Chloe Zhang
Plastic is a synthetic or semisynthetic material composed primarily of polymers. The defining characteristic of plastic is its plasticity, which allows it to be moulded, extruded, or pressed into a diverse range of solid forms. Plastics are usually classified by the chemical structure of the polymer's backbone and side chains. The two major categories of plastic are thermoplastics and thermosets. Thermoplastics can be moulded and recycled, whereas thermosets cannot be remelted or recycled once they solidify. Polyethylene, or polyethene, is the most common plastic in the world and is used in a wide variety of products. Polyvinyl chloride (PVC) is the third-most widely produced synthetic plastic polymer and is used in construction and pipes due to its strength and durability. Polypropylene is another widely used plastic that is known for its flexibility, durability, and heat resistance. Polycarbonate is a strong and impact-resistant plastic used in products like DVDs and sunglasses. Acrylic is a transparent thermoplastic used as a lightweight alternative to glass. Other important groups of plastics include polyesters, silicones, polyurethanes, and halogenated plastics.
| Characteristics | Values |
|---|---|
| Composition | Synthetic or semisynthetic materials composed primarily of polymers |
| Defining characteristic | Plasticity, which allows them to be moulded, extruded, or pressed into a diverse range of solid forms |
| Other properties | Low weight, durability, flexibility, chemical resistance, low toxicity, and low-cost production |
| Production | Made from natural gas and petroleum, or renewable resources like polylactic acid |
| Production (1950-2017) | 9.2 billion metric tons |
| Production (2023) | Over 400 million metric tons |
| Types | Polyethylene, Polyvinyl chloride (PVC), Acrylic, Polycarbonate, Polypropylene, Polyethylene Terephthalate (PET), Polyurethane, Polyester, Silicone, Halogenated plastics, Thermoplastics, Thermoset plastics, Thermoplastic elastomers, Heterchain polymers, etc. |
| Uses | Packaging, construction, plumbing, electrical cable insulation, clothing, medical tubing, laboratory equipment, automotive parts, food containers, beverage bottles, garden hoses, insulating food containers, shatterproof windows, compact discs, DVDs, sunglasses, police riot gear, optical devices, etc. |
| Environmental impact | Serious environmental damage due to slow decomposition rate (up to 500 years) |
| Recycling | Less than 10% of plastic is recycled; some types like PVC and PC are hard to recycle |
| Health impact | Hazardous materials may be produced under extreme conditions like extreme heat; PC has been linked to health problems in humans and animals |
| Positive traits | Durability, resistance to biodegradability, flexibility, strength, impact resistance, shatter resistance, transparency, etc. |
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What You'll Learn
Thermoplastics and Thermoset plastics
Plastic is a broad term for a wide range of synthetic or semi-synthetic materials composed primarily of polymers. The plasticity of plastics allows them to be moulded, extruded, or pressed into a diverse range of solid forms.
Two of the basic forms of plastic are thermoplastics and thermosetting plastics (thermosets). Thermoplastics are the most common plastic polymers for the injection moulding process. They can be heated, cooled, and reshaped repeatedly without altering their chemical structure. Thermoplastics have a lower melting point compared to thermosets. They do not form any chemical bond when curing, making them remouldable and recyclable.
Thermosets, on the other hand, undergo a chemical change when heated, forming irreversible bonds that set their shape permanently. This process is irreversible, meaning once thermosets have been set, they cannot be melted or reshaped. Thermosets are typically hard, strong, and have excellent resistance to heat and chemicals. They are produced with resin transfer moulding (RTM) or reaction injection moulding (RIM). During this process, cross-linking occurs between the polymers, forming an irreversible and unbreakable bond.
Thermoplastics and thermosets have their unique advantages and applications. Thermoplastics, for example, are used in optical devices and products, with acrylic being a common thermoplastic. Acrylic is a transparent, lightweight, and shatter-resistant alternative to glass. In contrast, thermosets like silicon are widely used in the electrical industry due to their excellent stability, flexibility, tear strength, and chemical resistance, making them ideal for electrical wire insulation.
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Polyvinyl chloride (PVC)
PVC can be modified by adding plasticizers such as phthalates, which make it softer and more flexible. This form of PVC is used in plumbing products, electrical cable insulation, clothing, medical tubing, and other similar products. Medical and food-contact products must meet U.S. Food and Drug Administration (FDA) regulations. PVC is also used in the production of clear vinyl, which is used for tamper-resistant over-the-counter medications and shrink wrap for consumer products.
PVC is known for its strong resistance to chemicals, sunlight, and oxidation from water. It is also lightweight and durable, making it suitable for a wide range of applications. PVC can be usefully modified by chlorination, which increases its chlorine content to or above 67%. Chlorinated polyvinyl chloride (CPVC) is produced by chlorination of an aqueous solution of suspension PVC particles, followed by exposure to UV light.
PVC was synthesized in 1872 by German chemist Eugen Baumann. However, it was not until 1926 that Waldo Semon and the B.F. Goodrich Company developed a method to plasticize PVC by blending it with various additives. Despite its many uses and advantages, PVC has come under scrutiny due to concerns about the health and environmental impacts of certain additives, such as lead compounds and phthalates.
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Polyethylene terephthalate (PET)
Polyethylene terephthalate, commonly known as PET, is a thermoplastic polymer resin of the polyester family. It is one of the most commonly used plastics, with an annual production of 56 million tons in 2016.
