Above: Metal coil coating is the logical next step for radiation curing. PPG is among coatings suppliers ready to invest in it.
December, 2023- Radiation-curable coatings boost performance, extend life cycle of downstream metal products
Industrial coatings made with advanced polymers, pigments and additives perform a variety of important functions, including UV protection, chemical and corrosion resistance, and durability that can extend the life cycle of a product. When it comes to performance, the quality of the coating formulation is only half of the equation. Coatings have to be cured properly, with strict adherence to the specifications provided by the coatings manufacturer. Curing, or cross linking, is the chemical process that enables a coating to harden and bond to a substrate. A good curing process is essential for chemical resistance, color, resiliency and other mechanical and performance properties. Curing methods vary based on the specific chemistry of the protective coating.
Conventional solvent-borne, waterborne or powder coatings typically cure with air or heat, but alternative solutions use radiant energy sources to initiate the cross-linking process that transforms a coating into a solid, protective finish in mere seconds.
Radiation curable (rad-cure) coatings were introduced in the wood products industry more than 50 years ago.
Due to innovations in formulations, processes and equipment, these high-technology solutions are widely used on substrates such as vinyl, composites and even pre-painted metal; this last application is an emerging frontier for rad-cure solutions.
WHAT IS RADIATION CURING?
The two primary types of coatings in this class use ultraviolet or electron beam (EB) radiation to cure, each with its own performance attributes. UV-curable formulations expose the coating to light, initiating photochemical reactions that trigger rapid polymerization of materials in the coating with little to no volatile organic emissions.
In terms of design flexibility— with modifications to formulations, processes and curing equipment— all sheen levels can be achieved, from deep matte to very high glosses with smooth leveling
EB formulations use high-energy electrons that interact with the coating material to cure. The technology offers excellent adhesion and requires no heat source to cure.
The application of UV light from lamps or EB radiation from special EB devices converts liquid paints to solids that fully harden and bond with the substrate in seconds, or even less. This, in turn, results in a hard, durable finish.
Unlike thermal curing, radiation curing is performed at or near ambient temperature using photons and electrons to polymerize materials onto a substrate. Often called cold curing, this method is attractive to manufacturers that use wood, plastic and other heat-sensitive materials because these substrates can be damaged when exposed to higher temperatures required for rapid curing of most conventional coatings.
UV and EB solutions can be used on various substrates that can be rolled, such as paper, PVC foils or coils, because curing is immediate and can be applied with a variety of methods, including roller coating, spraying and vacuum coating
. The advantages of UV- and EB-curable coatings—including their performance, sustainability and efficiency benefits—are driving rapid adoption of these solutions for a growing range of applications. Recent advancements in the technology are making it available for a wider range of industries, including metal coil coating applications.
SUSTAINABILITY
One factor driving global growth in UV/ EB cure coatings is rising interest in more sustainable products, as well as energy and manufacturing efficiency.
Most conventional liquid coatings rely on solvents to dissolve or disperse the ingredients such as pigments, resins and additives. Rad-cure solutions allow manufacturers to use 100 percent solid solutions that contain no solvents, which leads to low- to no-VOC emissions and lower flammability risks.
UV/EB cure coatings are also considered to be more energy efficient because curing is nearly instantaneous, and less energy is needed for drying from UV lamps or electron beams compared to conventional thermal drying ovens.
With increasing regulatory oversight of VOC emissions and efforts to lower energy consumption, radiation curing technology is poised to meet current and future environmental challenges and to grow in prominence in the coil coatings industry.
ADVANTAGES
FINE FINISH
Benefits of ultraviolet and electron beam curable coatings include:
• Fast curing in seconds
• No loss of coating thickness during curing
• Reduced emissions and waste
• Streamlined finishing processes with reduced downtime and quicker handling
• Enhanced durability with excellent hardness, abrasion- and chemical-resistance
• Great aesthetics with clear, high gloss finishes
• Reduced flammability risks
• Reduced energy consumption with minimal to no VOC
TIME AND EFFICIENCY
Productivity, and ways to improve turnaround time and cost efficiency, are keys to success in manufacturing. The finishing process can play a significant role in these efforts. Conventional liquid coatings require time and temperature to cure. Typically, lower temperatures require more time to dry and cure the paint.
