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Examining UV-curable inkjet printing

May 1st, 2008 / By: / Feature, Graphics

UV-curable inkjet printing has been available for a while now, and the number of printers using UV-curable inks consistently grows. Yet, in inkjet printing, the total number of printers is very small compared to solvent and other types of inkjet printing. Although UV-curable inkjet is exciting and seems to have the potential for growth, there are still many obstacles that the technology needs to fully overcome to become the major printing system choice.

In the last few years, printer and ink manufacturers have made great strides in improving the quality, speed, and versatility of this printing system. It’s worth our while to take a more detailed look at what UV-curable inkjet is, and its advantages and disadvantages compared to other types of inkjet printing available in the market.

In a nutshell

UV-curing ink is a reactive ink that cures using a light source that emits UV radiation. The inks are composed of colorant or pigments, monomers, oligomers or reactive binders, photo initiators, and some small amounts of other ingredients that perform functions to control physical and printing aspects. The inks are printed on the substrate using a Piezo print head and immediately cured or partially cured (and then cured to completion later) using an ultraviolet light.

UV-curing inks require a high-intensity source of ultraviolet light to initiate a chemical reaction, curing the ink almost instantaneously. The ultraviolet light forms a small part of the electromagnetic spectrum, which ranges from radio waves at the long-wave end, to x-rays and gamma rays at the short-wave end. The curing is actually a chemical reaction that chemically bonds the monomers and oligomers, forming a complex polymeric chemical structure. Hybrid UV-curable inks work the same way, except the chemical reaction is different, allowing a more complete cure.

Obstacles overcome

Printer and ink manufacturers had to overcome many obstacles. UV curing is a complex technique that required a perfect balance of curing time, strength of the UV radiation, and amount of ink deposited. If a proper cure level is not reached, adhesion between the colors and substrate would suffer. If UV inks are cured too much, then they can become brittle and flake off. These basic obstacles have now been overcome.

Assessing UV

Since the liquid portion of the ink is reacted in the curing process and becomes solid, the ink is considered to be 100 percent solid. Since it does not contain any solvents, the printer manufacturers can claim that it has zero volatile organic compounds (VOCs).

Since the liquid portion of the ink is reacted in the curing process and becomes solid, the ink is considered to be 100 percent solid. Since it does not contain any solvents, the printer manufacturers can claim that it has zero volatile organic compounds (VOCs).

By listing the ingredients as 100 percent reactive, claiming zero VOC is allowed. In traditional solvent printing, the majority of the ink is solvent, and most of it evaporates. Thus, in comparing UV-curing ink to traditional solvent printing ink, the VOCs in a UV curing system are probably insignificant, so there is no need to report them. In most UV-curable systems, a solvent is used to clean and maintain heads. One would think that this would also add to VOCs.

More study needed

A comprehensive study needs to be done in this area to determine the actual carbon footprint of the UV-curable printing system. At this point, it is not known if a complete study has been conducted. It would be wise to note that the printer carbon footprint can be drastically reduced by venting the printer through a HEPA (high-efficiency particulate air) and carbon filter. These are commercially available and are said to reduce VOCs by up to 98 percent.

Again, a comprehensive study would need to be done to evalute the actual VOCs discharged in an average printing day, and comparing a UV-curable system and a solvent system vented appropriately through HEPA and carbon filters.

On the plus side …

The biggest advantages of UV-curable inks are that they allow for greater durability, without lamination, of printed images, and they print on a greater variety of substrates. Most notably, UV inks allow high-quality direct printing on poster board, foam board, and other rigid substrates. In addition, UV inks allow direct printing on vinyl sheets, banners, wallpaper, textiles, and many other types of coated and uncoated media.

The questions remain: Does it technically outperform all other types of printing? Will the printer who purchases a UV system have an advantage over other printers who haven’t yet purchased a UV printer? Obviously, as in most things, it isn’t that clear cut.

On the other hand …

Most people who have UV printers seem to be very satisfied with the diverse capabilities of the UV inks and printers. On the other hand, ask printers who print on vinyl using solvent inks vs. UV inks, and you may find that many still prefer to print using regular solvent ink as opposed to UV-curable ink when printing on simple vinyl signage and banners. Some believe that the colors are brighter and the printers cost less, and it’s easier.

Also, solvent inks usually are preferred for highly abrasive applications. UV-curable inks have come a long way in being pliable, resistant to cracking on flexible substrates, and adhesion to these substrates, but they still have a muting effect on color and gloss on some of the substrates. For some printers this is not a problem, but for others, the loss of color gamut and intensity is not worth the cost of purchasing the UV printers.

Textiles, in particular

The advantages of printing signage on textiles are numerous. Textiles are usually less expensive and easier to handle. Having a UV-curable printer that allows roll-to-roll printing gives the printer the ability to print on various textiles. There are coated or treated textiles available in the market today from several companies. These textiles work very nicely. Having the coating or treatment increases the colorand adhesion.

UV-curable inks usually are capable of printing on other textiles as well, but are not as robust as the coated textiles. One of the biggest issues in UV-curable printing on textiles is the loss of the fabric handle. The UV-curable ink tends to stiffen the fabric feel. This may not be an issue for some signage, but can be a problem for stretch-type fabrics, apparel, and architectural textiles.

The same is essentially true for the solvent pigment system printing on textiles. Solvent pigment systems usually require a coated textile, and the print isn’t as robust as it is on vinyl. In textile digital printing, there exist more choices then just solvent and UV curable. One of the most predominant is sublimation transfer, whether oil, solvent, or water based. These methods by far yield the best color gamut and vividness on polyesters and do not change the feel of the fabric. Because it is a transfer process printed on paper first, a transfer press is needed. The water-based dye sub usually is limited in its ability to print extra-wide format.

A drawback in dye sublimation is outdoor light fastness, for which the UV-curable system is much better. For indoor applications and short-exposure outdoor applications, the transfer sublimation solution is superior. Fabrics transferred with dye sublimation are also washable and useful for printing apparel.

Other direct-print methods for printing on numerous textiles are also available. They always require some sort of fixing with heat or steam. Some also require or work better with coated or treated fabrics. The colors are usually good and most are washable or need to be washed.

Still improving

UV-curable inks have come a long way and are still improving. The advantage of direct printing on numerous substrates is by far the greatest strength of UV printing. The VOCs emitted in the UV-curing system are nonexistent or fewer than in most types of printing, but it seems that more work needs to be done to examine exactly how “green” the system is. Nevertheless, this is desirable because the VOCs are reduced enough or eliminated so that reporting them may notbe necessary.

There are challenges for UV-curing or radiation-curing systems in the future. Developing higher curing speeds under lower power would be preferable to increase the speed and decrease the cost of the units. This can also help to increase the color and strengths on other various substrates. UV curing does not work as well on many substrates for which other printing technologies are better suited, such as general textiles.

But as the manufacturers claim, it is a versatile system that works best on poster boards and various coated substrates with a wide color gamut and good adhesion, and works generally acceptably on many more. UV-curing inkjet is an excellent choice for many applications and, for many, the system of choice. For others, it may not be the ideal solution.

Daniel Slep, Ph.D, is director of technology at Hilord Chemical Corp., Hauggauge, N.Y. (www.hilord.com). He will be speaking about inks at “Fabric Graphics—Your Environmental Edge,” a symposium at IFAI Expo ’08, October 21, 2008.

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