Maya Ellison
Plastic is a polymeric material that can be moulded or shaped, with properties such as low density, low electrical conductivity, transparency, and toughness. The chemical structure of plastics is determined by polymers, which are macromolecules made up of a large number of similar structural units bonded together. These polymers are formed through a process called polymerization, which results in a three-dimensional network of long individual polymer chains. The properties of plastics are influenced by their chemical structure, including the length and arrangement of polymer chains, as well as the presence of additives and functional groups. Understanding the chemical structure of plastics is crucial for their classification, degradation behaviour, and potential health impacts.
| Characteristics | Values |
|---|---|
| Chemical composition | Polymers with long chains of carbon atoms, with or without oxygen, nitrogen, or sulfur atoms attached. |
| Molecular structure | Macromolecules with repeating units bonded together, forming a three-dimensional network of chains. |
| Polymerization | A chemical reaction that creates polymer chains through addition or condensation reactions. |
| Intermolecular bonding | Chains are entangled and held together by intermolecular forces such as Van der Waals forces and hydrogen bonding. |
| Crystallinity | Plastics can be semi-crystalline or amorphous, affecting their melting behavior and mechanical properties. |
| Modifiers | Additives like fillers, flame retardants, and plasticizers can modify properties but may also pose health and environmental risks. |
| Degradation | Resistant to natural degradation, but recent studies show ocean degradation is faster than expected due to environmental factors. |
| Physical properties | Low weight, durability, flexibility, chemical resistance, low toxicity, and low cost of production. |
| Production | Estimated 9.2 billion metric tons produced between 1950 and 2017, with over 400 million tons in 2023 alone. |
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What You'll Learn
- Plastic is a polymeric material that can be moulded or shaped
- Polymers are macromolecules built from many similar structural units bonded together
- Plastics are classified by their chemical synthesis, physical properties, and reactions to substances
- Polymerization results in multiple polymer chains made up of repeating units
- The chemical structure of plastic determines its fate, such as its degradation
Plastic is a polymeric material that can be moulded or shaped
There are two basic types of polymerization reactions — addition and condensation. Addition polymerization is the formation of polymers from monomers containing a carbon-carbon double bond through an exothermic addition reaction. The fundamental differences between the properties of these different types of polymers are attributable to the varying functional groups within the molecular structure. These differences include mechanical, thermal and chemical resistance properties. As such, it is important to select the correct type of plastic based upon the requirements of the application.
The chemical structure of plastic determines its fate. For example, polymers' marine degradation takes much longer due to the saline environment and cooling effect of the sea, contributing to the persistence of plastic debris in certain environments. The polymerization process is a chemical reaction, and while carefully controlled, there is some inherent variation. This results in polydispersity, or polymer chains of unequal length. Because of this, commercial plastics have polymers with a molecular weight distribution. Simply put, molecular weight distribution represents the relative amounts of polymers of different molecular weights to comprise a given specimen of that material.
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. The vast majority of polymers are formed from chains of carbon atoms, with or without the attachment of oxygen, nitrogen or sulfur atoms. These chains comprise many repeating units formed from monomers. Each polymer chain consists of several thousand repeating units. The backbone is the part of the chain that is on the main path, linking together a large numbers of repeat units.
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Polymers are macromolecules built from many similar structural units bonded together
Plastics are polymeric materials that can be moulded or shaped. They are classified based on their chemical structure, engineering behaviour, and physical properties. The chemical structure of plastics, particularly the polymer's backbone and side chains, influences their properties and degradation. The backbone of a polymer refers to the main path of the chain, linking together a large number of repeat units. The side chains are molecular groups that hang from the backbone and influence the polymer's properties.
The polymerization process results in polydispersity, or polymer chains of unequal length, leading to commercial plastics having polymers with a molecular weight distribution. The molecular weight distribution represents the relative amounts of polymers of different molecular weights within a given specimen of material. This distribution impacts the material's properties, such as ductility, impact resistance, strength, and stiffness.
The structure of a polymer can be linear macromolecules with a single unbranched chain or branched macromolecules with a main chain and side chains. The type of polymer morphology, such as the arrangement and ordering of polymer chains, influences the macroscopic physical properties of the polymer. Disordered polymers, such as those with a high degree of branching, tend to form amorphous structures.
The versatility of plastics, combined with their unique properties, has led to their widespread use in various products, including automobile interiors, compact discs, and packaging materials. However, the chemical structure of plastics also contributes to their durability and resistance to natural degradation processes, leading to environmental concerns.
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Plastics are classified by their chemical synthesis, physical properties, and reactions to substances
Plastics are classified in various ways, including by their chemical synthesis, physical properties, and reactions to substances.
Chemical Synthesis
Plastics are synthetic polymers, which are large molecules made up of many repeating units formed from monomers. The synthesis of polymers occurs through a process called polymerization, which involves bonding monomer molecules together through a chemical reaction. There are two basic types of polymerization reactions: addition and condensation. Addition polymerization involves the formation of polymers from monomers containing a carbon-carbon double bond through an exothermic reaction.
Plastics can be classified by the chemical process used in their synthesis, including condensation, polyaddition, and cross-linking.
