Why does wet paper tear so easily, while wet cloth gets tougher?
Paper and cloth are both made of cellulose. Yet wet paper weakens while wet cloth grows stronger. The difference is not the material but where the strength comes from: bonds between fibers, or the way the fibers are tangled.
Wet paper tears at the slightest touch. Yet a wet cotton rag or a pair of wet jeans is tougher than when dry. Both only met water, but one weakens and the other strengthens. And here is the surprise: paper and cotton are made of the very same material, cellulose. Why would the same material behave in opposite ways before water?
It is tempting to think paper is weak and cloth is strong, but that is not the answer. Dry paper is quite tough too. Even a single newspaper sheet resists tearing against the grain. The difference is not how strong the material is, but where that strength comes from. Paper's strength and cloth's strength have different sources. So when the same guest, water, arrives, it pulls out a support pillar in one and lends a hand in the other.
So even with the same cellulose, the outcomes diverge. Paper entrusted its strength to bonds between fibers, so the water that breaks those bonds becomes its weakness. Cloth entrusted its strength to the tangle of fibers, so water leaves it unshaken and the swollen fibers tighten the tangle further. What the strength was built on decided the fate before water.
Top: pick a material and add water to watch the strength meter move in opposite directions. Bottom: tap a material to look inside and see where its strength comes from.
Picture how paper is made. Short cellulose fibers pulled from wood are loosened in water into a pulp, then spread thin and dried. As it dries, the short fibers stick directly to one another wherever they touch, through a chemical handle called the hydrogen bond. Almost all of paper's strength comes from these fiber-to-fiber bonds. It is like a stack of paper scraps held by glue. When water enters, trouble begins. Water molecules carry the same handle, so they wedge into the spots where fibers were holding each other and take those spots over. The bonds between fibers come undone, and the short fibers let go of one another. The glue has dissolved, so the slightest pull tears it. Cloth is a different story. Cotton thread is made by twisting many long fibers together, and that thread is then woven crosswise into fabric. Cloth's strength comes not from chemical bonds between fibers but from a physical tangle, long twisted and tightly woven. So even when water touches the hydrogen-bond sites, the structure does not collapse. If anything, each fiber drinks in water and swells, growing thicker in cross section and pressing its neighbors more tightly together. The tangle grows firmer. That is why wet cotton is around twenty percent stronger than dry cotton.
This difference shows up plainly in how we choose things. Maps and packaging that must not tear in the rain are made by adding a special agent so the paper's fiber bonds do not loosen in water. The opposite goes for toilet paper, deliberately made so its bonds come apart quickly on contact with water, scattering rather than clogging the pipes. There is a reason rags are made of cotton too. They are forever wet and tugged as you wash and wring them, but since they toughen when wet, they do not wear out easily.