Chloe Zhang
Plastic is a polymeric material that can be moulded or shaped, usually with the application of heat and pressure. The largest application of plastics is in packaging materials, but they are also used in construction, textiles, consumer goods, transportation, electronics, and machine parts. Plastics are derived from natural, organic materials such as cellulose, coal, natural gas, salt, and crude oil. Crude oil is the principal source of carbon for modern plastic, but an array of variants are manufactured from renewable materials. Most plastics contain organic polymers, formed from chains of carbon atoms, with or without oxygen, nitrogen, or sulfur atoms. These chains comprise many repeating units formed from monomers.
| Characteristics | Values |
|---|---|
| Composition | Organic polymers, mostly formed from chains of carbon atoms, with or without oxygen, nitrogen or sulfur atoms |
| Production | Synthetic plastics are derived from crude oil, natural gas or coal. Biobased plastics come from renewable products such as carbohydrates, fats and oils |
| Properties | Low density, low electrical conductivity, transparency, and toughness |
| Uses | Packaging, food containers, household products, textiles, construction, consumer goods, transportation, electronics, machine parts, optics, medical devices |
| Environmental Impact | Plastics in the ocean decompose due to exposure to environmental conditions, releasing toxic chemicals such as bisphenol A. However, due to the volume of plastics in the ocean, decomposition has slowed down |
| Recycling | The recycling rate of PET bottles and jars was 29.1% in 2018, and the rate for HDPE natural bottles was 29.3%. Overall, the recycling rate for plastics is relatively small at 8.7% in 2018 |
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What You'll Learn
Organic polymers
Plastics are polymeric materials that can be moulded or shaped, often by applying heat and pressure. 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 of a polymer is the part of the chain that links together a large number of repeat units. To customise the properties of a plastic, different molecular groups called side chains hang from this backbone. These side chains are usually 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.
Polymers are substances composed of macromolecules, which are molecules of high relative molecular mass. The structure of macromolecules essentially comprises multiple repetitions of units derived from molecules of low relative molecular mass. Polymers are larger molecules formed by covalently joining many monomer units together in the form of chains. The word 'polymer' comes from the Greek words 'poly', meaning 'many', and 'mer', meaning 'repeating unit'.
Plastics are classified by the chemical structure of the polymer's backbone and side chains. Important groups classified in this way include acrylics, polyesters, silicones, polyurethanes, and halogenated plastics. Synthetic plastics are derived from crude oil, natural gas, or coal, while biobased plastics come from renewable products such as carbohydrates, fats, and oils.
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Crude oil, natural gas, coal, and other natural materials
Plastic can be categorised into two types: synthetic and biobased. Synthetic plastics are derived from crude oil, natural gas, or coal. These are fossil fuels that are made up of carbon, hydrogen, nitrogen, sulphur, oxygen, and other minerals. The generally accepted theory is that hydrocarbons, which are the primary component of fossil fuels, are formed from the remains of living organisms called plankton. Over time, these tiny plants and animals were buried deeper beneath the heavy layers of sediments in the Earth's mantle, where they decomposed without oxygen and transformed into oil and gas. Crude oil and natural gas then penetrate the rocks, ultimately accumulating in reservoirs. Oil and natural gas wells are found at the bottom of oceans. Coal, on the other hand, mainly originated from dead plants.
Synthetic plastics are the most common type of plastic in use today due to the ease of manufacturing methods involved in the processing of crude oil. However, the growing demand for limited oil reserves is driving the need for newer plastics derived from renewable resources. In Europe, only about 4–6% of oil and gas reserves are used for plastic production, with the rest going towards transport, electricity, heating, and other applications.
The process of creating synthetic plastics begins with the extraction of crude oil and natural gas, which are then refined through processes such as distillation and polymerisation to yield useful chemicals, including monomers, which are the building blocks of polymers. Polymers are the primary component of plastics, and they are formed from chains of carbon atoms, with or without the attachment of oxygen, nitrogen, or sulfur atoms. These chains, known as backbone chains, consist of several thousand repeating units formed from monomers. To customise the properties of a plastic, different molecular groups called side chains are attached to the backbone. The structure of these side chains influences the characteristics of the resulting polymer.
