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Superhydrophobic Coating for Anti-fouling of Chinese Paintings Xiangwei Kong, Jing Zhang, Qi Xuan, Junjie Lu, and Jie Feng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01423 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018
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Superhydrophobic Coating for Anti-fouling of Chinese Paintings Xiangwei Kong, Jing Zhang, Qi Xuan, Junjie Lu, Jie Feng* College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, 310014, China *Corresponding authors. Prof. Feng Jie, E-mail:
[email protected] ABSTRACT： ：In past 10 years, many studies on superhydrophobic (SH) papers have been reported. However, these works rarely carried out on Chinese Paintings. Compared with pure paper, the complexity of Chinese black ink and pigment caused by including animal glue, an emulsion agents and fixatives allowing for the nanoparticles to disperse in water, negatively affects the formation of SH coatings. In this work, we report the fabrication of SH coatings using fluorine silicon sol containing epoxy group on Chinese Paintings. Briefly, SiO2 nanoparticles were mixed with KH560 and FAS-17 and formed fluorine silicon sol. Then the sol was coated on the Chinese Paintings. After drying, the SH coating was created. It can protect Chinese Paintings and prevent them being fouled by various liquids. Six types of beverages and ink can be repelled by the as-prepared SH coatings. Moreover, Chinese black ink can also be washed away from the SH coatings using water, without leaving any trace of ink behind. The SH coatings did not affect the appearance and internal structure of the Chinese Paintings. Furthermore, due to covalent link, the SH coatings have good mechanical properties with regards to bending tests (120 times), finger pressing tests and friction tests. The SH coatings also show excellent high temperature resistance. This study has important applications for anti-fouling of Chinese Paintings or oil paintings from dirty water. KEYWORDS: Superhydrophobic; Coating; Chinese Painting; Anti-fouling
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INTRODUCTION Over the past 20 years, superhydrophobic (SH) surfaces with a contact angle (CA) greater than 150° and a sliding angle (SA) less than 10°, have attracted extensive attention due to their potential applications in self-cleaning, anti-icing, anti-fouling, oil-water separation, and anti-infrared irradiation.1-5 Paper possesses a wide range of applications and is used extensively in our daily lives; however, being composed of plant cellulose fibers, paper can be easily contaminated by liquid or other pollutants. In past 10 years, much research has focused on preparing SH paper using different methods to improve its water/moisture-proofing, self-cleaning, anti-fouling, etc.6-10 Many SH studies have focused on the use of printing or filter paper including printed or colorful ones but with few reports concerning Chinese Painting, oil painting, or painting on clothes. SH treatment can greatly enhance the anti-fouling and water-resistant properties of Chinese Paintings. Chinese Paintings have a long historical standing, originating during the Tang Dynasty in China (618-907 AD).11 Chinese Paintings are easily polluted by various liquids and how to maintain their original state has been the focus of our research. Compared with blank Xuan paper, which possesses many hydroxyl groups, the composition of the surface of colored Chinese Paintings is more complex. Chinese Paintings are painted with Chinese black ink and pigments. The former consists mostly of carbon black, animal glue and water.12 The later mostly consist of organic or inorganic coloring substances and animal glue. Additionally, preservatives and perfumes are often added to them. Animal glue is rough-wrought gelatin, acting as an emulsion agents and fixatives allowing for the nanoparticles to disperse in the water.13 However, the animal glue remaining in Chinese black ink and pigments after drying increases the difficulty of SH treatment for Chinese Paintings. In this paper, we focus on the SH treatment of Chinese Paintings, mainly using fluorine silicon sol following spraying, heating and drying treatment to achieve SH property. When water, ink and beverages are dropped onto the surface of the treated 2
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samples, they are all repelled by the SH surfaces leaving no trace of their presence. When the surface of the treated samples is contaminated with Chinese black ink, the ink can be quickly washed away with water without leaving any residual traces. The appearance of the Chinese Paintings is not affected by SH treatment. Therefore, our research allows for the practical application of SH coatings to Chinese Paintings for improving anti-fouling and water-proofing performance.
