PMHS Coatings on Bamboo ... - MDPI

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Fabrication of Hydrophobic ZnO/PMHS Coatings on Bamboo Surfaces: The Synergistic Effect of ZnO and PMHS on Anti-Mildew Properties Jingbing Chen, Yunyun Ma, Huiping Lin, Qingzhu Zheng, Xinxiang Zhang , Wenbin Yang * and Ran Li * College of Material Engineering, Fujian Agriculture and Forestry University, Fuzhou 350108, China; cjb8190925@163.com (J.C.); my448662507@163.com (Y.M.); 18450077642@163.com (H.L.); zjcf0911@163.com (Q.Z.); xxzhang0106@163.com (X.Z.) * Correspondence: fafuywb@163.com (W.Y.); szylr@163.com (R.L.); Tel.: +86-591-8371-5175 (W.Y. & R.L.) Received: 15 November 2018; Accepted: 27 December 2018; Published: 28 December 2018

Abstract: Poor mildew resistance has limited the application of bamboo materials. In this work, ZnO/PMHS coatings were fabricated on bamboo timber surfaces by hydrothermal synthesis method and hydrophobic modification with poly(methylhydrogen)siloxane (PMHS). The surface chemical characteristics and microstructure of the bamboo before and after modification were investigated using Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and water contact angle (WCA). The morphology results indicated that ZnO on the surface of bamboo forms flower-like aggregations. The WCA of ZnO/bamboo increased from 65◦ to 142◦ after PMHS hydrophobic modification, indicating that the hydrophobicity of bamboo was significantly improved. The original bamboo had poor anti-mildew properties when exposed to Trichoderma viride, Aspergillus niger, and Penicillium citrinum. After depositing ZnO onto the surface of the bamboo, the anti-mildew properties were significantly improved. Furthermore, it was found that after PMHS hydrophobic modification, ZnO/bamboo had excellent anti-mildew properties when exposed to the three mold hyphae, which indicated that PMHS hydrophobic modification had a synergistic effect on the anti-mildew properties of bamboo with a ZnO coating. Keywords: bamboo; anti-mildew; ZnO; poly(methylhydrogen)siloxane; synergistic effect

1. Introduction Bamboo, the second most abundant natural resource, has a long history of utilization. It has many advantages, such as a short growth cycle, high strength, high toughness, recyclability, and simple processing. Moreover, products made from bamboo are environmentally friendly and biodegradable [1]. Bamboo is widely used for furniture [2], building materials [3], scaffolding [4] and handicraft [5]. Unfortunately, when used in outdoor environments, bamboo is easily infected by mildew due to its high sugar, starch and protein contents [6], which strongly reduces the durability of bamboo products. To overcome this drawback, physical and chemical modifications have been carried out [7,8]. The physical methods include microwave and high-temperature treatment [9,10]. However, these traditional physical methods are not completely effective. Chemical mold-resisting agents are also commonly used for bamboo preservation. The chemical compounds used include pentachlorophenol, alkaline copper quaternary compounds (ACQ), chromated copper arsenate (CCA), and so on [11–13]. However, pentachlorophenol is harmful to human health. As a result, it has been banned as a wood (and bamboo) preservative in some countries [14,15]. Also, the potential toxicity of chromium, arsenic, and halogens to ecosystems should be taken into account [16]. Therefore, it is desirable to investigate

