Paper is a thin material primarily used for writing, printing, packaging, and wrapping. It is produced by pressing together moist fibers extracted from cellulose-based materials like wood, cotton, hemp, and bamboo. Paper is characterized by its lightweight, flexibility, and absorbent properties.
Paper interacts with water in several ways. When exposed to moisture, paper can absorb water, dissolve, lose strength, change dimensions, grow mold, and deteriorate over time. However, paper can also be restored and recycled through processes involving water. This article will explore the various reactions and relationships between paper and water.
Absorption
Paper absorbs water readily due to the capillary action of its fibers. Capillary action describes the ability of a liquid to flow against gravity in narrow spaces such as the spaces within a porous material like paper. This works because the adhesive intermolecular forces between the liquid and the material’s surface are stronger than the cohesive forces between the liquid molecules. As a result, the liquid is pulled up into the porous material (https://scienceprojectideasforkids.com/paper-absorbs-water/).
The cellulose fibers that comprise paper effectively wick water through tiny gaps between the fibers via capillary action. When paper makes contact with water, the water first binds to the hydrophilic cellulose fibers on the surface. Then the water molecules are drawn through the entire paper along the spaces between the fibers, even against gravity. This allows the paper to rapidly absorb and distribute liquid through its entire structure (https://www.pinterest.com/pin/how-paper-absorbs-water–423197696217307655/).
Dissolving
Some paper dissolves in water. This is due to the sizing chemicals used in paper production. Sizing chemicals like rosin, starch, and polyvinyl alcohol are added to paper to reduce its absorbency. Without sizing, paper would soak up liquids like a sponge. The sizing chemicals make the paper more resistant to water. However, they are water-soluble themselves and will dissolve when the paper is submerged in water. This causes the fibers in the paper to come apart and disintegrate over time (https://www.ozmo.io/how-fast-does-paper-dissolve-in-water/). The rate of dissolving depends on the type and amount of sizing used. Lightly sized papers with minimal chemical treatment will dissolve faster. Heavily sized papers like watercolor paper take much longer to break down in water.
Strength
Paper loses a significant amount of its strength when soaked in water. This is because paper fibers form hydrogen bonds with each other, which provide strength to the dry sheet. When paper is immersed in water, the water molecules disrupt and replace many of the fiber-fiber hydrogen bonds, leaving only 2-3% of the paper’s dry strength according to Vitale (1992).
As Tejado (2010) explains, “Surprisingly the strength of wet paper is still poorly understood. Here we show that the traditional explanation of the strength of wet paper is incorrect.” The old theory was that wet paper retained some strength due to areas of close fiber contact, but Tejado’s research found that hydrogen bonding is the primary source of wet strength.
According to the Abbey Newsletter on Papermaking (2007), “It is found, for example, that the tensile strength of paper is reduced in all liquids. In the more inactive liquids, this occurs…\” without swelling. In more active liquids, reduction in strength is greater because, in addition, the paper swells.”
Dimensional changes
When paper is exposed to water, the cellulose fibers that make up the paper absorb the water and swell, causing the paper to expand slightly in size. This effect is most noticeable in the width of the paper, as the cellulose fibers are oriented more across the sheet than along its length. As the fibers swell with absorbed water, the width of the sheet increases. However, the length changes very little since there are fewer fibers oriented that way. This anisotropic swelling is due to the fiber orientation created during the papermaking process.
The amount of expansion depends on the type of paper. Some papers are engineered to have very little dimensional change when wet, while other types like newsprint can expand 3-4% in the width. In general, papers with more refining of the fibers show less dimensional change with water. Fillers like clay can also restrain swelling. The fiber source also matters – softwood fibers swell more than hardwood.
The swelling is reversible when the water evaporates and the fibers shrink back to their original size. So paper that gets wet will expand while damp but then return to near its original dimensions once dried. However, there can be some irreversible changes if the swelling and shrinking occurs repeatedly over time. This wetting and drying cycle can fatigue the fibers, causing gradual dimensional changes.
