Rubber compounding — Zinc oxide and carbon black are added to crude natural rubber before vulcanization primarily to improve which property?

Introduction / Context:
Fillers and additives are essential in rubber compounding. Carbon black is the most common reinforcing filler, while zinc oxide participates in the vulcanization system and contributes to mechanical performance. Recognizing their primary role helps optimize formulations.

Given Data / Assumptions:

• Matrix: natural rubber before sulfur vulcanization.
• Additives: zinc oxide and carbon black.
• We are asked for the principal performance benefit.

Concept / Approach:
Carbon black provides reinforcement: increased tensile strength, tear resistance, abrasion resistance, and improved fatigue life. Zinc oxide acts as an activator in sulfur vulcanization, improving crosslink efficiency and contributing to final strength. Together, they significantly enhance the mechanical properties relative to unfilled gum rubber.

Step-by-Step Solution:
Assess carbon black: classic reinforcing filler → stronger, more durable rubber.Assess ZnO: activator improving crosslinking → better modulus and tensile properties.Thus, primary effect: improved strength and reinforcement.

Verification / Alternative check:
Comparative tensile and abrasion data show large performance gains with carbon black/ZnO compared to unfilled compounds.

Why Other Options Are Wrong:

• Elasticity only: reinforcement affects multiple properties, not just elastic recovery.
• Plasticity: mastication and oils/plasticizers handle processability, not primarily CB/ZnO.
• Weathering only: carbon black can aid UV resistance, but the core reason is reinforcement.
• Color/gloss: carbon black blackens; aesthetics are secondary to reinforcement.

Common Pitfalls:
Assuming ZnO is only a pigment; ignoring its activator role in vulcanization.

Final Answer:
Strength and reinforcement (tensile and abrasion resistance)