Self Assembling Fibre Reinforcement Processes
January 11th, 1994
Paper to International Conference on Textile Reinforced Composite Engineering, Bolton Institute of Higher Education, 11th-13th January, 1994.
S F Bush
Synopsis
Broadly, reinforcement may be introduced into polymeric materials either by mixing discrete fibres more or less at random with polymer before injecting into a shaping mould or by constructing a predetermined textile form from staple or continuous fibres and then bringing the polymer, or more usually thermoset resin, into contact with it. The first method readily lends itself to all the high speed automatic processes which have been devised for unreinforced thermoplastics, but carries the twin penalties of:
- short fibres (typically 0.3 to 0.6 mm) which are arranged by the adventitious action of the flow field, rather than by the need to match the imposed strains in the finished artefact, and
- a sharp drop in the notched impact strength by comparison with virgin matrices.
The second method on the other hand can give large increases in tensile strength and stiffness in directions which can be closely matched to the imposed strains, but at the expense of extended manufacturing times and therefore cost.
The paper describes a set of concepts which are designed to marry the high-speed advantage of the first method to the advantages of the textile-like reinforcements obtained with the second method. The concepts provide the means by which specially compounded granules each containing bundles up to 2000 10-20 micron diameter 6-20 mm long glass filaments embedded in the host polymer prior to injection into a mould or extrusion through a die. By means of flow shaping elements placed within the injection nozzle or immediately before the extrusion die, the separated filaments are caused to assemble themselves into semi-coherent mat-like reinforcement structures which are not seriously disrupted when carried by the melt into the mould or through a die. Such structures are readily observable in the solid artefact by burning off the polymer to leave a three dimensional fibre structure which faithfully reflects the artefact shape.
The paper will summarise the mechanical properties obtainable from these Self Assembing FIbre REinforcement (SAFIRE) processes. Generally it is found that by comparison with virgin polypropylene for example the goal of achieving improved tensile strength, stiffness and notched impact strength together can be achieved.
SAFIRE technology has now entered the commercial domain and some of the resultant practical mouldings and pipe extrusions will be briefly described.