Alexei Volkov
Polymers are chemical compounds whose molecules are very large, often resembling long chains made up of a seemingly endless series of interconnected links. The repeating units in polymers are often carbon and hydrogen, and sometimes oxygen, nitrogen, sulfur, chlorine, fluorine, phosphorus, and silicon. Plastics are a specific type of synthetic polymer with a large molecular mass where the structure is mostly linear – they resemble spaghetti with long chains. The first synthetic plastic was created in 1909 for telephone and electrical components and was known as Bakelite. The raw materials for plastic are natural gas and petroleum, which are processed to yield useful chemicals, including monomers, which are the basic building blocks of polymers. The monomers are then joined through a process called polymerization to form polymers. The arrangement of polymers in plastic can be amorphous or crystalline. Amorphous molecules are arranged randomly and are intertwined, whereas crystalline molecules are arranged closely and in a discernible order.
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What You'll Learn
Polymer structure: Thermoplastics vs. thermosets
Polymers are formed by covalently joining many monomer units together in the form of chains. The arrangement of these chains can be altered to change the shape and properties of the resulting plastic.
Thermoplastics
Thermoplastics are a type of plastic that can be repeatedly melted and moulded into new forms. They are characterized by their meltability and the presence of repeat units, or unit cells, which are the smallest identical sections of the chain. The repeat units of thermoplastics can vary in size, ranging from a single carbon atom and two hydrogen atoms in polyethylene to 38 or more atoms in nylons. Thermoplastics maintain their chemical structure whether they are in a liquid or solid form, allowing for their reusability and sustainability. However, they may stretch and weaken under long exposure to heat, and their manufacturing process is often more intensive and costly compared to thermosets.
Thermosets
Thermosets, or thermosetting polymers, can only be melted and moulded once. After solidifying, they retain their shape permanently and will decompose rather than melt if reheated. Thermosets do not deform, warp, or lose their shape in extreme temperatures, making them ideal for machinery used in varying climates. They are also low-viscosity and easy to work with at room temperature, and they carry a lower health hazard during the moulding process as they do not release toxic fumes. Additionally, thermosets are more cost-effective to manufacture, as they can utilize flexible moulds and require less heat and pressure during the injection moulding process.
Both thermoplastics and thermosets have their advantages and unique properties, making them suitable for different applications. The choice between the two depends on the specific requirements of the product or application.
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Plasticity: The ability to deform without breaking
Plasticity is a material's ability to undergo some degree of permanent deformation without breaking or failing. This property is important in forming, shaping, extruding, and other hot and cold working processes. All materials exhibit some degree of plasticity, but the extent of this varies. For example, mild steel and rubber are both elastic materials, but steel is more elastic than rubber.
The plasticity of a material is defined as its ability to perform its required function over a lengthy period under normal conditions of use without excessive maintenance or repair. This property is essential for the creation of solid objects with various shapes, such as through moulding, extrusion, or pressing.
In the context of polymers, plasticity refers to the ability of a polymer to survive the temperature and pressure during the moulding process without breaking. Polymers are formed by covalently joining many monomer units together in the form of chains. Monomers are the basic building blocks of polymers and are created during the refining process that transforms crude oil into different petroleum products. The monomers are then combined to form polymers or plastics.
The spatial arrangement of atoms in these polymer chains can be adjusted to change the performance properties of the resulting plastic. This is achieved by changing the chemical structure of the backbone, using copolymers, binding different elements and compounds to the backbone, and utilising crystallizability. Additionally, the inclusion of additives during the manufacturing process can further alter the properties of the plastic.
By tuning the various parameters, such as the type of monomers and their arrangement, the shape, molecular weight, and other chemical and physical properties of the polymer can be customised. This allows plastics to be designed with specific properties for particular applications. For example, thermoplastics, a common type of plastic, are characterised by their ability to melt and take shape. They can be repeatedly moulded as they do not undergo a chemical change in composition when heated.
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Polymer morphology: Amorphous vs. crystalline
Polymers are formed by covalently joining many monomer units together in the form of chains. These chains can be arranged in two ways, resulting in two types of polymer morphologies: amorphous and crystalline.
Amorphous Polymers
Amorphous polymers have a loose structure where the chains are arrayed randomly throughout the material, resulting in no long-range order. These polymers are typically transparent and glass-like, with the molecules oriented randomly and intertwined. This causes them to have a range of melting temperatures. Amorphous polymers are easy to thermoform and possess better dimensional stability, superior impact strength, and excellent resistance to hot water and steam. They also have good chemical resistance and stiffness. Examples of amorphous polymers include polystyrene and ABS.
Crystalline Polymers
Crystalline polymers, on the other hand, have an orderly arrangement of chains that form stacks of folded chains called lamellae, bringing long-range order to the polymer. These polymers exhibit organized and tightly packed molecular chains. Crystalline polymers have sharp melting points and are challenging to thermoform. They possess excellent chemical resistance, stiffness, and strength, as well as good toughness and a low coefficient of friction. Examples of crystalline polymers include HDPE and polypropylene.
