There are about 2000 different plants in the world that can produce polymers similar to natural rubber, and different kinds of rubber are obtained from 500 plants, but the Brazilian rubber tree is of real practical value. When the surface of the rubber tree is cut, the milk duct in the bark is cut off and the milk flows out of the tree. The latex collected from rubber trees is diluted, acid cured, washed, tabletted, dried and packaged to obtain natural rubber on the market. According to the different methods of making rubber, natural rubber can be made into cigarettes, air-dried films, crepes, technical dry rubber and concentrated latex.
natural rubber
Standard rubber or granular rubber is a new type of natural rubber developed in 1960s. In the past, general traditional products, such as cigarette flakes, crepe flakes and air-dried flakes, hindered the development of natural rubber in classification methods and manufacturing methods. Therefore, Malaysia began to implement the standard rubber plan in 1965, and developed the production of granular rubber based on the classification of physical and chemical properties of raw rubber. Standard rubber refers to rubber classified according to mechanical impurities, plastic retention rate, initial plastic value, nitrogen content, volatile matter, ash content, color index and other physical and chemical performance indicators. The standard rubber packaging is also relatively advanced, generally packed with polyethylene film and clearly marked. The bag is light in weight and easy to move. The packing weight in Malaysia is 33.3kg, and that in China is 40kg.
The classification of standard rubber is more scientific, so this classification method was quickly accepted by major natural rubber producing countries and international standardization organizations, and the classification standards of standard rubber were formulated one after another. These standards are roughly the same, but they are not completely consistent. For example, ISO2000 stipulates five grades, while our national standard GB 808 1-87 stipulates four grades. [ 1]?
physical features
Generally, it is a flaky solid with a relative density of 0.94, refractive index 1.522, elastic modulus of 2 ~ 4 MPa, softening temperature 130 ~ 140℃, viscosity 150 ~ 160℃ and 200℃. High elasticity at room temperature, slightly plasticity, crystallization and hardening at low temperature. Good alkali resistance, but not strong acid resistance. Insoluble in water, lower ketones and alcohols, but soluble in nonpolar solvents such as chloroform and carbon tetrachloride. [ 1]?
Natural quality
Usually, the natural rubber we refer to refers to the natural latex collected from Brazilian rubber trees, which is made into elastic solids through curing, drying and other processing procedures. Natural rubber is a natural polymer compound with cis-1, 4- polyisoprene as the main component, and its molecular formula is (c5h8) n. Its rubber hydrocarbon (cis-1, 4- polyisoprene) content is above 90%, and it also contains a small amount of protein, fatty acids, sugar and ash.
Physical properties of natural rubber. Natural rubber has high elasticity at room temperature, slightly plasticity, very good mechanical strength, low hysteresis loss and low calorific value during repeated deformation, so its bending flexibility is also good, and because it is nonpolar rubber, its electrical insulation performance is good.
Chemical characteristics of natural rubber. Because of unsaturated double bonds, natural rubber is a substance with strong chemical reaction ability. Light, heat, ozone, radiation, bending deformation, copper, manganese and other metals can promote the aging of natural rubber. Aging resistance is the fatal weakness of natural rubber. However, natural rubber with antioxidants can still be used as usual after being exposed to the sun for two months.
Medium resistance of natural rubber. Natural rubber has good alkali resistance, but it is not resistant to strong acid. Because natural rubber is nonpolar rubber, it can only resist some polar solvents, but it will swell in nonpolar solvents, so its oil resistance and solvent resistance are very poor. Generally speaking, hydrocarbons, halogenated hydrocarbons, carbon disulfide, ethers, higher ketones and higher fatty acids can dissolve natural rubber, but their solubility is affected by the degree of plasticization, while lower ketones, lower esters and alcohols are non-solvents of natural rubber. [ 1]?
Microstructure of natural rubber
The structure of natural rubber is mainly macromolecular chain structure, molecular weight and its distribution and aggregation structure. The structural unit of macromolecular chain of natural rubber is isoprene, and the macromolecular chain is mainly composed of polyisoprene. The content of rubber is above 97%, and there are aldehyde groups on its molecular chain, with one on each macromolecular chain on average. It is the condensation or reaction between aldehyde group and protein decomposition product that forms branching and crosslinking, which increases the viscosity of rubber during storage and makes natural rubber macromolecules.
It is speculated that the macromolecular end of natural rubber is generally dimethyl allyl and the other end is pyrophosphate group. There are few end groups, aldehyde groups and polymeric elements in the molecular chain. As far as the molecular weight and its distribution of natural rubber are concerned, its molecular weight range is very wide. According to foreign reports, most of them have molecular weights around 30,000, and the strength of raw natural rubber, rubber compound and vulcanized rubber is relatively high, and the strength of general natural rubber can reach 3 MPa.
The main reason for the high mechanical strength of natural rubber is that it is a self-reinforced rubber series. During stretching, the macromolecular chains will be oriented along the stress direction to form crystals, and the grains will strengthen the amorphous macromolecules. The reason why the strength is high and does not swell is the close agglutination of tiny particles in its internal structure. [ 1]?
natural rubber
Variety classification
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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. * * *, those that fail to reach Grade 5 are classified as 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 content, ash content and color index. Among them, the impurity content is the leading index, which is divided into five grades: 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). [ 1]?
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. [ 1]?
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. [ 1]?