materials Article

Facile Preparation of a Robust and Durable Superhydrophobic Coating Using Biodegradable Lignin-Coated Cellulose Nanocrystal Particles Jingda Huang 1,2 , Siqun Wang 1,2, * 1 2

*

ID

and Shaoyi Lyu 1, *

Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing 100091, China; [email protected] Center for Renewable Carbon, University of Tennessee, Knoxville, TN 37996, USA Correspondence: [email protected] (S.W.); [email protected] (S.L.); Tel.: +1-865-946-1120 (S.W.)

Received: 12 July 2017; Accepted: 11 September 2017; Published: 14 September 2017

Abstract: It is a challenge for a superhydrophobic coating to overcome the poor robustness and the rough surface structure that is usually built using inorganic particles that are difficult to degrade. In this study, a robust superhydrophobic coating is facilely prepared by using commercial biodegradable lignin-coated cellulose nanocrystal (L-CNC) particles after hydrophobic modification to build rough surface structures, and by choosing two different adhesives (double-sided tape and quick-setting epoxy) to support adhesion between the L-CNC particles and the substrates. In addition to excellent self-cleaning and water repellence properties, the resulting coatings show outstanding mechanical strength and durability against sandpaper abrasion, finger-wipe, knife-scratch, water jet, UV radiation, high temperature, and acidic and alkali solutions, possessing a wide application prospect. Keywords: superhydrophobic coating; lignin-coated cellulose nanocrystal (L-CNC); mechanical strength; durability

1. Introduction Wettability is a very important characteristic and has a significant influence on actual applications for a solid material surface. At first found in nature, such as in lotus leaves [1], superhydrophobic surfaces exhibit many outstanding functions, such as self-cleaning [2,3], anti-icing [4], anti-fog [5] and corrosion-resistant [6,7] functions, and have attracted much attention. An artificial superhydrophobic coating can be created on a hydrophilic surface by building suitable roughness and lowering surface free energy. Following decades of development, there have been many reports on the preparation of a superhydrophobic coating applied to different materials surfaces, such as wood [8], clothes [9,10], metal [11–13], filter paper [14], glass [15], and plastic [16]. Different preparation methods have also been employed, such as sol-gel [17], immersion [18], electrochemical deposition [19], spraying [20,21], etching [22,23], chemical vapor deposition (CVD) [24], and electrospinning [25]. However, superhydrophobic coatings still suffer from much obsession. In particular, poor abrasion resistance is a recognized challenge and directly limits their further application. Beyond doubt, robustness has become a hot topic with respect to the study of superhydrophobic coatings. A variety of methods are presented to solve the poor abrasion resistance. For example, in a report by Reference [26], polydimethylsiloxane (PDMS) was used as an interlayer between a superhydrophobic SiO2 layer and a glass substrate to supply adhesion. The resulted coating showed good abrasion resistance and could withstand nine cycles of sandpaper (150 grit) abrasion under a weight of 50 g. It was reported [27] that a superhydrophobic coating was prepared by spraying a mixture of SiO2 nanoparticles modified with fluoroalkylsilane. The resulting coating was mechanically robust and could withstand 200 cycles of sandpaper (2000 grit) abrasion under a 2-kg force. It could be Materials 2017, 10, 1080; doi:10.3390/ma10091080

