38

1. INTRODUCTION

In textile wet processing the production sequence is pretreatment, dyeing, printing, and finishing. Sometimes

mercerization is done additionally for cotton goods either as pretreatment process for dyed materials or as finishing

process for white goods. Mercerization has great impact on luster, moisture regain, chemical and dye absorbency,

dimensional stability, strength of cotton goods. In this project work, mercerization was done as a finishing process

for white cotton woven fabrics and observed its impact on brightness, moisture absorbency and strength. There are

several factors are involved in mercerization which control the ultimate results of mercerization. During this

experiment, some major mercerization parameters like temperature, alkali concentration and tension were varied,

which play the vital role during mercerization. After mercerization, the brightness, strength and moisture absorbency

of the samples were measured and the results were evaluated in context to the variables. Finally the results obtained

in this project work were graphically represented and the impacts of the variables were observed.

1.1 objectives

To know the influences of mercerization parameters on mercerization effects.

To observe the different physical & chemical changes of cotton fabric after mercerization.

To determine the effect of tension, temperature and alkali conc. on moisture regain, strength, and

brightness of cotton fabric.

To analyze the different results, compare them graphically & find out some relations among them.

38

THEORETICAL PART

2. COTTON FIBRE

38

Cotton is the most important natural fibre and it accounts for about 50% of the total fibre production of the world. Cotton fibre is obtained from the seed of the botanical family Gossypium. It is a cellulosic fibre, which is actually the most pure natural form of cellulose.

2.1 Structure of cotton fibre

Microscopic examination of cotton fibres reveals that they are single cells with a closed up but open at the end where they were cut from the seed. They have the appearance of flat, twisted ribbons. This characteristic shape develops as the cotton fibres dry out and collapse in the open boll. The fibre cross-section has a bean shape and often shows the presence of a central canal or lumen.

The morphology of a cotton fibre is extremely complex. Each fibre is composed of different layers, cuticle, primary wall, secondary wall and a lumen.

The cuticle, or outer cell wall, is relatively hydrophobic. It contains some cellulose but accompanied by fats and waxes. It will be broken and more or less removed during processing to render the fibres more water absorbent.

Beneath the cuticle is the primary cell wall composed of criss-crossed fibrils of cellulose and containing some pectin.

The next layer inside this, the secondary wall, constitutes the bulk of the fibre. It is built up of successive layers of fibrils. These are long structures, in each growth layer, that spiral around the fibre in a helical manner. From time to time, the spirals reverse direction and are responsible for the characteristic convolutions of the cotton fibre that develop on first drying. The fibrils, in turn, are composed of smaller micro fibrils, the smallest being a combination of cellulose molecules.The lumen, the cavity that may remain after the protoplasm in the cell interior has evaporated, has proteins, coloring matter and minerals deposited on its walls

2.2 Chemical composition of cotton

38

Table: Chemical composition of cotton fibreComponent Main location Relative amount (%)Cellulose Secondary wall 86.8Oils, waxes Cuticle 0.7Pectin Primary wall 1.0Carbohydrates Primary wall 0.5Proteins Lumen 1.2Salts Lumen 1.0Water 6.8Other 2.0

2.3 Chemical Structure of Cotton

The chemical composition of cotton, when picked, is about 94% cellulose; in finished fabrics is it 99 percent cellulose. Cotton contains carbon, hydrogen, and oxygen with reactive hydroxyl groups. Glucose is the basic unit of the cellulose molecule. Cotton may have as many as 10,000 glucose monomers per molecule. The molecular chains are arranged in long spiral linear chains within the fiber. The strength of a fiber is directly related to chain length.

Fig: chemical structure of cellulose.

Hydrogen bonding occurs between cellulose chains in a cotton fiber. There are three hydroxyl groups that protrude from the ring formed by one oxygen and five carbon atoms. These groups are polar meaning the electrons surrounding the atoms are not evenly distributed. The hydrogen atoms of the hydroxyl group are attracted to many of the oxygen atoms of the cellulose. This attraction is called hydrogen bonding. The bonding of hydrogen's within the ordered regions of the fibrils causes the molecules to draw closer to each other which increases the strength of the fiber. Hydrogen bonding also aids in moisture absorption. Cotton ranks among the most absorbent fibers because of Hydrogen bonding which contributes to cotton's comfort. These hydrogen bonds also hold several adjacent cellulose chains in close alignment to form crystalline areas called micro fibrils. Between the crystalline regions (about 70%) in cotton, disordered amorphous regions are found. Penetration of dyes and chemicals occur more readily in these amorphous reasons.

The chemical reactivity of cellulose is related to the hydroxyl groups of the glucose unit. Moisture, dyes, and many finishes cause these groups to readily react. Chemicals like chlorine bleaches attack the oxygen atom between or within the two ring units breaking the molecular chain of the cellulose.

2.4 Important physical properties of cotton

2.4.1 Tensile strength

38

Cotton is a moderately strong fibre; tenacity is 26.5 - 44.1 cN /tex (3 – 5 g/den) and tensile strength 2800-8400kg/cm2 (40,000-120,000 lb/in2). A unique property of cotton is that it shows greater strength in the wet state than dry.

