Material Analysis
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Precipitation and blooming caused by interaction between stabilizer and lubricant
Fenómeno do problema
After 7 days of storage, PVC transparent sheets using calcium-zinc stabilizers show white powdery precipitate (blooming phenomenon) on the surface, the product transparency decreases and it feels sticky to the touch, failing to meet food packaging appearance requirements.
Análise das causas
The low molecular weight metal soaps (zinc stearate/calcium stearate) in calcium-zinc stabilizers, after exceeding compatibility limits with lubricants (stearic acid, paraffin wax), migrate to the surface; the plasticizer DOP has a carrying-out precipitation effect on metal soaps; environmental temperature changes accelerate the migration of small molecular substances to the surface; the total amount of components in the formulation exceeds the resin absorption capacity.
Solução
Reduce the low molecular weight lubricant dosage and switch to high molecular weight ester lubricants such as GMS compounded with PETS; control the calcium-zinc stabilizer addition amount to no more than 3 phr; add 0.5-1 phr anti-precipitation agent (silicon dioxide or diatomaceous earth) as an absorption carrier; reduce the plasticizer dosage or select plasticizer varieties with better compatibility.
Conclusão principal
The essence of the precipitation problem is the compatibility and migration balance of components in the formulation; it can be effectively suppressed by selecting high molecular weight additives and controlling the total addition amount.
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