Broadening Color Shade Range of Rubia tinctorum L. Natural Colorants on Wool Fibers via Combination of Metal Mordants: Color Characteristics and Fastness Studies

ABSTRACT This study aims to extend color shade range of Rubia tinctorum L. (Dyer’s madder) natural dye on wool textiles via binary combination of Al, Sn, Cr, Fe, Cu, Ni, Co, Zn mordants. Wool yarns were premordanted, dyed with 75.0%owf madder dye, and characterized for their color strength, colorimetric parameters, and color fastness. Nature of single and binary metal cations substantially affected color shades, color strength, and fastness properties due to different interactions of dye, metal mordants, and wool polymer chains. This resulted in the development of novel color shades having different lightness, enhanced color strength, and color fastness properties. Combination of mordants had a synergistic effect on dye uptake which enhanced color strength values. Moreover, with the aid of binary metal mordants, colors of some legally restricted mordants like chromium can be reproduced which is important from ecological viewpoint. Binary mordanted samples exhibited superior color fastness. Results represent that this viable method can extend the color gamut of madder natural dye on wool with satisfactory color fastness and lessen the hazards associated with the use of some toxic metal mordants. SUMMARY A simple method was used to extend the color shade range of Dyer’s madder on wool yarns Wool yarns were premordanted with binary combinations of different metal mordants and dyed with Dyer’s madder New color shades were obtained on wool yarns Binary mixed metal mordanted dyed wool showed improved dye uptake and color fastness

Nevertheless, compared to synthetic dyes, the color gamut of natural dyes is still narrow being limited to several tones and hues (Gulrajani, Srivastava, and Goel 2001) which necessitates the exploring facile, viable, and economic routes to extend the color shade range of natural dyes. Some recent studies have addressed this issue by using a combination of metal mordants in dyeing with several natural dyes Khan et al. 2015;Khan, Islam, and Mohammad 2017;Rather et al. 2018Rather et al. , 2020aRather et al. , 2020bSafapour and Rather, 2022).
It is expected that binary combination of metal mordants having different degrees of complex/ coordination ability would effectively assist in the extension of the color gamut of anthraquinonebased madder dyes on wool fibers. To the best of our knowledge, up to date, there is no systematic study in this context. Therefore, for the first time, 28 binary combinations of 8 metal salts of aluminum, tin, chromium, iron, copper, nickel, cobalt, and zinc were applied on wool fibers through premodanting procedure with the aim to develop a simple route for extension of color shade of madder dye on wool fibers; then color characteristics (L*, a*, b*, C*, h°, and K/S) and fastness properties (light, washing, and dry/wet rubbing) of dyed wool were assessed. The results have been discussed in terms of the possible relationship between dye structure, the complex formation (coordination) ability of different metal ions, and the resultant wool-metal-dye interactions.

Materials and methods
Commercial scoured 100% wool yarn (20/4 Nm), used as pile yarn for production of hand-made carpet, was used. Premium madder (Rubia tinctorum L.) root fine powder as source of natural dye was purchased from local market in Yazd, Iran. Different metallic salts such as aluminum sulfate (Al 2 (SO 4 ) 3   wool. All chemicals and reagents used were of laboratory reagent grade. Distilled water was used for preparation of solutions, mordanting and dyeing tests.

Mordanting procedure
Prior to mordanting, wool yarns were scoured by a solution containing 5 g/L nonionic detergent at 60°C for 30 min, and liquor ratio (L:R) of 40:1, and dried at room temperature. Next, wool yarns were premordanted in an open beaker using different metal salts (8 single and 28 binary mixtures) using L:R of 50:1 at boil (93°C) for 60 min. Afterwards, mordanted samples were removed, rinsed thoroughly with water several times to remove unfixed surface mordants, dried at ambient temperature, and used in dyeing process. Table 1 shows detailed information about the type, ratio, and code of mordanted samples.

Dyeing procedure
Dyeing of wool yarns was performed in weak acidic medium (pH 4 adjusted by acetic acid), L:R 40:1, 75.0%owf (on weight of fiber) madder as natural dye. The weighed crude dye powder was added to dye bath at room temperature and kept at boil for ca. an hour to effectively extract natural dyes into solution, then cooled down to room temperature. Next, the mordanted wool yarns were introduced into the extracted dye solution (dyeing bath) and the temperature was raised to boil (ca. 93°C) within 30 min and kept for 60 min at boil under continuous stirring until dyeing was completed. Finally, the dyed yarns were cooled down, removed, rinsed several times to remove unfixed surface dyes, air dried, and analyzed.

Color parameters assay
The colorimetric values of dyed samples were determined using a surface reflectance color-eye XTH spectrophotometer, X-Rite Inc., using D65 illumination and 10° standard observer. The spectrophotometer was equipped with software which were able to automatically calculate CIEL*a*b* colorimetric parameters as well as color strength (K/S). The colors are given in CIEL*a*b* coordinates: L* corresponds to the brightness (100=white; 0=black), a* to the red -green coordinate (positive=red; negative=green), b* to the yellow -blue coordinate (positive=yellow; negative=blue), C* to color purity or vividness -dulness (100=vivid; 0=dull), and h 0 to hue angle. The color strength (K/S) is directly proportional to the concentration of the dye within the fibers (Sadeghi-Kiakhani et al. 2020). For each sample, four individual measurements were performed, averaged, and reported.