2.4.2 Elongation

Cotton does not stretch easily. It has an elongation at break of 5-10 per cent.

2.4.3 Elastic Properties

Cotton is a relatively inelastic, rigid fibre. At 2 per cent extension it has an elastic recovery of 74 percent, at 5 per cent extension, the elastic recovery is 45 per cent.

2.4.4 Specific gravity 1.54.

2.4.5 Moisture regain 8.5%

2.4.6 Luster

Cotton fibres have a natural luster which is due, in part, to the natural polish on the surface. The smooth, hard primary coat of cellulose contains waxes which no doubt contributes to the luster of the fibre. This surface-smoothness, however, is not the only factor as well; a high luster is provided by fibres of nearly circular cross-section and with fewer convolutions such as those produced when cotton is mercerized.

3. MERCERIZATION

The treatment of cellulosic textile in yarns or fabric form with a concentrated solution of caustic alkali whereby the fibers are swollen, the strength & dye affinity of the materials are increased & their handle is modified. The process takes its name from its discoverer, John Mercer (1844). The additional effect of enhancing the luster by stretching the swollen materials while wet with caustic alkali & then washing off was discovered by Horace Lowe (1889). The modern process of mercerization involves both swelling in caustic alkali & stretching to enhance the luster, to increase color yield & cotton yarn strength.

Mercerization is the process of subjecting a vegetable fiber to the action of a fairly concentrated solution of a strong base so as to produce great swelling with resultant changes in fine structure, dimensions, morphology and mechanical properties. Usually cotton goods are treated with 15-25% w/v caustic soda solution (55-65 oTw) at a temperature of 15-25oC during mercerization.

3.1 Changes occur in mercerization

Physical changes

Lengthwise shrinkage and swelling axially

38

Untwisting of fiber Fiber cross section becomes circular due to swelling Increase the tensile strength Decrease the extension at break Increase luster and smoothness of fiber

Chemical changes

Breaks the hydrogen bonds and weak Vander Waals forces Formation of soda cellulose Increase adsorption of dyestuffs and chemicals Increase hygroscopicity

3.1.1 Swelling of cotton during mercerization

Mercerization is based on the swelling action of the concentrated aqueous solutions of sodium hydroxide on cotton.

In cellulose there are three dipolar hydroxyl groups in the molecular chain lying at regular intervals along the chains on the surface of the molecule. In some places lateral hydrogen bonds are formed between the chains and in other places, the hydroxyls are non-bonded. In mercerization, cotton yarn or fabric is treated with about 24% caustic soda solution at about 18C for 30 sec- 2 min. this caustic soda solution contains dissolved sodium hydroxide as Na+ and OH- ions. When cotton is entered into caustic soda solution, Na+ and OH- ions can easily diffuse into the amorphous region of the fibre and get hydrogen bonded with the accessible hydroxyl groups of cellulose. Because of the presence of these ions in the amorphous region, the chain molecules try to vibrate with longer amplitude, when some of the hydrogen bonds and other weaker bonds between the adjacent chain molecules on the fringe of the crystalline region are ruptured and the unbound molecular fragments vibrate with still longer amplitude and further NaOH molecules diffuse and get bound to the liberated hydroxyl groups. In other word, cellulose chain molecules acquire greater degree of movement. As a result, the fiber swells laterally and shrinks longitudinally.

This swelling action depends on the concentration of caustic soda. As the concentration of alkali increases, the extent of swelling passes through a maximum and then decreases. Alkali is preferentially absorbed by the cellulose from the solution when heat is liberated. So the extent of swelling decreases with an increase in the temperature of the solution.

3.2 Types of Mercerization

3.2.1 Cold Mercerization

38

Cold mercerization process is normally carried out by treating yarn or fabric with 20-25% caustic soda solution for 30-180 sec at a temperature between 15 and 20°C after treatment, the materials is washed to remove excess caustic soda. The material, which is then in a relaxed state, is further washed and finally washed with dilute acid to remove the remaining alkali.

The mercerization conditions, i.e. concentration, temperature, dwell time in alkali, etc., are varied in accordance with the particular effect required on the processed fabric. Althrough normal mercerizing leads to an improvement in luster, tensile strength, dye absorption, coverage of dead cotton and dimensional stability.

3.2.2 Hot Mercerization

In the hot mercerizing penetration of caustic soda into the textile structure & fiber self is extremely rapid, thorough, and uniform in effect. The fiber and textile structure become more pliable and less elastic then when saturated with cold concentrated caustic solution. Shrinkage of the fabric is much less then that occurring in the cold process. If necessary the fabric can be considerably overstretched to get improved luster, tensile strength, dimensional stability.

Hot mercerizing produces better luster, high tensile strength and improved dimensional stability then cold mercerization for two main reasons. Firstly owing to thorough of the hot caustic soda into the fabric and fiber structure a far greater proportion of the cellulose is modified.

Secondly in the presence of concentrated caustic soda solution at an elevated temperature, the fabric becomes highly plastic and less elastic and so is capable of being readily stretched, leading to improvement of the properties of the fabric being considered. Extent of the change of these properties depending of the degree of stretch. For example greater than normal stretch will lower the affinity of dyes, because this is affected by the degree of internal orientation of molecular structure.

