Strengthening Rotationally Moulded Products: Development of a Rotomouldable Thermoplastic Composite

Rotational moulding is a process with a range of unique benefits, currently experiencing significant growth within industry. A primary limitation of the process, however, is the restricted range of suitable materials and the subsequent limitations placed upon the mechanical performance of rotationally moulded products. The aim of this research project was to develop a rotational moulding material which possessed substantially improved mechanical properties over conventional, rotationally moulded polyethylene grades. This was attempted through the development of a fibre-reinforced thermoplastic composite which was amenable to the rotational moulding process. Although the development of rotationally moulded composites has been previously reported in literature, these materials have largely proven to be unsatisfactory in terms of mechanical performance. The composite material developed here is distinct from those previously reported in that it utilises melt-compounded composite granules of a significantly larger size than are normally considered suitable for rotational moulding. These granules are intended to be moulded as an inner layer within a rotationally moulded product, with the outer moulded layer formed from conventional, unfilled polymer powder. In this way, the average length of the fibres within the moulded article is maximised while retaining the benefits of the melt-compounding process and giving an excellent external moulded finish. In order to achieve a sufficient degree of composite granule flow and coalescence during rotational moulding, it was found necessary to base the composite granules on high melt-flow, injection moulding grades of polypropylene and polyethylene. The most successful of the resultant composite materials was shown to be superior to the previously utilised rotational moulding grade of unfilled, high-density polyethylene by approximately 65% in offset yield strength (0.5% offset) and over 95% in tensile modulus