Dyeing of Wool with Sappan Wood Natural Dye Using Metal Salts for Enhancement in Color and Fastness Properties

ABSTRACT In this study, we explore the dyeing properties of the extract of sappan wood natural dye for coloration of wool yarn to find the potential use of natural dyed wool yarn in the development of eco-friendly textiles. Different characterizations were used to know the chemistry of sappan wood dye and its interaction with metal salts and wool. Interestingly, we found that metal salts, which are called as mordants in reference to unmordanted dyed sample, play an important role in fixing the dye molecule on wool as well as in creating different shades with a variety of hues and tones. The typical color of fabric dyed with extracts of sappan wood without mordant varies from yellowish to red. Light yellowish shades were obtained in case of aluminum sulfate-mordanted samples, whereas yellowish shades were obtained in case of ferrous sulfate mordant. Light colorfastness of the premordanted dyed samples was relatively better than that of the unmordanted dyed samples. The wash fastness of all samples mordanted as well as unmordanted shows color change fairly good to good level, whereas the color staining on wool and cotton was found to be negligible.


Introduction
Synthetic colorants' major output from textile industries are hazardous and pose alarming threat to environment as well as human beings.Some azo dyes release aromatic amines that are carcinogenic, presenting a risk of cancer to users of articles dyed with these dyes as well as causing allergy.Thus, they have restricted use in some countries.In this regard, many processes and products used have been questioned and reevaluated due to social and environmental damages associated with them, and new alternatives for production and consumption have been explored by designers and researchers (Niinimäki and Hassi 2011).Since natural dyes are generally less allergenic and toxic than synthetic dyes and generate wastewater that can be treated by biodegradation, this family of colorants has been increasingly contemplated as an environmentally less impactful alternative to certain synthetic dyes.Natural dyes not only find their use in textile coloration but also have remarkable applications in other areas, such as antimicrobial (Adeel et al. 2022;Bechtold et al. 2003;Flax et al. 2022;Khan et al. 2012;Mansour andAli 2021, 2023;Shahid et al. 2012;Yusuf et al. 2015).Natural dyes obtained from renewable sources are eco-friendly to human health as well as for environment (Bhattacharya and Shah 2000;Samanta and Agarwal 2009;Sewekow 1988).These materials are biodegradable and are used for different purposes (Grifoni et al. 2009;Lee, Hwang, and Kim 2009;Singh et al. 2005).However, there are some problems associated with the use of natural dyes in dyeing, such as low exhaustion of dyes and poor fastness of dyed fabrics (Fatemeh et al. 2012).Metal salts called mordants are incorporated in textile dyeing to overcome these problems.Mordants form a ternary complex with the dye molecule and fiber, which results in good fastness property as well as depth of color into the fiber (Bhattacharya and Shah 2000).
Sappan wood is widely distributed in India, Burma, Thailand, and China (Flora of China 1988).In India, it is commonly called as Patang and mainly present in Madhya Pradesh, Orissa, West Bengal, and Southern parts of country.According to the literature survey, there are various studies on dyeing with sappan wood on different substrates.Plyosai and Nattida used the water and ethanol extracts of this dye to study dyeing properties on cotton and silk fabrics.Isolated and characterized the major dyestuff component from this plant.Lee et al. (2008) fabricated a nontoxic natural dye from an extract of sappan wood using microemulsion method.Besides textile coloration, the dried heartwood of this plant has been used as a traditional ingredient of food or beverages and in the treatment of various diseases like tuberculosis, diarrhea, dysentery, skin infections, and anemia (Sireeratawong et al. 2010;Toegel et al. 2012)).In this study, the bark of sappan wood was used as a natural dyeing agent for textile coloration (Figure 1a).The major coloring component of sappan wood is brazilin, but its oxidized product brazilein is also reported in this plant.Brazilin (C.I. 75280) shown in Figure 1b is a homoisoflavonoid compound (Cardon 2007), whose molecular weight is 286.98 (Lillie 1977).Brazilin is used as red dye for histological staining (Bae et al. 2005) and is readily oxidized by contact with atmospheric oxygen or other chemical oxidants to brazilein with a loss of two hydrogen atoms to form a carbonyl.The purpose of the present study is to explore the dyeing, fastness, and colorimetric properties of sappan wood natural dye.As synthetic dyes are harmful, this research was aimed to show the alternate source of wool dyeing by natural dyes.In addition to dyeing, fastness, and colorimetric properties, the detection of color components in dye and the chemistry of interaction of dye, fiber, and mordants were also studied by UV-visible and FTIR spectral studies.So far, less research has been done on wool dyeing with sappan wood bark; this research will be interesting in providing mankind a good clothing and floor covering.

