Omar Khalid
Plastics are a range of synthetic or semisynthetic materials composed primarily of polymers. They can be classified in several ways, including by chemical structure, polarity, and application. The most common classification system groups plastics by number, with seven types in total. These numbers can be found on plastic products and indicate the type of plastic used. Some types of plastic are easily recyclable, while others are hazardous to human health and the environment.
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What You'll Learn
Chemical structure and polarity
Plastics are classified into three types based on their chemical structure, polarity, and applications. The chemical structure and polarity of plastics are closely related to their temperature behaviour, and they can be broadly categorized into thermoplastics, thermosets, and elastomers.
Thermoplastics are plastics that can be easily deformed and bent upon heating. They include PVC, nylon, polythene, and polyethylene, which is the most common plastic on Earth. Polyethylene, also known as polyethene, can be manufactured in varying densities, giving it unique physical properties for different applications. Thermoplastics do not undergo chemical changes when heated and can be molded repeatedly. They are used in food and drink packaging, grocery bags, milk jugs, and agricultural pipes due to their strong ability to prevent oxygen from entering and spoiling the product inside.
Thermosets, on the other hand, cannot be softened again by heating once they are moulded. Examples of thermosets include Bakelite, used for electrical switches, and melamine, used for floor tiles.
Elastomers are plastics with good mechanical resistance to high temperatures, particularly up to 150°C. Examples of elastomers include polyimide (PI), polysulfone (PSU), and polyethersulfone (PES).
The chemical structure of plastics is primarily composed of polymers, which are long chains of carbon atoms with attached oxygen, nitrogen, or sulfur atoms. These chains are formed from monomers, which act as building blocks. The structure of the polymer backbone and side chains determines the properties of the plastic, such as hardness, density, tensile strength, and thermal resistance.
The polarity of a plastic refers to the presence of polar groups or functional groups within its molecular structure. These polar groups can form hydrogen bonds with other molecules, affecting the solubility, melting point, and other physical properties of the plastic. The polarity of plastics also influences their classification as either polar or non-polar, which is important in determining their interactions with other substances and their suitability for specific applications.
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Physical properties
Plastic is a synthetic or semi-synthetic material that usually comes from petrochemicals or other sources like cellulose and starch. Plastics are polymers, which are long chains of molecules that can be moulded into shape. The versatility of plastics is one of the reasons for their widespread use. They are available in a wide range of colours and textures, from smooth and shiny to rough and matte, and can even be made flexible or transparent.
Plastics can be classified by their physical properties, including hardness, density, tensile strength, thermal resistance, and glass transition temperature. One important classification of plastics is the degree to which the chemical processes used to make them are reversible. Thermoplastics, for example, can be moulded repeatedly as they do not undergo chemical changes when heated. Thermoplastics tend to be less stiff than thermosetting plastics, and are therefore softer and more malleable. Examples of thermoplastics include polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC). On the other hand, thermosetting polymers can melt and take shape only once. If reheated, they decompose rather than melt. Examples of thermosetting polymers include epoxy resin, polyimide, and Bakelite.
Another way to classify plastics is by the chemical structure of the polymer's backbone and side chains. Important groups classified in this way include the acrylics, polyesters, silicones, polyurethanes, and halogenated plastics. Furthermore, plastics can be classified by the chemical process used in their synthesis, such as condensation, polyaddition, and cross-linking.
Plastics are also categorised into commodity plastics and engineering plastics. Commodity plastics are chosen for their low cost and ease of manufacturing, whereas engineering plastics can compete with die-cast metals in plumbing, hardware, and automotive applications. Engineering plastics are more robust and are used to manufacture products such as vehicle parts, building and construction materials, and some machine parts. High-performance plastics are a category of polymers that exhibit superior properties to commodity and engineering plastics. These plastics can withstand high temperatures, are highly resistant to chemical corrosion and degradation, have excellent mechanical and electric properties, and are lightweight and extremely versatile.
