From Cotton to Cellulose Acetate

Full transcript

Cotton is nearly pure cellulose — and so is wood pulp. Cellulose is a polymer, which simply means a long chain: thousands of sugar rings snapped together, end to end, stretching on and on. Zoom in on just one of those rings. Every single ring in the chain carries three little groups called hydroxyls — an oxygen holding a hydrogen, written OH — and those tiny groups are the key to everything. Those OH groups reach across and grab the OH groups on the neighbouring chain. Millions of tiny handshakes, chain gripping chain gripping chain — all the way down the line, locking everything together. That iron grip is why your cotton t-shirt survives the dryer. Heat it and it scorches before it ever softens. It will not melt, it will not dissolve. Wonderful in a shirt — completely useless in an eyeglass frame. So about a hundred and fifty years ago, chemists picked the lock. They soaked cellulose in acetic acid and acetic anhydride — vinegar's angry cousin — and bulky acetyl groups shoved the OH groups right off their seats. Just like that, the handshakes are gone. The chains can slide past each other. The material softens with heat, dissolves in acetone, takes a shape and holds it. That is cellulose acetate — cotton's toughness without cotton's stubbornness. At this stage it is just a fine powder. Now we feed that powder: plasticizer for flexibility, acetone to loosen everything up. Knead it all together and you get something that looks and behaves exactly like bread dough. Which is why my workshop is called the Optical Kitchen — that is the joke, but it is also just completely true.