Saminathan Ratnapandian

10


0
400 450 500 550 600 650 700
Wavelength (nm)
Figure 3.6 Reflectance curves of cotton fabrics dyed with A. catechu


Reflectance curves for A. nilotica
45
40

Reflectanc (%)
35
30
25
20
15
10
5
0
400 450 500 550 600 650 700
Wavelength (nm)
Figure 3.7 Reflectance curves of cotton fabrics dyed with A. nilotica
The change in colour according to mordant is consistent with the polygenetic behaviour of some natural dyes. Such dyes are claimed to yield up to 16 colours from a single
natural dye depending on the mordant used [32]. This has been attributed to the formation of metal complexes (chelates) by the functional groups (tannins) present in the dyes derived from Acacia family [31, 94, 115, 135, 136]. Although a simple depiction of this interaction is shown in Figure 3.8, the polymeric characteristic of poly- phenolic dyes precludes exact determination of the features of the metal complex [115].

Figure 3.8 Schematic of metal complex formation [115]

Norkus et al. [137] outline the interaction between textiles and heavy metal ions to occur mainly by:

1. 
   intercalation into the fibre matrix
   
2. 
   adsorption onto the fibre
   
3. 
   formation of chemical bonds with reactive groups of material; and
   
4. 
   formation of complexes with dissolved hydrolysis products.
   

Figure 3.9 Equatorially coordinated copper with cellulose [138]

Figure 3.9 is a lattice structure with equatorially coordinated metal (copper) acting as a bridge between cellulose molecules, as proposed by Ajiboye and Brown [138]. Such a structure is possible when the cellulose molecules are loosely held in a gel. When cellulose exists in a relatively rigid state, as in cotton, only partial complexes can be formed. The inherent polymeric character of condensed tannins (Figures 2.3 and 2.4) favours the formation of multi-dentate three-dimensional isomeric structures.
Consequently when cotton is dyed using a condensed tannin based dye in conjunction with metallic mordants, a highly complicated structure that incorporates metal, dye and fibre evolves. Under these conditions, although the presence of metal (Mⁿ⁺) can be detected, definitive characterisation of the final resultant structure is inhibited.
However, differences in structure become evident by changes in the shade obtained.
The role of the metal was highlighted when different dyes in combination with the same metal yielded near-equivalent shades. This can be readily identified by the almost parallel reflectance curves in Figures 3.6 and 3.7 for the two dyes in combination with iron (II) sulfate. On the other hand, the same dyes in combination with copper (II) sulfate yielded reflectance curves that are different due to dissimilar dye-fibre-mordant complex structures. This indicates that the metal is responsible for the ultimate structure of the dye-metal-fibre complex.

2. ATR–FTIR results

The FTIR spectra for the two dyes, Caspian and Thar, are given in Figure 3.10. Table
3.4 lists the peak assignments for the spectra. The combination of peaks around 767 and 1500 cm^–1 can be attributed to aromatic ring breathing mode and CH out-of-plane deformation with two adjacent free hydrogen atoms respectively, indicating the prominent presence of procyanidin [139]. The other peaks are consistent with those reported for polyphenols [140]. A larger number of peaks are identified between the wavelengths of 1600 to 1280 in the spectrum for Thar enabling a distinction to be made between the two dyes.
Table 3.4 FTIR band assignment