Poly(vinyl chloride) (PVC) processing: why are stabilisers added during manufacture and compounding?

Introduction / Context:
PVC is a versatile thermoplastic used in pipes, cables, profiles, and films. However, the polymer's chlorine-bearing backbone is susceptible to dehydrochlorination when exposed to heat and shear in processing. Stabilisers are therefore an essential additive family in PVC compounding.

Given Data / Assumptions:

• PVC is processed at elevated temperatures where HCl elimination can start.
• Thermal degradation leads to discoloration, embrittlement, and viscosity changes.
• Commercial stabilisers include Ca–Zn, organotin, and mixed metal systems.

Concept / Approach:
Dehydrochlorination forms conjugated polyenes along the backbone, causing rapid autocatalytic degradation (HCl catalysis). Stabilisers scavenge HCl, replace labile chlorine sites, and inhibit chain unzipping, thereby preserving molecular weight and mechanical properties.

Step-by-Step Solution:
Identify the main degradation pathway: heat-induced HCl loss from PVC.Recognise stabiliser action: HCl scavenging and substitution at labile sites.Conclude the primary purpose: prevent thermal degradation.

Verification / Alternative check:
Processing windows for PVC widen markedly when appropriate stabiliser packages are used; colour hold and torque stability tests demonstrate effectiveness.

Why Other Options Are Wrong:
Impact and elasticity depend on plasticisers and impact modifiers, not primarily stabilisers.Reducing melt viscosity or glass transition is the role of plasticisers and processing aids.Crystallinity changes are not the main stabiliser function.

Common Pitfalls:
Confusing stabilisers with plasticisers or lubricants; assuming a single additive can address all property targets.

Final Answer:
To prevent thermal degradation during processing and service