What is Cellulose Accetate, and Why?
A Plastic Film Made Out of Cotton??!!

Author: Patricia DePra: Westfield State College

Well, no, not really. Cotton is mostly cellulose, which is a polymer of glucose and grown by special plants. Glucose molecules can be attached to each other in a number of different ways, by different types of chemical bonds.

Within one chain of cotton cellulose, the glucose molecules are arranged so that the polymer is the most extended that it can be. Each glucose unit has three OH groups (hydroxyls) that can hydrogen bond to adjacent chains. This strong binding between chains (called intermolecular forces) makes for strong properties: cotton is tough, which is why we make clothes out of it. This same structure for cellulose is found in trees and most plants. Cellulose in trees is combined with two other components, hemicellulose (to fill in the empty places) lignin (the glue that binds it all together).

The result is a very stiff and strong material that can hold up the tallest trees. (Which, as a matter of fact, it does! The major structural component of wood is cellulose.)

  Chains of cellulose are kind of like uncooked spaghetti: extended and lined up next to each other. Imagine these sticks of spaghetti much, much longer, and stuck to each other, too.

Because of this hydrogen bonding between the extended chains of cellulose, it's hard for any kind of solvent to squeeze in. Cotton just isn't soluble, really, which is good for us and plants: wouldn't want our clothes (and trees) dissolving and washing away when it rains. So,if we want to use it, we have to find a way to take the fibers from a cotton plant and spin those nature-made fibers into thread and other things.

What would happen if we took away the ability of cellulose to hydrogen bond? The polymer chains would be much more free to move around and become all un-tangled up. That is, the couldn't interact with a solvent better and less with themselves.

  Picture a bowl full of cooked spaghetti and you'd be pretty close to what a polymer is like without hydrogen bonding (or any other kind of intermolecular interactions that are relatively strong).

There's not much that we can do with raw cotton in terms of processing it, other than to make thread or yarn. But, that tangled up mess of polymer chains can also be made into films. Just imagine taking a big handful of that cooked spaghetti and smooshing it around on a plate to give a flat, smooth layer or film. You can't do that with the raw stuff.

So, how do we take away the hydrogen bonding? We have to block the OH group with something that won't hydrogen bond. A relatively cheap and easy way to do this is to replace them with acetate groups (or react them using chemistry). We use acetic acid to do this.

Acetic acid is what gives vinegar its sour taste, but soaking the cotton balls in vinegar won't work (don't even bother -- you'll just make cotton balls that smell like pickles!). We'll have to go into the lab and use 100% acetic acid (also called "glacial acetic acid) and acetic anhydride. (Acetic anhydride is just two acetic acid molecules put together with the loss of a water molecule. It's kind of a sneaky way to have extra-concentrated and more reactive acetic acid.)

  After all of the acid is added, it looks like the cotton is dissolving. What's really happening is a chemical reaction. As the cellulose OH groups are replaced with acetate groups, it all becomes more soluble. The chains spread apart, and get less tangled up. Now THIS is something that we can work with!

We haven't put the names together yet, but you probably guessed what this new stuff is -- cellulose acetate! (insert here info on what it's used for)

Cellulose acetate isn't soluble in water (which makes sense -- there's nothing on the cellulose acetate chain that can hydrogen bond with water). We use this property to purify it. When we stir water into the reaction mixture, the cellulose acetate precipitates out as a solid. After filtering it and pouring pure water over it to wash away the excess acid, we have solid cellulose acetate.

  Cotton balls before the reaction,
with precipitated cellulose acetate.

Now, up above we mentioned that each glucose linked up in a cellulose molecule has three OH groups available. Substituting one, two, or three of those OHs with acetates will result in different properties of the polymer films. As a matter of fact, cellulose acetate is often identified by "percent acetate". If it has at least 92% of its OH groups replaced by aceates, it can legally be called "cellulose triacetate." (I'm not kidding, there are laws about this!)

  Clear films can be cast from a solution of cellulose triacetate and methylene chloride (CH2Cl2). One of the uses of cellulose acetate is in candy wrappers.

Industrial uses of cellulose acetate include fibers (ever see "acetate" written on a clothes label? That's cellulose acetate!) and cigarette filters. Can you find other uses by searching the web, or (Heaven forbid!) going to the library? List all the uses you can find and send them to us. We'll put them together for everyone to see.

Copyright ©2000 | Department of Polymer Science | University of Southern Mississippi

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