Why is it that a wet tissue or piece of cardboard or paper tear more easily? How does water affect the structural integrity of paper products?


Why is it that a wet tissue or piece of cardboard or paper tear more easily? How does water affect the structural integrity of paper products?

In: Chemistry

Paper products are made from wood fibers that are basically matted together in random orientations like felt. As the paper dries, little hairs and random kinks in the fibers cause them to link up like tangled hair. Pressing/rolling operations enhance this effect.

Paper fiber is very hydrophilic, meaning it can soak up water easily and will soak up far more than its own weight in water. As the water wets the paper, capillary action pulls the water into the spaces between fibers and forces them apart. At the same same time, the fibers themselves absorb water, swelling up.

Between the swelling and the capillary action, the fibers are simply not as able to cling together.

Water covers the paper fibers making them slippery and it can move in between fibers causing expansion. This expansion means that fibers that were previously touching and providing structural integrity are now free floating. The reason water doesn’t cause paper to instantly disintegrate is that even with the reduction in friction there is still quite a bit of entanglement in the fibers.

The fibers that make up those materials are held together by hydrogen bonds. Now don’t let the name mislead you – hydrogen bonds are not actually chemical bonds. Rather, they are intermolecular forces that rely on molecular polarity. Polarity can basically be described by the distribution of electrons that can make a part of a molecule partially positive or partially negative. So the partially positive part of one fiber is attracted to the partially negative part of another, and thus we get a sheet of fibers held together by hydrogen bonds.

Now introduce water. One of the most important properties of water is its polarity, so when you put these often hydrophilic (or water-loving) fibers in the presence of water, the partially charged parts of these fibers will form hydrogen bonds with the water instead. Thus, it takes much less force to actually pull these fibers apart after it has been exposed to water as there are fewer attractions holding the fibers together!

Paper is like felt, but made from wood. When it gets wet it swells up like a sponge. This pulls all the little bits apart.

Follow up question: then how does a paper towel or plate not disintegrate the moment it gets wet?

The internal fibers are lubricated by the water and can slide past each other easier, with less friction.

Rub your finger on a dry mirror/table. Wet the surface and try again. You’ll see what I am talking about.

Paper and cardboard have tree fibers in them, like your hair. When they’re dry it’s harder to untangle them so they keep their shape. When they’re wet they become very slippery and loose so they don’t hold up.


Please ask away!

I’m going to try and add on to some of the questions I’ve seen in this thread.

-The main question-

When it comes to papermaking, there are generally two important things to consider when it comes to what we call “wet strength” or how well the paper product can stay together when wet. Those are the fibers – or bits of trees or plants that are used to make the paper – and chemicals added to the paper as it is made.

The fibers are made of long chains of sugars we call cellulose. They are different between different types of trees and plants. Softwoods, like Pine Trees, tend to have longer fibers whereas Hardwoods, like Maple, have shorter fibers. They serve different purposes. Long fibers tend to help paper be strong, while short fibers help fill in holes in the paper to improve things like printing on the surface with a printer.

Now when paper is made, think of these fibers like different sized noodles of sorts. On a paper machine, they will orient themselves in the direction the machine moves, or essentially will line up next to each other as if they were still dry spaghetti noodles in the box. They are parallel, though not perfectly so. BUT this is why sometimes it is easier to tear a piece of paper one way than the other. If you think of the dried handful of spaghetti before you put it in the pot to cook, right, think of trying to break the pasta to fit it into the pot. You try to bend the bundle in half to break it. It is pretty tough to do when you have a lot of pieces of spaghetti together. This is the same idea for paper. When you rip the paper one way and it is easier than the other way, then you can tell the paper has what we call “directionality”. It is harder to tear the paper when you rip against the direction the fibers are oriented (think of the spaghetti) than it is to tear along the direction the fibers are facing.

Another factor to paper strength is how these fibers interact with each other. Cellulose is stronk because these fibers have multiple points where they can interact with what is called hydrogen bonding. A few people already explained this above so I won’t go into too much detail. But I like to think of it sometimes like this: a bunch of people are standing around a next to each other, not necessarily touching but still hard to run through if you randomly tried to run up to the group. Enter water, the disruptor of this group. I like to think of it as a sopping wet dog that just came in from the rain. It runs up to the group all happy because the dog loves you and let’s face it, you love dogs too (in this scenario). Then the dog does what dogs do when sipping wet: they shake the water off. Now you and the people in your group are hitting the deck, trying to hide so you don’t get covered in wet dog water and wet dog smell. The group has essentially broken apart.

This is kinda what happens with the fibers when wet. The water – that wet dog – disrupts their rather close hangout and causes fibers to move apart from one another. It isn’t so much a repelling force, more like it gets in the way, and as this dog shakes more and more, it is harder for you and the other people in the group to stay together. You still love the dog and don’t blame it for your separation.

Now there are ways to change these fibers or their interactions so that they don’t get as disrupted by water. Two ways, generally. One is to refine and the other is to add chemicals.