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be seen thatinorganic inorganicparticles particlessuch suchasasSiO SiO2are are generally generally used used as as the seen that the main main materials materialsto tobuild buildaarough rough 2 surface structure. surface structure. Recently, Recently,our ourresearch researchgroup grouphas hasmade madesome someprogress progressin inthe thepreparation preparationof ofsuperhydrophobic superhydrophobic coatings using biodegradable materials. We successfully prepared a cellulose coatings using biodegradable materials. We successfully prepared a cellulosenanofibril nanofibril(CNF) (CNF) superhydrophobic coating by CVD after spraying [28], but the abrasion resistance superhydrophobic coating by CVD after spraying [28], but the abrasion resistancestill stillrequires requires significant Additionally,the theCVD CVDmodification modification method is not convenient to in use in significant improvement. improvement. Additionally, method is not convenient to use some some areas such as building walls or wide panels. Cellulose nanocrystal (CNC) is usually isolated areas such as building walls or wide panels. Cellulose nanocrystal (CNC) is usually isolated from from cellulose anhydrolyzation acid hydrolyzation method and stored in theofform of an aqueous cellulose fiber fiber via anvia acid method [29,30][29,30] and stored in the form an aqueous solution solution wherecould CNCsrepel could repel each other due negative to their negative surface charges. Due high to their high where CNCs each other due to their surface charges. Due to their strength strength and environmental friendliness, CNCs are often used as a reinforcement material. As is and environmental friendliness, CNCs are often used as a reinforcement material. As is known, the known, the reinforcement effect will be limited, unless individual CNC can be evenly dispersed. reinforcement effect will be limited, unless individual dried CNCdried can be evenly dispersed. However, However, the large quantities of hydroxyls on their surfaces would lead to CNC aggregation afterit the large quantities of hydroxyls on their surfaces would lead to CNC aggregation after drying, and drying, and it will be very difficult to disperse the aggregated CNCs again. will be very difficult to disperse the aggregated CNCs again. InInthis research, individual drieddried CNC CNC is undesired. Instead, the lignin-coated cellulose nanocrystal this research, individual is undesired. Instead, the lignin-coated cellulose (L-CNC) particles formed from the L-CNC aggregation are required. Besides natural nanocrystal (L-CNC) particles formed from the L-CNC aggregation are required. being Besides being biodegradable, L-CNC particles are of high strength due to the high crystallinity of CNCs themselves natural biodegradable, L-CNC particles are of high strength due to the high crystallinity of CNCs [31]. A suitable necessaryisfor a superhydrophobic coating [32]. Unlike some coatings themselves [31].roughness A suitableisroughness necessary for a superhydrophobic coating [32]. Unlike some [33,34] where different sizes of particles are needed to build a rough surface structure, the L-CNC coatings [33,34] where different sizes of particles are needed to build a rough surface structure, the particles possess irregular sizes and unsmooth surfaces and could supply suitable roughness by L-CNC particles possess irregular sizes and unsmooth surfaces and could supply suitable roughness themselves. Also,Also, the the L-CNC particle surfaces have many hydroxyl groups and by themselves. L-CNC particle surfaces have many hydroxyl groups andare arebeneficial beneficialtoto hydrophobic modification. To fix the L-CNC particles to a substrate surface, an adhesive hydrophobic modification. To fix the L-CNC particles to a substrate surface, an adhesiveisisneeded. needed. ItIt was reportedthat thata robust a robust superhydrophobic coating be prepared using double-sided was reported superhydrophobic coating couldcould be prepared using double-sided adhesive adhesive or spray adhesive as binders between TiO2 particles with hydrophobic modification and or spray adhesive as binders between TiO2 particles with hydrophobic modification and substrates [34]. substrates [34]. Inspired by this article, in this study, a quick-setting epoxy resin and a double-sided Inspired by this article, in this study, a quick-setting epoxy resin and a double-sided tape are used to tape are used to supply adhesion. supply adhesion. The preparation process is simple, as shown in Figure 1. First, hydrophilic L-CNC particles are The preparation process is simple, as shown in Figure 1. First, hydrophilic L-CNC particles are modified using a low-energy reagent. And then, the hydrophobic L-CNC particles are completely modified using a low-energy reagent. And then, the hydrophobic L-CNC particles are completely covered onto the substrate surface, which was previously treated with adhesive. Lastly, a wooden covered onto the substrate surface, which was previously treated with adhesive. Lastly, a wooden bar is used to press the L-CNC particles onto the adhesive. The resulting coatings show excellent bar is used to press the L-CNC particles onto the adhesive. The resulting coatings show excellent superhydrophobic properties, as well as high mechanical strength and durability. Furthermore, the superhydrophobic properties, as well as high mechanical strength and durability. Furthermore, L-CNC particles are biodegradable, non-toxic, and not harmful to environment, and the quick-setting the L-CNC particles are biodegradable, non-toxic, and not harmful to environment, and the epoxy or double-sided tape can increase the processing efficiency without requiring a cumbersome quick-setting epoxy or double-sided tape can increase the processing efficiency without requiring a process. cumbersome process.

Figure 1. Sketch of the procedure to prepare the composite superhydrophobic coating. Figure 1. Sketch of the procedure to prepare the composite superhydrophobic coating.

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2. Results and Discussion 2.1. Surface Morphology Morphology of 2.1. Surface of the the Coatings Coatings As As shown shown in in Figure Figure 2a, 2a, the the double-sided double-sided adhesive adhesive tape tape surface surface is is smooth smooth and and cannot cannot supply supply enough roughness. When the L-CNC particles are directly scattered onto the double-sided adhesive enough roughness. When the L-CNC particles are directly scattered onto the double-sided adhesive surface tape coating surface and and rolled rolled with with the the wooden wooden bar, bar,the theL-CNC L-CNCparticles/double-sided particles/double-sided tape coating (marked (marked as as D coating) is formed (as shown in Figure 2b). Under the action of the wooden bar, some of L-CNC D coating) is formed (as shown in Figure 2b). Under the action of the wooden bar, some of L-CNC particles adhesive and and thus thus lose lose the the function function to to build build surface surface roughness. roughness. particles are are entirely entirely buried buried inside inside the the adhesive Other inin thethe adhesive. The restrest do do notnot make contact withwith the Other L-CNC L-CNC particles particlesare areonly onlypartially partiallyburied buried adhesive. The make contact adhesive. As can be seen from Figure 2c, the L-CNC particles are irregular and have a diameter of the adhesive. As can be seen from Figure 2c, the L-CNC particles are irregular and have a diameter approximately 2–20 µm. The high-magnification image shows that the L-CNC particle surface has of approximately 2–20 μm. The high-magnification image shows that the L-CNC particle surface has many protuberances instead Therefore, the L-CNC particle particle itself itself many nanoscale nanoscale gaps gaps and and protuberances instead of of being being smooth. smooth. Therefore, the L-CNC is a micro-nano hierarchical structure, which is one of necessary conditions for a superhydrophobic is a micro-nano hierarchical structure, which is one of necessary conditions for a superhydrophobic coating. Similar to to Figure Figure 2a–c, 2a–c, Figure Figure 2e–g 2e–g show show that the pure pure epoxy coating. Similar that the epoxy surface surface is is also also smooth, smooth, and and the the compounding pattern between the L-CNC particles and epoxy in the L-CNC/epoxy coating (marked compounding pattern between the L-CNC particles and epoxy in the L-CNC/epoxy coating (marked as Figure 2d,h 2d,h show show cross-sections cross-sections of of the the D as E E coating) coating) is is the the same same as as that that of of the the D D coating. coating. Figure D and and E E coatings; different regions distinguished between between the the glass glass slide, slide, adhesives, adhesives, coatings; the the different regions are are able able to to be be clearly clearly distinguished and The thickness thickness of of the the D D and and E E coatings coatings is is estimated estimated to to be be approximately approximately 120 120 μm µm and L-CNC L-CNC particles. particles. The and 30 µm, respectively. and 30 μm, respectively.

