Maya Ellison
Plastic is everywhere, from single-use items like grocery bags to durable goods like car bumpers. But how is it made? Plastic is derived from natural, organic materials such as cellulose, coal, natural gas, salt, and crude oil. Crude oil and natural gas are extracted from the ground and transported to a refinery, where they are turned into multiple products, including ethane and propane, the foundation of plastics. The process of turning these raw fossil fuels into plastic involves refinement, cracking, and polymerisation or polycondensation. In the refinement stage, oil and gas are refined to obtain the specific chemicals needed to make plastic—hydrocarbons. Cracking 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, in a polymerisation reactor, monomers such as ethylene and propylene are linked together to form long polymer chains. These polymers are then melt-blended and transformed into finished or semi-finished products through extrusion or moulding processes.
| Characteristics | Values |
|---|---|
| Main ingredient | Polymers |
| Material | Synthetic or semisynthetic |
| Plasticity | Molded, extruded, or pressed into solid objects of various shapes |
| Derived from | Natural gas, petroleum, and a small fraction from renewable materials |
| Examples of renewable materials | Carbohydrates, fats, oils, and polylactic acid |
| Plastic production | 9.2 billion metric tons between 1950 and 2017 |
| Plastic production in 2023 | Over 400 million metric tons |
| Plastic pellets | Building blocks of plastic manufacturing |
| Plastic pellet issues | Easily spilled, mistaken for food by wildlife, and can absorb toxic chemicals |
| Plastic types | Polyethylene terephthalate (PETE or PET), high-density polyethylene (HDPE), polyvinyl chloride (PVC), low-density polyethylene (LDPE), polypropylene (PP), polystyrene or styrofoam (PS), and other miscellaneous plastics |
| Plastic use | Packaging, water bottles, durable medical equipment, toys, auto parts, and many other things |
| Plastic waste | 8-12 million tons of plastic enter oceans annually |
| Plastic pollution | 99% of plastic is produced from fossil fuels |
| Plastic recycling | Only ~9% of all plastics are recycled globally |
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What You'll Learn
Plastic pellets are the building blocks of plastic manufacturing
Plastic is a synthetic material derived from crude oil, natural gas, coal, or renewable products such as carbohydrates, fats, and oils. The production of plastic involves extracting and refining these raw materials, converting them into useful chemicals, and then polymerizing them. Polymerization is a process in which monomers like ethylene and propylene are linked together to form long polymer chains through a chemical reaction.
The versatility of plastic pellets allows them to be transformed into a wide range of products through techniques like injection molding, extrusion, or blow molding. They can be used to create single-use plastics like forks, takeaway containers, and bags, as well as durable items such as toothbrushes, toys, and car bumpers. The ability to mold and shape plastic into various forms has led to its widespread use in packaging, containers, automotive parts, and electronics.
While plastic pellets are essential in plastic manufacturing, their environmental impact cannot be overlooked. Spills and improper disposal contribute to the growing problem of plastic pollution, endangering ecosystems and public health. As a result, there is a growing emphasis on corporate transparency and accountability in preventing and remediating pellet spills, as well as exploring sustainable alternatives to traditional plastics.
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Crude oil and natural gas are extracted and transported to a refinery
The extraction and refining of crude oil and natural gas contribute significantly to CO2e emissions, with an estimated 108 million metric tons of CO2e attributed to plastic production annually outside the United States. This environmental cost is a concern, given the world's growing reliance on plastic.
Once separated, the gas and oil are transported via pipelines, either by land to a nearby refinery or to a tanker ship. If transported by ship, the gas is liquefied before being loaded and unloaded. Refineries are typically located near coastlines to facilitate the transport of these resources.
At the refinery, the gas and oil undergo a series of processes to transform them into the desired products. Distillation is a crucial step, where the crude oil is heated and separated into different fractions based on their boiling points. These fractions are then selectively reconfigured into new products, such as fuels for transportation, heating, or feedstocks for chemicals and plastics.
Further processing techniques, such as cracking, isomerisation, reforming, and alkylation, are applied to the fractions. Cracking, for instance, involves using heat, pressure, catalysts, and sometimes hydrogen to break down large hydrocarbon molecules into smaller ones. These smaller molecules, such as ethylene and propylene, are essential for plastic production.
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Refinement and cracking of hydrocarbons
The production of plastics begins with the distillation of crude oil in an oil refinery. This process involves heating the oil in a furnace and then sending it to a distillation unit, where it separates into lighter components called fractions. Each fraction is a mixture of hydrocarbon chains, which differ in the size and structure of their molecules. One of these fractions, naphtha, is crucial for plastic production.
