UV Coatings: A Cure for Wood Alternatives
Wood will probably always be the material of choice for many construction applications, such as decking, siding, fencing, railings and window and door trim. But alternative materials like wood plastic composites, cement fiber and vinyl substrates have made considerable inroads in the past decades. In the field, each has its advantages over wood; however, some also are susceptible to the same deterioration caused by weathering and use patterns that plague wood. Manufacturers, along with academic and industry research facilities, continue to search for ways to produce even better value-added products at a lower cost. They are being cheered on by companies in the distribution and retail business, who constantly seek improved product offerings.
One of the options being examined to meet the challenges for all three materials is the application of UV-cured coatings. UV-cured coating systems are factory-friendly, as they emit no volatile organic compounds (VOCs). They also are nonflammable, produce no hazardous waste and replace other types of coating systems that require far more floor space. In the laboratories and on the test benches of today’s coatings professionals, as well as in materials development labs, new UV-cure coatings and application techniques are being perfected that might dramatically enhance the field performance of modern wood alternatives.
Wood Plastic CompositesHomeowners love their decks, as do hotel, restaurant and office building developers who add decks for function space and eye appeal. As a result, decking is considered a $4.6 billion industry, and decking material demand is expected to reach 3.6 billion lineal feet in four years.
Virtually every deck at one time was constructed of wood, but wood decks must be refinished or replaced regularly. Wood plastic composites (WPC) became extremely popular in the ’90s as an alternative decking and railing material, and annual sales have continued to increase for both the new construction and the remodeling market.
WPC is produced by creating a blend of plastic, wood flour and/or certain fibers, which is extruded into profiles with many shapes, sizes and lengths, including dimension lumber-type boards. These profiles can be sawed, nailed, screwed and otherwise worked like lumber, with the inherent strength to be used for applications such as decks and fencing. Today, the composite decking and railing market is more than $1 billion annually. While WPC products are more durable than wood, they tend to exhibit poor fade, scratch, stain, abrasion and mold/mildew resistance over time. WPC decks also can retain heat during hot weather.
Several new approaches are being tested that incorporate the application of a UV-curable coating that will adhere to the surface of the WPC. This coating, available in virtually any color and gloss level, provides long-term resistance to fading, increases scratch, stain, abrasion and mar resistance, and prevents mold and mildew growth. It is also available in a clearcoat for application over composites containing pigments.
Historically, extruded WPC boards could not be painted. During the extrusion process, the thermoplastic in the composite rises to the surface, preventing any coating from adhering. As a result, WPC boards are produced to closely replicate the appearance of wood by adding pigments to the composite mixture before extrusion. New UV coating approaches being developed incorporate specialized formulation practices to produce a coating that will adhere to the composite. Depending on the formulation of the composite, a pretreatment step such as fluoroxidation, plasma, corona or flame might be needed. The coating can be applied by vacuum, roll or spray techniques and then cured instantly with UV lamps so that it can be handled immediately. There is no need for any forced-air drying equipment or production floor space to dry the boards.
Not only are these coatings highly durable and moisture-resistant, but their broad color palette also offers architects, designers and builders a range of creative options. Also, as the years pass, decks produced with this process can be repainted a totally different color if the home or building owner desires.
An innovative pretreatment option* gaining a great deal of attention is to feed the extruded composite through a chamber, where it is exposed to a reactive gas atmosphere that oxidizes the WPCs surface so it will accept the coating (see Figure 1). The board next is fed through a UV coating chamber, where a patent-pending UV coating can be applied to some or all sides of the board in a single pass, depending on the preference of the manufacturer. It then is cured instantly with UV lamps.
This system has the potential to coat WPC at 100 feet per minute (fpm) and can be installed inline with the extrusion line so that boards can be treated and coated continuously. The manufacturer also has the option of locating the system offline or in an entirely different location to allow for the inventory of extruded boards in several locations, which then can be treated and coated as customer orders arrive. This latter option could be increasingly important as the market becomes accustomed to the ability to order WPC boards in different colors. Each order can be quickly treated, coated with the desired color and shipped.
The process is environmentally friendly. The pretreatment uses small amounts of reactive gas at any given time and is conducted at less than atmospheric pressure, eliminating chemical emissions. All process gases are completely neutralized. The coating is a 100% solids coating containing no solvents; as a result, it contains no flammable elements, or volatile organic compounds (VOCs) or hazardous air pollutants (HAPs) to be emitted into the atmosphere.
