Supercritical CO2 Assisted TiO2 Preparation to Improve the UV Resistance Properties of Cotton Fiber

Cotton fiber is favored by people because of its good moisture absorption, heat preservation, soft feel, comfortable wearing and other excellent performance. In recent years, due to the destruction of the ozone layer, the intensity of ultraviolet radiation at ground level has increased. Cotton fiber will degrade under long time ultraviolet irradiation, which limits the outdoor application of cotton fiber. In this study, titanium dioxide (TiO2) particles were prepared on the surface of cotton fibers with the help of supercritical carbon dioxide (SCCO2) to improve the UV resistance of cotton fibers. The effects of SCCO2 treatment on the morphology, surface composition, thermal stability, photostability and mechanical properties of TiO2 were studied by Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, thermogravimetric analysis, UV-VIS spectroscopy, and single fiber test. The results showed that TiO2 particles were generated on the fiber surface, which reduced the photo-degradation rate of cotton fiber. This is because TiO2 can absorb UV rays and reduce the absorption of UV rays by the cotton fiber itself. The synthesis process of SCCO2 is simple and environmentally friendly, which provides a promising technology for the synthesis of metal nitrogen dioxide on natural plant fibers.


Introduction
Cotton fabric has excellent moisture absorption, heat preservation and air permeability, which is widely favored by people, and is also widely used in medical treatment and various high-end clothing products. In recent years, because the ozone layer is destroyed gradually, the ultraviolet radiation on the ground increases. Under ultraviolet irradiation, cotton fiber is prone to photo-oxidation reaction and aging phenomenon, which not only affects the color of the fabric, but also seriously affects the mechanical properties of the fabric, reduces the wear resistance of the fabric, and increases the ultraviolet radiation of human skin [1].The US Environmental Protection Agency estimates that ozone depletion will cause some new negative health effects by 2075, such as accelerated skin aging, photo dermatosis, erythema, and severe skin cancer [2]. Therefore, the need to provide UV protection is strong, and cotton textiles play an important role in this regard.
The anti-photoaging property of cotton fiber can be improved by adding antioxidants on the surface of cotton fiber. Titanium dioxide (TiO 2 ), which has high chemical stability, high refractive index [3][4][5][6], non-toxicity and strong light catalytic activity, has attracted much attention in recent years due to its potential applications in the energy and environmental fields [7,8]. TiO 2 has excellent UV shielding, infrared absorption and photocatalytic properties, and its application on cotton fibers can give cotton fibers a variety of functions such as antibacterial, self-cleaning, and UV protection. As a general liquid-phase based method for the synthesis of particles and inorganic network oligomers, sol-gel method has Degreasing experiments were carried out on cleaned cotton fibers by adding 400 mL NaOH and 0.08 g osmolyte to 80 g cotton. The degreasing work is divided into three stages: the first stage is heating up, heating up to 90 °C for 30 min; The second stage was the heat preservation stage, and the temperature was controlled at 90 °C for 50 min. In the third stage, warming was continued at 100 degrees for 10 min. The fourth stage is cooling down.

