UV Curing Theory
Ultraviolet curing is a photochemical process in which an abundant amount of UV energy, produced by a mercury discharge lamp, is focused at monomers through cross linking or polymerization. The sensitizer present in the monomer absorbs UV radiation at a rapid rate and initials the reaction in the monomer, producing a hard and dry surface. The rate or speed of the curing process depends on the following:
(A) Chemical Compound - Each monomer will cure at a different rate, depending upon the composition and the amounts of sensitizer, pigment and the chemical additives.
(B) Thickness of Coating - The thickness of a specific coating is not directly proportional to exposure time. The amount of UV energy inside a layer of coating decreases exponentially with depth. If 70% of the UV energy is absorbed in the top .001" of coating, then 70% of the remainder or 7% of the initial amount will be absorbed into the second .001" of coating. Thus, a two fold increase in thickness requires a ten fold increase in UV intensity.
(C) Amount of UV per Unit Surface - Normally, the curing speed will increase with the amount of UV energy per unit surface at a non-linear rate. If a 200 watt per inch mercury lamp was increased to 400 watts per inch, the curing speed could increase ten fold. In addition, special Superior metal halide lamps can trigger the receptor at an even faster rate with a standard 200 watt per inch lamp. The charts in Figure 1 depict the flexible control that Superior technology has developed over the years. We can pinpoint the UV energy produced by our curing lamps to your exact specifications.
The sensitizer should absorb UV in the range which is not absorbed by the monomer or pigment. The wavelength produced by a Superior medium pressure mercury lamp or Superior metal halide lamp should coincide with the wavelength absorbed by the sensitizer. The continuous light spectrum produced by Superior lamps in the 200 to 440 nanometer range propels technology to a new plateau of efficiency.
Radiation emitted from our mercury lamp depends on many factors. In a gas discharge lamp, the output is a function of the atomic structure of the gas molecules, their temperature, pressure of the gas vapor, and quality of materials used in the basic construction. Superior lamps utilize only the finest raw materials available. Second best will not do.
Ozone is a by-product of ultraviolet producing lamps. If your application requires the absence of ozone gases, our engineering department effectively responds with a special grade of quartz that prevents the 254 nanometer line from reaching oxygen in the atmosphere, thereby preventing the production of ozone. Please bear in mind that all effective lines below 254 nanometers are blocked which might be required to start your reaction.
Highlights Industrial Curing in the 200-400nm range
The UV Spectrum - Once the basic concept of radiation has been understood, how ultraviolet radiation fits into the scheme of things, and what UV Curing contains, then, one can easily understand the advantages and disadvantages of various light sources. Standard Superior curing lamps display the following characteristic in Figure 3 displayed below.
Spectrum of a Mercury Vapor Lamp
The far ultraviolet lies between 200mn and 300mn and is classified as Germicidal or UV-C. The middle ultraviolet lies between 280nm and 320nm is called Erythema (suntan) or UV-B. The near ultraviolet lies between 320nm and 400nm and is commonly called Black Light (long ultraviolet) or UV-A.
The Superior medium pressure lamp builds vapor pressure as the power and temperature start to increase. As shown below in Figure 4, when the power input increases, so does the efficiency (ratio of total radiation to power input). From 100 watt per inch to 300 watt per inch, the efficiency is increased by 12%
TOTAL RADIATION EFFICIENCY
|Input Power (watt/inch)||Efficiency R/P %|
This efficiency further evolves from the highly skilled technicians that receive years of critical training and their adaptability to construct flawless linear lamps. Our Superior 400 watt per inch lamp will more than double the cure rate when compared to two lamps running at 200 watt per inch each.
The spectral output does not shift as the lamp power is increased from 100 to 200 to 300 watts per inch. There are a multitude of medium pressure mercury lamps varying in shape, size, diameter, length and power output. Please note that a Superior lamp operating on a properly designed stabilized ballast has a power conversion from line input to lamp output of 92%.
Advantages of UV Curing over Conventional Heat Curing
The heat curing process of solvent evaporation creates environmental pollutants. These pollutants known as Volatile Organic Compounds (VOC) are the number one contributor to the destruction of the ozone layer in our upper atmosphere. The reactive process of specialized UV curing inks when exposed to UV light in the 320 to 400 nm range, eliminates VOC's thereby reducing the amount of hazardous waste disposal and potential release of chemical waste into the atmosphere. UV curing is considered one of the most “Green” technologies in the printing industry.
Quality and Diversity of Product Offering
Improved gloss, better scratch and abrasion resistance, better adhesion are just of few of the benefits UV Curing offers to achieve outstanding print graphics. Cool Cure UV Systems redirects heat in the IR range to allow printing on heat sensitive substrates, such as films, allowing printers to offer a broader range of products.
The instantaneous reactive process of UV curing results in up to 50% improved production speeds over heat curing systems.
The smaller footprint of the UV Curing equipment combined with eliminating the need for space required to store product with long cure times, significantly increases plant capacity allowing for greater utilization of equipment.
Improved Competitive Positioning
Faster speeds, decreased production costs and increased production space results in direct cost savings and significantly improves printers' competitive position in the marketplace.