3.3 Yarn mercerization

In yarn mercerization, yarn should be singed to remove those nap hairs which are not twisted into the yarn structure & hence project from the surface. Mercerization carried out on dry or wet condition. With wet yarns, it is essential that the greater part of the water be removed evenly before mercerization.

For warp mercerization, there may be 6000 to 10,000 ends parally wound into cylindrical balls. It is important that, they must be wound at equal tension. Alkali concentration must be constant & high enough to ensure the required degree of swelling.

Wetting of cotton in grey state is not easy & this may be overcome by wetting the warp in boiling water, cooling & squeezing prior to mercerization or by wetting agent. After mercerization, yarn is squeezed & passed into warm water which de-swells cotton. Finally treatment with sulphuric acid (1-3%)is given to remove the last traces of alkali. Then it is washed & dried.

Sewing threads, embroidery threads may be mercerization in hank form which is cross wound at uniform tension & spread on to two parallel rollers of the mercerizing machine. The machine give tension by arm, rotate roller in liquor & raise –lower the yarn into & out of the bath. Yarn is allowed to shrink & tension being applied later

3.4 Fabric mercerization

38

Fabric must be singed, desized to reduce time of wetting, contamination of alkali & raise in temperature. It may be mercerized before or after scouring. After mercerizing in the grey state, it is not necessary to remove the alkali completely, since the cloth can go forward into kier where residual alkali is utilized. Howe ever, mercerizing in the grey state presents some difficulties, namely, slower penetration & contamination of alkali. Before the wet cloth is immersed into alkali, it must be evenly mangled. Bleached or half bleached cloths usually mercerized dry. It is done by chain mercerization machine or by chainless mercerization machine.

3.5 Slack mercerization

Slack mercerization is a finishing treatment of cotton. Cotton is treated with cold caustic soda. ‘Slack mercerization’ ‘mercerized loose’ and ‘mercerization without tension ‘mean free or complete shrinkage of cotton fibers or textile structures in sodium hydroxide solutions of sufficiently high concentrations.

Tension normally applied during mercerization restricts shrinkage, swelling and conversion of crystalline areas into amorphous ones. If no tension is applied, the maximum decrease in the degree of crystallinity and also a decrease in the orientation of the crystallites would result. Hence the maximum effect of caustic soda would be obtained on fibers where no restrictions on swelling or shrinkage would be present.

Slack mercerization treatments were carried out using 25% sodium hydroxide solutions containing 1-3% wetting agent. The duration of immersion and temperature of wash water were varied.

4. MERCERIZATION CHEMICALS

4.1 Sodium hydroxide

Sodium hydroxide (NaOH), is a caustic metallic base. It is used in many industries, mostly as a strong chemical base in the manufacture of pulp and paper, textiles, drinking water, soaps and detergents and as a drain cleaner.

Pure sodium hydroxide is a white solid available in pellets, flakes, granules, and as a 50% saturated solution. It is hygroscopic and readily absorbs water from the air, so it should be stored in an airtight container. It is very soluble in water with liberation of heat. Molten sodium hydroxide is also a strong base, but the high temperature required limits applications. A sodium hydroxide solution will leave a yellow stain on fabric and paper.

Sodium hydroxide is predominantly ionic, containing sodium cations and hydroxide anions. The hydroxide anion makes sodium hydroxide a strong base which reacts with acids to form water and the corresponding salts.

Caustic soda plays a vital role in various stages of textile wet processing like scouring, bleaching, mercerizing, dyeing and printing. It is also used in production of regenerated cellulosic fibre (e.g., viscose rayon).

However mercerization is generally carried out with concentrated solution of NaOH. As it is a secondary standard material, the purity must be checked before use. There are several ways to express the concentration of NaOH solution like %w/w, %w/v, g/l, degrees Baume(oBe) and degrees Twaddle (oTw). In mercerization, NaOH acts as a swelling agent which changes the structure and morphology of cotton fibre.

4.2 Mercerizing Wetting agents

38

The use of a suitable wetting agent in the mercerizing solution overcomes many of the difficulties faced in mercerizing, especially grey mercerizing.

The wetting agent-

Should have high wetting power.

Should not be preferentially absorbed by the fiber being treated.

Should be suitable over a wide range of sodium hydroxide concentration.

Should be suitable in strong caustic soda solution.

Should not give rise excessive foaming or turbidity in the solution or deposits on the fiber and machine parts.

Should be easily removed completely from the fibers.

Should not discolor the fibers permanently.

Should be available at an economical price etc.

There are two types of mercerizing wetting agents,

Cresylic

Non -cresylic.

Mixture of o-,m- and p- cresols also called cresylic acid hence the name cresylic type are not soluble in water, but dissolve in strong caustic soda solutions. These are found to be stable wetting agents in these solutions.

Non-cresylic wetting agent includes sulphated lower aliphatic alcohols such as hexyl alcohol and octyl alcohol (specifically, 2-ethyl hexyl alcohol).

Commercial cresylic wetting agents are available in the form of dark brown liquid with a strong phenolic odour while the commercial non-cresylic wetting agent is a clear yellow-brown liquid. These are used in the recommended range of 10-20 gm/liter for grey cloth and 3-5 gm/liter for bleached cloth.