Synthesis of TiO2 in SCCO2
The experiment was carried out in a stainless-steel supercritical reactor, which was equipped with a booster pump to push CO2 into the reactor. The reactor device is shown in Figure 1. TiO2 preparation on cotton fiber surface is divided into two steps: (1) 20 mL of TBT precursor was placed on the bottom of the reactor, and then the defatted cotton fiber was placed on the stainless steel frame in the reactor to separate it from the TBT precursor. The temperature was adjusted to 100 °C and pressure was set to 10 MPa, and the response time was set for 2 h. Because supercritical CO2 fluid (SCCO2) has the function of carrying, swelling and dissolving, TBT can be dissolved and carried to the fiber surface, and the resulting fiber is called CF-TBT. (2) 30 mL ethanol, 0.1 mL glacial acetic acid, and 3 mL deionized water were mixed and stirred at the bottom of the reactor, and the CF-TBT fiber obtained in the first step was placed on the stainless steel frame in the reactor. Adjust the temperature to 100°C and pressure to 10 MPa again, and the reaction time was 2 h. TiO2 was prepared on the fiber surface by in situ hydrolysis of the precursor TBT deposited on the fiber surface. After the reaction, the fibers were washed with acetone, ethanol, and deionized water and dried, and the resulting fiber is called CF-TiO2.The schematic illustration of a two-step process is shown in Figure 2. TiO 2 preparation on cotton fiber surface is divided into two steps: (1) 20 mL of TBT precursor was placed on the bottom of the reactor, and then the defatted cotton fiber was placed on the stainless steel frame in the reactor to separate it from the TBT precursor. The temperature was adjusted to 100 • C and pressure was set to 10 MPa, and the response time was set for 2 h. Because supercritical CO 2 fluid (SCCO 2 ) has the function of carrying, swelling and dissolving, TBT can be dissolved and carried to the fiber surface, and the resulting fiber is called CF-TBT. (2) 30 mL ethanol, 0.1 mL glacial acetic acid, and 3 mL deionized water were mixed and stirred at the bottom of the reactor, and the CF-TBT fiber obtained in the first step was placed on the stainless steel frame in the reactor. Adjust the temperature to 100 • C and pressure to 10 MPa again, and the reaction time was 2 h. TiO 2 was prepared on the fiber surface by in situ hydrolysis of the precursor TBT deposited on the fiber surface. After the reaction, the fibers were washed with acetone, ethanol, and deionized water and dried, and the resulting fiber is called CF-TiO 2 .The schematic illustration of a two-step process is shown in Figure 2.

Characterizations
The surface functional groups were studied using Fourier transform infrared spectroscopy (FTIR) with a diamond attenuated total reflectance. The surface morphology and chemical compositions of the CF and CF-TiO2 were determined. The morphology of the pretreated cotton fibers was observed using a JEM-6390LV scanning electron microscope (SEM) (JEOL, Tokyo, Japan) at 15 kV. The samples were coated with platinum using a vacuum sputter coater before they were observed. X-ray diffraction (XRD, Smartlab9Kw, Tokyo, Japan) was used to determine the crystalline phase of CF and CF-TiO2. The measurement was conducted with a Cu Kα radiation source (λ = 1.5406 Å) with 40 kV and 200 mA), followed by a scanning range of 5.0°-90.0° at a speed of 5°/min. A UV-visible (UV-

Characterizations
The surface functional groups were studied using Fourier transform infrared spectroscopy (FTIR) with a diamond attenuated total reflectance. The surface morphology and chemical compositions of the CF and CF-TiO 2 were determined. The morphology of the pretreated cotton fibers was observed using a JEM-6390LV scanning electron microscope (SEM) (JEOL, Tokyo, Japan) at 15 kV. The samples were coated with platinum using a vacuum sputter coater before they were observed. X-ray diffraction (XRD, Smartlab9Kw, Tokyo, Japan) was used to determine the crystalline phase of CF and CF-TiO 2 . The measurement was conducted with a Cu Kα radiation source (λ = 1.5406 Å) with 40 kV and 200 mA), followed by a scanning range of 5.0 • -90.0 • at a speed of 5 • /min. A UV-visible (UV-Vis) spectrophotometer (UV3600, Shimadzu, Japan) equipped with an integrating sphere was used to measure the absorbance of the fibers in the wavelength range of 200-450 nm. The cotton fibers were cut into powders. Then, the powder of the fiber was pressed into a thin layer at the center of the sphere and the measurements were performed. A thermogravimetric analyzer (TGA550, TA, New Castle, DE, USA) was used to measure the thermal stability of the fibers. Temperature ramp measurements were conducted under a nitrogen atmosphere from 30 to 900 • C at 20 • C·min −1 . A simple artificial accelerated aging tester was built with a UV lamp (UVB 280-315 nm, 40 W, lamp length of 1220 mm, Baoxiang Lighting Technology Co. Ltd., Guangzhou, China) to study the effect of UV aging over time on the mechanical properties of the fibers. The distance between the fiber sample and the UV lamp was 20 cm. The fibers were placed in parallel rows in the sample tray which was exposed to 40 W/m 2 and held at a relative humidity of 60% for 192 h, according to the Chinese Standard GB/T 14522-93. The mechanical properties of single fibers of cotton fiber were measured by high-precision plant staple fiber mechanical properties tester (JSF08, Powereach, Shanghai, China). The tensile strength and elastic modulus of single fiber were measured. The measuring range of the sensor is 980.7 mN, the tensile load accuracy is 0.01 µN, and the tensile rate is 0.05 mm /min. The tensile strength, elastic modulus, and elongation at break were calculated according to the formula in GB/T 35378-2017. The results were analyzed by Excel and origin software. Figure 3 is the measurement chart of fiber mechanical properties, Figure 3a

