Cost Reduction in Dyeing of Denim Garments by Using Natural Indigo Dyes

Last Updated on 22/03/2021

Cost Reduction in Dyeing of Denim Garments by Using Natural Indigo Dyes

Mustaque Ahammed Mamun
Department of Textile Engineering
Dhaka University of Engineering & Technology (DUET)
Email: mamuntex09@gmail.com

1. INTRODUCTION
A renewed international interest has arisen in natural dyes due to increased awareness of the environmental and health hazards associated with the synthesis, processing and use of synthetic dyes. Textile processing industry is one of the major environmental polluters as the effluent from these industries contains a heavy load of chemicals including dyes used during textile processing. There are two main ways to limit the environmental impact of textile processing. One is to construct sufficiently large and highly effective effluent treatment plants, and the other way is to make use of dyes and chemicals that are environment friendly. Natural dyes, when used by themselves have many limitations of fastness and brilliancy of shade. However, when used along with metallic mordants they produce bright and fast colors. Therefore, instead of using unsustainable technology for producing colors one can use mild chemistry to achieve almost similar results. The rich biodiversity of our country has provided us plenty of raw materials, yet sustainable linkage must be developed between cultivation, collection and their use.

Natural dyes can produce special aesthetic qualities, which, combined with the ethical significance of a product that is environmentally friendly, gives added value to textile production as craftwork and as an industry. Natural dyes are known for their use in coloring of food substrate, leather, wood as well as natural fibers like wool, silk, cotton and flax as major areas of application since ancient times. Natural dyes may have a wide range of shades, and can be obtained from various parts of plants including roots, bark, leaves, flowers, and fruit. Since the advent of widely available and cheaper synthetic dyes in 1856 having moderate to excellent color fastness properties, the use of natural dyes having poor to moderate wash and light fastness has declined to a great extent. However, recently there has been revival of the growing interest on the application of natural dyes on natural fibers due to worldwide environmental consciousness.

Although this ancient art of dyeing with natural dyeing with natural dyes withstood the ravages of time, a rapid decline in natural dyeing continued due to the wide available of synthetic dyes at an economical price. However, even after a century, the use of natural dyes never erodes completely and they are still being used. Thus, natural dyeing of different textiles and leathers has been continued mainly in the decentralized sector for specialty products along with the use of synthetic dyes Recently, a number of commercial dyers and small textile export houses have started looking at the possibilities of using natural dyes for regular basis dyeing and printing of textiles to overcome environmental pollution caused by the synthetic dyes.

2. LITERATURE REVIEW

2.1 Denim
Denim originated in the ancient textile centre of Nimes, France and was called “serge de Nimes”. Columbus is reported to have used denim for his sails on the Santa Maria. At approximately the same time in India, the sailors of Dhunga, were using denim material for their pants, which became known as dungarees. Later on, the word “jeans” came into existence from Genoa, Italy, where working men wore denim trousers. In 1849, Levi Strauss, an unsuccessful miner in the Californian gold rush in the U.S.A, became rich by making denim pants for the more successful miners. The first western “Levis” jeans were born. Over the next hundred years they remained working pants. About 1947, denim started to move into sportswear and rainwear. Since 1960, the jeans business has undergone a transformation from tough, cheap clothing to a fashion-conscious market. In 1970 the American youth adopted denim as their favorite material.

2.2 History of Indigo
The blue dye indigo has been used in India for about the last 4000 years. It was derived from the plant Indigofera tinctoria. Phoenican traders and migrating peoples gradually introduced this dye to the Mediterranian area and then spread to Europe. In Northern Europe from the Bronze Age (2500 – 850 BC) people used a blue dye, waod from the plant Isatis tinctoria. It has since been discovered that this plant contains the chemical indigo, but due to other compounds in the plant it is not a ‘pure’ blue like the Indigofera. Indigo is a vat dye. The plant was fermented and then treated with urine. The fibre was dipped into the colourless dye bath and then hung out in the sun to contain a blue insoluble dye on the fibre. In 1865, the German chemist Johann Friedrich Wilhelm Adolf von Baeyerbegan working with indigo. In 1880, his work resulted in the first synthesis of indigo and three years later the announcement of its chemical structure. BASF developed a viable manufacturing process that was in use by 1897, and by 1913 natural indigo had almost been replaced by synthetic indigo. In 2002, 17000 tons of synthetic indigo was produced worldwide.

2.3 Chemistry of Indigo
Today you will be synthesizing indigo using the Baeyer-Drewson reaction, which is an Aldol Condensation reaction shown in Figure 2.2. This is the method developed by J. F. W. Adolph von Baeyer in 1880 to produce the first synthetic indigo. This reaction works well for small scale reactions and is not used today in industry for producing large quantities of indigo.

Indigo is not soluble in water, so to dye cloth the indigo needs to be made into a water-soluble form. Therefore, indigo is called a vat dye. In this experiment, the insoluble indigo dye is synthesized and then reduced with sodium hydrosulfite (sodium dithionite), as shown in Figure 2.3, to the water soluble leucoindigo (sometimes called indigo white). When the clear yellow leucoindigo solution comes into contact with air it oxidizes back to the insoluble blue indigo compound.

