Omar Khalid
Plastic is everywhere. It's in our televisions, computers, cars, and even our houses. But not all plastics are created equal. In 1988, the Society of the Plastics Industry introduced a system to classify plastic resins into seven categories, each with unique properties and uses. These categories include commonly known plastics like Polyethylene Terephthalate (PET), High-Density Polyethylene (HDPE), and Polyvinyl Chloride (PVC). However, there are also thousands of other types, like polycarbonate, acrylic, and nylon, which fall into the Unallocated References category. Understanding these different plastics is crucial for making informed choices as consumers and reducing our environmental impact. Some plastics are easier to recycle than others, and some may even leach dangerous toxins. With growing awareness, we can move towards a more sustainable relationship with plastic, reusing and recycling it effectively.
| Characteristics | Values |
|---|---|
| Classification by chemical structure | The chemical structure of the polymer's backbone and side chains. Important groups include the acrylics, polyesters, silicones, polyurethanes, and halogenated plastics. |
| Classification by chemical process | Condensation, polyaddition, and cross-linking. |
| Classification by physical properties | Hardness, density, tensile strength, thermal resistance, and glass transition temperature. |
| Classification by resistance and reactions | Exposure to organic solvents, oxidation, and ionizing radiation. |
| Classification by engineering behaviour | Thermoplastics, thermosets, conductive polymers, biodegradable plastics, engineering plastics, and elastomers. |
| Thermoplastics | Polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC). Can be molded repeatedly without chemical change when heated. |
| Thermosets | Epoxy resin, polyimide, and Bakelite. Can melt and take shape only once, decomposing when reheated instead of melting. |
| Acrylic | Transparent, shatter-resistant alternative to glass. Can be made coloured, fluorescent, abrasion-resistant, bullet-resistant, UV-tolerant, non-glare, and anti-static. |
| Polycarbonate | Tough, stable, and transparent. Two hundred and fifty times stronger than glass, and thirty times stronger than acrylic. Easily worked, moulded, and thermo-formed or cold-formed. |
| Polyethylene | Can be manufactured in varying densities, giving it unique physical properties. |
| LDPE | Used in shopping bags, plastic bags, clear food containers, and disposable packaging. |
| MDPE | Used in gas pipes, shrink film, carrier bags, screw closures, etc. |
| HDPE | More rigid than LDPE and MDPE. Used in plastic bottles, piping for water and sewer, snowboards, boats, and folding chairs. |
| UHMWPE | Abrasion-resistant due to the extreme length of its polymer chains. Used in military body armour, hydraulic seals, bearings, and artificial ice skating rinks. |
| PET | Excellent chemical resistance to organic materials and water. Practically shatterproof and possesses a high strength-to-weight ratio. Easily recyclable. Contains antimony trioxide, a carcinogen. |
| PVC | Can be manufactured to be rigid or flexible. Well-known for its ability to blend with other materials. |
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What You'll Learn
Polyethylene Terephthalate (PET)
Polyethylene terephthalate, commonly known as PET, is the most common thermoplastic polymer resin of the polyester family. It is a polymer produced through the polymerization of ethylene glycol and terephthalic acid. PET is commonly recycled and has the digit 1 (♳) as its resin identification code (RIC). It is easily recyclable and has excellent chemical resistance to organic materials and water. It is practically shatterproof and possesses an impressive high strength-to-weight ratio.
PET is widely used in the fabrication of carbonated beverage bottles due to its high strength, toughness, good abrasion and heat resistance, low creep at elevated temperatures, good chemical resistance, and excellent dimensional stability. It is also used to make artificial fibres for textiles, commonly found in clothing tags and written as polyester. Fibres made from PET have outstanding wear resistance, low moisture absorption, and are very durable. Textile applications include blankets, bed sheets, comforters, carpets, cushioning in pillows, upholstery padding, and upholstered furniture.
PET is also used in thermoforming for manufacturing and in combination with glass fibre for engineering resins. In 2016, the annual production of PET was 56 million tons, with the biggest application being in fibres (over 60%), and bottle production accounting for about 30% of global demand. PET is also used as a 3D printing filament, as well as in the 3D printing plastic PETG (polyethylene terephthalate glycol). PETG is a clear amorphous thermoplastic that can be injection-moulded, sheet-extruded, or extruded as a filament for 3D printing.
PET is a strong gas and moisture blocker and a great deterrent to liquor and solvents. It is gaining market share as a garment fibre due to its reuse and recycling, as well as the significant surplus of post-consumer waste in the form of bottles and cans. However, the short useful life, large production volume, and non-biodegradability of PET have raised concerns about its environmental impact.
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Polyvinyl Chloride (PVC)
PVC was first 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 additives. PVC is produced by the polymerization of the vinyl chloride monomer (VCM), which can be derived from salt and ethylene. This process can be done through suspension polymerization, emulsion polymerization, or bulk polymerization.
PVC has been widely used in plumbing and drainage, replacing cast iron in waste pipes, drainpipes, gutters, and downspouts. It is also used in blood bags, medical tubing, wire and cable insulation, windshield system components, and packaging. PVC can be modified by chlorination to increase its chlorine content, creating chlorinated polyvinyl chloride (CPVC). This process enhances its properties and makes it suitable for specific applications.
