What is plastic made from?
Plastic is a material that every single person on this planet uses. It is found everywhere and it is long lasting. Plastic is economical to produce and is not a scarce resource. In the past, materials like ivory and turtle shells were used to make various items. These were finite resources. Plastic solved the issue of scarcity of such resources. Plastics are light in weight and demonstrate varying degrees of physical strength depending on the type of plastic. They can be transformed into an endless number of shapes and forms, and that makes it a very versatile material in the field of manufacturing.
The Raw Material
Plastics are made from the raw material called hydrocarbons. Hydrocarbons, in this case, are well-known materials such as oil, natural gas, and coal – all naturally occurring resources. The hydrocarbon materials are processed to form long chains of molecules. These chains form the plastic material that we know of today.
The individual molecules are called monomers. These monomers are linked together by bonds between themselves and repeat over and over to form chains that are collectively known as polymer. “Poly” means many and “mer” is shorthand for monomer. Hence the name polymer. You may have noted that various types of plastic names often begin with a “poly” (e.g. polycarbonate, polyester, etc.). Interestingly, we actually refer to plastic as if it is a single material. However, there are in fact several types of materials that are classified as plastic.
Polymers are mainly comprised of carbon and hydrogen atoms. Sometimes, atoms of oxygen, chlorine, fluorine, oxygen, etc. are also present depending on the type of plastic being manufactured.
Hydrocarbons are converted into monomers via a process called cracking. Examples of monomers include ethylene, ethylene glycol, styrene, propylene, etc. These monomers are then further processed with other chemicals and catalysts to get the polymer chains that represent the plastic material.
The Process of Polymerization
There are different types of polymerization processes, namely addition polymerization, condensation polymerization, and so on. Different polymers are made using different types of polymerization processes. In the process of addition polymerization, the polymer is formed by fastening monomers together in a daisy chain format. In the process of condensation polymerization, two small hydrocarbon chains are joined while removing a water molecule via condensation. Thus, what we get is a bigger hydrocarbon chain. This simple process can be repeated to get longer and longer polymer chains.
Catalysts or reaction inducing agents are used to make the reactions in polymerization happen. Catalysts simply increase the likelihood of a reaction and accelerate the rate of reaction. The catalysts may undergo temporary change during the reaction but return back to their original form once the reaction is complete. Ziegler-Natta catalysts are well-known for making plastic polymers. They were developed by a German chemist Karl Ziegler and his Italian colleague Giulio Natta. In fact, so significant was their development of these polymerization catalysts, that the landmark work won them the 1963 Nobel Prize in Chemistry.
As plastic has a wide variety of uses, certain physical and chemical properties are desired in the hydrocarbon monomers to get the necessary end product depending on the application. To get these specific properties or appearance, various other ingredients are added to the hydrocarbons before or after polymerization. For example, plasticizers such as phthalates are used to process polyvinyl chloride and make it soft. Butadiene increases the toughness of plastic products.
Colorants are added to give specific bright or dark colors to the polymer. Stabilizers are added to prevent the plastic material from degrading or breaking under sunlight/heat. Fillers or low-cost minerals are sometimes added to reduce the cost of the plastic because adding a filler would mean using fewer hydrocarbons, which are expensive fossil fuels.
Once the chemical reactions are done and the additives are added, what we get is a polymer material that is ready for further machining. The polymer is available in powder or grain/pellet form and is suitable for use in various types of manufacturing machines. The powder or pellets can either be injection molded, cast, spun, or fabricated to create the final shape of form that is desired.
Manufacturing and Machining Processes
Processes like injection molding and casting involve heating the polymer till it turns into liquid and then either molding it or shaping it while cooling it down. Thus, the liquid polymer solidifies and takes the shape of the mold as per the design of the end product. Such reheating and cooling can only happen if the polymer is thermoplastic.
That brings us to an important distinction within polymers. There are two types of plastics, thermoplastic and thermoset. Thermoplastics can be heated, cooled, and then reheated to make new products without significant degradation. This process is possible because the long chains in the polymer are held together by weak bonds which tend to break when exposed to high heat. Thermoplastics then reform the bonds again when cooled down. Materials like polyethylene, polyvinyl chloride, and polypropylene are examples of thermoplastics. They are easy to recycle and reuse.
Thermosetting plastics cannot be reheated and reshaped. Once they are manufactured, that is pretty much it. They get severely degraded if you attempt to reheat them. The main reason for this behavior is the fact that the molecular chains are held together by strong cross-linked bonds. This is unlike the weak bonds as seen in thermoplastic polymers. Examples of thermosetting plastics are polyurethane, epoxy resin, and polytetrafluoroethylene.
Besides injection molding and casting, there are other manufacturing methods used to make plastic products. Calendering is a process where the polymer is squashed between heavy rollers to make thin sheets and films of plastic. Extruding is used to squeeze polymer through a nozzle in order to make products like straw and pipe. Other similar processes involve forcing of the polymer through tiny sieves called spinnerets to make fine fibers that we see in toothbrushes and nylon stockings. Plastic bottles are made using a process called blow molding, where hot polymer is squirted through a nozzle.
Though we talk about plastic manufacturing and the various processes, it does not necessarily mean that all plastics are man-made. There are naturally occurring plastics as well. One example is cellulose, which is a natural polymer used for making the ubiquitous sticky tape.