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CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

Reactive dyes are becoming increasingly popular for dyeing cellulosic fibres because of their wide range of shades, brilliant colour, ease of application, and excellent wash fastness properties due to strong covalent bond. However, these dyes are known to have poor perspiration and light fastness properties (Imada et al 1994). The light fastness of different types of dyes on textile materials has been extensively discussed in a great number of technical papers, reviews and monographs (Terenin 1977), while only little attention has been paid to the light fastness of reactive dyes. The criterion of light stability is judged in terms of the magnitude of the change in the relevant properties at various exposure times (Brunnschweiler 1964). The light fastness of dyed textile materials is one of the most important characteristic for ready-made goods. The light fastness of the dyed goods is evaluated by the rate of dye destruction in the fibre by the exposure of an artificial light source which is similar to sun light. Light fastness is the fading of dyes due to the effect of light. Factors affecting light fastness are the intensity and spectral composition of the light used for exposure, the properties of dyed fibre, the dye concentration in the fibre, the dye reactivity, the state of the dye in the fibre, the nature of the bond between the dye and fibre, physical and the chemical constitution of the fibre. In combination shades, light fastness ratings are even lower than the lowest values of the individual dyes constituting in the mixture. There is very limited study on the light fastness due to varying factors such as cotton yarn linier density, fabric structure, pretreatment methods, dyeing methods and after treatments.

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2.2 COTTON

Today, cotton is the most widely used fibre in apparel. The modern textile industry covers different consumer sectors such as apparel textiles, household textiles, medicinal textiles and technical textiles. The production of garment includes numerous steps starting from raw cotton. The journey of cotton from fields to consumer is described below.

a) Cotton

b) Spinning (Yarn manufacturing)

c) Weaving or knitting (Fabric Manufacturing)

d) Pretreatment

e) Dyeing and/or printing

f) Goods preparation

g) Consumer

The production of cotton starts with cotton harvesting and converting it into yarns by processes like ginning and spinning. Then the yarns are made suitable for weaving by sizing. Sizing makes the wrap yarns stronger and reduces the friction during weaving.

The resulting textiles are known as grey fabrics (Karmakar 1999). Grey fabrics are not ready to use, because of their hydrophobic nature (water repellent) and unwanted colours. Therefore, grey fabrics undergo a wet pretreatment consisting of a chain of chemical treatments that alters the properties of cotton fabric, converting fabrics from hydrophobic to hydrophilic and making them brighter in terms of colour. Thereafter, fabric is dyed and or printed before the final apparel production. Finally the cloths go to consumer via the outlets (Rouette 2000).

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2.3 COTTON YARN AND DIAMETER

Yarn is an assemblage of fibres twisted together. Yarn formation methods were originally developed for spinning of natural fibres including cotton, linen, wool and silk. Since the overall physical characteristics of the fibres and processing factors needed differed from fibre to fibre, separate processing systems were developed for different fibres.

Cotton count is a measure of linear density. In the English count, Ne is the number of hanks (840 yard or 770 m) of skein material in 1 pound (0.454 kg). This is an indirect system, higher the count numbers finer the yarn. In the United States cotton counts between 1Ne and 20Ne are referred to as coarse counts. A regular single-knit T-shirt can be between 20Ne and 40Ne; fine bed sheets are usually in the range of 40Ne to 80Ne. The number is now widely used in the staple fibre industry. Yarn count below 12Ne is used for denim, home furnishing, bed sheets and curtains. Finer count is useful in shirting’s and ladies wear. Yarn diameter is calculated by the following formula

Yarn diameter in inches = (2.1)

Yarn diameter increases as cotton count number decreases in this indirect system of count calculation. Whereas density in grams per cubic centimeter and the diameter is in mm.

(2.2)

2.4 COTTON FABRIC

Cotton yarn can be knitted or woven into cloth. Depending upon form of interlacing of warp and weft yarn fabric weaves classified as plain and twill

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weaves. A plain weave produces fabrics like poplin, cambric and broadcloth. A twill weave is more durable and used in denim, khaki and gabardine.

2.4.1 Plain Fabric

The simplest of all weaves is the plain weave. Each filling yarn passes alternately over and under the warp yarns. Each warp yarn passes alternately over and under the filling yarns. Some examples of plain-weave fabric are crepe, taffeta, and muslin. The surface of the fabrics is very smooth and even.

2.4.2 Twill Fabric

A weave that repeats on three or more ends and picks & produces diagonal lines on the face of the fabric. Twill weave is characterized by diagonal rib (twill lines) on the face of the fabric. These twill lines are produced by letting all warp ends interlace in the same way but displacing the interlacing points of each end by one pick relative to that of the previous end. In twill weave line moves sinisterly (Right - Left, Z twill) and dextrally (Left - Right, S twill). The surface of the fabric is wavy and uneven. Figure 2.1 shows the woven fabric structure.

(a) Plain Fabric (b) Twill Fabric

Figure 2.1 Woven fabric structure

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2.4.3 Single Jersey

Single jersey fabric is a type of knit textile made from cotton or a

cotton and synthetic blend. Some common uses for jersey fabric include

t-shirts and winter bedding. The machine gauge 10 needles per cm are

commonly used. The fabric weight is varying from 160 grams per square

meter. The fabric is warm, flexible, stretchy, and very insulating, making it a

popular choice for the layer worn closest to the body. Jersey also tends to be

good lustre, soft, smooth, even and comfortable to wear.

2.4.4 Pique

Pronounced “PEEK”, this is the fabric that is most associated with

the original Lacoste Alligator Polo shirt. The construction is designed to pull

moisture from the skin and wick it into the air, keeping the fabric and the

wearer relatively dry and cool. If combination of knit and tuck stitch is equal then

it is called a pique fabric. Such as 1 knit and 1 tuck or 2 knit or 2 tuck and if

combination is not equal and it is called as Lacoste fabric. Due to the structure

the surface of the fabric is rough and wavy causes dull look. The grams per

square meter of fabrics are varying from 160 to 240. Figure 2.2 shows the knitted

fabric structure.

(a) Single jersey (b) Pique

Figure 2.2 Knitted fabric structure

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2.5 PRETREATMENT FOR COTTON

Scouring removes waxes and impurities from the fabric and has an

influence on the dye uptake depending on the amount of alkaline used.

Treatment of cotton with higher quantities of alkaline (mercerising) has a

more marked effect on the physical and chemical properties of the cotton

fibre. The average moisture regain of the fibre increases by almost 25% to

10.5% at 65% relative humidity and 20°C. Mercerising also results in

physical changes to the cotton fibre that gives added value to the final

product. If the fabric is under tension during the mercerisation the fibre is

prevented from shrinking during the swelling process. Surface lustre is

developed, in part due to the changes that take place in the fibre cross-section.

The fibre loses its kidney shape and becomes more circular, thus increasing

the surface reflective properties. Increased hydrogen bonding between the

molecular chains also occurs that gives an increase in fibre strength of

approximately 20%. Since the fibre swells dramatically during the treatment,

the fibrils in both the crystalline and non-crystalline regions become more

accessible to the penetration of moisture. Thus the relative moisture

absorbency increases. This increase in moisture absorbency increases the

comfort factor of a typical cotton garment. At the same time the dye-ability of

the fibre also increases, so that a lower quantity of dye is required for a given

shade depth. Not all cotton fabrics are mercerised. Therefore the experimental

work in this study was restricted to cotton fabrics that had not been

mercerised.

2.5.1 Grey Boiling

Grey boiling with non-ionic surfactants is preferred as they are

stable in alkaline medium at high temperature. The use of non-ionic

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surfactants in the grey boiling helps for wetting out the fabric. Since water

used for the wet processing and fabrics have hardness compounds,

sequestering agent has to be used in scouring process. Grey boiling was

carried out with 1 g/l sequestering agent and 2 g/l non-ionic surfactant. The

process is carried out at 80°C for 20 min followed by cold wash for 10 min.

2.5.2 Enzymatic Treatment

Enzymatic scouring is the latest development in the pretreatment

part of cotton. The alkaline pectinase is commonly used for this treatment.

The main advantages of this treatment are eco-friendly due to the very less

usage of alkaline. For bioscouring, 2% OWF (On the Weight of Fabric)

Enzyme was used. A buffer solution was used to set scouring-bath at a

favourable pH for enzyme to act. The pH of the scouring bath is 8-9

according to the type of enzyme used in the process. 2 g/l non-ionic surfactant

was used. Scouring was carried out at 55°C for 40 min, then hot wash at 80°C

for 10 min and then neutralizing with acetic acid for 15 min.

2.5.3 Alkaline Scouring and Semi Bleaching

Alkaline scouring is a common industrial method of pretreatment

for cotton with sodium hydroxide. This process removes the non-cellulosic

impurities in cotton and improves the water absorbency and dyeability. Semi

bleaching is very famous industrial method of pretreatment for cotton with

alkaline and peroxide. This process removes the non-cellulosic impurities in

cotton and improves the water absorbency and dyeability. Though different

scouring materials are used in the textile industry like NaCO3, Ca(OH)2 etc.,

alkaline (NaOH-sodium hydroxide) is mostly used for the scouring.

Conventional chemical scouring is done in hot (90°C-100°C) NaOH solution

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for 45-60 minutes. This condition depends upon the quality of scoured fabric

required. Moreover, different agents are used such as reducing agents,

detergent, sequestering agent (also called chelating agents or sequestrant), and

wetting agent. Sequestering agent reduces the water hardness, reducing agent

prevent oxidation of cellulose by air oxygen at high pH, detergent acts as

emulsifier to assist in removing waxy substances and wetting agent reduces

the surface tension of water help fibres to swell.

2.5.4 Bleaching

The natural fibre and fabrics even after scouring still contain naturally

occurring colouring matter. This yellowish and brown discolouration may be

related to flavones pigment of the cotton flower. The climate, soil, drought

and frost can also cause various degrees of yellowness. Tips of leaves or

stalks coming in contact with the moist ball after opening will cause dark

spots and discolouration. Discolouration may also come from dirt, dust, and

insects or from harvesting or processing equipment in the form of oils and

greases. The object of bleaching is to produce white fabrics by destroying the

colouring matter with the help of bleaching agents with minimum degradation

of the fibre (Shenai 1991). The bleaching agents either oxidize or reduce the

colouring matter which is washed out and whiteness will be obtained.

In the later stage of twentieth century, the time required for

bleaching dropped steadily from months to days and days to hours. Now-a-

days, manpower required for average plant is declined considerably and the

cost of bleached finished product is also reduced. This technical break-

through will continue in the future also and it will reduce the cost of bleaching

further. Efforts were made to optimize time, temperature and concentration of

hydrogen peroxide, whiteness, weight loss of substrate (Shore 2002).

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Bleaching of textile material is a commercial, chemical process, which can be

defined as “Destruction of natural colouring matters to impart a pure

permanent and basic white effect suitable for the production of white finishes,

level dyeing and desired printed shade with the minimum or without

diminishing the tensile strength”.

Hydrogen peroxide is stable in acidic medium. Bleaching occurs by

the addition of alkaline or by increase the temperature. Hydrogen peroxide

liberates per hydroxyl ion (HO2-) in aqueous medium and chemically behaves

like a weak dibasic acid. The per-hydroxyl is highly unstable and in the

presence of oxidizable substance (coloured impurities in cotton), it is

decomposed and thus bleaching action takes place. Sodium hydroxide

activates hydrogen peroxide because H+ ion is neutralized by alkaline which

is favorable for liberation of O2.

