Why Does Calcium Zinc Stabilizer Turn Yellow Immediately? And Sudden ‘Zinc Burn’ Black Screen? What Causes It?

Calcium-zinc stabilizer is not a single substance, but a group of substances.Calcium stearate (CaSt₂), zinc stearate (ZnSt₂)A complex synergistic system primarily based on stabilizers, supplemented by a large amount of auxiliary stabilizers.

Their core working mechanism can be compared to an exciting "team battle."

🛡️ First Line of Defense: Zinc Salts — "Efficient Vanguard" (Rapid Response but Easily Sacrificed)

Zinc (Zn)It is the "vanguard" of the reaction, characterized by an extremely fast reaction rate.

MissionRapidly capture the HCl generated from PVC decomposition and preferentially replace the unstable allylic chlorine atoms, thereby preventing chain reactions at the source.

- Reaction equation：  

  ZnSt₂ + 2HCl → ZnCl₂ + 2HSt 

(Zinc stearate + hydrogen chloride → zinc chloride + stearic acid)

- CrisisBy-product generatedZinc chloride (ZnCl₂) Strong Lewis acidIt is the degradation of PVC.Powerful catalystOnce the zinc salts are depleted, a large accumulation of ZnCl₂ can trigger intense "zinc burning," causing the material to instantly turn black and disintegrate.

🛡️ The second line of defense: calcium salts — the "solid defender" (provides lasting neutralization and buffering)

Calcium (Ca)It is the "defender" of the reaction, and its characteristic isThe reaction rate is slow, but it is lasting and stable.

- Mission OneContinuously absorbs HCl to support the zinc salt.

CaSt₂ + 2HCl → CaCl₂ + 2HSt

Mission Two (Most Critical)：Capturing lethal ZnCl₂Calcium salts can undergo a double decomposition reaction with ZnCl₂, converting the highly catalytically active ZnCl₂ into harmless CaCl₂ and regenerating ZnSt₂.Effectively delay the occurrence of "zinc burning"。  

CaSt₂ + ZnCl₂ → CaCl₂ + ZnSt₂

This reaction is the cornerstone for the existence of the calcium-zinc system!

⚖️ The Art of Balance: Calcium-Zinc Ratio

- High zinc ratioEarly staining is good (whiter), butThe "zinc burning" time has been brought forward.The duration of thermal stability is short.

- High calcium contentThe safety is high, and the "zinc burning" time is delayed, butThe initial coloration is more yellow.

- KeyLooking for the best oneCa/Zn ratio…to achieve a balance between initial hue and long-term stability. Generally, the zinc content in rigid products must be controlled with particular caution.

Relying solely on calcium-zinc soap is simply not enough to cope with complex processing environments.A highly efficient calcium-zinc system relies 70% on the synergistic formulation!

The following additives are its "game-changing allies":

1. β-diketones - "Special Demining Experts"

- FunctionCan take the lead in efficiently replacing the most unstable chlorine atoms in PVC molecules, fundamentally preventing degradation from occurring.

- Effect：Greatly improve the initial coloring ability.Allowing the product to be pure white and translucent from the very beginning. This is an essential core additive in high-end calcium-zinc formulations.

2. Epoxy types (such as ESBO epoxy soybean oil) — "the all-rounder"

- Function OneAbsorb HCl.

Function TwoReact with unstable chlorine atoms to form a stable chlorohydrin structure.

- Function ThreeIn synergy with zinc soap, it can inhibit the catalytic activity of ZnCl₂.

- Additional BenefitsIt is also an excellent plasticizer and lubricant.

3. Phosphite Esters - "Metal Chelating Agents & Antioxidants"

Function OneChelation (encapsulation) of harmful metal ions (such as ZnCl₂) weakens their catalytic activity.

- Function TwoDecompose hydrogen peroxide to block the oxidative degradation pathway.

- Function ThreeImprove the transparency and color of the product.

4. Polyols - "Zinc Burn Inhibitor"

- FunctionThrough coordination interaction, the catalytically active ZnCl₂ is directly "passivated," providing a safety barrier for the system.

5. Antioxidants — "Long-lasting Guardians"

FunctionTo prevent the oxidative degradation of PVC under the combined effects of heat and oxygen, and to enhance long-term thermal stability.

6. Hydrotalcite-type Compounds — High-Temperature-Resistant “All-Purpose Fire Extinguisher” Bodyguard

Function 1It can both absorb HCl and neutralize acidic substances.

Function 2"Capture" foreign heavy metal ions (anti-pollution), with super strong stability at high temperatures.

- Function ThreeAt high temperatures, it can still "cover" the flames caused by PVC decomposition (inherently flame-retardant), making it suitable for use in cable materials and other products that require high heat resistance.

Designing a calcium-zinc formula is like playing a game of chess; you need to consider the whole situation.

🧪 1. Determine the core calcium to zinc ratio

- General typeThe Ca/Zn ratio is usually between 2:1 and 4:1.

- High-demand typeMore complex multi-metal systems (such as the addition of small amounts of aluminum, magnesium, etc.) may be needed for further optimization.

🧪 2. Construct a compound additive system.

- Improve initial hue: Requiredβ-Diketone。

- Improve long-term stabilityMandatoryPhosphite esters and polyols。

- Soft goods：Epoxidized Soybean Oil (ESBO)It combines plasticizing and stabilizing properties, making it the preferred choice.

- Hard productsNeeds to be given priority considerationThe balance of the lubrication systemTo prevent the impact of the lubricity of calcium-zinc soap on processing.

🧪3. Coordinate with the entire recipe system

- LubricantCalcium-zinc soap itself has a lubricating effect and needs to be carefully balanced with external lubricants (such as wax) and internal lubricants (such as GMS) to prevent blooming.

-  Calcium carbonate (CaCO₃) itself has a slight neutralizing effect on HCl and can serve as an inexpensive "auxiliary stabilizer," but high loading will affect the efficiency of the heat stabilizer.

- Impact modifierIt usually does not affect thermal stability, but the cost and performance need to be evaluated as a whole.

It is no longer a "green alternative" used out of necessity, but rather a product carefully designed and formulated.Collaborative system。  

Its development process is a microcosm of the PVC industry's transformation towards being greener, more advanced, and higher value-added.

When encountering initial yellowing during production, sudden zinc burning, or insufficient weather resistance, don't simply blame "calcium-zinc not working," but delve into the details of your formulation instead.

Is my calcium-to-zinc ratio reasonable?

Have I added enough of my β-diketone and phosphite?

Is my lubrication system compatible?

Understood this "golden partnership"Internal Logic and Collaborative ArtOnly then can you truly take control of the future of eco-friendly PVC formulations.