The polymorphic composition of long-chain branched polypropylene processed by injection and compression molding

Abstract

The impact of injection molding processing parameters, including mold temperature, injection speed, and holding pressure, on the supermolecular structure of long-chain branched polypropylene (LCB-PP) was investigated. Commercial LCB-PP was processed under different conditions, with four sets of processing parameters applied. Holding pressure was varied from 30–70 MPa by 10 MPa in the P-SET, mold temperature was raised from 40 to 120 °C in 20 °C steps in the T-SET, and injection speed increased from 20 to 140 mm/s in 30 mm/s steps in S-SET1 and S-SET2, with mold temperatures at 40 or 120 °C. The polymorphic composition of the specimens was analyzed using wide-angle X-ray scattering, demonstrating a strong dependence on the processing parameters. The skin of the specimens showed an increase in β-phase content with higher mold temperature and injection speed. Conversely, growing holding pressure slightly suppressed β-phase content and promoted the formation of the γ-phase, which was predominantly manifested in the core of the specimens. The polymorphic composition, with both β-and γ-phases present in notable amounts, had a beneficial effect on the impact strength of the specimens.