To minimize dimensional changes from humidity, acid-free papers made for archival records are often buffered with an alkaline compound like calcium carbonate. The alkalinity helps to set the dimensions of the paper and reduce swelling and shrinking.1
Coating
Some papers have coatings to resist water. There are different types of coatings that can be applied to paper to make it water resistant. Some common coatings include:
Fluorochemical coatings – These contain fluoropolymers that create a hydrophobic barrier. They chemically bond to the paper fibers to repel water and oil (Zeng et al., 2020).
Wax coatings – Wax derived from paraffin, beeswax, carnauba and other sources can be used to coat paper. The wax fills in pores and creates a water-repellent barrier (Li et al., 2018).
Silicone coatings – Silicones can be used to create a crosslinked, flexible coating that repels water. The coatings are durable and stable (Basak, 2024).
Starch coatings – Modified starches can be applied and cured to make greaseproof coatings. They are biodegradable and renewable (Li et al., 2018).
Coatings protect the paper from water damage and prevent ink from running or smearing when exposed to moisture. They allow paper products like food packaging, labels, and postcards to be used even in wet conditions.
Mold growth
Wet paper provides an ideal growing environment for mold. Mold spores are ubiquitous in indoor and outdoor air. When paper becomes damp or wet, these spores can begin to grow and spread. Mold requires moisture, nutrients, warmth, and oxygen to grow. Wet paper provides all of these conditions and allows mold colonies to become established.
If paper remains wet for an extended period, visible mold growth will begin to occur within days or weeks. This mold can appear in various colors like black, gray, green, or white. The fungi release enzymes and acids that break down and digest the paper, causing permanent damage and deterioration over time. Mold also releases spores that can cause allergic reactions or other health issues.
According to a 2010 indoor air quality assessment report, papers left damp from an open window showed visible mold growth just days or weeks after becoming wet. This demonstrates how quickly mold can develop on wet paper given the right conditions (Massachusetts Rehabilitation Commission, 2010). Maintaining dry conditions is critical to preventing mold issues with paper materials.
Massachusetts Rehabilitation Commission. (2010). Indoor Air Quality Assessment. Massachusetts State Government Documents.
Restoration
Wet paper can sometimes be restored. There are a few techniques for attempting to restore wet paper documents or books.
One option is to place the wet paper in a sealed plastic bag and put it in the freezer. The cold temperature prevents further damage and buys some time. When ready to restore, take the paper out and let it thaw slowly at room temperature. The frozen water in the fibers will turn back to liquid and can then be dried gently with blotting and pressing (Source).
Another technique is to microwave wet paper in short bursts of 10-15 seconds. This rapidly heats the water molecules in the paper and causes them to evaporate. Care must be taken not to overheat the paper. After microwaving, the paper should be pressed flat immediately with a heavy book or other weight to flatten it as it dries (Source).
With careful, gradual drying, gently flattening, and patience, wet paper often can be restored close to its original condition.
Recycling
Wet paper is harder to recycle. When paper gets wet, the fibers begin to break down and become shorter. This makes the paper low quality and not as useful for recycling into new paper products. Mold can also grow on wet paper before it gets recycled. Recycling facilities want clean, dry paper in order to maintain quality control over the recycled paper they produce. According to Ridwell, wet paper runs the risk of contaminating other paper meant for recycling if it is not kept separate and dry. Wet paper essentially becomes a contaminant in the recycling process if it is mixed in with clean, dry paper. The best practice is to allow wet paper to fully dry before placing it into a recycling bin to avoid these challenges.
Conclusions
Water can have dramatic effects on paper through various chemical and physical interactions. When paper absorbs water, the fibers swell and stretch, causing dimensional changes like curling and cockling. Too much water causes paper to dissolve and fall apart through hydrolysis of cellulose fibers. Even small amounts of moisture allow mold to grow, which stains and weakens paper. However, the strength and durability of modern coated papers provide some resistance to water damage. Careful drying and restoration can save wet paper. While recycling reclaims fibers, water typically degrades paper, underscoring the need to keep it dry for preservation. Understanding paper’s reactivity with water allows us to handle it appropriately to avoid permanent harm.