Alexei Volkov
Plastic is a broad category of materials that are made from polymers, which are large molecules formed by linking chains of smaller molecules called monomers. The most common monomers used in plastic production are ethylene and propylene, which are derived from the distillation of crude oil or natural gas. Other sources of monomers include coal, salt, cellulose, and renewable resources like polylactic acid. The specific ingredients or monomers used in plastic determine its unique properties, such as transparency, strength, and flexibility. While plastic has become ubiquitous due to its adaptability, durability, and low cost, its slow decomposition rate has led to widespread environmental concerns.
| Characteristics | Values |
|---|---|
| Main Ingredient | Crude oil, natural gas, or coal |
| Other Ingredients | Cellulose, salt, carbohydrates, fats, oils, natural latex rubber, proteins from milk, eggs, blood |
| Plastic Type | Synthetic plastics, biobased plastics |
| Production Process | Distillation, polymerisation, polycondensation |
| Plastic Properties | Transparency, deformation under stress, strength, ductility, resistance to moisture, stretchiness, machine washability, stain interaction |
| Plastic Structure | Polymers, monomers, side chains |
| Plastic Uses | Packaging, construction, pipes, medical devices, automobiles, furniture, toys, insulation, solar panels, windmill blades |
| Environmental Impact | Slow decomposition rate, plastic pollution, microplastics, marine garbage patches, incineration, recycling |
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What You'll Learn
Plastic is a large class of materials
Plastic is a huge class of materials with a wide range of applications. The word 'plastic' comes from the Greek 'plastikos' and the Latin 'plasticus', meaning 'fit for moulding' or 'capable of being moulded into various forms'. This refers to the material's malleability or plasticity during manufacture, which allows it to be moulded, extruded, or pressed into a diverse range of solid forms.
Plastics are derived from natural, organic materials such as cellulose, coal, natural gas, salt, and crude oil. Crude oil, a complex mixture of thousands of compounds, is processed through distillation into groups of lighter components called fractions. Each fraction is a mixture of hydrocarbon chains, which differ in terms of the size and structure of their molecules. One of these fractions, naphtha, is crucial for plastic production. Two main processes are used to produce plastics: polymerisation and polycondensation. In a polymerisation reactor, monomers such as ethylene and propylene are linked together to form long polymer chains.
The defining characteristic of plastics is their plasticity, or malleability, which allows them to be moulded into a diverse range of solid forms. This adaptability, combined with their low weight, durability, flexibility, chemical resistance, low toxicity, and low-cost production, has led to their widespread use around the world. Plastics have had major benefits for mankind, from medical devices to lightweight construction materials.
However, plastics are also the basis of widespread environmental concerns due to their slow decomposition rate in natural ecosystems. Most plastic produced has not been reused, with much of it ending up in landfills or as plastic pollution. Particular concern is focused on microplastics, which create marine garbage patches. Nevertheless, initiatives such as the Circular Plastics Alliance aim to foster the sustainable use of plastics and promote a circular economy.
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Polymers and monomers
Plastics are derived from natural, organic materials such as cellulose, coal, natural gas, salt, and crude oil. Crude oil is a complex mixture of thousands of compounds and needs to be processed before it can be used. The production of plastics begins with the distillation of crude oil in an oil refinery. This separates the heavy crude oil into groups of lighter components, called fractions. Each fraction is a mixture of hydrocarbon chains (chemical compounds made up of carbon and hydrogen), which differ in terms of the size and structure of their molecules.
One of these fractions, naphtha, is the crucial compound for the production of plastics. Two main processes are used to produce plastics – polymerisation and polycondensation – and they both require specific catalysts. In a polymerisation reactor, monomers such as ethylene and propylene are linked together to form long polymer chains.
Plastics always consist of large molecules called polymers. Polymers, in turn, consist of many identical small particles that are strung together like a chain. These individual small particles are called monomers. The length of these polymer chains determines the properties of plastic. For example, the length of the polymer chain for polyethylene (PE) determines the hardness of this type of plastic.
