Basic knowledge of rubber

Rubber refers to highly elastic polymer materials with reversible deformation, which are elastic at room temperature and can undergo significant deformation under small external forces. After removing external forces, they can return to their original state. Rubber belongs to completely amorphous polymers, with a low glass transition temperature (T g) and often a large molecular weight, exceeding several hundred thousand. Rubber products are widely used in various aspects of industry or daily life.

In 1770, British chemist J. Priestley discovered that rubber could be used to erase pencil handwriting, and the material used for this purpose was called rubber, a term that has been used to this day.

1、 The source of rubber

Rubber is divided into two types: natural rubber and synthetic rubber.

Natural rubber is extracted from plants such as rubber trees and rubber grass and processed into gum;

Synthetic rubber is obtained through polymerization reactions of various monomers.

Natural rubber mainly comes from three leaf rubber trees. Initially, rubber trees grew in South America, but after artificial transplantation, there are also a large number of rubber trees planted in Southeast Asia. In fact, Asia has become the most important source of rubber. When the skin of this rubber tree is cut open, it will secrete a large amount of milky white juice containing rubber emulsion when damaged (such as the bark of the stem being cut open), called latex. The latex is condensed, washed, formed, and dried to obtain natural rubber.

Rubber made from silver chrysanthemum gum can reduce sensitivity. In addition, fig trees and some plants in the Euphorbiaceae family can also provide rubber.

Germany attempted to obtain rubber from these plants during World War II due to the cut off of rubber supply, but later switched to producing synthetic rubber.

Synthetic rubber is made by artificial synthesis methods, and different types of rubber can be synthesized using different raw materials (monomers). From 1900 to 1910, chemist C D. Harris determined that the structure of natural rubber is a high polymer of isoprene, which opens up a way for artificial rubber synthesis. In 1910, Russian chemist SV Lebedev (1874-1934) used sodium metal as an initiator to polymerize 1,3-butadiene into sodium butadiene rubber. Since then, many new synthetic rubber varieties have emerged, such as butadiene rubber, chloroprene rubber, butadiene styrene rubber, and so on.

The production of synthetic rubber has greatly exceeded that of natural rubber, with butadiene styrene rubber being the largest producer.

Rubber is the basic raw material of the rubber industry, widely used in the manufacturing of tires, hoses, tapes, cables, and other various rubber products

2、 Composition of rubber

Natural rubber is made from latex, and a portion of the non rubber components contained in latex remain in solid natural rubber. Natural rubber generally contains 92% -95% rubber hydrocarbons, with non rubber hydrocarbons accounting for 5% -8%. Due to different production methods, production areas, and even different rubber picking seasons, the proportion of these components may vary, but they are basically within the range.

Protein can promote the vulcanization of rubber and delay aging. On the other hand, proteins have strong water absorption, which can cause rubber to absorb moisture, mold, and reduce insulation. Proteins also have the disadvantage of increasing heat generation.

Acetone extracts are some advanced fatty acids and sterols, some of which act as natural antioxidants and accelerators, while others can help disperse powdered additives during mixing and soften raw rubber.

Ash mainly contains salts such as magnesium phosphate and calcium phosphate, with a small amount of metal compounds such as copper, manganese, and iron. Because these variable valence metal ions can promote rubber aging, their content should be controlled.

The moisture content in dry rubber does not exceed 1% and can evaporate during processing. However, if the moisture content is too high, it not only makes the raw rubber prone to mold during storage, but also affects the processing of the rubber, such as the tendency of the compounding agent to clump during mixing; During the rolling and extrusion process, bubbles are easily generated, while during the vulcanization process, bubbles or sponge like structures are produced.

Composition of synthetic rubber: Synthetic rubber is a polymer formed by polymerization of dienes and olefins using petroleum and natural gas as raw materials.

3、 The structure of rubber

Linear structure: The common structure of unvulcanized rubber. Due to its high molecular weight, the macromolecular chains exhibit random curling curves and clusters without external forces. When external forces act, withdraw, the entanglement degree of the thread changes, the molecular chains rebound, and a strong tendency to recover is generated, which is the origin of high elasticity of rubber.

Branch chain structure: aggregation of branch chains of rubber macromolecules to form gel. Gel is unfavorable to the properties and processing of rubber. When mixing rubber, various compounding agents often cannot enter the gel area, forming a local blank, which can not be reinforced and cross-linked, becoming the weak part of the product.

Cross linked structure: Linear molecules are connected to each other through the bridging of some atoms or atomic groups, forming a three-dimensional network structure. As the vulcanization process progresses, this structure continues to strengthen. In this way, the free movement ability of the chain segment decreases, plasticity and elongation decrease, strength, elasticity and hardness increase, compression permanent deformation and swelling degree decrease.

