Keynote paper to the Polymer Processing Society Euro Meeting, Gothenburg, Sweden, 19th-21st August 1997.
S F Bush
Introduction
For many years two forms of fibre reinforced polymers have been predominant: thermoplastic granules with predispersed short glass fibres in the length range around 0.5-1.5 mm and thermosets used with preformed mat like structures made up of glass fibres of length 25 mm upwards.
To varying degrees glass fibre mats or preforms may be prepositioned in a mould to provide reinforcement in the directions of the stresses applied to the made artefact. In the case of the short fibre thermoplastics however the flow fields set up by the die or moulds exercise a major adventitious influence on the distribution and orientation of the fibres. In particular for flows with a predominant velocity component in one direction as in extrusion and many mouldings, the fibres tend to be disposed very largely in that direction, which will only rarely be the direction of major applied stress in the solid artefact.
In recent years, thermoplastics compounds with much longer glass fibres, in the range 6-13 mm, have become widely available. While earlier predictions1 of explosive growth in sales of these long fibre reinforced compounds have not so far been realised, in particular applications they do show considerable advantage over their short-fibre counterparts(2, 3). The principal reason for their advantage is their ability to form mat-like structures in the melt which then persist into the solid state. These can readily be seen by burning off the polymer. We thus get something like the reinforcing structures of the preformed thermoset compounds, without the time-consuming necessity of a separate processing step.
The significance of this mat-forming property is two-fold: it greatly increases the melt strength and it provides a means of overcoming the tendency of the die or mould to align the fibres in directions which are undesirable from an end-use point of view. Thus in blow moulding we need fibres oriented at right angles to the parison extrusion direction so that during the stretching phase the moulding doesn’t pull apart. Likewise in the extrusion of reasonably isotropic sheet, we need as many fibres oriented in the transverse direction as in the flow direction, and for pipe extrusion arguably twice as many. In injection moulding isotropy is generally desirable, particularly if the part is subject to temperature variations. While some transverse orientation is often obtained adventitiously, to obtain the lace-like isotropic fibre separation, specific fibre management devices of the types previously described(2, 5) are necessary in most applications.
References
[1] Plastics Brief 15 23, (1991)
[2] Bush S F, Harland W G and Bilgin S, Pipe Extrusion with Rotating Die Systems, 6th Plastics and Rubber Institute Pipes Conference, York, England, 25th-27th March 1985
[3] Gibson A G et a, ECCM-I Conf. (24-29 Sept. 1985)
[4] Bush S F, US Patent, 5,264,261 (1993)
[5] Bush S F, Yilmaz F B, Zhang PF, Impact Strengths of Injection Moulded Polypropylene Long Glass Fibre Composites, Inst Mats 6th Int Conf on Fibre Reinf Comp, Newcstle, England, 29-31 March 1994, 24 139-147
[6] Torres F, Private Communication, UMIST (1997)
[7] Bush S F, Ademosu O K, Blackburn D R, Yilmaz F B, Zhang P F, Relationship of impact strength, tensile strength and the fiber-matrix interface in varieties of polypropylene long-glass fiber injection moldings, Poly Proc Soc, 9th Ann Mtg, Akron, USA, paper 6-28 (1994)
[8] Bush S F and Tonkin J D, Polymer packaging as an environmental benefactor, Antec Technical Papers 43 3081-86 (1997)