With bi-functional reactive dyes, the dyeing cycle for soybean fibre is the most commercially viable, and with this dyeing cycle, the mechanical, wash fastness and light fastness properties of soybean fibre are also satisfactory, inferArnab Sen and Dr M L Gulrajani.
In the pursuit of a natural but inexpensive substitute for wool and silk, various regenerated protein fibres have been developed in the past, like those based on casein, peanut, zein, gelatin and collagen. However, these fibres were not found to be commercially viable. Among the regenerated protein fibres of plant origin, soybean fibre, originally developed in 1935 , seems to have attained highest commercial success till date. It is environment-friendly and has certain unique properties like biodegradability, non-allergic, microbiocidal and anti-ageing properties. It combines softness and featheriness similar to cashmere and has silk-like lustre. Its blends with wool, silk, cotton and spandex are being produced. Soybean blended with cashmere imparts the fabric the uniqueness of smoothness, lightness, warmth and damp permeation.
In a recent study, dyeing of soybean fibres with acid and metal complex dyes has been carried out. In this study, the exhaustion and realisation values of these dyes were found to vary substantially depending on the conditions of dyeing. The wash fastness of the acid dyed fibres was found to be very poor. This might have been due to poor degree of penetration of the dye in the fibre and weak dye-polymer interaction due to lesser amount of functional groups present compared to wool and silk. With some cationic fixing agents, the fastness properties were found to improve substantially. Reactive dyes are known to possess good wash fastness properties, and have also been successfully dyed on protein fibres. Hence, a need was felt for studying the reactive dyes on soybean fibre and to evaluate total dye fixation as well as the fastness properties.
The prime objectives of this work were as follows:
1. To evolve a dyeing cycle for the dyeing of soybean fibres with reactive dyes by varying the salt concentration and consequently studying the exhaustion and total dye fixation values with bi-functional reactive dyes vis-a-vis wool and silk.
2. To study the mechanical properties of the dyed fibres.
3. To study the extent of formation of cross links of bi-functional reactive dyes on the dyed fibres by evaluation of degree of solubilisation, thus affecting the wash and light fastness properties.
Figure 1 shows the surface structure and cross-section of soybean fibre. The cross section is seen to be irregular, dumbbell-shaped and have island-in-the-sea structure.
Soybean fibre has been reported to have moisture absorption of 3.73% and air permeability of 1.79% [1,4]. Its UV-resistance has been reported to be higher than cotton, silk and wool. It loses about 10% strength on exposure to UV-light for 120 hours.
In this study, one homo-bi-functional and one hetero-bi-functional reactive dye were used at differing shade depth of 2, 4 and 6%. The chemical structures of these two dyes are as in Figures 2 and 3.
Dyeing was carried out at 90°C and pH 7 with material to liquor ratio of 1:50, as per the dyeing cycle as shown in Figure 4.
Exhaustion percentages were determined using the calibration curves obtained from the spectrophotometer with the help of a blank dye bath diluted from 0.08 gpl to 0.002 gpl. For calculating the fixation percentage, 0.25 g of dyed yarns were treated with 50 ml solution of 500 gpl urea, 28.1 gpl sodium dihydrogen phosphate, 38.7 gpl disodium hydrogen phosphate and 5 gpl Lissapol N at pH 7 for 5 min at boil. The dyed and stripped wool and silk samples were then dissolved in 5% caustic soda (NaOH) solution. For soybean, 15% caustic soda solution was used. About 0.05 g of each sample put in 50 ml solution at boil till they dissolved completely, leaving it coloured. The time of dissolution was 15 - 20 min for silk and wool whereas for soybean, it was as long as 100 min. The calculations involved were as follows:
Exhaustion percentage (%E):
%E = (C1 – C2)/C1 X 100,
where C1 and C2 are the dye bath concentrations before and after dyeing.
Fixation percentage (%F)
%F = [(k/s)2/(k/s)1] X 100,
where (k/s)2 and (k/s)1 are the colour strengths of the dyed samples after and before stripping of the unfixed dyes.
