Spectroscopy is an important technique used in Chemistry Topics to analyze the interaction of matter with electromagnetic radiation.
Classification and the Characteristics of Synthetic Fibres
A very thin, thread-like strand from which cloth is made, is called a fibre. Fabric means cloth. Fabric is made by weaving or knitting long, twisted threads called ‘yarn’ made from fibres. The clothes which we wear are made of fabrics. Fabrics are made from fibres obtained from ‘natural’ or ‘artificial’ sources (synthetic sources). Thus, all the fibres can be divided into two groups :
- Natural fibres, and
- Synthetic fibres.
The fibres obtained from plants and animals are called natural fibres. Cotton, flax, jute, wool and silk are natural fibres. Cotton, flax and jute fibres come from plants whereas wool and silk come from animals.
The synthetic fibres are made by human beings. Rayon, nylon, polyester and acrylic are synthetic fibres. We have studied the natural fibres in Classes VI and VII. In this Class we will study synthetic fibres. Before we go further and discuss synthetic fibres in detail, we should know the meaning of the term ‘polymer’. This is described below.
A polymer is a ‘very big molecule’ formed by the combination of a large number of small molecules. The small molecules (of chemical compounds) which join together to form a polymer are called ‘monomers’. The monomers which make a polymer may all be of the ‘same compound’ or of ‘two different compounds’ (see Figures). The word ‘polymer’ comes from two Greek words ‘poly’ meaning ‘many’ and ‘mer’ meaning ‘units’. So, a polymer is made of many small ‘repeating units’ (of chemical compounds) called monomers.
Polymers are of two types : Natural polymers and Synthetic polymers. Cotton, wool and silk are natural polymers. For example, cotton fibre is made of a natural polymer called cellulose. Cellulose is a polymer which is made up of a large number of small glucose molecules (or glucose units) joined one after the other (see Figure). The walls of all the plant cells are made up of cellulose. So, wood contains a large amount of cellulose polymer. Thus, polymers occur in nature too. Nylon, polyester, acrylic, polythene, polyvinyl chloride (PVC),
bakelite, and melamine are synthetic polymers (or man-made polymers). For example, nylon fibre is made up of nylon polymer in which two different types of molecules (or monomer units) are combined alternately to form long chains (see Figure). Please note that the term ‘synthetic’ means made by humans in an industrial process (and not occurring naturally). We will now discuss synthetic fibres.
The man-made fibres produced from chemical substances are called synthetic fibres. Synthetic fibres are made in industry by the chemical process called ‘polymerisation’. A synthetic fibre is a long chain of small units joined together. Each small unit is a chemical compound (called organic compound). Many, many such small units join together one after the other to form a very large single unit called polymer. It is this man-made polymer which forms synthetic fibres. Thus, a synthetic fibre is a polymer made from the molecules of a monomer (or sometimes two monomers) joined together to form very long chains (see Figure). Synthetic fibres are also known as man-made fibres or artificial fibres.
Types of Synthetic Fibres
Depending upon the type of chemicals used for manufacturing synthetic fibres, there are four major types of synthetic fibres (or man-made fibres). These are :
- Polyester, and
Rayon is a man-made fibre made from a natural material called cellulose (obtained from wood pulp). Nylon, polyester and acrylic are fully synthetic fibres which do not require a natural material (like cellulose) for their manufacture. These fully synthetic fibres are prepared by a number of processes by using raw materials (or chemical compounds) of petroleum origin, called petrochemicals. We will now study all these synthetic fibres, one by one. Let us start with rayon. Before we do that, we should know the meaning of the term ‘wood pulp’. Wood pulp is a soft, wet mass of fibres obtained from wood. Wood pulp contains a large amount of natural polymer called ‘cellulose’.
We have studied in Class VII that silk is a natural fibre obtained from silkworms. The fabric (or cloth) made from natural silk fibres is very costly. But the beautiful texture (feel, appearance, shine) of natural silk fabrics fascinated everyone. So, attempts were made to make silk artificially which would be cheaper than natural silk. Towards the end of 19th century, scientists were successful in obtaining fibres having properties similar to that of silk. This fibre was called rayon. Rayon is often regarded as artificial silk.
Rayon is a man-made fibre prepared from a natural raw material (called cellulose) by chemical treatment.
The cellulose required for making rayon is obtained from ‘wood pulp’. So, we can also say that rayon is obtained by the chemical treatment of wood pulp (which contains cellulose). Rayon is produced as follows:
- Wood pulp is dissolved in an alkaline solution (sodium hydroxide solution) to form a sticky liquid called ‘viscose’.
