The Salt-Free Dyeing on Cotton: An Approach to Effluent Free Mechanism; Can Chitosan be a Potential Option?

The present study utilizes salt free dyeing on cotton which would not con tribute to environmental pollution an undertaken to explore promising approach to reduce cost of dye process. The salt free dyeing used to dye cotton. In conventional Method of dyeing of cotton with reactive dyes, alkali P H is should maintain in the dye bath. This method requires more electrolytes for exhaustion and alkali for fixation. In this paper the fibre modification technique based on polyacrylamide was discussed. When the fabric is treated with polyacrylamide (chitosan), the primary hydroxyl groups of cellu lose is (part ially) modified into amide groups, which intern leads the cellulose to act like as wool fibre and hence rea ctive dyes can be dyed on cotton at neutral P H in the absence of electrolyte and alkali.


Introduction
Cotton fibres are widely applied in text ile industry due to its excellent properties of hygroscopicity, air permeability, biodegradab ility, no static electricity, etc. The dyeing of these fibres are generally done with reactive dyes due to its brillian cy, variety of hue, h igh wet fastness, convenien t usage and high applicability. These reactive dyes contain a react iv e g roup, either a haloheterocycle o r an act iv ated double bond, that, when applied to a fibre in an alkaline dye bath, forms a chemical bond with hydro xy l g roup on the cellu losic fibre [1]. In recent years, reactive dyes maintain the largest annual consumpt ion in the wo rld among the dyes used for which establishes its important status in the dye manufacture industry. But some problems, such as low dye utilizat ion, large amount of electrolyte used and high volume of wastewater discharged, always exist in the application of reactive dyes. The dyeing of one kilogram o f cotton with reactive dyes demands from 70 to 150 litre water, 0.6-0.8 Kg NaC1 and fro m 30 to 60 g dy estuffs [2]. Du e to these problems this class of dyes is the most unfavorable one from the ecological point of view, these effluents produced gives high values of BOD/ COD (Bio log ical o xygen demand / Chemical o xygen demand ) and increases salin ity o f the rivers affects the delicate b iochemistry of aquatic life. More t han 80,000 t o ns o f react iv e dy es are p ro du ced and consumed each year, making it possible to quantify the total amount of pollution caused by their use. So, most researchers Chitosan will be used for modificat ion of cotton fabric and then fabric will be dyed with conventional dyeing procedure without use of salt [2,3]. There are certain advantages in salt free dyeing over the conventional dyeing technique, for example; • Elimination of salts an electrolyte.
• Low volu me of water requirement during the wash off process.
• To increase the reactivity of react ive dyes.
• To increase the wash fastness and rubbing fastness of pretreated sample.
• To increase the fabric crease recovery and flexu ral rig idity environ mentally friendly. • Significant savings in process costs.

Techni ques Used for Salt-Free Dyeing Process
The follo wing way are generally used to achieve the salt-free dyeing over cotton fabric using reactive dyes by modifying the cotton surface after introducing some new functional group with having reactive dye-affin ity.

Modification of Reactive Dyes
It is reported that by using reactive dyes containing two or more reactive groups rather than one reactive group per dye M echanism;Can Chitosan be a Potential Option? mo lecule fixation can be increase fro m 60% to 80%. In practice this method can be productive owing to the detrimental effect that additional group can have on migrat ion, lead ing to lower fixation, especially at h igher depths. Most suitable of this type of dye is Cibacron LS range. Two hetrobifunctional groups mixed to form mu ltifunctional reactive dye. It utilizes several different types of reactive groups. Monoflourotriazine and monochlorotriazine in co mb ination with sulphone can be used. Due to h igher mo lecular weight of this dye it function effectively In presence of reduce amount of salt but its mo lecular weight is high so migration capacity is lo w. So it is applied in p resence of small amount of electrolyte for only 30 minutes at neutral condition for 90⁰C followed by addition of alkali to effect fixation, generally over 60 minutes but at 70⁰C to p ro mote fixat ion. M igration and leveling occur in higher temperature but the fixation occurs slowly at lower temperature. In present days novel bis-sulphatoethyl sulphone are vailable in the market which consists of two ureido groups. Build up well in absence of electrolyte, here ureido group help to increase substantively leading to lower use of salts [3,4].

