Schematic diagram
In order to prepare uniform and stable nano-ZnO dispersion, the dispersant triton and titanate coupling agent were mixed with the nano-ZnO solution, and then the mixture was ultrasonically emulsified for 1h. Cotton fabric was finished with nano-ZnO dispersion by impregnating to improve the thermal conductivity of fabric. As revealed by particle size analysis, the size of nano-ZnO dispersion varied from 300 to 800nm, and the average nano particle size was 458 nm (Fig. 1a). The PDI (dispersion index) was a uniform and stable dispersion of 0.159. As shown in Fig. 1b, SEM images of nano-ZnO, which shows the uniformity of the nano-ZnO particles.
Then, single-sided discontinuous hydrophobic finishing was performed on the cotton fabric by screen printing. The resulted fabrics with a resembling river branches hexagon water-transfer-channel which greatly improved the water transport performance of the fabric (Fig. 1d). Figure 1c illustrates the preparation process of discontinuous hydrophobic/hydrophilic Janus fabric. The improvement of the thermal conductivity of cotton fabric will accelerate the evaporation of sweat on the surface of cotton fabric, which is beneficial to the improvement of the thermal and wet comfort of the fabric. As the preparing method is easy to practice, it contributes a novel approach to the fabrication of directional water transport textiles.
Characterization of Nano-ZnO Cotton Fabric
A commercial cotton fabric was used as the substrate due to its suitable thickness and denseness. According to the SEM image (Fig. 2a, b), the surface of cotton fibric is smooth (Fig. 2a). As shown in Fig. 2b, the cotton fiber is covered by a layer of nano-ZnO particles, which increases the roughness of the cotton fiber, indicating the success in loading the nano-ZnO particles onto the cotton fabric. The chemical compositions of the nano-ZnO coated cotton fabric surface were examined through the EDS analysis. As shown in Fig. 2(c-e), elemental mappings of the nano-ZnO cotton fabric show that the distribution of C and Zn is uniform across the whole nano-ZnO cotton fabric surface, demonstrating that nano-ZnO are well dispersed on the cotton fabric.
As displayed in Fig. 2f, FT-IR spectrogram of cotton fabric and nano-ZnO cotton fabric. The peak at 3337 cm− 1 was derived from -OH stretching vibration, and 1168 cm− 1, 1104 cm− 1, 1058 cm− 1, and 1023 cm− 1 appeared in the spectrum of cotton fabric were derived from -C-O-C- stretching vibration, which constitute the characteristic absorption peaks of cotton fabric. After using titanate coupling agent to finish cotton fabric with nano-ZnO, two new characteristic peaks appeared.(Liu et al. 2012) The bands at 1718 cm− 1 were associated with stretching vibration of phospholipid bond formed by the reaction of the coupling agent and the -OH on the cotton fabric,(Salla et al. 2012) respectively. In addition, the absorption peak around 816 cm− 1 is corresponding to the bending vibration of the C-H in the para-substituted benzene ring of the coupling agent, indicating that nano-ZnO was loaded in the cotton fabric. The crystalline structures of nano-ZnO fabric were identified by X-ray diffraction (XRD). The XRD patterns of nano-ZnO fabric are described in Fig. 2g. Cotton fabric has corresponding cellulose I structure at 2θ = 14.89°, 16.39°, and 22.62° characteristic diffraction peaks. After finishing with nano-ZnO, not only the characteristic diffraction peaks of cellulose I-type structure appeared on the cotton fabric at 2θ = 14.89°, 16.39°, 22.62°, but also nano-ZnO (100), (101), (110) crystal planes appeared at 2θ = 31.79°, 36.37°, 56.67°,(Wang et al. 2017, 2018a) which proved that nano-ZnO was finished on the cotton fabric.
