Ethan Clarke
Plastic is a material that has become integral to our everyday lives, from bicycle helmets to food containers. Its defining characteristic, plasticity, allows it to be moulded, extruded, or pressed into a wide range of solid forms. Plastics are derived from fossil fuels such as crude oil, natural gas, and coal, which are processed and refined to yield monomers. These monomers, such as ethylene and propylene, are then linked through polymerisation to form polymers, which are the basis of plastics. The chemical structure of these polymers can vary, leading to different types of plastics with distinct properties. While plastics offer numerous benefits, their production and use have raised concerns about environmental and health hazards due to the presence of certain chemical additives.
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Plasticity and Polymers
Plasticity, or plastic deformation, is the ability of a solid material to undergo irreversible changes in shape in response to applied forces. In the context of polymers, plasticity refers to the ability of these long-chain molecules to deform without breaking. This property is essential for the moulding process, where polymers are subjected to temperature and pressure to create plastic products.
Polymers are large molecules formed by linking together many smaller molecules called monomers. The monomers used in plastic production are typically derived from crude oil, natural gas, and coal. During the refining process, these raw materials are converted into useful chemicals, including monomers such as ethylene, propylene, and butene. These monomers contain double bonds, allowing carbon atoms to react and form polymers through a process called polymerisation.
The polymerisation process involves joining monomers together to form long, repeating chains of carbon atoms. These polymer chains can be customised by attaching molecular groups called side chains, which influence the properties of the resulting plastic. The structure of the polymer backbone and side chains are used to classify important groups of plastics, including acrylics, polyesters, and polyurethanes.
The plasticity of polymers is influenced by various factors, including temperature and the application of external forces. For example, when a polymer is ripped or torn, energy is put into the material, causing the polymer chains to attempt to align and making continued deformation easier. While this process involves some bond-breaking, the plastic retains its chemical characteristics.
The behaviour of polymers under stress can be modelled using plasticity models, such as isotropic hardening plasticity and kinematic hardening plasticity. These models can predict the response of polymers to forces and help engineers understand the yielding point beyond which deformation becomes irreversible. However, the accuracy of these models varies, and they may be more challenging to calibrate for certain materials, such as polymers.
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Chemical Reactions
Plastic is a polymer, a chemical compound formed by long chains of carbon atoms, with or without the attachment of oxygen, nitrogen, or sulfur atoms. The defining characteristic of plastic is its plasticity, or ability to deform irreversibly without breaking. This plasticity is influenced by the structure of the polymer's side chains, which are molecular groups attached to the monomers (the basic building blocks of polymers) before they are linked together.
The chemical process of creating plastic involves converting raw material molecules into monomers such as ethylene, propylene, and butene. These monomers have double bonds, allowing carbon atoms to react and form polymers. During polymerisation, hydrocarbon monomers are linked together by a chemical mechanism to produce polymers. This process generates thick, viscous substances known as resins, which are used to make plastic products. For example, ethylene monomers, when subjected to heat, pressure, and a catalyst, form long, repeating carbon chains that become a plastic resin called polyethylene.
The chemical changes in plastic can be observed when it is ripped or torn. While plastic retains its chemical characteristics after being ripped, the process of tearing it involves breaking bonds and forming new ones. This is because, when a polymer is mechanically deformed, energy is put into the material, causing the polymer chains to attempt to align and making continued deformation easier. To continue tearing the polymer, the van der Waals forces holding the chains together must be overcome.
Plastics can be classified by the chemical processes used in their synthesis, such as condensation, polyaddition, and cross-linking. They can also be classified by their resistance and reactions to various substances and processes, such as exposure to organic solvents, oxidation, and ionizing radiation. The degree to which the chemical processes used to make plastics are reversible is another important classification factor. For instance, thermoplastics do not undergo chemical change when heated and can be moulded repeatedly.
The chemical composition of plastic can also impact its environmental impact. Plastic fragmentation into microplastics and nanoplastics allows chemical additives to spread in the environment, potentially persisting and bioaccumulating in organisms. According to a review by the US EPA, out of the thousands of chemicals associated with plastic packaging, 68 were ranked as "highest for human health hazards" and 68 as "highest for environmental hazards".
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Raw Materials
The raw materials used to produce plastics are mostly derived from the natural world, including cellulose, coal, natural gas, salt, and crude oil. However, with a global focus on a circular economy, plastics are increasingly being produced from waste, renewable materials, and CO2.
Sources of Raw Materials
Crude oil, a complex mixture of thousands of compounds, is a significant source of raw materials for the plastic industry. The distillation of crude oil in refineries separates it into lighter components called fractions, with naphtha being crucial for plastic production. This process involves heating the oil in a furnace and then sending it to a distillation unit.
