Guide to UV Measurement
Significance of Wavelength
Most UV curing involves two UV wavelength ranges at work simultaneously (three, if we include IR). Short wavelengths work on the surface; longer waves work more deeply in the ink or coating. This is principally the result of the fact that short-wavelength energy is absorbed at the surface and is not available to deeper layers. Insufficient short-wave exposure may result in a tacky surface; insufficient long-wavelength energy may result in adhesion failure. Each formulation and film thickness benefits from an appropriate ratio of short and long-wavelength energy.
The basic mercury bulb emits energy in both ranges, but its strong emission in the short wavelengths make it particularly useful for ‘clear’ coatings and thin ink layers. Higher absorptivity materials such as adhesives and screen inks are often formulated for longer wave cure using long wave photoinitiators. These materials are cured with bulbs containing additives, along with mercury, that emit UV much richer in the long-wave UV. These longer-wave bulbs also emit some short-wavelength energy, which is often sufficient to assist with surface cure.
In more extreme applications, which may require cure of materials with heavy loading of difficult pigments like titanium dioxide or applications which require curing through plastic or glass, long wave curing is essential because these materials block short wavelength UV almost entirely.