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Facile method for the preparation of superhydrophobic cellulosic paper
Applied Surface Science 496 (2019) 143648
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Full length article
Facile method for the preparation of superhydrophobic cellulosic paper ⁎
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Brian Musikavanhu, Zhijun Hu , Ratidzo Lisah Dzapata, Yinchao Xu, Peter Christie, Daliang Guo, Jing Li Key Laboratory of Recycling and Eco-treatment of Waste Biomass of Zhejiang Province, Zhejiang University of Science and Technology, Hangzhou 310023, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Crystalline structure Nanoparticles Superhydrophobic surface Cellulosic paper
We presented a three-step method for the preparation of superhydrophobic cellulose filter paper. It involved ZnCl2 pre-treatment to activate the cellulose fibers, a SiO2 sol-gel process to increase surface roughness and grafting of hexadecyl trimethoxysilane groups to chemically enhance the water repellency. The water contact angle of the modified paper reached 154.8°. XRD analysis confirmed silica as the product from the sol-gel process. EDS and SEM analysis showed that pre-treatment results in greater silica adhesion and retention on the paper surface. The modified paper proved to be self-cleaning and resistant to staining. Further tests under various conditions showed that ZnCl2 pre-treatment improved the durability of the superhydrophobic paper. It was concluded that pre-treatment of the filter paper prior to superhydrophobic coating results in a disruption in the cellulose crystalline structure that favours greater silica adhesion and retention.
1. Introduction Cellulose is the most abundant biopolymer available and it can be modified using various techniques to produce different functional materials with outstanding chemical, thermal and mechanical properties compared to their synthetic counterparts [1,2]. Paper is derived from cellulose and is used in a variety of applications. It is recyclable and is prepared from renewable sources [3]. Paper as a substrate has attracted considerable interest because of its properties such as lower relative costs, good surface features, relative abundance and numerous applications such as packaging and medical devices [4,5]. Numerous other applications are possible if cellulosic paper is modified to superhydrophobic form [6,7]. Superhydrophobicity is a phenomenon where a surface has a water contact angle (WCA) > 150°, whereas a sliding angle (SA) < 10° is a measure of the self-cleaning properties [8]. Applications of superhydrophobic surfaces include water repellency [9], separation and self-cleaning surfaces [10]. Lithography, template methods, plasma treatment, layer-by-layer (LBL) deposition, hydrogen bonding, colloidal assembly and chemical deposition are some of the techniques used when preparing superhydrophobic materials [11]. For cellulosic paper, superhydrophobicity is generally achieved by manipulating the physical and chemical nature of the surface. Chemical manipulation involves lowering the surface energy with chemicals such as fluorocarbons, silicones, and organic and inorganic compounds. Physical manipulation involves inducing roughness on the paper
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surface, and one way of doing this is by coating it with a material with a micro or nano structure such as silica [8]. Methods of fabrication include dipping, spin-coating or spraying a surface with a solution, suspension or an emulsion of hydrophobic nanoparticles and pigments [1,12,13]. Various nano-structures have been studied but silica is relatively effective. The Stober process is the most common method for the synthesis of silica, where tetraethyl orthosilicate (TEOS) is the silicon source, ammonia is used as the catalyst and alcohol acts as the solvent [14,15]. Silica nanoparticles themselves, however, are hydrophilic because of hydroxyl groups on their surface. Therefore, hydrophobic materials such as fluorosilanes [16–18] and alkane silanes [19–21] are normally used to complete the modification process. Most hydrophobic/oleophilic materials used in industry are based on fluorinated compounds and non-renewable synthetic materials. However, fluorinated hydrophobic materials that contain long perfluoroalkyl chains are environmentally unfriendly because they resist degradation [22,23]. New environmental regulations promote the use of environmentally friendly hydrophobic materials based on renewable substances. Li et al. [6] adopted a simple spray-coating process using octadecyl trichlorosilane (OTS)-modified SiO2 nanoparticles (~50 nm in diameter) in an ethanol suspension to create a superhydrophobic and superoleophilic coating on paper. The process, however, involved manually grinding the silica into fine powder using a mortar. Zhang et al. [7] used rather large OTS-modified silica particles (~243 nm) in a
Corresponding author. E-mail addresses: [email protected] (Z. Hu), [email protected] (Y. Xu).
https://doi.org/10.1016/j.apsusc.2019.143648 Received 26 April 2019; Received in revised form 25 June 2019; Accepted 10 August 2019 Available online 12 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 496 (2019) 143648
B. Musikavanhu, et al.
The silica obtained was applied onto the ZnCl2 pre-treated filter paper surface using a brush and the paper was allowed to dry at room temperature for 12 h. After drying the filter paper was soaked in a 0.6% (w/ v) HDTMS and ethanol solution for 2 h. The paper was then dried in a hot air oven at 120 °C for 1 h and was labelled WPT. Filter paper without ZnCl2 pre-treatment also underwent the same modification process and was labelled NP.
tetrahydrofuran solution containing polystyrene to prepare a superhydrophobic coating on filter paper. The process was rather long as it involved 12 h of ageing, 12 h of magnetic stirring after addition of octadecyl trichlorosilane and another 12 h of vacuum drying. MedinaSandoval et al. [24] used a procedure similar to the one proposed in this paper. They used a sol-gel process for the synthesis of silica before soaking in a 3.5% (w/v) HDTMS-ethanol solution for 24 h. The product was hydrophobic paper with a WCA of 120°. Shortcomings of current methods include poor adhesion and retention of silica on the fiber surface and poor durability when subjected to different conditions. There is a strong commercial interest in improving the quality of superhydrophobic cellulose paper for various applications. A possible solution for this might be a modification of the Stober method to functionalize the silica with specific groups and activating the cellulose fiber to expose more hydroxyl groups for the formation of stronger bonds between silica and fibers. According to some literature, zinc chloride (ZnCl2) has been used for cellulose hydrolysis and has been found to be a good swelling agent. Zn2+ ions penetrate and disrupt the crystalline regions of cellulose into amorphous form, resulting in swelling. This swelling causes the cellulose fibers to become softer and tougher [25]. If cellulosic paper is pretreated with zinc chloride before any surface modification, the resulting shift in the orientation of the fibers may result in improved reactivity during further processing. The utilization of ZnCl2 pre-treatment to enhance the modification treatment efficiency and improve the quality of superhydrophobic cellulosic paper has not been reported. This study aimed to use ZnCl2 to enhance the modification treatment efficiency for the purpose of fabricating durable superhydrophobic paper with improved silica adhesion and retention. The superhydrophobicity was evaluated by the WCA method. The morphological and chemical changes were characterized by scanning electron microscope (SEM), x-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and infra-red spectroscopy. In addition, the stability of cellulosic paper was analysed in different solutions.