The Influence of Rubber Structure

The impact of rubber reinforcement performance mainly focuses on tensile strength and tear strength. The general rule is that when the particle size is the same, high structured carbon black has a greater reinforcement effect on amorphous rubber, and generally has higher tensile strength and tear strength. The structure of rubber is still the most important factor affecting its conductivity, and the branched chain structure is prone to forming interwoven conductive pathways in rubber, which can improve its conductivity. The molecular chains of rubber can be crosslinked, and when the crosslinked rubber undergoes deformation under external forces, it has the ability to quickly recover and has good physical and mechanical properties and chemical stability.

4、 Characteristics of rubber

When rubber products are formed, they are subjected to high pressure compression. Due to the inherent cohesive force of the elastic body, extremely unstable shrinkage (the shrinkage rate of rubber varies depending on the type of rubber) often occurs during the molding and demolding process. It must take a period of time before it can be stabilized. So, at the beginning of a rubber product design, regardless of the formula or mold, careful calculation and coordination are necessary. If not, it is easy to cause product size instability, leading to a decrease in product quality.

2. Rubber is a hot melt thermosetting elastomer. Due to the different types and main bodies of sulfides, the temperature range for its molding and curing also varies significantly, and can even be affected by climate changes and indoor temperature and humidity. Therefore, the production conditions of rubber products need to be adjusted appropriately at any time. If not, there may be differences in product quality.

3. Rubber products are a mixture of rubber raw materials made by mixing them in an internal mixer. During the mixing process, a formula is designed based on the characteristics of the desired rubber product, and the required product hardness is determined. The product is produced and molded using a rubber flat vulcanization machine. After the product is formed, the final trimming treatment is carried out to ensure that the surface of the product is smooth and free of burrs.

4. Rubber product aging testing belongs to the category of aging testing. Rubber aging refers to the phenomenon where rubber and its products undergo changes in performance and structure due to the combined effects of internal and external factors during processing, storage, and use, resulting in the loss of useful value. Manifested as cracking, sticking, hardening, softening, powdering, discoloration, mold growth, etc.

5、 Classification of rubber

1. According to form: it is divided into block like rubber, latex, liquid rubber, and powder rubber, which are colloidal water dispersions of rubber;

Liquid rubber is an oligomer of rubber, usually a viscous liquid before vulcanization;

Powder rubber is the process of processing latex into powder form for the purpose of ingredient preparation and processing.

The thermoplastic rubber developed in the 1960s does not require chemical vulcanization and is formed using the processing method of thermoplastic plastics.

2. Rubber can be divided into two types based on usage: general and special types. It is an insulator that is not easily conductive, but if it is exposed to water or at different temperatures, it may become a conductor. Conductivity refers to the easy conduction of electrons within molecules or ions within a substance.

According to the source and method of raw materials, rubber can be divided into two categories: natural rubber and synthetic rubber. Among them, natural rubber accounts for one-third of the consumption, while synthetic rubber accounts for two-thirds.

4. According to the appearance of rubber, it can be divided into four categories: solid rubber (also known as dry rubber), emulsion rubber (abbreviated as latex), liquid rubber, and powder rubber.

According to the properties and uses of rubber: In addition to natural rubber, synthetic rubber can be divided into general synthetic rubber, semi general synthetic rubber, specialized synthetic rubber, and special synthetic rubber.

6. According to the physical form of rubber, it can be divided into hard rubber and soft rubber, raw rubber and mixed rubber, etc.

7. According to performance and use: it is divided into general rubber and special rubber.

6、 The development of rubber

The rubber industry is one of the important basic industries of the national economy. It not only provides people with essential daily medical and other light industrial rubber products, but also provides various rubber production equipment or rubber components to heavy industries and emerging industries such as mining, transportation, construction, machinery, electronics, etc. It can be seen that the rubber industry has a wide variety of products, and the backward industry is very broad.

In recent years, the rubber industry has made significant progress, with some sub sectors steadily rising and new sub sectors rapidly developing. However, at the same time, the rubber industry also faces issues such as environment, resources, disasters, and innovation.

The development prospects of China's rubber industry are broad. The product structure of the rubber industry will undergo significant changes, with an increase in new and updated products, an expansion in the application of new materials and processes, and significant progress in production technology.

