Now You Know Dyeing of Eri Silk with Vat Dyes

Dyeing of Eri Silk with Vat Dyes

Arunkumar.V
Indian Institute of Handloom Technology
Email: vijayan401arun@gmail.com

 

Chapter 1

Introduction:
Silk is known as the queen of all textile fibers because of its soothing luster and elegance. None of the natural or manmade fibers have been able to rival its versatility and share its beauty. Lustre, softness, elasticity, strength, drape, absorbency and affinity for dyes and its adaptability to various forms of twisting continue to meet a variety of market demand has made silk as a highly valued textile fibre. India produces all the four varieties of silks namely Mulberry, Eri, Tasar and Muga. Eri silk is the second largest variety of silk being produced in India. The production went up from 93 Metric Tons in 1950s to 1500 Metric Tons in the present senario; which is nearly 15 times more. Eri silk is a variety of wild silks and unlike other varieties it does not come in continuous filamet form. Eri cocoons are open mouthed in nature. Eri silk is having good aesthetic values cause of smoothness, luster, liveliness and improved thermal properties. Fine suitings, ladies wear, fine knit wears produced using Eri silk are having great market demand. [1]

Eri silk is important amongst non-mulberry silks in India. Besides possessing commercial value, it is well documented to possess high medicinal value as per the traditional folk knowledge. India is the sole producer of Eri silk at global level. In India the production of Eri silk is confined to north eastern states and very little is known about this silk in other parts of the country. Although detailed information is available on Eri silkworm rearing for the production of good quality cocoons, information on Eri silk fibre material and yarn characteristics is scanty. [1]

Dyeing is a process of application of colour to the textile material in scientific and systematic way. Normally the dye liquor consists of dye, water and auxiliaries. To improve the effectiveness of dyeing, heat is usually applied to the dye liquor. Although Acid dyes, Metal complex dyes, Reactive dyes are popular in silk industry. Some criteria like overall fastness properties of vat colour have been the attention of processsors to use them in dyeing of vanya silk. Genarally vanya silk is showing higher overall resistance compared to mulberry silks can be safely dyed with special criterion colours. Vat is basically an insoluble in water produces a good fastness properties compared to regular acid and metal complex dyes. Keeping eye on production of washable silk an attempt has been made to optimize dyeing of Eri silk with vat dyes. [2]

Large quantity of water is used in processing of silk. Liquor ratio, type of silk, & class of dyes are certain criteria will decide the effluent load. As quantity of water is used in vat dye is less compared to other process by controlling few parameters like pH, Time, and Temperature, concentration of Sodium hydroxide & sodium Hydrosulphite, we can reduce the BOD & COD.As wet processing industry is one of the industries in the textile sector which uses large quantity of water and this water after the process is released out without treatment will have a direct impact on the ecology due to the release of toxic and carcinogenic substances.

“Vat dyes” are a special class of dyes that work with a special chemistry. The name Vat was derived from the large wooden vessel from which vat dyes were first applied. Vat dyes provide textile material with the best color fastness of all the dyes in common use. Vat dyes are an ancient class of dye based on the original natural dye Indigo, which is now produced synthetically and its close chemical relative historic tyrain purple. Vat dyeing means dyeing in a bucket or vat it can be done whenever a solid even shade is required. [3]

Vat dyes probably the oldest dye known to man, is one of the most important members of this group.

• The Egyptians used natural indigo extracted from the plant ‘Indigofera tinctorie’ in 200 BC.
• The first synthetic indigo was introduced to the textile trade in 1897 & had the effect of completely replacing the natural product.
• Although the vat dyes may be divided into 3 chemical groups, they are similar in that they are insoluble in water & become water soluble when reduced in the presence of an alkali.[3]

Effect of alkaline condition on yarn strength:

• Formation of leuco compound is an essential process of reduction of the colour later by means of oxidation; colour is regained on to the fabric. This all happens in an optimal alkaline condition and a critical temperature between 50˚-60˚ C.
• Here optimization of parameters like concentrations of Sodium hydroxide and Sodium hydrosulphite were studied separately in gaining optimum colour strength. Further, effect of duration on dye uptake was also studied at different intervals. Similarly, dye uptake was observed at different temperature profiles (50˚-60˚ C). Finally all the parameter were taken together to gain the optimum colour strength on to the silk. Loss of strength is an unavoidable phenomenon when protein fibres are in alkaline conditions. As Eri silk is more alkali resistant than mulberry silk, an attempt been made to achieve maximum colour strength at minimum strength loss.

Chapter 2

Objectives of Project Work:

• To study the application of vat dyes on Eri silk with minimal strength loss.
• To optimise the process conditions such as the concentrations of sodium hydroxide and sodium hydrosulphite, duration of dyeing, dyeing temperature, etc. For vat dyes on Eri silk.
• To study the effect of process conditions such as the concentrations of sodium hydroxide and sodium hydrosulphite, duration of dyeing, dyeing temperature, etc. on mechanical properties of Eri silk.
• Evaluation of colour fastness of Eri silk yarn dyed with vat dyes.

2.1 Limitations:

• Did not use H2O2 (Hydrogen peroxide) there is likely hood that evaporation of the liquor followed by precipitation of dye to oxidation persists.
• It is necessary to conduct dyeing in airtight compartment, due to experimental limitation the reduced form of vat dye could not diffuse in to the fibre matrix.

Chapter 3

Literature Review:
This chapter aims at presenting the relevant reported researches available, pertaining to the present investing of the specific review related to the topic of “Studies on Application of Vat Dyes on Eri Silk” are very scanty. Hence, the reviews related to the topic are also been included and classified under the following sub headings.

3.1 Queen of Textiles and Its Uniqueness:
Silk fibre is known for its strength, fineness, lustre and elasticity. Silk used for manufacturing various dress materials and house old items that have good aesthetic value. The textile dictionary (1970) defines silk as a “Long Strand Reeled from Number of Cocoons”.

According to C.D. Koshy (1993) silk is the queen of textiles. Gulrajani (1993) says, silk is a strong, lustrous fibre extruded by certain kinds of moth and spiders. The cultivated silk variety is produced by the species bombyx mori. [2]

According to Sadov, et.al (1973) silk, in contrast to all the other natural fibres, does not have a cellular structure. In this respect, as well as in the way it is formed, it closely resembles artificial and synthetic fibres. [4]

According to Katherine Paddock Hess (1974) the characteristics of silk that have made it the question of fabrics for many centuries, have never been equalled in any other material. The beauty and elegance of this fibre will prevent it from disappearing from the textile field. Silk has never lost its luxury appeal. [5]

3.2 Introduction to Eri Silk:
Sericulture in India can be broadly classified in to two distinct sectors viz., mulberry and non-mulberry. Mulberry sericulture is practiced mainly in the states of Karnataka, Tamil Nadu, Andhra Pradesh, West Bengal and Jammu and Kashmir. Non-mulberry silk is mainly confined to the states of Bihar, Jharkhand, Orissa, West Bengal, Assam, Meghalaya and Andhra Pradesh. Of this Eri silk, mainly produced from the North Eastern states of India, positioned next only to Tasar silk from the commercial importance point of view.

The name Eri derives from the Assamese word ‘Era’, which means castor plant, the main food plant of this silkworm. Samia Cynthia ricini a multivoltine silkworm commonly called as ‘Eri silkworm’ is known for its white or brick-red Eri silk. It is distributed in Northeastern part of India, China and Japan. The primary food plant of this polyphagous insect is castor (Ricinus communis L.), but it also feeds on a wide range of food plants such as Heteropanax fragrans, Manihot utilissima, Evodia flaxinifolia, Ailenthus gradulosa etc.

The wild samia Cynthia ricini silkworm completes one to three generations per year depending on geographical position and climatic conditions of the region, however, up to six generations occur in the domesticated cultures. Populations of Samia Cynthia ricini, that have been commercially exploited and are present in different regions of north-east India show wide morphological and quantitative variations in characters such as silk content, larval weight, cocoon weight, cocoon shell weight and silk ratio. Eri silkworms were successfully acclimatized in America and Europe, but could not take firm hold.

Recent trends suggest a steady gaining popularity for non-mulberry silks in the export market due to their unique natural characteristics. Thus there is a strong need for increasing Eri silk production. Eri silk in addition to its fibre value, it is known for its traditional medicinal value in the north eastern states.

The wild Eri silkworm Samia Cynthia is generally uni, bi or trivoltine. The commercially exploited S.ricini is multivoltine and has eco-races like Nongpoh, Kokrajhar Red, Borduar local, Titabar local Sille, Dhanubhnga, Mendipathar and Khanapara based on the locations of their collection. Eri silk is a fibre available in cocoon form and is non continuous. It can be opened and cut to the required length. [6]

3.3 History of Dyeing:
The history can be traced back to at least 4000 years, as dyeing is one of the ancient crafts. It was a guarded secret until the middle of the last century; all dyes were of natural products, extracted from a variety of plants and few animal sources. The roots, stem, leaves, flowers and fruits of various plants supplied different colours like red, blue, black, brown and from animal source important red extracted from certain dried insects. The famous Tyrian purple was produced from molluscs found on Mediterranean Sea shores.

Mixing blue, yellow and red (Primary Colours) to produce any desired hue is usually considered a development of modern times made necessary for using colours in printing and photography as well as in dyeing. Fabrics found in the tombs of ancient Egypt reveal that the craft of dyeing was known at least as early as 2500 BC. It is believed to have originated in India (due to the name indigo) and have spread westward to Persia, Pheonicia and Egypt.

The mid-19th century was a time of great advances in the understanding of organic chemistry and the new dyestuff industries emerged with academic work and industrial scientists working hand in hand. Kekule and Couper independently postulated the tetra valency of carbon in 1858. Benzene cyclic structure was put forward by Kekule and many other significant contributions were made during this period. In 1856 an astonishing achievement by W H Perkin, who as a student of Royal College of Chemistry accidentally prepared violet substance that lead to the manufacture of violet colour dye.(Mauve dye). The First World War though delayed developments, but by the 1920’s the trend for improved fastness property lead to renewed interest in anthraquinone vat dyes of greater structural complexity i.e. Green vat dye, Caledon Jade Green by Scottish Dyes. 1925 saw the great discoveries in the history of colour chemistry. Since the 2nd World War in 1956 the most important development is the introduction of ICI of Procion Reactive dyes for Cellulose fibres.

