Maya Ellison
Many plastic names start with 'poly' because they are polymers, which are derived from the Greek prefix poly-, meaning many, and the suffix -mer, meaning parts. Polymers are created when small molecules, known as monomers, chemically combine to form a larger network of connected molecules. The most common plastic, polyethylene, is a polymer primarily used for packaging. It is derived from the polymerization of ethylene, a gaseous hydrocarbon. When heated, ethylene joins together into long, repeating carbon chains, forming polyethylene, a plastic resin.
| Characteristics | Values |
|---|---|
| Why do plastics start with "poly"? | The term "poly" comes from the Greek prefix "poly-", which means "many". Plastics are polymers, which are made up of many uniform molecules called monomers. |
| Examples of plastic polymers | Polyethylene (PE), polypropylene (PP), epoxy, polyester (PS) |
| How is plastic made? | Plastic is made by polymerization, which is the process of converting raw material molecules into monomers, which are then linked together to form polymers. |
| What are monomers? | Monomers are small molecules that combine chemically to form polymers. Examples of monomers include ethylene, propylene, and butene. |
| What are the characteristics of plastic polymers? | Plastic polymers can have varying characteristics depending on their type. For example, polyethylene has a low melting point and is flexible, while polycarbonate plastic is strong and impact-resistant. |
| What are the concerns associated with plastics? | Plastics have slow decomposition rates, leading to environmental concerns. They can also release gases, such as methane and ethylene, and contribute to pollution when not disposed of properly. |
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What You'll Learn
Plastic is a specific type of polymer
Plastics are synthetic polymers with a large molecular mass and a mostly linear structure. They are made from oil and have varying levels of durability. The first synthetic plastic, Bakelite, was created in 1909 for telephone and electrical components.
The most familiar plastic polymers are polyethylene (PE), polypropylene (PP), epoxy, and polyester (PS). These are derived from petroleum hydrocarbons and are used in a wide range of applications. However, they pose challenges in terms of end-of-life recycling and disposal due to their difficulty in breaking down.
The process of converting raw material molecules into monomers such as ethylene, propylene, and butene is an important step in plastic production. These monomers then undergo polymerisation, where they are linked together through chemical mechanisms to produce polymers.
The polymerisation process results in the formation of thick, viscous substances known as resins, which are used to create plastic products. For example, ethylene monomers, when subjected to heat, pressure, and catalysts, join together to form long, repeating carbon chains, resulting in a plastic resin called polyethylene (PE).
Polyethylene, or polythene, is the most commonly produced plastic. It is primarily used for packaging, including plastic bags, films, geomembranes, and containers. It has excellent chemical resistance and is non-toxic, making it a popular multi-use plastic. However, its chemical resilience also contributes to its persistence in the environment when improperly disposed of.
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Polymers are derived from crude oil and natural gas
Crude oil, also known as petroleum, is the principal source of carbon for modern plastic. It provides the necessary hydrocarbons for polymerization. However, it is important to note that crude oil is not the primary source of feedstock for plastic production in certain regions like the United States. Instead, natural gas and feedstocks derived from natural gas processing and crude oil refining play a more significant role in plastic production in these areas.
Natural gas, a fossil fuel, is another crucial feedstock in the production of polymers. It provides reactant gases such as ethane, propane, and butane, which are essential for the polymerization process. Additionally, natural gas processing yields hydrocarbon gas liquids (HGLs), which can be used as feedstock for petrochemical crackers or direct inputs into plastic manufacturing.
The process of converting crude oil and natural gas into polymers involves several steps. Firstly, the raw materials are heated, melted, and kneaded. Then, initiators are added to start the chain reaction and form polymers through polymerization. After processing with additives, the polymers are converted into strings and then ground into pellets. Finally, these pellets are melted and moulded into various plastic products with specific shapes, sizes, and colours.
While synthetic polymers are predominantly derived from crude oil and natural gas, there is a growing trend towards the development of biodegradable polymers. These biodegradable alternatives are produced from renewable resources such as starch, cellulose, lignin, proteins, and lipids. They address environmental concerns associated with waste disposal and the diminishing reserves of crude oil and natural gas.
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Polymers are made of uniform molecules
Polymers are substances composed of macromolecules, which are molecules of high relative molecular mass. They are formed by the polymerization of many small molecules known as monomers. This process involves linking together hydrocarbon monomers to produce polymers. Monomers are simple chemical units that have double bonds, allowing carbon atoms to react and form polymers.
The word "polymer" comes from the Greek words "poly," meaning "many," and "mer," meaning "part." Thus, a polymer is a substance made up of many parts. Monomers are the individual parts that make up a polymer.
