Called photopolymerization, the photochemical UV curing process relies on liquid monomers and oligomers that, when mixed with a small percentage of materials called photoinitiators and exposed to UV light, harden instantly to become tough clear coatings, decorative inks, and adhesives.
Technology Delivers Effective Colored UV Coatings
Ciba has been a pioneer in UV curing since the 1970s, when the company introduced Ciba™ IRGACURE® 651 as one of the first photoinitiators. Over the past 20 years, the company has focused its research on developing photoinitiators to meet evolving customer and industry needs. The result is the broadest range of photoinitiators available in the industry, with IRGACURE and Ciba™ DAROCUR® photoinitiators covering a wide UV and visible light absorption range. These products can be used to give balanced surface and through cure characteristics in clear and pigmented coatings.Ciba’s research priority today is developing photoinitiators for colored UV coatings, which are receiving increasing attention for indoor and outdoor applications. To this point, experience with UV-cured colored systems for outdoor applications has been fairly limited. Colored prints with a film thickness of a few micrometers have cured well, but the through-curing of thicker coatings has, historically, been difficult.
Bisacylphosphine oxide (BAPO) photoinitiators are changing the rules by opening new opportunities in this application area. BAPOs show excellent properties in through-curing of highly pigmented white lacquers as well as several millimeter thick glass fiber reinforced materials. They also effectively cure thicker coatings that contain colored pigments or even carbon black.
The company is meeting with customers to promote Ciba IRGACURE® 819 as the preferred BAPO photoinitiator for colored coatings. The recommended concentration range is from 0.5 to 1.5%, increasing with the pigment loading. A blend of IRGACURE 819 and an additional photoinitiator, such as IRGACURE 184, is recommended to get a balance of surface cure and through cure. IRGACURE 819 offers exceptional curing properties, along with a photobleaching effect that minimizes yellowing in coatings. The high efficiency photoinitiator is capable of through-curing thick sections of pigmented materials.
“Expanding upon our ability to help our customers cure thicker cross sections and highly pigmented systems, all at higher speeds, is a high priority for us, along with creating photoinitiators specifically designed for UV-curable powder coatings,” says Industry Market Manager Don Wostratzky.
The ability to cure thick-section composites and powder coatings represents the next steps on a continuum of innovation that is creating new UV curing solutions that meet productivity and performance needs in what were previously traditional industrial and automotive markets.
Solving Technical Challenges
Colored coatings present several technical challenges. Recommended concentrations for blue or green pigments are 3–10%. For yellow and red, higher concentrations of 7% to 12% are necessary because of their lower hiding power. Pastel colors, which contain a smaller amount of colored pigment together with titanium dioxide, are easier to cure.Carbon black as well as certain other colored pigments have strong hiding power. Pigments with a strong hiding power do not always influence the UV curing process. Surprisingly, coatings with strong colors and hiding power, like blue and green, are easier to cure than red and yellow, which have less hiding power. The color itself affects the degree of cure, as does the particle size of the pigment. Tightly packed fine particles make it more difficult for light to pass through compared to larger particles, which are more loosely packed. Because of this, Ciba recommends that formulators use only as much pigment as is needed for hiding the substrate. An unnecessary higher degree of pigmentation makes the UV curing process more difficult.
In general, Ciba’s researchers believe that through-curing is the crucial process for colored coatings, especially in a film thickness above 50 microns (2 mils). Insufficient through-curing is indicated by surface wrinkling, delamination (bubbles) or a wet bottom of the film. In some cases, a significant loss of gloss appeared during exposure (UV matting).
The pigment concentration per substrate area is crucial for through-curing ability, depending on the pigment loading of the formulation and the film thickness. A common cause of insufficient through-cure is excess loading of pigment per unit of substrate area. The simplest way to avoid this, according to Ciba researchers, is to decrease the film thickness and reach the necessary thickness by subsequent curing of thinner layers. This approach is possible if a reasonable thickness (for instance, 30 microns/1.2 mils) is curable at once. Otherwise, the BAPO content has to be increased.
With less reactive colored coatings, an influence of the substrate was observed in Ciba tests. On a bright substrate, cure performance is superior to that on a darker, non-reflective substrate. This is likely due to the reflective behavior of the material.
Fusion Delivers the Power of UV Light
Along with Ciba, Fusion UV Systems is also working to meet customer needs for UV curing advances. Fusion UV Systems supplies UV curing systems and UV-based process solutions. The company invented microwave lamp technology and pioneered its use for UV curing. Working with its customers, Fusion continues to take UV curing processes originally developed for the graphic arts industry and adapt them for an ever-increasing array of industrial uses. The company is focused on the needs for curing industrial inks, coatings or adhesives on paper, metal, plastic, glass, wood or any other material.Dick Stowe, director of Technical Communication, reports that Fusion has developed the lamp technology and materials necessary to cure thick composites. “With thick-section composite curing as a driver, we’re now concentrating on replacing traditional composites with UV-cured matrices,” he says.
