"Vulcanization" is named after the original natural rubber products were crosslinked with sulfur as crosslinking agent. With the development of rubber industry, various non-sulfur crosslinking agents can be used for crosslinking. Therefore, the more scientific meaning of vulcanization should be "cross-linking" or "bridging", that is, the process of linear polymers forming network polymers through cross-linking.
Chinese name: vulcanization mbth: vulcanization property: original natural rubber product object: rubber product use: realizing natural rubber crosslinking Raw materials: sulfur definition, overview, vulcanization conditions, vulcanization process, vulcanization method, equipment, vulcanization accelerator, peroxide vulcanization, definition: adding sulfur and carbon black to raw rubber and heating it under high pressure to make it vulcanized rubber. This process is called vulcanization. However, in order to realize the ideal vulcanization process, in addition to selecting the best vulcanization conditions, the selection of compounding agents, especially accelerators, is of decisive significance. With the increase of synthetic rubber varieties and the in-depth study of vulcanization methods and vulcanizing agents, it is found that many non-sulfur compounds also have vulcanization function. Therefore, this term has developed into an extended industrial term. After vulcanization, the inherent defects of rubber, such as low strength, small elasticity, cold hardness, hot adhesion and easy aging, are changed, and the wear resistance, swelling resistance and heat resistance are obviously improved, thus expanding the application scope. The chemical reaction between rubber macromolecules and cross-linking agent sulfur under heating and the process of cross-linking into three-dimensional network structure are summarized Vulcanized rubber is called vulcanized rubber. Vulcanization is the last working procedure of rubber processing, and molded rubber products with practical value can be obtained. In the network structure of rubber, sulfur crosslinking bonds (where the number of sulfur atoms n ≥1; The density of uncrosslinked sulfur atoms (X or Y) determines the vulcanization degree of rubber. The latter is judged by the macroscopic physical and mechanical properties of rubber compound or the change of rubber viscosity in process practice. Chemical vulcanization is the most commonly used vulcanization method in rubber industry, which can be divided into room temperature vulcanization and heating vulcanization. Heating vulcanization is the main production method of rubber products. After vulcanization, the physical and mechanical properties of rubber are obviously improved, the strength, elasticity and tensile modulus are increased, and the plastic deformation is reduced, becoming an insoluble elastomer (only limited swelling). Besides sulfur, peroxides, aliphatic or aromatic amines, sulfonates, aromatic diols and quaternary phosphonium (ammonium) salts can also be used as vulcanizing agents. The main factors affecting the vulcanization process are sulfur dosage, vulcanization temperature and vulcanization time. ① Sulfur consumption. The higher the dosage, the faster the vulcanization speed and the higher the degree of vulcanization that can be achieved. The solubility of sulfur in rubber is limited, and excessive sulfur will precipitate from the surface of rubber compound, commonly known as "sulfur spraying". In order to reduce sulfur spraying, it is required to add sulfur at the lowest possible temperature or at least below the melting point of sulfur. According to the use requirements of rubber products, the sulfur content in soft rubber is generally less than 3%, in semi-hard rubber it is generally around 20%, and in hard rubber it can be as high as over 40%. ② Vulcanization temperature. If the temperature is higher than 10℃, the curing time will be shortened by about half. Because rubber is a poor heat conductor, the vulcanization process of products is different with different temperatures in different parts. In order to ensure a relatively uniform degree of vulcanization, thick rubber products are generally vulcanized at low temperature for a long time. ③ Curing time. This is an important link in the vulcanization process. The time is too short and the degree of vulcanization is insufficient (also known as sulfur deficiency). Over time, the degree of vulcanization is too high (commonly known as persulfate). Only a proper degree of vulcanization (commonly known as positive vulcanization) can ensure the best comprehensive properties. Vulcanization conditions Vulcanization process can be divided into four stages, each with its own characteristics. By measuring the tensile