What is Plastic?
Plastic is a polymer just like synthetic fibers which can be molded into desired shape and size when soft and can be hardened to produce durable articles. The term ‘Plastic’ was derived from the word ‘Plastikos’ which means ‘to mould’ in Greek, and it is used to refer to a wide range of semi-synthetic or synthetic organic polymers. Different types of plastics are known to possess different physical and chemical properties. Many articles like chairs, tables, buckets, toys, balls, etc are made of plastic material.
Properties of Plastics
They are light in weight and are chemically stable.
Easily moulded into different shapes and sizes.
Good insulation and low thermal conductivity.
Good impact resistance and they do not rust.
Good transparency and wear resistance.
Poor dimensional stability and can be easily deformed.
Low processing cost.
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Different Types of Plastic
Plastics are of Two Types. They are
Thermoplastics
Thermosetting Plastic
Thermoplastics
The term ‘thermoplastic’ refers to plastics that do not undergo any chemical changes when subjected to high temperatures. These plastics do not undergo any changes in their chemical structures and chemical compositions when subjected to heat and can be changed into a soft state and remolded multiple times. Examples: Polystyrene, Teflon, Acrylic, Nylon, etc.
Thermosetting Plastics
They are also known as thermosets, and are plastics that can be molded only once and do not change shape on applying heat. These plastics can only be molded once and they cannot be softened on further heating. These plastics undergo degradation and become damaged when exposed to a large amount of heat. Examples: Vulcanized rubber, Bakelite, Polyurethane, Epoxy resin, Vinyl ester resin, etc.
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Polyethylene
It is one of the most commonly used thermoplastics in the world. The primary application of polyethylene (polythene) is in packaging. It is often used to make plastic bags, bottles, plastic films, containers, etc.
Polyethylene Properties
They exhibit strong creep when placed under a persistent force.
The mechanical strength of polyethylene is comparatively lower than other plastics.
They are very good insulators of electric current.
The rigidity and the hardness of the polymer are relatively low.
They exhibit strong creep when placed under continuous force.
Structure of Polyethylene
Polyethylene is made by the reaction of multiple ethylene molecules in the presence of a catalyst to break the double bond and connect the carbon atoms into a chain.
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Types of Polyethylene
Polyethylene can be classified into different types based on the plastic density and the degree of branching in its structure. Different types of polyethylene exhibit different mechanical properties and melting point of plastic.
High-density polyethylene (HDPE)
High-density cross-linked polyethylene (HDXLPE)
Medium-density polyethylene (MDPE)
Cross-linked polyethylene (XLPE)
Low-density polyethylene (LDPE)
Uses of Polyethylene
It is used in packaging products.
It is used for cable jacketing because it is a good insulator of electric current.
LDPE is also used in pipes, fittings, garbage bags, food packaging, etc.
HDPE is also used in ropes, fishing and agricultural nets, and industrial fabrics.
Recycling of Plastic
It is hard to overstate the importance of plastic recycling. They become mixed with other chemicals or materials if they are not recycled at the appropriate time, making recycling more difficult and causing pollution.
They are non-biodegradable and are not degraded by microbial action.
To circumvent this, biopolymers or biodegradable polymers must be used.
FAQs on Properties of Plastics
1. What are plastics and how are they made?
Plastics are a type of synthetic polymer, which means they are very large molecules made by linking together many smaller, repeating units called monomers. This process of joining monomers into long chains is called polymerisation. Most plastics are derived from organic materials like crude oil and natural gas.
2. What is the main difference between thermoplastics and thermosetting plastics?
The key difference lies in their response to heat after being formed.
- Thermoplastics can be repeatedly softened by heating and hardened by cooling, allowing them to be remoulded. Examples include polyethylene (PE) and polyvinyl chloride (PVC).
- Thermosetting plastics undergo a permanent chemical change when heated and set. They cannot be remelted or reshaped; reheating will cause them to char or degrade. Examples include Bakelite and epoxy resins.
3. What are the key general properties that make plastics so useful?
Plastics are highly valued for a combination of useful properties:
- Lightweight: They have a high strength-to-weight ratio.
- Durability: They are resistant to corrosion, chemicals, and water.
- Insulation: They are poor conductors of both electricity and heat, making them excellent insulators.
- Mouldability: They can be easily moulded into complex shapes and sizes at a low cost.
- Versatility: They can be made rigid or flexible, and opaque or transparent.
4. Can you provide examples of common plastics and their everyday uses?
Certainly. Here are a few common plastics and their applications:
- Polyethylene (PE): Used for making plastic bags, packaging films, bottles, and toys.
- Polyvinyl Chloride (PVC): Used for pipes, window frames, flooring, and electrical cable insulation.
- Polypropylene (PP): Used for containers, car parts, carpets, and laboratory equipment.
- Polystyrene (PS): Used for disposable cups, insulation, and protective packaging foam.
5. Why are plastics considered excellent electrical and thermal insulators?
Plastics are excellent insulators due to their molecular structure. The electrons within their molecules are tightly held in covalent bonds and are not free to move around. Since the flow of electricity and the efficient conduction of heat both rely on the movement of free particles (like electrons in metals), the lack of these mobile electrons in plastics makes them highly resistant to conducting both electric current and heat.
6. If plastics are so durable, why is their non-biodegradable nature a major environmental problem?
The very chemical stability that makes plastic durable is precisely why it is an environmental hazard. The term 'non-biodegradable' means that microorganisms like bacteria and fungi cannot break the plastic down into simpler, natural compounds. As a result, once discarded, plastics persist in the environment for hundreds or thousands of years, accumulating in landfills and oceans, causing long-term pollution and harming wildlife.
7. What are plastic additives and why are they necessary in manufacturing?
Plastic additives are substances mixed into the base polymer to improve its properties or reduce costs. They are necessary because the raw polymer might not have all the desired characteristics for a specific application. For example, plasticisers are added to increase flexibility (like in PVC cables), stabilisers are added to protect against degradation from UV light or heat, and colourants provide specific colours.
8. How can two items made from the same plastic, like polyethylene, have very different properties?
This is due to differences in the polymer's molecular structure. Even with the same monomer (ethylene), the arrangement of the polymer chains can vary significantly:
- High-Density Polyethylene (HDPE) has linear, unbranched chains that pack together tightly. This makes the material rigid and strong, ideal for milk jugs and pipes.
- Low-Density Polyethylene (LDPE) has branched chains that prevent tight packing. This results in a more flexible and less dense material, used for plastic bags and films.