While the level of light or energy needed to cure UV/EB coatings varies depending on film thickness and chemistry type, these technologies contribute to higher productivity from faster finishing since the curing process does not require the evaporation of water or solvents. Another advantage is that no thermal ovens are needed for curing since the polymerization reaction takes place at room temperature, which can produce cost and time savings.
These alternative technologies cure in seconds, with immediate conversion rates that are very high (more than 90 percent). Parts can also be handled almost immediately for packing, storage and shipping, which reduces the need for floor space to hold inventory at various stages of the finishing process.
In multilayer systems, it is also sometimes possible to consider wet-on-wet application in order to further reduce the length of the line and increase productivity. With the electron beam solutions, a single cure is possible with up to three different layers of wet-on-wet paint of 80 microns each. With EB curing, paint thickness and opaque film have a negligible impact on curing as electron beams are powerful enough to penetrate the film. Curing can even be done at high speed.
In contrast to EB cure solutions, UV curing for wet-on-wet application can be more challenging when the paint is not clear or transparent as it is more difficult for light to penetrate the paint.
BONDING STRENGTH
Radiation curable coatings deliver on protective performance by offering exceptional hardness, high scratch and chemical resistance and high flexibility.
These formulations are often mistakenly believed to be brittle, but that is no longer the case. In the technology’s early years, these solutions were ideal for flat surfaces that didn’t require “flexible” paint. The latest rad-cure coatings meet the requirements for PVC rolls, for instance, that call for good bonding strength and resistance to twisting and bending.
Because UV/EB coatings are made with 100 percent solids and do not contain water or solvent, these solutions maintain the same film thickness before and after the cure, with no loss in weight. This is because everything happens during the nearly instantaneous polymerization process, representing a key advantage of these next-generation technologies.
DESIGN FLEXIBILITY
In terms of design flexibility—with modifications to formulations, processes and curing equipment— all sheen levels can be achieved, from deep matte to very high glosses with smooth leveling. This type of cure is also suitable for colors and digital prints. While both technologies offer distinct advantages, the decision between UV and EB curable coatings comes down to a variety of factors, including initial equipment start-up costs, application requirements and substrates.
EXPANDING APPLICATIONS
While wood manufacturing remains one of the primary industries for UV/EB cure technologies, research and development efforts are making these cutting-edge coatings available for a broader range of applications and industries, including coil steel and aluminum.
While the appeal of radiation-curable solutions is primarily driven by temperature- sensitive substrates, interest among coil applicators is growing primarily due to energy efficiency advantages, low VOC content and lower carbon footprint. This is due to the elimination of high-temperature gasfired ovens needed to cure traditional protective coatings.
With radiation curing, coils manufacturers can expect to reduce their processing costs. Historically, the shape or design of the substrate was a major consideration in whether UV/EB technology was an option. The application of radiation curable coatings was more suitable for flat, linear shapes. New formulations, equipment and processes, however, are making the technology more applicable for complex 3D shapes, which is creating new opportunities in industries like automotive and for general finishes.
A RAD-CURE OUTLOOK
The use of UV/EB curable coatings is growing as manufacturers and applicators from an everexpanding array of industries transition to these technologies. The resulting benefits include energy savings, performance advantages, productivity improvements and the potential to lower harmful air pollutant emissions.
With ongoing investments in R&D, advancements in curing equipment and newer generations of rad-cure coatings will continue to drive adoption of these solutions.
TECHNOLOGY COMPARISON
PPG’s Raycron ultraviolet and electron beam curable coatings require much less energy than conventional coatings using heat, and cure in only seconds. They can be made from 100 percent solids, but also with a mix of waterborne and solvent- borne formulations.
PPG was an early pioneer of these advanced formulations. The company’s extensive history in UV/EB technologies is helping the global coatings manufacturer accelerate the development of new radiation- cured coatings, including solutions that will offer the durability and performance needed for exterior applications. Solutions on the horizon also include coatings that offer wider gloss level ranges, including popular matte finishes. Authors: Emeline Firmin, global technical manager, radiation cure products, and Marisol Rodriguez, global technical director, industrial product development PPG,
Authors: Emeline Firmin, global technical manager, radiation cure products, and Marisol Rodriguez, global technical director, industrial product development.
PPG, http://ppgindustrialcoatings.com/
