Chloe Zhang
Plastics are an incredibly useful and versatile material, with applications across a wide range of industries. However, not all plastics are created equal, and their environmental impact varies greatly. Plastics can be broadly classified into two groups: thermoplastics and thermosets. Thermoplastics, such as polyethylene and polypropylene, are known for their flexibility and ability to be remelted and remoulded. On the other hand, thermosets retain their shape once moulded and are difficult to recycle. Beyond these two groups, there are numerous other types of plastics, including engineering plastics and commodity plastics, each with unique properties and applications.
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What You'll Learn
Thermoplastics and thermosets
There are two major groups of plastics: thermoplastics and thermosets. Both have distinct characteristics and applications.
Thermoplastics
Thermoplastics are a type of plastic polymer that can be heated, cooled, and reshaped repeatedly without altering their chemical structure. They are solid at room temperature but become soft upon heating and eventually turn fluid. This is due to crystal melting or the crossing of the glass transition temperature. Importantly, thermoplastics do not form any chemical bonds during the curing process, allowing them to be remoulded and recycled. They are traditionally popular for their high-quality finish and are used in a wide range of applications, such as LEGO bricks, which are made from acrylonitrile butadiene styrene (ABS).
Thermosets
Thermosets, on the other hand, are a type of plastic polymer that undergoes a chemical reaction when heated, resulting in the formation of irreversible chemical bonds and a three-dimensional network of bonded molecules. This process is what sets their shape permanently, and once formed, thermosets cannot be easily melted or reshaped. Thermosets are typically hard and strong, with excellent resistance to heat and chemicals. They are often used to manufacture products with a long life, such as electrical wire insulation, due to their ability to retain their strength and geometry when exposed to elevated temperatures.
Similarities and Differences
While thermoplastics and thermosets are distinct types of polymers, they share some similarities. For example, polyester can exist as both a thermoplastic and a thermoset. Additionally, both types of plastics are used in injection moulding processes, with the choice between the two depending on the specific product requirements. The critical difference between them lies in their behaviour during and after the curing process, particularly in their response to heat. Thermoplastics have a lower melting point compared to thermosets, making them more suitable for recycling. However, thermosets have greater physical properties, such as higher structural integrity and resistance to impact and deformation.
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Polyethylene terephthalate (PET)
There are two major groups of plastics: thermoplastics and thermosets. Polyethylene terephthalate (PET) is the most common thermoplastic polymer resin of the polyester family. It is a clear, durable, and versatile plastic that is widely used in packaging, textiles, and 3D printing applications.
PET is produced through the polymerization of ethylene glycol and terephthalic acid. It is commonly recycled and has a resin identification code (RIC) of 1. The recycling process for PET is more straightforward than for thermoset plastics because it can be melted and reshaped, whereas thermosets retain their shape once moulded.
In packaging, PET is used for food and drink containers, particularly carbonated beverage bottles, due to its high strength, toughness, and excellent dimensional stability. It is also a strong gas and moisture barrier, making it suitable for protecting products from spoilage. PET is the most widely recycled plastic in the world, and using recycled PET reduces environmental impact by lowering energy consumption and greenhouse gas emissions.
In textiles, PET is used to create artificial fibres with outstanding wear resistance, low moisture absorption, and durability. These fibres are commonly referred to as polyester and are used in clothing, blankets, bed sheets, carpets, and upholstery. PET is gaining market share in the garment industry due to its reuse, recycling, and the availability of post-consumer waste.
PET is also utilized in 3D printing, where it is known as PETG or PET-G (polyethylene terephthalate glycol-modified). This modified version of PET can be injection-moulded, sheet-extruded, or extruded as a filament for 3D printing. PETG is a clear amorphous thermoplastic that can be coloured during processing and is used in various industrial applications, including automotive and aeronautical sectors.
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Polyvinyl chloride (PVC)
History
German chemist Eugen Baumann synthesized PVC in 1872. In the early 20th century, Russian chemist Ivan Ostromislensky and Fritz Klatte of the German chemical company Griesheim-Elektron attempted to use PVC in commercial products, but difficulties in processing the rigid, sometimes brittle polymer thwarted their efforts. Waldo Semon and the B.F. Goodrich Company developed a method in 1926 to plasticize PVC by blending it with various additives, including the use of dibutyl phthalate by 1933.
