LESSON 1 PREPARATORY PROCESS FOR YARN AND

FABRICS

STRUCTURE

1.0 OBJECTIVES

1.1 INTRODUCTION

1.2 NECESSITY TO REMOVE IMPURITIES

1.3 BASIC PRINCIPLE OF PREPARATORY PROCESSES

1.4 THE CLEANING PROCESSES FOR COTTON YARNS AND FABRICS

1.4.1 DESIZING 1.4.2 SCOURING 1.4.3 BLEACHING

1.5 CLEANING OF PRODUCTS MADE FROM FIBRES OTHER THAN

COTTON

1.6 ASSIGNMENTS

1.6.1 CLASS ASSIGNMENTS 1.6.2 HOME ASSIGNMENTS

1.7 SUMMING UP

1.8 POSSIBLE ANSWERS TO SELF-CHECK QUESTIONS

1.9 TERMINAL QUESTIONS

1.10 REFERENCES AND SUGGESTED FURTHER READING

1.11 GLOSSARY

1. PREPARATORY PROCESSES FOR YARNS AND

FABRICS

Textile substrates contain impurities in one form or another which need to be removed before subjecting them to any chemical processing operation like dyeing. For removing these contaminations, certain pre-treatments like desizing, scouring and bleaching are required which would be covered in this lesson. The two subsequent lessons of this unit will deal with dyes and dyeing.

1.0 Objectives

After going through this lesson, you will be able to know about the;

• Necessity to remove impurities;

• Basic principle of preparatory processes;

• Cleaning processes for cotton yarn and fabrics; and

• Bleaching of products made from fibres other than cotton.

1.1 Introduction

Textile substrates contain impurities which must be removed before they are subjected to chemical processing operations like dyeing, printing etc. Once these impurities are removed; the ability of the substrate to absorb the colouring matter improves. In addition, some of these processes may make the material aesthetically more acceptable. The three important preparatory processes are desizing, scouring and bleaching.

The role of bleaching (Fig. 1.1) is of paramount importance for cotton fibre since it removes the natural creamy colour of cotton and makes it white. Therefore the emphasis in this lesson will be on cotton. However other fibres will be briefly considered:

Raw cotton contains various impurities. The natural impurities include waxes, proteins, pectic substances, colouring matter and residual seeds. In addition to this, during the process of conversion of

Fig. 1.1 Bleaching of fabric

fibres to yarn and yarn to fabrics, it gets contaminated with various added impurities.

The added impurities include lubricants, spin finishes, traces of metals and the sizing ingredients (mainly starch). These impurities must be removed. The scouring and bleaching of yarns and fabrics are essential preparatory processes before they are dyed or printed. Also if a size is applied to the yarn or fabric during mechanical processing, it must be removed by desizing before scouring and bleaching.

1.2 Necessity to Remove Impurities

The removal of natural waxes is essential to develop good water absorbency for uniform dyeing and printing. The removal of impurities (Fig. 1.2) like pectin, proteins and mineral matters is required to improve the dyeability and visual appearance.

In addition to these the intrinsic colour of cotton is creamish due to the presence of natural colouring matter. Its removal enhances the whiteness of cotton considerably. The removal of residual seeds is also essential; if they are not removed, tiny black spots would be visible over the fabric. The size that is present in woven fabric must also be removed as it interferes with the effectiveness of the process used for the removal of natural impurities. Therefore desizing is the first preparatory process for woven fabrics. The other important preparatory processes are scouring and bleaching.

Self-Check Questions

1. What kind of natural impurities do the raw cotton fibres contain? List them.

2. Why is the removal of natural waxes necessary?

3. Why is it essential to remove the residual seeds?

4. Write the name of added impurities.

Fig. 1.2 Removal of impurities manually

1.3 Basic Principle of Preparatory Processes

Both natural and added impurities are insoluble in water. Preparatory processes are designed to convert them into water soluble form. Since the chemical nature of these impurities differs from each other, the chemicals and process parameters to render them water soluble also differ.

