The macromolecule of rubber undergoes a chemical reaction with the crosslinking agent sulfur under the heating of a rubber extrusion production line, and cross-links into a three-dimensional network structure. The vulcanized rubber is called vulcanizate. Vulcanization is the last step in rubber processing, and it is possible to obtain shaped rubber products with practical value. In the rubber network structure, the density of sulfur crosslinks (where the number of sulfur atoms is n ≥ 1; the number of uncrosslinked sulfur atoms is Sx or Sy) determines the degree of vulcanization of the rubber. The latter is judged in the process practice by the macroscopic physical and mechanical properties of the rubber or the change in rubber viscosity. Chemical vulcanization is the most commonly used vulcanization method in the rubber industry. It is divided into room temperature vulcanization and heat vulcanization. Heat vulcanization is the main production method of rubber products. After vulcanization, the physical and mechanical properties of the rubber are significantly improved, the strength, elasticity, and tensile modulus are increased, and the plastic deformation is reduced to become an insoluble elastomer (only limited swelling). In addition to sulfur, compounds such as peroxides, fats or aromatic amines, sulfonates, aromatic diols, and quaternary ammonium (ammonium) salts can be used as vulcanizing agents.
The main factors affecting the vulcanization process are the amount of sulfur, vulcanization temperature and vulcanization time. ① Sulfur dosage. The larger the amount, the faster the vulcanization speed, and the higher the degree of vulcanization that can be achieved. The solubility of sulfur in rubber is limited. Excess sulfur will be precipitated from the rubber surface, commonly known as "sulfur spray". In order to reduce the sulfur injection phenomenon, it is required to add sulfur at the lowest possible temperature, or at least below the melting point of sulfur.
According to the curing conditions, it can be divided into three types: cold curing, room temperature curing and hot curing. Cold vulcanization can be used for the vulcanization of thin film products. The products can be immersed in a carbon disulfide solution containing 2% to 5% sulfur chloride, and then washed and dried. During room temperature vulcanization, the vulcanization process is carried out at room temperature and normal pressure, such as using room temperature vulcanized rubber (mixed rubber solution) for bicycle inner tube joints and repair. Thermal vulcanization is the main method of vulcanizing rubber products. According to the different curing media and curing methods, thermal curing can be divided into three methods: direct curing, indirect curing and mixed gas curing. ① Direct vulcanization, put the product directly into hot water or steam medium for vulcanization. ② Indirect vulcanization, the product is vulcanized in hot air. This method is generally used for some products with strict appearance requirements, such as rubber shoes. ③Mixed gas vulcanization, first use air vulcanization, and then use direct steam vulcanization. This method can overcome the shortcomings of steam vulcanization that affect the appearance of the product, and can also overcome the disadvantages of long vulcanization time and easy aging due to the slow heat transfer of hot air.