Color fastness assay
Wash fastness was measured using ISO 105 C06 C2S:1994 (E) standard method. The washing was conducted for 30 min at 60°C, rinsed with cold water, air dried, and analyzed with gray scale. The dry and wet rub fastness test was performed according to ISO105-X12:1993 (E) standard method using a Crockmeter. The staining on the white test cloth was evaluated according to the gray scale. Light fastness test was evaluated according to ISO 105 B02:1988 (E) standard method using xenon arc lamp. Irradiated samples were then analyzed with a blue scale. Figure 2 shows the effect of single metal mordanting on color strength values of madder dyed wool yarns. It is evident that metal mordanting play a key role in dyeing and colorimetric results; that is, mordants not only increase the amount of dye uptake by wool fibers but also affect the shape of the K/S spectra differently. Fe, Cr, and Cu mordants almost affect all visible region and are more effective than other mordants. These behaviors is related to the type of mordant and the extent and degree of its complex forming ability with dye molecules and wool polymer chains. In terms of K/S values, Sn has the best performance (K/S = 27.09). Alizarin as the main colorant of the madder lacks dissociable groups like sulfonic acid or carboxylic acid moieties, and therefore the ionic interaction between dye and wool polypeptide chains is very low particularly in acidic dyeing medium. The existence of metal cations on the mordanted wool can substantially improve dye uptake by wool due to the coordination bonds formed between dye and metal mordants, thus resulting in higher color strength values. The possible interaction mechanism between dye and metal mordanted wool dyed in acidic medium has been proposed in Figure 3 as per Fain, Zaitsev, and Ryabov (2004).

Color strength (K/S) values
Moreover, according to Figure 4, it is seen that some samples mordanted with binary combination of metal mordants possess unexpectedly higher K/S values than their parent single mordants. According to theoretical calculations, it is expected that the color strength values in binary combinations lie between K/S of single mordants; however, in some cases, the color strength of mixed metal mordanted samples is higher than the average values. For example, color strength of Sn (1.5%)+Zn (2.5%) is 28.07 which is higher than Sn (3%) with K/S = 27.09 and Zn (5%) with K/S = 24.91. Although diffusion coefficients of the metal cations are not equal and their absorption rate will differ from each other, K/S value of the binary combinations is not even between single states, suggesting that there are some synergistic effects in binary combination of the metal mordants which lead to better dye uptake by wool fiber. This statement is valid in majority of binary mixed mordants. It can be concluded that there is possibility to combine different single mordants with the aim to improve the color strength of the final dyed yarns. This synergistic effect can be exploited to reach higher color strengths without using extra metallic salts which are usually toxic for environment. This finding demonstrates that application of binary combination of metal salts is more economical than single mordants.

Colorimetric values
Colorimetric values of 8 single and 28 binary mixed metal mordanted dyed samples as well as images of dyed samples are presented in Table 2. It is clearly seen that all color parameters are affected by single and binary combination of metal mordants and 36 different color shades are developed. Indeed, the nature and amount of the metal mordant substantially affect the color shade of the dyed samples. This can be ascribed to the extensive interactions of metal mordants having different coordination ability with wool polypeptide chains and numerous anthraquinone-based natural colorants of madder. Binary mixed metal mordanted samples exhibit extensive variations in color coordinates and develop new color shades and tones. In single metal mordanted samples, the lowest and highest lightness values (L*) are related to Fe (17.78) and Sn (31.84), respectively. Generally, compared to unmordanted wool, some single metal mordanted samples including Cr, Cu, Co, Ni, and Fe exhibit lower lightness values, while some others      like Al and Sn show higher L* values. Among the mordants used, Al, Sn, and Zn salts are colorless while Cr, Fe, Cu, Ni, and Co salts are colored. Interestingly, mordanting with colored transition metal cations such as Cr, Fe, Ni, Cu, and Co leads to lower lightness values (darker shades). It seems that absorption bands of the metal cations in the visible region combine with the absorption bands of the dyes, resulting in the broadening of the absorption peaks, thus absorbing more wavelengths of lights and resulting in darker colors with lower lightness values. However, colorless metal cations such as Al and Sn slightly increase lightness and brightens color shades. Accordingly, in binary mixed metal mordanted samples, the lowest lightness value (18.31) is related to Fe (2.5%) + Co (2.5%) combination, both of which are colored cations and the highest lightness (33.35) is related to Al (2.5%) + Sn (1.5%) mixtures, both of which are colorless cations. This observation is in accordance with previous hypothesis observed in single metal mordanted samples. Generally, it can be concluded that whenever both of the cations belong to the colored cations, the color lightness would be low and the resultant shade would be darker; similarly, whenever both cations are colorless, the obtained shade would be brighter and possess high lightness value. a*-b* color coordinate plots of single and binary mixed metal mordanted wool yarns are shown in Figure 5. It can be clearly seen that a large variety of color shades located in yellow-red region are obtained using single and binary mixtures of metal mordants in wool dyeing with madder dye. Indeed, the type and quantity of metal mordants in a particular combination greatly assist in the development of a wide range of red color shades on wool yarns with different hues and tones. Generally, the presence of Sn and Al cations in the mixture leads to brighter shades with high yellow and red components, while the presence of Fe and Cu usually darkens color shades and decreases a* and b* values. These results indicate that the desired color shades can be simply obtained by manipulating dyeing auxiliaries (metallic salts, their types and ratios). Another interesting result from Figure 5 is the possibility of color matching of Cr mordanting by Fe+Cu combination. As previously discussed, Cr is a hazardous pollutant, that its usage is nowadays restricted by legal communities. To obtain its peculiar color shade, the combination of Fe and Cu can be used that both of which comparatively have lower toxicity than Cr.