Extraction of dihydropyran natural colorant from sappan wood
The raw material of powdered sappan wood bark natural dye was taken in an aqueous solution using M:L (material to liquor) ratio 1:20 and kept for 12 h.The aqueous solution was heated three times at simmering point (91-93°C) for 20 min with occasional stirring, cooled, and filtered through a clean cotton cloth.The filtrate obtained was extracted from liquid sappan wood bark dye and was used for dyeing wool samples.The extracted samples were analyzed using a UVvisible spectrometer.

Spectral studies
The maximum absorbance wavelength (λ max ) of the extracted dye from sappan wood bark fruit was evaluated in aqueous solution using Perkin Elmer Lambda-40 double-beam UVvisible spectrophotometer.The UV-visible spectrum was obtained in the region 200-700 nm.Fourier transform infrared spectroscopy (FTIR) of samples was recorded on a Bruker Tensor 37 FT-IR spectrophotometer ranging from 4000 to 500 cm −1 .Discs were prepared by cutting samples of both unmordanted and premordanted dyed woolen yarn into fine pieces and grinding them with KBr, which was used as internal standard.

Mild scouring of woolen yarn
Before the application of mordants, woolen yarn samples were soaked in nonionic detergent solution (5 mL/L) as pretreatment to enhance surface wettability.

Mordanting process
Woolen yarn samples were mordanted by premordanting method using several mordants such as 10% (o.w.s.) aluminum sulfate and 5% (o.w.s) ferrous sulfate.Mordanting was done for 60 min with M:L ratio of 1:40 at 90°C.Mordanted woolen yarn samples were rinsed with tap water to remove superfluous mordants.

Dyeing
Dye stock solution was prepared by dissolving 93 g of sappan wood red dye in 3.72 L of water.The dyeing with red natural dye was performed at seven concentrations, 1, 3, 5, 8, 10, 15, and 20% (o.w.f).The dyeing experiments were carried out using an M:L ratio of 1:40 in separate dyeing baths with manual agitation at neutral pH in different concentrations of dye at 90°C for 60 min.In order to get uniform dyeing, samples were manually stirred regularly (after every 5 min).Finally, dyed samples were washed with 5 mL/L nonionic detergent (Safewash Wipro), rinsed with tap water, and dried in shade.

Evaluation of color characteristics
The CIE L*a*b* coordinates and color strength (K/S) values of dyed and mordanted dyed samples were evaluated using Gretag Macbeth Color-Eye 7000A Spectrophotometer connected to a computer with installed software of MiniScan XE Plus.The color strength (K/S) in the visible region of the spectrum (400-700 nm) was calculated based on the Kubelka-Munk equation (1).
where K is the adsorption coefficient, R is the reflectance of the dyed sample, and S is the scattering coefficient.
Chroma (c*) and hue angle (ho) were measured using following equations:

Fastness testing
The light fastness of dyed woolen yarn samples was conducted on digi light NxTM having watercooled Mercury Blended Tungsten lamp as per test method AATCC 16E-1993 (2004), similar to ISO 105-B02:1994 (Amd.2:2000).The wash fastness of dyed woolen yarn samples was measured in Launder-o-meter as per the ISO 105-C06:1994 (2010)-Test No. B1M specifications.The samples were assessed for staining on white adjacent fabrics (wool and cotton).Dry and wet rub fastness of the dyed woolen yarn samples were tested using a Crock-meter as per Indian standard IS 766:1988 (Reaffirmed 2004) based on ISO 105-X12:2001 by mounting the fabric on a panel and giving 10 strokes for both dry and wet rub fastness tests.