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novel efficient and non-toxic anti-mildew agents for bamboo materials. Recently, nano-metals and their oxides, such as TiO2 , CuO, ZnO, and Ag have been widely prepared by various methods and applied to improve anti-mildew properties [17–21]. These nano-metals and their oxides have been reported to be more active than the non-nano ones, which has been attributed to their higher surface area and ability to display unique physical and chemical properties. For instance, Devi et al. [22] demonstrated that a wood/polymer composite treated with TiO2 had improved mechanical stability, UV stability, and antimicrobial properties. TiO2 deposited onto the surface of bamboo at low temperatures by the sol-gel method has improved the anti-mildew properties of bamboo [19]. Xie et al. [23] also reported that nano copper particles imparted wood excellent mildew resistance to wood. It was shown that nano ZnO can be fabricated successfully on bamboo surfaces by the hydrothermal synthesis method, resulting in improved resistance of bamboo timber against mold [24]. Among the aforementioned nano materials, nano zinc oxide (ZnO) has received more and more attention due to its high chemical stability, non-toxicity, low cost, and good anti-mildew properties [25]. Another problem limiting the application of bamboo is its easy absorption of water from environment [26]. In recent years, research on hydrophobic surfaces has advanced considerably. For instance, bamboo or wood has been modified by stearic acid [27], 1H, 1H, 2H, 2H–perfluoroalkyltriethoxysilane (POTS) [28], and fluoroalkylsilane (FAS) [29], improving its hydrophobicity. However, most of the modifiers used were fluorine-containing, and therefore very expensive. The modification processes for the other compounds are complicated and time-consuming, which have limited their practical applications for the hydrophobic surface improvement of bamboo. It has been reported that hydrophobic surfaces can inhibit the adherence of bacteria, decreasing the incidence of infection [30,31]. It has been reported that hydrophobic wood can have improved water resistance and mildew inhibition, but the anti-mildew effect was not significant [32]. Although a hydrophobic surface can reduce the adherence of bacteria and water, it does not have the function of sterilization, leading to poor anti-mildew properties for hydrophobic modified bamboo [31,32]. It is interesting to investigate the synergistic effect of hydrophobic surface with antimicrobial agents, because it will afford bamboo good resistance to both water absorption and microbial degradation. Our previous studies revealed that poly(methylhydrogen)siloxane (PMHS) is an excellent hydrophobic agent for wood [33]. This is due to the special chemical structure of PMHS. PMHS possesses hydrophobic methyl groups and abundant –Si–H groups. The former will significantly lower the surface energy of the modified surface, while the latter can react with –OH groups on the substrate surface through dehydrogenation reactions [34,35]. In this work, ZnO/PMHS coatings were fabricated on the surface of bamboo to improve its anti-mildew properties, and the synergistic effect of ZnO and PMHS was also investigated. The results indicated that the ZnO/PMHS coated bamboo possessed excellent anti-mildew properties against three mold fungi, Trichoderma viride, Aspergillus niger, and Penicillium citrinum. 2. Experimental Section 2.1. Materials Bamboo was produced by China Resources Bamboo Co., Ltd., Zhejiang, China. The bamboo specimens were processed into blocks with the dimensions of (50 ± 1) mm (length) × (20 ± 0.5) mm (width) × (5 ± 0.5) mm (thickness), then washed several times with deionized water and ethanol to remove surface contamination and dried at 80 ◦ C for 24 h. Anhydrous sodium hydroxide (NaOH) and zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O) were provided by Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. Zinc acetate, n-hexane, and methanol were purchased from Tianjin Zhiyuan Chemical Reagent Co. Ltd., Tianjin, China. Sodium dodecyl sulfate (SDS) and hexamethylenetetramine (HMTA) were supplied by Aladdin, Shanghai, China. PMHS with a reactive hydrogen content of 1.5% and Kastredt catalyst (platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane) with a Pt content of 2000 ppm were obtained


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from Chenguang Research Institute of Chemical Industry (Chengdu, China). All agents were used as received, without further purification. Aspergillus niger, Penicillium citrinum, and Trichoderma viride were provided by Academy of Sciences Quality Inspection Biotechnology Co. Ltd. (Beijing, China). 2.2. Preparation of ZnO-Coated Bamboo Timber In a typical synthesis process [29,36], a methanol solution of sodium hydroxide (NaOH, 0.12 M) was added slowly to a methanol solution of Zn(CH3 COO)2 ·2H2 O (Zn(Ac)2 , 0.04 M) with a volume ratio of 1:1. Subsequently, the resulting mixed solution was stirred for 30 min at 60 ◦ C using a magnetic stirrer (DF-101S, Yuhua Instrument Co. Ltd., Gongyi, China), until a homogeneous and stable colloid solution formed. Bamboo specimens were coated by the ZnO colloid solution through a repeated dip-coating process. In detail, the bamboo slices were immersed in the ZnO colloid solution for 10 min. Next, the bamboo slices were dried at 150 ◦ C for 10 min inside a drying oven. The dip-coating and drying procedures were repeated three times to obtain multilayer films. The reaction solution for hydrothermal treatment was then prepared by the following procedure: equimolar aqueous solutions (0.2 M) of zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O) and hexamethylenetetramine (C6 H12 N4 , HMTA) were prepared in a vessel under constant stirring, while a 0.16 M sodium dodecyl sulfonate (NaC12 H25 SO3 , SDS) solution was added. After stirring for 30 min to form a clear solution, the mixed solution was transferred into a Teflon-lined autoclave with the bamboo samples. The hydrothermal treatments were performed at 95 ◦ C for 3 h. The samples were then washed several times with distilled water (DI water) and dried at 80 ◦ C for 48 h in a vacuum oven. The ZnO/bamboo samples are denoted as zinc oxide modified samples. 2.3. Surface Modification of ZnO/Bamboo The modifier solution was obtained by mixing 2.5 g PMHS, 47.5 g n-hexane and 2 drops of Kastredt catalyst. ZnO/bamboo samples were dipped into the modifier solution for 3 min. The modified bamboo samples were finally washed three times with n-hexane to remove the PMHS chains that were not covalently bonded onto the bamboo surface. Thereafter, the specimens were heated at 60 ◦ C for 20 min. After PMHS modification, the samples were named PMHS-ZnO/bamboo. 2.4. Characterization Scanning electron microscopy images of the bamboo samples were observed by Supra55 (scanning electron microscopr, SEM, Zeiss, Oberkochen, Germany) at an accelerating voltage of 20 kV, attached to an X-ray energy dispersive spectrometer (EDS, Genisis XM, Oxford, UK). EDS microanalysis was conducted to obtain information on the elemental compositions of the samples. Before scanning process, a thin layer of gold was sputtered onto sample surfaces to enhance their electron conductivity. The changes in the crystalline structures of bamboo surface before and after modification were characterized using X-ray diffraction (XRD, D/max-UItima III, Rigaku, Tokyo, Japan). The scattered radiation was detected in the range of 5◦ –80◦ at a scanning rate of 2◦ s−1 . The chemical composition of the modified and unmodified bamboo surfaces were determined by Fourier-transform infrared spectroscopy (FTIR, Bruker Tensor 27, Bruker Optik GmbH, Ettlingen, Germany). For each sample, the absorbance spectrum was measured from 4000 to 400 cm−1 at a 4 cm−1 resolution for 32 scans. The water contact angle of bamboo was measured using a goniometer (Krüss DSA100, Krüss GmbH, Hamburg, Germany) at room temperature, with average values taken from of five measurements. 2.5. Test of Anti-Mildew Properties The test of anti-mildew properties for bamboo was carried out according to the standard of GB/T18261-2013 [37] (test method for anti-mildew agents in controlling wood mold and stain fungi). Six dimensioned bamboo samples were tested with each fungus. The bamboo samples were exposed to fungi growing on potato media, which allowed mildew growth under a controlled temperature of

