Material Drying
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PP Regrind-Induced Embrittlement
Problem Phenomenon
After increasing regrind ratio, part toughness drops significantly, breaking easily on drop or bending; meanwhile melt index abnormally rises and flowability improves.
Cause Analysis
After multiple heat processing cycles, PP molecular chains break and molecular weight drops, manifesting as increased flowability but degraded mechanical properties; pulverized material contains much fine powder with uneven plasticization, and if regrind contains moisture or impurities, performance fluctuation is further amplified.
Solution
①Control regrind ratio within 30%—use virgin material for critical load-bearing parts as much as possible; ②sieve regrind to remove fines and mix thoroughly and uniformly with virgin material; ③re-dry regrind; ④appropriately lower melt temperature and shorten residence time to reduce secondary degradation; ⑤establish a ledger correlating regrind batches with properties (melt index/impact).
Key Takeaway
PP regrind requires 'control ratio, control heat history, control drying' simultaneously, monitored with melt index and impact data; better flowability does not equal better usability—it is often exactly the signal of molecular weight drop and performance degradation.
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