AbstractWork was carried out to develop a process for incorporating fibre reinforcement in rotational moulding. Various types of mouldings were tried, ranging from flat sheet mouldings to two shot cube mouldings. As flat sheet mouldings did not result in proper composites, they were abandoned at an early stage in the project. Single shot cube mouldings with short reinforcement did not lead to composites either as the reinforcement migrated towards the inner surface of the moulding. Two shot mouldings proved more successful since a second shot of powder was added during the process to encapsulate the reinforcement. After an initial focus on polyethylene it was decided to use polypropylene as matrix material since contacts with industry showed that the technology for reinforcement of polypropylene is much further developed.
Various shapes of reinforcement were used in conventional rotational moulding, ranging from chopped fibres to woven fabric. Mouldings were tested for flexural properties, impact properties and Inter Laminar Shear Strength. The best results were found for woven PP-Twintex fabric and PP/glass melt impregnated tape in polypropylene. In the latter case, the Flexural Modulus measured was the same as the Flexural Modulus calculated according to the Theory of Laminates.
It proved beneficial for the mechanical properties if some polypropylene was already included in the reinforcement. An improvement in wet through and wet out can be achieved, which is shown by the good properties achieved when using commingled Twintex or melt impregnated tape. The theoretical explanation for this was found in Darcy’s law. Based on Darcy’s law it was concluded that the only way to achieve sufficient impregnation was to use physical pressure on the molten polypropylene. Processing methods were developed for pressure bag and vacuum bag assisted rotational moulding resulting in Twintex parts with impact and flexural properties superior to those of polyethylene and polypropylene. A mathematical study showed that it was more economically viable to achieve these properties in mouldings using Twintex than with a theoretical increase in material use of polyethylene or polypropylene.
|Date of Award||Dec 2002|