Ink Technology
Thursday, July 1, 2010
Improvements in Eco/Mild/Light Solvent Inks
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This illustration depicts the molecular structure of cyclohexanone, a highly flammable liquid that is part of the ketone group (flash point of 111°, and a vapor pressure of 5.2 mm Hg at 77°). It is commonly used in true solvent ink formulations and is considered a volatile organic compound (VOC). By comparison, diethylene glycol diethyl ether—a flammable substance belonging the ether group—is commonly used in mild solvent ink formulas. But with a higher flash point (130°), and a vapor pressure of 0.54 mm Hg (at 77°), it’s significantly less volatile than substances in the ketone group.
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Several years back a few new terms were introduced into the wide format inkjet world: eco-solvent, light-solvent and mild-solvent. The terms, which some people will say is more a marketing concept than a scientific one, does convey the idea that there exists a class of wide-format inkjet ink that is not aqueous, not UV, but a solvent–just not a real strong solvent.
A LITTLE CLARITY
As of this writing, there isn’t an official or widely accepted definition for mild solvent inks as a class. Instead, the industry uses a variety of terms and brand names to convey product descriptions.
In a glossary found on the Flaar Reports website (www.wide-format-printers.org) edited by S&DG technical advisor Nicholas Hellmuth, Ph.D., the term eco-solvent is described as “a made-up name that attempts to make nasty solvent ink more socially acceptable.” Hellmuth prefers to group all eco-, light-, and mild-solvent inks together and to refer to them all as “mild solvent,” which for simplicity’s sake I will also follow here, knowing that things like this never fall into neat little categories. The differences between individual mild solvent inks can be dramatic, so it would not be accurate to think of all mild solvents as being equally mild.
For example, Reed Hecht, product manager, Epson Professional Imaging says, “Epson’s UltraChrome GS ink has the environmental benefits of a mild solvent, but when adhering to media, it acts more like a hard solvent.”
Still, it’s useful to have a “mild solvent” concept, if not an official classification because these inks do occupy that place in the market between relatively safer but less durable aqueous inkjet inks and full-blown highly durable solvent inkjet inks.
Ken Kisner, President, INX Digital Americas says the company typically uses regulatory definitions to define its products and has “developed definitions internally that assure we are not green washing or misleading our customers.”
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Formulas for eco-solvent inks have improved vastly over the years. Mutoh’s entire line of ValueJet printers, which are often used for outdoor applications such as this colorful wrap job, employ Mutoh’s Eco-Ultra eco-solvent inks.
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“Epson does not differentiate ink based on the element components but instead on the market segment and the application needs it meets,” says Hecht. “For example, if it can be used for outdoor signage and meets the requirements of longevity and adhesion to outdoor signage media, then it is true solvent ink.”
BASIC CHEMISTRY
Nomenclature aside, mild solvent ink is more than just a marketing ploy. As a whole, mild solvent inks are relatively less harmful to the environment and relatively less objectionable to the human beings who handle them than full-strength true solvent inks are. But “relatively” is the operative word here. It’s a matter of differentiating quantities and percentages, and relative volatility of the solvents used in ink formulations. There’s a trade-off involving a number of factors such as color gamut, ink adhesion, print speed, dry time, media compatibility and outdoor durability–as well as overall environmental impact.
The Flaar Reports glossary goes on to explain “eco-solvent ink (and most mild-solvent) must lack cyclohexanone to be called eco-solvent.”
Cyclohexanone, it turns out, is a highly flammable colorless liquid that belongs to the ketone group and has a flash point of 111 degrees (F). With a vapor pressure of 5.2 mm Hg at 77 degrees (F), it would be considered a volatile organic compound (VOC). However, compared to methyl ethyl ketone (MEK), an ingredient found in some high-speed solvent inkjets that has a flash point of 26 degrees (F) and a vapor pressure of 100 mm Hg at 77 degrees (F), cyclohexanone is—relatively speaking—not as volatile.
By comparison, diethylene glycol diethyl ether, a substance that belongs to the ether group, makes up large (but varying) percentages of a number of mild solvent inks. It is also a highly flammable colorless liquid, but with a slightly higher flash point of 130 degrees (F). With a vapor pressure of 0.54 mm Hg at 77 degrees (F), it’s significantly less volatile than substances in the ketone group.
(Sources: http://cameochemicals.noaa.gov and MSDS information found on various printer and ink manufacturers’ websites.)
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Recently Roland has developed Metallic Silver Eco-Sol Max inks, a silver metallic mild solvent ink for use in the Roland XC-540MT inkjet printer. When used with the printer’s other colors, it can produce metallic prints in many shades. The ink actually gave rise to a major hardware improvement—an ink recirculation system.