H2O2 +NaOH Na+ +HO2- +H2O (2.3)

H2O2- H++HO2- OH HO2

-+H2O (2.4)

However, at higher pH (above 10.8) the liberation of HO2- ion is so

rapid. So, it becomes unstable with the formation of oxygen gas which has no

bleaching property.

If the rate of decomposition is very high, the unutilized HO2 may

damage the fibre. A safe and optimum pH for cotton bleaching lies in 10.5 to

10.8 whereas the rate of evolution of per hydroxyl ion is equal to the rate of

consumption (Saravanan & Ramachandran 2010). At higher pH, hydrogen

peroxide is not stable and hence a stabilizer is frequently added in the

bleaching bath (Abdul & Narendra 2013).

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2.5.5 Mercerization

Cotton can be made to have somewhat the lustre of silk if it is given

a treatment called Mercerization. During mercerization cotton fibres swell and

untwist and thus present a better reflecting surface for light. At the same time,

the tensile strength and elasticity are increased by mercerization treatment.

Mercerization of fabrics is performed using NaOH with the concentration

normally being in a range of 25 to 30° Be, at low temperatures (15 to 25°C).

Tension is applied to the fabrics in the vertical direction with a tension

cylinder, and in the horizontal direction with a clip stenter. The processing

time by the cylinder and the stenter in total is 30 to 60 seconds.

To prevent the fabrics from shrinking after going through the

stenter, the NaOH concentration in the fabrics needs to be decreased

sufficiently (down to 7° Be or lower) when the fabrics leave the stenter. Also,

since the piling on thick fabrics in a wet state leaves creases on the fabrics, the

thick fabrics need to be dried promptly.

2.6 COLOURANT

Colourants are characterised by their ability to absorb visible light.

Since 1900, numerous coloured chemical compounds have been synthesised

and established in practical use. Colourants are generally classified into dyes

and pigments although in some instances the terms are used synonymously

(Zollinger 1992). The basic difference between the two types is their particle

size and solubility in the polymer medium. Ideal pigments are normally

materials with a large particle size, which are insoluble in the medium in

which they are applied, while dyes are molecules that are soluble.

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Even in these early times it was known that different colours and

hues could be obtained through the use of different metals with a single dye

chromophore. Some of the earliest dyes were a luxury such as Murex and

Purpura (Tyrian Purple) and yet, unfortunately, extremely unstable to light.

Cave drawings such as those in Altamira, Spain demonstrate that inorganic

pigments were used in prehistoric times. Pigments and dyes are widely used

in the colouration of polymer materials for many commercial applications.

2.6.1 Textile Dyes

Dyes can be said to be coloured, ionized and aromatic organic

compounds which shows an affinity towards the substrate to which they are

applied. They are generally applied in a solution that is aqueous. Dyes may

also require a mordant to improve the fastness of the dye to the material on

which they are applied.

2.6.2 Classification of Dyes

Dyes may be classified according to their chemical structure or by

the method by which they are applied to the substrate. The dye manufacturers

and dye chemists prefer the former approach of classifying dyes according to

the chemical type. The dye users, however, prefer the latter approach to of

classification according to application method. Classification by application

or usage is the principal system adopted by the Colour Index (C.I.). The

classification of dyes according to their usage is summarized in Table 2.1,

which is arranged according to the C.I. application classification

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Table 2.1 Classification of textile dyes

Class Principal substrates Method of application Chemical Types Acid Nylon, wool, silk,

paper, inks and leather

Usually from neutral to acidic dye baths

Azo (including premetalized), anthraquinone, triphenylmethane, azine, xanthene, nitro and nitroso

Azoic components and composition

Cotton, rayon, cellulose acetate and polyester

Fibre impregnated with coupling component and treated with a solution of stabilized diazonium salt

Azo

Basic Paper, polyacrylonitrile, modified nylon, polyester and inks

Applied from acidic dye baths Cyanine, hemicyanine, diazahemicyanine, diphenylmethane, triphenylmethane, azine, xanthene, acridine, oxazine, azo and anthraquinone,

Direct Cotton, rayon, paper, leather and nylon

Applied from neutral or slightly alkaline baths containing additional electrolyte

Azo, phthalocyanine, stilbene and oxazine

Disperse Polyester, polyamide, acetate, acrylic and plastics

Fine aqueous dispersions often applied by high temperature/pressure or lower temperature carrier methods; dye may be padded on cloth and baked on or thermofixed

Azo, anthraquinone, styryl, nitro and benzodifuranone

Fluorescent brighteners

Soaps and detergents and all fibres, oils, paints and plastics

From solution, dispersion or suspension in a mass

Stilbene, pyrazoles, coumarin and naphthalimides

Mordent Wool, leather and anodized aluminum

Applied in conjunction with Cr salts

Azo and anthraquinone,

Oxidation bases

Hair, fur and cotton Aromatic amines and phends oxidized on the substrate

Aniline black and indeterminate structures

Reactive Cotton, wool, silk and nylon

Reactive site on dye reacts with functional group on fibre to bind dye covalently under influence of heat and pH (alkaline)

Azo, anthraquinone, phthalocyanine, formazan, oxazine and basic

Sulphur Cotton and rayon Aromatic substrate vatted with sodium sulfide and reoxidized to insoluble sulfer-containing products on fibre

Indeterminate structures

Vat Cotton, rayon and wool

Water-insoluble dyes solubilized by reducing with sodium hydrogen sulfite, then exhausted on fibre and re-oxidized

Anthraquinone (including polycyclic quinines) and indigoids

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2.6.3 Specialities of Textile Dyes

Textile dyes speciality is that should have affinity towards textile

fibre and have good overall fastness. Pigments are sometimes used to colour

cotton fabrics, however they are not considered to be dyes. They are

completely insoluble in water and have no affinity for cotton fibres. Some

type of resin, adhesive, or bonding agent must be used to fix them to the

cotton fibre. Typically, they exhibit good colour fastness to light and poor colourfastness to washing.

Direct dyes are water soluble and categorized into the surface

bonding type dye because they are absorbed by the cellulose. There is no

chemical reaction, but rather a chemical attraction. The affinity is a result of

hydrogen bonding of the dye molecule to the hydroxyl groups in the cellulose.

After the dyestuff is dissolved in the water, a salt is added to control the

absorption rate of the dye into the fibre. Direct dyes are fairly inexpensive and

available in a wide range of shades. Typically, they exhibit good light fastness

and poor wash fastness. However, by applying a fixing agent after dyeing the wash fastness can be improved drastically.

Vat, sulphur, and naphthol dyes are fine suspensions of water

insoluble pigments, which adhere to the cotton fibre by undergoing an

intermediate chemical state in which they become water-soluble and have an

affinity for the fibre. Typically, vat dyes exhibit very good colour fastness

properties. Sulphur dyes are used to achieve a low cost deep black. They

exhibit fair colour fastness properties, although the lighter shades tend to have

poor light fastness. Naphthol dyes are available in brilliant colours at low

cost, but application requirement limit in their use. They exhibit a good light

fastness and wash fastness, but poor rubbing fastness.

Reactive dyes attach to the cellulose fibre by forming a strong

covalent (molecular) chemical bond. Bright shades and excellent wash

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fastness properties are the trademark of reactive dyes. Two concern regarding

reactive dyes are their susceptibility to damage from chlorine. Another is that

lighter shades tend to have reduced light fastness properties. The azo reactive

dyes are also often poor light fastness, consequently the photo decomposition

processes of such systems have been studied by (Allen & McKellar 1980).

2.7 REACTIVE DYES

Reactive dyes, as their name implies, chemically react with the

fibre to form a strong linkage that gives rise to high performance to wet

treatments such as laundering. Today they are the largest single range of dyes

used for the dyeing of cotton fibres and their blends. They are also very

important for producing bright shades and high wash fastness. The revolution

in reactive dye usage has been brought about by a steady reduction in the

costs of manufacture. It is made possible by the production of larger batch

sizes and improved yields during the manufacture.

2.7.1 History and Development of Reactive Dyes

The earliest reactive dye (1932) produced was Supramine Orange R

(Lewis 1992) (C.I. Acid Orange 30), It was not clearly understood at that time

why because this particular dye has excellent wash fastness on wool.

Subsequent research showed that the high wash fastness was due to the

chlorine group which formed a covalent bond with the amino (-NH2) group in

the wool fibre via a neucleophilic substitution reaction. In 1937 a German

patent was lodged that indicated it was possible to attach dyes to the wool

fibre by covalent bonding. Various chemicals had already been tried that

could react with the hydroxyl groups in cellulose. However, the very severe

reaction conditions that had to be employed led researchers to the then

concluded that the dye-fibre reaction with cellulose was not practical or

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commercially achievable. Hence the various wool dyes that are capable of

forming covalent bonds with cellulose were not considered at that time.

The first truly reactive dyes for cotton were developed by Rattee

and Stephens at ICI England in 1954. These first cotton fibre reactive dyes

were based on dichlorotriazine groups (Ahmed 1995). When dyed under

alkaline conditions (approximately pH 10.0) the resultant dyeing’s had

excellent wash fastness. The alkalinity caused a reactive chlorine atom on the

triazine ring to be substituted by an oxygen atom from the cellulose hydroxyl

group. The alkaline also caused acidic dissociation of some of the hydroxyl

groups in the cellulose allowing the cellulosate ion (Cell O2-) to react with the

dye, as illustrated in Figure 2.3 (Broadbent 2001).

Figure 2.3 Reaction mechanisms between the triazine ring and cellulose chain

2.7.2 Structure and Classification of Reactive Dyes

There are many reactive groups that have been used in the

manufacture of reactive dyes but most reactive dyes have the structural

features, represented diagrammatically in Figure 2.4. Some or all of these

features may be present more than once in the dye molecule, as in the case of

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bi- functional or poly-functional reactive dyes. The solubilising groups are

usually sulphonic acids and they typically range in number from one to four,

depending on the raw materials used for the synthesis of the dye, the overall

size of the dye molecule and the intended application method.

Figure 2.4 Structure of reactive dye

Where high substantivity (the attraction between the dye and a

substrate) for the fibre is desirable (e.g. for batch-wise exhaustion) a low

number of solubilising groups should be present within the dye structure; the

reverse is found the case low substantivity is required, for example, in

continuous processes such as pad-batch process. It is possible to use almost

any chromophore group in the reactive dye class. The only structural features

required are at least one sulphonic acid group to ensure adequate water

solubility and a site that a bridging group (such as an amino group) can bond

in order to link in the reactive group.

Therefore, reactive dye ranges can incorporate, for example, mono-

azo, di-azo, metallised mono- and dis-azo, anthraquinone and phthalacyanine

chromogens. Bridging groups attach the reactive group to the chromophore,

but are not always necessary. Typical bridging groups are amino (-NH-),

substituted amino and amide linkages (-NHCO-). The bridging group can bear

some influence on the reactivity, substantivity and stability of the reactive

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dye. The dye chromophore is that part of the chemical structure of a dye that

gives a colour. In reactive dyes the dye chromophore has at least one fibre

reactive group added (Figure 2.4.). This distinguishes reactive dyes from acid

and simple direct dyes. The number and type of the reactive groups present in

the dye determines its degree of reactivity and hence the dyeing conditions as

shown in Table 2.2.