2.4 How to Choose Natural Indigo
The type of dried indigo you purchase can have an impact on your dye vat, and the color of the items dyed. I prefer to use natural indigo that has already been powdered. The powdered form is easier to work with, but natural indigo also comes in lumps and cakes. If you purchase non-ground indigo, you will need to grind it. I highly recommend natural indigo over artificial indigo because it produces better colors. Historically, when artificial indigo became available in the nineteenth century, customers sought out goods dyed with natural indigo because it was considered superior. Natural indigo really is better – it can create a deeper blue, and, since it is not water soluble, you can wash natural indigo over and over without any color loss.

2.5 Needed Equipment for Indigo Dyeing
You will also need a jar or water bottle with a tight lid, a 2-gallon bucket with a tight lid, a pair of latex (or similar material) gloves, lye, thiourea dioxide (also known as color remover) and potassium hydroxide. Lye can be easy to find at a home improvement store being marketed as drain cleaner. For the thiourea dioxide and indigo, check online with Dharma Trading company or Aurora Silk. Both sites supply natural indigo dyeing products.

2.6 How to Prepare Indigo Stock
Next, prepare your “stock.” Mostly fill your jar or bottle with hot water. I just use water from the hot tap of the sink. Then, add 1 1/2 teaspoons of lye and 2 or more teaspoons of indigo powder. If you want your textiles to be sky blue, go with 2. If you want them to be a dark, midnight blue, use up to 5 teaspoons. I usually use 2 or 3. Then, screw the lid on tightly and shake for 2 minutes. Next, add 2 teaspoons of thiourea dioxide, recap and shake for an additional minute.

2.7 How to tell if Indigo Stock is Correct
I know it is difficult to tell in the photo because I used a green water bottle, but the stock should not be completely blue. It should have a bubbly, coppery blue/green/purple appearance. If it looks funky, you’re on the right track! Make sure the lid is on well to prevent extra unnecessary exposure to air and simply let the stock sit for at least 15 minutes. I usually let it rest for 30-40 minutes.

2.8 How to Prepare Indigo Vat
After your stock is ready, you can prepare the vat. I just put my 2-gallon bucket in the sink and fill it up with hot water. Then, you add 1/8 teaspoon of lye and stir until dissolved. I like using a big metal spoon, but you can use plastic, too. Do not use a wooden spoon to stir the indigo because the wood will actually absorb the dye. Next, add 1/2 teaspoon of potassium hydroxide and 1 teaspoon of thiourea dioxide.

The next part is a little difficult. You definitely want to wear your gloves. I only have the wrist-length gloves, but a pair of elbow-length gloves would be better. You must add the stock to the vat while introducing as little air as possible. I usually submerge the bottle, at least partially, and then remove the cap. Because indigo will only dye a material when it is deprived of oxygen, allowing excess exposure to air can ruin your vat. The vat will probably have a top foamy layer that turns blue. This is okay. However, the bulk of the liquid should be a deep green and the vat should have a coppery sheen. If your vat is blue, you can try to salvage it by mixing in more thiourea dioxide, but you may simply want to start over. After closing up your vat, let it rest for at least 15 minutes. Then, you are ready to dye.

It basically doesn’t bind at all. Wool, cotton, linen and silk are all fairly easy to dye with indigo. No mordant (a presoak solution that improves color retention with some natural dyes) is required when you dye with indigo. However, you should make sure to wash your fabric or other items before dyeing to remove any chemical finish that may have been applied in the manufacturing process. You should also soak whatever you want to dye in warm water for several minutes before dyeing. This encourages an even application of color. Make sure to wring the item out lightly before dunking it in the dye vat. Unlike most natural dyes, the length of time the items spend in the vat does not dictate the richness of the color. An item should only be submerged for a few minutes at a time. If you want a deeper or darker color, simply re-submerge it after it has dried. I usually hold the cloth in the vat to make sure it remains submerged. When you are ready to remove the item from the vat, try to wring it out a little while it is still submerged. When you remove it from the vat, attempt to avoid letting the runoff dye flow back into the vat. This dye is useless for re-dyeing after it has been exposed to air, so letting it run back into the vat is detrimental.

3. RESEARCH DESIGN

3.1 Diagram of Methodology
In order to compare by using synthetic reducing agent ( sodium dithionite) and natural reducing agent ( i.e made from date, apple, banana) in case of natural indigo dyeing on denim garments. After developing, both samples were taken. Then the different properties of both samples were measured and compared the results. Also, considering cost effective and eco-friendly aspect between two process.

3.2 Methods

3.2.1 Preparation of Natural Dye
Natural dyes are collected in hard particle form. Then it makes into powder form by grinder. After that it is converted into paste form with hot water by using blender. Finally prepare stock solution of natural dyes. For example, 10% stock solution. Preparation of Natural Reducing Agent from Date At first date is collected from market. Then separate seeds, husk etc. of date. About 200 gm date is taken into a pot by using eclectic balance to prepare one liter of reducing agent. Heat is raised at boiling temperature and cooling at 30⁰C. After that filtration is done and store in a suitable jar with flap.