Advantages
• Low amount of salt required. In this process a new fiber modification technique based on cationic acrylic copoly mer is retreated with cotton fiber because it believed that pre-treated of cellulosic fiber with Poly mer to offer an opportunity for increasing both the substantivity and reactivity of fibers towards reactive dyes under neutral conditions. The nature o f a react ive poly mer resin is such that it may react with nucleophilic sites in cellu losic fibers or in the poly mer itself, thus fixing the polymer to the substrate. During subsequent dyeing, further reactions between the polymer and the dyestuff, the fiber and the dyestuff, and the fibre and the poly mer and can be expected to take place, forming cross -lin k within the fibers.

Processing of Chitosan
The preparation of chitosan is very easy as shown by

Princi ple Invol ved In Chitosan Dyeing Process on Cotton Fabric
The pretreated cotton with poly mer is generally used for the chitosan dyeing process for a cotton fabric. The pretreatment of cotton fabric with polyacrylamide demonstrates the introduction of functional amino groups which increase the substantivity and also the reactivity of cotton. The cationic charged amino groups may be involved in the adsorption of anionic chro mophore of react ive dyes. The improved dye ability is postulated due to the presence of amide groups (-CONH 2 ) available fro m the polyacrylamide which also tents to imp rove the reactivity of cellulosic substrate. The attachment of the dye molecules onto the partially-modified cellu losic substrate is by covalent bonding since no dyes strips out from the dyed sample ( fig.  3). This is also indicative through the fastness properties wash fastness. The fastness values of all such dyed samples are quite satisfactory and comparable with those of conventional dyed samples. The dry crease recovery angle values of the polymer treated samples are 80 o while that of conventional dyed sample is 68 o . Therefore, as expected, the polymer treated dyed samples indicate an improvement in the wrin kle recovery.
A high level of dye exhaustion on the treated fabric can be achieve in the absence of salt and alkali at a temperature as low as (No rmally at 60-80℃) that used in the conventional dyeing process. Further increases in temperature may improve dye bath exhaustion, but only to a limited extent. However higher temperature (90-100 ℃ ) are generally recommended for dyeing modified fabrics to obtain better penetration and fixation of dye [5,6].

Advantages
• Pretreat ment of cotton with polyacrylamide enhances the possibility of dyeing cotton at neutral pH.
• Such pretreatment, as applied through paddrycure process, brings about some chemical changes in the treated fabric.
• Fastness properties are adequate and quite comparable with conventionally dyed samples.
• The wrinkle resistance of the dyed fabric also improves.
• The dyeing of cotton with reactive dyes using polyacrylamide in the dye bath improves the dye ability of cellu losic fabrics with reactive dyes and reducing effluent discharge [7].

Using CHPTAC
Chemically cationized cotton is usually produced by the etherifying reaction of cotton with the tert iary amino or quaternary ammoniu m cationizing reagents, especially quaternary ammoniu m cationizing reagents ( fig. 4), such as 2,3-epo xypropyltrimehtylammoniu m chloride. This compound is usually formed in situ fro m the reaction of sodium hydro xide with 3-chlo ro-2-hydro xypropyltrimethyla mmon iu m chlo ride (CHPTAC) [12].

Pre-treatment of Cotton wi th CHPTMAC
The solution consisting of 35 g/l of 3-chloro -2-hydroxy propyl trimethyl ammoniu m chloride (CHPTMAC) (65 % w/w) and 15 g/l of sodium hydro xide was applied to the well-p repared cotton fabric. The wet pick up was 100 % and to min imize the hydrolysis of reactant, the alkali was added to the bath just before application. The fabric was stored at room temperature fo r 24 hr, and then rinsed several times with water and finally neutralized with a d ilute acet ic acid (1g/l). Reaction of CHPTMA C with Cellu lose 2,3epoxypropyltrimethylammoniu m ch loride (CHPTMAC) was prepared in situ by the reaction of 3-chlo ro-2-hydro xypropyltrimethylammon iu m chloride (EPTMA C) with alkali as shown in Scheme 1. EPTMAC will react with alcohols under alkaline conditions to form ethers (step 2) and will thus produce a modified fiber when it reacts with cotton as shown in step 3. As a result, cotton will have cationic dye sites covalently bound to the poly mer chains. These dye sites will strongly attract anionic reactive dyes, enabling the use of these dyes without the large amount of electrolyte ( fig. 5).