Table 2.2 Reactive groups and their dyeing temperatures

Reactive group Reactivity Dyeing temperature °C

Dichlorotriazine High 25-40

Monochlorotriazine Low 80-85

Monofluortriazine Moderate 40-60

Trichloropyrimidine Low 80-95

Dichloroquinoxaline Low 50-70

Difluorchloropyrimidine Moderate to high 30-50

Vinylsulphone Moderate 40-60

(a) Dichlorotriazine (b) Monochlorotriazine (c) Monofluortriazine

(d)Trichloropyrimidine (e)Dichloroquinoxaline (f)Difluorchloropyrimidine

Dye-SO2-CH=CH2

(g) Vinylsulphone

Figure 2.5(a-g) Reactive group structure

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Higher fixation efficiency could possibly be obtained by incorporating

additional reactive groups into the dye molecule. Figure 2.5 shows the

reactive group structure. However, this can have a detrimental effect on the

dyeing properties such as migration and can lead to lower build-up of the final

shade.

The Monochloro-s-triazine, Bis (Monochloro-s-triazine) and Bis

(Monofluoro-s-triazine) dyes are widely used in textile dyehouses. All the

above dyes are stable for 60°C and 85°C wash fastness tests, but for 98°C

wash fastness, only Bis (Monofluoro-s-triazine) dyes are stable (Gorensek

1999). These developments have resulted in the introduction of more

advanced reactive navy blue dyes that offer a better overall light fastness

properties. The majority of black dyes, however, remain mixtures still based

on C.I. Reactive Black 5. One of the advantages of the vinylsulphone

structure is that it contains a masking group (OSO3Na) attached to the two

methyl groups. This masking group increases the dyes resistance to hydrolysis

during the early stages of the dyeing process and is not removed or

deactivated until the alkali is added at the fixation stage.

2.8 COLOUR INDEX

The Colour Index (C.I.) is one of the options for identifying dyes.

The C.I. lists of all the dyes disclosed and registered (with the C.I.) by

dyestuff manufacturers, giving their fastness properties, uses, hues and in

many cases the chemical constitution (including the molecular structure) of

the main colourant that they contain.

It is worth noting that some of the major European dyestuff

manufacturers have chosen not to disclose some of their dyes or dye ranges to

the Colour Index to reduce the chance of them being copied. Under each C.I.

generic name, there is a list of all the different trade names under which that

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dye is sold by various manufacturers. It should not be assumed that all dyes

listed under a given C.I. name are actually identical, as they are often not.

2.9 EXHAUSTION AND FIXATION PROPERTIES OF

REACTIVE DYES

2.9.1 Dye Substantivity

The term ‘substantivity’ was originally derived from popular

substantive dyes (Direct dyes) and refers to the ability of a dye to be taken up

from a liquid medium onto a textile fibre and set on it (Rouette 2000). The

quantitative measurement of the force with which the dye is captured by the

fibre is determined as ‘affinity’. However, substantivity is often used as a

qualitative description of the affinity of a dye for a particular fibre.

The substantivity of a dye generally depends on the extent of its

solubility, molecular size and structure. Substantivity is favoured by the

formation of multiple dye-fibre bonds (Gordon et al 2007). In reactive dyeing

of cotton, these bonds are hydrogen bonds and covalent bonds. Thus, reactive

groups also exert a significant effect on the substantivity.

2.9.2 Dye Exhaustion

In exhaust dyeing, the fibre starts absorbing the dye as soon as it is

immersed into the dye liquor. As a result, the concentration of dye in the dye

bath decreases gradually (Broadbent 2005c). The shift of dyes towards the

fibre is generally referred as exhaustion. The degree of dye bath exhaustion as

a function of time describes the rate and extent of the dyeing process. For a

single dye, the exhaustion is expressed as the mass of dye taken up by the

fibre divided by the total mass of the dye originally used in the dye bath of

constant volume (Rouette 2000) (Equation 2.4).

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(2.5)

where, Co and Cs are the concentration of dye in the dye bath initially and at

the end of the process, respectively.

2.9.3 Dye Diffusion

The penetration of a dye into the fibre polymer structure from the

dye-fibre interface is known as dye diffusion. Fick’s second law states that the

rate at which the dye diffuses across a unit area in the fibre is proportional to

the concentration gradient across that area, the proportionality constant being

the diffusion coefficient (Broadbent 2005c). The coefficient of diffusion is a

parameter used in most fundamental studies on dye diffusion. The extent of

dye diffusion as a percentage of the total dye on the fibre has not been

generally reported.

2.9.4 Dye Migration

The mobility of dye molecules within the fibre is referred to be dye

migration. The extent of this mobility depends mainly on dye substantivity

and dye-fibre bonding. In the case of dyeing cotton with reactive dyes,

covalently-fixed dyes cannot migrate during the dyeing process. Accordingly,

the dye cannot diffuse into the fibre when it is fixed on the surface of the fibre (Imada et al 1992b).

2.9.5 Role of Electrolyte

In dyeing of cotton with anionic (direct or reactive) dyes, the role of

the cation of an electrolyte has been widely reported as the reduction or even

extinguishing of the negative charge built-up (the zeta potential) on the fibre

in an aqueous media (Guo et al 1993, Shore 1995 & Noah et al 1986). The

negative charge on the fibre is not required because it repels anionic dye

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molecules in the dye bath. Iyer et al (1987) have studied the effect of three

different Group 1A metal chlorides (i.e. lithium chloride, sodium chloride and

potassium chloride) in the dyeing process. They found that increased dye

exhaustion was obtained with increasing size of the alkaline metal cation:

Potassium (K+) > Sodium (Na+) > Lithium (Li+). Potassium chloride give the

highest dye exhaustion and the lithium chloride provided the lowest. This

supported the previous work by Nango et al (1984), where they proposed the

similar order, i.e. Caesium (Cs+) > K+ > Na+. The increase in disrupting

effect of electrolytic cations on the water molecules around the dye molecules

with the increase in the size of the cation.

Noah et al (1986) extended the work by includes Group 2A metals

(calcium (Ca2+) and magnesium (Mg2+)), aluminium (Al3+), and other

cations. They found that electrolytes of Group 2A alkaline earth metals

outperformed for dye exhaustion comparing to Group1A alkaline metals.

However, many other studies have shown that using calcium or magnesium

salts is not favourable in the dyeing processes (Yeung & Shang 1999, Patra &

Gupta 1995, Jain & Mehta 1991 & Bradbury et al 1992). This is because these

salts tend to promote dye aggregation and increase water hardness. The

aluminium salts did not support the dye exhaustion adequately. This is

probably due to the fact that their large trivalent cation tends to form insoluble

aluminium-dye complexes. Nango et al (1984) also looked at the effect of

different anions of the electrolyte but found that there was no significant

change in the dye uptake. However, Noah et al (1986) later obtained different

depths of shade with different electrolytic anions (chloride and sulphate) in

dyeing with direct dyes. They achieved deeper shades with the chloride

counter-ion. Most studies on the role of electrolyte cations and anions have

been carried out for exhaust dyeing uptake or adsorption.

31

2.9.6 Dye Fixation

There are three main ways in which dye molecules can become

attached (fixed) to the cotton fibre: mechanical retention, physical bonding

and chemical reaction (Hamlin 1999). Vat, sulphur and azoic dyes are fixed

principally with mechanical retention, the dye molecules are trapped in an

insoluble pigmentary form within the fibre polymer system. Direct dyes are

fixed with physical hydrogen bonding and van der Waal’s forces. Reactive

dyes are fixed mainly by reaction with the fibre polymer leading to the

formation of covalent bonds.

Dye fixation is generally determined as an estimate of the average

proportion of dye actually fixed on a textile fibre (Rouette 2000). The lower

fixation levels of reactive dyes are essentially due to unavoidable dye

hydrolysis during dyeing (Shukla 2007). There have been various analytical

ways for estimating the extent of dye fixation and dye hydrolysis. Today, the

percentage of dye fixation is usually determined by using absorbance

measurements of dye bath solution and/or colour strength measurements of

the fabric during dyeing (Lewis 2007 and Chattopadhyay et al 2007).

2.10 REACTIVE DYE APPLICATION PROCEDURES FOR

COTTON

The dyeing procedures for this class of dyes may be divided into

two major groups of immersion exhaustion (exhaust dyeing) and continuous

(pad-batch and pad-humidity fix dyeing) processes.

2.10.1 Exhaust Dyeing

Exhaust dyeing is a process of immersion of the fabric in the

dyebath, transfer of the dye to and its gradual diffusion into the fibre, so that

32

the dyebath concentration decreases. In the typical exhaust dyeing of cotton

with reactive dyes, the first phase of dyeing is carried out under neutral pH

conditions to allow dye exhaustion and diffusion (Broadbent 2005c). This

promotes uniform colouration. Sodium chloride or sodium sulphate is often

present initial stage itself or added gradually to the dyebath during this phase,

to promote exhaustion. The temperature of the dyebath may also be gradually

increased to aid penetration of the dye molecule into the fibres and assist

uniform migration of the dye molecule.

Fixation of the dye is then achieved by adding a suitable alkaline to

the dyebath, either at one step or gradually, to activate the cellulose anions.

The reaction phase of the dyeing occurs over 30–60 min with typical dyeing

temperatures within the range from 30 to 90°C, depending upon the type of

reactive group and its reactivity. The fixation process results in additional dye

transfer to the fibre, which is often referred as secondary exhaustion (Imada &

Harada 1992a and Srikulkit & Santifuengkul 2000). The secondary dye

exhaustion and dye-fibre reaction then progress until no further dye is taken

by the fibre. The important parameters in exhaustive reactive dyeing are the

liquor-to-fibre ratio, temperature, pH and time. Dye bath curve explained in

chapter 3.2.2.1.

2.10.2 Washing-off

After completion of the dye exhaustion and dye-fibre reaction

phases, the fabric contains covalently-bonded dye, absorbed but unreacted

dye, and the hydrolysed dye. The unreacted and hydrolysed dyes are generally

referred to as unfixed dyes. The fabric also contains the residual electrolytes

and alkali. The unfixed dye is weakly trapped within the fibre through

hydrogen bonds and vander Waal’s forces which can desorb easily during

washings of the dyed cotton textiles by the consumer. In other words, the

presence of unfixed dye in a reactive dyed fabric gives poor washing fastness.

33

Thorough washing-off after dyeing of reactive dyed cotton is therefore

essential to remove all the unfixed dye, residual electrolytic and alkali. This

washing-off is a series of thorough rinsing including boiling with a detergent.

This needs large amounts of good quality water (Shukla 2007). In traditional

reactive dyeing, about three quarters of the total water consumed is required

for washing-off phase (Knudsen & Wenzel 1996).

2.10.3 Pad Dyeing

The lowest possible liquor-to-fibre ratio in exhaust dyeing is 3:1

with ultra-low-liquor ratio dyeing machines. However, pad dyeing extends

this further, to the range of 1:1 to 0.5:1. Thus, the dye absorption and fixation

is significantly enhanced further with the pad dyeing processes. Another

advantage of fully-continuous pad dyeing is the mass production of fairly

large fabric lots. Continuous pad impregnation is a process where a fabric is

passed in open-width form through a small bath (trough) containing dye

solution and then through the pressure squeezing rollers to remove the excess

liquid evenly. The amount of dye solution taken as the percentage of mass of

the fabric is the pickup percentage. The impregnation and uniform squeezing

together are called padding and the device is known as a padding mangle. For

continuous pad dyeing, fixation must be rapid (30–120 s) and usually involves

heating (by baking or steaming) of the impregnated fabric. The exception to

this is the cold pad-batch process, where the fabric is padded with dye and

alkali and then batched at ambient temperature for

6–24 hours. The cold pad-batch dyeing is also referred to as a semi-

continuous dyeing because of such a prolonged fixation time. The fabric is

finally subjected to a thorough washing-off after the dye fixation step. This is

usually done on the continuous washing range. The washing-off procedures,

used in this research are given in section 2.10.2. Padding is the most

important process of continuous and semi-continuous dyeing (Hunger 2003).

