Understand Types And Basic Properties of Rubber Materials

Understand Types and Basic Properties of Rubber Materials

Rubber is a highly elastic polymer material with reversible deformation, which is elastic at room temperature, can produce large deformation under small external force, and can return to its original state after removing the external force. Rubber is a completely amorphous polymer with a low glass transition temperature (Tg) and a large molecular weight, greater than several hundred thousand.

The types of rubber are natural rubber and synthetic rubber. Natural rubber is extracted from the rubber plant tree and has the advantages of good elasticity, high strength, good insulation, small deformation and easy processing. Synthetic rubber is synthetic, there are many types, such as styrene butadiene rubber, nitrile rubber, maleic rubber, ethylene propylene rubber, butyl rubber, neoprene rubber, acrylate rubber, hydrogenated nitrile rubber, chlorosulphonated polyethylene rubber, fluoroelastomer, silicone rubber, fluoro-silicone rubber and so on.

I. Natural Rubber NR

Natural rubber (NR) is biosynthesized polyisoprene in the rubber tree. As early as 1826, Faraday first determined the chemical formula of the natural rubber molecule as C5H8. However, its molecular structure and content were not confirmed until the advent of infrared spectroscopy and high-resolution nuclear magnetic resonance instruments. It was found that the molecular structure of natural rubber is cis-1,4-polyisoprene, with a content as high as 99%. The density of NR's raw rubber is 0.9–0.95 g/cm³, and its chemical structure is shown below:

The main properties of natural rubber NR are as follows:

1. Mechanical properties: NR's molecular chains are amorphous and flexible at room temperature, giving it excellent elasticity with an elongation of up to 1000%. Its highly regular molecular structure allows the chains to orient and crystallize under stress, providing self-reinforcement. After vulcanization, its tensile strength can reach 15–20 MPa, increasing to 25–30 MPa with carbon black reinforcement. NR also exhibits high tear resistance, with a tear strength of 98 kN/m.

2. Aging performance: The carbon-carbon double bonds in NR's molecular chains make it susceptible to oxidation and cracking by oxygen and ozone in the air, leading to aging.

3. High and low-temperature resistance: NR has a glass transition temperature (Tg) of approximately -72°C, allowing it to maintain elasticity at low temperatures. It begins to soften upon heating, decomposes around 200°C, and undergoes intense decomposition at 270°C.

4. Medium resistance: As a non-polar substance, NR dissolves easily in non-polar solvents and oils, making it unsuitable for use with gasoline, benzene, and other media.

5. Electrical properties: NR's non-polar nature gives it good insulation with a volume resistivity of 10¹⁵–10¹⁷ Ω·cm. However, vulcanization introduces polar substances like sulfur and accelerators, reducing its insulation properties.

6. Other properties: NR also offers good wear resistance and fatigue resistance, making it widely used in automotive tires, suspension systems, and various cushion blocks.

II. Ethylene Propylene Rubber EPDM

Ethylene propylene rubber, synthesized from ethylene and propylene monomers, is a synthetic rubber developed after the introduction of the Ziegler-Natta stereoregular catalytic system. It falls between general-purpose and specialty rubbers. Based on its molecular monomers, it can be divided into binary ethylene propylene rubber (EPM, made from ethylene and propylene) and ternary ethylene propylene rubber (EPDM, incorporating a small amount of non-conjugated diene). The non-conjugated dienes used in EPDM include 1,4-hexadiene, dicyclopentadiene, and ethylidene norbornene, resulting in different EPDM types. EPDM, synthesized from ethylene, propylene, and a third monomer, has a raw rubber density of 0.85–0.87 g/cm³. Besides tire rubber, EPDM is the most extensively used rubber in the automotive industry. Its chemical structure is depicted below:

The main properties of EPDM are as follows:

1. Mechanical properties: EPDM's main chain lacks double bonds, with only a few in the side chains, making it a saturated rubber. The absence of polar substituents allows the molecular chains to remain flexible across a wide temperature range, providing good elasticity, second only to NR among general-purpose rubbers.

