Tone on Tone Printing Effect on Cotton Using Caustic Soda (NaOH)

Last Updated on 12/07/2023

Tone on Tone Printing Effect on Cotton Using Caustic Soda (NaOH)

Yibeltal Getachew
Institute of Technology for Textile, Garment and Fashion Design
Bahir Dar University, Ethiopia
Email: g.yibeltal@yahoo.com

1. Introduction

1.1 History:
The history of printing started around 3000 BC with the duplication of images. The use of round “cylinder seals” for rolling an impress onto clay tablets goes back to early Mesopotamian civilization before 3000 BC, where they are the most common works of art to survive, and feature complex and beautiful images. In both China and Egypt, the use of small stamps for seals preceded the use of larger blocks. In Europe and India, the printing of cloth certainly preceded the printing of paper or papyrus; this was probably also the case in China. The process is essentially the same – in Europe special presentation impressions of prints were often printed on silk until at least the seventeenth century. [1]

The earliest woodblock printed fragments are from China. They consist of printed flowers in three colors on silk. They are generally assigned to the Han Dynasty so date before 220 BC. The technology of printing on cloth in China was adapted to paper under the influence of Buddhism which mandated the circulation of standard translations over a wide area, as well as the production of multiple copies of key texts for religious reasons [2]

Textile printing was known in Europe, via the Islamic world, from about the 12th century, and widely used. However, the European dyes tended to run, which restricted the use of printed patterns. Fairly large and ambitious designs were printed for decorative purposes such as wall-hangings and lectern-cloths, where this was less of a problem as they did not need washing. When paper became common, the technology was rapidly used on that for woodcut prints. Superior cloth was also imported from Islamic countries, but this was much more expensive. [3]

As early as the 1630s, the East India Company was bringing in printed and plain cotton for the English market. By the 1660s British printers and dyers were making their own printed cotton to sell at home, printing single colors on plain backgrounds; less colorful than the imported prints, but more to the taste of the British.

Textile printing was introduced into England in 1676 by a French refugee who opened works, in that year, on the banks of the Thames near Richmond.

During the last two decades of the 17th century and the earlier ones of the 18th new works were started in France, Germany, Switzerland and Austria; but it was only in 1738 that calico printing was first, practiced in Scotland, and not until twenty-six years later that Messrs Clayton of Bamber Bridge, near Preston, established in 1764 the first print-works in Lancashire, and thus laid the foundation of the industry.

In early days of the industry down to the latter half of the 20th century, the productions of the French printers in Jouy, Beauvais, Rouen, Alsace-Lorraine, &c., were looked upon as representing all that was best in artistic calico printing.

1.2 Definition:
Printing is the process of applying colour to fabric in definite patterns or designs. In properly printed fabrics the colour is bonded with the fiber, so as to resist washing and friction. Textile printing is related to dyeing but, whereas in dyeing proper the whole fabric is uniformly covered with one colour, in printing one or more colors are applied to it in certain parts only, and in sharply defined patterns. [1] or the localized application of color on fabrics. In printing textiles, a thick paste of dye or pigment is applied to the fabric by appropriate mechanical means to form a design. The color is then fixed or transferred from the paste to the fiber itself, maintaining the sharpness and integrity of the design. In a multicolor design, each color must be applied separately and in proper position relative to all other colors. Printing is one of the most complexes of all textile operations. [1]

1.3 Printing Methods:
In printing, wooden blocks, stencils, engraved plates, rollers, or silk-screens can be used to place colours on the fabric. Colourants used in printing contain dyes thickened to prevent the colour from spreading by capillary attraction beyond the limits of the pattern or design. [1]

Traditional textile printing techniques may be broadly categorized into following styles:

