Ethan Clarke
Plastics are synthetic or biobased materials that have become a global necessity, with annual production exceeding 300 million tons. They are composed of organic polymers, which are large molecules formed by covalently joining many monomer-repeating units together in the form of chains. The backbone of a polymer chain is the part of the chain that links together a large number of repeat units. These chains are typically made up of carbon atoms, with or without the attachment of oxygen, nitrogen, or sulfur atoms. The structure of these chains, including the backbone and side chains, determines the properties of the polymer and, by extension, the plastic.
| Characteristics | Values |
|---|---|
| Definition | Polymer is a substance composed of macromolecules. |
| Chemical composition | Polymers having only aliphatic (linear) carbon atoms in their backbone chains or heterochain polymers containing atoms such as oxygen, nitrogen, or sulfur in their backbone chains, in addition to carbon. |
| Plasticity | The property of a material that can deform irreversibly without breaking. |
| Types | Synthetic plastics are derived from crude oil, natural gas, or coal. Biobased plastics come from renewable products such as carbohydrates, starch, vegetable fats and oils, bacteria, and other biological substances. |
| Uses | Construction, medical devices, packaging, automobiles, furniture, and toys. |
| Production | The annual production of plastic exceeds 300 million tons, with 400 million metric tons produced in 2023 alone. |
| Environmental impact | Plastic pollution and slow decomposition rate in natural ecosystems are major environmental concerns. Bioremediation and the use of fungi and bacteria are potential solutions for plastic degradation. |
| Classification | Plastics can be classified by their chemical composition, physical properties (hardness, density, tensile strength, thermal resistance, glass transition temperature), and resistance/reactions to various substances and processes. |
| Synthesis | Condensation polymerization, polyaddition, cross-linking, and other chemical processes. |
| Structure | Polymers are formed by covalently joining many monomer units together in chains. The backbone is the part of the chain that links together a large number of repeat units. |
| Degradation | Changes in properties such as tensile strength, color, shape, or molecular weight due to environmental factors like heat, light, chemicals, oxygen, and enzymes. |
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What You'll Learn
Plastic backbone composition
Plastic is a polymeric material that can be moulded or shaped. Most plastics contain organic polymers, which are formed from chains of carbon atoms, with or without oxygen, nitrogen, or sulphur atoms. These chains comprise many repeating units formed from monomers. Each polymer chain consists of several thousand repeating units. The backbone is the part of the chain that is on the main path, linking together a large number of repeat units.
The backbone of a polymer is made up of carbon-carbon bonds, with or without the attachment of oxygen, nitrogen, or sulfur atoms. These atoms are known as side chains or branches and are attached to the monomers before they are linked together to form the polymer chain. The structure of these side chains influences the properties of the polymer. For example, polyethylene has alkyl groups as its side chains.
Plastics can be classified into two categories based on the chemical composition of their backbones:
- Aliphatic (linear) carbon atom polymers: These polymers have only carbon atoms in their backbone chains. Examples include polypropylene, polyethylene, polyvinyl chloride, and polystyrene.
- Heterchain polymers: These polymers contain atoms such as oxygen, nitrogen, or sulfur in their backbone chains, in addition to carbon. Examples include polycarbonate and polyurethane.
The susceptibility of a polymer to degradation depends on its structure, including the backbone. Polymer degradation occurs when the bonds in the backbone break due to environmental factors such as heat, light, or the presence of certain chemicals. This process can be intentional, as in biodegradation and recycling, or unintentional, as in the case of environmental degradation.
The versatility of plastics in terms of their physical and chemical properties, as well as their ease of manufacturing, has led to their widespread use in various industries, including packaging, construction, automobiles, and medical devices. However, the accumulation of synthetic polymers in the environment has become a pressing global challenge, with a growing demand for the development of biodegradable plastics and alternative renewable resources.
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Plastic synthesis
Plastic is 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 to create plastic. The production of plastics begins with the distillation of crude oil in an oil refinery, which separates the heavy crude oil into lighter components called fractions. Each fraction is a mixture of hydrocarbon chains, which differ in terms of the size and structure of their molecules.
The process of turning raw fossil fuels into plastic involves three main steps: refinement, cracking, and polymerization. First, oil and gas are refined to obtain the specific chemicals needed to make plastic—hydrocarbons. Then, large hydrocarbon molecules are broken down into smaller molecules in a process called cracking. This is an energy-intensive process where steam cracker furnaces reach extremely high temperatures to break the molecular bonds, resulting in smaller molecules like ethylene and propylene.
Finally, these smaller molecules are reassembled into long chains, often with the aid of a catalyst, in a process called polymerization to produce plastics. Polymerization is the process of combining many small molecules known as monomers into a covalently bonded chain or network. During this process, some chemical groups may be lost from each monomer. The backbone of a plastic is the part of the chain that is on the main path, linking together a large number of repeat units. Different molecular groups called side chains hang from this backbone, influencing the properties of the polymer.
During processing, various additives may be mixed with the polymer to enhance properties such as strength, flexibility, colour, and resistance to UV light or fire. Additives can include plasticizers, stabilizers, fillers, pigments, and flame retardants. The end result of polymerization typically creates a “resin,” which can take many different forms. These resins then undergo high temperatures, pressure, and cooling, often resulting in long, solid strands that are then cut into plastic pellets, or “nurdles.”