- Viscose is forced to pass through the tiny holes of a metal cylinder (called spinneret) into a solution of sulphuric acid when a silk like thread of rayon is formed.
Since rayon is made from naturally occurring polymer (cellulose) present in wood pulp, therefore, rayon is neither a fully synthetic fibre nor a fully natural fibre. It is a semi-synthetic fibre. Rayon is different from truly synthetic fibres because it is obtained from a natural material (wood pulp). Although rayon is obtained from a natural resource called wood pulp, yet it is said to be a man-made fibre. This is because it is obtained by the chemical treatment of wood pulp in factories. Rayon fibre is chemically identical to cotton but it has shine like silk. Since rayon resembles silk in appearance, therefore, rayon is also called artificial silk. Rayon is cheaper than natural silk and can be woven like silk fibres. Rayon can also be dyed in a variety of colours.
Uses of Rayon
- Rayon is used in textile industry for making clothing like sarees, blouses, dresses, socks, etc.
- Rayon (mixed with cotton) is used to make furnishings such as bed-sheets, curtains, blankets, etc.
- Rayon (mixed with wool) is used to make carpets.
- Rayon is used in medical field for making bandages and surgical dressings.
- Rayon is used in tyre industry for the manufacture of tyre cord.
Nylon is a synthetic fibre. In fact, nylon is the first fully synthetic fibre made by man without using any natural raw materials (from plants or animals). It was made in the year 1931. The chemical compounds (or monomers) used in making nylon are now obtained from petroleum products called petrochemicals. Actually, nylon is made up of the repeating units of a chemical called an ‘amide’. So, nylon is a polyamide (which is a polymer). The name NYLON comes from the fact that it was developed in New York (NY) and London (LON). Nylon is a thermoplastic polymer (which can be melted by heating). Molten nylon is forced through the tiny holes in a spinneret to make nylon fibres (or nylon threads), or cast into desired shapes.
Some of the important properties of nylon fibres are as follows : Nylon fibres are very strong, fairly elastic, lightweight and lustrous. Nylon fibres absorb very little water, so clothes made of nylon are easy to wash and dry. Nylon is wrinkle resistant. Nylon fibres have high abrasion resistance (high wear and tear resistance), so they are very durable (long lasting). Nylon is not attacked by moths and ordinary chemicals. Due to all these properties, nylon fibres have become very popular for making clothes.
Uses of Nylon
- Nylon is used for making textiles (fabrics) like sarees, shirts, neck-ties, tights, socks and other garments.
- Nylon is used in making curtains, sleeping bags and tents.
- Nylon is used in making ropes, car seat-belts, fishing nets, tyre cord, strings for sports rackets and musical instruments, bristles for toothbrushes and paint brushes. Nylon is used for making parachutes and ropes for rock climbing. All these uses of nylon are due to the high strength of nylon fibres. A nylon thread is actually stronger than a steel wire of similar thickness.
Polyester is another synthetic fibre. Actually, polyester is the general name of synthetic fibres which contain many ester groups. Polyester (poly + ester) is made up of the repeating units of a chemical called an ‘ester’ (Esters are the organic chemical substances which give fruits their sweet smell). We can now say that polyester is a synthetic fibre in which the polymer units are linked by ester groups. Terylene is a popular polyester fibre. The chemical compounds (or monomers) used in making polyester fibres are made from petroleum products called petrochemicals. Like nylon, polyester is also a thermoplastic polymer. When molten polyester is forced through the tiny holes of a spinneret, then thin polyester fibres (or polyester threads) are formed. The polyester yarn can be woven to make fabrics.
Most of the properties of polyester fibres (like terylene) are similar to those of nylon. Polyester fibres are, however, stronger than nylon fibres. Polyester fibres are also softer than nylon fibres. Since polyester fabric is strong, wrinkle resistant, easy to wash and dry, not attacked by moths and ordinary chemicals, and has high abrasion resistance, it is quite suitable for making dress materials. This is why we see many people around us wearing polyester shirts and other dresses. Sometimes, natural fibres (such as cotton or wool) are mixed with polyester (or terylene) to make blended fabrics. Blended fabrics are sold by the names like polycot (or terrycot) and polywool (or terrywool), etc. As the name suggests, these fabrics are made by mixing (or blending) two types of fibres. For example, polycot is a mixture of polyester and cotton. Similarly, polywool is a mixture of polyester and wool.