Fastness properties
Durability of color on textiles in different end-use conditions so-called "color fastness" is an important factor to be assessed to ensure the quality of the colored products. In this study, three important fastness tests, i.e., light, washing (staining on cotton fabric, staining on wool fabric, and color change), and rubbing (dry and wet), have been assessed. Color fastness ratings for all 75.0%owf madder dyed samples (unmordanted, single metal mordanted, and binary mixed metal mordanted) are shown in Table 3. It is evident that compared to unmordanted wool, all mordanted samples exhibit higher light fastness values. Generally, due to low stability of natural chromophores, the light fastness ratings of most natural dyes are weak to moderate. Compared to unmordanted wool, mordants have improved the durability of madder natural colorants against light and all the mordanted samples showed improved light fastness ratings of 6-7. In this study, the nature of the metal mordants and their interactions between wool, metal salts, and dye molecules are the factors affecting light fastness. In fact, metal coordination with dye molecules reduces the electron density on the dye and results in higher resistance against photochemical reactions. Actually, when a colorant absorbs light, it gains energy and moves to higher energy levels. The excited states may degrade the dye molecules due to photochemical and photo-oxidation reactions. Metal complex formation with the dye structure quenches the excited states and improves the light fastness of dye molecules (Cristea and Vilarem 2006;Manian, Paul, and Bechtold 2016;Safapour and Rather, 2022). Among single metal mordants, some of them like Cr, Fe, and Cu have better performance in the improvement of light fastness of madder dye; however, binary combinations of these mordants give much better results, e.g., Fe+Cu exhibits excellent light fastness rating of 8. Overall, majority of binary combinations demonstrate very good light fastness ratings of ≥6. These findings indicate the fact that metal mordants have a significant role in the assessment of the light fastness of madder colorants and binary mordanting gives rise better results.
From Table 3, it is seen that depending on the type of the mordants, the dyed samples possess different wash fastness ratings ranging from moderate rating of 2 to excellent rating of 5 on gray scale. Among the single mordanted samples, Cu and Cr exhibited the highest washing fastness (rating 4) while among the binary metal mordanted samples Al(2.5%)+Cr(0.5%) possessed the highest rating of 4-5 on gray scale. The fastness ratings to dry and wet rubbing for all color shades with single and binary metal mordants from Table 3 show a good to very good ratings of 3-5 on gray scale. Generally, dry rubbing fastness for binary metal mordanted samples is almost the best. There is no single mordanted sample to have dry rubbing fastness rating of 5 while there are some binary combinations such as Fe+Cu, Fe +Sn, Fe+Cr, and Cu+Cr which exhibit excellent rubbing fastness ratings of 5.

Conclusion
In this study, the potential of eight metal salts and their binary combinations have been investigated to extend the color shade of madder natural dye on wool yarns. Premordanted wool yarns were dyed with Rubia tinctorum L. root extracts as natural colorant. Metal mordanting resulted in development of different color shades with improved color strength (dye uptake) and color fastness ratings. In binary combination of metal mordants, although the concentration of each cation was half of the single mordanting, same or better results were obtained, indicating that the binary combination of metal mordants had a synergistic effect on the color strength of the dyed samples. For example, Sn+Zn binary mordanted samples exhibited higher K/S values compared to wool premordanted with parent single mordants. In addition, a*-b* plots revealed that binary combinations of the cations substantially developed new color shades. The presence of some cations like Sn generally increased yellowness, redness, and lightness of color while some others like Fe usually decreased these values. It is also observed that the manipulating mordant types and their combination can be used as a tool for ecofriendly color matching. For example, a combination of Fe and Cu can successfully color-match Cr mordanted samples; in this way, the dyer can replace the combination with chromium mordants which are legally restricted due to its high toxicity. Furthermore, investigation of the correlation between lightness and metal mordant types revealed that colorless mordants such as Sn and Al usually resulted in higher lightness values while colored mordants such as Co, Ni, Fe, Cr, and Cu generally create duller colors with low lightness values. Finally, binary mordanted samples generally possessed enhanced color fastness, particularly light fastness. These findings indicated that the application of binary combination of metal mordants could be a suitable, economical, and eco-friendly method to surpass the limitation of natural dyes having comparatively narrower color shade ranges.