Coating lets ink-jet prints dry quickly.

Digital cameras, computers, and ink-jet printers can turn home offices into color photo labs. To make high-quality images that dry almost instantly and don't smear, ink-jet printers need specially coated photographic paper and transparency sheets. A new report describes a coating that enables the newest papers to give sharp, bright images.

Bulent E. Yoldas, formerly of Carnegie Mellon University in Pittsburgh, developed a coating by combining an inorganic compound, a polymer, and an organic binder. Placed on paper or plastic, this thin film, less than 20 micrometers thick, immobilizes water-based ink droplets in about 30 seconds. "By the time it comes out of the printer, the print is dry," Yoldas says.

Ink-jet printers shoot tiny, closely spaced ink droplets onto a surface (SN: 4/10/99, p. 232). Color printers generate a spectrum of hues by combining cyan, magenta, yellow, and black. Since the printer deposits as many as 600 dots per inch of watery ink, prints on most surfaces take from 3 to 5 minutes to dry--plenty of time to get smudged by a stray thumb or the next page to print.

The earliest photo papers developed for ink-jet printing used a gelatin coating much like the ones on traditional photo paper, says Mike Sproviero, manager of research and development at Hewlett-Packard's ink-jet division in San Diego. "These work pretty well but tend not to have a good drying time," he says.

Newer, porous coatings made of inorganic materials dry immediately, quickly soaking up the ink like a sponge, but they have other drawbacks. The ink diffuses into the film, so the dots don't have sharp edges. Also, "there's a finite amount of ink [the film] can absorb," says Sproviero. "If you put too much ink down, it will flood. The colors will bleed together and look horrible."

The films developed by Yoldas and those now used by Hewlett-Packard work in a different way. The polymer molecules absorb the water in the ink, causing the film to swell like a balloon. The film can expand to seven times its original thickness. Since the water is securely bound to the polymer, the ink is effectively dry.

Eventually, the water evaporates, leaving the colored dyes on the film's surface. "The brilliance of the color stands out," says Yoldas. In contrast, colors in the porous coatings lose their intensity because most of the dyes rest beneath the surface. He describes the mechanism underlying the newest films in the June JOURNAL OF MATERIALS RESEARCH.

Although Yoldas' films give sharp, glossy prints, the images fade within days when exposed to ultraviolet light.

The images printed on Hewlett-Packard's quick-drying photo paper last only 2 years--several years less than they persist on gelatin-coated paper, says Sproviero.

Fast fading may be acceptable for temporary overhead transparencies but not for photos, Yoldas says. Changing the chemistry of the ink may make instant-dry images more colorfast.