3.1. Surface properties analysis of cotton fabric
The two-dimension grazing incidence X-ray diffraction measurements were used to observe the crystallinity of cotton fiber. And the 2D-GIXD scattering patterns were depicted in Fig. 3a − f, the cotton fibers crystallinity was decreased as indicated by the weakening of scattering intensity (Li et al. 2016). The weakening of lamellar peak also meant the increase of amorphous state of cotton fiber (Mai et al. 2016). To further study the change of the crystal structure of the treated cotton fabric, the above diffraction patterns were integrated and the corresponding curve shown in Fig. 3g was obtained. Cellulose I is a mixture of cellulose Iα and cellulose Iβ of two crystal forms, cellulose Iα mainly existed in bacterial cellulose and seaweed cellulose, while cellulose Iβ mainly existed in the fibers of higher plants(Liang et al. 2021b; French 2014). Cellulose I was a parallel chain structure. The original samples showed the characteristic diffraction of cellulose Iβ, and the diffraction peaks appeared at 2θ = 14.7 °, 16.8 °, 20.5°, 22.7 ° and 34.8 °, corresponding to the diffractions of the (1–10), (110), (012/102), (200) and (040) crystal planes. Weak diffraction peaks appeared at 2θ = 12.1 ° and 20.1 ° corresponding to the (1–10) and (110) lattice planes respectively after treatment, which was typical characteristics for cellulose Ⅱ(Sebe et al. 2012). In the progress of alkali treatment, sodium hydroxide entered the amorphous region of the cotton fiber and separated the crystallites in the cotton fiber, resulting in the structure of Na-cellulose Ⅰ. After the alkali treatment, Na-cellulose Ⅰ was rinsed with water to remove alkali, and then Na-cellulose Ⅰ was converted to Na-cellulose Ⅳ (hydrate form of cellulose II) (Sarko 2021; Nishiyama. et al. 2000). After drying and removing water, the crystal structure of cellulose II with antiparallel structure is formed(Langan. et al. 2001). However, due to the existence of tension, the penetration of sodium hydroxide solution in the highly ordered crystal structure of cotton fiber was limited. Therefore, the cellulose Ⅰ was not completely transformed into cellulose Ⅱ during the treatment. As shown in Fig. 3i, the original samples exhibited the crystalline form of cellulose Ⅰ. The crystal structure of treated cotton fiber is a mixed structure of cellulose Ⅰ and cellulose Ⅱ.
To better investigate the changes of cotton fiber crystallinity after treatment, Eq. (4) was used to calculate the crystallinity of cotton fiber(Zhang et al. 2021), and the results were shown in Table 1.
Table 1
The content of crystalline zone and amorphous zone of treated cotton fiber.
Treated Time (s)
|
Crystal zone content (%)
|
Amorphous region content (%)
|
0
|
73.9
|
26.1
|
10
|
65.1
|
34.9
|
20
|
63.5
|
36.5
|
30
|
62.5
|
37.5
|
60
|
59.3
|
40.7
|
90
|
58.5
|
41.5
|
As the cellulose molecular chain was a highly ordered structure, this ordered structure was a network formed by a large number of intramolecular and intermolecular hydrogen bonds, resulting in high crystallinity of cotton fiber. From Table 1, the crystallinity of the treated cotton fiber was reduced from 73.9–58.5%. This result implied that alkali could reduce the crystallinity of cotton fibers through breaking the degree of order of cellulose macromolecular chains.
To further investigate the surface properties of cotton fibers, FTIR spectra of the cotton fabrics, before and after alkali treatment, were demonstrated in Fig. 3h. All samples treated by alkali showed the similar spectral curves, indicating that no new groups were produced after treatment. The stretching of the strong hydrogen bond -OH near 3455 − 3210 cm− 1 is universally observed in all spectra(Tarbuk et al. 2014). The bands at around 3455–3410 cm− 1 and 3375–3340 cm− 1, which were assigned to O3H···O5 and O2H···O6 intramolecular hydrogen bonds(Schwanninger et al. 2004). The intermolecular hydrogen bonding of O6H···O3 in cellulose are generally shown 3310–3230 cm− 1(Duchemin 2015; Remadevi et al. 2018). The maximum absorption peak of the OH stretching vibration of the treated cotton fiber was transferred to a higher wavenumber. The crystal structure of cellulose was transformed from cellulose I to cellulose II by high concentration alkali treatment(Oh et al. 2005). Moreover, the alkali treatment reduced the OH stretching vibration mainly caused by intramolecular hydrogen bonds.
