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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
30
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).
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