Dyeing and UV Protective Properties of Chitosan-Modified Cotton Fabric Treated with Black Rice Extract

ABSTRACT Black rice is known as a health-promoting food for its abundant content of anthocyanins. The main objective of this paper is to get functional and eco-friendly materials dyed with black rice extract. In this research work, chitosan-modified cotton fabric was dyed with the black rice extract, and the fabric’s CIELab color characteristic values (L*, a*, b*, C*), color strength (K/S) value, and UPF value were investigated closely. The K/S value and UPF value of dyed samples depend on temperature, time, and pH. It is worth noting that the acid medium favored the dyeing to obtain a purple-red color and achieve a larger K/S value and UPF value. The results showed that chitosan-modified cotton fabric dyed with black rice extract had good UV resistance and color fastness.


Introduction
Cotton fiber is the most widely used natural plant fiber in the world. It is the most important textile fiber so far, accounting for about half of the total fiber used in the world. It has the advantages of good warmth retention, soft handle, easy cleaning, wear resistance, washing resistance as well as good dyeing performance, which makes it popular with people around the world (Hou et al. 2021;Zhou et al. 2019). It is generally accepted that natural dye dyeing cotton fabric has low color content and poor color fastness. This is because the dyes are mainly adsorbed to cellulose fibers through van der Waals forces and hydrogen bonds, while the low molecular weight of natural dyes makes them have a poor affinity for cellulose, resulting in low dye uptake, low formation rate, and poor color fastness. To improve the uptake rate and color fastness of natural dyes on cotton fabric, the common methods are cationization of cotton fabric before dyeing (Baaka et al. 2017(Baaka et al. , 2019Giacomini, de Souza, and de Barros 2020;Ke, Zhu, and Chowdhury 2021a;Nakpathom et al. 2018;Rym, Farouk, and Bechir 2016;Ticha et al. 2016), dyeing with mordant (Davulcu et al. 2014;Diarsa and Gupte 2020;El-Zaher et al. 2019;Hong et al. 2012;Hossain et al. 2021;Ke, Zhu, and Chowdhury 2021b;Naveed et al. 2020;Shahidi 2016) and enzyme pretreatment before dyeing (Benli and Bahtiyari 2015;Nam Sik et al. 1996;Samant et al. 2020;Tsatsaroni and Liakopoulou-Kyriakides 1995;Vankar, Shanker, and Verma 2007).
It was found that chitosan is the only kind of basic polysaccharide that contains polar groups -OH and -NH 2 in its molecule. It is a natural cationic modifier and can be used to improve the dyeing depth of acid dyes, disperse dyes, and anionic dyes (Feng and Zhang 2001;Mansour and Ali 2019;Wang and Gao 2003). After chitosan treatment, the number of -NH 2 groups on the surface of cotton fabrics increases, so the affinity of natural dye for cotton fabric is improved (Haji 2017;Hao et al. 2017;Liu et al. 2019;Lou, Gong, and Zhang 2017;Peng and He 2019;Zhang and Wang 2019;Zhu et al. 2019). In addition, chitosan can also endow cotton fabric with certain antibacterial and crease resistance (Li et al. 2020;Rahman Bhuiyan et al. 2017;Sadeghi-Kiakhani, Tehrani-Bagha, and Safapour 2018;Zhang et al. 2003;Zhou and Kan 2014).
Black rice is dark in appearance and rich in nutrition. Black rice known as the "King of rice" is an ideal health food raw material. The main component of black rice pigment is anthocyanin (Kong, Wang, and Cao 2008;, and the main type of anthocyanins is cyanidin-3-glucoside which belongs to the family of flavonoids and has strong nutritional value and health care function (Li, Zhang, and Zong 2019;Seo et al. 2013;Xue and Xu 2020;Zhou, Lv, and Juan 2013). Moreover, it was found that flavonoids have a double bond, conjugated double bond, or triple bond structures, which can absorb ultraviolet energy. It is reported that black rice pigment has two characteristic absorption peaks in the ultraviolet region of 265-275 nm and the visible region of 465-560 nm (Xue 2009;Zhang et al. 1999). Therefore, black rice pigment has a certain antiultraviolet ability. However, black rice pigments are currently used primarily to dye protein fabrics rather than cotton fabrics (Jia et al. 2015;Yan 2013;Yang 2018;Yu et al. 2019). If black rice pigment is applied to cotton fabric, the dyed sample not only obtains natural color but also has certain biological health care functions as well as anti-ultraviolet properties.
As mentioned above, cotton and chitosan are harmless to human skin, and the black rice pigment is a food-grade substance with medical and health care functions. By effectively combining the three, ecological green functional textile fabrics can be obtained. Based on this, the cotton fabric was first modified with chitosan and then dyed directly with black rice pigments. The sample was soft purplered color with good UV resistance.