5. FACTORS AFFECTING MERCERIZATION

Tension

Temperature

Caustic soda concentration

Time of treatment

Washing condition

Use of wetting agent

Rate of drying

Construction of yarn

6. EFFECT OF MERCERIZATION ON MOISTURE REGAIN

38

6.1 Moisture regain

Moisture regain is defined as the ratio of weight of water present in a material to the oven dry weight of the material. The ratio is usually expressed as a percentage. LetOven dry weight =DWeight of water= W Moisture regain= RSo, R= W/D x 100%The regain of textile materials appears to depend upon the relative humidity rather than the actual amount of water vapour present, so it is convenient to describe a given atmosphere in terms of relative humidity rather than absolute humidity. Since the relative humidity affects the regain of a textile material, and since the properties of the material are influenced by the regain, it is necessary to specify the atmospheric conditions in which testing should be carried out.

6.2 Reasons for increasing moisture regain

The moisture regain depends on the change of the molecular orientation in the interior of the fiber. It is well known that during the mercerizing process molecular structure tends to become decrystallized and the canals or spaces within the cellulose structure become more uniform. It is the reason that mercerized fabric take on more water, have higher regains and are more easily wet out then those are non mercerized cotton.

Concentration of NaOH. %(W/V)

Moisture regain%

10oC 18oC 31oC

5 6.97 6.79 6.8010.04 7.09 7.11 7.1216 7.65 7.54 7.5625.10 8.73 8.72 8.6829.75 8.83 8.78 8.70

33.47 8.93 8.82 8.71

Table: Moisture regain values of cotton yarn mercerizes at different temperatures & concentrations.

Due to caustic soda penetration, many hydrogen bonds are broken and it is estimated that that the number of available hydroxyl groups (-OH) are increased by about 25%. Mercerization thus decreases the crystalline part or increases the amorphous region of the fiber. Thus in the amorphous part of the fiber is directly related to moisture sorption. Moisture is assumed to be absorbed by suitable groups in the amorphous region & on the surface of crystallites. When mercerization is carried out under tension, the change in the crystalline portion is comparatively lower than that without tension. Standard cotton have moisture content about 7%, mercerized cotton with tension has about 9% and that without tension about 11%.

Higher concentration of alkali produce better swelling resulting greater amorphousness that’s why moisture regain of mercerized cotton increases with increase of sodium hydroxide concentration.

The moisture also increases with decrease in the temperature of mercerization at a given sodium hydroxide concentration as higher temperature causes lower swelling.

6.3 Measurement of moisture regain

38

First the sample has to be weighted in its original condition, to dry it, and then weight it again the oven dry weight, is defined as the constant weight, obtained by drying at temperature of 105+/-3oC, in B.S 1051:1964. The drying condition specified a recommendation to use a ventilated drying oven with a positively induced air current when successive weighting at 20 min differ by less than 0.05 %, it may be assumed that the constant weight has been reached.

To prevent moisture absorption during the transfer from the oven to the balance the weighting should be done with the samples remaining in the balance or the material is weighted in tared stopped containers.

6.4 Moisture testing oven

The manufacturer of testing instrument offers moisture testing ovens which conforms standard set out in B.S 1051:1964 and at the same time simplifies the work. The major specifications of moisture testing oven are given below:

1) Forced hot air thermostatically controlled.

2) Automatic balancing.

3) Weighting is carried out inside the heated chamber.

4) Preheating unit for partially drying before transferring to the main oven.

7. EFFECT OF MERCERIZATION ON LUSTER (BRIGHTNESS)

7.1 Lustre & Brightness

Lustre is the visual property of something that shines with reflected light. If a beam of light falls on a surface, it may be reflected specularly or diffusely or in a combination of both. The total visual appearance resulting from these reflections determines the luster of the material.

Brightness is the intensity of light reflected or emitted by something. Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. The brightness of an object is the amount of light it reflects, the more it reflects the brighter it will be.

7.2 Reasons for increasing luster during mercerization

When cotton is viewed using optical microscope, it is found that three forms, unmercerized, mercerized without tension and mercerized with tension are vastly different. Unmercerized cotton has a general appearance of being a flat ribbon with spiral twists; its surface is rough and non uniform. Its cross section is irregular and ear shaped while the lumen, the central canal, is broad, irregular and resembles a collapsed tube. With such a twisted structure, the light falling on cotton fibers gets reflected irregularly & as a result there is no luster in cotton fibers

38

A B

Fig: Electron microscope image of cotton fibre (x2200) A. natural B.mercerized cotton

when cotton is mercerized without tension its general appearance is observed to be much rounder with little or no twist, and its surface is much smoother then to compared with that of an unmercerized fiber fibers are more uniform with high magnification, approximately 500X,they appear to be creased and wrinkled. A mercerized cotton fiber cross section is oval and the lumen is contracted but not collapsed.

When mercerized with tension, the general appearance of cotton changes and similar to that of cylindrical glass rod. Its surface is very smooth and is completely free from folds and creases and its cross section is circular with the lumen being contracted or compressed to a slit. The smooth & more regular surface structure enables it to reflect incident light more evenly. This results in increasing the luster of the fibre.

