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Release Liners: A Worldwide Special ReportEB/UV Curing Drying

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Recent years have shown an increase of interest in the use of electron beam and ultraviolet light for curing/drying inks and coatings. The advantages of reduced emissions, lower energy costs, and increased speed have fueled this interest.

Electron Beam (EB)
The electron beam (e-beam) is generated in the same manner as a picture is generated on your television tube or a computer cathode ray tube (CRT) screen but at much higher voltages.

In a CRT electrons are emitted from a heated cathode in the electron gun. They are accelerated and focused to strike the luminescent screen as a fine point. Between the electron gun and the screen, deflecting plates or deflecting coils control the up-and-down and left-to-right motion of the beam. The beam of electrons thus can be made to sweep horizontally across the face of the tube.

With an industrial e-beam, the principle of operation is the same. However, the voltages employed are much greater, and a thin metal foil (usually titanium) acts as a window that allows the electrons to pass through and strike the coating or film (replacing the luminescent screen mentioned above). When these high-energy electrons strike the coating, they can break and form new chemical bonds, resulting in a cured coating. How far these electrons penetrate into the coating depends upon the potential difference between the cathode and anode—i.e., how high the voltage is set. The greater the difference, the faster the acceleration and the greater depth of penetration.

This ability to penetrate to various depths, together with insensitivity pigments and high throughput, has made e-beams particularly useful in curing both highly pigmented and thick coatings. It also has been used successfully in the processing of radiation-curable silicones for release coatings. In this case, however, a lot of the energy is “wasted” as it passes through the thin coating (usually 1 micron). Recent advances in the industry have allowed EB units with lower voltage to be produced.

Depth is not the only important factor in EB cure; dose also must be controlled. Dose refers to the amount of electrons delivered over an area (the term is “megarad”). This is controlled by the current (amps) used to run the equipment. In the case of thick coatings, it is normal to use 4–6 mR. There are applications in film cross-linking and specialty laminations where this dose is much higher. In the case of thin films, such as silicone release coatings, a lower dose will give sufficient cure—usually 1.75–2.5 mR.

EB equipment, due to simple design, is best suited to long-run, high-throughput applications. Highly pigmented systems can be radiation cured only by this method. Speeds more than 1,000 mpm have been achieved. A lot of stops/starts tend to shorten the life of both the window and the gun due to thermal stress of heating and cooling.

A byproduct of EB emission is the creation of x-rays. The equipment is designed to shield and contain these, and they are monitored constantly. The x-rays are caused as the extra electrons pass through the coating, substrate, etc., and strike the back plate. The higher the voltage, the more likely this is to occur.

Ultraviolet Light (UV)
Ultraviolet is defined as electromagnetic radiation with wavelengths between 4,000 angstrom units (Å), the wavelength of violet light, and 150 Å, the length of x-rays. Often it is divided into further categories based on wavelength, UV-A (400 nm to about 315 nm), UV-B (about 315 nm to about 280 nm), and UV-C (about 280 nm to 150 nm).

In the coatings industry, this energy normally is created through the use of mercury vapor bulbs. Mercury in the bulb is excited through the use of electrical energy until it “glows.” As it relaxes, it gives off the UV light we are looking for. The electrical energy can be applied directly by creating an arc between the two ends of the bulb. Alternatively, it can be used to create microwaves that are directed through the bulb and excite the mercury.

For most applications the “arc” and “microwave” lamps can be used interchangeably. However, some differences can be seen in thin coatings (1 micron or less) at higher speeds (>300 mpm). This is especially true in coatings with fewer reactive groups such as silicone acrylates (used in release coatings). Arc lamps seem to be more efficient at the higher speeds with these types of compounds.

There are some other differences between the two systems. Arc lamps need to warm up for a few minutes in order to create the arc. Microwave lamps need no warm-up time to create the microwave energy.

Of course, the choice of photoinitiator is the determining factor for cure. They actually convert the UV energy into chemical reaction. There are many choices, and many factors—including absorption characteristics of substrate and coating; how well it mixes with the coating in question, out-gassing properties, etc—determine that choice. Bulbs also can be doped to shift the output wavelengths when necessary.

In comparison to EB, UV equipment design seems better suited for shorter runs and many formulation/substrate changes. Speeds in excess of 400 mpm are common in production, and it is possible to achieve much higher speeds. Price is another difference, with EB being in the order of 5x or more costly. However, the new, low- voltage EB will change this considerably.

Applications in Silicone Release
Our expertise is, of course, the silicone chemistry used for release coatings. We have seen both EB and UV successfully used for the cure of these materials. There are some geographic differences, with EB cure capturing a much larger share in the North American market than in Europe. The general worldwide trend is toward UV, probably due to versatility and cost issues. Also, importantly, UV equipment now is readily available with an inerted module, which is necessary to cure silicone acrylates (oxygen inhibits the cure). This inerting improves the consistence of silicone epoxy cure by controlling moisture levels in the cure chamber.

EB and UV also have different effects on release properties. EB, having much higher energy input, tends to create a much different cross-link structure. Cure is not limited to the acrylate groups, as the electrons also can abstract hydrogen from the chain itself or even cause chain scission/recombination. The resulting release levels tend to be much higher than similar formulations cured by UV.

UV is a much “gentler” cure, specific to the acrylate groups. The UV energy is used indirectly, with the photoinitiator being cleaved by the resulting radicals actually initiating the reaction. Resulting release levels are, therefore, lower and tend to become more consistent with aging. Cure, as defined by Subsequent Adhesion, can vary depending on formulation and equipment differences but normally will be greater than 90%. Another measure of cure is to look for extractables. The exact method for this test still is extremely variable, but good processing should result in 3% or less extractables.

As mentioned above, new EB technology with lower voltage is more readily available. Initial testing shows the effects to be more UV-like—lower release levels and more consistent aging. This makes sense, as dose can be better utilized: The electrons are not penetrating as much due to lower voltage potential. Further studies are underway.

Since 1989, Thomas Hohenwarter has worked in the Radiation Curable Technical Service group at Goldschmidt Chemical Corp., Hopewell, VA. He was technical service manager and currently is market manager for Radiation Curable Silicones. Hohenwarter graduated with a BS in biology from Elizabethtown College; he also attended the Graduate School for Organic Chemistry at Shippensburg University.

Gary Freelin is the technical manager for Degussa Goldschmidt Chemical Corp. and has been with the company for the past five years.

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