3.2.2 Preparation of Natural Reducing Agent from Apple
At first apple is collected from market and chopping into small pieces with knife. One-liter natural reducing agent can get by synthesis 200-gram apple. Apple is taken into a pot with one-liter normal water. Temperature raised at 100⁰C and running about 30 minutes. After that filtration is done and store in a suitable jar with flap.

3.2.3 Preparation of Natural Reducing Agent from Banana
At first banana is collected from market and chopping into small pieces with knife. One-liter natural reducing agent can get by synthesis 200-gram banana. Banana is taken into a pot with one-liter normal water. Temperature raised at 100⁰C and running about 30 minutes. After that filtration is done and store in a suitable jar with flap. Preparation of Calcium Hydroxide (Lime) must be grinded into powder form and make paste form with hot water. After that filtration is done and store in a suitable pot.

3.2.4 Mehthod-1 (Natural Indigo Dyeing Process by Using Sodium Dithionite)

Indigo stock solution – Depending on the quantity you want to make soluble (reduced form).

Example: To reduce 10g of indigo

Prepare two pots as follows:

Pot A: Prepare a solution depending on the quantity to reduce

• Weight Indigo: 10g
• Pour in a jar with a lid of: 200 ml
• Add Alcohol (ethanol): 8-10g (methylated spirits may be used) homogenize until getting a consistent paste.

Pot B:

• Weight Water – heat up to 50°C (»120°F) 120 g
• Add Sodium carbonate, and then mix 10g
• Add Sodium hydrosulfite 10g

IMPORTANT:

ALWAYS pour sodium carbonate or sodium hydrosulfite IN water. Never reverse order.

GENTLY mix (avoid air in the solution). Close the pot. Allow 10 min at 50°C (»120°F) in order for ingredients to dissolve. Pour slowly pot B into pot A. Keep closed with gentle mixing. Allow 30 min at 50°C (»120°F) for the reduction process to complete. A well reduced stock solution must turn into a greenish-yellow color.

During this time, we recommend to get to steps 1 and 2.

1. Fibers must be rinsed in mild water with soap prior to dye bath
2. Prepare the dye bath (water containing 5 g/L of sodium hydrosulfite + 2 g / L of sodium carbonate). Heat up to 50°C (»120°F)
3. GENTLY pour the indigo stock solution into the dye bath, without using the sediment at the bottom (you also can determine and use the exact quantity necessary for your dye bath). If possible, set pH to 8-9.
4. Immerse the fibers and dye at 50°C (»120°F) with gentle mixing. Keep the dye tank closed. Allow fibers to cool into the bath at the end.
5. Remove dyed fibers from the bath. Quickly move into the air or under a tap water stream in order to make the oxidation of indigo. This will develop the blue colour.
6. In order to lower alkalinity on fibers it may be useful to make an acidic bath (pH 4 or 1 spoon of white vinegar per 10 L)
7. Rinse with mild water and soap, leave them drying If you want a more intense shade, you may dye again your fibers (humid) from the step.
8. Make sure first that the indigo bath is still under reduced form (greenish-yellow colour). If necessary, add a little sodium hydrosulfite and sodium carbonate and leave them dissolve. Washing and care of your natural dyed textiles.

3.2.5 Method -2 (Synthetic Indigo Dyeing Process)

Sample-1 (Recipe)

• Vat dye: 10 gm/L
• Sodium dithionite: 8 gm/L
• Sodium hydroxide: 8 gm/L
• Sodium chloride: 3 gm/L
• Wetting agent: 1.5 gm/L
• Sequestering agent: 1.5 gm/L
• Leveling agent: 1.5 gm /L
• Time: 55 minutes
• Temperature: 70⁰C
• M: L: 1: 20

Sample-2 (Recipe)

• Vat dye: 10 gm/L
• Sodium dithionite: 4 gm/L
• Zinc powder: 4 gm/L
• Sodium hydroxide: 8 gm/L
• Sodium chloride: 3 gm/L
• Wetting agent: 1.5 gm/L
• Sequestering agent: 1.5 gm/L
• Leveling agent: 1.5 gm /L
• Time: 55 minutes
• Temperature: 70⁰C
• M: L: 1: 20

3.2.6 Method-3 (Natural Indigo Dyeing Process with natural reducing agent)

Sample-1 (Recipe)

• Natural dye: 10 gm/ l
• Natural reducing agent (Date): 200 ml/ l
• Ca (OH) 2: 15 gm/ l
• Reducing Temperature: 80⁰C
• Dyeing Temperature: 30⁰C
• Time: 30 minutes

Oxidation is done by tap water for 5-10 minute until yellowish shade is diminishes.

Sample-2 (Recipe)

• Natural dye: 10 gm/ l
• Natural reducing agent (Date): 200 ml/ l
• Ca (OH) 2: 10 gm/ l
• Reducing Temperature: 80⁰C
• Dyeing Temperature: 30⁰C
• Time: 30 minutes

Oxidation is done by tap water for 5-10 minute until yellowish shade is diminishes.

