Synthetic rubber and elastomers

Emulsion and solution SBR, polybutadiene, NBR, butyl, EPDM and neoprene manufacture, plus compounding and sulphur vulcanisation, with recycle and Fox Tg calculations.

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Why it matters

India is a large tyre producer and also grows natural rubber, yet most tyres, hoses, seals and belts need synthetic elastomers that natural rubber cannot match in oil, heat, ozone or gas resistance, or in supply stability. Synthetic rubber plants sit downstream of the steam cracker's C₄ cut, and they illustrate emulsion, solution and cationic polymerisation along with the compounding and vulcanisation that turn a gum into a usable product.

Key ideas

What makes an elastomer. Long, flexible chains with a glass transition temperature (Tg) well below the use temperature, little crystallinity at rest, and a light network of crosslinks so that the material snaps back after large deformation. Raw (unvulcanised) rubber is sticky and flows under load; vulcanisation fixes the network.

Monomers. 1,3-Butadiene is extracted from the steam-cracker C₄ cut by extractive distillation with polar solvents such as NMP, DMF or acetonitrile. Styrene comes from ethylbenzene dehydrogenation; acrylonitrile from propylene ammoxidation; isobutylene from the C₄ raffinate; chloroprene from butadiene chlorination.

Main synthetic rubbers.

  • Emulsion SBR (E-SBR), the largest volume: butadiene + styrene (about 23.5 wt % bound styrene) in water with soap emulsifier. The "cold" process runs at about 5 °C with a redox initiator (hydroperoxide + iron salt + reducing agent) and a chain-transfer agent (mercaptan) to control molecular weight. Polymerisation is stopped at about 60–70 % conversion with a shortstop, because higher conversion gives branching and gel. Unreacted butadiene is flashed off and styrene steam-stripped, both recycled. The latex is blended with extender oil if needed and coagulated with acid and salt, then washed, dewatered and dried as crumb. Cold SBR has better properties than the older "hot" (about 50 °C) process.
  • Solution SBR (S-SBR) and polybutadiene (BR): anionic polymerisation with butyl-lithium in a hydrocarbon solvent ("living" chains, narrow distribution, controllable microstructure), or Ziegler–Natta catalysts (Nd, Co, Ni) for high-cis BR. S-SBR gives low rolling resistance in "green" tyres; BR gives abrasion resistance and resilience.
  • Nitrile rubber (NBR): emulsion copolymer of butadiene and acrylonitrile (about 18–50 % ACN). The polar nitrile groups make it resistant to swelling by oils and fuels; higher ACN raises oil resistance but also Tg. Hydrogenated NBR (HNBR) adds heat resistance.
  • Butyl rubber (IIR): isobutylene with 1–3 % isoprene (to provide double bonds for curing), cationic polymerisation with AlCl₃ in methyl chloride at about −100 °C; the reaction is extremely fast and the low temperature controls molecular weight. Very low gas permeability: inner tubes and tyre inner liners (halobutyl).
  • EPDM: ethylene + propylene + a small amount of a non-conjugated diene such as ethylidene norbornene (ENB), made in solution with Ziegler–Natta or metallocene catalysts. The saturated backbone resists ozone and weather; the diene puts unsaturation in a side group for sulphur curing. Uses: automotive seals, roofing.
  • Polychloroprene (neoprene, CR): emulsion polymerisation of chloroprene; flame, oil and weather resistance.
  • Synthetic polyisoprene (IR): Ziegler–Natta or lithium catalysts give high-cis structure close to natural rubber.

Compounding and vulcanisation. Rubber is mixed (internal mixers) with fillers (carbon black or silica with silane coupling agents for reinforcement), process oils, antioxidants and antiozonants, and the cure system: sulphur, accelerators (sulphenamides, thiazoles), activators (zinc oxide + stearic acid). It is shaped (extrusion, calendering, moulding) and vulcanised at about 140–180 °C, forming mono-, di- and polysulphide crosslinks between chains. Crosslink density sets modulus and hardness; too little gives poor strength, too much gives brittleness (ebonite at very high sulphur). Peroxide curing is used for saturated rubbers like EPM.

Formulas

Mole fraction of styrene in SBR: x_S = (w_S / M_S) / (w_S / M_S + w_B / M_B)

  • w: mass fraction; M_S = 104.15 g/mol (styrene), M_B = 54.09 g/mol (butadiene).

Fox equation: 1 / Tg = w₁ / Tg₁ + w₂ / Tg₂

  • Tg in kelvin; w: mass fractions of each monomer in the copolymer. An estimate for random copolymers.

Monomer fed per pass = monomer polymerised / X, recycle = fed − polymerised (when unreacted monomer is fully recovered)

  • X: per-pass conversion.

Worked examples

Example 1 (standard): monomer recycle in E-SBR. A plant makes 1 t of SBR (23.5 wt % styrene); polymerisation is stopped at 60 % conversion of both monomers, and unreacted monomers are fully recovered. Find the butadiene and styrene charged and the amounts recycled.

  1. Bound butadiene = 765 kg; bound styrene = 235 kg.
  2. Butadiene charged = 765 / 0.60 = 1275 kg; recycled = 1275 − 765 = 510 kg.
  3. Styrene charged = 235 / 0.60 = 391.7 kg; recycled = 391.7 − 235 = 156.7 kg.

Answer: charge 1275 kg butadiene and 392 kg styrene; recycle 510 kg butadiene and 157 kg styrene.

Example 2 (GATE level): composition and Tg estimate. For SBR with 23.5 wt % styrene, find the mole fraction of styrene and estimate Tg with the Fox equation, taking Tg(polystyrene) = 100 °C and Tg(the polybutadiene segment) = −85 °C.

