The Solvent Ink Story

Ink chemists are the unsung heroes behind the growth and diversity of wide-format digital printing applications.

While the first 36"-wide aqueous-ink inkjet printers in the 1990s gained traction for outputting photo enlargements and engineering drawings, inkjet chemists were already working on digitally jettable versions of the solvent and UV-curable inks that screen printing companies used to produce short runs of outdoor-durable signs, vehicle decals, industrial labels, and promotional products.

Solvent inks generated interest in inkjet-printed outdoor graphics by eliminating the time and expense of laminating large-format prints made with aqueous inks or electrostatic toners.

When start-up companies such as Idanit, Vutek, Signtech, and Nur introduced their first 16'-wide solvent-ink inkjet printers, the devices intended to print billboards were also used to explore creative new forms of outdoor advertising such as bus wraps and truck-side advertising. Raster Graphics introduced the solvent-ink “Arizona Digital Screen Press” so smaller sign shops could digitally print full-color outdoor signs that didn’t require lamination.

Unfortunately, those first aggressive solvent inks limited the opportunity to promote wide-format digital printing as an environmentally friendly alternative to the chemically intensive screen-printing processes.
The first solvent inks contained high levels of volatile organic compounds (VOCs) that polluted the air as they evaporated and caused health and safety issues for press operators.

Since then, continuing shifts in environmental, health, and safety regulations have resulted in three broad subcategories of solvent inks: “true” solvent, “mild” solvent, and “light” solvent (eco-solvent) inks.

As a result, advanced eco-solvent inks make it possible to produce outdoor-durable graphics in non-industrial settings. For example, some printers enable novices in homes, schools, shops, and offices to print pro-quality indoor and outdoor window graphics and other signs, stickers, decals, posters, and wall art, as well as graphic transfers for heat-applied apparel decorating.

Here’s a quick overview of each category, advances in other ink types, and what wide-format inkjet printer buyers should consider.

True Solvent

Also known as “hard solvent” inks, these fast-drying inks use aggressive solvents to help the inks adhere to the inexpensive uncoated vinyl that large-format graphic screen printers use. The hard solvent inks are specifically formulated so that inkjet prints on uncoated vinyl wouldn’t fade, peel off, slough off, or rinse off in outdoor weather.

“Hard solvent inks were designed to dissolve any of the plasticizers that exuded from the vinyl,” explains inkjet manufacturing consultant Dr. Ray Work, who has a PhD in chemistry. Plasticizers are the chemical additives that keep pliable, flexible vinyl from becoming brittle.

“The solvents physically cleaned plasticizers from the surface so that the binders in the ink could penetrate the surface and cause a physical bond with the vinyl,” Work says. Without a strong physical bond to the uncoated substrate, it’s like the ink would have sloughed off due to abrasion.

“Cheap vinyls were loaded with plasticizers,” recalls Work. “Some vinyl manufacturers used different amounts of plasticizers in the winter than in the summer because vinyl would get more brittle in the winter.”

“For many years, hard solvent inks were the only solution for durable outdoor prints,” recalls Shahar Turgeman, the founder and owner of STS Inks. Solvent inks provided excellent adhesion to a wide range of substrates and could last up to five years outdoors without lamination.

But because of their high levels of VOCs, solvent inks were unsuitable for producing indoor graphics. The printed graphics would continue to emit VOCs and a noticeable odor for weeks.

Those first solvent-ink printers could be used only in manufacturing facilities that were well-equipped to minimize the fumes, flammability potential, and employee exposure to harmful chemicals.

Specific procedures for ink handling and disposal had to be followed. And preventive maintenance was a must because true solvent inks degraded the performance of the printheads and ink delivery systems.

Mild Solvent Inks

“Mild solvent inks are less aggressive than true solvent inks, but they still contain VOCs. These inks are often used to print on vinyl and other synthetic materials.” says Marco Zanella, global business development director for inkjet, INX Europe.

“Mild solvent inks strike a balance between durability and environmental concerns,” says Turgeman. “They offer good adhesion and outdoor durability of two to three years without lamination, while being less harmful to the environment and health than hard solvent inks.”

Vehicle wraps and other graphics printed with mild-solvent inks should be laminated if they require higher levels of lightfastness and scratch resistance. But the prints can’t be laminated until all solvents in the inks have evaporated (outgassed). Otherwise, the laminating film’s adhesive won’t bond to the print properly.

Light Solvent Inks

Light solvent inks are also known as “eco-solvent” inks because they are specifically formulated to meet or exceed stringent environmental regulations.

