Vulcanization is also called cross-linking and curing.Add in rubberVulcanizing agentandacceleratorUnder certain temperature and pressure, the linear macromolecule can be transformed into three-dimensional network structure.Since sulfur was first usedNatural rubberIt is cross-linked, so it is called vulcanization.
"Vulcanization" was named after the original natural rubber products were crosslinked with sulfur as the crosslinking agent. With the development of the rubber industry, a variety of 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 forming a network of linear polymers through cross-linking.
Adding sulfur, carbon black, etc. into raw rubber and heating it under high pressure can turn it into 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 the selection of accelerants, is of decisive significance.With the increase of the variety of synthetic rubber and the in-depth study of vulcanization methods and vulcanization agents, it has been found that many non sulfur compounds also have vulcanization effects.Therefore, the term has developed into an extended industrial term.The vulcanized rubber has changed its inherent defects such as low strength, small elasticity, cold hardness and hot stickiness, and easy aging. Its wear resistance, swelling resistance, and heat resistance have been significantly improved, expanding its application range.
Introduction to Chemistry
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Rubber macromolecules occur with cross-linking agent sulfur under heatingchemical reactionThe process of cross-linking into a three-dimensional network structure.The vulcanized rubber is called vulcanizate.Vulcanization is the last process in rubber processing, which can be finalized and has practical valueRubber products。In the network structure of rubber, sulfur cross-linking bond (sulfurAtomic numbern≥1；The number of sulfur atoms not crosslinked is SxOr Sy）The density of determines the degree of vulcanization of rubber.The latter is based on the macroscopic physical and mechanical properties of rubber or rubberviscosityTo judge the change.
Chemical vulcanization is the most commonly used vulcanization method in the rubber industry. There are 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 significantly improved, the strength, elasticity and tensile modulus are increased, and the plastic deformation is reduced, becoming an insoluble elastomer (only limited swelling).In addition to sulfur,peroxideFats or aromatic amines, sulfonates, aromatic diols and quaternary phosphonium (ammonium) salts can be used as vulcanizing agents.
Vulcanization conditions
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Vulcanization conditions
The main factors affecting the vulcanization process are the amount of sulfur, vulcanization temperature and vulcanization time. ①Sulfur consumption.The higher the dosage, the faster the vulcanization speed and the higher the degree of vulcanization can be achieved.The solubility of sulfur in rubber is limited, and excessive sulfur will be separated from the rubber surface, commonly known as "sulfur injection".In order to reduce sulfur injection, 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, sulfurSoft rubberGenerally, the dosage in the semi hard glue is not more than 3%, the dosage in the semi hard glue is generally about 20%, and the dosage in the hard glue can be more than 40%. ②Vulcanization temperature。If the temperature is 10 ℃ higher,Vulcanization timeAbout half.Because the rubber is poorThermal conductorThe vulcanization process of the product is different due to the temperature difference of each part.In order to ensure a more uniform degree of vulcanization, thick rubber products are generally vulcanized for a long time by gradually heating up and low temperature. ③Cure time.This isVulcanization processThe important link of.The time is too short, and the degree of vulcanization is insufficient (also called sulfur deficiency).The time is too long, and the degree of vulcanization is too high (commonly known as over sulfur).Only the appropriate degree of vulcanization (commonly known as positive vulcanization) can ensure the best comprehensive performance.
Vulcanization process
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It can be divided into four stages, each with its own characteristics.
By measuring the constant tensile strength of rubber material (orVulcanizer）It can be seen that the whole vulcanization process can be divided into four stages: vulcanization induction, pre vulcanization, positive vulcanization and over vulcanization (for natural rubber, it is vulcanization reversion).
Curing time - constant tensile strength
vulcanizationinduction period（ScorchTime), the crosslinking has not yet started, and the rubber has good fluidity.This stage determines the scorching property and processing safety of the rubber compound.At the end of this stage, the rubber begins to cross link and loses its fluidity.The difference between the length of vulcanization induction period andRaw rubberIt is related to its nature and mainly depends on the additives used, such as the delayacceleratorIt can obtain longer scorching time and higher processing safety.
After the curing induction period is the pre curing stage of crosslinking at a certain speed.The degree of crosslinking during the pre curing period is low, even at the later stageVulcanizateThe tensile strength and elasticity of the composites can not reach the expected level, but the tear and dynamic fracture properties are better than those of the corresponding positive vulcanization.
Diagram of rubber molecular changes during vulcanization
After reaching the positive curing stage, various physical properties of the vulcanizate reach or approach the optimal point, or reach the comprehensive balance of properties.
After the positive vulcanization stage (vulcanization flat area), it is the over sulfur stage. There are two situations: the natural rubber has "reversion" (the constant elongation strength decreases), and most synthetic rubber (exceptButyl rubberThe constant elongation strength continues to increase.