3.4 Mechanism of Dyeing on Silk:
Recent times have seen an entrenchment in the dyestuffs industry with elimination of many dyes in existing ranges as the improvement of synthetic routes established different ranges and more profits.

Four principal properties in the dyes are important. They must have:

1. Intense Colour
2. Solubility in an aqueous solution
3. Ability to adsorb and retain by the fibre
4. Ability to withstand treatment after fibre fixation – Fastness property.

The Textile fibres are composed of molecules that are very long and flexible. All such fibrous molecules can be said to be polymeric in nature, consisting of hundreds to thousands of identical chemical repeat units joined end to-end form polymeric chains held together by inter chain forces and bonds. Although chemical structure has a marked effect on dyeing properties, the microphysical state of the fibre influence the dye ability by controlling the access of water and dye molecules into the internal areas of the fibre structure. The chain-packing characteristics of the linear polymeric molecules are explained in terms of models showing arrangement in different degrees of order varying from completely disordered or amorphous condition to partially oriented crystalline condition. Both orientation and crystallinity influence the kinetics and equilibrium uptake of water and dye molecules by fibres. However the degree of orientation in natural fibres cannot be controlled which can be done in case of synthetic fibres.

By looking into theoretical aspects of dyeing the chemical interaction responsible for fibre-dye bonds are broadly of four types. These are:

• Hydrogen Bonds
• Vander Waals forces
• Electrostatic or Ionic forces
• Covalent bonds

Rate of successful dyeing process can be phased into three principal stages:

• Diffusion of dye in the dye bath to the surface of the fibre.
• Adsorption of dye at the surface of the fibre.
• Diffusion of dye from fibre surface to its interior.

Dyeing is the process of colouring textile materials by immersing them in an aqueous solution of dye. Normally, the dye liquor consists of dye, water and auxiliaries. To improve the effectiveness of dyeing, heat is usually applied in dye liquor. Theory of dyeing explains the interaction between dye, fibre, water and dye auxiliaries.

It explains:

• Forces of repulsion, which are developed between dye molecule and water.
• Forces of attraction, which are developed between dye molecule and fibre.

These forces are responsible for the dye molecules leaving the aqueous dye liquor entering and attaching them to the polymer of the silk fiber. Silk is an organic compound and develop a slight negative charge or potential when immersed in an aqueous solution. Acid dyes are generally, sodium salts of sulphonic acids and are negatively charged. Since the dye molecule and silk fibre both become slightly negatively charged in an aqueous solution.

There is a tendency for the dye and the fibre to repel each other. Sufficient energy has to be built up in the dye liquor to overcome this repulsive force and allow the dye and silk fibre to be attract to one another, so that the dye molecule can enter the polymer system of the silk fibre.Water, in addition to dissolving the dye, acts as the medium through which the dye molecules are transferred into the silk fibre. The polar group in the dye molecule attract water molecule and thus the dye dissolves. This attraction between water and dye is undesirable, as the dye restrict leaving the water and entering the fibre. Heating of dye liquor causes water to dissociate and tend to repel the organic dye molecule to a greater extent, ensuring readier uptake of the dye molecule by the silk polymer.Although these dyes are popular it is very difficult to achieve overall fastness of the silk fabric during wet as well as dry condition.

3.5 Vat Colours:
Vat colours have the best fastness. In vat there is no reaction, the molecule size of vat dyes is large it is made smaller by reducing and then again assembled to its original size by oxidising. Once they are inside the fiber they cannot come out.

Dr. Tanveer Hussain in his work on “Important Considerations in Dyeing with Vat Dyes” defined vat dyes applied to Mercerized cotton gives a higher rate of dyeing as compared to un-mercerized cotton which in turn gives higher rate than the grey material. At low temperature, the rate of exhaustion is low which might promote levelness but the rate of diffusion is also low. At high temperature, the rate of exhaustion is high which might decrease levelness but the rate of diffusion is high. Maximum exhaustion, penetration and levelness can be obtained by starting the dyeing at low temperatures in the leuco stage and slowly raising the temperature. Some dyes may not be stable to very high temperatures, so the stability of dyes to temperature must be taken into account. The reducing efficiency of sodium hydrosulphite in caustic soda solutions at high temperatures decreases rapidly in the presence of air. The higher the liquor ratio, the slower is the rate of dyeing. Most of the dyes exhaust more rapidly at low concentrations, increasing the risk of unlevel dyeing in light shades. Some have the same rate of dyeing irrespective of the concentration. The higher the concentration of electrolyte, the higher is the rate of dyeing. [8]

In the article “Evaluation of Vat dye Solubility Using Derivative Spectrophotometry” E.Ekrami, M.Ahmadi Kamarposhti, H.Tayebi, A.Yosefi and H.Goodarzian explained that Spectrophotometry evaluation and control of vat dye solutions usually encounters with serious difficulties due to the presence of light scattering insoluble dyes. In the present study virtual absorptions of light scattering dyestuff particles were eliminated through differentiation of spectra data and in this way the prerequisite of Beer-Lambert law validity was prepared. The calibration graphs were constructed by measuring the visible absorbance spectra of different concentrations of completely reduced vat dyes.subsequently vat dye solubility was evaluated by determination of the extent of solute (leuco) dye concentration.This research is aimed to illustrate the applicability of derivative spectrophotometry in order to determine and control of vat dye solutions.The use of strongly alkaline solutions pH (12-14) for vatting and dyeing limits the use of most vat dyes to cellulosic fibres. The developed approach can be used in order to exact determination of the vat dye concentration in dyeing solutions and consequently achieving the optimized reduction condition in vatting process. [9]

Peter keusch in his work on “Dyeing with vat dyes” defined that vat dyes are insoluble in water and incapable of dyeing fibres directly.Reduction in alkaline medium produces the water soluble alkali metal salt of the dye.In the reduced form the dye is applied to impregnate the fibre. The dye absorbed on the fibre is converted to its original water insoluble form by a subsequent reoxidation. Vat dyes have excellent wash and light fastness properties. [10].

Here “Technologies of vat dyeing process their application & properties” by AVM Chemical Industries, given a relevant information about application of vat dyes on cotton is also made available, same class of dyes are used for experimentation over Eri Silk & optimization of the process.

3.5.1 Application of Indigo on Cotton:
Since dyeing is carried out at low temperature a good preliminary scour is necessary to make the material easily permeable. The dye vessel is filled with soft water and the dissolved oxygen is removed by the addition of 1 oz / 100 gallans of sodium hydrosulphite. The required amount of reduced indigo is added from the stock vat and the soods are immersed in the dye liqour at 20 to 25 º C (68 to 77 º F) and agitated for 15 minutes. It is important that a machine or method handling should be used in which the goods are totally immersed to prevent premature oxidation from taking out, the excess which are exposed to the air. At the end of 15 minutes the goods are taken out, the excess liquor is squeezed back, and leuco compound is oxidized by exposure to air. The first dip will only give a pale blue and the sequence of operations is repeated 2, 3, 4 or 5 times until the necessary depth of shade is obtained. Cellulose has not great affinity for the leuco compound of indigo and heavy shades must there fore be built up the successive immersions because an excessive concentration of the dye in the liquor leads to unsatisfactory rubbing fastness. Exhaustion can be improved by the addition of 5 to 40 percent of common salt. According to the depth of shade and the liquor ratio. Deep shades are built up by successive in a series of liquor of increasing indigo and thus the first bath might, for example, contain 0.3 g/l of reduced indigo and concentration would increase until, in the sixth, it is 3 to 4 g / l a counter flow system may be used, the first bath being certified from the second, and so on, all addition of reduced indigo being made to the final liquor. When the dyes goods have been exposed to air for large enough for oxidation to be complete, they are second through to remove any insoluble indigo blue deposited on the surface of the fibers. [11]

Islem Chaari and Fakher Jamoussi in the article “Application of activated carbon for vat dye removal from aqueous solution” explained that the adsorption of a vat dye, namely, Indanthrene blue RS (C.I. vat blue 4) on to Activated Carbon (AC) in aqueous solution was studied in a batch system with respect to contact Time, pH, and Temperature. The adsorbent employed was characterized by X-ray diffraction, infrared spectroscopy and specific surface area and point of zero charge were also estimated. The effect of contact time on dye adsorption by AC showed that the equilibrium was reached after a contact time of 10 min. The optimum pH for dye retention was found 7.3 for Activated Carbon. The equilibrium adsorption data were analysed using the Langmuir and Freundlich isotherms. The adsorption capacity for Activated Carbon was found 20.28 mg g-1. The effect of temperature on the adsorption was also investigated; adsorption of Indanthrene Blue RS is an endothermic process.

Vat dyes account for about 15% of total consumption of textile dyes. They exhibit good fastness to light, acid, alkali and solvents, and they mainly used in dyeing cotton fibres. Vat dyes cause environmental concerns when released in industrial wastewaters due to their carcinogenic health effects. Vat dyes, like indanthrene or Indigo are practically insoluble in water, but can be reduced in the presence of an alkali and a reducing agent, like sodium hydrosulphite (Na2S2O4) through a reduction reaction.These water insoluble vat dyes are converted to soluble anthrahydroquinone compounds ( leuco dye), which have a certain affinity to cellulosic fibers. This convertion is represented in reaction. The reduced dyestuff penetrates into the fiber (decreasing concentration of dyestuff in dye bath) and is reoxidized on the fiber back to the insoluble form which remains fixed in place. The reducing property of sodium hydrosulphite is due to the evolution of hydrogen when dissolved in water or sodium hydroxide (NaOH). [12]

P.Santhi & J.Jeyakodi Moses in the article “study on different reducing agents for effective vat dyeing on cotton fabric” explained that vat dyes is the most popular dye used for coloration of cellulosic fibres about 120000 tons of vat dyes are being used annually. Vat dyes are practically insoluble in water, but can be converted in to water soluble form by reduction with a strong reducing agent sodium hydrosulphite and solubilizing agent sodium hydroxide. The reduced dyestuff penetrates in to the fibre and it is reoxidized on the fibre back to the insoluble form which remains fixed in the fabric. The use of sodiumhydrosulphite is being criticized for the formation of non environment friendly decomposition products such as sulphite, sulphate, thiosulphate and toxic sulphur .Therefore, many attempts are being made to create alternate for the sodiumhydrosulphite that causes less pollution. [13]

