In the United States, the world’s largest consumer of petroleum, roughly 5% of total petroleum used goes to the production of plastics. That works out to approximately 330 million barrels of oil per year. Estimates are difficult to come by and fluctuate a great deal based on the source, but the basic message is that most plastic is made from petroleum. In the past, plant oils were used to make plastics, but the process is more difficult and expensive than using petroleum. How is oil made into plastic?
Monomer vs. Polymer
Plastic is a polymer, which means it is nothing more than a long chain of smaller units hooked together. A good example of a polymer is polyethylene. Ethylene (ethane) is the monomer and, when lots of these are connected end-to-end in a chain by a chemical reaction that breaks the double bond, a polymer is formed.
Monomers are obtained by refining petroleum. They are usually found naturally in oil or can be easily manufactured through simple chemical processes. It is important that the monomers be pure because contamination prevents them from polymerizing to form a chain and can also lead to weak or brittle plastic in the end.
Polymerization is the process of connecting monomers together in a chain. There are two basic methods of polymerization, addition and condensation. Note that when the same monomer is used in a reaction, it is called a polymer, but when two or more different monomers are used to make one chain, the resulting product is called a copolymer.
A condensation reaction is one that produces water (or sometimes an alcohol). Condensation polymerization is not only used to manufacture plastics, but is also used by living organisms to make large molecules like DNA, RNA, proteins, fat (triglyceride), starch, glycogen, and more. In fact, without condensation polymerization, it is a good bet that there would not be any life on earth.
In producing plastic, condensation is used to manufacture things like polyesters and Nylon. The basic process is outline by the following reaction where R represents any number of carbon atoms and n represents any whole number.
Addition reactions do NOT involve the loss of any other atom or molecule, so what goes into the reaction, stays in the reaction (kinda like Vegas). Polymers formed by this process include polypropylene, polyvinyl chloride, Teflon, polystyrene, and many types of rubber. The general reaction is as follows.
The major difference between addition polymerization and condensation polymerization is the quality of the plastic. As was mentioned, condensation reactions occur in many biological organisms, which is a good hint as to the properties of polymers formed using this reaction. Namely, they are biodegradable. Condensation polymers are biodegradable because the carbon-carbon bonds in these molecules tend to be polar (charge is not shared equally. Polar bonds are more reactive then non-polar bonds and so are more easily broken down.
Addition polymers are not very biodegradable, which has led to serious environmental problems. They are also very difficult to recycle because they don’t “come apart” easily to reform the monomers they were made from. This means they really can’t be reused.
Thermoset vs. Thermoplastic
Now that we know the basics of how plastics are made, let’s look at the two basic categories of plastic. The first is referred to as thermoset plastic. These plastics, after being heated and shaped, are cooled and become very hard. They then cannot return to their original form and are generally very hard. These are plastics you find in car parts or airplanes and include things like polyesters, polyurethanes, and epoxys.
Thermoplastics, on the other hand, remain soft after they cool. They can be remolded and changed and are the plastics that we are most familiar with in water bottles, bags, etc. Thermoplastics include polyethylene and polypropylene. The ability of these plastics to be remolded is what makes them so useful. They can be manufactured in one place (as pellets) and then shipped anywhere to be remolded into a product of just about any shape.
Many plastics are synthesized as pellets, which are produced from longer fibers. The process is really quite simple. A polymerization reaction is carried out continuously, producing a long, thin fiber of plastic. This fiber is then extruded (pulled) into a standard form and cut into tiny slices. These slices are called pellets and can be shipped anywhere they need to go to be reheated and made into a final product.
Pellets can be heated and reformed into products in several ways. As follows:
- Extrusion – The pellets are forced through a small opening (shapes vary) to make a long, continuous strand. This is howplastic wraps and sheets are made.
- Injection Molding – The pellets are forced, under high pressure, into a mold that has a specific shape. The mold is cooled and out comes a plastic cast in the same shape. This is how containers and tubs are made.
- Blow Molding – Pellets are heated and compressed into a tube that is forced into a mold. Air is then blow into the hot plasticto force it out against the sides of the mold. The mold is then cooled and the final cast is released. This is how most plastic bottles are made.
- Rotational Molding – In this case, the mold is rotated with heated pellets inside of it, which are slowly cooled over time. This is how large items, like toys, furniture, and garbage cans are made.
In some cases, plastics need special properties like color, fire resistance, antistatic coatings, etc. Pigmentation for color can occur either during final production or afterwards. All other special additions are done after final production in a process called post production. Things that occur in post production include:
- Welding (plastic can be attached through plastic welding techniques)
- Coating (Color, static, light properties)