What is Rubber?
Rubber refers to a highly elastic polymer material that exhibits reversible deformation. At room temperature, it is elastic and can undergo significant deformation under minimal external force, returning to its original shape once the force is removed. Rubber is classified as an entirely amorphous polymer, with a low glass transition temperature (Tg), and often has a very high molecular weight, typically above several hundred thousand.
Rubber is generally divided into two categories: natural rubber and synthetic rubber. Natural rubber is obtained from the latex secreted by the Hevea brasiliensis tree (rubber tree), which is processed through coagulation and drying. It accounts for about 40% of rubber production. Synthetic rubber, on the other hand, is made from raw materials such as natural gas, coal, and petroleum. Through chemical processes, monomers are obtained, which are then polymerized into highly elastic compounds within a specific temperature range. Synthetic rubber can be used as a substitute for natural rubber, accounting for about 60% of rubber production.
Natural Rubber Vs. Synthetic Rubber
The term “rubber” originates from the indigenous word cau-uchu, meaning “the tree that weeps.” Natural rubber is produced from the latex of the rubber tree, which is harvested by tapping the tree and coagulating the latex through various processes. In 1770, British chemist J. Priestley discovered that rubber could be used to erase pencil marks, and this use led to the material being called “Rubber,” a term still used today.
The molecular chains of rubber can be cross-linked, and when rubber undergoes deformation under external force, its cross-linked structure allows it to recover quickly, giving it excellent physical, mechanical, and chemical stability. Rubber is a fundamental raw material in the rubber industry, widely used in the manufacture of tires, hoses, belts, cables, and other rubber products. The Hevea brasiliensis tree provides the bulk of commercial rubber. When the tree’s bark is cut, it secretes a large amount of latex, which contains rubber emulsions.
Other plants, such as the fig tree and certain species of Euphorbiaceae, can also produce rubber. During World War II, Germany attempted to extract rubber from these plants when their rubber supply was cut off but eventually shifted to synthetic rubber production. The original rubber trees grew in South America, but through artificial transplantation, large quantities of rubber trees are now cultivated in Southeast Asia, which has become the most important source of rubber.
Natural rubber refers to elastic solid material made from latex harvested from the rubber tree (Hevea brasiliensis), processed through coagulation, drying, and other procedures. At room temperature, natural rubber exhibits high elasticity, slight plasticity, excellent mechanical strength, low hysteresis loss, and minimal heat generation during repeated deformation. As a non-polar rubber, it also has excellent electrical insulation properties. Additionally, natural rubber has greater structural regularity, higher green strength, and faster vulcanization rates. Today, natural rubber has become a key raw material in many industries, including the manufacture of tires, gloves, rubber carpets, and more.
Synthetic rubber is a man-made, highly elastic polymer. Also known as synthetic elastomers, it is one of the three main types of synthetic materials, with production volume second only to synthetic resins and synthetic fibers. Synthetic rubbers are typically classified by their monomer types, including natural rubber (NR), polybutadiene rubber (BR), nitrile rubber (NBR), styrene-butadiene rubber (SBR), chloroprene rubber (CR), isoprene rubber (IR), butyl rubber (IIR), hydrogenated nitrile rubber (HNBR), ethylene propylene rubber (EPM/EPDM), and fluoroelastomers (FPM).
Characteristics of Rubber Products
- Rubber products are molded under high pressure because the elastic material lacks a cohesive method to eliminate internal forces. Upon molding and removal from the mold, rubber often experiences unstable shrinkage (the shrinkage rate varies depending on the type of rubber) and requires time to stabilize. Therefore, both the formulation and mold design must be carefully calculated and adjusted during the design phase of rubber products. Otherwise, the product’s dimensions may be unstable, resulting in low product quality.
- Rubber is a thermoplastic and thermoset elastomer, while plastics are thermoplastic and cold-setting. Due to the variety of vulcanizing agents, the temperature range for rubber molding and curing can vary significantly and can even be affected by changes in climate, room temperature, and humidity. Therefore, production conditions for rubber products must be carefully controlled to prevent product quality issues.
- Rubber products are made from mixed rubber compounds, which are designed according to the specific requirements of the rubber products and the desired hardness. Products are shaped using rubber plate vulcanizing machines. After molding, the final step is edge trimming, ensuring that the product’s surface is smooth and free from burrs.
- Rubber aging tests fall under the category of aging trials. Rubber aging refers to the process where rubber and rubber products undergo changes in performance and structure due to internal and external factors during processing, storage, and use. As a result, they lose their functional value. Signs of aging include cracking, adhesion, hardening, softening, powdering, discoloration, and mold growth.
