UV curing (ultraviolet curing) is the process by which ultraviolet light is used to initiate a photochemical reaction that generates a crosslinked network of

polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...

s. UV curing is adaptable to

printing Printing is a process for mass reproducing text and images using a master form or template. The earliest non-paper products involving printing include cylinder seals and objects such as the Cyrus Cylinder and the Cylinders of Nabonidus. The ...

, coating, decorating, stereolithography, and in the assembly of a variety of products and materials. In comparison to other technologies, curing with UV energy may be considered a low-temperature process, a high-speed process, and is a solventless process, as cure occurs via direct

polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...

rather than by evaporation. Originally introduced in the 1960s, this technology has streamlined and increased automation in many industries in the manufacturing sector.

Applications

UV curing is used in applications where there is a need for converting or curing inks, adhesives, and coatings. UV-cured adhesive has become a high speed replacement for two-part adhesives, eliminating the need for solvent removal, ratio mixing, and potential life concern. It can be used in the flexographic, offset, pad, and

screen printing Screen printing is a printing technique where a mesh is used to transfer ink (or dye) onto a substrate, except in areas made impermeable to the ink by a blocking stencil. A blade or squeegee is moved across the screen to fill the open me ...

processes, where UV curing systems are used to polymerize images on screen-printed products, ranging from T-shirts to 3D and cylindrical parts. It is used in fine instrument finishing (guitars, violins, ukuleles, etc.), pool cue manufacturing and other wood craft industries. Printing with UV curable inks provides the ability to print on a very wide variety of substrates such as plastics, paper, canvas, glass, metal, foam boards, tile, films, and many other materials. Other industries that take advantage of UV curing include medicine, automobiles, cosmetics (for example artificial fingernails and gel nail polish), food, science, education, and art. UV curable inks have met the requirements of the publication sector on a variety of papers and boards.

Advantages of UV curing

A primary advantage of curing with ultraviolet light is the speed at which a material can be processed. Speeding up the curing or drying step in a process can reduce flaws and errors by decreasing time that an ink or coating spends wet. This can increase the quality of a finished item, and potentially allow for greater consistency. Another benefit to decreasing manufacturing time is that less space needs to be devoted to storing items which can not be used until the drying step is finished. Because UV energy has unique interactions with many different materials, UV curing allows for the creation of products with characteristics not achievable via other means. This has led to UV curing becoming fundamental in many fields of manufacturing and technology, where changes in strength, hardness, durability, chemical resistance, and many other properties are required.

Types of UV curing lamps

Medium-pressure lamps

Medium-pressure mercury-vapor lamps have historically been the industry standard for curing products with ultraviolet light. The bulbs work by sending an electric discharge to excite a mixture of mercury and

noble gas The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low che ...

es, generating a plasma. Once the mercury reaches a plasma state, it irradiates a high spectral output in the UV region of the

electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...

. Major peaks in light intensity occur in the 240-270 nm and 350-380 nm regions. These intense peaks, when matched with the absorption profile of a photoinitiator, cause the rapid curing of materials. By modifying the bulb mixture with different gases and

metal halides Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, su ...

, the distribution of wavelength peaks can be altered, and material interactions are changed. Medium-pressure lamps can either be standard gas-discharge lamps or electrodeless lamps, and typically use an elongated bulb to emit energy. By incorporating optical designs such an elliptical or even aconic reflector, light can either be focused or projected over a far distance. These lamps can often operate at over 900 degrees Celsius and produce UV energy levels over 10 W/cm².

Low-pressure lamps

Low-pressure mercury-vapor lamps generate primarily 254 nm 'UVC' energy, and are most commonly used in

disinfection A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than s ...

applications. Operated at lower temperatures and with less voltage than medium-pressure lamps, they, like all UV sources, require shielding when operated to prevent excess exposure of skin and eyes.

UV LED

Since development of the aluminium gallium nitride LED in the early 2000s, UV LED technology has seen sustained growth in the UV curing marketplace. Generating energy most efficiently in the 365-405 nm 'UVA' wavelengths, continued technological advances, have allowed for improved electrical efficiency of UV LEDs as well as significant increases in output. UV LED lamps generate high energy directed to a specific area which strengthen the uniformity. Benefiting from lower-temperature operation and the lack of hazardous mercury, UV LEDs have replaced medium-pressure lamps in many applications. Major limitations include difficulties in designing optics for curing on complex three-dimensional objects, and poor efficiency at generating lower-wavelength energy, though development work continues.

References

{{Reflist Curing agents Ultraviolet radiation