1. Hand block printing: In this process, a design is drawn on, or transferred to, a prepared wooden block. It is the earliest, simplest and slowest of all methods of printing.
2. Direct printing: in which colourants containing dyes, thickeners, and the mordant or substances necessary for fixing the colour on the cloth are printed in the desired pattern. The printing of a mordant in the desired pattern prior to dyeing cloth; the color adheres only where the mordant was printed.
3. Resist dyeing: In which a wax or other substance is printed onto fabric which is subsequently dyed. The waxed areas do not accept the dye, leaving uncoloured patterns against a coloured ground.
4. Offset printing: It is a widely used printing technique where the inked image is transferred (or “offset”) from a plate to a rubber blanket, then to the printing surface. When used in combination with the lithographic process, which is based on the repulsion of oil and water, the offset technique employs a flat (planographic) image carrier on which the image to be printed obtains ink from ink rollers, while the non-printing area attracts a film of water, keeping the non-printing areas ink-free.
5. Discharge printing: In which a bleaching agent is printed onto previously dyed fabrics to remove some or all of the colour.
6. Tone On Tone: Tone on tone refers to a printed fabric that is made by combining different shades and tones of the same color. Tone on tone fabrics often appear to be solid when viewed from a distance, but their printed motifs become recognizable on closer inspection.

Resist and discharge techniques were particularly fashionable in the 19th century, as were combination techniques in which indigo resist was used to create blue backgrounds prior to block-printing of other colors. Most modern industrialized printing uses direct printing techniques. [1]

In this work tone –on –tone printing is the major concern.

1.4 Tone-On-Tone Printing:
Tone-on-tone printing is a very subtle method of imaging. The term refers to printing a color on top of the same or very similar color (i.e. the same “tone”). It could be black on black or any color printed on top of the same color. The extra layer of ink makes the second image visible, but only very subtly. It could also mean black ink printed on a black piece of paper or fabric, for example (or any color printed on the same color of paper or fabric). It’s more popular in the screen printing industry where the difference in color tones is generally more noticeable or it refers to a printed fabric that is made by combining different shades and tones of the same color. Tone on tone fabrics often appears to be solid when viewed from a distance, but their printed motifs become recognizable on closer inspection. Tone on tone fabrics is popular with quilters, because they add subtle, visual texture to a quilt without the busyness of a multicolor print. [2]

2. Objective of the Study
To carry out both dyeing and printing at the same time by using the available resources such as time, dye, auxiliaries and caustic soda most economically on cotton fabric.

To investigate the effect of caustic soda on dye printing.

To obtain a design printed on a fabric by the dye itself absorbed deeply.

2.1 Relevance to Industry and Country:
Applying tone on tone printing effect on textile products has got a remarkable economical advantage to any one engaged in supplying printed textile products to the local market. Apart from this , the cost effective nature of tone on tone printing would make product suppliers free from using pigments like in the case of conventional printing processes.

Simplicity and less requirement of energy to carry out design printings makes tone on tone printing chosen to prepare as many products as possible in a very short period of time.

Uniqueness of the printing effect makes the products very competitive in the existing market. So that product suppliers would be very beneficiary. The customers will also find the tone on tone printed designs useful due to their durability of fixation on the product. Buying of domestic products in turn has got an economical advantage to our country.

2.2 Statement of the Problem:
The conventional printing mechanisms are not cost effective enough like tone on tone printing in such a case that they uses pigment, mordant and design in printing systems e.g silk screen in an inflexible manner. Here, once we used the silk screen for a specific design, preparation of another silk screen would be a must. This uses extra consumption of energy, material and time.

Although the conventional printing mechanisms use mordants and other fixing agents in order to help the pigment fix on the surface of the fabric, the design area or printing has a short life cycle before the entire body of the product. This would in turn dissatisfy customers.

Currently, inadequate varieties of printed textile products are available in the market. According to the excessively evolving paradigms. Of customers unique and variety of products should be supplied to the market to satisfy needs.