2.4. Chemical characterization Water contact angle (WCA) measurements were made using a DSA100 drop shape analyser (Krüss GmbH, Hamburg, Germany). A 5-μl drop of water was dropped on each sample to measure the WCA. The WCA of each sample was determined five times and the average was calculated. Surface morphology was analysed using a Phenom ProX desktop scanning electron microscope (Thermo Fisher Scientific, Eindhoven, Netherlands). The elemental composition of the paper was determined using an energy-dispersive spectrometer (EDS, JED-2300, JEOL, Tokyo, Japan) on SEM apparatus. An Ultima IV diffractometer (Rigaku, Tokyo, Japan) was used for x-ray diffraction (XRD) on Cu Kα radiation basis (λ = 1.54 Å, scanning rate of 2° min−1 at 40 kV and 20 mA). A V70 (Bruker, Billerica, MA) IR spectrometer was used to analyze the chemical composition of the paper. Thermogravimetric analysis (TGA) was conducted with a TGA Q5000 (TA Instruments, New Castle, DE) using nitrogen at 50 ml min−1 and a heating rate of 10 °C min−1, starting at room temperature and rising to 500 °C. 3. Results and discussion 3.1. Proposed concept for ZnCl2-pretreatment assisted superhydrophobic modification of cellulosic paper
Filter paper was cut into 4 × 2 cm pieces. The pieces, each 0.885 g, were placed in 50 ml of 66 wt% ZnCl2 solution at 85 °C for different times ranging from 1 to 10 min. The paper was removed and washed with ethanol and distilled water to remove residual zinc. The paper was then dried on a steam cylinder dryer.
ZnCl2 was used as a swelling agent to disrupt the crystalline regions of the cellulose, partially converting them into amorphous form [25,26]. Crystalline cellulose is comprised of long chains that are ordered and packed adjacent to one another via strong intermolecular and intramolecular hydrogen bonds [27]. In contrast, amorphous cellulose has shorter chains of different lengths, linked to crystalline structures by weaker hydrogen bonds [28]. Therefore, the large surface area of amorphous cellulose makes it significantly more reactive than the crystalline region [29]. The amide on the silica has greater bond strength with the –OH groups on the fibers than the O-H… O hydrogen bond that would otherwise exist if there was no functionalization [30,31]. The hypothesis was that ZnCl2 pre-treatment would result in an increase in the adhesion and retention of silica on the paper surface. At the same time, APTES was applied as a modification to the Stober method to functionalize the silica with –NH2 groups. The procedure used for the preparation of superhydrophobic paper is shown in Fig. 1. As shown in Fig. 1, we conferred superhydrophobic properties on the cellulose substrate using a ZnCl2-pre-treatment initial step to activate the cellulose fiber. This was followed by a sol-gel process aimed at synthesizing the silica for application on the fiber surface. A HDTMSethanolic solution was then used to attach alkyl chains (C16) on the SiO2 film to improve the bonding between hydroxyl groups and silica and chemically enhance the water repellence of the surface. A control experiment consisting of the same modification process but without ZnCl2-pre-treatment was also conducted for comparison purposes.
2.3. Superhydrophobic modification
3.2. Water contact angle and sliding angle measurements
Nanosilica was prepared using a modified Stober method [14,15]. In brief, 9 ml of TEOS was added to 9 ml of ammonia and 90 ml of ethanol in a round-bottomed flask. The flask was placed in a water bath at 50 °C and the mixture was stirred at 200 rpm. After 1 h, 1 ml of APTES was added and the reaction was allowed to continue for a further 30 min.
WCA and SA measurements were made to ensure that the paper qualified as superhydrophobic. Fig. 2a shows the WCA measurement for the WPT sample after 7 mins of pre-treatment, and Fig. 2b shows the interaction of the water drop with the modified (WPT) paper surface. The WPT sample had a WCA of 154.8° after 7 min of pre-treatment. Raw
2. Methods 2.1. Materials Filter paper was purchased from Hangzhou Special Paper Co., Ltd (Hangzhou, China), with pore size ranging from 15 to 20 μm. Zinc chloride (ZnCl2) was purchased from Xilong Scientific Co., Ltd (Shantou, Guangdong, China). Ammonia (NH3) was purchased from Pingyaoheshun Chemicals (Hangzhou, China). Hexane (C6H14) was purchased from Kelong Chemical Co., Ltd (Chengdu, China). Hexadecyl trimethoxysilane (HDTMS, C19H42O3S) was purchased from Quanxi Chemical Co., Ltd (Nanjing, China). 3-aminopropyltriethoxysilane (APTES, C9H23NO3Si) and tetraethyl orthosilicate (TEOS, Si(OC2H5)4) were purchased from Huakai Resin Co., Ltd (Jining, China) and ethanol was procured from Huipu Chemical Co., Ltd (Hangzhou, China). 2.2. Zinc chloride pre-treatment
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Fig. 1. Schematic diagram of the procedure for the preparation of superhydrophobic paper.
Fig. 2. Images of a) WCA measurement of the WPT sample and b) interaction of a water droplet with the modified superhydrophobic paper. SA (º)
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Fig. 3. Variation of WCA and SA with pre-treatment time.
filter paper is so hydrophilic that the measured contact angle was virtually 0°. The NP sample had an average WCA of 148.8°, which lower than the 150° required to qualify as superhydrophobic. Fig. 3 shows that there was a steady increase in the WCA as pre-treatment time increased until it peaked at 7 min, after which a slow decline was observed. The reason for this might be that ZnCl2 penetrates the inner structure of the paper fibers, inducing swelling and disrupting the intramolecular and intermolecular hydrogen bonds, resulting in inconsistencies within and outside the paper fibers. The crystalline regions were disrupted and partially converted into amorphous form. This may have resulted in the exposure of more hydroxyl groups for hydrogen
Fig. 4. FT-IR spectra of paper samples at different stages of the modification process.
