Enhancing digital ink-jet printing patterns quality through controlling the crystallinity of cotton bers

Cotton bers have a high crystallinity, which makes a large number of reactive hydroxyl groups blocked and therefore affects the ink-jet printing performance of reactive dyes on cotton fabrics. In this work, the alkali treatment was employed to adjust the structure of cotton bers. The crystallinity of treated cotton bers reduced from 73.9–58.5%, and the breaking strength did not decrease compared with original cotton ber. Thus, the accessible reactive hydroxyl groups and the wettability were enhanced for treated cotton bers, which promoted the penetration of inks into the bers. The optimal K/S value of 23.47 was achieved for treated cotton fabrics which was higher than that of untreated cotton fabrics (17.15). Meanwhile, the printed fabrics displayed good washing fastness, rubbing fastness and glossiness. This work provides an effective way for improving the utilization of dye solution and producing high-quality cotton fabric digital printing products.


Introduction
Cotton has been well known as one of the most popular agricultural renewable resources because of its excellent hand feel, moisture absorption and air permeability (Zhang et (Pransilp et al. 2016). However, most of the current studies only focus on the pretreatment of cotton fabrics, and the in uence of cotton ber crystallinity on the ink-jet printing performance has not been studied.
It has been reported that alkali treatment could change the internal structure and physicochemical properties of cellulose. Wakida, T et al. used sodium hydroxide/liquid ammonia to treat cotton fabrics, and found that with the decrease of the crystallinity of cotton bers, the water absorption rate and dye utilization rate increased (Wakida et al. 2000). Poondodi, G. R et al. treated regenerated cellulose bers by sodium hydroxide swelling method. The results showed that the dyeing performance of regenerated cellulose yarn was effectively improved after alkali treatment (Poongodi et al. 2019). Gao et al. pointed out that using NaOH/urea aqueous solution system to treat waste cotton fabric can change its morphology and reduce its crystallinity (Gao et al. 2020). As a consequence, it is feasible to change the morphology and chemical properties of cotton bers through alkali solution treatment, thus to improve the inkjet printing performance. However, the related research in this area has not been reported yet.
In this work, the effects of cotton ber crystallinity on color performance and ink penetration of cotton fabric inkjet printing were investigated. The crystallinity of cotton ber could be controlled by adjusting the alkali treatment time. The crystal structure of cotton bers was studied by 2D-GIXD scattering patterns. The changes of surface groups were characterized by FTIR spectra. The morphology changes of cotton fabric were observed by the scanning electron microscope (SEM). Meanwhile, contact angle and capillary effect were measured to characterize the wettability of cotton fabrics before and after treatment, and the wetting process of ink drops on the fabric surface was observed. The color strength and color fastness of printed cotton fabrics were evaluated to test the ink-jet printing performance.
Moreover, the breaking strength and glossiness of the cotton fabrics were also tested. Based on this study, the alkali treated fabric was identi ed as a good substrate for digital inkjet printing, which provided an effective way to produce high-quality printed cotton products.

Preparation of dye solutions
The

Alkali treatment
The cotton fabric with tension was soaked in a 250 g/L NaOH aqueous solution at room temperature for 10 s, 20 s, 30 s, 60 s, 90 s, and then immersed in 80°C water to remove extra alkali solution. Finally, the cotton fabric was neutralized by 2 g/L sulfuric acid.

Sodium alginate pretreatment
All cotton fabrics were soaked and rolled with a solution containing sodium alginate, urea, sodium bicarbonate and sodium m-nitrobenzene sulfonate at 90 % pickup, and dried at 100°C (Tang et al. 2020).

Inkjet printing system
The inkjet printing system used in this research under laboratory conditions was supplied by Shanghai Ruidu Optoelectronics Technology Co., Ltd. As shown in Fig. 2, the inkjet printing system consists of a nozzle, a movable platform, a light source and a high-speed camera. When inkjet printing, the cotton fabric was adhered to a movable platform, and the nozzle can move up and down. By controlling the movement of the nozzle and the platform, patterns can be generated on the cotton fabric (Zhang et al.

Surface analysis
The Fourier transform infrared (FTIR) spectroscopy of cotton fabrics was carried out using a Nicolet iS 10 instrument (Thermo Fisher Scienti c, US). The two-dimension grazing incidence X-ray diffraction(2D-GIXD) of cotton fabrics were measured by two-dimensional grazing incidence wide-angle scattering (Xenocs company, France). A scanning electron microscope (SEM, Phenom Pure) was used to observe the morphological changes of cotton bers treated with high concentration alkali solution under 5 kV accelerating voltage.

