Paper to the Institute of Materials 4th International Conference on Automated Composites, University of Nottingham, 6th-7th September 1995, pp 134-241.
S F Bush with E S Erdogan
Abstract
Broadly, reinforcement may be introduced into polymeric materials either by mixing discrete fibres into a polymer before injecting the fibre containing melt into a shaping mould, or by constructing a predetermined textile form from staple or continuous fibres and then bringing it into contact with the polymer or resin. The first approach readily lends itself to all the high speed automatic processes which have been devised for unreinforced thermoplastics, but carries the penalty of inefficient reinforcement. The fibres are short (typically 0.3 to 0.6 mm), distributed and oriented more by the adventitious action of flow in the mould than in the directions needed to resist imposed stresses and strains.
The second approach 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 this reinforcement efficiency is obtained at the expense of extended manufacturing times and therefore cost.
This paper describes results obtained from processes designed to marry the high-speed advantage of the first approach to the reinforcement efficiency advantage of the second approach. By means of specially designed fibre management devices, well-wetted glass filaments, typically 6-15 mm long, are caused to assemble themselves into semi-coherent mat-like reinforcement structures which are not seriously disrupted when carried by the polymer melt into a mould or 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’s shape.
Earlier papers (1, 2, 3, 4) describe the features of the fibre-mat reinforcing structures and the mechanical properties obtainable from these Self Assembling Fibre Reinforcement (SAFIRE ©) processes. The present paper describes the heat distortion (HDT) temperatures (BS2872 method 121A) obtained from glass fibre reinforced polypropylene injection moulded plaques, with and without SAFIRE fibre management devices, at three different glass fibre concentrations.
The results show that there is a skin-core effect, though smaller than with short-fibre compositions. As a result of this effect the measured HDTs are a few percent greater in the transverse direction than in the flow direction. This low degree of anisotropy is similar to that found previously for tensile3 and impact4 strengths and is a product of the fibre-mat structures obtained(1, 2).
The general result is that for polypropylene the HDT rises by about 6 oC for every 1% fibre volume. This is of considerable commercial significance for polypropylene since mass-produced, largely isotropic, high speed long fibre reinforced mouldings, thermoformings, and extrusions can now be obtained with heat distortion temperatures of around 120 oC (with 25% glass w/w reinforcement) instead of 55 oC for the unreinforced polymer, greatly extending the range of potential applications.
References
[1] BUSH S F, Control of Fibre Structures in Melt Extrusion, 36th Conf. Can. Soc. Chem. Eng., Sarnia, Canada, 6-10 Oct. 1986, paper 32d.
[2] BLACKBURN D R and ADEMOSU O K, Factors affecting Fibre-matrix Contacting in Fibre-filled Granules, Poly. Proc. Soc. IX Ann. Mtg., Manchester, England, 5-8 Apr. 1993, paper 06-14.
[3] BUSH S F, Factors Affecting the Tensile Strength of Long-fibre Reinforced Injection Mouldings, Poly. Proc. Soc. Eur. Mtg., Prague, 21-24 Sep. 1992, paper 6-06.
[4] BUSH S F, YILMAZ F B and ZHANG P F, Impact Strengths of Injection Moulded Polypropylene Long Glass Fibre Composites, VI Fibre Reinforced Composites Conference, Newcastle, England, 29-31 Mar. 1994, paper 5.