Adhesives from return labels cause considerable losses in the production of recycled paper

Adhesives hold the world together, as well as countless paper products. But adhesives are one of the biggest problems in paper recycling. A paper mill has just been confronted with a major complaint after some damage caused by adhesive residues remained undetected during quality control of the finished paper. These adhesives came from return labels from a large online mail order company and had presumably ended up as misprints in large quantities in the paper for recycling. Such mass incidents in particular can often only be discovered after the fact, after the damage has already occurred – and they expose products that otherwise contribute to the adhesive load again and again without being detected.

In most cases, the optical sensors find the defects: Sticky contaminants on the drying cylinders and the doctor blades that clean these cylinders repeatedly lead to damage in production – for example, pieces fall onto the paper web and stick it to the next layer during reeling. When rewinding and cutting to the format required by the customer, it then happens: a hole tears into the web at the glued point and the web tears. The affected layers are removed and clean paper is applied.

Small crumbs of adhesive that are deposited on the paper fabric before the dryer section lead to thin spots and holes in the web – this often results in breaks in the paper machine and costly production interruptions. Too many sticky impurities are one of the most common reasons that make an additional wire change necessary. Together with the downtime, the costs can quickly reach five figures.

This is why paper mills invest up to 3 million euros per year in various measures to remove sticky contaminants in the recycling process: firstly with screens that can separate even larger, undivided pieces – which is why polyurethanes and hot melt adhesives are usually no problem. Smaller particles are separated using centrifugal forces (“cleaners”). However, it becomes difficult with dispersions that disperse (redisperse) together with the fibres in the water and only reappear in the dryer section when the water evaporates. Various substances are said to help remove dispersed adhesives from the water, but so far there is no magic bullet.