Types and basic properties of rubber

Posted on : Jan 31 , 2023 ， By  GREFEE

Natural rubber (NR) does not have melting point, which is usually be heated and softened gradually. It is completely softened at 130℃~140℃, becomes viscous fluid at 150 ℃~160 ℃, starts to decompose at 200 ℃, and starts to decompose rapidly at 270 ℃. Under normal temperature, natural rubber is elastic and gradually become hard as the temperature goes down. The elasticity is greatly reduced when it is under  0℃, and become brittle when it is under 70℃. The frozen natural rubber can recover its elasticity when heated to normal temperature.

1. The elastic modules of elastic NR is around 3-6MPa and can stretch 1000% without breaking. 2. Hardness: natural rubber is easy to crystallize under external tension and has strong self compensation. It has large molecular weight and high mechanical strength. The breaking strength of pure rubber vulcanizate is 16.7~28.4MPa, and it can reach 24.5~34.3MPa after carbon black reinforcement. Natural rubber has low heat generation after repeated deformation, and has good flex fatigue resistance

General synthetic rubber I. SBR

1. SBR is an unsaturated adhesive with less double bonds than NR. However, benzene ring reduces the activity of double bond. Therefore, SBR vulcanization speed is slower than NR. SBR is not easy to scorch and is resistant to heat, oxygen, ozone, and sun aging.

2. SBR is non-crystallizable. Its strength is about 2MPa without reinforcement, which reaches 17~24MPa after carbon black is reinforced. The benzene ring causes large friction. The elasticity, strength, bending crack resistance and cold resistance of SBR are inferior to those of NR. In case of multiple deformation, it is difficult to process due to large heat generation, large rolling and extrusion deformation. Poor adhesion, difficult to fit and shape. But SBR has good wear resistance and air permeability. 4. SBR is non-polar, soluble in hydrocarbon solvents, and oil resistant, but better than NR. Its acid and alkali resistance, dielectric property are almost same with NR 5. Low cost, SBR is mainly used for producing tires and other insulating products, like rubber hoses, cables, rubber shoes, etc.

 Cis polybutadiene BR

BR is the abbreviation of 1,4-polybutadiene. Its dosage is second only to SBR and can be divided into high cis BR, medium cis BR and low cis BR.

1. BR molecular arranges regularly and the elasticity is better than NR, which is considered as the best one. It is the most cold resistant rubber. The Tg of high cis BR is – 105 ℃, In addition, it has excellent wear resistance, low heat generation and good flexibility resistance.

2. BR is unsaturated. It is similar to NR, and can react with sulfur and oxygen. There is no methyl beside the double bond, so the double bond activity is lower than NR. The vulcanizate NR is slow, but  the aging resistance and heat resistance are good.

 3. BR is crystalline. The crystallization temperature is lower than NR, so its pure rubber strength is low, and it is reinforced with carbon black. The breaking strength and tear strength of BR are lower than those of NR.

4. BR is non-polar, soluble in hydrocarbons. It is not resistant to oil, but has good affinity for oils and reinforcements. The utility model can be filled with oil and carbon black in large quantities. The physical properties are reduced, thereby reducing the cost. 5. Not only the processing property and adhesion of BR are poor, it is also sensitive to changes in temperature and humidity, so it is often used together with NR. BR is easy to slide on wet and slippery surfaces, and the cold fluidity of raw rubber is large. BR is mainly used in the tire industry and other wear-resistant products, such as rubber hose, rubber shoes, etc

Isoprene rubber IR

1.IR is the abbreviation of polyisoprene.IR is similar to NR. However, the gel content in IR is low, and it contains less impurities, and is good at texture.

2.Great processing performance. IR is easy to plasticize, and can even eliminate the plasticizing process.

3.Low extrusion shrinkage in semi finished products & good liquidity

4.IR has better elasticity than NR. Generate a low amount of heat, good crack resistance. However, the constant elongation stress, tensile strength and hardness are slightly low, and the tear resistance is poor, so the roller is easy to stick when mixing rubber.

5.The non rubber constitutes in IR is extremely low. Its water resistance, electric insulation, and aging resistance are better than those of NR.

6.The ozone resistance and the weather resistance can be greatly improved when IR and CR are used together. At the same time, the breaking strength, hardness and compression deformation are improved largely. IR is used in truck tyres, aircraft tyres, medical products, adhesives, rubber shoes and impregnated products, etc. IR can be used for all products with NR.

