Anti-mold, self-cleaning superhydrophobic bamboo fiber/polypropylene composites with mechanical durability

Bamboo fiber/polypropylene composites (BPCs) have been widely used in buildings, interior decoration, and automobile components. However, pollutants and fungi can interact with the hydrophilic bamboo fibers on the surface of Bamboo fiber/polypropylene composites, degrading their appearance and mechanical properties. To improve their anti-fouling and anti-mildew properties, a superhydrophobic modified Bamboo fiber/polypropylene composite (BPC-TiO2-F) was fabricated by introducing titanium dioxide (TiO2) and poly(DOPAm-co-PFOEA) onto the surface of a Bamboo fiber/polypropylene composite. The morphology of BPC-TiO2-F was analyzed by XPS, FTIR, and SEM. The results showed that TiO2 particles covered on Bamboo fiber/polypropylene composite surface via complexation between phenolic hydroxyl groups and Ti atoms. Low-surface-energy fluorine-containing poly(DOPAm-co-PFOEA) was introduced onto the Bamboo fiber/polypropylene composite surface, forming a rough micro/nanostructure that endowed BPC-TiO2-F with superhydrophobicity (water contact angle = 151.0° ± 0.5°). The modified Bamboo fiber/polypropylene composite exhibited excellent self-cleaning properties, and a model contaminant, Fe3O4 powder, was rapidly removed from the surface by water drops. BPC-TiO2-F showed excellent anti-mold performance, and no mold was on its surface after 28 days. The superhydrophobic BPC-TiO2-F had good mechanical durability and could withstand sandpaper abrasion with a weight load of 50 g, finger wiping for 20 cycles, and tape adhesion abrasion for 40 cycles. BPC-TiO2-F showed good self-cleaning properties, mildew resistance, and mechanical resistance, giving it promising applications for automotive upholstery and building decoration.


Introduction
Biobased plastic composites formed by blending biomass resources with thermoplastics have attracted attention for applications in buildings, decoration, automotive interiors, and decking (Bengtsson et al., 2007;Kuo et al., 2009;Durmaz et al., 2021;Guo et al., 2021) because of their broadly-distributed resources, low cost, lightweight, high toughness, sound insulation, and shock absorption properties (Rowell, 2007;Kuo et al., 2009;Ren et al., 2020;Vedat and Fatih, 2020). Bamboo fiber (BF) is an important biomass resource, with rapid renewability, wide availability, biodegradability, and outstanding mechanical properties (Yu et al., 2014). The tensile strength and Young's modulus of bamboo fibers can reach as high as 1.  GPa, which are much higher than those of most other plant fibers, such as wood, flax, jute, and sisal (Yu et al., 2011;Sobczak et al., 2012). BF is a good substitute for wood fiber, carbon fiber, and glass fiber for preparing biomass plastic composites. Therefore, to add value to BF and reduce the use of petroleumbased plastics, bamboo fiber/polypropylene composites (BPCs) have been extensively used in various fields, such as buildings, interior decoration, and automobile components (Fei et al., 2016;Wang et al., 2019;Jamaludina et al., 2020). Therefore, BPCs need to withstand damage by water, mildew fungi, and mechanical wear during use.
Large amounts of hydrophilic fibers are exposed on the BPC surface, although most BFs are entrapped by polypropylene (PP). Those fibers are hygroscopic and rich in proteins, sugars, starches, and other nutrients (Liese, 1987;Shalbafan et al., 2013), which lead to contamination by mold and fungi that may reduce mechanical properties and accelerate aging. Therefore, it is necessary to modify BPCs to achieve hydrophobicity and self-cleaning properties to prolong their service life.
Numerous studies have shown that physical and chemical modification of bamboo fibers or plastic matrixes can improve the performance of BPCs; however, physical treatment methods generally consume large amounts of energy, while chemical modification processes usually produce toxic wastewater (Zhang et al., 2018;Sanjay et al., 2019). Therefore, it is better to adopt a method that consumes less energy, less wastewater, and can easily form mildew-proof, superhydrophobic, water-resistant, and self-cleaning layers on the surface of BPCs. On superhydrophobic surfaces with a water contact angle (CA) higher than 150° (Wang S. et al., 2011;Budunoglu et al., 2011;Wang et al., 2012;Nguyen-Tri et al., 2019), pollutants or dust can be easily removed by the rolling of water droplets. Generally, superhydrophobic surfaces can be obtained by a combination of rough surfaces and low-surface-energy substances (Windeisen et al., 2007;Xu and Cai, 2008;Yang and Deng, 2008;Zimmermann et al., 2008;Wang S. L. et al., 2011;Ellinas et al., 2011;Lu et al., 2015;Lin et al., 2016;Mrad et al., 2018;Wu et al., 2018;Li et al., 2020;Lu et al., 2022). It is still challenging to produce BPCs with anti-mold, self-cleaning, and mechanical durability through a simple and convenient process.
In this paper, we aimed to fabricate superhydrophobic, selfcleaning, and anti-mold BPCs as well as mechanical durability via a facile immersion method that introduced titanium dioxide on the BPC surface via hydrogen bonding. The low surface energy of rough micro/nanostructured copolymers with fluorinated units imparted the BPC with superhydrophobicity. The modified BPC exhibited excellent self-cleaning and anti-mildew properties with an easy-tooperate approach, showing great potential for applications in automotive upholstery and building decoration.
Poly(DOPAm-co-PFOEA) was synthesized using DOPAm and PFOEA (Lin et al., 2016;Lu et al., 2021). DOPAm (0.08 g) was dissolved in 2 mL DMF. Then, 0.008 g AIBN and 1 mL FOEMA were added. Four freeze-thaw cycles were performed to remove O 2 . After reacting at 70°C for 6 h, the solution was dropped into methanol to produce a white precipitate. Poly(DOPAm-co-PFOEA) was obtained after the precipitate was dried under a vacuum for 24 h.

