Why Does Paper Get Weaker When Wet? Unraveling the Chemistry Behind Its Vulnerability to Moisture

The susceptibility of wet paper to tearing is evident when a drink spills on your desk's paperwork or a dinner napkin is placed on a damp surface.

The delicate nature of wet paper, even with the slightest water drop, seems to permanently compromise its strength. However, the explanation for why paper tears more easily when wet is rooted in its chemical structure.

Understanding the composition of Paper

Paper originates from trees, underscoring the importance of conservation due to the environmental repercussions of tree cutting in its production. In essence, a sheet of paper is made from wood.

Wood is a diverse substance, consisting of around 40-50% micro-fibrils of cellulose and 15-25% hemicellulose, bound together by the natural adhesive lignin. The process of creating paper from wood involves the initial conversion of raw wood into 'pulp,' a mixture of cellulose fibers, water, lignin, and specific chemicals. Subsequent processes shape the pulp into the familiar paper form.

The term 'cellulose' frequently arises in discussions related to materials like cotton, wood, and dried hemp. It represents the most prevalent organic polymer globally, characterized by a molecular structure composed of numerous similar units bonded together.

The chemical structure of cellulose, a crucial element in paper, is responsible for the paper's resilience when dry and its increased fragility in moist conditions.

The chemical structure of cellulose incorporates multiple hydroxyl groups (-OH), forming robust hydrogen bonds with oxygen atoms on the same or adjacent chains. These hydrogen bonds, known for their strength, play a pivotal role in maintaining the cohesion of cellulose chains and contribute to the observed sturdiness of paper.

Why Paper Disintegrates When Wet?

Paper is fundamentally composed of cellulose fibers that are intertwined through small hook-like irregularities on individual cellulose strands and are connected by hydrogen bonds, which are crucial interactions in chemistry that play a pivotal role.

Certain chemical bonds exhibit magnet-like behavior, with one end slightly positive and the other slightly negative. The positive end of one molecule is attracted to the negative end of another, holding them together.

Molecules with oxygen bonded to hydrogen, such as water (H2O), are particularly prone to hydrogen bonding. Cellulose polymers, strands of repeating chemical units forming paper, are covered with oxygen-hydrogen handles along their entire length.

Marko Kolari, a research and development fellow at Kemira in Finland, explained the process of tearing dry paper, emphasizing the need to overcome intermolecular forces, friction, and fiber entanglements.

When paper is wet, the fiber matrix swells, fibers detach, and the paper weakens. Water disrupts crucial hydrogen bonds in cellulose fibers, forming its own hydrogen bonds, facilitating fiber separation and paper tearing.

Different paper products respond to water uniquely due to additives during the papermaking process. The cellulose fiber is similar across products, but diverse properties arise from various additives.

Strength is a key focus for manufacturers, achieved through dry strength additives like potato starch. Applied as a gel, starch forms a robust barrier around cellulose fibers, enhancing paper strength. However, even strengthened cardboard succumbs to moisture as starch dissolves in water, rapidly diminishing added strength.