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28 °C and relative humidity of 95%–98%. Samples were evaluated daily for mildew growth. Mildew 28 ◦ C and relative humidity of 95%–98%. Samples were evaluated daily for mildew growth. Mildew growth on bamboo surfaces was visually rated using grading standards of infection, shown in Table growth on bamboo surfaces was visually rated using grading standards of infection, shown in Table 1. 1. The final grade was determined by taking the average infected area of six bamboo samples exposed The final grade was determined by taking the average infected area of six bamboo samples exposed to to the same fungus. The lower the infection value, the better the efficiency of treatment. the same fungus. The lower the infection value, the better the efficiency of treatment. Table 1. The grading standards for the infection vof bamboo samples by mold fungi. Table 1. The grading standards for the infection vof bamboo samples by mold fungi. Grading Description Grading Description 0 No hypha or mold 0 No hypha or mold 1 The surface infection area of the sample is less than ¼1 1 The surface infection area of the sample is less than /4 The ½1/2 1/and 2 2 Thesurface surfaceinfection infectionarea areaof ofthe thesample sampleisisbetween between¼ 4 and 1 3 3 Thesurface surfaceinfection infectionarea areaof ofthe thesample sampleisisbetween between½/and 2 and The ¾3/4 3 4 4 Thesurface surfaceinfection infectionarea areaofofthe thesample sampleisisgreater greaterthan than¾/4 The

3. Results and Discussion 3.1. Morphology of of ZnO ZnO Coatings Coatings on 3.1. Morphology on Bamboo Bamboo Surface Surface Scanning electron microscopy microscopy (SEM) (SEM) was was used used to to observe observe the the morphology morphology of of bamboo bamboo surfaces. surfaces. Scanning electron The surfacemorphologies morphologiesofofbamboo bamboo samples before treatment are shown in Figure 1. The surface samples before andand afterafter treatment are shown in Figure 1. There There are numerous pits on the surface of the original bamboo samples, as revealed in Figure 1a. are numerous pits on the surface of the original bamboo samples, as revealed in Figure 1a. As shown As shown1b, in Figure 1b, the covered withlayers various of which ZnO, inthe which ZnOformed sheets in Figure the surface is surface coverediswith various of layers ZnO, in ZnOthe sheets formed flower-like aggregations constructed in a hierarchical manner [29]. In Figure 1c, the surface flower-like aggregations constructed in a hierarchical manner [29]. In Figure 1c, the surface of the of theisZnO less rough after treatment with PMHS. This may be to due to long the long chain molecule of ZnO less isrough after treatment with PMHS. This may be due the chain molecule of the the PMHS, which filled gaps between ZnO sheets.InInaddition, addition,the thepoor poorelectrical electrical conductivity conductivity of PMHS, which filled thethe gaps between ZnO sheets. of PMHS on ZnO surface probably contributed to the indistinct morphology of the PMHS-ZnO/bamboo PMHS on ZnO surface probably contributed to the indistinct morphology of the PMHS-ZnO/bamboo samples. However, the the surface surface of of PMHS-ZnO/bamboo was still still very very rough rough compared compared to to that that of samples. However, PMHS-ZnO/bamboo was of original bamboo. original bamboo.

Figure 1. Scanning electron microscope images of untreated and treated bamboo surfaces: (a) original Figure 1. Scanning electron microscope images of untreated and treated bamboo surfaces: (a) original (untreated) bamboo; (b) ZnO/bamboo; (c) PMHS-ZnO/bamboo. (untreated) bamboo; (b) ZnO/bamboo; (c) PMHS-ZnO/bamboo.