Preparation of bamboo fiber/ polypropylene composites (BPCs)
BF and PP were alternately put into a mold (200 mm × 200 mm × 4 mm) using a layer-by-layer method. The weight ratio of BF and PP in BPC was 1: 1. A BPC with a density of 0.85 g/cm 3 was obtained by pressing the specimen at 190°C under a pressure of 8 MPa for 20 min.

Modification of bamboo fiber/ polypropylene composites (BPCs)
Titanyl oxysulfate (0.8 g) was added to 30 mL of deionized water. The pH of the solution was adjusted to 6 using 0.3 mol/L acetic acid solution. BPC was immersed in the solution and stirred at 70°C for 4 h. After the reaction, the composite was dried under a vacuum at 60°C for 24 h to obtain TiO 2 -modified BPC (BPC-TiO 2 ). Then, BPC-TiO 2 was soaked in 15 mL AK-225 solution containing 3.3 mg/mL poly(DOPAm-co-PFOEA) for 24 h and dried under vacuum at 60°C for 12 h to obtain superhydrophobic BPC (BPC-TiO 2 -F).
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Characterization
X-ray photoelectron spectroscopy (XPS) was conducted using a Thermo Scientific ESCALAB 250Xi instrument (Waltham, MA, United States) with a standard Al-Kα X-ray source. The energy resolution of the spectrometer was set to a pass energy of 30.0 eV for survey scans at 0.05 eV per step and a 150.0 eV pass energy for region scans at 1.0 eV per step. The X-ray beam operated at an acceleration voltage of 13 kV and a current of 25 mA. Spectra were calibrated to the C 1 s peak at 285.0 eV.
Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy was performed on a Perkin-Elmer Spectrum 2000 FTIR spectrometer (Norwalk, CT, United States), which was equipped with an MKII Golden Gate, single reflection ATR System from Specac Ltd., London, United Kingdom. BPC, BPC-TiO 2 , and BPC-TiO 2 -F were pressed against the ATR crystal to obtain a sufficient signal. The spectra were measured in the wavenumber range of 4,000-600 cm -1 at a resolution of 4 cm -1 with 32 scans.
The surface morphologies of the composites before and after modification were observed using a Hitachi S-4800 field emission scanning electron microscope (FESEM) from Tokyo, Japan. To enhance the sample conductivity before measurements, galvanic platinum deposition (5 mA) was conducted by spraying the samples for 110 s.
The wettability of BPC and BPC-TiO 2 -F was determined by a water drop shape analyzer of KRUSS DSA 30 from Germany. The contact angle (CA) was tested by dropping a 2 μL water droplet on the surface of BPC and BPC-TiO 2 -F. Five different locations on the same sample were recorded.