During polymerization, chemical groups may be lost from each monomer. For example, in the polymerization of PET polyester, the monomers are terephthalic acid and ethylene glycol, but the repeating unit in the polymer is —OC—C6H4—COO—CH2—CH2—O—. This corresponds to the combination of the two monomers with the loss of two water molecules.
Polymers can be natural or synthetic. Natural polymers include proteins, which are polymers of amino acids, and nucleic acids, which are polymers of nucleotides. Synthetic polymers include polyethylene, polypropylene, polystyrene, and polyvinyl chloride (PVC).
Polyethylene, the simplest synthetic polymer, is made up of just one kind of monomer—ethylene, which consists of two carbon atoms and two hydrogen atoms. It is formed when thousands of ethylene molecules are joined end-to-end through a process of heating and the creation of free radicals. This results in a long chain of molecules known as polymerization.
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Polymerisation involves linking hydrocarbon monomers
Polymerisation is a chemical reaction in which product molecules can grow indefinitely in size, provided that the necessary reactants are available and the involved monomers possess the appropriate functionalities. In polymer chemistry, polymerisation involves reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. Polymerisation can proceed according to two different mechanisms: condensation polymerisation and addition polymerisation.
Addition polymerisation involves the successive addition of monomer molecules to the reactive end of a growing polymer chain. In this process, an active free radical is created by a chemical. This free radical is quite unstable but highly reactive due to its unpaired electrons. Once formed, the free radical reacts with another monomer radical, resulting in the formation of another monomer and kickstarting a chain reaction. Within a fraction of a second, tens of millions of monomers can be added. The reaction concludes when two free radical ends encounter each other and bond, forming a large molecule.
Condensation polymerisation, on the other hand, involves independent reaction steps between functional groups of monomer units, typically containing heteroatoms such as nitrogen or oxygen. Most condensation polymers are also considered step-growth polymers, as a small molecule like water is lost when the polymer chain is lengthened. However, exceptions exist, such as polyurethanes, which are formed without the loss of water or other volatile molecules.
In the context of plastics, polymerisation involves linking hydrocarbon monomers together through chemical mechanisms to produce polymers. This process generates thick, viscous substances known as resins, which are used to create plastic products. For instance, ethylene, a gaseous hydrocarbon, can be subjected to heat, pressure, and a catalyst, causing it to join into long, repeating carbon chains. These joined molecules, known as polymers, form a plastic resin called polyethylene (PE). PE is then processed in factories to create plastic pellets, which can be melted into various final products.
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Polyethylene is the most common plastic
Many plastics start with "poly" because they are polymers, which are long chains of repeating carbon-based molecules. One of the most common polymers is polyethylene, which is made through the polymerization of ethylene.
Polyethylene, also known as polythene and often abbreviated as PE, is the most common plastic in the world. It is a light, versatile synthetic resin primarily used for packaging, including plastic bags, plastic films, geomembranes, and containers such as bottles, cups, and jars. It is also used in the automotive industry for fuel tanks, under-hood components, and interior trim due to its impact resistance and fluid stability. Additionally, polyethylene can be slit or spun into synthetic fibres or modified to exhibit elastic properties similar to rubber.
As of 2017, over 100 million tonnes of polyethylene resins were being produced annually, accounting for 34% of the total plastics market. Polyethylene is so widely used because it can be manufactured in varying densities, giving it unique physical properties for different applications. It is also durable, chemically resilient, and non-toxic.
However, polyethylene's chemical resilience also makes it a long-lived pollutant when disposed of improperly. When exposed to ambient solar radiation, polyethylene produces trace amounts of the greenhouse gases methane and ethylene. The plastic type that releases gases at the highest rate is low-density polyethylene (LDPE), which is commonly used in packaging film, trash and grocery bags, agricultural mulch, wire and cable insulation, squeeze bottles, toys, and housewares. High-density polyethylene (HDPE), on the other hand, is used in more durable applications such as milk jugs, detergent bottles, shampoo bottles, and industrial containers due to its excellent chemical resistance and impact strength.
Despite their recyclability, the environmental impact of polyethylene plastics remains a concern.
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Frequently asked questions
'Poly' comes from the Greek prefix 'poly-', meaning 'many'. Plastics are polymers, which are made up of a long chain of uniform molecules called monomers.
Some examples include polyethylene (PE), polypropylene (PP), and polyester (PS).
Plastics are formed through the polymerisation of monomers, which are usually derived from petrochemicals.
Plastics have a wide range of applications, including packaging, construction, medicine, automobiles, and electronics.
No, not all polymers are plastics. Plastics are a specific type of synthetic polymer with a large molecular mass and a mostly linear structure.
