Recycling revamped

By Simon Firth

Successful digital printing research isn’t just about creating new kinds of ink that adhere well to different kinds of media, says HP Labs researcher Hou T. Ng.

“On the one hand,” says Ng, “we’re trying to improve our ink’s adhesion, but on the other we want our technology to be environmentally sustainable. So we want to be able to take these inks off the media as well.”

That turns out to be a fairly complex challenge. For one thing, the media to which inks both need to stick to and be removable from can vary from coarse, thick cardboard to glossy, wafer-thin high quality office writing paper.

But media printed by many different methods – from home inkjet and laser printers, to commercial digital presses and conventional offset lithography – all gets recycled in the same waste stream. So new digital inks need to be removable by a process that also successfully ‘deinks’ other printing products at the same time.

Ng and several colleagues in HP's Commercial Print Engine laboratory, headed by Eric Hanson, have been researching HP’s newest digital printing technology – liquid electrophotographic printing (LEP) – and in the process have been investigating how best to deink LEP printed media.

While conducting their research, the team quickly established that LEP ink could be removed by conventional methods. But they also established that changes to the conventional recycling process could massively improve the efficiency of LEP deinking.

And their research had an added bonus. What they came up with, says Ng, "was a baseline deinking process that will work with a wide range of inks and print media."

Ng outlined that process in a presentation to the Society for Imaging Science and Technology’s 25th International Conference on Digital Printing Technologies in Louisville, Kentucky. Most significantly, it promises a 'universal' solution that the recycling industry could adopt with relative ease - and one that, if adopted, could make paper-recycling both a cheaper and more environmentally-friendly process.

Improving the technology of paper recycling

Paper recycling typically involves a complex series of stages: shredding the mixed paper waste, forming a pulp by adding liquid chemicals, screening the pulp several times, removing the ink in a flotation cell, bleaching the recovered pulp, thickening it and then storing the pulp in a tank prior to reusing it as a constituent of new paper.

In their research, Ng and colleagues Manoj Bhattacharrya and Laurie Mittelstadt found that huge improvements could be made to the quality of deinking just by making changes in two key stages: pulping and flotation.

Two kinds of chemicals are mixed together in the pulping stage. Ethoxylated fatty alcohol (EFA) interacts with the ink and fibers, separates them and prevents the ink from reattaching. EFA also aggregates fine ink specks and disaggregates larger ones, making the job of the second chemical, an anionic surfactant, easier.

Anionic surfactants interact with the ink particles and create an ink speck-filled foam that can be skimmed off in the flotation stage, leaving the clean paper fibers to be recovered.

When the two are placed together, though, they tend to interact with each other, leaving some specks attached to the fibers. But if the EFA is added in during pulping and then the surfactant is put in at the floatation stage, the HP team found, LEP deinking is strikingly more successful.

Breakthrough deinking chemistry

This new process does more than just deink LEP inks better, says Ng. "It gives us a universal deinking chemistry that caters to digital commercial prints and inkjet dye-based ink." Furthermore, Ng reports, “Our most recent data shows that we were able to use this deinking chemistry for offset media, like newsprint, which is one of the hardest media to deink.”

Another advantage is that the process works at a neutral pH level while many conventional recycling methods use a highly alkaline treatment that is far less environmentally benign. And because it doesn’t require that paper mills adopt new chemicals or substantially alter the equipment they use in recycling paper, Ng is hopeful that the new process will be fairly easy to adopt on an industrial scale.

Tackling the challenges of deinking with advanced molecular modeling

Deinking research is very much a part of HP Labs’s mission to advance the state-of-the-art by doing a deep dive into the science and technology of digital inks.

The deinking team’s next challenge, Ng says, will be to “dive deeper into creating more ways to tweak this new process from both a chemical and a process perspective. We are using, for example, advanced molecular modeling techniques to understand how these chemicals interact with inks, with fibers, and with water.”

And the challenge of deinking won’t go away. New inks are continually being developed both by HP Labs and by the scientists in HP’s Imaging and Printing Group (IPG) business unit with whom Labs researchers regularly collaborate – inks that need to remain attached for as long as they are useful, and yet be just as easy to remove when you need them gone.