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.
