Natural rubber can be divided into solid natural rubber (film and particle rubber) and concentrated latex according to its morphology. In daily use, solid natural rubber accounts for the vast majority.
According to the different manufacturing processes and shapes, films can be divided into cigarette glue, air-dried film, white wrinkled film and brown wrinkled film. Tobacco flake adhesive is the most representative variety of natural rubber, and it has always been a kind of adhesive with large dosage and wide application. Tobacco flake adhesive is generally classified according to appearance, which is divided into six grades: super grade, first grade, second grade, third grade, fourth grade and fifth grade. , can not reach the fifth grade is classified as equal external glue.
Granular rubber (that is, standard rubber) is classified according to internationally unified physical and chemical properties and indicators. These physical and chemical properties include impurity content, initial plastic value, plastic retention rate, nitrogen content, volatile matter content, ash content and color index, among which impurity content is the leading index, which is divided into five grades according to the amount of impurities, such as 5L, 5, 10, 20 and 50.
The contract delivery grades of natural rubber in Shanghai Futures Exchange, China's futures trading market, are domestic first-class standard rubber SCR5 and imported tobacco flake rubber RSS3, among which domestic first-class standard rubber SCR5 is also commonly known as No.5 standard rubber, and all quality indexes of Natural Rubber GB/T 808 1 ~ 1999 issued and implemented by the State Bureau of Technical Supervision are implemented. Imported cigarette rubber RSS3 implements the International Standard for Grade Quality and Packaging of Natural Rubber (Green Paper) determined by the International Rubber Quality and Packaging Conference (version 1979).
Main applications
Natural rubber has a series of physical and chemical characteristics, especially its excellent resilience, insulation, water repellency and plasticity. After proper treatment, it also has valuable properties such as oil resistance, acid resistance, alkali resistance, heat resistance, cold resistance, pressure resistance and wear resistance, so it is widely used. For example, rain boots, warm water bags and elastic bands used in daily life; Surgical gloves, blood vessels and condoms used in the medical and health industry; All kinds of tires for transportation; Industrial conveyor belts, conveyor belts and acid alkali resistant gloves; Irrigation and drainage hoses and ammonia bags for agriculture; Sounding balloons for meteorological measurement; Sealed shockproof equipment for scientific experiments; Aircraft, tanks, cannons and gas masks for national defense; Even high-tech products such as rockets, artificial earth satellites and spaceships are inseparable from natural rubber.
perform
Modern scientific research results prove that ordinary natural rubber is polymerized from isoprene.
(1) Chemical properties of natural rubber Natural rubber is unsaturated rubber, which easily reacts with vulcanizing agent (structural reaction), oxidative cracking of bromine with oxygen and ozone, chlorination and chemical reaction with halogen, and chemical reaction with catalyst and acid.
However, because natural rubber is a polymer compound, it has the reaction characteristics of olefinic organic compounds, such as slow reaction speed, incomplete and uneven reaction, and coexistence of various chemical reactions (such as oxidative cracking reaction and structured reaction).
Among all kinds of chemical reactions of natural rubber, the most important ones are oxidative cracking reaction and structural reaction. The former is the theoretical basis of raw rubber plasticizing, and it is also the reason for the aging of folic acid gum. The latter is the theoretical basis of vulcanization of raw rubber. Chlorination, cyclization and hydrogenation of natural rubber can be used to modify natural rubber.
(2) Natural rubber has excellent comprehensive physical and mechanical properties. Natural rubber has good elasticity at room temperature. This is because the molecular chain of natural rubber is amorphous at room temperature and has good flexibility. Its density is 0.9 13g/cm, its elastic modulus is 2-4MPa, which is about one third of that of steel, and its elongation is 300 times that of steel, with the maximum of 1000%. In the range of 0- 100 degrees, the resilience of natural rubber can reach more than 50%-85%.
(3) Thermal aging natural rubber is a high elastomer at room temperature, and its glass transition temperature is -72 degrees. After heating, it softens slowly, and 130- 140 degrees begins to flow, and it begins to decompose around 200 degrees, and it decomposes violently at 270 degrees.
(4) Medium-resistant medium refers to oil, liquid chemicals, etc. Natural rubber is resistant to cyclohexane, gasoline, benzene and other media, but insoluble in polar acetone and ethanol. , insoluble in water, resistant to 10% hydrofluoric acid, 20% hydrochloric acid, 30% sulfuric acid, 50% sodium hydroxide, etc. Potassium permanganate, potassium dichromate, etc., which are not resistant to strong acid and strong oxidation.
(5) Good processability Natural rubber is easy to plasticate, mix, calender, extrude and shape. Because of its high relative molecular weight, wide distribution of equivalent molecular weight and easy breakage of molecular chain, there are a certain number of gel molecules in raw rubber.