Naphtha is a volatile mixture of liquid hydrocarbons obtained from the distillation of crude oil. It is composed of C5 to C10 hydrocarbons. To produce plastics, naphtha is decomposed thermally at high temperatures (around 800°C) in a steam cracker, resulting in light hydrocarbons called olefins and aromatics.
Steam cracking is a process that uses high temperatures and pressures to break down the long chains of hydrocarbons without a catalyst. In the case of naphtha, it is cracked into products such as propylene and heptane, which are then used to make polypropylene. Catalytic cracking, on the other hand, adds a catalyst to the process, allowing it to occur at lower temperatures and pressures.
The olefins and aromatics produced from cracking naphtha include ethylene, propylene, butane, butadiene, benzene, toluene, and xylene. These small molecules are then linked together to form long molecular chains called polymers. This process is known as polymerization, where monomers (such as ethylene and propylene) are chemically bonded into chains, resulting in higher molecular weight hydrocarbons (polymers).
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Polymerisation and polycondensation processes
Polymerisation and polycondensation are two processes used to produce plastics from natural resources such as cellulose, coal, natural gas, salt, and crude oil.
Polymerisation
Polymerisation is a process in the petroleum industry where light olefin gases (gasoline), such as ethylene, propylene, and butylene (monomers), are converted into higher molecular weight hydrocarbons (polymers). This happens when monomers are chemically bonded into chains. There are two different mechanisms for polymerisation:
- Addition polymerisation: This occurs when one monomer connects to the next one (dimer) and the dimer connects to the next one (trimer), and so on. This process is facilitated by the introduction of a catalyst, typically a peroxide. Common examples of addition polymers include polyethylene, polystyrene, and polyvinyl chloride.
- Condensation polymerisation: This process involves joining two or more different monomers by removing small molecules such as water.
Polycondensation
Polycondensation is a polymer formation process that involves linking small molecules (monomers) together, accompanied by the elimination of byproducts such as water and alcohols. Polycondensation of lactic acid, for example, connects carboxyl and hydroxyl groups, producing a water byproduct.
Polycondensation methods have been developed to address the challenge of removing byproducts from the highly viscous reaction mixture. These methods include azeotropic polycondensation (AP) and solid-state polymerisation (SSP). AP efficiently removes water by using appropriate azeotropic solvents, manipulating the equilibrium between the monomer and polymer. SSP, on the other hand, is a method where the polymerisation reaction is carried out in the solid state, avoiding the need to remove byproducts from a viscous mixture.
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Thermoplastics and thermosets
Plastic is derived from crude oil, natural gas, coal, or renewable products such as carbohydrates, fats, and oils. Over 99% of plastic is produced from fossil fuels. The process of turning these raw materials into plastic involves refinement and cracking. Large hydrocarbon molecules are broken down into smaller molecules, such as ethylene and propylene. Plastic pellets are the building blocks of plastic manufacturing and are used to create a wide range of products, from single-use plastics to durable items.
Thermosets, on the other hand, are materials that can be heated and molded only once. After curing, thermosets remain in a permanent solid state and do not melt or deform when exposed to high temperatures. Common examples of thermosets include epoxy, silicone, polyurethane, and phenolic. Thermosets have advantages over thermoplastics in terms of aesthetics, structure, cost, and labour. They are low-viscosity and easy to work with at room temperature, and they carry a lower health hazard since they do not release toxic fumes during the molding process.
The choice between thermoplastics and thermosets depends on the specific requirements of a manufacturing project. Thermoplastics offer the advantage of remolding and reshaping, while thermosets provide heat resistance and structural stability. Additionally, some materials like polyester can be found in both thermoplastic and thermoset versions.
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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.
The process of making plastic involves three main steps: refinement, cracking, and polymerisation or polycondensation. In the first step, oil and gas are refined to obtain the specific chemicals needed to make plastic—hydrocarbons. In the second step, large hydrocarbon molecules are broken down into smaller molecules in a process called cracking. In the third step, polymerisation or polycondensation, monomers such as ethylene and propylene are linked together to form long polymer chains.
Plastic pellets are the building blocks of plastic manufacturing. They are small, lightweight, and cheap to manufacture and transport.
There are a variety of plastic manufacturing processes, including compounding, extrusion, moulding, and 3D printing. Compounding involves melt-blending different blends of materials and then pelletising the mixture. Extrusion or moulding then transforms these pellets into finished or semi-finished products. 3D printing processes include fused deposition modelling (FDM), stereolithography (SLA), and selective laser sintering (SLS).
The two main types of plastic are thermoplastics and thermosetting plastics (also known as thermosets). Thermoplastics can go through multiple melt and solidification cycles without significant degradation, while thermosetting plastics remain in a permanent solid state after curing and decompose when heated.