The characteristics of the rubber industry determine that when a country's rubber industry matures, the prosperity of the industry will maintain a strong correlation with the overall economic operation: the length of its development cycle is equivalent to the length of the country's economic cycle, and the trend is in the same direction; However, due to the fact that the rubber industry belongs to the basic industry, its cyclical changes are slightly earlier than those of the economic cycle. In addition, due to the fact that the rubber industry is at the forefront of the national economic production chain, the amplitude of its cyclical fluctuations is smaller than that of the industries at the end of the industry chain, and also smaller than the amplitude of the entire economy. Therefore, from the perspective of industrial investment, the mature rubber industry is closer to the income oriented investment industry. China's rubber processing industry is in a period of vigorous development, and the rubber industry in various regions not only accelerates the process of China's industry, but also drives a good situation of economic construction and development in various regions. The regions with relatively developed rubber industry in China include Yunnan, Guangdong, Juxian County, Shandong, Hebei, and other places.

7、 Rubber processing

This process includes basic processes such as plasticizing, mixing, rolling or extrusion, forming, and vulcanization. Each process has different requirements for the product and is accompanied by several auxiliary operations.

In order to add various required blending agents to rubber, raw rubber first needs to undergo plasticization to improve its plasticity; Then, carbon black and various rubber additives are uniformly mixed with rubber through mixing to form a rubber material; The rubber material is pressed out to form a certain shape billet; Then combine it with textile materials (or metal materials) that have been rolled, glued or coated to form semi-finished products; Finally, after vulcanization, the plastic semi-finished product is made into a highly elastic final product.

For products with high precision requirements, such as oil seals, O-rings, seals, and other rubber products, edge trimming and deburring processing are also required. The available methods include manual edge trimming, mechanical edge trimming, and frozen edge trimming.

Manual edge trimming: high labor intensity, low efficiency, and low qualification rate.

Mechanical edge trimming: mainly includes punching, grinding wheel edge grinding, and circular knife edge trimming, suitable for specific products with low precision requirements.

Freezing edge trimming: A specialized freezing edge trimming machine equipment that uses liquid nitrogen (LN2) to make the rough edges of the finished product brittle at low temperatures, and uses specific frozen particles (pellets) to strike the rough edges to quickly remove them. The efficiency of frozen edge trimming is high, the cost is low, and it is suitable for a wide range of products, which has become the mainstream process standard.

8、 Quality testing of rubber

There are strict quality requirements for rubber materials and products, such as tensile strength, elastic modulus, elongation, aging resistance, etc. Rubber products are used in high-precision fields, and these parameters are often very demanding.

In the early stages of rubber development in our country, a Rubber Products Committee was established, responsible for rubber research and development, academic research, quality supervision, and other work.

1. Medium resistant weight gain experiment

Samples can be taken from the finished product, soaked in one or several selected media, and weighed after a certain temperature and time. The type of material can be inferred based on the weight change rate and hardness change rate.

For example, soaking in 100 ℃ engine oil for 24 hours, the quality and hardness change rate of nitrile rubber NBR, fluorine rubber, and chloroprene rubber CR are very small, while natural rubber NR, ethylene propylene rubber EPDM, and butadiene styrene rubber SBR increase weight by more than twice and have a significant change in hardness, with significant volume expansion.

2. Hot air aging experiment

Take a sample from the finished product and place it in an aging box for one day to observe the phenomenon after aging. Gradually heating up can be achieved through graded aging. For example, CR chloroprene rubber, NR natural rubber, SBR styrene butadiene rubber will all fracture at 150 ℃, NBR nitrile rubber, EPDM ethylene propylene rubber also have elasticity. Ordinary nitrile rubber NBR will brittle fracture when raised to 180 ℃; At 230 ℃, hydrogenated nitrile rubber HNBR will also brittle, while fluorine rubber and silicone still have good elasticity.

3 combustion method

Take a small sample and burn it in the air. Observe the phenomenon.

Generally speaking, fluorine rubber and chloroprene rubber CR are self igniting, and even if the flame is ignited, they are much smaller than natural rubber NR and ethylene propylene rubber EPDM. Of course, if carefully observed, the combustion state, color, and odor will also provide us with a lot of information. For example, when NBR/PVC is mixed with glue, the fire splashes randomly when there is a source of fire, as if there is water, and it extinguishes automatically when the fire is off. The smoke is thick and sour. It should be noted that sometimes adhesives with added flame retardants but without halogens can also self extinguish after ignition, which requires further inference through other methods.

4. Specific gravity measurement

Use an electronic scale or analytical balance, accurate to 0.01 grams, plus a cup of water and a strand of hair.

Generally speaking, fluorine rubber has the highest specific gravity, above 1.8, and chloroprene rubber CR is also mostly above 1.3. If the specific gravity is significantly higher, these adhesives can be considered.

5 Low temperature method

Take samples from the finished product and create a suitable low-temperature environment using dry ice and alcohol. Soak the sample in a low-temperature environment for 2-5 minutes and feel the softness and hardness at the selected temperature. For example, below minus 40 ℃, compared to silicone and fluorine adhesives, which are also resistant to high temperature and oil, silicone is relatively soft.