Ultraviolet curing Principle

Ultraviolet curing is a process in which UV energy produced by a mercury discharge lamp is absorbed by a sensitizer, causing a reaction in the monomer which makes it hard and dry. The rate of the curing process depends on the following:

1. Chemical compound - Each monomer cures at a different rate, depending on the amounts and compositions of sensitizer, pigment, and chemical additives.

2. 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 in the second .001" of coating. Thus, a two-fold increase in the thickness requires a ten-fold increase in UV intensity.

3. Amount of UV per Unit Surface - Normally, the curing speed will increase with the amount of UV energy per unit surface at the non-linear rate. If a 200 watt per inch mercury lamp was increased to 400 watt, the curing speed could increase ten fold. In additional, special metal halide lamps can trigger the receptor at an even faster rate with a standard 200 watt per inch lamp. The chart on Figure 1 demonstrates the flexible control of Innovative's lamps. 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 medium pressure mercury lamp or metal halide lamp should coincide with the wavelength absorbed by the sensitizer.

UV Curing

The types of radiation emitted from our mercury lamps depend on many factors. In a gas discharge lamp, the output is a function of the atomic structure of the gas molecules, their temperature, and the pressure of the gas vapor.

If you require the absence of ozone gases, our engineering department can respond with a special grade of quartz that prevents the 250 nanometer line from reaching oxygen in the atmosphere. Please bear in mind that all effective lines below 254 nanometers are blocked, which may interfere with the curing of some inks. Figure 2 shows the basic area of interest for industrial curing.

The U V Spectrum
Ultraviolet refers to all electromagnetic radiation with wavelengths in the range of 10 to 400 nanometers, or frequencies from 7.5E14 to 3E16 Hz.
The UVA range is wavelengths from 315 to 400 nanometers. Wavelengths from about 345 to 400 nm are used for "Blacklight" effects (causing many fluorescent objects to glow) and are usually very slightly visible if isolated from more visible wavelengths. Shorter UVA wavelengths from 315 to 345 nM are used for suntanning.
UVB refers to wavelengths from 280 to 315 nanometers. These wavelengths are more hazardous than UVA wavelengths, and are largely responsible for sunburn. The ozone layer partially blocks these wavelengths.
Strangely, UVB lasers are considered less hazardous than UVA lasers, since UVB is more easily absorbed by various fluids and tissues in the eye and cannot reach the retina in significant amounts. UVB also does not penetrate as deeply in the skin as UVA. However, the deadliest types of skin cancer (malignant melanomas) start in the epidermis, an upper layer of the skin. UVB is largely blamed for these cancers, although shorter UVA wavelengths are considered possibly cancer-causing.
UVC refers to shorter UV wavelengths, usually 200 to 280 nM. Even shorter wavelengths from 10 to 200 nM are usually considered separately as "Vacuum Ultraviolet" since they are absorbed by air, although these wavelengths are also considered a shorter range of UVC. Wavelengths in the UVC range, especially from the low 200's to about 275 nM, are especially damaging to exposed cells. Such shortwave UV is often used for germ killing purposes

How a UV Lamp Works
UV-light is generated in a mercury discharge lamp: a quartz tube with mercury vapor, an inert gas, two electrodes and insulators for suspension.

The mercury generates radiation between 200 and 400 nm with peaks at 254 nm, 310 nm and 366 nm. The lower wavelengths are cut off by the quartz, which does not transmit any radiation below 230 nm.

Each atom consists of a nucleus, around which a number of electrons float in fixed orbits. By adding energy (electricity) the electron is brought in a higher orbit. Each element shows a tendency to go back to its original condition. The electron will fall back in its former orbit: the excess energy is emitted as a photon.

The most commonly used UV-lamp is the medium-pressure mercury-arc lamp or MPMA-lamp. It can be manufactured in lengths of some mm to more than 2 meter. The life span of these lamps varies from 1000 to 2500 hours.

This lamp is made of quartz because this is the only material that transmits UV-light and at the same time endures high temperatures of 6 to 800¡ÆC. This lamp will expand little and does have a high melting temperature (from 1100¡ÆC).

The electrodes are made from tungsten: the process to manufacture them is extremely complex. Tungsten is used because the temperature of the curve may rise up to more than 3000¡ÆC.

The connection between the electrode and the wire is accomplished with a molybdenum plate that can expand together with the quartz when warming up and still does endure the high voltages.

The lamp finally is hung up to ceramic insulators.

As the supply current is often insufficient to power a MPMA-lamp, our lamps employ transformers. Two types are used: inductive and fixed-power transformers. Standard ballast is also used up to 5 kW capacity.

Medium pressure UV lamps
In a high pressure lamp, the average kinetic energy of free electrons (also known as electron temperature) is only slightly higher than the temperature of the gas in the discharge. Both temperatures are similar, and one can refer to a general temperature of the discharge. The electron temperature is higher in order for the electrons to have the net effect of transferring energy to the gas.

It is quite fair to think of the gas or vapor as a thermal radiator, which is usually spectrally selective and radiating mainly in specific spectral lines.
Typically, the discharge diameter is over 50 times the mean free path of an electron. There is generally no non-thermal, non-optical, non-mechanical interaction between the discharge and any container it is in, except at the ends of the discharge.

In order to efficiently produce light, the power input must be much greater than the heat conduction from the discharge, which is usually near or over 10 watts per centimeter of discharge length. Therefore, power input to a high pressure lamp is usually near or over 20 watts per centimeter of discharge length.
The above can all be satisfied even if the pressure is less than 1 atmosphere. It can even be satisfied in a nearly practical lamp with a pressure of .1 atmosphere.

High pressure lamps are often referred to as High Intensity Discharge lamps, or HID lamps.

In a low pressure lamp, the electron and gas temperatures are very different, and the pressure is generally below .05 atmosphere. Power input is generally near or less than 1 watt per centimeter.

Low pressure mercury UV lamp
In a low pressure lamp with mercury vapor as an active ingredient, the mercury vapor is mixed with an inert gas, often neon or argon. The mercury vapor's pressure is usually well under 1/1000 atmosphere, or a fraction of a mm. of Hg. The mixture is generally 1 percent or even as little as .1 percent metal vapor, 99 to 99.99 percent inert gas.

The desired spectral output generally results from atomic transitions that terminate in the atom's "ground" or unexcited state. This means that most of the metal vapor atoms, since they are not excited, can easily absorb this radiation. Therefore, you don't want too much mercury vapor. The inert gas largely determines electrical characteristics, mainly by controlling the mean path traveled by electrons between collisions. The gas also reduces collisions of electrons, ions, and excited mercury vapor atoms into the lamp's walls.