The coatings
These come in the form of liquid paint, film or powder coatings.
Films
There are 4 types: polyvinyl chloride, polyvinyl fluoride, thermoplastic acrylics and polyethylene terephthalate (PET).
Their advantages include high flexibility and suitability for deep drawing. The more recently co-extruded qualities also have very high gloss and good hardness.
Certain films have a primarily exterior role (resistant to rain, sun, heat, etc.) while others are resistant to fire, stains, abrasions or aggressive chemicals.
Films are used where very high aesthetic quality is required. Variations in film thickness, colours and finish (smooth, structured or printed) can be achieved.
Liquid paints
There are two types of paints used for coil, based on either thermosetting or thermoplastic resins. The resins can be subdivided into 8 major categories: alkyds, acrylics, epoxies, polyesters, polyurethane, silicone-modified resins, polyvinyl chlorides and polyvinyl fluorides.
Paints used in coil coating
Coil coatings must have with the following three properties:
- Easy application to give uniform appearance and defect-free films even on the fastest production lines (up to 220m/min);
- A very short cure time during which the pigment colours remain stable (time spent in the oven can vary between 5 to 60 seconds depending on the line; the substrate temperature is between 200°C and 260°C according to the paint used);
- Flexibility to enable the coated metal to be bent without cracking or loss of adhesion of the paint film.
To view a table giving an estimate of the relative properties of the main types of coil coatings, click here.
Since no single paint system as yet is best for all requirements, the choice of system is determined according to a combination of life expectancy and commercial considerations.
A two-coat paint system is normal. The first coat is the primer, usually applied directly on to the pre-treated metal surface. The primer will probably contain anti-corrosive pigments to protect the metal substrate from corrosion. The second coat of finishing paint provides the colour and appearance of the final coated system, and plays a large part in the system’s weathering and mechanical performance.
The dry film thickness of the coatings are usually between 5 to 35 microns for a primer and 15 to 200 microns for finishes and thickness is determined by the paint qualities chosen and the precoated metal’s final usage.
Each type of coating has its own specific advantages, be it thickness, gloss, hardness, flexibility, durability in harsh weather or resistance to chemical attack. The choice of the most suitable system must be based on its usage and expected performance, and should always be agreed between the producer and the client. Some coatings are applied in a single layer, to either one or both sides, while others are used with a primer in multi-layered systems.
Powder coatings
In some sheet metal markets small banks of guns have been used satisfactorily for several years to powder coat flat sheets before forming.
The problem with scaling up to a full size, full speed coil coating line is that a continuous coil line is faster, and applying powder evenly and consistently to a rapidly moving strip of coil is not easy. If conventional guns are used, banks of around 40 are required and if one gun blocks, the whole length of strip can be ruined. The process needs to be under tight control. Beckers is a European market leader in the formulation and manufacture of powder for coil coating.
Two alternatives to spray for coil application are being developed. The 'PowderCloudTM', as its name implies, applies powder to the coil strip as it passes through a charged cloud of powder coating. The principles of the 'PowderCloud™' system are the same as corona application: same-charge powder particles are repelled from each other and attracted to the earthed strip.
The other alternative is the electromagnetic brush (EMB) system. This uses similar technology to that of a laser printer or photocopier. In principle this is a controllable system, which could produce unlimited special effects, and could deliver low film thicknesses.
Powder has real attractions for coil coaters. Being solvent-free, powder coatings can potentially be cured quickly using infra red or induction heating, offering shorter, less capital-intensive coating lines with quick change-over times. So, once again, the challenge comes back to the formulating chemist to develop products that will retain their high performance specifications after curing at, for instance, 270°C in well under a minute.
As the European leader in both liquid and powder coating systems for coil, you can be sure that Beckers will continue to develop competitive powder systems for pre-finishing metal.