- 1 History and Characteristics of Plastics
- 1.1 Types of Plastics
- 1.2 1. Thermoplastics
- 1.3 2. Thermosetting Plastics
- 1.4 Thermoplastics and Thermosetting Plastics Differ in Structure
- 1.5 Useful Properties Of Plastics
- 1.6 (i) Plastic is Chemically Unreactive
- 1.7 (ii) Plastics are Poor Conductors of Heat and Electricity
- 1.8 (iii) Plastics Can be Moulded into Different Shapes
- 1.9 (iv) Plastics are Quite Cheap and Easily Made
- 1.10 (v) Plastic is Light, Strong and Durable
The study of reaction mechanisms is crucial in Chemistry Topics as it provides insights into the step-by-step pathways of chemical transformations.
History and Characteristics of Plastics
We will now discuss plastics.
We use a large number of articles (or things) made of plastics in our everyday life. Some of the articles made of plastics which are used by us in our everyday life are plastic bags (polythene bags), water bottles, buckets, mugs, water tanks, water pipes, ballpoint pens, combs, toothbrushes, toys, shoes, tea-strainers,
cups, plates, chairs, tables, insulation of electric wires, covers of electric switches, plugs, sockets and bulb- holders, etc. (see Figure). Some of the parts of radio, television, refrigerator, cars, buses, trucks, scooters, trains, aeroplanes, ships and spacecrafts are also made of various types of plastics. The list of things made of plastics and used in homes, transport and industry is endless. Plastic articles are available in all possible shapes and sizes.
A plastic is a synthetic material which can be moulded (or set) into desired shape when soft and then hardened to produce a durable article (the term ‘plastic’ means ‘easy to mould’). Like synthetic fibres, plastics are also polymers. This means that plastics consist of very long molecules made by joining many small molecules together. The starting materials for plastics are obtained from petroleum products called ‘petrochemicals’. Some of the examples of plastics are : Polythene, Poly-Vinyl Chloride (PVC), Bakelite, Melamine and Teflon. Nylon is also a plastic. The major properties and uses of some of the plastics are given below.
Polythene (poly + ethene = polythene). Polythene is a plastic obtained by the polymerisation of a chemical compound known as ethene. Polythene is tough and durable. Polythene is used in making polythene bags (plastic bags), waterproof plastic sheets, bottles, buckets and dustbins. Polythene is also used for packaging.
Polyvinyl chloride (commonly known as PVC) is a strong and hard plastic. It is not as flexible as polythene. PVC is used for making insulation for electric wires, pipes, garden hoses, raincoats, seat covers, etc.
Bakelite is a very hard and tough plastic. Bakelite is a poor conductor of heat and electricity. Bakelite is used for making the handles of various cooking utensils (such as frying pans and pressure cookers, etc.). Bakelite is used for making handles of cooking utensils because
- it is a poor conductor of heat, and
- it does not become soft on getting heated (It is a thermosetting plastic).
Bakelite is also used for making electrical fittings such as electric switches, plugs and sockets, etc. Bakelite is used for making electric switches, plugs and sockets, etc., because
- it does not conduct electricity, and
- it does not become soft on getting heated.
Melamine is a plastic which can tolerate heat better than other plastics and resists fire. Melamine is used for making floor tiles, unbreakable kitchenware (cups, plates, etc.), ashtrays and fire-resistant fabrics. Melamine is a fire-resistant plastic. The uniforms of fire-men have a coating of melamine plastic to make them fire-resistant. Special plastic cookware made of melamine is used in microwave ovens for cooking food. In microwave ovens, the heat cooks the food but does not affect the plastic vessel.
Teflon is a special plastic on which oil and water do not stick. Oil and water do not stick on teflon plastic because it has a slippery surface. Teflon also withstands high temperatures. Teflon is used for giving non-stick coating on cookwares (like non-stick frying pans). Teflon is also used for making soles (bottoms) of electric irons.
Plastics are used extensively in the health care industry. For example, plastics are used in the packing of tablets, for making syringes, doctors gloves, threads for stitching wounds, and a number of medical instruments.