3.2. Chemical Structure of the Bamboo Surface 3.2. Chemical Structure of the Bamboo Surface The main chemical elements of the bamboo surface before and after modification were analyzed TheEDS mainspectra chemical elements of bamboo, the bamboo surface before and after modification were analyzed by EDS. of the original ZnO/bamboo, and PMHS-ZnO/bamboo are presented by EDS. EDS spectra of the original bamboo, ZnO/bamboo, and PMHS-ZnO/bamboo are presented in Figure 2A. As shown in Figure 2A(a), for the original bamboo, only the C and O elements could be in Figure 2A. As shown Figure and 2A(a), for therespectively. original bamboo, only the C andrevealed O elements could be observed, accounting forin58.37% 41.63%, The ZnO/bamboo the presence observed, accounting for 58.37% and 41.63%, respectively. The ZnO/bamboo revealed the presence of Zn element in addition to the C and O elements, which confirmed the loading of the bamboo of Zn element in (Figure addition2A(b)). to the In C addition, and O elements, which confirmed thealso loading of which the bamboo surface with ZnO a small amount of sulfur was found, could surface with ZnO (Figure 2A(b)). In addition, a small amount of sulfur was also found, which be attributed to the addition of a surfactant (SDS). Finally, after the PMHS treatment, silicon wascould also be attributed to the 2A(c). addition of indicates a surfactant Finally, after the PMHS treatment, siliconbamboo. was also observed in Figure This the(SDS). existence of PMHS on the surface of the coated observed in Figure 2A(c). indicates the existence of PMHS and on the surface of the coated bamboo. The FTIR spectra for This the original bamboo, ZnO/bamboo, PMHS-ZnO/bamboo are shown The FTIR spectra for the original bamboo, ZnO/bamboo, and PMHS-ZnO/bamboo are shown in − 1 in Figure 2B. The original bamboo (Figure 2B(a)), had an absorption peak at 1036 cm , belonging −1, belonging to Figure 2B. The original bamboo (Figure 2B(a)), had an absorption peak at 1036 cm − to skeletal vibration of the C–O–C from the pyranose ring [38]. The absorption bands at 1742 cm 1 skeletal of the from ring [38]. The absorption at 1742stretching cm−1 and and 1423vibration cm−1 could be C–O–C attributed to the the pyranose C=O stretching vibration and C–Hbands asymmetric −1 1423 cm could be attributed to the C=O stretching vibration and C–H asymmetric stretching


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vibration, [36].REVIEW Compared with the original bamboo, the FTIR spectrum of ZnO/bamboo Coatingsrespectively 2018, 8, x FOR PEER 5 of 10 showed a distinct absorption peak at 460 cm−1 , as shown in Figure 2B(b), which could be assigned vibration, respectively [36]. the original thespectrum FTIR spectrum of ZnO/bamboo to the stretching vibration of Compared the Zn–Owith groups [29]. Inbamboo, the FTIR of PMHS-ZnO/bamboo −1, as shown in Figure 2B(b), which could be − 1 , which showed a distinct absorption peak at 460 cm tocould (Figure 2B(c)), there were three strong absorption peaks at 2851, 2921, and 2964 cm assigned the stretching vibration of the Zn–O groups [29]. In the FTIR spectrum of PMHS-ZnO/bamboo (Figure 2B(c)), be attributed to the –CH2 and –CH3 stretching vibration respectively [34]. In addition, Si–O–Si and there were three strong absorption peaks at 2851, 2921, and 2964 cm−1, which could be attributed to Si–C absorption bands were found for PMHS at 799 and 1255 cm−1 , respectively [39]. These results the –CH2 and –CH3 stretching vibration respectively [34]. In addition, Si–O–Si and Si–C absorption confirmed that the PMHS chains had been successfully grafted onto ZnO/bamboo surface. bands were found for PMHS at 799 and 1255 cm−1, respectively [39]. These results confirmed that the To further the crystalgrafted structure chemicalsurface. composition of the bamboo surface PMHS chainsascertain had been successfully ontoand ZnO/bamboo before and after ZnO and PMHS modification, XRD was employed. Thebamboo typicalsurface reflection planes To further ascertain the crystal structure and chemical composition of the before ◦ , 16◦ , and 34◦ , respectively of the crystalline domain of cellulose were found at 2θ angles of 22 and after ZnO and PMHS modification, XRD was employed. The typical reflection planes of the crystalline of cellulose were found at 2θin angles of 22°, 16°, and 34°, respectively 2C(a)) [40]. (Figure 2C(a))domain [40]. For ZnO/bamboo, shown Figure 2C(b), in addition to the (Figure characteristic peaks of ◦ , 34.4 ◦ , 56.6◦ , For ZnO/bamboo, shown in Figure 2C(b), addition to peaks the characteristic peaks of◦cellulose, there cellulose, there were also characteristic ZnOindiffraction at 2θ = 31.8 , 36.2◦ , 34.4 ◦ , and characteristic ZnO diffraction peaks at 2θ(101), = 31.8°,(102), 34.4°,(110), 36.2°, 34.4°, 62.8°, 66.4°, and 67.9°, 62.8◦were , 66.4also 67.9◦ , attributed to (100), (002), (103),56.6°, (201), and (004) respectively, attributed to (100), (002), (101), (102), (110), (103), (201), and (004) respectively, for the diffraction of good for the diffraction of ZnO crystals with a hexagonal structure. These characteristic peaks are in ZnO crystals with a hexagonal structure. These characteristic peaks are in good agreement with the agreement with the standard card (JCPDS card NO. 36-1451). These findings confirm the formation of standard card (JCPDS card NO. 36-1451). These findings confirm the formation of ZnO on the ZnO on the bamboo surface. Furthermore, the structure was not changed, because there was only a bamboo surface. Furthermore, the structure was not changed, because there was only a thin PMHS thin layer PMHS on the surface ZnO modification after PMHS(Figure modification on layer the surface of ZnO afterofPMHS 2C(c)). (Figure 2C(c)).

