A Simple and Efficient Method to Fabricate Superhydrophobic Wood with Enhanced Mechanical Durability

Abstract

The poor durability and complex production process are two tough challenges for the practical application of superhydrophobic wood. In this work, high-mechanical-resistance superhydrophobic wood was fabricated by a one-step hydrothermal vacuum dipping method using SiO₂ nanoparticles (SiO₂ NPs) in combination with vinyltriethoxysilane (VTES). The as-prepared superhydrophobic surfaces exhibited water contact angles (CAs) greater than 152° and water sliding angles (SAs) less than 3°. It also exhibited robust stability and durability in harsh conditions, including finger wiping, water brushing, intense sandpaper abrasion, and severe ultrasonic cleaning. The superhydrophobic surface was created by the random distribution of oligomer-wrapped SiO₂ NP spheres having different sizes. Further testing showed that the SiO₂ NPs were firmly fixed on the wood substrate via chemical bonding, which contributed to the high wear resistance. The modification method developed in this work provides a simple and efficient route to fabricate large-scale, mechanically stable, and durable superhydrophobic surfaces for advanced engineering materials.

1. Introduction

Being renewable and environmentally friendly, having superior physical strength as well as being aesthetically pleasing, wood has been widely used in mankind′s daily lives in various applications, including construction, indoor decoration, furniture, and flooring [1]. One of the major disadvantages of untreated wood is hydrophilicity as a result of abundant hydroxyl groups in whole mass. Hygroscopic wood is vulnerable to attack by fungi and mildew, resulting in decay. By converting the hydrophilic hydroxyl groups into hydrophobic groups via chemical modification, the water resistance and dimensional stability of wood can be significantly improved. This will be beneficial to increase their durability and extend their service life [2,3,4].

The design of the superhydrophobic surface on wood is a promising method to solve the hygroscopicity problem [5]. To our knowledge, the superhydrophobic surface can be achieved by the following two approaches: (1) the creation of a suitable hierarchical roughness, and (2) the chemical modification of rough surfaces with low surface energy materials [6,7,8]. In order to create the suitable hierarchical roughness, methods such as sol-gel [9], layer-by-layer assembly [10,11], chemical vapor deposition [12], and electrochemical processes [13] have been reported. Low surface energy materials such as fluorosilane [14], plasma and organosilanes [15], and perfluorinated compounds [16] have been used to perform chemical modification. Although artificial superhydrophobic surfaces have been fabricated on various materials via the above-mentioned methods, the build processes are tedious and time-consuming. In addition, some of these methods are expensive and only applicable to small flat surfaces or specific homogeneous materials.

In this work, we investigate if SiO₂ NPs modified by low surface energy vinyltriethoxysilane (VTES) can efficiently generate a superhydrophobic surface via a one-step hydrothermal vacuum dipping process. In detail, can SiO₂ NPs be used to create hierarchical roughness and can VTES act as the adhesive for fixing SiO₂ NPs on a wood surface? Finally, we study if the resulting superhydrophobic wood can display robust mechanical stability and durability in harsh conditions.

2. Materials and Methods

2.1. Materials

All the chemicals were supplied by Kebaiao Co., Beijing, China, and were used without further purification. Recently cut Populus euramericana (cv. ‘I-214′), which is one of the most common tree species in North China, was collected. The tree ages were about four to five years. Wood blocks with dimensions 20 mm longitudinal (L) × 20 mm tangential (T) × 20 mm radial (R) were obtained from the sapwood sections and dried at 60 °C for 24 h until mass constancy. The following abbreviations are used: natural wood (W), nano-SiO₂-treated wood (W_SiO2), and nano-SiO₂/VTES-treated wood (W_SiO2/VTES).

2.2. Preparation of Superhydrophobic Surfaces on Wood Substrate

Superhydrophobic surfaces using SiO₂ NPs were fabricated on wood substrates as follows. (1) 100 mL of 30% (w/w) silica sol and 100 mL of 97% (w/w) VTES were added into 800 mL distilled water and stirred magnetically for 30 min, and then the resultant solution was subjected to an ultrasonic treatment for 1 h in a KQ-600DE computer numerical controlled (CNC) ultrasonic device (Kunshan Ultrasonic Instruments Co., Ltd., Kunshan, China) with an output power of 100 W. (2) The wood samples were placed in an engineered vacuum drier. The specimens were evacuated for 30 min (−0.1 MPa). (3) The obtained homogeneous mixture was transferred into the above vacuum drier and this vessel was persistently sealed and heated to 35 °C for 2.5 h. (4) Finally, the vessel was left to cool to room temperature. The prepared superhydrophobic wood were removed from the solution and dried at 80 °C until mass was constant. The morphology and microstructure were observed using a scanning electron microscope (SEM) (Hitachi SU-8020, Tokyo, Japan) operating at 3.0 kV. Fourier-transform infrared (FTIR) spectra were acquired by the KBr pellet technique (120-mesh particle size) using Bruker Tensor 27 (Bruker Optics, Ettlingen, Germany) with a 4 cm⁻¹ resolution (32 scans were accumulated). For X-ray diffraction (XRD), the instrument XRD 6000 (Shimadzu, Kyoto, Japan) was used (graphite monochromatic with Cu Kα radiation). The patterns were obtained between 5° and 40° 2θ with 0.05° steps and a scan speed of 2°∙min⁻¹. Water contact angles (CAs) and sliding angles (SAs) were measured using a 5 μL distilled water droplet by a CA analyzer (JC2000D, Powereach, Shanghai, China) at room temperature.