Despite its versatility and widespread use, PVC has come under scrutiny due to concerns about the leaching of chemicals, such as lead compounds, into drinking water. Studies have linked exposure to vinyl chloride monomer (VCM) with certain types of occupational cancers in workers in the polyvinyl chloride industry. As a result, regulatory authorities have set strict standards for PVC products, especially those used for drinking water and food-contact applications.
PVC plays a critical role in healthcare, with PVC blood-collection bags enhancing ambulatory medicine and forming the foundation for modern blood banks. It is also used in tamper-resistant over-the-counter medications and shrink wrap for consumer products. In construction, PVC is used in cladding, windows, roofing, fencing, decking, wall coverings, and flooring.
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Polypropylene (PP)
Polypropylene, commonly abbreviated as PP, is a versatile, low-cost, semi-rigid, and tough thermoplastic polymer. It was first polymerized in 1951 by Italian scientists Giulio Natta and Karl Ziegler, who independently discovered the catalysts enabling its production. Their work earned them the 1963 Nobel Prize in Chemistry.
PP is derived from the monomer propylene and can be manufactured through a process called chain-growth polymerization. This process involves propylene monomers undergoing polymerization to form the polypropylene polymer structure. It is the lightest polymer among commodity plastics and exhibits excellent chemical resistance. PP is available in homopolymer and copolymer forms, with the homopolymer being a general-purpose grade used in healthcare, automotive, packaging, electrical applications, and pipes. The copolymer is further divided into random copolymers and block or impact copolymers, produced by polymerizing propene and ethane.
Polypropylene is widely used across industries due to its characteristics, including its low density, durability, strength, flexibility, and heat resistance. It is used in automotive parts, packaging, consumer appliances, and medical devices. In the automotive industry, PP is used for battery casings, trays, bumpers, interior details, instrumental panels, and door trims. Its chemical resistance and ability to withstand sterilization make it valuable in the medical field. Additionally, PP's waterproof properties are advantageous in the marine sector.
PP can also be used as a fibre in products such as tote bags, ropes, twine, carpets, and upholstery. It is soft, malleable, and has a low melting point, making it suitable for injection moulding. Its adaptability to different fabrication techniques, flexibility, and lightweight nature have earned it the moniker of the 'steel' of the plastic industry.
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Polycarbonate (PC)
Polycarbonate is commonly used in safety equipment such as safety goggles, face shields, and helmets due to its excellent impact resistance and transparency. It is also used in optical lenses for eyeglasses, cameras, and automotive headlight lenses because of its optical clarity and lightweight nature. In the automotive industry, polycarbonate is well-suited for sputter deposition or evaporation deposition of aluminium without the need for a base coat. It is also used in decorative bezels, optical reflectors, and sunroofs.
Polycarbonate is widely used in 3D FDM printing, producing durable and strong plastic products with a high melting point. It is commonly used in the production of compact discs, DVDs, and Blu-ray discs. Polycarbonate is also used in advertising applications such as signs, displays, and poster protection.
Polycarbonate has some disadvantages, such as its low scratch resistance and susceptibility to ultraviolet degradation (yellowing). It is also not suitable for direct food contact due to concerns about the potential leaching of bisphenol A (BPA), a compound listed as a potential environmental hazardous chemical. However, BPA-free polycarbonate options are available.
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High-Density Polyethylene (HDPE)
Industrial production of HDPE from ethylene can be achieved through the Ziegler-Natta polymerization method or the Phillips slurry process. The Ziegler-Natta process involves using catalysts like titanium tetrachloride with gaseous ethylene to form HDPE. On the other hand, the Phillips slurry process utilizes silica-based catalysts in conjunction with a hydrocarbon and polyethylene slurry mixture to produce HDPE. The choice of synthesis method influences the microstructure and properties of the resulting HDPE. For instance, the Phillips Slurry process yields HDPE with less branching and more precise molecular weights, while the Ziegler process offers greater flexibility in the type of polyethylene produced.
Owing to its high strength-to-density ratio, HDPE finds applications in various industries. It is commonly used in the production of sturdy plastic bottles, corrosion-resistant piping, geomembranes, and plastic lumber. Specifically, HDPE is suitable for oil-resistant bottles, milk bottles, jugs, shampoo bottles, and bleach bottles. Additionally, HDPE is used in cell liners for sanitary landfills, providing a chemical-resistant barrier to prevent soil and groundwater pollution. It is also favoured in pyrotechnics for mortars due to its tendency to rip or tear instead of shattering into shrapnel during malfunctions. Furthermore, HDPE is used in cutting boards, agricultural pipes, playground equipment, and lids, showcasing its versatility.
HDPE is highly recyclable, contributing to its environmental benefits. It is accepted at most recycling centres worldwide and can help reduce plastic production by up to 50%. Its malleability, rigidity, strength, and corrosion resistance make it a sustainable and affordable alternative to heavier materials. HDPE's nonporous nature also enhances food safety by preventing bacterial growth, making it suitable for food applications. Overall, HDPE is a versatile, strong, and cost-efficient plastic with a wide range of applications.
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