3.1 SiO2 powder characterization
Fig.3 (a) shows the SiO2 particles uniformly dispersed in the anhydrous ethanol solution. Fig.3 (b) is an enlarged view of one SiO2 particle with slightly larger size. The measured particle size is 6.758 μm, which is a loose and porous spherical structure. However, the spherical structure is beneficial to reduce the surface energy of the paper film surface, thereby it is hydrophobic. Fig.3 (c) shows the hydrophobic effect of silica powder. NaOH, H2SO4, NaCl, Distilled water (D H2O), and Deionized water (DI) all have a large contact angle on the surface of silica, which proves that the dioxide Silicon powder has good hydrophobic property.
3.2 Morphological characterization of paper mulch under different processing methods
By observing Fig.4 (a) and (b), the fiber structure on the surface of the paper mulch is intertwined to form a network structure with a certain mechanical strength. The shape and outline of the fiber can be clearly observed. After further observation, it was found that the surface of the paper mulch was not smooth, and there are many gaps between the fibers. By zooming in, it was found that the width and length of the fiber are different. An interferometric three-dimensional profiler is used to measure the surface roughness of paper mulch. Fig.4 (c) is a three-dimensional cloud map of the surface roughness of the paper mulch. By calculating the average surface roughness, the arithmetic average height of Sa is 5.784 μm. The highest and lowest height difference (Sz) reached to 64.893 μm. Observing Fig.4 (d), it is found that the silica powder is evenly covered on the surface of the paper mulch. By Fig.4 (e), a part of the SiO2 powder is filled into the voids of the fibers on the surface of the paper mulch, and a part is covered on the fiber surface of the paper mulch. As shown in Fig.4 (f), the measured Sa of SiO2/paper mulch is 6.637 μm. The SiO2/paper mulch is compared with the untreated paper mulch. As the SiO2 powder is covered on the surface of the paper mulch, the roughness increases.
We conducted an EDS analysis on the paper mulch. As shown in Fig.5, the surface elements of the untreated paper mulch are mainly C and O. These are also the main components of the fibers. After SiO2 treatment, the percentage of the C, and O on the surface of the paper mulch is reduced, and a small amount of Si element is added to the paper mulch which proves SiO2 has covered on the surface of the paper mulch.
Observing Fig.6 (a) and (b), it is found that part of the fiber structure was broken after the paper mulch was immersed in an acidic solution for 24 hours, and a small part of the fiber was corroded by acid. But the shape and contour of the fiber can still be identified. Therefore, the mechanical properties of the fiber are reduced. Compared with the unsoaked original paper mulch, the roughness of the paper mulch soaked in the acid solution for 24 hours has increased. The Sa of the original paper mulch soaked in H2SO4 with a pH of 4.6 for 24 hours was 6.641 μm. It can be seen from Fig.6 (c) that the measured Sa increased to 7.975 μm after the paper mulch was immersed in the acidic solution for 48 hours. Observing Fig.6 (d), it is found that most of the fibers on the surface of the SiO2/paper mulch have been corroded by the acid solution. Meanwhile some of the fibers are broken and fall off and the outline of the fibers has gradually blurred.
It can be seen from Fig.6 (e) that after the SiO2 treated paper mulch is soaked in an acidic solution for 24 hours, the SiO2 particles on the surface have a little shedding and a little dent is produced. Shown in Fig.6(f), the acidic solution has contacted the fibers and corroded the fibers, which increasing the roughness of the SiO2/paper mulch. It can be seen from Fig.6 (g) and (h) that after 48 hours of H2SO4 soaking, due to the acid corrosion of the fiber, some of the raised SiO2 has fallen off.
As shown in the results of Fig.7 (a) and (b), it was found that the Si element on the surface of the SiO2/paper mulch film soaked for 24 h was reduced. It is consistent with the result that the surface of the SiO2/paper mulch covered by SiO2 particles has a little peeling off in the SEM image. Observing Fig.7 (c) and (d), it is found that the Si element content of the SiO2/paper mulch surface immersed for 48h is less than that after 24h. Part of the SiO2 has fallen off, but SiO2 still adheres to the SiO2/paper mulch.
Fig.8 (a) showsthat after the original paper mulch (PM) is soaked in an alkaline solution for 24 hours, the surface fibers are deformed. This phenomenon reduces the mechanical strength of the paper mulch. Since the paper mulch did not break or fall off, the quality loss was not obvious. Comparing Fig.8 (b) and (c), it is found that the surface roughness of the paper mulch increases after being immersed in the alkaline solution for 48 hours. Observing Fig.8 (d), it is found that part of the fiber structure has changed from a strip-like fiber structure to a sheet-like fiber structure. The result shows that the fiber structure has undergone significant corrosion. Observing Fig.8 (e), it is found that the fiber structure of the SiO2/paper mulch has not changed significantly after being immersed in an alkaline environment for 24 hours. The reason for this result may be that SiO2 is an acidic oxide. At room temperature, SiO2 reacts slowly with NaOH to produce Na2SiO3 and H2O. The SiO2 particles covered on the paper mulch can protect the fiber structure of the paper mulch. Comparing Figure 8 (f) and (g), it is found that the surface roughness of the paper mulch increased after being soaked for 48 hours. Also, it is found that the Si element on the surface of the SiO2/paper mulch is reduced by Fig.8(h). It shows that the SiO2 particles on the surface of the SiO2/paper mulch have fallen off.