3. Literature Review
Mercerization of cellulosic fibers, this method was patented in 1850 by the English calico printer John Mercer and hence forth this process has been called as mercerisation. Under the action of concentrated alkaline solutions chemical, physico-chemical and structural modifications of cellulose take place. Chemical reactions lead to the formations of alkali cellulose, physical reactions, to intensive swelling of fibres and structural reactions, to a change in the arrangement of units in the cellulose macromolecule.

Mercerisation is one of the most important processes of finishing cotton materials. It imports gloss to the fibre, increases its hygroscopicity, strength and improves its dye affinity. The mercerizing process consists in treatment of cellulosic materials with concentrated solutions of caustic soda at a temperature of 15 to 18°C. Mercerized cellulose is hydrated cellulose, i.e., a product which from the chemical point of view is identical to the original cellulose, but differing from it in physical properties.

Concentrated solutions of caustic soda cause considerable swelling of cotton fibre; the changes in cellulose physical properties being irreversible. When the fibre swells, its volume undergoes considerable changes; at maximum water absorption, the cross section of cotton fibre is increased by 40 to 50% with inconsiderable increase in length (about 1 to 2%). The size of pores in the fibrous, material is considerably increased.

Adsorption of caustic soda by cotton fabric Application of Caustic Soda swells cotton. Whereas a higher concentration of caustic soda is required at higher temperatures, same level of swelling is obtainable with lower concentrations at lower temperatures. When stretch forces are applied to counter the swelling and the caustic is washed off in the stretched condition the cotton retains the new dimensions. John Mercer was the first to observe the phenomenon of swelling in strong caustic soda and extensive work on this process by later workers led to what has come to stay as mercerization.

Swollen fibre tends to become cylindrical in shape and the lumen or the central cavity tend to get reduced in size. Due to stretch de -convolution of the cotton fibre is also facilitated – these two physical changes enable efficient and even reflection of incident light, thus improving luster and brilliance. Also much of the crystalline regions are converted to amorphous state and therefore, cotton becomes more permeable providing easy and freer access for various dyes and chemical solutions to diffuse /penetrate in to the substrate. The strength also improves considerably.

The term stretch here should be understood as the stretch force applied to offset the normal shrinking process during swelling to the extent possible without rupture to the matured fibres. The immature cotton gets ruptured due to swelling and along with the dead cotton gets dissolved in the mercerizer strength caustic Soda. Over the years on the basis of cost benefit considerations the approach to mercerization concentration/temperature profiles have undergone changes but the principle of swelling and stretch has remained the same.

3.1 Dyeing of Cotton:

3.1.1 Direct dyes:
Direct dyes are so called because they dye cellulosic such as cotton wit out the need for a mordant. Direct dyes are anionic in nature. These dyes are water soluble, even though some of them require the presence of sodium carbonate to dissolve; their solubility increases with an increase in the number of solublizing groups and with temperature, and decreases in relation to molecular weight.

Direct dyes can be classified based on the chromophore chemical composition and equalizing properties. The two chemical classes to which direct dyes belong are those containing the azo groups (azoic dyes) and those containing the thiazole ring (thiazoilic dyes). Based on their equalizing properties the direct dyes can be classified as self leveling, average leveling (salt controllable) and poor leveling (temperature controllable). There are also other parameters for classification such as in relation to further treatments, continuous dyeing, high temperature dyeing and so on.

Direct dyeing of cotton fabrics is influenced by different factors. Among the factors are action of temperature, action of electrolytes (neural and alkaline), liquor ratio and agitation. More interest is towards the electrolytes effect on the direct dyeing process. Generally speaking electrolytes used in the direct dyeing process affect the amount of dye exhaustion at equilibrium. While the neutral electrolytes have an increasing effect the alkaline electrolytes perform the opposite.

The mechanism of dye-fiber interaction involved between direct dye and cotton fiber is by means of secondary bonds. The general procedure followed in the dyeing process is dye dissolution, the dyeing itself and subsequent treatments. The direct dyeing process can be carried out both in batch and continuous systems.