3. Results and Discussion

3.1. The Investigation of Formation Mechanism

Superhydrophobic surfaces were successfully fabricated via a one-step hydrothermal vacuum dipping method proposed in this paper. The functionalization process was performed by the interaction of hydroxyl groups on the surface of the SiO₂ NPs with ethyoxyl from VTES. The sizes of the SiO₂ particles of W_SiO2 and the superhydrophobic particles of W_SiO2/VTES obtained were observed under SEM images. Figure 1a shows that the stacks of SiO₂ NPs as well as the shape (sphere) and size (100 nm) of a single SiO₂ can be seen clearly. Figure 1b shows that the average size of superhydrophobic particles was 10 μm. With supporting evidence from the FE-SEM images, it can be concluded that the particle size increased to the micro level from the nanoscale level. This probably attributed to a high degree of oligomer formation on the SiO₂ NPs’ surface [17].

Figure 1c displays the FTIR spectra of W_SiO2/VTES at ambient temperature from 4000 to 400 cm⁻¹. The peaks at 3422 cm⁻¹ and 1602 cm⁻¹ can be assigned to –OH stretching and bending vibrations, respectively, which are mainly chemical bonds interacting within the SiO₂ NPs and hydrophilic wood substrate and enabling the fabrication of rough surfaces [18,19]. The clear decrease in the intensity of these two peaks was observed due to the formation of ether linkages between SiO₂ NPs and wood substrates. The peak at 788 cm⁻¹ can be assigned to Si–O stretching, arising from the SiO₂ particle [20]. The peaks at 1647 cm⁻¹ and 750 cm⁻¹ were due to C=C and Si–C vibrations, respectively [21]. The origin of C=C and Si–C groups was attributed to the presence of VTES. The band at 2903 cm⁻¹ is a C–H stretching vibration and alkyl groups like –CH₂ were mainly responsible for the superhydrophobicity [22]. The peak at 1050 cm⁻¹ can be assigned to Si–O–Si asymmetric stretching vibrations [23]. –O–C₂H₅ groups were replaced with Si–OH groups due to the hydrolysis of VTES. So, there were an amount of –OH groups on hydrolyzed VTES surfaces and they can react with each other to form Si–O–Si bonds immediately. The Si–O–Si bond with –OH groups can further react to form short oligomer chains. The reaction between VTES and SiO₂ particles occurred via –OH groups on their surface. As a result, VTES and SiO₂ particles were chemically bonded through the Si–O–Si bonds. The probable reactions resulting in the oligomer formation on the SiO₂ surface are shown in Figure 2.

3.2. X-ray Powder Diffraction (XRD) Analysis

Figure 3 shows the XRD patterns of the W, W_SiO2, and W_SiO2/VTES. As shown in Figure 3, similar diffraction peaks at 15.6° 2θ (101), 22.0° 2θ (002), and 34.6° 2θ (040) appear in the W, W_SiO2, and W_SiO2/VTES, which originated from the crystalline region of the cellulose in the wood [24]. According to the results of experimentation, SiO₂ NPs and SiO₂ NP/VTES treatment do not change the crystalline structures within the cell wall. The crystallinity of the W was 26.5%, while the result for the W_SiO2/VTES was 20.2%. The diffraction peak intensity and the relative crystallinity decreased in W_SiO2/VTES due to the amorphous SiO₂/VTES present in the composite.

3.3. Mechanical Durability of the Superhydrophobic Wood Surfaces

The superhydrophobic performance of W_SiO2/VTES after ultrasonic washing (40 KHz, 100 W) for 60 min was investigated. Figure 4a shows the change in CAs and SAs as an influence of ultrasonic washing time for the W_SiO2/VTES. It can be observed that the CAs of the surface remained almost constant around 151°, and SAs exhibited only a slight increase, up to 12°, after ultrasonic washing for a duration of 60 min. This indicated that the SiO₂/VTES complex coating was ultrasonic resistant, and the outgoing adhesive properties of the hybrid coating was therefore durable enough to withstand the ultrasonic impact without destroying its water resistance.

In order to further investigate the mechanical stability of the robust superhydrophobic W_SiO2/VTES, a sandpaper (1500 mesh) abrasion test was carried out. As shown in Figure 4b, the superhydrophobic surface of W_SiO2/VTES was placed on sandpaper, and then different weights (0 g, 100 g, 200 g, and 500 g) were applied to the top of W_SiO2/VTES. Finally, the W_SiO2/VTES burdened with different weights was horizontally pushed for 15 cm along the ruler. It was apparent that the wood surface maintained its superhydrophobic performance in the range from 0 g to 200 g but lost its superhydrophobicity when the weight increased to 500 g (the CAs value was 140° and SAs value was 14°). The sandpaper abrasion test indicated that the as-prepared superhydrophobic wood surface had some limitations for uploading weights and overloading impaired the superhydrophobic wood surface. However, the damaged superhydrophobic wood surfaces still had a strong water resistance.

4. Conclusions

In summary, a durable and mechanically robust superhydrophobic surface using SiO₂ NPs combined with VTES modification was successfully fabricated through a facile, one-step hydrothermal vacuum dipping method. The superhydrophobic surface exhibited exceptional water repellency, having high CAs larger than 152° and low SAs less than 3°. And this high water-resistance performance was attributed to hierarchical roughness (stacks of SiO₂ NPs) and low surface energy material (VTES). The superhydrophobic surface on the wood substrate was durable enough to withstand long ultrasonic washing while retaining its superhydrophobicity. In addition, the superhydrophobic surface possessed good mechanical resistance to sandpaper abrasion. Such a durable and mechanically robust superhydrophobic wood surface could widely expand the range of possible applications of wood-based materials.