3.1 Colorimetric properties
Figure 2 shows the colour difference of the dyed fabric which increases with the increase of pH as well as the concentrations of CHPTAC. The result is in agreement with the fact that the cationiser helps in the bonding between tea dye and cellulose. Although samples dyes in pH 9 and pH 11 showed uneven shade which improved in case of pH 12 and pH 13. Sample 8 at pH 13 showed the best result (ΔE = 46.27) among all the samples. However, the mordanted dyed fabric shows a slightly better value compared to that of bleached dyed fabric but all the cationized dyed fabric (sample 1-8) have much higher values of ΔE .
CIELab, CIELch values were illustrated in the Figure 3 and Figure 4. The illustrations show the effect of pH and concentration of CHPTAC on the values of CIE L*, a*, b*, and CIE L*, c* and h. In case of L (Light-dark), it was found that as the pH increases, lightness decreases, indicating a more uptake of dye resulting in deeper shade. The CIELch value also showed similar lightness values as well as the chroma and hue values of different samples. Sample 7 of pH 13 showed 23.45 chroma and 67.65 hue values where bleached cotton showed 3.58 chroma and 88.01 hue though hue did not affect the whiteness as the chroma was very less.
In Figure 5, a*-b* plot shows that all the values were between red (+a) and yellow (+b). The bleached sample was near to center values (towards white). Bleached dyed and 3% alum dyed samples also showed average values. Among all, pH 9 and pH 11 showed comparatively lower coordinates than pH 12 and 13. In every pH, there is an increase of coordinates with a few exceptions. The highest redness value was 8.92 for sample 7 at pH 13. However, the trend line illustrated the uniformity of the values.
From the reflectance value, the colour strength values were determined which is illustrated on the figure for different pH with respect to different cationiser concentrations. The figure 6 shows that the K/S values were highest for pH 13 (Sample 8: 6.63) and lowest for pH 9 (Sample 1: 1.26) which also increases with the increase of cationiser amount from sample 1(20 g/L) to sample 8 (55 g/L). Due to lower and non-uniform K/S values at pH 9 and pH 11, further characterisations of dyed samples were conducted only for pH 12-13.
3.2 Dye exhaustion percentage
Depending on the concentrations of before dyeing and after dyeing, the exhaustion% of two different pH had been evaluated. It was clear that the K/S value profoundly depends on pH of cationisation. The figure 7 shows that pH 13 has better exhaustion than pH 12. Where every specific sample in both pH shows proportional relation with cationiser amount from sample 1 (25g/L) to sample 8 (55 g/L). The highest amount of exhaustion was 88.61% for sample 8 at pH 13 which is in agreement with the discussion based on computational analysis in later section. The lowest value was 31.4 for sample 1 of pH 12. However, bleached dyed and 3% alum mordanted samples have very poor exhaustion% (0.7475% and 3.607% respectively). The result indicates that the pH and concentration of cationiser have great impact on dye exhaustion thus colouration of textiles. The higher the pH the higher the percentage of dye exhaustion and again the higher the concentration of cationiser, the higher the percentage of dye exhaustion to a certain limit.
The IR spectra of bleached, mordanted and cationised cotton fabric were shown in Figure 8. The bleached cotton showed the O-H spectrum of cotton fabric at 3334.98 cm-1 which is also showed by mordanted tea dyed and cationised tea dyed fabric with little left (3335.02 cm-1 ) and right shift (3334.97 cm-1) respectively. The peak of 2899.70 cm-1 showed the (alkane) C–H stretching vibration, 1314.39 cm-1 was assigned to O-H bending vibration, 1427.48 cm-1 showed C=O vibration, 1160.70 cm-1 showed S=O stretching sulfone, 1030.08 cm-1 and 1054.34 cm-1 showed S=O stretching sulfoxide, 558.59 cm-1 and 663.05 cm-1 showed halogen chloro compound.
The canonized fabric shows relatively narrow O–H stretching peaks at 3334.97 cm-1, aliphatic C–H stretching at 2899.29 cm-1 and C–N stretching vibration at 1160.98 cm-1 . Moreover, the sharp peak of 1427.76 cm-1 refers to the existence of quarternary ammonium groups.