Wettability of Nano-ZnO Cotton Fabric and Printed Cotton Fabric
The moisture wicking performance of the fabrics were enhanced by adjusting the surface wettability. Through the video obtained from the experiment, we observed and compared the water droplet wetting of cotton fabric and nano-ZnO cotton fabric from0 s to 0.15 s. The water contact angle of cotton fabric and nano-ZnO cotton fabric are measured, and the results are shown in Fig. 3(a-c). It is shown that cotton fabric has strong hydrophilicity. In Fig. 3c, it can be seen that the cotton fabric after nano-ZnO pretreatment also has strong hydrophilicity, indicating that nano-ZnO pretreatment will not affect its hydrophilicity, and will not affect the next step of discontinuous hydrophobic printing.
Through single-sided discontinuous hydrophobic screen-printing method, the inner side (the side contacting the skin) is partially hydrophobic and the outer side (the side that does not touch the skin) is hydrophilic. In the same way, the wettability of printed cotton fabric was observed and compared with the video obtained from the experiment. Water contact angle of discontinuous hydrophobic area and discontinuous hydrophilic area are measured, and the results are shown in Fig. 3(d-f). Cotton fabric after hydrophobic treatment possess extremely strong hydrophobicity while other parts without hydrophobically finishing have strong hydrophilicity. As shown in Fig. 3e, water contact angle of those hydrophilic parts decreases dramatically to 0° in the first 0.2 s. On the contrary, cotton fabric after hydrophobic treatment remains 130° in the first 0.2 s and decrease slightly in 1 minute or even longer. Single-sided discontinuous hydrophobic screen-printing makes the inner and outer layers of the cotton fabric own different moisture absorption, which provide a differential capillary effect and realize the unidirectional moisture transport function of the cotton fabric. The sweat produced by the pores of the human body transfers from the inner surface of the cotton fabric to the outer surface and evaporates rapidly. Thus, discontinuous hydrophobic screen-printing fabric possess the ability of one-way water transport.
Moisture Wicking Property of Printed Cotton Fabric
Regarding the fact that different hydrophilic-hydrophobic proportion of the cotton fabric surface would affect the transfer speed of sweat and the one-way water transport ability, the screen-printing patterns with different hydrophilic and hydrophobic ratios are designed. More vividly, once the sweat excreted from human skin touches the inner side, it transfers along the hydrophilic area preferentially which represents the gap width of hexagon water-transfer-channel, just like a river branch. In the discussion of hydrophobic screen-printing section, C stands for cotton fabric, C-Z stands for cotton fabric finished only with nano-ZnO, and C-X% stands for cotton fabrics with single-sided discontinuous hydrophobic treatment, C-Z-25%, C-Z-50%, and C-Z-75% respectively represent the cotton fabric treated with single-sided discontinuous hydrophobic treatment after nano-ZnO finishing (X% is the percentage of hydrophilic area to hydrophobic area). C-Z-H represents the cotton fabric treated with single-sided continuous hydrophobic treatment after nano-ZnO finishing.
Moisture wicking functionality of discontinuous hydrophobic/hydrophilic Janus fabric was tested by M290 liquid moisture management tester, and the results are shown in Table 1. Wetting time is the time when the fabric starts to be wetted. The higher the hydrophilicity of the cotton fabric, the faster it will be wetted. Since cotton fabrics inherently have good hydrophilicity, the wetting time of the fabrics before and after hydrophobic finishing is relatively fast. The wetting time of C-Z, C-Z-75%, C-Z-50%, C-Z-25% and C-Z-H, indicates that the fabric has the ability to quickly absorb moisture. The water absorption rate also reflects the ability of the cotton fabric to absorb water. For C-Z-75%, C-Z-50%, C-Z-25% and C-Z-H, the top surface shows lower water absorption rate than the bottom surface, this is because C-Z-75%, C-Z-50%, C-Z-25%, C-Z-H have undergone hydrophobic treatment, and the treated hydrophobic area make the water absorption rate of the fabric decline. According to the water absorption rate, single-sided discontinuous hydrophobic screen-printing fabric has a better ability to absorb moisture. The max wetted radius reflects the diffusion capacity of moisture in the cotton fabric. In comparison with cotton fabric, the max wetted radius of C-Z-75%, C-Z-50%, C-Z-25%, C-Z-H have been improved. The spreading speed of liquid water is the diffusion rate of fabric when wetting to the maximum wetted radius, which reflects the rapid drying ability of the cotton fabric. The different spreading speed of top surface and bottom surface indicate that the cotton fabric can quickly evaporate water, which is beneficial to moisture transfer and evaporation in the air. Comparing results of C-Z-75%, C-Z-50%, C-Z-25%, all the samples have the directional water transport property. Moreover, it was worth mentioning that C-Z-50% has the largest difference in spreading speed between the top surface and bottom surface which implying that sample C-Z-50% has a preferable directional water transport property. The one-way transport capability is the ratio of the water content difference between the top surface and bottom surface to the total test time, which reflects the directional water transport property directly. Cotton fabric does not possess a directional water transport property with the one-way transmission index of 19.58. After single-side continuous hydrophobic finishing, the one-way transmission index increases to 45.97. However, C-Z-75%, C-Z-50%, C-Z-25% reach 94.35, 147.26, 63.57 respectively. As for C-Z-50%, the one-way transmission index was highest impling that C-Z-50% possess an enhanced directional water transport property.