In the article “The Chemistry and Manufacture of Vat Dyes”, Robert J. Baptista, explained that Vat dyes, which include indigo and anthraquinone-based dyes, are chemically complex dyes which are insoluble in water. They must first be reduced to the leuco form in an alkaline solution of sodium hydrosulphite before application to the cotton or rayon fiber. Air oxidation fixes the dye strongly on the fiber, resulting in excellent wash-fastness and light-fastness. The vat dyes were one of the most significant textile dye inventions in the 20th century. [14]

G.S.Egerton “some aspects of the Photochemical Degration of Nylon. Silk, and Viscose Rayon” given that the vat dyes that are particularly effective in increasing the degradation of undyed cotton that is simultaneously irradiated in their vicinity.The experiments took the form of irradiating a sheet of yarn composed of alternate dyed and undyed threads not in contact with one another .The dyed threads used included nylon,silk and viscose rayon yarns dyed with various vat dyes .Many of the dyed yarns have a considerable effect up on the degradation of adjacent undyed yarns of cotton or silk,but the same dyed yarns generally have only a small or negligible effect up on the degradation of undyed yarns of nylon or viscose rayon.These results are parently due to the greater resistance of the latter textile materials to oxidation by the volatile agent (Hydrogenperoxide),which is responsible for the “Action at a distance”. In some cases –for example, dyed nylon-there is evidence that although the volatile agent is produced by the dye in a relatively large amount it is not responsible for the major proportion of the degradation of the dyed yarn.The effect of humidity on the rate of photochemical degradation of textile materials dyed with active vat dyes appears to be closely related to the erase with which the textile fibre is oxidized by the volatile agent.

These results are interpreted in terms of a theory in which the photochemical degradation of the textile material is represented as the result of oxidation by activated oxygen and or hydrogenperoxide.The relative effectiveness of these two oxidation processes depends up on the particular experimental conditions and the nature of the textile fibre. [15]

Robin J.H.Clark, D.Sc.,Christopher J Cooksey, M.Sc.,Marcus A.M Daniels,M.Sc., Robert Withnal, in the article “Indigo,woad and Tyrian purple:important vat dyes from antiquity to the present” states that the use and history of indigo ,woad and Tyrian purple,which are among the most ancient of dyes.The emphasis is on the chemistry of the production and synthesis of the dyes ,together with some reference to the use of their spectroscopic properties as an aid to their identification in art facts.[16]

Here in the “Application of vat dyes “ we genally got to know that if a dye is not soluble in water, as is the case with vats, it may be applied to the fabric as a dispersion by a padder. Once the insoluble vat dye has been uniformly applied to the fabric surface, usually with the aid of special dispersing agents (detergents), it can be solubilized by reaction with a reducing agent, e.g., sodium hydrosulphite ("hydrose", Na2S2O4) in dilute NaOH. Once it has been converted to its soluble (LEUCO) form, the vat dye can penetrate into the cotton fibers. After adequate time for penetration to occur, the fabric is withdrawn from the bath and oxidized by air or an oxidizing agent such as sodium perborate or hydrogen peroxide. This process is schematically represented below.

Figure 3.1: Conversion of vat insoluble form to soluble form

Before chemical reducing agents were readily available, vat dyes were converted to their soluble leuco form by fermentation of organic matter in wood tubs called vats. This method of reduction and application is the source of the name for this class of dyes. Once the vat dyes have been regenerated inside the fiber, they are very insoluble. This accounts for their excellent wash fastness. Because they can be applied as dispersion by padding, solubilized by reduction, and finally reoxidized when inside the fibers, vats are well- suited to continuous dyeing operations. Such treatments exhibit a number of advantages:

• Very efficient use of the dye.
• The insoluble vat is very evenly distributed over the fabric surface, leading to level dyeing.
• Continuous processes are normally more economical processes than batch processes.[17]

Vat dyes, so called because indigo, the first member belonging to this class of dyes was dyed on textile materials in wooden vats (tubs) in ancient days, water-insoluble coloured compounds. As such they cannot be directly applied to be converted in to a water-soluble form, having sufficient affinity for fibres. During the dyeing process, it is this soluble form, of the dye that is applied on cotton, followed by reconversion of the soluble form into original insoluble form. As a result, the insoluble dye is trapped in the fibre substance and come out during soaping or any other wet treatements, there by ensuring excellent washing fastness. Most of the vat dyes are extremely fast to light; in fact, the brilliance and depth of shades produced from some of the vat dyes on cotton fabrics lost longer than the fabrics themselves i.e., the dyed fabric may lose its strength on prolonged usage and exposure to sunlight, but the brilliance and depth of shade remain unaltered.Subsequently it was found that these dyes absorbed light energy from sunlight and in the presence of atmospheric oxygen and moisture caused an accelerated degradation or tendering of cotton fabrics on which they were present. These two dyes were then withdrawn from the market and are not manufactured any more. This property is called photochemical tendering activity of vat dyes [18].

The name “vatting” which once meant using natural fermentation processes in a vat to produce the reducing conditions to make the dye soluble. Indigo, the blue of blue jeans is a common vat dye. Vat dyes, with the notable exception of indigo are generally very lightfast and washfast. Many have very good resistance to chlorine bleach. Multiple applications of dye may be required to build strong shades because of limited substantivity of the colour. Sulfur dyes use processes similar to vat dyes, but are distinguished by their sulfur content. Some modern vat dyes are supplied in already-reduced soluble form. [19]

3.6 Physical and Chemical Properties of Fibre:

3.6.1 Microscopic Features:
A microscopic observation of the Eri silk shows continuous striations along the fibre. The cross section shows two elongated triangles facing each other on the flat side with rounded corners surrounded by sericin.

3.6.2 Breaking Load:
The maximum load / force, supported by a specimen in a tensile test carried to rupture. It is usually expressed in Newton or kilogram.

3.6.3 Tenacity:
The silk filament is strong compared to other natural fibres. This strength is due to its linear beta-configuration and highly crystalline polymer system. These two factors permit many more hydrogen bonds to be formed in a much more regular manner. When wet, silk loses its strength. This is due to water molecules hydrolyzing a significant number of hydrogen bonds and in the process weakening the silk polymer.

3.6.4 Elongation:
Silk is considered to be more plastic than elastic because it is a crystalline structure does not permit the amount of polymer movement, which could occur in a more amorphous system. Hence, if the silk material is stretched excessively, the polymers, which are already in a stretched state (a beta-configuration), slide past each other. The process of stretching ceases, the polymers do not return to their original position, but remain in their new positions. This disorganizes the polymer system of silk, which is seen as a distortion and wrinkling or creasing of the silk textile material. The handle of the silk is described as a medium and its crystalline polymer system imparts a certain amount of stiffness to the filaments. This is often misinterpreted, in that the handle is regarded as soft, because of the smooth, even and regular surface of silk filaments.

3.6.5 Hygroscopic Nature:
Silk has a crystalline polymer system and is less absorbent than wool. The greater crystallinity of silk polymer system allows fewer water molecules to enter than does the amorphous polymer of wool. The other hygroscopic properties of silk are rather similar to those of wool.

3.6.6 Thermal Properties:
Silk is more sensitive to heat than wool. This is considered to be partly due to the lack of any covalent cross links in the polymer system of silk, compared with the disulphide bonds which occur in the polymer system of wool. The existing peptide bonds, salt linkages and hydrogen bonds of the silk polymer tend to break down once the temperature exceeds 100˚C.

Chemical Properties:

3.6.7 Effect of Acids:
Silk is degraded more readily by acids than is wool. This is because, unlike the wool polymer system with its disulphide bonds, there are no covalent cross-links between silk polymers. Thus perspiration, which is acidic, will cause immediate breakdown of the polymer system of silk. This is usually noticed as a distinct weakening of the silk textile material.

3.6.8 Effect of Alkalies:
Alkaline solutions cause the silk filament to swell. This is due to partial separation of the silk polymers by the molecules of alkali. Salt linkages, hydrogen bonds and vander waal’s forces hold the polymer system of silk together. Since these inter-polymer forces of attraction are all hydrolyzed by the alkali, dissolution of the silk filament occurs readily in the alkaline solution. It is interesting to note that initially this dissolution means only a separation of the silk polymers from each other. However prolonged exposure would result in peptide bond hydrolysis, resulting in a polymer degradation and complete destruction of the silk polymer. 

3.6.9 Effect of Sunlight and Weather:
The resistance of silk to the environment is not as good as that of wool. This lower resistance is mainly due to the lack of covalent crosslink’s in the polymer system of silk

3.7 Classification of Vat Dyes:
Vat dyes are classified into four groups

• IK vat dyes
• IW group of vat dyes
• IN vat dyes
• IN special vat dyes

a) IK vat dyes: (cold dyeing, k=kalt /cold in german)
They have optimum affinity at very low temperature i.e., 20˚-30˚C and requires minimum addition of causticsoda and sodium hydrosulphite.

b) IW (warm dyeing) group of vat dyes:
They require moderate caustic soda conditions. These dyes are vatted and exhausted at comparatively low temp (40˚-50˚C) the dye baths are exhausted in the presence of moderate some of exhausting agent.

c) IN (normal dyeing group):
These have maximum affinity for cellulose at 60˚C and require relatively high concentration of caustic soda in dye bath and exhaustion of dye bath is also achieved even without the addition of common salt but these dyes use certain retarding agents during dyeing because of their greater affinity for dyeing.

d) IN special dyes:
They include specific dyes like blacks, which require relatively higher concentration of caustic soda, sodium hydrosulphite and also much higher vatting and dyeing temperature.

It is also classified based on the

• Classification based On Chemical Constitution
• Methods Of Application

Under this there are subdivisions as follows,

a) Classification by chemical constitution:

• Anthraquinonoid Vat Dyes
• Indigoid Vat Dyes
• Sulphurised Vat Dyes

1) Anthraquinonoid vat dyes:
They cover a large range of shades and these dyes have good substantivity in water soluble leuco form and display superior washing fastness properties these are not suitable for application to protein fibres.

2) Indigoid vat dyes:
These dyes are characterized by presence of chromophore and these are derivatives of other indigo tin or thio indigo and form pale yellow solution under alkaline condition these dyes are widely used for dyeing blue denims.