Characterizations
The surface functional groups were studied using Fourier transform infrared spectroscopy (FTIR) with a diamond attenuated total reflectance. The surface morphology and chemical compositions of the CF and CF-TiO2 were determined. The morphology of the pretreated cotton fibers was observed using a JEM-6390LV scanning electron microscope (SEM) (JEOL, Tokyo, Japan) at 15 kV. The samples were coated with platinum using a vacuum sputter coater before they were observed. X-ray diffraction (XRD, Smartlab9Kw, Tokyo, Japan) was used to determine the crystalline phase of CF and CF-TiO2. The measurement was conducted with a Cu Kα radiation source (λ = 1.5406 Å) with 40 kV and 200 mA), followed by a scanning range of 5.0°-90.0° at a speed of 5°/min. A UV-visible (UV-Vis) spectrophotometer (UV3600, Shimadzu, Japan) equipped with an integrating sphere was used to measure the absorbance of the fibers in the wavelength range of 200-450 nm. The cotton fibers were cut into powders. Then, the powder of the fiber was pressed into a thin layer at the center of the sphere and the measurements were performed. A thermogravimetric analyzer (TGA550, TA, New Castle, DE, USA) was used to measure the thermal stability of the fibers. Temperature ramp measurements were conducted under a nitrogen atmosphere from 30 to 900 °C at 20 °C·min -1 . A simple artificial accelerated aging tester was built with a UV lamp (UVB 280-315 nm, 40 W, lamp length of 1220 mm, Baoxiang Lighting Technology Co. Ltd., Guangzhou, China) to study the effect of UV aging over time on the mechanical properties of the fibers. The distance between the fiber sample and the UV lamp was 20 cm. The fibers were placed in parallel rows in the sample tray which was exposed to 40 W/m 2 and held at a relative humidity of 60% for 192 h, according to the Chinese Standard GB/T 14522-93. The mechanical properties of single fibers of cotton fiber were measured by high-precision plant staple fiber mechanical properties tester (JSF08, Powereach, Shanghai, China). The tensile strength and elastic modulus of single fiber were measured. The measuring range of the sensor is 980.7 mN, the tensile load accuracy is 0.01 μN, and the tensile rate is 0.05 mm /min. The tensile strength, elastic modulus, and elongation at break were calculated according to the formula in GB/T 35378-2017. The results were analyzed by Excel and origin software. Figure 3 is the measurement chart of fiber mechanical properties, Figure 3a

FTIR Analysis
In order to study the binding mode of TiO 2 particles and cotton fiber, Fourier infrared spectroscopy ATR method was used to further characterize CF and CF-TiO 2 samples. Curve a in Figure 4 is the infrared spectrum of raw cotton fiber, and curve b is the infrared spectrum of CF-TiO 2 .The peak value at 3344 cm −1 is the intramolecular hydrogen bond stretching vibration of cellulose in CF. The peak value at 2917 cm −1 is the asymmetric stretching vibration of CH 2 of non-cellulose of CF [30]. The H-O bending vibration peak is at 1639 cm −1 .
In order to study the binding mode of TiO2 particles and cotton fiber, Fourier infrared spectroscopy ATR method was used to further characterize CF and CF-TiO2 samples. Curve a in Figure 4 is the infrared spectrum of raw cotton fiber, and curve b is the infrared spectrum of CF-TiO2.The peak value at 3344 cm −1 is the intramolecular hydrogen bond stretching vibration of cellulose in CF. The peak value at 2917 cm −1 is the asymmetric stretching vibration of CH2 of non-cellulose of CF [30]. The H-O bending vibration peak is at 1639 cm −1 . The vibration peak at the corresponding position of curve a in Figure 4 can also be observed at the same position of curve b, and the vibration peak intensity of 1063 cm −1 , 2917 cm −1 and 3344 cm −1 is obviously enhanced and tends to shift towards the higher wave number. This is because in the process of TiO2 synthesis on the surface of cotton fiber, a large number of hydroxyl groups contained on the surface of TiO2 formed hydrogen bonds with the hydroxyl groups on the surface of CF, which strengthened the intensity of the vibration absorption peak of CF-TiO2 [31]. At the same time, the supercritical-treated cotton fiber (Figure 4b) showed a new absorption peak at 616 cm −1 [32] compared with the untreated CF (Figure 4a), which was just within the Ti-O bond absorption peak interval, indicating that TiO2 particles were successfully prepared on the surface of the cotton fiber.