34

Dye build-up, levelness and evenness on the fabric largely depend on this

step. Factors of concern being wetting of the fabrics are dwell time in the

padding mangle, type of fibre, construction of the fabric and pickup

percentage, Preferential adsorption of dyebath components because of

substantivity of dyes and reaction of reactive dyes in the dyebath.

2.10.4 Pad-Batch Process

Pad-batch dyeing process is the most economical of all pad dyeing

processes for the reactive dyeing of cotton (Aspland 1992). This process is

more economical than exhaust dyeing, mainly due to minimal energy

requirements. This process involves padding the fabric with a dye solution

containing a suitable alkaline system and then winding up the padded fabric

onto a suitable roller (Broadbent 2005b and Shore 1995). For dye fixation, the

fabric wound on the roller is batched for 6–24 hours at ambient temperature.

This process is therefore often called cold-pad-batch dyeing. For dye fixation

at ambient temperature, the dyes must have adequate reactivity. The dyes of

low reactivity are not preferred for this process. During batching, the roller

should preferably be rotating at low speed to avoid drainage of the internal

liquid within the batch. In order to avoid evaporation from the exposed

surfaces and edges of the roll, the fabric is wrapped with the winder end-cloth

around the entire roll and covered with a plastic film. After batching, the

fabric must be thoroughly washed to remove unfixed dye and residual

chemicals. This is done either on a continuous washing range or on a batch

dyeing machine. If the fabric is wound on a perforated beam, the washing-off

can be carried out using a beam dyeing machine. The fabric from this process

is claimed to have a better handle and surface appearance because it is not

continuously circulating around as the fabric does in exhaust dyeing

machines. Also prolonged fixation at ambient temperature often results in

better dye diffusion.

35

2.10.5 Pad-Humidity Fix Processes

During the first half of the twentieth century, the textile chemical

industry focussed its energies, resources on product and process innovations.

As a result, a phenomenal improvement in product quality was observed.

Unfortunately, little attention was paid to the consequences that the

introduction of new chemicals and new processes might have on the

ecological balance of the environment. Thus, by dumping chemical effluents

the eco-logical balance of nature was disturbed slowly (Schlaeppi 1998). In

recent years, the realisation of the need for controlling pollution through

industrial effluent has grown and all efforts are being made by governments

all over the world to draw up or to tighten the legislations pertaining to the

controls on the types and extent of pollutants that could be passed on to

nature. To reduce the usage of chemicals in dyeing concept is pad-humidity

fix process (Chavan 2001).

The pad-humidity fix concept has been developed jointly by

Monforts and Zeneca colours to provide a simple, rapid and economical

continuous colouration process with minimum chemical usage. In this

process, the reactivity of the dyestuff is exploited together with the drying

behaviour of the fabric in such way that optimum colour yields can be

achieved without the use of large and aggressive volumes of alkali.

Only reactive dyestuffs with high reactivity can be used for the

Pad- humidity fix process. Dyestuffs employed in the development of this

process. These are commercially available and also used in other dyeing

processes. The dye attaches itself to the fibre under mild fixing conditions.

After a short air passage, the dye is padded uniformly and squeezed fabric is

transported directly to the dryer (hot flue) where the fabric remains in the

chamber continuously for two to three minutes at approx. 25 volume % steam

content. These conditions are quite sufficient to fix the dyestuff. In general,

36

reactive dyestuffs require alkaline and long dwell times for fixation, e.g. in

the cold pad-batch process; alkali, urea and high temperature in the pad-dry

thermo-fixing process or salt, steam and temperature in the pad-steam

process. However, since highly reactive dyestuffs are used in the Pad-

humidity fix process, even a weak alkali (sodium bicarbonate), a short dwell

time (2-3 minutes) and a low fabric temperature are sufficient for dyestuff

fixation (Khot & Lende 2011).

2.11 FINISHING TREATMENT

A Softener is a chemical that alters the fabric handle in such a way

that it is more pleasing to touch. The better pleasing feel is a combination of a

smooth sensation, characteristic of silk and of the material being less stiff.

The softened fabric is fluffier and has better drape. Drape is the ability of a

fabric to follow the contours of an object. In addition to aesthetics (drape and

silkiness), softeners improve abrasion resistance, increased tearing strength,

reduced sewing thread breakage and reduced needle cutting when the fabric is

sewn in to a garment. Because of these functional reasons, Softeners act as

fibre lubricants and thus reduce the coefficient of friction between fibres,

yarns, and between a fabric and an object too. Certain softeners will diminish

the light fastness of some direct and reactive dyes.

2.11.1 Softener Selection

The physical state of the softener/lubricant will govern the

corresponding handle of a fabric. Low viscosity lubricants are responsible for

soft, pliable silky feel while solid waxes provide low coefficient of friction

without changing the fabric's handle. The softener material's initial colour

and/or propensity to develop colour when heated or aged must be considered

when selecting the class of material to use. The softener material's smoke

point may cause processing problems. Fabric odours may be caused by certain

37

class of softener materials. Softeners can alter the shade of the fabric. Some

react with the dye to change its light fastness properties while some will cause

the shade to become darker (the same phenomenone that makes wet fabric

look darker). Softeners can be responsible for poorer rubbing fastness by

dissolving surface dye. Some may migrate onto adjacent light coloured yarns

and thus redering them stained.

2.11.2 Softener Classification

Softeners are divided into three major chemical categories

describing the ionic nature of the molecule, namely Anionic, Cationic and

Non-ionic. Nearly all surfactants are softeners; however, not all softeners are

surfactants. Surfactants are two-ended molecules, one end being lyophilic and

the other hydrophilic. The lyophile is usually a long hydrocarbon chain, the

essence of most lubricants. The ionic portion is responsible for water

solubility, (a necessary feature for applying the softeners) and as will be

discussed later, in how the molecule aligns itself at the fibre surface. This

section is devoted to describe the chemical structures of important softeners,

some of their properties and their fabric uses. It is worth to remember that the

same chemical structure may describe a surfactant used for other purposes

such as detergents, wetting agents, emulsifying agents etc.

2.11.3 Anionic Softeners

Anionic softeners and/or surfactant molecules have a negative

charge on the molecule which comes from either a carboxylate group (-COO-), a

sulfate group (-OSO3-) or a phosphate group (-PO4-). Sulfates and

sulphonates make up the bulk of the anionic softeners. Some phosphates are

to be lesser extent, so the carboxylates are used as softeners. Anionic

softeners impart pliability and flexibility without making the fabric feel silky.

They are used extensively on mechanically finished fabrics for example

38

napped, sheared or Sanforized. A good napping lubricant, for example,

provides lubrication between the fabric and the napping wires yet at the same

time provides a certain amount of cohesiveness between fibres. If the fibres

are too slippery, the napping wires will overly damage the yarn. Sulphonated

oils (eg. Turkey Red Oil) imparts a soft raggy handle, sulphonated tallow a

full waxy hand and sulphonated fatty esters a smooth waxy hand. Most

anionic softeners show good stability towards heat and some are resistant to

yellowing. Anionic softeners do not interfere with foam finishes in fact they

are deleterious for foam finishing. Anionic softeners have good rewetting

properties and are preferred for those fabrics that must adsorb water such as

bath towels.

2.11.4 Cationic Softeners

Cationic softeners are ionic molecules that have a positive charge

on the large part of the molecule. The important ones are based on nitrogen,

either in the form of an amine or in the form of a quaternary ammonium salt.

The amine becomes positively charged at acidic pH and therefore functions as

a cationic material at pH below 7. Quaternary ammonium salts (hereafter

referred to as QUATS), retain their cationic nature at all pH. Some important

types will be described in this section. An important quality of cationic

softeners is that they exhaust from water onto all fibres. When in water, fibres

develop a negative surface charge, setting up an electronic field for attracting

positively charged particles. These forces cause the cationic softener to

deposit in an oriented fashion, the positive end of the softener molecule is

attracted to the fibre surface forcing the hydrocarbon tail to orient outward.

The fibre now takes on low energy, nonpolar characteristics; therefore, the

fibre has the lowest possible coefficient of friction. Cation based softeners are

highly efficient softeners. The ionic attraction causes complete exhaustion

39

from baths and the orientation on the fibre surfaces allows a monolayer to-be

as effective as having more lubricant piled on-top.

Cationic softeners impart very soft, fluffy, silky handle to most of

the fabrics at very low levels of add-on. It will also exhaust from dyebaths

and laundry rinse baths making them very efficient materials to use. Further it

will exhaust from acidic solutions. Cation improves tear resistance, abrasion

resistance and fabric sewability. Cationic softener also improves antistatic

properties of synthetic fibres. They are compatible with most resin finishes.

They are good for fabrics to be napped. They are incompatible with anionic

auxiliary chemicals. They have poor resistance to yellowing. They may

change dye shade or affect light fastness of some dyes. They retain chlorine

from bleach baths. They adversely affect soiling and soil removal and may

impart unwanted water repellency to some fabrics.

2.11.5 Non-Ionic Softeners

Non-ionic softeners can be divided into three sub categories,

ethylene oxide derivatives, silicones and hydrocarbon waxes based on

paraffin or polyethylene. The ethylene oxides based softeners, in many

instances are surfactants and can be tailored to give a multitude of products.

Hydrophobes such as fatty alcohols, fatty amines and fatty acids are

ethoxylated to give a wide range of products. Silicones too can be tailored to

give several different types of products. Polyethylene wax emulsions, either

as high density or as low density polymers, are commercially available.

Different types of emulsifiers can be in making the emulsions so that the

products can be tailored to meet specific needs. This section will discuss some

of the more important non-ionic surfactants.

Silicones are water clear oils that are stable to heat and light and do

not discolour the fabric. They produce a slick silky handle and are preferred

40

for white goods. They improve tear and abrasion resistance and are excellent

for improving sewing properties of fabrics. Amino functional silicones

improve durable press performance of cotton goods. Epoxy functional silicon

is even more durable. The silicones are water repellent which make them

unsuitable as towel softeners. Silicone softeners are expensive compared with

fatty softeners. Amino functional silicones get discoloured with heat and

ageing. They may interfere with redyeing when salvaging off quality goods.

2.12 FIXING

Reactive dyes have a great usage due to wide colour range and

greater colour fastness rating. Most of the wet processing industries are using

reactive dyes on textiles for colouration purposes. Reactive dyes have good

water solubility. They posses various reactive groups. During the process of

dyeing of cellulosic material they react chemically with the fibre substrate to

form a co-valent bond in presence of alkali in such a way it becomes a part of

the fibre itself. Reactive dyes are the only class of dyes which form co-valent

bond with the fibre molecules therefore the colour fastness characteristics are

better. After treatment is carried out in reactive dyeing to remove the weakly

bonded unfixed dyes from the fibre substrate. However, fixing agent may be

induced after reactive dyeing to improve the stability of previously formed

co-valent bond between dye as well unfixed dye and fibre molecules.

2.12.1 Reason for Fixer in Direct Dyed Fabric

The main disadvantage of direct dyes is their poor wash fastness.