3.4 Colour fastness
The Table 1 shows that cationised dyed fabric samples (2-8) have better colour fastness to washing, rubbing, perspiration and light than bleached dyed or mordanted dyed samples. Colour fastness to washing for samples dyed at pH 13 are better than that of samples at pH 12. Moreover, with the increasing amount of CHPTAC at every pH. The fastness to staining was mostly excellent where colour fastness was moderate to very good. Rubbing fastness for pH 12 and 13 was also shown in the Table 1. The dry rubbing fastness values were good to excellent for all the samples where wet rubbing fastness was average to good. Here wet rubbing fastness was poor due to moisture content present in both dyed fabric samples and crocking cloth which increases the frictional coefficient. However, the bleached dyed sample showed also good to excellent fastness due to lower depth of colour, as indicated by K/S value. The perspiration fastness in Table 1 was found from moderate to excellent and colour fastness to staining are mostly excellent, particularly at pH 13. The fastness values prove that with the increasing the amount of cationization, bonding between tea pigments and cotton fibers through CHPTAC also increases, which was explained with DFT calculation in later section.
The light fastness of the dyed fabrics was good to very good for the sample 6-8 of each pH where the other samples 1-5 were moderate to good. The outcome proves to relatively stable chromophore group of the tea extracts.
Table 1 Colour fastness to washing, perspiration, rubbing and light of dyed fabric at different pH of cationisation and different CHPTAC concentrations. Three types of sample fabrics are shown in the Table 1, they are bleached cotton sample, mordanted sample and cationized sample (1-8).
|
Wash fastness
|
Rubbing fastness
|
Perspiration fastness
|
Light fastness
|
Sample
|
colour
|
Staining
|
colour
|
Staining
|
Dry
|
Wet
|
Dry
|
Wet
|
colour
|
Staining
|
colour
|
Staining
|
|
|
Bleached
|
3-4
|
4-5
|
|
|
4-5
|
4-5
|
|
|
3-4
|
4
|
|
|
4-5
|
|
Mordanted
|
3-4
|
4
|
|
|
4-5
|
4
|
|
|
3-4
|
4
|
|
|
5
|
|
|
pH 12
|
pH 13
|
pH 12
|
pH 13
|
pH 12
|
pH 13
|
pH 12
|
pH 13
|
1
|
3-4
|
4-5
|
3-4
|
4-5
|
4-5
|
4
|
4-5
|
3-4
|
3-4
|
4
|
3-4
|
3-4
|
4-5
|
5
|
2
|
3-4
|
4-5
|
4
|
4-5
|
4-5
|
4-5
|
4-5
|
4
|
4
|
4
|
3-4
|
3-4
|
4-5
|
5
|
3
|
4
|
4-5
|
4
|
4-5
|
4-5
|
4
|
4-5
|
4
|
4
|
4-5
|
4
|
4
|
4-5
|
5
|
4
|
4
|
4-5
|
4
|
4-5
|
4-5
|
4
|
4-5
|
3-4
|
3
|
4-5
|
4
|
4
|
4-5
|
5-6
|
5
|
4
|
4-5
|
4-5
|
4-5
|
4-5
|
4
|
4-5
|
3-4
|
4
|
4-5
|
3-4
|
3-4
|
4-5
|
5-6
|
6
|
4
|
4-5
|
4-5
|
4-5
|
4-5
|
3-4
|
4-5
|
3-4
|
3-4
|
4-5
|
3-4
|
3-4
|
5
|
5-6
|
7
|
3-4
|
4-5
|
4-5
|
4-5
|
4-5
|
3-4
|
4-5
|
3-4
|
3-4
|
4-5
|
4
|
4
|
5
|
5-6
|
8
|
3-4
|
4-5
|
4-5
|
4-5
|
4-5
|
4-5
|
4-5
|
3-4
|
4-5
|
4-5
|
4-5
|
4-5
|
5
|
5-6
|
3.4 pH of cationisation
As mentioned earlier, cationisation increases dyeability of cotton fabrics, although the appropriate pH for cationsation is the one of the most important factor, which is in agreement with the following computational study.