Sample-3 (Recipe)

• Natural dye: 10 gm/ l
• Natural reducing agent (Date): 200 ml/ l
• Ca (OH) 2: 5 gm/ l
• Reducing Temperature: 80⁰C
• Dyeing Temperature: 30⁰C
• Time: 30 minutes

Oxidation is done by tap water for 5-10 minute until yellowish shade is diminishes.

Sample-4 (Recipe)

• Natural dye: 5 gm/ l
• Natural reducing agent (Date): 200 ml/ l
• Ca (OH) 2: 15 gm/ l
• Reducing Temperature: 80⁰C
• Dyeing Temperature: 30⁰C
• Time: 30 minutes

Oxidation is done by tap water for 5-10 minute until yellowish shade is diminishes.

Sample-5 (Recipe)

• Natural dye: 5 gm/ l
• Natural reducing agent (Date): 200 ml/ l
• Ca (OH) 2: 10 gm/ l
• Reducing Temperature: 80⁰C
• Dyeing Temperature: 30⁰C
• Time: 30 minutes

Oxidation is done by tap water for 5-10 minute until yellowish shade is diminishes

Sample-6 (Recipe)

• Natural dye: 5 gm/ l
• Natural reducing agent (Date): 200 ml/ l
• Ca (OH) 2: 5 gm/ l
• Reducing Temperature: 80⁰C
• Dyeing Temperature: 30⁰C
• Time: 30 minutes

Oxidation is done by tap water for 5-10 minute until yellowish shade is diminishes.

Sample-7 (Recipe)

• Natural dye: 10 gm/ l
• Natural reducing agent (Date): 250 ml/ l
• Ca (OH) 2: 20 gm/ l
• Reducing Temperature: 90⁰C
• Dyeing Temperature: 50⁰C
• Time: 15 minutes

Blend Method Sample-8 (Recipe)

• Natural dye: 5 gm/ l
• Synthetic dye: 5 gm/l
• Sodium dithionite: 4 gm/l
• Sodium hydroxide: 4 gm/l
• Natural reducing agent (Date): 100 ml/ l
• Ca (OH) 2: 8 gm/ l
• Sodium chloride: 3 gm/l
• Reducing Temperature: 80⁰C
• Dyeing Temperature: 50⁰C
• Time: 50 minutes

Oxidation is done by tap water for 5-10 minute until yellowish shade is diminishes.

Sample-09 (Recipe)

• Natural dye: 10 gm/ l
• Natural reducing agent (Date): 200 ml/ l
• Ca (OH) 2: 8 gm/ l
• Reducing Temperature: 80⁰C
• Dyeing Temperature: 30⁰C
• Time: 30 minutes

Oxidation is done by tap water for 5-10 minute until yellowish shade is diminishes.

Sample-10 (Recipe)

• Natural dye: 10 gm/ l
• Natural reducing agent (Date): 200 ml/ l
• Ca (OH) 2: 12 gm/ l
• Reducing Temperature: 80⁰C
• Dyeing Temperature: 30⁰C
• Dyeing Time: 30 minutes

Oxidation is done by tap water for 5-10 minute until yellowish shade is diminishes.

Sample-11 (Recipe)

• Natural dye: 10 gm/ l
• Natural reducing agent (Apple): 200 ml/ l
• Ca (OH) 2: 15 gm/ l
• Reducing Temperature: 80⁰C
• Dyeing Temperature: 30⁰C
• Time: 30 minutes

Oxidation is done by tap water for 5-10 minute until yellowish shade is diminishes.

Sample-12 (Recipe)

• Natural dye: 10 gm/ l
• Natural reducing agent (Banana): 200 ml/ l
• Ca (OH) 2: 155 gm/ l
• Reducing Temperature: 80⁰C
• Dyeing Temperature: 30⁰C
• Time: 30 minutes

Oxidation is done by tap water for 5-10 minute until yellowish shade is diminishes.

3.2.7 Dyeing Procedure (Vat dye)

Vat dyes dyeing process
Vat dyes do not dissolve in water, while when reduced to be leuco salt by reducing agent under alkaline conditions, they can dissolve in water and get feature of immediacy with cellulose fibers, which is the way to achieve the purpose of dyeing. Then stable shade and good color fastness would come out via oxidation and soaping.

Reduction of dyes
Insoluble reducing dye will change into soluble leuco. Different levels of the best staining methods are brought out according to the reduction dyes’ reduction potential and level of its immediacy to fibers.

Dye-uptake of leuco
The leuco is absorbed by fiber and then it is diffusing into the fiber. It is necessary to use soft water or softened water in dyeing process. Hereby it is suggested to add the water softener for 1g/L to some poor-quality cotton fabric. The water softener would not only absorb calcium, magnesium ions, but also the iron, copper ions which are absorbed by the pipe into the dye bath, and finally keep the solubility of leuco. The sodium sulfate could increase the dyeing absorption rate, and surely it is also a good promoter. So, it can be added into the dye bathes with medium or low immediacy according to the demand.

Leuco Oxidizing
The leuco absorbed by fiber is oxygenated and changed into the former insoluble vat dyes, and then it colors. Scour them in 2-3 baths with software before oxygenation, avoiding to be oxygenated with alkali. Hydrogen peroxide is most suitable for suspension padding, while after oxygenation, it should be in soap boiling immediately without washing. The sodium perborate is mild in very widely use. It can be bathed with the next procedure (referring to soap boiling).