EXPERIMENTAL SECTION Materials 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (FAS-17) was purchased from Sicong New Material Co., Ltd (Quanzhou, China). 3-(2, 3-Epoxypropoxy) propyltrimethoxysilane (KH560) and Methyltriethoxysilane (MTES) were obtained from Aladdin Reagent Co., Ltd (Shanghai, China). SiO2 nanoparticles (SP-30) were purchased from Wanjing New Material Co., Ltd (Hangzhou, China). The pigments with brand Marie were purchased from Shanghai Siic Marie Painting Materials Co., Ltd (Shanghai, China). Chinese black ink was purchased from Shanghai Zhouhuchen and Caosugong Brush and Ink Co., Ltd (Shanghai, China). Xuan paper, ink, brush pen, mineral water, red tea, orange juice, coca cola, milk and coffee were purchased commercially. The Chinese Paintings used in this paper were drawn by Mrs. Zhao Hongyan on Xuan paper with Chinese black ink and pigments. All the water was deionized, and water was dyed with methylene blue to optimize vision in some experiments. All the other reagents were used as received from general companies. Preparation of SH coating The SH coatings were prepared according to the following general procedures: Firstly, 2 g of SiO2 nanoparticles, 75 mL ethanol, 75 mL water were added to a beaker, then stirred for 10 min with a magnetic stirrer; secondly, 2 mL FAS-17, 2 mL KH560 (or MTES) were added to the beaker, and then the mixtures were further stirred for 20 min; thirdly, all of them were milled for 5 hours at a speed of 375 rpm in a ball-milling machine (QM-SP2, Nanda Instrument Plant, China). Subsequently, 3
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fluorine silicon sol was created. Then it was sprayed on the Chinese Paintings three times. Finally, the Paintings were placed horizontally and dried at 120 oC for 40 min. The preparation process for the SH coatings is shown in Scheme 1. The reaction principle for the preparation of the fluorine silicon sol on the Chinese Paintings is shown in Scheme 2a-c. Briefly, KH560 and FAS-17 were first hydrolyzed and formed silanol, then the silanols condensed and formed fluorine silicon sol. After drying, the fluorine silicon sol formed fluorine silicon gel on the Chinese Paintings. Scheme 2d shows the reaction mechanism between the epoxy groups of the fluorine silicon sol and the imino groups of animal glue on the Chinese Paintings.
Scheme 1. Preparation process for SH coatings on the Chinese Paintings
Characterization The water CAs and SAs of the as-prepared coatings were measured by contact angle measurement system (Dataphysics OCA35, Germany) with 4 µL water droplets. The CAs were the averaged value of five different measuring points on each coating. The surface morphology and elements contents of the coatings were observed and measured by field emission scanning electron microscope (FE-SEM, FEI, Nova Nano 450) and energy dispersive spectrometer (EDS, Oxford X-MaxN80), respectively. For measuring the anti-fouling performance of the coatings, six different beverages including mineral water, red tea, Coca Cola, orange juice, milk and coffee were slowly poured on to the treated samples, respectively. Moreover, the general ink and Chinese black ink were also dropped on the slightly tilted samples. 4
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Mechanical durability tests contain three parts. (1) Bending test: the treated sample was bent upwards and downwards (from -90o to 90o), which was defined one bend. (2) Finger touch test: the treated sample was touched by bare finger. (3) Friction test: finger rubbed on the treated sample. In the high temperature resistance test, the treated samples were put in the oven at different temperatures for 1 min. Optical photos and Movies were recorded during above experimental process. OCH 3
O
Si OCH 3
O
+
O
OCH 3 F OC 2H 5 Si OC 2H 5
C 2H 5O
FF
F F F
O
Si OH + OH
OH
FF F F F
+
3C2H 5OH
(b)
F F
FF F F F F F
F
F
HO Si F
OH
FF F F F
F
F
R=
O
OR
O
(c)
F
F R R R R R O Si O Si O Si O Si O Si O O O O O O Si O Si O Si O Si O Si R R R R R
F F F F F F
F
OH
F F
FF
F
OH HO Si
F + 3H 2O
OH
O
(a)
OH
FF F F F F F F
OH Si OH
O
3H2 O
F F F F F F F F
F F F F F F F N HO O
R R R R R O Si O Si O Si O Si O Si O O O O O O Si O Si O Si O Si O Si R R R R R
H
+ N
R R R R O Si O Si O Si O Si O Si O O O O O O Si O Si O Si O Si O Si R R R R R
(d)
Scheme 2. The reaction principle of fluorine silicon sol on the Chinese Paintings