2. Aging performance: As a saturated rubber, EPDM boasts high chemical stability and the best aging resistance among general-purpose rubbers, particularly against ozone. For instance, sealing materials exposed to 200×10⁻⁸ ozone concentration for 72 hours show no cracking.

3. High and low-temperature resistance: EPDM has a glass transition temperature of approximately -60 to -50°C, maintaining elasticity at low temperatures. It can be used long-term at 120°C and intermittently between 150–200°C.

4. Medium resistance: As a non-polar rubber, EPDM resists various polar solvents (e.g., alcohols, acids, strong bases, and esters) and exhibits excellent water and steam resistance, making it suitable for water control components and new energy vehicle battery packs using water cooling.

5. Electrical properties: Like NR, EPDM's low polarity grants it good electrical insulation with a volume resistivity of 10¹²–10¹⁵ Ω·cm. Its low water absorption ensures maintained insulation even after water immersion.

6. Other properties: EPDM offers excellent foamability and compression resilience. Foam EPDM is used in seals like window and battery pack gaskets. However, its low strength requires reinforcement for practical use, and it has poor self-adhesion and inter-adhesion, posing processing challenges.

III. Chloroprene Rubber CR

Chloroprene rubber is synthesized from 2-chloro-1,3-butadiene monomers. It appears milky white, beige, or light brown and is one of the early-developed synthetic rubbers with a high raw rubber density of 1.11–1.13 g/cm³. Research on chloroprene rubber began in 1906, and it was industrialized by Carothers et al. in 1931. Its chemical structure is shown below:

CR is primarily produced via emulsion polymerization using water as the medium, rosin soap as the emulsifier, and potassium persulfate as the initiator. As a general-purpose rubber, CR offers good mechanical properties and weather resistance, flame resistance, oil resistance, sunlight resistance, ozone resistance, and chemical corrosion resistance, making it widely used in rubber shoe soles, weather-resistant products, and coatings.

The main properties of CR are as follows:

1. Mechanical properties: CR is self-reinforcing. The chlorine atoms in its molecules enhance intermolecular forces due to their high electronegativity, making CR a polar rubber. Its highly regular structure and easy crystallization result in mechanical properties close to NR, with a tensile strength of 20–25 MPa.

2. Aging properties: Despite the carbon-carbon double bonds in its main chain, the chlorine atoms in the side chains reduce double bond activity, improving molecular stability and providing good ozone and aging resistance.

3. High and low-temperature resistance: CR has a glass transition temperature of around -40°C. However, its highly regular and polar molecular structure limits molecular movement. At low temperatures, deformation is difficult to recover, and brittleness may occur, indicating poor cold resistance. Its typical use temperature is above -30°C. CR's heat resistance surpasses NR but is inferior to EPDM. It can be used long-term at 100°C, with short-term use up to 150°C.

4. Medium resistance: CR's strong polarity grants it good oil and non-polar solvent resistance, making it suitable for oil pipes and dust covers.

5. Electrical properties: The presence of polar chlorine atoms results in poor insulation with a volume resistivity of 10¹⁰–10¹² Ω·cm, making CR unsuitable for high electrical insulation requirements.

6. Other properties: CR exhibits excellent flame retardancy due to its halogen content. It is preferred for rubber parts around new energy vehicle battery packs with high flame retardancy requirements.

IV. Nitrile Rubber NBR

NBR is produced via low-temperature emulsion copolymerization of butadiene and acrylonitrile monomers. The acrylonitrile content generally comes in five ranges: 18–24%, 25–30%, 31–35%, 36%, 41%, and 42–46%. Its raw rubber density is 0.96–1.20 g/cm³. Its chemical structure is shown below:

Due to its excellent oil resistance, NBR is widely used in traditional automotive fuel systems, such as fuel hoses, fuel tank liners, and oil seals. In new energy vehicles, where battery packs replace engines and fuel systems, NBR's application is relatively reduced.

The molecular structure of NBR contains strong polar cyano groups, making it a polar rubber. It offers excellent resistance to non-polar and weakly polar oils and solvents (e.g., gasoline, aliphatic oils, and vegetable oils). However, its volume resistivity is only 10⁸–10⁹ Ω·cm, making it the poorest in electrical insulation among rubbers. NBR has a glass transition temperature of -20 to -10°C, with colder temperatures leading to brittleness. The polar groups enhance the stability of unsaturated bonds in the molecular structure, providing slightly better heat resistance than NR. Its long-term use temperature is 100°C, with a maximum of 125°C. The presence of unsaturated bonds also results in poor ozone resistance.