The sorption of direct dye by cotton is not a permanent and irreversible

process. The dye may be removed from the cotton fibre in successive

washings with fresh Water. Darker shades can be reduced in depth quickly

after only a few water Washings. How quickly the colour change occurs

depends mainly on the affinity of the dye to the fibre. Sulphonic acid groups

41

are present on the direct dye molecule to impart aqueous solubility, thereby

facilitating the application from the aqueous phase. Sulphonic groups,

however, reduce the affinity of the dye towards the cellulosic substrate. This

decreased affinity is due to two factors

1) Both the cellulosic fibres and the direct dye have negative

charges in the aqueous medium.

2) The sulphonate group increases the dye-water interaction and

therefore decreases the dye-fibre attraction.

To improve the wash fastness of the reactive dyed fabric, the

application of formaldehyde, cationic, and metallic salts fixatives are the most

common approaches. Formaldehyde improves colour fastness through

crosslinking reactions. Improvement in the fastness properties occur during

the reaction of two dye molecules with one molecule of formaldehyde

through the formation of a methylene bridge. It is also possible that one

formaldehyde molecule could simultaneously react with one dye molecule

and one hydroxyl group of cellulose. Both reactions could decrease the dye

desorption from the fibre. Because of its high reactivity, formaldehyde is one

of the most effective fixatives for direct dyes. However, due to the health

problems associated with formaldehyde, there is a market demand for non-

formaldehyde fixing agents.

An alternative method utilizes nitrogen containing organic

compounds that couple with the dye to increase the molecular weight and

reduce solubility of the dye. The interaction between the dye and the fixative

agent is mainly due to the ionic attraction from the positively charged

nitrogen and the negatively charged dye ion. This salt linkage neutralizes the

negative charge on the dye and decreases the water solubility of the dye.

42

Enlargement of the dye molecule inside the fibre also makes it more difficult

for the dye to be released by the fibre.

The cupric cation, Cu2+, could behave like the organic nitrogen

containing fixatives, forming insoluble copper salts with the anionic dye

molecules. Direct dyes containing two hydroxyl groups in the ortho position,

adjacent carbon positions in a benzene ring, can react with copper salts to

produce less-soluble metallic chelates. Copper salts are not environmentally

desirable, but the after treatments can be tailored to improve both wash and

light fastness properties of some reactive dyes without producing enough free

copper ion in the effluent to cause problems. Copper salts are used primarily

with heavy browns, navy and black shades. Although the treated dyestuffs

have improved light fastness, blue shades exhibit a green cast. Other

approaches such as coupling with diazonium salts, forming metal complex

with metals other than copper, or the treatment with potassium bichromate are

also used for the improvement of colourfastness of dyed fabrics (Yang &

Carman 1996).

2.13 IMPORTANCE OF COLOUR FASTNESS AND DURABILITY

Today’s consumer is more sophisticated than ever. They are

conscious not only of style and comfort, but also of easy care and durability.

They demand a high quality product. Market studies show that consumers

make many purchase choices based on colour. Therefore, a fabric’s ability to

retain its original colour is one of the most important properties of a textile

product. The colour fastness or colour retention of cotton textiles is influenced

by a number of variables that occur both pre-consumer and post-consumer.

This report summarizes how variations in raw materials, chemicals,

manufacturing processes and consumer practices all have an effect on the

performance characteristics of a fabric. Manufacturers must understand how

43

the many variables affect colourfastness to achieve the ultimate goal of

consumer satisfaction.

2.13.1 Colour Fastness

Colourfastness is defined by the American Association of Textile

Chemists and Colourists as “the resistance of a material to change in any of its

colour characteristics, to transfer its colourant(s) to adjacent materials, or

both, as a result of the exposure of the material to any environment that might

be encountered during the processing, testing, storage, or use of the material”.

In other words, it is a fabric’s ability to retain its colour throughout its

intended full life cycle. There are many types of colourfastness properties that

must be considered to provide the consumer with an acceptable product. The

American Association of Textile Chemists and Colourists has over thirty test

methods that evaluate different colourfastness properties. These include, but

are not limited to wash, light, rubbing, dry cleaning, perspiration, abrasion

and heat. The type of product being manufactured determines which types of

colourfastness are important and therefore which test methods are relevant.

For example, upholstery fabrics must have excellent light fastness and

rubbing fastness properties, whereas wash fastness is important for clothing

fabrics. Manufacturers must know a fabric’s intended end use in order to

make processing decisions that will produce a product of acceptable

performance.

Dye selection must be based on desired performance criteria,

manufacturing restrictions and the costs that a market can bear for each end

product. Every dye has unique colourfastness properties. Some dyes are

known for their excellent wash fastness characteristics while some others are

known for their light fastness properties. The structure of the dye, the amount

of dye, its method of bonding to the fabric and dyeing procedures all

contribute to a dye’s performance characteristics. Dye combinations in a

44

specific formulation must also be evaluated for their effect on colourfastness.

Heavy shades often have reduced fastness properties. When high concentrations

of dye are required, proper rinsing and washing off procedures are essential.

However, due to entrapped dye particles within the cellulose structure, some

unbound dye molecules can still remain and contribute to colour loss and dye

transfer.

2.14 EFFECT OF LIGHT ON DYED MATERIAL

Textile dyes when applied to fabrics are subject to the action of a

range of outside influences like light, gases such as nitrogen oxides and

moisture which may contain dissolved atmospheric chemicals. Reactive dyes

have high fastness to wet treatments but the performance of a reactive dye to

light is critical. Early studies on light fading (Gebhard 1910) showed that

cotton fabrics dyed with direct dyes did not fade when exposed under

vacuum. From these experiments it was concluded that light fading was an

oxidative process. Light fading is normally carried out by exposing the dry

dyed fabric to a specific light source. The majority of home textiles are often

exposed to daylight in a wet condition following laundering. Line drying

under sunlight is known to result in accelerated fading. The swelling of the

fibre by water is one probable contributing factor. This swelling allows

increased diffusion of the oxygen and other chemicals contained in the water

to penetrate the fibre micro structure and influence the rate of fading. Dyes

are complex chemical structures. Further, they can contain impurities, isomers

of the main structure, as well as un-reacted intermediates. The dye structure

may also contain substituent groups that are affected by light in their own

right. Many dyes consist of several related structural isomers that can

influence a number of the properties of the dye such as build-up, rate of

dyeing, colour, fastness properties as well as the degree of fixation on the

fibre. The dye may exist in various states of aggregation within the fibre due

45

in part to the differences within the fibre microstructure. It is also possible

that certain chemical structures can combine with oxygen under the action of

light to form peroxides. Under these circumstances light has a strong

accelerating effect on the formation of peroxides and hence accelerate the rate

of fading.

2.15 FACTORS AFFECTING THE LIGHT FASTNESS

PROPERTIES OF DYED COTTON

The factors that influence the light fading of textiles can be

summarised below. Light source intensity, wavelength distribution, exposure

time, surface temperature of the fabric, moisture content of the fabric,

composition of the surrounding atmosphere, the chemical and physical structure

of the textile fibre, the degree of dye aggregation within the fibre structure,

presence of metal ions either bound in the dye or present as impurities on the

fibre etc.

2.15.1 Photofading and Light Stability of Dyed and Pigmented

Polymers

There are four principal factors which can influence the photostability

of commercial dyed and pigmented polymer systems (Allen 1983).

The intrinsic chemical and physical nature of the polymer

Environment in which the system is used,

The chemical and physical nature of the dye/pigment

The presence of antioxidants and light stabilisers.

The first factor is related to the interaction between UV light and

unpigmented polymer. Here, hydroperoxides and carbonyl groups formed

46

during the manufacturing and processing operations will absorb the harmful

radiation and then undergo photochemical reactions leading to breakdown of

the polymer. These processes are in turn controlled by the physical properties

of the polymer, e.g. morphology and sample thickness, both of which can

influence the diffusion of oxygen through the substrate. The chemical nature

of the substrate itself is also a very important factor in determining the

influence that many dyestuffs may have on polymer stability. In many polar

matrices, such as nylon and cellulose, photo reduction of the dye can result in

sensitized oxidation of the polymer as mentioned previously. In the textile

world this process is known as 'phototendering'.

The second factor is the effect of the environment in which the

system is used, temperature, humidity, oxygen and UV content of the light

source. An increase in any of these will result in an increase in the rate of

photochemical degradation. The effect of humidity has attracted widespread

attention and for many polymers with high moisture regains (e.g. nylon,

cellulose), absorbed moisture tends to swell the polymer chains, thus enabling

oxygen to diffuse more readily through the matrix (Giles & Forrestor 1980).

The third factor, and certainly one of the most important, is related

to particle size distribution, presence of surface treatments, the chemical

structure of the dye or pigment, and the chemical bonding that may be

involved between the colourant and the polymer. All four will dramatically

influence the stabilising or destabilizing effects of the dye or pigment on the

polymer. An increase in pigment particle size and the presence of a surface

treatment may, for example, reduce photocatalytic activity. Chemical

structure is probably the most important among all the four (Allen 1983).

The fourth factor and then one which is least understood from a

commercial and scientific point of view, is the nature of the interaction with

antioxidants and UV stabilisers. These are always used in commercial

47

polymers and may inhibit photocatalytic activity and/or enhance

photostability of the polymer and/or dye. Alternatively, the pigment could

reduce the stabilising action of the additive by adsorption onto its surface, an

effect which is little understood, but one which is of concern to many pigment

manufacturers (Allen et al 1987).

One of the major development in dye fading was the establishment

of some method of standardisation for dye and pigment stability especially

with regard to textiles.

2.15.2 Physical Factors Affecting Light Fastness

The following variables were must be monitored during light

fastness testing are explained by Samantha & James (2001).

a. Spectral distribution of the light sources

b. Intensity of the light source

c. Distance of light source from samples

d. Relative humidity

e. Specimen preparation

f. Duration of the test

g. Ambient temperature

h. Sample temperature

2.15.2.1 Spectral light sources

The light fastness properties of dyes are often assessed under

accelerated test conditions utilizing xenon arc lamp or high-pressure mercury

lamps. A number of commercial systems are available for this purpose(Giles

& Forrestor 1980). In dye fading, the nature of the light source is critical in

48

controlling the rate. The UV content and the heat of the light source can

accentuate dye fading and wherever possible should be removed. Some dyes

and pigments are also sensitive to visible light in their main long wavelength

absorption band (Allen 1994).

2.15.2.2 Atmospheric composition

The effect of atmosphere is a rather mixed and complex subject and

often relates more toward the importance of singlet oxygen ('02) in dye fading.

Numerous articles have dealt with this somewhat controversial subject and

will be covered later. However, it is worthwhile noting that while oxygen is

found to promote the fading of dyes in solution, studies on polymer films and

textiles generally show that oxygen impairs fading due to quenching of the

excited state of the dye by ground-state molecular oxygen. In the latter case it

should also be realised that the polymer will also photooxidise and under

these conditions will preferentially react with the oxygen on the surface of the

material (Allen 1994).

2.15.2.3 Humidity

The importance of humidity in dye photofading is another complex

subject which is closely interrelated to the influence of oxygen and the role of

singlet excited oxygen. Generally, an increase in humidity will decrease the

light stability of a dye but the effect is dependent very much on the nature of

the polymer (Giles & Forrestor 1980).