Figure 2. (a) Surface morphology of (a) the double-sided tape; (b) the D coating; (c) the L-CNC Figure 2. (a) Surface morphology of (a) the double-sided tape; (b) the D coating; (c) the L-CNC particles, particles, with a high-magnification inset; (d) the cross-sections of the(e)Dthe coating; (e)morphology the surface with a high-magnification image inset;image (d) the cross-sections of the D coating; surface coating; (g) thewith L-CNC particles, with a image highmorphology of the epoxy; quick-setting of the quick-setting (f) the Eepoxy; coating;(f)(g)the theEL-CNC particles, a high-magnification magnification image inset; and (h) the cross-sections of the E coating. inset; and (h) the cross-sections of the E coating.

2.2. Wettability 2.2. Wettability Figure 3a,b show that the untreated glass slide and the adhesive surfaces are hydrophilic and Figure 3a,b show that the untreated glass slide and the adhesive surfaces are hydrophilic and the water droplets (dyed blue with methylene) could spread out, resulting from a large amount of the water droplets (dyed blue with methylene) could spread out, resulting from a large amount of hydroxyl groups in the area. The water droplets on the glass slide treated with modified L-CNC hydroxyl groups in the area. The water droplets on the glass slide treated with modified L-CNC particles could form spheres and rolled off easily with only slight shaking, and the static water contact particles could form spheres and rolled off easily with only slight shaking, and the static water contact angle (WCA) of the D coatings reach 163.5° and that of the E coating being 161.2°, while the slide angle (WCA) of the D coatings reach 163.5◦ and that of the E coating being 161.2◦ , while the slide angle (SA) of the D or E coating is less than 10°. This benefits from large amounts of F originating angle (SA) of the D or E coating is less than 10◦ . This benefits from large amounts of F originating from 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFTS), which is currently known to have the from 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFTS), which is currently known to have the lowest lowest energy. As is known, it is difficult for liquid to infiltrate a material surface that has a lower energy. As is known, it is difficult for liquid to infiltrate a material surface that has a lower surface free surface free energy than the liquid. According to the Cassie wetting state, there is a thin layer of air energy than the liquid. According to the Cassie wetting state, there is a thin layer of air in the interface in the interface between the water and the substrate. Compared with a smooth surface, the actual between the water and the substrate. Compared with a smooth surface, the actual contact area between contact area between water droplets and the solid surface is smaller and there is less adhesion in a superhydrophobic surface, allowing the water droplets to roll off easily.

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water droplets and the solid surface is smaller and there is less adhesion in a superhydrophobic surface, Materials 2017, 10, 1080 4 of 13 allowing the water droplets to roll off easily. Figure 3c 3c shows the FTIR FTIR spectra spectra of of the the L-CNC L-CNC particles particles before before and modification. The The low low Figure shows the and after after modification. surface free energy is mainly attributed to the C–F bonds from PFTS. The stretching vibration of surface free energy is mainly attributed to the C–F bonds from PFTS. The stretching vibration of the the −1 −1 C–F bonds C–F bonds led led to to two two new newpeaks peaksat at1236 1236and and1144 1144cm cm−1.. Three Three new new peaks peaks at at 1014, 1014,899, 899,and and847 847cm cm−1 were formed from thethe dehydration of were caused caused by by the thestretching stretchingvibrations vibrationsofofthe theSi–O Si–Obonds bondsthat thatwere were formed from dehydration the hydroxyls between the PFTS after hydrolysis and the L-CNC particle surfaces. This suggests that of the hydroxyls between the PFTS after hydrolysis and the L-CNC particle surfaces. This suggests PFTSPFTS couldcould be effectively grafted to thetoL-CNC particle surfaces. that be effectively grafted the L-CNC particle surfaces.

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Figure Figure 3. 3. Status Status of of water water droplets droplets on on (a) (a) the the untreated untreated glass glass slide, slide, the the double-sided double-sided tape, tape, and and the the glass glass slide treated with D coating, and (b) the untreated glass slide, the epoxy, and the glass slide treated L-CNC particles particles before before and and after after modification. modification. with E coating; (c) FTIR spectra of the L-CNC

2.3. Oil and and Self-Cleaning Self-Cleaning 2.3. Water Water Repellence Repellence in in Air Air or or Oil As immersed in in water As shown shown in in Figure Figure 4a,b, 4a,b, when when entirely entirely immersed water and and taken taken out, out, the the untreated untreated parts parts of of the the glass glass slides slides and and the the adhesive adhesive surfaces surfaces clearly clearly present present residual residual water, water, but but there there is is no no contamination contamination in the treated treated parts parts with with both both the D and and E E coatings. coatings. This in the the D This proves proves their their outstanding outstanding water-repellence. water-repellence. This finding is not limited in air, but the coating also shows good water-repellence This finding is not limited in air, but the coating also shows good water-repellence in in oil oil (n-dodecane), (n-dodecane), which is of great significance for applications such as bearings and gears [34]. As shown in which is of great significance for applications such as bearings and gears [34]. As shown in Figure Figure 4c,d, 4c,d, on the untreated part of the glass slide, water droplets spread out and become a thin layer. on the untreated part of the glass slide, water droplets spread out and become a thin layer. However, However, on the treated treated part, part, the the water waterdroplets dropletsremain remainspheres sphereswithout withoutspreading spreadingout, out, even when the coating on the even when the coating is is polluted by the oil. polluted by the oil. Self-cleaning very important important application application for for aa superhydrophobic superhydrophobic surface. surface. As in Self-cleaning is is aa very As shown shown in Figure the untreated untreated part part of of the the glass glass slide, slide, water water droplets droplets spread spread down down instead instead of rolling Figure 4e–g, 4e–g, in in the of rolling away, resulting in larger pollution. However, on the treated part, the water droplets form spheres away, resulting in larger pollution. However, on the treated part, the water droplets form spheres and and roll roll away. away. The The hydrophilic hydrophilic dust dust is is attached attached to to the the rolling rolling water water droplets droplets surface surface and andtaken takenaway, away, leaving a clean rolling trace. leaving a clean rolling trace.