There are two groups of plastics, thermoplastics and thermosets. Thermoplastics become soft upon heating, which means that in principle they can be melted again and shaped in a mould. Thermoplastics include film/wrap, bottles, clothing, polystyrene and many more products. Thermosets, on the other hand, can no longer be melted down once they have been produced. They remain hard even after heating and are found in electrical sockets and the hulls of sailing boats.
Synthetic polymers include polystyrene (PS), polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP) and polyvinyl chloride (PVC). Copolymers occur when different monomers are linked together, for example, to make Nylon (polyamide).
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Synthetic plastics and biobased plastics
Plastic is derived from polymers, which are created by linking chains of molecules (monomers) to form a large molecule. The monomers are usually ethylene and propylene, which are obtained from the distillation of crude oil. Crude oil is a complex mixture of thousands of compounds, which need to be processed before they can be used to create plastic. The process of distillation separates the heavy crude oil into groups of lighter components, called fractions. Each fraction is a mixture of hydrocarbon chains, which differ in terms of the size and structure of their molecules. One of these fractions, naphtha, is the crucial compound for the production of plastics.
Synthetic plastics are derived from crude oil, natural gas, or coal. The production of synthetic plastics involves two main processes: polymerisation and polycondensation. Both of these processes require specific catalysts. In polymerisation, monomers are linked together to form long polymer chains. Synthetic plastics played a major role during World War II, with the US tripling its production of plastics between 1940 and 1945.
Biobased plastics, on the other hand, are derived from renewable products such as carbohydrates, fats, and oils. They can also be produced from CO2 and renewable raw materials. The development of biobased plastics was spurred by the environmental movement in the 1970s. Bioplastics are typically manufactured from bio-based polymers, which are derived from biomass. These bioplastics can have a lower carbon footprint than fossil-based plastics and can exhibit advantageous material properties. They are also compatible with existing recycling streams and some offer biodegradation. However, they may also have negative agricultural impacts, compete with food production, have unclear end-of-life management, and be more costly.
As of 2018, bioplastics represented approximately 2% of global plastic output. The most widely used bioplastic is thermoplastic starch, constituting about 50% of the bioplastics market. Bioplastics are used for disposable items such as packaging, crockery, cutlery, pots, bowls, and straws. They also have applications in the pharmaceutical sector, where pure starch-based bioplastic is used for drug capsules.
Overall, both synthetic and biobased plastics have their advantages and disadvantages. Synthetic plastics are more widely used and have a variety of applications, but they contribute to environmental pollution. Biobased plastics have the potential to be more sustainable and environmentally friendly, but they currently represent a small portion of the global plastics output and face challenges such as cost and performance.
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The process of making plastic
The main ingredients used in the production of plastics are natural, organic materials such as cellulose, coal, natural gas, salt, and crude oil. Crude oil is a complex mixture of thousands of compounds and needs to be processed before it can be used.
The production of plastics begins with the distillation of crude oil in an oil refinery. This separates the heavy crude oil into groups of lighter components, called fractions. Each fraction is a mixture of hydrocarbon chains (chemical compounds made up of carbon and hydrogen), which differ in terms of the size and structure of their molecules. One of these fractions, naphtha, is the crucial compound for the production of plastics. Two main processes are used to produce plastics – polymerisation and polycondensation – and they both require specific catalysts.
In a polymerisation reactor, monomers such as ethylene and propylene are linked together to form long polymer chains. Dimers can then join to form tetramers (four units) and so on. These byproducts are necessary to be removed for the success of the reaction. The most common byproduct is water, which is treated and disposed of easily. Byproducts can also be valuable raw materials that are recycled back into the feedstream.
Nurdles are little plastic pellets made from the resins created in the polymerization process. The process to do so is through melting and cooling operations. Once these lentil-sized pellets are produced, they are shipped from a petrochemical refining facility to manufacturing facilities where they are melted down and formed into a final product. Manufacturers compound, mix, and melt the plastic pellets with other ingredients to very specific recipes. When followed, these recipes determine the characteristics and properties of the plastic product. The melted plastic is then formed into shape by plastic forming machinery, which is determined by the application of the product.