Materials
Cotton knitted fabric (Weight per square meter:151.52 g/m 2 , Course density: 75 ring/5 cm, Wale density: 121 ring/5 cm, thickness: 0.82 mm) was bought from Yuantong Textile City, which has been pretreated and has an 87% whiteness and a capillary of 9.3 cm/30 min. Black rice is purchased online and produced by Shenyang Xinchang Trading Co., Ltd. Chitosan(Degree of deacetylation: 80-95%) was provided by Shanghai McLin Biochemical Technology Co., Ltd.

Extraction of pigment from black rice
Black rice was pulverized in a grinder and then sifted through a 70-mesh sieve. 30 g of black rice powder was added to in beaker containing 900 g of distilled water at a solid-liquid ratio of 1:30, followed by ultrasonic treatment at 40°C for 25 min to obtain the initial extract. Then the initial extract was filtered with a 400 mesh nylon strainer. After filtering, under the same extraction conditions, the residue of black rice powder was extracted to obtain the secondary extract. Then the primary extract and the secondary extract were mixed to obtain the black rice pigment mother liquor, which was put into the refrigerator for use. When the extract was heated and evaporated, the remaining material is then dried and ground to obtain powder particles. The yield of black rice extract can be obtained by dividing the mass of pigment powder by the initial mass of black rice powder. Here, The yield of black rice extract was 16.81%. While in the experiment, black rice extract was directly used for dyeing instead of black rice pigment powder.

Modification of cotton fabric with chitosan
Cotton fabrics were immersed in a 2% acetic acid solution containing 7 g/L of chitosan at 80°C for 30 min with the liquor ratio of 1:30. Subsequently, the samples were washed thoroughly with 70°C hot water and cold water respectively, and then dried for use.

Dyeing of chitosan-modified cotton fabric with black rice pigment
Dyeing of chitosan-modified cotton fabric was carried out in a beaker. A certain concentration of NaCl (0 g/L, 5 g/L, 10 g/L, 15 g/L, 20 g/L, 25 g/L, 30 g/L) was added to dyeing solution containing certain volume of black rice extract (5 mL, 10 mL, 15 mL, 20 mL, 25 mL, 30 mL). 1 g of cotton fabric was put into 30 mL of dye liquor and dyed for a certain time(20 min, 30 min, 40 min, 50 min, 60 min) at different pH values (2,4,6,8,10,12) and temperatures(60°C, 70°C, 80°C, 90°C, 100°C) with the liquor ratio of 1:30. After dyeing, the samples were washed with 70°C hot water and cold water respectively and then dried.

Color characteristics of dyed fabrics
The color strength of the dyed fabric is usually expressed by its K/S value. K/S value, CIE L*(Lightness value), a*, b*, and C* (Saturation value) values of the dyed samples were measured in the wavelength range of 360-750 nm by X-Rite Color Eye 7000A spectrophotometer. Parameters were large aperture (25 mm) and D 65 daylight with a 10° standard observer. Each sample was folded twice for testing. Each sample was randomly measured three times at different positions, and the average value was taken as the final measurement value. The K/S value was calculated by the Kubelka -Munk equation. The equation is as follows. (1) Where R is the reflectivity factor, K and S are the absorption coefficient and the scattering coefficient, respectively.
Comparing the dyed cloth with the undyed white cloth, CIE1976 Lab was used to calculate color difference ΔE between the dyed sample and undyed white cloth. The color difference ΔE here is different from the color difference ΔE used in color matching. The smaller the color difference between the dyed sample and the standard sample during color matching, the higher the precision of the dyed sample. While ΔE here reflects the difference between the dyed sample and undyed white cloth. The higher ΔE means a more significant color shifting relative to the undyed white cloth. From another angle, the color difference ΔE here can indicate the color depth of the dyed cloth. In other words, the color difference value ΔE can support the change of K/S value to a certain extent. The equation is as follows.

