Why Too Much Ca-Zn Stabilizer Can Cause PVC “Burning” Instead of Protection

In PVC processing, it is often assumed that more stabilizer means better stability. However, overdosing Ca-Zn stabilizer can actually trigger thermal degradation and blackening, also known as “zinc burn.”

There are four key mechanisms behind this phenomenon:

Self-Catalyzed Decomposition

Metal soaps in Ca-Zn stabilizers, such as zinc stearate, can decompose at high temperatures. When added excessively, they generate acidic by-products and free acids, which accelerate PVC degradation. In particular, excessive zinc compounds form zinc chloride (ZnCl₂), a strong degradation catalyst responsible for blackening.

Uncontrolled Lubrication Chain Reaction

Ca-Zn stabilizers also serve as lubricants. Overuse reduces shear efficiency during extrusion, forcing operators to raise temperature or screw speed to compensate. This leads to uneven plasticization and unstable “raw spots,” as well as sudden melt temperature spikes due to excessive shear heat.

Formulation Balance Disruption

Too much stabilizer can compete with other lubricants for adsorption, disturb the lubrication balance, and react with pigments or fillers to form dark-colored compounds. It may also cause additive migration and carbon build-up at die lips, resulting in surface scorching.

Threshold Reversal Effect

The stabilizing efficiency of Ca-Zn systems reaches a saturation point at around 3–5 phr. Beyond this point, excess stabilizer molecules may promote crosslinking and gel formation, creating hot spots that initiate charring.

Key Takeaway

PVC processing is a precise balance of chemistry and engineering. 

The stabilizer dosage should be adjusted dynamically according to resin type, processing equipment, and production parameters. More stabilizer does not always mean better results — excessive use can lead to both chemical degradation and mechanical instability.

It is recommended to determine the optimal dosage through torque rheometer testing. 

In most cases, maintaining 2.5–5 phr of composite Ca-Zn stabilizer achieves the best balance between performance and cost.