ARTICLE pubs.acs.org/IECR
Scale-Up of a Reaction Chamber for Superhydrophobic Coatings Based on Silicone Nanofilaments Georg R. J. Artus and Stefan Seeger* Institute of Physical Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland ABSTRACT: The facile and cheap large-scale production of superhydrophobic surfaces is one of the major challenges to exploit the commercial potential of strongly water-repellent materials. Here, we present the scale-up of a gas-phase reaction process for coating different materials with silicone nanofilaments and rendering them thereby superhydrophobic. As compared to the lab-scale equipment, the chamber volume of the pilot plant is larger by a factor of 1300, and the maximum sample dimension is ∼2 m. Design and technical issues of the pilot plant are presented. The achieved contact angles above 150° and sliding angles below 20° compare well to those achieved on the lab scale. Coated samples with dimensions on the order of meters such as fabric or window glass are presented.
’ INTRODUCTION During the last 15 years, superhydrophobic and self-cleaning surfaces have attracted a lot of interest. Such surfaces could find many applications such as in water-repellent and stain-resistant textiles,14 fouling protection,5 self-cleaning glass panes for windows or solar cells,6,7 self-cleaning paints for facades,6 drag reduction,8,9 droplet adhesion and droplet transport,1012 or antisnow or anti-icing coatings.1315 Inspired by natural examples of superhydrophobic surfaces, a vast amount of artificial superhydrophobic surfaces and coatings has been developed.1620 By extending the design strategies for superhydrophobic surfaces, even superoleophobic surfaces have been realized.21,22 Although highly water-repellent and self-cleaning surfaces hold high commercial potential, only few products are on the market.6,23,24 A general reason for this may be the insufficient long-term durability of superhydrophobic surfaces. Contamination with oleophilic substances or mechanical damage of the surface topography may lead to deterioration of the self-cleaning and water-repellent properties.25,26 Yet even for applications without the danger of mechanical wear or contamination, most of the published methods of fabricating such superhydrophobic products are not suitable for large-scale production or commercialization. For example, lithographic or spin-coating techniques are restricted to planar substrates of limited size, roughening of hydrophobic bulk materials is restricted to the very materials and cannot be used as coating, often high temperatures or other harsh conditions during the coating process restrict its applicability, many coatings modify the optical properties of the substrate material in an undesirable way, and many coating techniques are too complicated to be incorporated into an already existing production chain or they are simply too expensive due to material or apparatus costs. Indeed, only for a few proposed methods of fabricating superhydrophobic surfaces large-scale production seems to be realistic at all or has been realized effectively.23 During the past years, we have developed a silicone nanofilament (SNF) coating by which surfaces can be rendered highly superhydrophobic.2729 Basically, the chemical reaction consists of hydrolyzation of trichloromethylsilane (TCMS) by water to r 2011 American Chemical Society
silanol species, which then condensate yielding a polysiloxane. Complete condensation would lead to polymethylsilsesquioxane as product according to the following sum equation, although incomplete condensation is probable.30 nMeSiCl3 þ 3n=2H2 O f ½MeSiO3=2 n þ 3nHCl During the condensation process, quite unique nanofilaments are formed on the surface. Figure 1 shows a high magnification electron microscopy image of the SNFs on glass. Several mechanisms for the formation of these filaments have been proposed in the literature.30,31 The superhydrophobicity of the coating is well described by the CassieBaxter wetting mechanism.32 The specific surface topography of the SNF provides the necessary surface roughness, whereas the hydrophobic nature is imparted by the silicone material.19 The coating is a promising candidate for large-scale production for several reasons: The educts are readily available TCMS (1 L ≈ 70 h) and water. The coating can be conducted as a simple one-step CVD process either in the gas phase or in toluene, both at room temperature and at normal pressure. Either method allows arbitrary sample shapes. Many technologically interesting materials can be coated, for example, glass, silicon, titanium, aluminum, ceramics, polyester, polypropylene, polyethylene, or silicone.2,27,28 The coating is perfectly transparent, even antireflective.27 Because of the general inertness of silicones, the coating provides good chemical and environmental durability.33,34 A further advantage of the coating is its chemical composition: It is free of fluorine. Perfluorinated compounds are often used to achieve a low surface energy necessary for superhydrophobicity, but recently this class of compounds has raised environmental and toxicological concerns.35,36 Further chemical modification of the SNF-coating is also possible, for example, to introduce wettability patterns12,37 or superoleophobicity after Received: September 16, 2011 Accepted: December 20, 2011 Revised: December 16, 2011 Published: December 20, 2011 2631
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occurs within minutes.28 However, for optimum water repellence, reaction periods of more than 1 h are advantageous. After removal, the samples are rinsed with water. The small amounts of gaseous byproducts can be fed into the in-house exhaust gas system without further precautions. Annealing. Optionally, an annealing step can follow the coating process. This is not a necessity for achieving superhydrophobicity, although it improves contact and sliding angles. Annealing has also been shown to improve the chemical durability.33,34 Heat-resistant samples are annealed at 200 °C in air for at least 2 h. Figure 1. Scanning electron microscopy image of SNFs on glass coated in the gas phase. The bar corresponds to 500 nm.
