How is plastic created?
In: Technology
Hydrocarbons are the components of carbon and hydrogen. Without getting into deep details, carbon has 4 “links” it can use to link to other atoms, hydrogen has one. So, methane is 4 hydrogens connected to one carbon, CH4. Ethane is 2 carbons linked with a single bond, and 6 hydrogens occupying the remaining bonds, CH3-CH3. Hexane is six carbons, and hydrogens on all remaining links. CH3-CH2-CH2-CH2-CH2-CH3. Such chains can get pretty long; the longer they are the denser the substance as the chains start tangling up – up to 5 carbons it’s gas, up to a couple dozen it’s increasingly dense liquid, above that it’s solid.
Now the simple chains is not the only way these carbons can link. Ethylene is 2 carbons that are linked together with 2 bonds – H2C=CH2. These double links are kinda “unstable”, given the right circumstances (temperature, pressure) they’ll break leaving a single link – and two “open” linkages that will seek any chemical they could bond to, and even break a pre-existing weaker bond in a chemical they find, to attach to.
Now introduce the perfect conditions and a tiny bit of very reactive chemical (say, chlorine, Cl) to ethylene. The chemical breaks the bond, attaches to one of the “links” and leaves the other one dangling, seeking something to connect to. From your CH2=CH2 you get Cl-CH2-CH2-*
And what it has around is a lot of ethylene with weak double link, to break, attach to, leaving a single link – grabs the nearest one and… Cl-CH2-CH2-CH2-CH2-* …and the situation repeats, the chain grows until it happens to meet the end of another dangling chain or another chlorine atom (we added very little of it!) or some impurity to terminate the chain. This is called polymerization, such self-propelling linking of a massive amount of atoms into long chains.
And as said before, the chains tangle with each other, become less mobile, the substance becomes a solid – known as polyethylene, a pretty common plastic.
If you use a different chemical, you can get plastics of different properties – say, propylene, CH2=CH-CH3, will have these small “CH3” pieces dangling off every other node after polymerization (into polypropylene), tangling with everything, making a plastic that is more rigid, more brittle, less stretchy, with different properties. There’s a lot of such starting chemicals of different properties, and you can fine-tune the parameters further with additives, amount of catalyst that starts (and terminates) the polymer chains, temperature, pressure and other stuff that comprises a huge segment of material science.
Now how are these base substances made? The processes vary a lot, but usually involve heating crude oil (which is a mix of random, various length hydrocarbon chains, and other chaotic variants of hydrocarbons) until it breaks into small particles, separating them through distillation and possibly treating with other substances to achieve exactly the components you want. For example, ethylene will be produced by superheating longer hydrocarbons and then distilling it away from all the other compounds that got created in the process (most of which will find other uses).
In general, plastics are long chains of molecules. The chemicals that make up the links and how the links are assembled and shaped, effect the chemical and mechanical properties of the plastic. The way the individual links (monomers) get assembled into chains (polymers) is complicated and varies for just about every type, but typically include some sort of catalyst (chemical to make the reaction go faster) in order to produce the plastic. After the resin is made, it is formed into whatever is needed.
Latest Answers