  1. Moles per 100 g: styrene 23.5 / 104.15 = 0.2256; butadiene 76.5 / 54.09 = 1.4143.
  2. x_S = 0.2256 / (0.2256 + 1.4143) = 0.1376.
  3. Tg₁ = 373.15 K (≈ 373 K), Tg₂ = 188 K.
  4. 1 / Tg = 0.235 / 373 + 0.765 / 188 = 0.000 630 + 0.004 069 = 0.004 699 K⁻¹.
  5. Tg = 212.8 K = −60.3 °C.

Answer: x_S ≈ 0.138; Tg ≈ −60 °C (measured values for E-SBR are around −50 to −55 °C, so Fox is a reasonable first estimate).

Common mistakes

  • Saying vulcanisation adds sulphur "to harden" rubber; it creates crosslinks that give elasticity and strength.
  • Confusing emulsion polymerisation (a process technique) with addition polymerisation (the mechanism); E-SBR is both.
  • Running E-SBR to high conversion; gel and branching form, so a shortstop is used.
  • Using Celsius in the Fox equation.
  • Thinking EPDM's diene is in the backbone; it is in a side group, leaving the backbone saturated.

For GATE CH

Expect matching of rubber ↔ monomers ↔ polymerisation type ↔ key property (SBR tyres, NBR oil seals, IIR inner tubes, EPDM weather seals), the cold vs hot SBR process and shortstop, cationic butyl polymerisation at low temperature, and the purpose of fillers, accelerators and activators. Numericals: monomer balances with conversion and recycle, composition conversions, Fox Tg estimates.

Quick check

  1. Why is cold SBR stopped at about 60 % conversion?
  2. What gives NBR its oil resistance?
  3. Why is a little isoprene added in butyl rubber?
  4. What are ZnO and stearic acid for in a cure recipe?

Answers: 1. to avoid branching and gel formation that harm processing and properties; 2. polar nitrile (–C≡N) groups from acrylonitrile; 3. to provide double bonds for sulphur vulcanisation; 4. they are activators for the accelerated sulphur cure.

Interview questions

All Chemical Technology interview questions

Try answering each one aloud before you open it.

  1. 1.What is synthetic rubber, and how does it differ from natural rubber?Concept

    Synthetic rubber is a man-made elastomer produced through the polymerization of monomers such as butadiene and styrene. Unlike natural rubber, which is derived from the latex of rubber trees, synthetic rubber can be tailored to have specific properties by altering its chemical structure. This allows for greater versatility in applications, such as improved resistance to heat, oil, and abrasion.

  2. 2.Explain the process of vulcanization and its significance in the production of elastomers.Concept

    Vulcanization is a chemical process that involves adding sulfur or other curatives to rubber to form cross-links between polymer chains. This process enhances the elasticity, strength, and durability of the rubber, making it more suitable for industrial applications. Vulcanized rubber is less sticky and more resistant to temperature changes and chemical exposure.

  3. 3.What are the primary types of synthetic rubber, and what are their typical applications?Concept

    The primary types of synthetic rubber include styrene-butadiene rubber (SBR), nitrile rubber (NBR), and ethylene propylene diene monomer (EPDM). SBR is commonly used in tires and conveyor belts due to its abrasion resistance. NBR is used in fuel hoses and gaskets because of its oil resistance. EPDM is used in weather seals and roofing membranes due to its excellent weather and ozone resistance.

  4. 4.Why is styrene-butadiene rubber (SBR) commonly used in tire manufacturing?Application

    SBR is commonly used in tire manufacturing because it offers a good balance of properties such as abrasion resistance, aging stability, and cost-effectiveness. It provides better traction and wear resistance compared to natural rubber, making it ideal for use in the tread of tires. Additionally, SBR can be easily blended with other types of rubber to enhance specific properties.

  5. 5.What would happen if vulcanization is not performed on rubber?Application

    If vulcanization is not performed, the rubber would remain in its raw, sticky state, lacking the necessary mechanical properties for most applications. It would be less elastic, more prone to deformation, and would degrade quickly under heat and stress. This would limit its usefulness in products that require durability and resilience, such as tires and industrial seals.

  6. 6.How does the addition of fillers like carbon black affect the properties of synthetic rubber?Application

    The addition of fillers like carbon black enhances the mechanical properties of synthetic rubber by increasing its tensile strength, abrasion resistance, and durability. Carbon black also improves the rubber's resistance to UV radiation and heat, making it more suitable for outdoor applications. Additionally, it can improve the rubber's electrical conductivity, which is beneficial in certain applications.

  7. 7.Explain why nitrile rubber (NBR) is preferred for applications involving exposure to oils and fuels.Application

    NBR is a copolymer of butadiene and acrylonitrile, and the polar nitrile (C≡N) groups make the polymer chemically unlike non-polar hydrocarbons, so oils and fuels swell it very little ('like dissolves like'). Oil resistance rises with acrylonitrile content (about 18-50%), but so does Tg, so high-ACN grades lose low-temperature flexibility. That is why NBR is used for fuel hoses, O-rings and oil seals, and hydrogenated NBR is chosen when heat resistance is also needed.

  8. 8.What are the environmental considerations in the production and disposal of synthetic rubber?Concept

    The production of synthetic rubber involves the use of petrochemicals, which can have environmental impacts such as greenhouse gas emissions and resource depletion. Disposal of synthetic rubber products can also pose challenges, as they are not biodegradable and can contribute to landfill waste. Recycling and developing more sustainable production methods are important considerations to mitigate these environmental impacts.

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