Light solvent inks have the lowest VOC content among the three categories. These inks use a combination of mild or biodegradable solvents along with pigments or dyes as colorants. They use advanced ink chemistry to achieve good adhesion and durability on a range of indoor and outdoor media including some inkjet canvases and inkjet photo papers.

Several iterations of eco-solvent inks have been developed to comply with ongoing changes in regulations related to Volatile Organic Compounds; Hazardous Air Pollutants (HAPS); HazCom Standards; occupational safety, and health (OSHA); Waste Disposal regulations; Restriction of Hazardous Substances (RoHS); and REACH human health and environmental guidelines in Europe.

“Eco-solvent inks today contain significantly less highly volatile organic compounds than the hard solvent inks that were initially developed in the late 1990s,” confirms Zanella. “They are much more environmentally friendly.”

“But even though newer eco-solvent inks are very good,” Work believes the term “solvent” in “eco-solvent” raises fears of hazardous chemicals and health risks.

To dispel negative perceptions, eco-solvent ink manufacturers have had some inksets independently evaluated to meet UL GREENGUARD GOLD standards. The GREENGUARD GOLD certification means that emissions from the solvent are low enough that graphics can be safely used in health-sensitive environments such as schools and healthcare facilities.

Competition from Other Ink Types

As solvent inks were being refined for use in non-industrial settings, other types of inks for outdoor-durable graphics entered the market.

At drupa 2008, HP introduced water-based latex inks that enabled print service providers that produce outdoor-durable graphics to meet some of the stricter environmental policies being adopted in the EU and states such as California.

While latex inks included some mild solvents to clean the surface of substrates, the solvents weren’t intended to help the inks penetrate the substrate surface. Instead, latex inkjet primers use an optimizer layer to prepare the substrate’s surface for printing. A radiant heating system melts a latex polymer over the pigments to keep the colorants from sloughing off.

Epson and Roland, two leading suppliers of mild-solvent and eco-solvent printers, now offer wide-format inkjet printers that use resin inks. These durable water-based inks work similarly to latex inks and can be used not only with non-porous outdoor sign materials, but also with uncoated paper, wallcoverings, fabrics, and other indoor substrates.

“Eco-solvent inks consist of pigments suspended in a solvent base. Latex inks and resin inks are made up of pigments suspended in a water-based emulsion,” explains Zanella. He suggests that whoever is marketing latex and resin inks may be trying to divert the end user’s focus from the carrier to the binder in the ink: “This is a move dictated by marketing.”

The first UV-curable inkjet inks were introduced at drupa 2000 on an Inca Digital flatbed printer that would enable print service providers to output graphics directly on rigid signboards instead of mounting graphics printed of adhesive vinyls.

These UV-curable inks couldn’t print on flexible, heat-sensitive materials such as vinyl, and were prone to cracking when stretched. But thanks to innovations in specialty ink chemistries and LED curing, UV inks are now available for many outdoor applications, including thermoformed signs and truck graphics.

What Inkjet Ink Buyers Should Know

There are no standard definitions for “true solvent,” “mild solvent,” and “light solvent/eco-solvent” inks. That’s because not all true solvent inks, mild solvent inks, or eco-solvent inks are alike. Solvent inks in all three categories have been reformulated in recent years to work in modern printheads and reduce fumes and safety concerns.

“One of the biggest myths about solvent and eco-solvent inks is that they are all harmful to the environment and to human health. This is not true,” says Zanella. “Eco-solvent inks are specifically designed to be safer for the environment and for users. They contain significantly less VOCs and have a lower odor than traditional solvent inks.”

“There’s a misconception that all solvent inks are extremely harmful and have no place in eco-conscious printing,” says Turgeman. “Some solvent inks have been reformulated to reduce VOCs and can be used safely in well-ventilated environments.”

For now, it’s best to read Material Safety Data Sheets (MSDS) to determine if the ink requires any special handling, ventilation, or disposal procedures. Following the manufacturer’s guidelines for printer maintenance can prolong the life of the printhead and ink lines.

The lack of a standard definition of eco-solvent inks has led to confusion. “Different manufacturers may have varying definitions and criteria for their eco-solvent products,” says Turgeman. He notes that some people confuse eco-solvent inks with water-based inks, or assume that all eco-solvent inks are entirely environmentally friendly: “While eco-solvent inks are generally less harmful than hard solvent inks, some still contain chemicals that have environmental and health impacts. They are not ‘green’ the same way that water-based inks are.”

Plus, some eco-solvent inks are not completely odorless. Some odors can be detected in poorly ventilated spaces.