Total Dye Fixation (%T):
%T = (%E X %F)/100
The total dye fixation (%T) refers to the dye chemically bound to the sample relative to the dye applied to the sample.
The mechanical properties of all yarn samples were tested in a CRE machine. A gauge length of 50 mm was used with jaw speed of 50 mm/min. The load cell used was 10 kg. Wash fastness of the dyed samples was tested as per the ISO-3 standard test procedure. The light fastness was tested in a Xenotester as per AATCC 16.
The degree of solubilisation gives an idea of the extent of formation of cross links. For silk, a solution of calcium chloride, ethanol and water was used in the molar ratio of 1:2:8. 100 mg of dyed silk yarns were treated with 50 ml of the solution at 70°C for 20 min. The residues were treated with 0.5 N sulphuric acid, washed with distilled water and bone dried at 105°C and weighed after being kept in room temperature and relative humidity for 24 hours. For soybean fibre, 50 mg of dyed sample was treated with 50 ml of a solution of calcium chloride, ethanol and water in the molar ratio of 1:1:4 at 95°C for 6 hours. The residues were then treated with 0.5 N sulphuric acid, washed with distilled water and dried at 105°C and weighed.
For wool, both alkali and urea bisulphate solubility were evaluated. About 100 mg of dyed wool samples were treated with 50 ml of 1M NaOH solution at 65°C for 5 min. The residues were then filtered, neutralised with 0.5 N sulphuric acid, washed with distilled water, bone dried at 105°C and weighed after conditioning for 24 hours.
For urea bisulphate solubility test, 50 g of urea and 3 g of sodium metabisulphite were taken and the solution made to 100 ml. About 50 mg of dyed wool samples were treated with 50 ml of the solution at 65°C for 1 hour. The residues were then filtered, treated with 3% urea solution, washed with distilled water and bone dried at 105°C. They were then weighed after being kept in room temperature for 24 hours .
Results & discussion
The yarn samples were dyed at six different salt concentrations, starting from 0 to100 gpl, at intervals of 20 gpl each, at 90°C and pH 7. They were then evaluated for %E, %F and %T. The values for %T of all the fibres dyed with the two bi-functional reactive dyes are given in Table 1.
Optimisation of salt concentration
From Table 1, it can be seen that the increment in total dye fixation percentages of all the fibres tend to decrease sharply after a certain salt concentration. So, it was presumed that for each fibre, there is a particular value of the optimum salt concentration, beyond which the increase in salt concentration has no significant effect on the dye absorption. In case of protein fibres, absorption of reactive dyes into the fibre is governed not only by exhaustion, but this has also been related to the degree of fixation .
Although exhaustion is a function of salt concentration, in case of protein fibres, fixation is also affected by exhaustion of the reactive dyes. So, in order to find out the optimum salt concentration, the Total Dye Fixation, %T, which is a product of %E and %F, was studied. In order to estimate the optimum salt concentration for dyeing of soybean fibre, %T per unit salt concentration was calculated as in Table 2. From these values, %T per unit salt was plotted against salt concentration as in the Figures 5 and 6.
Table 2: Total Dye Fixation, %T, per unit salt concentration for soybean fibre
It could be observed that the curves followed a second order equation in almost all cases with a linear portion initially and another beyond a certain range of salt concentration. This region corresponds to the range of diminishing return. Tangents were drawn at the two almost linear regions of the curves, and their point of meeting gave an estimate of the optimum salt concentration. In order to estimate the optimum salt concentration, two tangents were drawn from these two almost linear portions of each curve. In all cases, it was found that the optimum salt concentration lay at around 50 - 60 gpl.
Only soybean and silk yarns dyed with salt concentrations of 60, 80 and 100 gpl were tested, as they gave highest %T values. The results are in Table 3.
Table 3: Comparative study of Tenacity (cN/Tex) of soybean and silk dyed at different shade depths
From the data in Table 3, it could be concluded that shade depth of dyes have a negative effect on the tenacity of soybean fibre.
