According to EN 13523-0, a prepainted metal (or coil coated metal) is a ‘metal on which a coating material (e.g. paint, film…) has been applied by coil coating’. When applied onto the metallic substrate, the coating material (in liquid, in paste or powder form) forms a film possessing protective, decorative and/or other specific properties. In 40 years, the European prepainted metal production has multiplied by 18.
Metal The choice of metallic substrate is determined by the dimensional, mechanical and corrosion resistance properties required of the coated product in use. The most common metallic substrates that are organically coated are:
Hot dip galvanised steel (HDG) which consists of a cold reduced steel substrate onto which a layer of zinc is coated via a hot dip process to impart enhanced corrosion properties onto the base steel. Galvanized mild steel (GMS) can be used as balustrade and handrail of staircase, pipe, etc. Other zinc-based alloys are coated onto steel and used as a substrate for coil coating, giving different properties. They give improved corrosion resistance in particular conditions. Electro-galvanised (EG) coated steel consists of a cold reduced substrate onto which a layer of zinc is coated by an electrolytic process. Cold reduced steel (CR) without any zinc coating Wrought aluminium alloys Many other substrates are organically coated: zinc/iron, stainless steel, tinplate, brass, zinc and copper.
Coil coating Coil coating is the continuous and highly automated industrial process for efficiently coating metal coils. Because the metal is treated before it is cut and formed, the entire surface is cleaned and treated, providing tightly-bonded finishes. (Formed parts can have many holes, recessed areas, valleys, and hidden areas that make it difficult to clean and uniformly paint.) Coil-coated metal (often called prepainted metal) is often considered more durable and more corrosion-resistant than most post painted metal. Annually, 4.5 million tons of coil-coated steel and aluminum are produced and shipped in North America, and 5 million tons in Europe. In almost every five-year period since the early 1980s, the growth rate of coil-coated metal has exceeded the growth rates of either steel and/or aluminum production.
Process The definition of a coil coating process according to EN 10169:2010 is a ‘process in which an (organic) coating material is applied on rolled metal strip in a continuous process which includes cleaning, if necessary, and chemical pre-treatment of the metal surface and either one-side or two-side, single or multiple application of (liquid) paints or coating powders which are subsequently cured or/and laminating with permanent plastic films’. The metal substrate (steel or aluminum) is delivered in coil form from the rolling mills. Coil weights vary from 5-6 tons for aluminum and up to about 25 tons for steel. The coil is positioned at the beginning of the line, then unwound at a constant speed, passing through the various pre-treatment and coating processes before being recoiled. Two strip accumulators at the beginning and the end of the line enable the work to be continuous, allowing new coils to be added (and finished coils removed) by a metal stitching process without slowing down or stopping the line.
The coil coating line The continuous process of applying up to three separate coating layers onto one or both sides of a metal strip substrate occurs on a coil coating line. These lines vary greatly in size, with widths from 18 to 60 inches (46 to 152 cm) and speeds from 100 to 700 feet per minute (0.5 to 3.6 m/s); however, all coil-coating lines share the same basic process steps. A typical organic coil coating line consists of decoilers, entry strip accumulator, cleaning, chemical pretreatment, primer coat application, curing, final coat application, curing, exit accumulator and recoilers. The following steps take place on a modern coating line:
Mechanical stitching of the strip to its predecessor Cleaning the strip Power brushing Surface treatment by chemical conversion Drying the strip Application of primer on one or both sides Passage through the first curing oven (between 15 and 60 seconds) Cooling the strip Coating the finish on one or both sides Passage through the second curing oven (between 15 and 60 seconds) Cooling down to room temperature Rewinding of the coated coil
Coatings Available coatings include polyesters, plastisols, polyurethanes, polyvinylidene fluorides (PVDF), epoxies, primers, backing coats and laminate films. For each product, the coating is built up in a number of layers. Primer coatings form the essential link between the pretreatment and the finish coating. Essentially, a primer is required to provide inter-coat adhesion between the pretreatment and the finish coat and is also required to promote corrosion resistance in the total system. The composition of the primer will vary depending on the type of finish coat used. Primers require compatibility with various pretreatments and top coat paint systems; therefore, they usually comprise a mixture of resin systems to achieve this end. Backing coats are applied to the underside of the strip with or without a primer. The coating is generally not as thick as the finish coating used for exterior applications. Backing coats are generally not exposed to corrosive environments and not visible in the end application.
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