Butyl rubber (IIR)

Butyl rubber is an elastomer polymerized by isobutene and a small amount of isoprene at low temperature (- 98 ℃~- 103 ℃). There is a little content of double bond. The lower the degree of unsaturation, the slower the vulcanization rate of rubber. However, it is resistant to corrosion and aging. The butyl rubber with unsaturation of 1.6%~2.0% has large molecular weight and fast curing speed, which is the largest amount of butyl rubber. The appearance is white, cold flow, relative density is 091~0.92, molecular weight is 3W~8.5W, tensile crystallization.

1.Good air permeability.  The  airtightness of butyl rubber is the best, which is 8 times higher than that of natural rubber.

2.High chemical stability. Low unsaturation, excellent heat aging resistance, weather aging resistance, especially ozone resistance and acid and alkali resistance. The ozone resistance is higher than NR by more than 10 times. It has good cold resistance and can be froze without crystallization. The Tg is – 73 ℃. It is non-polar and has little swelling in polar solutions, like alcohol and ketone. However, it is not resistant to hydrocarbon solvents.

3.Excellent electrical performance. Butyl rubber has better electrical insulation and corona resistance than general synthetic rubber. Also, the water absorption is low. In addition, it can be used as insulating material. The volume resistance can be up to 1016 Ω. cm, 10~100 times higher than that of general synthetic adhesive.

4.It has good shock absorption. Low elasticity and great ability to absorb vibration. It has good shock absorption at – 30 ℃~-+50 ℃.

Disadvantage:

1.Poor processing performance, slow vulcanizing, not  be apt to use with other rubber. The adhesion and oil resistance are poor. It generates high amount of heat. The elasticity under normal temperature is low.

2.Applications: manufacturing inner tube, water tube, air tube, radiation proof gloves and products with high air tightness requirements. Chemical corrosion-resistant containers and insulating skin and sheath of wires and cables. All kinds of heat and water resistant sealing gaskets and shock absorber.

3. Modification: Halogen is used for modification. Bromination and chlorination are added to slow the curing speed and poor adhesion of butyl rubber. It can be used together with NR/SBR/CR, and other rubbers. The advantages of it include better air tightness, ozone resistance, chemical corrosion resistance and other mechanical properties.

 Ethylene propylene rubber

The ethylene propylene rubber is obtained through the directional copolymerization of main monomers, including ethylene propylene diene monomer (EPM) and ethylene propylene diene monomer (EPDM). EPM is a saturated rubber that is copolymerized by ethylene and 20%~30% propylene.There is no double bond in the molecular chain. It cannot be vulcanized with sulfur, but only cross-linked with peroxide. Its application range is limited due to the slow curing and expensive curing agent. EPDM is the third monomer introduced into ethylene propylene copolymer, like ethylene norbornene, dicyclopentadiene, etc. By doing so, it can be sulfurized with sulfur, and its properties are similar to those of EPM. It is amorphous, and the relative density is 0.86~0.88. There is no polar substituent in the molecule.

1.Great aging resistance. Excellent ozone resistance, far better than that of butyl rubber and CR. Good weather resistance, no cracks after exposure to sunlight for 3 years, NR150 days, SBR 5 days, 70 days. Has good aging resistance and can be used at 150 ℃ for a long term. The aging resistance is the best in general rubber.

2.Chemical resistance. It is resistant to various polar chemicals, acids and alkalis. Poor resistance to hydrocarbon oil.

3.Excellent electrical insulation and exceed than that of butyl rubber, suitable for high pressure of 45~65KW.

4.Good impact and cold resistance. The rebound rate can reach 50%~60%, second only to NR and BR. The elasticity retention property is good at low temperature, and it becomes hard at – 57 ℃ and brittle at – 77 ℃.

Disadvantage

The curingprocess rate is 3 to 4 times slower than the general curing speed.  Mixing with other unsaturated adhesives is not suggested. Poor self adhesion and mutual adhesion, difficult adhesion, and poor processing performance. Easy accessible and cheap raw materials, mainly used for rubber hose, adhesive tape, auto parts, wires and cables, etc.

chloroprene rubber (CR)

CR is a polymer of chloroprene. It is light yellow or brown elastomer. There are three categories:

1.Sulfur regulated type (G type) sulfur or sulfur containing organic compounds (thiuram) are used as regulators in manufacturing. The most commonly used type is GN-A, which is equivalent to domestic general-purpose rubber.

2. Non sulfur regulating type, the mercaptan compound is used as regulator during polymerization, no sulfur. It is equivalent to the domestic 54-1 CR3 and other types, which have special performance and special use. It mainly includes bonded CR, chloropropylene rubber, chlorobenzene rubber, gel CR, etc.