Table 1
Moisture management test of cotton fabric with different hydrophilic and hydrophobic ratios
Samples | Wetting times(s) | Absorption rate(%/s) | Max wetted radius(mm) | Spreading Speed(mm/s) | One way transport capability |
Top Surface | Bottom Surface | Top Surface | Bottom Surface | Top Surface | Bottom Surface | Top Surface | Bottom Surface |
C-Z | 2.044 | 0.403 | 27.22 | 22.79 | 15 | 15 | 3.48 | 11.62 | 19.58 |
C-Z-75% | 0.325 | 0.325 | 23.98 | 25.34 | 10 | 10 | 2.01 | 12.93 | 94.35 |
C-Z-50% | 6.575 | 0.325 | 16.41 | 22.19 | 10 | 10 | 1.31 | 12.78 | 147.26 |
C-Z-25% | 3.528 | 0.325 | 23.85 | 26.54 | 10 | 15 | 1.96 | 13.26 | 63.57 |
C-Z-H | 2.747 | 0.325 | 22.72 | 25.46 | 10 | 10 | 2.26 | 12.87 | 45.97 |
Suitable screen-printing patterns make the fabric's directional water transport property better. When the hydrophobic area is less occupied, the cotton fabric does not have the directional water transport property. While the hydrophobic area is too large, its directional water transport property will be affected. As a result, it cannot transport the moisture from the inner surface of the cotton fabric to the outer surface quickly. In conclusion, it was obvious that hydrophobic pattern played strong part in improving the performance of directional water transport.
The curves in Fig. 4 were the moisture content curves of the upper (inner) surface and the lower (outer) surface of different samples. The blue curve represents bottom (Outer side) surface while the green curve refers to the top (Inner side) surface. For the raw cotton fabric, the water content curves on both sides almost overlap, which means that the blank cotton fabric does not possess the performance of directional water transmission. It can be seen from Fig. 4(b-e) that the water content of the outer side has always been much higher than that of the inner side. It is supposed that water can transport more quickly from hydrophobic side to hydrophilic side. As a result, water will spread in multiple directions to increase the diffusion area. The upper surface moisture content of C-Z-50% increased remarkably to 517% in the first 20 s, and then gradually decreased to 450%. But the maximum moisture content of the bottom surface was 330%, which was lower than that of the upper surface. The great difference of moisture content between upper surface and bottom surface implied that C-Z-50% possessed an improved directional water transport property, which is consistent with the data in the Table 1.
The moisture permeability results of cotton fabrics with different hydrophilic/hydrophobic ratios are shown in Table 2. The result of cotton fabric is 326.61 g/(m2๒h). With the hydrophobic screen-printing area increases, the moisture permeability of the printed cotton fabric gradually decreases. Due to the large number of hydroxyl groups on the cellulose molecule, the cotton fabric has a higher moisture regain rate and good moisture absorption performance. After finishing with hydrophobic agent, some of the hydroxyl groups on the cellulose molecules are blocked, resulting in a drop of moisture regain. Compared to the best directional water transport property of C-Z-50%, the slight change in moisture permeability can be ignored.
