Effect of mould temperature on the formation of α/β polypropylene blends in injection moulding

Abstract

Polypropylene was doped with 1% pimelic acid/calcium stearate to stimulate the growth of β-phase PP crystals to form α/β polypropylene blends in injection moulding. The effect of mould temperature on the composition of the blends was investigated. The degree of crystallinity of β-phase PP was found to increase with mould temperature from 20.6% at 60°C to 32.7% at 140°C while the crystallinity of α-phase PP dropped from 34.8% to 27.0%. When same moulding process was used, the overall degree of crystallinity of the α/β blends was similar to that of the pure samples which contained α-phase crystals only. For mould temperatures above 110°C, the β-phase crystals exhibited a distinctive fusion peak near 150°C in the differential scanning calorimetry (DSC) analysis. When the mould temperature was below 110°C, however, the β-fusion peak began to separate into small peaks from 131 to 152°C as a result of crystals formed under relatively rapid cooling conditions near the mould surface. In addition to the diffraction peaks of the α-phase PP, WAXS showed one extra peak at 29 = 16.0° corresponding to the {3 O 0} plane of the β-phase crystals.