The Remarkable Technology of Coil Coating

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The coil coating process (see Diagram 1) provides a way to rapidly apply a variety of paints in a uniform, controlled, continuous and quality fashion at line speeds varying from 100 to 700 feet per minute with the paint being cured in 15-45 seconds to achieve a Peak Metal Temperature of 390 to 480°F depending on the paint and metal thickness. In the coil coating process, the metal coil is first unwound, cleaned and pre-treated, one or more layers of paint are applied on a flat continuous sheet, heat cured, cooled and rewound for shipment. At the fabricator, it is then cut to the desired size and formed into its finished shape.

Since the metal is first painted and then formed into a part, coil coatings are frequently referred to as pre-paint. Pre-paint refers to the process of painting metal before fabrication rather than painting a formed article.  The coated product must not only be able to be cured quickly, it must also be flexible and resistant to pressure mottling as coated coils can weigh several tons.

Primary coil coated metal substrates include:

• Steel
  • Cold rolled and hot rolled steel
  • Galvanized steel (zinc coated steel &#; hot dip and electrogalvanized)
  • Galfan (~95% zinc and 5% aluminum coated steel)
  • Galvalume (43.5% zinc, 55% aluminum-with 1.5% silicon coated steel)
• Aluminum

Compared to most other application methods, coil coating is nearly 100% efficient from a paint usage standpoint. As opposed to a spray-applied coating, a coil-coated product offers uniform film thickness rather than the thicker films on edges, corners and bends that are more typical of spray-applied coatings.

Benefits of coil coating:

• Consistent quality
• High speed
• Nearly 100% efficient, therefore lower cost
• High quality
• Lower environmental impact
• Wide range of metal types
• A wide variety of coating types available
• Flexibility

Topcoats are applied by reverse roll coat in which the applicator roll travels in the reverse direction of the strip and thus provides a smoother film with fewer defects. Primers and backers are normally applied by direct roll coating. Some lines also apply coil coatings using an extruder or via a solid block of paint with a softening point such that it can be applied smoothly when heated.

Coil coating physical types include solvent borne, waterborne, powder and solid block paints.

Schematic 1 &#; Paint layers in a typical exterior surface

Table I &#; Synopsis of Coil Coating Options

The total global market size of coil coatings is in the range of $3.5 billion (USD) with polyester coatings making up greater than 60% of the total. The types of paint curing employed in the coil industry include thermal, infrared, induction and UV cure. By and large, the vast majority of coil coatings are cured using gas-fired ovens. Accordingly, the remaining portions of this article will focus on thermal-cured coil coatings.

Coil primers and backers are normally applied much thinner than spray-applied liquid or powder coatings, dip or electrocoat paints. Applied primer dry film thicknesses are normally in the range of 4-6 microns, whereas topcoats are normally applied to provide a dry film thickness of 18-20 microns.

Special consideration when formulating coil coatings include adjustments in the resin technology, crosslinking, solvent system, catalyst, surface modifiers and lubricants to accommodate proper cure, flow and leveling, and minimization of surface defects. Once cured, coil coatings can offer excellent handling, physical, chemical and environmental properties. Table II details typical materials utilized in a polyester coil coating.

Table II &#; Raw Materials (on Resin Solids) used in a Typical Polyester Coil Topcoat (1.2:1.0 Pigment:Binder, Dupont R960 Titanium Dioxide)

In summary, coil coating technology has grown more rapidly than more conventional application technologies and is projected to grow at a rate in excess of 5% per annum due to a variety of benefits that include cost savings, versatility, environmental, performance and quality issues.

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Numerous finishes redefine what is possible for fabricators and designers

There are various options for applying paint to metal. In the field it is common to see spray or brush applications; in the factory, you can spray, powder coat or coil coat. However, when it comes to the performance requirements of today&#;s metal architecture, products like metal composite materials almost exclusively use the coil coating process to apply finishes that look practically flawless and can last.

Coil coating is the method by which a metal coil is coated in a continuous process prior to fabrication. The metal coil is brought through an uncoiler, a line-loading accumulator, processed and painted according to requirements, brought through a finish line exit accumulator and then rewound on a recoiler. These coils are finished and cured in a continuous process. Though traditionally used for aluminum substrates, this procedure can be applied to other metals, such as steel.

According to the National Coil Coating Association, prepainted metal &#;holds up better over time than post-painted surfaces ...&#; With robust quality assurance programs, coil-coated metal has a clear advantage with regards to consistency. Continuous manufacturing also allows for lower costs and greener manufacturing with reduced waste, increased efficiency, and the collection of VOCs.

Methods of Coating

Application of paint to metal has moved beyond spray systems, which are now more typical in a single-sheet batch process. Continuous coil coating manufacturing utilizes either a roll coating or a die coating method. Reverse roll coating, common in metal manufacturing, applies paint with two rolls running in opposite directions. A metering roll loads and transfers paint to an applicator roll, which then applies the coating to the substrate. This allows for good control over the paint thickness but leaves the panel susceptible to &#;roping&#; marks, striations that form at the edges of the brush resembling a ship&#;s wake.

Die coating is a precision coil coating process that maintains the advantages of high-volume, continuous process coating, but with even greater paint control. Paint is applied directly to the substrate through a slot die extrusion system.

Paint thickness is meticulously controlled in a single, seamless application. Both the paint system and the coil require exacting quality standards in the die coating process. The result is the smoothest coating possible with the added bonus of being more sustainable because of the reduction in waste.

Paint Systems

Paint systems evolved over the years as the industry began to require more from finishes than polyester and urethane paints could provide. The only paint systems capable of meeting the most commonly referenced architectural performance specifications laid out in AAMA are fluoropolymers. The unique chemical composition provides exceptional weatherability and durability.

The first generation of fluoropolymer paint introduced in was polyvinylidene fluoride (PVDF). This paint system requires an additional acrylic resin to formulate PVDF into a coating, with the highest performance dispersion rate at 70 percent PVDF and 30 percent acrylic resin. As the ratio of PDVF to acrylic drops, so does the paint performance. The paint is thermoplastic, melting at high temperatures, however no chemical cross-linking occurs. This makes PVDF susceptible to remelt. PVDF is a relatively low-gloss finish. As measured by a 60-degree gloss meter, it is limited to a 30 to 40 range. Because of the added acrylic, color palettes are limited and bright colors can be a challenge.

The second generation of fluoropolymer paint introduced in is fluoroethylene-vinyl ether (FEVE). This paint system is thermoset, chemically cross-linking the fluorocarbon components so that under high pressure and temperature they will not remelt. FEVE is a clear resin, allowing for brighter and more vibrant finishes and has a smooth surface capable of a gloss rate as high as 70.

Advancements in coil coating and paint systems have enabled a nearly limitless palette of finishes that can last for decades. The introduction of metal composite material (MCM) into the architectural landscape was another revolutionary innovation. MCM is versatile and easy to fabricate into complex forms without the weight or other issues, such as oil canning found in solid plate metal. Over the last 40 years, these advancements have radically redefined what is possible for fabricators and designers.

Jim Moses is the technical services manager at Mitsubishi Chemical Composites America&#;s ALPOLIC MCM division, Chesapeake, Va. He has more than 40 years in the construction manufacturing, engineering and operations field, and is an active member of industry associations.

This article was featured in Metal Construction News.

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