Abstract
Rubberwood, which can be regarded as a natural polymer composite, presents many difficulties in the process of material modification. In this study, a convenient method was developed to impart superhydrophobicity to the rubberwood surface. SiO2-PS composite film was formed on rubberwood surface by simple short-term impregnation. The obtained superhydrophobic rubberwood exhibited a water contact angle (WCA) of ~155.6° at room temperature. The superhydrophobic coating surface provides the abilities of self-cleaning and anti-bacteria for rubberwood. Combined with a small amount of 3-iodo-2-propyl-butyl carbamate (IPBC), the control effect of the four fungi reached 75% (Botryodiplodia theobromae), 100% (Trichoderma viride), 95% (Penicillium citrinum), and 100% (Aspergillus niger), respectively. The developed method herein features environment-friendly raw materials, facile processing, and large-scale fabrication. It provides a new solution for long- term timber protection processing.
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A Facile Method of Superhydrophobic Coating on Rubberwood to Improve its Anti-Mildew Performance
Pengyao Lu,a Hong Yun,a,* Weiwei Zhang,a Dengyun Tu,a Chuanshuang Hu,a and Banyat Cherdchim b
Rubberwood, which can be regarded as a natural polymer composite, presents many difficulties in the process of material modification. In this study, a convenient method was developed to impart superhydrophobicity to the rubberwood surface. SiO2-PS composite film was formed on rubberwood surface by simple short-term impregnation. The obtained superhydrophobic rubberwood exhibited a water contact angle (WCA) of ~155.6° at room temperature. The superhydrophobic coating surface provides the abilities of self-cleaning and anti-bacteria for rubberwood. Combined with a small amount of 3-iodo-2-propyl-butyl carbamate (IPBC), the control effect of the four fungi reached 75% (Botryodiplodia theobromae), 100% (Trichoderma viride), 95% (Penicillium citrinum), and 100% (Aspergillus niger), respectively. The developed method herein features environment-friendly raw materials, facile processing, and large-scale fabrication. It provides a new solution for long- term timber protection processing.
Keywords: Superhydrophobic surface; Self-cleaning; Anti-bacteria; Rubberwood
Contact information: a: South China Agricultural University, College of Materials and Energy, Guangzhou, China; b: Faculty of Sciences and Industrial Technology, Prince of Songkla University , Muang district, Surat Thani84000, Thailand; * Corresponding author: 975805019@qq.com
INTRODUCTION
The rubber tree (Hevea brasiliensis) is widely planted in subtropical regions, such as Thailand, Malaysia, and Indonesia, as an economic tree species (Sik et al. 2010). Rubber plantations account for about 18% of global forest plantations (ITTO 2009). However, the use of rubberwood is limited because the wood is readily deteriorated by mold fungi, decay fungi, and insects (Ratnasingam et al. 2012). In order to expand the application range of rubber wood, it is important to protect the wood from degradation, mildew, and insects. For instance, studies have been conducted on utilizing essential oils to provide protection of rubberwood against decay (Matan 2012; Jantamas et al. 2016). To protect wood from mold fungi, many methods have been applied to deal with mold attack. At present, the anti-mold treatment of wood is mostly carried out together with the wood anti-degradation treatment, because moldy wood is often also susceptible to fungal attack. In the past, one such method for treating wood used chromated copper arsenate (CAA), which belongs to a group of inorganic waterborne preservatives. According to the relative proportions of metals, three CCA formulations are designated as three types A, B, and C. Though CCA was widely considered as the most effective wood protectant (Hingston et al. 2001), it has been banned all over the world due to its harm to humans and the environment (Balasoiu et al. 2001; Zagury et al. 2003).
As a substitute for CAA, the preservatives of copper as the main agent, including ammoniacal copper quaternary (ACQ), copper azole (CA), and copper dimethyldithio-carbamate (CDDC), show good protective effects on wood. Copper-based preservatives have been widely studied (Chung et al. 2005; Ye et al. 2015). However, copper ions will be gradually lost during the process, which will also cause serious pollution to the environment. Analogously, antiseptic boron compounds have not been shown to significantly affect the decay resistance of wood, and they present favorable environmental characteristics (Nami et al. 2007). However, borates are easily leached when treated woods are located in a relatively high humidity environment (Baysal et al. 2006). Several strategies have been proposed to resolve the above leaching problem of inorganic preservatives (Ratajczak and Mazela 2007; Obanda et al. 2008; Thévenon et al. 2010), such as micronized copper (McIntyre and Freeman 2009) and boron fixative agent (Mohareb et al. 2011). Although a large number of organic mold inhibitors, such as 4,5-dichloro-2-octyl-isothiazo-lone (DCOIT) (Feng et al. 2017), tebuconazole 3-iodo-2-propyl-butyl carbamate (IPBC) (Muin and Tsunoda 2003), etc. (Sun et al. 2012), have been developed, they have problems of being degraded by microorganisms (Wallace et al. 2008). Meanwhile, they could also be decomposed under high humidity conditions (Schultz et al. 2007).