V. Acrylate Rubber ACM/AEM

Acrylate rubber (ACM), an elastomer copolymerized from acrylate monomers, features a saturated main chain and polar ester side groups. Its excellent cost-effectiveness—better oil and aging resistance than NBR and superior mechanical properties to fluorosilicone rubber, coupled with a lower price than fluorine and silicone rubbers—makes it widely used in high-temperature and oil-resistant environments. However, its poor processing performance, mold adhesion, and low-temperature resistance limit its application. The molecular structure of acrylate rubber is shown below:

In 1975, DuPont introduced an improved version of ACM, polyethylene/acrylate rubber (AEM), which enhances processing and low-temperature performance but at a higher cost than ACM.

The main properties of acrylate rubber are as follows:

1. Mechanical properties: ACM offers excellent mechanical performance with a tensile strength of 10–15 MPa but has poor elasticity, with an elongation of 100%–300%.

2. Aging properties: ACM exhibits exceptional aging and ozone resistance, comparable to fluorine and silicone rubbers.

3. High and low-temperature resistance: ACM has better high-temperature resistance than EPDM and can be used short-term below 200°C. However, its low-temperature resistance is poor, with a brittle temperature of -20 to -10°C, limiting its application.

4. Medium resistance: The polar groups in ACM provide good oil resistance, especially to hot oil. For example, after immersion in standard oils No. 1, 2, and 3 at 150°C for 70 hours, performance changes are minimal. However, the ester groups make it susceptible to hydrolysis, resulting in poor water and steam resistance.

5. Electrical properties: The polar groups lead to average electrical insulation, restricting its use in electrical applications.

6. Other properties: ACM also demonstrates good air permeability, tear resistance, and wear resistance. In automotive rubber products, it is primarily valued for its excellent oil and high-temperature resistance, making it widely used in shaft seals, oil seals, O-ring seals, and gaskets.

AEM, as an improved version of ACM, inherits the excellent performance of ACM and improves some of the shortcomings of ACM, and the performance comparison of the two is shown in Table 1.

VI. Fluoroelastomer FKM

Fluoroelastomer (referred to as FKM in Europe and FPM in the US) is a synthetic polymer elastomer containing fluorine atoms on its main or side chains. Various types exist based on monomer structure, with vinylidene fluoride rubber being the most common, as shown below:

Development of fluoroelastomer began during World War II to meet military needs. China started developing several FKM types in 1958, primarily polyolefinic ones like Type 23, Type 26, Type 246, and nitroso types. In recent years, FKM has evolved into four main categories: fluorinated olefins, nitroso, perfluoroethers, and fluorinated phosphazenes. Its applications have expanded from defense and military fields like aviation and aerospace to civilian sectors, significantly contributing to national defense and economic development.

Among all synthetic rubbers, FKM, renowned as the “King of Rubber” for its superior comprehensive performance, has the following main properties:

1. Mechanical properties: FKM offers excellent mechanical performance with a tensile strength of 10–20 MPa but has poor elasticity, with an elongation of 150%–350%.

2. Aging properties: FKM has exceptional aging and odor resistance. For example, DuPont's Vion-type FKM retains good performance after a decade of natural storage.

3. High and low-temperature resistance: FKM boasts the best high-temperature performance among elastomers, with long-term use up to 250°C and short-term use below 300°C. However, its cold resistance is poor, losing elasticity and becoming brittle below -25 to -15°C.

4. Medium resistance: FKM's high chemical stability grants it the best medium resistance among elastomers, providing excellent resistance to organic media (fuels, solvents), inorganic acids, and strong oxidizing agents (e.g., hydrogen peroxide).

5. Electrical properties: FKM's electrical properties vary with its molecular structure. For instance, Type 23 FKM has better electrical performance than Type 26. Temperature also affects its electrical properties, with insulation decreasing as temperature rises. Thus, FKM is unsuitable as a high-temperature insulating material.