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Figure 4. Water repellence properties of (a) the D and (b) the E coatings; status of the water droplets Figure 4. Water repellence properties of (a) the D and (b) the E coatings; status of the water droplets on on the untreated glass slide and the glass slide treated with the D coating from (c) a top-down view the untreated glass slide and the glass slide treated with the D coating from (c) a top-down view and and (d) an eye level view; (e–g) the self-cleaning test of the D coating. (d) an eye level view; (e–g) the self-cleaning test of the D coating.

2.4. Mechanical Strength 2.4. Mechanical Strength 2.4.1. Sandpaper Abrasion Test 2.4.1. Sandpaper Abrasion Test Abrasion resistance is a key determinant of working life for a superhydrophobic coating. The Abrasion resistance is a key determinant of working life for a superhydrophobic coating. sandpaper abrasion test is the most common test method. One abrasion cycle is shown in Figure 5a,b. The sandpaper abrasion test is the most common test method. One abrasion cycle is shown in It is easy to comprehend that if there is no adhesive between the L-CNC particles and the substrates, Figure 5a,b. It is easy to comprehend that if there is no adhesive between the L-CNC particles and the L-CNC particles would not be fixed and easily scraped off. In the coating treated with adhesives, the substrates, the L-CNC particles would not be fixed and easily scraped off. In the coating treated the L-CNC particles without contact with adhesive could also be easily removed. So, adhesives are with adhesives, the L-CNC particles without contact with adhesive could also be easily removed. So, crucial for the stability of the coatings. In addition to adhesives, the thickness of the coatings and the adhesives are crucial for the stability of the coatings. In addition to adhesives, the thickness of the strength of the particles also contribute to the abrasion resistance. In the report by Reference [35], a coatings and the strength of the particles also contribute to the abrasion resistance. In the report by thin superamphiphobic surface could be created on paper by plasma processing and vapor phase Reference [35], a thin superamphiphobic surface could be created on paper by plasma processing and silanization of fluorosilane. In this research, the superhydrophobic coating is thicker, resulting from vapor phase silanization of fluorosilane. In this research, the superhydrophobic coating is thicker, the large size of the L-CNC particles and the high thickness of the double-sided adhesive and quickresulting from the large size of the L-CNC particles and the high thickness of the double-sided adhesive setting epoxy layer. As is known, a surface is defined as superhydrophobic when it exhibits a WCA and quick-setting epoxy layer. As is known, a surface is defined as superhydrophobic when it exhibits larger than 150° and an◦ SA smaller than 10° [36]. As shown in Figure 5c,d, after exposure to a 100-g a WCA larger than 150 and an SA smaller than 10◦ [36]. As shown in Figure 5c,d, after exposure to a weight and a sheet of sandpaper (320 grit), the D coating could not withstand three abrasion cycles 100-g weight and a sheet of sandpaper (320 grit), the D coating could not withstand three abrasion (total 60 cm) and the E coating could not withstand two cycles (total 40 cm) until the SA was larger cycles (total 60 cm) and the E coating could not withstand two cycles (total 40 cm) until the SA was than 10°. With◦ the increase of the abrasion cycles, the WCA decreases and the SA presents an larger than 10 . With the increase of the abrasion cycles, the WCA decreases and the SA presents increasing trend, resulting from the severe damage and wastage of the rough surface structure (as an increasing trend, resulting from the severe damage and wastage of the rough surface structure shown in Figure 5e–h). (as shown in Figure 5e–h). The abrasion resistance of the D coating is a little better than that of the E coating. This is mainly The abrasion resistance of the D coating is a little better than that of the E coating. This is mainly attributed to the different adhesives used. For the E coating, the adhesive is epoxy resin, which is a attributed to the different adhesives used. For the E coating, the adhesive is epoxy resin, which is a type of thermosetting resin with high hardness after fast curing. As shown in Figure 5i, during the type of thermosetting resin with high hardness after fast curing. As shown in Figure 5i, during the sandpaper abrasion, the L-CNC particles in the E coating appear as one of two conditions. One is that sandpaper abrasion, the L-CNC particles in the E coating appear as one of two conditions. One is that the L-CNC particles whose small fraction is buried in the adhesive are easily shaved and uprooted the L-CNC particles whose small fraction is buried in the adhesive are easily shaved and uprooted due due to insufficient adhesion, leaving a pit on the adhesive surface. The other is that the L-CNC to insufficient adhesion, leaving a pit on the adhesive surface. The other is that the L-CNC particles particles whose large fraction is buried in the adhesive are not uprooted. The latter benefits from a whose large fraction is buried in the adhesive are not uprooted. The latter benefits from a higher higher adhesion and could withstand the impact of the sandpaper, but the particle surfaces would also be abraded. The two conditions would damage the surface structure and cause a change in wettability.

