Natural vs Synthetic Rubber: Molecular Structure, Properties & Selection Criteria

Rubber is the second-largest polymer sector in India after plastics. India is the fourth-largest natural rubber (NR) producer globally (Kerala, Tamil Nadu plantations), and the Indian synthetic rubber market (SBR, EPDM, NBR, IIR) is growing at 6–8% CAGR driven by automotive tyres, seals, hoses, and industrial rollers. Apollo Tyres (Gurgaon), MRF Ltd (Chennai), Freudenberg (Pune), and Trelleborg (Vadodara) are major Indian rubber product manufacturers. Understanding the structural differences and performance trade-offs between NR and synthetic rubbers is foundational for rubber compounding and product selection.

• Compare the molecular structure of natural rubber (cis-1,4-polyisoprene) vs. synthetic rubbers.
• Explain strain-induced crystallisation (SIC) in NR and its effect on tensile strength.
• Distinguish SBR, NBR, EPDM, IIR, and CR by monomer composition and key properties.
• Select rubber type based on service environment (oil, heat, ozone, weather).
• Identify ISO 1629 (rubber nomenclature) and ASTM D2000 (rubber classification) standards.

3.1 Natural Rubber (NR) — cis-1,4-Polyisoprene

Natural rubber is harvested as latex from Hevea brasiliensis trees. Its molecular structure is predominantly (>98%) cis-1,4-polyisoprene — the cis double bond creates a kinked chain geometry preventing packing, giving rubber its characteristic low $T_g$ (−73°C) and exceptional elasticity.

Key structural feature: Gutta-percha is the trans-1,4-polyisoprene isomer — rigid, crystalline, used in golf balls and submarine cable insulation. Same monomer unit, different stereoregularity, completely different physical properties.

Strain-Induced Crystallisation (SIC): Under tensile strain, NR chains align and crystallise. This SIC acts as a self-reinforcing mechanism, giving unfilled NR tensile strengths of 25–35 MPa — much higher than SBR without carbon black (SBR unfilled ≈ 2–3 MPa).

3.2 Major Synthetic Rubbers

3.3 Oil Resistance and the Nitrile Effect

In NBR, the acrylonitrile (ACN) content controls oil resistance vs. low-temperature flexibility:

Selection rule: For fuel tank seals (petrol/diesel contact, Arctic conditions) → use medium ACN (28–33%) NBR. For jet fuel hydraulic seals → use high ACN (40–45%) NBR or FKM (fluoroelastomer).

3.4 ASTM D2000 / SAE J200 Classification

Rubber products for automotive use are classified by:

• Type: Maximum service temperature (A=70°C, B=100°C, C=125°C, D=150°C, E=175°C, F=200°C, G=225°C)
• Class: Oil resistance in ASTM Oil #3 (volume swell after 70h): A-K scale

Example: EPDM O-ring for coolant hose is classified D4 (service to 150°C, low oil swell). NBR fuel seal is BF (service to 100°C, very low oil swell in fuel).

Problem: An automotive transmission oil seal must operate continuously at 130°C and contact gear oil (ASTM Oil #3 equivalent). Volume swell must be <40%. Select appropriate rubber and justify.

Solution:

Recommendation: FKM (Viton) is the primary choice for 130°C transmission oil seals — outstanding oil resistance plus temperature headroom. ACM is a cost-effective alternative if FKM is cost-prohibitive.

MRF Ltd (Chennai) — India's largest tyre manufacturer — consumes ~120,000 MT/year of NR from Kerala plantations and SBR, BR from IPCL Vadodara (now Reliance Industries). Their tyre tread compound is 60% NR + 40% SBR for optimal wear/grip balance.

Freudenberg Sealing Technologies (Pune) manufactures EPDM coolant hoses, NBR transmission seals, and FKM fuel system seals for Maruti Suzuki, Tata Motors, and Mahindra. Their NBR grades use 34–38% ACN for the oil-temperature balance required by Indian engine specifications.

• cis-1,4-polyisoprene: Molecular structure of NR; cis double bond gives low $T_g$ and elastic character.
• SIC (Strain-Induced Crystallisation): NR self-reinforcement mechanism under strain — gives unfilled tensile strength of 25–35 MPa.
• SBR: Lowest-cost synthetic rubber; best abrasion resistance; poor oil resistance.
• NBR: Oil-resistant rubber; ACN content controls oil resistance vs. low-temp flexibility trade-off.
• EPDM: Best ozone/weather/heat resistance; cannot be used in oil or fuel service.
• IIR (Butyl): Best gas impermeability — used exclusively for tyre inner liners and flaps.

1. ISO 1629:2013 — Rubber and latices — Nomenclature
2. ASTM D2000 / SAE J200 — Classification system for rubber materials for automotive applications
3. ISO 1817 — Rubber vulcanizates — Determination of the effect of liquids
4. ASTM D412 — Tensile properties of vulcanised rubber
5. IS 7888:1975 (BIS) — Specification for natural rubber grades (RSS, TSR, Crepe)

1. Explain why unfilled NR has tensile strength of 25–35 MPa while unfilled SBR has only 2–3 MPa.
2. A pump seal contacts diesel fuel at 80°C. ACN content of NBR is 33%. Is this suitable? What would you change?
3. Explain why EPDM cannot be used for oil seals despite its excellent temperature and weather resistance.

Q1. Natural rubber is predominantly:

• A) trans-1,4-polyisoprene B) cis-1,4-polyisoprene C) 1,2-polybutadiene D) Polyisobutylene

Q2. Strain-induced crystallisation (SIC) in NR explains:

• A) Its poor ozone resistance B) Its high gas impermeability
• C) Its very high tensile strength without carbon black reinforcement D) Its low $T_g$

Q3. Higher ACN content in NBR results in:

• A) Better low-temperature flexibility B) Better oil/fuel resistance but worse low-temperature flexibility C) Better ozone resistance D) Higher tensile strength

Q4. Which rubber has the best gas impermeability and is used for tyre inner liners?

• A) NR B) SBR C) IIR (Butyl Rubber) D) EPDM

Q5. India's largest tyre manufacturer is:

• A) MRF Ltd B) Apollo Tyres C) CEAT D) JK Tyres