Fusion is working with its customers to demonstrate how control of certain lamp variables, including wavelength and irradiance levels, can achieve real advantages in UV curing of 3-D materials. Mirroring market trends seen by Ciba, Stowe sees UV-curable powder coatings as an area of tremendous business potential, allowing powder coatings to be used in areas that can’t tolerate the traditional powder bake cycles. This has broad potential in automotive and appliance applications.
Fusion is also using its UV equipment advances to drive continued change in the manufacturing paradigm. For example, with UV powder coating, UV curing now allows manufacturers to put a finish on a material after it has been assembled, without damage to the product’s internal components or performance. “We’re focusing on the design and configuration of lamps for 3-D curing,” Stowe says. “For the past 30 years, UV curing has been considered great for flat, linear applications. Future growth will demand curing heavier thicknesses, and more durable coatings on 3-D components.”
UCB Chemicals
Hand in hand with the photoinitiator and equipment advances coming from Ciba and Fusion are resin innovations pioneered by UCB Chemicals. As part of the worldwide UCB Group, UCB Chemicals is building on decades of experience in developing resins to match highly specialized coating requirements, particularly in solvent-free and waterborne technologies. The company specializes in Ebecryl® UV/EB curable resins, which are recognized throughout the industry for their superior finish and durable performance.UCB is focused on developing resins with improved wetting characteristics to meet pigment coating application needs. The market is strong for resins that can be used in UV-cured coatings for kitchen cabinets, windows and blinds, baseboards, and molding applications. A key market trend for UCB involves the need for resins that can contribute to improved scratch and abrasion resistance, particularly for use on flat panels and parquet flooring. Such resins have to be able to stand up to heavy traffic demands. Another area of innovation involves development of resins specifically targeted to meet needs in UV-cured plastics, such as eyeglass lenses and cosmetic packaging.
Along with Ciba and Fusion, UCB is responding to strong customer interest in the UV curing of 3-D materials. To meet market needs, UCB is developing resins specifically for use in vacuum coating and spray applications. According to Steve Awadalla, market manager-Industrial Coatings, this requires designing lower viscosity resin systems.
The company, which has long been a leading supplier of polyester powder resins to the metal coatings market, is now applying this technology to UV-curable powder coatings. “We’re building on our technical leadership to come up with UV-curable powder coatings for heat-sensitive substrates,” says Awadalla. “This has exciting application potential in pre-assembled parts such as heat-sensitive motor components.”
“All this activity underscores the fact that UV curing is poised for yet another period of strong growth, as Ciba, Fusion, UCB and other companies develop innovations that bring this ‘new’ technology to traditional industries that have not been able to fully utilize UV advantages up until this point,” says Ciba’s Don Wostratzky. “There’s a strong sense throughout the industry that we haven’t seen anything yet compared to the growth that lies ahead.”
For more information on UV curing, contact Ciba Specialty Chemicals, 540 White Plains Road, Tarrytown, NY 10592; phone 800/431.1900; fax 914/785.4253; visit www.cibasc.com.
UV Curing Technology
Prior to the latest innovations, which are bringing new performance advantages to the curing of thick-section composites, coatings manufacturers first embraced UV-cured coatings and inks for their value as cost-effective alternatives to solventborne products. Conventional heat and air drying evaporates solvents, reducing initial laydown of coatings by more than 50% and sometimes emitting hazardous pollutants (see Figure). Rather than driving off solvents with heat or high-velocity air drying, UV cures instantly – literally in the blink of an eye. There is no loss of coating thickness and nothing to evaporate. The entire amount of laydown as a liquid remains in the final cured product.UV curing meets demands for increased productivity, lower costs and greatly reduced pollutant emissions. UV curing is recognized as an enhancement technology, one that allows manufacturers to make their products more attractive and durable. Finished products exhibit exceptional stain, abrasion and solvent resistance coupled with superior toughness. UV-cured coatings and inks can be applied to any substrate – paper, wood, metal, leather, vinyl, plastic or glass.
Manufacturers who once used heat to make things harder, brighter and more scratch resistant now find that UV curing does everything heat can do, only better, including the following examples.
- A wide web printer, for example, significantly increased production and reduced the length of the process line by converting from hot air ovens to UV curing.
- A national two-piece can manufacturer increased line speeds by 50% with UV curing.
- Automobile manufacturers and hundreds of OEMs use UV curing systems to make automotive parts ranging from headlamps and flexible circuits to wood grain interiors and acrylic tail lens assemblies.
What UV Curing Can Do for You
- Higher quality products
- Instant drying
- Faster line speeds
- In-line processing
- Cure on demand
- Immediate work-and-turn
- Reduced cleanup
- No pollution controls needed
- Lower energy consumption
- Less floor space for equipment
- Less downtime for maintenance
- Use on heat-sensitive substrates