Soaping treatment
The stable shade and good color fastness would come out via soap boiling. Through high-temperature soap boiling, the floating color can be removed and the crystallization of vat dyes on fabric would be enhanced, so the fastness is increased, the stable and full color is achieved. The soap lotion which is compounded by nonionic and containing chelating agents plays a better role in color light promoting.

3.2.8 Dyeing Procedure with Natural Indigo Dyes by Using Natural Reducing Agent (Date)

Recipe:

• Natural dye: 10 gm/ l
• Natural reducing agent (Date): 200 ml/ l
• Ca (OH) 2: 15 gm/ l
• Reducing Temperature: 80⁰C
• Dyeing Temperature: 30⁰C
• Time: 30 minutes

Process
At first prepared the solution of natural dyes, natural reducing agent and calcium hydroxide maintaining all required parameters. Then solution is taken in a dyeing bath and temperature of the dyeing bath raised at 80⁰C. The well reduced solution turns change into green-yellowish color. Then solution is cooled at 30⁰C. After that fabric is immersed in the dye bath for 30 minutes at 30⁰C. Then oxidation is done by using tape water about 10 minute. The dyed sample change into indigo from yellowish. Then drying in electric oven.

The diagram of total pretreatment and dyeing process was as below

3.3 Quality Parameters
The different quality parameters of finished fabric were measured in laboratory of the factory as well as the laboratory of the department of Textile Engineering DUET, Gazipur following appropriate procedure.

3.3.1 Color Fastness to Rubbing Test

Test Method: ISO 105×12

Principle:
This test is designed to determine the degree of color which may be transferred from the surface of a colored fabric to a specify test cloth for rubbing (which could be dry and Wet).

Equipments:

1. Crock meter
2. Grey scale
3. Stop watch
4. Color matching cabinet

Size of Fabric:
Fabric sample size 14 Cm × 5 Cm (one warp direction/wale direction and other weft/course direction).

Procedure:

• Lock the test specimen (sample fabric) onto the base of the crock meter.
• Using the spinal clip, set 5Cm ×5Cm of the white cotton fabric to the finger of the crock meter.
• Lower the covered finger on the test sample.
• Turn hand crank at the rate of the one turn per second.
• Remove the white rubbing test cloth and evaluate with grey scale.

Evaluation
In this stage compare the contrast between the treated and untreated white rubbing cloth with grey scale and rated 1 to 5.

3.3.2. Color Fastness To Perspiration

Test Method: ISO 105-EO4

Principle:
Test specimens of colored sample fabrics are wet out in perspiration solution, and subjected to fixed mechanical pressure and allowed to dry slowly at a slightly elevated temperature, in contact with multi-fiber fabric, and any color transfer is evaluated.

Apparatus:

1. Multifiber fabric
2. Gray scale for staining
3. Gray scale for color change

Table 3.1 Recipe for perspiration test

Procedure:

• Cut a specimen 10cm × 4cm of the dyed material.
• Immerse test specimen in freshly prepared solution for 30 minutes.
• Immerse multifiber test fabric (about the same size as the test specimen) in freshly prepared solution for 30 minutes.
• Take out a test specimen and a multifiber test fabric from the solution and squeeze them enough so the excess solution is removed.
• Place all 21 plates into the unit (perspiration tester or perspirometer) regardless of number of specimens. After final plate is in position, set top of tester in place. Set 12.5 KPa. Weight on top. Lock pressure plate in place by turning thumb screws. Remove weight and place the unit lying on its side in oven.
• Place in oven at 37°C for 4 hours.
• Remove tester from oven and separate multifiber fabric and test fabric.
• Then the multifiber fabric and test fabric allow to dry at 210C +/-10C.
• Both specimens are then assessed for colour change of the test fabric and staining of the adjacent fabric.

3.3.3 Washing Fastness Test

Test Method: ISO 105C06C2S
The ability of a dyed fabric to retain its original shade against fade during washing is known as wash fastness.

The ISO wash fastness test conditions

Procedure:
Cut a specimen 10cm x 4cm of the dyed material. Most wash fastness tests are carried out in enclosed 2000ml vessels that are rotated at a constant speed 40±2 rpm and at a constant temperature in a wash wheel. The fabric adjacent material and wash liquor are placed into the test vessel. Some tests require the addition of stainless-steel balls or discs to the wash liquor to increase the severity of the test.

3.3.4 Light Fastness Test

Test Method: ISO B02

Principle:

1. A test specimen together with blue wool standards is exposed to the light from a mercury ballast tungsten fluorescent lamp (MBTFL) fading lamp.
2. The color fastness of specimen is assessed by comparing its fastness with that of 1-8 Blue Wool standard

Procedure:
This test measures the resistance to fading of dyed textiles when exposed to daylight. The tested specimen is exposed for a certain time (12h, 20h, 24h, 36h, 72h, etc. or as customer demand) under the light, and compared the changes with the original unexposed specimen. These changes are assessed by numerical expression which is made by blue light fastness scale. In textiles, there are two sets of blue wool reference standard in use. Those used in Europe are identified by the numerical designation 1 to 8. They have range from 1 (very low light fastness) to 8 (very high light fastness). The blue wool references used in America are identified by L2, L3, L4, L5, L6, L7, L8 and L9.

