Why Does Paper Tear More Easily When Wet?

An ordinary sheet of paper is not a solid slab. It is a thin network of interlaced fibres. When dry, neighbouring fibres sit closely together, while hydrogen bonding, friction and other interactions help transfer a pulling force across the sheet. For a tear to advance, many of these connections must be separated or broken in sequence.

When water enters the paper, the fibres absorb it and swell. Water also makes the contacts between fibres easier to slide apart. The paper has not simply “dissolved”; most fibres are still present, but the network that once shared the load has loosened. If pulling begins at a small nick along the edge, stress concentrates at the crack tip and the tear can spread quickly. Uneven wetting can also create weak patches, so the tear need not follow a straight line.

Kitchen towel and teabag paper can still function while wet, but they are not counterexamples. Fibre length, sheet structure and wet-strength treatments can preserve more connections in water. Comparing a dry corner and a wet corner of the same copier paper is therefore a fairer way to isolate water’s effect.

https://pmc.ncbi.nlm.nih.gov/articles/PMC4882072/
https://www.paperonweb.com/paperpro.htm