Why does paper tear so easily when wet?
― What holds paper together without glue
Paper that feels sturdy when dry can suddenly tear apart with barely any force at all once it gets wet. You may have felt this with tissue paper, a receipt, or a soggy newspaper. Paper's fibres were never glued together in the first place. That fact is the key to why wet paper turns weak.
Pull on a dry sheet of copy paper with both hands and it won't tear easily. But wet that same sheet and pull the same way, and it rips apart with almost no effort.
You may also have seen a receipt left in a washing machine come out as a pile of soggy paper crumbs after the wash.
Why does paper's very nature change so drastically just from touching water?
Paper is a mass of fibres (pulp) drawn from wood and other sources, and it's held together with no glue at all — the fibres bond directly to one another.
When paper gets wet, water molecules wedge into the bonds between fibres and steal them away, so the fibres come apart much more easily.
Let's look, step by step, at the identity of this "invisible bond" that makes paper what it is.
Paper is a "web" of tangled fibres
Paper is made by taking cellulose fibres (pulp) drawn from wood and other plants, breaking them down finely in water, spreading them into a thin flat sheet, and drying it. Ordinary paper contains no "glue" bonding the fibres together.
Even so, paper holds together as a single sturdy sheet because the fibres bond directly to each other wherever they touch.
Hydrogen bonds hold the fibres together
Cellulose molecules carry many hydroxyl groups (–OH) attached along their length. Where fibres press tightly together during papermaking, the hydroxyl groups of neighbouring fibres attract each other through hydrogen bonds.
A single hydrogen bond is never very strong on its own, but stacked up across the countless points where fibres touch, they add up to give the whole sheet real strength. It's essentially the same kind of force that links water molecules together, as described in why ice floats on water.
They grip each other directly through the "invisible hands" of hydrogen bonds.
Water muscling in steals that bond
Water molecules, like cellulose's hydroxyl groups, are also good at forming hydrogen bonds. When paper gets wet, water molecules wedge into the gaps between fibres and replace the hydrogen bonds that once held the fibres together with bonds to the water instead.
Because water is a liquid, the bonds it forms are constantly breaking and re-forming, so they can't hold the fibres firmly in place. As a result, the fibres slide past each other more easily, and the whole web-like structure loosens. This is thought to be the main reason paper weakens when wet.
On top of that, the cellulose fibres themselves swell as they absorb water. This change in fibre shape is also thought to contribute to the overall drop in strength.
Drying brings back some strength ― but not all
When wet paper dries out, the water molecules leave and some of the hydrogen bonds between fibres re-form, restoring some of the strength. But it rarely returns to its original strength.
This is thought to happen because fibres that swelled after absorbing water set into a different shape as they dry. This phenomenon is sometimes called "hornification," and it's considered one reason why recycled paper made from waste paper tends to be somewhat weaker than paper made from virgin pulp.
Banknotes, receipt paper, and some kitchen paper towels are sometimes pre-treated with special chemicals called wet-strength agents, which keep the paper from losing strength when wet. Precisely because ordinary paper is weak when wet, manufacturers deliberately add such treatments for uses where that matters.
Try it yourself
- Cut two pieces of copy paper or kitchen paper to the same size
- Leave one dry; dip the other in water and shake off the excess
- Pull on both slowly, using about the same amount of force
- Confirm that the wet one tears with far less force
- Try comparing different types of paper, such as tissue paper and kitchen paper — you'll notice they differ in how well they resist water
Some kitchen paper towels are deliberately made to resist water, so they may not weaken as dramatically as copy paper.
Summary
Paper is a "web" of fibres bonded directly to each other through hydrogen bonds. When it gets wet, water molecules wedge into those bonds and steal them, letting the fibres slide past each other more easily, so its strength drops sharply. Drying restores some of the bonds, but because the fibres' shape has changed, the paper never fully returns to its original strength.
What holds paper together isn't glue.
It's countless handshakes too small to see.
The same relationship between cellulose fibres and water is also at work in nature. Why do pine cones close when wet and open when dry? explores how fibres that absorb water bend the scales, opening and closing the cone.
Want to know more? ― Terms, numbers, and how this connects to textbooksWe label how advanced each part is, from middle-school science to open research questions
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Chemistry"
- High school+Covered in high-school "Chemistry," or treated as advanced/sidebar content in textbooks
- UniversityNot covered in high school — content from university-level specialist courses (pulp and paper science)
- ResearchNot yet settled even at university level — topics researchers are actively investigating
Middle schoolTerms: the vocabulary of paper
- Cellulose: The fibrous substance that makes up plant cell walls. The main raw material of paper.