SEM Analysis
The surface morphology of cotton fiber was studied by scanning electron microscope. SEM images of CF and CF-TiO2 are shown in Figure 5. It can be seen that the untreated fibers have a smooth and clean surface. Compared with untreated CF, the surface of modified CF-TiO2 (Figure 5b) is rough with many obvious particles on the surface. The vibration peak at the corresponding position of curve a in Figure 4 can also be observed at the same position of curve b, and the vibration peak intensity of 1063 cm −1 , 2917 cm −1 and 3344 cm −1 is obviously enhanced and tends to shift towards the higher wave number. This is because in the process of TiO 2 synthesis on the surface of cotton fiber, a large number of hydroxyl groups contained on the surface of TiO 2 formed hydrogen bonds with the hydroxyl groups on the surface of CF, which strengthened the intensity of the vibration absorption peak of CF-TiO 2 [31]. At the same time, the supercritical-treated cotton fiber (Figure 4b) showed a new absorption peak at 616 cm −1 [32] compared with the untreated CF (Figure 4a), which was just within the Ti-O bond absorption peak interval, indicating that TiO 2 particles were successfully prepared on the surface of the cotton fiber.

SEM Analysis
The surface morphology of cotton fiber was studied by scanning electron microscope. SEM images of CF and CF-TiO 2 are shown in Figure 5. It can be seen that the untreated fibers have a smooth and clean surface. Compared with untreated CF, the surface of modified CF-TiO 2 (Figure 5b) is rough with many obvious particles on the surface. In order to further determine the distribution of TiO2 on the surface of cotton fabric, SEM combined with EDX-mapping was used to analyze the surface of CF-TiO2 ( Figure 6). It can be obviously observed from Figure 6a that there are particles on the surface of cotton fiber. We performed anEDX analysis of particle number 13 in Figure 6a and confirmed that the particle was TiO2 from the energy chromatogram. The Ti peak in Figure 6b was not obvious due to the relatively low TiO2 load. XPS analysis further confirmed the existence of Ti atoms on the surface of modified cotton fiber.Combined with the FTIR analysis, we can conclude that these particles are TiO2. In order to further determine the distribution of TiO 2 on the surface of cotton fabric, SEM combined with EDX-mapping was used to analyze the surface of CF-TiO 2 ( Figure 6). It can be obviously observed from Figure 6a that there are particles on the surface of cotton fiber. We performed anEDX analysis of particle number 13 in Figure 6a and confirmed that the particle was TiO 2 from the energy chromatogram. The Ti peak in Figure 6b was not obvious due to the relatively low TiO 2 load. XPS analysis further confirmed the existence  In order to further determine the distribution of TiO2 on the surface of cotton fabric, SEM combined with EDX-mapping was used to analyze the surface of CF-TiO2 ( Figure 6). It can be obviously observed from Figure 6a that there are particles on the surface of cotton fiber. We performed anEDX analysis of particle number 13 in Figure 6a and confirmed that the particle was TiO2 from the energy chromatogram. The Ti peak in Figure 6b was not obvious due to the relatively low TiO2 load. XPS analysis further confirmed the existence of Ti atoms on the surface of modified cotton fiber.Combined with the FTIR analysis, we can conclude that these particles are TiO2.