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Thermoplastics
Some common thermoplastics include polypropylene, polyethylene, polyvinyl chloride (PVC), polystyrene, polyethylene terephthalate (PET), and polycarbonate (PC). Polypropylene, or PP, is a flexible thermoplastic known for its durability, heat resistance, and acid resistance. It is used in various applications, including laboratory equipment, automotive parts, medical devices, and food containers. Polyethylene, or PE, can be manufactured in varying densities, giving it unique physical properties. It is the most common plastic on Earth and is used in a wide range of products.
Polyvinyl chloride (PVC) is considered one of the most hazardous plastics due to the potential leaching of toxic chemicals such as bisphenol A (BPA), phthalates, lead, and mercury. However, it is widely used in construction materials, doors, windows, bottles, and packaging. Polycarbonate (PC) thermoplastics are strong and impact-resistant, with inherent design flexibility. They are easily worked, moulded, and thermoformed for applications such as electronic components, automotive parts, and security glazing.
Other notable thermoplastics include polyoxymethylene (POM), also known as acetal or polyacetal, which is used in precision parts requiring high stiffness, low friction, and dimensional stability. Polyether ether ketone (PEEK) is a colourless organic thermoplastic with good abrasion resistance, low flammability, and low smoke and toxic gas emission properties. Polylactic acid (PLA), derived from renewable resources such as corn starch or sugarcane, is a compostable thermoplastic commonly used in 3D printing.
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Thermosets
Plastic can be classified into three main types: thermoplastics, thermosets, and elastomers. Thermosets are a type of polymer that can only be melted down once. Unlike thermoplastics, once a thermoset has been melted, cooled, and set into a defined shape, it will burn if heated again. When high temperatures are applied to this material, the molecules become permanently bonded.
Thermosetting plastic has both electrical and thermal insulating properties, making it a very lightweight and technically advantageous material. While it cannot be remelted or remoulded, it can undergo machining processes.
In terms of toxicity, PVC (polyvinyl chloride) is considered the most hazardous plastic due to its potential to leach toxic chemicals such as bisphenol A (BPA), phthalates, lead, dioxins, mercury, and cadmium. Several of these chemicals are linked to cancer, allergic reactions in children, and hormonal disruptions in humans.
Polyethylene, on the other hand, is the most common plastic on Earth and can be manufactured in varying densities, giving it unique physical properties for a wide range of applications. Polyethylene terephthalate (PET) is a specific type of polyethylene with excellent chemical resistance to organic materials and water, and it is easily recyclable. It is used in clothing fibres, food and liquid containers, engineering resins, and carbon nanotubes, among other products.
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Elastomers
Natural rubber is one of the most important elastomers in the plastic industry. It is derived from a renewable source and has excellent elasticity, producing very little internal heat. Synthetic rubbers are also classified as elastomers, including nitrile, silicone, polyurethane, Viton, and neoprene.
In terms of their behaviour at high temperatures, elastomers are distinct from thermoplastics and thermosets. Thermoplastics can be melted and remoulded multiple times, while thermosets can only be melted down once as their molecules are permanently bonded. Elastomers, on the other hand, rarely melt and tend to return to their initial state after deformation.
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Frequently asked questions
There are several ways to classify plastics. One way is according to their chemical structure, polarity, and applications. Plastics can be classified as thermoplastics, thermosets, and elastomers. Another way is 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, and thermal resistance.
Thermoplastics include PVC, nylon, polythene, and polyethylene terephthalate (PET). PET is the fourth-most produced synthetic plastic and is used for food and liquid containers, engineering resins, and carbon nanotubes.
Thermosets include Bakelite, which is used for electrical switches, and melamine, which is used for floor tiles.
According to their applications, plastics can be classified as standard plastics or commodities, and engineering plastics. Examples of standard plastics include polyethylene (PE) and polypropylene (PP). Engineering plastics include polyamide (PA) and polycarbonate (PC).