fluorination.22 As for most superhydrophobic surfaces, the mechanical durability of the coating has to be improved. Nevertheless, there exist many potential applications where abrasion or mechanical wear is no problem, for example, in microfluidics, drag reduction, droplet transport or manipulation, single-use devices, filtering, chromatography, catalysis, or protein adsorption.38 Because of the promising properties of the SNF-coating and the simplicity of the coating process, we decided to develop and test potential applications of superhydrophobicity or self-cleaning beyond the laboratory scale. Here, we report on the scale-up of the gas-phase coating process to the size of a pilot plant and the achieved coating results.
’ LABORATORY-SCALE COATING PROCESS In this section, we introduce the core steps of the coating procedure on the lab scale as they are the basis for the design of the pilot plant. Further details are described elsewhere.27,29 Pretreatment of Samples. Most of the samples need to be cleaned and some activated prior to the coating step. Glass and silicon are cleaned and activated using a commercially available alkaline cleaning solution in an ultrasound bath at 50 °C. Chemically inert materials such as polypropylene or silicone are activated in an oxygen plasma. Polyester or cotton textiles do not need any pretreatment. Coating Procedure. The coating on the laboratory scale is conducted in a glass desiccator with a volume of 6.5 L and a diameter of 20 cm as coating chamber. The silane TCMS, typically between 200 and 400 μL, maximal 550 μL (1.7, 3.4, and 4.7 mmol, respectively), is transferred into an Eppendorf cap under inert gas atmosphere and placed in a special holder inside the coating chamber together with the samples. The water content in the chamber is adjusted by flushing the chamber with moistened air or nitrogen at room pressure. The desired humidity level is regulated by mixing appropriate flows of dry and saturated gas. The relative humidity is monitored by a hygrometer kept at constant temperature. Typically, the humidity ranges between 20% and 50% at room temperature, corresponding to 1.4 and 3.5 mmol of water for the volume of 6.5 L. After adjustment of the humidity, the chamber is closed. The reaction is started by releasing the silane via a magnetic trigger or an air pressure driven system. Diffusion inside the reaction chamber is sufficient for an even distribution of the silane within the coating atmosphere. Special carrier gases or heating or cooling of the substrate are not needed. The reaction can proceed unattended at room temperature. The growth of silicone nanofilaments
’ DESIGN OF THE PILOT PLANT On the basis of the laboratory process, the pilot plant was designed as follows. Pretreatment of Samples. For cleaning and activation of substrates, a custom-made ultrasound bath was installed in the coating lab. The inside dimensions are 2 m 1.5 m 0.75 m (W D H), allowing the activation of large glass panes. For plasma activation, a large-scale plasma chamber for sample sizes up to 2.4 m 1.1 m is available. A double trolley overhead crane with a maximum load of 500 kg facilitates the handling of heavy samples. Size of the Coating Chamber. The maximum size of the pilot plant was limited by the available laboratory space and also the unfortunately quite low dimensions of doors and the freight elevator in our university building. The inside dimensions of the coating chamber are 1.88 m 2 m 2.2 m (W H D), resulting in a chamber volume of 8.3 m3. This is an increase in volume by a factor of ∼1300 as compared to the laboratory equipment. The outside dimensions are 2.08 m 2.2 m 2.4 m. The dimensions of the door to the chamber are 2 m 1.3 m. Because of the size, we abandoned any additional temperature control for the chamber. Material. The chamber is constructed with a double-walled rib reinforced polyvinylchloride (PVC) casing. PVC was chosen as constructing material for several reasons: It is inert to all chemicals in the coating atmosphere, it provides sufficient mechanical stability, it is allowed by fire prevention regulations, it is a lightweight and low cost material, and, most important, it can be welded. By welding of prefabricated parts just as large to fit through the doors, the chamber could be assembled on site. The welding technique reduces also the necessity of using seals or gaskets, which would be potential and dangerous leaking points. The total weight of the chamber amounts to approximately 600 kg. One sidewall is equipped with a window and a pair of gloves similar to those in a drybox