Radiation curable coatings consist of polymer layers cured by exposure to radiation of ultraviolet light (UV) or electron beam (EB) irradiation. This method allows for rapid processing as compared to techniques such as film drying or thermosetting coatings. Radiation curing produces composite materials at ambient temperatures. The process enables the addition of silica nanoparticles into radiation curable resins for creating elements resistant to scratches and abrasions. Such materials are usable as sealants or clear films in inks, electronics, and adhesives.

By contrast to thermally cured coatings, the diluent (also known as a solvent) and resin are replaced with a reactive liquid substance where pigment, in addition to other additives, are dispersed or dissolved. This enhances throughput and conductivity. Radiation curable coatings offer productivity and cost saving advantages including eliminating the dependence on massive ovens, extended pot life, and reduction in equipment footprints. Such coatings permit replacement of complex, multiple component films with simpler single layer options of high quality. Relevant equipment is easy to install without any solvents required. They accommodate high production rates.

The advantages provided by these products extend to many fields, including wood, paper, optics, electronics, and plastics industries. This technology is considered "green" due to factors, such as reduced energy consumption, low volatile organic component, and high chemical resistance.

Disadvantages include the expense of acquiring necessary raw materials and potential health hazards, along with adhesion difficulties stemming from rapid coating shrinkage. In addition, three-dimensional objects are difficult to cure.

History and Current Trends

Radiation curable coatings were first introduced in the 1970s. Since then, due to their advantages over traditional versions, their use spread to the production of wood coatings, overprint varnishes, and automotive headlight films. Certain unique applications, such as fiber optic cables, engage the technology as well.

At present, these products comprise a very small percentage of the market. However, this is expected to change with the increasing interest in environmentally friendly technology. Other fields where radiation curable coatings are either gaining traction in the marketplace or undergoing testing for potential usage include:

• Nanoparticles: Used to improve properties such as rigidity in clear wood films.
• Stereolithography: Involves formation of three-dimensional parts from photosensitive resins.
• Waterborne systems: Serve as dual cure coatings and substitutes for solvent-borne systems.
• Printed electronics: Comprise electrical devices made by printing.

Types

Types of radiation curable coatings include:

• Resins
• Photoinitiators
• Additives

The coatings consist of a myriad of different components, such as:

Oligomers: Reactive substances of low molecular weight are subject to further polymerization. When polymerized, or cured, oligomers serve to determine the coating's fundamental properties. Oligomers present in the layers include:

• Acrylic acrylate
• Silicone acrylate
• Polyester acrylate
• Urethane acrylate
• Epoxy acrylate
• Polyether acrylate

Monomer: These molecular substances are low in weight. They reduce viscosity levels of radiation curable coatings. Their behavior resembles solvents in paint. Monofunctional monomers have a single function that is to lower viscosity. Multifunctional monomers serve additional functions such as forming polymer networks for cross-linking oligomers as radiation curing occurs.

Photoinitiators: The substances absorb light, either UV or visible, and start cross-linking reactions. The trigger for the reactions is the generation of free radicals occurring in polymerized systems or cationic photoinitiated systems.

Additives: The products modify and enhance different functions including pigments, viscosity control fillers, wetting agents, and various additives for paints.

How Radiation Curable Coatings Work

Radiation curable coatings are created by the substitution of light for heat in the curing activity. The layers employ UV energy, visible light, or low energy electrons (EB). This contrasts with traditional methods using thermal, oxidative, or evaporative techniques. The radiation curing process requires additional elements for coating formulations. These include photoinitiators that react with the light as well as reactive resins to cure with the photoinitiators.

Ultraviolet curing: UV curing engages medium-pressure mercury vapor lighting sources or gas-filled lamps without electrodes generating ultraviolet radiation with a wavelength ranging from 200-400 nanometers. The lamps are semi-elliptical or parabolic to focus the light on the curing coating. UV lamps come with shields to protect workers from radiation.

Electron beam curing: The method involves electrons bombarding a coating surface. This forms free radicals essential for the polymerization reaction. An electron beam emanating from an electron gun accelerates and directs electrons emitting from a cathode surface with high vacuum. The equipment includes protection from ionizing radiation for workers. The following two electron accelerator types are utilized:

• Point cathode electron beam
• Linear cathode electron beam

Innovative applications have been found for radiation curable coatings in areas such as metal and appliance films. It is possible to add particular attributes, such as smoothness, gloss, hardness, and other features, using UV curing. With UV formulations consisting of 100 percent solids, it is not necessary to disperse resins and additives using solvent or water. As a result, UV-cured systems do not have any settling or sedimentation tendencies.

Household appliances require coatings with the ability to withstand excessive stress and resist fingerprint marks. UV-curing offers the capability to produce coatings resistant to thermal wear, thereby avoiding a "plastic" appearance as seen with thermal coatings. With a high degree of polymeric crosslinking, a radiation curable coating offers enhanced surface rigidity as well as resistance to scratches. At the same time, they allow sufficient flexibility for bending of the surface.

Applications

Radiation curable coatings cover a diverse scope of applications, including:

• Graphic design
• Overprint varnishing
• Metal substrates
• Flooring
• Printing
• Packaging
• Furniture
• Photovoltaics
• Appliances
• Steel pipes
• Beverage cans

Selecting Radiation Curable Coatings

Radiation curable coatings are available in multiple forms with a plethora of potential uses. Check the manufacturer's specifications to ensure the selected product supports the intended use.

Image credits:

European Coatings | Master Bond