A primer on UV-cured coatings - Woodshop News

A primer on UV-cured coatings

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As you are no doubt aware, a primary goal of the coatings industry during the last several decades has been to reduce the amount of solvents released to the atmosphere. These are called VOCs (volatile organic compounds) and, effectively, they include all the solvents we use except acetone, which has a very low photochemical reactivity and has been exempted as a VOC solvent.

But what if we could eliminate solvents altogether and still get good protective and decorative results with a minimum of effort?

That would be great — and we can. The technology that makes this possible is called UV curing. It has been in use since the 1970s for all sorts of materials including metal, plastic, glass, paper and, increasingly, for wood.

UV-cured coatings cure when exposed to ultraviolet light in the nanometer range at the low end or just below visible light. Their advantages include significant reduction or complete elimination of VOCs, less waste, less floor space required, immediate handling and stacking (so no need for drying racks), reduced labor costs and faster production rates.

The two important disadvantages are high initial cost for the equipment and difficulty finishing complex 3-D objects. So getting into UV curing is usually limited to larger shops making fairly flat objects such as doors, paneling, flooring, trim and ready-to-assemble parts.

The easiest way to understand UV-cured finishes is to compare them with the common catalyzed finishes with which you are probably familiar. As with catalyzed finishes, UV-cured finishes contain a resin to achieve build, a solvent or substitute for thinning, a catalyst to initiate the crosslinking and bring about the curing and some additives such as flatting agents to provide special characteristics.

A number of primary resins are used, including derivatives of epoxy, urethane, acrylic and polyester.

In all cases these resins cure very hard and are solvent- and scratch-resistant, similar to catalyzed (conversion) varnish. This makes invisible repairs difficult if the cured film should get damaged.

UV-cured finishes can be 100 percent solids in liquid form. That is, the thickness of what is deposited on the wood is the same as the thickness of the cured coating. There’s nothing to evaporate. But the primary resin is too thick for easy application. So manufacturers add smaller reactive molecules to reduce the viscosity. Unlike solvents, which evaporate, these added molecules crosslink with the larger resin molecules to form the film.

Solvents or water can also be added as thinners when a thinner film build is desired, for example, for a sealer coat. But they aren’t usually needed to make the finish sprayable. When solvents or water are added, they must be allowed, or made (in an oven), to evaporate before the UV curing begins.

The catalyst

Unlike catalyzed varnish, which begins curing when the catalyst is added, the catalyst in a UV-cured finish, called a “photoinitiator,” doesn’t do anything until it is exposed to the energy of UV light. Then it starts a quick chain reaction that links all the molecules in the coating together to form the film.

This process is what makes UV-cured finishes so unique. There is essentially no shelf- or pot life for the finish. It remains in liquid form until it is exposed to UV light. Then it cures totally within a few seconds. Keep in mind that sunlight can set off the curing, so it’s important to avoid this type of exposure.

It might be easier to think of the catalyst for UV coatings as two parts rather than one. There’s the photoinitiator already in the finish — around 5 percent of the liquid — and there’s the energy of the UV light that sets it off. Without both, nothing happens.

This unique characteristic makes it possible to reclaim overspray outside the range of the UV light and use the finish again. So waste can be almost totally eliminated.

The traditional UV light is a mercury-vapor bulb together with an elliptical reflector to collect and direct the light onto the part. The idea is to focus the light for the maximum effect in setting off the photoinitiator.

In the last decade or so LEDs (light-emitting diodes) have begun replacing the traditional bulbs because LEDs use less electricity, last much longer, don’t have to warm up and have a narrow wavelength range so they don’t create nearly as much problem-causing heat. This heat can liquify resins in the wood, such as in pine, and the heat has to be exhausted.

The curing process is the same, however. Everything is “line of sight.” The finish only cures if the UV light strikes it from a fixed distance. Areas in shadows or out of the light’s focus don’t cure. This is an important limitation of UV curing at the present time.

To cure the coating on any complex object, even something as nearly flat as a profiled molding, the lights must be arranged so they strike every surface at the same fixed distance to match the formulation of the coating. This is the reason that flat objects form the great majority of projects that are coated with a UV-cured finish.

The two common arrangements for UV-coating application and curing are flat line and chamber.

With flat line, the flat or nearly flat objects move down a conveyor under a spray or roller or through a vacuum chamber, then through an oven if necessary to remove solvents or water and finally under an array of UV lamps to bring about the cure. The objects can then be immediately stacked.

In chambers, the objects are usually hung and moved along a conveyor through the same steps. A chamber makes possible the finishing of all sides at once and the finishing of non-complex, three-dimensional objects.

Another possibility is to use a robot to rotate the object in front of UV lamps or hold a UV lamp and move the object around it.

Suppliers play key role

With UV-cured coatings and equipment, it’s even more important to work with the suppliers than with catalyzed varnishes. The main reason is the number of variables that must be coordinated. These include the wavelength of the bulbs or LEDs and their distance from the objects, the formulating of the coating and the line speed if you are using a finishing line.

If you are interested in exploring UV curing in greater depth, here are three good sources:

• RadTech International (www.radtech.org) is the trade association for UV and electronic beam technology

• Lawrence C. Van Iseghem, president of Van Technologies in Duluth, Minn., has written a great introductory article, “Wood Finishing with UV Curable Coatings,” published in the RadTech Report. It’s available online by searching for the title.

• The State University of New York has archived more than a dozen informative webinars dealing with UV-LED curing at www.esf.edu/outreach/uvebwebinar.

This article originally appeared in the September 2014 issue.

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