Spectral studies
The UV-vis absorption spectroscopy is a vital tool, which provides information of organic compounds on their water solubility, stability, and UV absorption characteristics which are closely related to the application properties.In this section we will discuss the UV-vis absorption spectra shown in Figure 2 of red dye extract of sappan wood bark.Brazilin has two small conjugated systems of six atoms, so only low-wavelength (UV) light is absorbed and the molecule is colorless when pure.On the other hand, the oxidized form, i.e., brazilein, the number of π electrons increases to 14 and resulted the absorption of light at longer wavelengths and the molecule becomes visibly red colored.As depicted in Figure 2, the spectra shows three peaks: two peaks at 254 and 280 nm are in visible region, which correspond to the presence of polyphenols, and the third peak at 540 nm, which corresponds to the detection of brazilin and brazilein (Lioe, Adawiyah, and Anggraeni 2012).
The FTIR spectra of simple wool fiber, red dye extract of sappan wood, dyed wool fiber, mordanted wool fiber, and mordanted dyed wool fiber are shown in Figure 3a-e.TheFTIR spectra of sappan wood dye (Figure 3b) show peaks at 3273, 1633, 1504, and 1064 cm −1 responsible for the -OH stretching, -C=O stretching, -C=C, and -C-O ester present in the components of this dye.Figure 3a shows the FTIR of simple wool fiber; it is clearly shown in the figure that the absorption peaks are mainly due to peptide bond of polypeptide chain (Khan et al. 2011).IR spectral bands as I, II, and III amide bands arise due to the oscillations of atoms in peptide bond (Bae et al. 2005).Stretching and bending vibration of N-H group of amides I appear at 3280 cm −1 and 1640 cm −1 , respectively.The C-O stretching vibration band appears in the region between 1630 and 1670 cm −1 , probably overlapped with amide I vibration (N-H bending) (Ebrahimi and Gasht 2015;Wojciechowska, Włochowicza, and Wesełucha-Birczynska 1999).It is seen in Figure 3c-e that there is alteration of intensity and shifting of peaks in case of C-N stretching frequency of amide I and amide III bands dyed wool fiber at 3270 cm −1 , 1239 cm −1 , and 1065 cm −1 , which indicates the involvement of amine groups in the interaction between fiber, mordant, and dye molecules.Low-intensity peaks were found in premordanted dyed woolen yarn samples.The samples premordanted with alum, ferrous sulfate, and stannous chloride mordants show shifting of peaks with amide III at 1230 cm −1 and CN stretch of amide III band at 1150 cm −1 , which is attributed due to the formation of complex between metal ions and wool fiber.