Table 2
Moisture permeability test of cotton fabric with different hydrophilic and hydrophobic ratios
Samples | Weight before moisture absorption M1/g | Weight after moisture absorption M2/g | Weight difference(g) | Moisture permeability g/(m2.h) |
C-Z | 154.641 | 155.564 | 0.923 | 326.61 |
C-Z-75% | 154.862 | 155.769 | 0.907 | 320.95 |
C-Z-50% | 155.150 | 156.045 | 0.895 | 316.70 |
C-Z-25% | 155.481 | 156.369 | 0.888 | 314.23 |
C-Z-H | 153.789 | 154.606 | 0.817 | 289.10 |
With respect to the air permeability, cotton fabric reaches 206.70 mm/s. As the ratio of hydrophilic and hydrophobic areas decrease, the hydrophilic area decreases, and the air permeability of printed cotton fabric reduces correspondingly (Fig. 5b). This is due to the hydrophobic finishing of the cotton fabric affect its air permeability. Compared to the best directional water transport property of C-Z-50%, the slight change in air permeability can be ignored.
Thermal Property of Printed Cotton Fabric
The thermal conductivity of printed cotton fabric was tested by heating pad and FLIR infrared camera. The heating pad was set at 39°C to ensure that the fabric on the heating pad was maintained at about 37°C. The self-made microenvironment was used to simulate the microclimate of human skin. The temperature of cotton fabric and printed cotton fabric were measured with a FLIR infrared camera every 10 s until the temperature of the sample was maintained in a stable temperature range.
As shown in Fig. 6(b, c), printed cotton fabric heats up faster than the cotton fabric because the nano-ZnO possess certain thermal conductivity. Simulating the human skin environment, printed cotton fabric owns a higher heat absorption rate. As shown in Fig. 6a, the temperature change of two curves after 40 s is not obvious, because the two fabric has reached a balance with the surrounding thermal environment. However, printed cotton fabric obtains a higher surface temperature after the balance, which also proves the improvement of the thermal conductivity of the cotton fabric after nano-ZnO finishing. On the other hand, it means that compared with the cotton fabric, printed cotton fabric can take away more heat from the skin through thermal conduction in the same time.
UV Protective Property of Nano-ZnO Cotton Fabric
Nano-ZnO has good ultraviolet resistance, thus nano-ZnO cotton fabric possess certain UV protective property. Through UV resistance test of nano-ZnO fabric, the UPF (ultraviolet protection factor value), UVA (sunlight ultraviolet radiation with wavelength of 315 nm-400 nm) and UVB (sunlight ultraviolet radiation with wavelength of 280 nm-315 nm) transmittance are analyzed (Fig. 7). The results show that the UV transmittance of cotton fabric is higher, which indicates that the cotton fabric has no UV protective property. As the content of nano-ZnO in dispersion increases gradually, the UV transmittance values of the nano-ZnO cotton fabric decreases correspondingly. When the content of nano-ZnO in dispersion reaches to 20 g/L, the UV protection factor (UPF) values of cotton fabric maintain a certain range of balance, which means the adsorption of cotton fabric and nano-ZnO particles is saturated. In order to avoid unnecessary consumption, the optimal mass concentration of nano-ZnO is 20 g/L. Nano-ZnO has the function of scattering and absorbing ultraviolet rays. When irradiated by ultraviolet rays in sunlight, electrons in the valence band of ZnO are excited, and hole-electron pairing effects will occur. Thus, it has the function of absorbing ultraviolet rays. Compared to the wavelength of ultraviolet light, the particle size of nano-ZnO is very small, so it can scatter ultraviolet light in all directions. This scattering law of ultraviolet light conforms to the Raylieigh light scattering law. Nano-ZnO not only has good thermal conductivity, but also has certain UV protective property, which makes nano-ZnO play an important role in the development of functional textiles.