Wood has a strong hydrophilic nature, which not only leads to the size change of wood after exposure to water, but it also makes wood more susceptible to degradation by fungi and decay fungi (Kocaefe et al. 2015). In recent years, research on preparation of superhydrophobic structures has been widely carried out (Liu et al. 2015; Rao et al. 2016; Jia et al. 2018; Tu et al. 2018).
The superhydrophobic interface not only has the functions of water repellency, self-cleaning, anti-freezing, but also has certain antibacterial function. Superhydrophobic surfaces showing high resistance to bacterial contamination have been studied by many researchers (Heinonen et al. 2014; Feng et al. 2015). Superhydrophobic interfaces do not contain germicidal materials that destroy microorganisms. Instead, they prevent bacterial growth by reducing adhesion. Superhydrophobicity reduces adhesion between bacteria and a solid surface, making it easier to remove bacteria before they form a thick biofilm on the surface (Kartal et al. 2007; Zhang et al. 2013). In addition, superhydrophobic surfaces are found to have low protein adsorption and easy deproteinized properties (Koc et al. 2008). However, the existing methods of constructing hydrophobic structures are complex. In addition, there are no reports about the use of hydrophobic structures in conjunction with other antifungal agents.
Herein, we report a simple method to fabricate mechanically durable, self-cleaning, and multifunctional superhydrophobic surfaces on rubberwood by impregnating the specimens in the PS/THF solution and SiO2/acetone emulsion (Fig. 1). Superhydrophobic rubberwood is fabricated by nano-/micron- composite structure, which is combined with organic mildew inhibitor (IPBC) to improve its mildew resistance. Hydrophobic silica nanoparticles were partially embedded into PS micro-protrusion to construct robust superhydrophobic surfaces on the wood through a simple dissolution and resolidification method, which improved its water repellency and anti-biodeterioration performance. In addition, there is a process of simulating the loss process of mold inhibitor to demonstrate the auxiliary role of superhydrophobic interface in preventing wood. The film prevents moisture from contacting the wood while reducing the loss of fungicide in order to improve the wood’s anti-degradation and anti-bacteria performances.
Fig. 1. Schematic diagram of super hydrophobic coating structure on rubber wood
EXPERIMENTAL
Materials
Rubber wood was purchased from Dongguan Jilong timber market. 3-iodo-2-propynyI butylcarbamabte (IPBC, MW=281.09), polystyrene (PS, MW=104.14), and nano fumed silica (SiO2, hydrophobic type-260, MW=60.08, 99.8%, particle size: 7-40 nm) were purchased from Aladdin Chemical Reagent Co. (Shanghai, China). Tetrahydrofuran (THF,99%) and acetone (99.5%) were purchased from Guangzhou chemical reagent factory (Guangzhou, China).
Fabrication of nanocomposite surfaces on wood substrates
Rubber wood samples (20x20x20 mm) were dipped in the IPBC-EtOH solution, which was stirred for 2 h at a solid/liquid (g/mL) ratio of 1:1000, for 10 min at room temperature and pressure. Five grams PS and 50 mL THF were mixed in a 100 mL glass beaker for 20 min to form a homogeneous solution. Then the wood samples were dipped in the pre-PS solution for 5 s and kept at room temperature for 3 h to evaporate THF. SiO2 and acetone were stirred with a magnetic stirrer for 2 h. The emulsion was, respectively, divided into five different concentrations of 0.4, 0.8, 1.2, 1.6, and 2.0 wt%. The above wood samples were soaked in the emulsion again. Finally, the above samples were soaked in the emulsion for 5 s and then placed at room temperature for 3 h to obtain a nano-micro hierarchical structure.
Characterization
The water contact angle (WCA) on the surface was measured using a contact angle system OCA (Dataphysics, OCA20, Germany) at ambient temperature. The WCA was recorded 10 s after a droplet of deionized water (5 μL) was placed on the surface. The microscopic morphology of the superhydrophobic surfaces was observed using filed-emission scanning electron microscopy (FE-SEM, Hitachi SU-70, Japan) in high vacuum mode at a working voltage of 2.0 kV. Prior to the observation, thin aurum films were sputtered on all samples by using a low-conductivity sputtering coater (Hitachi E-1010, Japan) to improve conductivity.
