The wood coating industry initially embraced UV-curing technology as a way to increase both productivity and performance, and more recently as a means to save energy and meet environmental regulations. Initially, all applications were based on 100% solids UV-curable products, coating mainly flat panels by roller coater. During the 1990s, UV-curable polyurethane dispersions in water (UV-PUD) were developed, and PVC floor coating was the first industrial application. The flexibility, adhesion and stain resistance of these coatings made this new technology very successful. The low viscosity and the very low VOC of these dispersions also made them very attractive for spray, curtain and vacuum applications onto wood.
Field-applied wood floor coatings, until recently however, have required conventional curing. Polyurethanes are the leading type of field-applied floor coatings. Others include conversion varnishes, oils and waxes. Today, there are several UV-curable wood floor coatings in varying stages of commercial development, including 100% solids and waterborne coatings.
UV coatings have been increasingly used in the concrete tile industry for similar performance reasons as in the wood coating industry. All conventional concrete coatings are multi-component systems, and the two-component epoxy/amine and urethane (isocyanate/polyol) are the most common. These epoxy and urethane coatings require more than one day before return to service. Faster curing systems include polyaspartic and methyl methacrylate coatings, which can be returned to service in hours instead of days. However, the pot life of these fast-curing systems dramatically compromises the open time necessary for proper application, and can result in wasted product and deficiencies in appearance and product performance. The rate of cure of multi-component systems is also limited by temperature, and are often unacceptable for refrigerated end uses or cold weather application. Other disadvantages of conventional concrete coatings include high volatile organic content and odor, and lack of exterior durability, ease of cleaning and abrasion resistance. UV-curable concrete coatings can address many of the shortcomings of these conventional concrete coatings.
UV, From Factory Applied to Field-Applied Coatings
Applying and curing a coating in a factory is a well-controlled process. Moving this process to the field introduces many uncontrolled variables, which means that a robust coating and cure process are needed.
Substrate variations are more common in the field, where floor composition, surface treatment, roughness, porosity and contamination are all potential challenges. The substrate is also larger than the UV cure unit (unlike in the factory).
UV leakage at the sides of the curing equipment can prematurely cure the coating at the edges of the cure path. This may cause changes in the surface appearance in this area. Also, care must be taken to ensure complete cure of all areas. Overlap criteria for the UV-curing unit are typically provided by the coating supplier to assist in achieving complete cure.
Field-applied UV-curable coatings are applied like conventional floor coatings using a roller or T-bar applicator. This application method can result in coating thickness variations.
Finally, the UV-cure unit is mobile and moves over the substrate. The distance from the UV-cure unit to the substrate and the speed of the unit may both vary during the UV-curing process. The equipment for field-applied applications has undergone many modifications and improvements, addressing safety issues or process reproducibility. In general, however, it is the coating formulation that must be robust enough to overcome all of these challenges.
Waterborne UV Field Applied Wood Coatings
Waterborne UV systems are preferred for field-applied wood coatings because the conventional wood coatings are mostly waterborne. This allows the same application method to be used, along with the same drying times and sanding procedures. In addition to being similar to conventional wood coatings, the UV waterborne wood coating dries tack-free to the touch, making possible quick sanding and recoating. After the last wood coating has been applied, the UV cure process gives a fully cured coating that can be put back into service immediately. These properties allow floor coatings to be completed in one day, eliminate many post-coat defects for the contractor, and minimize downtimes for the floor owner.
UV Field-Applied Concrete Coatings
Concrete coatings can be used on floors that are exposed to various and sometimes severe conditions, such as strong chemicals in factories, pickle juice in grocery stores, buggy traffic in retail stores, and hot tires in garages and warehouses. Under even these conditions, the coating is expected to maintain a good appearance and performance. Ideally the coating should also not yellow during cure or use. These properties are needed in both clear and pigmented concrete floor coatings. Hot tire pickup resistance is important for garage and warehouse applications. Coatings without this resistance are softened by the heat and water from tires on automobiles or forklifts. Then under pressure from the weight of the vehicles, the plasticizers in the tires bond the tire to the coating. The end result is coating removal, and/or black tire marks on the coating. Coatings with higher crosslink density tend to perform better in these applications than coatings that are less cross linked. Coatings tests demonstrate that tires do not stick to the coatings, and can be removed remarkably easily. The dry tire leaves no marks.
Zippering in UV Field-Applied Concrete Coatings
One of the drawbacks of using UV-curable field-applied coatings for concrete is an appearance aberration called “zippering”. This aberration has the appearance of a zipper. This zippering is due to the UV light leakage from the edges of the UV-curing equipment, and to the differential in shrinkage of the UV-curable coating at the surface and in the depth. When the UV light leaks from the edges of the equipment, it prematurely cures the surface of the coating at the edges. Upon fully curing this section of the coating, the surface and the depth of the coating have differences in shrinkage, and the zippering occurs.
There are several approaches to minimize or eliminate zippering. Equipment choice is very important, and it is important to test the coating with the equipment that will be used in the field. Equipment manufacturers have made recent improvements, including the use of flexible shielding to block light leakage. Proprietary improvements in lamp design have also been made, and have shown better performance. Coating properties such as thickness, viscosity and shrinkage affect the formation of zippers, and should be controlled. The choice of monomers, oligomers, photoinitiators and additives all impact these coating properties, and should also be controlled. Pigmented systems can be even more challenging, because the photoinitiator choice is impacted by the UV absorbance of the pigments.
The photoinitiator package plays a major role in eliminating zippering. However, not only zippering must be considered when designing the photoinitiator package. Other performance parameters affected by photoinitiator choice are surface cure so that there are no wheel marks, depth of cure so that adhesion and performance are adequate, the number of times that an overlap cure can occur without affecting intercoat adhesion, and the open time of the uncured coating.
A totally different approach to eliminating the zipper effect is the use of texturing to hide/prevent the zippers. Fillers, particles, etc. that are transparent to UV light can be added to the coating to provide a textured surface, which does not show zippering. Aluminum oxide and sand are two examples of fillers that can be used. These fillers can also be used to adjust the coefficient of friction (COF) of the coating.
"The commercial use of UV-curable field-applied floor coatings continues to grow, but has been slowed by some performance deficiencies," says David Helsby. Yet, recent product and formulation developments have successfully addressed some of these deficiencies in wood and concrete coatings. RadTech's bi-annual congress is the ideal platform to learn about what innovative solutions UV and EB curing radiation has in store, and this for a many more industrial markets and applications.
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