Table 3.2 Description of the light fastness grades

Light source may be used-

1. Daylight B01
2. Xenon Arc B02
3. Mercury tungsten fluorescent lamp (MTFL) etc.

3.3.5 Measurement Reflectance by Spectrophotometer
The wavelengths of the light reflectance (of light) by an opaque objects or transmittance of light through a transparent object describe the color of the object. The spectrophotometer measures color by measures the reflectance or transmittance of the light. Spectrophotometer has three essential parts – a light source, a monochrometer and a detector.

Cloth reflectance was measured from each cloth at the wavelengths between 400 nm and 700 nm intervals by a spectrophotometer CM-3700d. The result is analyzed by a CIELAB color system (JIs Z 8729 [19994]). Here the values of a* and b* indicate the direction of color, that is, +a* denotes the red direction, -a* denotes the green direction, +b* denotes the yellow direction, and –b* denotes the blue direction. C* and L* denote chroma and lightness, respectively. The hue angle (hab) is defined as the angle of the anticlockwise movement from an axis of +a* direction, that is, +a*=00, +b*=900, -a*=1800, and –b*= 2700. The values of C* and hab are calculated from a* and b* as

C* = (a*² + b*²)1/2

hab= tan^-1(b*/a*)

A mean and standard deviation of twelve measurements were calculated. Standard deviations were standard deviation L* = 0.0053, standard deviation a* = 0.00114 and standard deviation b*= 0.0057.

R denotes reflectance. K/S values were calculated according to the formula of Kubelka and Munk.

K/S= (1-R)²/2R

3.4 Materials and Equipments

3.4.1 Materials

• Plain fabric (construction EPI = 117, PPI= 76, warp count = 33, weft count = 27 fabric width = 59 inch )
• Twill fabric (construction EPI = 120, PPI= 62, warp count = 12, weft count = 8 fabric width = 60 inch )
• Natural indigo dyes
• Synthetic indigo dyes
• Sodium dithionite
• Sodium hydroxide
• Calcium hydroxide
• Natural reducing agent
• Zinc powder
• Ferous sulphate
• Sodium carbonate
• Ethanol
• Hydrogent peroxide
• Wetting agent
• Sequestering agent
• Levelling agent
• Detergent
• Acetic acid
• Peroxide killer
• Stabilizer

3.4.2 Equipments

• Dyeing pot
• Thermometer
• Striker
• Burner
• Beaker
• Cylinder
• Electric balance
• Sample dyeing machine
• Light box
• Spectrophotometer
• Light fastness tester
• Dryer
• Crockmeter
• Perspimeter

4. COSTING

4.1 Costing of Synthetic Indigo Dyeing Process

Table 4.1 Dyes and chemical cost

25 L solution required to dye 1 kg garments

Cost of dyeing = 25× 112.09
= Tk. 2802.25

Energy costing for synthetic indigo

Gas burner release = 0.97 cubic feet (CCF)
Supplier charges = 60 Tk./CCF
Cost of gas burner = 60×0.97 Tk./hr = 0.97 Tk./Min

Synthetic indigo requires = 65 minutes
So total cost of gas = 65 × 0.97 = Tk. 63

4.2 ETP Costing

a) Chemical Costing-

Table 4.2 Costing of chemical for processing waste water in ETP

[Source: TEX EUROPE (BD) LTD, GAZIPUR]

b) Manpower Cost (Per Hour):

i) Operator = 2×7000 = Tk. 14,000
ii) Helper = 3×4000 = Tk.12,000
iii) In charge = 1×30000 = Tk. 30,000

Total Cost = Tk. 56,000
Cost per hour = Tk. 49.27

c) Electricity Cost per Hour:

The calculated Unit Electricity Cost is Tk. 9.00.
Supplied Amp for ETP is 150,

Then, Total KW for an hour = (√3 VI cos⁡θ)/1000
= (√3×440×150×0.8)/1000
= 91.45 KWH

Electric Bill Per Hour = 91.45×9 = Tk.823 (approx.)

d) Cost required for an hour to run ETP:

Cost Per Hour = (Chemical Cost + Manpower Cost + Electric Cost)
= 2056.9 + 49.67 + 823
= Tk. 2929.5

e) Treatment Cost (Tk. / m3) = Tk. 73.24

f) Treatment Cost (Tk. / L) =Tk. 0.07324

Synthetic indigo requires 35 L water for ETP treatment.

Cost of ETP = 35 × 0.07324

= 2.56 Tk./ Kg

[Source: 
1. The man power from Ananta Denim Technology
2. The price of electricity (Tk./KWH) is taken from Bangladesh Energy Regulatory Commission.]

