However, there are a number of drawbacks to this approach and Imhotep has developed alternative technology to overcome them.
To fully exploit the advantages of thermoplastic composite materials Imhotep believes that any polymer and fibre combination should be able to be used. The main problems with using different and therefore potentially incompatible polymers are poor adhesion between the reinforcing section and the rest of the profile, and differential shrinkage of the constituent parts. This second factor could lead to delaminations and porosity in the reinforced extrusion affecting product performance and possibly product life. Poor adhesion between the reinforcing section and the profile would lead to a reduced strength in the finished part. An further drawback is that the fibre volume fraction of the composite precursor materials is set by the manufacturers, which could lead to less efficient use of the materials in the finished profile.
The new technology allows the use of a variable amount of fibre within the extruded profile. As a result, the process overcomes the potential problems of using different polymers by using the same polymer throughout the profile. In addition, cost savings are possible as the profile is made from the raw materials, polymer granules and fibres, and is not reliant on the use of more expensive intermediate composite materials.
There are other possible advantages to the process technology compared to conventional pultrusion. In thermoset composites, there is a processing limitation on the minimum fibre volume fraction due to shrinkage and heat generated by the chemical reaction that occurs in the resin. Composites based on thermoplastics do not suffer from this major limitation, allowing reinforcement levels of 0% to about 65% by volume across the profile. This means that the technology provides the means produce profiles with the mechanical performance of anything between an extruded profile to a pultruded composite.
By careful design of the profile, it is possible to restrict the area of reinforcement to the region where it provides the most mechanical benefit, with the rest of the profile being formed by the cheaper polymer. Figure 3 compares the effective stiffness of the profile as given by the product of Young's modulus and the second moment of area of the profiles, EI. The first example, Figure 3a, is uniformly reinforced with glass fibres across the profile as produced by the conventional pultrusion technology. In Figure 3b, the reinforcement is restricted to near the outer edges of the profile where it gives the greatest contribution to the overall stiffness. As can be seen the overall stiffness is very similar even though the reinforcement level is greatly reduced. In Figure 3c, the profile is reinforced using carbon fibres, which have a much higher modulus, and the level of reinforcement required to give the same overall stiffness is much lower. Even though carbon fibres are much more expensive than glass fibres this need not be reflected in the profile cost. It could also provide a further advantage in terms of reduced weight. The potential to many weight-sensitive applications in automotive and other sectors is clear. For example, reducing the mass of sail batons batons in racingyachts enables the counter mass in the keel to be reduced dramatically by between 10 to 100 times the reduction in on mast mass- 1 kg off the mass up the mast could save up to 100 kg of the mass of the keel.
An emerging area for this 
new
technology, and one perhaps with the most to offer, is the manufacture
of environmentally friendly composites. The use of recycled plastic
in conventional extrusion is often restricted due to the deterioration
in mechanical properties when compared to the virgin material.
As Imhotep's new technology produces profiles that rely on the
continuous fibres to provide their strength, the use of recycled
plastics is not a disadvantage, so 100% recycled plastic 
can
be used and it can have 50 times the mechanical performance of
the virgin polymer. At the end of their life, profiles made using
this technology can be granulated to form a product suitable
for producing reinforced injection moulded parts.
The first recycled thermoplastic matrix composite product to be produced by Imhotep was a partially reinforced recycled polypropylene rod, Figure 1, which shows how the product can be richly coloured. It should be possible to generate any type of finish found in conventional extrusion. It has a flexural modulus 20 times that of an extruded polypropylene rod of the same diameter, and, due to the polymer coating, if the rods do break, they do so in a benign way without producing long shards or splinters. This has gained a favourable response from customers used to traditional fibreglass. The difference in behaviour between the two types of composite can be seen in Figure 4. Behaviour up to the yield point is similar, but after yield, the behaviour changes - the thermoplastic composite does not suffer the rapid drop in load seen in thermoset composites. Instead it appears to undergo plastic deformation, more commonly seen in pure thermoplastics and metals. Handling the product is also less problematic because of the polymer rich surface.
So far this product has found a niche in the horticultural market, and developments such as construction and other aspects of the leisure industry are also being investigated. For applications that require the aesthetics of an extrusion and the mechanical performance of a fibreglass pultrusion Imhotep's technology for thermoplastic composite profiles could provide the answer.
Further reading:
- Thermoplastic Aromatic Polymer Composites, F.N. Cogswell, Butterworth Heinemann 1992
- Reinforced Plastics Handbook, John Murphy, Elsevier Advanced
Technology, ISBN 1 85617 348 8