2.15.2.4 Temperature

As expected for any chemical reaction an increase in temperature

increases the fading rate of a dye. Experiments on this parameter have been

utilised mainly to study the effects of oxygen diffusion and aggregation (the

49

ability of the dye/pigment to exist in either single particles or to form

agglomerates) on dye light stability. In one study (Giles et al 1974) the

activation energies for dye fading on a series of polymer films were found to

follow the order wool < cellulose acetate < cellulose triacetate < nylon which,

in fact, is the inverse of the order of moisture regain. This suggests that the

ease of diffusion of moisture and/or oxygen is initial factors. However, both

these factors can influence the state of aggregation of the dye in the respective

polymer and a closer examination of the results indicates that this may well be

the case (Allen 1994).

2.15.2.5 Aggregation and dye concentration

Much evidence has accrued over the years to indicate that the

aggregation or physical state of the dye in a polymer substrate is an important

parameter in controlling photofading (Giles & Forrestor 1980). Generally,

aggregated dyes exhibit a much higher light fastness than fully dispersed

dyes. Dyes in more amorphous polymers tend to display a higher light

fastness than when present in a crystalline polymer. This has been confirmed

with experiments where the porosity of regenerated cellulose was altered

(Giles & Haslam 1978). Associated with dye aggregation is the filter effect

which becomes more dominant as the concentration of the dye in the polymer

is increased. In cellulose, dyes appear to build-up in extended multilayers

rather than as discrete particles. Under these conditions the inner layers of dye

would be protected from the incident light through attenuation by the outer

layers of dye and local fading will occur only on the surface of the polymer.

However, it may be argued that dye fading would still be evident even though

it may only be on the surface. This effect is demonstrated (Baxter et al 1955a)

for three types of situation

(a) The dye distribution is uniform with dye concentration and

there is a constant rate of fading.

50

(b) The particle size is increasing and the fading rate is

decreasing.

(c) Unsymmetrical particle growth decreasing the dye fading rate

in the direction of illumination (Baxter et al 1955b).

It is known that dyed-polymer systems are much more complex.

Many polymer systems contain a variety of impure chromophores produced

during manufacture, such as hydroperoxides and carbonylic groups, which

may interact with the dye and reduce its light fastness at low concentrations.

As the dye concentration increases, it quenches the activity of the

chromophores and hence the dye stability increases correspondingly. The

photofading rate of acid yellow dye in nylon-6,6 film is seen to decrease with

increasing concentration (Allen et al 1992). The polymer stability was also

found to be increase with increasing in dye concentration and this correlated

well with the ability of the dye to quench the photoactive , -unsaturated

carbonyl chromophores in the polymer (Allen et al 1992).

2.16 CHEMICAL STRUCTURE OF THE DYE AND THE LIGHT

FASTNESS

The light fastness is influenced by internal factors, the chemical and

the physical state of dye, the dye concentration and the nature of the fibres.

The chemical structure of a dye molecule is divided in to two parts: the main

skeleton (chromophore) and the substituent groups (auxochromes). In general,

the skeleton seems to determinate the average light fastness properties of a

dye, while substituent groups usually alter the light fastness properties of a

particular dye within a class in minor ways. Flavonoid compounds are not

very light fast, anthraquinones and indigoids are noted for their excellent light

fastness. However, the light fastness of anthraquinones decrease as the

number of hydroxyl substituent groups increase. Another aspect of chemical

51

structure which affect the light fastness is the symmetry of the dye molecules:

symmetrical dye molecules usually exhibit greater light fastness than non-

symmetrical dye molecules, and larger dye molecules generally provide faster

dyeing than smaller ones. The physical state of dye is generally more

important than the chemical structure. The more finely dispersed the dye is

within the fibre, the more rapidly it will fade. Fibres with large aggregates of

dye are lighter fast, since a smaller surface area of the dye is exposed to air

and light.

A useful way of probing the interrelationship between the physical

state of a dye within a fibre and its light fastness is by examination of fading

rate curves. In 1965 Giles described five types of fading rate curves which are

typical of synthetic dyes shown in Figure 2.6.

Type I fading rate which decreases steadily with time, but rarely

occurs in practice; the dye is probably molecularly dispersed throughout the

fibre.

Type II fading initially occurs at rapid rate followed by slower

fading at a constant rate; dyes are present in aggregates inside the fibre

substrate. Most synthetic dyes exhibit a type II fading rate curve.

Type III fading rate curve is characterized by a linear or constant

rate of fading. This type of fading occurs most often with pigments and fast

dyes that form larger aggregates inside the fibres.

Type IV fading is rate initially dark, followed by a slow fading rate.

This type of fading occurs in a few fast dyes.

Type V fading rate steadily increases with time and is observed

with azo dyes on cellulose; there is a continued breakdown of large dye

52

particles to small dyes particles. Fading rate curves can be useful because they

can give qualitative information about the physical state of dye within the

fibre. They may also be useful in determining colourant formulation or dye

concentration necessary to match faded materials. For these reasons, the

fading rates of numerous synthetic dyes and some pigments have been studied

(Crews & Reagan 1987).

Figure 2.6 Light fading rate curve

The light fastness of a dyed fibre usually increases with increasing

in dye concentration, the main cause being an increase in average size of the

sub microscopic particles which the dye forms in the fibre (Giles 1965).

The Light fastness of the dyed textiles is related to the chemical

structure and physical characteristics of the fibre. Cumming et al (1956)

attributed the fading on cellulose to an oxidative process. On protein fibres

the process has a reductive nature. It is explained by Cristea & Vilarem

(2006) stated that indigo is much more light resistant on wool than on cotton.

An oxidative pathway is involved in the fading of indigo dyed cotton. As

fading on non-protein substrates is reductive, the indigoid chromophore which

is resistant to photo reduction shows high fastness on wool (Cristea &

Vilarem 2006). External factors such as the source and the intensity of

illumination, temperature, humidity and the atmospheric pollution can affect

the reaction as well (Crews & Reagan 1987).

dE

53

The light source (the nature of the incident light) is very important

during the photofading process; dyes are faded mainly by visible radiation,

while dyes of high light fastness are faded mainly by UV radiation. Gantz &

Sumner (1957) stated that UV radiation is a major factor in the fading of the

more light fast dyes, particularly yellows and oranges. Use of a UV filter has

been shown to afford some protection to natural dyes (Cristea & Vilarem

2006). Under the normal conditions of exposure to light, both temperature and

humidity affect the rate of fading of dyed textile materials. It was found that a

drop in relative humidity from 65 to 45% had very little effect, but a further

decrease up to 25% caused a significant reduction in fading (Egerton &

Morgan 1970). Atmospheric contaminants, such as sulphur dioxide and

oxides of nitrogen and ozone, are known to react with dyes even in the

absence of light. Presence of substances like starch and gums might

accelerate the fading process (Gupta 1999).

UV absorbers are additives used to prevent the photodegradation of

polymeric materials by UV-rich sunlight and artificial light. These additives

absorb UV radiation and reemit it as fluorescent or infrared radiation. The

energy of the excited molecule which causes photodegradation is released as

thermal energy (Lappin 1971). The UV absorbers must have the following

characteristics: high absorptivity of the radiation between 290 nm and 400

nm, stability to long-term light exposure, molecular dispersion for optimum

screening activity, and chemical inertness to other additives in the substrate.

Researchers have investigated the usefulness of UV absorbers in reducing

colour alteration from two general perspectives: direct application of UV

absorbers to fibres and use of UV filtering materials over the light sources

(Woeppel & Crews 1990). The major classes of commercial ultraviolet-

radiation stabilizers include derivatives of 2-hydroxybenzophenone,

2-(2H-benzotriazol-2-yl)-phenols, phenyl esters, substituted cinnamic acids

and nickel chelates. Gordon et al (1961) studied the use of ultraviolet light

54

absorbers for protection of wool against yellowing. They demonstrated that

the substituted benzophenones are effective in protecting wool; the maximum

protection was obtained from water-soluble UV absorbers, using dye-bath

techniques. Without going into the history of UV absorbers or reviewing the

many structures that have been studied, it can be stated that substituted

benzophenones are the most effective compounds that have been found to

date. Since a hydroxyl group ortho to a carbonyl is characteristic of stable

ultraviolet absorbers, we could conclude that this structure is associated with

the stability and internal energy conversions of these molecules (Gantz &

Sumner 1957). Gordon et al (1961) studied the use of ultraviolet light absorbers for protection of wool against yellowing.

2.17 ANTIOXIDANTS

An antioxidant is a molecule that inhibits the oxidation of other

molecules. Oxidation is a chemical reaction that transfers electrons or

hydrogen from a substance to an oxidizing agent. Oxidation reactions can

produce free radicals. In turn, these radicals can start chain reactions. When

the chain reaction occurs in a cell, it can cause damage or death to the cell.

Antioxidants terminate these chain reactions by removing free radical

intermediates, and inhibit other oxidation reactions. They do this by being

oxidized themselves, so antioxidants are often reducing agents such as thiols,

ascorbic acid, or polyphenols.

Antioxidants, also called inhibitors of oxidation, are organic

compounds that are added to oxidizable organic materials to retard auto

oxidation and, in general, to prolong the useful life of the substrates.

Relatively few chemical classes are effective as antioxidants. Those in

common use today are hindered phenols, secondary aromatic amines, certain

sulfide esters, trivalent phosphorous compounds, hindered amines, metal

dithiocarbamates and metal dithiophosphates. Antioxidants are classified as

55

either radical trapping (chain breaking) or peroxide decomposing, terms that

describe the mechanism by which they function. Nikel sulphonate

interamolecular singlet oxygen quenching group display enhanced

photochemical stability towards visible light (Oda 2001a). As radical trapping

antioxidants, we can cite gallic acid and its esters, tocopherols and some

vegetal polyphenols (quercitin, myricetin, vanillic acid, cafeic acid, ferulic

acid). As synergists, we can cite ascorbic acid (vitamin C) and erythorbic

acid. As metal chelating agents, we can cite citric acid, lactic acid and lecithin

(Gordon et al 1961).

2.17.1 Determination of Antioxidants Strength

Usage of antioxidants like gallic acid, vitamin C and cafeic acid

absorb the oxygen radicals available for photo degradation. Oxygen Radical

absorbance capacity (ORAC) is the measure of antioxidants strength. Several

antioxidants assays have been developed over the years and they all use an RO

generator. A relatively simple but sensitive and reliable method of quantitating

the ORAC of antioxidants in serum using a few l is described. In this assay

system, -phycoerythrin ( -PE) is used as an indicator protein, 2,2 -azobis(2-

amidinopropane) dihydrochloride (AAPH) as a peroxyl radical generator, and

6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (a water-soluble

vitamin C analogue) as a control standard.

Results are expressed as ORAC units, where 1 ORAC unit equals

the net protection produced by 1 M vitamin C. The uniqueness of this assay

is that total antioxidants capacity of a sample is estimated by taking the

oxidation reaction to completion (Guohua 1993).One major benefit of using

the ORAC method is that it takes into account samples with and without lag

phases of their antioxidants capacities.

56

2.18 STUDIES IN THE LIGHT FASTNESS OF REACTIVE DYES

A lot of complaints are coming from the consumers about the poor

light fastness properties of dyed materials(Imada et al 1994). It is well-known

fact that the colour of dyed textiles fades on exposure to light. The rate at

which a dye fades is governed by the following factors (Krichevskii et al

1975).

