UV curing

The main components of a UV curing solution includes resins, monomers, and photoinitiators. Resin is an oligomer that imparts specific properties to the final polymer. A monomer is used as a cross-linking agent and regulates the viscosity of the mixture to suit the application. The photoinitiator is responsible for absorbing the light and kickstarting the reaction, which helps control the cure rate and depth of cure. Each of these elements has a role to play in the crosslinking process and is linked to the composition of the final polymer.[12]

Medium-pressure mercury-vapor lamps have historically been the industry standard for curing products with ultraviolet light.[13] The bulbs work by sending an electric discharge to excite a mixture of mercury and noble gases, generating a plasma. Once the mercury reaches a plasma state, it irradiates a high spectral output in the UV region of the electromagnetic spectrum. 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, 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 mercury-vapor lamps generate primarily 254 nm 'UVC' energy, and are most commonly used in disinfection 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.

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,[14] 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.[15] Benefiting from lower-temperature operation and the lack of hazardous mercury,[16] 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.

Radical Polymerization is used in the curing of acrylic resins in the presence of UV in the industry.[17] Light energy from UV breaks apart photoinitiaters, forming radicals. The radical then react with the polymers, forming polymers with radical groups that then react with additional monomers. The monomer chain extends until it reaches another polymer and reacts with the polymer. Polymers will form with monomer bridges between them, thus leading to a cross-linked network.

Cationic polymerization is used in the curing of epoxy resins in the presence of UV in the industry.[17] Light energy from UV breaks apart photoinitiaters, forming an acidic which then donates a proton to the polymer. The monomers then attach themselves to the polymer, forming longer and longer chains leading to a cross-linked network.