RESULTS AND DISCUSSION Surface wettability and morphology We measured water CAs and SAs of the coated samples on colored and colorless regions. Figure 1 shows that the coated sample had excellent superhydrophobicity: the water droplets retained a spherical shape (Figure 1a and Figure 1d) with a CA of 154.2° and a SA of 8.5° on the colored region of the sample, and a CA of 154.0° and a SA of 8.8° on the colorless region. When the water droplets were dropped on the 5
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uncoated sample, the water droplets spread and diffused quickly (Figure 1b and (Figure 1c). When the coated samples with Chinese black ink and colored pigments were immersed in water with methylene blue and then withdrawn from the water (Figure S1 of Supporting Information and Movie S1), no traces of methylene blue can be observed on the sample surfaces. When many water droplets are placed on the coated samples, they roll off without leaving any trace (Figure S2 and Movie S1). Ejected water bounced off the surface of the coated sample (Movie S1). These results show that the treated samples consisting of Chinese black ink and pigments had excellent superhydrophobicity. The presence of Chinese black ink and pigments on the Xuan paper had no effect on the SH coatings.
a
b
c
d
Figure 1. (a) Water drops on the coated sample; (b) water drops spread and diffused on the uncoated sample; (c) magnified (b); (d) magnified (a).
Figure 2 shows the SEM micrographs of the coated and uncoated samples on colorless regions at different magnification. As shown in Figure 2a, the structure and distribution of the coated sample are clearly visible, which is like that of the uncoated sample shown in Figure 2c. Thus, the SH coatings have little influence on the structure of the samples. Figure 2b shows the rough structure of the micron and nanometer particles more clearly, and the coat consists of silica particles with a diameter of approximately 30 nm (inset in Figure 2b). Figure 2d shows that the 6
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surface of the uncoated sample was relatively smooth and contained no silica nanoparticles
a
b 100nm
c
d 100nm
Figure 2. SEM micrographs of (a-b) coated sample and (c-d) uncoated sample on colorless
regions with different magnification. To achieve an SH effect on the surface of the samples, we designed three sets of comparative experiments with different reagent formulations. The first group is the original formulation shown in the experimental section 2.2. The second group involved replacement of KH560 with MTES, and the third group underwent removal of KH560. Colorless and colored regions were tested respectively in each group. Table 1 shows the CAs and SAs of the different tests. Changing the formulation had no significant effect on the CAs, irrespective of the samples being colored or colorless, but it had a significant influence on the SAs. When the KH560 is replaced with MTES or in the absence of KH560, the SAs of the colored samples all exceed 70°, but change only slightly for the colorless samples. This is because on the colorless region, there are many hydroxy groups (-OH), which can react with silanol groups. The fluorine silicon sols either from single FAS-17 or from mixture of FAS-17 and KH560, they can both form firm coatings on colorless region. So KH560 did not remarkably impact the sliding angle of the coatings on the colorless region. 7
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However, on the colored region of Chinese Paintings, a thin layer of carbon black (or other pigment particles) and animal glue exists, which makes the hydroxy groups (-OH) disappear or decrease in number. Animal glue consists of proteins formed through hydrolysis of the collagen from skins, bones, tendons, and other tissues. Like gelatin, animal glue proteins consist of large amount of imino groups (-NH).14 As shown in Scheme 2d, the imino groups can react with the epoxy groups of KH560 because of the active hydrogen in the imino groups. As shown in Table 1, except for the epoxy group, it is difficult for other reagents to react with the imino groups on the colored region. Although the animal glue is relatively hydrophilic, the fluorine silicon sol containing epoxy groups can react with the imino groups and form a film covering the surface of the samples, changing the surface from being hydrophilic to SH. Table 1. CAs and SAs of regions treated with SiO2 nanoparticles and different reagents formulation
region
contact angle /°
sliding angle /°
FAS-17+KH560
colorless
154.0±1.5
8.8±0.6
FAS-17+KH560
colored
154.2±1.6
8.5±0.5
FAS-17+MTES
colorless
151.2±1.2
12.6±1.1
FAS-17+MTES
colored
150.8±1.7
﹥70(a)
FAS-17
colorless
152.4±1.0
9.6±0.8
FAS-17
colored
151.8±1.2
﹥70(a)
(a): the SAs exceeds the detection limit of the instrument.