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adhesion and could withstand the impact of the sandpaper, but the particle surfaces would also be abraded. conditions would damage the surface structure and cause a change in wettability. MaterialsThe 2017,two 10, 1080 6 of 13

Figure 5. (a,b) Demonstration of one abrasion cycle; changes curves of WCA and SA of (c) the D Figure 5. (a,b) Demonstration of one abrasion cycle; changes curves of WCA and SA of (c) the D coating and (d) the E coating during abrasion; surface status of (e,f) the D coating after 12 abrasion coating and (d) the E coating during abrasion; surface status of (e,f) the D coating after 12 abrasion cycles and (g,h) the E coating after eight abrasion cycles; illustration of the L-CNC particles before cycles and (g,h) the E coating after eight abrasion cycles; illustration of the L-CNC particles before and and after abrasion in the (i) E coating and (j) the D coating. after abrasion in the (i) E coating and (j) the D coating.

For the D coating, the double-sided tape is a pressure-sensitive adhesive which has high viscosity temperature. Duringtape the sandpaper abrasion, the D coating also has appears two For the at D room coating, the double-sided is a pressure-sensitive adhesive which high in viscosity conditions (as shown in Figure 5j). One is that within a certain number of abrasion cycles, the L-CNC at room temperature. During the sandpaper abrasion, the D coating also appears in two conditions (as particles whose is in contact with the adhesive would not the be wiped Instead, shown in Figure 5j).small One fraction is that within a certain number of abrasion cycles, L-CNCaway. particles whose some of them sink deeper and move forward a certain distance undersome the impact the small fraction is inwould contact with the adhesive would not be wiped away. Instead, of themofwould sandpaper due to the high viscosity of the double-sided adhesive. The other is that the L-CNC sink deeper and move forward a certain distance under the impact of the sandpaper due to the high particles large fraction is in contact with the adhesive are notparticles moved awhose long distance due toisa in viscosity of whose the double-sided adhesive. The other is that the L-CNC large fraction stronger adhesion, but the particle surfaces are also abraded. These two conditions also to contact with the adhesive are not moved a long distance due to a stronger adhesion, but thelead particle damage of the surface structure, but the damage is lighter than that of the E coating within the same surfaces are also abraded. These two conditions also lead to damage of the surface structure, but number of abrasion cycles. This is contributed to the high adhesion and excellent flexibility of the the damage is lighter than that of the E coating within the same number of abrasion cycles. This is double-sided adhesive. When the D coating is treated with the sandpaper, the double-sided adhesive contributed to the high adhesion and excellent flexibility of the double-sided adhesive. When the D could transform and reduce some of the sandpaper’s impact. However, for the E coating, the quickcoating is treated with the sandpaper, the double-sided adhesive could transform and reduce some of setting epoxy, a thermosetting resin, cannot transform and buffer the sandpaper’s impact. the sandpaper’s impact. However, for the E coating, the quick-setting epoxy, a thermosetting resin, Thus, under the same abrasion conditions, the D coating shows better resistance to abrasion than the cannot transform and buffer the sandpaper’s impact. Thus, under the same abrasion conditions, the D E coating. coating shows better resistance to abrasion than the E coating. 2.4.2. Finger-Wipe and Knife-Scratch Test 2.4.2. Finger-Wipe and Knife-Scratch Test The D coating was used as the tested sample and scraped three times at different areas with a The Dfinger. coating used as the sample and scraped threehas times at different areaswhich with a gloved Thewas results show thattested the superhydrophobic property no obvious change, gloved finger. The results show that the superhydrophobic property has no obvious change, demonstrates the strong micro structure of the coating surface. Upon continuing to scratchwhich the demonstrates theastrong structure of not the coating surface.orUpon to scratch the coating with knife, micro the coating was been scraped torn,continuing which demonstrates thatcoating the double-sided adhesive is able to supply high adhesion.

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with a knife, the coating was not been scraped or torn, which demonstrates that the double-sided adhesive is able to supply high adhesion. Materials 2017, 10, 1080 7 of 13 2.4.3. Water JetJetTest 2.4.3. Water Test It Itis isvaluable to imitate imitateheavy heavyrain raintotoscour scourthethe and E coatings. valuablefor foroutdoor outdoor applications applications to DD and E coatings. AsAs shown in Figure 6a,b, the WCA shows a decreasing trend and the SA has an increasing trend. shown in Figure 6a,b, the WCA shows a decreasing trend and the SA has an increasing trend. ◦ After five jet jet cycles andand dried, the the D coating exhibits a WCA less than 150 .150°. Figure 6c shows the surface After five cycles dried, D coating exhibits a WCA less than Figure 6c shows the surface morphology the Dafter coating five jet The cycles. The L-CNC particles in aarea unitare area are morphology of the D of coating fiveafter jet cycles. L-CNC particles in a unit reduced reduced and there are even some large areas of double-sided adhesive exposed. This is because the and there are even some large areas of double-sided adhesive exposed. This is because the L-CNC L-CNCwith particles onlyfraction a small fraction in the adhesive are washed bylarge the large impact force particles onlywith a small in the adhesive are washed awayaway by the impact force of the of the jet, distance and the distance the adjacent particles thesebecome areas become wider water jet,water and the betweenbetween the adjacent L-CNC L-CNC particles in theseinareas wider and thus and thus the surface structure is damaged. It is also possible for water droplets to contact to the the surface structure is damaged. It is also possible for water droplets to contact to the hydrophilic hydrophilic adhesive surface, a decrease of However, the WCA. However, after the five jetand adhesive surface, resulting in aresulting decreaseinof the WCA. after the same fivesame jet cycles cycles and drying, the E coating still maintains a WCA larger than 150°. As shown in Figure 6d, there drying, the E coating still maintains a WCA larger than 150◦ . As shown in Figure 6d, there are more are more L-CNC particles than the D coating in the same unit and until eight jet cycles, after which L-CNC particles than the D coating in the same unit and until eight jet cycles, after which the E coating the E coating does not show a WCA lower than 150°, proving that the quick-setting epoxy is more does not show a WCA lower than 150◦ , proving that the quick-setting epoxy is more water-tolerant water-tolerant and could supply stronger adhesion than the double-sided adhesive, which would and could supply stronger adhesion than the double-sided adhesive, which would shrink and make shrink and make poorer contact with the L-CNC particles due to the water jet. poorer contact with the L-CNC particles due to the water jet.