CR has standardized structure, elongation crystallization and high tensile strength of pure rubber.

1.Great aging resistance and heat resistance，Cl absorbs electrons and can shield the external effect that improves aging performance, especially the weather resistance and ozone performance. It is only next to the ethylene propylene rubber and butyl rubber among the general rubber. Its heat resistance is almost equivalent to NBR. The color is likely to change after being exposed under the strong sunlight, so it is not suitable to make light colored or transparent products.

2. Good combustion resistance. A large amount of hydrogen chloride is released during combustion, which is only carbonized, does not delay combustion. In addition, it has good self extinguishing property. The flame resistance is the best of general rubber.

3. Oil resistance and chemical corrosion resistance: in addition to aromatic and chlorinated hydrocarbon oil, it is stable in other solvents. Its oil resistance is better than NR and SBR, but lower than NBR. It is resistant to the ordinary inorganic acid and alkali, but not resistant to concentrated sulfuric acid and concentrated nitric acid.

4. Good adhesion CR is the material with excellent adhesion performance.

5.Good air permeability. CR has great air permeability, which is only next to butyl rubber, NBR is better than NR, SBR and BR.

6. CR can be vulcanized with metal oxides (MgO, ZnO, etc.).

Disadvantage

Regular structure, easy to crystallize, but the molecular chain contains Cl, which brings the following disadvantages.

 1.Low storage stability  ，Generally, CR is likely to become hardened and modified during the storage process. No more than one year under 20℃, and no more than half year under 30℃, but the non vulcanized adjustable 54-1 type can be stored for 40 months under 30℃.

2. Poor performance at low temperature

It is likely to crystallize and hardened under low temperature.

3. Sensitive temperature change

Easy to stick roll and scorch if is processed in 70~90℃. Large shrinkage.

Application

CR is mainly applied into producing heat resistant conveyor belt, oil resistant and chemical resistant rubber hose, container lining, rubber roll and rubber sheet. Wire and cable, outer sheath and door and window sealant strip are also made. Some can be used as adhesives.

 Nitrile butadiene rubber (NBR)

NBR is an elastic copolymer polymerized by butadiene and acrylonitrile lotion.

According to the level of content, butyronitrile can be divided into five grades: ultra high, medium high, medium low, 42~46%, 36~41%, 31~35%, 25~30%, 18~24%. However, the elasticity is reduced if the content of acrylonitrile is high and oil resistance is good. Thus, the acrylonitrile content is generally controlled between 15~50% for the oil resistance and elasticity. There are generally three grades of domestic products: NBR-40, NBR-26 and NBR-18, which are equivalent to the foreign medium and high content grades. Side nitrile group (- CN), which makes its performance excellent.

1.NBR is amorphous rubber. The tensile strength and tear strength are very low, 3~4MPa, but the strength after carbon black reinforcement reaches 24.5~30MPa.

2. Good oil resistance. The higher the content of acrylonitrile, the better the oil resistance. The oil resistance is only next to the polysulfide rubber, acrylate rubber and fluororubber. However, the oil resistance to aromatic hydrocarbon oil and chlorinated hydrocarbon oil is poor.

Disadvantage:

1.Poor elasticity and cold resistance. The higher the content of acrylonitrile, the poorer the elasticity.

2.Poor electric insulation.  NBR has the poorest electricity insulation among all kinds of rubbers, which can be used for requiring to export static electricity to avoid places that may cause fire, such as textile roller.

3.Poor ozone resistance. Generally, the ozone resistance needs to be improved with the ozone resistant agent or PVC.

4.Poor acid resistance. NBR is not resist to the corrosion of nitric acid, concentrated sulfuric acid, hypochlorite, etc. However, the alkali resistance is better than NR.

Application

NBR is widely applied into different kinds of rubber products with oil resistance. The Nitrile-40 is generally used for rubber products directly contacting with oil, like the oil seal, oil pipe, chemical container lining, gasket, etc. Nitrile – 26 is generally used for oil resistant rubber hose, oil tank, printing cots, oil resistant gloves, etc.

 Hydrogenated nitrile rubber (HNBR)

HNBR is a kind of highly saturated elastomer that is obtained from the specially hydrotreated NBR. The HNBR has good oil resistance to fuel, lubricant, aromatic solvent, and also has great thermal resistance, chemical corrosion resistance, ozone resistance, and compression set resistance due to the highly saturated elastomer (good resistance to freon, acid, alkali). In the meantime, it also has high strength, tearing performance, wear resistance, which is the one of the best rubbers with high comprehensive performance.

