Extrusion & Post
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Insufficient thermal stability of calcium-zinc stabilizers in recycled material reprocessing
Problem Phenomenon
When a calcium-zinc stabilizer formulation containing 30% PVC recycled material undergoes secondary extrusion, the product color significantly darkens, with the yellowness index rising from 4 to 12, and irregular black specks and burnt particles appear, with the product downgrade rate exceeding 40%.
Cause Analysis
The original stabilizer in the recycled material has been partially consumed (consumed by reaction with HCl during the first processing), and the residual stabilizer amount is insufficient to protect the secondary processing; the recycled material already contains initial degradation products with conjugated double bond structures that accelerate secondary degradation; uneven mixing of recycled material and fresh material leads to local stabilizer insufficiency.
Solution
Additionally supplement calcium-zinc stabilizer, adding 0.5-1 phr according to the recycled material proportion; add thermal stabilizer synergist (hydrotalcite or zeolite-type HCl absorber) 0.5-1 phr; use a high-speed mixer to uniformly mix the recycled material, fresh material and additives at 120°C; control the recycled material proportion ≤30%; add 0.2 phr phosphite antioxidant to eliminate already generated peroxides.
Key Takeaway
Systems containing recycled material require additional stabilizer supplementation to compensate for the consumption in the first processing; hydrotalcite-type HCl absorbers are significantly effective as supplementary stabilizers.
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