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bonding with silica, resulting in greater silica adhesion and retention on the paper surface. After 7 min there was a gradual decline in WCA and the explanation may be that the paper started to degrade and suffer damage to its core structure. This would reduce its ability to retain silica. Sliding angle (SA) measurements were also taken and the secondary axis on Fig. 2 shows how SA varied with the pre-treatment time. The NP sample, along with those that received pre-treatment for one and two min, recorded sliding angles of 10°. ZnCl2 pre-treatment from six to nine minutes produced the lowest SA (6°). This may also be attributed to the differences in the surface roughness of the samples. 3.3. FTIR analysis Fig. 4 shows the FT-IR spectra of the paper at different stages of the modification process. The different chemical changes were monitored by following the formation of specific bonds on the samples. The raw, silica-coated and WPT paper show absorption bands at 3300–3600 cm−1, which may be attributed to the stretching of hydroxyl groups [32,33]. After addition of ZnCl2, silica and HDTMS alkyl units, further changes in the surface chemical composition were observed. For the WPT and silica-coated samples, 2922 cm−1 and 2849 cm−1 are two low intensity bands that are characteristic of symmetrical and asymmetrical stretching vibrations of the -CH2 units of alkyl hydrocarbon chains of HDTMS and APTES [24,34]. The band at 1345 cm−1 indicates CeC and CeO skeletal vibrations on the cellulose core structure. In the case of the paper that underwent ZnCl2 treatment, the 449 cm−1 band represents the stretching vibrations of the ZneO bond. On the WPT and silica-coated samples, there is spectral overlap at 3300–3400 cm−1 as the NeH stretch is weaker than the OeH stretch in the same region. The band at 1610–1650 cm−1 is assigned to bending vibrations of aliphatic amine (NeH) groups [34]. The bands at 797 cm−1 and 1101 cm−1 indicate Si-O-Si symmetric and asymmetric stretching vibrations, respectively, indicating the presence of silica [33,35,36]. The intensity of this band signifies the extent of the modification of the surface by HDTMS and silica.
Fig. 5. TGA analysis of the paper samples at different stages of the modification process.
3.4. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) The thermal degradation properties of raw and modified filter paper were analysed. Fig. 5 shows the thermogram for the samples. For all the samples, there was an initial weight loss of about 6% at temperatures between 50 and 110 °C. This weight loss is attributed to the loss of water. The raw filter paper showed another weight loss starting at about 320 °C and this is attributed to the degradation of the cellulose. This weight loss continued up to the final temperature of 500 °C. In this range the overall weight loss was 81.9%, leaving a residual weight of 12.1% for raw cellulose. ZnCl2 pre-treated paper without any further modification had degradation starting at 318 °C. The residual weight after this was 19.7% and the overall weight loss was 74.3%. Degradation of the NP sample started at 284 °C and continued up to 500 °C. The residual mass after this was 32.4% and the overall weight loss during this was 61.6%. Superhydrophobic paper (WPT) had degradation starting at 315 °C and continued up to 500 °C. The residual mass after this was 33.4% and the overall weight loss during this was 60.6%. The weight loss for raw paper came about as a result of the loss of water and gases, primarily CO and CO2. The 12.1% residual mass for raw paper at 500 °C is attributed to carbon remaining in the form of char. The excellent thermal stability of cellulose is primarily a result of the orientation of repeat D-glucose units [24]. The samples, WPT, NP, ZnCl2 only and raw filter paper had complete degradation at 379, 368, 398 and 402 °C respectively. The continuing weight loss after that was due to degradation of char. The WPT paper had the highest residual weight because of the added weight of silica. The NP sample had a lower residual weight because it retained less silica than the WPT sample. These results are slightly different from those obtained by Medina-Sandoval
Fig. 6. DSC analysis at different stages of the modification process.
Fig. 7. XRD diagram for raw, ZnCl2 pre-treated and superhydrophobic (WPT) paper.
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Fig. 8. SEM images of a) raw filter paper b) ZnCl2-treated paper c) silica-coated paper without ZnCl2 pre-treatment at 1000× d) ZnCl2 pre-treated, silica-coated paper (WPT) at 1000× e-f) WPT sample g) nanosilica particle size distribution.
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Fig. 9. EDS-SEM spectra of a) raw filter paper b) WPT paper and c) NP paper.
the cellulose fibers to such an extent that this outweighs the insulating effect of silica. DSC analysis was conducted and Fig. 6 illustrates the thermal
et al. [24]. The expectation was that the silica would act as a thermal insulator and prolong the onset of degradation by at least 5 °C but instead the reverse occurred. The explanation may be that ZnCl2 weakens 6
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WPT
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at 11.2° for the superhydrophobic (WPT) paper was assigned to (0−12) which signalled HDTMS. Several other small peaks at 20.8°, 36.5°, 38.2°, 40.8°, 42.1° and 44.4° were observed, corresponding to (200), (110), (012), (111), (200) and (201), indicating that the material is SiO2 (quartz) nanoparticles [39]. The two secondary peaks start to merge whilst the principal peak becomes smaller and this is evidence of the crystalline regions being changed into amorphous form [40].
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SEM images of untreated filter paper are shown in Fig. 8a. The fibers were slightly ordered with varying degrees of entanglement with each other. Fig. 8b shows the fiber after pre-treatment with ZnCl2. There was clear loosening of the fiber entanglements, and hence more of the surface was open for interaction with silica. There also appeared to be some minor abrasions on the fiber surface, resulting in inconsistencies and roughness on the filter paper surface. The resulting orientation, together with the hydrogen bond interactions between the hydroxyl groups of the cellulose and the NH2-functionalized silica increased the silica adhesion and retention on the paper surface. Fig. 8c and d show the difference in the amount of silica on the paper with pre-treatment and that without pre-treatment. Fig. 8e and f shows the WPT surface after modification. Fig. 8g shows the particle size distribution of the nanosilica particles ranging between 118 nm and 143 nm. This size distribution ensures that the surface is rough.
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3.7. EDS analysis 125.0 Isopropyl alcohol
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Energy dispersive spectroscopy (EDS) was carried out for elemental identification and quantitative analysis of the different samples. Fig. 9a shows that the main constituents of raw filter paper were carbon (kα = 0.3) and oxygen (kα = 0.5). The WPT sample in Fig. 9b shows an additional peak at kα = 1.8, characteristic of silicon. The silicon atoms were introduced when the nanosilica was applied onto the paper and also during HDTMS grafting. The intensity of the carbon signal shows a significant decline whilst that of oxygen has a sharp increase and this is because of the additional oxygen atom in the silica. Fig. 9c shows the NP sample having silica content of 43.55 wt%, compared to 49.06 wt% for the WPT sample. This difference in the silica content is a result of the different surface properties of the two samples which bring about different retention capabilities.
Fig. 11. WCA vs. solvents for the WPT and NP samples.
