Role of peptizers in rubber compounding and processing: Why are peptizers such as aromatic mercaptans (e.g., thiophenes) added to raw rubber during mastication on mills and internal mixers?

Introduction / Context:
Raw rubber has high molecular weight and entanglement density, resulting in very high viscosity. Before compounding and shaping, it is “masticated” to lower viscosity. Chemical peptizers accelerate chain scission under shear and heat, improving processing efficiency.

Given Data / Assumptions:

• Additives: aromatic mercaptans/thiophenic peptizers.
• Operation: milling or internal mixing at elevated temperature.
• Goal: improved processability (lower viscosity, better filler dispersion).

Concept / Approach:
Peptizers form reactive species that facilitate chain cleavage during mechanical working. This reduces molecular weight and Mooney viscosity, allowing faster mixing, improved filler wetting, and lower energy consumption. They do not primarily act as antioxidants or vulcanization accelerators (though overall mixing times can change when viscosity drops).

Step-by-Step Solution:
Identify processing challenge: raw rubber too viscous.Apply peptizer function: promote chain scission under shear/heat.Effect: reduced viscosity, easier processing and mixing.

Verification / Alternative check:
Mooney viscosity measurements before/after peptizer addition show measurable reductions, demonstrating improved processability.

Why Other Options Are Wrong:

• Protection against oxygen/ozone: that is the job of antioxidants/antiozonants.
• Reducing vulcanization time/agent: primarily done by accelerators/activators, not peptizers.
• Increasing viscosity: opposite of peptizer intent and effect.

Common Pitfalls:

• Confusing peptizers with accelerators or antioxidants.
• Using excessive peptizer, which can over-reduce molecular weight and harm properties.

Final Answer:
to reduce viscosity so that processing is easier