For further demonstrating above speculation, the contents of elements on colored and colorless regions both treated with different fluorine silicon sol were measured by EDS (Table 2). It can be found that N element from animal glue do appear on the colored regions. Moreover, the deeper black brought the higher N content. Fluorine silicon sols containing epoxy groups anchored much more F element. This is the main reason why KH560 brings small sliding angle on the colored region and is therefore important for the realization of superhydrophobicity on Chinese Paintings. 8
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Table 2. Contents of elements on regions treated with silica and different silicon sol formulation
region
C /wt%
O /wt%
N /wt%
F /wt%
Si /wt%
sum /%
\
colorless
54.34
45.55
0
0
0.11
100
FAS+KH560
colorless
33.31
45.60
0
7.32
13.77
100
FAS
colorless
38.46
44.62
0
9.26
7.66
100
\
black
63.25
34.25
2.19
0
0.31
100
75.36
19.78
4.74
0
0.12
100
\
(a)
black
\
red pigment
43.64
43.04
4.24
0
9.08
100
FAS+KH560
black
49.03
35.69
2.15
9.27
3.86
100
FAS
black
65.41
27.28
2.51
2.64
2.16
100
(a) This black region was drawn deeper than other black regions. \ regions without treatment.
In the work performed by Xiong, D. S. et al.,10 a silica sol, which was prepared using tetraethylorthosilicate (TEOS) and trimethylethoxysilane (TMES), was poured on the colorful printing paper to form an SH surface with superior stability against deformations and humidity. Maybe the colorful paper they used has no animal glue on surface thus silica sol formed from TMES would be enough. Fang, Z. Q. et al.9 prepared durable SH paper by first coating a thin layer of starch on pristine paper and immediately spraying hydrophobic silica nanoparticles. Compared with these two work, it is obvious that KH560 is necessary in forming SH coating on Chinese Paintings colored region due to existing of animal glue, which decreases amount of hydroxyl groups and amount of FAS-17 or FASA-17/MTES fluorine silicon sol. Optical transparence The transparency of SH coatings is very important and directly affect the appearance of the substrate, especially when the SH coatings are applied to glass.15-17 The high transparency of SH coatings applied to Chinese Paintings allows the appearance of the Painting to remain unchanged. In our test, we separated a Chinese Painting into two parts. As shown in Figure 1, one part was coated with the fluorine silicon sol (Figure 1a) and the other part was left uncoated (Figure 1b). Compared with Figure 1b, the color of the sample in Figure 1a has not changed significantly; the 9
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coherence between Figure 1a and Figure 1b is very good. The two separated parts retain the appearance of the original, complete Painting. The SH coatings for Chinese Paintings therefore exhibit excellent transparency. According to the Wenzel and Cassie models,18, 19 surface roughness is beneficial to superhydrophobicity.20 However, the rough surface will cause light diffuse reflection, which generally reduce the transparency of the coating.21 Therefore, the roughness of the coating surface should be maintained within a certain range, to ensure transparency requirements for practical applications. Figure 3 shows the effect of amounts of silica on the samples. Following an increase in the silica content, there is a gradual increase in the CAs and a gradual decrease in the SAs. When the content of silica is increased to 0.30 mg/cm2, the CA is 152.4° and the SA is 9.3°. However, when the content of silica is greater than 0.46 mg/cm2, white spots appear on the surface of the samples. The surface becomes increasingly white as the content of silica increases (inset in Figure 3).22 Silica can improve superhydrophobic properties, but excessive silica will alter the appearance of the samples. Therefore, we sprayed the samples three times to give them a coating with silica content between 0.30 mg/cm2 and 0.46 mg/cm2 to achieve superhydrophobicity and excellent appearance. 160 18 155
Contact angle Sliding angle
145
16 o
14
140
12
135 10 130 8
125 120 0.0
Sliding angle ( )
150
o
Contact angle ( )
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6 0.5
1.0
1.5
2.0 2
Content of silica (mg/cm ) Figure 3. The CAs and SAs of the coatings as a function of the contents of silica.