Uses
PVC is widely used in the building and construction industry to produce door and window profiles, pipes (both drinking and wastewater), and siding or weatherboarding. It has largely replaced the use of cast iron for plumbing and drainage, being used for waste pipes, drainpipes, gutters, and downspouts. PVC is also used for wire and cable insulation, as well as in the healthcare, electronics, and automobile sectors. In healthcare, PVC is used for blood bags, tubing, and medical devices.
Properties
PVC is a durable plastic with strong resistance to chemicals, sunlight, oxidation from water, impact, weather, and corrosive environments. It is also lightweight and easy to process. However, PVC is difficult to recycle and should be avoided whenever possible.
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Polycarbonate (PC)
Plastics are divided into two major groups: thermoplastics and thermosets. Polycarbonate (PC) is a type of thermoplastic polymer. It is a high-performance, tough, amorphous, and transparent material. PC is widely used as an engineering plastic due to its unique characteristics.
Polycarbonate is known for its high impact strength and resistance to fracture. This property makes it ideal for safety equipment such as goggles, face shields, and helmets. Its optical clarity and lightweight nature make it suitable for optical lenses, camera lenses, and eyewear lenses. The automotive industry also utilizes polycarbonate for headlight lenses, interior components, and sunroofs.
Polycarbonate has a wide range of applications due to its durability and UV resistance. It is commonly used for outdoor signs, display screens, advertising materials, and compact discs. Its transparency and impact resistance make it a popular alternative to glass. PC is also used in blends, such as PC/ABS, PC/PET, and PC/PMMA.
In the medical field, polycarbonate grades that comply with ISO 10993-1 and USP Class VI standards are utilized. These grades can be sterilized using steam, gamma radiation, or ethylene oxide (EtO). Aliphatic polycarbonates have improved biocompatibility and degradability, making them suitable for nanomedicine applications.
Polycarbonate is produced through the reaction of bisphenol A (BPA) and phosgene COCl2. However, concerns have been raised about the potential leaching of BPA from polycarbonate products, leading to the development of BPA-free alternatives.
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Engineering plastics
There are two major groups of plastics: thermoplastics and thermosets. Engineering plastics are a type of thermoplastic polymer that offers better mechanical and thermal properties than commodity plastics. They are more expensive than standard plastics and are produced in lower quantities, making them ideal for smaller objects or low-volume applications.
One of the key engineering plastics is acrylonitrile butadiene styrene (ABS), which is known for its good strength, stiffness, and toughness. ABS is used in a wide range of consumer applications, including housings for electronics, appliances, and power tools. It is also used in car bumpers and dashboard trim. Another important engineering plastic is nylon, which is widely used in high-performance applications across industries, including automotive, power tools, and office furniture.
Polycarbonate is another type of engineering plastic with a unique combination of properties. It is highly resistant to impact, making it ideal for applications where impact resistance is a critical factor, such as the cockpit canopy of fighter jets. Engineering plastics have gradually replaced traditional materials such as metal, glass, and ceramics in many applications due to their superior strength, weight, and manufacturing ease.
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Frequently asked questions
The two major groups of plastics are thermoplastics and thermosets.
Thermoplastics are the most common type of plastic. They can be easily melted and reshaped, so they are often used to manufacture products with a long life. Examples include polyethylene, polypropylene, PET, PVC, and acrylic.
Thermosets retain their shape once they have been moulded. They cannot be easily melted and reshaped, so the recycling process for these plastics is more involved. Thermosets are often used in products with long life cycles.
Polycarbonates (PC) are a well-known example of thermosets. They are used to build strong and tough products like lenses for sunglasses, eyeglasses, riot shields, and compact discs.
Plastics are classified by the chemical structure of their polymer backbone and side chains. They can also be classified by their physical properties, such as hardness, density, and tensile strength, or by their resistance to various substances and processes.
