Rubber is a highly elastic polymer material with reversible deformation, elastic at room temperature, can produce large deformation under the action of a small external force, and can be restored to its original state after removal of external force. Rubber belongs to completely amorphous polymer, its glass transition temperature (T g) is low, molecular weight is often large, more than hundreds of thousands. Rubber products are widely used in all aspects of industry or life.
In 1770, the English chemist J. Priestley discovered that rubber could be used to erase pencil writing. At the time, the material used for this purpose was called rubber, and the word has been used ever since.
First, the source of rubber
Rubber is divided into natural rubber and synthetic rubber two kinds.
Natural rubber is made from gum extracted from rubber tree, rubber grass and other plants.
Synthetic rubber is obtained by polymerization of various monomers.
The main source of natural rubber is the three-leaf rubber tree, which originally grew in South America, but after artificial transplantation, a large number of rubber trees are also grown in Southeast Asia. In fact, Asia has become the most important source of rubber. When the skin of this rubber tree is cut, it will be injured (such as the bark of the stem is cut) when it secretes a milky white juice containing rubber emulsion, called latex, latex after condensation, washing, molding, drying to obtain natural rubber.
Rubber made from guile gum reduces sensitivity. In addition, FIG trees and some plants in the Euphorbia family also provide rubber.
Germany tried to obtain rubber from these plants when its supply was cut off during World War II, but later switched to producing synthetic rubber.
Synthetic rubber is made by synthetic method, using different raw materials (monomers) can be synthesized into different kinds of rubber. In 1900-1910, the chemist C.D. Harris determined that the structure of natural rubber was a polymer of isoprene, which opened the way for synthetic rubber. In 1910, Russian chemist SV Lebedev (Lebedev, 1874-1934) used metallic sodium as initiator to polymerize 1, 3-butadiene into sodium butadiene rubber. After that, there were many new synthetic rubber varieties, such as butadiene rubber, neoprene rubber, styrene butadiene rubber and so on.
The production of synthetic rubber has greatly exceeded that of natural rubber, among which styrene butadiene rubber is the largest.
Rubber is the basic raw material of rubber industry, widely used in the manufacture of tires, hose, tape, cables and other rubber products
Two, the composition of rubber
Natural rubber is made from latex, and some of the non-rubber components contained in latex remain in the solid natural rubber. General natural rubber contains 92-95% rubber hydrocarbon, non-rubber hydrocarbon accounts for 5%-8%. Because of different manufacturing methods, different places of origin and different seasons, the proportion of these ingredients may be different, but basically within the scope.
Protein can promote rubber vulcanization, delay aging. On the other hand, protein has strong water absorption, can cause rubber moisture absorption mold, insulation decline, protein also increase the shortcomings of heat generation.
Acetone extract is a number of higher fatty acids and sterols, some of which play the role of natural antioxidant and accelerator, and some can help the powder complex disperse in the mixing process and play a softening role on raw gum.
Ash mainly contains salts such as magnesium phosphate and calcium phosphate, and a small amount of metal compounds such as copper, manganese and iron. Because these variable metal ions can promote rubber aging, their content should be controlled.
The moisture in the dry glue is not more than 1%, which can be volatilized in the process of processing. However, when the moisture content is too much, it will not only make the raw glue easy to mold in the storage process, but also affect the processing of rubber, such as mixing the compound is easy to group; Calendering, extrusion process is easy to produce bubbles, vulcanization process produces bubbles or spongy.
Composition of synthetic rubber: Synthetic rubber is a polymer made of petroleum and natural gas, with diolefin and olefin as monomers.
Three, the structure of rubber
Linear structure: Common structure of unvulcanized rubber. Due to the large molecular weight, without external force, macromolecular chains appear random coil curve group. When external forces act and withdraw forces, the entanglement degree of the wire group changes, the molecular chain rebounds and has a strong tendency to recover, which is the origin of the high elasticity of rubber.
Branched structure: Aggregation of branched chains of rubber macromolecular chains to form gels. Gel is bad for rubber properties and processing. In the process of mixing, various compounds often can not enter the gel area, form local blank, the formation of reinforcement and crosslinking, become the weak part of the product.
Crosslinked structure: Linear molecules are connected to each other by the bridging of atoms or groups of atoms, forming a three-dimensional network structure. The structure is strengthened as the curing process progresses. In this way, the free movement of the chain segment decreases, the plasticity and elongation decrease, the strength, elasticity and hardness increase, the compression permanent deformation and swelling degree decrease.
