Something New Under the Sun

We have been focusing on technology developments in both substrates and inks as the industry works to improve performance, reduce cost and to become more people- and earth-friendly. A major move forward in this area is a new technology recently disclosed by Hewlett-Packard in advance of their official planned product announcement at the big drupa tradeshow in Europe this summer.

It is very unusual for HP to present a technology in front of a product introduction. However, in this case we believe it promises to be one of the most important new innovations to be introduced to the industry in many years. HP calls the technology “HP Latex Ink.” The potential ramifications of this technology are significant and we can all look forward to its having a very positive impact on the safety and earth friendliness of inkjet printing technology used in industrial applications.

WHY LATEX INKS?
HP states their objective very simply. They want to provide the industry with a water-based ink and printing system that is a clean and safe alternative to mild/eco-solvent printing at a competitive performance, cost and level of productivity. While others try to provide less hazardous solvent inks and solvent inks made from renewable resources, HP has chosen to move to the least hazardous solvent available: water!

This move may be the end-game in industrial inkjet printing. While inks made with biodegradable solvents provide an improvement, they still are composed mainly of flammable solvents. And while UV-curable inks have no VOC emissions, they are hazardous in their liquid form and are made from very active chemicals. HP Latex Inks are not flammable, offer very low VOC emissions and are not hazardous in the liquid or solid form. This looks like a real winner.

WATER ON VINYL?
We all know that if you put water on untreated, uncoated solvent vinyl (the most widely used substrate in wide-format industrial printing today), there is nothing to absorb it and the water will run right off and make a puddle. Water doesn’t even wet vinyl. In addition, ordinary water-based pigmented inkjet inks don’t contain the polymer binders, like mild solvent inks do, that are needed to form a film when dry. The polymer binders are needed to encapsulate the pigments and give the printed images durability.

Traditional water-based inks require a coating on the media to provide the needed durability. Furthermore, we have all heard for years that thermal inkjet technology cannot tolerate large amounts of polymers because they burn onto the resistor.

The HP Latex Ink technology will be offered for printers with HP Wide Scan Printheads. These MEMS-based printheads employ thermal inkjet technology rather than piezo inkjet technology. HP and its ink partners say they have developed a printer and ink system using water-based inks that:

  • Wets vinyl
  • Can be dried on uncoated vinyl fast enough to avoid a puddle
  • Have thermal inkjet compatible binders which do not remain on the resisters (which would ruin the printheads)
  • Will form a film when dry and bond to the vinyl

Wow! Truth to tell, it sounds too good to be true.

THE PRINTING PROCESS
First, the HP Latex Ink vehicle (carrier) contains about 30 percent co-solvents (similar to those used in desktop printers) and 70 percent water. Dispersed pigments and latex polymer particles are added to the vehicle along with other additives, like surfactants (to control surface wetting) and ingredients to keep the resistors clean. The ink is jetted onto the vinyl or other compatible media (media currently used with mild-solvent printers). The liquid film of ink on the media is exposed to radiant heaters and an airflow in what HP refers to as the Print Zone.

Clearly, the surfactants and co-solvents in the ink are designed to wet the vinyl and to soften it, allowing the latex polymer to bond to the vinyl when dried and cured. On HP’s new printer, the Print Zone is the region of the platen where ink drops are jetted onto the print media, immediately under the scanning printheads. The temperature is not sufficient to cure the ink, so that the ink which might be dried on the printhead will not be cured and can be easily cleaned off by the service station.

HP’s two-step Latex Ink curing process:

a) In the Print Zone radiant heat and forced airflow evaporates most of the water and the liquid film condenses to a viscous mixture of co-solvents, latex polymer particles and pigment particles. High viscosity in the ink film immobilizes the polymers and colorant to set the dot size and minimize coalescence and bleed with dots in neighboring print locations.

b) When the media advances into the Curing Zone area a second dryer evaporates the co-solvents allowing the latex polymer particles to coalesce into a continuous polymer film that encapsulates the pigments.

In the Print Zone, the water is evaporated and the liquid film condenses to a viscous mixture of co-solvents, latex polymer particles and pigment particles. High viscosity in the ink film immobilizes the polymers and colorant to set the dot size and minimize coalescence and bleed with dots in neighboring print locations to give a sharp, high-resolution image. As the printed media advances out of the Print Zone and into the Curing Zone a second dryer evaporates the co-solvents allowing the latex polymer particles to coalesce into a continuous polymer film that encapsulates the pigments and strongly bonds to the vinyl.

As the print leaves the Curing Zone a continuous latex film (encapsulating the pigments) forms on the vinyl surface. No additional drying of the print is needed because virtually all of the ink vehicle has evaporated.

HP indicates that its Latex Ink formulation, along with the time and temperature of the heaters in both the Print Zone and Curing Zone, are critical to achieving the desired high image quality (using drops as small as 12 picoliters) and image durability. When the printed media leaves the printer, virtually all of the ink vehicle, including the co-solvents and the water, have evaporated. There is no smell from the ink and no residual solvent smell from the printed vinyl as is frequently the case with mild-solvent inks.

THE PRINTING SYSTEM
HP is withholding printer specifications until drupa (May 29-June 11) but they have released some important information. The printer will feature printheads and optical aligning systems that are major developments in themselves. The new HP Wide Scan Printing Technology is scalable and multiple printheads can be assembled into scanning arrays. Each printhead is 4.25" wide, contains five nozzle plates and offers 10,560 nozzles — that’s 1,200 nozzles per inch!

HP says the heads require no manual daily maintenance and are user-replaceable. No more service calls to replace a printhead like most piezo printers require. Each printhead supports two colors of inks. So, three printheads would be required for a six-color printer. That is a total of 31,680 nozzles! In a two-pass, unidirectional print mode (suitable for outdoor graphics) HP claims the printer will print about 800 square feet per hour and in a four-pass, unidirectional print mode (also suitable for indoor graphics, according to HP) speeds of about 400 square feet per hour are predicted.

In addition, the HP Optical Media Advance Sensor will allow the media to be advanced in precise steps allowing the printheads to place the drops very accurately. This is critical in avoiding banding.

COLOR AND DURABILITY
The color gamut provided by the HP Latex Inks is very similar to HP’s mild solvent inks, which have proven to be adequate for most applications. There are some subtle differences, but proper profiling will make them essentially indistinguishable. The polymer combined with light fast pigments give excellent lightfastness results.

Up to five years in-window un-laminated, and up to 10 years laminated are predicted on a range of media. HP says outdoor prints achieve display permanence up to three years un-laminated and up to five years laminated and are scratch-, smudge- and water-resistant on a range of media. This performance is predicted to be comparable to solvent ink technology. Media choice is up to the user, but the ink and printing system is designed to use media currently used for solvent printing.

CONCLUSIONS
HP appears to have a big winner with this new technology. The ability to deliver a system solution with this many new technologies and to deliver a safe and earth-friendly printing solution to replace solvent printing in many applications deserves to be applauded. Now we can only wait until Drupa to see the printer itself and determine whether the price/performance ratio meets industry needs. We will be writing more about these important technologies as more information becomes available.