The photochemistry of the dye molecule

The state of the dye in the fibre

The reactivity of the dye

The chemical and physical nature of the substrate

The method and conditions of dyeing

The nature of bond between the dye and the fibre

The presence of foreign substances in the substrate

The atmospheric conditions during testing

The illumination

Phototropic dyes change their colour when exposed to sunlight and

reverts to their original colour in the dark. These dyes are advantageously

used in increasing the light fastness of the dye. An increase in the electron

mobility of the dye molecule makes the dye non-phototropic and decreases

the light fastness (Shenai 1993). Antherton & peters found out that dyes

containing NO2 (meta or para) or para phenyl- COCH3 groups have

anonymously low light fastness. This was infact confirmed by Desai & Giles

(1949). A primary amino group leads to low light fastness and acetylation

increase the light fastness (Venkataraman 1952). Mounier has mentioned that

azo dyes oxidise during fading where most of the oxidizing agents act as

57

sensitizers. Baxter et al (1957) have suggested that the fading of aggregated

dyes are very slow while molecularly dispersed dyes fade at a faster rate. This

is so because whenever a dye is in molecularly dispersed form, every

molecule is equally accessible to the illumination and chemical reactants.

Kissa (1971) has suggested that dye-dye interactions between adjacent dyes

molecules covalently bonded to cellulose sometimes cause the fading of

reactive dyes. Kissa (1971) has found out that the electrolyte used in reactive

dyeing influence the light fastness properties. Here Glauber’s salt assisted

light fastness is higher than sodium chloride.

The light fastness is a function of concentration of the dyes on

weight of the material, (i.e) depth of the shade. Pale shades usually have a

lower light fastness compared to the deep shades. The fastness of a dye may

vary depending on fibre which is applied. High humidity usually accelerates

fading, but the quantitative effect varies widely and depends both on the fibre

and the dye. The light fastness of a particular dye may also vary depending up

on the source of light used (i.e) sun light, xenon arc lamp, mercury tungsten

filament lamp, etc. some dyes are fast to the sun light, but fade on the

exposure to UV light of a quartz mercury lamp (Nandy 1998). Fading by the

action of day light is mainly due to radiant in every visible region (Giles &

McKay 1963). Combination shades gives the light fastness rating whose value

was lying between the two individual dyes. However, there were still samples

whose light fastness ratings were even founded to be lower than the

individual dyes (Nandy 1999).

The improvement of light fastness of reactive dyes on textile

materials is extremely important, considering the fact that the dye fastness

yielded by reactive dyes is superior to that of dyes of other classes

(Krischevskii 1968). Many independent studies have been made on the

chemistry of the light fading of dyed cellulosic materials. The dyes on these

58

materials are converted to their fading products upon exposure to sunlight

primarily by a complex photo oxidative mechanism where oxygen and water

are involved (Weissbein & Coven 1960). UV light induced unimolecular

decomposition and visible light induced photo-oxidatition and visible light

induced photo oxidation are the two most important pathways.

Dye + UV Light Bleaching (2.6)

Dye + O2 + Light Bleaching (2.7)

Light induced oxidative degradation of vinyl sulphonyl reactive azo

dye was dominant in the presence of air (Vig et al 2007). Photodegradation of

wet vinyl sulphonyl reactive dyes occurred under the action of light induced

singlet oxygen. Auto-oxidation reaction of dyes is generally considered to

occur on exposure to UV radiation and prevented by the addition of UV

absorbers or antioxidants such as hindered phenols or naphthylamines

(Oda 2001b). These conclusions arise from work carried out over the last two

hundred years, which is briefly summarised below; more extensive reviews

are available elsewhere (Gebhard 1910). Early work in the 19th century

suggested that removal of oxygen prevented photofading (Okada et al 1998,

Okada 1997). This effect was crystallised by Gebhard in 1910 shows that

coloured fabrics (probably direct dyed cotton) under vacuum did not

photofade (Heerman 1924). From these experiments it was concluded that

photofading was an oxidative process. Although Gebhard realised that both

UV and visible light were emitted by the sun, it is not clear if the glass vessels

used in his studies were transparent to UV and therefore UV effects may have

been overlooked. However in 1924, Heerman clearly showed that dyes could

be readily photodegraded by UV light (Egerton & Morgan 1971b). Following

these seminal works many authors have sought to fully understand the

59

chemistry and reactive species involved in photofading. Notably, Egerton in a

series of papers showed that reactive oxygen species, ROS, were produced by

irradiation of dyed fabrics and these were capable of destroying dyes

(Wilkinson et al 1993). The nature of the ROS, i.e. singlet oxygen, hydrogen

peroxide, superoxide radicals, hydroxyl radicals or peroxy radicals was not

defined. Subsequently a large amount of work has been done on how these

species might be formed during irradiation and the damaging effect they have

on dyes. Most attention has been paid to singlet oxygen, 1O2, which can be

formed by the quenching of excited states of dyes by the triplet ground state

of oxygen (Wilkinson et al 1995).

light quench3 12 2dye dye* O dye O (2.8)

Many model studies have shown that singlet oxygen is very

reactive towards dyes (Griffiths & Hawkins 1977 and Jansen et al 1999),

although its importance is unclear. Recent quantitative work has suggested

that its role in photofading of azo-dyes is quite small (Bandara & Kiwi 1999

and Yamaguchi & Sasaki 2001). The quenching of excited states of dyes by

oxygen has also been shown to lead to the formation of the superoxide radical

and destruction of the dye (Okada et al 1990a).

light electron transfer32 2dye dye* O dye O (2.9)

The so-formed superoxide could then react and destroy further dye

molecules (Wilkinson et al 1993). Clearly, the exact mechanism by which a

dye photo fades will critically depend on the dye and fibre type. Much of the

literature studied employed model systems such as cellulose films or aqueous

solutions and it is not clear how the results relate to real world cases.

60

On exposure of reactive dyes on cellulose in aerated water, many

vinylsulphonyl (VS) dyes undergo oxidative fading (Okada et al 1990c),

while some monochloro-triazinyl (MCT) dyes undergo reductive or oxidative

fading. Some MCT dyes on cellulose undergo reductive fading on exposure in

deaerated water irrespective of non-addition of substrate (Okada et al 1990d).

The photofading behaviour of VS dyes on cellulose, or their

potential fading properties can be easily evaluated by an accelerated testing

method for the photostability of dyes (Okada et al 1990b), where the relative

fading of VS dyes under defined conditions are examined.

In the Okada et al (1992a) study the fading behaviour of 11 MCT

dyes on cellulose is examined by exposure in the presence of oxygen and

substrate under wet conditions. Cotton fabrics and cellulose films dyed with

these dyes were also irradiated under atmospheric dry conditions and the

fading behaviour was analyzed in terms of their potential photochemical

properties. The effect of migration time in the exhaustion dyeing on the initial

fading of dyed fabrics as well as the dye concentration effect of dyed films are

investigated. How these MCT dyes on cotton fabrics and cellulose films

manifest a different fading behaviour under wet conditions is also analysed.

Thus, by analysing the fading behaviour of 11 MCT dyes on dry

cellulose, it was shown that some dye properties were very operative in the

fading behaviour. But the increase in light fastness by the compensation effect

may not be desirable, because the fading of dyes may be severely influenced

by the change of environmental conditions. Thus, reactive dyes have

potentially diverse properties which are sometimes contradictory to each other

and suffer various kinds of photodecomposition on cellulose. The photofading

of dyes on cellulose must, therefore, be considered in the chemistry of dyes

and their interaction with the surroundings (Okada et al 1992b).

61

The photofading of commercial reactive dyes on cotton is due to

both UV and visible light, with the relative importance being determined by

the dye type. For the very popular azo/hydrazone dyes, visible light is the

most important contributor under normal lighting conditions. Oxygen is

required for visible but not UV fading of the dye indicating two mechanisms

of photodegradation. Wetting the fabric does not greatly increase the rate of

photofading, except at high (>9) or low (3<) pH. It would be interesting and

informative to expand the current study to include other dyed substrates such

as paper, wool and polyester, to see if similar factors control their photofading

(Batchelor et al 2003).

On exposure of cellulose film dyed with reactive dyes in various

aqueous solutions, oxidative and/or reductive fading occurred depending

primarily upon the chemical structure of dyes and secondarily upon the

environmental conditions. The addition of substrate promoted reductive fading

or suppressed oxidative fading, while an increase in the concentration of

oxygen gave the reverse effects, the promotion of oxidative fading and the

suppression of reductive fading. However, the potential photochemical

properties of reactive dyes were manifested in the fading behaviour on

exposing in various aqueous solutions, although they were influenced by the

environmental conditions. On exposure of dry dyed cellulose, the rates of

fading were considerably lowered. Some dyes on cotton fabrics suffered a

large initial fading from the large surface and of the fibres. With an increase in

the dye concentration on dry cellulose, an improvement of light fastness was

observed, which was almost explained by the filter effect, while the rates of

fading for the dyes with very high photosensitivity was in contrast, increased.

Some dyes with high photo reactivity showed slow fading, probably due to

the compensation effect of conflicting properties.

62

2.19 UV ABSORBERS

A UV absorber is a molecule that is incorporated within a host

polymer and which absorbs ultraviolet rays efficiently and converts the

energy into relatively harmless thermal energy without itself undergoing any

irreversible chemical change, or inducing any chemical change in the host

molecules. Typical UV absorbers are the 2-hydroxybenzophenones,

2-hydroxyphenylbenzotriazoles and 2-hydroxyphenyl-s-triazines, which

convert electronic excitation energy into thermal energy via a fast, reversible, intramolecular proton transfer reaction (Allen et al 1981d).

UV absorbers are technically important in preventing

photodegradation of polymers with or without organic pigments. Dyes

containing a built-in ultraviolet absorber moiety such as 2-hydroxybenzophenone

have been used as reactive dyes for cotton (Moura et al 1997), and dyes

containing a 2,4-dihydroxybenzophenone residue have been described for

polypropylene fibres containing nickel. Similar approaches have been used

for cationic dyeing, and for example used 2,2t-dihydroxy-4,

4t-dimethoxybenzophenone on the cationically dyed poly (m-phenylene

tere phthalimide) fibres, to obtain improved colour yields and light fastness.

2.19.1 Other Additives

Because of the commercial incentive to improve the photostability

of dye polymer systems, innumerable additives have been examined over the

years on a purely empirical basis, and as a consequence interesting examples

of enhanced light fastness have emerged where the mechanism of stabilization

by the additive is not clear. More recent examples of such investigations are

summarised collectively in this section.

Allen et al (1981a) studied the influence of o- and p-phenylphenol

on the light fastness of commercial anthraquinone dyes in an aerobic solution.

63

The effect of these compounds is interesting since they are used as carriers in

the dyeing of polyester fibres, and some disperse dyes have their light fastness

impaired by carriers. In this case, they were protective and the authors

suggested that this effect may be due to a screening effect (these carriers have

high absorbance in the UV) (Allen et al 1981b).

Triphenylmethane cationic dyes, formerly used on protein and

cellulosic polymers, now it is used for polyacrylonitrile substrates because of

their acceptable light fastness properties on these substrates (Allen et al

1981c).

2.19.2 Determination of Ultraviolet Absorber Strength

Ultraviolet protective factor measures the effectiveness of textile

fabrics in protecting the human skin from ultraviolet radiations. It is expressed

as the ratio of extent of time required for the skin to show redness (erythema)

with and without protection, under continuous exposure to solar radiation.

The UPF is calculated using the Equation (2.10).

MEDprotectedskin UPF = MEDunprotectedskin

(2.10)

where, MED is the minimal erythemal dose or quantity of radiant energy

needed to produce the first detectable reddening of skin after 22 ± 2 hours of

continuous exposure (Das 2010).

According to the following classification system, a UPF range of

15-24 gives good protection, a UPF of 25-39 very good protection and a UPF

of 40-50 gives more excellent protection (Paluszkiewicz 2005 and Algaba

2002).

