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在加工或存储过程中，PVC管道中无法解释的发黄、变红和脆化的原因是什么？
Created on 09.22
During the subsequent processing or storage of PVC products, unaccountable yellowing and embrittlement often occur. Many practitioners encounter such a confusion: why do calcium-zinc stabilizers always "fail" in the middle and later stages of use? In fact, the core cause of this phenomenon is often the "zinc burning" problem that has plagued the industry for years.
1. Causes of the "Zinc Burning" Phenomenon
When PVC is heated during processing, zinc soap, as the core component of calcium-zinc stabilizers, can quickly capture the hydrogen chloride (HCl) released from the PVC molecular chain, providing excellent initial stability for the products.
However, the key issue lies in the by-product generated from the reaction—zinc chloride (ZnCl₂). ZnCl₂ is a typical Lewis acid with extremely strong catalytic activity. It not only significantly accelerates the breakage of the PVC molecular chain but also promotes the further release of HCl, forming a vicious cycle of "degradation - catalysis - more severe degradation".
As the processing progresses, ZnCl₂ accumulates continuously, and its catalytic effect increases exponentially. Eventually, the PVC material undergoes sudden blackening and degradation in the middle and later stages of processing or storage, manifested as yellowing and reddening in appearance, and a decline in mechanical properties (such as embrittlement).
In short, the essence of "zinc burning" is that after zinc soap completes its initial stabilizing effect, the by-product ZnCl₂ generated thereby becomes a "catalyst" that accelerates the degradation of PVC.
2. Solutions to Inhibit the Catalytic Effect of ZnCl₂
The core of solving the "zinc burning" problem is not to prevent the normal reaction between zinc soap and HCl (which is crucial for ensuring initial stability), but to timely control the impact of ZnCl₂ and block its catalytic degradation effect.
Common solutions in the industry involve combining with auxiliary stabilizers such as calcium soap, achieving control through two pathways:
Neutralizing part of HCl: reducing the reaction amount between zinc soap and HCl, thereby slowing down the formation rate of ZnCl₂;
Pre-treating ZnCl₂: efficiently neutralizing or inerting ZnCl₂ before it initiates its catalytic effect, completely eliminating its threat to PVC degradation.
Therefore, the performance of calcium-zinc stabilizers essentially depends on the control effect on the catalytic action of ZnCl₂. If you have different opinions or optimization suggestions, please feel free to leave a message for communication. Thank you for reading!
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