The influence of rubber structure
The influence of rubber reinforcing properties is mainly focused on the tensile strength and tearing strength. The general rule is as follows: when the particle size is the same, the reinforcement effect of high structure carbon black on non-crystalline rubber is great, and generally has higher tensile strength and tearing strength. The rubber structure is also the most important factor affecting the conductive property. The chain-dendritic structure is easy to form an interwoven conductive path in the rubber, which will improve the conductive property. The molecular chain of rubber can be crosslinked. When the rubber is deformed by external force after crosslinked, it has the ability to recover quickly and has good physical and mechanical properties and chemical stability.
Four, the characteristics of rubber
1. When rubber products are formed, after large pressure compression, because of the cohesion of the elastomer can not be eliminated, when forming the mold, often produce extremely unstable shrinkage (the shrinkage rate of rubber, because of different rubber types and differences), must be after a period of time, in order to be gentle and stable. Therefore, at the beginning of the design of rubber products, no matter the formula or mold, it is necessary to carefully calculate and cooperate, if not, it is easy to produce product size instability, resulting in product quality decline.
2 rubber is a hot-soluble thermosetting elastomer, due to the sulfide type of the main body is different, its forming curing temperature range, there is also a considerable gap, even because of climate change, indoor temperature and humidity. Therefore, the production conditions of rubber products need to be adjusted moderately at any time. If not, it may produce differences in product quality.
3 Rubber products are made of rubber raw materials after rubber mixing machine as raw materials, in the rubber rubber products according to the characteristics of the design of the formula, and set the required product hardness. The product is molded by rubber vulcanizing machine. After molding, the product is finally treated with flying edge to make the surface of the product smooth and burr free.
4 Rubber products aging test belongs to the category of aging test, rubber aging refers to the rubber and products in the process of processing, storage and use, due to the comprehensive effect of internal and external factors caused by the change in the performance structure, and then the loss of use value. It is cracked, sticky, hardened, softened, powdered, discolored, mildew and so on.
Five, rubber classification
⒈ According to its morphology: it is divided into massive raw rubber, latex, liquid rubber and powder rubber for rubber colloidal water dispersion;
Liquid rubber is rubber oligomer, not vulcanized before the general viscous liquid;
Powder rubber is the processing of latex into powder form, to facilitate the mixing and processing of production.
Thermoplastic rubber, developed in the 1960s, was formed by the process of thermoplastic instead of chemical vulcanization.
B. Rubber can be divided into two categories according to the use of general type and special type. It is an insulator and does not conduct electricity easily, but may become a conductor if it is exposed to water or at different temperatures. Conduction is concerned with the ease of conduction of electrons in molecules or ions within a substance.
⒊ According to the source and method of raw materials, rubber can be divided into natural rubber and synthetic rubber. Among them, natural rubber consumption accounts for 1/3, synthetic rubber consumption accounts for 2/3.
It can be divided into four categories: solid rubber (also known as dry rubber), emulsion rubber (referred to as latex), liquid rubber and powder rubber.
⒌ According to the performance and use of rubber: in addition to natural rubber, synthetic rubber can be divided into general synthetic rubber, semi-general synthetic rubber, special synthetic rubber and special synthetic rubber.
According to the physical form of rubber, rubber can be divided into hard glue and soft glue, raw glue and mixed glue, etc.
According to its performance and use, it is divided into general rubber and special rubber.
Six, the development of rubber
Rubber industry is one of the important basic industries of national economy. It not only provides light industrial rubber products such as daily medical necessities for People's Daily life, but also provides various rubber production equipment or rubber parts to heavy industries and emerging industries such as mining, transportation, construction, machinery and electronics. Visible, rubber industry has a wide variety of products, backward industry is very broad.
In recent years, the rubber industry has been a lot of development, the subdivision industry has been stable and rising, the new rubber subdivision industry is developing rapidly, but at the same time, the rubber industry also has environment, resources, disaster, innovation and other problems.
The development prospect of rubber industry in China is broad. The product structure of rubber industry will have great changes, new products, replacement products, new materials, new technology application expansion, production technology has obvious progress.
