Sofia Martinez
Plastic is a synthetic material that has become integral to modern life, from food packaging to automotive parts. The chemistry of plastic involves the extraction and refining of fossil fuels, such as crude oil and natural gas, to create hydrocarbon monomers. These monomers are then linked through polymerisation to form long chains of polymers, which are moulded and hardened to create solid plastic products. The development of plastics has progressed from natural materials like rubber to synthetic polymers like polyethylene, with chemists like Leo Baekeland, who invented Bakelite, contributing to advancements in polymer chemistry.
| Characteristics | Values |
|---|---|
| Raw materials | Crude oil, natural gas, coal, carbohydrates, fats, oils, cellulose, salt |
| Chemical composition | Carbon, hydrogen, other elements |
| Chemical processes | Polymerisation, polycondensation, condensation polymerisation, compounding, vulcanisation |
| Polymers | Polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyethylene terephthalate, epoxy, nylon, polyimide, Bakelite |
| Properties | Low density, low electrical conductivity, transparency, toughness, plasticity |
| Uses | Packaging, building and construction, medical devices, light-weight construction materials, automobile interiors, CDs |
| Environmental concerns | Slow decomposition rate, plastic pollution |
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What You'll Learn
Plastic's chemical origins: crude oil, natural gas, coal, and bio-based sources
Plastic is derived from fossil fuels, which are composed of carbon, hydrogen, nitrogen, sulphur, oxygen, and other minerals. These fossil fuels are primarily crude oil, natural gas, and coal. Crude oil, a mixture of thousands of compounds, is accessed by drilling and pumped to the surface of the Earth. It is then heated in a furnace and sent to a distillation unit, where it separates into lighter components called fractions. One of these fractions, naphtha, is a crucial compound for plastic production. The monomers produced during this process are the building blocks of polymers, which are long chains of molecules that make up plastics.
Natural gas, another significant feedstock for plastic production, can be used directly or derived from crude oil refining. In the United States, natural gas is the primary source of feedstock for plastics, while crude oil is the major source of feedstock for the petrochemical industry. The flexibility of the petrochemical industry allows for the use of various feedstocks, including alkanes and refinery olefins such as propylene, ethylene, and butylenes.
Coal, the third fossil fuel in the group, mainly originates from dead plants. It is another source of hydrocarbons, which are crucial for plastic production.
In addition to these fossil fuel sources, there are also bio-based plastics derived from renewable products. These include carbohydrates, fats and oils, starch, bacteria, and other biological substances. An example of a plant-based polymer is polylactic acid (PLA), which is commonly used in 3D printing. PLA is obtained from corn starch through fermentation, producing lactic acid, which is then polymerised to create polylactic acid. The development of bio-based plastics is driven by the growing demand for limited oil reserves and the need for renewable resources.
The first fully synthetic plastic, Bakelite, was invented in 1907, and the development of plastics accelerated with the discovery of vulcanization to harden natural rubber in 1839. After World War I, advancements in chemistry led to an explosion of new forms of plastics, with mass production beginning in the 1940s and 1950s. Between 1950 and 2017, an estimated 9.2 billion metric tons of plastic were produced, with more than half of this amount made since 2004.
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Polymerisation: how monomers form polymers
Polymerisation is a process in which small molecules called monomers chemically react to form large chainlike or network molecules called polymers. The monomer molecules may be alike or different compounds. The formation of stable covalent chemical bonds between the monomers sets polymerisation apart from other processes.
The two main classes of polymerisation are step-growth and chain-growth. In step-growth polymerisation, pairs of reactants of any length combine at each step to form a longer polymer molecule. The average molar mass increases slowly, and long chains form only late in the reaction. Most step-growth polymers are also condensation polymers, which are formed with the loss of a small molecule such as water. Common examples of condensation polymers include polyester and nylon.
In chain-growth polymerisation, the only chain-extension reaction step is the addition of a monomer to a growing chain with an active centre such as a free radical, cation, or anion. Once the growth of a chain is initiated by the formation of an active centre, chain propagation is usually rapid by adding a sequence of monomers. Chain-growth polymerisation involves the linking together of unsaturated monomers, especially those containing carbon-carbon double bonds. The pi-bond is lost by the formation of a new sigma bond.
Addition polymerisation is a process involving many small, unsaturated monomers combining to form one large polymer molecule. The alkenes ethene and propene are two important feedstocks in the petrochemical industry that can also be used to make addition polymers. Polymers made in this way are called addition polymers. The monomers used to make other addition polymers are drawn in a similar shape to ethene, for example, propene.
The world's first fully synthetic plastic was Bakelite, invented in New York in 1907 by Leo Baekeland, who coined the term "plastics". Many other types of plastics are produced today, such as polyethylene, which is widely used in product packaging, and polyvinyl chloride (PVC), used in construction and pipes due to its strength and durability.
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Condensation polymerisation: the role of catalysts
Condensation polymerisation is a critical aspect of modern manufacturing processes. It involves joining two or more different monomers, which are the basic building blocks of polymers. This process requires a catalyst for the reaction to occur between adjacent monomers. The monomers join together to form polymers, with the removal of small molecules such as water. This is known as step growth, where you can add an existing chain to another chain.