ΔE ¼
ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi Where L 0 , a 0 and b 0 are the color characteristic values of undyed white cloth, L, a and b are color characteristic values of dyed samples.

UV protection of dyed fabrics
According to GB/T 18,830-2009, the UPF value of the sample was tested on a UV-2000F fabric ultraviolet transmittance tester, which was purchased from Labsphere in America. Each sample was randomly measured four times at different positions, and the average value of the UPF, UVA transmittance and UVB transmittance of the dyed samples were taken for use.

SEM analysis
Scanning electron microscopy (SEM) was used to observe the surface characteristics of fibers.

FTIR analysis
Infrared spectroscopy was used to determine whether the fiber was successfully modified by chitosan.

Colorfastness testing
Colorfastness to wet and dry rubbing of dyed cotton samples was evaluated according

Mechanism of chitosan-modified cotton dyeing with black rice extract
As shown in Figure 1, the color yield of the chitosan-modified cotton fabric was higher than that of untreated cotton fabric, indicating that the chitosan contributed to the adsorption and fixation of black rice pigment on cotton fabric. Figure 1 displaces the probable mechanism of attachment of black rice dye on chitosan-treated cotton. There may be an electrostatic attraction between cellulose, chitosan, and black rice pigment because of the different chemical groups in their structures, as shown in Figure 1. Scanning electron microscopy (SEM) morphologies of pristine cotton and chitosan-modified cotton fabrics were observed in Figure 2. The surface of unmodified cotton fabric is relatively smooth Figure 1. Probable mechanism of generating electrostatic attraction force among cellulose, chitosan and dye. and flat, while there are chitosan deposition particles on the surface of chitosan-modified cotton fiber, as shown in Figures 2(a,b). Figure 3 shows the infrared spectrums of pure cotton fiber and chitosanmodified cotton fiber. For pure cotton fiber, it can be seen that there is a wide absorption peak at 3450 cm −1 , which corresponds to the stretching vibration of O-H. The absorption peak at 2820 cm −1 corresponds to the stretching vibration of aliphatic C-H. The narrow and strong absorption peak at 1080 cm −1 is the stretching vibration peak of C-O-C on the cellulose chain. In the infrared spectrogram of chitosan-modified cotton fiber, the stretching vibration peak of N-H appeared at 3420 cm −1 , which overlaps with the stretching vibration peak of O-H, thus the absorption peak became wider. Three new absorption peaks appeared at 1647 cm −1 , 1575 cm −1 and 1375 cm −1 , which correspond to the phthalide I band, the phthalide II band, and the phthalide III band respectively (Cai and Guang 2011;Cao et al. 2008). Since only the chitosan has a phthalide bond, this result indicates that the cotton fiber was successfully modified by chitosan. The results in Figures 2 and 3 show that the modification experiment successfully realized the deposition of chitosan on cotton fabric.
As can be seen in Figure 3, for the curve of chitosan-modified fiber with black rice, the peaks at 1165 cm −1 and at 1058 cm −1 are generated by stretching vibration of phenolic hydroxyl group and  C-O stretching vibration of m-alcohol hydroxyl group, respectively. The characteristic absorption peaks of benzene ring appear at 1639 cm −1 and 1429 cm −1 . The peaks at 893 cm −1 , 707 cm −1 , 665 cm −1 indicate the out-of-plane bending vibration of C-H when the benzene ring is substituted, indicating the existence of phenolic hydroxyl groups. Figure 4 represents the effect of pH value on K/S value at 550 nm and UPF value of dyed fabric. Both of them showed a downward trend as the pH value increased. It was found in the experiment that the solution showed color changes from purple-red to red, colorless, and yellow-green with different pH values. The results in Figure 4 could be explained that under acidic conditions, the protonation of amino group on chitosan can make the surface of cotton fabric positively charged, improve its affinity with the active components in black rice extract, and increase the dyeing rate and K/S value of dyed cotton fabric. With the increase of pH value, the amino group on cotton fabric could not be protonated, and the surface of cotton fabric showed electronegativity in the dyeing bath, which reduced the affinity of cotton fabric for the active colored components in black rice, leading to the decrease in the dye uptake rate and K/S value (Liao et al. 2017;Zhou, Yang, and Tang 2016). Table 1 lists the pictures of dyed samples at different pH values. Under the acid condition, the dyed fabric was purplish red. While under alkaline conditions, the dyed fabric exhibited gray-white color having blue light instead of red light, indicating that acid medium favored the dyeing to achieve good color depth. According to the research at present, It was found that black rice anthocyanins were sensitive to pH and could show various color changes with different pH values due to the tautomerization of anthocyanins in black rice pigments occurring at different pH values (Li et al. 2016;Zeng et al. 2020). The color change of dyed samples may be due to the anthocyanin tautomerism of black rice pigment at different pH values. Under acidic and neutral conditions, the active components in black rice extract had a high affinity for fibers, resulting in a high color yield of the fabric. While under alkaline conditions, active components with another structure have less affinity for the fiber, resulting in a decrease in the color yield of fabrics. The stronger the alkalinity, the smaller the color yield of the fabric. Hence, acidic and neutral conditions are more favorable for dyeing. As can be seen from Table 1, the ΔE value decreased as pH increased, and the change of ΔE value was similar to that of K/S value (in Figure 4), indicating a gradual decrease in the amount of pigment on the fabric. The L* value of the dyed fabric gradually increased, while the C* value gradually decreased, indicating that the visual brightness of dyed fabric continued to increase and the saturation decreased continuously, which corresponded to the declining trend of K/S value of the dyed fabric in Figure 4. The value of a* kept decreasing in the positive value range, and the b* value changed from a negative value to a positive value, indicating that the red light of dyed fabric continued to weaken, while the yellow light on the fabric gradually weakened and turned to blue light, which was consistent with the pictures of the samples.