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3.8. Stability of the modified paper
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4 The stability of the WPT and NP samples in different conditions was investigated. The samples were soaked in different NaOH- and HClbased pH solutions at room temperature for 40 h. The samples were then rinsed in ethanol and dried at 100 °C for 40 min. Fig. 10 shows the WCA results. The WCA of the WPT sample remained above 150° when the paper was subjected to moderate pH solutions (pH 3–11). However, when exposed to extreme pH conditions (pH < 3 and pH > 11), the WCA decreased to below 150°. At pH 2 the WCA declined to 145.2° and the paper turned to a dark greyish colour. At pH values of 12 and 14 the WCA declined to 149.6° and 138.2°, respectively. Overally, the NP sample showed sharper drops in WCA compared to the WPT sample. Superhydrophobicity was lost at extreme pH conditions for all the samples. The other test involved soaking the samples in different organic solvents at room temperature for 40 h and, again, rinsing in ethanol and drying at 100 °C for 40 min. Fig. 11 shows the changes in WCA after soaking in these solvents. The superhydrophobicity of the WPT sample was maintained although there were small drops in WCA. The NP sample again showed larger drops in WCA. The variation in the contact angles of the two samples can be attributed to their different silica retention capabilities. For all the samples the sharpest drop was observed when they were immersed in acetic acid.
Fig. 12. WCA vs washing time for the WPT and NP samples.
behaviour of the various samples. Raw filter paper showed an endothermic peak in the temperature range 330–400 °C, peaking at 361 °C. This can be attributed to cross-linking reactions being activated at such temperatures leading to the release of volatile compounds [37,38]. An endothermic peak occurring just after 400 °C may be attributed to degradation of char. The ZnCl2 only, WPT and NP samples had exothermic peaks at 324, 340 and 360 °C, all because of crosslinkage reactions as reported by Langley et al. [38]. 3.5. X-ray diffraction (XRD) analysis XRD analysis was conducted to observe changes in crystallinity of the cellulose and also to confirm the presence of key molecules, particularly silica. XRD patterns of raw and modified filter paper are shown in Fig. 7 and were analysed with the aid of Jade version 6. The peaks at diffraction angles 14.9°, 16.8° and 22.7° were assigned to (110), (110) and (200) crystal planes of cellulose respectively. The very low peak at 16.6° corresponds to (002) and is attributed to residual ZnCl2. The peak 7
Applied Surface Science 496 (2019) 143648
B. Musikavanhu, et al.
Fig. 13. Images of (a and b) raw filter paper before and after immersion in muddy rain water (c and d) WPT paper during and after immersion in muddy rain water, e) Interaction of oil and water droplets with the WPT paper, f) raw paper with dirt g) wetted raw paper, h) WPT paper after self-cleaning.
4. Conclusions
Repeated and continuous washing in a solvent can result in physical damage to a surface. The samples were tested for physical durability by continuously washing in ethanol for up to 4 h. In the process, a conical flask containing the samples and ethanol was mounted onto a Zhichu ZHTY-70 table-type constant temperature shaker running at a speed of 175 rpm at 40 °C. WCA results are shown in Fig. 12. The WPT sample lost its superhydrophobicity after 4 h of continuous washing. NP samples showed sharper drops in WCA compared to the WPT samples. The large decline in WCA of the NP samples was largely because of poor adhesion and retention of the silica on the paper. This was also confirmed by EDS analysis of the samples before and after washing, where the WPT was shown to have lost less silica after washing compared to the NP sample (see Fig. A.1-A.6 of the supplementary information).
Superhydrophobic paper was successfully prepared with a maximum WCA of 154.8 °C. Pre-treatment with ZnCl2 prior to modification resulted in greater silica adhesion and retention. Less HDTMS (0.6% (w/v)) was used and the grafting time was also reduced compared to previous studies. Pre-treatment with ZnCl2 also increased the durability of the paper as evidenced by the paper maintaining its superhydrophobicity when exposed to various organic solvents. The paper also retained its superhydrophobicity when exposed to a wide range of pH conditions (pH 3–11). The paper also showed resistance to staining and demonstrated self-cleaning properties. Acknowledgements
3.9. Real-life applications
This work was funded by Zhejiang Provincial Key research and development project (Grant No. 2019C01156), Zhejiang Provincial 26 County Green Special project (Grant No. 2019C02042), Zhejiang Provincial Public welfare technology research project (Grant No. LGG19C160001), National Natural Science Foundation of China (Grant No. 21808209).
Resistance to staining by water-based fluids is an important feature of superhydrophobic materials. Fig. 13a and c show raw and modified (WPT) filter paper being immersed in dirty rainwater. Fig. 13b shows raw filter paper wetted and stained by the muddy rain water. Fig. 13d shows the WPT paper after immersion in the rainwater showing no wetting or staining. Another application of superhydrophobic surfaces is in the separation of oil and water mixtures. The modified paper was superhydrophobic and oleophilic and because of these properties it is suitable for the separation of oil and water mixtures. Fig. 13e below shows how oil and water behave on the WPT surface. Oil passes through but water is repelled. The self-cleaning properties of the modified paper were also studied. Soil particles were applied to raw filter paper (Fig. 13f) and WPT paper. Water was then poured on the surface in a slow steady flow. Fig. 13g shows the water and soil particles wetting and attaching onto the surface of the raw paper. In contrast, Fig. 13h shows that as the water flowed on the surface of the modified paper, it carried away the soil particles leaving a clean surface. This shows that the superhydrophobic paper also possesses self-cleaning properties.
