Manufacturing efficiency increases by reducing the time, materials, and energy to produce goods at the lowest cost possible (Baumann, 2022). One of the primary goals of a manufacturer is to continually speed up production and increase efficiencies, especially in their painting process. For example, when painting a guitar, the typical process might involve painting it, baking it for 20 minutes, allowing it to cool down, then moving it to the next stage. The entire procedure could take days before the finished instrument is fully cured and ready to ship. Increasing efficiency and throughput has become crucial when it comes to reducing the overall painting time. Ultraviolet (UV) light may be the answer to solve this efficiency problem by decreasing the cure time and enabling manufacturers to get their products to market faster. But what exactly is UV light?
In addition to radio and television signals, visible light, ultraviolet rays, and X-rays are all part of the electromagnetic spectrum. The range, measured in nanometers, has longer wavelengths toward the infrared and shorter wavelengths toward the ultraviolet. The naked eye can only see visible light between approximately 400 and 700 nanometers. The part of the spectrum used for UV curing ranges from 40-400 nanometers (Khutpale, 2022), with most coatings designed to cure within the 200-400 nanometer range.
DoctorUV.Com (n.d.) explains that the ultraviolet spectrum has four distinct ranges. The various ranges provide unique properties to the finished coating:
- UVV 395 – 450 nanometers. UV in this wavelength provides the depth of cure.
- UVA 320 – 390 nanometers. UV coatings generally respond to the UVA range more than the others and provide the vast majority of the energy needed for cure.
- UVB 280 – 320 nanometers. UV in this wavelength provides the coating toughness.
- UVC 200 – 280 nanometers. UV in this range provides surface hardness and scratch resistance.
How Fast Is UV Cure?
Visible light travels 186,000 miles a second, and UV light is no different. In a UV light curing process, parts are transported down a conveyor line, painted, and subjected to UV light. This exposure to UV light produces an instantly cured coating. A typical UV conveyor line runs at speeds of approximately 600 feet per minute or almost seven miles per hour. The output speed is staggering compared to conventional production paint lines, which can run between five and ten feet per minute.
Where is UV Used?
UV-cured products are more prevalent than you may realize. Expecting to reach 5.7 billion dollars by 2031, the global market for UV-curable products is expanding to almost every market that uses coatings (MarketWatch, 2022). Due to its unique process characteristics, virtually any substrate can be UV-coated (Shukla et al., 2004). Potato chip packages, magazines, and cereal boxes all use UV coatings. In addition to these paper products, other products include guitars, flooring, steel pipe, cell phones, and electronic circuit boards. Plastic, metal, leather, and glass are all painted with UV coatings (Shukla et al., 2004).
According to the American Dental Association (2021), the use of UV curing has revolutionized the field of dentistry with its’ speed and efficiency. The automotive industry has found that using UV coatings can significantly improve processing throughput when coating; headlamp lenses, lighting reflector housings, acrylic tail lenses, under-hood components, and wheels (Heraeus, n.d.). Why are so many of these everyday items UV cured? Because curing with UV light is fast. Extremely fast. Typical cure times for UV-cured coatings take seconds compared to minutes or hours. So why not cure all coatings with UV light?
UV Coating Composition
UV coatings are unique materials specifically designed to react within the UV spectral range. They are different from other coatings because they use light rather than heat energy to cure. The recipe, or list of ingredients, is unique in making UV coatings. According to Shaw (2013), the four primary raw material groups in a typical UV coating are:
Oligomers typically give the UV coating most of its basic properties. Generally, they are thick resins, similar to molasses, and are available in various types depending on their end use. Shaw (2013) briefly describes the most used acrylated oligomers:
- Aliphatic Urethane – flexible and suitable exterior weatherability
- Aromatic Urethane – flexible, lower cost, poor exterior weatherability
- Epoxy – fast curing, chemical resistant, workhorse
- Polyester- good adhesion and low viscosity
- Silicone – good heat resistance and flexibility
Monomers, especially monofunctional ones, are commonly used to lower the viscosity of the coating. Without them, the finished paint would be much too thick to use. Multifunctional monomers can also help reduce viscosity, but their most significant contribution is to make the final coating harder, more durable, and more chemically resistant (Radtech, 1995).