3) Sulphurised vat dyes:
These dyes have good all round fastness properties hydrogen blue is a classic example of this kind of dye these are intermediate between sulphur and vat dyes.

b) Classification by method of application:
Depending on difference in sensitivity to process conditions and other properties the vat dyes differ among themselves with respect to the following,

1. Ease of dissolution
2. Affinity in the leuco state
3. Rate of dyeing
4. Dye uptake
5. Requirement of electrolyte
6. Ease of oxidation
7. Sensitivity to reduction and oxidation
8. Stability to temperature and time during vatting, dyeing. In the light of factors mentioned above for the sake of simplicity.

3.8 Preparation of Leuco Vat Dye Solution:
The vat dyestuff powder is taken in a separate vessel and made in to a paste with Turkey red oil (the some weight of dyestuff to be taken) and add some hot water (50-60 deg C). The caustic soda is first added and then sodiumhydrosulphite is added, and allows standing for 10-20 minutes with occasional stirring, the complete vatting taking place.In above 10-20 minutes the dyestuff will be reduced completely and going to solution. This can be seen by the clearness of the solution and charecteristics of the vat colour. The vatting stage temporarily alters the original colour of the dye (reduced colour).Most vat dyes are sold in insoluble oxidized form. The first operation therefore, consists of reducing to the lecuo compound and dissolving the latter in alkali, a process commonly referred to as vatting. The classical nature vat dyes such as Indigo and Tyrian purple were reduced in fermentation vat.

3.9 Application of Vat Dyes:
Genarally, the application of vat dyes to textile materials involves four distinct steps.

3.9.1 Vatting
In which the insoluble commercial dye is reduced and solidifies (vatting) by using sodium hydro sulphite (hydrose) and sodium hydroxide (NaOH).

3.9.2 Dyeing
In which the soluble sodium salt of the leuco vat dye is absorbed by the textile material from an alkaline reducing medium in the presence of either a retarding agent or an exhausting agent depending on the rate of dyeing.

3.9.3 Oxidation
In which the soluble form of the dye absorbed by the fibre, is reconverted in to the original insoluble dye by atmospheric oxygen (Airing) or by the use of “chemical oxidation” (that is involving the use of a chemical like sodium per borate or potassium dichromate or Hydrogen peroxide)..

3.9.4 After Treatment
Soaping off in the dyed material is subjected to a treatment either boiling soap or other detergent solution in order to get a proper tone by way of aggregation of smaller dye particles in to bigger one and also to get the optimum fastness, especially rubbing fastness by removing the surface deposited dye particles.

Table 3.1 Composition of Eri Silk Fibre

3.10 Computer Colour Matching (CCM)

• Version: CIE Reflectance method.
• Instrument: Datacolour 650.
• Type : Dual spectrophotometer with Diffuse elimination at 8˚ viewing with CIE publication No1 5.2 colorimetry with pulse xenon filter to approximate D65 of Sphere diameter 6”
• Spectral analyzer: proprietary SP 2000 analyzer with dual 650 diode array high resolution holographic grating.
• Range: 360 nm-760 nm.
• Operating environment: 5˚- 40˚ C, 20-85% RH, noncondensing.
• Interface : RS 232-9600/19200 baud USB

3.10.1 Colour Measurement
The spectrophotometer measures the colour and this information is transferred to the computer connected to it. The measurement of colour is done by the spectrophotometer by scanning it at four different directions by mounting the yarn on it. This information transferred to the computer is used to complete the computation procedure according to the programs present in the software. The strength of the dye measured is presented in the form of a graph in which the K/S value is plotted against the concentration of the dye. [20]

Qualitative measurement of dye uptake can be made optically by measuring light absorption characteristics of the dye remaining in the dye bath or by measuring changes in the light reflectance from the dyed material. Nowadays this measurement is done with the help of Spectrophotometer wavelength ranging from 400 – 700 nm.

3.10.2 Light Reflectance And Dye Concentration (Kubelka- Munk Function)
The increase in the dye concentration on a textile fabric results in a decrease in the reflectance, which is most marked at the wavelength usually corresponding closely to the λmax of the dye in solution. The reflectance is measured against a reference white (Barium Sulphate or Magnesium Oxide), which should be non-linear function of dye concentration. Suggestions that gave approximately linear variation with concentration include,

• The reciprocal of the reflectance
• The simplified KUBELKA-MUNK function and
• Other functions, which include corrections for specular and surface reflection.

The relationship between the reflectance (R) and the concentration of the dye (c) for a thick opaque pattern is given by the Kubelka – Munk equation:

K / S = (1 – R)² / 2R = f (R)

The colour strength as K/S values were calculated within the computer from the absorption values measured on a Tex flash UV/visible spectrophotometer at the wavelengths of maximum absorption (max). The K/S values were calculated using the Kubelka- Munk equation i.e.,

K / S = (1 – R)² / 2R , Where R is the observed reflectance.[20]

In textile industry, the desired colour is obtained by mixing 3 or 4 dyes to match perfect match to customer’s samples using minimum amount of chemicals and colorants and also ensuring desired fastness property. This job is normally attended by professional colour technologists, who by his remarkable ability, decides the shade. In textile industries, an expert dyeing master maintains the records of his experience in ‘Shade Bank’ and select one of the colour recipe which may be close to the standard. The colour master then, makes necessary changes by trial and error methods to obtain the exact match. In past this task was comparatively easy with limited number of natural colours and fibres. The colour matching has now become more difficult due to increase in manmade fibres, multi blend fibres and almost endless number of dyes available in the market. With all these parameters, the arbitrary selection of colour recipe, which may be most economical and give better quality product, is almost impossible. In last two decades the researches in colour science, advancement in optical technology and computer science have resulted into the instruments which can give number of colour recipe for a given sample. This new technology enables the dyeing masters to work out several colour recipes and to select the most appropriate recipe depending on the cost of production and quality of products required. This technique is known as Computer Aided Colour Matching (CACM) or Computer Colour Matching (CCM). The introduction of computer colour matching to any industry should not be considered as replacement of expert colourists by the instrument, but it is aimed to provide a powerful tool in their hands to improve the quality of the products and to reduce the cost of production by minimizing the rework and rejections. [21]

The colour goods manufacturers faced problem in producing the products of specified colour as defined by customer. Usually the traditional method was to prepare the laboratory by trial and error methods and the sample visually analysed to match the standard and proceed for mass production.

To overcome this, in recent years the visual assessment is replaced by computer colour matching operated at the producers’ plant. The technique is usually based on the calculation of reflectance data using previously determined calibration coefficients for the dyed or pigments to be used, and comparing the predicted values with those of the sample to be matched. This utilises calibration data based on the Kubelka-Munk analysis of reflectance data. In the present version of the program the values along with corresponding values are printed at every cycle in the iteration up to ten cycles, with the correction coefficients being available if required at the end.

3.11 Evaluation of Colour and Colour Fastness:
Evaluation of Colour Fastness of textiles has been developed by AATCC. Testing is divided into two parts.

• First part is the exposure or treatment, which may be duplication of treatment that the specimen might be subjected to in actual use or it may be, simulated actual use condition in laboratory.
• The other part is the evaluation of colour change done instrumentally or visually.

3.11.1 Colour Fastness Properties:
The change in colour and the staining of the fabric (white adjacent fabric) is assessed as ratings. These ratings are over a range of numbers or it is a 5-step scale called the ‘gray scale’. The scale illustrates the colour differences corresponding to fastness rating of 5, 4, 3, 2, 1 and half step ratings as 4-5, 3-4, 2-3, and 1-2. Thus in all 9 steps rating could be given to these colour changes and staining. These ratings are valid for all fastness tests. However in the case of Light fastness these grading are based on ‘Blue standards’ and the rating are on 8-step scale (that is from 8-1 and corresponding half step ratings). Higher the rating better is the fastness of the colour. Thus a rating of 5 (8 in case of light) is excellent and the rating of 1 is to be inferred as very poor fastness.

3.12 Determination of Biochemical Oxygen Demand:
The most important in effluent analysis is BOD. Water at 20˚C contains about 9 mg/l of dissolved oxygen (D.O). When water contains organic matter, micro organism can utilize this for growth by consuming the oxygen. Where organic matter is present in sufficient quantity to use up most of the dissolved oxygen a state of pollution exists. BOD is a measure of the polluting tendency of water. Oxygen is demanded in effluent for the oxidation of inorganic and organic matter. Demand of oxygen by organic matter in sewage/effluent is known as BOD or “Biochemical oxygen demand”.

3.13 Determination of Chemical Oxygen Demand:
COD is a measure of the oxygen required to oxidize unstable materials in a sample by means of dichromate in an acid solution. To measure the content of organic matter of an effluent, normally COD test is carried out. In COD test a strong chemical oxidizing agent is used in acid medium and the oxygen equivalent of the organic matter is determined. The test is normally carried out using K2CH2O7 at given temperature in presence of some catalyst like silver sulphate. [22]

Chapter 4

Material and Methods
The experiments were conducted at Central Silk Technological Research Institute, Central Silk Board, Bengaluru, in wet processing laboratory, an accredited laboratory by NABL. Yarn supplied by the Central Silk Board has been used for the “studies on application of vat dyes on Eri Silk”,

4.1 Materials

Table 4.1: Yarn Parameters

As shown details in the table, undyed Eri silk sample had been used for the application of vat dyes for standardizing the optimized parameters of dyeing vat dyes on Eri silk.

4.2 Selection of Dyes:
Selection of vat dyes was made based on the regular usage by the industry after discussion and observation. The vat dyes for the yarn with the Colour Index numbers given according to respective colours which are as follows.

1. Corovat J.Green (Colour Index Nr: CI VAT GREEN 1)
2. Corovat Red (Colour Index Nr: CI VAT RED10)
3. Corovat Darkblue (Colour Index Nr: CI VAT BLUE 20)
4. Corovat Black (Colour Index Nr: CI VAT GREEN 9)

Figure 4.1: Structure of C I Vat Green 1

  Figure 4.2: Structure of C I Vat Red 10

Figure 4.3: Structure of C I Vat Blue 20

 Figure 4.4: Structure of C I Vat Green 9

4.3 METHOD

Raw silk
↓
Pretreatment (wetting agent+Water) for 15 minute
↓
Preparation of leuco compound (reduction of dye compound)
↓
Dipping of sample
↓
Oxidation (Natural)
↓
Soaping
↓
Washing
↓
Dry under shade

Figure 4.5: Flow Chart of Vat Dyeing

All these processes were done in steel vessels on temperature controlled electrical hot plate. The required quantity of water, dyes and chemicals were measured correctly with proper maintenance of Temperature, pH and Time. The material entered was pre-soaked to ensure uniform action in the bath. Calibrated balance, pH meter and thermometer with cleaned vessels, beakers, measuring jars were used with at most care to avoid any contamination. The dye solution was prepared at the time of dyeing to avoid the errors during dyeing.