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[24] C.F. Medina-Sandoval, J.A. Valencia-Dávila, M.Y. Combariza, C. Blanco-Tirado, Separation of asphaltene-stabilized water in oil emulsions and immiscible oil/water mixtures using a hydrophobic cellulosic membrane, Fuel 231 (2018) 297–306. [25] L. Yang, G. Li, F. Yang, S.M. Zhang, H.X. Fan, X.N. Lv, Direct conversion of cellulose to 1-(furan-2-yl)-2-hydroxyethanone in zinc chloride solution under microwave irradiation, Carbohydr. Res. 346 (2011) 2304–2307. [26] A.S. Amarasekara, C.C. Ebede, Zinc chloride mediated degradation of cellulose at 200°C and identification of the products, Bioresour. Technol. 100 (2009) 5301–5304. [27] N. Yoshiharu, L. Paul, H. Chanzy, Crystal structure and hydrogen bonding system in cellulose I α from synchrotron X-ray and neutron fiber diffraction, [J], J. Am. Chem. Soc. 125 (2003) 14300–14306. [28] R.H. Newman, J.A. Hemmingson, Carbon-13 NMR distinction between categories of molecular order and disorder in cellulose, Cellulose 2 (1995) 95–110. [29] Y. Yu, H. Wu, Significant differences in the hydrolysis behavior of amorphous and crystalline portions within microcrystalline cellulose in hot-compressed water, 2010, , [J], Ind. Eng. Chem. Res. 49 (2010) 3902–3909. [30] F. Biedermann, H.J. Schneider, Experimental binding energies in supramolecular complexes, Chem. Rev. 116 (2016) 5216–5300. [31] J. Emsley, Very strong hydrogen bonding, Chem. Soc. Rev. 9 (1980) 91. [32] A.M. Youssef, S. Kamel, M. Elsakhawy, M.A. El Samahy, Structural and electrical properties of paper-polyaniline composite, Carbohydr. Polym. 90 (2012) 1003–1007. [33] N.M. Mahmoodi, S. Khorramfar, F. Najafi, Amine-functionalized silica nanoparticle: preparation, characterization and anionic dye removal ability, Desalination 279 (2011) 61–68. [34] H.-T. Lu, Synthesis and characterization of amino-functionalized silica nanoparticles, Colloid Journal 75 (2013) 311–318. [35] C.-T. Hsieh, W.-Y. Chen, F.-L. Wu, W.-M. Hung, Superhydrophobicity of a three-tier roughened texture of microscale carbon fabrics decorated with silica spheres and carbon nanotubes, Diam. Relat. Mater. 19 (2010) 26–30. [36] E. Vinogradova, M. Estrada, A. Moreno, Colloidal aggregation phenomena: spatial structuring of TEOS-derived silica aerogels, J. Colloid Interface Sci. 298 (2006) 209–212. [37] G. Brancatelli, C. Colleoni, M.R. Massafra, G. Rosace, Effect of hybrid phosphorusdoped silica thin films produced by sol-gel method on the thermal behavior of cotton fabrics, Polym. Degrad. Stab. 96 (2011) 483–490. [38] J. Langley, M. Drews, R. Barker, Pyrolysis and combustion of cellulose. VII. Thermal analysis of the phosphorylation of cellulose and model carbohydrates during pyrolysis in the presence of aromatic phosphates and phosphoramides, J. Appl. Polym. Sci. 25 (1980) 243–262. [39] R. Nandanwar, P. Singh, F.Z. Haque, Synthesis and characterization of SiO2 nanoparticles by sol-gel process and its degradation of methylene blue, American chemical science 5 (2014) 1–10. [40] H.A. Foner, N. Adan, The characterization of papers by X-ray diffraction (XRD): measurement of cellulose crystallinity and determination of mineral composition, Journal of the Forensic Science Society 23 (1983) 313–321.
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Full length article
Facile method for the preparation of superhydrophobic cellulosic paper ⁎
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Brian Musikavanhu, Zhijun Hu , Ratidzo Lisah Dzapata, Yinchao Xu, Peter Christie, Daliang Guo, Jing Li Key Laboratory of Recycling and Eco-treatment of Waste Biomass of Zhejiang Province, Zhejiang University of Science and Technology, Hangzhou 310023, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Crystalline structure Nanoparticles Superhydrophobic surface Cellulosic paper
We presented a three-step method for the preparation of superhydrophobic cellulose filter paper. It involved ZnCl2 pre-treatment to activate the cellulose fibers, a SiO2 sol-gel process to increase surface roughness and grafting of hexadecyl trimethoxysilane groups to chemically enhance the water repellency. The water contact angle of the modified paper reached 154.8°. XRD analysis confirmed silica as the product from the sol-gel process. EDS and SEM analysis showed that pre-treatment results in greater silica adhesion and retention on the paper surface. The modified paper proved to be self-cleaning and resistant to staining. Further tests under various conditions showed that ZnCl2 pre-treatment improved the durability of the superhydrophobic paper. It was concluded that pre-treatment of the filter paper prior to superhydrophobic coating results in a disruption in the cellulose crystalline structure that favours greater silica adhesion and retention.
1. Introduction Cellulose is the most abundant biopolymer available and it can be modified using various techniques to produce different functional materials with outstanding chemical, thermal and mechanical properties compared to their synthetic counterparts [1,2]. Paper is derived from cellulose and is used in a variety of applications. It is recyclable and is prepared from renewable sources [3]. Paper as a substrate has attracted considerable interest because of its properties such as lower relative costs, good surface features, relative abundance and numerous applications such as packaging and medical devices [4,5]. Numerous other applications are possible if cellulosic paper is modified to superhydrophobic form [6,7]. Superhydrophobicity is a phenomenon where a surface has a water contact angle (WCA) > 150°, whereas a sliding angle (SA) < 10° is a measure of the self-cleaning properties [8]. Applications of superhydrophobic surfaces include water repellency [9], separation and self-cleaning surfaces [10]. Lithography, template methods, plasma treatment, layer-by-layer (LBL) deposition, hydrogen bonding, colloidal assembly and chemical deposition are some of the techniques used when preparing superhydrophobic materials [11]. For cellulosic paper, superhydrophobicity is generally achieved by manipulating the physical and chemical nature of the surface. Chemical manipulation involves lowering the surface energy with chemicals such as fluorocarbons, silicones, and organic and inorganic compounds. Physical manipulation involves inducing roughness on the paper
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surface, and one way of doing this is by coating it with a material with a micro or nano structure such as silica [8]. Methods of fabrication include dipping, spin-coating or spraying a surface with a solution, suspension or an emulsion of hydrophobic nanoparticles and pigments [1,12,13]. Various nano-structures have been studied but silica is relatively effective. The Stober process is the most common method for the synthesis of silica, where tetraethyl orthosilicate (TEOS) is the silicon source, ammonia is used as the catalyst and alcohol acts as the solvent [14,15]. Silica nanoparticles themselves, however, are hydrophilic because of hydroxyl groups on their surface. Therefore, hydrophobic materials such as fluorosilanes [16–18] and alkane silanes [19–21] are normally used to complete the modification process. Most hydrophobic/oleophilic materials used in industry are based on fluorinated compounds and non-renewable synthetic materials. However, fluorinated hydrophobic materials that contain long perfluoroalkyl chains are environmentally unfriendly because they resist degradation [22,23]. New environmental regulations promote the use of environmentally friendly hydrophobic materials based on renewable substances. Li et al. [6] adopted a simple spray-coating process using octadecyl trichlorosilane (OTS)-modified SiO2 nanoparticles (~50 nm in diameter) in an ethanol suspension to create a superhydrophobic and superoleophilic coating on paper. The process, however, involved manually grinding the silica into fine powder using a mortar. Zhang et al. [7] used rather large OTS-modified silica particles (~243 nm) in a
Corresponding author. E-mail addresses: [email protected] (Z. Hu), [email protected] (Y. Xu).
https://doi.org/10.1016/j.apsusc.2019.143648 Received 26 April 2019; Received in revised form 25 June 2019; Accepted 10 August 2019 Available online 12 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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The silica obtained was applied onto the ZnCl2 pre-treated filter paper surface using a brush and the paper was allowed to dry at room temperature for 12 h. After drying the filter paper was soaked in a 0.6% (w/ v) HDTMS and ethanol solution for 2 h. The paper was then dried in a hot air oven at 120 °C for 1 h and was labelled WPT. Filter paper without ZnCl2 pre-treatment also underwent the same modification process and was labelled NP.