Natural gas, derived from oil refining operations, is another essential raw material for the petrochemical industry. Coal, while less commonly used today, remains valuable for producing materials that convert methanol into plastic.
Conversion Processes
The Advanced Methanol-to-Olefins (MTO) technology uses coal and natural gas to create ethylene and propylene, which are essential monomers in plastic production. Cellulose, derived from softwood trees, is another raw material used to produce cellulosic plastics or bioplastics.
Monomers and Polymers
The raw materials are converted into monomers like ethylene, propylene, and butene, which have double bonds allowing carbon atoms to react and form polymers. Polymerisation involves linking these monomers together through chemical mechanisms to create polymers, which are thick, viscous substances known as resins.
For example, ethylene, a gaseous hydrocarbon, when subjected to heat, pressure, and a catalyst, forms long, repeating carbon chains, resulting in a plastic resin called polyethylene. This PE-based plastic is processed into pellets, melted, and cast into moulds to create various plastic products.
Cost Factors
Obtaining plastic raw materials can be expensive, and their prices are influenced by various factors. Supply and demand dynamics, transportation costs, labour costs, and supply chain disruptions can all impact the pricing of these materials.
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Additives
The use of additives in plastics can have both positive and negative impacts on the environment. On the one hand, additives can help prolong the life of plastic products, reducing the need for frequent replacement. This can lead to a decrease in the consumption of raw materials and the associated environmental costs of production. Additives can also enhance the durability and flexibility of plastics, making them more suitable for certain applications and potentially reducing waste.
However, the release of certain additives into the environment during the use, disposal, and recycling of plastic products has raised concerns. Some additives have been found to persist in the environment, bioaccumulate in organisms, and have adverse effects on human health and ecosystems. For example, a review by the United States Environmental Protection Agency (US EPA) identified 68 chemicals associated with plastic packaging that were ranked as "highest for human health hazards" and 68 chemicals ranked as "highest for environmental hazards".
The sound recycling of plastics is crucial for managing the environmental impact of additives. However, the current practice of simply remelting and fabricating used plastic into new items may not adequately address the complications caused by additives. More research and regulation are needed to ensure the safe use and disposal of plastics containing additives.
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Manufacturing
Plastic is a polymer that can be crafted into a variety of shapes and made to imitate natural substances. The first synthetic polymer was invented in 1869 by John Wesley Hyatt, who treated cellulose derived from cotton fibre with camphor. This discovery allowed humans to create materials that were not constrained by the limits of nature. The first fully synthetic plastic, Bakelite, was invented in 1907 by Leo Baekeland. It was durable, heat-resistant, and could be shaped or moulded into almost anything.
Today, most plastic is derived from crude oil and natural gas, although there is a growing demand for plastics made from renewable resources such as waste biomass or animal waste products. The process of refining crude oil involves separating it into different fractions, one of which is naphtha, the crucial compound for plastic production. The naphtha fraction contains hydrocarbons, which are chains of carbon and hydrogen atoms. These hydrocarbons can be further processed to yield useful chemicals, including monomers. Monomers are small molecules that serve as the building blocks of polymers.
During the manufacturing process, monomers are chemically bonded to form polymer chains through a process called polymerisation. This can occur through addition polymerisation, where one monomer connects to the next, or condensation polymerisation, where two or more different monomers are joined by removing small molecules such as water. The type of monomers used and the way they are arranged determine the properties of the resulting plastic, including its shape, molecular weight, and chemical and physical characteristics.
Plastics can be classified by the chemical processes used in their synthesis, such as condensation, polyaddition, and cross-linking. They can also be categorised by their physical properties, such as hardness, density, tensile strength, and thermal resistance. The chemical composition of plastics influences their performance, with elements like carbon, hydrogen, oxygen, or chlorine bonding to form unique structures. For example, oxygen atoms can determine whether a compound becomes ethyl methacrylate or ethyl acrylate. Additives are another critical aspect of plastic manufacturing, as they enhance the performance and appearance of the final product. However, some additives, such as bisphenol A (BPA) and phthalates, have raised concerns due to their potential impact on human health.
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Frequently asked questions
Plastic is made from polymers, which are long chains of carbon atoms, usually derived from crude oil and natural gas.
Plastic can undergo chemical changes when it is ripped or torn, as the polymer chains are pulled apart and some bond breaking occurs. Additionally, additives in plastic can degrade and form new compounds, which can have adverse effects on human health and the environment.
Plastics can be classified by the chemical processes used in their synthesis, such as condensation, polyaddition, and cross-linking. They can also be classified by their chemical structure, such as the backbone and side chains of the polymer.
Plastic is used in a wide range of products, including bicycle helmets, child safety seats, automotive airbags, cell phones, food packaging, and medical devices. Plastic is valued for its low weight, durability, flexibility, and low cost.