Anti-mold test
Aspergillus niger was used to determine the anti-mold performance of the composites. PDA powder (46.0 g) was dissolved in 1,000 mL of distilled water. After autoclaving at 115°C for 20 min, the PDA solution was poured into a Petri dish sterilized by ultraviolet light. After cooling, the activated A. niger was added into the culture medium using an inoculating needle and incubated at 28°C and a relative humidity of 85% for 7 days. The sterilized BPC and BPC-TiO 2 -F were put into a culture medium. Images of the samples were captured every day using a digital camera (Nikon, Japan).

XPS analysis
A schematic of the surface-modified BPC by TiO 2 particles and poly(DOPAm-co-PFOEA) is shown in Figure 1. TiO 2 particles were loaded in situ on the surface of BPC via hydrogen bonding (Guvendiren et al., 2008;Mahadik et al., 2010;Xu et al., 2012) with the hydroxyl groups of BF . Poly(DOPAm-co-PFOEA) was deposited on the surface of TiO 2 particles through catechol-Ti complexation (Zeng et al., 2010). The catechol groups in DOPAm bonded to the TiO 2 surface. The low-surface-energy fluorinated units in PFOEA covered the TiO 2 particle exterior. Thus, a rough micro/nanostructure was created on the BPC surface, which decreased the surface energy of the composites Frontiers in Chemistry frontiersin.org and endowed the BPC with superhydrophobicity (Lin et al., 2016). After modification, the appearance of BPC was unchanged. Figure 2 shows the wide-scan and C 1s narrow-scan XPS spectra of BPC, BPC-TiO 2 , and BPC-TiO 2 -F. The peaks at binding energies of 533.0 eV and 284.8 eV were attributed to O 1 s and C 1s (Li et al., 2015) in the BPC, as shown in Figure 2A, a. After TiO 2 particle deposition, a new peak of Ti 2p 3/2 at a binding energy of 459.1 eV appeared, indicating that TiO 2 particles were successfully coated on the surface of BPC by in situ synthesis, as shown in Figure 2A, b. After BPC-TiO 2 was modified by poly(DOPAm-co-PFOEA), a new F 1 s peak appeared at 690.1 eV, indicating that poly(DOPAm-co-PFOEA) was successfully introduced onto the surface of BPC-TiO 2 , as shown in Figure 2A, c.
As shown in the narrow-scan C 1 s spectrum of untreated BPC in Figure 2A, b, the peaks at binding energies of 284.8 eV and 286.4 eV were attributed to C-H/C-C and C-O, respectively. After BPC-TiO 2 was modified by poly(DOPAm-co-PFOEA), the C 1 s spectrum produced two new peaks at binding energies of 289.3 eV, 291.2 eV, and 293.4 eV, which belonged to the characteristic peaks of C=O, C-F 2 , and C-F 3 (Baidya et al., 2017) of poly(DOPAm-co-PFOEA), as shown in Figure 2B, c.
Changes in the atomic composition of the composites were analyzed by XPS, as shown in Table 1. Compared with BPC, BPC-TiO 2 showed a Ti content of 14.8% on the surface. The O/C ratio increased from 0.38 to 0.84. After being modified with poly(DOPAm-co-PFOEA), the Ti content in BPC-TiO 2 -F decreased to 0.7, while the F content increased to 48.9%, and the O/C ratio decreased to 0.17. These changes indicated that the surface of BPC was modified by poly(DOPAm-co-PFOEA).