- Pulp: Fibres extracted from wood and other sources, used as the raw material for paper.
- Hydrogen bond: A relatively weak attractive force between molecules, mediated by a hydrogen atom.
- Hydroxyl group (–OH): A part of a molecule where an oxygen atom is joined to a hydrogen atom. It readily forms hydrogen bonds.
High schoolChecking the numbers: just how much weaker is wet paper?
Let's use some example numbers to see just how much smaller the force needed to tear paper becomes when it gets wet.
Strength of wet paper ＝ strength of dry paper × fraction remaining
| Force needed to tear dry paper | Let's use 40 N (newtons) as an example |
| Fraction of strength remaining when wet | This varies by paper type, but a typical rough estimate is that it drops to around one-tenth (roughly 8%) of the dry strength |
| Force needed to tear wet paper | 40 × 0.08 ＝ 3.2 |
| Force to tear wet paper | about 3.2 N |
| How many times stronger dry paper is than wet | 40 ÷ 3.2 ＝ 12.5 |
| Strength ratio | about 12.5× |
| Fraction of strength lost | 100 － 8 ＝ 92 |
| Strength lost | about 92% |
By this estimate, simply getting wet drops the force needed to tear paper to nearly one-tenth of the dry value. Such a large drop makes sense given that most of paper's strength rests entirely on hydrogen bonds, which water easily disrupts.
※ The force values and the fraction of strength remaining are illustrative examples meant to convey the mechanism. Actual values vary widely depending on the type, thickness, and fibre composition of the paper.
High school+Why are hydrogen bonds so vulnerable to water?
Hydrogen bonds are known to be considerably weaker than covalent bonds and other bonds that directly link atoms within a molecule. That's precisely why when a flood of another substance that also readily forms hydrogen bonds — like water — arrives, it can easily displace the original bonds. The fact that paper's strength rests on hydrogen bonds, which are weak against water, directly explains why paper as a whole is so vulnerable to water.
University"Hornification": a concept from paper science
The phenomenon in which a fibre's original structure changes irreversibly after being wetted and dried once, reducing the number of sites available for hydrogen bonding, is known in pulp and paper science as hornification. Studies report that the more times waste paper is recycled, the further this hornification progresses, and the strength of the recycled paper tends to gradually decline.
ResearchWhat's still not fully understood
- The precise molecular-level changes that cause hornification are said not to be fully understood yet. Fine-scale structural changes within the fibres are thought to be involved, but research is ongoing toward building a quantitative predictive model.
- Most wet-strength agents used in banknotes and kitchen paper chemically reduce the paper's biodegradability and recyclability, which is a known drawback. Developing agents that resist water while remaining environmentally friendly (biodegradable, recyclable) is an active research topic in the paper industry.
- How far new plant-derived fibre materials, such as nanocellulose, can push paper's strength and functionality is also being studied in materials science.
Even for a material as old and familiar as paper, research continues into balancing strength with environmental performance.
Where this connects to textbooks, by level
| Level | Subject/unit | Where in this article |
|---|---|---|
| Middle school | Science: properties of matter | Basic terms — cellulose, pulp, hydrogen bonds |
| High school | Basic Chemistry: intermolecular forces | Estimating a force from a strength fraction |
| High school+ | Chemistry: chemical bonding | Difference in strength between hydrogen and covalent bonds |
| University | Pulp and paper science | Hornification and the strength loss of recycled paper |
| Research | Materials science / paper industry (ongoing) | Environmentally friendly wet-strength agents, nanocellulose materials |
- Explanatory material on inter-fibre bonding (fibre bonding) from resources related to the Japan Technical Association of the Pulp and Paper Industry (紙パルプ技術協会).
- Explanations of hydrogen bonding and intermolecular forces in high-school Basic Chemistry textbooks.
- Explanations of the hornification phenomenon in pulp and paper science research literature.
- Technical papermaking-industry materials on the role and uses of wet-strength agents.
- Materials-science literature on research trends in nanocellulose materials.
※ The strength figures and ratios are representative illustrative values meant to convey the mechanism. Actual values vary considerably by paper type and conditions.
※This article is a general-audience science explainer. For guidance on handling important documents or banknotes that have gotten wet, please check with the issuing body or relevant authority.