Omar Khalid
Plastic is a synthetic or semi-synthetic material that is derived from petrochemicals or natural sources such as cellulose and starch. It is composed primarily of polymers, which are large molecules made up of many smaller, repeating units called monomers. The monomers are mainly derived from the extraction and refining of crude oil and natural gas, which contain hydrocarbons—molecules consisting of hydrogen and carbon. These carbon-carbon bonds form the backbone of the plastic molecule, with other atoms such as oxygen, nitrogen, and sulphur attached to form different polymer chains. The polymer chains are entangled within each other and exhibit viscoelastic behaviour, resulting in time-dependent properties. The molecular structure of plastics can be semicrystalline or amorphous, with the former exhibiting a distinct melting transition and the latter softening upon heating. The fundamental differences in polymer structures give rise to a variety of plastic types with distinct properties, such as polyethylene, polypropylene, and polyvinyl chloride.
| Characteristics | Values |
|---|---|
| Composition | Synthetic or semisynthetic materials composed primarily of polymers |
| Molecular Structure | Amorphous or semicrystalline |
| Polymers | Large molecules (macromolecules) made up of many smaller molecules called monomers |
| Monomers | Ethylene, camphor/lime, cellulose nitrate, etc. |
| Polymer Chains | Multiple individual chains made up of repeating units; not covalently bonded but kept together by intermolecular forces |
| Polymerization | An exothermic process where monomers are linked; results in polydispersity, or polymer chains of unequal length |
| Molecular Weight Distribution | Represents the relative amounts of polymers of different molecular weights in a given specimen |
| Plasticity | The property that allows a material to be molded or deform irreversibly without breaking |
| Other Properties | Low weight, durability, flexibility, chemical resistance, low toxicity, low cost, ease of production, non-conductivity, biocompatibility, etc. |
| Raw Materials | Natural gas, coal, starch, cellulose, crude oil, salt, etc. |
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What You'll Learn
Plastic is a synthetic or semi-synthetic material
Plastic is a synthetic or semi-synthetic organic compound. It is a polymeric material that is manufactured through polymerization, where small molecules (monomers) combine to form long chains (polymers). The word 'plastic' comes from the Latin 'plasticus' and the Greek 'plastikos', both of which mean 'capable of moulding'. This is because plasticity, or the ability to be irreversibly deformed without breaking, is one of the key properties of plastic.
Plastics are high molecular weight organic polymers composed of various elements such as carbon, hydrogen, oxygen, nitrogen, sulphur, and chlorine. They can also be produced from silicon atoms (silicone) along with carbon. The backbone of a polymer used for the preparation of plastics consists mainly of carbon atoms. For example, the monomer of polyethylene ('polythene' in British English) is ethylene. Other elements, such as silicon, oxygen, and nitrogen, can also be present in polymer backbones.
The process of polymerization involves combining many small molecules (monomers) into a covalently bonded chain or network. During this process, some chemical groups may be lost from each monomer. The microstructure of a polymer describes the arrangement of these monomers within the polymer at the scale of a single chain. This microstructure determines the possibility for the polymer to form phases with different arrangements, such as crystallization or microphase separation. An important feature of a polymer's microstructure is its architecture and shape, which relates to the way branch points deviate from a simple linear chain.
Plastics are available in a wide range of colours and textures and can be molded into almost any shape. They can be made transparent, flexible, or rigid, and their unique properties make them suitable for a diverse range of applications. For example, polyethylene terephthalate (PET) is used for beverage bottles, polyvinyl chloride (PVC) for garden hoses, and polycarbonate for DVDs and sunglasses.
The first truly synthetic plastic was Bakelite, invented by Leo Baekeland in 1906. It was made from phenol and formaldehyde resin and exhibited high resistance to electricity, heat, and chemicals. Before the invention of synthetic plastics, materials such as wood, metal, glass, ceramics, and animal-derived materials were commonly used. The discovery of plastic can be traced back to 1839 when Charles Goodyear discovered vulcanization, a process that made rubber more resilient and elastic. This was followed by several advancements, including the creation of the first synthetic polymer (industrial plastic) by John Wesley Hyatt in 1869.
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Polymers are the large molecules that plastics are made of
Plastics are a type of synthetic or semi-synthetic polymer. They are composed of very large molecules, known as macromolecules, which are multiples of simpler chemical units called monomers. The process by which monomers link together into a polymer chain is known as polymerization.
The polymerization process is a chemical reaction that results in the formation of multiple individual polymer chains made up of repeating units. The continuously linked backbone of a polymer used for the preparation of plastics consists mainly of carbon atoms. However, polymers can also contain atoms such as oxygen, nitrogen, or sulfur in their backbone chains. Each polymer chain consists of several thousand repeating units, with different molecular groups called side chains hanging from this backbone. The structure of these side chains influences the properties of the polymer, such as its mechanical, thermal, and chemical resistance properties.
The inherent viscoelastic nature of polymeric materials produces movement within the polymer chains under conditions of applied stress, resulting in time dependency within polymeric materials. This means that the properties of a plastic material, such as strength and ductility, are not static but will decrease over time. This often leads to creep and stress relaxation within plastic materials.
Plastics can be categorized as being semicrystalline or amorphous based on their molecular structure. Semicrystalline polymers, such as polyethylene, undergo a distinct melting transition and have a melting point. Amorphous polymers, such as polystyrene, do not truly melt but soften as they are heated above their glass transition temperature. The glass transition temperature can be engineered by altering the degree of branching or crosslinking in the polymer or by adding plasticizers, which increase polymer flexibility.