for conducting manipulations inside the coating chamber in an easy and safe way, for example, releasing the silane or moistening the atmosphere. All flaps and pipes have a diameter of 125 cm and are made from PVC, too. Further Technical Aspects. While diffusion is fast enough for distribution of TCMS in the lab-scale setup, it certainly would not suffice in the pilot plant. Also, TCMS would enrich at the bottom of the chamber because of its higher weight as compared to air. Therefore, a ventilator is needed to constantly circulate and mix the coating atmosphere. The outlet of the chamber to the fan is located at the bottom of the chamber. The gas reenters the chamber from above via an interior inlet pipe with holes along its complete length of 2 m. Instead of the inlet pipe, other equipment for special coating purposes can be installed. 2632
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Figure 3. Photograph of the pilot plant.
Figure 2. Schematic presentation of the pilot plant. The exhaust gas outlets are connected to the exhaust gas system of the chemistry building. The air suction can be fed with dry and pressurized air or with air from the lab atmosphere. For circulation within a closed system during coating, only flaps 1 and 2 are open; for treatment of exhaust gases, flaps 36 are open.
The continuous line in figure 2 depicts the circuit of the gases during the coating process (flaps 1 and 2 open, flaps 36 closed). The desired humidity is adjusted depending on the humidity level in the lab. In case the humidity is too high, dry nitrogen is pumped into the chamber via the air suction at flap number 6. In case the humidity is too low, moistening the atmosphere is done via atomizing water from a spray bottle inside the chamber repeatedly. By constantly monitoring the relative humidity via a hygrometer, the desired value can be adjusted quite easily in both ways. The pilot plant can be operated from a central control unit. From here, the flaps can be actuated for the two states “closed system internal circulation” and “open system exhaust gas treatment” (continuous and dashed lines, respectively, in Figure 2). The washing tower can be activated by hand or automatically, the speed of rotation of the ventilator can be adjusted, and the water level in the washer can be monitored. To prevent leakage, we decided to run the coating chamber at a constant depression of 200 Pa relative to the laboratory pressure by use of a constantly controlled pump. A small single walled panel is introduced into the rear wall of the chamber, which holds connectors for the pump, hygrometer, and other optional equipment. Safety Issues. The area of the samples to be coated increases approximately by a factor of 200 as compared to the lab-scale coating. Thus, the 200-fold amount of silane and byproducts can be expected. For safety reasons, we extrapolated the upper limit of amounts of chemicals by the chamber volume ratio of 1300. Using the latter value and the maximum amount of 550 μL TCMS used in the lab-scale procedure results in 6.1 mol or 1 kg of TCMS. Assuming complete hydrolysis, this would correspond to 18.3 mol or 700 g of HCl. Clearly, these numbers demand special attention for the design of the pilot plant with regard to safety issues and air pollution regulations. The constant depression in the chamber provided by the vacuum pump minimizes the leakage of hazardous vapors. Constant pressure regulation via this pump is also necessary
for compensating rising pressures inside the chamber due to the evaporation of silane or water or due to varying temperature. Evaporation of 6.1 mol of TCMS corresponds to an increase in pressure of 1800 Pa. The same pressure rise would follow an increase in temperature of about 5.3 °C. This pressure overload could lead to enhanced leakage. Also, it corresponds to an additional weight per chamber wall of more than 700 kg, which would put the mechanical stability of the chamber at risk. Safety valves for excess and negative pressure in the chamber are integrated. For the disposal of byproducts, mainly hydrochloric acid and unreacted or partially hydrolyzed silane, a wet scrubber had to be included. The scrubber is run with a total of 150 L of sodium hydroxide solution at pH values between 9 and 13 for most efficient absorption of the acidic byproducts. The exhaust gas is pumped in a countercurrent flow through an attached spray tower. The flow during exhaust gas treatment is depicted in Figure 2 with a dashed line (flaps 1 and 2 closed, flaps 36 open). Figure 3 shows a photograph of the pilot plant in our lab.
