Materials
A 100 % grey cotton plain woven fabric obtained from Tata mills, Mumbai, was used throughout the experiment. The fabric had a yarn count of 40s and a gram per square metre (GSM) of 112. The chemicals, wetting agents, sodium hydroxide, hydrogen peroxide, peroxide stabiliser, glacial acetic acid, sulphuric acid, potassium iodide, iodine, Glauber’s salt and sodium carbonate were obtained from S.D. Fine chemicals and were of AR grade. Reactive dyes, Yellow HE6G and Navy Blue HER were obtained from Colorband India Pvt. Ltd and were used without any further purification.
Conventional method
Untreated fabric, also known as grey fabric, was desized, scoured and bleached in separate baths as per the procedure given in literature (Koushik and Josico 2003; Shore 1995; Tomasino 1992). All the processes were carried out in the lab scale Rota dyer (Rossari® Labtech, Mumbai, having a capacity of 1800 ml, 18 pots each with a capacity of 100 ml). The flow chart of the same can be seen from Fig. 1.
Pre-treatment of fabric was followed by dyeing. For this study, reactive HE dyes were used. In order to dye a cotton fabric using reactive HE dye with 1 % shade, 20 g l−1 sodium carbonate and 40 g l−1 Glauber’s salt are required. The time vs temperature graph of dyeing is shown below in Fig. 2.
The proposed recycle and reuse method
Scouring followed by bleaching of a desized fabric was carried out using the conventional recipes. The process bath of scouring and bleaching still containing unexhausted chemicals was recycled as it is without addition of any extra chemicals three times. The final, i.e. the third bath of both the scouring and bleaching processes were then mixed in 50:50, 70:30 and 30:70 proportions, respectively, and were reused to desize a new grey fabric. The fabrics thus desized with the recycled water were subjected to reactive dyeing. A flow diagram of the same can be seen from Fig. 3.
Testing and analysis
Testing of desized fabric
Iodine drop test (Tegewa test) is employed to check the desizing efficiency. The underlying principle for the same is the violet colouration of starch with iodine. Hence, the presence of starch remaining on fabric can be deciphered. A small sample of desized fabric was cut and immersed in a beaker containing a solution of potassium iodide and iodine for 1 min. The sample was then rinsed thoroughly under tap water, dabbed with filter paper and immediately compared with the Tegewa scale or Violet scale having a 1–9 rating. A rating of 1 denotes almost no removal of size and a rating of 9 indicates complete removal of size from the fabric. The commercially acceptable rating is 6–7 (Wurster et al. 1987).
Testing of scoured fabric
Absorbency of a fabric was performed by using AATCC Test Method 79-2000, where a drop of water is allowed to fall from a fixed height onto the taut surface of a test specimen. The time required for the reflection of the water drop to disappear is measured and recorded as wetting time. Five seconds or less is generally considered to represent adequate absorbency (Chehna et al. 2007).
Testing of bleached fabric
The bleached fabrics were evaluated for whiteness index using a computer colour matching system (SpectraScan 5100+) supplied by Premier Colorscan Instruments Pvt. Ltd. Evaluation for whiteness index was carried out using AATCC Test Method 110-2005. An average of two readings taken at two different positions of samples was used to calculate the indices (Chehna et al. 2007).
Presence of alkali in the scouring bath
In this test, the accurately measured volume of specimen was titrated against standard solution of sulfuric acid using a phenolphthalein indicator. The alkalinity measured is in terms of CaCO3 (mg/l) (American Public Health Associations et al. 1999).
$$ \mathrm{Alkalinity},\ \mathrm{mg}\ {\mathrm{CaCO}}_3/\mathrm{l}=\frac{(A)(N)\left(50,000\right)}{\mathrm{ml}\ \mathrm{sample}} $$
where
A: millilitre of the standard acid used,
N: normality of the standard acid.