XPS Analysis
In order to study the effect of supercritical fluid treatment on the surface of cotton fiber, the surface components of CF and CF-TiO2 were determined by XPS method. The results showed that the surface oxygen content of CF-TiO2 fiber was higher than that of untreated CF. As shown in Figure 8, C1s and O1s peaks appear at 284.8 and 531.6 eV, respectively. The CF-TiO2 fiber shows a new peak of titanium atoms at 457.37 eV compared to the untreated CF. The result of element content change is shown in Table 1. Compared with untreated CF, the content of C in CF-TiO2 decreased, the content of O and Ti increased, and the ratio of O: C increased from 0.1780 to 0.2705. The content of Ti reached 1.2%, indicating that TiO2 was successfully prepared on the surface.

XPS Analysis
In order to study the effect of supercritical fluid treatment on the surface of cotton fiber, the surface components of CF and CF-TiO 2 were determined by XPS method. The results showed that the surface oxygen content of CF-TiO 2 fiber was higher than that of untreated CF. As shown in Figure 8, C1s and O1s peaks appear at 284.8 and 531.6 eV, respectively. The CF-TiO 2 fiber shows a new peak of titanium atoms at 457.37 eV compared to the untreated CF. The result of element content change is shown in Table 1. Compared with untreated CF, the content of C in CF-TiO 2 decreased, the content of O and Ti increased, and the ratio of O: C increased from 0.1780 to 0.2705. The content of Ti reached 1.2%, indicating that TiO 2 was successfully prepared on the surface. results showed that the surface oxygen content of CF-TiO2 fiber was higher than that of untreated CF. As shown in Figure 8, C1s and O1s peaks appear at 284.8 and 531.6 eV, respectively. The CF-TiO2 fiber shows a new peak of titanium atoms at 457.37 eV compared to the untreated CF. The result of element content change is shown in Table 1. Compared with untreated CF, the content of C in CF-TiO2 decreased, the content of O and Ti increased, and the ratio of O: C increased from 0.1780 to 0.2705. The content of Ti reached 1.2%, indicating that TiO2 was successfully prepared on the surface.

Mechanical Properties
Mechanical properties of CF and CF-TiO 2 samples are shown in Table 2. The tensile strength and modulus of CF-TiO 2 treated with SCCO 2 increased by 6.37% and 7.80% compared with the untreated CF samples. The reason for this phenomenon may be that SCCO 2 fluid has a relatively strong washing effect, which can remove some impurities on the surface of cotton fiber. At the same time, SCCO 2 fluid can enhance the movement of the chain segment of cotton fiber, and further optimize the structure of the molecular chain of cotton fiber under the action of certain pressure and temperature. To a certain extent, the structural defects of the fiber are reduced, and the mechanical properties of cotton fiber are improved.   (Figure 9f), there were only some fine lines on the surface, without the above obvious etching and groove. Meanwhile, as shown in Figure 9, TiO 2 particles gradually become smaller with the continuous absorption of UV. Therefore, SCCO 2 assisted in situ preparation of TiO 2 on the surface played a role in slowing down photoaging of cotton fibers and played a protective role on fibers. Figure 9 shows the microscopic morphology (SEM) of unmodified CF and CF-TiO2 after 48 h and 196 h photo-aging. It can be seen that the surface of untreated CF ( Figure  9a) is relatively smooth. After accelerated aging with UV light for 48 h, the surface of untreated CF (Figure 9b) becomes rough with a small amount of etching and streaking. After 196 h accelerated aging, the untreated CF (Figure 9c) surface became rougher, with more etchings, furrows, and streaks. However, after 48 h accelerated aging of the treated CF-TiO2 (Figure 9d) sample, the surface of sample (Figure 9e) did not show obvious changes, and after 196 h accelerated aging of sample (Figure 9f), there were only some fine lines on the surface, without the above obvious etching and groove. Meanwhile, as shown in Figure 9, TiO2 particles gradually become smaller with the continuous absorption of UV. Therefore, SCCO2 assisted in situ preparation of TiO2 on the surface played a role in slowing down photoaging of cotton fibers and played a protective role on fibers.

