The Chemistry of Fibers – Protein/Plant... Soy, Milk, Chitin, Etc

So, a lot of people know that fibers are largely divided into two classes. Protein, and cellulose. Proteins are what make the strong flexible fibers in animals, hair, skin, muscles, even collagen. Cellulose is the main ingredient in plant cell walls. It’s what gives the plant its strength and shape.

Many of the fibers are thus obvious to categorize. Wool and alpaca and dog and bunny and goat are all animal fibers, and all protein fibers. Cotton, flax, hemp, and nettle, are all cellulose, plant fibers. But many fibers are not obvious, like chitin, or seem obvious but actually aren’t, like soy or even silk.

So the first thing to understand is the basic chemistry of a “protein” fiber or a “cellulose” fiber. How are they the same, how are they different?

Protein Fibers:

We’ll start with a protein fiber. Protein fibers are actually made of multiple layers of proteins bound to each other in different ways to make the many different types of fibers you’re familiar with. However, here’s the important part.

This is a simple, four amino acid (piece) protein. If you stuck a bunch of these together you’d have a “polypeptide,” a protein fiber. You can tell it’s four pieces, or amino acids, because I’ve conveniently labeled each amino acid’s “R group” with a number. That’s because what’s in the R group doesn’t really matter for this part of the discussion. They affect things like durability, crimp, and tensile strength, but not dyeing.

What’s most important for your dyeing purposes are those N-H bits, called “amine” groups (which is how amin-o acids get their name!). These amine groups are the signifying feature of a protein. And, if you remember my previous post on the difference between acid and reactive dyes, they’re the part that acid dyes bind to.

So, any fibre that’s made up of these amino acids, with their NH and NH2 groups, is a protein fiber, and will dye with acid dyes. Got it?

Cellulose Fibers:

Cellulose fibers have a different structure, with no amino groups. Celluose is made of strings of glucose sugar instead of amino acids. Like protein fibers, natural cellulose fibers are more complicated than this, with layers and such, but also similarly, this is the important part.

The important part here is those “OH” groups hanging off the edges, called “hydroxide” groups (’cause they have a HYDRogen and an OXygen). Those are the parts that interact with reactive dyes (when you put the fiber in a basic solution). So, any fiber made with cellulose molecules, is a cellulose fiber, dyed with reactive dyes. All pretty clear and straight forward, right?

“Odd” Fibers:

So, that’s the easy part. Now lets look at some weirder ones. We’ll start with soy. Soy is a plant, right? Of course! It’s where we get edamame and soy beans and tofu. Mmm tofu. Yay plants! So it should be a cellulose fiber, that dyes with reactive dyes, riiiight?

Well… no. (Bet you didn’t see that coming!)

You see, there’s another way to divide fibers, though it’s not as all-around helpful for things like dyeing. Synthetic and natural. That is, fibers which are used more-or-less “as is” and fibers which we have to create.

The obvious “fibers which are used more-or-less ‘as is'” are the wool/hair/fur/cotton fibers. We pluck or shear them off the animal or plant, clean them, and use them, simple. Slightly less obvious are the “bast” fibers which are cellulose fibers that naturally grow for strength and flexibility in some plants. Flax and hemp are excellent examples of bast fibers. To collect these we “rett” (partially rot, how’d you guess?) the rest of the plant away from the bast fiber, clean it up, and spin it. Examples include of bast fibers are flax/linen, hemp, ramie, some bamboo, nettle, wisteria, and milkweed.

So the “natural” fibers are the fibers which already exist and we just collect and use them.

The “synthetic” fibers are fibers that we have to make. Most people, when they think of “synthetic” think of things like acrylic, plastics. And, of course, acrylic is synthetic. But in this case, I use synthetic in its more complete sense of synthesized, or combined from two or more parts. Again, there are some fairly clear cut examples of this, and more confusing ones.

A clear example is milk, and a confusing one is the soy I mentioned above. We’ll start with milk.

Milk is clearly… not a fiber. And not particular fibrous to boot. If you’re like most non-chemists you probably wonder where in the heck they found fiber in milk! And you’d be right, there IS no fiber in milk! What there is, is casein, a protein made of many amino acids. In fact, it’s the majority of the protein in whey protein you can get as a dietary supplement. Another protein that looks roughly like…

So you string a bunch of those together, and you get a fiber! And, as you’d expect looking at the chemistry, this is a protein fiber, made with lots of amino (NH and NH2) groups and thus dyes with acid dyes.