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Figure 6. Change curves of the WCA and SA of (a) the D coating and (b) the E coating in the water jet Figure 6. Change curves of the WCA and SA of (a) the D coating and (b) the E coating in the water jet process; surface morphologies of (c) the D coating and (d) the E coating after the same five jet cycles. process; surface morphologies of (c) the D coating and (d) the E coating after the same five jet cycles.

2.5. Durability 2.5. Durability 2.5.1. UV Radiation Test 2.5.1. UV Radiation Test UV radiation is a very important detection index for durability. The D and E coatings were UV radiation is aradiation very important detection forWCA durability. The D and E coatings exposed to UV lamp for six days (144 h),index and the and SA were measured every 24 were h. exposed to UV lamp radiation for six days (144 h), and the WCA and SA were measured 24 h. As shown in Figure 7a,b, the results do not change significantly, which indicates that theevery L-CNC Asparticles shown have in Figure 7a,b, resultsresistance do not change which indicates that the L-CNC a good UV the radiation and the significantly, chemical components on the particle surfaces particles have a good UV radiation resistance and the chemical components on the particle surfaces show no change. show no change.

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2.5.2.Heat Heat Resistance Resistance Test Test 2.5.2. Considering potential potential applications in some Considering some high-temperature high-temperaturelocations, locations,the thesuperhydrophobic superhydrophobic coatings need need aa heat heat resistance resistance test. As coatings As shown shown in in Figure Figure 7c,d, 7c,d,the theDDand andEEcoatings coatingswere weretested testedatata a ◦ variety of temperatures. The WCAs and SAs showed no obvious change even at 170 °C, proving variety of temperatures. The no obvious change even at 170 C, provingthat that the coatings possess good heat stability, which benefits from thermal stability of the adhesives the coatings possess good heat stability, which benefits from thethe thermal stability of the adhesives and andL-CNC the L-CNC particles retain their morphologies. Atthe 190WCA °C, the WCA of the has D the particles that arethat ableare to able retaintotheir morphologies. At 190 ◦ C, of the D coating coating has an obvious reduction, but the E coating does not. To further test the thermal stability of an obvious reduction, but the E coating does not. To further test the thermal stability of the coatings, thebottom coatings, the bottom sidesslides of thewith glassthe slides withEthe D and were E coatings above an alcohol the sides of the glass D and coatings fired were abovefired an alcohol burner for for results 1 min. The results show that the loses D coating loses its superhydrophobicity, but the Emaintains coating 1burner min. The show that the D coating its superhydrophobicity, but the E coating maintains thisFor property. For theasDshown coating, shown 7e, the double-sided adhesive is this property. the D coating, in as Figure 7e, in theFigure double-sided adhesive is melted, which melted, which causes some of the L-CNC particles to sink, resulting in damage of the surface causes some of the L-CNC particles to sink, resulting in damage of the surface structure. However, for structure. However, forinthe E coating, shownstructure in Figure 7f,no theobvious surfacechange, structure has no obvious the E coating, as shown Figure 7f, theas surface has which is attributed change, which is attributed to the ultrahigh heat resistance of the epoxy. to the ultrahigh heat resistance of the epoxy.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 7. Change curves of the WCA and SA of (a) the D coating and (b) the E coating when exposed Figure 7. Change curves of the WCA and SA of (a) the D coating and (b) the E coating when exposed to to UV radiation; change curves of the WCA and SA of (c) the D coating and (d) the E coating at UV radiation; change curves of the WCA and SA of (c) the D coating and (d) the E coating at different different temperatures; surface morphologies of (e) the D coating and (f) the E coating after being temperatures; surface morphologies of (e) the D coating and (f) the E coating after being fired with an fired with an alcohol burner for 1 min. alcohol burner for 1 min.