Anti-mold test
In order to investigate the anti-mold performance of superhydrophobic rubberwood and the protective effect of superhydrophobic layer on IPBC, four groups of specimens, including control group (RW), superhydrophobic rubberwood (SRW), IPBC-treated rubberwood (IRW), and IPBC-treated superhydrophobic rubberwood (SIRW), were tested. Six wood samples of each type were used. Malt extract agar was used as the culture media for growing mold captured from air. Deionized water-mold solution was sprayed on the surface of specimens. In addition, IRW and SIRW primarily underwent a 72-h high humidity environment process (UVTest, ATLAS, Chicago, IL, USA), which was designed to spray pure water on the specimen surface for 5 min and then for the surface to be dried at 40 ℃ for 120 min. The proportion of the growth area of the mold on the surface of specimens was calculated as the criterion after 4 weeks.
RESULTS AND DISCUSSION
Hydrophobicity
As shown in Fig. 2, the average water contact angle (WCA) of the samples treated with PS was 126.1°, while that of the samples treated with 0.8% SiO2-acetone emulsion was 149.3°. When the concentration of silica was 1.6%, the WCA reached 155.6°. Therefore, the solid-liquid ratio of silicon dioxide particles to acetone is the key to controlling the microstructure and roughness of the hydrophobic surface, which is the basis for obtaining adjustable water repellency of treated wood.
Fig. 2. Variations of WCAs with silica nanoparticle contents in the acetone-SiO2 emulsion
The prepared wood surface not only exhibited superhydrophobicity, but also had self-cleaning properties. As shown in Fig. 3, carbon-powder contaminates on the surface could be easily removed by dripping water onto the wood surface. When water droplets fell onto a slightly sloping surface, they rapidly rolled off the superhydrophobic wood surface because of the small sliding angle of the surface and removed surface contaminants.
Fig. 3. Snapshots of the self-cleaning process on the superhydrophobic wood surfaces
Microstructure and Surface Morphology
Figure 4a presents the morphology of the wood substrate after the evaporation process using only PS-THF solution, and the image was shot by FE-SEM. The figure shows that a honeycomb membrane was formed on the surface of specimen, which constitutes a coarse structure of micron size, after the evaporation process. The micro-scale roughness of the wood substrate was increased by PS-THF. The parenchyma cells were concealed by the PS layer (~130°), which provides a degree of hydrophobicity. The wood substrate, however, was not thoroughly covered due to void structure. One serious drawback of this structure is that there are many pores in the membrane, which are the most likely locations for microorganisms to hide. Hence, the less soluble solvent acetone was used to redissolve the PS layer fixed on rubberwood surface to form a continuous dense film. Even at higher magnification, it is difficult to detect structural defects in the newly formed coating surface (Fig. 4b).
The contact angle of the coated rubberwood was 100°. Compared with PS-THF coating, this contact angle eliminates the pore structure, thereby reducing the probability of microbial adsorption. Although the pores of PS coating modified by acetone were reduced, their properties were also reduced accordingly. Therefore, nano-silica was embedded in PS coatings as an auxiliary, forming a new rough structure. In the process of acetone dissolving PS, the interface between the polymer and solvent behaved as a kind of viscous liquid. Thus, the silica nanoparticles added to acetone could be embedded in the interface overall or partially (Figs. 3c and 3d). A hierarchical structure was formed by reason of the partially-embedded nano-silica in relatively neater PS layer while the interface was resolidificated again. The addition of nanoparticles dramatically increased the roughness of specimens. Figures 4c and 4d show the morphology of the wood substrate subjected to the second dissolution-solidification using acetone. When the concentration of acetone-silicon suspension reached 1.6%, it can be seen that the rubberwood surface was covered almost completely with silica particles. This structure not only had good hydrophobicity, but also provided a more complete protection of the wood surface. A microscale obvious protuberance with multitude tiny peaks and valleys around it can be seen in Fig. 3d. The entire superhydrophobic layer was like a hill covered with trees, which provided an excellent multiple spatial structure, resulting in eminent hydrophobicity. Therefore, two times shaping treatment with PS-THF and silicon-acetone resulted in the formation of both micro- and nanohierarchical structures. Under the rough structure created in this study, water droplets in contact with the surface should be blocked by air retained in nano-grooves and particles, which prevents penetration of the water droplets into the wood surface. In addition, due to the absence of large-scale pores, microorganisms on the surface are easily carried away by water flow, making it difficult for them to reproduce and grow on the wood.