UV-Vis Analysis
As can be seen from Figure 10, the UV-Vis spectrum of CF is in the range of 200-600 nm, in which 250-400 nm is the ultraviolet absorption region and 400-600 nm is the visible absorption region. Compared with untreated CF, the UV absorption effect of treated CF-TiO 2 was significantly improved. CF-TiO 2 samples have strong absorption at 200-400 nm, but almost no absorption at 400-600 nm. As can be seen from the figure, the absorption value of CF-TiO 2 sample at 258 nm was increased by 21.51% compared with untreated CF. The reason for this phenomenon is as follows: the electronic structure of TiO 2 is characterized by an empty conduction band and a full valence band. The gap between the conduction band and the valence band is 3.0 eV, which is equivalent to the energy of a photon with a wavelength of 413 nm. When the wavelength of TiO 2 is not higher than 413 nm, the electrons in the valence band will absorb energy and produce a transition, forming two kinds of carrier fluids, thus playing the role of UV shielding [34].

UV-Vis Analysis
As can be seen from Figure 10, the UV-Vis spectrum of CF is in the range of 200-600 nm, in which 250-400 nm is the ultraviolet absorption region and 400-600 nm is the visible absorption region. Compared with untreated CF, the UV absorption effect of treated CF-TiO2 was significantly improved. CF-TiO2 samples have strong absorption at 200-400 nm, but almost no absorption at 400-600 nm. As can be seen from the figure, the absorption value of CF-TiO2 sample at 258 nm was increased by 21.51% compared with untreated CF. The reason for this phenomenon is as follows: the electronic structure of TiO2 is characterized by an empty conduction band and a full valence band. The gap between the conduction band and the valence band is 3.0 eV, which is equivalent to the energy of a photon with a wavelength of 413 nm. When the wavelength of TiO2 is not higher than 413 nm, the electrons in the valence band will absorb energy and produce a transition, forming two kinds of carrier fluids, thus playing the role of UV shielding [34].

Mechanical Properties
The CF and CF-TiO2 samples were subjected to UV aging for different times, and the mechanical properties of the fibers before and after modification were studied. Figure 11 shows the strength and modulus mechanical properties of CF and CF-TiO2 samples after

Mechanical Properties
The CF and CF-TiO 2 samples were subjected to UV aging for different times, and the mechanical properties of the fibers before and after modification were studied. Figure 11 shows the strength and modulus mechanical properties of CF and CF-TiO 2 samples after UV aging for 24 h, 48 h, 96 h, 144 h, and 196 h. With the increase of photoaging time, the strength and modulus of cotton fiber showed a downward trend, which indicated that ultraviolet radiation caused damage to the mechanical properties of cotton fiber. However, the decreased degree of CF-TiO 2 samples is lower than that of untreated CF, indicating that the preparation of TiO 2 on the surface of cotton fiber can effectively slow down the fiber photoaging rate.
It can be seen from Figure 11 that the tensile strength and modulus retention rates of CF-TiO 2 samples after UV irradiation for 196 h are 76.53% and 68.46%, respectively, which are 5-7% higher than those of untreated CF samples. These results are similarly attributed to the photostabilization of TiO 2 particles on the surface. TiO 2 absorb ultraviolet light, preventing the ultraviolet light from causingthe degradation of organic molecular bonds in cotton fiber. The volume of TiO 2 becomes smaller with the continuous absorption of UV light until it disappears. After that, UV light will continue to destroy the chemical bonds of cotton fibers, resulting in the decrease of fiber mechanical properties. These results indicated that in situ preparation of TiO 2 on the surface of cotton fibers alleviated the fiber photoaging rate.  Figure 12 shows the thermogravimetric analysis diagram of CF and CF-TiO2 samples. The first interval (100-200 °C) is the microweight loss stage, which is mainly the loss process of intermolecular bound water. The second stage is the thermal decomposition stage (400-600 °C), and the start time of fiber decomposition is basically the same. Due to the better thermal stability of TiO2, the residual mass of CF-TiO2 samples is higher than that of untreated CF fibers. The third zone (600-850 °C) is the stable stage of cotton fiber carbonization. The results show that the synthesis of TiO2 on the surface of cotton fibers has little effect on the thermal stability of the fibers.  Figure 12 shows the thermogravimetric analysis diagram of CF and CF-TiO 2 samples. The first interval (100-200 • C) is the microweight loss stage, which is mainly the loss process of intermolecular bound water. The second stage is the thermal decomposition stage (400-600 • C), and the start time of fiber decomposition is basically the same. Due to the better thermal stability of TiO 2 , the residual mass of CF-TiO 2 samples is higher than that of untreated CF fibers. The third zone (600-850 • C) is the stable stage of cotton fiber carbonization. The results show that the synthesis of TiO 2 on the surface of cotton fibers has little effect on the thermal stability of the fibers.

