If you really make an effort to separate your waste, you break down packaging into its components before throwing it away. As far as possible. You might be able to remove the paper band around the yogurt pot, but it's more difficult when it comes to the strawberry printed on it. Therefore, it should at least be ensured that the printing inks do not negatively influence or hinder the recycling process of the base material on which they are printed.
Deinking is the technical term for the removal of printing inks. If you want to recycle cardboard and paper and reuse their fibers as a raw material, a multi-stage preparation process is necessary. This includes wet preparation, in which the paper fibers are mechanically dissolved in an aqueous solution. In these individual steps, unwanted foreign bodies such as metals, foils, adhesives or printing inks, among other things, are separated out using air and chemicals.
Is unlimited recycling realistic?
It is widely believed that paper fibers can be recycled up to seven times, but researchers at the University of Graz and the Technical University of Darmstadt, Germany, have conducted a laboratory test and found that almost unlimited recycling is possible. 25 recycling cycles without significant changes in fiber length and strength properties - that conserves resources and energy in equal measure. To ensure that such circulation rates and good quality can also be achieved outside the laboratory in the future, the paper must be thoroughly freed of printing inks and coatings, among other things.
This is not only the case for graphic papers, but also for corrugated board or even folding cartons, pharmaceutical packaging, cigarette packs, because the water circuits of recycling plants for cardboard packaging are a closed circuit with permanent treatment of the water. Foreign substances contained in the water - e.g. from dissolving inks or adhesives - therefore accumulate more and more and can then get back into the recovered paper fibers and impair paper production.
Offset or flexo printing inks, solvent- or water-based, UV ink or varnish: the variety of inks and varnishes for printing packaging is huge. Which variant one chooses has a decisive impact on the recyclability of the printed material. "Inks and adhesives that are water-soluble can discolor the fibers via the water, adhesives can clump again during drying and flow into paper production as foreign substances - literally. On the one hand, such small lumps or particles impair the paper quality due to color spots or, on thinner papers, can cause the paper web to tear. This in turn is highly cost-intensive and risky, especially as the trend is towards thinner and thinner high-performance papers," says Pacoon Managing Director Peter Désilets.
This is because each of them has a different deinkability. To be able to recycle paper or cardboard optimally, the hydrophobic inks and varnishes must be separated from the fiber in the recycling process, using chemicals and air. In this way, they detach from the surface and are then skimmed off at the surface of the basins. And that's what successful recycling is all about first - separating and sorting all the components so they can be reused down the road.
The crux of UV inks
The aim of any packaging is to ensure that no substances migrate into the packaged food or products; this would be known as migration of the components of the printing inks and coatings. UV printing inks and varnishes present a particular challenge in the recycling process. Many of them currently still have particularly poor deinkability, plus they are usually mineral oil-based. But how does this happen?
Slow-drying inks are a hindrance to fast production. It is faster to apply a (UV) ink or varnish that is cured by UV. UV-curing inks do this under UV radiation, and highly reactive binders firmly crosslink all the components together. They form a plastic layer that is firmly bonded to the cellulose fiber and is correspondingly difficult to remove. This results in so-called dirt specks on the material to be recycled.
Faster production offers cost advantages, but this entails ecological disadvantages and the risk of these mineral oil-based substances migrating into the products. Low-migration inks therefore dry somewhat more slowly. "For a while, mineral oil ink had virtually disappeared from offset printing. Cost pressure then led to the mineral oil issue being reactivated again via UV curing. This also made deinking more difficult and the recycling quality suffered," says Désilets, regretting the development.
In the meantime, there are new UV flexo coatings and UV offset inks on the market, for example, that improve the recyclability of UV-coated commercial and packaging materials. These are deinkable solutions.
Flexible use in printing, poor deinkability
In addition to UV inks, liquid toner prints, many digital prints and flexographic inks are also poorly or not at all suitable for deinking fibers. The inks used must be as water repellent as possible, which is especially the case with the offset printing process. However, flexographic printing is often used for flexible packaging made of paper or plastic, and the trend towards individualized packaging is in turn leading to the growing popularity of digital printing. However, prints with the liquid toner Indigo, for example, cannot be deinked because they do not rest on the fiber but penetrate it. For example, such printing waste must not be disposed of in graphic waste paper because it inevitably causes discoloration of the fiber.
Colors on films strongly influence recyclate quality
How are gray or black recyclates actually produced in plastics recycling? Essentially through the mix of colors of the various plastics that are not deinked. Impurities such as colors, additives, adhesives, etc. on films are removed as far as possible in the recycling process - if they can be. Ultimately, however, many of these foreign substances end up in the extruder, where the plastics are melted. If these foreign substances gasify at around 250 °C, large quantities of gas are formed, odors are produced and ash discolors the plastics. They thus contaminate the recyclate by discoloration and odor, which is also harmful to health in the recycling process and puts the extruder under extreme pressure.
Cheap cellulose nitrate inks (also called nitrocellulose NC) are often used, which cannot withstand these temperatures. Tests with e.g. PU inks and adhesives have shown that these produce a much better quality recyclate. Wash-off inks would also favor this effect, but are applied by frontal printing, so must be protected against scratching. PU inks are more expensive, but would also offer only limited possibilities for finishing in intermediate printing. In addition, they would remain in the recyclate, which would permanently increase the density of the recyclate and thus - together with other foreign substances of a higher density - could lead to plastics that are actually recyclable, such as PE and PP, sinking and being burned in the float-sink process. "Using the example of colors and additives, it's nice to see how the last three to five percent of the packaging can compromise or even prevent the whole construct towards recyclability. We have been trying for years to anchor this topic in the companies. To this end, we are in constant exchange with the deinking experts and are also involved in projects to promote sustainable printing and recycling," explains Peter Désilets.
The roots of the deinking process go back to the 18th century, and a lot has happened since then. Today, more than ever, researchers are concerned with sustainability, and technical innovations can bring unlimited recycling within reach. Meanwhile, by choosing the most deinkable inks, producers can already contribute to higher-quality recycling. I think it will be a while before the industry is aware of the impact of inks on recycling and reacts. On the one hand, the trade-off of sustainability versus cost will influence the decision - as it has for years - on the other hand, the legislature could build pressure to classify NC paints as non-recyclable. That would bring a new dynamic to the market," Désilets hopes.
One thing we would like to mention at this point: all our references to the major hurdles in plastics deinking relate to mechanical recycling. New recycling technologies such as solvent-based and chemical or enzymatic recycling will probably bring new perspectives, but also new challenges.