3.1. FTIR and K/S Values Analysis
The molecular structures of the raw, direct, mordant, and cross-linked dyed cotton fabrics were analyzed using FTIR spectra, the results are shown in
Figure 2. The
K/S values and digital images of the dyed cotton fabrics are shown in
Table 1.
As shown in
Figure 2, the absorption peak at 3410 cm
−1 was assigned to –OH stretching vibration and the band at 2900 cm
−1 was due to C–H stretching vibration [
21]. It could be observed that all dyed cotton fabrics appeared a similar anthraquinone ring vibration peak of madder dye at 1597 cm
−1 [
2], indicating the madder dye could dye in the cotton fabrics. And all cross-linked dyed cotton fabrics appeared the ester carbonyl absorption peak near 1733 cm
−1, showing that the esterification reaction occurred between dye-polycarboxylic acid-cotton fabric [
22,
23]. It could be seen from
Table 1, the
K/S value of the direct dyed cotton fabric was 2.24, indicating that the madder dye had a poor affinity for the cotton fabric. The
K/S value of the alum mordant dyed cotton fabric was approximately 85% higher than that of the direct dyed cotton fabric, which indicated that the mordant dyeing enhanced the color strength of the madder dye on the cotton fabric. The
K/S values of the four cross-linked dyed cotton fabrics were higher than that of mordant dyeing, indicating that the polycarboxylic acids as cross-linking agents further enhanced the color strength. The same conclusion could also be obtained from digital images of the dyed cotton fabrics. As shown in
Table 1, compared with the direct dyed cotton fabric, the
K/S values of the cross-linked dyed cotton fabrics increased more than 160%, revealing that the polycarboxylic acid not only reacted with the hydroxyl group of cellulose, but also had the esterification reaction with the hydroxyl group of dye. The polycarboxylic acid as cross-linking agent played a bridging role to connect with the dye and cotton fabric. The cross-linking process of dye-polycarboxylic acid-cotton fabric is shown in
Figure 3.
Furthermore, the
K/S value of the CA cross-linked dyed cotton fabric was lower than that of the other three compounds cross-linked dyed cotton fabrics. This is due to the fact that when CA was used as a single cross-linking agent, under the condition of catalytic heating, three carboxyl groups in the molecular structure of CA formed only two five-membered cyclic anhydrides intermediates successively and esterified two hydroxyl groups at most with low esterification sites. And in the same time, the α-hydroxyl group in the CA molecule structure hindered the progress of the cross-linking reaction, resulting in low esterification efficiency [
14]. Therefore, the
K/S value of the CA cross-linked dyed cotton fabric was relatively low. While when the mixtures of dicarboxylic acids and CA were used as cross-linking agents, firstly, the two adjacent carboxyl groups of one polycarboxylic acid generated five-membered cyclic anhydride intermediate under the condition of catalytic heating, and then reacted with the α-hydroxyl in the other polycarboxylic acid molecular structure to form a tetracarboxylic acid structure, at the same time, the close to the α-hydroxyl group and the negative effect on esterification reaction was reduced. Finally, the new tetracarboxylic acid formed three five-membered cyclic anhydrides intermediates successively and they could esterify three hydroxyl groups at most, so it increased the reaction sites and improved the efficiency of esterification cross-linking [
15].
In addition, the SUA structure does not contain α-hydroxyl, the MLA structure contains one α-hydroxyl and the TTA structure contains two α-hydroxyl [
24]. The experimental results showed that the α-hydroxyl hindered the esterification reaction, so the esterification reaction efficiency and the
K/S value of the CA+SUA cross-linked dyed cotton fabric was the best, followed by CA+MLA, and CA+TTA was the lowest.
3.2. XRD Analysis
Crystal structures of the raw, direct, mordant, and cross-linked dyed cotton fabrics were analyzed by XRD and the crystallinities of the cotton fabrics were calculated by splitting and fitting the crystalline superimposed peaks and amorphous superimposed peaks of the diffraction curves with the Gaussian function [
25]. The results are shown in
Figure 4 and
Table 2.
The dye solution can only spread into the amorphous and the edges of crystalline areas in the dyeing process.
Figure 4 showed that the diffraction peaks of the raw and dyed cotton fabrics appeared all around 14.8°, 16.5°, 22.7°and 34.3°. These four diffraction peaks were formed, respectively, by (101),
, (002) and (040) of the cellulose crystal plane diffraction [
26], and compared with the raw cotton fabric, the X-ray diffraction peak position of the dyed cotton fabrics had almost no change. It could be seen from
Table 2 that compared with the raw cotton fabric, the crystallinities of the dyed cotton fabrics were changed approximately ±3%, indicating that dyeing processes had little effect for the crystallinity. XRD results showed that the direct, mordant and cross-linked dyeing mainly occurred on the active surface or amorphous areas, and retained the original crystal of the cotton fabrics [
27].
3.3. Dyeing Property and Color Fastness Analysis
The color characteristic values, the hue angles (
h°), and color differences (Δ
E) of the direct, mordant and cross-linked dyed fabrics were tested. These results are shown in
Table 3.