Biobased plastics, on the other hand, are derived from renewable sources such as carbohydrates, starch, vegetable fats and oils, bacteria, and other biological substances. They are created through processes such as rotational molding, casting, film blowing, spinning, and 3D printing.
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Additives and chemicals
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. Plastics usually contain organic polymers, which are formed from chains of carbon atoms, with or without the attachment of oxygen, nitrogen, or sulfur 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 links together a large number of repeat units. To customize the properties of a plastic, different molecular groups called side chains hang from this backbone. They are usually attached to the monomers before the monomers themselves are linked together to form the polymer chain. The structure of these side chains influences the properties of the polymer. Plastics are usually classified by the chemical structure of the polymer's backbone and side chains.
Additives are chemicals blended into plastics to improve their performance or appearance. For example, additives can be used to make plastics more resistant to degradation by sunlight, or to make them more flexible. Some additives are also used to improve the processing of plastics, such as lubricants, which help the plastic flow better during processing. Other additives can be used to improve the properties of plastics, such as impact strength or heat resistance.
Some common additives in plastics include:
- Plasticizers: These are chemicals that are added to plastics to make them more flexible, soft, or elastic. They are commonly used in polyvinyl chloride (PVC) to make it more flexible.
- Stabilizers: These additives help to protect plastics from degradation caused by heat, light, or oxygen. They can also help prevent discoloration and loss of strength in plastics.
- Lubricants: These additives are used to reduce friction and improve the processing of plastics. They can also be used to improve the surface finish of plastic products.
- Impact modifiers: These additives are used to improve the impact strength of plastics, making them more resistant to cracking or breaking.
- Antioxidants: These chemicals are added to plastics to prevent oxidation and degradation caused by heat and light. They can also improve the long-term stability of plastics.
- Fillers: Fillers are additives that are used to reduce the cost of plastics or to improve their properties. They can be inorganic or organic materials, such as calcium carbonate, talc, or glass fibres.
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Plastic resins
The process of creating plastic resins involves first breaking down hydrocarbons with high heat in a process known as cracking. Then, the resulting compounds are formed into chains, known as polymers. Different chains and polymers allow resin companies to create plastics with different characteristics, such as varying densities, strengths, and levels of flexibility.
There are seven major types of plastic resins, each with unique physical characteristics, end uses, and recyclability:
- HDPE (High-Density Polyethylene): Made from petroleum, HDPE has a high melting point and strong intermolecular force and tensile strength. It is commonly used for sturdy containers and can support significant weight.
- LDPE (Low-Density Polyethylene): Derived from oil, LDPE has lower tensile strength and density than HDPE but is more resilient. It is flexible, tough, and almost unbreakable, used in products like food storage containers and bags.
- PP (Polypropylene): PP is used for food packaging and products like bottle caps and drinking straws. It is less tough than HDPE but more flexible than LDPE, with good resistance to fatigue and a high melting point.
- PVC (Polyvinyl Chloride): PVC contains a high percentage of chlorine, requiring less petroleum. It is chemically resistant and durable, commonly used for storing chemicals, oils, and shampoos, as well as products like pipes, electrical wire insulation, and bags.
- PET (Polyethylene Terephthalate): PET is lightweight, strong, and impact-resistant, used for beverage bottles, cooking oil bottles, and food packaging. It is naturally transparent and can withstand microwave and oven temperatures.
- PS (Polystyrene): PS is made from petroleum and can be transparent or coloured. It has limited flexibility and is used for products like bottle caps, drinking straws, yogurt cups, and food containers.
- Other: Any plastic resin type developed after 1988 falls into this category, including biodegradable, photo-sensitive, and plant-based plastics.
