Sulfur is the most commonly used vulcanizing agent, along with selenium and tellurium in the same family. There are two types of sulfur, rhombic and amorphous (or insoluble sulfur). The former is a cyclic structure containing 8 sulfur atoms, S8. The latter is actually a metastable polymer with a molecular weight of 100,000-300,000. It is insoluble in most solvents and rubber, so it is called "insoluble sulfur". Sulfur is generally obtained from natural sulfur-containing ores through dry distillation, cooling and crystallization; it can also be obtained by separating hydrogen sulfide from petroleum refining waste gas, and then recovering it from hydrogen sulfide. After processing, different varieties of sulfur can be obtained for use in the rubber industry. The properties of various types of sulfur are as follows:
1. Powdered sulphur Block sulphur can be obtained as powdered sulphur by crushing and deacidification. The fineness of sulfur used in rubber is generally below 700 mesh, and the special variety is about 600 mesh.
2. Sedimentation of sulfur Sulfur and oxygen calcium oxide are heated together to form polysulfide calcifications, and dilute sulfuric acid is added to make the sulfur settle out. It is called sedimentary sulfur, the average particle size is i-5um, and it has excellent dispersibility in rubber and is used for advanced rubber products.
3. Colloidal sulfur, also known as highly dispersive sulfur. In the presence of a dispersant, grind the powdered sulfur or precipitated sulfur with a ball mill or a colloid mill to make a viscous substance, then dry and pulverize to make a colloidal sulfur with a particle size of 1--3um, suitable for latex products .
4. Surface treatment Sulfur is coated with a layer of oil or polyisobutylene and other substances on the surface of sulfur particles, which is conducive to dispersion. Generally, 2.5% oil and sulfur are used to make ointment.
5. Insoluble sulfur is also called elastic sulfur. The crystalline sulfur is heated to 200-250 degrees Celsius, in a molten state, and rapidly cooled at a temperature of 20 degrees to obtain a transparent elastomer chain polymer sulfur. Due to the supercooling operation, it maintains a solution state and can remain stable for a long time below -30 degrees. But above room temperature, it gradually becomes a rhombic structure and assumes a ring S. Especially noticeable at 90-100°C. The transfer conditions of insoluble sulfur are: stable for 24 hours at room temperature, stable for 1.5 hours at 78°C, and immediately transformed at 105°C. The use of insoluble sulfur can prevent blooming and shorten the vulcanization time. It is suitable for high temperature and short time vulcanization.
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