To reduce the computational cost, cellulose monomer (β-glucose) was used for calculation instead of cellulose. CHPTAC is bonded with β-glucose and this glucose-CHPTAC dimer then interacts with theaflavin at its two different positions to form a glucose-CHPTAC-theaflavin complex A and complex B (see Figure 9). To get insight into each type of the bonding with theaflavin in complex, the binding energies for glucose-CHPTAC-theaflavin complexes were calculated separately for A and B (see Figure 9) within DFT framework with B3LYP/3-21G basis sets, using the below equation.
From the above equations (1) and (2), it was found that the binding energy (BE) in between glucose-CHPTAC dimer and theaflavin in complex A is less, thus more stable than that in complex B. It can be explained by n-σ* hyperconjugation that is directly estimated using the second order perturbation energy from the NBO analysis. From QTAIM molecular graph of complex A and B, the strong interaction between glucose-CHPTAC dimers and theafalvin can be also understood. Although there are several interactions taking place between different donor and acceptor orbitals of CHPTAC and theaflavin, NBO shows that major channels for electron transfer in complex A is from lone pair orbital of chloride ion of CHPTAC to antibonding orbital of O1-H8 of theaflavin (nCl→σ*O1-H8) which initiate charge flows between two fragments (see Table 2). However the bonds between dimer and theaflavin were strengthened further by several other interaction like nO2→σ*C1-H2 and nO2→σ*C2-H4. Thus the electrostatic nature of bond between dimer and theaflavin in complex A is dominating (see Figure 9). But in case of complex B, no direct contribution of chloride ion was found, rather several interactions from lone pair orbitals of O3 and O4 of theaflavin to respective antibonding orbitals of σ*C2-H4, σ*C3-H6 and σ*C1-H2 of CHPTAC were significant (see Table 2) which is associated with covalent nature of bonds between dimer and theaflavin in complex B. Nevertheless, in both complexes A and B, the bonding between dimer and theaflavin are sufficiently strong which could be the possible reason for their higher colour fastness to washing and rubbing.
Table 2. The 2nd order perturbation energies from NBO analysis and binding energies (BE) in complexes A and B at the level of B3LYP/6-31G.
Complex, A
|
|
Complex, B
|
DE
|
Kcal/mol
|
|
DE
|
Kcal/mol
|
DE(nO2→σ*C1-H2)
|
3.16
|
|
DE(nO4→σ*C3-H6)
|
3.36
|
DE(nO2→σ*C2-H4)
|
4.20
|
|
DE(nO4→σ*C1-H2)
|
1.76
|
DE(nCl→σ*O1-H8)
|
29.39
|
|
DE(nO3→σ*C2-H4)
|
4.19
|
DE(nCl→σ*C5-H7)
|
3.45
|
|
--
|
--
|
DETotal, NBO
|
40.2
|
|
DETotal,NBO
|
9.31
|
DEBE
|
-24.30
|
|
DEBE
|
-9.65
|
From aforementioned discussion, it can be suggested that more probable structure is complex A than that of complex B as the former has higher stability (DEBE = -24.3 Kcal/mol). It is because of electrostatic nature of bonds resulting from the reaction between theaflavin and cotton when it is cationised (See Figure 10). Hydrated cotton has a very weak negative charge that affords almost no affinity towards theaflavin. So, more cationisation of cotton indicates more bond formations with available theaflavin from tea extract which have higher bond strength as well. And enhanced cationisation is only possible when there will be adequate amount of epoxypropyl tri-methylammonium chloride (EPTAC) present to take part in reaction (Figure 10). The more NaOH reacts with CHPTAC, the more EPTAC is formed to react with cellulose [31] and later forms bond with the theaflavin [32], [33]. Karnik et. al [34] argued that at higher pH, cellulose is also more accessible to react with EPTAC. So, percentage of dye exhaustion can be expected higher in alkaline environment, i.e. higher pH of cationisation of cotton.