Effects of Plant-Based Bio-Mordants and Metallic Mordants on Washing Fastness and Sun Protective Quality of Madder Dyed Silk

Xinqi Yin^a1, Nengjie Pan^a1, Tao Huang^a, Khai Ly Do^b,c, Asim Mushtaq^d*, Miao Su^a,c*

^aCollege of Textile Science and Engineering (International Silk Institute), Zhejiang Sci-Tech University, Hangzhou 310018, China

^bSchool of Materials Science and Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, China

^cShengzhou Innovation Research Institute, Zhejiang Sci-Tech University, Shaoxing 312369, China

^dInstitute for Intelligent Bio/Chem Manufacturing (iBCM), Zhejiang University-Hangzhou Global Scientific and Technological Innovation Center, Hangzhou 311200, China

* Corresponding authors’ emails: sumiao@zstu.edu.cn, asimmushtaq@zju.edu.cn

¹ These authors contributed equally.

Abstract

In a ground-breaking study, madder dye extract and innovative mordants converge to create an eco-friendly dyeing revolution for silk, unlocking enhanced UV protection. Through FTIR and UV-Vis spectroscopy, the presence of potent anthraquinone components is revealed, while FTIR and XRD analyses confirm the silk's integrity amidst mesmerizing transformations observed via SEM. Remarkably, three samples achieve an impressive UPF greater than 39, ensuring robust defence against harmful rays. This research not only champions the sustainable use of natural materials but also presents a thrilling alternative to traditional metallic mordants, proving that nature's power can redefine the future of functional textiles.

Keywords: Madder, natural mordant, eco-friendly, UV protection, functional Silk

1. INTRODUCTION

Nature provides a plethora of plant-derived extracts obtained from various sections of plants, including roots, flowers, fruits, seeds, and leaves. The organic molecules present in these botanical extracts make them suitable for fabric dyeing as they offer the desired hues [1]. Natural dyes also add functional values to the coloured substances, for instance, antimicrobial, insect-repellent, antioxidant, and sun protective performances [2]. Significantly, madder (Rubia cordifoliaL.) has been a prominent organic colourant throughout history, known for its ability to produce many colours of red because of the presence of anthraquinone molecules, such as purpurin, alizarin, and rubiadin [3].

Bio-mordants are introduced to the process of dyeing with natural dyes to augment the intensity and durability of colours, while also minimising the need for hazardous metallic mordants. Tannins, which are organic molecules that precipitate protein, are mostly identified as hydrolysable tannins, such as ellagitannins and gallotannins [4]. Tannins serve as a bio-mordant by enhancing the affinity of colourants to textile fibres by their strong binding to fibres via their hydroxyl (–OH) groups [5]. Nevertheless, tannins yield distinct colours that potentially alter the ultimate shades of dyed textiles.

The present study examines the impact of various bio-mordants on the dyeing and sun protective performance of madder dye extract when applied to silk. Myrobalan, gallnut, and pomegranate peel extracts are utilised as bio-mordants due to their abundant tannin contents. To conduct a comparative analysis on the efficacy of natural and chemical mordants, the dyeing procedure utilises alum and copper mordants. The colouring characteristics of silk are assessed based on its K/S, colour coordinates, and washing fastness. Their sun protective capability is determined by UPF and UV transmittance rate. This study proposes the use of bio-mordants instead of chemical mordanting agents in green textile dyeing.

2. EXPERIMENTAL METHODOLOGY

2.1. Materials

Madder roots, dried pomegranate peels, myrobalan, and gallnuts were purchased from a Chinese herbal store (Hangzhou, China). Acetic acid (99.7 %, v/v), potassium aluminium sulphate [K₂SO₄Al₂(SO₄)₃.24H₂O], and copper (CuSO₄.5H₂O) were supplied by Macklin (Shanghai, China). The materials were used as they were supplied. Standard silk (white colour) was provided by Tianxia Silk Co. Ltd. (Hangzhou, China).