Dyeing of Fabric with CHPTMAC
Dyeing of pre-t reated cotton was carried out in the absence of salt by the conventional method. The results showed that all of the dyes gave an excellent exhaustion with 1% o.w.f. and most of them showed a high fixat ion with good levelness exhaustion, fixat ion and color strength values for the most dyes on the treated cotton is higher than those on the untreated cotton. The fixation values varied from dye to dyes. In these dyeing systems, the strong attraction between the cationic dye sites on the modified cotton and the anionic dyes existed which led to obtain a very high exhaustion rates without addition of electrolytes to the dye-bath. The color strength of most of the dyeing on the treated cotton was often twice and in some cases up to four times of untreated cotton [8,9]. However, these high color strength values might be partially attributed to surface dyeing or ring dyeing. The wash fastness of the cat ionized dyed cotton was similar to that of the untreated dyed fabrics and the cross staining results were good The excellent light fastness of the reported dyeing on cationic cotton is particularly striking, since cationic pretreat ments, as a general rule, are known to impair light fastness. The cationic pretreatment used here is nonpolymeric in nature and can be expected to penetrate the cotton fiber prior to fixation. The rubbing fastness of both sets of samp les was good. The observation of the dyed samples also showed a good level of uniformity. An enhancement of the color strength is expected when the dye concentration is increased, since a greater nu mber o f dye mo lecules would be availab le in the vicinity of the cotton cellu lose at higher concentrations. Unlike untreated cotton, however, the dye build-up on the treated cotton is limited by its saturation adsorption value which is related to the amount of EPTMAC applied during pretreatment. It can be seen fro m Figure 1 that the cotton treated with EPTMA C obtained good color strength ( fig. 6) at all depth of shades below 4.5 % o.w.f. Above this level, there is no increase in color M echanism;Can Chitosan be a Potential Option? strength with increasing the dye concentration. However, the color strength on the treated fiber dyed with 5 % o.w.f. dye is similar to that of the co lor strength of an 8 % o.w.f. conventional dyeing.  The effect of salt on the dyeing of the quarternized cotton is shown in Figure 4. In the dye concentrations applied to the fabrics well belo w the saturation limit of the treated fibers, the degree of exhaustion decreased with increasing salt concentration [10] and for the 2 % o.w.f. Cibacron Red HF, the treated cotton gave 60 % exhaustion of anionic dye in the absence of electrolyte, but the exhaustion droped to 45 % after adding 20 g/l of sodium sulphate. This unfavorable effect was because of the sulphate anions which have a significant affinity for qurternized cellu lose and they were able to compete with dye anions for the quaternary sites in the modified fibers with increase in salt concentration [11], as nearly all the quaternary sites are occupied and the fiber became neutrally or even negatively charged ( fig.7). In this way dye adsorption was the same as the untreated cotton.

Grafting of Cellulosic Material: Using MAPTAC
Graft poly merization of the cationic mono mer methacrylo l amino propyltrimethyl ammoniu m chloride (MAPTAC), on to scoured cellulose onto scoured cellulose was carried out in bleaching process, aiming to modify the fibre using a single bath. The extent of MAPTAC fixat ion on cellu lose was measured .The bleaching performance of hydrogen peroxide in presence of the modify ing agent was found to slightly reduce. The modified b leached cotton fabric was then dyed with a commercial react ive dye in the absence of salt. The dye uptake and colour strength of the modified fabric was markedly increased with an increase in concentration of MAPTAC. Th is was attributed to the presence of cationic group of MAPTAC which play a crucial role in attracting the an ionic dyes fro m the dye bath [12]. Cationic mono mer wh ich contains C-C double bond can be grafted using redox init iator. Fixation of MAPTAC achieved by graft poly merizat ion using potassium per sulphate as redox init iator, it o xidizes the cellulose hydroxyl group, produce cellulose hydroxyl free rad ical, wh ich in itiate the graft poly merization, the enhanced dye ability of the modified cotton fabrics were attributed to the presence of cationic groups which have large affin ity for anionic dyes. The treatment occurs at 75℃ and MLR is 1:20 for 45mins, 5g/l hydrogen peroxide,2g/l NAOH,1g/l sodiu m silicate added and the temperature raised to 95℃ . This temperature is maintained for 30min before fab ric is removed fro m the solution [13].