Thermogravimetric Analysis (TG)
The first interval (100-200 °C) is the microweight loss stage, which is mainly the loss process of intermolecular bound water. The second stage is the thermal decomposition stage (400-600 °C), and the start time of fiber decomposition is basically the same. Due to the better thermal stability of TiO2, the residual mass of CF-TiO2 samples is higher than that of untreated CF fibers. The third zone (600-850 °C) is the stable stage of cotton fiber carbonization. The results show that the synthesis of TiO2 on the surface of cotton fibers has little effect on the thermal stability of the fibers.

Illustration of Modification
According to the above results and discussion, the model of the modified cotton fiber process is shown in Figure 13. With SCCO2 as the carrier, TBT as the TiO2 precursor can be dissolved and carried to the surface of cotton fiber and deposited on the surface of the fiber when CO2 is decompressed. In the second step, TiO2 particles were synthesized on the surface of cotton fiber by ethanol hydrolysis. TiO 2+ is absorbed by a large number of hydroxyl groups on the surface of cotton fiber, and then formed through hydrolysis and condensation to grow into TiO2 particles [35]. In addition, TiO2 has hydrogen bonds with

Illustration of Modification
According to the above results and discussion, the model of the modified cotton fiber process is shown in Figure 13. With SCCO 2 as the carrier, TBT as the TiO 2 precursor can be dissolved and carried to the surface of cotton fiber and deposited on the surface of the fiber when CO 2 is decompressed. In the second step, TiO 2 particles were synthesized on the surface of cotton fiber by ethanol hydrolysis. TiO 2+ is absorbed by a large number of hydroxyl groups on the surface of cotton fiber, and then formed through hydrolysis and condensation to grow into TiO 2 particles [35]. In addition, TiO 2 has hydrogen bonds with the hydroxyl group of CF molecular chains, and provides secondary inter-fiber forces, which contribute to improving mechanical properties. the hydroxyl group of CF molecular chains, and provides secondary inter-fiber forces, which contribute to improving mechanical properties.

Conclusions
In this study, a two-step method for in situ synthesis of TiO2 on the surface of cotton fibers was proposed to improve the photoaging resistance of cotton fibers. FTIR, SEM, DEX, and XPS results showed that TiO2 particles were successfully prepared on the surface and that the TiO2 particles were bonded together by hydrogen bonding by the reaction with hydroxyl groups on the cotton fiber surface. In addition, SEM and mechanical properties test results show that after UV aging simulation of cotton fiber, the surface of CF-TiO2 samples is smoother than that of untreated CF samples under the same light time, and the fracture strength and modulus of CF-TiO2 samples are higher than that of untreated CF samples. This is because TiO2 can absorb UV rays and reduce the absorption of UV rays by the cotton fiber itself. These findings suggest that the process of preparing particles on the surface of cotton fibers assisted by SCCO2 is one of the important methods

Conclusions
In this study, a two-step method for in situ synthesis of TiO 2 on the surface of cotton fibers was proposed to improve the photoaging resistance of cotton fibers. FTIR, SEM, DEX, and XPS results showed that TiO 2 particles were successfully prepared on the surface and that the TiO 2 particles were bonded together by hydrogen bonding by the reaction with hydroxyl groups on the cotton fiber surface. In addition, SEM and mechanical properties test results show that after UV aging simulation of cotton fiber, the surface of CF-TiO 2 samples is smoother than that of untreated CF samples under the same light time, and the fracture strength and modulus of CF-TiO 2 samples are higher than that of untreated CF samples. This is because TiO 2 can absorb UV rays and reduce the absorption of UV rays by the cotton fiber itself. These findings suggest that the process of preparing particles on the surface of cotton fibers assisted by SCCO 2 is one of the important methods to improve fiber properties and may be a potential application for oxide modified CFs and other natural fibers.