4.3.1 Pretreatment of Eri Silk:

Table 4.2: Pretreatment recipe for Eri silk

Procedure
Eri silk yarn will contain some resudial gum 2 to 3% (Y.C.Radhalakshmi etal) and some impurities add as well as inherent need to taken care before dyeing.pretreated yarn show uniform dye uptake in case of all dyes. Luster of pretreated sample is found better than yarn that dyed without pretreatment. Turkey red oil dissolved in required amount of water, enters the material at 40˚C, raise the temperature according to the given temperature, worked for 30mints taken out the material then washed in the cold water and removed the excess water, then finally dried under the shade.

4.4 Optimization of Influencing Parameters on Dye Uptake:

1. Role of pH (combined effect of sodium hydroxide and sodium hydrosulphite)
2. Role of Temperature
3. Role of Time Intervals

Procedure followed for arriving the optimization of the above parameters for the application of vat dyes on Eri silk.Hence final optimization of these parameters can be concluded based on the results obtained from the strength parameter, colourfastness, colour measurement results and also statistically it has been checked and concluded as the optimized parameters for the application of vat dyes on Eri silk.

4.4.1 Kinetic Study Of Vat Dye On Eri Silk With Constant Temperature, Time And Concentration Of Sodium Hydrosulphite At Different Concentration Of Sodium Hydroxide

Table 4.3: Details of the experimental trails to illustrate the role of sodium hydroxide in dye absorbency at constant Time, Temperature and concentration of sodium hydrosulphite.

Procedure:
As per the weight of material, the bath is adjusted to the material-to-liquor ratio. The dye percentage (1-5%) shade samples are dyed. Enter the pre-soaked silk materials to the different vat baths, time, concentration of sodiumhydrosulphite, temperature is kept constant .The concentration of sodium hydroxide is varied as 3.2,4 ,4.8 gpl, Then material is removed according to the given time, washing treatment has given using required amount of soap and soda ash mentioned in the above table and dried it under shade.

4.4.2 Kinetic Study Of Vat Dye On Eri Silk With Constant Temperature, Time And Concentration Of Sodium Hydroxide At Different Concentration Of Sodium Hydrosulphite

Table 4.4: Details of the experimental trails to illustrate the role of sodium hydrosulphite in dye absorbency at constant Time, Temperature and concentration of sodium hydroxide.

Procedure:
As per the weight of material, the bath is adjusted to the material-to-liquor ratio. The dye percentage (1-5%) shade samples are dyed. Enter the pre-soaked silk materials to the different vat baths, time, concentration of sodium hydroxide, temperature is kept constant. The concentration of sodiumhydrosulphite is varied as 6.66, 13.33, 20 gpl, Then material is removed according to the given time, washing treatment has given using required amount of soap and soda ash mentioned in the above table and dried it under shade.

4.4.3 Kinetic Study Of Vat Dye On Eri Silk With Constant Time, Concentration Of Sodium Hydroxide And Sodium Hydrosulphite At Different Temperature.

Table 4.5: Details of the experimental trails to illustrate the role of Temperature in dye absorbency at constant time and known concentration of sodium hydroxide and sodium hydrosulphite.

Procedure
As per the weight of material, the bath is adjusted to the material-to-liquor ratio. The dye percentage (1-5%) shade samples are dyed. Enter the pre-soaked silk materials to the different vat baths, time, concentration of sodium hydroxide, sodium hydrosulphite is kept constant .The temperature of the bath is varying as 40˚, 50˚, 60˚C., Then material is removed according to the given time, washing treatment has given using required amount of soap and soda ash mentioned in the above table and dried it under shade.

4.4.4 Kinetic Study Of Vat Dye On Eri Silk With Known Concentration Of Sodium Hydroxide And Sodium Hydrosulphite At Different Time Intervals

Table 4.6: Details of the experimental trails to illustrate the role of time in dye absorbency at constant temperature and known concentration of sodium hydroxide and sodium hydrosulphite.

Procedure
As per the weight of material, the bath is adjusted to the material-to-liquor ratio. The dye percentage (5%) shade samples are dyed. Enter the pre-soaked silk materials to the different vat baths,The concentration of sodium hydroxide and sodium hydrosulphite also kept constant keeping one temperature 40˚C as contant time is varied as 30,45,60 mints, again its repeated for 50˚ C and 60˚ C . Then material is removed according to the given time, washing treatment has given using required amount of soap and soda ash as mentioned in the above table and dried it under shade.

4.5 Tests Conducted for Silk Fabrics

4.5.1 Physical Tests
In this experiments Eri silk yarn used, yarn was tested for before and after dyeing. The physical and mechanical tests were carried out and test methods adopted are listed below.

Table 4.7: Test methods adopted for testing of yarn

Instron Tensile Testing Machine 5500R

Principle

• CRE (Constant Rate Of Extension)
• Automatic Data Capturing option

Scope
The standard prescribes method for constant rate of traverse or determination of Breakingload & Elongation at break of yarn using constant rate of load, constant rate of extension machines.since for any fibre types breaking load is approximately propotional to the linear density, strands of different sizes are compared by converting the observed breakingload to breaking tenacity (Milli Newton/tex).

• Option A-straight
• Option B-knotted
• Option C-looped

Procedure
Set the clamps of the testing machine so that the distance between the nips of the clamps along the specimen axis (Including in any portion in contact with snubbing surfaces) 500 ±2 mm with the help of preliminary specimens, set the machine so that the specimen breaks with in 20±5 sec. but if the machine is CRT type, set it at a rate of traverse of 300±15 mm/mint. Take a yarn, discard a first few meters of it and secure its one end in the jaws of one clamp in such a way that the twist does not change. Place the other end in the other clamps, apply the required pre-tension from this free end to remove any slack or kink without appreciable stretching and secure it in the jaws of the clamp.

Here for Eri silk used option A (straight) method. Mount the specimen as directed in using a pretension of 0.50±0.05 CN/tex. Operate the machine, carry the test to rupture and record the Breaking Load and Elongation at break. If the specimen slips/breaks in the jaws or breaks with I 5 mm from the edge of the jaws, the result shall be discarded and another test specimen taken in there of.

Breaking Load
It’s the ratio of breaking strength in kg to the number of observations.Measured in grams (or) the maximum load/ force supported by a specimen in a tensile test carried to rupture it is usually expressed in grams.

Elongation
Silk is considered to be more plastic than elastic because it is a crystalline structure does not permit the amount of polymer movement, which could occur in a more amorphous system. Hence, if the silk material is stretched excessively, the polymers which are already in a stretched state slide past each other. The process of stretching ceases, the polymers do not return to their original position, but remain in their new positions. This disorganizes the polymer system of silk, which is seen as a distortion and wrinkling or creasing of the silk textile material. The handle of the silk is described as a medium and its crystalline polymer system imparts a certain amount of stiffness to the filaments. This is often misinterpreted, that the handle is regarded as soft, because of the smooth, even and regular surface of silk filaments.

Tenacity
The silk filament is strong compared to other natural fibres. This strength is due to its linear beta-configuration and highly crystalline polymer system. These two factors permit many more hydrogen bonds to be formed in a much more regular manner. When wet, silk loses strength. This is due to water molecules hydrolyzing a significant number of hydrogen bonds and in the process weakening the silk polymer.

4.5.2 Colour Measurement
The spectrophotometer measures the colour and this information is transferred to the computer connected to it. The measurement of colour is done by the spectrophotometer by scanning it at four different directions by mounting the yarn on it. This information transferred to the computer is used to complete the computation procedure according to the programs present in the software. The strength of the dye measured is presented in the form of a graph in which the K/S value is plotted against the concentration of the dye.

Qualitative measurement of dye uptake can be made optically by measuring light absorption characteristics of the dye remaining in the dye bath or by measuring changes in the light reflectance from the dyed material. Nowadays this measurement is done with the help of Spectrophotometer wavelength ranging from 400 – 700 nm.

4.5.3 Chemical Tests
Different chemical tests were carried out for Era silk, the details about tests and instruments used are shown in table.

Table 4.8 Instruments used for testing

Colour Fastness of Textile Material to Washing

Method: Is 687: 1979 (Wash Test No: 1)

Equipment: Launderometer/Washing Fastness Tester (Capacity of the each container: 500 ml & the rotor speed are 40 ± 2 revolutions /minute.)

Figure 4.6: Launderometer

Sampling

• Draw a sample representing the lot.
• Take test specimen from the sample, covering all the colours of dimensions 10 cm x 4 cm.
• Take two adjacent fabrics each measuring 10 cm x 4 cm. One piece of same kind of fabric and the second piece based on the below mentioned table.

Table 4.9: Recommended Adjacent Fabric

Procedure
Place the test specimen between two adjacent fabrics and sew along four sides to form a composite specimen.

• If the test material is yarn, then form a layer of parallel lengths and if the test material is loose fibre, then comb and compress the fibre (Ensure the mass of the test specimen shall be approximately equal to half of the combined mass of the adjacent fabrics).
• Prepare soap solution, which contains 5 grams soap per litre of water. Heat the soap solution to 40 ± 2°C.
• Place the test specimens in separate containers of the launderometer.
• Add the soap solution to the container so as to have 1: 50 liquor ratio. Treat the test specimens for 30 min at 40 ± 2°C.
• Remove the composite specimens, Rinse it twice in cold water and then cold running tap water for 10 min and then squeeze it.
• Remove the stitch along the two long sides and one short side.
• Open out composite specimen and dry in air at room temperature with the three pieces in contact only along the remaining line stitching.
• Evaluate the change in colour of the treated test specimen and degree of staining of two pieces of adjacent fabrics with the help of grey scales. Record the ratings for all specimens and calculate the average rating in change in colour and staining on adjacent fabric.
• Report the Colour Fastness ratings for change in colour and staining on adjacent fabrics.