Uses of Polyester Fibres
- The most important use of polyester (like terylene) is in making fabrics for sarees, dress materials and curtains. Polyester mixed with cotton (called polycot or terrycot) is used for making shirts, trousers and other dresses. Polyester mixed with wool (called polywool or terrywool) is used for making suits,
- Polyester is used for making sails of sail-boats. The polyester sails are light, strong, do not stretch and do not rot in contact with water.
- Polyester is used for making water hoses for fire-fighting operations.
- Polyester is used for making conveyer belts.
From the above discussion we conclude that synthetic fibres have become very popular. The synthetic fibres have become very popular because they are strong and elastic, and have low water absorption. Synthetic fibres are lightweight, long lasting and extremely fine. They are wrinkle resistant, chemically unreactive and not attacked by moths or common chemicals. Due to these properties, synthetic fibres are much more superior to natural fibres like cotton, wool and silk.
PET is a very familiar form of polyester. PET is the abbreviation of the synthetic polymer called ‘Poly-Ethylene Terephthalate’. PET can be made into a fibre or a plastic. In discussing synthetic fibres, PET is generally referred to as ‘polyester’ while the term PET is usually used for the plastic form. PET as a plastic is very lightweight. It is naturally colourless with high transparency. PET is strong and impact- resistant. As a plastic, PET is replacing materials like glass. Unlike glass, PET is shatterproof. PET is used for making bottles, jars and utensils. For example, PET bottles are used for fizzy drinks and PET jars are used for storing sugar, salt, spices and rice, etc., in our homes (see Figure). PET is also used for making thin films Figure. All these containers are made of PET. and many other useful products. It is clear that polyester fibres and PET bottles and jars are made of the same material.
Acrylic is a synthetic fibre. Acrylic fibre is made from a chemical called ‘acrylonitrile’ by the process of polymerisation. Acrylic is lightweight, soft and warm with a wool-like feel. Acrylic retains its shape, resists shrinkage and wrinkles. It can be dyed very well. Acrylic fibres are strong and durable. Acrylic absorbs very little water so it has ‘quick-dry’ quality. Acrylic fibres are resistant to moths and most chemicals.
Due to its wool-like feel, acrylic fibre is often used as a substitute for wool. The wool obtained from natural sources (like sheep) is quite expensive. Acrylic offers a less expensive alternative to natural wool. So, the clothes made from acrylic are relatively cheaper but more durable than those made from natural wool. Many of the sweaters which the people wear in winter, and the shawls and blankets which people use, are actually not made from natural wool, though they appear to be made from wool. They are made from synthetic fibre called acrylic. Acrylic fibre is used for making sweaters, shawls, blankets, jackets, sportswear, socks, furnishing fabrics, carpets and as lining for boots and gloves.
Characteristics of Synthetic Fibres
Synthetic fibres have unique characteristics which make them popular dress materials. The important characteristics (or properties) of synthetic fibres are given below:
1. Synthetic fibres are very strong. On the other hand, natural fibres like cotton, wool and silk have low strength.
2. Synthetic fibres are more durable. Synthetic fibres have high resistance to abrasion (wear and tear). Due to this, the clothes made of synthetic fibres are very durable (long lasting). On the other hand, natural fibres like cotton, wool and silk have low abrasion resistance due to which the clothes made of natural fibres are not much durable. They do not last long.
3. Synthetic fibres absorb very little water. Due to this, the clothes made of synthetic fibres dry up quickly. On the other hand, natural fibres like cotton, wool, and silk absorb a lot of water. So, the clothes made of natural fibres do not dry up quickly.
4. Synthetic fibres are wrinkle resistant. Due to this, the clothes made of synthetic fibres do not get crumpled easily during washing or wear. They keep permanent creases. On the other hand, natural fibres like cotton, wool and silk are not wrinkle resistant. So, the clothes made of natural fibres get crumpled easily during washing and wear.
5. Synthetic fibres are quite lightweight. On the other hand, natural fibres are comparatively heavy.
6. Synthetic fibres are extremely fine. So, the fabrics made from synthetic fibres have a very smooth texture. On the other hand, natural fibres are not so fine. Due to this, the fabrics made from natural fibres do not have a very smooth texture.
7. Synthetic fibres are not attacked by moths. On the other hand, natural fibres are damaged by moths.
8. Synthetic fibres do not shrink. So, the clothes made of synthetic fibres retain their original size even after washing. On the other hand, natural fibres shrink after washing.