Types of Rubber
1. Natural Rubber (NR)
Natural rubber is primarily made of polyisoprene (rubber hydrocarbon) and contains small amounts of proteins, water, resin acids, sugars, and inorganic salts. It has excellent elasticity, high tensile strength, good tear resistance, and electrical insulation properties. It also has good wear resistance, drought resistance, and processability, and easily bonds with other materials. In terms of comprehensive performance, it outperforms most synthetic rubbers.
However, natural rubber has some drawbacks, including poor resistance to oxygen and ozone, making it prone to aging and degradation. It also has poor oil and solvent resistance, low resistance to acid and alkali corrosion, and limited heat resistance. Its typical operating temperature range is approximately -60°C to 80°C. Natural rubber is commonly used in the production of tires, rubber footwear, hoses, belts, electrical insulation, cable sheaths, and other general-purpose products. It is especially suitable for manufacturing vibration dampers, engine shock absorbers, machine mounts, rubber-metal suspension components, diaphragms, and molded products.
2. Butadiene Rubber (BR)
Polybutadiene rubber is made by polymerizing butadiene into a cis-structure. The advantages of BR include excellent elasticity, wear resistance, good aging resistance, and excellent low-temperature performance. It also generates little heat under dynamic load and bonds well with metals.
However, its drawbacks include lower strength, poor tear resistance, and poor processing performance and self-adhesion. Its typical operating temperature range is about -60°C to 100°C. BR is often used in combination with natural rubber or styrene-butadiene rubber (SBR) to make tire treads, conveyor belts, and special cold-resistant products.
3. Nitrile Butadiene Rubber (NBR)
NBR is a copolymer of butadiene and acrylonitrile. It has excellent resistance to gasoline and hydrocarbon oils, second only to polysulfide rubber, acrylate, and fluoroelastomers, and superior to most other general-purpose rubbers. It also exhibits good heat resistance, air tightness, wear resistance, and water resistance, with strong adhesive properties.
However, NBR has poor cold resistance, ozone resistance, and acid resistance. Its elasticity and strength are lower, and its electrical insulation properties and resistance to polar solvents are also inferior. The typical operating temperature range for NBR is approximately -30°C to 100°C. It is primarily used to manufacture oil-resistant products like hoses and seals.
4. Styrene Butadiene Rubber (SBR)
SBR is a copolymer of butadiene and styrene. Its performance is similar to natural rubber, and it is the most widely produced general-purpose synthetic rubber. SBR offers superior wear resistance, aging resistance, and heat resistance compared to natural rubber. It is also more uniform in texture.
However, SBR has lower elasticity, poor flexural and tear resistance, and poor processing performance, particularly with low green strength and poor self-adhesion. Its typical operating temperature range is about -50°C to 100°C. SBR is mainly used as a substitute for natural rubber in the production of tires, rubber sheets, hoses, footwear, and other general-purpose products.
5. Chloroprene Rubber (CR)
Chloroprene rubber is made by polymerizing chloroprene (2-chloro-1,3-butadiene). This rubber contains chlorine atoms in its molecular structure, which gives it excellent resistance to oxidation, ozone, and fire. It is also non-flammable and self-extinguishing once the flame is removed. CR has good oil, solvent, acid, and alkali resistance, along with excellent aging resistance, air tightness, and better physical mechanical properties than natural rubber.
However, CR has poor cold resistance, relatively high specific gravity, higher costs, poor electrical insulation, and difficulties in processing such as adhesion and mold sticking. Additionally, its raw material stability is poor, making it difficult to store. The typical operating temperature range for CR is approximately -45°C to 100°C. It is used to make cables and protective covers requiring ozone resistance and high aging resistance, oil-resistant hoses, belts, chemical-resistant linings, fire-resistant underground mining rubber products, as well as various molded products, seals, gaskets, and adhesives.
6. Isoprene Rubber (IR)
Isoprene rubber is made from the polymerization of isoprene monomers, and its chemical composition and stereostructure are similar to natural rubber, so its properties are very close to those of natural rubber. For this reason, it is often referred to as synthetic natural rubber. IR has most of the advantages of natural rubber, including good aging resistance. However, its elasticity and strength are slightly lower than that of natural rubber, and its processing performance is poor and costs are higher.
The typical operating temperature range for IR is about -50°C to 100°C. It is used as a substitute for natural rubber to make tires, footwear, hoses, belts, and other general-purpose products.
