Rubber technology: which substance is most commonly used as a vulcanisation agent for unsaturated rubbers?

Introduction / Context:
Vulcanisation is the process of crosslinking rubber chains to improve elasticity, heat resistance, and mechanical strength. The chemical used for crosslinking—particularly in natural rubber and SBR—is central to performance and processing.

Given Data / Assumptions:

• We consider traditional, widely-used systems for unsaturated rubbers like NR and SBR.
• Catalysts/activators/accelerators may be present, but the question asks for the primary vulcanising agent.

Concept / Approach:
Sulphur is the most common vulcanising agent for unsaturated rubbers, forming sulphidic crosslinks between polymer chains. Accelerators (e.g., sulfenamides, thiazoles) and activators (ZnO, stearic acid) control cure rate and crosslink structure. Bromine and platinum are not standard vulcanising agents for these rubbers; alumina is an inorganic filler, not a curative.

Step-by-Step Solution:
Identify material class → unsaturated rubber (NR/SBR).Recall standard cure → sulphur crosslinking with accelerator/activator system.Choose sulphur as the principal vulcanising agent.

Verification / Alternative check:
Tyre compounds and industrial rubber recipes universally cite sulphur cures or sulphur donors, with well-known cure kinetics and crosslink types (mono-, di-, polysulphidic).

Why Other Options Are Wrong:
Bromine: not a typical vulcanising agent for NR/SBR.Platinum: used as a catalyst in silicone (addition) cures, not for NR/SBR sulphur vulcanisation.Alumina: filler/reinforcement, not a cure agent.

Common Pitfalls:
Confusing silicone rubber cure (platinum-catalysed hydrosilylation) with sulphur vulcanisation of diene rubbers.

Final Answer:
Sulphur