3.1.2 Reactive dyes: 
The principal structure of reactive dyes consists of solubilizing group (S), chromophore (C), bridge (B), reactive group (RG) and a leaving group (X) attached to the reactive group.

Based on their reaction with cellulose the reactive dyes are classified as cellulose ester formers and cellulose ether formers. In the case of cellulose ester forming reactive dyes the reaction takes place by means of nucleophilic substitiution mechanism. Dichlorotriazinyl and monochlorotriazinyl types of reactive dyes are catagorized under this group. In the case of cellulose ether forming reactive dyes the reaction takesplace by means of nucleophilic addition mechanism. Vinyl sulphone dyes are grouped under this category.

There are reactive dyes called bifunctional to differentiate with the above monofunctional dyes where there is more than one triazinyl dye and/or vinylsulphone dye functional group in the structure. The dyes can be homofunctional and heterofuctional.

Reactive dyes are anionic in nature. The dyes are soluble in water. In neutral solution they have substantivity towards cellulose similar to low affinity direct dyes. In this neutral phase physical adsorption and possibly some hydrogen bondings takes place. Exhasution is better in the presence of a neutral electrolyte added in the neutral adsorption phase. The reaction between cellulose and reactive dyes is a covalent bond which is responsible for the good wash fastness characteristics in the dyeing of cotton fabrics with the reactive dyes. This covalent bonding takes place in alkaline phase by the addition of alkali in the dyebath.

Besides dye – fibre covalent bonding the dyes at the same time can undergo hydrolysis with water to form hydrolysed dye. Hydrolysis results in shade inconsistency and wash fastness problems. The question of hydrolysis increases importance as the dyeing conditions demand longer liquor ratios and dyeing cycles. Thus in reactive dyeing of cotton fabrics parameters need to be adjusted in favour of dye – fibre covalent linkage.

The fundamental parameteres which describe the dyeing performance of a reactive dye are diffusion property, substantivity and reactivity. In general substantivity and diffusion operate in opposite direction and highly substantive reactive dyes have low rate of diffusison. The higher the reactivity, the lower is the minimum substantivity required for dyeing from long liquor. The change in substantivity with temperature is the inverse to change in reactivity and diffusion with temperature. The relative reactivities of different type of reactive dyes with cellulose are of the following order: Dichlorotriazinyl > Vinylsulphone > Monochlorotriazinyl. Based on the reactivity of functional groups in their structure, reactive dyes can also be classified as cold, warm and hot brand dyes.

In addition to the above factors the reactive dyeing process of cotton fabrics is influenced by the state of the material and the application conditions such as temperature, duration of dyeing, dyebath conditions etc. Due to low affinity of the dyes and hence requirement for shorter liquor ratio the dyeing of cotton with reactive dyes is preferebly carried out using continous systems. Mostly reactive dyeing is carried out by a pad – batch semicontinous system.

Excess use of salt may possibly cause dye aggregation and lower migration, which leads to un- level dyeing. Insufficient use of salts may cause insufficient color yield to poor fixation. Fujimura [6] and his co-followers in 1997 described the effect of adding methods of neutral and alkali salts on concentration change of dye-bath and dyeing efficiency in dyeing cotton fabric with reactive dyes. This study was with real time dyeing bath detection system. Not only the adding methods, but also the nature of dye used gave big effect on dyeing.

Similarly in 1998, Yamaguchi [7] and co­workers introduced the methodology of the dyeing of cross-spum cotton and wool fabric with reactive dyes. Xie [8] and studied the property of bi functional reactive dyes with sulfato ethyl sulfone group and monofluorotriazine group for the cellulose of poplin. The dye had better dyeing property for cellulose than that of conventional reactive dyes having only one reactive group. The property applying conditions of the bifunctional reactive dyes for cellulose were, temperature of fixing the dye 50-60°C, dosing of Na2CO3, and 50-60g/L NaCl. The use of sodium edate in dyeing of cotton/wool blend with hetero bi-functional reactive dyes was studied by Reda M.

