristics of the resulting products due to changes in the reactivity to vulcanization (activation energy, pre-exponential multiplier of kinetic constants) of the rubber mixture.
The closest to the proposed method is the method of controlling the vulcanization process, in which, synchronously with the actual vulcanization process according to the boundary conditions, based on temperature measurements on the surface of the metal mold, the temperature in the vulcanized products is calculated using a grid electrical model, the calculated temperature values are set on a vulcameter, on which parallel to the main During the vulcanization process, the kinetics of non-isothermal vulcanization of a sample from the batch of rubber mixture being processed is studied; when a given level of vulcanization is reached, control commands are generated on the vulcanization meter for the product vulcanization unit [AS USSR No. 467835]. The disadvantages of the method are the great complexity of implementation in the technological process and the limited scope of application.
The objective of the invention is to increase the stability of the characteristics of manufactured products.
This goal is achieved by the fact that the vulcanization time of rubber products on the production line is adjusted depending on the time of obtaining the maximum shear modulus of the rubber mixture during vulcanization of samples of the processed rubber mixture in laboratory conditions on a rheometer and the deviation of the tensile modulus of rubber in the manufactured products from the specified value.
The proposed solution is illustrated in Figs. 1-5.
Figure 1 shows a functional diagram of a control system that implements the proposed control method.
Figure 2 shows a block diagram of a control system that implements the proposed control method.
Figure 3 shows the time series of the tensile strength of the Jubo coupling, produced at OJSC Balakovorezinotekhnika.
Figure 4 shows characteristic kinetic curves for the moment of shear of rubber mixture samples.
Figure 5 shows a time series of changes in the duration of vulcanization of rubber mixture samples to 90% of the achievable shear modulus of the vulcanizate.
The functional diagram of the system that implements the proposed control method (see Fig. 1) shows the stage of preparation of the rubber mixture 1, the vulcanization stage 2, the rheometer 3 for studying the kinetics of vulcanization of rubber mixture samples, the mechanical dynamic analysis device 4 (or tensile testing machine) for determining rubber tensile module of finished products or satellite samples, control device 5.
The control method is implemented as follows. Samples from batches of the rubber mixture are analyzed on a rheometer and the values of the vulcanization time, at which the shear moment of the rubber has a maximum value, are sent to the control device 5. When the reactivity of the rubber mixture changes, the control device adjusts the vulcanization time of the products. Thus, disturbances are processed according to the characteristics of the initial components, affecting the reactivity of the resulting rubber mixture. The tensile modulus of rubber in finished products is measured by dynamic mechanical analysis or on a tensile testing machine and is also sent to the control device. The inaccuracy of the resulting adjustment, as well as the presence of changes in the temperature of coolants, heat exchange conditions and other disturbing influences on the vulcanization process are worked out by adjusting the vulcanization time depending on the deviation of the tensile modulus of rubber in manufactured products from the specified value.
The block diagram of the control system that implements this control method and is presented in Fig. 2 includes a control device of the direct control channel 6, a control device of the feedback channel 7, an object for controlling the vulcanization process 8, a transport delay link 9 to take into account the length of time for determining the characteristics of the rubber of finished products , a comparison element of the feedback channel 10, an adder 11 for summing up the vulcanization time adjustments via the direct control channel and the feedback channel, an adder 12 for taking into account the influence of uncontrolled disturbances on the vulcanization process.
When the reactivity of the rubber mixture changes, the estimate τ max changes and the control device via direct control channel 1 adjusts the vulcanization time in the technological process by the value Δτ 1.
In a real process, the vulcanization conditions differ from the conditions on the rheometer, therefore the vulcanization time required to obtain the maximum torque value in the real process also differs from that obtained on the device, and this difference changes over time due to the instability of the vulcanization conditions. These disturbances f are processed through the feedback channel by introducing a correction Δτ 2 by the control device 7 of the feedback loop, depending on the deviation of the rubber module in the manufactured products from the specified value E set.
Transport delay link 9, when analyzing the dynamics of the system, takes into account the influence of the time required to analyze the characteristics of the rubber of the finished product.
Figure 3 shows the time series of the conditional breaking force of the Juba coupling, produced by OJSC Balakovorezinotekhnika. The data show a wide range of products for this indicator. The time series can be represented as the sum of three components: low-frequency x 1, mid-frequency x 2, high-frequency x 3. The presence of a low-frequency component indicates the insufficient efficiency of the existing process control system and the fundamental possibility of building an effective feedback control system to reduce the spread of parameters of the finished product according to its characteristics.
Figure 4 shows characteristic experimental kinetic curves for the shear moment during vulcanization of rubber mixture samples, obtained on an Alfa Technologies MDR2000 rheometer. The data shows the heterogeneity of the rubber mixture in terms of reactivity to the vulcanization process. The spread in time to reach the maximum torque ranges from 6.5 minutes (curves 1.2) to more than 12 minutes (curves 3.4). The spread in the completion of the vulcanization process ranges from not reaching the maximum torque value (curves 3.4) to the presence of the over-vulcanization process (curves 1.5).
Figure 5 shows the time series of vulcanization time to the 90% level of maximum shear moment, obtained by studying the vulcanization of rubber mixture samples on the MDR2000 Alfa Technologies rheometer. The data shows the presence of a low-frequency variation in the curing time to obtain the maximum shear moment of the vulcanizate.
The presence of a large scatter in the mechanical characteristics of the Juba coupling (Fig. 3) indicates the relevance of solving the problem of increasing the stability of the characteristics of rubber products in order to increase their operational reliability and competitiveness. The presence of instability in the reactivity of the rubber mixture to the vulcanization process (Fig. 4, 5) indicates the need to change the time during the vulcanization process of products made from this rubber mixture. The presence of low-frequency components in the time series of the conditional breaking force of finished products (Fig. 3) and in the vulcanization time to obtain the maximum shear moment of the vulcanizate (Fig. 5) indicates the fundamental possibility of increasing the quality indicators of the finished product by adjusting the vulcanization time.
The above confirms the presence in the proposed technical solution:
Technical result, i.e. the proposed solution is aimed at increasing the stability of the mechanical characteristics of rubber products, reducing the number of defective products and, accordingly, reducing the specific consumption rates of initial components and energy;
Essential features consisting in adjusting the duration of the vulcanization process depending on the reactivity of the rubber mixture to the vulcanization process and depending on the deviation of the rubber tensile modulus in finished products from the specified value;