Time for another look?
By Jerry Davis
Curing under a nitrogen blanket offers both advantages and disadvantages. Sometimes, the higher degree of polymerization brought about by nitrogen curing has a dramatic and positive effect on the finished product. In other cases, it offers little benefit and may even have adverse results. When considering this process, it's critical to weigh job requirements to see if they justify using nitrogen UV curing.
As stated earlier, curing without the presence of oxygen leads to optimum polymerization and a very hard, scratch-resistant coating. This is an extremely important function of a hardcoated film or selective clearcoat.
The texturing capability of nitrogen curing came about when developers discovered that UV wavelengths in the C-band (250-270 nm) can, at very low power settings, produce a deep cure in the ink film. The UVC polymerizes the ink at the substrate level between 0.3-0.5 mil deep, while leaving the ink's surface uncured <B>(Figure 1)</B>. Because of subtle molecular attachments at the interface between the cured base and uncured surface, following the initial UVC exposure with UVA exposure (350-380 nm) to complete the surface cure leads to a buckling of the surface. The buckling results in a distinct texture, sometimes referred to as "wrinkle" or "craze," which can be readily identified upon inspection under a 50-power loupe.
Characteristics of the texture, such as density and definition, are functions of the following:
UVC exposure dosage UVC lamps are little more than low-pressure germicidal lamps, similar to those used for air and water disinfection. While low in overall UV output, they are very efficient producers of short UV wavelengths needed for subsurface polymerization. Providing the correct UV dosage to the ink film is a function of energy over time; therefore, the faster the belt, the greater number of lamps required. But nitrogen-curing systems rarely need more than four. In all cases, the fewer bulbs that can be used to achieve the subsurface cure, the better.
UVA exposure dosage UVA lamps provide the main cure. Typically, these lamps are the normal mercury vapor type, although longer-wavelength mercury-iron lamps sometimes can be advantageous. Ideally, the power supply in the curing unit should be able to operate either type of UVA bulb.
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