tetrahydrofuran solution containing polystyrene to prepare a superhydrophobic coating on filter paper. The process was rather long as it involved 12 h of ageing, 12 h of magnetic stirring after addition of octadecyl trichlorosilane and another 12 h of vacuum drying. MedinaSandoval et al. [24] used a procedure similar to the one proposed in this paper. They used a sol-gel process for the synthesis of silica before soaking in a 3.5% (w/v) HDTMS-ethanol solution for 24 h. The product was hydrophobic paper with a WCA of 120°. Shortcomings of current methods include poor adhesion and retention of silica on the fiber surface and poor durability when subjected to different conditions. There is a strong commercial interest in improving the quality of superhydrophobic cellulose paper for various applications. A possible solution for this might be a modification of the Stober method to functionalize the silica with specific groups and activating the cellulose fiber to expose more hydroxyl groups for the formation of stronger bonds between silica and fibers. According to some literature, zinc chloride (ZnCl2) has been used for cellulose hydrolysis and has been found to be a good swelling agent. Zn2+ ions penetrate and disrupt the crystalline regions of cellulose into amorphous form, resulting in swelling. This swelling causes the cellulose fibers to become softer and tougher [25]. If cellulosic paper is pretreated with zinc chloride before any surface modification, the resulting shift in the orientation of the fibers may result in improved reactivity during further processing. The utilization of ZnCl2 pre-treatment to enhance the modification treatment efficiency and improve the quality of superhydrophobic cellulosic paper has not been reported. This study aimed to use ZnCl2 to enhance the modification treatment efficiency for the purpose of fabricating durable superhydrophobic paper with improved silica adhesion and retention. The superhydrophobicity was evaluated by the WCA method. The morphological and chemical changes were characterized by scanning electron microscope (SEM), x-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and infra-red spectroscopy. In addition, the stability of cellulosic paper was analysed in different solutions.
2.4. Chemical characterization Water contact angle (WCA) measurements were made using a DSA100 drop shape analyser (Krüss GmbH, Hamburg, Germany). A 5-μl drop of water was dropped on each sample to measure the WCA. The WCA of each sample was determined five times and the average was calculated. Surface morphology was analysed using a Phenom ProX desktop scanning electron microscope (Thermo Fisher Scientific, Eindhoven, Netherlands). The elemental composition of the paper was determined using an energy-dispersive spectrometer (EDS, JED-2300, JEOL, Tokyo, Japan) on SEM apparatus. An Ultima IV diffractometer (Rigaku, Tokyo, Japan) was used for x-ray diffraction (XRD) on Cu Kα radiation basis (λ = 1.54 Å, scanning rate of 2° min−1 at 40 kV and 20 mA). A V70 (Bruker, Billerica, MA) IR spectrometer was used to analyze the chemical composition of the paper. Thermogravimetric analysis (TGA) was conducted with a TGA Q5000 (TA Instruments, New Castle, DE) using nitrogen at 50 ml min−1 and a heating rate of 10 °C min−1, starting at room temperature and rising to 500 °C. 3. Results and discussion 3.1. Proposed concept for ZnCl2-pretreatment assisted superhydrophobic modification of cellulosic paper
Filter paper was cut into 4 × 2 cm pieces. The pieces, each 0.885 g, were placed in 50 ml of 66 wt% ZnCl2 solution at 85 °C for different times ranging from 1 to 10 min. The paper was removed and washed with ethanol and distilled water to remove residual zinc. The paper was then dried on a steam cylinder dryer.
ZnCl2 was used as a swelling agent to disrupt the crystalline regions of the cellulose, partially converting them into amorphous form [25,26]. Crystalline cellulose is comprised of long chains that are ordered and packed adjacent to one another via strong intermolecular and intramolecular hydrogen bonds [27]. In contrast, amorphous cellulose has shorter chains of different lengths, linked to crystalline structures by weaker hydrogen bonds [28]. Therefore, the large surface area of amorphous cellulose makes it significantly more reactive than the crystalline region [29]. The amide on the silica has greater bond strength with the –OH groups on the fibers than the O-H… O hydrogen bond that would otherwise exist if there was no functionalization [30,31]. The hypothesis was that ZnCl2 pre-treatment would result in an increase in the adhesion and retention of silica on the paper surface. At the same time, APTES was applied as a modification to the Stober method to functionalize the silica with –NH2 groups. The procedure used for the preparation of superhydrophobic paper is shown in Fig. 1. As shown in Fig. 1, we conferred superhydrophobic properties on the cellulose substrate using a ZnCl2-pre-treatment initial step to activate the cellulose fiber. This was followed by a sol-gel process aimed at synthesizing the silica for application on the fiber surface. A HDTMSethanolic solution was then used to attach alkyl chains (C16) on the SiO2 film to improve the bonding between hydroxyl groups and silica and chemically enhance the water repellence of the surface. A control experiment consisting of the same modification process but without ZnCl2-pre-treatment was also conducted for comparison purposes.
2.3. Superhydrophobic modification
3.2. Water contact angle and sliding angle measurements
Nanosilica was prepared using a modified Stober method [14,15]. In brief, 9 ml of TEOS was added to 9 ml of ammonia and 90 ml of ethanol in a round-bottomed flask. The flask was placed in a water bath at 50 °C and the mixture was stirred at 200 rpm. After 1 h, 1 ml of APTES was added and the reaction was allowed to continue for a further 30 min.
WCA and SA measurements were made to ensure that the paper qualified as superhydrophobic. Fig. 2a shows the WCA measurement for the WPT sample after 7 mins of pre-treatment, and Fig. 2b shows the interaction of the water drop with the modified (WPT) paper surface. The WPT sample had a WCA of 154.8° after 7 min of pre-treatment. Raw
2. Methods 2.1. Materials Filter paper was purchased from Hangzhou Special Paper Co., Ltd (Hangzhou, China), with pore size ranging from 15 to 20 μm. Zinc chloride (ZnCl2) was purchased from Xilong Scientific Co., Ltd (Shantou, Guangdong, China). Ammonia (NH3) was purchased from Pingyaoheshun Chemicals (Hangzhou, China). Hexane (C6H14) was purchased from Kelong Chemical Co., Ltd (Chengdu, China). Hexadecyl trimethoxysilane (HDTMS, C19H42O3S) was purchased from Quanxi Chemical Co., Ltd (Nanjing, China). 3-aminopropyltriethoxysilane (APTES, C9H23NO3Si) and tetraethyl orthosilicate (TEOS, Si(OC2H5)4) were purchased from Huakai Resin Co., Ltd (Jining, China) and ethanol was procured from Huipu Chemical Co., Ltd (Hangzhou, China). 2.2. Zinc chloride pre-treatment
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Fig. 1. Schematic diagram of the procedure for the preparation of superhydrophobic paper.