Types of Plastics
Plastics are of two types : Thermoplastics and Thermosetting plastics. We will now discuss these two types of plastics in detail, one by one. Let us start with thermoplastics.
Some plastics get soft (or melt) when heated, and hard again when they are cooled. Such plastics can be made soft and hard again and again. A plastic which can be softened repeatedly by heating and can be moulded into different shapes again and again, is called a thermoplastic. Thermoplastics are flexible so they can be bent easily (without breaking). Thermoplastics are also known as ‘thermosoftening’ plastics. Some of the examples of thermoplastics are : Polythene and Polyvinyl chloride (PVC).
If we take a plastic bottle (polythene bottle) and add quite hot water in it, the plastic bottle gets deformed— its shape changes and becomes irregular. This happens because the bottle is made of a thermoplastic (like polythene) which becomes soft on getting heated by hot water and changes shape. This activity shows that the articles made of thermoplastics become soft on heating. Let us now take a plastic bottle and press it by applying the force of our hands. We will find that the plastic bottle bends easily. This shows that the articles made of thermoplastics bend easily. In other words, we can say that thermoplastics are flexible. In fact, thermoplastics are used for making those articles which do not get too hot, and are flexible. Thermoplastics are used for making insulation of electric wires and cables, various types of plastic containers (plastic bottles, plastic jars, etc.), combs, toys, plastic bags, raincoats, seat covers, bristles of
brushes, packaging materials and chairs. Thermoplastics are used for making the insulation of electric wires because
- they do not conduct electricity, and
- they are flexible. Some of the articles (or objects) made of thermoplastics are shown in Figure.
2. Thermosetting Plastics
There are some plastics which get soft only once—the first time they are heated after being made. When such plastics are heated for the first time, they become soft (or melt) and can be moulded to make an article of any desired shape. On cooling, this article becomes very hard and rigid. When this plastic article is heated again, it does not become soft at all. A plastic which once set, does not become soft on heating and cannot be moulded a second time, is called a thermosetting plastic.
Once set in a given shape and solidified, a thermosetting plastic cannot be re-softened or re-moulded. Thus, an article (or object) made of thermosetting plastic will retain its original shape permanently, even on heating. Thermosetting plastics are also known as thermosets. Some of the examples of thermosetting plastics are : Bakelite and Melamine. Thermosetting plastics are hard and rigid. Thermosetting plastics are not flexible. Due to this, thermosetting plastics cannot bend. When an article made of thermosetting plastic is forced to bend, it breaks.
If we take a discarded electric switch and put it in hot water for some time, we will find that the electric switch does not become soft. This is because an electric switch is made of a thermosetting plastic (called bakelite). This activity shows that thermosetting plastics do not become soft on heating. If we try to bend an electric switch by applying the force of our hands, we find that it does not bend at all. This shows that the articles made of thermosetting plastics are hard and rigid. The articles made of thermosetting plastics are not flexible. They do not bend at all. Thermosetting plastics are used for making those articles which may get too hot during use and are hard and rigid (so that they do not bend at all). Thermosetting plastics are used for making handles of cooking utensils (such as frying pans, pressure cookers, etc.), plates, cups,
floor tiles, electrical fittings (such as electric switches, plugs and sockets), ballpoint pens and telephone instruments. Some of the articles made of thermosetting plastics are shown in Figure.
Thermosetting plastics are used for making the handles of cooking utensils (such as frying pan handles and pressure cooker handles) because
- they do not soften on getting heated, and
- they are poor conductors of heat.
Thermosetting plastics are used for making electrical fittings such as electric switches, plugs and sockets, etc., because
(i) they do not become soft on getting heated, and
(ii) they do not conduct electricity. Please note that the handle and bristles of a toothbrush cannot be made of the same plastic because the handle of a toothbrush has to be hard and rigid whereas the bristles of a toothbrush have to be soft and flexible. This means that the handle of a toothbrush should be made of a thermosetting plastic whereas its bristles should be made of a thermoplastic.
