Alexei Volkov
Plastic is a synthetic or biobased substance that is manufactured using a variety of chemicals and additives. Synthetic plastics are derived from crude oil, natural gas, or coal, while biobased plastics are made from renewable sources such as carbohydrates, starch, vegetable fats, and oils. The process of creating plastics involves polymerization, where hydrocarbon monomers are linked together to form polymers, which are then used to create plastic products. Additives are also commonly mixed with the polymer resin to enhance its properties, such as toughness, flexibility, and color. Nitric acid is one of the essential substances used in the production of plastics, among other chemicals like PTFE (Teflon), polyethylene, polypropylene, and PVC.
| Characteristics | Values |
|---|---|
| Substances important in making plastics | Nitric acid, hydrocarbon monomers, polyethylene, polystyrene, polypropylene, polyvinyl chloride, and polytetrafluoroethylene (Teflon) |
| Plastic types | Synthetic, biobased, bioplastics, thermoplastics, thermosets, conductive polymers, biodegradable plastics, engineering plastics, elastomers |
| Additives | Stabilizers, plasticizers, dyes, cadmium, chromium, lead, mercury, octabromodiphenyl ether, pentabromodiphenyl ether, alkylphenols |
| Plastic properties | Hardness, density, tensile strength, thermal resistance, glass transition temperature, colour, flexibility, elasticity, toughness, chemical resistance |
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What You'll Learn
Nitric acid is essential for making plastics
Nitric acid, also known as aqua fortis or spirit of niter, is a corrosive liquid inorganic compound with the formula HNO3. It is a crucial substance in the process of plastic production.
Plastics are derived from synthetic or biobased sources. Synthetic plastics are primarily obtained from crude oil, natural gas, or coal, while biobased plastics are produced from renewable resources, such as carbohydrates, starch, vegetable fats, oils, bacteria, and other biological substances. Nitric acid is essential in the synthesis of synthetic plastics, which constitute the majority of plastic products in use today.
During the production of plastics, nitric acid serves as an oxidizing agent and plays a vital role in creating organic compounds. It undergoes decomposition, breaking down into oxides of nitrogen and water. This process is integral to the chemical reactions necessary for the formation of plastic polymers.
The versatility of nitric acid extends beyond its role in plastic production. It is also used in the creation of dyes, fertilizers, and explosives. In the manufacturing of dyes, nitric acid contributes to the development of vibrant and durable colors. In agriculture, it is instrumental in producing ammonium nitrate fertilizers, enhancing crop growth and yield. Additionally, nitric acid is a key component in the production of explosives like nitroglycerin and TNT.
While nitric acid is essential for making plastics, it is important to recognize that plastics are not sold as pure substances. They are often mixed with various additives, such as stabilizers, plasticizers, and dyes, to improve their lifespan, workability, or appearance. These additives can be classified by their physical properties, such as hardness, density, tensile strength, and thermal resistance.
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Synthetic vs biobased plastics
Plastics are classified based on the chemical processes used in their synthesis, their physical properties, and their resistance and reactions to various substances and processes. They can be categorised as synthetic or biobased.
Synthetic plastics are derived from crude oil, natural gas, or coal. They are the most common type of plastic in use today due to the ease of manufacturing methods involved in processing crude oil. However, the growing demand for limited oil reserves is driving the need for newer plastics from renewable resources. The production of synthetic plastics often involves harmful chemicals, and their waste can be burned, releasing hazardous substances such as acid gases and ash.
Biobased plastics, on the other hand, are derived from renewable products such as carbohydrates, starch, vegetable fats and oils, bacteria, and other biological substances. They are marketed as a sustainable alternative to conventional plastics. Bioplastics like PLA (polylactic acid) are particularly valuable for foodservice packaging where compostability is a key requirement for end-of-life disposal. However, little is known about the chemicals they contain and the safety of these compounds. Studies have shown that bioplastics and synthetic plastics have similar toxicity profiles.
The development of bioplastics was initially hindered by cheap oil prices, which drove the continued production of synthetic plastics. However, the environmental movement of the 1970s spurred interest in bioplastics, and the first bioplastics company was founded in 1983. Today, bioplastics represent approximately 2% of global plastic output, and their output is steadily increasing alongside that of fossil plastics.
While bioplastics offer a more sustainable solution, they are not inherently superior in terms of environmental friendliness. The environmental benefits of bioplastics depend on their entire life cycle, from resource to end-of-life, and their ability to fit into a circular economy model. Additionally, the production of bioplastics raises the question of whether we should be using food sources to create materials.
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Polymerisation process
Polymerisation, also referred to as polymer synthesis, is a chemical reaction that involves converting low-molecular-weight monomers into high-molecular-weight polymers. Monomers are the building blocks of polymers, and they chemically combine to form larger molecules or macromolecules. Hundreds of such macromolecules collectively constitute a polymer.
The polymerisation process can be performed in two ways: addition polymerisation and condensation polymerisation. In addition polymerisation, monomers are added together in a long chain, with one monomer connecting to the next in the presence of a catalyst, forming chain growth polymers. Common examples of addition polymers include polyethylene, polystyrene, and polyvinyl chloride. On the other hand, condensation polymerisation involves joining two or more different monomers by removing small molecules like water. This process is known as step growth, where an existing chain is added to another chain. Examples of condensation polymers include polyester and nylon.
