Extrusion & Post
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Initial yellowing problem of PVC extruded products
Problem Phenomenon
After extrusion processing of PVC rigid pipes with a calcium-zinc stabilizer system, a yellow tint appears on the product surface, the degree of discoloration increases over time, and the color deepens under ultraviolet light.
The initial product color is yellowish and cannot meet pure white or natural color requirements, affecting the product appearance quality grade.
Cause Analysis
The initial thermal stabilization efficiency of calcium-zinc stabilizers is relatively lower than that of lead salt systems; at extrusion temperatures of 160-185°C, they fail to completely capture the HCl released from PVC; the addition amounts of auxiliary stabilizers (epoxidized soybean oil, phosphite) in the formulation are insufficient; the processing temperature is set too high, leading to accelerated initial degradation rate.
Solution
Optimize the calcium-zinc stabilizer formulation by increasing the β-diketone initial coloring inhibitor dosage by 0.1-0.3 phr; incorporate 3-5 phr epoxidized soybean oil as an auxiliary stabilizer; reduce the first barrel zone temperature by 5-10°C; add 0.5-1 phr TiO₂ to the formulation to raise the whiteness baseline.
Key Takeaway
The initial coloring control of calcium-zinc stabilizers requires the synergistic cooperation of multiple auxiliary stabilizers such as β-diketones and epoxidized soybean oil; through formulation optimization and processing temperature adjustment, initial yellowing can be effectively suppressed.
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