Now, back to soy. Soy is, indeed, a plant, but the thing to know about soy fiber is that it is not a bast fiber. That is, the soy fiber doesn’t come out of the stalk of the plant as a fiber. Instead, soy fiber, like milk fiber, is a synthetic fiber, actually made from the bean. The leftovers of the bean after they make other products have a high quantity of the amino acid lysine in them. So, they take the amino acid lysine, stick it all together in strings, and you again get…

A protein molecule all stuck together into long fibers, and thus a protein fiber that dyes with acid dyes, even though it came from a plant!

The Odd Case of Synthetic Cellulose:

Many of the other “new” fibers are in this synthetic class, but are made slightly differently than soy and milk. Seacell, Viscose, Tencel, Rayon, and Lyocell are all examples of what is called “regenerated cellulose.” These terms can be a bit confusing as they’re all essentially the same fiber with variations in word choice or fiber origin.

Basically, they take an already existing cellulose (wood, seaweed) and then break it down and string it back together into fibers to look like this again.

Bamboo is an interesting example in that it exists in two forms. There is “bamboo rayon” which is a regenerated form of bamboo and far far more common. However, bamboo does also have a bast fiber that can be harvested and used. Both are still cellulose fibers, of course, so will dye with reactive dyes.

“Combination” Fibers:

Finally, two of the odder examples of fibers… silk and chitin. We’ll start with chitin.

Chitin fiber, also called crab fiber, comes from exactly that. The chitin (shell) of crabs. It could also come from the chitin in insect shells or shrimp shells or any other arthropod. So… clearly not a cellulose fiber, right? Since there’s no cellulose in a crab.

So what is “chitin” actually made of? Well, the chemical answer is “polymerized N-acetylglucosamine.” Now, what do you notice about that long, complicated word? The first thing with any complex chemistry word is to split it into it’s pieces.

So… “polymerized” means it’s a bunch of something all strung together, interesting but not useful. “N” well that’s boring, useful to a biochemist to know where it came from, but not for anyone else. Ok, what else? There’s “acetyl,” “glucos,” and “amine.” Ah, now these could be useful.

Depending on your memory, you’ll have already locked onto the second two words, “glucos” which looks a lot like “glucose” and “amine.” Good job, those are the important parts to us.

But now you’re thinking, “But Gnome, wait, glucose is what you said cellulose is made of! And amines are what you said proteins are made of! What are you trying to pull here?!”

Well… I also told you chitin was weird, didn’t I? Because you’re right, glucose is what cellulose is made of, and amines are what proteins are made of! And “glucosamine” is a glucose with an amine group! Ok… so… lets draw out the chemistry and see if that tells us anything, ok?

Chitin fiber is made of strings of this. You’ll notice overall it looks a lot like the cellulose fiber, and in fact you can actually make half-cellulose half-chitin fibers! But remember how I said what the important parts of the protein and cellulose fibers were in terms of dyeing?

In pink are the OH, hydroxide, groups that are useful in reactive dyeing. In green are the NH2, amine, groups that are useful in acid dyeing. So you can see that chitin is odd in that it will bind with both acid and reactive dyes. But you can also see that it will bind better with reactive dyes, as there are more hydroxide binding sites. And you can guess that overall it will behave like a regenerated cellulose fiber.

Now, silk. Similarly odd. Silk is an animal fibre, but unlike most animal fibers, it’s not a hair/wool/fur fiber. Instead, silk is formed of protein polymers layered on top of each other. Each repeating motifs of this sequence…

If you look at it the same way as we looked at the chitin fiber, you can see that there’s a lot of amine (NH and NH2) and only one hydroxide (OH) per stretch of silk. So, in reverse of chitin, silk dyes more easily (and usually more deeply) with acid dyes though reactive dyes will also bind to it.


So, that’s the dyeing chemistry of fiber, with a focus on how they differ and the odd fibers.

Protein fibers are made of amino acids and include: wool, alpaca, dog, goat, rabbit, milk, silk, and soy.

Cellulose fibers are made of strings of glucose and include: cotton, flax/linen, ramie, bamboo, seacell (seaweed), tencel (wood), hemp, wisteria, nettle, and milkweed.

Chitin dyes as if it was a cellulose fiber, even though it’s not cellulose. Silk dyes as the protein fiber you’d expect but will also dye like a cellulose fiber though not well.

So! Any questions?

~That's all for now!