2.2. Methods

2.2.1. Madder dye extraction

The dried madder roots were first ground into powder and added to ultrapure water in a 1:10 (w/v) ratio and heated at 80 ºC for 120 min while ultrasonic waves were running concurrently. Afterwards, filter paper sheets were employed to acquire the pure madder dye solution.

A part of the prepared madder dye solution was pre-frozen at -80 ºC for 24 h, followed by a freeze-drying process at -47 ºC for 71 h. The lyophilized dyestuff was kept for an FTIR test.

2.2.2. Silk mordanting

The mixtures of gallnut/myrobalan/pomegranate peel extract and distilled water were separately prepared (1:30, w/v) by heating them for 120 min at 70 ºC. Silk was immersed into these solutions for 1 h at 60 ºC (1:30, w/v). Finally, the processed silk was removed and place in one oven for 15 min at 70 ºC to facilitate drying.

The mixtures of alum/copper and distilled water were individually prepared (1:25, w/v) by agitating them for 60 min at 25 ºC. Silk was immersed into these solutions for 1 h at 60 ºC (1:30, w/v). Finally, the processed silk was removed and place in one oven for 15 min at 70 ºC to facilitate drying.

2.2.3. Silk dyeing

Pristine silk and the silk that underwent the mordant pre-treatment were individually soaked in the madder dye solution (1:40 w/v). The dyeing work was carried out in an electrically operated dye equipment at 70 ºC for 1 h. The dyed fabrics were slowly dried in the air (25 ºC). The original silk was labelled as control. The silk dyed without pre-treatment was tagged Rubi-1. The silk samples dyed with alum and copper mordants were Rubi-2 and Rubi-3, respectively. The silk mordanted using pomegranate, myrobalan, and gallnut bio-mordants and dyed were named Rubi-4, Rubi-5, and Rubi-6, respectively.

2.3. Characterizations

2.3.1. Sample characterization

 FTIR data of madder extract and silk was recorded using one FTIR spectrometer (Nicolet iS20, MA, USA). Each sample underwent thirty-two scans in total, utilizing the attenuated total reflectance (ATR) mode for the silk samples and the KBr pellet method for the turmeric extract. The scans had a spectral resolution of 4 cm?¹ and covered a wavenumber range of 500-4000 cm?¹.

A double-beam UV-vis spectrophotometer (MAPADA P7, China) was used to record the UV-vis spectrum of the turmeric extract, covering a spectral range of 200 to 900 nm.

The XRD profiles of the studied silk samples were analysed by utilising one D2 Phaser XRD equipment (Brooke, Germany). The experiment utilised Cu Kα radiation at a scan rate of 1 second, with an applied voltage of 30 kV and a current of 10 mA. The step size for each measurement was 0.02 degrees. The inter-atomic distance (d-spacing) was calculated by following the Bragg’s equation:

d-spacing =  / 2 sin

where  represents the X-ray wavelength of 1.54 Å and 2 stands for X-ray scattering angle ranging from 5 to 60 degrees.

Silk surface morphology and elemental mapping was captured by taking advantage of a SEM machine (Bruker, Germany).

2.3.2. Colour characterization

The colour strength of silk specimens was recorded using the SF600 Plus colourimeter (Data Color, USA). The K/S value presenting the colour strength was measured via the calculation:

K/S = (1 - R)² / 2R

where R presents the decimal fraction of fabric, K exhibits the absorption coefficient, and S is attributable to the scattering coefficient.

Simultaneously, the colour coordinates (L*a*b*) of the Commission Internationale de l’Eclairage (CIE) were noted down from the computer screen during the testing process. In particular, L* reflects the lightness, a* presents the range from red (positive value) to green (negative value), and b* presents that from yellow (positive value) to blue (negative value) in the colour space.