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Anti-fouling and self-cleaning performance The surface of Chinese Paintings is hydrophilic. When they contact with water, drinks or other liquids, they soon become soaked. Their strength decreases rapidly thus they are destroyed easily. The surface of Chinese Paintings will absorb the colored matter of the beverages, which cannot be easily removed. Anti-fouling and water-proofing performances are therefore very important for the protection of Chinese Paintings. Different beverages have been used to test the repellency of the coatings in our test. As shown in Figure 4 and Movie S2, six different beverages (mineral water, red tea, Coca Cola, orange juice, milk, coffee) were slowly poured on to the treated samples，respectively. They all slide away from the surface quickly, leaving the surface of the treated samples without any trace of beverages.
a
b
c
d
e
f
Figure 4. Liquid-proof test. Six different beverages were poured on to the treated samples, including (a) mineral water, (b) red tea, (c) Coca Cola, (d) orange juice, (e) milk, (f) coffee.
Due to the fast-drying and adhesive performance of ink and Chinese black ink, once they are spilt on Chinese Paintings, the appearance of Chinese Paintings will be seriously fouled. Ink-proofing performance is therefore important for Chinese Paintings. We tested the effect of ink drops on the surface of the coated sample. As 11
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shown in Figure 5 and Movie S3, when the sample is slightly tilted, the ink drops readily roll off the surface without leaving any trace of the ink.
b
a
c
Figure 5. Ink-proof test. (a) The treated sample leaned on the plastic container; (b) ink dropped on sample; (c) ink leaving the sample rapidly.
Compared with ink, Chinese black ink and pigments are more likely to adhere to the surface of Chinese Paintings because they possess better diffusion behavior and absorbability. The SH coatings also showed excellent resistance to Chinese black ink. Figure 6a shows that the uncoated sample leaves an obvious trace of Chinese black ink after cleaning for 40 s (Movie S4). The uncoated sample can be easily broken because the sample is soaked and its strength has decreased. Figure 6b shows that the Chinese black ink on the coated sample can be washed away with water without wetting and polluting the surface (Movie S4).
0s
5s
20 s
40 s
0s
1s
4s
6s
(a)
(b)
Figure 6. Test for resistance to Chinese black ink. Washing process for the samples with (a) and without (b) SH coatings.
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Chinese Paintings generally absorb lots of dust when they are placed indoors or outdoors for a long time. The self-cleaning performance allows the Chinese Paintings to be washed with water without wetting them. The rough structures from the fluorine silicon sol reduce the contact area between the coating and the dust so that the dust can be removed by the water drops.23 As shown in Figure S3, cement powder can be washed away completely without leaving behind any trace of contamination, thus showing excellent self-cleaning performance. Mechanical durability analysis Poor mechanical stability limits the application of SH coatings. Chinese Paintings are often rolled-up and unfolded for storage and use, thus the mechanical stability of the SH coatings is very important.9, 10 Figure 7 shows the change in the CAs and SAs of the coated Chinese Painting in the bending test that is performed 120 times; bending upwards and then bending in the opposite direction constitutes one bend (inset in Figure 7). The surfaces of the samples maintain SH property (CA >150°, SA