There are several plastic forming methods, each used for a specific reason, although they can sometimes be used in combination to make more complex finished parts. Plastic injection molding is a high-volume manufacturing method where a liquid plastic resin is injected into the empty cavity of a forming die, taking on the shape of its interior surface. When the resin is cooled and solidified, the forming die opens, the finished part is ejected, and the process is repeated.
Rotational molding also uses a mold tool, consisting of a core side and a cavity side. The plastic powder is first poured into the cavity of the mold, and then the mold is placed in an oven. While being heated, the mold is slowly rotated on two axes. Gravity is used to stick the plastic to the tool walls and build up the correct thickness. Finally, the mold is removed from the oven and slowly cooled to prevent warpage. Full cooling can take several minutes, after which the tool is opened and the part is removed for the next cycle. Rotational molding is ideal for making large, hollow, or concave shapes, often for outdoor use, such as canoes, tubs, or water storage tanks.
Another method is blow molding, which is the most common way to make thin-walled, inexpensive containers like disposable drinking cups or bottles. During injection blow molding, gas pressure is used to force the molten resin into a mold cavity. The process is easily controlled and repeatable and is commonly used for transparent plastic drinking bottles. It makes for excellent surface quality but it’s not ideal for thin walls.
RIM is a process mostly used in the automotive industry because it produces lightweight parts that have a rigid skin. This skin is easily painted to make body panels, dashboards, and other car parts. However, thermoforming plastics won’t work in this process. Instead, this process requires thermosetting plastic. Thermosetting plastics undergo an irreversible chemical reaction inside the mold. This usually causes them to expand like foam, filling a mold cavity. When the chemical reaction is done, the plastic sets into its final form.
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Uses of plastic
Plastics are derived from natural, organic materials such as cellulose, coal, natural gas, salt, and crude oil. Synthetic plastics are derived from crude oil, natural gas, or coal, while bio-based plastics come from renewable products such as carbohydrates, fats, and oils. The production of plastics involves the distillation of crude oil into fractions, which are then converted into simpler hydrocarbons through cracking. These hydrocarbons are essential for preparing various products, including plastics.
Plastics have a wide range of uses across numerous sectors due to their versatility, durability, and unique properties. They are used extensively in transportation, including automobiles, trains, planes, ships, and even space stations. Plastics are combined with other materials to form structural elements in vehicles, skateboards, rollerblades, and bicycles. They also play a crucial role in packaging, with plastic containers, bottles, and cartons commonly used for food and beverages.
In the home, plastics are prevalent in the kitchen, with plastic chairs, countertops, linings in non-stick cookware, and plumbing. Plastics are also used in furniture, electronics, and insulation. They contribute to sustainability by making products lighter, reducing fuel consumption during shipping, and providing thermal insulation in houses. Additionally, plastics are used in renewable energy technologies such as solar panels and windmill blades.
Plastics have revolutionized industries with innovations like shape-memory polymers, light-responsive polymers, and self-healing polymers. They are used in building and construction, textiles, consumer products, electrical and electronic devices, and industrial machinery. The ability to mold, laminate, or shape plastics allows for their use in various forms, such as pipes, boxes, and films. However, the durability of plastics poses disposal challenges, and efforts are being made to promote sustainable practices and recycling in the plastics industry.
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Frequently asked questions
The main ingredient in plastics is carbon. Plastics are polymers, which are large molecules made by linking chains of smaller molecules called monomers. Most polymers are formed from chains of carbon atoms, with or without oxygen, nitrogen or sulphur atoms attached.
Monomers are small molecules that are not yet part of a chain. They are linked together to form polymers. Examples of monomers include ethylene and propylene.
Polymers are large molecules that are shaped like long strings. They are formed by linking together many smaller molecules called monomers. The links between the monomers make polymers strong and durable.
Some examples of polymers that are commonly used in plastics include polyethylene, polyvinyl chloride (PVC), nylon, polyester, polyurethane, and silicone.