As shown in
Table 3,
L* corresponds to brightness (100 = white, 0 = black), the darker the surface color depth of the dyed cotton fabric, the less bright it is [
28]. It could be seen from
Table 1 that compared with the direct dyed cotton fabric, the
K/S values of the mordant and cross-linked dyed cotton fabrics had been effectively improved, so the
L* values were reduced. The
a* value is the red-green coordinate (+ve = red, –ve = green), The
b* value is the yellow-blue coordinate (+ve = yellow, –ve = blue), and the
C* value is color saturation (100 = vivid, 0 = dull) [
29,
30]. Compared with the direct dyed cotton fabric, the
a*,
b* and
C* values of the mordant and cross-linked dyed cotton fabrics all increased, indicating that the color of the dyed cotton fabrics shifted to the red and yellow axis, and the color saturation increased.
h° value is the chromaticity angle of the dyed cotton fabric, the range is 0~360° and 0/360–90° is defined as the red–yellow range, 90–180° is defined as the yellow–green range, 180–270° is defined as the green–blue range, 270–360/0° is defined as the blue–red range [
31].
Table 3 showed the
h° values of all dyed cotton fabrics were in the range of 0~90°, they were in the overlapping area of red and yellow, so the dyed cotton fabrics appeared orange.
The color levelness of the cotton fabric is evaluated according to the Δ
E value. The Δ
E value is less than 1, which means that the cotton fabric has great leveling property [
32]. As shown in
Table 3, the Δ
E value of the direct dyed cotton fabric was 1.08, and it was more than 1, illustrating that its leveling property was poor. While the Δ
E values of the mordant and cross-linked dyed cotton fabrics were less than 1, indicating their levelness properties were improved, and the Δ
E value of the CA+SUA cross-linked dyed cotton fabric was the smallest (only 0.41), so its leveling property was the best. Since the madder dye has low molecular weight and poor affinity for the cotton fabric, the dye is only combined with cotton fabric through weak van der Waals in direct dyeing. Washing and soaping will easily cause the dye to fall off or shift on the surface, resulting in poor leveling property. The coordination bond formed by Al
3+ ions can strengthen the bonding force between the dye and cotton fabric, and the leveling property was improved compared with the direct dyeing. During the cross-linked dyeing, the polycarboxylic acid and the dye were completely dissolved in the dye solution, and they were uniformly absorbed on the cotton fabric surface through the rolling method. At the same time, the strong chemical bonds between the dye and cotton fabric prevented the dye migrating and falling off during the curing and washing [
33], therefore cross-linked dyeing further improved the leveling property of the cotton fabrics.
Washing, rubbing and light fastness were tested for the direct, mordant, and cross-linked dyed cotton fabrics, the results are given in
Table 4.
It could be seen from
Table 4 that compared with the direct dyed cotton fabric, the rubbing and washing fastness of the mordant and polycarboxylic acid cross-linked dyed cotton fabrics significantly improved. And the rubbing and washing fastness of the cross-linked dyed cotton fabrics increased by 4 levels or above. This can be attributed to the strong ester bonds of dye-polycarboxylic acid-cotton fabric, and in the process of rubbing and washing, the hydrolysis degree of the ester bonds was relatively little, so the rubbing and washing fastness obviously improved.
Natural dyes have poor light fastness. According to the reported results [
5], under the visible light and ultraviolet light irradiation, the madder dye obviously fades on the cotton fabric. Therefore, it is particularly important to improve the light fastness of the madder dyed cotton fabric. As shown in
Table 4, the light fastness of the direct dyed cotton fabric was only 1~2 levels, and the light fastness of the mordant dyed cotton fabric was not significantly improved. But the light fastness of the cross-linked dyed cotton fabric with polycarboxylic acid obviously increased, and in particular, the light fastness of CA+SUA cross-linked dyed cotton fabric reached 4~5 levels.
In order to intuitively study the light resistance of the dyed cotton fabrics, the dyed cotton fabrics were placed under a xenon arc lamp to simulate the intensity of sunlight at 12 noon, and the humidity and temperature of the room were maintained at 35% and 30 °C, respectively. The results of the
K/S and
h° values of the dyed cotton fabrics after 12 h and 24 h of light are shown in
Table 5.
From
Table 5, the direct and mordant dyed cotton fabrics began to fade after 12 h of exposure to simulated sunlight, and after 24 h, their
K/S values decreased by 28.9% and 31.2%,
h° values shifted to the yellow axis by 17.33° and 17.85°, respectively. And the color tonality of the direct and mordant cotton fabrics obviously changed. This is due to the fact that the madder natural dyes are easily oxidized and decomposed at exposure to sunlight, and the coordination bonds formed by Al
3+ ions have poor light stability [
5], leading to the poor light fastness of the direct and mordant dyed cotton fabrics. Furthermore, the structure of alizarin is decomposed to a variety of carboxyl compounds at sunlight condition, which increased the acidity of the dyed cotton fabrics. The alizarin structure appears yellow under acidic condition, so the colors of the dyed cotton fabrics were shifted to yellow after exposure to simulated sunlight. But the
K/S and
h° values of the cross-linked dyed cotton fabrics did not obviously change after 24 h of exposure to sunlight. They showed great light fastness, and the CA+SUA cross-linked dyed fabric had the best effect (4~5 levels). The great light resistance in the presence of the dyed cotton fabrics was explained by the fact that the diffusion of oxygen and moisture become difficult in the polycarboxylic acid cross-linked structure, which negatively affects the initiation of the photofading mechanism [
13,
34].