The morphology of cotton fibers treated by high concentration alkali at different times were observed by scanning electron microscopes (SEM). As illustrated in Fig. 4a and Fig. 4g, the cross section of original cotton fiber was flat waist shaped with a large cell cavity. The longitudinal direction of untreated cotton fiber had natural distortion and rough surface. As for the cotton fabrics treated by high concentration alkali solution, sodium hydroxide solution diffuses rapidly into the fiber. Because of the deconvolution, cotton fibers changed from natural twisted band structure to rod structure with smooth surface.(Liang et al. 2021b). As shown in Fig. 4b-4f, the cross section of cotton fiber gradually changed from flat oval to round with the increase of treatment time, and the cell cavity gradually became smaller, and finally reduced to a line. As illustrated in Fig. 4h-4l, the surfaces of the cotton fibers became a little smooth after treatment, and the vertical natural torsion gradually disappeared.
The glossiness of the treated cotton fabrics treated was shown in Table 2. All the treated cotton fabrics exhibited excellent glossiness than original fabric. The results clearly indicated that the glossiness of cotton fabrics might be related to morphological structure. Combined with Fig. 4, the cross section of the cotton fibers treated with alkali solution changed from ear shape to round shape, the wrinkles on the surface of the fiber disappeared and the surface became smooth. As a result, the treated fibers exhibited an improvement in light reflection, bringing much better glossiness.
Table 2
Glossiness of cotton fabric was treated at different times.
Treated Time(s)
|
0
|
10
|
20
|
30
|
60
|
90
|
Glossiness
|
2.15
|
2.41
|
2.43
|
2.45
|
2.44
|
2.50
|
3.2. Effect of crystallinity on ink deposition morphology
Figure 5a and Fig. 5b showed the wettability of cotton fabric through the contact angle and capillary effect measurement. It could be seen from Fig. 5a that the treated cotton fabric obtained a better capillary effect. The wicking height of original cotton fabric was 65 mm in 30 minutes. In comparison, the wicking height of the treated cotton fabric increased with the increase of high concentration alkali solution treatment time. The wicking height changed little when the treatment time reached 60 s. The reason was that the crystallinity of treated cotton fiber decreased and the amorphous region increased. The increase of amorphous region led to the increase of accessible hydrophilic groups in the fiber, and water molecules could rapidly form hydrogen bonds with hydrophilic group, which led to the increase of wicking height. At the same time, cotton fibers could be regarded as a porous medium, and liquid could be transported in the porous medium (Zhu et al. 2019; Zhao et al. 2021). After alkali treatment, the fiber swelled and the gap between fibers became smaller, which was helpful to improve the wicking height.
The droplet spread rapidly on the fabric due to the capillary pressure and hydrogen bonding after contacting the cotton fabric. As depicted in Fig. 5b, the contact angle of cotton fabrics gradually decreased with the increase of treatment time. After the treatment, the accessible hydroxyl groups on the fiber surface increased and the hydrogen bond between the droplet and the fiber surface was enhanced (Song et al. 2021). As the fiber swelled, the fiber gap became smaller and the capillary pressure increased, the droplet diffusion on the fabric accelerated. Therefore, under the action of hydrogen bond and capillary pressure, the contact angle of droplet on the treated cotton fabric gradually decreased and finally reached an equilibrium state.
As shown in Fig. S2, the diffusion of ink drops on cotton fabric was mainly divided into two parts: the first was that the ink drops fall on the fabric, and the second was that the ink drops wet the cotton fabric (Josserand and Thoroddsen 2016; Rioboo et al. 2002). This process mainly included the spreading and penetration of ink droplets on the fabric, and finally stable deposition on the fabric surface to form a line pattern (Zhang et al. 2020a). The wetting state of droplets on the fabric was observed, and the deposition state of droplets on the fabric before and after treatment was further investigated, as shown in Fig. 5c-5e. After the droplets hit the fabric, they permeate and spread instantly (Mhetre et al. 2010). The wetting speed of the droplets on the treated cotton fabric was obviously accelerated. Combined with the results in Fig. 3, it was believed that with the decrease of crystal area, the penetration of droplets into the fiber amorphous area increased after the droplets impacted on the fabric surface, which accelerated the penetration of droplets perpendicular to the fabric surface. That also meant that the increase of amorphous area, the combination of cotton fabric and dye ink increased, leading to the improvement of dye utilization (Xie et al. 2020).
