High Performance UV Curing Systems
UV Light Energy
UV energy is commonly referred to as "light" because it conforms to the optical rules of visible light. In the electromagnetic spectrum, UV is located in the higher frequency range:
Light energy is composed of individual particles known as "photons". Planck's law describes the energy level of a photon of a particular frequency as:
E = hv
Where E = energy of the photon, h =Planck's constant, and v = frequency
As frequency increases, a point is reached in the electromagnetic spectrum where photons have enough energy to power photochemical reactions. This is what gives UV light its commercial usefulness. There are a number of organic compounds which absorb in the UV region and which have the chemical capabilities to use this energy to promote a photochemical reaction. The most common photochemical reaction is photosynthesis - where green plants absorb visible light photons from the sun and convert carbon dioxide and water to carbohydrates.
For convenience sake, the upper end of the electromagnetic spectrum is specified by wavelength. Infra Red is specified in units of wavelength called "microns" ( 10-6 or one millionth of a meter). Visible and UV are specified in "nanometers" ( 10-9 or one billionth of a meter)
The UV region is divided into four parts. VUV, or Vacuum UV, has a wavelength of 100 to 200 nanometers (nm). These wavelengths are not useful in UV curing, because atmospheric gases ( notably Oxygen and Nitrogen ) strongly absorb these wavelengths. UVC (200 - 280 nm) has the most energetic of the wavelengths used in UV curing. Photons in the UVC are important for surface cure and promote surface properties such as hardness, stain resistance, and abrasion resistance. Photons in the UVB (280 - 320 nm) contribute to bulk cure and photons in the UVA (320-400 nm) promote through cure, especially with thicker film layers.
Visible light has also found commercial use in curing. Light in the 400-420 nm region (violet) has been used to cure white pigmented coatings. Light in the 440-460 region (blue) has been used in dental offices for tooth repair.
The choice of the right output spectrum is very important for successful curing applications. Not only must the lamp output match the absorption spectrum of the photoinitiator, but the effects of pigments and other additives must be taken into consideration. In general terms, the thicker or more heavily pigmented the UV curable layer is, the longer the wavelength should be. This is because longer wavelengths tend to penetrate deeper.
There are three commonly used lamp spectra for UV curing. The most common is the Mercury spectrum, also known as the "H" spectrum. This is produced by using only Mercury as the fill material of the lamp. The Mercury spectrum output has a series of peaks distributed throughout the UV spectrum and is used as a general purpose lamp. Most printing applications use the Mercury spectrum. Strong output in the UVC region makes the Mercury spectrum the lamp of choice where surface cure properties are very important. An example of this is UV curing on vinyl flooring.
When other additives are mixed with the Mercury fill, the output spectrum of the lamp changes dramatically. The "D" spectrum is formed when Iron halides are mixed with the Mercury. This spectrum has most of its output in the UVA region. The Iron lamp is used in applications which require curing of a thick layer of UV material.
When Gallium halides are mixed with the Mercury, the Gallium ( "V" ) spectrum is produced. This lamp has a characteristic purple hue, due to the location of most of its output in the violet region (400 - 425 nm) of the visible spectrum. It is used to cure thick white pigmented coatings. This is because the lamp output very closely matches a cure window in the coating formed by the absorption of the photoinitiator and the transmission curve of the white Titanium Dioxide pigment.
For spectral output charts of these lamps, click the "UV Spectra" button on the navigation bar to the left.
Other metal halides or combinations of metal halides are used for special applications. An example is Indium halide which has a strong output peak in the 440 - 460 nm (blue) region. This spectrum is used in dental offices to cure coatings on teeth.
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