7.3 Factors that affect lustre

The lustre of mercerized cotton depends on various factors:

Cross-section of fibre

Staple length of the fibre

Wall thickness of the fibre

Concentration of alkali (sodium hydroxide)

Temperature

Percent stretch

Yarn construction

Yarn twist

Doubling of yarn

Degree of singeing

Application of tension

Rate of drying

38

7.3.1 Cross-section

Lustre of the fabric is due to the regular reflection of light incident on the fibre surface, which depends on the cross-section of the fibre.

7.3.2 Staple length of cotton fibre

It has been found that long staple length cotton produces better lustre than short staple length cotton after mercerization under identical conditions. In the raw state, long staple length cotton has a cross-section more approaching a circle than in the case of short staple length cotton.

7.3.3 Temperature of mercerizing solution

Lustre values produced after mercerization at different temperature are given below:

It is seen that if mercerization is carried out at 7.5oC using refrigeration, it is costlier than when it is carried out at 17oC which results in very little change in lustre. Most economical mercerization may b carried out at 17oC. In countries like Bangladesh, the average room temperature is 30-35oC, the use of refrigeration is essential.

Lower temperature cause better swelling which further provide better smoothness & better surface structure. As a result better luster is found.

7.3.4 Concentration of alkali

Lower the temperature, lower the alkali concentration needed to produce maximum lustre. Thus if mercerization is carried out at 0oC a saving of sodium hydroxide may be achieved. However, the cost of refrigeration has to be met with.

Lustre values obtained when the alkali concentration was varied at 15oC are given below:-

Temperature (oC) Lustre

7.5 76

17.0 71

25.0 57

35.0 40

Alkali concentration(Tw) Lustre value

0 26

20 28

25 44

45 70

52 76

58 64

38

It is seen that the lustre produced increases with increasing alkali concentrations, reached maximum at 52oTw & then decreases with further increase in alkali concentration.

Higher concentration will cause better swelling of fibre which further provide better smoothness & better surface structure. As a result better luster is found.

7.3.5 Percent stretch

Percent stretch affects lustre. When a cotton hank is mercerized in an unrestricted state & then stretched to different extents, the resulting lustre values are given below:-

It is seen that maximum lustre is obtained when the hank is stretched 100%.

7.3.6 Yarn construction

Lustre produced by mercerization depends on the construction of the yarn. Single yarn cannot be successfully mercerized, while doubled yarn can be. Keeping the fibre staple length & fibre wall thickness constant, lower the twist per inch, higher the lustre because of the greater penetration of sodium hydroxide into the fibre. In the doubled yarn, the twist must be in the same direction (either S-twist or Z-twist).

7.3.7 Singeing

Yarn to be mercerized is always gassed (singed) to produce maximum lustre. The yarn should be gassed when it is to be used as sewing thread or for making organdie fabric. The presence of short fibre on the surface decreases lustre & is removed during singeing.

7.3.8 Tension

Maximum lustre is obtained when the tension is sufficient to bring the material back to its original state & any increase in tension above this does not increase lustre. The lustre obtained by impregnating & washing under tension is the same as impregnating loose & washing under tension, but more force is required in the second case. Greater lustre is produced by the maximum tension, but this high tension reduces the dye affinity & elasticity of the mercerized sample. It is seen that maximum lustre is obtained when the hank is stretched 100%.

As a caustic solution of sufficient concentration to cause mercerization to take place enters the fiber, the fiber swells. As the fiber swells, the fiber shrinks in length. Because of the fact that there is no tension, fiber surfaces, and while much smoother, still show residual creases and wrinkles. The creases and wrinkles scatter light as it falls on the fiber

surface and, therefore, luster does not occur to the same degree as when tension is applied.

Stretch% Lustre value

0 35

40 57

70 65

100 71

38

7.3.9 Rate of drying

Lustre produced also depends on the rate of drying after mercerizing & washing. Thus when cotton yarn is mercerized, washed, hydro-extracted & dried slowly; it acquires a higher lustre value than when it is dried rapidly.

7.4 Measurement of brightness

Brightness can be measured by Spectrophotometer.

7.4.1 Spectrophotometer

A spectrophotometer is an instrument which is used to measure the intensity of electromagnetic radiation at different wavelengths. Important features of spectrophotometers are spectral band width and the range of absorption or reflectance measurement. Spectrophotometers are generally used for the measurement of transmittance or reflectance of solutions, transparent or opaque solids such as polished gases or glass. They can also be designed to measure the diffusivity on any of the listed light ranges in electromagnetic radiation spectrum that usually covers around 200 nm-2500 nm using different controls and calibrations

7.4.1.1 Working principle of spectrophotometer

Light source, diffraction grating, filter, photo detector, signal processor and display are the various parts of the spectrophotometer. The light source provides all the wavelengths of visible light while also providing wavelengths in ultraviolet and infra red range. The filters and diffraction grating separate the light into its component wavelengths so that very small range of wavelength can be directed through the sample. The sample compartment permits the entry of no stray light while at the same time without blocking any light from the source. The photo detector converts the amount of light which it had received into a current which is then sent to the signal processor which is the soul of the machine. The signal processor converts the simple current it receives into absorbance, transmittance and concentration values which are then sent to the display.