Figure 2. (A) EDS spectraand andelement, element, (B) and (C)(C) XRD patterns of (a)oforiginal Figure 2. (A) TheThe EDS spectra (B) FTIR FTIRspectra, spectra, and XRD patterns (a) original bamboo, (b) ZnO/bamboo, and (c) PMHS-ZnO/bamboo. bamboo, (b) ZnO/bamboo, and (c) PMHS-ZnO/bamboo.

3.3. Hydrophobicity of ModifiedBamboo BambooSurfaces Surfaces 3.3. Hydrophobicity of Modified Water contact angle (WCA)was was used used to to characterize of treated bamboo Water contact angle (WCA) characterizethe thehydrophobicity hydrophobicity of treated bamboo surfaces. Figure 3 presents the WCA of original untreated bamboo, ZnO/bamboo, and PMHS-ZnO/bamboo surfaces. Figure 3 presents the WCA of original untreated bamboo, ZnO/bamboo, and PMHS-ZnO/ samples. Figure 3a presents an image of water droplets on a radial section of original bamboo. The bamboo samples. Figure 3a presents an image of water droplets on a radial section of original WCA of original bamboo was only 12°, which indicates that unmodified bamboo is very hydrophilic bamboo. The WCA of original bamboo was only 12◦ , which indicates that unmodified bamboo is and therefore the water drop could spread over a large surface area. This is because unmodified verybamboo hydrophilic and large therefore the water drop could spread over aunmodified large surface area. This is because contains amounts of hydroxyl groups. Therefore, bamboo can easily unmodified bamboo large amounts of the hydroxyl Therefore, unmodified absorb water fromcontains the environment, which aids growthgroups. of mold on the bamboo surface. Asbamboo shown can easily water the environment, which aids the growth ofindicating mold on an theimprovement bamboo surface. in absorb Figure 3b, after from the deposition of ZnO, WCA of ZnO/bamboo was 65°, in hydrophobicity. after ofmodification PMHS, the WCA As shown in Figure 3b,Most afterimportantly, the deposition ZnO, WCAwith of ZnO/bamboo was of 65◦PMHS, indicating ZnO/bamboo was dramatically increased to 142°, indicating that ZnO-PMHS/bamboo surfaces an improvement in hydrophobicity. Most importantly, after modification with PMHS, the WCA of became very hydrophobic. It is well-known thattogood hydrophobicity can not only inhibit PMHS-ZnO/bamboo was dramatically increased 142◦surface , indicating that ZnO-PMHS/bamboo surfaces

became very hydrophobic. It is well-known that good surface hydrophobicity can not only inhibit the

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adhesion of bacteria, but alsobut keep from absorbing water from the the environment. Both the adhesion of bacteria, alsobamboo keep bamboo from absorbing water from environment. Bothofofthese Coatings 2018, 8, x FOR PEER REVIEW 6 of 10 effects are beneficial to the anti-mildew properties of bamboo. these effects are beneficial to the anti-mildew properties of bamboo. the adhesion of bacteria, but also keep bamboo from absorbing water from the environment. Both of these effects are beneficial to the anti-mildew properties of bamboo.

3. Water contact angle of (a) original bamboo,(b) (b)ZnO/bamboo, ZnO/bamboo, and (c)(c) PMHS-ZnO/bamboo. FigureFigure 3. Water contact angle of (a) original bamboo, and PMHS-ZnO/bamboo.