El-Shishtawy [9] et al, They reported that the compounds of the present invention exhibit increased Exhaustion (E), Fixation (F) and Efficiency (T) values and provide improvements in terms of reducing spent dyestuff in effluent, increasing dye affinity to the substrate, increasing the efficiency of the dye-substrate covalent reaction, ability to carry out the long-liquor dyeing process at room temperature as well as at elevated temperatures, and simplifying the post dyeing “soaping off process” traditionally associated with fiber reactive dyes.

Bredereck et al. [10, 11] have shown that bulky molecules and dyes better diffuse into fibers. It is believed that this higher proportion of larger pores and greater longitudinal extent of pores between the microfibrels is responsible for this phenomenon [12].

3.1.3 Vat dyes:
Vat dyes in particular give dyeings on cellulosic fibres with the best overall fastness Properties, Its application involves reduction to the water-soluble leuco compound, dyeing the cotton and re-oxidation of the leuco dye in the fibres to the insoluble pigment, the three basics steps involved in vat dyeing. Vat dyes are water-insoluble pigments. They are called dyes because chemical reduction in alkaline solution converts the pigment into a water-soluble leuco form with substantivity for cotton. The vat pigment and the leuco compound

Often have quite different colors – blue and pale yellow in the case of Indigo – so the progress of the reduction is often easy to observe. After dyeing with the leuco compound, the pigment is regenerated in the dyed cotton by oxidation. The overall process are reduction of the pigment to the soluble leuco compound, a process called vatting, absorption of the leuco compound by the cotton during dyeing and oxidation of the absorbed leuco compound in the cotton, reforming the insoluble pigment inside the fibers.

The use of strongly alkaline solutions (pH 12–14) for vatting and dyeing limits the use of most vat dyes to cellulosic fibers.

3.2 Raw Materials:

1. Cotton gray fabric [160g/m2]
2. Sodium hydroxide (Caustic soda)
3. Thickener:high viscosity sodium alginate or CMC( carboxyl-methyl cellulose)
4. Leveling agent
5. Direct dye
6. Salt (NaCl)
7. Weighing balance
8. Scissors
9. Ruler
10. Stirrer
11. Stove
12. Measuring cylinders
13. Beakers and other accessories
14. Stencils, block

3.3 Procedure:
The details of material and methods involved in the experiment are present under the following headings.

Preparation of fabric:
The fabric size that is prepared is: 6x4in.

Printing a design on a fabric:
The design can be printed on a fabric by using stencil, block and sketches.

Sodium hydroxide (caustic soda):
# 25g of 100% of NaOH in 100ml of water.

Purity of sodium hydroxide in lab is- 81%. Therefore, 30.864g of NaOH is mixed in 100ml of water. The sample fabric weighs -5.38g.

The paste prepared as follows:

30.864g……………………100ml NaOH
5.38g………………………X
X = 5.38*100/30.864
= 17.43ml

Therefore: 30.864g of NaOH dissolved in 17.43ml of water.

Thickeners:
High viscosity sodium alginate is added in to the solution of the paste until it attains the required viscosity and then painting the design area by using the paste solution

Wet the fabric with cold water before it’s immersed in the dye solution.

3.4 Preparation of Dye Solution:

3.4.1 Direct dye:
To prepare the dye solution the dye is mixed with a small quantity of cold water until homogenous paste is obtained. Then enough hot water (about 80°C) is added to the paste to dissolve the dye completely. Finally, leveling agent is dissolved in the dye solution (for dyes difficult to level). We use the following chemical recipe:

Table 3.1 chemical recipe of direct dye

Experiment-1

1. Direct dye
0.5% of dye = 0.5*5.38/100
= 0.0269g

2. NaCl: 5g
3. Leveling agent: 0.3g
4. 161.4ml of water
5. Sodium hydroxide: 30.864g

Experiment-2

1. Direct dye
0.5%of dye = 0.5*5.38/100
= 0.0269g

2. NaCl: 10g
3. Leveling agent: 0.3g
4. 161.4ml of water
5. Sodium hydroxide: 30.864g

Experiment-3
#the sample fabric weighs -9.96g.