Effect of pH value on dyeing property and UV resistance
From the experimental results, although acid media is beneficial to obtaining higher color strength, dyeing at around pH 4 is a very sensible choice given that cotton fabrics may be damaged under strongly acidic conditions.

Effect of NaCl concentration on dyeing property and UV resistance
As shown in Figure 5, the addition of sodium chloride slightly increased the K/S value and UPF value of dyed fabric. When the NaCl concentration ranged from 0 to 20 g/L, the K/S value and UPF value of dyed fabrics increased slowly but changed little. However, the K/S value and UPF value of dyed fabrics decreased when NaCl concentration exceeded 25 g/L. This may be because the presence of sodium ions neutralizes some of the negative surface potentials of cotton. Too much salt, however, could cause the dye molecules to suddenly clump onto the fabric. As a result, dye accumulated on the surface of the fibers was washed away during washing, resulting in less color depth. As shown in Table 2, when the concentration of sodium chloride ranged from 0 to 30 g/L, the L* value of dyed fabric first decreased and then increased, on the contrary, the values a* and C* first increased and then decreased. From another perspective, this result shows that black rice pigment has good salt tolerance, which is consistent with the results reported in the literature (Lei 2013). The UVA and UVB transmittance values of all dyed fabrics were less than 4%, and the UPF values were more than 60 in Figure 5, compared to the UPF value of undyed fabric at 24.6, indicating that dyed fabrics had excellent UV resistance. Existing research results found that the main chemical components and activity analysis of black rice pigment are mainly composed of a variety of anthocyanins. Three sesquiterpenes, three vitamins, six steroids, seven organic acids, 19 flavonoids, 41 alkaloids, 109 amino acids or polypeptides (including eight essential amino acids) and other compounds have been identified in the water extract of black rice. These active components in the black rice extract likely give fabric excellent UV resistance.
As shown in Table 2, the ΔE value increased very little as the NaCl concentration ranged from 0 to 20 g/L, the difference between the maximum and minimum values of ΔE is less than 3. In Table 2, there was no significant difference in the brightness values of the cloth samples with different concentrations of NaCl, which agreed with the results in Figure 5. The result shows that the effect of NaCl is not prominent in dyeing in terms of the change of corresponding index value, therefore, NaCl can be omitted from the dyeing process.

