Question

The main purpose of vulcanization of rubber is to

• A. decrease elasticity
• B. break crosslinks in molecular chains
• C. increase crystallinity
• D. introduce crosslinking to improve strength

Hint:

Remember:
- Unvulcanized rubber \(\rightarrow\) Thermoplastic behavior (linear chains, slides easily, sticky when warm).
- Vulcanized rubber \(\rightarrow\) Thermosetting elastomer (crosslinked 3D network, does not melt, highly resilient).

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
This question asks for the primary chemical and mechanical purpose of the vulcanization process in raw elastomer (rubber) processing.

Step 2: Key Formula or Approach:
Raw natural rubber consists of long, linear polymer chains of polyisoprene. At room temperature, these chains can slide past one another easily under stress, resulting in low mechanical strength, high tackiness (sticky), and poor thermal stability. Vulcanization is a chemical process that addresses these issues by forming crosslinks between these polymer chains.

Step 3: Detailed Explanation:

• Mechanism of Vulcanization:
- Developed by Charles Goodyear, vulcanization involves heating raw rubber with sulfur (typically \( 1-5\text{ wt}\% \)) and organic accelerators.
- The sulfur atoms react with the double bonds in the polyisoprene chains, forming covalent sulfur bridges (crosslinks) that link adjacent polymer chains together.
- This creates a three-dimensional network structure.

• Effects on Mechanical Properties:
-

Strength and Stiffness: The crosslinks prevent the chains from sliding past each other permanently, significantly increasing the tensile strength, stiffness, and wear resistance of the rubber.
-

Elasticity: When stretched, the chains uncoil, but the crosslinks pull them back to their original configuration once the load is released. This improves the rubber's elastic recovery and resilience.
- It also reduces tackiness and makes the rubber resistant to temperature variations and chemical swelling.


Step 4: Final Answer:
The primary purpose of vulcanization is to introduce covalent crosslinks between polymer chains to dramatically improve its mechanical strength.
Therefore, option (D) is the correct choice.