Science of Craft: 003 The Structure of Cotton Fiber

Surely you've heard the fairytale of Rumpelstiltskin. Some chick finds herself in a predicament that requires her to turn hay into gold thread otherwise she'll be executed. Luckily some woodland spirit named Rumpelstiltskin comes to her aid in exchange for the distressed damsel's first born. Things turn out pretty much how one might expect. Now, if you know about bast fibers, you might be heckling our heroine from the sidelines. Congrats, you are the real life Rumpelstiltskin. Certainly, you're not going to be making anything out of hay, but our girl is probably mistaking flax for hay. But she's not the only idiot. The king who wanted her to make gold thread was only ever going to get linen, which is a wonderful material in its own right. Too bad they didn't have Pinterest back then, where they could have learned to class anything up with a spritz of metallic Krylon. Anyway, I grew up enjoying Shelly Duvall's Fairytale Theater, and looking back on this scene (10:54), it's especially hilarious to me now. What a good show.

Now, try suspending judgement on our poor heroine for a moment and try empathizing with her incredulity about spinning plant fiber. It does seem pretty weird, but it's amazingly doable as long as you can find a way to get at the cellulose that makes up every plant. In case you forgot, here's what I'm talking about.

One cellulose unit

This is a unit of cellulose, which has such wonderful properties which human beings have found many uses for. It's part of what makes up the fiber in our diet, it's what makes up the lumber we use, the pulp in our paper, and the plant fibers that we spin. However, there are a couple of factors, polymerization and crystallinity, that make any given source of cellulose better suited for one application or another.

Polymerization

Take a look at that cellulose unit again and notice the oxygens (in red) seemingly just hanging off the ends. That's where the cellulose units attach to each other. These attachments are particularly strong bonds known as covalent bonds. That's when a couple of atoms actually share electrons with each other. For cellulose in particular, the connection between the sugar subunits is called a glycosidic bond, and they always occur on the first and fourth carbons, connecting unit to unit, to another unit, and another, and so on, which makes a polymer. Polymers are molecules made of long chains of smaller units repeated together. A good analogy for polymers would be pop-beads. The beads would represent individual atoms and the "pop" part would be the shared electrons.

The longer the polymer, the better it is for whatever you're doing with it. When making paper, using long cellulose fibers makes a sturdier, more flexible paper. You've experienced this if you've ever noticed the difference between drying your hands on recycled paper towels and using some premium brand like Brawny or Cottonelle. Whenever we recycle paper, those polymer chains get damaged and broken, thereby shortening them. I once had a student insist on using the crummy brown paper towels to make recycled handmade paper. Besides the fact the she wasn't really recycling anything by unwinding the roll of fresh unused paper towels, the paper came out extremely weak and friable, just like I told her it would. There's only so many times you can recycle cellulose fibers until they are too short to do anything with.

It may not be surprising that paper makers often add cotton to their paper pulp to enhance the quality of their paper. I know my interviewers must have been impressed with the cotton stock I printed my resume on. Anyhow, I'm saving the short fibers left over from my cotton spinning and will see if they help a all with my paper making. Of course, the longest fibers are more desirable for spinning, but cotton is notoriously short (usually no more than an inch) compared to animal fibers which intimidates a lot of people, but obviously it's got a long enough staple length because people have been spinning it a very long time. One just needs to use different techniques and make a few adjustments to existing equipment to spin cotton, and someday I will go on at length about it.

Crystallinity

I hope you haven't been equating a single cellulose polymer with a single cellulose fiber. Cellulose fibers are actually a much more complex arrangement of many polymers bonded together more or less in parallel. The more parallel and tightly packed the polymers are, the higher the degree of crystallinity is. This makes cotton pretty strong, especially when wet, a feature animal fibers can't claim.

The hydroxyl groups that are key in dyeing of the fiber, as we have learned, but they are also what gives cotton its high degree of crystallinity, because they are so very attracted to each other. This is due to something called polarity which creates the condition for hydrogen bonds to form. This won't be difficult to understand if you can stick with me. Ha. You know how opposites attract? That's all that's really going on here, and in this case the opposites are negative and positive charges. The oxygen has a slight negative charge and the hydrogen has a slight positive charge.

This unbalance didn't just happen for no reason. Oxygen is a greedy asshole, and in bonding covalently with hydrogen, doesn't share the electron quite fairly. It's like if you were to split lawn mower ownership with your neighbor, and yet it spends more time in your neighbor's garage than yours. The lawnmower represents the electron in this analogy, keep up with me here. Anyhow, electrons have a negative charge, and because oxygen is bogarting the electrons, the oxygen ends up being more negative and the hydrogen ends up being more positive.

If the hydroxyl group from a cellulose polymer encounters a hydroxyl from another molecule, the positive hydrogen is going to be attracted to the negative oxygen of the other hydroxyl group. This attraction forms the hydrogen bond. It's not as strong as a covalent bond, but considering how many potential hydrogen bonds could be formed all up and down a cellulose fiber, overall, that's going to make a pretty tough fiber. The more hydrogen bonds form with adjacent cellulose fibers, the more closely organized in parallel the polymers become. This is crystallinity.

A hydrogen bond between the hydrogen in a
hydroxyl unit and the oxygen in water.

You know what else has Hs and Os bonded in it? Oh come on, do I have to spell it out for you? H two Ohhhhhhhh, now you get it don't you? Now remember, oxygen is an asshole, even in water. It has a slightly negative charge and water's hydrogens have a slightly positive charge. When water meets a cellulose fiber, the water becomes attracted to the hydroxyl groups in the cellulose. That's why I have 100% cotton bath towels. Nothing dries your butt crack better.

Fiber Structure

If you've got good eyes, it's pretty easy to tease out a single cotton fiber and look at it. That is not one polymer chain. It's not even a bunch of polymer chains hydrogen bonded together. One cotton fiber is actually many layers of crystalline cellulose chains arranged in different ways, and this is what makes cotton so splendid and easy to work with. Some of the layers are twisted, which makes the fiber strong. The fiber structure of cotton is pretty consistent, having a diameter of about 12-20 micrometers. The result is a fiber that is smooth, soft, and not irritating like wool is. Fine Merino is comparable.

Cotton is so great. It comes right off the plant this way. You can't just start spinning trees you know, even though they contain a lot of cellulose. In order to "spin trees" you have to remove all of the lignins and other compounds to get at the cellulose, and by that time you just have a bunch of unspinnable pulp left over, so then you'll have to chemically polymerize the pulp, which will never be as long or strong as cotton polymers. What I just described there is how rayon is made. Through some physical effort, bamboo, and bast fibers like linen and hemp, can be processed into spinnable fibers. Cotton, though, is a clear winner for me.

If you want to know more, here are a couple of more technical pages on the structure of cotton. Have at it!

Cotton Inc.

Barnhardt Cotton