ATR-FTIR spectrometry
The ATR-FTIR spectra of the BPC surface modified by TiO 2 particles and poly(DOPAm-co-PFOEA) are shown in Figure 3. The absorption peak at 1,033 cm -1 was the C-O stretching vibration peak of the BF skeleton. This peak changed little after TiO 2 modification but weakened after being treated by poly(DOPAm-co-PFOEA). For BPC-TiO 2 -F, a new absorption peak at 1735 cm -1 appeared, which was the stretching vibration peak of C=O groups. The absorption peaks at 1,148 cm -1 , 1,203 cm -1 , and 1,236 cm -1 were associated with various stretching modes of -CF 2 and -CF 3 groups (Baidya et al., 2017). The absorption peak at 665 cm -1 was due to a combination of rocking and waging vibrations of -CF 2 groups (Tingting et al., 2005). The IR results also indicated that poly(DOPAm-co-PFOEA) was introduced on the surface of BPC.

SEM analysis
SEM images of the micromorphology of BPC, BPC-TiO 2 , and BPC-TiO 2 -F surfaces are shown in Figure 4. BF was embedded in PP ( Figure 4A), and some fibers protruded from the BPC surface. After being treated with titanyl oxysulfate, TiO 2 particles in the diameter range of 0.3-1 μm were formed on the surface of the BPC ( Figure 4B). The BPC surface formed a micro/nanostructure after modification, and the recesses of this structure can trap air. When a liquid was dropped onto the surface of BPC-TiO 2 -F, the air in the micro/nano structure acted as a buffer that reduced the contact area between the liquid and the solid, thereby achieving superhydrophobicity Bayer, 2017).

Wettability
As shown in Figure 5, the wettability of BPC, BPC-TiO 2 , and BPC-TiO 2 -F was characterized by water contact angle measurements. The contact angle of BPC was 119.6°± 1.9°( Figure 5A), and this value decreased to 0°after being modified with TiO 2 particles ( Figure 5B). This indicated that the introduction of TiO 2 particles increased the hydrophilicity, which might be due to the increased number of hydroxyl groups. When BPC-TiO 2 was modified by poly(DOPAm-co-PFOEA), the contact angle reached 151.0°± 0.5°( Figure 5C), indicating that poly(DOPAm-co-PFOEA) was introduced onto BPC-TiO 2 to produce a superhydrophobic surface.

Self-cleaning property
Self-cleaning is an important property of superhydrophobicity and can indicate the anti-fouling ability of BPC surfaces. The selfcleaning behavior of BPC and BPC-TiO 2 -F was characterized by a  dirt-removal test using Fe 3 O 4 as a contaminant (Baidya et al., 2017), as shown in Figure 6. Black Fe 3 O 4 was sprinkled on the surface of BPC and BPC-TiO 2 -F and then rinsed with water. For the unmodified BPC, the water droplet wrapped the black powder and adhered to the BPC surface ( Figure 6A). The contact angle of the unmodified BPC was less than 150°, so the water droplets showed good wettability to the surface and spread. In contrast, the powder on the modified BPC was rapidly removed by water, and no powder residue was observed after being rinsed, as shown in Figure 6B. The above phenomena show the self-cleaning behavior of BPC-TiO 2 -F. The combined Wenzel-Cassie model (Marmur, 2004;Ma et al., 2012;Peng et al., 2022) ( Figure 5D) is a reasonable explanation for the self-cleaning property. The surface with a micro/nano-hierarchical roughness and low surface energy  Frontiers in Chemistry frontiersin.org 06 was formed on the BPC after modification. The concave-convex on the rough surface trapped air well, decreasing contact between water droplets and modified BPC, which endowed the modified BPC with a remarkable self-cleaning capability.

