Saturday, October 18, 2014

Ever wonder where polystyrene comes from?

Recently I come across information about the styrene monomer from which polystyrene is made. It turns out that styrene, like almost everything in the petrochemical industry, comes from a mix of ethylene, which comes from ethane, and benzene, which comes from hexane. And ethane and hexane both come from petroleum. Let's look at each in turn.

Ethane is a simple hydrocarbon with 2 carbons and a full slate of 8 hydrogen atoms:

The picture illustrates carbon's gregarious nature; it likes to have 4 friends. Hydrogen is a little more stand-offish and really only wants one friend. So ethane has two carbons, each of which has three hydrogen atoms and the other carbon atom as friends. Chemists write this as C2H6.

However it turns out that you can't do much with ethane as it stands, except maybe mix it with propane (C3H8) to make a fuel for the barbeque (which is of course an excellent thing to use it for, and which therefore merits it a mention as one of my favourite molecules, especially when accompanied by an ethanol-enhanced beverage). But if we take advantage of the fact that carbon can decide to be really, really BFF with another carbon atom and make what is called a double bond, we can strip off two of the hydrogen atoms in ethane to make ethylene, C2H4:

Here each carbon has two hydrogen friends, and the remaining two friends are taken care of through the BFF, double-bond relation with the other carbon, which counts for two friends.

Next is hexane. If ethane consists of two carbon atoms and a full complement of hydrogen, hexane consists of six carbon atoms and the usual full complement of hydrogen atoms, in this case fourteen (C6H14):

Now comes the really interesting part. It turns out that you can bend hexane around into a hexagon, creating three double bonds and eliminating most of the hydrogen atoms, thusly:

You will note that each carbon atom has four connections: one to a hydrogen atom, one to a carbon atom, and two to a second carbon atom (its BFF). This molecule is known as benzene, C6H6. Now drawing this is a complicated business, and some smart fellow (probably an engineer, being busy people they like shortcuts) decided that you could represent the benzene ring much more simply, thusly:

Now here comes the fun part: by replacing one of the hydrogen atoms around the ring with a link to an ethylene molecule, you get ethenyl-benzene, a.k.a. the styrene monomer (C8H8):

Now I have brought up the word "monomer", which means one ('mono') unit ('mer'); string a bunch of these together and you get a polymer, in this case polystyrene which is made up of a large number of styrene monomer units.

So there you go: from black gooey crude to a clear, hard plastic material that melts at fairly high temperatures and that has excellent stiffness and hardness while being easy to mold or shape into all kinds of interesting things, like for instance a 1/24 scale model of the Mini that won the Monte Carlo Rallye in 1967, driven by Paddy Hopkirk.

Originally posted 18 October 2014, update 21 January 2015

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