Photoinitiators are extremely important to the performance of a UV coating. Photoinitiators absorb UV light at specific wavelengths to start the polymerization of the oligomer and monomer (Shaw, 2013). Polymerization, or crosslinking, causes the coating to cure like a conventional baked coating (Shukla et al., 2004) but without the added heat and energy costs associated with an industrial oven. The types and concentrations of various photoinitiators strongly influence the rate of cure and cured film properties.
Performance and appearance improvement are the primary uses for coating additives. Some of the most common additives are added to lower gloss, decrease foam, improve adhesion, or make the surface slicker. They are generally in the formulations at low concentrations, but even a little can significantly affect the overall quality of the coating.
- Curing speed is very fast, resulting in high production speeds and shorter production lines.
- Compliant technology. UV cure coatings are typically 100% solids and do not contain solvents that can harm the environment. Without solvents, there are no flammability concerns.
- In addition to being durable, UV coatings also resist chemicals, abrasion, and marring.
- Due to crosslinking, UV coatings have good early corrosion and humidity resistance compared to ambient or low-temperature cure systems.
- Due to the UV cure mechanism, energy is invested only in the curing reaction, not in the heating of the part.
- Like the cost of a Ferrari compared to a pickup truck, the improvement in production speed comes at a price. Raw material costs for UV coatings are typically three or four times higher than most conventional solvent or water-based products.
- For optimal performance, the substrate should be clean, dry, dirt-free, and dust-free.
- Worker safety. There are safety concerns associated with high-intensity UV light. Injuries can occur even after a few minutes of unprotected work.
- In the absence of UV light, a UV coating will not dry. Production halts if bulbs blow or equipment malfunctions.
The improved thruput is worth the extra cost for most companies using UV coatings, but there is more to this technology than faster production speeds. Most UV coatings are solvent-free and better for the environment when compared to solvent and many water-based coatings. Inherent UV coating properties include good corrosion and humidity performance and superior abrasion, mar, and chemical resistance (Shaw, 2013). For any organization that applies industrial coatings, improving production efficiency is well worth investigating UV coatings, especially if they need to improve manufacturing efficiency.
American Dental Association. (2021). Dental curing lights. ADA. https://www.ada.org/resources/research/science-and-research-institute/oral-health-topics/dental-curing-lights
Baumann, A. (2022). Manufacturing efficiency: How to measure and improve production. Amper. https://blog.amper.xyz/manufacturing-efficiency
DoctorUV.Com. (n.d.). The ABC’s of UV. http://www.doctoruv.com/documents/pages/What%20is%20the%20difference%20between%20UV%20ABC.pdf
Heraeus. (n.d.). UV curing in the automotive industry. https://www.heraeus.com/en/hng/industries_and_applications/uv_technology/uv_curing_in_the_automotive_industry.html
Khutpale, C. (2022). Exploitation of UV curing coating technology for OEM industries. Paintindia, 72(4), 68–70.
MarketWatch. (2022). UV curable resins & formulated products market size, share, industry outlook, growth, trends, opportunity, forecast 2022-2031. https://www.marketwatch.com/press-release/uv-curable-resins-formulated-products-market-size-share-industry-outlook-growth-trends-opportunity-forecast-2022-2031-2022-09-06
Radtech. (1995). UV/EB curing primer (4th ed.). RadTech International North America.
Shaw, J. G. (2013). Advantages of UV curing in composite manufacturing [Conference session]. uv.eb WEST 2013, Redondo Beach, CA, United States. https://fdocuments.net/document/advantages-of-uv-curing-in-composite-manufacturing-of-uv-curing-in-advantages.html?page=1
Shukla, V., Bajpai, M., Singh, D. K., Singh, M., & Shukla, R. (2004). Overview of basic chemistry of UV-curing technology. Pigment & Resin Technology, 33(5), 272–279. https://doi.org/10.1108/03699420410560461