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As with all inkjet inks, mild solvent ink chemistry is a soup of ingredients similar to those named above, mixed with various proprietary ingredients, that provides optimum performance when used in specific printers.
“There are many considerations when designing an inkjet formulation,” says Kisner. “A good example is one must confirm the selection of raw materials is compatible with the various components to the machine. There are many chemicals that a chemist can choose that will, for example, reduce a printhead’s life due to the corrosive behaviors of the raw materials that were selected.”
You don’t have to be a chemist to understand that there must be vast differences in how these inks perform compared to other kinds of inks and to each other. Not to be trite, but as they say, “It is what it is, and it ain’t what it ain’t.” Or put another way, “The printers and ink are fine for what they are,” says Hellmuth.
And what they are is a solution that enables small sign shops with limited space to, as Oransky says, “get the kind of durability that their print buyers and sign buyers want without having to go to full-on hard solvent systems that require venting of dangerous fumes.”
But it wasn’t always necessarily so. Early complaints were threefold: dry time, media compatibility and durability.
Roland’s Eco-Sol Max, for example, is in its third generation and those concerns have been largely addressed with new ink formulas and redesigned printers.
“Early generations simply didn’t dry as fast,” says Oransky. “In those days you had to buy fairly specialized media, so you found yourself paying more money for substrates. The compatibility of the current set is much greater, to the point where you can run a huge range of off-the-shelf vinyl and typical sign materials without having to go to specialty materials that are coated or treated for that ink set.”
The third concern has always been and continues to be outdoor durability, which has been improving quite a lot since the early days as well.
Similarly, Hecht says that Epson’s UltraChrome GS ink “is engineered with new formulations, compounds, elements, and pigments to emit fewer harmful VOCs and minimize the impact on the environment and those professionals working with it.” He says additionally, the new GS ink has virtually no odor and that advances in ink chemistry have not impacted color gamut, ink adhesion, or media compatibility.
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This Morgan 8 sports car, winner of the Street Art category of this year’s S&DG Auto Art Contest, was printed by The Wrap Shoppe, Phoenix, using an Epson GS6000 printer and Epson’s UltraChrome GS mild solvent inks.
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“With Epson’s UltraChrome GS ink, we have seen improvements in every part of the ink chemistry,” Hecht says. “Solvent ink evolution has led to a range of hardware improvements. In the case of the Epson Stylus Pro GS6000, we’ve eliminated the need for external dryers, filters, and expensive exhaust systems to control caustic fumes. In addition, media has improved with the advent of newer types of media for different signage applications, such as wall film, textured wallpaper, and smoother adhesive backed vinyls like SuperCast from Avery.”
Heater systems are another area of improvement. Oransky points out that Roland started out with single stage heaters that were added onto the chassis of printers that had been designed for aqueous inks.
“By adding heating systems and beefing up some of the plumbing, we could start using them for early generations of eco-solvents. But now we’re building printers from the ground up that have two- and three-stage heating systems. So that’s something we’re building into the printer in a much more fundamental way rather than making it an add-on.”
But Roland’s most recent technical development has been its Metallic Silver Eco-Sol Max ink, which is a silver metallic mild solvent ink for use in the Roland XC-540MT inkjet printer.
The ink actually gave rise to a major hardware improvement, which in turn makes silver inkjet ink practical: an ink recirculation system.
“We’d been looking at the formulation of this silver ink for quite some time and discovered fairly early on that the tricky part was not in fact the chemistry,” says Oransky. “There was some difficulty to that, but the tricky part was designing a mechanical system that would allow it to run effectively and with a minimum amount of user intervention.”
ANALYSIS
Hecht believes that solvent ink is still the backbone of the printing industry. “It is the standard for vehicle wraps, banners and signs, and now, with advances in quality by the Epson Stylus Pro GS6000, the new standard for reproduction fine art,” He says. “UV technology and latex technology have specific applications in which they excel, such as UV being great for rigid substrates. However, neither have established that they can stand up to the durability, image quality, color gamut, and user experience that solvent printers are capable of today as a tried and proven technology.”
Andrew agrees. “The popularity is established. It’s a proven and reliable technology and it’s somewhat mature. And, it’s cost effective for end users to produce outdoor durable output on inexpensive materials, using a relatively inexpensive ink in a confined environment. Every technology has its lifespan. If we look far enough off, everything is going to be replaced. But I think in the short- to mid-term, eco-solvent has a lot of life left in it.”
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