’ COATING RESULTS The simplicity and scalability of the coating procedure is underlined by the fact that first successful coatings could be achieved after only a few test cycles. Because of the higher demand in time and resources, we did not conduct a complete optimization of all coating parameters for each kind of substrate for the pilot plant. So the reported coating parameters should be understood as coarse experienced data. In turned out that the optimum relative humidity values correspond well to those used in the lab-scale process. For glass, the optimum relative humidity is ∼30%; for fabrics depending on the kind of material, it ranges between 25% and 50%. The amount of silane scales roughly according to the area of the substrate to be coated. For glass, an increase in substrate area by a factor of 100 (200 cm2 to 2 m2) as compared to the lab-scale process requires an increase in amount of silane by a factor of 230 (0.35 to 80 mL). For polyester fabric, an increase in area by a factor of 240 (20 cm2 to 4.8 m2) requires the 330-fold amount of silane (0.35 to 100 mL). On glass, contact angles above 150° can be achieved already after 30 min of coating time. However, for better contact angles and low sliding angles, longer coating times are necessary similar to the lab-scale coating process. Table 1 summarizes coating parameters and analytical data on microscopic glass slides coated on both sides with the lab-scale equipment and in the pilot plant. Contact and sliding angles are very similar for both processes. As reported earlier, annealing improves the sliding angle much 2633
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Table 1. Comparison of Coating Parameters and Analytical Data for Coatings Performed with Laboratory Equipment and with the Pilot Planta pilot plant
lab scale
temperature/°C
24 ( 1
23 ( 1
rel. humidity/%
28 ( 2
35 ( 2
contact angle/deg
160.2/164.3
161.9/164.3
sliding angle/deg
18.3/7.0
16.8/9.5
transmittance at 400 nm/% transmittance at 700 nm/%
93.9/94.0 98.3/98.4
94.8/95.2 94.4/94.7
a
The two values given for the analytical data refer to measurements before and after thermal annealing.
Figure 5. Impact of a water jet on an inclined and coated glass pane of size 1 m 1 m as photographed from below. The repellent behavior of the glass is obvious by the formation of droplets instead of a film-like spreading of the water as expected on uncoated glass. The transparency of the glass is clearly visible. Figure 4. Scanning electron microscopy image of silicone nanofilaments on glass coated in the pilot plant. The bar corresponds to 500 nm.
