Effect of mould temperature on the formation of alpha/beta polypropylene blends in injection moulding

Polypropylene was doped with 1% pimelic acid/calcium stearate to stimulate the growth of beta-phase PP crystals to form alpha/beta polypropylene blends in injection moulding. The effect of mould temperature on the composition of the blends was investigated. The degree of crystallinity of beta-phase PP was found to increase with mould temperature from 20.6% at 60 degrees C to 32.7% at 140 degrees C while the crystallinity of alpha-phase PP dropped from 34.8% to 27.0%. When same moulding process was used, the overall degree of crystallinity of the alpha/beta blends was similar to that of the pure samples which contained alpha-phase crystals only. For mould temperatures above 110 degrees C, the beta-phase crystals exhibited a distinctive fusion peak near 150 degrees C in the differential scanning calorimetry (DSC) analysis. When the mould temperature was below 110 degrees C, however, the beta-fusion peak began to separate into small peaks from 131 to 152 degrees C as a result of crystals formed under relatively rapid cooling conditions near the mould surface. In addition to the diffraction peaks of the alpha-phase PP, WAXS showed one extra peak at 2 theta = 16.0 degrees corresponding to the {3 0 0} plane of the beta-phase crystals.