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research 
In this, my last article for 2005, I will update a couple of the research projects that I have chronicled over the past three years Exactly two years ago, I wrote of one of the Centre’s major projects to de-ink and recycle paper, with an eye on the chemical and physical processes currently employed to do that. This project has now become even larger, as more resources of the CRC have been focused onto the various aspects of the process.

To remind you of the components of the project, the parts now under examination are to:
• develop a method of measuring how much ink is removed from the paper pulp;
• establish the most effective environment to assist ink removal;
• separate ink and pulp most efficiently,
• study the influence of ink composition;
• establish whether paper surface modification would assist ink separation.

Clearly the rest of the study would not be worth much if we could not establish a baseline, and it was to this end that work has been conducted by PhD student Jolly Pan, under the expert supervision of Dr Loi Nguyen.

The most effective method of removing ink in the laboratory has been to use two concentric cylinders, with a printed sample attached to the outer wall of the inner cylinder, and a suspension of paper fibres in water as the filler between the print and the inside wall of the outer cylinder. The inner cylinder can then be rotated against the stationary outer shell for a chosen time, and the measurement taken. Controllable variables are the gap distance, the pulp viscosity, the time of exposure, and the torque applied to
the system.

The device was also used to examine the effect of various chemical additives to the slurry and to the inks.

Once the removal technique had been perfected, the next task was to find a method of measurement of ink removal that was both simple and reproducible. Three techniques have been evaluated, the first to measure the reflectivity of the ink and the substrate with a spectrophotometer, and then to calculate the absorption coefficient (k) of the ink and paper. From the Kubelka Munk theory on mechanism of light scattering and absorption in solid matrices, the effective residual ink concentration in typical recycled paper can be calculated from the obtained k value. At low concentrations the residual ink in parts per million is ~ 106* k.

The second method is to use a scanner of high resolution, and by software manipulation, relate the ink concentration with the ratio of number of black to white pixels.

The third method is to obtain infra-red spectra of paper samples to mark the concentration of functional chemical groups from the ink before and after de-inking.

To date, the most effective method has proven to be the first, which is pretty much how intuition would have it. (Isn’t it nice when real science backs up what logic predicts?) A number of experiments have been conducted to test the method to its limits, and this work has now confirmed the technique as the one by which future tests will be quantified.

The influence of chemical additions to influence the behaviour of paper in the de-inking process has also been investigated, and once again the logical answer has been proven. Up to a certain level, de-inking aids are effective, but above particular limits, the effect is reversed – too much additive has a clear negative effect on the ink removal process. This is because the effectiveness of such materials depends on charge neutralisation, (as paper pulp is anionic), and if too much is present, the charge of the whole mess can be reversed, gluing the ink to the fibres even more closely.

As I wrote two years back, the effect of the formulation of litho printing ink, especially for newspapers, is one area of the de-inking process that has received little or no attention from ink manufacturers, as their task has been seen to be to produce a material that will lithograph at high speed, will set quickly with minimal turner marking, will not mark readers or their clothes, and costs the absolute minimum that print companies are prepared to pay.

However, having been an inkie for well over 40 years, the way inks go together is hardly a secret, and one suggested addition of a simple catalyst to black ink formulation tested so far has increased the amount of ink that can be removed from aged print, albeit at a slightly slower rate. More work is to be done to speed up the ink removal rate and to establish optimal concentrations and chemical moities, but the direction appears to be valid.

Since I last wrote, a pilot-scale bubble chamber separation cell has been designed and built for around $100,000 at Monash’s Department of Chemical Engineering to the design of our PhD student Richard Markowski, (also one of Dr Nguyen’s students). Richard is now very close to finishing his thesis on the mechanism of fibre and ink separation.

Finally, the effect of paper surface modification on ink removal is being approached by two of the Centre’s Post Doctoral Fellows – Dr Soo Ooi at CSIRO with supervisor Dr Nafty Vanderhoek, with support from Dr Shannon Notley, at ANU supervised by Associate Professor Tim Senden. This is the “newest” part of the process, and a lengthy programme stretches ahead, including looking at increasing the hydroxyl group concentration on the fibres to influence the hydrophilicity of the paper, and so perhaps make ink separation easier. Stay tuned for some mighty interesting developments there.

Another project that continues to uncover some fascinating insights into the lithographic process is being pursued by PhD student Fuping Liu, supervised by our litho mechanics expert, Dr Wei Shen (refer July 2005 article).

Fuping has been studying the behaviour of fountain solution emulsions as they are delivered to the litho plate and thence to the blanket and substrate. The approach is to use a confocal microscope (June 2003) to measure the three dimensional shape and size of emulsion droplets, and to follow their progress through the print process.

In order to make the emulsions suitable for confocal viewing, a fluorescent dye that is soluble only in the ink vehicle, and another that is soluble only in the fountain solution are essential in order to identify the two phases. The discovery of such a pair has been pivotal to the work, and the developments that have come to date.

Fuping has shown that emulsions form droplets that are essentially sperical, and these split out of the film applied to the plate before hitting the blanket, and migrate to the non-image area of the plate surface.

Also, by suspending a piece of litho plate from a very accurate balance, and bringing the plate downward toward a drop of ink, he has demonstrated that ink will actually “jump up” to the image area of the plate surface in the presence of fountain solution, thus confirming the mechanism proposed in the July 2005 article.

This work promises to uncover more of the science of the lithographic process, and future columns are likely to reveal more of this fascinating clarification of what has largely been known but not necessarily understood.