Vulcanisation effects: which property of rubber is NOT increased by vulcanisation with sulfur or equivalent cross-linking systems?

Introduction / Context:
Vulcanisation creates cross-links between rubber chains, transforming a soft, tacky gum into an elastic, durable material. Understanding what improves and what diminishes with curing is essential for compounding and selecting service conditions for rubber goods like tires, seals, and belts.

Given Data / Assumptions:

• Vulcanisation introduces chemical cross-links (e.g., sulfur bridges).
• Network formation restricts flow, reduces tack, and increases modulus.
• Additives (fillers, antioxidants) may accompany the cure system.

Concept / Approach:
Cross-linking generally increases modulus, tensile strength (up to an optimum), resilience, heat and oxidation resistance (with suitable systems), and solvent resistance (insolubility/swelling control). However, hardness increases while “softness” decreases. The cured network is less plastic and less tacky than the raw gum. Mass may increase slightly due to added sulfur/fillers, while elasticity and environmental resistance improve relative to the uncured state.

Step-by-Step Solution:

Verification / Alternative check:
Shore hardness curves vs. cure time show increasing hardness with cure. Swelling tests show decreased solvent uptake after cross-linking, confirming increased network integrity.

Why Other Options Are Wrong:

• Oxidation/heat resistance: improved with proper cure/antioxidants.
• Weight & strength: mass may rise due to additives; tensile often increases to an optimum.
• Elasticity/insolubility: network enhances elastic recovery and reduces solubility.

Common Pitfalls:
Over-curing can embrittle and reduce tensile; the question asks the general effect, where softness does not increase.

Final Answer:
Softness