Résumé : |
In coating technology, many desirable properties result from the chemical crosslinking (or curing) of coating polymers, after the wet coating has been applied to a product. There are a number of general curing methods available to coatings technologists to turn a liquid coating formulation into a solid film. In industrial finishing, for example, coatings may be passed through a heated oven in order to force them to cure at an appropriate rate for the manufacturing process (typically 10-20 minutes). Radiation curing provides an essentially zero-VOC, energy-efficient alternative to conventional curing methodologies. In contrast with thermally induced polymerisation, radiation curing can be extremely fast, but it is largely only suitable for flat substrates that can move beneath the lamp system or for applications requiring spot curing.
Radiation curing essentially involves a polymerisation process initiated by the absorption of energy in the form of light or other radiation, and by far the most common form of radiation curing is UV curing, which uses the energy from ultraviolet (UV) radiation, and allows the liquid coating to be handled in normal lighting conditions. The ultraviolet region of the electromagnetic spectrum encompasses radiation with wavelengths from about 100 nm to about 400 nm, where we start moving into the visible light region. For UV curing purposes, the most commonly used lamp system is the mercury arc lamp (or comparable electrodeless lamp), but UV LEDs (which are more energy efficient and have less health & safety-related issues than mercury lamps) are increasingly used. Once excited, mercury emits UV radiation, with particularly strong emission lines at 254 nm, 313 nm, 366 nm, and towards the blue part of the visible spectrum, 404 nm and 436 nm. UV LED light systems have predominantly monochromatic (single wavelength) emissions, which can be at 365 nm, 385 nm, 395 nm or 405 nm, depending on the LED system. I suspect most of us have (sadly) been exposed to radiation curing in the dental surgery, where a blue light was used to initiate the polymerisation (or hardening) of a dental filling.
UV curing a coating, however, is not as simple as just shining UV light at it ! The chemistry of the coating formulation needs to be coordinated with the light source. This short article aims to give a brief introduction to UV curing and some of the interesting and unique issues arising from the technology. |