more than the contact angle.33,34 Contact angles above 160° and sliding angles below 10° prove the highly superhydrophobic properties of the coating. A difference in the result of both processes is observed already by eye. The glass slides out of the pilot plant appear slightly opaque, whereas the lab-scale coating results in perfectly transparent slides. This difference in optical properties is confirmed by the transmittance values as measured by UV/vis spectroscopy. At 400 nm, the lab-scale samples show better transmission by roughly 1%, whereas the samples coated in the pilot plant are nearly perfectly transparent at a wavelength of 700 nm. Here, the transmittance is higher than 98%. As compared to uncoated glass, all samples reduce the reflectivity: The corresponding transmittance values of uncoated glass are 91.5% and 92.2% for 400 and 700 nm, respectively. Figure 4 shows an electron microscopy image of SNF on glass that was coated in the pilot plant. The diameters of the filaments range from 30 to 50 nm, and the length is at least several hundred nanometers as observed for the lab-scale process (compare Figure 1).27 Large Samples. Several large samples with surface areas in the range of square meters could be coated successfully. The largest piece of fabric coated up to now had dimensions of 3.2 m 1.55 m. Also, we have been able to coat a complete tent for two people (approximately 2 m 1.2 m 1 m). Because of the large sample size, a direct measurement of analytical data such as transparency, contact, or sliding angles mostly is impracticable. However, inspection by eye or lateral rinsing with water reveals uniform superhydrophobic properties across the area of the samples presented here. Also, small standard samples added as reference confirm a successful coating process. Figure 5 shows water rolling or sliding off a superhydrophobic glass pane of size 1 m 1 m. The water-repellent properties and
Figure 6. A puddle of 100 mL of water held on a coated polyester fabric of size 1.4 m 1.55 m. The curvature of the puddle at the edges clearly indicates superhydrophobicity. The plastron underneath the drop is clearly visible. After the water is moved around the surface of the fabric, it collects small dirt particles at its surface (inset), indicating the selfcleaning effect.
the transparency of the glass are obvious to the eye. We also coated the interior side of glass capillaries of length 1.5 and 2.2 mm in diameter by connecting them directly to the gas inlet in the pilot plant. This way the coating gases were pressed through the capillary. Here, the contact angle can be determined along the complete length of the capillary by measuring the capillary depression as a function of the immersion depth into water. The derived contact angles varied between 152° and 170°. The large deviations are probably explained best by a varying diameter of the nonprecision capillary. Nevertheless, the contact angles prove the superhydrophobic coating at the interior walls of the capillaries. Figure 6 shows 100 mL of water in the deepened middle of a polyester fabric of size 1.4 m 1.55 m. 2634
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Figure 7. Photograph of the shoulder of a SNF-coated polyester suit coat in artificial rain. The droplets bounce off, and the textile remains dry.
The curvature at the edge of the puddle indicates the high contact angle. A silvery layer underneath the water is clearly visible. This effect is due to total reflection of light at the so-called plastron, a gas layer between water and substrate indicating the superhydrophobicity of the textile. The water puddle can be moved easily around the fabric. After that, small dust or dirt particles adhere to the water surface (white spots in inset of Figure 6). These are collected by the water from the fabric indicating the self-cleaning effect. Finally, we successfully coated a coat of a polyester suit. Figure 7 shows the shoulder of the suit coat in artificial rain. The water drops bounce off, and the fabric is not wetted.
’ SUMMARY We have presented a pilot plant for rendering large-area substrates superhydrophobic by coating them with silicone nanofilaments in the gas phase. The coating chamber has a volume of 8 m3, which corresponds to a scaling factor of ∼1300 as compared to the lab-scale coating equipment. The necessary coating conditions compare well to those of the lab-scale process. Samples of square meters in size made out of glass or polyester fabric have been coated successfully. On glass, an antireflective coating with contact angles above 150° and sliding angles below 20° could be achieved as for the lab-scale process. This result demonstrates the potential of the silicone nanofilament coating for large-scale production. The pilot plant offers the possibility for further product research or scientific experiments where large superhydrophobic areas are necessary. ’ MATERIALS AND METHODS Materials. As standard glass samples, microscope glass slides (Menzel, Braunschweig, Germany) of the dimensions 26 mm 76 mm 0.15 mm were used. For the large-scale coating, a standard window glass of size 1 m 1 m 5 mm was purchased at a local glazier. The polyester suit coat was purchased in a local clothes shop. Methyltrichlorosilane (97%, ABCR, Germany) was used as received. Cleaning and Activation. All glass samples were treated with ultrasound in a 10% solution of “deconex 11 universal” (Borer Chemie AG) at 50 °C, rinsed with deionized water, and dried under a nitrogen flow. Small samples were sonicated for 0.5 h in an Elmasonic X-tra 70H (Elma Hans Schmidbauer GmbH & Co. KG, Singen, Germany) at 35 kHz. The large glass pane was sonicated for 0.5 h at 40 kHz and 0.5 h at 100 kHz in a custommade ultrasound bath (KKS Ultraschall AG, Steinen, Switzerland).