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2.5.3. Acid and Alkali Resistance Test 2.5.3. Acid and Alkali Resistance Test In the acid and alkali resistance tests, as shown in Figure 8a, the WCAs only showed a slight In the acid and resistance tests, as shown in Figure 8a, the onlycomponents showed a slight change, proving that alkali the coatings could retain the surface structure andWCAs chemical and change, proving that the coatings could retain the surface structure and chemical components and that that they had good resistance to acidic and alkali corrosion. To further test the coatings, water they hadwith good resistance to 14 acidic alkali corrosion. further test the coatings, water droplets droplets pH = 1, 7, and wereand used to measure theTo WCA of the coatings; a measurement was with pH = 1,37,min and up 14 were measure theare WCA of theincoatings; a measurement wasthe taken every taken every to 12used min.toThe results shown Figure 8b,c; the WCA of coatings 3 min upover to 12time. min.At The resultsdroplet are shown Figure 8b,c; WCA of theonly coatings decrease overfrom time. decrease a water within pH = 1, and thethe WCA shows a reduction of 3.6° ◦ ◦ ◦ At a water droplet pH =At 1, aand the droplet WCA shows only= a14, reduction of 3.6 from 158.6 to 155.0 158.6° to 155.0° afterwith 12 min. water with pH it has only a reduction of 3.2° from ◦ ◦ ◦ after 12 Atafter a water droplet withatpH = 14, itdroplet has only a reduction 3.2 shows from 159.4 to 156.2 after 159.4° tomin. 156.2° 12 min. Lastly, a water with pH = 7, itofalso a similar reduction ◦ from 161.0◦ to 12 min. Lastly, at a water droplet with pH = 7, it also shows a similar reduction of 4.7 of 4.7° from 161.0° to 156.3° after 12 min. The reason for this small reduction of the WCAs is possibly ◦ 156.3 12 min. reason for this smallthemselves reduction of theafter WCAs is possibly that theingravity of the that theafter gravity of theThe water droplets makes sink a long time, resulting an increase water droplets makes themselves sink after a long time, resulting in an increase of the contact area of the contact area between the droplets and the coating surface. This demonstrates good acid and between the droplets and the coating surface. This demonstrates good acid and alkali resistance. alkali resistance.

Figure8.8.(a) (a)WCAs WCAsof ofwater waterdroplets dropletswith withdifferent differentpH pHon onthe theDDcoating; coating;change changein instatus statusof ofwater water Figure dropletswith with(b) (b)pH pH==1;1;(c) (c)pH pH==7;7;and and(d) (d)pH pH==1414over overtime timeon onthe theDDcoating. coating. droplets

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3. Materials and Methods 3.1. Materials BioPlus-L™ Crystals, a kind of lignin-coated cellulose nanocrystals (L-CNC) (brown powder, lignin content ~3–6 wt %, 4.3% moisture) were purchased from American Process Inc., Atlanta, GA, USA, and dried in an oven for two hours under 110 ◦ C before use. The quick-setting epoxy (25 mL, two-component, clear, cures in an hour) was purchased from the J-B Weld company, Sulphur Springs, TX, USA. The double-sided tape (2” × 20 m, long-term hold, Hongxin Lattice) was purchased from Amazon, Seattle, WA, USA. Toluene (99.8%, extra dry, anhydrous) and 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFTS, CF3 (CF2 )5 (CH2 )2 SiCl3 , 97%) as a low energy reagent and anhydrous ethanol (200 proof) were purchased from Fisher Scientific, Waltham, MA USA. Glass slides (7.5 × 2.5 cm) were used as substrates and were washed for 5 min using distilled water in an ultrasonic cleaning machine and dried before use. 3.2. Preparation of L-CNC Surperhydrophobic Coating 3.2.1. Hydrophobic Modification of L-CNC Particles Two grams of L-CNC particles were added to 18 g of toluene and stirred for 10 min, followed by adding 1 g of PFTS and continuing to stir for 5 h. Then, the L-CNC particles were separated by a centrifuge and washed with anhydrous ethanol three times to remove the residual toluene and dissociative low-energy reagent, and finally were dried for 30 min at 60 ◦ C. 3.2.2. Quick-Setting Epoxy Modulation and Double-Sided Tape Handling The two components of quick-setting epoxy were squeezed out with a quality ratio of 1:1 at the same time and then mixed and stirred for about 1 min. The double-sided tape was cut to the size of 2.5 cm × 2.5 cm. 3.2.3. Hydrophobic L-CNC Particles and Adhesive Composite The modulated epoxy was first smeared onto the substrate surface with a gluing surface of 2.5 cm × 2.5 cm, or the double-sided tape of 2.5 cm × 2.5 cm was stuck onto the substrate surface. Then, the adhesive surface was completely covered with the modified L-CNC particles scattered with a sieve (200 meshes), followed by rolling the particles with a smooth wooden bar for more than 1 min. The composite superhydrophobic coatings were successfully prepared. 3.3. Characterization The chemical composition of the composite coating was analyzed using Fourier transform infrared spectroscopy (FTIR, Perkin-Elmer, Waltham, MA, USA, scanning 16 times in the range of 4000~600 cm–1 ). A Zeiss Auriga SEM/FIB crossbeam workstation (Oberkochen, Germany) was chosen to observe the surface morphology of the coating at an accelerating voltage of 3 kV. WCA and SA measurement: The water contact angle (WCA) of the coating was obtained by a Drop Shape Analysis System (EasyDrop, Germany KRUSS, Hamburg, Germany). As reported in our previous paper [28], a self-made slide angle (SA) measuring station was built by simply inserting a “T” iron frame with a smooth panel into the superposed center between a protractor and a strip of hollow aluminum. As shown in Figure 9a, the smooth panel could be rotated by 180◦ and an SA value could be directly read from the protractor. Each WCA or SA was the average value of five measurements at different points using a 4~8 µL water droplet.

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Figure Figure 9. 9. (a) (a) Self-made Self-made slide slide angle angle measuring measuring station; station; (b) (b) knife-scratch knife-scratch and and (c) (c) water water jet jet sketch. sketch.

