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Grab your coat, and score

Pre-coated finishes, once seen only on 'big shed' projects, are now finding their way into more small-scale applications In previous issues we have touched on finishes that are either post-applied (ie after construction) or an integral part of the chemical composition of the metal (eg stainless steel).

One area that is becoming increasingly commonplace, with the continuing growth in the off-site construction industry, consists of pre-coated finishes, usually on a metallic substrate. Once the preserve of the 'big shed' beloved of Martin Pawley, pre-coated metal is now equally at home on the home.

Pre-coating is one of those concepts which has an alluring simplicity in the abstract. It would be the easiest thing in the world to spray an even, aesthetically pleasing layer on to a coil of galvanised steel - wouldn't it? It is actually very difficult to spray an even coat of anything on to anything. Anyone who has wielded a can of spray mount in the pre-deadline early hours can attest to the ease (or not) with which a relief map of the Himalayas can be conjured up from what was previously a perfectly flat piece of paper and card. It is the same with paint, whatever its composition.

The use of pre-coated metal isn't particularly new. A form of corrugated steel with a bitumastic pre-applied paint has been around for years, but its widespread adoption can be traced initially to the farming industry, where perhaps it would not be impolite to suggest, the aesthetic qualities of the material were secondary to the potential longevity of a plastic coat on a galvanised substrate. Colours were derived from BS 381 which, dating from 1930, was primarily concerned with the standardisation of camouflage. The Ruralclad range of coatings is still available, but it was with the demise of asbestos as a cladding material that the real revolution in coatings was to happen.

The increase in demand for the material was coupled with a need for more colour variation, as traders sought to incorporate elements of their corporate identity into the building envelope.

There are four basic coating types available, all of them based on organic long-chain polymers. The most usual substrate for these coatings is metal, usually steel or aluminium.

The most familiar of the coating types available is polyvinyl chloride (PVC). This is also known as Plastisol or specific trade names such as HPS200 by Corus. PVC coatings are applied in relatively high thicknesses of100-200µm. Plastisols are flexible so the substrate can be bent after painting.Additionally, a thickness of 200µm means the finish can be embossed, and this further improves scratch resistance. Polyvinylidene fluoride - PVDF or pvf2 - is another commonly used coating type. It has a typical thickness of 2530µm and layers can be built up to achieve special colour and surface effects.

Polyesters (PE) and polyurethanes (PU or PUR) are also used. Then there are supplierspecific finishes such as Celestia, and attributes like Assure by Corus, which is a finish incorporating Microban.

The coating process is complex and varies slightly according to the chemistry used, but certain attributes are always present. Coating is a continuous process, analogous in some ways to newspaper printing.

The substrate material, usually a coil of metal, is contained in a pay-off coil at the beginning of the process. In order to ensure continuity, successive coils are stitched together. This in itself is an operation fraught with technical difficulties. The stitch has to be strong and flat, and it has to be done in such a way as not to upset the continuity of the rest of the process. In order to achieve this, metal from the coil is fed into an accumulator. This is basically a series of rollers around which the metal is fed; over, under, etc. The rollers are then pulled apart vertically resulting in - when the accumulator is full - a long, continuously moving ribbon of zig-zagging metal several metres in height. This gives several minutes of supply into the line, should there be a problem at the start end.

Before the painting processes, the coil is degreased and a process known as passivation is applied. Passivating ensures paint adhesion and also has anti-corrosive properties (see previous MetalWorks).

The now clean and pre-treated coil passes through a prime coating stage, which uses two heads to coat each side. The primed metal goes on to be stove-dried, quenched with cold water and dried again. This then moves to the main process section. Here the main finish is applied, that is the pigment, its binder and any special properties to be imparted.

Paint is mixed in the 'paint kitchen' and pumped to the coating heads which apply the finish to one or both sides of the strip. If the finish is applied to one side only, then a backing coat is given to the reverse at the same time. The resulting wet film of paint is cured in a continuous furnace at final metal temperatures up to 250infinityC. The paint polymerises at these temperatures and the carrier solvents vaporise and are burnt off. This burning-off process is used to power the curing furnace. It also ensures that volatile organic compounds are reduced to a minimum.

The now-painted metal is passed through a tension leveller which corrects any heatinduced curve in the strip, and finally the metal is re-coiled at the end. During the painting process, gauges will have ensured the paint layer is even and to the required depth.

Substrates As mentioned, the type of substrate can vary, but the most commonly coated metal, particularly in construction, is steel. If aluminium is used there will be no need to provide extra corrosion resistance, but steel must be treated before painting.

At Corus Colours both corrosion protection and painting occur on the same site. The processes for both are, superficially at least, the same. Galvanising, which remains the most commonly applied anti-corrosion treatment, uses the same coil/accumulator beginning as the paint line but, after degreasing, the metal strip plunges into a bath of molten zinc.

The sheet, pulled vertically out of the zinc, passes through a series of slots over which air is blown at high speed. This dries and regulates the thickness of the zinc. The dried coated metal is then tension-levelled and coiled.

The ultimate protection would combine the attributes of aluminium, the corrosion products of which inhibit further degradation, with those of zinc, which in conjunction with exposed steel and water sets up an electrochemical reaction. This combination is available under the trade name Galvalloy.

Galvalloy is a eutectic alloy of zinc and 5 per cent aluminium. The aluminium is held in solution in the zinc (there is no chemical combination of the two metals).

Colours and finishes All of the common colour-specification systems can be used to describe a colour accurately. For building purposes BS 5252 persists in certain areas, but other systems - NCS and RAL - are equally applicable and in more widespread use, particularly in the cladding industry. Supplier names for colours are, more often than not, direct translations of BS, RAL or NCS colours. Colour can also be enhanced by embossing, particularly with Plastisol systems.

Metallic finishes are becoming much more popular. These are achieved by adding aluminium flakes to the coating to reflect and scatter the light falling on the surface.

Sizes of flakes can be altered to achieve various degrees of sparkle (from a soft sheen to a sort of diamond dust effect). Recent developments in metallic coatings have increased the thickness of the paint layer, which imparts a depth to the metallic effect. Celestia is an example of a product using this extra thickness.

Colour fashions play some part but most of the colours available have been around for a long time (Goosewing Grey is a derivative of Light Aircraft Grey, while Raven used to be RAF Blue Grey. ) However, in response to fashions in cladding - recently the use of weathering steel cladding in several buildings (MetalWorks Summer 2003) - ranges of colours are available that mimic the appearance of natural surfaces and metals.

The future of coatings We already have bactericidal, infra-red reflecting and anti-static coatings. It is not hard to imagine the use of specialist coatings capable of colour change by varying the charge across the surface, or the coating that uses piezoelectric crystals to switch things on and off.

Heating or cooling might be incorporated into the finish, and display systems and security systems may also, one day, be part of the wall, floor or ceiling. Basically, if you can apply it in a film there is probably a coating application for it.

The following are registered trademarks: Ruralclad, HPS 200, Celestia, Assure, Galvalloy, Microban

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