8. EFFECT OF MERCERIZATION ON TENSILE STRENGTH

8.1 Tensile Strength

The maximum tensile force recorded in extending a test piece to breaking point. This is the load at which the specimen break, usually expressed in grams weight or pounds weight.

8.2 Reasons for increasing tensile strength during mercerization

During mercerization of cotton fibers, swelling occurs to a much greater extent & it’s molecular structure become decrystallized & the canals or spaces within the cellulosic structure become more uniform. Cotton fiber allign themselves in a regular way leading to an increase in the hydrogen bond formation. The major reason can be an alleviation of internal stresses and the deconvoluting of the fibers in the fabric during swelling process. Tensile strength increases with the increase in swelling. Mercerization increases cohesion between individual cotton hairs & this closer embedding of the hair in the yarn not only increase the strength but makes it more uniform in strength. Because of the longitudinal shrinkage & lateral swelling of yarn, fabric shrunk & the yarn appeared closer together with an increased thickness, polymer chain minimizes the weak link in the fiber which helps to increase strength.

38

8.3 Factors affecting tensile strength

When cotton fiber, yarn, cloth is mercerized its strength increases by 10-50%. The tensile strength increases depends on –

Alkali concentration

Temperature of impregnation

Tension in the fabric

Time of impregnation

Construction of yarn

8.3.1 Alkali concentration

It is seen that as the alkali concentration increases, the tensile strength also increases, reaches a maximum value at 52 0Tw & then decreases with further increase in alkali concentration

Dependence of tensile strength (breaking load) on the alkali concentration:

Swelling depends on hydration of alkali ion. Greater degree of hydration will give greater increase in swelling. Degree of hydration increase with concentration of alkali but decrease with temperature. Thus degree of hydration influences swelling which further influence tensile strength.

Alkali concentration , 0Tw Tensile strength, gm

25 247

35 287

45 288

52 302

60 282

38

8.3.2 Temperature of impregnation

Again, lower the temperature of mercerization, greater is the tensile strength (breaking load of the yarn) as seen from the table:

Dependence of tensile strength (breaking load) on the temperature:

Primary & secondary hydroxyl group in cellulose are the basis for the high hydrogen bonding & orientation found in cotton fibers. During mercerization, hydroxyl groups independently dissociate to the extent of alkali sorption. The result is that there is an osmotic pressure increase which causes water to enter the fiber until such time as the osmotic pressure is in balance with the restraining or elastic force of the swollen fiber. If the osmotic pressure has increased to the maximum which can be contained by the primary wall of the fiber, the resultant pressure can realign the fibrillar structure & cause the orientation of fiber to be changed permanently. Experiments have shown that mercerization causes the osmotic pressure to increase when mercerization is carried out at any temperature. If temperature of mercerizing media decreases, osmotic pressure increases.

So , it is clear that lower temperature will cause higher osmotic pressure & higher osmotic pressure will give higher swelling. Higher swelling will convert crystallign region into amorphous region where molecular alignment will be more uniform which produce better strength.

8.3.3 Tension in the fabric

Mercerization, both slack & with tension , increase strength uniformity along the fiber length , but mercerized fiber with tension shows greater gain in strength than that of without tension.

Internal pressures caused by the swelling, are much less during tensionless processing than when cotton is processed using high tension. Changes in the interior portions of the fiber are the direct result of internal osmotic pressure causing changes in the molecular configuration of the cotton fiber. When warp and filling tensions are applied, shrinkage tends to be reduced, internal pressures are increased causing the changes to be more profound both on the morphological structure.

8.3.4 Time of impregnation

Provided minimum time for maximum swelling to take place is given, the increase in time of contact of material with alkali does not seem to effect the tensile strength. Usually, cloth is treated for 30 sec & yarn for 50 sec.

8.3.5 Construction of yarn

For long staple fiber yarn, greater the twist, greater is the tensile strength of the mercerized material. Grey yarn with soft doubling twist gives stronger yarn.

Temperature,oC Tensile strength , gm

7 302

17 276

30 253

38

8.4 Methods of determining tensile strength

Basically, two methods are used to observe the effect of tensile forces on textile specimen-

Constant rate of loading (C.R.L.)

Constant rate of extension (C.R.E.)

8.4.1 Constant rate of extension (C.R.E.)

In this principle the top jaw is not fixed but needs a certain amount of movement in order to operate the load indicating mechanism. The movement relative to the bottom jaw prevents extension of specimen at an absolutely constant rate. Nevertheless bottom jaw traverse downward at a constant rate.

8.4.2 Tests

Grab test

Strip test

8.4.2.1 Grab test.

A tensile test in which only the central portion of the width specimen is held in jaws.

8.4.2.2 Strip test.

Laboratory method where the strip of the fabric cut along the yarn to a specific width. Here strip size is equal to the width of the clamp.

