UV-curable technology is one of the fastest growing markets in the paint and coating industry. In 2008, a market analysis for the UV coating technology anticipated a world growth between 8% and 13% on average over the next five to seven years [1
]. In 2011, UV/EB-formulated product usage in North America was 120,000 metric tons. For wood finishes only, UV/EB product usage went from 14,900 metric tons in 2001 to 23,200 metric tons in 2011 [2
]. In 2013, according to the Radtech biennial market survey [3
], the percentage of UV/EB formulated product usage by volume was 19% for wood, which represents an annual growth rate of 6.3% for wood stains/sealers and 5.8% for flooring.
The reasons for the rapid and steady growth of UV-curable coatings are numerous. The most commonly cited are the following: low VOC emissions, excellent mechanical and chemical resistance, and fast curing/drying. Currently, most UV systems for flat line wood coating operations are Hg arc lamps and are operated at either 200 or 300 W/inch [4
]. However, lamp requirements are closely related to the formulation to be cured and its reactivity. The latter depends on two main factors: the type of acrylate used and their functionality. Cure speed is higher for epoxy acrylate followed by polyester acrylate, urethane acrylate, and finally unsaturated polyester acrylate [5
]. A high concentration of monofunctional monomer used as thinner strongly reduces the coating cure speed. At the opposite, highly functionalized monomers increase coating cure speed and increase cured film resistance to abrasion. Recent work has led to advances in improved outdoor weathering, adhesion increase, faster cure speeds, and the use of UV-Light emitting diodes (UV-LEDs) [3
]. Replacement of conventional mercury or gallium arc lamps by UV-LEDs has intensified. According to Radtech, the main motivations for users to switch for UV-LED technology are their suitability for heat-sensitives substrates, their energy efficiency, LED lifetime, and their instant on/off capability. UV-LEDs also present significant environmental benefits (ozone free, workplace safety, UV-A wavelength range) as well as advanced capabilities (through cure, compact equipment, controlled curing intensity) [6
]. All this put together makes it an environmentally friendly technology which has the potential to significantly reduce the carbon footprint of the UV-curing technology. UV-LEDs have been commercially available for over 10 years but with recent developments, i.e.
, increased energy output and lower initial cost, they have become commercially viable for several industries, including the wood coating one.
Light-emitting diodes (LEDs) are semiconductor light sources. When the excited electrons relax, they emit energy in the form of photons. The wavelength of the emitted photons depends on the material used for the construction of the diode. The output of the LEDs is one very narrow band (± 10 nm) where 96% of the energy is emitted. At the opposite, conventional arc lamps present several peaks distributed throughout the UV spectrum (UVA, UVB, UVC, UVV). This explains why UV-LEDs are 60%–80% more efficient than conventional mercury lamps at a given wavelength [7
]. Moreover, UV-LEDs do not emit infrared energy (700 nm to 1 mm), which is the contributor to the heat buildup, since the energy of the UV-LED is concentrated in a narrow region. As stated previously, changing to UV-LEDs also reduces environmental impact. Indeed, each UV-LED lamp consumes on average 50% less energy than an equivalent arc lamp which could be explained in part by an instant on/off (no warm-up time) and no unnecessary wavelength [8
]. In addition, UV-LEDs do not contain harmful compounds such as mercury and do not produce ozone or any other harmful gases. Their consumption power is much lower than the one of UV-mercury lamps and neons and their service life is significantly longer; UV LED lamps last over 20,000–50,000 h of run time as compared to a mercury lamp running ~2000 h total [6
]. This technology also offers energy and cost saving due to the performance of the lamp and the system; lower surface temperature, smaller lamp, and less maintenance and downtime which lead to a higher productivity rate. According to the literature, with the integration of this technology in a coating line, it is possible to achieve energy savings on the order of 30% and even more when a UV-LED can replace several conventional UV arc lamps [9
All this said, UV-LED technology still presents some major drawbacks. Still according to Radtech [3
], the main factors that may be limiting the use of UV-LED are, (1) the lack of suitable curable materials; (2) high investment costs; (3) technical limitations of existing equipment (output, cooling, size, wavelength). Suitable curable materials should be more easily available in the next few years as photoinitiators adapted to UV LED technology [10
] are currently under development. Purchase price of UV LEDs is still higher than the one of conventional lamps although the development of the technology and the increased market shares are both reversing this trend. As for the technical limitations, UV-LEDs are sensitive to heat and the use of high power diodes often requires the addition of a cooling circuit. Their power is still low compared with conventional lamps and as a result UV-LEDs’ coating polymerization is slower than with traditional UV lamps. Lamp manufacturers are working on the development of more powerful lamps with should improve curing shortly.
UV-curing has always been a technology of choice for wood flooring products [4
]. However, traditional UV lamps still emit little heat which can be problematic for products that are packaged immediately after the coating operations, such as wood flooring, as wood is a heat sensitive material and is prone to cracking/splitting. Since UV-LEDs emit no infrared energy, recent studies showed that they provide significant advantages compared to conventional UV-curing method for wood coating applications [8
]. The purpose of our work was to compare the mechanical properties of UV-curable high solids and UV-curable water-based coatings formulated for wood flooring after UV-LED curing and UV mercury curing (mercury lamps microwave). A technological evaluation was also carried out on the heat emissions for the two technologies.
In this study, the UV-LED curing efficiency and mechanical properties of UV water-based formulations were evaluated. For comparison, some formulations were cured with a UV-mercury lamp. The irradiation of the UV-mercury lamp can be 5 to 12 times higher than the one of UV-LED lamp depending on the position of the lamp and the conveyor speed. Thereby, the surface temperature of the flooring strips obtained with UV-LED lamp is an average of 12 °C lower than when a UV-mercury lamp is used.
Preliminary tests showed that the intensity of the UV-LED lamp is not high enough to cure UV high-solids coatings at a conveyor speed of 1.5 m/min, which is a conveyor speed significantly lower than for a standard UV-curing flooring line. Uncured monomer residues increase the VOC emissions. However, the UV-LED lamp possesses sufficient intensity to cure clear UV water-based coatings. The main reason is that UV high-solids are in liquid form before curing compare to the UV water-based which, once the water is evaporated, there is coalescence resulting in fewer chemical links to be created. Water-based coatings cured under UV-LED lamps have a coating hardness and an abrasion resistance lower than the coating cured under the UV-Mercury lamps. There is still some work to do in order to bring the UV-LED technology to the same level of UV-mercury curing such as developing photoinitiators that are best suited for this type of technology, i.e., clear after curing. The development of UV-LED lamps with higher irradiation is also necessary.