Materials choice for inner tubes of automobile tyres: Given the need for very low air permeability and good oxidation resistance, which rubber is most suitable for manufacturing inner tubes?

Introduction / Context:
Inner tubes must retain air over long periods and resist oxidative aging. The polymer's gas permeability and chemical stability are decisive. Among common elastomers, one material is preferred for its exceptionally low permeability to gases such as oxygen and nitrogen.

Given Data / Assumptions:

• Functional requirement: minimal gas diffusion through the tube wall.
• Environmental requirement: resistance to oxygen and ozone.
• Candidate rubbers: SBR variants, NBR, and IIR (butyl).

Concept / Approach:
Butyl rubber (isobutylene-isoprene copolymer) has a tightly packed, saturated backbone that yields very low gas permeability. It is therefore the standard for inner tubes and air bladders. SBR (including “cold” or emulsion versions) and NBR have higher permeabilities and are less suitable for long-term air retention.

Step-by-Step Solution:
Compare gas permeability among candidates.Identify IIR as the lowest-permeability elastomer commonly used.Select butyl rubber for inner tubes.

Verification / Alternative check:
Permeation data tables consistently show IIR permeation coefficients well below SBR and NBR. Butyl inner tubes are standard in automotive and bicycle applications.

Why Other Options Are Wrong:

• SBR (cold or emulsion): higher gas permeability; used in tread/sidewall blends, not inner tubes.
• NBR: oil-resistant but not optimized for air impermeability.

Common Pitfalls:

• Choosing rubbers for unrelated strengths (e.g., oil resistance) instead of permeability.
• Ignoring the effect of unsaturation on oxidative stability.

Final Answer:
butyl rubber (IIR)