For centuries, paper has been a cornerstone of communication and creativity, yet it still holds surprises for us.
Among these is a common yet puzzling phenomenon: why does paper printed on one side with an inkjet printer begin to curl after a short period, even when it seemed perfectly flat right after printing?
Researchers Alexander Maaß and Ulrich Hirn from Graz University of Technology have uncovered the answer, shedding light on this everyday mystery.
Their discovery, published in the journal Materials & Design, points to the solvents in the inkjet ink as the culprits. Specifically, the solvent glycerol, contained in the ink, gradually moves through the paper towards the unprinted side.
This migration process causes the cellulose fibers on the unprinted side to swell, leading to the curling of the paper.
To observe this phenomenon, the team conducted experiments using standard A4 printer paper, which they sprayed on one side with a mixture of water and glycerol. Over the course of a week, they used a laser scanner to monitor the curvature of the paper sheets.
They found that while paper sprayed with just water stopped curling after 24 hours, the curling continued for a full week on the paper treated with the water-glycerol mix.
This ongoing curling is attributed to glycerol’s property of not evaporating under normal conditions, unlike water, causing it to steadily migrate from the sprayed side to the opposite side.
Further investigation involved separating the paper into thin layers and measuring their glycerol content. This process revealed a gradual decrease of glycerol in the layers closest to the sprayed side and an increase in the lower layers.
Since glycerol, like water, makes cellulose fibers swell, this imbalance in the paper’s structure caused the observable curling effect.
Addressing this issue poses a challenge. Glycerol, despite its role in causing paper to curl, is integral to the functioning of inkjet inks, providing properties essential for the printing process.
One potential solution proposed by Ulrich Hirn involves either replacing glycerol with an alternative solvent or treating the unprinted side of the paper with a similar amount of glycerol to counterbalance the effect.
This research not only solves a longstanding puzzle but also opens avenues for improving the quality of printed materials, ensuring that the pages we print remain as flat and presentable as intended.
The research findings can be found in Materials & Design.
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