7. Isobutene Isoprene Rubber (IIR)
Butyl rubber is a copolymer of isobutene with a small amount of isoprene or butadiene. Its greatest feature is its excellent air tightness, ozone resistance, aging resistance, and good heat resistance (long-term working temperatures can be below 130°C). It can also withstand inorganic strong acids (like sulfuric acid and nitric acid) and common organic solvents. It has good vibration and damping properties, as well as excellent electrical insulation.
However, its drawbacks include poor elasticity, poor processing performance, slow vulcanization, low adhesion, and poor oil resistance. The typical operating temperature range for IIR is about -40°C to 120°C. It is mainly used in the manufacture of inner tubes, balloons, electrical cable insulation, chemical equipment linings, vibration-damping products, heat-resistant conveyor belts, and heat-resistant, aging-resistant fabrics.
8. Hydrogenated Nitrile Butadiene Rubber(HNBR)
HNBR is a copolymer of butadiene and acrylonitrile, obtained by hydrogenating the double bonds in the butadiene portion of NBR. HNBR offers high mechanical strength and wear resistance. When crosslinked with peroxides, it also exhibits better heat resistance compared to NBR, while retaining similar performance to nitrile rubber.
Its main disadvantage is its higher cost. The typical operating temperature range for HNBR is about -30°C to 150°C. It is mainly used in the production of oil-resistant and high-temperature-resistant sealing products.
9. Ethylene Propylene Rubber(EPM/EPDM)
Ethylene propylene rubber is a copolymer of ethylene and propylene, generally divided into binary (EPM) and ternary (EPDM) types. Its key features include excellent ozone resistance, ultraviolet (UV) stability, weather resistance, and aging resistance, making it one of the best general-purpose rubbers. It also exhibits good electrical insulation, chemical resistance, and impact elasticity, along with low specific gravity and good filler compatibility. It can withstand temperatures up to 150°C and resist polar solvents such as ketones and esters, but it is not resistant to aliphatic hydrocarbons or aromatic hydrocarbons. Its mechanical properties are slightly inferior to natural rubber but better than SBR.
The main drawbacks of EPM/EPDM are its poor self-adhesion and poor inter-adhesion, making bonding difficult. The typical operating temperature range is approximately -50°C to 150°C. EPDM is used in manufacturing chemical equipment linings, electrical cable sheaths, steam hoses, heat-resistant conveyor belts, automotive rubber products, and other industrial products.
10. Fluoro Carbon Rubber (FPM）
Fluoroelastomers (FPM), commonly used types include FPM-26 and FPM-246. FPM-26 is a copolymer of vinylidene fluoride and hexafluoropropylene, while FPM-246 is a copolymer of vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene. Fluoroelastomers offer outstanding heat resistance, oil resistance, acid resistance, alkali resistance, aging resistance, and electrical insulation. They are also flame-retardant and have low permeability to gases. However, their low-temperature performance is inferior.
The typical operating temperature range for FPM is -40°C to 250°C, with short-term exposure up to 300°C. Fluoroelastomers are suitable for manufacturing high-temperature, oil-resistant sealing parts, hoses, tapes, and fuel tanks, although they are relatively expensive.
Rubber Product Manufacturing Process
There is a wide variety of rubber products, but the manufacturing process is generally the same. The basic manufacturing process for rubber products made from solid rubber or raw rubber as the raw material includes three main steps: plastication, mixing, and molding. Of course, basic processes such as raw material preparation, finished product finishing, inspection, and packaging are also necessary. The rubber processing process mainly addresses the contradiction between plasticity and elasticity. Through various techniques, elastic rubber is transformed into plasticized rubber, then mixed with various additives to make semi-finished products, which are then vulcanized to turn them back into elastic rubber with good physical and mechanical properties.
1. Plastication
Raw rubber is elastic but lacks the necessary plasticity for processing, making it difficult to work with. To improve its plasticity, plastication is required. This helps to evenly disperse the additives in the raw rubber during mixing and also improves the permeability of the rubber into fiber materials and the flowability during the rolling and molding processes. The process of breaking down the long chain molecules of raw rubber to create a plasticized material is called plastication. There are two main methods for plastication: mechanical plastication and thermal plastication.
- Mechanical Plastication: This is done at a relatively low temperature, where mechanical extrusion and friction in a plastication machine degrade and shorten the long-chain rubber molecules, changing the rubber from a highly elastic state to a plastic state.
