Like Water for Solvents

Theodore Lustig -- graphic arts online, 1/1/2001

Since their introduction two decades ago, water-based inks have found applications in flexographic, gravure, and letterpress inks.

But the next logical step, lithographic inks that are also water-washable, had, until now, eluded ink research scientists. Substituting water for solvents to clean inks from offset presses would be a major step in improving pressroom environments and addressing worker safety.

POD advantages

At last year's National Printing Ink Research Institute (NPIRI) annual technical conference, a presented paper dealt with the development of commercially viable water-based, water-washable, waterless lithographic inks. They should be particularly useful to waterless printers for short-run, print-on-demand offset color work.

The introduction of waterless printing in the 1980s freed printers from many of the constraints imposed by two-fluid (ink plus fountain solution) oil-based inks. However, as late as 1992 the use of offset water-based inks had not reached commercialization, principally because neither attaining stability on conventional offset rollers nor how to make a water-based ink with high-enough viscosity and tack had been solved.

Additionally, there were severe problems with the then-available lithographic plates. A wide range of press problems evidenced themselves when printers switched from solvent-developed plates to those that were water-developed, not the least of which was image blinding.

There also existed the high cost of available plates, a problem that has since been somewhat mitigated by competitive entrants into the market as well as technological innovations.

For use with washable lithographic inks, new plates will use aluminum as the image area and silicone as the non-image area. This restores the balance between surface energies of the image and non-image areas, resulting in much improved stability and a dramatically reduced evaporation rate, both of which are achieved by chemical means.

Although the heat of the vaporization of water is considerably higher than that of typical ink solvents, total energy consumption has been calculated as actually being lower.

Use with existing presses

The novelty of this new ink system, say researchers, is that it can be used with existing offset printing units. Because the inks themselves are water-based, organic cleaning solvents are eliminated, as blankets and ink distribution rollers can be washed off with plain tap water. This also achieves two environmental goals: removal of volatile organic compounds from the pressroom, and elimination of spray powders in sheetfed jobs when infrared dryers are used.

Another desirable property is the absence of ink misting during press runs. This unwanted phenomenon is a direct consequence of the ink transfer process, occurring when two inked rollers rotate at high speeds, causing fragments of ink to coalesce into droplets that are propelled as a mist into the immediate atmosphere.

It has also been established that this ink mist acquires an electric charge, the magnitude of which is relative to press speed and the rate of ink separation. It is theorized that, even at high speeds, the much higher dielectric constant of water-based inks is the reason for the reported complete absence of misting.

Increased opportunities

These new inks should offer increased opportunities for the growth of waterless lithography, which until now has had a limited market due to plate availability, the cost of solvents, and the necessity to control roller temperatures during printing-all problems that now seem to be solved.

Suggested economies include lower plate costs, higher productivity, less start-up waste, some energy conservation, and decreased labor costs, the latter coming from time saved during wash-up.

These new types of waterless inks are based on entirely different chemistries than conventional waterless products. The polymers used provide the needed balance of both ink and print, including drying and setting speeds, gloss (both low and high), rub resistance, and cost through the potential for increased productivity.

Furthermore, say researchers, the new materials introduced into the formulations open unlimited possibilities for future research and development work.

These water-based, water-washable inks are examples of how the ink industry and its suppliers continue to keep pace with advances in printing processes. To receive the full-text NPIRI conference paper, contact the National Association of Printing Ink Manufacturers at (732) 855-1525.