strength of rubber (or vulcanizer), it can be seen that the whole vulcanization process can be divided into four stages: vulcanization induction, pre-vulcanization, normal vulcanization and over-vulcanization (for natural rubber, it is vulcanization recovery). During the curing induction period (scorch time) when the curing time is constant and the tensile strength is constant, the crosslinking has not yet started, and the fluidity of the compound is good. This stage determines the scorch performance and processing safety of the compound. At the end of this stage, the compound begins to cross-link and loses fluidity. The length of vulcanization induction period is not only related to the nature of raw rubber, but also depends on the additives used. For example, a longer scorching time can be obtained by using a delay accelerator, and it has higher processing safety. After the induction period of vulcanization, it is the pre-vulcanization stage of crosslinking at a certain speed. The crosslinking degree in the pre-vulcanization stage is low. Even in the later stage, the tear strength and elasticity of vulcanizate can not reach the expected level, but the tear and dynamic cracking properties are better than the corresponding normal vulcanization. After the molecular change diagram of rubber in vulcanization reaches the normal vulcanization stage, the physical properties of vulcanized rubber reach or approach the optimal point respectively, or reach the comprehensive balance of properties. After the normal vulcanization stage (vulcanization flat zone), it is the over-vulcanization stage. There are two situations: natural rubber "reverts" (tensile strength decreases), and the tensile strength of most synthetic rubber (except butyl rubber) continues to increase. For any rubber, vulcanization not only produces cross-linking, but also breaks the production chain and molecular chain due to factors such as heating. This phenomenon runs through the whole vulcanization process. In the over-vulcanization stage, if cross-linking is still the main method, the rubber will harden and the tensile strength will continue to rise. On the contrary, rubber will soften, that is, return to its original state. According to vulcanization conditions, vulcanization methods can be divided into three types: cold vulcanization, room temperature vulcanization and hot vulcanization. Cold vulcanization can be used to vulcanize film products. Soak the product in carbon disulfide solution containing 2% ~ 5% sulfur chloride, then wash and dry. When curing at room temperature, the curing process is carried out at room temperature and pressure, such as the joint of bicycle inner tube and the normal temperature vulcanized rubber slurry (mixed rubber solution) for repair. Thermal vulcanization is the main method of vulcanization of rubber products. According to the different vulcanization media and vulcanization methods, thermal vulcanization can be divided into three methods: direct vulcanization, indirect vulcanization and mixed gas vulcanization. (1) Direct vulcanization, that is, the product is directly vulcanized in hot water or steam medium. (2) Indirect vulcanization, i.e. the product is vulcanized in hot air. This method is generally used for some products with strict appearance requirements, such as rubber shoes. (3) Mixed gas vulcanization: air vulcanization is adopted first, and then direct steam vulcanization is adopted. This method not only overcomes the shortcomings that steam vulcanization affects the appearance of products, but also overcomes the shortcomings of slow heat transfer, long vulcanization time and easy aging of hot air. The above vulcanization methods all belong to batch production, and some rubber products with unlimited length can be continuously vulcanized, such as salt bath vulcanization, boiling bed vulcanization, microwave or high frequency vulcanization, drum vulcanization of adhesive tape and rubber plate, etc. In addition to sulfur vulcanization, rubber products can also adopt sulfur-free vulcanization and high-energy ray vulcanization, but their application fields are limited. Equipment Flat Curing Machine The main curing equipment in China mainly includes flat curing machine, traditional cylinder curing machine, new circular curing machine and international advanced cylinder curing machine. In China, flat vulcanizer and traditional cylinder vulcanizer are mainly used. In 2006, the 1725 hydraulic vulcanizer developed by Guilin Rubber Machinery Factory passed the appraisal organized by the Science and Technology Department of Guangxi Zhuang Autonomous Region. The appraisal committee agreed that the product filled the domestic gap and its performance reached the international advanced