These plastic resins are used in a wide range of applications, from packaging and food containers to construction, textiles, consumer goods, and healthcare. The largest application is in packaging, with 36% of global production used for this purpose in 2015.
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Decomposition and environmental impact
Plastic pollution is a pressing issue, with plastic waste generated at a rate of approximately 400 Mt per year. This waste is persistent in the environment, with decomposition taking anywhere from 100 to 1,000 years or more, depending on various factors. The persistence of plastic waste in the environment has led to its presence in every ecosystem on the planet, from the Antarctic tundra to tropical coral reefs.
Plastics are derived from crude oil, natural gas, or coal, and they contain organic polymers formed from chains of carbon atoms, often with attached oxygen, nitrogen, or sulfur atoms. These chains comprise many repeating units formed from monomers, with each polymer chain consisting of several thousand repeating units. The properties of plastics are determined by the structure of these chains and the attached molecular groups, or side chains.
The environmental impact of plastics is significant, with plastic pollution posing a particular threat to the marine environment. Marine species are at higher risk of ingesting plastic, suffocating, or becoming entangled in plastic waste. More than 1,500 species in marine and terrestrial environments are known to ingest plastics, and microplastics have been found in human livers, kidneys, and placentas. Carcinogenic chemicals from plastic products can also leach into tap water, potentially causing developmental, reproductive, neurological, and immune disorders.
To address plastic pollution, microbial degradation has emerged as a significant area of research. Microbial biodegradation is considered one of the main ways to effectively deal with plastic pollution. This process involves microorganisms using carbon sources in organic matter to metabolize, producing non-toxic byproducts and providing energy for the microorganisms. Biodegradation can also reduce the harm of additives to plastics and improve the ecological environment. Studies have focused on enzymes isolated from bacteria, such as PETase and cutinases, to develop better enzymes for addressing plastic waste. Additionally, microbial species capable of degrading plastics, such as a strain of Flavobacterium that digests certain byproducts of nylon manufacture, have been discovered.
The degradation rates of plastics vary depending on the type of plastic and the environmental conditions. For example, high-density polyethylene (HDPE) bottles in the marine environment have an estimated half-life of 58 years, while HDPE pipes can take up to 1200 years to degrade. Media estimates for the degradation of plastic bags range from 10 to 20 years or 500 to 1,000 years, while plastic bottles are reported to take over 70 to 450 years. These discrepancies highlight the need for better experimental studies and standardized reporting of degradation rates.
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Frequently asked questions
Plastics are made from natural materials such as cellulose, coal, natural gas, salt, and crude oil. Crude oil is the principal source of carbon for modern plastic, but an array of variants are manufactured from renewable materials.
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, which differ in terms of the size and structure of their molecules. One of these fractions, naphtha, is crucial for the production of plastics. The next step is to convert the long-chain hydrocarbons into hydrocarbons that can be turned into chemicals for the preparation of plastic. Two main processes are used to produce plastics: polymerisation and polycondensation.
Plastic products include beverage bottles made of polyethylene terephthalate (PET), garden hoses made of polyvinyl chloride (PVC), food containers made of foamed polystyrene, and shatterproof windows made of polymethyl methacrylate. Plastic is also used in construction, consumer goods, transportation, electronics, and textiles.
Plastics enable us to do more with less and contribute to sustainability. They help protect the environment by reducing waste, lowering greenhouse gas emissions, and saving energy. Plastic packaging extends the shelf life of fresh foods and beverages, reduces food and packaging waste, and allows for more efficient shipping. Plastic insulation and sealants make homes more energy-efficient and reduce heating and cooling costs. Lightweight plastics in cars can increase fuel efficiency.
Plastics can be harmful to the environment if they are not properly recycled or disposed of. Plastics in the ocean can decompose due to exposure to the sun, rain, and other environmental conditions, releasing toxic chemicals such as bisphenol A. However, the increased volume of plastics in the ocean has slowed down decomposition. Some plastic products, such as disposable diapers and fishing lines, can take hundreds of years to degrade.