Presence of hydrogen peroxide in the bleaching bath
AATCC Test Method 102-2002 was used to determine hydrogen peroxide in the bleach bath. A specimen is acidified with sulphuric acid and titrated with standardised potassium permanganate solution. The concentration of hydrogen peroxide is calculated using the volume and normality of the permanganate solution used (Chehna et al. 2007).
$$ \%\ {\mathrm{H}}_2{\mathrm{O}}_2=\frac{\left(V\mathrm{t}-V\mathrm{b}\right)\left(N\mathrm{t}\right)(0.017)(100)}{W\mathrm{s}} $$
where
Vt: volume of titrant in millilitres,
Vb: volume of titrant in millilitres for blank sample,
Nt: normality of the titrant solution,
Ws: specimen mass, in grams.
Presence of alkali in the bleach bath containing peroxide
This test is carried as per AATCC Test Method 98-2002. A weighed specimen of the bleach bath is titrated with a standardised solution of sulfuric acid using Phenol Red indicator or to the pH range 6.8–8.4 on a pH metre. The total alkali, expressed as %NaOH, is calculated based on the weight of the bath (Chehna et al. 2007).
$$ \%\ \mathrm{Total}\ \mathrm{a}\mathrm{lkali},\ \mathrm{a}\mathrm{s}\ \mathrm{NaOH}=\frac{\left(\mathrm{ml}\right)(N)(0.040)(100)}{W} $$
where
ml: the number of millilitres of the sulphuric acid solution required,
N: the normality of the sulphuric acid solution,
0.040: the milliequivalent weight of sodium hydroxide,
W: mass of the specimen.
Colour depth (in terms of K/S value and colour strength) and tone of the colour (in terms of L*, a* and b* values)
Dyed samples were evaluated for the depth of the colour by determining K/S values as well as colour strength using a computer colour matching system (SpectraScan 5100+) supplied by Premier Colorscan Instruments Pvt. Ltd. An average of two readings was taken at two different positions of samples and was used to calculate the reflectance values and KubelkaMunk K/S function which is given by
$$ \frac{K}{S}=\frac{{\left(1-R\right)}^2}{2R} $$
where
R: reflectance at complete opacity,
K: absorption coefficient,
S: scattering coefficient.
The dyed samples were also evaluated in terms of L*, a*, b*, C, h and dE. The colour difference (dE) can be visualised as the distance between the standard or reference and the sample points plotted in the L* a* b* colour space. L* values have a range of 0–100, 100 indicates lightness or white and 0 indicates darkness or black. Negative values of ‘a*’ indicate green while positive values indicate red, whereas negative values of ‘b*’ indicate blue and positive values indicate yellow. The L* a* b* system adopted from the CIE in 1976 is the most widely used system today.
$$ \mathrm{d}\mathrm{E}=\sqrt{\left(D{L}^2+D{a}^2+D{b}^2\right)} $$
where
DL: L (sample) − L (standard),
Da: a (sample) − a (standard),
Db: b (sample) − b (standard).
From the CIELAB coordinates, one can compute C (chroma) and h (hue). The chroma or saturation (C) is the distance between achromatic point and colour and is calculated from ‘a*’and ‘b*’ using the following equation:
$$ C=\sqrt{\left({a^{*}}^2+{b^{*}}^2\right)} $$
The point a = 0 and b = 0 lies at the centre of the chromaticity diagram. The lower is the value of ‘a*’ and ‘b*’, the higher is the achromaticity and the lower is the purity. On the other hand, the higher the values of ‘a*’ and ‘b*’ (ignore the negative signs), the purer or more saturated or brighter is the colour.
$$ h={ \tan}^{-1}\frac{b^{*}}{a^{*}} $$
Positive values of chroma (or saturation) indicate more saturated colour (i.e. brighter) and negative values indicate less saturated colour (i.e. duller). Hue is an angle measured in degree and can also be termed as purity of colour (Sule 1997).
Colour fastness evaluation of dyed samples
Colour fastness to washing was performed in a Laundrometer by using ISO 105-C03 (1989). Colour fastness to rubbing was done on a Crockmeter by using ISO 105 X12 (2001). Light fastness of dyed samples was evaluated on a Q-sun light fastness tester by following AATCC 16E method which is similar to ISO 105-A02 (2013).