1. Direct dye-
0.5%of dye = 0.5* 9.96/100
= 0.0498g

2. NaCl: 10g
3. Leveling agent: 0.5g
4. 298.8ml of water
5. Sodium hydroxide: 32.270607g

Dyeing Cycle/Procedure: Carry out the dyeing process according to the following dyeing cycle.

• At ‘A’ start with the prepared dye solution in a beaker with the fabric sample immersed in it.
• At ‘B’ add 1/5th of the total amount of the neutral electrolyte (sodium chloride or sodium sulphate) required.
• At ‘C’ add 4/5th of the total amount of the neutral electrolyte (sodium chloride or sodium sulphate) required.
• At ‘D’ rinse the sample twice with cold water.
• Dry the dyed fabric

3.4.2 Reactive dye:
Table 3.2 Chemical recipe of reactive Dye

• DCT – Dichlorotriazinyl
• MCT – Monochlorotriazinyl
• VS – Vinylsulphone
• BF – Bifunctional

Experiment-4
# The sample fabric weighs –[8.56g]

1. Reactive dye- DCT
0.5%dye = 0.5*[8.56]/100
= 0.0428g

2. NaCl-40g
3. Sodium carbonate-5g
4. Sodium hydroxide-27.735g
5. 256.8 ml of water

Experiment-5
# The sample of fabric weigh -5.38g

1. Reactive dye-MCT
0.5% dye = 0.5*[5.38]/100
= 0.0269g

2. NaCl-45g
3. Sodium carbonate-15g
4. Sodium hydroxide-17.43g
5. 161.4 ml of water

Experiment-6
# The sample of fabric weigh -[12.8g]

1. Reactive dye-VS
0.5% dye = 0.5*[12.8]/100
= 0.064g

2. NaCl-50g
3. Sodium carbonate-5g
4. Sodium hydroxide-41.47g
5. 384 ml of water

Dyeing Cycle/Procedure:
After preparing the dye solution and placing the fabric sample in the dye bath follow the following dyeing cycles given for the different kinds of dyes used in the experiment.

• Start dyeing with the bath containing dye solution and fabric sample at 30°C.
• Continue for 15 minutes at 30°C and add half of the predissolved Glauber’s salt.
• Continue further for 15 minutes at 30°C and add the remainder of the salt.
• Continue dyeing for 15 minutes at 30°C and add the soda ash solution.
• Continue dyeing for 45 – 60 minutes at 30°C and carry out washing.

• Start dyeing with the bath containing dye solution and fabric sample at 50°C.
• After 15 minutes add half of the predissolved Glauber’s salt.
• Continue dyeing while rising the temperature to 80 – 85°C {the remainder of the salt is added in the intermediate as shown by the dyeing cycle above}.
• At the dyeing temperature (80 – 85°C) continue dyeing for 10 min and add soda ash solution.
• Continue dyeing for 30 – 60 min and carry out washing

• Start dyeing with the bath containing dye solution and fabric sample at 30°C (A).
• Continue for 10min and add the predissolved Glaubler’s salt /common salt (B) and then continue for 20 minutes.
• After 20 min add the predissolved alkali (C) and raise the temperature to 60°C in 30 minutes.
• Continue dyeing at 60°C for 30-60 minutes.
• At D carry out rinsing.