Refining is like this. Say you were in that scenario with the wet dog. Now instead of just standing near other people in a group, several of you are holding hands too. Refining essentially helps the fiber branch out a bit – like spreading your fingers out – so that you have more places to connect to other people. Think of if you stood up straight with your arms to your sides and your legs together compared to standing with your arms stretched out and your legs spread out. It is easier for you to touch other people (fibers) and get yourself tangled up in their arms like a romance novel if your arms are stretched out than if they are at your sides. This has some benefits. One of them is that the paper is stronger with refining; it is even harder to run through your group of people if you all have your arms linked together.

Now here comes the big wet boi to shake that doggy water all over. With some effort, you’re able to eventually break away from the group in an attempt to avoid that doggy water. But it will be harder for you to break apart, so you’re going to get hit by some of the water but it won’t be enough to fully get you away from the others until you untangle yourselves.

This is also why longer fibers are stronger than shorter fibers: you have the ability to connect with more fibers the longer you are than if you’re le heckin short.

This is why newsprint gets wet and breaks up easily than, say, cardboard (paperboard) boxes. The latter has much more longer fiber than the shorter fibers because it needs to be strong enough to carry that stuff you probably don’t need from Amazon to your house without spilling it all on the street.

But speaking of cardboard (paperboard), there’s another thing that makes it and other things like paper towels strong (other than the way cardboard has multiple layers to give it strength) and that is chemicals. Not like “I’m going to touch this paper and die” chemicals. No. Otherwise you’d never live past your first slice of pizza. But these chemicals are also why boxes hold dog food from chewy and paper towels don’t fall apart the way toilet paper does when wet (and why you should never flush paper towels!!).

So these fibers we have been talking about, both y’all and short, have a negative charge to them. Think of two people who don’t know each other very well in that group example with the dog. Generally they just want to be with other people they know but those friends are busy chatting about Fortnite or something. So you two are kinda just there, ya know, part of the group, still hard to tackle and still affected by the wet dog, but you’re in the group because of other factors, like the room is small so there isn’t much space for you to move around or the atmosphere is chill so you don’t MIND being in the group but part of you would rather Netflix and eat cookie dough in your bed. The nice atmosphere is that hydrogen bonding kinda keeping you there in the group, but if it wasn’t there, you’d probably push away from the other people. Negative charges push away negative charges. Fibers need something to keep them together.

Now your mutual friend pops up to the pair of you in the group. They know you both LOVE Stranger Things. So they mention “huh, what do you think of what happened at the end of the season with Hopper?”. And you two instantly connect and start ranting about your theories and the clues in the final episode that point to what may have happened and Eleven feels and what not. This person connected you two fibers much stronger than you would have been without them.

That is what chemicals do in the paper. They connect fibers together with a stronger bond to help the fibers stay together under the stress and strain of being hurled off the FedEx truck into a puddle on your front step. Now you two are so far into the Stranger Things rabbit hole that you don’t even notice the wet dog enter the room. You are unphased when the dog shakes and gets wet doggy water all over you. Your bond is strong enough to withstand the water and continue talking about how Hopper is probably doing this or that.

This is why paper towels don’t break up in the water and toilet paper does. Paper towels have that Stranger Things bond whereas toilet paper is like you showed up at the party and your ex ended up there as well and of course your mutual friends dragged you to the party without telling you so you’re both just standing there awkwardly waiting for something to happen to get you o u t o f t h e r e but you’re still floating in the group because your mutual friend was your ride but the second that dog comes in and shakes you get the heckin out of there and call an uber because my LORT you just wanna go home. But you gotta make sure you wait for that wet dog to come over to you sometimes because otherwise you can’t leave the group and it just feels awkward to just leave because of your ex since it is your other friend’s birthday and you promised you two would go out to celebrate. (Hence why toilet paper clogs eventually break down once enough water has disrupted the fibers in the sheet).

So that’s that. Other things I’ve noticed so far:

– Cotton doesn’t break down as easily in water cause the fibers are le heckin long compared to tree fibers like think of that one hair that you always miss when you shave and it just seems invincible like why won’t you fall like the others and that is because it long and stronk

– Paper morphs when it gets wet and dries. Think of that wet dog. Your group initially flees and separates but when the dog leaves and is no longer a threat of covering you with wet doggy water, your group slowly goes back together again. Maybe you’re standing in a different position. Maybe you’re closer together than before, or you can’t find Ryan so your group shape is different, or you’re shriveled together. This is what happens to the fibers when they are wet and then dry again, hence why paper gets wrinkly and deformed based on what it once looked like. There’s a specific term for it that is escaping my mind at the moment, but essentially the arrangement of fibers (the people in your group) changes from what it originally was, hence the change in paper texture.

There are lots of other things I know about paper if you have any other questions but boy howdy I really enjoyed this. Thank-you ❤️

Looked into this in my college chemistry course. If you’re able, look at dehydrated, “regular”, and wet cardboard under a microscope.
Dehydrated is not at all flexible, and you will be able to watch it snap when squeezed instead of flexing (if you’re lucky and good at micro-scoping like our professor)

Regular, you will be able to see the fibers flex and then return to their shape unless you deliberately snap them / crease them.

To answer your question…In contrast, wet cardboard’s fibers could barely hold themselves up in between cross sections, much less actually stand up as a group. They were bloated, and heavy, and when folded the sections more slumped in any direction instead of what you’d consider “folding” They were just too heavy.

And how was Jackie Chan able to bend steel bars of his jail cell by urinating on a shirt?