Effect of temperature on dyeing property and UV resistance
As can be seen from Figure 6, with the increase of dyeing temperature, the K/S value of dyed fabric decreased continuously, while the UPF value first decreased and then increased slightly, and finally remained almost unchanged. This is because the UPF value of the same fabric is related to the color characteristics of the fabric and the amount of UV-resistant substances (Han et al. 2009;Zhang, Zhou, and Shi 2017). The reasons may be that on the one hand, black rice pigment was unstable at high temperatures, and the decomposition rate of dye was faster than the complexation rate of dye and fabric. On the other hand, at high temperatures, due to the poor binding force between dye and fabric,  part of the dye fell off the fabric. All of these eventually led to a decrease in K/S and UPF values (Zeng, Zhang, and Jin 2014;Zhou and Yu 2012). As shown in Table 3, at 60°C, cotton fabric dyed has the smallest L* value, the largest ΔE, a* and C* values, corresponding to the largest K/S value in Figure 6. Hence, 60°C was the most suitable dyeing temperature.

Effect of time on dyeing property and UV resistance
It is shown in Figure 7 that the K/S value and UPF value of the dyed fabric increased firstly and then decreased when the dyeing time ranged from 20 to 60 min. It was apparent that dyeing for 40 min gave   the highest color depth. It may be that the dye uptake of black rice pigment increased gradually from 20 to 40 min, which improved the K/S value and UPF value. When the dyeing time exceeded 40 min, the pigment adsorbed on the fiber reached saturation and no longer increased. Moreover, the longterm heating during the dyeing process would cause the hydrolysis and degradation of the black rice pigment, and the K/S and UPF values of the fabric gradually decreased. As shown in Table 4, fabric dyed for 40 min had the smallest L* value, largest ΔE, a*, and C* values, which corresponded to the maximum value of K/S in Figure 7. The highest apparent depth and excellent UV resistance can be obtained by dyeing for 40 min, thus it was advisable to choose 40 min for dyeing.

Dyeing property and UV resistance of cotton fabric under the optimum conditions
The cotton fabric was immersed in 7 g/L chitosan solution at 80°C for 30 min, and the treated fabric was washed with 70°C hot water and cold water respectively, and then dried for use. 30 g of chitosan- modified cotton fabric was immersed in 900 mL of NaCl-free black rice extract at 60°C and pH 4.0 for 40 min. After dyeing, the samples were washed and dried. Then the color characteristic values, UV protective properties, and color fastness of the fabrics are obtained in Tables 5 and 6 respectively. Images of real samples before and after the test were shown in Figure 8. As shown in Table 5, UPF value is greater than 90, and the transmittance value of UVA and UVB were all less than 3%, indicating that the sample had good UV protection performance. Meanwhile, except for acid perspiration fastness, the rubbing fastness, washing fastness, light fastness, and alkaline perspiration fastness of the dyed fabric could reach level 3 in Table 6. As can be seen in Figure 8, the blue light on the sample increases after acid perspiration test. This may be because the acid perspiration affects the structure of some chromophores in the black rice pigment, which makes the absorption wavelength of the pigment move toward the long wave direction, leading to a little color change of the dyed cloth. On the whole, the black rice pigment had good adhesion to the chitosan-modified cotton fabric, and the dyed fabric could meet wearing requirements.

Conclusions
Chitosan-modified cotton fabrics were dyed with black rice extract to obtain functional and environment-friendly clothing materials. It is well known that the affinity of black rice pigments for cotton fabrics is lower than that for silk and wool fabrics. Therefore, we increased the absorption of black rice colorants on cotton fabrics by chitosan modification. It was found that factors such as pH value, dyeing temperature, dyeing time, and sodium chloride concentration all have an impact on the dyeing effect and the UV resistance of the fabric. Among them, the pH value has the greatest impact on the dyeing effect because it not only affects the color depth of the sample but also changes the sample's color. Finally, purple color was obtained on chitosan-modified cotton fabric. The samples dyed under optimal conditions possess good color fastness. Although the K/S value of the dyed sample is small, it shows excellent UV resistance compared to the undyed sample.

Informed consent
Informed consent was obtained from all individual participants included in the study.