Ethan Clarke
Plastics are a range of synthetic or semisynthetic materials that contain polymers as their main ingredient. They are lightweight, durable, flexible, nontoxic, and inexpensive to produce, which has led to their widespread use around the world. While there are hundreds of types of plastic, they can be broadly categorized into seven groups based on their chemical structure, each with unique properties and applications. The functional groups within the molecular structure of these polymers give rise to the fundamental differences in properties between the different types of plastics, such as mechanical, thermal, and chemical resistance.
| Characteristics | Values |
|---|---|
| Number of functional groups | Hundreds |
| Number of types | 7 |
| Most common type | Polyethylene |
| Most dangerous to human health | PVC |
| Functional groups | PETE, HDPE, PVC/Tygon + plasticizer, Teflon, PP, PS |
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What You'll Learn
- Plastic polymers are formed from chains of carbon atoms, with or without oxygen, nitrogen, or sulfur atoms
- The unique molecular structure of plastics results in different functional groups
- Functional groups determine the diversity of physical properties in plastics
- The seven types of plastic are categorised by the chemical structure of the polymer's backbone and side chains
- The most common plastics are polyethylene, polyvinyl chloride (PVC), polystyrene, and polycarbonate
Plastic polymers are formed from chains of carbon atoms, with or without oxygen, nitrogen, or sulfur atoms
The structure of plastic polymers is often described as a backbone with side chains attached. The backbone is the main path of the chain, linking together the repeating units. The side chains are different molecular groups that hang from the backbone and influence the properties of the polymer. For example, the side chains can make a plastic more flexible or rigid, change its melting point, or give it specific chemical or physical characteristics.
The monomers that make up the polymer chains are typically derived from hydrocarbons, which are most often obtained from crude oil, natural gas, and coal. These fossil fuels are composed of carbon, hydrogen, nitrogen, sulfur, and oxygen elements, among other minerals. Through various chemical reactions, these hydrocarbons can be processed into monomers such as ethylene, propylene, and butene.
The type of monomers used, as well as their arrangement in the polymer chains, play a crucial role in determining the final properties of the plastic. For instance, polyethylene (PE) can be manufactured in varying densities, resulting in unique physical attributes. High-density polyethylene (HDPE) is known for its strength and rigidity, while low-density polyethylene (LDPE) exhibits a thinner and more flexible design.
Additionally, the versatility of plastic polymers allows them to be designed for specific applications. For example, polyvinyl chloride (PVC) can be made rigid for use in construction or flexible for plumbing and electrical insulation. Acrylics, polyesters, silicones, polyurethanes, and halogenated plastics are also important groups that are classified based on the chemical structure of their backbone and side chains.
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The unique molecular structure of plastics results in different functional groups
Plastics are classified by the chemical structure of the polymer's backbone and side chains. The backbone of a polymer chain is the main path that links together a large number of repeating units formed from monomers. The side chains are molecular groups that hang from the backbone and influence the properties of the polymer. These side chains can be customized to modify the characteristics of a plastic.
One example of a plastic with unique functional groups is polyvinyl chloride (PVC). PVC is the world's third-most widely produced synthetic plastic polymer and comes in two basic forms: rigid and flexible. In its rigid form, PVC is commonly used in the building and construction industry for pipes, doors, and windows. When mixed with plasticizers, PVC becomes softer and more flexible, making it suitable for plumbing, wiring, electrical cable insulation, and flooring. The versatility of PVC allows it to replace traditional building materials such as wood, metal, and concrete. However, despite its advantages, PVC is challenging to recycle and should be avoided whenever possible.
Another example of a plastic with distinct functional groups is polyethylene terephthalate (PET), commonly known as PETE. PET is widely used for food and beverage packaging due to its ability to prevent oxygen from spoiling the product inside. It is lightweight, strong, and transparent, making it ideal for beverage bottles, food jars, and polyester fabrics. PET bottles are the most widely recycled plastic globally, with a positive track record in curbside recycling programs.
In summary, the unique molecular structure of plastics, characterized by different functional groups, results in a wide range of physical properties. These functional groups determine the mechanical, thermal, and chemical resistance properties of plastics, making it essential to select the correct type of plastic for specific applications. Understanding the correlation between the molecular structure and performance of plastics is crucial in various industries.
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Functional groups determine the diversity of physical properties in plastics
The wide range of synthetic or semisynthetic materials that we call plastics are derived from polymers, which 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 links together a large number of repeat units. To customize the properties of a plastic, different molecular groups called side chains hang from this backbone. These side chains are usually attached to the monomers before the monomers themselves are linked together to form the polymer chain. The structure of these side chains influences the properties of the polymer.
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. For example, the density of polyethylene gives the final plastic unique physical properties. As a result, polyethylene is used in a wide variety of products.