Fiber Cement SidingCement siding has been used in construction since the early 1900s, but new design and production technology developed during the past decade or so has enabled manufacturers to introduce high-performance siding to meet the more stringent performance and creative demands of the construction business.
Fiber cement siding is a durable, water-, termite- and flame-resistant siding product that is produced by blending cement, sand and cellulose fibers and extruding it into siding boards, which are cured with pressurized steam to enhance their strength and stability. The cellulose fibers prevent cracking. The boards can be embossed with wood patterns or other designs.
Fiber cement siding can be painted without any pretreatment to alter the surface structure. A two-coat process is used, with a primer applied first, followed by a coating to add the desired color. In most instances, the primer is applied at the factory. Depending on the desire of the architect or contractor, the primed board can be shipped to the construction site where the builder paints it to match the desired color scheme of the building being completed. By purchasing primed siding, the contractor is assured that the siding will not absorb moisture while awaiting installation.
Occasionally a manufacturer might ship uncoated siding for priming and coating on-site. On-site coating requires a third step - washing the board before the primer is applied to clean away any dirt and debris that adhered during shipment or on-site storage. Following cleaning, the board must be allowed to dry completely before the primer is applied
Some manufacturers might offer precoated siding in a range of designs and colors and will apply both the primer and the topcoat in the factory.
A patent-pending, UV-cured coating system is available that will allow manufacturers to adopt a one-step coating system regardless of whether they are producing primed only or color-coated siding. For producers applying a color coating in the factory, UV coating systems eliminate the need to apply primer first. The UV topcoats, available in any desirable color, adhere directly to the board without the need for a primer. If a manufacturer wishes to apply primer only before shipping, a UV-cured primer is available that will still accept the topcoat in the field. Not only is the UV coating highly durable and weather-resistant, but it also can be repainted with a different color in future years if the home or building owner chooses.
When the UV coating approach is adopted, the factory is able to dispense with its previous primer and paint application systems, reducing production, materials and labor costs, as well as freeing up floor space. If the UV system is replacing a spray system using solventborne coatings, VOC emissions are immediately eliminated, and indoor air quality is improved. UV-cured boards can be handled immediately for fast shipment of inventory.
Vinyl SubstratesVinyl siding, fencing, decking and trim products, first introduced decades ago, have continued to grow in popularity among developers, builders and homeowners. Today, vinyl siding is used on a large number of new homes and is the most popular siding in the replacement market.
The initial material cost of vinyl compared to wood is slightly higher in most regions of the country, but the increasing cost of wood is quickly narrowing the gap. And when the lifetime cost of virtually maintenance-free vinyl is compared to the cost of maintaining and/or replacing wood, then vinyl becomes a highly competitive option.
Vinyl for construction materials is produced from a compound consisting of polyvinyl chloride (PVC) resins plus a variety of stabilizers, coloring and processing aids, UV inhibitors, and plasticizers. This compound is fed through an extruder, where it is exposed to heat and pressure to become pliable vinyl. The vinyl is then fed through a die in the shape of the finished product. Vacuum technology is used to keep the vinyl in the proper shape, and water is used to cool it into its final rigid state.
Some producers create vinyl with a single extrusion process, in which, the raw materials are spread throughout the product. More recently, some producers have adopted a co-extrusion process that creates a finished product with two layers of PVC. The outer layer contains a denser concentration of the essential elements, such as the UV inhibitors. The inner layer is essentially the same as the outer layer but with a lowered concentration of UV inhibitors and color pigment. This vinyl is highly resistant to weather and abrasion while having a long lifespan. Like WPC, colors and hues have historically been created by adding pigments during the extrusion process.
A patent-pending UV-curable coating system has been developed that allows vinyl producers to eliminate the coextrusion process and feed the extrusion directly into a UV coating chamber. Here, a coating in virtually any color can be applied. The cured coating is highly resistant to weather, temperature, sunlight and abrasion.
If the producers prefer to provide color by including pigment in the initial extrusion material, a clear UV coating can be applied to provide the necessary protection.
Ultimately, whether wood, WPC, fiber cement or vinyl is used at a construction site is the decision of the architect, the contractor or the building owner. Regardless of the material, high-quality appearance, color and field performance are becoming the norm thanks to modern UV coating technologies.
For more information, visit www.finishesunlimited.com. More information about InFin Coating Technology can be found online at www.paintablecomposites.com.
*InFin™ Coating Technology, jointly developed by Inhance/Fluoro-Seal Ltd. and Finishes Unlimited.