The wash fastness at 4% shade depth was evaluated. Besides shade change, the staining on cotton and silk fabrics was evaluated for soybean and silk, whereas for wool, staining on cotton and wool fabrics were evaluated. The fastness ratings were given using the AATCC grey scale. The results obtained for soybean and silk are listed in Table 4.
From the results, it could be observed that for soybean, Remazol Black B133 showed excellent wash fastness whereas Reactive Navy Blue BFN exhibited moderate fastness. For silk, both the dyes exhibited excellent wash fastness properties in almost all cases. For wool, the ratings were good to excellent for shade change with both the dyes, whereas those for staining on cotton were moderate to good for Remazol Black B133 and poor to moderate for Reactive Navy Blue BFN.
The light fastness of Remazol Black B133 and Reactive Navy Blue BFN on soybean, silk and wool at 4% shade depth were evaluated as per AATCC 16 method. The results obtained are shown in Table 5 as follows:
The light fastness of samples dyed with Remazol Black B133 was found to be good to excellent for wool and moderate for soybean and silk. For Reactive Navy Blue BFN, the light fastness was found to be good with soybean and wool, whereas for silk, it was found to be moderate.
Evaluation of formation of cross links
It has been reported that the bi-functional reactive dyes are capable of formation of cross links when applied to silk . In this study, the possibility of formation of cross links for the two bi-functional reactive dyes applied to soybean, silk and wool have been evaluated. When a polymeric material is dissolved in a solvent, due to polymer-solvent interaction, the intermolecular bonds of attraction are broken up and the polymeric molecular chains are individualised so that they go into solution. Formation of cross links among the polymer molecular chains diminish the degree of solubilisation of the material due to greater number of intermolecular bonds. So, degree of solubilisation provides an idea about the extent of cross links formed. The results for degree of solubilisation of soybean and silk are listed in Table 6 and for wool in Table 7.
Compared to the undyed specimen, the degree of solubilisation decreased after dyeing with both the dyes in case of all the fibres. This may be due to:
i. Blocking of the solubilising groups in the fibre by the dye.
ii. Formation of cross links among the polymeric chains, effectively increasing the intermolecular bonds.
In all cases, solubility decreased with increase in shade percentage, indicating an increase in the extent of cross link formation. This was because bi-functional dyes are capable of formation of cross links with soybean, silk and wool, and the extent of their formation increases with shade percentage, the increase being at a higher and more uniform rate with silk than other fibres. Also, Remazol Black B133 is capable of formation of cross links to a larger extent than Reactive Navy Blue BFN, because the two reactive sites in it are situated at a larger distance from each other than those in Reactive Navy Blue BFN, where they are very close to each other. Being at such a distance apart, they have greater possibility to interact with the reactive side groups of the polymeric molecular chains and form cross links.
The formation of cross links of the dye molecules with the reactive groups in the fibres ensures better wash fastness properties.
From the above results, it could be concluded that with bi-functional reactive dyes, the dyeing cycle for soybean fibre as shown in figure is the most commercially viable. The pH to be maintained is 7, the required temperature is 90°C, and the optimum salt concentration lies within the range of 50 - 60 gpl. With this dyeing cycle, the mechanical, wash fastness and light fastness properties of soybean fibre are also satisfactory, as discussed above. The drawbacks of acid and metal complex dyes on soybean fibre could be largely overcome.
It was also observed that with increase of shade depth, the increment in total dye fixation of soybean fibre was higher than silk and wool, indicating that if deep shades of a particular colour are needed using bi-functional reactive dyes, soybean fibre is more economical than silk and wool, due to less wastage of dye. This increases the commercial acceptance of soybean fibre for deeper shades all the more.
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Note: For detailed version of this article please refer the print version of The Indian Textile Journal November 2009 issue.
Mr Arnab Sen
Assistant Professor & Centre Coordinator-Textile Design
Department of Fashion & Textiles
National Institute of Fashion Technology
Dr (Prof) M L Gulrajani
Department of Textile Technology
Indian Institute of Technology