In the industrial process of vulcanisation, which element is mainly used to cross link and strengthen natural rubber?

Introduction / Context:
Natural rubber by itself is soft, sticky and loses its useful properties over a wide temperature range. Charles Goodyear discovered that heating rubber with certain chemicals can greatly improve its strength, elasticity and resistance to heat and wear. This process is called vulcanisation. A common general knowledge and chemistry question asks which element is primarily responsible for this transformation.

Given Data / Assumptions:

• The process mentioned is vulcanisation of rubber.
• The options include nickel, copper, sulphur, graphite and chlorine.
• We assume ordinary natural rubber, which is mainly polyisoprene.
• The question focuses on the main element used, not minor additives.

Concept / Approach:
Vulcanisation involves forming cross links between individual polyisoprene chains in natural rubber. These cross links limit the movement of the chains relative to each other and greatly improve mechanical properties. Sulphur atoms form bridges, often called sulphur cross links, between the double bonds or adjacent positions on the polymer chains. Metals like nickel and copper, carbon in the form of graphite and halogens like chlorine do not play this primary cross linking role in classical vulcanisation of rubber.

Step-by-Step Solution:
Step 1: Recall that natural rubber is mainly cis 1,4 polyisoprene, which has carbon carbon double bonds in the polymer chain.Step 2: In vulcanisation, rubber is heated with sulphur or sulphur containing compounds.Step 3: Sulphur atoms form cross links between polymer chains, turning a soft and sticky material into a tougher, more elastic network.Step 4: This cross linked rubber has improved resistance to heat, abrasion and deformation, which is why it is used in tyres.Step 5: Compare this role with that of nickel, copper, graphite and chlorine, none of which are standard cross linking agents in the basic vulcanisation process.Step 6: Conclude that sulphur is the main element used in vulcanisation of rubber.

Verification / Alternative check:
Chemistry textbooks often define vulcanisation as heating natural rubber with about 3 to 5 percent sulphur. Advanced formulations may include accelerators and activators, but sulphur is always highlighted as the key element that forms cross links. Trade descriptions of tyres and rubber goods sometimes mention sulphur vulcanisation specifically, reinforcing this association. No similar usage is described for nickel, copper, graphite or chlorine in this context.

Why Other Options Are Wrong:
Option a, nickel, is a metal used in alloys and catalysts but not as the main vulcanising agent for rubber. Option b, copper, has many electrical and structural uses but is not the key element in vulcanisation. Option d, graphite, is a form of carbon used as a lubricant and in electrodes, not as a cross linking agent for rubber. Option e, chlorine, is used in making some synthetic rubbers and plastics but is not the principal element in classical vulcanisation of natural rubber.

Common Pitfalls:
Sometimes learners over generalise and think any additive in rubber processing could be the main vulcanising agent. Others may confuse the role of carbon black, which is added as a reinforcing filler, with the cross linking agent. To avoid this, remember that carbon black improves strength and wear resistance through reinforcement, while sulphur changes the chemical structure by forming cross links between chains.

Final Answer:
In the vulcanisation of rubber, the main element used to cross link and strengthen the polymer chains is sulphur.