Total costing of synthetic indigo dyeing process = (cost of dyes + cost of energy +cost of ETP)
= (2802.25+63+2.56)
=2867.81 Tk./Kg

Total costing of synthetic indigo dyeing process =Tk. 286781 /100 kg

4.3 Costing of Natural Indigo Dyeing Process with Sodium Dithionite

Table 4.3 Dyes and chemical cost

25 L solution required to dye 1 kg garments

Cost of dyeing = 25× 83.4
= Tk. 2085

4.4 Energy costing for natural indigo with synthetic reducing agent

Gas burner release = 0.97 cubic feet (CCF)

Supplier charges = 60 Tk./CCF

Cost of gas burner = 60×0.97 Tk. /hr
= 0.97 Tk. /min

Natural indigo with sodium dithionite required = 40 mins

Total cost of gas = 40 × 0.97
= Tk. 38.8

ETP Costing

Cost of ETP = 2.56/ kg

Total costing of natural indigo with synthetic reducing agent
= (Cost of dyes + cost of energy +cost of ETP)
= (2085 +38.8 + 2.56)
= Tk. 2126.36

Total costing of natural indigo with synthetic reducing agent= Tk. 212636 / 100 Kg

4.5 Costing of Natural Indigo Dyeing Process with Natural Reducing Agent

Table 4.4 Dyes and chemical cost

Similarly cost of dyeing = 45.2 × 25
=1130 Tk./Kg

4.6 Energy costing for natural indigo with natural reducing agent

Gas burner release = 0.97 cubic feet (CCF) /hr.

Supplier charges = 60 Tk./CCF

Cost of gas burner = 60×0.97 Tk. /hr
= 0.97 Tk. /min

Natural indigo with natural reducing agent = 10 min

Total cost of gas = 10 × 0.97= Tk. 9.7

Total costing of natural indigo with natural reducing agent,
= (Cost of dyes + cost of energy +cost of ETP)
= (1130 + 9.7 + 0)
= Tk. 1139.70

Total costing of natural indigo with natural reducing agent = Tk. 113970 / 100 Kg

Cost saving against synthetic indigo dyeing process per 100 kg = (286781- 113970)
= Tk. 172811

Cost saving against natural indigo dyeing process with sodium dithionite per 100 kg,
= (212636- 113970)
= Tk.98666

5. ECOLOGICAL ASPECT

5.1 Sodium Dithionite
Sodium dithionite has strongly reducing properties and decomposes/disproportionates rapidly in aqueous media (especially under acidic conditions and under oxygen consumption) to sulfite, SO₂ and sodium thiosulfate (Na₂S₂O₃) as major decomposition products (BASF AG, 1988a).

According to Hofmann and Rüdorff (1969) and Holleman and Wiberg (1995) (see also BASF AG,1988a), this process can roughly be described by the following equations:

2Na₂S₂O₄ + H₂O → Na₂S₂O₃ + 2NaHSO₃ (anaerobic conditions) (1)
Na₂S₂O₄ + O₂ + H₂O → NaHSO₄ + NaHSO₃ (aerobic conditions) (2)

Under aerobic conditions and with low concentrations, reaction (2) is favoured.

The formation of hydrogen sulfite (HSO_3-) and hydrogen sulfate (HSO_4-) lowers the pH of the media and accelerates the process of decomposition strongly. Therefore, to keep solutions of dithionite stable for several days, they need to be cooled, kept in an alkaline state by excess of NaOH and oxygen has to be excluded. According to the literature overview of Münchow (1992) the following principal decomposition patterns can be described for dithionite in relation to pH ranges at temperatures between 0°C and 32°C for 0.0025 molar solutions.

5.2 Using of Sodium Dithionite from the Environmental Point of View
During industrial use as reductive substance, sodium dithionite is oxidized to sulfate, going to Wastewater/hydrosphere. During use as consumer product (color remover) it is oxidized to sulfate. Remaining product is rapidly hydrolyzed and oxidized in wastewater and wastewater treatment plants. The chemical is currently of low priority for further work. The chemical possesses properties indicating a hazard for the environment. These hazards do not warrant further work as they are related to acute toxicity which may become evident only at very high exposure levels. They should nevertheless be noted by chemical safety professionals and users. Sodium dithionite dihydrate is very sensitive towards atmospheric oxygen in the finely crystalline state and oxidizes under heat development: the heat of oxidation can lead to ignition, e.g. upon contact with moisture. The anhydrous salt decomposes exothermically in air on prolonged heating above 90°C (decomposition/oxidation products: sodium sulfate (Na₂SO₄) and sulfur dioxide (SO₂)). Above ca. 150°C, in exclusion of air, vigorous decomposition occurs, yielding mainly sodium sulfite (Na₂SO₃), sodium thiosulfate (Na₂S₂O₃), sulfur dioxide (SO₂) and a small amount of sulfur. Because of decomposition on heating, boiling point and melting point are not relevant. The vapour pressure is negligible and the Henry constant is near to zero due to the ionic character of the inorganic salt. Biodegradation or elimination tests are not appropriate for the inorganic substance. Hydrolysis occurs within hours at pH 7 and room temperature. There is no indication of a bioaccumulation potential. Main hydrolysis products are thiosulfate (S₂O₃ 2-) and sulfite (SO₃ 2-). Small amounts of sulfur and sulfide (S2-) have been detected during oxygen-free hydrolysis. Oxygen dissolved in water is consumed by dissolved sodium dithionite. Final oxidation products are sulfate (SO₄ 2-) and sulfite (SO₃ 2-).