3.4. Schematic Representation of Surface Modification 3.4. Schematic Representation of Surface Modification

Figure 3. Water contact angle of (a) original bamboo, (b) ZnO/bamboo, and (c) PMHS-ZnO/bamboo.

on above the above experimentalresults, results,aaschematic schematic diagram proposed mechanism of of BasedBased on the experimental diagramofofthe the proposed mechanism 3.4. SchematicofRepresentation of Surface Modification deposition ZnO and PMHS modification of the bamboo surface is shown in Figure 4. The surface deposition of ZnO and PMHS modification of the bamboo surface is shown in Figure 4. The surface of of bamboo has many hydroxyl ZnO aseeds were loaded onof the the bamboo by Based on the above experimental schematic diagram thesurface proposed mechanism of bamboo has many hydroxyl groups.groups. ZnOresults, seeds were loaded on the surface of theofbamboo by soaking soaking the bamboo in the ZnO colloid solution (Figure 4a). The ZnO seeds were bonded onto the deposition ZnO andcolloid PMHS solution modification of the4a). bamboo is shown Figure 4.onto The surface the bamboo in of the ZnO (Figure The surface ZnO seeds wereinbonded the surface surface of the bamboo by hydrogen bonds. Thereafter, theloaded bamboo was soaked three times to create of bamboo has many hydroxyl groups. ZnO seeds were on the surface of the bamboo of the bamboo by hydrogen bonds. Thereafter, the bamboo was soaked three times to create by a dense a densethe ZnO seed layer onZnO the colloid bamboosolution surface.(Figure Then, as is The shown inseeds Figure 4b, bonded the hierarchically soaking bamboo in the 4a). ZnO were onto the ZnO seed layer on the bamboo surface. Then, as is shown in Figure 4b, the hierarchically structured structured ZnO coating fabricated on Thereafter, the surface the of the bamboo the hydrothermal surface of the bamboo bywas hydrogen bonds. bamboo wasby soaked three times tomethod. create ZnO coating was fabricated onPMHS the surface of the bamboo by the–CH hydrothermal As shown As shown in Figure 4d, the chain contains hydrophobic 3 groups and method. a large quantity of in a dense ZnO seed layer on the bamboo surface. Then, as is shown in Figure 4b, the hierarchically Figure 4d, the–Si–H PMHS chainThe contains hydrophobic –CH groups and large quantity of reactive reactive reactive –Si–H bonds can 3of participate in aaby variety of chemical reactions, structured ZnO bonds. coating was fabricated on the surface the bamboo the hydrothermal method.–Si–H especially with vinyland hydroxyl-containing materials. The –CH 3 groups provide the PMHS bonds. The reactive –Si–H bonds can participate in a variety of chemical reactions, especially As shown in Figure 4d, the PMHS chain contains hydrophobic –CH3 groups and a large quantitywith of with very low surface energy. As shown in Figure 4e, the surface of ZnO prepared by the hydrothermal vinyl-reactive and hydroxyl-containing materials. –CHcan provide the PMHS with very low surface –Si–H bonds. The reactive –Si–HThe bonds participate in a variety of chemical reactions, 3 groups method contains large hydroxyl groups. Karstedt catalyst is the complex compound of energy. As shown in aFigure 4e, the of surface of ZnO prepared by the hydrothermal contains especially with vinylandamount hydroxyl-containing materials. The –CH 3 groups provide themethod PMHS with platinum and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane, which has very high catalytic activity for very low surface energy. As shown in Figure 4e, the surface of complex ZnO prepared by the hydrothermal a large amount of hydroxyl groups. Karstedt catalyst is the compound of platinum and dehydrogenation between –Si–Hofbands of PMHS andKarstedt hydroxylcatalyst groupsisofthe ZnO at roomcompound temperature. method contains a large amount hydroxyl groups. complex of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane, which has very high catalytic activity for dehydrogenation Therefore, it was inferred that the bamboo surface would be grafted with numerous hydrophobic platinum and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane, which has very high catalytic activity for between –Si–H bands of simple PMHSmethod and hydroxyl groups of ZnO at room temperature. Therefore, it was PMHS chains by this (Figure 4f). As a result the hydrophobicity of room ZnO/bamboo could dehydrogenation between –Si–H bands of PMHS and hydroxyl groups of ZnO at temperature. inferred that the bamboo surface wouldmodification. be grafted with numerous hydrophobic PMHS chains by this be significantly by PMHS Therefore, it wasimproved inferred that the bamboo surface would be grafted with numerous hydrophobic simple method (Figure 4f). As a result the hydrophobicity of ZnO/bamboo could be significantly PMHS chains by this simple method (Figure 4f). As a result the hydrophobicity of ZnO/bamboo could improved by PMHSimproved modification. be significantly by PMHS modification.

Figure 4. Schematic of (a) deposited ZnO seed; (b) the hydrothermal reaction generates ZnO; and (c) reaction of ZnO and PMHS; (d) the PMHS chain; (e) the ZnO; and (f) the PMHS-ZnO..

