Effect of nucleating agents and cooling rate on the microstructure and properties of a rotational moulding grade of polypropylene

Rotational moulding consists of coating the inside surface of a metal mould with a layer of plastic by rotating the mould, firstly in an oven and then in a cooling bay to induce solidification to the desired part shape. As the rotational speeds are slow (typically about 10 rev/min), the resulting hollow articles are practically stress free. The primary material used for rotationally moulded parts is polyethylene but there is an increasing interest in using polypropylene to provide stiffer, higher temperature products. Unfortunately the slow cooling combined with the slow crystallisation rate of polypropylene results in brittle mouldings with coarse spherulites. Since the inner surface of the plastic is in contact with air during moulding, degradation is also likely to occur. In order to improve the mechanical properties of the rotationally moulded polypropylene, α and β nucleating additives were added. The effect of using faster cooling rates was also studied. It was found that heterogeneous nucleation, both of β and α spherulites, did not improve the ductility of the samples. However, when fast cooling was used, the impact strength of the polypropylene improved markedly, independent of the presence of nucleating additives. In the rotationally moulded polypropylene parts, the fast cooling could only be applied to the outer surface of the mould, which led to asymmetric cooling. This resulted in severe warpage, and uneven morphology. This problem should be overcome by using fast cooling on both the inside and outside surfaces of the plastic.