For any rubber, vulcanization not only produces cross-linking, but also produces chain linking and molecular chain breaking due to heat and other factors.This phenomenon runs through the whole vulcanization process.In the over sulfur stage, if the cross-linking is still dominant, the rubber will harden and the constant elongation strength will continue to rise; on the contrary, the rubber will become soft, which meansReturn to the original
Vulcanization method
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According to the curing conditions, it can be divided into cold curing, room temperature curing and hot curing.Cold vulcanization can be used forfilmVulcanization of products, products containing 2%~5%Sulfur chlorideOfcarbon disulfideImmerse in solution, then wash and dry.During room temperature vulcanization, the vulcanization process is carried out at room temperature and normal pressure, such as using room temperature vulcanized rubber slurry（Rubber mixtureSolution) for cyclingInner tubeJoints, repairs, etc.Thermal vulcanization is the main method of vulcanization of rubber products.According to different vulcanizing media and methods, thermal vulcanization can be divided into direct vulcanization, indirect vulcanization and gas mixture vulcanization. ①Direct vulcanization: put the product directly into hot water or steam medium for vulcanization. ②Indirect vulcanization: products are vulcanized in hot air. This method is generally used for some products with strict appearance requirements, such as rubber shoes. ③For gas mixture vulcanization, air vulcanization is used first, and then direct steam vulcanization is used.This method can not only overcome the disadvantages of steam vulcanization affecting the appearance of products, but also overcome the disadvantages of long vulcanization time and easy aging due to slow heat transfer of hot air.[1]
The above vulcanization methods are batch production, and some rubber products with unlimited length can be continuously vulcanized, such as salt bath vulcanizationFluidized bedVulcanization, microwave or high-frequency vulcanization, adhesive tape and rubber plateDrum vulcanizerVulcanization, etc.In addition to sulfur vulcanization, rubber products can also be vulcanized without sulfur and with high energyradialVulcanization, but its application is limited.
equipment
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Flat curing press
The main domestic vulcanization equipment mainly includesFlat curing press, traditional round mold cylinder curing pressNew round mold curing pressInternational advanced drum vulcanizer and other equipment;Flat vulcanization andTraditional round mold cylinder curing press。[2]
In 2006,Guilin Rubber Machinery FactoryThe 1725 hydraulic curing press developed has passed theGuangxi Zhuang Autonomous RegionIdentification organized by the Department of Science and Technology.The appraisal committee agreed that this product filled the gap in China, and its performance reached the international advanced level. It is an ideal equipment for vulcanizing high-grade radial truck tires, and has strong competitiveness in the international market.
The product has the following characteristics: the force of the rack is reasonable, and each curing chamber has an independent frame to bear the mold closing force;Good rigidity, small deformation of upper crossbeam and base;The central mechanism is novel in design and original, and can automatically align with the tire loading and unloading manipulator to improve thepositioning accuracy ；The curing chamber has a novel structure, and the lower steam chamber contains the junction of the upper steam chamber
Individual curing machine
Structure, changing the traditional sealing form, thus improving the sealing effect, service life and safety;The mold flexible block is installed in the lower steam chamber, which improves the service life and safety of the mold, and facilitates the cleaning of the mold;The components of the hydraulic system are all advanced and reliable components, ensuring smooth operation;The electrical system adopts internationally advanced control elements, and the opening and closing positions of the equipment, the lifting positions of the manipulator, and the lifting positions of the upper ring of the central mechanism adoptLinear displacement sensorControl to ensure the positioning accuracy and reliability;International advanced grating safety monitoring technology is adopted to ensure the safety of operators.This product has passedeuropean unionThe vulcanizing chamber has passed the EU pressure vessel certification.
Vulcanization accelerator
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Vulcanization acceleratorIt is a kind of auxiliary agent that can accelerate the curing reaction speed, shorten the curing time, reduce the curing temperature, reduce the amount of curing agent, and improve or improve the physical and mechanical properties of vulcanizates.According to its nature and chemical composition, it can be divided into two categories: inorganic accelerator and organic accelerator.Inorganic accelerator was first used, but due to its small promotion effect and poor performance of vulcanizate, it is now basically replaced by organic accelerator.along withsynthetic rubberWith the continuous development of varieties and uses, the existing promoters have various names. According to the chemical structure of promoters, they are usually divided intoThiazoleClass, thiuram class, hyposulphonateamideClass A, Guanidine, Dithioaminoformic acidsalt, aldehydes and aminesXanthic acidSalts andThiourea。Tradition containing secondary amine groupAccelerator NOBS DIBS, TNTD, TETD, etc. are considered to be potentially harmful to health, so look for alternatives to these promoters or develop noneNitrosamineThe problem of accelerators has attracted international attention.New accelerator N launched by American Verizon-Tert butyl-2-DoubleBenzothiazolesecondSulfonamideIt is the best substitute for traditional accelerator containing secondary amine group.[3]
Peroxide vulcanization
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The vulcanization systems used in industry are mainly peroxides (2,5-dimethyl-2,5-di (tert-butylperoxy) hexane, DCP, etc.). The vulcanization mechanism is mainly divided into: (1) peroxides are decomposed into two active free radicals;(2) The generated active free radical attacks the active hydrogen atom or unsaturated double bond on the rubber chain;(3) Two active rubber chains collide to form a cross-linking network.The degree of vulcanization of rubber is related to the decomposition degree and rate of peroxide.Chatterjee et al. compared the effect of three peroxide vulcanizing agents with different half-life, namely DCP, 3,3,5,7,7-pentamethyl-1,2,4-tricyclohexane (PMTO) and cumyl hydrogen peroxide (CHP), on PDMS/PA12Thermoplastic vulcanized rubberThe results show that PMTO is the best vulcanizing agent when the processing temperature is 190 ℃.This is mainly because PMTO has a long half-life, which can fully vulcanize silicone rubber to obtain thermoplastic vulcanized rubber with high sulfur degree.
For peroxides with short half-life, if the curing rate is too fast, it is easy to cause local vulcanization, and the silicone rubber cannot be fully vulcanized, thus affecting the product performance.Mani et al found that PDMS/PA12 with DCP as curing agentThermoplastic elastomerIn the dynamic vulcanization process ofTetramethylpiperidineOxygen (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, which is mainly generated with the generated polymer active free radicalsgraft copolymer To prevent collision between polymer active radicals from generating network structure.