Fig. 2. Images of a) WCA measurement of the WPT sample and b) interaction of a water droplet with the modified superhydrophobic paper. SA (º)
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filter paper is so hydrophilic that the measured contact angle was virtually 0°. The NP sample had an average WCA of 148.8°, which lower than the 150° required to qualify as superhydrophobic. Fig. 3 shows that there was a steady increase in the WCA as pre-treatment time increased until it peaked at 7 min, after which a slow decline was observed. The reason for this might be that ZnCl2 penetrates the inner structure of the paper fibers, inducing swelling and disrupting the intramolecular and intermolecular hydrogen bonds, resulting in inconsistencies within and outside the paper fibers. The crystalline regions were disrupted and partially converted into amorphous form. This may have resulted in the exposure of more hydroxyl groups for hydrogen
Fig. 4. FT-IR spectra of paper samples at different stages of the modification process.
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bonding with silica, resulting in greater silica adhesion and retention on the paper surface. After 7 min there was a gradual decline in WCA and the explanation may be that the paper started to degrade and suffer damage to its core structure. This would reduce its ability to retain silica. Sliding angle (SA) measurements were also taken and the secondary axis on Fig. 2 shows how SA varied with the pre-treatment time. The NP sample, along with those that received pre-treatment for one and two min, recorded sliding angles of 10°. ZnCl2 pre-treatment from six to nine minutes produced the lowest SA (6°). This may also be attributed to the differences in the surface roughness of the samples. 3.3. FTIR analysis Fig. 4 shows the FT-IR spectra of the paper at different stages of the modification process. The different chemical changes were monitored by following the formation of specific bonds on the samples. The raw, silica-coated and WPT paper show absorption bands at 3300–3600 cm−1, which may be attributed to the stretching of hydroxyl groups [32,33]. After addition of ZnCl2, silica and HDTMS alkyl units, further changes in the surface chemical composition were observed. For the WPT and silica-coated samples, 2922 cm−1 and 2849 cm−1 are two low intensity bands that are characteristic of symmetrical and asymmetrical stretching vibrations of the -CH2 units of alkyl hydrocarbon chains of HDTMS and APTES [24,34]. The band at 1345 cm−1 indicates CeC and CeO skeletal vibrations on the cellulose core structure. In the case of the paper that underwent ZnCl2 treatment, the 449 cm−1 band represents the stretching vibrations of the ZneO bond. On the WPT and silica-coated samples, there is spectral overlap at 3300–3400 cm−1 as the NeH stretch is weaker than the OeH stretch in the same region. The band at 1610–1650 cm−1 is assigned to bending vibrations of aliphatic amine (NeH) groups [34]. The bands at 797 cm−1 and 1101 cm−1 indicate Si-O-Si symmetric and asymmetric stretching vibrations, respectively, indicating the presence of silica [33,35,36]. The intensity of this band signifies the extent of the modification of the surface by HDTMS and silica.
Fig. 5. TGA analysis of the paper samples at different stages of the modification process.
3.4. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) The thermal degradation properties of raw and modified filter paper were analysed. Fig. 5 shows the thermogram for the samples. For all the samples, there was an initial weight loss of about 6% at temperatures between 50 and 110 °C. This weight loss is attributed to the loss of water. The raw filter paper showed another weight loss starting at about 320 °C and this is attributed to the degradation of the cellulose. This weight loss continued up to the final temperature of 500 °C. In this range the overall weight loss was 81.9%, leaving a residual weight of 12.1% for raw cellulose. ZnCl2 pre-treated paper without any further modification had degradation starting at 318 °C. The residual weight after this was 19.7% and the overall weight loss was 74.3%. Degradation of the NP sample started at 284 °C and continued up to 500 °C. The residual mass after this was 32.4% and the overall weight loss during this was 61.6%. Superhydrophobic paper (WPT) had degradation starting at 315 °C and continued up to 500 °C. The residual mass after this was 33.4% and the overall weight loss during this was 60.6%. The weight loss for raw paper came about as a result of the loss of water and gases, primarily CO and CO2. The 12.1% residual mass for raw paper at 500 °C is attributed to carbon remaining in the form of char. The excellent thermal stability of cellulose is primarily a result of the orientation of repeat D-glucose units [24]. The samples, WPT, NP, ZnCl2 only and raw filter paper had complete degradation at 379, 368, 398 and 402 °C respectively. The continuing weight loss after that was due to degradation of char. The WPT paper had the highest residual weight because of the added weight of silica. The NP sample had a lower residual weight because it retained less silica than the WPT sample. These results are slightly different from those obtained by Medina-Sandoval
Fig. 6. DSC analysis at different stages of the modification process.
Fig. 7. XRD diagram for raw, ZnCl2 pre-treated and superhydrophobic (WPT) paper.
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Fig. 8. SEM images of a) raw filter paper b) ZnCl2-treated paper c) silica-coated paper without ZnCl2 pre-treatment at 1000× d) ZnCl2 pre-treated, silica-coated paper (WPT) at 1000× e-f) WPT sample g) nanosilica particle size distribution.
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Fig. 9. EDS-SEM spectra of a) raw filter paper b) WPT paper and c) NP paper.
the cellulose fibers to such an extent that this outweighs the insulating effect of silica. DSC analysis was conducted and Fig. 6 illustrates the thermal
et al. [24]. The expectation was that the silica would act as a thermal insulator and prolong the onset of degradation by at least 5 °C but instead the reverse occurred. The explanation may be that ZnCl2 weakens 6
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WPT
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at 11.2° for the superhydrophobic (WPT) paper was assigned to (0−12) which signalled HDTMS. Several other small peaks at 20.8°, 36.5°, 38.2°, 40.8°, 42.1° and 44.4° were observed, corresponding to (200), (110), (012), (111), (200) and (201), indicating that the material is SiO2 (quartz) nanoparticles [39]. The two secondary peaks start to merge whilst the principal peak becomes smaller and this is evidence of the crystalline regions being changed into amorphous form [40].
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SEM images of untreated filter paper are shown in Fig. 8a. The fibers were slightly ordered with varying degrees of entanglement with each other. Fig. 8b shows the fiber after pre-treatment with ZnCl2. There was clear loosening of the fiber entanglements, and hence more of the surface was open for interaction with silica. There also appeared to be some minor abrasions on the fiber surface, resulting in inconsistencies and roughness on the filter paper surface. The resulting orientation, together with the hydrogen bond interactions between the hydroxyl groups of the cellulose and the NH2-functionalized silica increased the silica adhesion and retention on the paper surface. Fig. 8c and d show the difference in the amount of silica on the paper with pre-treatment and that without pre-treatment. Fig. 8e and f shows the WPT surface after modification. Fig. 8g shows the particle size distribution of the nanosilica particles ranging between 118 nm and 143 nm. This size distribution ensures that the surface is rough.