Figure 4. 4. Schematic (a)deposited deposited ZnO the hydrothermal reactionZnO; generates Figure Schematic of (a) ZnO seed;seed; (b) the(b) hydrothermal reaction generates and (c) ZnO; reaction of ZnO and PMHS; (d) the(d) PMHS chain; (e) the ZnO; andZnO; (f) theand PMHS-ZnO.. and (c) reaction of ZnO and PMHS; the PMHS chain; (e) the (f) the PMHS-ZnO.


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anti-mildew properties limit bamboo’s application, as mentioned above. Therefore, it is 3.5.Poor Anti-Mildew Properties very important to develop mildew-resistant bamboo. The mildew-resistances of the original bamboo, Poor anti-mildew properties limit bamboo’s application, as mentioned above. Therefore, it is ZnO/bamboo, and PMHS-ZnO/bamboo during a 4 weeks incubation trial are presented in Figure 5. very important to develop mildew-resistant bamboo. The mildew-resistances of the original bamboo, The original bamboo samples were infected by Trichoderma viride on the 3rd day (Figure 5a). On the 8th ZnO/bamboo, and PMHS-ZnO/bamboo during a 4 weeks incubation trial are presented in Figure 5. day, the area of original bamboo infected by Trichoderma viride was more than 75%, and the infection was The original bamboo samples were infected by Trichoderma viride on the 3rd day (Figure 5a). On the rated as 4 on the grading scale (Table 1). After 4 weeks of testing, original bamboo samples were totally 8th day, the area of original bamboo infected by Trichoderma viride was more than 75%, and the covered by Trichoderma 6a), suggesting that original bamboo had very poorbamboo resistance infection was rated asviride 4 on(Figure the grading scale (Table 1).the After 4 weeks of testing, original against Trichoderma viride. This is because bamboo is rich in nutrients and easily absorbs water from the samples were totally covered by Trichoderma viride (Figure 6a), suggesting that the original bamboo environment. These factorsagainst provide the growth conditions mold.bamboo ZnO/bamboo not infected had very poor resistance Trichoderma viride. This is for because is rich inwas nutrients and byeasily Trichoderma viride over the first 19 days, but hyphae appeared on the surface on the 20th day. After absorbs water from the environment. These factors provide the growth conditions for mold. 4 weeks of testing, grading valueviride was 1over andthe only 5%19ofdays, the bamboo area was infected ZnO/bamboo wasthe notinfection infected by Trichoderma first but hyphae appeared on (Figure 6d). This means an anti-mildew effect, and growth of Trichoderma viride the surface on the 20ththat day.ZnO/bamboo After 4 weekshad of testing, the infection grading value was 1 and only 5% was inhibited the bamboo The reason delayed mold of ZnO modified ofobviously the bamboo area wason infected (Figuresurface. 6d). This means thatfor ZnO/bamboo hadgrowth an anti-mildew effect, and growth of Trichoderma viride was obviously on the bamboo surface. The for bamboo has been suggested by previous scholars inhibited [17,30]. ZnO nanoparticles destroy thereason membrane delayed ZnObe modified bamboo hasdirect been suggested bybetween previousZnO scholars [17,30]. ZnO wall of themold mold,growth whichofmay due in part to the interaction nanoparticles and themembrane. membrane During wall of28 the mold, which may there be due in no part to the direct thenanoparticles surface of thedestroy bacterial days of observation, was Trichoderma viride ZnOsamples nanoparticles thePMHS-ZnO surface of the bacterial During 28the days of oninteraction the surfacebetween of bamboo coatedand with (Figure 6g).membrane. This indicates that PMHS observation, there was no Trichoderma viride on the surface of bamboo samples coated with PMHSmodification had a synergistic or additive effect on the anti-mildew properties of bamboo with ZnO ZnO (Figure This indicates theon PMHS modification had a synergistic or additive effectenergy, on deposition. This6g). is because PMHS that chains the ZnO/bamboo surface greatly reduce its surface the anti-mildew properties of bamboo with ZnO deposition. This is because PMHS chains on the which inhibits the bamboo from absorbing water from the environment and gives the bamboo better ZnO/bamboo surface greatly reduce its surface energy, which inhibits the bamboo from absorbing anti-mildew properties. In addition, the hydrophobic surface may also reduce the adhesion of mold. water from the environment and gives the bamboo better anti-mildew properties. In addition, the Similarly, as shown in Figure 5b,c, original bamboo also showed no resistance to Aspergillus niger hydrophobic surface may also reduce the adhesion of mold. Similarly, as shown in Figure 5b,c, and Penicillium citrinum. The ZnO/bamboo had improved anti-mildew resistance to Aspergillus niger original bamboo also showed no resistance to Aspergillus niger and Penicillium citrinum. The and Penicillium citrinum, while PMHS-ZnO/bamboo possessed excellent anti-mildew resistance to ZnO/bamboo had improved anti-mildew resistance to Aspergillus niger and Penicillium citrinum, while Aspergillus niger and Penicillium it can be concluded that the ZnO/PMHS coated PMHS-ZnO/bamboo possessedcitrinum. excellentTherefore, anti-mildew resistance to Aspergillus niger and Penicillium bamboo possessed excellent anti-mildew resistance to various molds, including Trichoderma viride, citrinum. Therefore, it can be concluded that the ZnO/PMHS coated bamboo possessed excellent antiAspergillus niger, and citrinum. mildew resistance to Penicillium various molds, including Trichoderma viride, Aspergillus niger, and Penicillium citrinum.

