Chloe Zhang
Plastic is a polymer, a large molecule made of smaller molecules called monomers. These monomers are derived from raw materials found in nature, such as natural gas, oil, and plants. The process of manufacturing plastic from these raw materials involves three main steps: extraction and refinement of the raw materials, “cracking” of the refined materials, and polymerization. Cracking is an energy-intensive process where steam cracker furnaces reach extremely high temperatures to break down large hydrocarbon molecules into smaller molecules like ethylene and propylene. These monomers are then combined through polymerization to form plastic. Despite its convenience, plastic has a significant environmental impact, from the extraction of fossil fuels to the pollution caused by plastic waste.
| Characteristics | Values |
|---|---|
| Raw Materials | Natural gas, oil, plants, coal, animal-waste products, waste biomass, carbohydrates, fats & oils |
| Extraction | Fossil fuel companies use large-scale mining and drilling operations to extract raw materials |
| Refinement | Raw materials are refined into ethane and propane, which are the foundation of plastics |
| Cracking | Ethane and propane are treated with high heat to be converted into monomers such as ethylene and propylene |
| Polymerisation | Monomers combine to form polymers, which are large molecules made of smaller ones |
| Plastic Pellets | Also known as nurdles, these are the building blocks of plastic manufacturing, shipped to facilities to be melted and formed into products |
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What You'll Learn
Extraction of raw materials
The extraction of raw materials is the first step in the process of manufacturing plastic. The raw materials used to make plastic are derived from fossil fuels, such as crude oil, natural gas, and coal. In the United States, gas is the primary source of plastic production, while outside the US, oil is the main feedstock.
Crude oil is a complex mixture of thousands of compounds and needs to be processed before it can be used to make plastic. The first step in this process is distillation, which separates the heavy crude oil into groups of lighter components called fractions. One of these fractions, naphtha, is crucial for plastic production. It is a volatile mixture of liquid hydrocarbons obtained by the distillation of crude oil.
Natural gas, another raw material, is also obtained through oil refining operations. It is a major source of carbon emissions, contributing to the environmental cost of plastic production.
Other raw materials used in plastic manufacturing include coal, plants, carbohydrates, fats, and oils. These renewable resources are increasingly being used to create biobased plastics due to the growing demand for limited oil reserves.
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Refining process
The refining process is a critical stage in the transformation of raw materials into plastic products. This stage involves multiple steps to convert crude oil and natural gas into useful chemicals and monomers, which are the building blocks of polymers.
Firstly, the raw materials, including crude oil and natural gas, undergo distillation in an oil refinery. This process separates the heavy crude oil into lighter components called fractions, which are mixtures of hydrocarbon chains. One crucial fraction is naphtha, a mixture of hydrocarbons obtained from the distillation of crude oil.
Subsequently, the fractions are further processed to break down their hydrocarbon chains. High temperatures and pressures are applied, sometimes in conjunction with catalysts, to break these chains into smaller molecules. For instance, ethane and propane, derived from the fractions, can be cracked to produce ethylene and propylene. These monomers, with their double bonds, serve as the foundational units for polymer formation.
The monomers then undergo polymerisation, facilitated by catalysts, to form long polymer chains. These polymers are essentially plastic resins, such as polyethylene (PE), which exhibit high mouldability when heated. The resins are processed into plastic pellets or "nurdles", which are then shipped to manufacturing facilities.
At the manufacturing facilities, plastic compounding occurs, where the pellets are compounded, mixed, and melted with other ingredients according to specific recipes. This process determines the characteristics and properties of the final plastic product. The melted plastic is then formed into various shapes and sizes using injection moulding or extrusion machines, depending on the desired application.
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Cracking
There are two types of cracking: steam cracking and catalytic cracking. Steam cracking involves high temperatures and pressures to break the long chains of hydrocarbons without a catalyst. Temperatures can reach between 1380°F and 2,010°F, or even as high as ~800°C in some cases. Catalytic cracking, on the other hand, uses a catalyst, allowing the process to occur at lower temperatures and pressures.
During the cracking process, the molecular bonds of large hydrocarbons are broken, resulting in smaller molecules like ethylene and propylene. Ethylene, for example, is created when ethane "cracks" under high temperatures of around 1,500 degrees Fahrenheit. This process causes the bonds in the ethane molecule to stretch, weaken, and eventually break, forming ethylene.
Ethylene molecules then form long chains to create polyethylene, also known as PE. PE is a versatile plastic resin that can be modified to meet the requirements of different types of plastic applications, whether rigid or pliable. PE-based plastics are further processed in factories to create plastic pellets, which are the building blocks of plastic manufacturing. These plastic pellets are then melted and formed into specific shapes to create a wide range of plastic products.
The cracking process is a critical step in the production of plastics, especially when using natural gas as a feedstock. Natural gas, composed primarily of methane, contains ethane molecules that can be separated and "cracked" to produce the essential ingredients for plastic manufacturing. This process is facilitated by ethane cracker plants, which harness the potential of natural gas liquids (NGLs).
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Condensation polymerisation
Dimers can further join to form tetramers (four units), and this process continues to create larger structural units, which are typically polymers. These polymers are formed from polyfunctional monomers, which are usually prepared by the reaction between two types of bifunctional symmetric monomers. The two monomers, denoted as XaaX and YbbY, can yield the -aabb- repeating units without issues on sequence control. Here, "aa" and "bb" represent symmetric monomer substrates, while "X" and "Y" represent the leaving groups.
An example of condensation polymerisation is the reaction between a dibasic acid and a glycol, resulting in the formation of a polyester with water as the byproduct. Another important class of condensation polymers is polyesters, which are formed from the reaction of a carboxylic acid and an alcohol. An example is polyethyleneterephthalate, commonly known as PET plastic.
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Melting and forming
Plastic pellets, also known as nurdles, are the building blocks of plastic manufacturing. These pellets are created from the resins produced during the polymerisation process. The process involves melting and cooling operations, resulting in lentil-sized pellets.
The plastic pellets are then shipped from petrochemical refining facilities to manufacturing facilities. Here, they are melted down and formed into specific products. Manufacturers combine, mix, and melt the plastic pellets with other ingredients according to specific recipes. These recipes determine the characteristics and properties of the final plastic product.
The melted plastic is then formed into shape by plastic forming machinery, with the technique used depending on the application of the product. Common techniques include injection moulding, extrusion, and blow moulding.
Plastic pellets are small and lightweight, making them easy to transport and an integral part of the global plastic supply chain. However, their small size also means they can be easily spilled or dumped during transportation, contributing to plastic pollution in the environment.
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Frequently asked questions
The raw materials used to make plastic include natural gas, oil, and plants. Crude oil and natural gas are extracted from the ground and transported to a refinery.
During the refining process, natural materials are turned into products like ethane and propane, which are the foundation of plastics.
The next step is called "cracking," where ethane and propane are treated with heat to turn them into ethylene and propylene.
Ethylene and propylene are monomers or small molecules that serve as the building blocks of polymers or plastics.
Ethylene and propylene are combined to create different polymers or plastics. These plastics are then melted and formed into specific shapes.