The extent to which polymers are semicrystalline or amorphous is determined by their chemical structure, including polymer chain length and functional groups. The ordered arrangement of the molecular structure associated with crystallinity results in melting when a sufficient temperature is reached. Because of this, semicrystalline polymers such as polyethylene, polyacetal, and nylon will undergo a distinct melting transition and have a melting point. Amorphous polymers, including polystyrene, polycarbonate, and poly(phenyl sulfone), will not truly melt but will soften as they are heated above their glass transition temperature.
In summary, the diversity of physical properties in plastics is determined by functional groups.
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The seven types of plastic are categorised by the chemical structure of the polymer's backbone and side chains
Plastics are classified by the chemical structure of the polymer's backbone and side chains. The backbone is the part of the chain that links together a large number of repeat units. To customize the properties of a plastic, different molecular groups called side chains hang from this backbone. The structure of these side chains influences the properties of the polymer.
Plastics are a wide range of synthetic or semisynthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded, or pressed into solid objects of various shapes. Most plastics contain organic polymers, which are formed from chains of carbon atoms, with or without the attachment of oxygen, nitrogen, or sulfur atoms. Each polymer chain consists of several thousand repeating units.
Polymers are large molecules or macromolecules that are a combination of many subunits. They can be naturally found in plants and animals (natural polymers) or can be human-made (synthetic polymers). Different polymers have unique physical and chemical properties, which is why they are used in everyday life.
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 seven types of plastic are categorised by the chemical structure of the polymers' backbone and side chains. Important groups classified in this way include the acrylics, polyesters, silicones, polyurethanes, and halogenated plastics.
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The most common plastics are polyethylene, polyvinyl chloride (PVC), polystyrene, and polycarbonate
Plastics are synthetic or semisynthetic materials that use polymers as their main ingredient. They are typically classified by the chemical structure of the polymer's backbone and side chains. The most common plastics are polyethylene, polyvinyl chloride (PVC), polystyrene, and polycarbonate.
Polyethylene
Polyethylene is the most common plastic on Earth. It can be manufactured in varying densities, with each density giving the final plastic unique physical properties. For example, low-density polyethylene (LDPE) is thinner and more flexible, and is used in shopping bags, plastic bags, and clear food containers. On the other hand, high-density polyethylene (HDPE) is more rigid and is used in products such as plastic bottles, piping, and folding chairs. Ultra-high-molecular-weight polyethylene (UHMWPE) is even more abrasion-resistant due to the extreme length of its polymer chains.
Polyvinyl Chloride (PVC)
PVC is the third-most widely produced synthetic plastic polymer. It can be manufactured to possess rigid or flexible properties. Rigid PVC is commonly used in construction materials, doors, windows, and bottles, while flexible PVC is used in plumbing, electrical cable insulation, and clothing. PVC is known for its ability to blend with other materials. However, it is difficult to recycle and has been associated with serious health and environmental risks.
Polystyrene
Polystyrene (PS) is commonly used for food containers, egg cartons, disposable cups and bowls, packaging, and bike helmets. It is lightweight, durable, flexible, nontoxic, and inexpensive to produce. However, when exposed to hot and oily food, polystyrene can leach styrene, which is considered a brain and nervous system toxicant. It also has a low recycling rate.
Polycarbonate
Polycarbonate is a tough, stable, and transparent plastic that is as clear as glass but much stronger. It can be easily worked, moulded, and thermo-formed or cold-formed. Polycarbonate is commonly used in products such as sunglasses, police riot gear, DVDs, and greenhouses. It is also known as Lexan, Makrolon, and Makroclear. However, polycarbonate has fallen out of favour in recent years due to its association with bisphenol A (BPA), a potential environmental hazard.
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Frequently asked questions
There are hundreds of types of plastic, each with its own unique functional groups. However, plastics are usually classified by the chemical structure of the polymer's backbone and side chains. Some of the important groups classified in this way include acrylics, polyesters, silicones, polyurethanes, and halogenated plastics.
Some examples of plastics with distinct functional groups include PETE, HDPE, PVC/Tygon + plasticizer, and Teflon.
Functional groups are molecular groups that hang from the backbone of a polymer chain and influence the properties of the polymer.
Understanding the functional groups of plastics is crucial because the fundamental differences in the properties of polymers are due to the varying functional groups within their molecular structure. These differences include mechanical, thermal, and chemical resistance properties.
The functional groups of plastics can impact their infrared (IR) spectroscopy, with plastics showing different IR signatures due to their different functional groups. For example, PETE may show characteristic ester peaks in the IR spectrum, while PVC/Tygon with plasticizers may present a distinct signature related to chlorine absorption.
