New water-based inks—designed to perform as an alternative to solvent and UV-curable inks—offer comparable durability on numerous substrates, including vinyl-based media.

For more than 13 years, inkjet ink chemists have been working to develop inks and inkjet printing systems that would enable water-based inks to print with good quality and durability onto uncoated vinyl. I presented a paper in 1999 in Barcelona, Spain, discussing the research being done at DuPont, and the promise it held. DuPont filed its first patent on this technology in 1997. 

At that time, water-based inks were thought to be the “holy grail” for vinyl printing—if it could be done. Needless to say, it has been a challenge, but one well worth accepting because virtually everyone believed that the replacement of solvent inks with water-based inks in this application would be a big winner in the marketplace. Early attempts to commercialize this technology included the “Vinyl Jet” system from Encad, and more recently Lumocolor inks from Staedtler. However, they have not been embraced by the industry. 

 

With the introduction of HP’s Latex water-based inks two years ago, and now with Austrian manufactured Sepiax water-based inks introduced in the U.S. by Graphics One we now have working examples that indeed water-based inks can print directly onto vinyl in a commercially successful way. 

 

I thought it might be helpful to discuss the pros and cons of these two very similar ink formulation approaches as alternatives to solvent/mild solvent/eco-solvent inks and UV-curable inks. As is usually the case with new technology introductions, there are promises of great performance but little emphasis on the performance limitations. Here I will try to explain how these inks work, how they differ from other ink types as well as some of the limitations of these technologies.

 

Media must be heated to a certain temperature to perform well with these new inks, usually requiring calibration of the printing system so that the correct temperatures can be achieved without damage to the media.

A UV-CURE / SOLVENT ALTERNATIVE 

UV-curable inks and all types of solvent inks pose certain hazards in their use, whether it is from the reactive ingredients in uncured UV inks or the solvent vapors from solvent-based inks. Both Latex and Sepiax ink technologies attempt to avoid these hazardous materials and replace them with less hazardous materials and water. The objective is to do this while still providing good performance on uncoated vinyl as well as other non-porous media not designed for water-based inks. Vinyl substrates dominate solvent inkjet printing and are a mainstay in rigid media as well. So, first and foremost, these inks must work on vinyl and give performance competitive with solvent and UV inks if they are to become attractive alternatives. 

 

With solvent-based inks, small amounts of active solvents are incorporated in the inks that soften the vinyl and dissolve the plasticizer in flexible vinyl substrates, allowing the binders in the ink to bond to the polymeric vinyl structure when the solvent is evaporated. 

 

UV-curable inks in their liquid form are similar to solvent inks in that they contain components that perform like solvents. These components are made principally of organic molecules (monomers and oligomers) that act like a solvent—softening the vinyl and dissolving plasticizers—allowing the same types of solvent-like bonding mechanisms. But rather than being evaporated, as with solvent inks, they cure when exposed to UV light, thus rendering them inert.

 

HP’s Latex ink printing system is designed to work with HP’s Latex printer series, including the HP Designjet L25500 shown here.

The Sepiax inks, which are compatible with Epson piezo printheads and require only one heating station on the printer, can be made to work with a range of commercially available printers, including the Mimaki JV3 model shown here.

THE CHALLENGES OF WATER

With water-based inks, the first problem on non-porous substrates is coalescence (the merging of the individual ink droplets into a puddle once on the substrate). To avoid this with these inks, the printing speed must be slow enough to allow the substrate to be heated enough to quickly evaporate enough water. Rapid evaporation of the water prevents coalescence from occurring. Control of the temperature is critical since ink spot size can vary if the temperature varies. Since water takes a lot of energy to evaporate compared with organic solvents in solvent inks, this is a significant issue. 

 

In addition, bringing a variety of different media to the required temperature usually requires calibration of the printing system so that the correct temperatures can be achieved without damage to the media. Hence, with the HP Latex inks, HP branded media work best because the printer has already been calibrated to work with the various HP media types. Performance on non-HP media may prove problematic unless the printer is adjusted to optimize performance. 

 

HP’s Latex inks are about 70% water, and they require two heating zones—built into the HP Latex printer series. The first heating zone evaporates the water, and the second evaporates the co-solvents, which require a higher temperature—about 125 degrees Fahrenheit. The two zone temperatures and the printing speed need to be optimized for each media type being printed in order to obtain the best results. 

 

The Sepiax inks are about 50% water. Sepiax requires the substrate to be heated to at least 125 degrees Fahrenheit at the time of printing to flash off most of the water and render the ink dry to the touch. Since only a single heating zone is required, and since the inks have been made compatible with common Epson piezo printheads, Sepiax inks can be made to work in a number of commercially available printers. Once the print system is optimized for the inks and specific media, the performance is said to be excellent and most materials can be printed to give acceptable performance with this ink. 

 

Sepiax inks have been made to successfully print onto a range of unusual media, including tissue paper, shown in this test print.

WHEN WATER-BASED INKS BECOME SOLVENT INKS

Both Latex and Sepiax inks contain a significant percentage of co-solvent, as much as 50%. These co-solvents are organic solvents similar to those used in water-based office inkjet printers. When the water is removed, the remaining ink is converted into a kind of solvent ink. This solvent is removed in the second stage heating process in the HP Latex printers; and for Sepiax most of it is flashed off with the water making the ink dry to the touch and the remainder evaporates over the next day or so. The key to achieving the adhesion is the solvent ink stage in the drying process. The solvent—along with surfactants, or wetting agents—enable the ink to cut through the plasticizer in the vinyl’s surface, allowing it to bond to the vinyl much like the aggressive components in conventional solvent inks. 

 

With UV-curable inks, the monomers and oligomers that make up most of the UV liquid inks act as a solvent in the liquid state, performing the same function before being cured with UV light. For water-based inks, once the co-solvents are gone from Latex and Sepiax inks, the binder (latex or resin) and pigment combination forms a film on the media that can provide comparable durability to many solvent or UV-curable inks. 

 

CONCLUSIONS

While these new inks show promise as an alternative to solvent and UV inks, they have a number of drawbacks, particularly with respect to the calibration of the heating requirements and its effect on printing speeds with different media. And in the case of Sepiax, I see the lack of a printer designed specifically to run these inks as a drawback. And, despite some of the media hype, these inks don’t adhere well to everything and don’t run in every Epson printhead-based printer without add-on hot air heaters and reduced printing speeds. 

 

For example, with Sepiax inks, glass must be coated or treated to achieve good adhesion, and Graphics One provides a service to test your media to see if Sepiax sticks to it; and with Latex inks, a “treatment” or coating must be applied to Polyethylene and Polypropylene film substrates to achieve adequate adhesion. Still, treatments are not uncommon, and many substrates must be coated or treated to properly perform with both solvent and UV-curable inks.

 

Still, both HP and Sepiax have made a good start in developing a new class of inkjet inks that avoid many of the negative characteristics of solvent/mild solvent/eco-solvent and UV-curable inks while giving the user output that is competitive with these technologies. 

 

I’m certain work will continue to further develop these water-based ink technologies to overcome their shortcomings. Both companies deserve kudos for their efforts in giving us more environmentally friendly inkjet printing solutions for outdoor printing applications.