Wettability
The wettability of cotton fabric before and after alkali treatment was characterized by measuring contact angle and capillary effect. The static contact angle of cotton fabric was measured by using Dataphysics-OCA 25(Germany). A drop (ethylene glycol) was injected into the cotton fabric. The volume of the drop was 3 µL and the injection speed was 0.5 µL/s. At the same time, this method was used to simulate the wetting process of ink drops on cotton fabric. The fabric before and after treatment was cut into 3 cm *25 cm strips, and then one side of the cotton fabric was immersed in deionized water to observe the climbing height of deionized water on the cotton fabric at different times.
2.6.4. Glossiness 3 nh glossmeter was used to test the glossiness of cotton fabrics before and after treatment, and each fabric was measured at six different locations, and then obtained the average of the data.

Breaking strength and color fastness
The breaking strength was measured by electronic fabric strength tester (Shanghai Sansi Experimental Instrument Co., Ltd). A Sw-12A washing color fastness tester (Wuxi Textile Instrument Co., China) was used to measure the washing fastness according to ISO 105-C10:2007. The rubbing fastness was tested by Q238BB rubbing colorfastness tester (Gellowen Co., Ltd., UK) based on GB/T 3920 − 2008.

Surface properties analysis of cotton fabric
The two-dimension grazing incidence X-ray diffraction measurements were used to observe the crystallinity of cotton ber. And the 2D-GIXD scattering patterns were depicted in Fig. 3a 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 (Sebe et al. 2012). In the progress of alkali treatment, sodium hydroxide entered the amorphous region of the cotton ber and separated the crystallites in the cotton ber, resulting in the structure of Nacellulose . After the alkali treatment, Na-cellulose was rinsed with water to remove alkali, and then Nacellulose was converted to Na-cellulose (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 ber was limited. Therefore, the cellulose was not completely transformed into cellulose during the treatment. As shown in Fig. 3i, the original samples exhibited the crystalline form of cellulose . The crystal structure of treated cotton ber is a mixed structure of cellulose and cellulose .
To better investigate the changes of cotton ber crystallinity after treatment, Eq. (4) was used to calculate the crystallinity of cotton ber ), and the results were shown in Table 1. 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 ber. From Table 1, the crystallinity of the treated cotton ber was reduced from 73.9-58.5%. This result implied that alkali could reduce the crystallinity of cotton bers through breaking the degree of order of cellulose macromolecular chains.
To further investigate the surface properties of cotton bers, FTIR spectra of the cotton fabrics, before and after alkali treatment, were demonstrated in Fig. 3h The morphology of cotton bers 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 ber was at waist shaped with a large cell cavity. The longitudinal direction of untreated cotton ber had natural distortion and rough surface. As for the cotton fabrics treated by high concentration alkali solution, sodium hydroxide solution diffuses rapidly into the ber. Because of the deconvolution, cotton bers 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 ber gradually changed from at oval to round with the increase of treatment time, and the cell cavity gradually became smaller, and nally reduced to a line. As illustrated in Fig. 4h-4l, the surfaces of the cotton bers 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 bers treated with alkali solution changed from ear shape to round shape, the wrinkles on the surface of the ber disappeared and the surface became smooth. As a result, the treated bers exhibited an improvement in light re ection, bringing much better glossiness.  After alkali treatment, the ber swelled and the gap between bers 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 ber surface increased and the hydrogen bond between the droplet and the ber surface was enhanced ). As the ber swelled, the ber 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 nally reached an equilibrium state.
As shown in Fig. S2, the diffusion of ink drops on cotton fabric was mainly divided into two parts: the rst 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 nally 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 ber 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).

Study on inkjet printing performance of cotton fabric
The color strength of the cotton fabric directly re ects 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 ber, making the dye molecules more easily penetrate into the bers. At the same time, the crystal area in the ber decreased and the amorphous region increased, leading to the increase of the reaction sites in the bers. 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 ). 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 bers. 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 ber. 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 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 ). 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 ber 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

Conclusions
In this work, the effect of crystallinity on ink-jet printing performance of cotton fabric was investigated. The crystallinity of cotton bers could be modi ed by controlling the alkali treatment time. The treated cotton bers crystallinity was reduced from 73.9-58.5%, and therefore the treated cotton bers had more reaction sites with dyes. Compared with the original cotton ber, the alkali treated cotton ber swelled, which increased the contact area between dyes and bers. Meanwhile, its wettability was also enhanced, which bene tted for the ink drops to penetrate into the ber. Therefore, a higher K/S value of 23.47 was achieved for the treated cotton fabrics compared with the original sample with a K/S value of 17.15. This work provides an effective way to improve the utilization rate of reactive dyes and therefore the printing performance for cotton fabric in digital ink-jet printing.  Schematic diagram of inkjet printing system