Physical Properties
Plastics can be classified by their physical properties, such as hardness, density, tensile strength, thermal resistance, and glass transition temperature. Density, for example, is an important factor in the mechanical properties of polyethylene, with higher densities leading to stronger intermolecular forces and increased tensile strength. The degree of crystallinity in plastics also affects their physical properties, with semicrystalline polymers having a distinct melting point, while amorphous polymers will soften but not melt when heated.
Reactions to Substances
Plastics can be classified by their resistance and reactions to various substances and processes, such as exposure to organic solvents, oxidation, and ionizing radiation. For instance, the chemical structure of most plastics makes them resistant to degradation, but recent studies have shown that ocean plastics decompose faster than previously thought due to exposure to environmental conditions, releasing toxic chemicals.
Other Classifications
Other classifications of plastics are based on qualities relevant to manufacturing or product design, such as thermoplastics, thermosets, conductive polymers, biodegradable plastics, engineering plastics, and elastomers. Thermoplastics, for example, can be molded repeatedly as they do not undergo a chemical change when heated, while thermosets can only melt and take shape once, after which they remain solid.
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Polymerization results in multiple polymer chains made up of repeating units
Plastics are polymeric materials that can be moulded or shaped. This property of plasticity, often combined with other characteristics such as low density, low electrical conductivity, transparency, and toughness, allows plastics to be made into a wide variety of products. Polymers are macromolecules that are built up from a large number of similar structural units bonded together. These structural units are often called chains, and polymers consist of repeating units, similar to links.
The process of converting a starting chemical called a monomer to a long-chain molecule, or polymer, is called polymerization, or polymer synthesis. Polymerization results in multiple individual polymer chains made up of repeating units. These chains are entangled within each other, and are held together by intermolecular forces, such as Van der Waals forces, hydrogen bonding, and dipole interactions. The polymerization process differs according to the chemical composition and structure of the starting monomer. There are two basic types of polymerization reactions: addition and condensation.
Addition polymerization is the formation of polymers from monomers containing a carbon-carbon double bond through an exothermic addition reaction. In this process, monomers combine with their structure remaining unchanged. In condensation polymerization, the resulting polymer is less massive than the two or more monomers that combined to form it. In chain-growth polymerization, one monomer is added at a time until a polymer is formed. In step-growth polymerization, at least two different monomers participate in the reaction between dissimilar chemical compounds that are part of the monomer molecules.
The repeat units of a range of polymers, together with the monomer units from which they are derived, can be seen in the example of polyethylene. The simplest repeat unit consists of two carbon atoms linked to four hydrogen atoms. The difference between the monomer and the repeat unit is the loss of the double bond in the former, which gives the chain-linked repeating group. The molecular masses of both the monomer and unit are identical at 28. The number of repeat units in a chain is specified by the degree of polymerization, but a more commonly used measure is the chain molecular mass.
The chemical structure of plastics determines their fate. For example, polymers' marine degradation takes much longer due to the saline environment and the cooling effect of the sea, contributing to the persistence of plastic debris in certain environments.
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The chemical structure of plastic determines its fate, such as its degradation
Plastics are polymeric materials that can be moulded or shaped, with other properties such as low density, low electrical conductivity, transparency, and toughness. Polymers are macromolecules that are built up from a large number of similar structural units bonded together, often called chains. These chains are made up of thousands of repeating units formed from monomers. The backbone of the chain is the main path that links a large number of these repeat units.
The polymer chains can be semicrystalline or amorphous, and this is determined by their chemical structure, including polymer chain length and functional groups. The semicrystalline polymers will undergo a distinct melting transition and have a melting point, while amorphous polymers will not melt but will soften as they are heated. The difference between semicrystalline and amorphous molecular arrangements has an implication on the mechanical properties of the material, particularly temperature dependency.
The chemical structure of most plastics renders them durable and resistant to many natural degradation processes. However, plastics do degrade by a variety of processes, the most significant of which is usually photo-oxidation. The rate of degradation varies significantly, with biodegradation taking decades, while some industrial processes can completely decompose a polymer in hours. Polymer degradation is the reduction in the physical properties of a polymer, such as strength, caused by changes in its chemical composition.
Biodegradable plastics are prone to degradation due to their suitable physico-chemical properties, such as low molecular weight, high flexibility, and more functional groups. The chemical structure of biodegradable plastics includes ester linkages that make them susceptible to breakdown by microbial enzyme systems. Pretreatment through physical and chemical methods can also make plastics more prone to degradation by altering their structural and morphological characteristics, such as reducing their molecular weight, rupturing chemical bonds, and enriching functional groups.
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Frequently asked questions
Plastic is a polymeric material that can be moulded or shaped. The chemical structure of plastic is determined by the polymer, which is made up of a large number of similar structural units bonded together. These units are often called chains and consist of repeating units, similar to links. The polymer chains can be of unequal length, resulting in polydispersity.
Polymers are macromolecules that are formed through a process called polymerization. This process involves bonding monomer molecules together through a chemical reaction, resulting in a three-dimensional network of long individual polymer chains. These chains are entangled within each other and are held together by intermolecular forces.
The chemical structure of plastics determines their degradation process. The most significant degradation process for plastics is photo-oxidation. Plastics in the ocean can also decompose due to exposure to the sun, rain, and other environmental conditions, releasing toxic chemicals such as bisphenol A.
