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 th e rotational speeds are slow (typically about 10 rev/min), the resulting ho llow articles are practically stress free. The primary material used for ro tationally moulded parts is polyethylene but there is an increasing interes t in using polypropylene to provide stiffer, higher temperature products. U nfortunately the slow cooling combined with the slow crystallisation rate o f polypropylene results in brittle mouldings with coarse spherulites. Since the inner surface of the plastic is in contact with air during moulding, d egradation is also likely to occur. In order to improve the mechanical prop erties of the rotationally moulded polypropylene, alpha and beta nucleating additives were added. The effect of using faster cooling rates was also st udied. It was found that heterogeneous nucleation, both of beta and alpha s pherulites, did not improve the ductility of the samples. However, when fas t cooling was used, the impact strength of the polypropylene improved marke dly, independent of the presence of nucleating additives. In the rotational ly moulded polypropylene parts, the fast cooling could only be applied to t he outer surface of the mould, which led to asymmetric cooling. This result ed in severe warpage, and uneven morphology. This problem should be overcom e by using fast cooling on both the inside and outside surfaces of the plas tic. (C) 2001 Kluwer Academic Publishers.