Anti-mold effect
It is essential to evaluate the effect of mold on BPC products because BF is susceptible to mold attack (Xu et al., 2013). Aspergillus niger is a mold that is commonly used to test the anti-mold capacity because it can quickly grow on a starch-based medium Liu et al., 2022;Peng et al., 2022). Therefore, it was chosen in the anti-mold experiment, as shown in Figure 7.
Because of the presence of hydrophilic BF, the surface of BPC could be signed with water-soluble black ink ( Figure 7A), while the surface of BPC-TiO 2 -F could not, indicating its superhydrophobicity ( Figure 7B). After 7 days of cultivation, the marked ink became blurry, indicating that the surface of the unmodified BPC was hydrophilic, and water vapor wetted the ink during the mold culture process. As expected, white hyphae of molds were observed on the 14th day, showing that the hyphae invaded the unmodified BPC. The surface of the BPC was completely covered by mold on the 28th day, showing that the BPC had weak anti-mold activity. The unmodified BPC was easily attacked by mold fungi due to hygroscopic hydroxyl groups on the surface of BF, which provided essential conditions for mold growth (Feng et al., 2019). On the contrary, no mold was observed on the surface of the BPC-TiO 2 -F after 28 days of cultivation, indicating that the superhydrophobic surface kept the BPC-TiO 2 -F in a relatively dry state because the water droplet could not remain on its surface. This prevented mold colonies from forming. Thus, the growth of mold on the surface of BPC-TiO 2 -F was prevented.

Mechanical abrasion resistance
Mechanical durability is a critical requirement for the practical applications of BPC-TiO 2 -F. In this study, the mechanical durability of BPC-TiO 2 -F was characterized by sandpaper abrasion, tape adhesion, and finger wiping Zhou et al., 2022). Changes in the water CA were recorded after every five abrasion cycles, as shown in Figure 8A. In Figure 8B, BPC-TiO 2 -F was tested against sandpaper abrasion and was loaded with a weight of 50 g. After 20 abrasion cycles, the water CA remained greater than 150°, displaying excellent abrasion resistance. After 50 abrasion cycles, the water CA dropped to 141.0°. For finger wiping ( Figure 8C), the water CA dropped to 149.8°and 146.2°after 20 and 50 abrasion cycles, respectively. For tape adhesion ( Figure 8D), the tape had difficulty attaching to the surface of BPC-TiO 2 -F due to its superhydrophobicity. After 40 abrasion cycles, the water CA remained higher than 150°. Although the superhydrophobic BPC-TiO 2 -F surface was obtained by a physical impregnation method, the hydrophobic layer was fixed via hydrogen bonding and chelation, which are physical forces. Therefore, BPC-TiO 2 -F retained its strong mechanical durability.

Conclusion
In summary, a superhydrophobic BPC with self-cleaning and anti-mold properties was fabricated. TiO 2 particles were in situ loaded onto BPC via hydrogen bonding. Through catechol-Ti complexation, poly(DOPAm-co-PFOEA) with a low surface energy was deposited on the TiO 2 particles, forming rough micro/nanostructures on the BPC surface. The contact angle of BPC-TiO 2 -F reached 151.0°± 0.5°, indicating a superhydrophobic surface. The modified superhydrophobic BPC had good selfcleaning performance, which protected contaminants. BPC-TiO 2 -F exhibited superior anti-mold properties, and no mold colonies appeared on the surface of BPC-TiO 2 -F. Superhydrophobic BPC-TiO 2 -F could withstand a 50 g load of sandpaper and finger wiping for 20 cycles and tape adhesion abrasion for 40 cycles. BPC-TiO 2 -F, with self-cleaning, anti-mildew, and mechanical durability properties shows excellent potential for applications in automotive upholstery and building decorations.

Data availability statement
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.