3.2. TG analysis
Figure 2b reveals that the TG curve of latex film. It can be seen from Fig. 2b, the thermal degradation of the adhesive film is mainly divided into the following stages: the decomposition of small molecules and the volatilization of solvents occur from 0℃ to 250℃, the temperature range of 250 ~ 350°C is ascribed to the decomposition of the hard section, the range of 350 ~ 430°C is attributed to the fracture decomposition of the soft section. After 450°C, the film has almost completely degraded, and the quality has not changed. Among them, the maximum thermal decomposition temperature of the hard section of WPU is 342°C, and the maximum thermal decomposition temperature of FWPU increased by 18°C, while the maximum thermal decomposition temperature of the soft section of the two remained basically unchanged. Compared with pure WPU film, due to the bond energy of C-F is higher than that of C-C, the thermal stability of FWPU film is improved to some certain extent, so the maximum decomposition temperature of the film is increased, and the heat resistance of the film is improved. It is important to note that too much F13 can affect the thermal stability of the WPU film.
3.3. XRD analysis
As shown in Fig. 2c, the XRD spectrum of WPU shows a wide diffraction peak, which is the characteristic diffraction peak of amorphous polyurethane, indicating that the crystallinity of WPU molecules is slightly worse. The hard section amorphous diffraction peak can be seen at 2θ = 21°, and the FWPU shows a decreasing trend in crystallinity compared to the WPU curve. The addition of F13 destroys the original ordered structure of WPU and reduces the crystallization performance. Due to the high resistance of F13, the cross-linked structure of the molecules is strengthened, which prevents the tight stacking of molecules, breaks the amorphous structure of the molecules themselves, and makes the molecular arrangement less regular (Lu et al. 2008).
3.5. SEM analysis
Surface morphology of WPU and FWPU-10 was observed through scanning electron microscopy (SEM) images as shown in Fig. 3. The longitudinal section of WPU is a relatively complete and ordered scale structure, while the scale structure of FWPU-10 is small and disordered. The reasons are as follows: On the one hand, the difference in the dissolution parameters of F13 and polyurethane leads to phase separation of the adhesive film, and this incompatibility is reflected in the fracture of the adhesive film, so a larger scale structure appears at the fracture. On the other hand, fluorine aggregates onto the surface to form phase separation. Therefore, it can also be proved that the hydrophobic performance of the modified WPU is increased.
The SEM images of the hemp fabric and the treated hemp fabric are shown in Fig. 3. In Fig. 3e, the surface of the hemp fabric is rough and accompanied by impurities. Compared with Fig. 3e, Fig. 3f shows that the surface of the treated hemp fabric is covered with polyurethane film, the surface is smooth, the fiber is bonded, and the friction between the fibers is reduced.
3.6. TEM analysis
TEM is used to characterize the microstructure of latex particles. The images of microscopic morphology of FWPU-0 and FWPU-10 emulsion are shown in Fig. 4. The results show that the latex particles have a stable morphology and uniform particle size. The surface of the latex particles is smooth and there is no significant aggregation (Fig. 4a). The particles are irregularly shaped, with slight adhesion of the particles, but still evenly dispersed (Fig. 4b). This phenomenon indicates that the high proportion of F13 enhances the morphology of microphase separation and promotes agglomeration to a certain extent.
3.7. Contact angle analysis
It can be seen from Fig. 5a that the water contact angle of FWPU-0 film is 75.81°, and the water contact angle of FWPU-10 is 105.95°. The results showed that with the increase of F13 amount, the hydrophobic angle of the film gradually increased, and the hydrophobic angle is relatively maximum when the amount of F13 reaches 10%. In the process of PU film formation, the surface energy of each part of the film varies greatly, and the F-containing segments with low surface energy migrate and aggregate to the membrane surface, thus improving the hydrophobicity of FWPU film.