Because of the high-water solubility at 20 °C of 182 g/l (value related to formula Na₂S₂O₄) and 219 g/l (related to formula Na₂S₂O₄ * 2H₂O) respectively, for hydrated sodium dithionite, aquatic environment is the target compartment. Sodium dithionite is expected not to be stable in soil because of its rapid decomposition in water and the reaction with oxygen.

From acute toxicity test to fish, 96-hr LC50 was 62.3 mg/l. For algae), 72-hr ErC50 was 206 mg/l and 72-hr NOErC was 62.5 mg/l (corresponding values for biomass are 135 and 62.5 mg/l respectively; nominal concentration). For Daphnia magna, the acute toxicity value of 48-hr EC50 was 98.3 mg/l, and the chronic value of 21-day NOEC was > 10 mg/l. Due to oxygen concentrations < 1 mg/l at test start in high test concentrations in the fish and acute daphnia test, it cannot be excluded that the effect values found in these studies are at least partly caused by oxygen deficiency. A PNEC of 0.1 mg/l for the aquatic organisms was calculated from the chronic value (NOEC for daphnia > 10 mg/l) using an assessment factor of 100.

5.3 Natural Reducing Agent
In many parts of the world chemicals are used to quickly prepare an indigo vat. Sodium hydrosulfite or thiourea dioxide are both commonly used as reducing agents. A reducing agent removes the oxygen from a solution. In doing this, the reducing agent also takes the oxygen from the indigo molecule. With the oxygen removed, indigo becomes soluble in water at room temperature. A reducing agent is necessary to make an indigo solution. Without it the powdered indigo is suspended in water but not actually dissolved. What is the difference between a suspension and a solution? A fish is suspended in the ocean. But salt is dissolved in the ocean. You can see the fish (which remains distinct) you cannot see the salt (which has dissolved by being broken down into separate components).Many natural substances will behave as reducing agents. From the environmental point of view, particularly the textile dyeing process constitutes a major pollution problem due to the variety and complexity of chemicals employed. In most industrial vat and indigo, dyeing processes, all of them are reduced mainly using sodium dithionite. This process produces large amounts of hazardous by-products which increase the costs for waste water treatment. Hence, many attempts are being made to replace the environmentally unfavorable sodium dithionite by ecologically more attractive alternatives, such as natural reducing agent.

5.4 Calcium Hydroxide
Calcium hydroxide is one component of a cyclical process, where calcium carbonate (CaCO₃) is either heated or calcinated to a temperature of 1100° C, forming calcium oxide (CaO). Through the addition of water via hydration or slaking, calcium oxide becomes calcium hydroxide (Ca(OH)₂). The final piece is when carbon dioxide is added to the calcium hydroxide, reproducing calcium carbonate and starting the whole cycle When dissolved in water, hydroxides dissociate to produce free hydroxide ions (thus raising the pH of the solution) and the counter metal cations:

Ca (OH)₂ = CaO + H₂O

The hydroxide ion may then react with any free H+ or any acidic species in the system, forming water:

OH- + H+ = H₂O

The solubility of Ca(OH)₂ in solution is 1.85 g/L H₂O; this solubility is affected by pH, temperature, and the presence of aquatic species in the solution. Increased pH lower solubility because a higher OH- concentration lowers the available solid hydroxide that can dissociate into free OH- ions and metal ions. With increased temperature, the calcium hydroxide solubility decreases (ESIS, 2000).

5.6 Using of Calcium Hydroxide from the Environmental Point of View
Calcium hydroxide dissociates slowly in water into form calcium oxide and water. When calcium hydroxide comes into contact with carbon dioxide it creates an insoluble form of calcium carbonate. Acute ecotoxicity tests with fish showed LC50’s ranging from 33.9 mg/L to 240 mg/L. After calcium hydroxide is used in the treatment of wastewater and biosolids, a great deal of it will be discharged with the remaining wastewater into natural aquatic ecosystems. Calcium hydroxide would inevitably come in contact with carbon dioxide during this process and leave behind an insoluble form of calcium carbonate. In aquatic ecosystems, calcium hydroxide binds to phosphorous, making it unavailable in the system. This lowers the risk of large-scale algae blooms in lakes. Calcium has a very low toxicity, and the amount being emitted is relatively low in comparison to the natural background of calcium in water. Based on the available data, the use of calcium hydroxide in the treatment of wastewater has no adverse effect on the aquatic ecosystem.

6. CONCLUSION:
The goal of the research is the use of non-toxic and eco-friendly natural dyes on textiles has become a matter of significant importance because of the increased environmental awareness in order to avoid some hazardous synthetic dyes. However, worldwide the use of natural dyes for the coloration of textiles has mainly been confined to craftsman, small scale dyers and printers as well as small scale exporters and producers in the large scale sector for general textiles owing to the specific advantages and limitations of both natural dyes and synthetic dyes. Dealing with high valued eco-friendly textile production and sales. To fulfill the above advantages of natural dyes we feel this research.

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Mazharul Islam Kiron

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

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