2.3.3. Washing fastness test

The fastness of the dyed silk was assessed through washing, rubbing, and light exposure tests. For washing fastness evaluation, the dyed silk underwent a 30-minute wash at 40 ºC using standard detergent, following ISO 105-C06:2010 standard.

2.3.4. UV-protection test

According to the AATCC 183-2000 standard, a UV Transmittance Analyzer (UV-2000F, Labsphere, USA) was used to test the UPF, UV-A transmittance, and UV-B transmittance of the undyed and dyed silk [6]

The UPF, UV-A transmittance rate, and UV-B transmittance rate of the tested samples were calculated by following the formulations:

UPF =  /

UV-A transmittance rate =  /  (%)

UV-B transmittance rate = 100  /  (%)

Here, E (l) stands for the relative erythema spectral effectiveness, while dl stands for the bandwidth, S(l) presents the spectral irradiance, whereas T(l) reflects the fabric average spectral transmittance, and l exhibits the wavelength. The UPF is categorized into poor (lower than 15), fair (from 15 to 24), good (from 25 to 39), and excellent (greater than 39) categories

3. RESULTS AND DISCUSSIONS

The FT-IR profile (Figure 1a) provides clarification on the presence of primary functional groups in the madder dye extract. A wide spectral band at 3325 cm^-1 was credited to the stretching vibration of the O–H bond, which is a characteristic of the hydroxyl group. One peak at 2923 cm^-1 suggested the existence of an aromatic group in the dye constituents. The wavenumbers 1582 and 1035 cm^-1 likely corresponded to the appearance of the C=O and C–C stretches, individually. Two minor peaks at 820 and 740 cm^-1 suggested that they might be owing to the C–H bends of the aromatic rings [7,8].

Figure 1b depicts the absorption aspect of the madder dye in the UV-visible ranges. The dye extract was found to have the highest absorbances at the wavelengths of 224 and 277 nm. This outcome aligned with the regions of absorption for purpurin and alizarin, declaring that the prominent presence of these two compounds had a substantial influence on the UV-vis absorption of the dye material [9].

Figure 1c displays a comparison of the fibre structure of silk before and after colouration, as determined by their FT-IR data. The control had broad absorption peaks in the wavenumber range of 3300 - 3000 cm^-1, which might likely be attributable to the stretching vibrations of the N–H bonds in the amide group. The presence of the amide was also revealed through the peak at 1633 cm^-1, which was probably assigned to the stretching of the –C=O bond. The peak at 1522 cm^-1 might belong to the bending of the N–H bond in the amine group. The minor peaks observed in the wavenumber region of 1500 - 900 cm^-1 were feasibly responsible for the bending of alkane (C–H) and the stretches of amine (C–N) [10,11]. The identical peaks were examined in the dyed samples, proposing that the fibre composition of silk remained unaltered after being treated with mordant and dyestuff.

The XRD spectra reflects the secondary structure of silk (Figure 1d). Both the control and treated samples had identical diffraction patterns at the 2θ angles. Particularly, two distinct crystalline peaks, characteristics of the beta sheets found in silk and valuable for assessing silk crystallinity, have been discovered as (210) and (002) lattice planes at the 2θ angles of 20.4° and 24.8°, individually. It is plausible to presume that the interaction between the dye and silk did not take place in the crystalline area but in the amorphous region [12].

Fig. 1. (a) FT-IR profile of madder dye extract, (b) UV-vis absorbance of madder dye extract, (c) FT-IR data of silk, (d) XRD profiles of silk.

Figure 2 illustrates the morphology and elemental content of silk. Figure 2a displays a clear surface of the control silk yarn, devoid of any clusters or imperfections. The elemental mapping of this sample reflects the presence of carbon (C), nitrogen (N), and oxygen (O) elements exclusively. The silk samples, which have been dyed directly and treated with natural mordants and dyed using madder extract, likewise exhibit a surface smoothness without any damage or modification. Three elements, including C, N, and O were also found in these samples (Figure 2b-e). However, the elemental mapping of the Rubi-3 sample unveiled the appearance of sulphur (S) and copper (Cu) (Figure 2f). The elemental content of the Rubi-2 sample included potassium (K), alum (Al), and sulphur (S), alongside the fundamental elements typically present in silk (Figure 2g). This outcome validates the bonding of metallic mordants to silk yarn.

