MIT Technology Review

Tuesday, March 13, 2012

Laser Erasers Gently Remove Ink from Paper

The trick could reduce the carbon emissions and energy usage associated with recycling paper.

Using laser light to remove toner from printed paper could lead to a new type of Xerox machine capable of both printing and "unprinting." A new technique developed by researchers in the U.K. can remove ink without damaging the paper—which would mean that a single piece of paper could be reused up to five times before being condemned to the recycling bin.

Just as printing a page uses heat to deposit ink, the unprinting process uses heat to remove it. Julian Allwood, head of the low-carbon materials processing group at the University of Cambridge, who devised the technique with doctoral researcher David Leal-Ayala, says the trick is to do so without causing any heat damage to the underlying cellulose fibers. "What we're trying to do is vaporize [the ink] very rapidly."

Previously, Allwood and Leal-Ayala demonstrated that abrasion with sandpaper can remove toner ink. "It works, but you can't avoid thinning the paper, so the number of times you can reuse it is limited," says Allwood. Using laser light to ablate the paper avoids this problem, he says. 

Ultrashort (nanoseconds-long) pulses of green laser light are absorbed by the relatively dark toner, whereas the cellulose fibers are almost transparent to the pulses and so are left unaffected. The only danger is that the heated toner will transfer heat to the paper, says Allwood, but using the short pulses ensures that the toner vaporizes before it has a chance to.

Publishing their results today in the journal the Proceedings of the Royal Society A, the pair showed that the new method produces, at worst, half the carbon emissions of recycling, and, at best, nearly one-twentieth.

There is already a huge industry devoted to removing ink from paper as part of the recycling process, says Carinna Parraman, deputy director of the Centre for Fine Print Research at the University of the West of England in Bristol. De-inking is used to improve the quality of cellulosic material in recycled paper, she says. But to remove the ink without recycling the paper offers great benefits, since recycling uses an enormous amount of energy and resources, particularly water.

Precisely how the technique could be incorporated into a photocopier or printer remains to be seen, says Leal-Ayala. Although printing already involves heating toner, removing it requires higher temperatures. So it is not clear whether the same laser could be used to both print and unprint.