The More Stable, the More "Burnt"? Excessive Stabilizers Cause Problems Instead: 4 Causes + 1 Key Suggestion

创建于10.16

Those working in PVC processing may have encountered this situation: to prevent product degradation, they intentionally added more stabilizers, only to find the products became "burnt" and turned black. This is not because the stabilizers are ineffective, but because "excess" disrupts the processing balance. Today, we will explain the underlying reasons in plain language and provide a practical suggestion.

I. Why Does Excessive Stabilizer Lead to "Burning" Instead?

The role of stabilizers is to inhibit the high-temperature degradation of PVC. However, when the dosage exceeds the critical point, four mechanisms will trigger problems in reverse. Essentially, it is the dual effect of "chemical degradation" and "physical processing out of control".

1. Stabilizers Undermine Their Own Role: Generating "Degradation Catalysts" at High Temperatures

Calcium-zinc composite stabilizers, which are commonly used, contain a component called "metal soaps" (such as zinc stearate). At normal dosages, they protect PVC, but when added excessively:

During high-temperature processing, they decompose into acidic by-products, which in turn accelerate PVC degradation.

Especially when zinc soaps are excessive, zinc chloride (ZnCl₂) is generated — a "powerful catalyst" for PVC degradation that directly causes "zinc burning" and blackening of products.

2. Lubrication Failure: Triggering a Chain Reaction of "Temperature Surge"

Stabilizers are not just "stabilizers"; they also have inherent lubricating functions. Excessive lubrication due to over-adding stabilizers leads to:

Insufficient "shear force" of plasticizing equipment, making it impossible to knead PVC material evenly.

To ensure good plasticization of the material, operators are forced to increase temperature and speed. This either results in "undervulcanized material" (poor local plasticization, low thermal stability, and easy burning) or excessive heat generated by equipment friction, causing the melt temperature to suddenly soar and the material to burn black directly.

3. Formula Imbalance: Disrupting the Function of Other Additives

PVC formulas rely on "teamwork". Excessive stabilizers "compete for resources" and "disrupt functions":

They compete with lubricants to adsorb on PVC molecules, disrupting the normal lubrication system.

They react with fillers and pigments to form dark-colored compounds, making the products look "burnt".

Excess stabilizers also precipitate and coke at equipment parts like dies, then stick to the products.

4. Threshold Reversal: "Doing More Harm Than Good" Beyond the "Saturation Point"

The stabilizing effect of stabilizers has a "saturation point". Beyond this point:

Unused "free stabilizer molecules" promote the "cross-linking reaction" of PVC.

The "gel particles" formed after cross-linking become the starting point of "coking" in subsequent processing, eventually turning into black spots.

II. Core Conclusion: PVC Processing Is an "Art of Balance", Not "The More Stable, the Better"

In a nutshell: The dosage of stabilizers is not "the more the better", but must match three factors — PVC resin type (different resins have different degradation characteristics), equipment parameters (such as the shear force and temperature range of extruders), and process conditions (such as processing temperature and time). Blindly increasing the dosage will trigger both "chemical degradation" and "physical processing out of control", ultimately leading to "burning".

III. Practical Suggestion: How to Find the "Optimal Dosage" of Stabilizers?

There is no need to rely on trial and error based on experience; a professional tool can directly measure it — a torque rheometer. It simulates the temperature and shear environment in actual processing, and judges whether the stabilizer dosage is appropriate by observing torque changes.

According to industry experience: For composite stabilizers used in most PVC products, controlling the dosage between 2.5-4 phr not only ensures a stable effect but also avoids "burning", achieving the highest cost-effectiveness.

If your production line has encountered similar problems, you can first test whether the current stabilizer dosage exceeds the balance value. You are also welcome to leave a message in the comment section to share the "small PVC processing troubles" you have encountered, so we can exchange solutions together.

If you need dosage references for stabilizers in specific products (such as PVC pipes and profiles), our company can provide relevant analysis.

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