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.
Physically, it is the process of transforming plastic rubber into elastic rubber or hard rubber.
The meaning of "vulcanization" includes not only the actual crosslinking process, but also the method of producing crosslinking.
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 in rubber processing, and molded and practical rubber products 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 (Sx or Sy) determines the vulcanization degree of rubber.
In process practice, the latter is judged by the macroscopic physical and mechanical properties of rubber compound or the change of rubber viscosity.
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 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, 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.
By measuring the tensile strength of rubber (or vulcanizer), we can see 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 induction period of vulcanization (scorch time), crosslinking has not yet started, and the compound has good fluidity.
This stage determines the scorch and 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 promoter, and it has higher 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 reaching the positive 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 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 sulfur-over stage, if crosslinking 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.
Methods According to curing conditions, individual curing presses can be divided into three types: cold curing, room temperature curing and thermal curing.
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 normal temperature and pressure, such as bicycle inner tube joints and room temperature curing 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 the application scope is limited.
Equipment Flat vulcanizer Rubber general vulcanizing equipment can be generally divided into four categories: vulcanizing tank, flat vulcanizer, monomer vulcanizer and injection vulcanizer.
Vulcanization tanks can be divided into vertical and horizontal types. The former is mostly used for vulcanization of tires, while the latter is mostly used for vulcanization of rubber hoses, cables and rubber shoes. Flat vulcanizer is used to vulcanize rubber sheets, adhesive tapes and some model products; Individual vulcanizers are mostly used to vulcanize automobile tires and power tires; Application of injection vulcanizer in rubber shoes and seals production.
For rubber model products, its molding, setting and vulcanization are carried out at the same time.
For products with unlimited length (such as adhesive tape, adhesive tape, etc.). ), they can be produced by a continuous vulcanizer.
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 vulcanization chamber has a novel structure, and the structure that the lower steam chamber contains the upper steam chamber changes the traditional sealing form, improving 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 workers.
At present, the product has passed the EU's' CE' certification, and the vulcanization chamber has passed the EU's pressure vessel certification.
Accelerator Vulcanization Accelerator Vulcanization Accelerator is an auxiliary agent that can accelerate the vulcanization reaction, shorten the vulcanization time, lower the 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 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 NOBS, DIBS, d and TETD containing secondary amine groups
It is considered to be potentially harmful to health, so the problem of finding substitutes for these accelerators or developing non-secondary accelerators 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.
In addition, Monsanto, Goodyear and Uni royle in the United States, Bayer and Rheinland Chemical Company in Germany have also produced various non-toxic composite accelerators.
China Zibo Ying Hui Rubber Additives Co., Ltd. successfully developed primary amine-based non-carcinogenic accelerator NS, which created conditions for replacing secondary amine-based carcinogenic accelerator.
Nanjing Chemical Plant has made gratifying progress in the development of non-carcinogenic accelerators, but the main problem is that the development of various non-subtype accelerators and the research and substitution of NS accelerators are slow, which is far from developed countries.
The technological mode of thermal vulcanization: reactive injection molding vulcanization: refers to the impact mixing of two or more components under high pressure (14-20MPa), and then the evenly mixed materials are quantitatively injected into the mold cavity with a certain temperature by an injection molding machine for vulcanization molding into products. Because the polymerization of raw materials and the molding of products are integrated in the production process, the mixing speed is fast and the mixing quality is high, and the production steps are optimized. And the production efficiency is improved. For example, the mixture of polyol and isocyanate enters the mixing chamber through a metering pump at a pressure of about 65,438 0.4 MPa, resulting in turbulence in the mixing chamber. After fully mixing, it enters the mold cavity through the injection port and is vulcanized at 80-65,438+000℃ for a certain time. Room temperature vulcanization: The process of vulcanizing products at room temperature and pressure is usually called room temperature vulcanization, such as curing adhesives at room temperature. Usually, two components are made: accelerator and inert compounding agent are mixed with solvent to make one component, and rubber and vulcanizing agent are mixed in proportion when used. Room temperature vulcanized mucilage is often used to repair the joints of belt vulcanized rubber or industrial rubber sheet products. Cold vulcanization method: vulcanization method is also called sulfur monochloride solution vulcanization method, that is, semi-finished products are immersed in carbon disulfide, benzene or carbon tetrachloride solution containing 2-5% sulfur monochloride for a certain period of time to complete vulcanization; Electron beam radiation continuous vulcanization: different from microwave vulcanization, rubber macromolecules can be crosslinked at room temperature, and electron beam can be directly ionized and activated to produce crosslinking reaction; The trimming and trimming of rubber mould products make the surface of products smooth and meet the requirements. After some special processes, such as surface treatment, the performance of special products has been improved. Products such as ordinary canvas conveyor belt with fabric skeleton should be heated, stretched and cooled under inflation pressure after vulcanization to ensure the stability of product size and shape and good service performance.