Why Does Paper Tear More Easily When It’s Wet? Explained by Experts

KOMPAS.com – When you accidentally spill water on document paper, you can observe that the paper is more brittle and prone to tearing than when it is dry.

So why does paper tear more easily when it’s wet?

“Basically, paper is just cellulose fibers—the natural polymer molecules of wood, which are woven together to form sheets,” says Charlotte Scott-Parker, research and development specialist at James Cropper Paper Mill in the UK.

“In a plain sheet of paper, these fibers are interlocked with each other through tiny hook-like irregularities in the individual strands of cellulose. However, they are also bonded to each other through hydrogen bonds,” he added, as quoted Live ScienceSaturday (2/9/2023).

Hydrogen bonds are one of the most important interactions in chemistry. Those chemical bonds can behave like magnets, with one end slightly positive and the other end slightly negative.

As with a true magnet, the forces of attraction are opposites, so the positive end of one molecule is attracted towards the negative end of another nearby molecule, and this attractive force brings the two together.

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Paper tears easily when wet

So what to do with paper?

Molecules that contain oxygen bonded to hydrogen, including water, are particularly susceptible to this type of interaction known as hydrogen bonding.

And it happens that the cellulose polymer that makes up the paper is covered with oxygen-hydrogen along the fibers.

On a chemical level, water disrupts the important hydrogen bonds that hold cellulose fibers together.

Because water also contains very important oxygen-hydrogen bonds, it begins to form its own hydrogen bonds with the cellulose, blocking the binding of other fibers.

With less interaction between the individual cellulose polymers, it becomes easier to separate the fibers, so less force is required to tear the paper.

“So if you make the paper wet, the fiber matrix will swell, the fibers will start to loosen, and they will start to lose strength and tear more easily,” explained Marko Kolari, researcher and development officer at Kemira, a pulp and paper chemical company in Finland.

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Not all paper is the same

However, not all paper is created equal as there are various types of paper.

“Cellulose fiber is pretty much the same in all of these products, but the properties are very different and varied,” says Kolari.

The way that various papers respond to water is due to the additives added during the paper-making process.

The paper industry has a myriad of chemical tricks to improve the properties of paper products, and one of the most important features manufacturers focus on is strength.

“If you want a strong material like packaging boxes, we need to strengthen the fiber matrix. We do this by using additives like potato starch,” explains Kolari.

This layer of natural compounds is applied to the surface of the paper as a gel and forms a strong barrier around the interwoven cellulose fibers when they dry.

This tough starch surface acts as a scaffold and gives the paper a major increase in strength.

However, even toughened cardboard is not immune to the damaging effects of moisture. If it gets wet, the material will also lose its strength very quickly.

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2023-09-04 01:30:00
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