Paper printed on one side begins to curl after a delay of up to several days. Researchers at TU Graz discovered that this is because the solvents in the ink migrate to the unprinted side of the paper over time.

Although humans have been using paper for at least 2000 years, it still holds a few mysteries for us. For example, there was an unknown reason why paper printed on one side with an inkjet printer, although initially flat after printing and drying, would begin to curl after a few hours or days.

Alexander Maas and Ulrich Hearn from the Institute of Bioproducts and Paper Technology at Graz University of Technology (TU Graz) have now discovered why. This is because the solvents contained in the ink migrate towards the unprinted side of the paper over time. This causes the cellulose fibers on the non-printing side to swell and the paper begins to curl.

The degree of curl of the paper is measured with a laser scanner

For their experiments, the researchers sprayed model ink consisting of water and glycerin solvent onto one side of standard A4 printer paper. They then observed the warping of sheets of paper using a laser scanner for a week. While paper sprayed with water no longer curled after 24 hours, the process continued for a week with paper sprayed with a mixture of water and glycerin. This is because glycerin, which, unlike water, evaporates only at very high temperatures, moves slowly but steadily from the sprayed side of the paper to the other.

Paper is divided into thin layers

The researchers managed to demonstrate this process by cutting paper samples into thin layers and determining the glycerol content in them. They found that glycerol constantly decreased in the atomized upper layers and increased in the lower layers. Because glycerin, like water, causes the cellulose fibers to swell, the volume of the different layers of the paper varied over the testing period. Layers closer to the eruption side shrank, while deeper layers were blown away. The visible result was curling of the paper, and this occurred continuously for a week.

Replacing glycerin with other solvents

“To solve the problem, glycerin can be replaced with other solvents. But it is not that simple, because glycerin gives important properties that make inkjet ink suitable for inkjet printing,” says Ulrich Hirn. Alternatively, the reverse side of the paper can be printed or treated with an appropriate amount of glycerin.