Plastics can be classified by the chemical process used in their synthesis, such as condensation, polyaddition, and cross-linking. They can also be classified by their physical properties, including hardness, density, tensile strength, thermal resistance, and glass transition temperature. Plastics are also classified by the chemical structure of their backbone and side chains, with important groups including acrylics, polyesters, silicones, polyurethanes, and halogenated plastics.
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Plastic properties
Plastic is a synthetic material composed of organic polymers. Polymers are substances or materials consisting of very large molecules or macromolecules, which are formed from chains of carbon atoms, with or without the attachment of oxygen, nitrogen, or sulfur atoms. These chains are made up of thousands of repeating units formed from monomers. The backbone of a polymer is the part of the chain that links together a large number of these repeat units.
The properties of plastics are numerous and vary depending on their applications. The backbone structure of a polymer influences its properties. For example, plastics with carbon-chain backbones are called aliphatic polymers, while those with oxygen, nitrogen, or sulfur atoms in their backbones, in addition to carbon, are called heterochain polymers. Heterchain polymers are generally stronger and used in engineering plastics like polycarbonate.
Plastics can be classified by their physical properties, such as hardness, density, tensile strength, thermal resistance, and glass transition temperature. 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. For instance, plastics vary in their susceptibility to degradation, which is the change in properties like tensile strength, colour, shape, or molecular weight due to environmental factors like heat, light, and the presence of certain chemicals.
Plastics also exhibit plasticity, which is the ability to be moulded or shaped, usually by applying heat and pressure. This property, combined with their low weight, durability, flexibility, chemical resistance, low toxicity, and low-cost production, has led to their widespread use. For example, plastics are used in automobiles (up to 20%), beverage bottles, construction materials, and packaging.
There are two primary forms of plastics: thermoplastics and thermosets. Thermoplastics can be moulded, melted, and remoulded repeatedly without any chemical bonding or change in their physical properties. On the other hand, thermosets are stronger than thermoplastics due to the irreversible chemical bonds formed during the curing process.
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Plastic uses
The backbone of plastics is formed by chains of carbon atoms, which may or may not have oxygen, nitrogen, or sulfur atoms attached. These chains are made up of several thousand repeating units formed from monomers. The versatility of plastics stems from the ability to mold, laminate, or shape them, and to tailor them physically and chemically.
Plastics are used across almost every sector, including packaging, construction, textiles, transportation, electronics, and industrial machinery. They are used in products ranging from bicycle helmets, child safety seats, and automotive airbags to cell phones and packaging for food and beverages. They are also used in medical devices, sewage systems, and light-weight construction materials.
Plastics are essential in transportation, used in seats, panels, and structural elements of vehicles. They are also combined with other materials as structural elements in transport such as skateboards, rollerblades, and bicycles. In the developing world, the applications of plastic may differ; for example, 42% of India's consumption is used in packaging.
Plastics are also used in clothing, furniture, and toys. They are used in automobile interiors, such as dashboards and seating, due to their light weight and durability. Plastic insulation, sealants, and other building products are also making homes more energy efficient while reducing heating and cooling costs.
While plastics have major benefits, they also pose environmental challenges due to their slow decomposition rate in natural ecosystems. Most plastics have not been reused or recycled, and they can create disposal problems as they do not easily corrode and can persist in landfills for hundreds of years. However, recycling and sustainable materials are helping to address these issues.
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Plastic production
The backbone of plastics is formed by chains of carbon atoms, with or without oxygen, nitrogen, or sulfur atoms attached. These chains are composed of thousands of repeating units formed from monomers. The polymerization process combines these monomers into a covalently bonded chain or network, resulting in a polymer.
Plastics are derived from natural, organic materials such as cellulose, coal, natural gas, salt, and crude oil. The production of plastics involves several steps, starting with the distillation of crude oil in an oil refinery. This process separates crude oil into lighter components called fractions, which are mixtures of hydrocarbon chains. One crucial fraction for plastic production is naphtha. The next steps involve either polymerization or polycondensation, both of which require specific catalysts. During polymerization, monomers like ethylene and propylene are linked to form long polymer chains.
Various additives can be mixed with the polymer during processing to enhance strength, flexibility, color, and resistance to UV light or fire. These additives can include plasticizers, stabilizers, fillers, pigments, and flame retardants. The polymerization process typically results in a "resin," which can take on various forms. These resins are then subjected to high temperatures, pressure, and cooling, often yielding long, solid strands that are cut into plastic pellets or "nurdles."
The plastic pellets are then transported to manufacturing facilities, where they are melted and formed into specific products. Manufacturers follow precise recipes to compound, mix, and melt the plastic pellets with other ingredients, determining the characteristics and properties of the final product. The melted plastic is then shaped by plastic-forming machinery according to its intended application.
The versatility of plastics, combined with their low weight, durability, flexibility, chemical resistance, low toxicity, and low-cost production, has led to their widespread use in numerous industries, including packaging, construction, automobiles, and medical devices. However, the environmental impact of plastic production, disposal, and climate change is a significant concern.
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Frequently asked questions
The backbone of a plastic is the part of the chain that is on the main path, linking together a large number of repeat units.
Repeat units, also known as monomer residues, are the distinct pieces of each monomer that are incorporated into the polymer.
Polymers are substances composed of macromolecules. They are made up of very large molecules, or macromolecules, that are constituted by many repeating subunits derived from one or more species of monomers.
