What Is the Compound Used to Make Synthetic Rubber?
Synthetic rubber is a man-made polymer that mimics the elasticity and durability of natural rubber. The main compound used to make synthetic rubber is butadiene, often combined with styrene or isoprene in a process called polymerization. For the most common type, styrene-butadiene rubber (SBR), the two monomers are butadiene and styrene. This combination accounts for over half of all synthetic rubber produced worldwide, used primarily in tires and industrial goods.
How Butadiene and Styrene Combine
Synthetic rubber starts with monomers, small molecules that link together into long chains. The two primary monomers are:
- 1,3-butadiene (C₄H₆) – a hydrocarbon gas derived from petroleum refining or natural gas processing.
- Styrene (C₈H₈) – an aromatic hydrocarbon also sourced from petroleum.
These monomers are mixed in a reactor with a catalyst (often a peroxide or an organolithium compound) to start the polymerization. The reaction can be done in emulsion (soapy water suspension) or in solution (using a solvent like hexane). Emulsion SBR is the older, more common method; solution SBR gives better control over the polymer structure and produces rubber with lower rolling resistance for fuel-efficient tires.
The ratio of styrene to butadiene changes the rubber’s properties. A typical passenger tire SBR contains about 23.5% styrene by weight. More styrene makes the rubber harder and less flexible; more butadiene keeps it soft and elastic.
The Role of Isoprene and Other Monomers
Not all synthetic rubber uses styrene. Polyisoprene rubber (IR) uses isoprene (C₅H₈) as its sole monomer. Isoprene is chemically similar to the monomer in natural rubber, so polyisoprene closely matches natural rubber in strength and elasticity. It is used in medical gloves, tubing, and shoe soles.
Other specialty rubbers use different monomers:
- Chloroprene (C₄H₅Cl) – used in neoprene, which resists oil and weathering.
- Ethylene and propylene – used in EPDM rubber for roofing and weatherstripping.
- Isobutylene and isoprene – used in butyl rubber for inner tubes and tire liners.
Each monomer combination produces a rubber with a specific balance of heat resistance, oil tolerance, flexibility, and cost.
The Polymerization Process Step by Step
Polymerization turns liquid monomers into solid rubber. The process has three main stages.
1. Reaction. Monomers are pumped into a reactor with a catalyst and a chain-transfer agent (which controls molecular weight). In emulsion polymerization, water and soap are added to form a latex. The reaction runs at temperatures between 5°C and 50°C, depending on the recipe. Cold SBR (run at 5°C) produces a more uniform polymer than hot SBR (run at 50°C).
2. Coagulation and Drying. After polymerization, the latex is mixed with an acid or salt solution to coagulate the rubber particles into crumb-sized solids. The crumb is washed, dewatered, and dried in a hot-air oven or extruder. The result is a dry, crumbly rubber that can be baled or shipped as powder.
3. Compounding. Before the rubber can be molded into a product, it must be mixed with additives. Compounding ingredients include:
- Carbon black – reinforces the rubber and improves abrasion resistance.
- Sulfur – used in vulcanization to cross-link polymer chains and give the rubber its elastic strength.
- Zinc oxide and stearic acid – activate the vulcanization process.
- Antioxidants – prevent the rubber from degrading in sunlight and ozone.
A typical tire tread compound might contain 100 parts rubber, 50 parts carbon black, 3 parts sulfur, 5 parts zinc oxide, and 2 parts antioxidant. The exact recipe depends on the performance needed.
How Synthetic Rubber Differs from Natural Rubber
Natural rubber comes from the latex of the Hevea brasiliensis tree. Its monomer is isoprene, and the polymer has a cis-1,4 structure that gives it high tensile strength and tear resistance. Synthetic rubber can be engineered to match or exceed these properties, but there are trade-offs.
| Property | Natural Rubber | SBR (Synthetic) |
|---|---|---|
| Source | Tree latex | Petroleum monomers |
| Tensile strength | Higher | Moderate |
| Heat resistance | Good up to 100°C | Better up to 120°C |
| Oil resistance | Poor | Poor (unless specialty grade) |
| Abrasion resistance | Good | Excellent |
| Cost | Variable | Lower and more stable |
Synthetic rubber is often preferred where consistency and volume matter. A tire plant might use 50–60% SBR in a passenger tire tread and 100% natural rubber in a truck tire sidewall, where flex fatigue resistance is critical.