PET is produced through the polymerization of ethylene glycol and terephthalic acid. It is known for its durability, mechanical strength, and transparency. Due to these properties, PET is widely used in packaging, textile production, and carbonated beverage bottles. It is also used in 3D printing and the fabrication of thin-layer products. PET is easily recyclable and has a resin identification code (RIC) of 1 (♳).
One of the unique characteristics of PET is its ability to exist in both amorphous (transparent) and semi-crystalline forms, depending on its processing and thermal history. When subjected to rapid cooling, it behaves as a transparent, amorphous thermoplastic, while slow cooling results in a semi-crystalline structure. This flexibility allows PET to be used in a variety of applications, such as injection molding, blown molding, and extrusion.
The excellent mechanical, thermal, and chemical resistance of PET, along with its dimensional stability, makes it a versatile material. It is a strong barrier against gases and liquids, including solvents and liquor. PET's high strength-to-weight ratio and shatterproof nature further enhance its applicability in various industries.
PET has added immense value to our daily lives, but there are also some drawbacks. The short useful life, large production volume, and non-biodegradability of PET have raised environmental concerns. Researchers are conducting life cycle assessments to investigate and address the environmental impact of PET packaging.
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Polypropylene (PP)
Polypropylene, also known as polypropene, is a thermoplastic polymer that is widely used across various industries. It is produced through the chain-growth polymerization of the monomer propylene and belongs to the group of polyolefins, with partial crystallinity and non-polarity. While polypropylene shares some similarities with polyethylene, it exhibits greater hardness and heat resistance. Its mechanical ruggedness, white colour, and high chemical resistance further distinguish it from other plastics.
Polypropylene has a range of desirable properties that make it a popular choice for many applications. It is durable, flexible, heat resistant, and acid resistant. Additionally, it has excellent electrical and chemical resistance at higher temperatures and does not exhibit stress-cracking. Its low density, compared to other commodity plastics, allows for the production of lighter parts with a reduced amount of plastic. Polypropylene's flexibility enables it to adapt to a wide range of fabrication techniques, making it a versatile material in manufacturing.
Polypropylene's versatility extends to its processing methods as well. It can be processed by almost all thermoplastic-processing techniques, including extrusion blow moulding, injection moulding, and general-purpose extrusion. The choice of processing method depends on the specific application and desired properties. For instance, glass-reinforced polypropylene increases strength, stiffness, and HDT but reduces impact resistance.
Polypropylene has found widespread use in the automotive, medical, and marine industries. In the automotive industry, it is used for battery casings, trays, bumpers, interior details, instrumental panels, and door trims. The medical field utilises polypropylene for its waterproof properties, flexibility, resistance to mould, bacteria, and chemical corrosion, as well as its ability to withstand steam sterilisation methods. Its waterproof nature also makes it valuable in the marine sector.
Polypropylene is also commonly used in plastic packaging, plastic parts for machinery and equipment, fibres, and textiles. It is a key component in promotional tote bags, shopping carriers, ropes, twine, tape, carpets, and upholstery. The flexibility and low melting point of polypropylene make it well-suited for the injection moulding process, where it can be used to create incredibly thin layers of plastic.
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Polyurethanes (PURs)
Polyurethanes are typically produced by reacting a polymeric isocyanate with a polyol. They contain two types of monomers, which polymerize one after the other, and are thus classed as alternating copolymers. Polyurethanes mostly consist of three building blocks: the soft segments, which impart flexibility; the hard segments, which are usually the combination of diisocyanates; and the chain extender, which also acts as a cross-linker. By varying the ratio of these building blocks, PURs of required properties such as chemical and mechanical compatibility, as well as hardness and softness, can be obtained.
Polyurethanes are used in a wide range of applications, including building insulation, refrigerators and freezers, furniture and bedding, automotive parts, coatings, adhesives, and sealants. They are also used in high-load-bearing applications such as bone scaffolds. The largest application of PURs is in packaging, consuming 146 million metric tons (36% of global production) in 2015.
In 2007, the global consumption of polyurethane raw materials was above 12 million metric tons, with an average annual growth rate of about 5%. Global production in 2019 was 25 million metric tons, accounting for about 6% of all polymers produced that year.
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Frequently asked questions
The two major categories of plastic are Thermoplastics and Thermoset. Thermoplastics are the most common type of plastic, making up 80% of plastics worldwide. They can be heated and cooled multiple times, making them recyclable and transformable. Thermoset plastics, on the other hand, cannot be remelted as they form irreversible bonds when heated.
Polyethylene Terephthalate (PET) is a commonly known Thermoplastic. It is used in beverage bottles and is known for its excellent chemical resistance and high strength-to-weight ratio. Polyvinyl Chloride (PVC) is another example. It is the third-most produced synthetic plastic polymer and comes in rigid and flexible forms.
Polyurethanes (PURs) are a type of Thermoset plastic. They are designated as commodity plastics due to their low cost, ease of manufacturing, and versatility. Polycarbonate (PC) is another example, known for its strength and transparency. However, due to its toxicity, PC has been banned in several countries for specific uses.
Plastics are synthetic or semi-synthetic materials composed primarily of polymers. They are characterized by their plasticity, which allows them to be molded, extruded, or pressed into various solid forms. This adaptability, combined with their low weight, durability, flexibility, chemical resistance, low toxicity, and low-cost production, has led to their widespread use.
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