The characteristics of the rubber industry determine that when the rubber industry of a country is mature, the boom situation of the industry and the operation of the whole economy will maintain a strong correlation: the length of its development cycle is equivalent to the length of the economic cycle of the country, the trend is the same; But because the rubber industry belongs to the basic industry, its cycle changes slightly ahead of the economic cycle changes. In addition, also because the rubber industry is at the front end of the production chain of the national economy, its cyclical fluctuation amplitude is smaller than that of the end industry chain, but also smaller than that of the whole economy. Therefore, from the perspective of industrial investment, the mature rubber industry is close to the income investment industry. China's rubber processing industry is in a period of vigorous development. The rubber industry in various places not only speeds up the process of China's industry, but also drives the good situation of economic construction and development. Juxian County in Shandong, Hebei and other places where rubber industry is developed are Yunnan, Guangdong.
Seven, rubber processing
This process includes plasticizing, mixing, calendering or extrusion, molding and vulcanization and other basic processes, each process has different requirements for products, respectively with a number of auxiliary operations.
In order to be able to add various required compounds to rubber, raw rubber first needs to be plasticized to improve its plasticity; Then through mixing the carbon black and various rubber additives and rubber evenly mixed into rubber; The rubber material is pressed out to make a certain shape blank; And then make it after calendering hanging glue or glue coated textile materials (or with metal materials) together forming semi-finished products; Finally, the plastic semi-finished product is vulcanized and made into a final product with high elasticity.
For products with high precision, such as oil seals, O-rings, seals and other rubber products, it is also necessary to carry out trimming and burring processing. The optional way is manual trimming, mechanical trimming and freezing trimming.
Manual trimming: labor intensity, low efficiency, low qualified rate.
Mechanical trimming: mainly punching, grinding wheel and round knife trimming, suitable for specific products with low precision requirements.
Frozen edging: Special frozen edging machine equipment, the principle is to use liquid nitrogen (LN2) to make the raw edge of the finished product brittle at low temperature, use specific frozen particles (pellets) to hit the raw edge, in order to quickly remove the raw edge. Frozen trimming has high efficiency, low cost and wide range of applicable products, which has become the mainstream process standard.
Eight, rubber quality detection
In rubber materials and products have strict quality requirements, such as tensile strength, elastic modulus, elongation, aging resistance and so on. Rubber products are used in the field of high precision, these parameters are often very demanding.
The rubber products committee was established in the early stage of Chinese rubber development, responsible for the research and development of rubber, academic, quality supervision and other work.
1. Experiment of weight gain resistance to medium
The finished product can be sampled, soaked in one or several selected media, weighed after a certain temperature time, and the type of material can be inferred according to the rate of change of weight and hardness.
For example, soaked in 100℃ oil for 24 hours, nitrile rubber NBR, fluorine rubber, neoprene rubber CR mass and hardness change rate is very small, and natural rubber NR, ethylene propylene rubber EPDM, styrene butadiene rubber SBR weight more than double and hardness change greatly, volume expansion is very obvious.
2. Hot air aging experiment
Take a sample from the finished product and put it in the aging box for one day to observe the phenomenon after aging. Can be graded aging gradually heating up. For example, at 150℃, neoprene rubber CR, natural rubber NR, styrene butadiene rubber SBR will be brittle, nitrile butadiene rubber NBR, ethylene propylene rubber EPDM and elastic. Rise to 180℃ ordinary nitrile rubber NBR will be brittle break; And at 230℃ hydrogenated nitrile gum HNBR will also be brittle, fluorine gum and silica gel still have good elasticity.
3. Combustion method
Take a few samples and burn them in the air. Observe the phenomenon.
Generally speaking, fluorine rubber, neoprene rubber CR from the fire, even if the fire is smaller than the general natural rubber NR, ethylene propylene rubber EPDM. Of course, if you look closely, the state of combustion, color, and smell can also tell you a lot. For example, NBR/PVC is used with glue. When there is fire, the fire splashes randomly, seems to be like water, and the smoke is thick and sour. It is important to note that sometimes added flame retardants but do not contain halogen gum will also self-extinguish from the fire, this needs to be further inferred by other means.
4 Specific Gravity Measurement
Use an electronic scale or analytical balance, accurate to 0.01 grams, plus a cup of water, a hair can be.
Generally speaking, the proportion of fluorine rubber is the largest, above 1.8, and the proportion of CR of neoprene rubber is more than 1.3, which can be considered as these rubber.
5 Low temperature method
Take a sample from the finished product and create a suitable low temperature environment with dry ice and alcohol. Soak the sample in a low temperature environment for 2-5 minutes, at the selected temperature to feel the degree of hardness. For example, below minus 40℃, the same high temperature and oil resistance is very good silicone and fluorine rubber, silicone is soft.