• Start dyeing with the bath containing dye solution and fabric sample at 30°C.
• After 20 minutes add half of the predissolved Glauber’s salt.
• Continue dyeing while rising the temperature to 60-65°C and at this point add the remainder of the salt as shown by the dyeing cycle. Continue dyeing
• After 15 minutes add soda ash solution and continue dyeing at 60-65°C.
• Continue dyeing for 60min and carry out washing.

Note: As per requirement soaping can be carried out using 5 gpl standard soap at boiling temperature for 20 min, then final rinsing is carried out by washing the fabric with hot water (50 – 60°C) for 5min and with cold water for 5 min.

4. Result and Discussion
The experiment shows that tone on tone printing using a direct dye –red color, the effect is more visible in the design area due to more absorption of dye on the fabric part which gets efficient caustic soda solution paste. The design was a picture of tiger on a fabric as shown below, the parts that is the pasted area is darker than the normal red parts which makes the image more visible.

Tone on tone cotton fabrics towards direct printing while toning was carried out using caustic soda on different concentration of sodium chloride then the treated fabric samples were subjected to printing followed by dying. It is observed that the color strength of prints enhance on the increasing of sodium chloride. Enhancement in the color strength is rather a manifestation of the increasing the attack of sodium hydroxide to be more adsorption of the dye to become more visible than the other parts of the fabric.

Experiment-1 show the effect of caustic soda on the fabric, on the design area which is painted by caustic soda in order to improve the absorption of dye that cause deep red than the normal color that is used on the dying of the fabric. i.e. the direct dye-red is more absorbed on the design area becomes better adsorption of dye on the specified place in order to give the shade difference on the fabric.

Experiment-2 shows the effect of caustic soda with the direct dye on the fabric, using direct dye –blue the effect also visible on the fabric design area due to the above reasons.

Experiment-3 shows the effect of caustic soda with the reactive dye on the fabric, reactive dying is using more sodium chloride (Glaubler’s salt / common salt) than the direct dying which helps the fixation of the dye of the fabric so this enhances the dye up take ratio more on the design area. Due to the presence of the more amount of salt which increase the adsorption of dye to fabric this cause bright shade depth of the color on the design area meanwhile the presence of more caustic soda which enhance the mercerization process higher than the rest part of the fabric on the specified place.

During the dyeing of cellulose with reactive dyes, Hydrogen ion of cellulose react with Chloro (Cl) or Sulfone (SO₃) of the reactive group and forms a strong acids like Hydrochloric Acid (HCl) or Sulphuric Acid H₂SO₄. Alkali (Soda Ash or Caustic soda) is important to neutralize this strong acid which will otherwise cease the reaction. Addition of alkali controls rate of reaction of the dyeing mechanism. Therefore it is important to add the alkali slowly. If pH of dyebath increases at full speed, reaction to the right goes very fast and hence patchy dyeing is inevitable.

…………………………………………Soda ash
Cellulose – O H + Cl – R – Dye      →     Cellulose – O – R – Dye + HCl

Role of Elektrolyte (Salt) with Reactive Dyeing, Reactive dyes for cotton have negatively charged active groups, they are anionic. When cotton or viscose fiber immersed into water, its surface due to hydroxly ions become also anionic, hence they- the dye particles and the cellulosic fiber-tend to repel each other. The addition of salt, creates an electical positive double layer which hides negative electrostatic charge of cotton surface. This allows the dye approach the fiber. If electrolyle is not spread-out uniformly on cotton surface, dye distribution will not be even also and patchy dyeing is unavoidable.

To better illustrate the concept of how the dye molecules, water, cloth and salt interact with one another in the reservoir, a simple sketch of the water molecules, cloth and dye molecules together [Figure 1] is below.

As one can see, the water molecules tend to attach themselves to the dye molecule as well as attaching themselves to the cloth. Also, many molecules of water exist between the dye molecule and the cloth. By using the operational zone identification, one can see where the useful operating zone (UFOZ) and the harmful operating zone exists (HFOZ). The zones also help to identify when the useful operational time and (UFOT) harmful operational time (HFOT) occur.