6. Other properties: The halogens in FKM's molecular structure provide excellent flame retardancy, which improves with higher halogen content. FKM is self-extinguishing upon flame removal. Given its performance characteristics, FKM is primarily used in high-temperature, oil-resistant, and specialty media applications, such as fuel pipes and engine seals.

VII. Silicone Rubber

Developed in the 1940s, silicone rubber (abbreviated as Q) initially involved dimethyl silicone rubber research. Now with over a thousand varieties, silicone rubber stands out for its molecular chain combining inorganic and organic characteristics, as shown below:

The main chain of silicone rubber alternates between silicon and oxygen atoms. The Si-O bond energy (370 kJ/mol) is significantly higher than that of the C-C bond (240 kJ/mol), endowing it with superior thermal stability compared to other synthetic rubbers.

Based on vulcanization methods, silicone rubber is divided into heat-vulcanized (HTV) and room-temperature-vulcanized (RTV) types. HTV, mainly high-molecular-weight solid rubber, resembles conventional rubber. RTV, primarily low-molecular-weight liquid rubber, includes common one-component or two-component silicone rubbers. Depending on side groups introduced, it can be dimethyl, methyl vinyl, methyl phenyl vinyl, or fluorosilicone rubber. The unique main chain of silicone rubber allows different side groups to significantly influence its mechanical, temperature, and processing properties.

The main properties of silicone rubber are as follows:

1. Mechanical properties: Silicone rubber has relatively low tensile properties among common rubbers, with a tensile strength of 4–7 MPa. However, it offers good compression resistance at high and low temperatures. When applying silicone rubber, the stress conditions of components should be carefully considered. To ensure longevity, components should avoid tensile forces whenever possible.

2. Aging properties: The main chain of silicone rubber is -Si-O-, lacking unsaturated bonds. The bond energy of Si-O is higher than that of C-C, providing excellent resistance to ozone, oxygen, and weather aging.

3. High and low-temperature resistance: Silicone rubber exhibits outstanding high and low-temperature resistance with a working temperature range of -100 to 350°C, the widest among automotive rubbers.

4. Medium resistance: Due to its special structure, silicone rubber is highly inert. Its resistance to liquid media primarily depends on the type of substituent groups. For example, nitrile silicone rubber offers good oil and polar solvent resistance.

5. Electrical properties: Silicone rubber's main chain resembles quartz in structure, giving it excellent electrical insulation.

6. Other properties: In addition to the above, silicone rubber offers good flame retardancy, high air permeability, and foamability. It is non-toxic, odorless, physiologically inert, and highly resistant to extreme temperatures, making it one of the superior synthetic rubbers.

Currently, silicone rubber plays an extremely important role in the field of biomedicine, such as in the manufacture of noise-reducing earplugs, artificial blood vessels, artificial lungs, etc., with very satisfactory results. Additionally, silicone rubber has a wide range of applications in the fields of sealants and adhesives.

In the field of fire safety, YINSU Flame Retardant has developed a series of flame retardant products suitable for rubber, adhesives, and other materials. For instance, the rubber flame retardant XJ-A2 and XJ-85M are specially designed for rubber materials. XJ-A2 is a highly efficient halogen-free flame retardant with excellent thermal stability, which can effectively improve the flame retardancy of rubber products without significantly affecting their mechanical properties. XJ-85M is also a halogen-free flame retardant that boasts good compatibility with rubber, ensuring uniform distribution within the rubber matrix and stable flame retardant performance. For adhesives and other bonding applications, the company has introduced the adhesive-specific organic phosphorus halogen-free flame retardant JS-FR and the MCA-coated halogen-free flame retardant MCA-B. JS-FR offers excellent flame retardant efficiency and is suitable for various adhesive systems, while MCA-B, with its unique coating technology, enhances the compatibility of flame retardants with adhesives, improving the flame retardant effect and maintaining the adhesive's bonding strength. These flame retardant products from YINSU Flame Retardant provide practical solutions for enhancing the flame retardant performance of rubber and adhesive products, meeting the safety requirements of different application scenarios.