Ethan Clarke
Plastic is a synthetic or semi-synthetic material composed primarily of polymers. The chemical makeup of plastics can change with the use of copolymers and the binding of different elements and compounds. Plastics are mostly carbon-based atoms, with carbon atoms linking to hydrogen, oxygen, nitrogen, chlorine, or sulfur. These atoms form long chains, resulting in a polymer termed 'thermoplastic'. Thermoplastics can be remoulded after heating and make up the vast majority of plastics. Other types of plastics include thermosetting polymers, biodegradable plastics, and high-performance plastics.
| Characteristics | Values |
|---|---|
| Molecular structure | Polymerization, crystalline/amorphous structure |
| Composition | Synthetic or semi-synthetic polymers, carbon-based atoms, other atoms (hydrogen, oxygen, nitrogen, chlorine, sulfur) |
| Plasticity | Ability to be molded, extruded, or pressed into solid forms |
| Properties | Low weight, durability, flexibility, chemical resistance, low toxicity, low-cost production |
| Types | Thermoplastics, thermosetting polymers, biodegradable plastics, engineering plastics, elastomers |
| Uses | Packaging, agriculture, construction, consumer goods, healthcare, body armor, aerospace, military |
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Polymers and polymerization
The chemical makeup of plastics is based on polymers, which are macromolecules built up from a large number of similar structural units bonded together. These structural units, known as monomers, are relatively small molecules that combine chemically to form a very large chain-like molecule, or a polymer. This process is called polymerization.
Polymers are often referred to as chains, as they consist of repeating units, similar to a chain's links. These repeating units are made up of the same monomer molecules, which can be all alike or represent different compounds. Most polymers contain carbon and hydrogen, and they can also include oxygen, chlorine, fluorine, nitrogen, silicon, phosphorus, and sulfur. The specific elements included in the polymer depend on the substances used in its creation.
Polymerization can occur through various reaction mechanisms, including step-growth and chain-growth. In step-growth polymerization, pairs of reactants of any length combine at each step to form a longer polymer molecule. This process increases the average molar mass slowly, and long chains only form late in the reaction. An example of a step-growth polymer is nylon, which is formed through the reaction of alcohol and carboxylic acid groups.
On the other hand, chain-growth polymerization involves the linking together of unsaturated monomers, particularly those containing carbon-carbon double bonds. This process results in the formation of high molecular weight alkanes. An example of a polymer formed through chain-growth polymerization is polyethylene.
There are also different types of polymerization processes, including addition polymerization and condensation polymerization. In addition polymerization, monomers add to each other in a way that results in the polymer containing all the atoms of the starting monomers. Condensation polymerization, on the other hand, involves the formation of a small molecule, often water, as a byproduct during the polymerization process.
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Carbon-based atoms
Plastics are mostly composed of carbon-based atoms. The carbon atom is unique in its ability to link to other atoms with up to four chemical bonds. In plastics, carbon atoms typically link to hydrogen, oxygen, nitrogen, chlorine, or sulfur.
When these atoms are bonded together in long chains, the resulting polymer is termed a 'Thermoplastic'. Thermoplastics are meltable and make up about 92% of plastics. They are so-called because they do not undergo a chemical change in their composition when heated and can, therefore, be molded repeatedly.
Thermoplastics are formed through a process known as polymerization, in which monomer molecules are bonded together through a chemical reaction. This results in a three-dimensional network of long individual polymer chains consisting of smaller repeated units. The chains are not covalently bonded to each other but are instead held together by intermolecular forces, such as Van der Waals forces, hydrogen bonding, and dipole interactions.
The monomers that make up the polymer chains contain double bonds between carbon atoms, allowing the carbon atoms to subsequently react to form polymers. The plastic behavior of polymers is influenced by their arrangement of molecules on a large scale.
The structure of plastics can be altered by adding additives, copolymers, and the chemical binding of different elements and compounds.
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Thermoplastics
Some common types of thermoplastic include polypropylene, polyethylene, polyvinyl chloride (PVC), polystyrene, and polycarbonate. Polyvinylidene fluoride (PVDF) is a thermoplastic with high chemical inertness and resistance, used in piping for aggressive chemicals and in construction, transportation, and electricity. Polyoxymethylene (POM), also known as acetal or polyformaldehyde, is an engineering thermoplastic with high stiffness, low friction, and excellent dimensional stability. It is used in precision parts such as gears and power tool casings.
Polylactic acid (PLA), a compostable thermoplastic derived from renewable resources such as corn starch or sugarcane, is commonly used in 3D printing. Polybenzimidazole (PBI) fibre is a synthetic fibre with a high melting point and exceptional thermal and chemical stability, making it useful for heat-resistant applications. Thermoplastic composites (TPCs) are a class of composite materials that consist of a thermoplastic polymer matrix reinforced with fibres of carbon, glass, or aramid, among other materials.
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Crystallinity
Semi-crystalline polymers contain sections of ordered structure, while amorphous polymers have an unorganised structure. The degree of crystallinity in polymers typically ranges between 10% and 80%, and polymers with higher crystallinity are often referred to as "semi-crystalline". The properties of semi-crystalline polymers are determined not only by the degree of crystallinity but also by the size and orientation of the molecular chains.
Crystallisation of polymers occurs through various mechanisms, including cooling from melting, mechanical stretching, and solvent evaporation. During crystallisation, the molecular chains of polymers fold together to form ordered regions called lamellae, which then compose larger spheroidal structures called spherulites. The crystallisation process affects the optical, mechanical, thermal, and chemical properties of the polymer. For example, the addition of particles to semi-crystalline polymers can change their mechanical properties, either strengthening or weakening the material.
The difference between semi-crystalline and amorphous molecular arrangements also impacts the mechanical properties of plastics, particularly their temperature dependency. Amorphous plastics generally exhibit a consistent modulus over a range of temperatures, while semi-crystalline plastics show modulus stability below the glass transition temperature and a decline between the glass transition temperature and the melting point.
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Additives
Some common additives include plasticizers, which are used to make polymers more flexible or rigid, fillers to make plastics cheaper, and oil composition to improve rheology. Other additives are used for electrical stability or to give polymers a certain capacity, such as photovoltaic agents for photolithography.
Additionally, some additives are used to improve the processing of plastics, such as internal lubricants that lower viscosity and heat dissipation, or processing aids that enable or ease the production of plastics, such as polymerization catalysts, solvents, or lubricants.
The use of additives in plastics has been associated with potential risks to human health, particularly in the case of plasticizers like phthalates and bisphenol A (BPA), which can leach from products and cause adverse health effects. As a result, there is a growing demand for circular plastics and additives that improve the processing, performance, and quantity of recycled plastics.
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Frequently asked questions
Plastics are synthetic or semi-synthetic polymers. Most polymers are formed from chains of carbon atoms, with or without attached oxygen, nitrogen, hydrogen, chlorine, or sulfur atoms.
Polymers are substances made of many repeating chemical units or molecules. They can be found in nature in the form of tar, shellac, tortoise shells, horns, tree saps such as amber and latex.
Plastics can be classified 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, thermal resistance, and glass transition temperature.
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