White PET masterbatch yellowing at 180°C is a significant issue often observed during processing of plastics and films. Here’s a detailed explanation of the causes and prevention strategies:
Causes of Yellowing
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Thermal Oxidation: At elevated temperatures like 180°C, PET and its additives can undergo thermal oxidation, leading to the formation of yellow chromophores. This is especially likely when oxygen is present during processing.
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Impurities and Additives: Impurities in raw materials or inadequate dispersion of titanium dioxide (TiO₂) pigment can accelerate yellowing. Certain additives, when over-oxidized (especially phenolic antioxidants), can turn yellow under heat.
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Choice and Dispersion of TiO₂: Suboptimal TiO₂ grades (large particle size or insufficient surface treatment) can cause yellowing or a yellow undertone in the finished product.
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Poor Quality Control: PET masterbatch made from lower grade or contaminated resin is more susceptible to discoloration when processed at high temperatures.
Prevention Strategies
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Use High-Purity TiO₂: Select a white masterbatch with high-purity, fine TiO₂ designed for PET applications. Proper surface treatment of TiO₂ improves stability and helps avoid yellowing.
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Incorporate UV Stabilizers & Antioxidants: Adding proper UV stabilizers and thermal antioxidants in the masterbatch can minimize both UV- and heat-induced yellowing.
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Optimize Processing Parameters: Avoid excessive heat and long residence time during processing; ensure temperature control to reduce thermal degradation.
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Moisture & Storage Control: Store masterbatch in a cool, dry place, and avoid exposure to sunlight and moisture. Moisture and UV exposure accelerate yellowing both during storage and use.
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Minimize Impurities: Ensure material cleanliness, especially with recycled PET, as impurities and contaminants (like acetaldehyde) can catalyze yellowing reactions.
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Use Additives Designed for Yellowing Prevention: Specialized masterbatch additives (such as PET enhancers from Clariant or ThermProtect from Ampacet) are available to reduce yellowing in PET, especially if using recycled content.
Key Takeaways
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The primary cause of yellowing in white PET masterbatch at 180°C is heat-induced oxidation, aggravated by material or additive quality and process conditions.
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Using high-quality TiO₂, applying UV stabilizers and thermal antioxidants, and controlling processing temperatures will help prevent discoloration.
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For products requiring the highest whiteness, choose masterbatch specifically formulated for PET at high processing temperatures.
By following these guidelines, you can minimize yellowing and maintain the desired appearance and performance of PET products.