At present, the HNBR  is mainly applied into aspects like oilfield and automobile industry. Its mainly supplier is the Ruiweng Company owning rich experience in the field of formulation and processing of hydrogenated nitrile rubber. According to different application fields, it is specialized in supplying  varieties of rubber compounds with excellent performance and stable quality, as well as various hydrogenated nitrile rubber sheets and molded products. With the rapid development of automotive and petroleum industries, it also requires great heat resistance, high temperature resistance, high pressure resistance, oxygen resistance and other characteristics. NBR is way far to meet up these requirements even though some of them have been replaced by the fluororubber. However, the price of fluororubber is high. Thus, people embarked to seek improving the performances of NBR. Hydrogenated nitrile rubber was successfully developed to meet this new demand.

The high temperature resistance of HNBR is 130~180 ℃, and the cold resistance is – 55~- 38 ℃. Compared with other polymers, HNBR has excellent mechanical properties, so it can better meet the requirements of the automobile industry. HNBR reinforced with ZnO/methacrylic acid (MAA) can be used to make V-belts, uniform V-belts, bottom rubber of multi-purpose V-rings, vibration isolators, etc. Moreover, it can also prepare sealing rings, seals, heat resistant pipes, etc. In oil drilling, it is required that rubber products must withstand high temperature, high pressure, acid, amine, H2S, CO2, CH4 and other steam tests. Various products made of HNBR can be acid resistant, oil resistant and solvent resistant. HNBR reinforced with ZnO/MAA can be used to make drilling protection box and piston for mud pump. In addition, paper washer made of HNBR by beating can be used as sealing washer in petroleum industry and automobile industry.

The heat resistance and radiation resistance of HNBR are higher than those of silicone rubber, fluororubber and polytetrachloroethylene. It is ideal material for producing various rubber seals in power stations, as well as for hydraulic pipes, hydraulic seals, cable sheaths for power stations, printing and textile rollers, weapon parts, and seals, coatings, fuel capsules for aerospace, etc. Moreover, HNBR latex also an be used as surface coating (painting), adhesive for textile, paper, leather, metal, ceramics, non-woven fiber, foaming rubber, impregnated latex products, etc. In addition, the comprehensive properties of HNBR reinforced with ZnO/MAA, peroxide and high wear furnace black are higher than those of ordinary HNBR. HNBR not only obtained excellent heat resistance (resistance to 150 ℃ high temperature) and ozone resistance, but also maintained the original excellent oil resistance of NBR. Its application field has broken through the traditional field of NBR, making chlorinated polyethylene, chlorosulfonated polyethylene and other special rubbers subject to certain impact.

Economically, the price of HNBR is still high. Meanwhile, the rigidity is low and the processing performance is good and the density is low, so it can add more filler. What’s more, the unit volume of HNBR is only half of that of fluororubber, so it will be a substitute for fluororubber and other special elastomers. Although, HNBR has been used for years and its application is continually enlarging. It is still underdeveloped domestically, so we should not miss the opportunity to seek better development of HNBR. According to the information, in Daqing, we have more than 2000 Phase-II wells need to use submersible pumps to pump oil at a well depth of 2000m and a temperature of about 140 ℃. If NBR spindles are used, the service life is only a few months or even shorter. If HNBR spindles are used, the service life can be more than one year. China’s Nanjing No. 1 Rubber Products Factory needs more than 30 tons of HNBR every year.

During the manufacturing of HNBR, the catalytic hydrogenation technology of NBR is essential. As we know, the technical conditions for the production of NBR by solution hydrogenation are harsh. Lanzhou Chemical Company has been embarked the program. Due to the low recovery rate of rhodium, the cost is relatively high. In recent years, factors, like separation, recovery and recycling of precious metal catalysts have been the focuses of the research on hydrogenation and hydroformylation of water-soluble two-phase catalysts. Once the application of such catalysts can be extended to the hydrogenation of polymers, they can be used for the hydrogenation of NBR. At present, we have synthesized two water-soluble phosphine ligands and are conducting a trial in this regard.

Special mixed rubber

 Silicone rubber VMQ

Silicone rubber is a kind of organic elastic polymer formed by condensation polymerization of various siloxanes. 1. Advantages: Silicone rubber is a kind of synthetic rubber with cold resistance and high temperature resistance due to the main chain structure of the molecular chain. Compared to other rubbers, its robust properties, like excellent ozone aging resistance, heat aging resistance, light aging resistance, weather aging resistance and good electrical insulation enable it the widest working range (- 100~300 ℃),. 2. Disadvantages: compared to other rubbers, its tensile strength, tear strength and wear resistance of vulcanization are much lower than those of NR and other rubber, and poor acid and alkali resistance, and high price. 3. Application: It is mainly used for various high temperature, low temperature, ozone and oil resistant sealing products and shock proof accessories used in aviation and industry, as well as insulation and medical supplies.