Hemp fabrics are among the most common fabrics due to their excellent wearability. However, the hemp fabrics easily absorb moisture and breed bacteria. Therefore, hemp fabrics with hydrophobic properties are highly needed in the functional textile industry. As shown in Fig. 5b, for pristine hemp fabric, the water droplet quickly spread and completely wetted the fabric, indicating that the pure hemp fabric was superhydrophilic as hemp fabric. The hydrophobic angle of the hemp fabric after WPU finishing is 83.71°. However, the contact angle of FWPU-coated hemp fabric dramatically increased to approximately 117.1°, which demonstrated that the FWPU had good hydrophobicity. The hydrophobicity of FWPU film is improved by the migration of fluorine groups between molecular chains to the surface of the film. As shown in Fig. 6, the water droplet could stand on the surface of modified hemp fabric. The finished hemp fabric is not only hydrophobic, but also resistant to milk, soy sauce, fruit juice, and other solutions.
Cellulose paper is a suitable substrate for a wide range of applications due to its low cost, ease of manufacture and disposal, flexibility and renewability. However, cellulose paper is superhydrophilic due to the large number of hydroxyl groups on the surface, especially when in contact with water, leading to a loss of strength. At the same time, the adsorption of water to cellulose paper can lead to the growth of bacteria. For the pristine cellulose paper, the water was quickly absorbed after dropping (Fig. 7a). However, the water droplet could stand on the surface of FWPU modified filter paper.
Figure 7a shows that the hydrophobic angle of the filter paper after coating with waterborne polyurethane is 89.09°. The hydrophobic angle of the FWPU coated filter paper could reach 112.77°. As shown in Fig. 7b, juice, methylene blue solution, soy sauce and milk are dribbled onto the filter paper, which can be easily permeated and dyed. FWPU-10 coated filter paper was found to resist the penetration by juice, methylene blue, soy sauce and milk (Fig. 7b). Quite interestingly, FWPU-10 coated filter paper exhibited a noticeable cleanability when fouled by juice, methylene blue, soy sauce and milk. Because of the introduction of fluorine in the emulsion, fluoropolymer is currently the main material of the "three prevention", its anti-fouling performance is excellent.
3.8. Mechanical performance analysis
Mechanical property is crucial parameter for practical application of textiles. The experimental results of breaking strength of hemp fabric are shown in Fig. 8a. The pristine hemp fabric has a breaking strength of 400 N. As expected, the introduction of FWPU onto the hemp fabric has a significant influence on the tensile deformation behavior of hemp fabric. As can be seen from Fig. 8a, the breaking strength of the hemp fabric shows a trend of increasing and then decreasing as the fluorine content increases. Because of the fluorine-containing segment is introduced into the polyurethane matrix, which increases the number of internal hydrogen bonds, increases the crystallinity of the hard segment, and enhances the intermolecular force. However, with the increase of F13 dosage, the steric hindrance effect increases, which affects the phase separation structure of polyurethane, resulting in the decline of the mechanical properties of hemp fabric after finishing. It is clear to see that the breaking strength of FWPU-10 coated hemp fabric is up to 491 N. Compared to pure hemp fabric, the breaking strength of FWPU-10 coated hemp fabric is increased by 23%. The mechanical strength of paper samples was tested (Fig. 8b). As with hemp fabric, the mechanical strength of the filter paper shows a trend of increasing and then decreasing as the fluorine content increases. Compared with pure filter paper, FWPU-10 coated filter paper is increased by 82%.
3.9. Crease recovery angle analysis
As shown in Fig. 8c, the crease recovery angle of the hemp fabric increases after the WPU finishing. With the increase of F13 dosage, the crease recovery angle increases gradually, which is attributed to the flexibility of C-F bond enhances the continuity between fibers. The more the -OH in the finishing agent combines with the -OH on the surface of the fabric fiber, the stronger the wrapping of the main chain C-C, thus improving the crease resistance and other properties. However, when the addition of F increases to 15%, the crease recovery angle of fabric decreases as the structure of the WPU is affected.
3.10. Wearing performance analysis
The test result of friction coefficient of finished hemp fabric is shown in Table 2. It is worth noting that the friction coefficient of the FWPU finished hemp fabric decreases, indicating that the surface of the FWPU hemp fabric is smoother, and the wear resistance of hemp fabric is improved.
Table 2
Test of the wearing performance of hemp fabric
Performance | Hemp fabric | WPU hemp fabric | FWPU hemp fabric |
Coefficient of dynamic friction | 0.541 | 0.506 | 0.499 |
Coefficient of static friction | 0.619 | 0.537 | 0.535 |