3.3. Study on inkjet printing performance of cotton fabric
The color strength of the cotton fabric directly reflects the distribution of dye molecules in inkjet printed fabrics. In ink-jet printing, the amount of dye in each area is certain, so the darker the color of printed cotton fabric means the higher the ink utilization. The effect of crystallinity on color intensity of inkjet printed cotton fabrics was investigated. As shown in Fig. 6a, the color strength of cotton fabrics increased with the increase of treatment time and the color intensity of the treated cotton fabric reached the maximum K/S value in 60 s. There was no obvious change when the treatment time was over 60 s. The reason may be that when the cotton fabric was treated with alkali for 60 s, the absorption capacity of cotton fabric to dyes reached the maximum. Based on the above research, it could be concluded that alkali treatment increased the wettability of cotton fiber, making the dye molecules more easily penetrate into the fibers. At the same time, the crystal area in the fiber decreased and the amorphous region increased, leading to the increase of the reaction sites in the fibers. Therefore, the color strength of the treated printed cotton fabric was high after steam washing. The schematic mechanism was shown in Fig. 6b.
The color data of inkjet printed cotton fabrics with reactive dye inks were shown in Table 3. L* and C* represent lightness and chroma, respectively (An et al. 2020a). It can be seen that the printed cotton fabrics treated for 60 s obtained the lowest L* values and the largest C* for all the cotton fabric samples, indicating that the cotton fabrics got the deepest colors. a* represents the degree of greenness(-) and redness(+), b* corresponds to the degree of blueness(-) and yellowness(+) and h° represents the hue angle (Li et al. 2021). At the same time, both a* and b* are positive, which means that orange is a mixture of red and yellow. These color data have a certain relationship with the dyes used. All in all, alkali treatment can improve the printing quality of cotton fabric through controlling the crystallinity of cotton fibers. Furthermore, the best color strength can be obtained by treating the cotton fabric for 60 s, and the color strength of printed cotton fabric has no obvious change by prolonging the treatment time.
As mentioned above, the color strength of ink-jet printing on cotton fabrics could be improved by controlling the crystallinity of cotton fiber. Figure 6c-6e showed the scanning images of inkjet printed cotton fabrics treated with alkali for different time under laboratory conditions. Compared with untreated cotton fabrics, the color strength of alkali-treated cotton fabrics improved in different extents. And the highest color strength was achieved after 60 s of treatment, and the trend of color change was consistent with Fig. 6a. To verify the feasibility of implementation in the factory, cotton fabrics with alkali treatment time of 0 s, 10 s and 60 s were printed with vega 5000 digital inkjet printing machine, as shown in Fig. 6f-6h, and the color change trend was consistent with the above.
Table 3
Color parameters of inkjet printed cotton fabrics.
Treated time(s)
|
L*
|
a*
|
b*
|
C*
|
h°
|
0
|
62.37
|
56.22
|
64.23
|
85.36
|
48.80
|
10
|
62.06
|
56.52
|
67.18
|
87.79
|
49.93
|
20
|
61.86
|
57.07
|
68.72
|
89.33
|
50.29
|
30
|
60.45
|
57.99
|
66.20
|
88.01
|
48.78
|
60
|
60.31
|
58.38
|
69.02
|
90.40
|
49.78
|
90
|
60.38
|
58.46
|
68.55
|
90.09
|
49.54
|
Table 4 showed the color fastness and breaking strength of different cotton samples. The range of color fastness was from 1 to 5, the larger the value of color fastness, the better the color fastness (An et al. 2020b). All printed products showed excellent color fastness to washing and rubbing, as all color fastness levels were higher than 4. Table 3 also indicated that cotton fabrics treated with alkali exhibit better mechanical properties than untreated fabrics. In the process of alkali treatment, the cellulose macromolecules were arranged neatly and the orientation of the fiber was increased due to the presence of tension. As a result, cellulose molecular chains could more synergistically resist the destruction of external forces, thus reducing the fracture phenomenon caused by stress concentration (Ahmed et al. 2017). Hence, the treated cotton fabric got a better breaking strength than original fabric.
Table 4
Color fastness and breaking strength of the cotton fabrics. a
Treated Time(s)
|
Washing Fastness
|
Rubbing Fastness
|
Breaking strength(N)
|
SC
|
CC
|
Dry
|
Wet
|
0
|
4–5
|
4–5
|
4–5
|
4–5
|
486.0
|
10
|
4–5
|
4–5
|
4–5
|
4–5
|
493.0
|
20
|
4–5
|
4
|
5
|
4–5
|
494.0
|
30
|
4–5
|
4–5
|
5
|
5
|
495.6
|
60
|
4–5
|
4–5
|
5
|
5
|
495.2
|
90
|
4–5
|
4
|
5
|
4–5
|
495.0
|
a SC = Staining to cotton fabric, CC = color change. |