3.4. Wearing Property Analysis
Usually, the polycarboxylic acids are used as zero formaldehyde cross-linking agents to enhance the durable pressing property of the cotton fabric, so the wrinkle recovery angle and breaking strength of the raw, direct, mordant, and cross-linked dyed cotton fabrics were tested. The results are shown in
Table 6.
Under stretching effect, macromolecular chains in the amorphous region of the cotton fabric are slipped, which causes the hydrogen bonds disassembly-reconstruction phenomenon. The newly formed hydrogen bonds prevent the macromolecular chains from returning to their original states and then forming irreversible wrinkles. It could be seen from
Table 6 that compared with the raw cotton fabric, the sum of the warp and weft WRA of the direct and mordant dyed cotton fabrics did not obviously change, indicating that the direct and mordant dyeing did not improve the wrinkle resistance of the cotton fabrics. But the WRAs of the cross-linked dyed cotton fabrics obviously increased, this is due to the fact the esterification reaction occurred between dye-polycarboxylic acid-cotton fabric during the cross-linked dyeing process, leading to the formation of a cross-linked network structure on the surface of the cotton fabric prevented the slippage of the macromolecular chains [
35,
36]. Therefore, the wrinkle resistances of the cross-linked dyed cotton fabric were improved, and the CA+SUA cross-linked dyed cotton fabric increased by 40% than that of the raw cotton fabric, it had the best wrinkle resistance.
As shown in
Table 6, compared with the raw cotton fabric, the breaking strengths of the direct and mordant dyed cotton fabrics had a smaller drop, both within 10%, while breaking strengths of the cross-linked dyed cotton fabrics obviously decreased, especially for the CA+SUA cross-linked dyed cotton fabric, the warp and weft breaking strengths were reduced by 22% and 21%, respectively. This is mainly due to the fact that the cellulose is not resistant to acid. Although the low concentration of polycarboxylic acid was used during the process of cross-linking dyeing, it also affected the breaking strength of the cotton fabric [
37]. In addition, the curing of the cotton fabric at 160 °C resulted in a certain loss of strength [
36].
UV radiation refers to the irradiation of two wavelength bands, including UVA (320–400 nm) and UVB (280–320 nm). Long-term exposure to UV radiation will cause skin aging and carcinogenesis [
38]. In order to evaluate the UV protective property of the raw and dyed cotton fabrics, the UV transmittances in the ultraviolet region (280~400 nm) and UPF values were tested, and the results are shown in
Figure 5 and
Table 7.
Figure 5 and
Table 7 showed that more than 50% of ultraviolet light passed through the raw cotton fabric, and its UPF value was only 13.35, but compared with the raw cotton fabric, the UV transmittances of the dyed cotton fabrics were greatly reduced and their UPF values were obviously improved. This reason is that the aromatic ring and conjugated structure of natural madder dye has a strong UV absorption ability. It also could be seen from
Figure 5 and
Table 7 that the average transmittance of the compound cross-linked dyed cotton fabrics in the UVA region were about 5%, and the UPF values were greater than 50. According to the requirements of AATCC-183-2014, when the UPF value > 40 and UVA < 5%, the fabrics have excellent UV resistance. The reason is that more dyes in the cotton fabrics had a better ability to absorb ultraviolet light [
5]. At the same time, the cross-linked network structure could effectively prevent the passage of ultraviolet light and reduce the transmittance of UV. The combination of the two effects made the cross-linked dyed cotton fabric had great UV resistance property.
In general, the natural dyes have certain antibacterial property [
39], so the antibacterial activities of the dyed cotton fabrics with madder had been tested. The
E. coli (Gram-negative) and
S. aureus (Gram-positive) as representative microorganisms were chosen in this study, and the results are shown in
Figure 6.
Figure 6 showed that the raw cotton fabric had no antibacterial activity for both
S. aureus and
E. coli. The antibacterial property of the dyed cotton fabrics was significantly improved compared with the raw cotton fabric. In particular, the antibacterial activity of the compound cross-linked dyed cotton fabrics increased by more than 60%. This is mainly due to the fact the anthraquinone structure of the madder dye was found to complex irreversibly with nucleophilic amino acids in proteins, leading to inactivation of the protein and loss of function, and hence it could inhibit the growth of both Gram-positive and Gram-negative bacteria [
39]. The SUA+CA cross-linked dyed cotton fabric had the highest color yield, so it had the best antibacterial property (about 70%).