Colour Fastness of Textile Material to Perspiration

Method: Is 971: 1983

Equipment: Perspirometer With Conditioning Cabinet
(The equipment consisting of a frame of stainless steel into which a weight piece of 5 Kgs & a base of 11.5 cms x 6 cms with glass or acrylic plates of same size & of 0.15 cms thickness).

Figure 4.7: Perspirometer with Conditioning Cabinet

Sampling

• Draw a sample representing the lot.
• Take test specimen from the sample, covering all the colours of dimensions10 cmÎ4 cm.
• Take two adjacent fabrics each measuring 10 cm Î 4 cm. One piece of same kind of fibre (Fabric) and the second piece made of the fibre as indicated in the following table.

Table 4.10: Recommended Adjacent Fabric for perspiration method

Place a test specimen between two adjacent fabrics and sew along one of the shorter side to form a composite specimen.

• If the test material is yarn, then form a layer of parallel lengths and sew along two opposite sides.
• If the test material is loose fibre, then comb and compress the fibre and sew along all four sides. Ensure the mass of the test specimen shall be approximately equal to half of the combined mass of the adjacent fabrics.
• Prepare at least two composite test specimen.

Procedure

Alkaline Solution Preparation
Prepare alkaline solution containing 0.125gm of Histidine (L-Histidunmonohydrochloride monohydrate & L-Histidine monohydrochloride, 1.25gm of sodium chloride crystals and 1.25gm of 12 H2O (pure sodiumphosphate dibasic [dodeca hydrate] ) Wet one of the composite specimens thoroughly in the alkaline solution at a liquor ratio of 1: 50 for 30 min at room temperature.

• Place the composite specimen between two acrylic resin plates. Keep the device in the oven for 4 hours at 37 ± 2°C.
• Remove the specimen and dry the specimen at a temperature of around 60°C.

Effect of Alkalies
Alkaline solutions cause the silk filament to swell. This is due to partial separation of the silk polymers by the molecules of alkali. Salt linkages, hydrogen bonds and vander waals forces hold the polymer system of silk together. Since these inter-polymer forces of attraction are all hydrolyzed by the alkali, dissolution of the silk filament occurs readily in the alkaline solution. It is interesting to note that initially this dissolution means only a separation of the silk polymers from each other. However, prolonged exposure would result in peptide bond hydrolysis, resulting in a polymer degradation and complete destruction of the silk polymer.

Acid Solution Preparation

• Prepare acid solution containing 0.125 gms of Histidine (L-histidunmonohydrochloride monohydrate & L-Histidine monohydrochoride, 1.25 gms of sodium chloride (pure sodium chloride crystals) and 0.55 gms of sodium di hydrogen phosphate
• Wet one of the composite specimens thoroughly in the acidic solution at a liquor ratio of 1: 50 for 30 min at room temperature.
• Place the composite specimen between two acrylic resin plates. Keep the device in the oven for 4 hours at 37 ± 2°C. Remove the specimen and dry the specimen at a temperature of around 60°C.
• Evaluate the change in colour of the treated test specimen and degree of staining of two pieces of adjacent fabrics with the help of grey scales. Record the ratings for all specimens and calculate the average rating in change in colour and staining on adjacent fabric.
• Assess the Colour Fastness ratings for change in colour and staining on adjacent fabrics for both acidic and alkaline medium

Effect of Acids
Silk is degraded more readily by acids than is wool. This is because, unlike the wool polymer system with its disulphide bonds, there are no covalent cross-links between silk polymers. Thus perspiration, which is acidic, will cause immediate breakdown of the polymer system of silk. This is usually noticed as a distinct weakening of the silk textile material.

Colour Fastness of Textile Material to Rubbing

Method: Is 766: 1988

Equipment: Crockmeter

Figure 4.8: Crockmeter

Sampling

• Draw a sample representing the lot (As agreed).
• Select test specimen from the sample.
• Cut two samples of 14 cm Î 5 cm , (One for wet and one for dry rubbing)

Procedure

• Place a test specimen in the device and clamp it.
• Cover the end of rubbing finger with dry rubbing cotton desized and bleached cloth of 5 cm x 5 cm.
• Rub the specimen to and fro in a straight line for 10 times
• Repeat the above procedure for the sample with wet rubbing cotton cloth (Dipped in distilled water and squeeze).
• Dry the wet rubbed specimen at room temperature.
• Evaluate the degree of staining of the rubbing cotton cloth with grey scale for staining.
• Report the ratings for dry staining and wet staining.

Colour Fastness of Textile Material to Artificial Light

Method: Is 2454: 1985 Equipment: Xenon Tester / Fadeometer

Figure 4.9: Xenon Tester / Fadeometer

Sampling

• Draw a sample representing the lot.
• Take test specimen from the sample, covering all the colours of dimensions 1 cm Î 4.5 cm and fix the test specimen on a card along with the eight blue standards.
• If the test material is yarn, then wind close together on a card or lay parallel and fix it to the card and if the test material is loose fibre, then comb and compress the fibre to give uniform surface and fix it to the card (Ensure the size of the specimen and standards are same and kept in the same plane which is essential in the case of Carpet testing).
• Cover the middle one third of the specimen and standard with the help of Opaque cardboard.

Procedure

• Mount the test specimen and the standard in the XENON TESTER.
• Check for the lowest intensity (1) level of the Xenon arc lamp to operate.
• Operate the xenon tester and expose the specimen and standard pattern simultaneously for 24 hours per day.
• Inspect the effect of light by removing the opaque cover frequently. If the change of the specimen is just perceived and equal to grey scale grade 4 -5, then note down the number of the standard pattern showing a similar change as Preliminary assessment.
• Continue the exposure until the contrast of the specimen or the standard pattern 7 is equal to grey scale grade 4.
• If the standard pattern 7 fades to this level before the specimen does, terminate exposure (Case 1).
• If the specimen fades to this level before the standard 7 does, continue the exposure by covering the left hand side one third of the specimen and the standard patterns with another opaque cardboard until the contrast of the specimen is equal to grey scale grade 3, and terminate the exposure.
• In respect of Case 1 the material under test may be have light fastness rating more than seven. Rate the specimen accordingly.
• The light fastness rating of the specimen is equal to the number of the standard, which shows similar change in colour.

If the light fastness is equal to or higher than 4, then preliminary assessment should also be reflected along with rating in the parenthesis when the preliminary assessment is equal to 3 or less, e.g. 6(3) where 3 is the preliminary assessment

4.6 Assessment of Colour Fastness Properties
Dyed samples were evaluated for the fastness of washing, rubbing, perspiration and light fastness properties. Colour fastness rating was given as per the standard methods. Wash, rubbing, perspiration and light fastness procedure were showed below and assessment is done by using grey scale

Grey Scales

Figure 4.10: Grey Scales

Two standard grey scales have been developed by AATCC for evaluating colour differences. One Grey Scale for evaluating the colour change (AATCC-EP1) and the other for evaluating the Staining (AATCC-EP2). Each grey scale consists of nine steps of colour difference represented by stationary pairs of grey clips. Grey scale ratings are compared with the difference observed between an unexposed specimen (original) and an exposed specimen with the difference observed between the two members of a stationary pair of clips on the grey scale. The original specimen and its corresponding exposed specimen are placed side by side.

The grey scale is placed beside the edges of the test specimen and original specimen and compared with the perceived differences represented between the pairs of clips on the grey scale. A rating is assigned based on this comparison of perceived differences in value.In the grey scale for Colour Change, each pair of colour chips includes a standard grey chip that is identical for each step of the scale, and a second grey scale chip whose value depends on the step in which is located. The nine steps are designated as 5, 4-5, 4, 3-4, 3, 2-3, 2, 1-2, and 1. In step 5 the two colour chips are identical (both the same standard shade of grey); then, with each step on the scale, the second grey chip is successively lighter.

A Grey Scale rating of 5 indicates no colour difference between two specimens (good Colour Fastness) and the extreme opposite, maximum colour difference between two specimens (poor Colour Fastness), is indicated by a rating of 1.The Grey Scale for Staining is organised and used in exactly the same manner as the Grey Scale for Colour Change, except that the clip that is constant in each of the nine pairs is standard white chip. In step 5, both colour chips are the standard white colour. The second chip is progressively darker in each successive step. The maximum difference between the two chips is at step 1, just as in the grey scale for Colour Change. The Grey Scale for Staining is used to evaluate the transfer of colour or staining of light coloured fabrics by darker materials.

Chapter 5

Results and Discussions

5.1 Evaluation of Strength Related Parameters:
Figure 5.1 Physical test result (strength values) of undyed sample and vat dyed sample with respect to different concentrations of sodiumhydroxide.

Breaking Load

Here as shown in the above graphs Breaking Load of the dyed sample increases as compared to controlled sample (undyed).In most of the cases it has been observed that there is no loss in the strength property of the yarn. But at the same time role of sodium hydroxide is quiet evidenced in reduction or migration of the colour on the dye bath.In all the dye shade percentages strength increases except in 5% shade the strength of the 3.2gpl concentration of sodiumhydroxide dyed sample strength decreases. Statistically there is significant difference.

Elongation

Here as shown in above graphs Elongation of the dyed materials increases as compared to controlled sample. In case of Elongation sodium hydroxide has not affected the strength of the Eri silk, Eri silk having higher resistance to the alkaline conditions.In all shade percentages there is continuous increase in the elongation takes place this shows that strength of the Eri silk has not affected by the respective concentrations of the sodium hydroxide.statistically there is significant difference.

Tenacity

Here as shown in the above graphs Tenacity increases as compared to controlled sample.In all the shade percentages the strength property showing better results, statistically there is significant difference.

(NOTE: All above graphs shown under in the figure 5.1)

Figure 5.2 Physical test result (strength values) of undyed sample and vat dyed sample with respect to different concentrations of sodiumhydrosulphite.

Breaking Load

Here as shown in the above graphs in most of the cases Breaking Load of the dyed sample increases as compared to controlled sample (undyed). In most of the cases 13.33 gpl concentration of sodiumhydrosulphite dyed sample giving better strength results.Statistically there is significant difference.It has been observed that there is no loss in the strength property of the yarn.

Elongation

Here as shown in above graphs Elongation of the dyed materials increases as compared to controlled sample.In all the graphs increase in the strength takes place. As concentration of alkali increases in this case it doesnot affect the strength property.statistically there is significant difference.