The specific polymerisation process depends on the chemical composition and structure of the starting monomer. For instance, in the case of ethylene, a gaseous hydrocarbon, polymerisation occurs when it is subjected to heat, pressure, and a catalyst, resulting in long, repeating carbon chains. These joined molecules form a plastic resin known as polyethylene (PE). The production of PE-based plastic involves processing the resin in a factory to create plastic pellets, which are then melted and moulded into the desired product.
The polymer industry employs various catalysts, and the selection of a particular catalyst influences the properties of the resulting polymer. Catalysts can be solid particles or suspended in a hydrocarbon or solvent, depending on the reactor type. Polymerisation is a highly exothermic reaction, requiring continuous cooling to prevent runaway reactions. To ensure safety, reactor systems are equipped with emergency quenches that activate if the temperature exceeds a predetermined setpoint.
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Additives and their controversies
Nitric acid is an essential substance in plastic-making. However, the process also involves the use of several other chemicals, collectively known as additives, which improve the final product's lifespan, workability, and appearance. These additives can include stabilizers, plasticizers, and dyes.
Additives are crucial in providing plastics with targeted optimum properties such as toughness, flexibility, elasticity, and colour. They also make plastics safer and more hygienic for specific applications. The number of additives in plastic varies substantially, ranging from 0.05 to 70% of the plastic's weight, depending on the polymer and the function of the additive.
While additives are essential in the production of plastics, they have also been a source of controversy. Some additives are potentially harmful and have been associated with adverse health effects at high exposure levels. For instance, compounds leaching from polystyrene food containers have been suspected of interfering with hormone functions and are considered possible human carcinogens. Other chemicals of concern include alkylphenols and phthalates, which can migrate into the environment and lead to human exposure through food packaging and other products.
Due to these concerns, certain jurisdictions have banned or restricted the use of specific additives. For example, the EU and the US have limited the use of particular phthalates in some applications, especially those intended for children.
The inappropriate use, disposal, and recycling of plastics can further lead to the undesirable release of additives, causing environmental and human health issues. Therefore, sound recycling practices are crucial to ensuring the emission of harmful substances is avoided.
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Plastic types and their applications
Nitric acid is a crucial substance in plastic production. It is used in the creation of organic compounds and is essential for making plastics.
Plastics are classified into several groups, including acrylics, polyesters, silicones, polyurethanes, and halogenated plastics. They can be categorised based on their synthesis processes, physical properties, and reactions to various substances. Some common types of plastics and their applications include:
- Polyethylene (PE): A versatile, durable, and cost-effective thermoplastic polymer derived from ethylene. PE is lightweight and mouldable, making it suitable for a wide range of applications. It has varying densities, resulting in unique physical properties. LDPE, LLDPE, and HDPE are different types of PE, each with distinct characteristics. LDPE is used for drip irrigation tubing in agriculture, while HDPE is used for water and gas distribution pipes due to its high resistance to chemicals and corrosion. PE is also used in construction, automotive, and medical fields.
- Polypropylene (PP): The second-most widely produced commodity plastic, known for its durability, heat resistance, and flexibility. PP is used in automotive parts, medical devices, laboratory equipment, and food containers. It is also used for living hinges due to its resistance to fatigue.
- Polyvinyl Chloride (PVC): The third-most widely produced synthetic plastic polymer, available in rigid and flexible forms. PVC is extensively used in construction for pipes, window frames, flooring, and electrical insulation. It is valued for its durability, heat and cold resistance, and lightweight properties. However, PVC is challenging to recycle.
- Polystyrene (PS): An inexpensive and easy-to-produce resin, commonly known as Styrofoam. PS is highly inflammable and can leach harmful chemicals when heated. It is used in beverage cups, disposable dinnerware, packing materials, and insulation.
- Polyethylene Terephthalate (PET): A recyclable plastic with excellent chemical resistance to organic materials and water. PET is widely used in food and beverage packaging due to its ability to prevent oxygen from spoiling the contents. It is also used in medical devices and equipment due to its good biocompatibility.
- Polycarbonates (PC): Tough, stable, and transparent plastics that are two hundred and fifty times stronger than glass. They are used in eye protection, such as lenses for sunglasses and goggles, as well as in electronics and compact discs. However, PC resins have come under controversy due to the potential environmental hazards associated with bisphenol A (BPA) leaching and decomposition issues.
- Acrylics: Lightweight, shatter-resistant, and transparent thermoplastics used as an alternative to glass. Acrylics can be made in various colours and finishes, such as fluorescent, abrasion-resistant, and bullet-resistant. They are commonly used in sheet form for mirrors, plexiglass, and security applications.
- Bioplastics: These are derived from renewable sources such as bacteria, soybeans, fish-skin waste, and algae. Bioplastics have been explored for medical applications, controlled drug release, packaging, and absorbable surgical sutures.
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Frequently asked questions
Nitric acid is considered essential in making plastics. It is a corrosive liquid inorganic acid (HNO3) used as an oxidizing agent and in making organic compounds.
Many other substances are added to plastics during the compounding stage to improve their lifespan, workability, or appearance. These include stabilizers, plasticizers, and dyes.
Some harmful additives used in plastic production include cadmium, chromium, lead, and mercury. These substances are regulated under the Minamata Convention on Mercury and are banned in many jurisdictions.
Some common plastic polymers include polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polyvinyl chloride (PVC), and polystyrene (PS).
Synthetic plastics are derived from crude oil, natural gas, or coal, while biobased plastics come from renewable sources such as carbohydrates, vegetable oils, and bacteria. The majority of plastic in use today is synthetic due to the ease of manufacturing methods involved in processing crude oil.