The role of catalysts in condensation polymerisation is crucial. Catalysts can help speed up the reaction rate, making the process more efficient. They also enable control over the properties of the polymer, allowing for adjustments in strength, flexibility, or heat resistance, depending on the desired application. This control is achieved by manipulating the conditions under which the reaction takes place, including the type of monomers used, the reaction temperature, and other factors.
The process of condensation polymerisation results in the formation of two products: the polymer chain and water. Water is produced when the two monomers join together. This type of polymerisation is essential in forming various products, including textiles, plastics, and packaging materials.
One example of a condensation polymer is polyester, formed from the reaction between ethane diol and hexanedioic acid. Another common condensation polymer is nylon, which is also a type of polyamide. Polyamides are formed from the reaction of carboxylic acid and an amine. These polymers are versatile and find applications in numerous industries.
The chemistry of condensation polymerisation allows for the tuning of polymer properties. By using different elements, changing monomer types, and rearranging their patterns, it is possible to modify the shape, molecular weight, and other chemical and physical properties of the resulting polymer. This adaptability ensures that plastics can be designed with specific characteristics suited for particular applications.
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Vulcanisation: hardening natural rubber
Plastic is a polymeric material that can be moulded or shaped, usually by applying heat and pressure. The chemistry behind making plastic involves the extraction and refining of raw materials, polymerisation, compounding, and finishing.
Extraction and Refining of Raw Materials
The first step in making plastic involves the extraction of raw materials, primarily crude oil and natural gas, but also coal. These fossil fuels are complex mixtures of thousands of compounds that need to be processed. The refining process involves heating the crude oil in a furnace and then sending it to a distillation unit, where it separates into lighter components called fractions. One crucial fraction is naphtha, which is essential for plastic production.
Polymerisation
Polymerisation is the process of linking hydrocarbon monomers together through chemical reactions to produce polymers. For example, ethylene, a gaseous hydrocarbon, can be subjected to heat, pressure, and catalysts to form long, repeating carbon chains called polyethylene (PE). This process generates thick, viscous substances known as resins, which are used to make plastic products.
Compounding and Finishing
In compounding, various blends of materials are melt-blended and then pelletised. These pellets are then transformed through extrusion or moulding processes into finished or semi-finished plastic products. The processing of polymers may also include the addition of plasticisers, dyes, and flame-retardant chemicals.
Vulcanisation is a specific process within the broader chemistry of making plastic that involves hardening natural rubber. Natural rubber has been used for thousands of years, but Charles Goodyear's discovery of vulcanisation in 1839 revolutionised its applications. Vulcanisation typically involves treating natural rubber with sulfur, creating cross-links between polymer chains and resulting in a more durable and elastic material. This process enhances the physical and chemical properties of natural rubber, making it more resistant to abrasion, corrosion, and heat. Vulcanisation has enabled natural rubber to be used in a wide range of products, from tyres and footwear to hoses and gaskets.
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Plastic's chemical future: environmental concerns and sustainability
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, which 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 crucial for plastic production. Two main processes are used to produce plastics: polymerisation and polycondensation.
The chemistry of plastic production has allowed humans to create materials with a wide range of properties, from medical devices to construction materials. However, the environmental impact of plastic production and disposal is a growing concern. Plastic pollution affects all land, freshwater, and marine ecosystems, contributing to biodiversity loss, ecosystem degradation, and climate change. It is estimated that 20 million metric tons of plastic litter end up in the environment every year, and this amount is expected to increase significantly by 2040.
To address these environmental concerns, a global plastics treaty is needed to reduce plastic production, phase out harmful chemicals, and adopt strong national plans. The EU's Zero Pollution Action Plan, for example, aims to reduce plastic litter at sea and microplastics released into the environment. Additionally, circularity practices throughout the lifecycle of plastics can help reduce greenhouse gas emissions, pollution, and waste.
While plastic production has environmental implications, it is important to note that evidence for the effects of plastic chemicals on human health is still limited. Further research is needed to understand the long-term impacts of exposure to chemicals that leach out of plastics, especially the complex mixtures of chemicals that people are exposed to in everyday life.
In conclusion, plastics have brought many societal benefits, and their future potential in medicine and technology is promising. However, the current approaches to plastic production, use, and disposal are not sustainable. To secure a sustainable future for plastics, global collective action is required to reduce plastic production, eliminate harmful chemicals, and promote circularity practices.
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Frequently asked questions
Plastics are synthetic or man-made polymers derived from natural, organic materials such as cellulose, coal, natural gas, salt, and crude oil.
The process of making plastics involves the distillation of crude oil in an oil refinery to separate it into lighter components called fractions. One of these fractions, naphtha, is crucial for plastic production. Then, monomers such as ethylene and propylene are linked together through polymerisation to form long polymer chains.
Monomers are molecules that serve as the basic building blocks of polymers. They are formed when chemists combine various atoms using chemical bonds.
Polymers are complex organic compounds produced by polymerisation, where small molecules combine to form large, chain-like structures. They can be moulded or shaped into various forms, including films or textile fibres.
Common examples of plastics include polyethylene, polypropylene, polyvinyl chloride (PVC), and polystyrene. The world's first fully synthetic plastic was Bakelite, invented in 1907.