38

EXPERIMENTAL PART

38

9. MACHINES & INSTRUMENTS

9.1 Band A wooden circular band of diameter 9 inch

Fig: Circular Band Fig: Fabric attached with band

9.2 Tensile tester Name: Universal strength tester (Titan)

Origin: England Company: James H. heal & Company Ltd

Standard: EN ISO 13934-1(100mm 100mmmin) Software: Version 6.1.4

Fig: Universal strength tester (Titan)

38

9.3 Spectrophotometer Name: Dual Beam Spectrophotometer Instrument: Data color 650 Software: USA Manufacture: China

Fig: Dual Beam Spectrophotometer

9.4 Oven Name : Binder Nenntemperature : 300oC

Manufacturer : Germany

Fig: Binder

38

10. RAW MATERIALS

10.1 Fabric specification

Fibre type : Cotton Fabric weaves : Plain Fabric construction : 60×60/110×70 GSM : 75

10.2 Chemical specifications

Sodium hydroxide: Purity : 98 %( Lab standard) Physical form : solid flakes

Mercerizing oil:

Name : NANOWET MRO Chemical composition : Sulphated alcohol Stability : Stable to conc. alkali (up to 32°Be´) Ionic nature : anionic Manufacture : Jrk colour & chemicals ind. (bd) ltd.

11. MERCERIZATION

11.1 Sample preparation

We have cut our required number of samples (12inch x 12inch) from fabric by scissor & scale.

11.2 Mercerization liquor preparation

We have prepared required concentration of liquor according to the recipe:

NaOH : 13% /18% /25%

Mercerizing oil : 4 g/l

Water : 650ml

Time : 1 min

Temp : 10oC/ 17oC/ 27oC

38

11.3 Mercerization parameters

We performed the experiment using the following parameters are given below:

No Concentration Temperature Tension

1 13% 10oC With tension

17oC With tension

27oC With tension

Without tension

2 18% 10oC With tension

17oC With tension

27oC With tension

Without tension

3 25% 10oC With tension

17oC With tension

27oC With tension

Without tension

11.4 Tension adjustment

Tension was applied to the fabric by attaching the fabric with a wooden circular band. Here tension was applied manually.

11.5 Temperature adjustment

We have maintained 10oC by using ice & performed mercerization immediately. Then we waited until the liquor temperature becomes 17oC & performed mercerization & again we waited until it becomes 27oC & done the same.

11.6 Mercerization procedure

Mercerization was performed within required temperature, time, and concentration with constant tension. Then fabrics were rinsed with cold water to remove traces of alkali completely. Any remaining alkali was neutralized with dilute acetic acid solution, followed by washing with cold water. Then fabrics were dried in oven at 95oC for 16 min.

38

12. TESTING PROCEDURE

The following tests were done:

Determination of moisture regain %

Determination of Brightness values

Measurement of Tensile strength

12.1 .Determination of moisture regain%

12.1.1 Atmospheric condition

Dry bulb temperature : 30oC

Wet bulb temperature : 29oC

Difference : 01

Relative humidity % : 93

12.1.2 Procedure

At first, we cut sample from both mercerized & unmercerized fabrics of same size. Then we weighted them by electric balance. Then we took oven dry weight of these samples. Then we calculated M.R. %. By using the following formula:

R= W/D x 100%

Here, Oven dry weight = D Weight of water = W Moisture regain = R

12.2 Determination of brightness values

Samples are prepared in 4 fold & set it in the measuring port of spectrophotometer. Then we evaluated brightness value of samples by using “Data Color Tools” software.

38

12.3 Measurement of tensile strength

12.3.1 Atmospheric condition

Dry bulb temperature: 26oC

Wet bulb temperature: 24oC

Difference: 02

Relative humidity %: 84

12.3.2 Sample size. (18cm x 5cm)

Rectangular, both in warp & weft direction.

12.3.3 Procedure

At first, we cut the fabric according to the size. Then we ensure proper gripping of samples in machine manually. We gave command via computer to the machine. We have measured both warp & weft strength in same manner by strip method.

38

RESULT & DISCUSSION

38

13. DATA ANALYSIS

13.1 Effects of alkali concentration on moisture regain

Comment

From the above graph, we found that moisture regain increases with the increase of concentration of NaOH. The increase in moisture regain is concerned with the decrystallization in the molecular structure of the fibre. As the concentration of alkali increases, the amorphousness of the fibre also increases and the canals or spaces within the cellulose structure become more uniform which causes more moisture absorption.

8.4725

7.4618

7.0213

6.660*

Moisture regain (%)Concentration of NaOH (%)

*unmercerized

13.1.1 Dependence of moisture regain (%) on Conc. of NaOH

38

13.2 Effects of temperature on tensile strength

The tensile strength of the unmercerized sample was: warp 420.36 N & weft 211.41 N

13.2.1 For 13% alkali concentration

Temperature Tensile strength (N)

Warp Weft10oc 456.98 232.36

17oc 451.03 226.32

27oc 448.28 219.55

13.2.1.1 Dependence of tensile strength on temperature

A) Warp B) Weft

38

13.2.2 For 18% alkali concentration

Temperature Tensile strength (N)

Warp Weft10oC 478.31 237.57

17oC 473.09 231.02

27oC 461.55 227.63

13.2.2.1 Dependence of tensile strength on temperature

A) Warp B) Weft

38

13.2.3 For 25% alkali concentration

Temperature Tensile strength (N)

Warp Weft

10oc 520.32 255.14

17oc 495.12 246.4

27oc 467.31 240.25

13.2.3.1 Dependence of tensile strength on temperature

A) Warp B) Weft

Comment

From the above graphs, we can say that the tensile strength decreases at both warp and weft direction with the increase of temperature within same alkali concentration (13%/18%/25%). Due to increasing temperature, less swelling occurs inside the fibre within same caustic soda concentration. As a result less hydrogen bonds is formed which causes lower tensile strength.