Table 4.1 operational zone identification

After the addition of salt and dye, Sufficient period of time should pass, for the even distribution of salt and the dye. This levelling period is between 30 and 45 minutes, depending upon, Circulation speed, liquor ratio and Primary exhaustion property of dyes.

If electrolyte is not spreaded out uniformly on cotton surface ,dye distribution will not be even also and patchy dyeing is unavoidable. After the addition of salt and dye, sufficient period of time should pass for the even distribution of salt and the dye.

This leveling period us between 30 and 45 minutes, depending up on:

• Circulation speed
• Liquor ratio
• Primary exhaustion property of dyes

Uniform dyeing results with acceptable fastness properties can be obtained by controlling this parameters properly.

Since sodium sulphate generates less sodium ion with respect to NaCl, cotton surface becomes less substantive. This is precondition for light shades. So sodium sulphate is preferred light shades, Blue 19 and Viscose fiber.

After the addition of salt and dye, at least 40 min should be elapsed, to obtain perfect dye distribution and migration. This excess amount of salt presence as compared to direct dye this facilitates the driving of dye molecules to the fabric surface, adsorption of the dye take up will takes place and uniform shade depth is formed except the design part. Due to the presence of caustic soda on the design area which enhances the mercerization process higher than the other part of the fabric on the specified place to give deep shade than the rest.

In reactive dying in order to see the effect of caustic soda on printing of cotton fabric, the following parameter should be controlled are:

A) Temperature: If the Temperature Increase, it cause an effect of Reactivity increases, Substantivity decreases, Migration improved, Hydrolization increased and Reactivity increases, which Result Low built-up and better levelling.

B) pH: If the PH Increase, it causes an effect of Reactivity increases, which Results Substantivity increases, Built-up increases, Migration improved and Hydrolization increased.

C) Electrolyte: High amount of Electrolyte, cause the effect of increases Substantivity, which results High built-up, Low migration, Low dye solubility and Substantivity increases.

D) Bath Ratio: Low Bath Ratio causes an effect of Less hydrolization and High economy, which intern Result in High build-up and less UN levelling. In case low bath ratio and high circulation dyeing machines, electrolyte concentration should be decreased. If bath ratio is increased, e.g. in sample dye units, salt amount should be increased. The rest experiment result shows the same effect as explained on the above.

5. Conclusion
Tone on tone printing of fabric is more advantages to save cost, dye and time, due to the presence of the caustic soda makes only the design area of the fabrics to be absorb more dye than the rest part. Printing is followed by dying this speed up and makes the processing step faster than the normal way.

The consumption of caustic soda is economical. The Concentrated solutions of caustic soda cause considerable swelling of cotton fibre this changes in cellulose makes its physical properties being irreversible which cause an increase the adsorption of dye to the fabric surface.

To prepare the design on the fabric a technical and conceptual person is need to facilitates the process faster but it is possible to create a design on a fabric by using methods of block and resist printing.

6. Future Scope of Work
Tone on tone printed cotton textile products has got a significant contribution to the product suppliers, by minimizing resource and material up take unlike the other printing techniques. Since time consumption of this printing techniques is less, its possible to print as many cotton textile products as possible in a very short period of time.

It is also be applied easily on screen printing without the use of pigment and binder. The existence of in adequate supply of variety of printed textile products in our country is one of the reasons for dissatisfaction of customers by the domestic product. Due to such merits, its highly recommended to apply tone on tone printing effects on cotton textile products like other printing techniques, in order to meet excessively evolving desires of the customer.

7. References

1. http://en.wikipedia.org/wiki/Textile_printing#Origins
2. http://quilting.about.com/od/quiltingglossaryt/g/toneontone.htm
3. http://www.bl.uk/onlinegallery/hightours/diamsutra/index.html
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Mazharul Islam Kiron

Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. Mr. Kiron is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.

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