Colorimetric properties
Table 1 shows the color parameters, viz., L*, a*, b*, c*, h°, and color strength (K/S) values of woolen yarn dyed with sappan wood red dye extracts.It is evident from the table that unmordanted woolen yarn samples dyed with sappan wood extract showed lower dye uptake than mordanted samples.It is clear from the table that as the concentration of dye increases from 1% to 20%, the K/S values also go on increasing, this is attributed to concentration gradient of dye on fiber via adsorption.Increase in dye bath concentration leads more dye transfer to the fabric, and thus a higher apparent depth of color occurs.Moreover, as we see in low concentrations of dye like 1-5%, the K/S values are low and L* values are high, which means that at low concentration of dye, the shades are lighter and bright.
On imparting the mordants to sappan wood dye, there is also alteration in color values.Darker shades were found in case of ferrous sulfate-mordanted samples, which means the samples have higher K/S and low L* values.On the other hand, alum mordanted samples show bright and light shades.Iron shows strong coordinate tendency which forms strong fiber-metal-dye complexes (Bhattacharya and Shah 2000;Bukhari et al. 2017) whereas alum mordants form weak coordination complexes with dye, although they tend to form strong bonds with the dye but not with the fiber; so, they block the dye and reduce the dye interaction with fiber (Bhattacharya and Shah 2000;Bukhari et al. 2017).The mechanism of interaction between mordants, dye, and wool fibers is shown in Figure 4.It is clear from the figure that metal salts form the bonds with the functional groups -OH and -C=O of sappan wood dye, on the one side, and, on the other side, it forms bonds with -NH and -C=O groups of woolen yarns.This results in the formation of ternary complex and is responsible for the high K/S values as well as good fastness properties.
Figures 5 and 6 represent the a*-b* plot of unmordanted and premordanted woolen yarn samples of sappan wood dye.It is evident from Figure 5 and Table 1 that all unmordanted dyed samples show color toward the yellow zone of color space diagram and all the samples show higher L values, which means lighter shades.On the other side, aluminum sulfate and ferrous sulflate mordants show color values in between the yellow and red zone of color space diagram.This shifting of color coordinates in   case of mordanted samples is attributed to the formation of ternary complex between dye, metal, and wool fabrics (Bhattacharya and Shah 2000).

Dye fastness evaluation
Table 2 shows the results of evaluating the fastness to washing, crocking, and light fastness based on the irradiation time according to the one of time of washing and the process of rubbing per sample 10 times.Table 2 shows that, on the application of mordants, there is increase in light fastness ratings of 5  on blue scale of mordanted dyed samples compared to unmordanted dyed samples, which have light fastness ratings of 5 on blue scale.The reason is the chelating ability of metals, which results in coordination complex between dye, metal, and wool fiber.The wash fastness of unmordanted and premordanted woolen yarn samples was found to be within the range of 3-5, which means fairly good to very good levels except one sample of iron mordant at low concentration of dye where it is poor.The results show that all samples mordanted as well as unmordanted show color change at fairly good to level (3-4), whereas the color staining on wool and cotton was found to be very good (5).The results of color fastness to crocking were found to be within the range 3-5, which means fairly good to very good levels in all dyed yarn samples.At low concentration of dye, the woolen yarn samples show better wash and rub fastness properties.This is attributed to the fact that at higher dye concentration, there is more leeching of color from dyed wool samples (Cotton and Wilkinson 1972).

Conclusion
The purpose of this study is to realize that dihydropyran-based natural colorants from sappan wood bark extract to act as alternative to synthetic dyes.Specifically, the combination of metal mordants with dye was selected to investigate the changes in color spaces of dyed wool and mordanted dyed wool, the possibility of wool dyeing, and the fastness to dyeing.Dyeing studies of the extracted compounds on wool revealed that the application of mordants and their various combinations changes colorimetric and fastness properties.It was found that higher color strength was obtained in case of mordanted woolen yarn samples as compared to woolen yarn samples.Light yellowish shades were obtained by aluminum sulfate mordant samples at high concentrations, whereas vibrant red and dark yellowish shades were obtained by stannous chloride and ferrous sulfate mordants, respectively.The light fastness results show that premordanted woolen yarn samples have better light fastness than unmordanted dyed woolen yarn samples.The wash fastness of all  samples mordanted as well as unmordanted shows color change fairly good to good level, whereas the color staining on wool and cotton was found to be negligible.At low concentration of dye, the woolen yarn samples show better rub fastness properties when the samples were subjected to crocking.Based on the observations and results, we can say that sappan wood dye can be a good source of natural dyes used textile industries for dyeing textiles.Moreover, obtained eco-friendly yarns with great tone and hue will benefit mankind in wearing environment-friendly clothes and designing carpets for home use.

Figure 1 .
Figure 1.(a) Sappan wood bark and dye.(b) Chemical structures of coloring component in sappan wood.

Table 1 .
Colorimetric properties of unmordanted and mordanted dyed wool samples.

Table 2 .
Fastness properties of unmordanted and mordanted dyed wool samples.