 Fluorosilicone compound (FVMQ)

Technical performance: 1. Fluorosilicone rubber is a modified silicone rubber. The methyl on the siloxane side chain of silicone rubber is replaced by r-trifluoropropyl.

2. Not only does it have the excellent oil and solvent resistance, but also good stability at room temperature and high temperature for aliphatic, aromatic and chlorinated hydrocarbon solvents, petroleum based fuel oils, lubricating oils, hydraulic oils and some synthetic oils (like diester lubricating oils, silicate ester hydraulic oils, etc.). in addition, it can also maintain a good level of elasticity. Under the condition of oil immersion, the maximum operating temperature can reach 180 ℃. It has good stability under normal temperature and high temperature, and can be used for a long time in the range of – 50 ℃~+200 ℃, and for a short time at 250 ℃. Main application: due to its high oil resistance and temperature resistance, fluorosilicone rubber is widely used in aerospace, aviation and automobile manufacturing industries.

 Fluoroelastomer (FKM)

FKM is an elastic polymer formed by polymerization or condensation of fluorine containing monomers, which is mainly polymerized by fluorine-containing olefin monomer.

Properties:

1. The most distinct one is its great thermal oxidation resistance, which can be comparable to silica gel. 2. It is highly resistant to chemical corrosion and has higher chemical stability than any other rubber for organic liquids, concentrated acids, high concentration hydrogen peroxide, and other strong oxidants. Disadvantages: low cold resistance, tensile strength decreases rapidly with the increase of temperature, high price, and poor processing property. Application: used to manufacture high temperature, corrosion and oil resistant products for aviation, rocket, automobile and chemical industry.

 Polyurethane rubber (PU)

Polyurethane rubber, simply known as PU, which is made by polycondensation of polyester or polyether and diisocyanate compounds. It can be divided into polyester type and polyether type according to the monomer. It can be divided into three types: pouring type, mixing type and thermoplastic type according to the processing method. Advantages: 1. High mechanical strength, abrasion resistance, more than any other known rubber, more than 4 times higher than NR and SBR. 2. The air tightness, elasticity and aging resistance are equivalent to IIR (butyl rubber). 3. Good oil resistance.

Disadvantages:

1. Poor thermal resistance, water resistance, and easy hydrolysis in water.

Application:

1. Mainly applied into products with extremely high requirements in wear resistance, strength, and oil resistance.

2. It is radiation resistant and can be used as protective articles for the space industry and the atomic energy industry.

3. The reaction of isocyanate with water can release CO2, which can make PU foam rubber 30 times lighter than water.

4. It has good mechanical performance and electrical insulation, and can be used for heat insulation, sound insulation and shock resistance.

 Polysulfide rubber

The main chain has rubber of – S-C-S-S-chain, which equips the rubber products with the best oil resistance, excellent ozone resistance and low air permeability. It also has good low temperature flexibility and adhesion to other materials. The disadvantages include poor strength, poor heat resistance, and expensive price.

 Acrylate rubber (ACM)

ACM is mainly the butyl acrylate or copolymer of ethyl acrylate and acrylonitrile, with high polarity and full saturation, good heat resistance and oil resistance. Good weather aging resistance and ozone resistance. However, the water resistance and steam resistance are low.

 Ethylene acrylate (AEM)

AEM is an ethylene copolymer with content of 8%~40% methyl acrylate. The appearance is milky white translucent solid. The melt flow rate is 2 ～ 6g/10min, Vicat softening point is 59 ℃. Advantages: good resistance to environmental stress cracking and excellent electrical performance. The extrusion bonding temperature is 316~322 ℃. Ethylene and methyl acrylate are used as raw materials. It is prepared by polymerization under high pressure with oxygen or peroxide as initiator. It can be blown into film for disposable gloves, medicine and food packaging, extruded hoses and profiles, blow molded toys, foaming foam board, etc.

 Chlorohydrin rubber

It is a polyether rubber made from epichlorohydrin. Advantages: 1. Owns the properties of saturated rubber and polar rubber, cold resistance, heat resistance, oil resistance, ozone resistance, combustion resistance, acid and alkali resistance and solvent resistance. 2. Affordable price and easily assessable raw materials.

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