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The polyester suit was activated in a Tetra 2800 plasma machine (Diener Electronic GmbH & Co. KG, Ebhausen, Germany) for 1 min using an oxygen pressure of 0.6 mbar. The polyester fabric was cut to a size of 1.4 m 1.55 m and used as received. Coating. Microscope slides were coated on the lab scale at 23 °C at 35% relative humidity using 522 μL of TCMS as described in the literature.27,29 For the microscope slides in the pilot plant, 80 mL of TCMS was used at 24 °C and 28% relative humidity, for the glass pane of size 1 m 1 m 100 mL TCMS at 25 °C and 34% relative humidity. For the suit coat, 100 mL of TCMS at 25 °C and 39% relative humidity was used, while for the polyester fabric, 24 °C, 25% relative humidity, and 80 mL of TCMS were used. Glass samples were left in the coating chamber overnight. The coating period for the fabric was 4 h, and for the suit it was 4.75 h. Annealing. Glass samples were annealed for at least 2 h at 200 °C at normal atmosphere in a drying oven. Analytics. Measurements of static contact angles and sliding angles were performed with 10 μL drops of deionized water using the Contact Angle System OCA and included software (Dataphysics, Filderstadt, Germany). Sliding angles were measured with a custom-made tilting device. For electron microscopy, the samples were sputter coated with gold/palladium or platinum. Electron microscopy images were taken at the Zeiss Supra 50 VP at the center for microscopy and image analysis of the University of Zurich at 2 kV acceleration voltage.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the center for microscopy and image analysis of the University Zurich for their support and for use of their facilities, and the Alfred-Werner-Legat of the University of Zurich for financial support. ’ REFERENCES (1) Deng, B.; Cai, R.; Yu, Y.; Jiang, H.; Wang, C.; Li, J.; Li, L.; Yu, M.; Li, J.; Xie, L.; Huang, Q.; Fan, C. Laundering durability of superhydrophobic cotton fabric. Adv. Mater. 2010, 22, 5473. (2) Zimmermann, J.; Reifler, F. A.; Fortunato, G.; Gerhardt, L.-C.; Seeger, S. A. Simple, one-step approach to durable and robust superhydrophobic textiles. Adv. Funct. Mater. 2008, 18, 3662. (3) Mahltig, B.; B€ottcher, H. Modified silica sol coatings for waterrepellent textiles. J. Sol-Gel Sci. Technol. 2003, 27, 43. (4) Michielsen, S.; Lee, H. J. Design of a superhydrophobic surface using woven structures. Langmuir 2007, 23, 6004. (5) Nosonovsky, M.; Bhushan, B. Superhydrophobic surfaces and emerging applications: Non-adhesion, energy, green engineering. Curr. Opin. Colloid Interface Sci. 2009, 14, 270. (6) Solga, A.; Cerman, Z.; Striffler, B. F.; Spaeth, M.; Barthlott, W. The dream of staying clean: Lotus and biomimetic surfaces. Bioinspiration Biomimetics 2007, 2, S126–S134. (7) Park, Y.-B.; Im, H.; Im, M.; Choi, Y.-K. Self-cleaning effect of highly water-repellent microshell structures for solar cell applications. J. Mater. Chem. 2011, 21, 633. (8) Ou, J.; Perot, B.; Rothstein, J. P. Laminar drag reduction in microchannels using ultrahydrophobic surfaces. Phys. Fluids 2004, 16, 4635. 2635
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