It is important to note that most crystalline polymers have amorphous regions, and the degree of crystallinity can vary from 0% (completely amorphous) to 100% (completely crystalline). Polymers can also exhibit both amorphous and crystalline characteristics simultaneously.
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Polymer size: The impact of large molecules
Polymers are typically large molecules, composed of many repeating subunits derived from one or more types of monomers. These monomers are the basic building blocks of polymers, formed through the refining of raw materials such as crude oil and natural gas. The monomers are then joined through a process called condensation polymerization, which involves removing small molecules like water and using catalysts to facilitate the reaction between adjacent monomers. The resulting polymers are long chains of molecules that can be moulded into various shapes, forming plastics.
The size of these polymer molecules plays a significant role in their properties and behaviour. Due to their large size, polymer molecules exhibit unique physical characteristics such as toughness, high elasticity, and viscoelasticity. They tend to form amorphous or semicrystalline structures rather than crystals. Their large molecular mass and coiled shape contribute to a slower dissolution process compared to low-molecular-weight molecules. Additionally, the driving force for mixing polymeric mixtures is entropy, resulting in lower miscibility compared to small molecule materials.
The molecular weight of polymers is a critical factor in their behaviour. Larger molecules in a sample contribute to a higher weight average, skewing the results. This weight average, or Mw, is calculated based on the weight fraction distribution of different-sized molecules. Polymers are also classified by the chemical structure of their backbone and side chains, with the structure of these side chains influencing the properties of the polymer.
The size and structure of polymers impact the performance properties of plastics. By adjusting the spatial arrangement of atoms on the backbone chains, manufacturers can customise the characteristics of plastics. For example, the inclusion of additives and the arrangement of links in thermoplastics can alter their processing, aesthetic, and performance attributes. The degree of crystallinity in polymers also affects their toughness and resistance to impact, with crystalline polymers generally exhibiting superior properties.
The bulkiness and flexibility of the mer unit within polymers have various effects. For instance, the melting point of thermoplastics increases with reduced chain flexibility and increased chain bulkiness and intermolecular bonding strength. Additionally, the backbone stiffness of a polymer influences its strength, impact toughness, and thermal properties. The exploration of nanotechnology further highlights the significance of polymer size. Nanomaterials, such as silver nanoparticles, can be incorporated into polymers to enhance their antimicrobial properties, physical strength, and catalytic capabilities.
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Polymer customisation: The role of additives
Polymers are the foundation of plastics, determining their physical and chemical properties. Polymer customisation through additives is an integral part of enhancing the properties and functionalities of polymer materials. Additives are chemical compounds that are added to polymers during the manufacturing process to improve their processing, mechanical, thermal, electrical, and aesthetic characteristics.
Types of Additives
There are various types of additives used in plastic manufacturing, including:
- Stabilisers prevent the degradation of polymers due to heat, light, or oxygen exposure.
- Plasticisers enhance flexibility, making the polymer less brittle.
- Fillers improve tensile strength and rigidity.
- Pigments add colour to the plastic.
- Flame retardants reduce flammability and prevent the spread of fire.
- Reinforcements provide resistance to microbial growth.
- Lubricants reduce friction during moulding and improve processability.
- Blowing agents create a cellular structure within the polymer, reducing density and providing unique textural properties.
- Antioxidants improve film packaging longevity.
- UV stabilisers protect against UV radiation.
Benefits of Additives
Additives play a significant role in improving the mechanical properties of polymers, increasing their strength, flexibility, and impact resistance. They enable polymers to be tailored for specific applications, enhancing their performance, safety, and durability. For example, flame retardants are commonly used in electronics, construction materials, and transportation to increase fire safety. Additives can also improve the aesthetic appeal of products, enhancing consumer satisfaction.
Limitations and Considerations
While additives offer numerous benefits, it is important to carefully select and use them to ensure their effectiveness and avoid potential drawbacks. For instance, melamine resins, a type of thermosetting polymer, can be customised with additives to enhance their properties and appearance. However, they have limitations, such as brittleness and the potential release of formaldehyde during the curing process. Therefore, a comprehensive understanding of the properties and functions of additives is crucial to achieving the desired results in plastic manufacturing.
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Frequently asked questions
Polymers are chemical compounds whose molecules are very large, often resembling long chains made up of a seemingly endless series of interconnected links.
Polymers are arranged in plastic in two ways: amorphous or crystalline. Amorphous molecules are arranged randomly and are intertwined, whereas crystalline molecules are arranged closely and in a discernible order. The higher the degree of crystallinity, the less light passes through.
Plastics are made by extracting raw materials such as crude oil and natural gas, which are then refined into different petroleum products. These products are then converted into monomers, which are the basic building blocks of polymers.