9. Synthetic fibres are less expensive and readily available as compared to natural fibres.
10. Clothes made from synthetic fibres are easier to maintain as compared to those made from natural fibres.
We will now describe an activity to compare the strengths of synthetic fibres with those of natural fibres. For this activity, we require threads of natural fibres (such as wool, cotton and silk) and of synthetic fibres (like nylon and polyester) which are of nearly the same thickness and of the same length.
Take an iron stand with a clamp. Take a woollen thread of about 50 cm length. Tie one end of the woollen thread to the clamp so that it hangs freely. Tie a pan to the lower end of the woollen thread (as shown in Figure 5) so that weights can be placed in it. Put a weight in the pan. Go on adding more and more weights in the pan till the woollen thread breaks. Note down the total weight required to break the woollen thread.
This weight indicates the strength of the woollen thread. Repeat this activity by using similar threads of cotton, silk, nylon and polyester. Note down the weights required to break all these threads, one by one. We will find that:
- Minimum weight is required to break the woollen thread showing that woollen thread has the minimum strength.
- More weight is required to break the cotton thread showing that the strength of cotton thread is greater than that of woollen thread.
- Still more weight is needed to break the silk thread, indicating that silk thread is stronger than the cotton thread.
- Much more weight is required to break the nylon thread showing that nylon thread has a greater strength than the silk thread.
- Maximum weight is needed to break the polyester thread indicating that the polyester thread is even stronger than the nylon thread.
Based on this activity, we can now arrange the natural fibres and synthetic fibres in the order of their increasing strengths as : Wool, Cotton, Silk, Nylon, Polyester. This activity tells us that the synthetic fibres (like nylon and polyester) are stronger than the natural fibres (like wool, cotton and silk).
We will now describe an activity to demonstrate the difference in the ‘water absorption’ property of synthetic fibres and natural fibres.
(i) Take a piece of cloth made of a synthetic fibre (like polyester cloth) of about half a square metre size. Also take another piece of cloth made of a natural fibre (like cotton cloth) of exactly the same size.
(ii) Take two mugs and put 500 mL of water in each mug. Soak the piece of polyester cloth in water taken in one mug for about 5 minutes. Soak the piece of cotton cloth in water taken in the other mug for an equal time.
(iii) Take out the soaked pieces of polyester cloth and cotton cloth from the two mugs and compare the volume of water which remains behind in each mug. We will find that more water is left behind in the mug in which polyester cloth was soaked. This shows that polyester cloth absorbs less water. Much less water is left behind in the mug in which cotton cloth was soaked, showing that cotton cloth absorbs much more water. From these observations we conclude that synthetic fibres (like polyester) absorb much less water than natural fibres (like cotton).
(iv) Spread the wet piece of polyester cloth and the wet piece of cotton cloth in sunshine so as to dry them. We will find that the wet piece of polyester cloth dries up rapidly but the wet piece of cotton cloth takes much longer time to get dried. From these observations we conclude that the wet synthetic fibres (like polyester) dry up quickly but the wet natural fibres (like cotton) do not dry up quickly. So far we have described the advantages of synthetic fibres. We will now describe some disadvantages of synthetic fibres (over the natural fibres).
A disadvantage of synthetic fibres is that they melt on heating. If a person is wearing clothes made of synthetic fibres and his clothes catch fire accidently, then the synthetic fibres of clothes melt and stick to the body of the person causing severe burns. This can be disastrous for the person concerned. We should, therefore, not wear synthetic clothes (made of nylon, polyester, etc.) while working in the kitchen or in a science laboratory. The natural fibres (like cotton, wool, etc.) do not melt on heating. So, it is quite safe to wear clothes made of natural fibres while working in the kitchen or in a science laboratory.
Another disadvantage of synthetic fibres is that the clothes made of synthetic fibres are not suitable for wearing during hot summer weather. This can be explained as follows : Synthetic fibres are extremely fine, so the clothes made of synthetic fibres do not have sufficient pores for the sweat to come out, evaporate and cool our body. Due to this, clothes made of synthetic fibres make us feel hot and uncomfortable during summer. Clothes made of natural fibres (like cotton) are more comfortable during summer. This is because the large pores of cotton clothes allow the body sweat to come out through them, evaporate and make us feel cool and comfortable. So, if we want to buy shirts for summer, we should buy cotton shirts and not the shirts made from synthetic materials (like polyester).
The manufacturing of fully synthetic fibres (like nylon, polyester and acrylic, etc.) is helping in the conservation of forests. This is because the fully synthetic fibres are manufactured from petrochemicals (obtained from crude oil ‘petroleum’), so no trees have to be cut down for making them. On the other hand, semi-synthetic fibres like rayon are made from wood pulp which requires cutting down of forest trees.