level. It is an ideal equipment for curing high-grade radial truck tires and has strong competitiveness in the international market. The product has the following characteristics: the frame is reasonable, and each curing chamber has an independent frame to bear the clamping force; Good rigidity, small deformation of upper beam and base; The central mechanism is novel and unique in design, and automatically aligns with the manipulator for loading and unloading tires, thus improving the positioning accuracy; The vulcanizing chamber has a novel structure, and the lower steam chamber is used to contain the upper steam chamber, which changes the traditional sealing form and improves the sealing effect, service life and safety; The movable block of the mold is installed in the lower steam chamber, which improves the service life and safety of the mold and facilitates the cleaning of the mold; All components of the hydraulic system adopt advanced and reliable components, running smoothly; The electrical system adopts international advanced control elements, and the position of mold opening and closing, the position of manipulator lifting and the position of lifting rings in the central mechanism are all controlled by linear displacement sensors, which ensures the positioning accuracy and reliability. Adopt international advanced grating safety monitoring technology to ensure the safety of operators. The product has passed the' CE' certification of the European Union, and the vulcanizing chamber has passed the pressure vessel certification of the European Union. Vulcanization accelerator of monomer vulcanizer Vulcanization accelerator is an auxiliary agent that can accelerate vulcanization reaction, shorten vulcanization time, lower vulcanization temperature, reduce the dosage of vulcanizing agent, and improve or improve the physical and mechanical properties of vulcanized rubber. According to its properties and chemical composition, it can be divided into two categories: inorganic accelerators and organic accelerators. Inorganic accelerators were used the earliest, but the properties of vulcanizates were poor because of their small promotion. Now it is basically replaced by organic accelerators. With the continuous development of synthetic rubber varieties and uses, the existing accelerators have various names. According to its chemical structure, accelerators are usually divided into thiazoles, Qiu Lan, sulfenamides, guanidines, dithiocarbamates, aldehydes and amines, xanthates and thioureas. Traditional accelerators containing secondary amine groups, such as NOBS, DIBS, TNTD, TETD, etc. It is considered to be potentially harmful to health, so the problem of finding substitutes for these accelerators or developing accelerators without nitrosamines has aroused widespread concern in the world. N- tert-butyl -2- bis-benzothiazole sulfenamide, a new accelerator introduced by American Wellesley Company, is the best substitute for traditional accelerators containing secondary amine groups. The main curing systems used in peroxide curing industry are peroxide (2,5-dimethyl-2,5-bis (tert-butylperoxy) hexane, DCP, etc. ), and the vulcanization mechanism is mainly divided into: (1) peroxide is decomposed into two active free radicals; (2) The generated active free radicals attack the active hydrogen atoms or unsaturated double bonds on the rubber chain; (3) Two active rubber chains collide to form a cross-linked network. The vulcanization degree of rubber is related to the decomposition degree and rate of peroxide. Chatterjee et al. compared the effects of three peroxide curing agents with different half-lives, such as DCP, 3,3,5,7,7-pentamethyl-1, 2,4-trioxyhexane (PMTO) and cumyl hydrogen peroxide (CHP), on the mechanical properties of PDMS/PA 12 thermoplastic vulcanizates. The results show that this is mainly due to the long half-life of PMTO, which can fully vulcanize silicone rubber and obtain thermoplastic vulcanized rubber with high vulcanization degree. For peroxide with short half-life, the vulcanization speed is too fast, which is easy to cause local vulcanization, and silicone rubber can not be fully vulcanized, thus affecting the product performance. Mani et al. found that during the dynamic vulcanization process of PDMS/PA 12 thermoplastic elastomer with DCP as vulcanizing agent, adding tetramethylpiperidine oxide (TEMPO) can prolong the vulcanization time, so that silicone rubber can be fully vulcanized and uniformly dispersed in PA 12. In this process, TEMPO actually acts as a polymerization inhibitor, mainly to generate graft polymers with the generated polymer active radicals, thus preventing the collision between polymer active radicals and generating a network structure.