Composites set new limits

Tom Shelley reports on a composite technology that combines low cost with exceptional performance, including fire resistance



Inspired originally by early developments in making 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.

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

The technology is the invention of George Elliott, and his Stratford on Avon based company, Curon. Many years ago, he used to work for Royal Ordnance, in the early days of their 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 the action of the ultra violet content of sunlight.

Unlike conventional GRP materials that contain about 70 per cent resin, Curon has less than 20 per cent, 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 Mr Elliott describes 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, George Elliott has his own special formulations, which he varies according to sheet thickness and intended function. All his ingredients can, however, 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 big advantages of a UV cure is that it is a cold process, avoiding possible thermal distortion effects, and is also 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. 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 on the other hand, 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.

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. We were shown how it was possible to use specially developed tools to form cable ducts and flanged panel enclosures, by a method which we thought to be particularly ingenious but we are unable to describe. It is also possible to make car panels and doors, parts for aerospace, light weight vehicle armour, and large diameter pipes and ducts. Finish is smooth and polished, despite the absence of a 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 thicknesses from 1mm to 6mm, although they can be supplied up to 25mm. Resin formulation is different for the thicker sections.

A fire resistant version has been developed able to withstand an oxy acetylene 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.

While having a secure local market supplying local roofers, the company 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, apart from all the other possible markets.

Curon Limited

Eureka says: This looks to be a massive breakthrough in composite technology possibly rendering what has until now been the expensive if weight saving option in car and enclosure panel design, the preferred low cost option in many cases

Pointers

* Material is twice as strong in compression as conventional GRP and much more resistant to cracking.

* It uses a cheaper set of ingredients, requiring much less resin, and employs very low cost manufacturing techniques

* Tooling is low cost and not subject to wear

Author
Tom Shelley

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Do you have any comments about this article?
is posible set this material to cure with visible ligth, to use in stereolithography ?

Comment ALVARO SAAVEDRA RIVEROS, 05/07/2012
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