By Fig.9, we find that the Si element on the surface of the SiO2/paper mulch after immersing for 24 hours in an alkaline environment is less than immersing in an acidic environment for same period. After soaking in the alkaline solution for 48 h, SiO2 was significantly reduced. The results showed that SiO2 and NaOH reacted when SiO2/paper mulch was soaked in the alkaline solution. At the same time, a large number of SiO2 particles adhere to the surface of the SiO2/paper mulch, and the surface is hydrophobic.
3.3 Hydrophobic performance test
According to the superhydrophobic surface wetting principle, it is known that the hydrophobic properties of a solid surface are directly related to its surface energy. SiO2 particles are attached to the surface of the SiO2 treated paper mulch, which reduces the surface energy of the paper mulch and makes it hydrophobic.
As shown in Fig.10 (a), the contact angle of the paper mulch after SiO2 soaking treatment is obviously large, increasing from 135.91° to 160.61°. Due to the low surface energy of hydrophobic SiO2 itself, it is coated on the surface of the paper mulch to reduce the surface energy of the paper mulch and then make the paper mulch showing a super hydrophobic effect. After soaking in acidic and alkaline solutions, the contact angles of paper mulch and SiO2/paper mulch show a downward trend. The 4.6 PM and 4.6 SPM after being immersed in acidic solution for 48 hours decreased by 14.94° and 12.94°, respectively. The contact angles of 8.5 PM and 8.5 SPM after immersing in alkaline solution for 48 hours decreased by 22.06° and 12.94°, respectively. By contrast, the contact angle decreased more after the immersion in alkaline solution than the immersion in acid solution. In addition, whether it was immersed in acidic or alkaline solutions for 48 hours, the contact angles of the SiO2 treated paper mulch are greater than 145°.
Fig.10 (b) shows the hydrophobic effect on the surface of the paper mulch under NaOH, H2SO4, NaCl, D H2O, DI droplets, indicating that the SiO2/paper mulch has good hydrophobic properties. Therefore, it can be seen that the SiO2/paper mulch can maintain good hydrophobicity in both acidic and alkaline environments. SiO2/paper mulch shows acid and alkali resistance.
Fig.11 (a) is the diagram of the bounce behavior of the SiO2/paper mulch surface after different treatments. The droplet falls from a height of 10 cm and can bounce multiple times on the surface of the sample. Combined with Fig.11 (b), it is found that as the droplet bounces several times, the spread diameter gradually decreases. Because the droplet is in the Wenzel model when it encounters the paper mulch, it causes energy loss. A small part of the droplets remained on the surface of the paper mulch, causing a large loss of capacity when the droplets bounce. The silicon dioxide attached to the surface of the SiO2/paper mulch is corroded and peeled off after immersion in the acid solution or alkali solution thatmakes the surface of the SiO2/paper mulch form a more complex structure and then the contact area between the droplet and the surface of the SiO2/paper mulch cut back. Observe Fig.11 (c), the maximum bounce height of SiO2/paper mulch is 3.2 mm. The maximum bounce height of the SiO2/paper mulch after being soaked in acidic solution for 24 h is 6.2 mm. The maximum bounce height of the SiO2/paper mulch after 48h soaking in acid solution is 5.0 mm. The maximum bounce height of the SiO2/paper mulch after soaking in alkaline solution for 24 h is 6.3 mm. The maximum bounce height of the SiO2/paper mulch after being immersed in alkaline solution for 48 h is 4.1 mm. Experiments show that after soaking in acid or alkaline solution, the bounce performance of the SiO2/paper mulch is improved.
3.4 The effect of pH on the quality of paper mulch
Observing Fig.12 (a) and (b), the mass-loss rate of paper mulch and SiO2/paper mulch in acid solution is rising. Comparing Fig.12 (a) and (b), it is found that the mass loss rate of paper mulch and SiO2/paper mulch in acid solution is significant. After being soaked in an acidic environment for 48 hours, the mass-loss of both paper mulch and SiO2/paper mulch reached more than 8%. But immersed in the alkaline solution, the mass-loss rate basically keeps below 1.5%. Because the paper mulch contains a lot of fibers, the 1,4-β-glycosidic bonds in the fibers are more sensitive to acid and will break in an acidic environment, causing a part of the paper mulch surface material to dissolve in the acid solution, and resulting in the quality of the paper mulch cut back [28]. However, when immersed in an alkaline solution, the quality loss of the paper mulch is mainly due to the reaction of silica particles with sodium hydroxide and the falling off of the sample surface.
3.4 The effect of pH on the mechanical properties of paper mulch
From Fig.13 (a) and (b), we find that as the soaking time increases, the mechanical properties of the paper mulch decrease. From Fig.13 (a), after the paper mulch is soaked in an acidic environment, the SiO2/paper mulch (SPM) has mild higher tensile properties than the original paper mulch (PM), which may be since SiO2 has a certain effect on the surface of the paper mulch, the reinforcing effect. From Fig.13 (b), it can be seen that after immersion in an alkaline environment, the tensile properties of SiO2/paper mulch (SPM) are much higher than that of original paper mulch (PM). This is mainly due to the slow reaction of SiO2 with sodium hydroxide. The paper mulch has a certain protective effect.