Those of us who have made our living manufacturing ink and applying ink to paper, seldom think of the processes required and the difficulty involved in removing the layers of ink from the substrate so that the fibre may be recycled into new paper production. But that is about to change.
With the guidance of our industry partner, Norske Skog Paper Mills, the world’s second largest manufacturer of newsprint, four projects to study ink removal and fibre re-use are now in progress.
The first, situated at ANU looks into fundamental studies of the interaction of ink and paper, the second investigates methods of modifying the surface of paper cellulose fibres (ANU and CSIRO), the others at Monash involve the development of an accurate measure of the amount of ink removed, and the chemical engineering challenges of separating the ink particles from the paper pulp, once the ink has been detached.
The simplified explanation of the current process for de-inking newsprint in order to recycle the fibre, involves firstly mashing the paper into pulp in water, adding sodium hydroxide (caustic soda), and cooking the two together to swell the paper fibres, and wash the ink particles free. The fibres then have to be bleached in hydrogen peroxide, as the action of the caustic turns them buff in colour.
The fibres are then washed in water, and bubbles are blown through the pulp to lift the ink particles to the surface for skimming off. Additives are also included in the mix to assist the ink particles to adhere to the bubbles.
This has been the process for a long time, and in the past the approach to increasing the speed or efficiency of the process has been to build bigger plants or to use more powerful motors on the pulpers.
To add a further layer of complexity, in my 43 years as an ink maker, I cannot remember any ink company employees ever devoting more than a few seconds thought to whether a new formulation would be easier or harder to de-ink than its predecessor. The minds were concentrated firmly on ink performance on the press, whether ink would mark on the machine, the rub resistance of the print (the dreaded bed-sheet smear problem), and most importantly for a commodity product, the selling price.
A news ink black comprises a dispersion of carbon pigment in a vehicle predominantly made up of mineral oils, plus some pigment wetting agent, a small amount of vegetable oil (usually semi-drying oils such as Soya or Canola), and some dissolved resin (either modified rosin or condensed hydrocarbon types).
These inks dry by soaking into the paper fibres, where they remain as a sticky, high viscosity fluid. Such rubbery layers of ink do not lend themselves to being readily separated from the fibres on which they come to rest, or of breaking into discrete particles to be carried upwards in a cloud of bubbles.
Our researchers at ANU are investigating how the ink layer wets its cellulose substrate, and are also combining with the scientists at CSIRO to study whether modifying the paper fibres by grafting on to them hydrophilic (water attracting) chemical groups will enable the water phase to get under the ink and so float it off.
These research projects are showing the way towards a modification of the method of detaching the ink, which leads nicely to the two projects at Monash.
A major investigation is in progress to study and redesign where necessary the equipment required to separate the detached ink particles. This study includes the efficiency of bubble generation and distribution, and the means by which ink particles may be picked up by the bubbles and carried to the surface of the tank.
One of our PhD students, Richard Markowski, is the principal researcher, and his work under the supervision of Dr Loi Nguyen and industry advisor Des Richardson, is providing some fascinating insights into the possible future of the separation process.
The remaining project is centred on the accurate measurement of the amount of ink that is removed. Not immediately apparent as such a vital step, but until a precise method is available, the results of the other projects cannot be quantified.
Research has already been conducted around the world to use colour measurement (spectrophotometry – more of this subject in a future article) as the instrumental yardstick, and the initial part of our work is centred on efforts to improve the present imprecise techniques that have been published.
When the test method has been nailed, variations in the processing conditions and the chemistry of de-inking will become the real focus of the project. A means of reducing the caustic, (or of even removing it altogether), is a prime objective.
When the relationship between fibre surface chemistry and the chemistry and operating conditions for optimal detachment and separation of ink particles are fully understood, the influence of the ink formulation will be studied as the grand finale.
The results of this final step could change forever the thinking processes of ink development chemists, and balancing ink application properties with ink removability may yet become the new measurement of their success.