Disadvantages of this Process
• Colour Fastness to Light Light fastness decreases with an increase in the amount of modifying agent, cat ionic g roup may act as photocatalyst because of lack of deactivating nitro group on the dye chromophore comb ine with prevention of dye aggregation by the presence of cationic groups, leading to acceleration of chromophore decomposition under the light [13].
• Ring Dyeing Since the compound carries the cationic group, the substantivity towards cotton fiber should be high enough to force the modifying agent to diffuse into inner portion of the fiber. By graft copoly merization we get high molecular weight poly mer. If the grafted co mpound will be high mo lecular weight its mig ration will be low but substantivity is high as results such compound remain on yarn surface and lead to ring dyeing. • Using dendrimers on cotton fabric surface In contrast to linear poly mers, dendrimers are highly branched, fractal-like macro molecules of well-defined, three-dimensional structure, shape and topology. Fro m their shapes are derived names such as arborols, cascade, or star-burst polymers. The synthesis typically proceeds in stages or generations in which both the extent of branching and the size of the dendrimer increase; synthesis can be closely controlled to achieve a given size, shape and internal structure. Dendrimers cannot engage in chain entanglements as do linear poly mers and thus are not useful in mechanical applications as are conventional linear poly mers; generally, dendrimers are viscous liquids at room temperature. Functional end groups impart to the dendrimer characteristic physical and chemical properties, such as solubility, compatibility with plastics, uptake of guest molecules and surface activity [10,11].

Dyeing
To highlight differences in dye uptake between the pre-treated and untreated cotton samples, the majority of dyeing were carried out using a competitive dyeing method (Fig. 8) in which a sample of dendrimer-t reated cotton and a sample of untreated cotton were dyed competitively in the same dye bath. All dyeing were carried out using a liquor ratio of 20:1. A fter dyeing, the samples were washed-off [10]. In addit ion, a modified method of dye application was used to investigate the application of dye without salt and/or alkali. Accordingly, these modified dyeing processes were carried out both unbuffered as well as at a buffered pH of 4.0 and 7.4.   It shows that for each of the three pH values used for dendrimer application, the color strength of the pretreated cotton was considerably higher than that of the corresponding untreated cotton (fig. 10).
The effect of p retreatment with varying amounts of dendrimer on color strength is shown in Fig. 11 and generally, color strength increased with increasing amount of dendrimer applied, although the observed increase ( fig. 12) in color strength was small [10].

Use of Org anic Salt: Sodi um Edate (S E)
Sodiu m edate was used in the dyeing of cotton fabrics with reactive dyes. Exhaustion behaviour of this organic electrolyte was co mpared with sodium sulphate. Also, using this electrolyte for reactive dye fixat ion was evaluated in comparison with sodium carbonate [14].

Dyeing Conditions
In a dyebath containing different amounts of SE (50 g/l) and 2% shade of CI Reactive Blue 19 o r CI Reactive Black 5 with liquor ratio 40:1, cotton fabric was added at 35℃ and the temperature was raised to 60℃ or 80℃ over 20 min for vinyl sulphone or monochlorotriazine dyes, respectively. After wh ich t ime the dyeing was continued at 60℃ for 30 min (mig ration phase). Then 20 g/l of sodium carbonate was added portion wise and the dyeing was maintained at 60℃ for further 45 min [15].

Conclusions
The purpose of this paper is to g ive a overall idea about the pre-treat ment of cotton with a chitosan could enhance the dye ability of the fibre with reactive dyes. The chitosan contains primary amino groups, with which theoretically, a reactive dye should be able to react under neutral/acidic P H conditions [16]. It is also decided to examine whether or not the chitosan could, under appropriate P H conditions, assume a positive charge and so permit ``salt-free'' dyeing.
Pretreat ment of cotton with chitosan enhances the possibility of dyeing cotton at neutral P H with various commercial reactive dyes and such pretreatment also brings about some chemical changes in the treated fabric. Fastness properties are adequate and quite comparable with conventionally dyed samples. The bending resistance of the dyed fabric also improves . The dyeing of cotton with reactive dyes using chitosan with cross -linker in the dye bath improves the dye ability of cellulosic fabrics with reactive dye [17], when dyeing the modified substrates; reactive dyes can be much more efficiently exhausted and fixed onto cellu losic fabrics under neutral conditions in the absence of salt and alkali. The modificat ions show an overall suitability for d ifferent reactive dyes. The modified dyeing do not suffer either fro m a significant drop in co lour strength, wash fastness or fro m duller shades.