We can tell whether a given plastic is thermoplastic or thermosetting plastic from the way it behaves on heating. If a given plastic article softens on heating, then it will be a thermoplastic. On the other hand, if the given plastic article does not become soft on heating, then it will be a thermosetting plastic. Moreover, thermoplastics are flexible and can be bent whereas thermosetting plastics are very hard and rigid which cannot be bent at all. Please note that the articles made of thermoplastics can be recycled whereas the articles made of thermosetting plastics cannot be recycled.
Thermoplastics and Thermosetting Plastics Differ in Structure
We will now discuss why thermoplastics can be softened by heat but thermosetting plastics cannot be softened by heat. This is due to the difference in their structure. It can be explained as follows : Both, thermoplastics and thermosetting plastics are made up of long chain molecules called polymers. In thermoplastics, the long polymer chains are not cross-linked with one another [see Figure (a)). Due to this, on heating, the individual polymer chains can slide over one another and thermoplastic material becomes soft and ultimately melts. On the other hand, in thermosetting plastics, the long polymer chains are cross-linked with one another [see Figure (b)]. These cross-links prevent the displacement (or sliding)
of individual polymer chains on being heated. Due to this, thermosetting plastics do not become soft on heating (or change their shape on heating) once they have been set into a particular shape. For example, polythene is a thermoplastic having linear polymer chains with no cross-linkages, so it becomes soft on heating. On the other hand, bakelite is a thermosetting plastic having long polymer chains connected through cross-links (or held strongly through cross-links), due to which it does not become soft on heating.
Useful Properties Of Plastics
Plastics have many useful properties which make them materials of choice for all sorts of uses. Due to these special properties, plastics have many advantages over the traditional materials like metals, wood, etc., for making various articles. The important properties of plastics which make them very useful materials are given below.
(i) Plastic is Chemically Unreactive
We know that metals like iron get rusted (or corroded) when left exposed to air and water (moisture). This is because metals are chemically reactive. Plastics are chemically unreactive. Plastics do not react with air and water. Due to this, plastics are resistant to corrosion. In other words, plastics are not affected by the weather. Plastics are also often unaffected by various chemicals (including acids and bases). Since plastics are unreactive and resist corrosion, the plastic containers are used to store various kinds of materials, including many chemicals.
(ii) Plastics are Poor Conductors of Heat and Electricity
Plastics do not conduct heat or electricity, so they can be used as ‘insulators’. Plastics, being poor conductors of heat, are used where heat is to be kept away from reaching our hands. For example, the handles of cooking utensils (like frying pans and pressure cookers) are made of plastic so that we can hold the hot cooking utensil safely (without getting our hands burnt). Since plastics are poor conductors of electricity, they are used as electrical insulators. For example, electric wires have plastic covering as insulation so as to protect us from electric current passing through them. The handles of screw drivers used by electricians are also made of plastic because it is an electrical insulator. Electric switches, plugs and sockets also have plastic covers.
(iii) Plastics Can be Moulded into Different Shapes
Since plastics can be easily moulded, they are used to make a large variety of articles (or objects) having different shapes and sizes such as buckets, mugs, furniture (chairs, tables, etc.), bags, sheets, slippers, electrical fittings, toys, combs, toothbrushes, etc. The list is endless.
(iv) Plastics are Quite Cheap and Easily Made
Plastics are generally cheaper than metals. Plastics can also be made much more easily than metals. Due to these properties, plastics are now widely used for making many of the household and industrial articles which were earlier made from metals. For example, the buckets used in our homes were earlier made from iron metal sheet but these days most of the buckets are made of plastics.
(v) Plastic is Light, Strong and Durable
Plastics have low density, so they are lighter than metals. Plastics also have good strength and they are durable (long lasting). It is because of the lower price, easy availability, lightweight, good strength, durability and corrosion-resistance of plastic that the plastic containers are preferred for storing food, water, milk, jams, juices, pickles, squashes and soft drinks, etc. Being lighter than metals, plastics are also used in cars, aircrafts and spacecrafts.