Tenacity

Here as shown in the above graphs Tenacity increases as compared to controlled sample. In 3% and 5% shade dyed samples resembles the controlled sample. Statistically there is significant difference. In 5% shade tenacity become reduced as compared to control sample here strength loss occurs.

(NOTE: All above graphs shown under in the figure 5.2)

Figure 5.3 Physical test result (strength values) of undyed sample and vat dyed sample with respect to different Temperatures.

Breaking Load:

Here as shown in the above graphs in most of the cases Breaking Load of the dyed sample increases as compared to controlled sample (undyed). In all the graphs the strength (breaking load) increase. This shows that, statistically there is significant difference. The vatting should takes place between the temperature 50˚C-60˚C.

Elongation

Here as shown in above graphs Elongation of the dyed materials increases as compared to controlled sample.as per the graphs strength become more at 50˚ C.statistically there is significant difference. Here in all the graphs better Elongation results was found.

Tenacity

Here as shown in above graphs tenacity become more compared to control sample.in some cases the strength may decreases because of yarn irregularities. As the temperature increases the strength may increase up to a point again its decreases .statistically there is a significant difference.

(NOTE: All above graphs shown under in the figure 5.3)

Figure 5.4 Physical test result (strength values) of undyed sample and vat dyed samples with respect to different intervals of time.

Breaking Load

Here as shown in above graphs the strength value become more as compared to control sanple. Especially for 30 minutes duration with different temperatures gives good strength results. As the durration increases the strength of the material may decrease.statistically there is significant difference.

Elongation

Here as shown in the above graphs the Elongation become more as compared to control sample. In all the cases the 30 mints duration dyed sample showing better strength results with respect to different temperatures. Statistically there is a significant difference.

Tenacity

Here as shown in graphs tenacity increases as compared to control sample.statistically there is significant difference.

(NOTE: All above graphs shown under in the figure 5.4)

5.2 One Way Anova
One way ANOVA was carried out to study the significance of the difference in the values of the parameters between the samples. The ANOVA tables are as given in the following.

Table 5.1: One way ANOVA for physical tests (strength parameters) with respect to sodiumhydroxide.

As shown in the above table all the parameters like breaking load, elongation and tenacity are not significant.

Table 5.2: One way ANOVA for physical tests (strength parameters) with respect to sodiumhydrosulphite

As shown in the above table breaking load and Elongation strength parameters are not significant.Tenacity strength parameter showing a significant difference.

Table 5.3: One way ANOVA for physical tests (strength parameters) with respect to Temperature

As shown in the above table all the strength parameters are not significant.

Table 5.4: One way ANOVA for physical tests (strength parameters) with respect to Time

As shown in the below table breaking load showing a significant difference. Elongation and tenacity strength parameters are not significant.

5.3 K/S Values of Vat Dyes on Eri Silk
In this adsorption study of vat dye on Eri silk were carried out to find the capacity of vat dye absorption on Eri silk yarn using different temperature with respect to time. The below graphs shows the clear picture of vat dye on Eri silk fabric at different Temperature, Time, Concentration Of sodium hydroxide and sodium hydrosulphite.

Figure 5.5: K/S value of Eri silk sample dyed with vat dye at different concentration of sodium hydroxide.

The above graphs shows the K/S values verses concentration of sodiumhydroxide.Here three different concentrations like 3.2 gpl,4 gpl, 4.8 gpl are used .The dyed sample with the concentration of 4.8 gpl showing more K/S value compared to others.

(NOTE: All above graphs shown under in the figure 5.5)

Figure 5.6: K/S value of Eri silk sample dyed with vat colours at different concentration of sodium hydrosulphite.

The above graphs shows the K/S values verses concentration of sodiumhydrosulphite. Here three different concentrations like 6.66gpl,13.33 gpl, 20 gpl are used .The dyed sample with the concentration of 13.33 gpl showing more K/S value compared to others. In all the cases additional dosage of sodiumhydrosulphite has reduced the migration of the colour and it has direct influence on the colourstrength.

(NOTE: All above graphs shown under in the figure 5.6)

Figure 5.7: K/S value of Eri silk sample dyed with vat dye at different temperatures

The above graphs shows the K/S values verses Temperature.Here varied three different temperatures during dyeing.The dyed sample with the temperature of 50˚ C showing better colourstrength when comparing with the standard dyed sample . There is a direct influence of the temperature on the reduction of the colour though it is evident that from the graphs at 40˚-50˚ C there is increase in the depth of the shade and at 60˚C again temperature has decresed in 4 and 5% shades.As leuco compound formation is temperature is 50˚-55˚ C the maximum colourstrength is attend in this range.

(NOTE: All above graphs shown under in the figure 5.7)

Figure 5.8: K/S value of Eri silk sample dyed with vat dyes at different intervals of time.

The above graphs shows the K/S values verses Time.Here varied three different time intervals during dyeing.The dyed sample with the time interval 60 mints showing better results at 50˚ and 60˚ C but as duration of the dyeing increases depth may increase but strength of the sample may affect because of the more duration exhaution takes place. So as compare to that 30 mints duration is suitable for vat dyeing on Eri silk.

Finally there is no change in the tenacity and elongation compared to the controlled sample.Although little difference in the colourstrength is observed in the above experiments.

(NOTE: All above graphs shown under in the figure 5.8)

5.4 Evaluation of Colourfastness at Different Dyeing Conditions

Table 5.5: Effect Of Sodium Hydroxide On Colourfastness Properties

Click on Image for large size

The ratings under colour fastness (3,3/4,4,4/5) are ranked as 1,2,3,4,5 etc..Then each rank is divided by highest rank assigned in the respective category. Further these values are totaled to arrive at the best treatment under each category.

1. In 1% shade, Eri sample dyed with 3.2 gpl concentration of sodium hydroxide showing best fastness properties among in case of wash, rubbing and perspiration fastness.
2. In 2% shade, Eri sample dyed with 4 gpl concentration of sodium hydroxide showing best fastness properties among in case of wash, rubbing and perspiration fastness.
3. In 3% shade, Eri sample dyed with 3.2 gpl concentration of sodium hydroxide showing best fastness properties among in case of wash, rubbing and perspiration fastness.
4. In 4% shade, Eri sample dyed with 4, 4.8 gpl concentration of sodium hydroxide showing best fastness properties among in case of wash, rubbing and perspiration fastness.
5. In 5% shade, Eri sample dyed with 4, 4.8 gpl concentration of sodium hydroxide showing best fastness properties among in case of wash, rubbing and perspiration fastness.

Among all the cases the vat dye samples showing best fastness properties.

Table 5.6: Effect Of Sodium Hydrosulphite On Colourfastness Properties

Click on Image for large size

The ratings under colourfastness (3,3/4,4,4/5) are ranked as 1,2,3,4,5 etc..Then each rank is divided by highest rank assigned in the respective category. Further these values are totaled to arrive at the best treatment under each category.

1. In 6.66 gpl concentration dyed samples except 1% shade all the samples preferring best fastness properties among in case of washing, rubbing, perspiration fastness.
2. In 13.33 gpl concentration dyed samples all the samples preferring good fastness properties especially in 4% and 5% shade dyed samples showing best fastness properties among in case of washing, rubbing, perspiration fastness.
3. In 20 gpl concentration dyed samples preferring good fastness properties, compare to all the samples in this concentration of sodiumhydrosulphite showing better fastness properties.

Table 5.7: Effect Of Temperature On Colourfastness Properties

Click on Image for large size

1. The ratings under colourfastness (3,3/4,4,4/5) are ranked as 1,2,3,4,5 etc..Then each rank is divided by highest rank assigned in the respective category. Further these values are totaled to arrive at the best treatment under each category.
2. Generally rate of reaction increases with increase temperature , where as leuco compound formation is temperature specific activity it happens between 50˚ to 55˚ C .Further increase temperature up to 60˚C and above has not shown any improvement in the dye absorbency.From the above table 5% shade with 40˚C preferring good fastness to wasing,rubbing,perspiration fastness.
3. At 50˚ C 4% shade dyed sample showing better fastness properties
4. At 60˚ C both 4% and 5% shade dyed sample showing better fastness properties.

Table 5.8: Effect Of Time On Colourfastness Properties

Click on Image for large size

The ratings under colourfastness (3,3/4,4,4/5) are ranked as 1,2,3,4,5 etc..Then each rank is divided by highest rank assigned in the respective category. Further these values are totaled to arrive at the best treatment under each category.

Here at 60˚ C 60 mints duration dyed sample showing better fastness properties among wash, rubbing, and perspiration fastness properties.

5.5 Kinetic Study Observations

5.5.1 Effect of Sodium Hydroxide on Dye Uptake
As per the results obtained, the effect of concentration of sodium hydroxide on colourstrength, fastness properties and mechanical properties of the dyed material it is observed that addition of sodiumhydroxide in the proportions of 3.2 gpl, 4gpl, 4.8 gpl with keeping time, temperature and concentration of sodiumhydrosulphite constant.where 4.8 gpl concentraton of sodium hydroxide gives optimum colourstrength, colourfastness and minimum strength loss. Hence standard concentration of sodium hydroxide is considered as 4.8 gpl and this concentration of alkali is applied for the experiments. Up to certain dosages sodiumhydroxide works better for reduction of the colour.It is evident from the experiments higher dosages of sodium hydroxide can dissolve the silk.

During the application of statistical tool for understanding the effect of sodium hydroxide on strength, Elongation and tenacity it is observed that 3.2 gpl dosages giving highest load is observed (minimal dosage) 4.8 gpl dosage giving breaking load is better than control sample. This may be due shrinkage of the sample.

Similarly increase in the case of tenacity and elongation properties has been observed during 4 gpl and 4.8 gpl dosages.combined results cross checked 4 and 4.8 gpl dosages are preferred as higher colourshade percentage, but 4.8 gpl dosage is preffered.

