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Composites Set New Limits

Posted on: Wednesday, 22 February 2006, 06:00 CST

By Shelley, Tom

Tom Shelley reports on a new composite technology that combines low cost with exceptional performance

Inspired originally by early developments in manufacturing composite armour plate, a dramatic new composite technology combines particulate and composite technologies, novel chemical formulations and ingenious low cost manufacturing techniques.

The result is a technology that was originally developed to produce rolls of material to seal flat roofs, which quickly cure in sunlight to produce rain barriers of exceptional durability.

Material inspired by the development of composite armour plate is stronger and stiffer than conventional GRP sheet and comes on a roll for UV curing

It has since been extended to produce cable ducts, large diameter pipe sections, enclosure and automotive body panels, train seats and aerospace parts.

The technology is the invention of George Elliott, and his Stratfordupon-Avon firm, Curon.

He originally worked for Royal Ordnance, in the early days of its development of composite armour, which seems to have inspired his idea for 'Curon Roofing', rolls of composite material which can be unrolled on leaking roofs and which cure under ultra violet sunlight.

Unlike conventional GRP materials that contain about 70% resin, Curon has less than 20%, but is somewhat stronger and stiffer. Some of the resin is replaced with a graded particulate (that we have been asked not to divulge) which Elliott described as "not a filler, but something one-third the price of the resin, which significantly adds to the final strength and generally enhances the properties of the material".

Although the base resins are fairly standard products supplied by DSM, Elliott has his own special formulations, which he varies according to sheet thickness and intended function. All his ingredients, however, can be mixed in a manner which reduces the possibility of entraining air bubbles. "The secret," Elliott told Eureka, "is in the mixing". All the particles, he said, have to be coated in order to avoid sites for crack initiation. Their grading is crucial, because small particles need to sit between large particles, locking the whole structure together, in the same manner as a well-designed concrete.

Because cracks have to find ways through the resulting labyrinth between the particles, the material is effectively much tougher and less prone to damage from chemicals or weathering.

It also has a doubled overall compressive strength and greatly increased stiffness, similar to that of carbon fibre, but associated with much lower cost. The presence of the particulates also reduces the effective coefficient of thermal expansion of the resin matrix, reducing the effect of thermal shock, while the tensile strength remains that of the glass fibres.

Apart from being able to cure in the sun, one of the great advantages of a UV cure is that it is a cold process, avoiding possible thermal distortion effects - and is fast. Heat curable composites usually take around five weeks to achieve full strength, but are often taken out of their moulds after only a day or two for production cost reasons, leading to a risk of distortion while the cure continues out of the mould.

Material inspired by the development of composite armour plate is stronger and stiffer than conventional GRP sheet and comes on a roll for UV curing

Because the process is exothermic, there is also the possibility of heat cracking. Curing of the outermost layers of Curon using UV, takes only a few seconds, and the whole process is complete in about two hours.

As supplied to roofers, the material is made up of a film layer, followed by the resin and particulate mixed matrix, fibre, more matrix and a top film. Provided it is kept indoors, it can easily be shaped and cut.

No tool wear

It can also be made into parts, cut and formed over low cost tooling, and UV cured by lamp in the factory. Because no force is required to form the material off the roll, tools can be made out of any rigid material and do not wear. Eureka was shown how it was possible to use specially developed tools to form cable ducts and flanged panel enclosures, by a method which was particularly ingenious but which we cannot describe.

It is also possible to make car panels and doors, parts for aerospace, lightweight vehicle armour and large diameter pipes and ducts. Finish is smooth and polished, despite using no gel coat.

Some customers require that instead of a top film, the top surface be made with double-sided adhesive, so formed panels can immediately be stuck on. Rolls are normally supplied in 1mm to 6mm thicknesses, although they can be supplied up to 25mm. Resin formulation is different for the thicker sections.

A fire resistant version has been developed which can withstand an oxyacetylene torch flame for 20 minutes, conforming to BS 6853, 1987. There is no surface spread of flame, no heat contribution from the sample and no toxic fumes.

The commercial designation of fire resistant grades is 'F-Max' followed by a number, which represents the grams of glass fibre per square metre. Tests have been undertaken by DSM Resins at the Fire Research Centre at Warrington to certify the fire resistance of the resins, which are to be followed by similar tests applied to finished 'F-Max' Curon parts. The aim is to certify the material for use in public transport, particularly underground trains.

The latest development is a spray-on technology involving spraying the resin in such a way as to entrain chopped glass fibres, followed by almost instant UV curing. A patent application is pending. For commercial reasons, we are unable to describe details at this time.

While having a secure local market supplying local roofers, the firm is looking to expand. It is therefore seeking development and large scale manufacturing partners. Enquiries from overseas indicate that sales to install and repair flat roofs and render them reliably leak-free, alone could be a vast business, aside from other markets.

Enter 440 for Curon at www.eurekamagazine.co.uk/enquiry

Large, quite complex shapes can easily be made using low cost tooling

Pointers

* The material is twice as strong in compression as conventional GRP and much more resistant to cracking

* It uses a cheaper set of ingredients that require less resin and uses low cost manufacturing techniques

* Tooling is low cost and not subject to wear

Copyright Findlay Publications Limited Feb 2006

Source: Eureka

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