Molds & Tooling
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PVC Processing Yellowing (Early Thermal Degradation)
Problem Phenomenon
White or light-colored PVC products turn yellow or milky, colored products show hue shift and brightness reduction; it is an early signal of thermal degradation, and continued development will turn brown and black.
Cause Analysis
PVC undergoes dehydrochlorination (HCl) reaction when heated, forming conjugated polyene structures on the molecular chain that show color; excessive temperature, too long residence time, insufficient or consumed stabilizer can all trigger it, and the released HCl autocatalytically accelerates degradation, presenting a chain reaction.
Solution
①Verify and reduce the actual melt temperature (measured, not displayed); ②Shorten the melt residence time in the barrel to reduce heat accumulation degradation; ③Check whether the type and dosage of heat stabilizer match the processing window; ④Calibrate temperature control instruments and thermocouples; ⑤Once yellowing occurs, stop immediately for troubleshooting, do not produce with defects.
Key Takeaway
PVC yellowing is a 'degradation alarm' rather than ordinary color difference: conjugated polyene coloring indicates de-HCl has already occurred, and it will autocatalytically worsen; temperature must be lowered immediately, the stabilizer system checked, dead corners cleaned — delaying one minute wastes one more share of material.
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