Plastics are known for their ability to be molded, extruded, or pressed into solid objects of various shapes. This adaptability, combined with their low weight, durability, flexibility, chemical resistance, low toxicity, and low-cost production, has led to their widespread use around the world. However, the slow biodegradability of synthetic polymers has also led to environmental concerns, as they can cause ecological deterioration and harm ecosystems.
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Plastics are categorised as semicrystalline or amorphous
Plastics are synthetic or semisynthetic materials composed primarily of polymers. Polymers are substances or materials that consist of very large molecules or macromolecules, formed from chains of carbon atoms, with or without attached oxygen, nitrogen or sulfur atoms. These chains comprise many repeating units formed from monomers. Each polymer chain consists of several thousand repeating units.
The microstructure of a polymer relates to the physical arrangement of monomer residues along the backbone of the chain. The microstructure determines the possibility for the polymer to form phases with different arrangements, for example through crystallization, the glass transition or microphase separation.
Amorphous polymers, on the other hand, have random, entangled chains with a coiled molecular structure. They do not immediately melt when heated and do not have a sharp melt point. Instead, they gradually soften as the temperature rises. Amorphous polymers are isotropic in flow, meaning they shrink uniformly in the direction of flow and transverse to flow, resulting in less shrinkage and less tendency to warp. They also tend to have better impact resistance but are more prone to stress cracking and have poor fatigue resistance.
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Plastic's molecular structure determines its properties
Plastics are synthetic or semisynthetic materials composed primarily of polymers, which are macromolecules made up of chains of carbon atoms, with or without the attachment of oxygen, nitrogen, or sulfur atoms. The chains are formed through a process called polymerization, which involves bonding monomer molecules together through a chemical reaction. The unique molecular structure of these polymers gives rise to the characteristic properties exhibited by plastics.
The molecular structure of plastics can be categorized as semicrystalline or amorphous. Semicrystalline plastics, such as polyethylene, polyacetal, and nylon, have a more ordered arrangement of molecules, resulting in a distinct melting point. On the other hand, amorphous plastics, including polystyrene, polycarbonate, and poly(phenyl sulfone), lack a highly ordered molecular structure and do not truly melt but soften when heated.
The fundamental differences in the properties of various polymers are due to the varying functional groups within their molecular structure. These differences include mechanical, thermal, and chemical resistance properties. For example, the presence of pendant methyl groups (CH3) attached to carbon atoms in polypropylene influences its characteristics. Additionally, the molecular weight distribution, or the relative amounts of polymers of different molecular weights, also affects the properties of plastics. A wider distribution generally leads to better ductility and impact resistance but reduced strength and stiffness.
The inherent viscoelastic nature of polymeric materials causes movement within the polymer chains under stress, resulting in time dependency. Consequently, the properties of plastic materials, such as strength and ductility, are not static and may decrease over time. Furthermore, the entanglement of polymer chains within polymeric materials, held together by intermolecular forces, contributes to their overall structure and behavior.
The characteristic properties of plastics can also be enhanced by incorporating additives such as fillers, reinforcements, anti-degradants, stabilizers, flame retardants, and plasticizers. However, it is important to note that the underlying attributes of a plastic material are determined by the polymer itself. Therefore, understanding the correlation between the molecular structure and performance of plastics is crucial for selecting the appropriate plastic for specific applications.
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Plastic is made from petrochemicals, cellulose, starch, etc
Plastic is a synthetic polymeric material that can be moulded or shaped, usually by applying heat and pressure. It is derived from crude oil, natural gas, or coal, with most plastic being synthetic due to the ease of manufacturing methods involved in processing crude oil. The word 'plastic' comes from the Ancient Greek 'plastikos', meaning "capable of being shaped or moulded".
Plastics can be divided into two categories based on their chemical composition. The first category is made up of polymers with only aliphatic (linear) carbon atoms in their backbone chains. The structure of polypropylene can be used as an example here; each carbon atom has a pendant methyl group (CH3) attached. The second category is made up of heterochain polymers, which contain atoms such as oxygen, nitrogen, or sulfur in their backbone chains, in addition to carbon. Most engineering plastics fall into this category, such as polycarbonate, whose molecules contain two aromatic (benzene) rings.
The polymerization process is a chemical reaction that results in the formation of multiple individual polymer chains made up of repeating units. These chains are entangled within each other and are influenced by intermolecular forces such as Van der Waals forces and hydrogen bonding. The inherent viscoelastic nature of polymeric materials produces movement within the polymer chains under conditions of applied stress, resulting in time dependency. This means that the properties of a plastic material, such as strength and ductility, are not static but will decrease over time.
Synthetic plastics are derived from petrochemicals, which use naphtha and natural gas from oil refining operations as their raw material. The first synthetic plastic was Bakelite, made from phenol and formaldehyde resin. However, the first man-made bioplastic was Parkesine, patented in 1856 by Alexander Parkes, made from cellulose nitrate. John Wesley Hyatt made a fortune with this invention in the 1860s, improving its malleability by adding camphor and renaming it Celluloid.
Bioplastics, on the other hand, are derived from renewable biomass sources such as vegetable fats and oils, corn starch, straw, woodchips, and recycled food waste. They are biodegradable and made of natural materials, providing eco-friendly alternatives for packaging and single-use items. Examples of bioplastics include starch, cellulose, and protein.
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