Desizing involves the removal of size which is primarily starch. Enzymatic treatment is needed to remove it. Scouring removes oils, fats, waxes, pectic substances and proteins by treatment with alkalis like caustic soda. Bleaching removes the colouring matter and also seed residues by treatment with an oxidizing agent like bleaching powder (calcium hypochlorite) or hydrogen peroxide (H2O2).

1.4 The Cleaning Processes for Cotton Yarns and Fabrics

The as-obtained cotton yarn with a natural creamish colour is called grey yarn and the cloth woven from it is called grey fabric. Sometimes the yarn is first dyed and then the cloth is woven using the coloured yarn. In the latter case, before dyeing, the yarn is subjected to scouring and bleaching while in the former case, before dyeing, the fabric is subjected to desizing, scouring and bleaching.

1.4.1 Desizing

If the fabric is woven from sized yarn, desizing (Fig.1.3) is essential before subjecting it to other treatments. For this, the fabric must be soaked in 0.5% aqueous solution of amylase enzyme for 8 hours ensuring that it is completely immersed in the solution. After the size has been removed, the fabric is subjected to a hot and cold water wash.

1.4.2 Scouring

As stated earlier, scouring (Fig.1.4) may be of either the yarn or of the desized fabric. For this an aqueous solution is prepared containing 2 grams per litre of Turkey red oil and 2% of caustic soda (NaOH) on the weight of the yarn or fabric. In addition small amounts of wetting agent and detergent are also added. The material is completely immersed in the solution and kept overnight. Before taking it out in the morning, it may be gently

Fig. 1.3 Desizing

Fig. 1.4 Scoured yarn

beaten with a wooden stick so that the impurities come out of the fabric. If the fabric is boiled for an hour, the cleaning is more effective. The material is then given a cold or hot wash and then sent for bleaching. When the operation is performed on a large scale, a kier is used for the scouring of Yarn in hank form at a temperature of 100º-130ºC for 4 to 6 hours.

1.4.3 Bleaching

As-grown cotton is creamy in colour. Bleaching removes the pale yellow colouring matter present in the cotton fibre and makes it white and lustrous. A paste of bleaching powder (calcium hypochlorite) (Fig. 1.5) is first made by taking 6% to 8% of bleaching powder on the weight of the dry yarn or dry fabric in a vessel with a wide mouth into which a small amount of water is poured. Wearing gloves and using rotatory motion of the hands, the whisking action leads to an aqueous paste of the bleaching powder. Then the paste is transferred to a larger vessel containing water and stirred vigorously by a wooden stick after which it is allowed to remain stationary for about 15 minutes. The froth or foam which appears at the top is removed gently with the help of a small container and what remains in the vessel appears like clear water. In fact the bleaching powder has dissolved and formed a clear solution. It may be transferred to another clean vessel in which more water can be added taking care to stir it properly. Then the material to be bleached may be immersed in the solution ensuring that no part of the yarn or fabric is outside the solution (Fig. 1.6a) and allowed to remain there for at least one hour. After taking it out, it may be washed and dried (Fig.1.6b).

The bleach solution can be reused a number of times refilling it with fresh bleach solution in between, so that the amount of chlorine in the solution is maintained as this will ensure that the degree of whiteness will be uniform.

Subsequent to bleaching, the textile material may be dipped in a vessel containing 0.25% solution of an optical brightening agent like ‘Tinopal’ this is only if the fabric is to be sold without dyeing.

Besides bleaching powder, sodium hypochlorite is also often used for bleaching. However, both these are not favored because of environmental concerns, as both

Fig. 1.5 Bleaching powder Fig. 1.6a Fabric for bleaching

Fig. 1.6b Bleached fabric

contain chlorine. Hydrogen peroxide and sodium perborate are more appropriate bleaching agents and are therefore more in use today.

Self-Check Questions

5. Which is the first preparatory process for grey woven fabrics?

6. What kind of impurities can be removed by the following processes?

• Desizing

• Scouring

• Bleaching

1.5 Cleaning of Products Made from Fibres Other than Cotton

Raw wool (Fig. 1.7) contains impurities such as grease, dirt and vegetable matter to remove which the fibres have to be thoroughly washed. A chemical treatment is sometimes necessary to eliminate burrs and other vegetable matter tangled in the fibres.