38

13.3 Effects of conc. on tensile strength

13.3.1 Dependence of Tensile strength (warp) on concentration of NaOH (%)

13.3.2 Dependence of Tensile strength (weft) on concentration of NaOH (%)

Comment

From the above graphs, we can say that the tensile strength increases at both warp and weft direction with the

increase of concentration of NaOH within same temperature (10 C/17 C/27 C). As the concentration of alkali

increases, more swelling takes place which results molecular alignment in more regular way leading to an increase in hydrogen bond formation. So tensile strength increases with the alkali concentration.

38

13.4 Effects of tension on tensile strength

13.4.1 for warp

13.4.1.1 Dependence of tensile strength (warp) on tension.

Conc. of NaOH (%) Tensile strength(N)

With tension Without tension

13 448.28 441.6

18 461.55 460.27

25 467.31 465.4

38

13.4.2 For weft

13.4.2.1 Dependence of tensile strength (weft) on tension

Comment

From the above graphs, we found that mercerization with tension shows slightly better tensile strength, for both warp & weft, than mercerization without tension. Tension increases molecular orientation in the amorphous region which provides additional strength.

Conc. of NaOH (%) Tensile strength(N)

With tension Without tension

13 219.55 215

18 227.63 221.27

25 240.25 226.33

38

13.5 Effects of temperature and conc. on brightness values

TemperatureBrightness values at different conc. of NaOH (%)

13% 18% 25%

10oC 70.22 71.24 72.46

17oC 68.85 70.21 71.31

27oC 67.09 68.72 70.67

27oC* 65.64 67.8 69.28

The brightness value of unmercerized sample was 63.65

13.5.1 Effects of temperature on brightness values

13.5.1.1 Dependence of brightness on temperature (for 13% concentration)

*Mercerization without tension .

38

13.5.1.2 Dependence of brightness on temperature (for 18% concentration)

13.5.1.3 Dependence of Brightness on temperature (for 25% concentration)

Comment

From the above graphs, we found that the brightness value decreases with the increase of temperature within same alkali concentration (13%/18%/25%). This fact can be clarified with the extent of swelling which decreases with the increase of temperature. So the cross section of the fibre becomes less circular and the surface structure becomes less smooth and regular when temperature of the alkali solution is increased. As a result brightness is decreased.

38

13.5.2 Effects of concentration on brightness values

13.5.2.1 Dependence of Brightness on conc. of NaOH (%)

Comment

From the above graph, we can say that the brightness value increases with the increase of concentration of alkali

within same temperature (10 C/17 C/27 C). When the concentration of alkali increases, more swelling takes place

in the fibre. As a result the cross section becomes more circular and the surface structure becomes more smooth and regular enabling it to reflect incident light more evenly. So brightness is increased

38

13.6 Effects of tension on brightness value

Conc. of NaOH (%) Brightness value

With tension Without tension

13 67.09 65.64

18 68.72 67.8

25 70.67 69.28

13.6.1 Dependence of brightness on concentration of NaOH (%)

Comment

From the graph, we found that mercerization with tension shows greater brightness values than mercerization without tension. Because of the fact that there is no tension, fiber surfaces, and while much smoother, still show residual creases and wrinkles. The creases and wrinkles scatter light as it falls on the fiber surface and, therefore,

luster does not occur to the same degree as when tension is applied.

38

14. LIMITATIONS

There are few limitations in this project. Such as:

There was no tensioning device to control the fabric tension precisely over the whole process. We could not do enough tests due to lack of same construction fabric. Uniformity of the strength of the supplied fabric was not good. We found strength variation in different

parts of the fabric. We could not maintain standard atmospheric condition. We were not able to maintain the mercerizing temperature perfectly. Fabric properties & construction were not well known. We were not able to control the liquor pickup of fabric. We were not able to determine the oven dry weight accurately.

15. SUGGESTIONS

This project could have been better if we had a tensioning device & moisture testing oven. More number of tests would give us more accurate results. Fabric of known properties like construction, strength, count and TPI for both warp and weft may help for better evaluation of the results. Standard atmospheric condition & liquor pick up control must be ensured for accurate result. Similar project work can be done for further investigation on the effects of mercerization parameters on dye absorbency and the brightness after dyeing.

16. CONCLUSION

In this project, we have learned about mercerization & its effect. We found effect of mercerization parameters variation over its performance. We have also learned about important cotton properties & there change after mercerization. We have learned about a project presentation, writing sequence, team work. This will enrich our analytical ability. All these will be very helpful for our knowledge; will increase our experience, skill & above all our future industrial job life.