Self-cleaning: The Thecoating coatingwith withdust dust surface tilted a small angle, droplets Self-cleaning: onon itsits surface waswas tilted at a at small angle, waterwater droplets were were added dropwise, and the situation the coating surface was observed. added dropwise, and the situation of the of coating surface was observed. Water-repellent in in oil: oil: The The coating coating was was immersed immersed into into oil oil (n-dodecane). (n-dodecane). Then, water water droplets droplets were dripped dripped dropwise dropwise onto onto the the surface. surface. were Sandpaper abrasion test: The method was similar to that in a previous previous report report [34]. [34]. The The coating coating engaged a linear movement forfor 10 against sandpaper (320 (320 grit) grit)was waspressed pressedwith withaa100-g 100-gweight weightand and engaged a linear movement ◦ and cmcm under external force. Then, thethe coating was horizontally wiggled at 90° moved another 10 10 under external force. Then, coating was horizontally wiggled at 90and moved another cm. This process was counted as one abrasion cycle. 10 cm. This process was counted as one abrasion cycle. hand, and and was was scraped scraped Finger-wipe test: The sample was kept stationary by pressing with one hand, with aa finger finger from from the the other other hand. hand. This This test test was was conducted conducted three threetimes timesin indifferent differentareas. areas. with Knife-scratch test: test: The Thesample samplewas wasscratched scratchedwith witha aknife knifealong along the intersecting lines shown Knife-scratch the intersecting lines as as shown in in Figure then WCA were measured. Figure 9b,9b, andand then thethe WCA andand SASA were measured. Water jet jet test: test: As shown in Figure 9c, the coating was tested using a water jet with a water flow Water of approximately approximately 55 m/s, m/s, aa water rate of water pressure of 50 kPa, and a standoff distance of 10 cm. The spray approximately 40 40 cm cm22, and each jet last for 5 s. The WCAs and SAs were measured after surface was approximately each water water jet, jet, and and the the coatings coatings were were dried dried again. again. ◦ C,°C, ◦ C,°C, ◦ C, °C, ◦ C, °C, ◦ C, Heat resistance resistancetest: test:The Thecoating coatingwas wasplaced placed oven with Heat inin anan oven with 90 90 110110 130130 150150 170 170 ◦ C for °C, and °C 2for h, and the WCAs measured, respectively. and 190 190 h, 2and thenthen the WCAs and and SAs SAs werewere measured, respectively. Acid and alkali resistance tests: The aqueous solution with pH 1-6 was deployed with 5 wt % HCl solution solution and and the the aqueous aqueous solution solution with with pH pH 8-14 was deployed deployed with with 5 wt % NaOH solution. Then, HCl the water droplets at pH 1 to 14 were were dripped dripped onto onto the the coating coating surface and the WCAs and SAs were were measured, respectively. respectively. measured, UV radiation radiation test: test: The samples samples were were continuously continuously exposed exposedunder underaa Blak-Ray Blak-Ray®® XX-15M UV bench UV lamp with with aa light light source source (UV-C (UV-CLAMP LAMPG15T8/SW, G15T8/SW, power: power: 15 W, W, wavelength: wavelength: 254 254 nm, nm, UV UV micro micro watts: watts: lamp 2 2 uW/cm at 49 uW/cm at11 M, M, Sankyo Sankyo Denki, Denki, Hiratsuka, Hiratsuka, Japan). The The distance between the sample and the light source was was 10 10 cm. cm. source 4. Conclusions L-CNC particles are environmentally friendly and suitable to build rough surface structure for a superhydrophobic superhydrophobic coating due to to their their irregular irregular shape shape and and surface surface morphology. morphology. A robust and and durable superhydrophobic coating can be successfully prepared by a facile method in which which the the hydrophobic L-CNC particles are dispersed with a sieve sieve on on the the adhesive adhesive (double-sided (double-sided tape tape and and quick-setting quick-setting epoxy) epoxy) surface surface and and then then rolled rolled with with a wooden wooden bar. bar. The resulted coatings coatings show excellent superhydrophobic andand water repellence in air or outstandingoutstanding mechanical superhydrophobicproperties properties water repellence in oil, airalso or possessing oil, also possessing strength against sandpaper abrasion, finger-wipe, knife-scratch, and water jet, as well as mechanical strength against sandpaper abrasion, finger-wipe, knife-scratch, and water jet,durability as well as against high temperatures, acid and alkali and UV radiation. The D coatingThe shows better durability against high temperatures, acidsolutions, and alkali solutions, and UV radiation. D coating sandpaper abrasion resistance and can be used some places whereplaces abrasion resistance the main shows better sandpaper abrasion resistance andin can be used in some where abrasionisresistance is the main consideration. Alternatively, the E coating possesses better water jet and heat resistance

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consideration. Alternatively, the E coating possesses better water jet and heat resistance and is suitable for use in areas with high temperatures and heavy rain. In view of the environmental performance of the L-CNC particles and the simple preparation, the coatings are of great potential application value. Acknowledgments: This work was financially supported by the Special Fund for Forest Scientific Research in the Public Welfare (No. 201504603) and Tennessee Experimental Station Project #TEN00510. We thank the Department of Food Science & Technology, the University of Tennessee for providing Drop Shape Analysis System. Author Contributions: S.W., S.L. and J.H. conceived and designed the experiments; S.W. and J.H. performed the experiments and analyzed the data; S.W. and S.L. contributed reagents/materials/analysis tools; J.H. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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