Fig. 2. Morphological images and elemental mappings of (a) control, (b) Rubi-1

Figure 3 displays the textile qualities of silk. The K/S value of silk dyed directly was 4.5, whereas those of the Rubi-2, Rubi-4, and Rubi-6 samples were 10.6, 6.4, and 5.3, respectively. This outcome declared the beneficial effect of alum, pomegranate peel extract, and myrobalan extract in enhancing the attachment of dye molecules to silk. In contrast, the K/S values of the Rubi-3 (3.4) and Rubi-5 (3.5) samples were even lower than that of the silk dyed without a mordant, demonstrating the weak bindings in the cases of copper and gallnut extract. Furthermore, the modest reduction in the K/S values of the coloured silk suggests that the dye complexes had lower water solubility and bigger molecular size, making them less likely to be easily washed away (Figure 3a). Subsequently, the measurement of the dyed silk using a greyscale resulted in a moderate level of colour change for all examined samples (Figure 3b).

Out of all the dyed samples, only the Rubi-2 sample had a considerably lower L* value (38.02) compared to the Rubi-1 sample (56.53). There was no significant difference in the L* value of the Rubi-1 sample compared to the other samples. This assumes that only alum mordant had a diminishing effect on the lightness of the coloured silk. Furthermore, the positive (+) values of a* and b* in all the dyed samples are seen as proof of the hues falling inside the CIE red and yellow spectrums (Figure 3c). The visual photographs of the dyed samples revealed a subtle change in colours before and after washing (Figure 3d and e). In addition, the marginal reduction in the bending lengths of the dyed silk compared to the control silk, suggests that the stiffness of silk fabrics remains mostly unaffected following the dyeing procedure (Figure 3f).

Figure 3g and h show a low UPF of only 5 and an exceeding UV transmittance rate for the control, declaring the inadequate sun protection provided by silk. Good UPF values were achieved in the Rubi-1 (37), Rubi-2 (29), and Rubi-6 (33) samples. The UPF 39+ was acquired from the Rubi-2 (44), Rubi-4 (42), and Rubi-5 (44) samples. In addition, the coloured samples exhibited remarkably low UV transmittance rates. This result clarified that the dyeing process considerably improved the sun protective capability of silk fabrics.

Fig. 3. (a) K/S values of silk before and after washing, (b) colour coordinates, (c) bending length of silk, (d) images of dyed silk before washing, (e) images of dyed silk after washing, (f) washing fastness scores, (g) UPF of silk, (h) UV transmittance percentages of silk.

4. Conclusions

In this study, a madder dye extract and various mordants were utilized for eco-friendly dyeing to enhance the UV protection of silk. The presence of anthraquinone components was confirmed using FTIR and UV-vis techniques. FTIR and XRD analyses showed that the silk fibre composition remained largely unchanged, although SEM revealed minor alterations in fibre morphology due to dye complexes on the silk surface. The results indicated satisfactory colour fastness during washing, with three samples achieving a UPF greater than 39. Given the growing trend of harnessing the functional properties of natural substances for environmentally friendly material fabrication, this study highlights the potential of organic extracts as sustainable agents for dyeing and functionalizing silk, demonstrating that nature-derived mordants can effectively replace metallic ones to enhance both fastness and UV protection in dyed textiles.

Acknowledgements

This work was supported by The National Key Research and Development Program of China (Grant number: 2019YFC1521301) and Zhejiang Science and Technology Projects of Cultural Relics Protection (Grant number: 2021016).

Declaration of interest statement

No potential conflict of interest was provided by the author(s).

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