# Eco-friendly salt/alkali-free exhaustion dyeing of cotton fabric with reactive dyes

### materials

A bleached cotton fabric in plain weave (160 g/m2, Ne 120/2, 69 end/cm, 22 picks/cm) supplied from El-Mahalla El-Kobra Company, Egypt, was scoured by boiling in a bath containing 5 g/L sodium carbonate and 2 g/L nonionic detergent ( Sera wash M-RK DyStar, Egypt) for 3 h, then rinsed with cold water and air-dried at ambient temperature. Four commercial reactive dyes, comprising one hetero-bifunctional monochloro-s-triazine/vinylsulphone (MCT/VS) dye (CI Reactive Yellow 145), one homo-bifunctional Bis(MCT) dye (CI Reactive Blue 160), one homo-bifunctional Bis(VS) dye (CI Reactive Black 5) and one monofunctional MCT dye (CI Reactive Red 24) were used in this investigation. These dyes were supplied by DyStar and Oh Young Industrial Co. Ltd., and used as received. The CI generic name and chemical structures of these dyes are illustrated in Table 1. All the dyestuff obtained were of commercial quality and used without further purification. Sodium sulphate anhydrous (SS), sodium carbonate (SC) and trisodium nitrilotriacetate (TNA) Fig. 1 was purchased from Fluka, Germany.

### Dyeing methods

The viability of using TNA method in reactive dyeing was studied, corresponding the optimum dyeing temperature/time during the exhaustion and fixation stages by varying the time from 0 to 60 min and temperature from 40 to 80 °C. The effect of adding TNA salt instead of the conventionally sodium sulphate and sodium carbonate was investigated at the same dyeing conditions.

A series of SS/SC-free TNA dyeings was produced using 2% shade of the dye at a liquor ratio of 1:40. The dyeing process was started at 40 °C with various amounts of TNA (0–70 g/L) were added for 30 min primary exhaustion time, unless otherwise specified. The dyeing process continued for a further 60 min, while the temperature was then raised to 40–80 °C, unless otherwise specified, to complete the secondary exhaustion and fixation stage. The SS/SC conventional dyeing of cotton fabric (reference sample) was carried by replacing trisodium nitrilotriacetate (TNA) with sodium sulfate (SS) 50 g/L and sodium carbonate (SC) 20 g/L. SS was added at 40 °C in two portions within 30 min, SC was added in two portions within 1 h. All the dyed fabric were rinsed with cold water, and the unfixed dye was washed using a solution of 2 g/L sodium carbonate and 2 g/L non-ionic detergent at LR 1:50 and boiling for 30 min (Fig. 2) .

The K/S, exhaustion and fixation of the dyed sample using TNA were compared with conventional dyed sample using SS 50 g/L and SC 20 g/L. The color strength (K /S) and the color coordinates of all dyed fabrics were expressed in the CIELAB color space system (often denoted as L*, a*, b* coordinates). From which the value of L* represent lightness or darkness of the sample (a higher lightness value represents a lower color yield); a* denote redness if positive value or greenness if negative; b* represent yellowness if positive or blueness if negative and C* specifies chroma and h denotes hue angle were also measured using a Hunter Lab UltraScan PRO spectrophotometer (USA) under illuminant D65, 10 standard observer.

The total color difference values ​​(ΔE*) between the TNA dyed sample and the conventional one was calculated using the Eq. (1):

$$Delta mathrm{E}*=sqrt{({Delta mathrm{L}*)}^{2}+{left(Delta mathrm{a}*right)}^{2 }+{left(Delta mathrm{b}*right)}^{2}},$$

(1)

where ΔL*Δa* and Δb* are the differences of L*a*b* color parameters corresponding to TNA and conventionally dyed samples, respectively.

The color strength (K/S) of dyed fabrics after washing was recorded using the technique of light reflectance by applying Kubelka–Munk Eq. (2) 31.

The reflectance (R) of the dyed fabrics was measured on Shimadzu UV2401 spectrophotometer (Japan).

$$K/S = frac{{left( {1 – R} right)^{2} }}{2R},$$

(2)

where R = Decimal fraction of the reflection of the dyed fabric, K = Absorption coefficient, and S = scattering coefficient.

The absorption spectra of the dye solutions before and after dyeing was recorded by a Shimadzu UV2401PC UV–Visible spectrophotometer at the value using a calibration curve previously obtained using known dye concentrations (g/L) in water for calculating the % of exhaustion and fixation of Dyes onto cotton fabrics.

The extent of exhaustion achieved for 2% (owf) dyeing on cotton fibers was determined using spectroscopic analysis of the dyebath before and after dyeing at different times. The calibration curve for each dye was determined by measuring the absorbance of the dye solution of known concentration. The percentage of the dyebath exhaustion achieved for each dye was calculated from the Eq.(3).

$$% E =frac{{A}_{1}-{A}_{2}}{{A}_{1}}times 100.$$

(3)

where A1 is the concentration of the dyebath before dyeing, and A2 is the concentration of the dyebath after the neutral exhaustion stage (primary exhaustion, E1) and/or the alkaline stage (secondary exhaustion, E2).

The determination of the dye fixation ratio (%F) was measured by stripping the dyed samples at boiling for 30 min (liquor ratio 1:50) in a boiling bath containing 2 g/L sodium carbonate SC and 2 g/L nonionic detergent until all unfixed dyed was removed. The dye fixation ratio (%F) was calculated as presented in Eq. (4).

$$% F =frac{{A}_{1}-{A}_{2}-{A}_{3}}{{A}_{1}-{A}_{2}} times 100,$$

(4)

where A3the concentration of dye extracted after boiling using a solution of 2 g/L sodium carbonate and 2 g/L non-ionic detergent for 30 min at boiling LR 1:50.

### COD, BOD, TDS and TSS measurements

Laboratory analyzes of the chemical oxygen demand COD, biochemical oxygen demand BOD and the total dissolved salt TDS of the residual dye bath was carried out in accordance with Standard Methods for Examination of Water and Wastewater [APHA, American Public Health Association Standard Methods for the Examination of Water and Wastewater, 23ed edition, Washington, D.C (2015)].

### Fastness testing

After washing-off using 2 g/L SC and 2 g/L nonionic detergent until all unfixed dyed was removed, a specimen of dyed cotton fabrics of 2% owf depth of shade were tested according to ISO standard test methods. The wash fastness test was assessed in accordance with the standard method ISO 105-C06 B2S (2010) using 4 g/L of ECE detergent, 1 g/L of sodium perborate, 25 steel balls) at 50 °C for 30 min and at a liquor ratio of 50:1. Fastness to acidic and alkaline perspiration was determined with a perspirometer set at specific pressure, temperature and time in accordance with ISO 105-E04 (2008). Any change in color of the dyed samples (Alt) and color staining on the adjacent undyed cotton (SC) and polyester (SP) fabrics was then assessed with the corresponding ISO gray scales for color change and staining rates. Light fastness was also assessed using a Xenon arc lamp test in accordance with ISO 105-B02 (2013).