Short and Long Term Behaviour of Long Glass Fibre Reinforced Polyolefins
March 24th, 1997
Paper to Institute of Materials Conference on Deformation and Fracture of Composites, 24th-26th March 1997.
S F Bush with J D Tonkin
Abstract
Despite the high levels of improved mechanical properties in glass fibre reinforced thermoset composites, difficulties and restrictions in processing methods have limited their application in manufacturing to high value added products. In addition, the prominence of green issues in the commercial environment is forcing the selection of readily recyclable composite materials that still offer enhanced specific properties. These factors have all contributed to the increased use of thermoplastic composites despite their lower glass concentrations and specific mechanical properties compared to thermoset composites. The term long glass fibre is generally used to describe filaments with a sufficiently high aspect ratio as to allow several fibre touches or near touches (interactions), even at relatively low fibre concentrations (e.g. fibre lengths approximately > 1 mm and preferably > 5 mm.
Under the generic title of SAFIRE (Self Assembling FIbre REinforcement) research has been successfully carried out into the production of long glass fibre granules, the development of static fibre management systems to optimise distribution and production of long fibre composites using conventional processing techniques (injection moulding, blow moulding, extrusion, and compression moulding).
Using results obtained from injection moulded tensile test specimens of different thermoplastic (PP, HDPE & LDPE) base polymers the following mechanical properties were observed:
- Tensile strengths improved by approximately 100% for a 7% glass v/v composite, with strengths of 68 MPa reached in a PP homopolymer composite.
- Contrary to expectation, the addition of long glass fibres to PP has given a significant increase (over 200%) in impact strength. Other materials such as recycled HDPE showed no significant change. Although glass fibres reduced the impact strength of LDPE it still remained high at almost three times that of recycled HDPE. The significance of the coupling agent on the fibres is shown by a doubling of impact strength in PP copolymer before glass fibres were added. When 7% glass was added to HDPE without coupling agent there was a 50% reduction in impact strength compared to the composite containing the coupling agent.
- Creep properties have also shown perhaps the larges improvement due to the presence of long glass fibres. At loadings of 7% glass v/v LDPE has shown a 400% increase in load bearing resistance and recycled HDPE a 200% increase in load bearing resistance.
In conclusion, it has been shown that at only 7% glass fibre concentrations the tensile strength, creep resistance and even the impact properties of certain polyolefins can be increased significantly. Even more important is the fact that these composites can be processed on conventional plastic processing equipment with only minimal modification to optimise properties. Finally, as the base polymer used in the matrix is thermoplastic the composite may be granulated and recycled.