Many eco-solvent inks offer excellent longevity and resistance to outdoor conditions. According to Turgeman, it’s a mistake to assume that eco-solvent inks are significantly less durable than true solvent inks. When used with certain types of high-quality media, some eco-solvent inks can outperform the excellent durability and scratch-resistance of prints produced with latex or resin inks.

Some eco-solvent printers produce expanded color gamuts. To expand the versatility and capabilities of eco-solvent inkjet printers used by sign shops and wrap companies, printer manufacturers such as Mimaki, Epson, and Roland offer wide-gamut ink sets that go beyond CMYK and light magenta and light cyan inks used for photo reproduction.

The Epson SureColor S80600 offers nine colors of UltraChrome GS3 ink that enable printer users to reproduce “98.2% of Pantone colors,” says David Lopez, product manager, Professional Imaging, Epson America. The ink set includes red, gray (light black), and orange, plus an optional white or silver ink.

According to Lopez, “It’s not uncommon for a sign shop to use two or three SureColor S80600 printers with the expanded color gamut to get the high-quality output. Having multiple units helps print shops achieve faster production turnaround times rather than using one faster printer that uses two sets of CMYK inks. A CMYK-only printer generates a smaller color gamut and lower image quality.”

The Mimaki CJV300 Plus eco-solvent inkjet printer/cutter offers 10 color options including silver. white, orange, and light black inks.

“White ink has exponentially expanded the number of substrates usable for digital printing,” explains Josh Hope, senior manager, Segment Marketing, Mimaki USA. For example, white ink base layers make it possible to print true, vibrant graphics on window clear films or colored vinyls.

“Orange ink is fantastic for expanding the color gamut and hitting tough spot colors such as PMS165 (a warm orange). Silver inks enable metallic effects to be printed. Metallic effects can be stunning when combined with CMYK inks.” When silver is combined with CMYK, it’s possible to achieve the look of other metallic colors such as gold or bronze.

Roland TrueVIS VG3-540 and VG3-640 eight-color large-format inkjet printer/cutters can use orange and green inks in various combinations with CMYK inks, white inks, and light black inks.

Each new formulation of eco-solvent inks brings new marketing challenges. If future regulations restrict the use of any of the solvents used in the current ink formulations, the term “eco-solvent” could expose the ink marketer to penalties for greenwashing. It could also lead to changes in the performance of the ink.

"Formulating inkjet inks can be very complicated because it’s hard to balance the many different requirements,” explains Work, who developed some of the first aqueous pigment inks for DuPont.

“Changing one ingredient to meet a new environmental regulation can affect how well the colorants adhere to an uncoated surface. Each new formulation of ink affects the adhesion.”

Ink performance expectations also evolve over time. The binders used in solvent inks (and other inkjet inks) must be flexible enough, so the dried ink doesn’t crack when the graphic is cut.

Tens of thousands of new- and old-model solvent-ink printers are still used. “True solvent printers are still in use in various industries,” says Turgeman. “But their prevalence has declined in favor of more environmentally friendly and versatile inkjet technologies that use eco-solvent, latex, and UV-curable inks.”

True solvent inks are mainly used to print only on vinyl, says Zanella. Because of their high levels of VOCs, they are typically used only for industrial applications.

Specialty ink suppliers such as INX, STS Inks, Sun Chemical, and Nazdar have developed replacement ink sets so that users of older solvent inkjet printers can reap the benefits of advanced solvent ink chemistries.

Plus, many true-solvent and mild-solvent inkjet printers now have bulk ink systems for lower ink costs and longer periods of unattended production.

Every business will have different requirements and working environments. The best bet for choosing the right printer is to consult with a reputable inkjet printer manufacturer that offers a full line-up of printers that use different types of inks.

Describe the range of applications you would like to produce, the types of materials you plan to use, and the environments in which you would like to print the graphics.

Also compare the total operating costs. For example, fusing latex or UV-curable inks to a substrate may require more energy than evaporating the gases from eco-solvent inks.

“In general, eco-solvent inks will require less drying time and therefore less power consumption at the printer,” says Hope.

“If you are considering using solvent or eco-solvent inks, it is important to do some research and choose the right inks for your needs,” advises Zanella. “You should also consult with a printing expert to get advice on how to safely use and dispose of these inks.”

As all types of products become safer and more environmentally friendly to meet the demands of new generations of consumers, marketers may not need to use terms such as “eco-solvent” to distinguish their products. Just as the term “digital photography” has simply become “photography,” it’s likely that
“eco-solvent” inks may eventually be known as “solvent” inks.