Figure Infectionvalues valuesover over28 28days days of of bamboo bamboo samples samples with against Figure 5. 5. Infection withdifferent differentsurface surfacetreatment treatment against Trichoderma viride, Aspergillusniger, niger,and and(c) (c)Penicillium Penicillium citrinum. citrinum. (a)(a) Trichoderma viride, (b)(b)Aspergillus


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Figure 6. The physical images of anti-mildew anti-mildew properties properties in in (a–c) (a–c) original original bamboo, bamboo, (d–f) (d–f) ZnO/bamboo, ZnO/bamboo, Figure and stain over over 28 28 days days of cultivation with different fungi types. and (g–i) (g–i) PMHS-ZnO/bamboo PMHS-ZnO/bamboo stain

4. 4. Conclusions Conclusion In In this this study, study, an an easy easy approach approach was wasdeveloped developedfor forfabrication fabricationofofhydrophobic hydrophobicZnO/PMHS ZnO/PMHS coatings on bamboo surfaces, based on a hydrothermal process and hydrophobic coatings on bamboo surfaces, based on a hydrothermal process and hydrophobic treatment. treatment. The The nano nano ZnO sheets were successfully grown on the surface of bamboo, where they formed a hierarchical ZnO sheets were successfully grown on the surface of bamboo, where they formed a hierarchical flower-like improved thethe hydrophobicity of bamboo by flower-like structure. structure.Further FurtherPMHS PMHSmodification modificationgreatly greatly improved hydrophobicity of bamboo ◦ . Deposition of ZnO onto bamboo surfaces could significantly dramatically increasing the WCA to 142 by dramatically increasing the WCA to 142°. Deposition of ZnO onto bamboo surfaces could improve the anti-mildew of properties the bamboo. A further PMHS modification of ZnO coated significantly improve the properties anti-mildew of the bamboo. A further PMHS modification of bamboo gave the bamboo excellent anti-mildew properties, indicating a good synergistic additive ZnO coated bamboo gave the bamboo excellent anti-mildew properties, indicating a goodor synergistic effect of nano-ZnO PMHS on anti-mildew properties ofproperties bamboo. of The innovative or additive effect ofand nano-ZnO andthe PMHS on the anti-mildew bamboo. The fabrication innovative of the ZnO/PMHS coating directly on the surface of the material can be used not only fabrication of the ZnO/PMHS coating directly on the surface of the material can be used for notbamboo, only for but also for etc.wood, Due toetc. its Due synergistic effect against mildew, it can effectively the service bamboo, butwood, also for to its synergistic effect against mildew, it can extend effectively extend life of bamboo products. the service life of bamboo products. Author Contributions: Conceived and designed the experiments, Y.M., J.C. and R.L.; Performed the experiments, Author Conceived the experiments, Y.M., J.C.W.Y.; andWriting-Original R.L.; PerformedDraft the H.L., Q.Z.Contributions: and J.C.; Formal Analysis, and Y.M.,designed J.C.; Contributed analysis tools, X.Z., experiments,J.C.; H.L., Q.Z. and J.C.; Formal Analysis, Y.M., J.C.; Contributed analysis X.Z., W.Y.;&WritingPreparation, Critically revised the article, J.C., Y.M., H.L., Q.Z., X.Z., W.Y. and R.L.; tools, Writing-Review Editing, W.Y., X.Z.;Draft Supervision, W.Y.,J.C.; R.L.Critically revised the article, J.C., Y.M., H.L., Q.Z., X.Z., W.Y. and R.L.; WritingOriginal Preparation, Review & Editing, W.Y., X.Z.; Supervision, R.L.and Technological Projects for University in Fujian Province by MajorW.Y., Scientific Funding: This work was supported (No. 2016H61010036), Science and Technology Extension Project of Fujian Forestry Department (No. 2018TG13-2), Funding: This work was supported by Major Scientific and Technological Projects for University in Fujian the National Natural Science Foundation of China (Grant No. 31700498), Natural Science Foundation of Fujian Province (No. (No. 2017J05072), 2016H61010036), Science and Technology Extension Project offor Fujian Forestry Department (No. Province Fujian Agriculture and Forestry University Fund Distinguished Young Scholars (No. xjq201726). 2018TG13-2), the National Natural Science Foundation of China (Grant No. 31700498), Natural Science Foundation of Fujian (No. 2017J05072), Agriculture and Forestry University Fund for Conflicts of Interest: TheProvince authors declare no conflict ofFujian interest. Distinguished Young Scholars (No. xjq201726).

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