Why Butadiene Is the Workhorse Monomer
Butadiene appears in more synthetic rubber types than any other monomer. It is the main component in SBR, polybutadiene rubber (BR), and nitrile rubber (NBR). Polybutadiene rubber is used in golf balls, conveyor belts, and as an impact modifier in plastics. Nitrile rubber, made from butadiene and acrylonitrile, resists oils and fuels and is used in hoses and gaskets.
Butadiene’s popularity comes from its low cost, high reactivity, and ability to produce polymers with a wide range of glass transition temperatures. By adjusting the polymerization conditions, manufacturers can make polybutadiene with high cis content (for elasticity) or high vinyl content (for higher stiffness).
Vulcanization: What Makes Rubber Elastic
Raw synthetic rubber is sticky and weak. It becomes useful only after vulcanization, a chemical process that cross-links the polymer chains. The cross-linking agent is usually sulfur, added at 1 to 5 parts per hundred rubber (phr) for soft rubber products and up to 30 phr for hard rubber (ebonite).
During vulcanization, sulfur atoms form bridges between carbon atoms on adjacent polymer chains. This network prevents the chains from sliding past each other permanently, so the rubber snaps back after stretching. The reaction is accelerated by chemicals called accelerators, such as mercaptobenzothiazole (MBT) or diphenylguanidine (DPG). Zinc oxide and stearic acid activate the accelerators.
The vulcanization temperature ranges from 140°C to 180°C, depending on the rubber type and the cure system. A passenger tire tread might cure for 10 to 15 minutes at 160°C. Overcuring makes the rubber brittle; undercuring leaves it tacky.
Peroxide Vulcanization
Some synthetic rubbers, especially silicone rubber and EPDM, are cross-linked with organic peroxides instead of sulfur. Peroxides generate free radicals that form carbon-carbon bonds between polymer chains. These bonds are stronger than sulfur bridges, giving the rubber better heat resistance. Peroxide-cured rubber can withstand continuous service temperatures up to 200°C, while sulfur-cured rubber typically tops out at 120°C.
Common Types of Synthetic Rubber and Their Uses
Beyond SBR, several other synthetic rubbers serve specific industries. Each uses a different monomer or combination of monomers.
Polybutadiene rubber (BR) is made from butadiene alone. It has very low rolling resistance and excellent abrasion resistance. Tire manufacturers blend BR with SBR to improve tread wear. BR also appears in shoe soles, golf ball cores, and conveyor belt covers.
Nitrile rubber (NBR) is a copolymer of butadiene and acrylonitrile. The acrylonitrile content (18% to 50%) determines oil resistance. High-acrylonitrile NBR resists gasoline and hydraulic fluids, making it the standard material for fuel hoses, gaskets, and oil seals. NBR gloves are common in laboratories and automotive workshops because they resist punctures and chemicals better than latex.
Chloroprene rubber (CR), sold as Neoprene, is made from chloroprene monomer. It resists ozone, sunlight, and moderate heat. CR is used in wetsuits, automotive belts, hose covers, and adhesives. Its flame resistance makes it suitable for conveyor belts in mining.
Butyl rubber (IIR) is a copolymer of isobutylene and a small amount of isoprene (about 2%). It has very low gas permeability, so it holds air inside tire inner liners and basketball bladders. Butyl rubber also absorbs vibration, making it useful for engine mounts and shock absorbers.
Silicone rubber (VMQ) uses a silicon-oxygen backbone instead of carbon. Its monomer is dimethylsiloxane. Silicone rubber stays flexible from -60°C to 230°C and resists UV and ozone. It is used in medical implants, baking pans, and electrical insulation.
Key Takeaways
- The primary compound used to make synthetic rubber is butadiene, often combined with styrene to form SBR.
- Polymerization links monomers into long chains; the process can be emulsion or solution based.
- Different monomers (isoprene, chloroprene, ethylene) produce rubbers with specific properties for tires, seals, and medical goods.
- Synthetic rubber is compounded with carbon black, sulfur, and other additives before being vulcanized into finished products.
- Butadiene is the most common monomer because it is cheap, versatile, and works in many rubber formulations.