64

The UPF increases with fabric density and thickness for similar

construction and is dependent on porosity (UPF = 100 / porosity). A high

correlation exists between the UPF and the fabric porosity but is also

influenced by the type of fibres. The relative order of importance for the UV

protection is given by % cover > fibre type > fabric thickness. Therefore

fabrics with the maximum number of yarns in warp and weft give high UPFs.

UPF values of 200, 40, 20 and 10 can be achieved with the percentage cover

factors of 99.5, 97.5, 95 and 90 respectively. The percentage UVR

transmission of a fabric is related to the fabric cover factor by (100 – cover

factor) and the UPF is given by UPF = 100 / (100-CF). To achieve a

minimum UPF rating of 15, the cover factor of the textile must be greater than

93%, and a very small increase in cover factor leads to substantial

improvements in the UPF of the textiles above 95% cover factor. In the case

of terry cloth, a high variability in UPF exists due to irregularities in the fabric

construction. Woven fabrics usually have a higher cover factor than knits due

to the type of construction (Saravanan 2007).

2.19.3 Effect of UV Absorber on Light Fastness

UV absorbers inhibit photo degradation of polymeric materials in

several ways. Some additives preferentially absorb most of the ultraviolet

radiation reaching the substrate and convert it to harmless infrared radiation.

Other additives function by interacting with the photoexcited molecule before

any other reaction occurs. By quenching the excited states of molecules, the

UV absorbers prevent or minimize polymer degradation and colour loss.

Chemical compounds suitable for use as UV absorbers must be strong

absorbers of ultraviolet radiation and stable to ultraviolet light. An effective

UV absorber should meet three criteria: it should absorb effectively

throughout the near UV region of the electromagnetic spectrum (290-400 nm,

but especially 350-400 nm), it must be UV-stable itself, and it must dissipate

65

the absorbed energy in such a manner as to cause no degradation or colour

change in the medium it protects. The most important chemical classes of UV

absorbers are hydroxy-benzophenone derivatives, benzotriazoles, and phenyl

esters. Minor chemical classes include cinnamic acid derivatives, s-triazines,

and other compounds with nitrogen acceptors. Some researchers found UV

absorbers to be very effective in reducing dye fading, while others found them

ineffective or detrimental. Maerov & Kobsa (196l) observed improvements in

light fastness of basic dyes on polyester in the range of 200 to 300% when

2,2’-dihydroxy, 4,4’-dimethoxybenzophenone was applied in the dye bath.

Coleman and Peacock (1958) found 24 to 86% improvement in the light

fastness of disperse dyes on acetate and reduced strength losses on nylon

when 2,2’-dihydroxy4,4’methoxybenzophenone was applied in an aqueous

dyebath. Gantz & Sumner (1957) contended that substituted benzophenone

absorbers markedly improve the light fastness of dyes faded by ultraviolet

radiation. Reinert and Thommen observed dramatic improvement in the light

fastness of pale shades of dyed nylon when an UV absorber was applied

during dyeing with another light fastness improver, but the effect was obvious

on dark shades only after prolonged exposure. On the other handle, Cegarra &

Ribe (1972) found only slight, but not statistically significant, light fastness

improvements of acid-dyed wool treated with 2,4-dihydroxybenzophenone-

2’-ammonium sulphonate applied in a dye bath.

Crews & Reagan (1987) found modest reduction of fading of some

natural dyes on wool, but observed increased fading of others when treated

with selected alkyl-hydroxybenzophenone absorbers by an immersion

procedure. Giles & McKay (1963) found little reduction of fading of direct

dyes on cotton and observed increased fading on modified polyester dyed

with methylene blue as a result of application of substantive UV absorbers

(fluorescent photostabilizers). Woeppel found no significant reduction in

colour change of acid-dyed nylon when three hydroxybenzophenone

66

absorbers were applied by immersion treatment. Lappin (1971) noted that

many compounds absorb radiation in the desired region, but do not provide

protection when applied to fibres. Furthermore, some compounds and some

dyes sensitize the fibres to ultraviolet radiation, increasing the amount of

fading and degradation. Gantz & Sumner (1957) note that the problem is

complex, and years of testing under practical conditions may be required to

prove the merits of a stabilizer system. Because of UV absorber type,

application method, dye class, shade depth and length of exposure varied in the

published studies. Regarding the beneficial effects of UV absorbers on consumer

textiles are limited, and the reasons for the contradictory reports about the

effectiveness of UV absorbers in reducing fading remain unclear. However,

Rush & Hinton (1979) senior scientist at Allied Fibres in dye applications and

dyeing technology, suggested that differences in depth of shade are likely to

be one reason for the contradictory reports. Others suggested that application

methods might contribute to the differing findings on the effectiveness of UV

absorber.

2.20 NON-ADDITIVE APPROACHES TO IMPROVING THE

LIGHT FASTNESS OF DYED POLYMERS

Many other approaches have been investigated for improving the

light fastness of dyed polymers which do not involve the use of additives, but

rather attempt to modify the physical characteristics of the system. Whilst

these approaches do not strictly fall into the scope of this review, for

completeness some of the more significant developments are considered here.

One possibility for altering the photodegradation characteristics of a

uidyepolymer composite is to alter the physical situation of the dye by

altering its intermolecular interaction characteristics. Thus the state of

aggregation of the dye, the strength of binding to the polymer, and the

location of the dye within the polymer can all influence the observed rate of

67

photodegradation. Sulphonated anthraquinone dyes are an important group of

dyes for dyeing wool, silk and polyamide fibres that generally show good

light fastness. According to early work of Giles, dyes with planar molecules

sulphonated at only one end are surface active, whereas symmetrically

sulphonated dyes are not, and the difference in surface activity of the two

classes. (Giles et al 1960).

Giles et al (1960) studied this expected to influence their light

fastness properties effect with acid dyes on collodion, ethylmethyl cellulose

and gelatin films and found that symmetrically sulphonated dyes showed

better light fastness than the asymmetrical dyes. Also, the light stability of a

dye improved when the number of sulphonic groups in the dye molecule

increased. These results seem to be related to the formation of aggregates.

Later, Shah & Jain (1984) reached similar conclusions for acid dyes

on nylon 6.6 but they suggested that the increase in light fastness of some

monoazo dyes with increasingly number of sulphonic groups may be

attributed to the increased strength of dye-fibre bonding. However,

Weatherall & Needles (1992) studied this effect on nylon and wool with six

acid dyes and concluded that dye aggregation on wool did play an important

role in light fastness improvement.

Recently, new methods to improve the light fastness of polymeric

materials have been described such as structural modifications of polymers or

mass dyeing of polyamide fibres. Similar improvement for cationic dyes is

obtained in aromatic polyamides, sulphonated aromatic dicarboxylic acid-

modified polyamides and polyesters.

68

2.21 CONCLUSION

Reactive dyes are the most recently developed amongst the major

dye classes. Improvements in the structure of reactive dye chromogens and in

the structure, selection and number of reactive groups have led to an increased

use of dyes. Reactive dyes are gradually becoming the main class of dyes for

cellulosic fibres and are used extensively to dye or print casual wear and

sportswear goods (Waheed & Ashraf 2003).

The ability of dyed polymers to withstand prolonged exposure to

sunlight without the dyestuff fading or undergoing physical deterioration is

largely determined by the photochemical characteristics of the dye itself.

Resistance of the material to a change in its colour characteristic on exposure

of the material to sun light or an artificial light source is known as light

fastness. Light fastness has been an active area of research for nearly

200 years (Kuramato et al 1996).

An increase in the electron mobility of the dye molecule makes the

dye non-phototropic and decreases the light fastness. Antherton & Peters

found out that dyes containing NO2 (meta or para) or para phenyl- COCH3

groups have low light fastness. A primary amino group leads to low light

fastness and acetylation increase the light fastness. Mounier has mentioned

that azo dyes oxidise during fading where most of the oxidizing agents act as

sensitizers. Baxter et al (1957) have suggested that the fading of aggregated

dyes are very slow while molecularly dispersed dyes fade at a faster rate. This

is so because whenever a dye is in molecularly dispersed form, every

molecule is equally accessible to the illumination and chemical reactants.

Kissa (1971) has suggested that dye-dye interactions between adjacent dyes

molecules covalently bonded to cellulose sometimes cause the fading of

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reactive dyes. Kissa (1971) has found out that the electrolytics used in

reactive dyeing influence the light fastness properties. With Glauber’s salt

light fastness is higher than with sodium chloride.

The fastness of a dye may vary depending on to which fibre it is

applied. High humidity usually accelerates fading, but the quantitative effect

varies widely and depends both on the fibre and the dye. The light fastness of

a particular dye may also vary depending up on the source of light used i.e.

sun light, xenon arc lamp, mercury tungsten filament lamp, etc. some dyes are

fast to the sun light, but fade on the exposure to UV light of a quartz mercury

lamp (Nandy 1998). Fading by the action of day light is mainly due to radiant

in every visible region (Giles & McKay 1963). The increase in testing

ambient temperature reduces the light fastness similarly very acetic or

alkaline pH environment of test sample reduces the fastness.

The light fastness of different types of dyes on textile materials has

been extensively discussed in a great number of Technical papers, reviews

and monographs while only little attention has been paid to the light fastness

of reactive dyes. On exposure to light, the possible photochemical reactions

that a dye molecule may undergo are varied and complex (Rastogi 2001).

Understanding these phenomena has been a challenge to researchers for some

time (Tripathi et al 2008).

The combined impact of sunshine and other outdoor factors such as

temperature, moisture content, and chemical composition of the atmosphere

induce photo-destruction of fibres and photofading of dyestuffs. The

processes that occur are mostly photo-oxidative and depend strongly not only

on the above-mentioned factors, but also on the chemical composition of

dyestuffs, their concentration and the type of media in which they are

distributed (Vassileva & Jeleva 2005).

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Application of the nickel complexes of phenyl ester UV absorbers

and phenolic anti-oxidants in a polymer substrate result in the improvement of

light fastness. The most positive protection was achieved by the presence of

nickel carboxylate grouping particularly the nickel salt of salicylic acid

salicylate (Oda 2004).

Many authors studied the chemistry and reactive species involved

in photofading. Notably, Egerton & Morgan (1971a) in a series of papers

showed that reactive oxygen species (ROS) were produced by irradiation of

dyed fabrics which were capable of destroying dyes. Antioxidants called

inhibitors of oxidation.

These organic compounds are added to oxidisable organic materials

to retard auto oxidation (Cristea & Vilarem 2006). The antioxidant absorbs

free singlet oxygen and thereby reduce the photofading. This activity of

antioxidant is measured by the unit of oxygen radical absorbance capacity

(ORAC) (Alam et al 2008). Antioxidants have been used only on natural dyes

dyed fabric for light fastness improvement.

Rich & Crews (1993) studied ability of UV absorber to reduce

fading of nylon coloured with acid dyes and concluded UV absorbers were

more effective at reducing colour change on lighter shades. Yang & Naarani

(2007) studied improvement of the light fastness of reactive inkjet printed

cotton.

Further, impact of testing atmosphere, effect of covalent bond and

admixture dyes in this regard were studied. The mechanisms by which the

dyes undergo photo degradation are thought to be complex processes.

However, most of the research papers on this subject suggest that

UV light induced decomposition and visible light-induced photo-oxidation are

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the two most important pathways of fading, as shown in Equations (2.6) and

(2.7) described by Batchelor et al (2003).