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3.7. EDS analysis 125.0 Isopropyl alcohol
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Energy dispersive spectroscopy (EDS) was carried out for elemental identification and quantitative analysis of the different samples. Fig. 9a shows that the main constituents of raw filter paper were carbon (kα = 0.3) and oxygen (kα = 0.5). The WPT sample in Fig. 9b shows an additional peak at kα = 1.8, characteristic of silicon. The silicon atoms were introduced when the nanosilica was applied onto the paper and also during HDTMS grafting. The intensity of the carbon signal shows a significant decline whilst that of oxygen has a sharp increase and this is because of the additional oxygen atom in the silica. Fig. 9c shows the NP sample having silica content of 43.55 wt%, compared to 49.06 wt% for the WPT sample. This difference in the silica content is a result of the different surface properties of the two samples which bring about different retention capabilities.
Fig. 11. WCA vs. solvents for the WPT and NP samples.
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4 The stability of the WPT and NP samples in different conditions was investigated. The samples were soaked in different NaOH- and HClbased pH solutions at room temperature for 40 h. The samples were then rinsed in ethanol and dried at 100 °C for 40 min. Fig. 10 shows the WCA results. The WCA of the WPT sample remained above 150° when the paper was subjected to moderate pH solutions (pH 3–11). However, when exposed to extreme pH conditions (pH < 3 and pH > 11), the WCA decreased to below 150°. At pH 2 the WCA declined to 145.2° and the paper turned to a dark greyish colour. At pH values of 12 and 14 the WCA declined to 149.6° and 138.2°, respectively. Overally, the NP sample showed sharper drops in WCA compared to the WPT sample. Superhydrophobicity was lost at extreme pH conditions for all the samples. The other test involved soaking the samples in different organic solvents at room temperature for 40 h and, again, rinsing in ethanol and drying at 100 °C for 40 min. Fig. 11 shows the changes in WCA after soaking in these solvents. The superhydrophobicity of the WPT sample was maintained although there were small drops in WCA. The NP sample again showed larger drops in WCA. The variation in the contact angles of the two samples can be attributed to their different silica retention capabilities. For all the samples the sharpest drop was observed when they were immersed in acetic acid.
Fig. 12. WCA vs washing time for the WPT and NP samples.
behaviour of the various samples. Raw filter paper showed an endothermic peak in the temperature range 330–400 °C, peaking at 361 °C. This can be attributed to cross-linking reactions being activated at such temperatures leading to the release of volatile compounds [37,38]. An endothermic peak occurring just after 400 °C may be attributed to degradation of char. The ZnCl2 only, WPT and NP samples had exothermic peaks at 324, 340 and 360 °C, all because of crosslinkage reactions as reported by Langley et al. [38]. 3.5. X-ray diffraction (XRD) analysis XRD analysis was conducted to observe changes in crystallinity of the cellulose and also to confirm the presence of key molecules, particularly silica. XRD patterns of raw and modified filter paper are shown in Fig. 7 and were analysed with the aid of Jade version 6. The peaks at diffraction angles 14.9°, 16.8° and 22.7° were assigned to (110), (110) and (200) crystal planes of cellulose respectively. The very low peak at 16.6° corresponds to (002) and is attributed to residual ZnCl2. The peak 7
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Fig. 13. Images of (a and b) raw filter paper before and after immersion in muddy rain water (c and d) WPT paper during and after immersion in muddy rain water, e) Interaction of oil and water droplets with the WPT paper, f) raw paper with dirt g) wetted raw paper, h) WPT paper after self-cleaning.
4. Conclusions
Repeated and continuous washing in a solvent can result in physical damage to a surface. The samples were tested for physical durability by continuously washing in ethanol for up to 4 h. In the process, a conical flask containing the samples and ethanol was mounted onto a Zhichu ZHTY-70 table-type constant temperature shaker running at a speed of 175 rpm at 40 °C. WCA results are shown in Fig. 12. The WPT sample lost its superhydrophobicity after 4 h of continuous washing. NP samples showed sharper drops in WCA compared to the WPT samples. The large decline in WCA of the NP samples was largely because of poor adhesion and retention of the silica on the paper. This was also confirmed by EDS analysis of the samples before and after washing, where the WPT was shown to have lost less silica after washing compared to the NP sample (see Fig. A.1-A.6 of the supplementary information).
Superhydrophobic paper was successfully prepared with a maximum WCA of 154.8 °C. Pre-treatment with ZnCl2 prior to modification resulted in greater silica adhesion and retention. Less HDTMS (0.6% (w/v)) was used and the grafting time was also reduced compared to previous studies. Pre-treatment with ZnCl2 also increased the durability of the paper as evidenced by the paper maintaining its superhydrophobicity when exposed to various organic solvents. The paper also retained its superhydrophobicity when exposed to a wide range of pH conditions (pH 3–11). The paper also showed resistance to staining and demonstrated self-cleaning properties. Acknowledgements
3.9. Real-life applications
This work was funded by Zhejiang Provincial Key research and development project (Grant No. 2019C01156), Zhejiang Provincial 26 County Green Special project (Grant No. 2019C02042), Zhejiang Provincial Public welfare technology research project (Grant No. LGG19C160001), National Natural Science Foundation of China (Grant No. 21808209).
Resistance to staining by water-based fluids is an important feature of superhydrophobic materials. Fig. 13a and c show raw and modified (WPT) filter paper being immersed in dirty rainwater. Fig. 13b shows raw filter paper wetted and stained by the muddy rain water. Fig. 13d shows the WPT paper after immersion in the rainwater showing no wetting or staining. Another application of superhydrophobic surfaces is in the separation of oil and water mixtures. The modified paper was superhydrophobic and oleophilic and because of these properties it is suitable for the separation of oil and water mixtures. Fig. 13e below shows how oil and water behave on the WPT surface. Oil passes through but water is repelled. The self-cleaning properties of the modified paper were also studied. Soil particles were applied to raw filter paper (Fig. 13f) and WPT paper. Water was then poured on the surface in a slow steady flow. Fig. 13g shows the water and soil particles wetting and attaching onto the surface of the raw paper. In contrast, Fig. 13h shows that as the water flowed on the surface of the modified paper, it carried away the soil particles leaving a clean surface. This shows that the superhydrophobic paper also possesses self-cleaning properties.
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