Very little natural wool is pure white, and bleaching is difficult owing to its essential sensitivity in this respect and is only done when absolutely necessary, not as a routine as with cotton. The fibre has good dye affinity and a whole range of colours can be applied with appropriate chemical control.

The mulberry silkworm spins white, yellow or yellow-green silk (Fig. 1.8), while wild silkworm spins brown to black brown silk. The enzyme-based bleaching agents tend to chlorinate the fibroin and are therefore usually not used. Hydrogen peroxide is the most preferred bleaching agent.

Synthetic fibres (Fig. 1.9) in general have a very different production history and unlike natural fibres, are produced from synthesized raw materials, resulting in fibres that are white in colour, thus they generally do not require a bleaching treatment. However, before dyeing a very mild

Fig. 1.7 Raw wool

Fig. 1.8 Raw silk

Fig. 1.9 Synthetic fibres

scouring treatment may be given to remove some added impurities like spin finishes, etc. Polyester fabrics are stabilized by heat setting before scouring. To make synthetics Ultra-white, they may be bleached.

Self-check Questions

7. What kinds of impurities are present in raw wool?

8. What is the colour of raw silk which is spun by:

• The mulberry silk worm

• The wild silk worm

Activity

1. You are provided some textile markings of scouring and bleaching treatments. Study their details and make presentation of the facts of the following:

• Natural fabric samples

• Synthetic fabric samples

1.6 Assignments

1.6.1 Class assignments

i) Visit a unit where cotton yarn and fabric are scoured and bleached.

1.6.2 Home assignments

i) Collect 5-7 samples of bleached and dyed fabrics made from (a)Natural fibres & (b) Synthetic fibres

1.7 Summing Up

The need for preparatory processes that precede dyeing process of yarns and fabrics is explained and the three important processes viz. desizing, scouring and bleaching for cotton are described. Product-wise the emphasis in this lesson is on cotton and process-wise on bleaching.

1.8 Possible answers to Self-Check Questions

1. The natural impurities that raw cotton fibres contain are waxes, proteins, pectins, colouring matter, and residual seeds etc.

2. The removal of natural waxes is essential to develop good water absorbency for uniform dyeing and printing.

3. The presence of residual seeds causes the tiny black spots visible on the fabric.

4. The added impurities include lubricants, spin finishes, traces of metals and the sizing ingredients (mainly starch).

5. Desizing.

6. Desizing involves the removal of size which is primarily starch

Scouring removes oils, fats, waxes, pectin substances and proteins

Bleaching –It removes the colouring matter and residual seeds.

7. Raw wool contains impurities such as grease, dirt and vegetable matter

8. The mulberry silk worm spins white, yellow or yellow green silk and the wild silk worm spins brown to black brown silk.

1.9 Terminal Questions

1. What is the importance of pretreatment of fabrics before dyeing?

2. Write an essay, explaining the preparatory processes.

3. Describe the Bleaching of products made from fibres other than cotton.

4. What is H2O2? What is its use?

5. Why do yarns and fabrics made from synthetic fibres not need a bleaching treatment before dyeing?

1.10 References and Suggested Further Reading

1. Chavan, R. B. (Ed.). 1999. Chemical Processing of Handloom Yarn and Fabrics. Dept of Textile Technology, I.I.T- Delhi.

2. Marsh, J. J. 1979. An Introduction to Textile Bleaching. B.I. Publications, India.

3. Trotman, E. R. 1968. Textile Scouring and Bleaching. Charles Giffen & Co, London.

1.11 Glossary

1. Calcium hypochlorite Hypochlorite of calcium; used as a bleaching agent, whitener

2. Impurities Unwanted particles

3. Insoluble A substance that does not dissolve

4. NaOH Caustic soda

5. Optical As seen with eye

6. Pretreatment The treatment done prior to any other process

7. Soluble A substance which dissolves

8. Uniform Unvarying, the same, equally distributed

9. Weaving To make a fabric by interlacing thread

10. Yarn A fine cord of twisted fibers (of cotton or silk or wool or nylon etc.) Used in sewing and weaving