5.5.2 Effect of Sodium Hydrosulphite on Dye Uptake
As per the results obtained ,the effect of sodiumhydrosulphite on colourstrength,fastness properties and mechanical properties of the dyed material it is observed that addition of sodiumhydrosulphite in the proportions of 6.66gpl,13.33gpl, 20gpl with keeping time,temperature and concentration of sodiumhydroxide as constant. Where in 13.33 gpl concentration of sodium hydrosulphite gives optimum colourstrength, colourfastness and minimum strength loss. Hence standard concentration of sodiumhydrosulphite is considered as 13.33 gpl and this concentration of alkali is applied for the experiments. Up to certain dosages sodiumhydrosulphite works better for reduction of the colour.It is evident from the experiments higher dosages of sodiumhydrosulphite will not help for exhaustion of the colour as bath will start converting to acid media due to formation of sulphuric acid.This may be one reason, silk can be dyed in alkaline condition at lower temperature where alkaline condition bath will not affect the strength of the material.

During the analysis effect of sodiumhydrosulphite on breakingload, elongation & strength, it is observed that tenacity is showing minimum difference with the mean of the averages at 13.33 gpl sodiumhydrosulphite similar case with breaking load.As usual elongation is not affected where as colourstrength is concerned at 13.33 gpl maximum colourstrength is observed compare to the other cases, so 13.33 gpl dosage of sodiumhydrosulphite is preferred.

5.5.3 Effect of Alkalanity on Dye Uptake
It’s the combined effect of sodium hydroxide and sodium hydrosulphite, addition of both the alkali shows different pH, here the initial pH will be 11-12 and the final pH will be 10-11. It shows that vatting should takesplace in alkaline condition. Dye bath pH need to be set to the optimum alkaline condition to ensure the formation of leuco compound.As reduction and oxidation are happening intermittently complete exhaustion of the dye liquor is not possible in vat dyeing

5.5.4 Effect of Temperature on Dye Uptake
As per the results obtained the effect of temperature on colourstrength, fastness properties and mechanical properties on the dyed material. It is observed that dyeing experiments were conducted at different temperatures 40˚C, 50˚C, 60˚C keeping time, concentration of sodium hydroxide and sodiumhydrosulphite as constant.where in 50˚C dyeing condition gives optimum colourstrength, colourfastness and minimum strength loss. Finally standard temperature is concluded as 50˚C and this temperature is maintained for the experiments. Generally rate of reaction increases with increase temperature, where as leuco compound formation is temperature specific activity it happens between 50˚ to 55˚ C .Further increase temperature up to 60˚C and above has not shown any improvement in the dye absorbency.

5.5.5 Effect of Time on Dye Uptake
As per the results obtained the effect of time on colour, strength, fastness properties and mechanical properties of the dyed material. it is observed that dyeing experiments which were conducted at different time intervals like 30 mints, 45 mints ,60 mints keeping temperature, concentration of sodium hydroxide and sodiumhydrosulphite as constant. Where in 30 mints dyeing condition gives optimum colourstrength, colourfastness and minimum strength loss. Hence standard time is taken as 30 mints and this time is maintained for the experiments.

Dye absorbency of vat dye on substrate is noncontinious process that is incremental with time as reduction and oxidation are happening in steps for every step reduction and oxidation there is increase in the depth of the shade.This phenomena will exist for 30 mints. Further enhancing of time has not showed any true increase of colour where diffusion reduced vat dye particles are observed on the surface of substrate which need a soap treatment to ensure the removal of these unfixed colour.

5.6 Analysis after Optimization

5.6.1 Dyeing
The following parameters are optimized for the application of vat dyes on Eri silk are as follows.

1. Sodium Hydroxide-4.8 gpl
2. Sodiumhydrosulphite- 13.33 gpl
3. Temperature-50˚ C
4. Time -30 mints

Using above optimized parameters final dyeing of vat dyes on Eri silk can be conducted for the following four colours.The selected colours are as follows.

1. Corovat Red (Colour Index Nr: CI VAT RED10)
2. Corovat Darkblue (Colour Index Nr: CI VAT BLUE 20)
3. Corovat Black (Colour Index Nr: CI VAT GREEN 9)

Table 5.9: Recipe for vat dye

Procedure
As per the weight of material, the bath is adjusted to the material-to-liquor ratio. The dye percentage (1-5%) shade samples are dyed. Enter the pre-soaked silk materials to the different vat baths. Time, concentration of sodium hydroxide and sodium hydrosulphite, temperature should be maintained as given in the above table. Then material is removed according to the given time, washing treatment has given using required amount of soap and soda ash mentioned in the above table and dried it under shade.

5.6.2 Test Results after Optimization

Strength Related Results

Figure 5.9 .physical test result (strength values) of undyed sample and red colour vat dyed sample

Breaking Load

Elongation

Tenacity

As per above tables in all the cases strength(breaking load and elongation) increases as compared to control sample in Corovat Red Colour, but tenacity become reduced as compared to control sample. From the above result we conclude that the optimized parameters conditions are suitable for application of vat dyes on Eri silk.

Figure 5.10 physical test result (strength values) of undyed sample and darkblue colour vat dyed sample

Breaking Load

Elongation

Tenacity

As per above tables in all the cases strength increases (breaking load and elongation), but no increase in tenacity is found with dyed samples as compared to control sample in Corovat Dark Blue Colour. From the above result we conclude that the optimized parameters conditions are suitable for application of vat dyes on Eri silk.

Figure 5.11 physical test result (strength values) of undyed sample and black colour colour vat dyed sample

Breaking Load

Elongation

Tenacity

As per above tables in all the cases strength increases (breaking load and elongation), but no increase in tenacity is found with dyed samples as compared to control sample in Corovat Black Colour. From the above result we conclude that the optimized parameters conditions are suitable for application of vat dyes on Eri silk.

Colour Measurement Results

In this adsorption study of vat dye on Eri silk were carried out to find the capacity of vat dye absorption on Eri silk yarn using optimized parameters.

Figure 5.12: K/S value of Eri silk sample dyed with vat dye at optimized condition (corovat red)

The above graphs show the K/S values for the 1-5% shades for the respective colours. As per above graphs there is an increase in the trend of the colour.

Figure 5.13: K/S value of Eri silk sample dyed with vat dye at optimized condition (corovat darkblue)

The above graph shows the K/S values for the 1-5% shades for the respective colours. As per above graphs there is an increase in the trend of the colour, variations are seen between the samples in their colour strength.

Figure 5.14: K/S value of Eri silk sample dyed with vat dye at optimized condition (corovat black)

The above graphs shows the K/S values for the 1-5% shades for the respective colours.As per above graphs there is an increase in the trend of the colour.

Table 5.10: Colour Fastness Results

Click on Image for large size

As per the above tables in all the colours preferring best fastness properties among in case of wash, rubbing perspiration and light fastness, when dyed with optimized conditions.

R-corovat red, DB-corovat dark blue, B-corovat black

CC – change in colour
SC – stain on cotton
SS- Stain on silk

Conclusions:

1. Concentration of sodium hydroxide and sodium hydrosulphite plays a major role during reduction of insoluble dyes.

2. Dye absorption on Eri silk depends up on the formation of leuco vat compound.

3. Vat dyes can be successfully dyed on Eri silk with minimum strength loss by maintaining optimal conditions of concentration of sodium hydroxide, sodium hydrosulphite, temperature and duration.
Breaking load, Tenacity and Elongation properties remain unaffected during dyeing process the causes may be adverse effect.

• Sodium hydroxide at lower temperature is insignificant.
• Eri Silk is more resistance to severe condition during the processing.
• Final treatment with mild acid to dyed substrate may also prevent strength strength loss.

5. Mainly strength of Eri silk will be severely affected in case of adverse condition like higher temperature and time.

6. As per the optimized condition got good colourmeasurement values.

7. Vat dyed samples dyed at various conditions were subjected wash, perspiration, rubbing and light test by following standard laboratory test method and following observations are made.

• Vat dyes on Eri silk are showing better wash, perspiration, and light fastness properties. Almost all the samples are possessing very good light fastness properties
• Wash fastness as well staining on adjacent cotton and silk goods during normal washing condition i.e., 5 gpl of soap at 40˚C was far excellent fastness laundering and hypochlorite bleaching was also observed found outstanding.
• Perspiration fastness of vat dyes were also found satisfactory under standard test condition
• Rubbing fastness both dry and wet rubbing was found excellent for soap treated samples.

Even pale shades of vat dyes also possess high performance to different fastness tests this may be because strong entroping of colour companion inside the fibre inspite of several application drawbacks. If proper dyeing method is followed vat dye can be used successfully in dyeing of Eri silk. These only class of dyes which will not possess any change in the tones during finishing as melting point if these class is higher.

* Role of pretreatment
Eri spun yarn will contain some residual gum 2 to 3% (Y.C.Radhalakshmi etal) and some impurities add as well as inherent need to take care before dyeing. Pretreated yarn show uniform dye uptake in case of all dyes. Luster of pretreated sample is found better than yarn that dyed without pretreatment.

* Role of sodiumhydrosulphite and sodium hydroxide
Up to certain dosages sodiumhydrosulphite and sodium hydroxide combination works better for reduction of the colour.

* Role of temperature
Generally rate of reaction increases with increase in temperature, where as leuco compound formation is temperature specific activity it happens between 50˚ to 55˚C. Further increase temperature up to 60˚C and above has not shown any improvement in the dye absorbency.

* Effect of time
Dye absorbency of vat dye on substrate is noncontinious process that is incremental with time as reduction and oxidation are happening in steps for every step reduction and oxidation there is increase in the depth of the shade. These phenomena will exist for 30 mints. Further enhancing of time has not showed any true increase of colour where diffusion reduced vat dye particles are observed on the surface of substrate which need a soap treatment to ensure the removal of these unfixed colour.

* Role of alkalinity (pH)
Dye bath pH need to be set to the optimum alkaline condition to ensure the formation of leuco compound. As reduction and oxidation are happening intermittently complete exhaustion of the dye liquor is not possible in vat dyeing.

Scope for Further Study:

1. In present study application of vat dye on Eri silk yarn is done by air oxidation method, further experiments may be conducted for chemical oxidation method using Sodium Perborate or Hydrogen Peroxide. Colour difference can be assessed using computer colour matching system.
2. Experiment can be set up for Eri silk fabric dyeing in closed winch ,where complete reduction and exhaustion can be achieved in closed condition effect of this method can compared with air oxidation method
3. Leuco vat formation is an essential process this can be achieved by using glucose dosages which will be an ecofriendly approach
4. Application of vat can be done in kera stage to high twisted yarn latter cross colour effect can be achieved by dyeing weft colour with acid dyes in production of chiffon, crepe, georgette fabric.

References

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