thermoplastic composite
rods
The brightly coloured sticks in Figure 1 look like extrusions, but they have the mechanical performance of a conventional pultrusion. They were made using a novel new one-stage process developed by Imhotep Ltd, a start-up company founded by two ex-chairs of the IoM's younger members committee. The low-cost process, which has been termed 'gel coated pultrusion' but is in fact more versatile than the name implies, produces unidirectional, continuously reinforced thermoplastic composite profiles comprising a high performance composite section and a polymer coating.
Fibre reinforced thermoplastics have been around for many
years and are finding increasing applications in a number of
areas, such as the automotive industry. The majority of these
applications are centred on injection moulded parts. Reinforced
extrusions have also been investigated but the work in this area
generally uses similar materials to those used in injection moulding.
By
tackling the traditional problems associated with using thermoplastics
as a matrix in fibre reinforced composite profiles with a range
of properties.
In a composite material the choice of length of the fibres is critical in getting the maximum increase in performance. Figure 2 illustrates how fibre length affects the mechanical performance of composites by comparing the flexural modulii of different types of glass reinforced polypropylene composites. The highest mechanical performance increase is achieved with continuous fibres. The technology developed by Imhotep uses continuous fibres along the length of the extruded part, giving the maximum performance advantage.
In conventional composites such as fibreglass and GRP, pultrusions use a thermoset resin matrix such as epoxy or polyester.
Thermoset resins are formed by a chemical reaction and as such cannot be remelted or reformed once set. They are also inherently brittle. Thermoplastics, on the other hand, such as polyethylene, acetal, polyamide, and polypropylene are tough and can be remelted and reformed as required. The main obstacle to their widespread use in the were their comparatively high melt viscosities of between 10 and 100 Nm-2 compared to between 0.2 and 2 Nm-2 for thermoset resins-which makes impregnation of the reinforcing fibres much more difficult. Poor impregnation of the fibres by the polymer translates into poor mechanical performance in the finished part. Achieving relatively void free thermoplastic matrix composites has required significant process development and optimisation.
These greater difficulties with impregnation mean that most thermoplastic composite manufacturing routes are based on a two stage process. A fibre reinforced polymer precursor material is prepared which is then processed, by standard polymer processing equipment, in the manufacture of the finished part. For fibre reinforced injection mouldings there are two generic types of reinforcement- short fibre (fibres of up to 3mm in length) and long fibre (fibres up to 13mm in length). These are available in a wide range of thermoplastic/fibre combinations. However, during processing by either injection moulding or extrusion, most fibres are damaged and their length can be reduced, adversely affecting the reinforcing capabilities of the fibres in the polymer.
For thermoplastic composites containing continuous fibres, precursor materials can and have been used in producing the finished composite profile or reinforced extrusion. A number of precursors based on continuous fibres are commercially available, for example commingled fibres-tows of continuous fibres of glass or carbon intermingled with continuous fibres of the polymer - and prepregs, reinforcement fibres impregnated with a polymer matrix in the form of thin sheets.
These precursors can then be incorporated into the extruded profile, giving the improvements in performance, or used in their entirety to produce a true thermoplastic composite profile.