- Thermal Plastication: This involves introducing hot compressed air into the raw rubber, which, under the influence of heat and oxygen, degrades and shortens the long-chain molecules, thus making the rubber plastic.
2. Mixing
To adapt to various use conditions and achieve different performance requirements, as well as to improve the performance of rubber products and reduce costs, various additives must be added to the raw rubber. Mixing is the process of combining plasticated raw rubber with additives and mechanically blending them in a mixing machine to ensure that the additives are fully and evenly dispersed throughout the rubber. Mixing is a crucial step in the rubber product manufacturing process. If the mixing is uneven, the rubber’s performance and the additive’s effect will not be fully realized, which can affect the product’s usability. The resulting material after mixing is called mixed rubber, which is a semi-finished product used to manufacture various rubber products, commonly referred to as rubber compound. This material is usually sold as a product and can be directly processed into molded, vulcanized rubber products. Depending on the formulation, mixed rubber comes in different grades and varieties with varying performance characteristics.
3. Molding
During the production of rubber products, various shapes and sizes can be pre-formed using rolling machines or extrusion machines. The molding methods include:
- Calendering Molding
This method is used to produce simple sheet or plate-shaped products. Mixed rubber is passed through a calendering machine to form sheets of a specific shape and size. Some rubber products, such as tires, rubber cloth, and rubber tubes, require a layer of rubber to be applied to textile fiber materials. This process is called coating or brushing the rubber. The coating process is typically completed on the calendering machine. Prior to calendering, the fiber materials must be dried and rubber-coated. Drying reduces the water content of the fibers to prevent bubbling due to moisture evaporation and increases the temperature of the fibers to ensure the quality of the calendering process. Rubber immersion before coating improves the bonding between the fibers and the rubber. - Extrusion Molding
This method is used for more complex rubber products, such as tire treads, rubber tubes, and rubber-coated metal wires. Mixed rubber with a certain level of plasticity is placed into the feed section of an extruder. The rubber is then forced through various die shapes under pressure from the screw, continuously forming the product. Before extrusion, the rubber must be preheated to make it soft and easier to extrude, ensuring a smooth surface and accurate dimensions for the rubber product. - Compression Molding
For rubber products with more complex shapes, such as rubber seals and gaskets, compression molding is used. In this process, the rubber material is placed between male and female molds, which are then heated to form the product.
4. Vulcanization/Curing
The process of converting plastic rubber into elastic rubber is called vulcanization. It involves adding a vulcanizing agent, such as sulfur or sulfur accelerators, to the semi-finished product made from raw rubber. The mixture is then heated and kept at a specified temperature in a vulcanizing chamber, where the linear molecules of the rubber form “cross-links” through sulfur, creating a three-dimensional network structure. This transforms the plastic rubber into highly elastic rubber with good physical and mechanical properties.
Because the cross-linking bonds are primarily formed by sulfur, the process is referred to as “vulcanization.” With the rapid development of synthetic rubber, there are now many types of vulcanizing agents, including organic polysulfides, peroxides, metal oxides, and more. Therefore, any process that transforms the plastic rubber with linear molecular structure into elastic rubber with a three-dimensional network structure is considered vulcanization.
Vulcanized rubber is also called soft rubber or “rubber” in everyday terms. Vulcanization is the most important process in rubber manufacturing. All rubber products must undergo vulcanization to achieve their ideal performance. Unvulcanized rubber has little practical use, and if the vulcanization degree is insufficient or the vulcanization time is too short, the rubber does not reach its optimal state, while over-vulcanization can significantly degrade its performance. Thus, controlling the vulcanization time during production is essential to ensure the rubber products achieve the best performance and the longest possible service life.
What Are the Applications of Rubber?
There is no unified scientific classification for rubber products. They are commonly divided into three categories: tires, industrial products, and daily necessities.
1. Rubber Products for Tires：
- Motor Vehicle Tires – Includes heavy-duty tires, car tires, engineering tires, industrial tires, agricultural machinery tires, motorcycle tires, etc.
- Non-Motor Vehicle Tires – Includes bicycle tires, rickshaw tires, cart tires, hand truck tires, etc.
- Specialty Tires – Includes aircraft tires, artillery tires, tank tires, etc.
2. Rubber Industrial Products:
- Belts – Conveyor belts, transmission belts, etc.
- Hoses – Fabric-reinforced hoses, braided hoses, coiled hoses, knitted hoses, special-purpose hoses, etc.
- Molded Products – Rubber seals, shock absorbers, etc.
- Compression Molding Products – Pure rubber tubes, door and window seals, various rubber profiles, etc.
