In conventional method of dyeing of cotton with reactive dyes, alkali PH should be maintained in the dye bath, which requires more electrolytes for exhaustion and alkali for fixation. In this paper, the fiber modification technique based on polyacrylamide was discussed. When the fabric is treated with polyacrylamide, the primary hydroxyl groups of cellulose is (partially) modified into a amide groups, which leads the cellulose to act like wool fiber and hence reactive dyes can be dyed on cotton at neutral PH in the absence of electrolyte and alkali.
In current practice, cellulosic fibers are predominantly dyed with reactive dyes in the presence of a considerable amount of salt and fixed under alkaline conditions. However, dye fixation efficiency on cellulosic fibers is generally low (varying from 50-90%). This, results in a highly colored dye effluent, which is unfavorable on environmental grounds. Furthermore, the high concentrations (40-100g/I) of electrolyte and alkali (5-20 g/I) required in cellulose fiber dyeing may pose additional effluent problems.
In this case, a new fiber-modification technique based on cationic acrylic copolymer. Pretreatment of cellulosic fiber with polymer is believed to offer an opportunity for increasing both the substantivity and reactivity of fibers towards reactive dyes under neutral conditions. The nature of a reactive polymer resin is such that it may react with nucleophilic sites in cellulosic fibers or in the polymer 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 fiber and the polymer can be expected to take place, forming crosslink within the fibers.
Materials and methods
For material, plain fabric of count 32 of warp and weft is used throughout the process. The weight of the fabric is 123 GSM, its cover factor is 21.63, and Ends/inch is 98, Picks/ inch is 64.
Dyestuff used are polyacrylamide resin, reactive dyes, Remazol Brill.Orange 3 R, Remazol Brill.Red 5B, Remazol Brill.Violet 5R, Remazol Turquiose Blue G, Procion Red M5B, Cibacron Green LS-3B, Cibacron Navy LS-G, Cibacron Red FN-R and Cibacron Yellow FN-R.
The material is first desized using enzyme, scoured with caustic soda and then bleached with hydrogen peroxide. The fabric neutralized and goes for dyeing.
Table 1 (Note: In case of vinylsulphone reactive dyes neutralization is very important before soaping. Neutralization is carried out with 0.5% Hcl at room temperature.) |
The dye bath is set with calculated amount of dye solution and water, MLR 1:20. Enter the wetted well bleached fabric into the bath, raise the temperature to 40℃ and work for 10 min then add the calculated quantity of salt in three portions at regular intervals (10 min). Raise the temperature to 50℃ and continue dyeing for 30 min, add the calculated quantity of soda ash and continue the dyeing for further 30 min. Finally take the material out and wash it with cold water and then give soaping treatment to remove the unfixed dyestuffs and chemicals. The dyeing temperature and recipe of various reactive dyes are shown in table 1.
Fiber modification technique
Treatment with polyacrylamide: pad the material with calculated quantity of polyacrylamide and water with 70% expression. After padding the material is dried at ambient temperature and then cured at 120℃ for 7 min.
Dyeing of pretreated fabric: set the bath with calculated amount of dye solution and water. Enter the pretreated fabric into the bath. Raise the temperature to a specified level at 1.5℃/ min, and dyeing continue at the set temperature for the further 60 min. Finally take out the material, soaped thoroughly and washed with cold water and dried.
Results
Determination of fastness to washing
The wash fastness was tested following ISO (standards) test no: 2
Determination of fastness to light
The light fastness was tested following ISO (standards) test no: 2
Table 2: Wash fastness values |
Table 3: Light fastness values |
Table 4: The color strength values |
Determination of color strength (k/s)
The color strength which is a measure using spectrophotometer using KUBELKA - MUNK FUNCTION.
K/S = (1 - R)2 /2R
Where, R = Reflectance value at wave length of maximum absorption.
K = Absorption co-efficient
S = Scattering co-efficient
Discussion
The probable mechanism for the fixation of reactive dye on the polymer treated cotton sustain may be expressed as follows:
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 chromophore of reactive dyes.
The improved dye ability is postulated due to the presence of amide groups (-CONH) available from the polyacrylamide which also tents to improve the reactivity of cellulosic substrate. The attachment of the dye molecules onto the partially-modified cellulosic substrate is by covalent bonding since no dyes strips out from the dyed sample. This is also indicative through the fastness properties wash fastness.
The fastness values (given in table 1 and 2) 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° while that of conventional dyed sample is 68°. Therefore, as expected, the polymer treated dyed samples indicate an improvement in the wrinkle recovery.
A high level of dye exhaustion on the treated fabric can be achieved in the absence of salt and alkali at a temperature as low as (normally 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 temperatures (90-100℃) are generally recommended for dyeing modified fabrics to obtain better penetration and fixation.
Conclusion
Pretreatment of cotton with polyacrylamide enhances the possibility of dyeing cotton at neutral pH with various commercial reactive dyes. Such pretreatment, as applied through pad-dry-cure 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 cellulosic fabrics with reactive dyes and reducing effluent discharge.
When dyeing the modified substrates, reactive dyes can be much more efficiently exhausted and fixed onto cellulosic fabrics under neutral conditions in the absence of salt. The modifications show an overall suitability for different reactive dyes. The modified dyeing do not suffer either from a significant drop in light fastness, wash fastness or from duller shades.
M. Subramanian Senthil Kannan currently works as Manager-Research & Development at Arvind Mills Limited in Ahmedabad, India, and R.Nithyanandan is a merchandiser at Francis Wacziarg Agencies (P) Ltd in Tirupur.
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