CHAPTER 2 LITERATURE REVIEW

Transcription

1 10 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.

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).

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. 2.4 COTTON FABRIC (2.2) 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

4 13 weaves. A plain weave produces fabrics like poplin, cambric and broadcloth. A twill weave is more durable and used in denim, khaki and gabardine 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 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

5 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 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

6 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 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

7 16 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 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 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 NaCO 3, 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

8 17 for 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 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-adays, manpower required for average plant is declined considerably and the cost of bleached finished product is also reduced. This technical breakthrough 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).

9 18 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 (HO 2 -) 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 O 2. H 2 O 2 +NaOH Na + +HO 2 - +H 2 O (2.3) H 2 O 2- H ++ HO 2 - OH HO 2 - +H 2 O (2.4) However, at higher ph (above 10.8) the liberation of HO 2 - 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 HO 2 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).

10 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.

11 20 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 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 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

12 21 Table 2.1 Classification of textile dyes Acid Class Principal substrates Method of application Chemical Types Azoic components and composition Basic Direct Disperse Fluorescent brighteners Mordent Oxidation bases Reactive Nylon, wool, silk, paper, inks and leather Cotton, rayon, cellulose acetate and polyester Paper, polyacrylonitrile, modified nylon, polyester and inks Cotton, rayon, paper, leather and nylon Polyester, polyamide, acetate, acrylic and plastics Soaps and detergents and all fibres, oils, paints and plastics Wool, leather and anodized aluminum Hair, fur and cotton Cotton, wool, silk and nylon Usually from neutral to acidic dye baths Fibre impregnated with coupling component and treated with a solution of stabilized diazonium salt Azo (including premetalized), anthraquinone, triphenylmethane, azine, xanthene, nitro and nitroso Azo Applied from acidic dye baths Cyanine, hemicyanine, diazahemicyanine, diphenylmethane, triphenylmethane, azine, xanthene, acridine, oxazine, azo and anthraquinone, Applied from neutral or slightly alkaline baths containing additional electrolyte 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 From solution, dispersion or suspension in a mass Applied in conjunction with Cr salts Aromatic amines and phends oxidized on the substrate Reactive site on dye reacts with functional group on fibre to bind dye covalently under influence of heat and ph (alkaline) Sulphur Cotton and rayon Aromatic substrate vatted with sodium sulfide and reoxidized to insoluble sulfer-containing products on fibre Vat Cotton, rayon and wool Water-insoluble dyes solubilized by reducing with sodium hydrogen sulfite, then exhausted on fibre and re-oxidized Azo, phthalocyanine, stilbene and oxazine Azo, anthraquinone, styryl, nitro and benzodifuranone Stilbene, pyrazoles, coumarin and naphthalimides Azo and anthraquinone, Aniline black and indeterminate structures Azo, anthraquinone, phthalocyanine, formazan, oxazine and basic Indeterminate structures Anthraquinone (including polycyclic quinines) and indigoids

13 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

14 23 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 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 (-NH 2 ) 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

15 24 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 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 O 2 -) 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 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

16 25 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, monoazo, 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

17 26 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 Monochlorotriazine Low Monofluortriazine Moderate Trichloropyrimidine Low Dichloroquinoxaline Low Difluorchloropyrimidine Moderate to high Vinylsulphone Moderate (a) Dichlorotriazine (b) Monochlorotriazine (c) Monofluortriazine (d)trichloropyrimidine (e)dichloroquinoxaline (f)difluorchloropyrimidine Dye-SO 2 -CH=CH 2 (g) Vinylsulphone Figure 2.5(a-g) Reactive group structure

18 27 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 (OSO 3 Na) 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

19 28 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 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 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).

20 29 (2.5) where, Co and Cs are the concentration of dye in the dye bath initially and at the end of the process, respectively 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 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) 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

21 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.

22 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) 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 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

23 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 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 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.

24 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) 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 ( 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 semicontinuous 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 Padding is the most important process of continuous and semi-continuous dyeing (Hunger 2003).

25 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 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.

26 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,

27 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 Padhumidity 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) 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 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

28 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 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 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

29 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 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

30 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 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