- Rubber Tape Products – Everyday protective tape products (e.g., raincoats), industrial tape products (e.g., mining air ducts), transportation storage products (e.g., oil tanks), Lifebuoy Products (e.g., liferafts), etc.
- Rubber Rollers – Printing rollers, paper-making rollers, dyeing rollers, etc.
- Rigid Rubber Products – Electrical insulating products (battery casings), chemical corrosion-resistant linings, microporous rigid rubber (microporous partitions), etc.
- Rubber Insulation Products – Electrical wires, cables, etc.
- Latex Products – Dipped products, sponges, extruded products, molded products, etc.
3. Rubber Products in Daily Necessities:
- Daily Sporting Goods – Rubber shoes, rubber balls, erasers, rubber bands, etc.
- Medical and Health Products – Equipment (various catheters, washing balls), protective gear, medical packaging components, medical rubber products for the human body, etc.
Among these, tires, rubber shoes, rubber bands, and rubber hoses hold significant shares in both output value and rubber consumption, and sometimes these are collectively referred to as “miscellaneous rubber products.”
Rubber products can be classified based on raw materials into dry rubber products and latex products. All rubber products made from dry rubber, such as tires, belts, rubber hoses, etc., are collectively called dry rubber products. These products account for over 90% of total rubber product output. Products directly made from latex, such as latex gloves, balloons, sponges, etc., are collectively called latex products and account for less than 10% of the total output of rubber products.
Rubber products can also be classified by production method into molded products and non-molded products. Rubber products formed by vulcanization in metal molds are collectively known as tire products, rubber seals, rubber shock absorbers, etc. However, in the rubber industry, molded products are typically understood to mean rubber products made in molds excluding tires. Products that are vulcanized without molding are collectively called belts, hoses, tapes, rollers, and other non-molded products. Some rubber products (such as rubber shoes) can be produced by both molded and non-molded methods.
Types of Available Rubber Products
1. Tensile Strength
Tensile strength indicates the maximum ability of a product to resist stretching damage. The rubber industry generally uses tensile strength as a standard to compare the vulcanized rubber from different formulations and control the quality of vulcanized rubber.
2. Tear Strength
Rubber tearing occurs when cracks or fissures in the material quickly open under stress. This is one of the key indicators to measure the destructiveness resistance of rubber products.
3. Durability in Harsh Conditions
Fixed elongation stress and hardness are important indicators of the stiffness of rubber materials. Both represent the effectiveness required to cause certain deformations in vulcanized rubber. For example, for seals, fixed elongation stress is related to larger stretching deformations, while hardness is related to smaller compression deformations.
4. Wearability and Tear Resistance
This resistance refers to the ability of vulcanized rubber to resist material loss due to surface damage caused by friction. This property is closely related to the service life of the rubber product.
5. Fatigue (Damage)
When vulcanized rubber is subjected to alternating stress (or strain), the changes in its structure and performance are called fatigue. As the fatigue process progresses, material damage occurs, known as fatigue damage.
6. Elasticity
The most valuable feature of rubber is its high elasticity. This high elasticity comes from the movement of the rubber polymer chain segments, which is entirely caused by the transformation of coiled molecules.
7. Maximum Stretch
A rubber product must have a high stretch limit to ensure it does not sustain damage during deformation, which is a necessary condition for achieving high tensile strength.
Lifetime of Rubber Products
- Natural Rubber (NR) – 5 to 10 years
- Polybutadiene Rubber (BR) – 5 to 10 years
- Nitrile Rubber (NBR) – 5 to 10 years, up to 15 years
- Styrene-Butadiene Rubber (SBR) – 5 to 10 years
- Chloroprene Rubber (CR) – 5 to 10 years
- Isoprene Rubber (IR) – 5 to 10 years
- Butyl Rubber (IIR) – 5 to 10 years
- Hydrogenated Nitrile Rubber (HNBR) – 15 years, up to 20 years
- Ethylene Propylene Rubber (EPM/EPDM) – Up to 15 years
- Fluoroelastomers (FPM) – 5 to 15 years, up to 20 years
How to Maintain Rubber Products
Rubber has a certain service life and will age over time. For storage, it should be kept in a place that is cool, dry, and away from direct sunlight. Additionally, it should be kept away from substances that contain strong acids or alkalis. Another method to extend the service life of rubber products is to apply talcum powder on their surface when they are not in use. Talcum powder, a white powder, is easy to purchase, cost-effective, and very effective in prolonging the life of rubber products.
