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Why Does Your PVC Product Suffer Severe Initial Discoloration? Look for the Cause in Calcium-Zinc Stabilizers

Created on 12.10
This is a classic and critical issue in PVC processing. The initial discoloration of calcium-zinc composite stabilizers stems from their unique stability mechanism—"preventive" and "absorptive" rather than the "rapid displacement-type" of traditional lead salts or organotins.
I. Root Causes of Initial Discoloration
Initial discoloration refers to PVC materials turning slightly yellow, pink, or even obviously red during the early processing stages (e.g., melting, plasticization). Its fundamental causes lie in the structural defects and instability of PVC resin itself.
1. Structural Defects of PVC Molecular Chains:
  • Allylic chlorine structure: The most unstable structure. Under heat and shear force, C-Cl bonds easily break to form chlorine free radicals. Once HCl is eliminated, the generated allylic structure further activates chlorine atoms on adjacent carbon atoms, triggering a "zipper-like" degradation chain reaction and forming conjugated polyene sequences.
  • Tertiary carbon chlorine atoms: Formed by chain transfer or branching points, their C-Cl bonds are weaker than secondary carbon chlorine atoms and prone to breaking, initiating degradation.
  • Oxygen residues: Oxygen remaining during polymerization or processing reacts with free radicals to form peroxy radicals, greatly accelerating oxidative degradation.
2. Stability Mechanism and Limitations of Calcium-Zinc Stabilizers:
  • Capture HCl (core function): Effectively absorbs HCl produced by PVC degradation, preventing its autocatalytic effect—their primary function.
  • Replace unstable chlorine atoms (critical but limited capacity):
Zinc soap (ZnSt₂): Rapidly replaces unstable chlorine atoms on PVC molecular chains to form relatively stable ZnCl₂. This is the core contradiction!
Calcium soap (CaSt₂): Cannot directly replace unstable chlorine atoms but reacts with ZnCl₂ to regenerate ZnSt₂ and CaCl₂—known as the "zinc burning" recovery reaction.
3. "Zinc Burning" Phenomenon and Initial Discoloration:
  • "Zinc burning": Generated ZnCl₂ is a Lewis acid and PVC degradation catalyst, sharply accelerating subsequent HCl elimination reactions.
  • Direct cause of initial discoloration: Conjugated polyene sequences catalyzed by "zinc burning" are the source of discoloration. Materials turn pale yellow when the number of conjugated double bonds (n) reaches 5-7, red at n=8-12, and darken to black with more n.
Summary of Root Causes: Initial discoloration of calcium-zinc stabilizer systems essentially arises because zinc soap, while exerting rapid stabilizing effects, produces the strongly catalytic byproduct ZnCl₂, triggering "zinc burning" and leading to premature and excessive formation of conjugated polyene structures.
II. Improvement Solutions
Addressing initial discoloration requires a systematic approach involving inhibiting "zinc burning", auxiliary stabilization, and processing optimization.
1. Optimize the Calcium-Zinc Stabilizer System Itself
  • Adjust calcium/zinc ratio
  • Use high-performance composite soaps:
Zinc soaps with greater steric hindrance (e.g., modified zinc soaps)
Synergistic metal soaps
2. Scientifically Use Auxiliary Stabilizers (Key to Improving Initial Discoloration!)
Auxiliary stabilizers are not primary stabilizers but exhibit strong synergy with calcium-zinc systems.
3. Optimize PVC Formulation
  • Select high-quality PVC resin: Resins with uniform molecular weight distribution, low impurities, few fish eyes, and low VCM residues have fewer inherent unstable structures.
  • Rational use of plasticizers: Reduce melt viscosity and processing temperature, minimizing shear heat to improve discoloration.
  • Impact of fillers: Alkaline fillers like calcium carbonate absorb small amounts of HCl, enhancing stability.
  • Balance of lubrication system: Proper internal/external lubrication balance is crucial for inhibiting initial discoloration.
4. Optimize Processing Technology
  • Lower processing temperature: Use the lowest possible temperature while ensuring plasticization.
  • Shorten residence time: Optimize screw speed and feeding rate to avoid prolonged material retention.
  • Prevent local overheating: Regularly inspect equipment to ensure normal operation of heating coils and temperature control systems, avoiding "hot spots" caused by equipment issues.
Through multi-level and multi-angle synergy, the initial discoloration of calcium-zinc composite stabilizers can be significantly improved, meeting or even exceeding the requirements of high-end products.
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