Patent Application
20090011222
1. Field of the Invention
This invention relates generally to the field of superhydrophobic surface coatings, and methods for forming same.
2. Description of Related Art
The Lotus Effect is named after the lotus plant, and was first used for technical applications by Professor Wilhelm Barthlott from the University of Bonn. The Lotus Effect generally refers to two characteristic properties: superhydrophobicity and self-cleaning, although in some instances, either one of these properties provide the benefits of the Lotus Effect.
Superhydrophobicity is manifested by a water contact angle larger than 150°, while self-cleaning indicates that loose (non-adhered) dirt particles such as dust or soot are picked up by a drop of water as it rolls off the surface, and are thus removed. The superhydrophobicity and self-cleaning properties of a Lotus Effect surface are illustrated in
TABLE 1 provides common definitions of liquid/surface phenomena related to ionized water. For example, it will be understood that the values will change with other liquids, such as saline solution, wherein in a low concentration saline solution, there is no appreciable effect, but in higher saline concentrations, the contact angle will be greater. Thus, these definitions are also applicable to liquids with low concentrations of salts and particulates such as those found in normally encountered environmental pollution environments.
In general, a Lotus Effect surface arises when both of the following factors are achieved: the surface is covered with low surface free energy materials, and has a very fine structure. Low surface free energy materials provide a relatively high contact angle. The contact angle is a measure of the wettability of a surface with water. Readily wettable (hydrophilic) surfaces have relatively small contact angles, and non-wetting (hydrophobic) surface have relatively large contact angles.
Regarding surface structure, surfaces that are rough tend to be more hydrophobic than smooth surfaces, because air can be trapped in the fine structures, which reduce the contact area between the liquid and the surface, or water and solid. It is recognized that when a water drop is placed on a lotus plant surface, the air entrapped in the nanosurface structures prevents the total wetting of the surface, and only a small part of the surface, such as the tip of the nanostructures, can contact with the water drop. For the lotus plant leaves, the actual contact area is only 2-3% of a droplet-covered surface. This enlarges the water/air interface while the solid/water interface is minimized. Therefore, the water gains very little energy through adsorption to compensate for any enlargement of its surface. In this situation, spreading does not occur, the water forms a spherical droplet, and the contact angle of the droplet depends almost entirely on the surface tension of the water. The relationship between the surface water contact angle and the surface structural geometry (Wenzel roughness) can be given in Cassie equation:
cos θA=rf1 cos θY+f1−1 Equation 1
where the parameter r is the ratio of the actual solid-liquid contact area to its vertical projected area (Wenzel roughness factor), θA is the apparent contact angle on the rough surface, and θY is the contact angle on a flat surface as per Young's equation, f1 is the solid surface fraction.
Although the Lotus Effect was discovered in plants, it is essentially a physicochemical property rather than a biological property. Therefore, it is possible to mimic the lotus surface structure. A Lotus Effect surface can be produced by creating a nanoscale rough structure on a hydrophobic surface with contact angles of over 90° (in situ bulk fabrication), coating thin hydrophobic films on nanoscale rough surfaces (surface fabrication), or creating a rough structure and decreasing material surface energy simultaneously (combination fabrication). To date, many methods have been developed to produce hydrophobic surfaces with nanoscale roughness.
Conventionally, a variety of methods have been developed to produce hydrophobic surfaces with nanoscale roughness. These methods include the fabrication of polymer nanofibers and densely packed aligned carbon nanotube films combined with fluoroalkylsilane coating, solidification of melted alkylketene dimer, anodic oxidation of aluminum with fluoroalkyltrimethoxysilane, immersion of porous alumina gel films in boiling water, mixing of a sublimation material with silica particles, and treating the fluorinated polymer film with different plasma techniques.
Superhydrophobic properties are desirable for many applications. For example, a durable superhydrophobic and self-cleaning coating would be invaluable from the high voltage industry to limit or prevent flashover, to the microelectromechanical systems (MEMS) industry to limit or prevent stiction, to the anticorrosion of metal coatings. Other applications for superhydrophobic surfaces are emerging all the time, such as the directed liquid flow in microfluidics, antifouling in biomedical applications, and transparent coatings in photovoltaics devices, just to name a few.
In regard to high voltage applications, superhydrophobic properties would help limit or even prevent the accumulation of contaminants on the surface of the insulators, which can produce a conductive layer when wet, which can then lead to an increase in leakage currents, dry band arcing, and ultimately flashover. Due to the self-cleaning properties of the surfaces, the contamination that is deposited on the surface can be easily picked up by water droplets falling or condensed on the surface.
The bulk of power delivery from the generating sites to the load centers is done by overhead transmission lines. To minimize line losses, power transmission over such long distances is more often carried out at high voltages (several hundred kV). The energized high voltage (HV) line conductors have to be physically attached to the support structures. Also, the energized conductors have to be electrically isolated from the support structures.
The device used to perform the dual functions of mechanical support and electrical isolation is the insulator. Since transmission lines are often in remote locations that are hard to reach, it is desirable that once a line has been constructed that it will work satisfactorily, without maintenance, for the expected life of the line, generally exceeding 30 years. The quality of raw materials, processing, design, and quality control of the insulator are all important.
In many parts of the world, insulator contamination has become a major impediment to the interrupted supply of electrical power. Contamination on the surface of insulators gives rise to leakage current, and if high enough, flashover. Conventional techniques have been applied to address this problem, including:
(1) Cleaning with water, dry abrasive cleaner, or dry ice can effectively remove loose contamination from insulator, but it is expensive, labor intensive and only a short term solution;
(2) Mobile protective coatings, including surface treatment with oils, greases and pastes, can prevent flashover, but have damaging results to the insulator during dry band arcing;
(3) Grease-like silicone coating components, usually compounded with alumina tri-hydrate (ATH), provide a non-wettable surface maintaining high surface resistance, and have been used as protective coatings for the past 30 years. A major strength of silicone grease lies in its ability to maintain a mobile water repellent surface, thereby controlling leakage current;
(4) Fluorourethane coatings were developed for high voltage insulators, but the field test was not successful, and its low adhesion to the insulators has been a problem; and
(5) Since 1970s, room temperature vulcanizing (RTV) silicone coatings have gained considerable popularity, and become the major products available in the market, such as Dow Corning's SYLGARD High Voltage Insulator Coatings (HVIC), CSL's Si-Coat HVIC, and Midsun's 570 HVIC. Service experience has indicated that of the various types of insulator coatings, the time between maintenance and RTV coating reapplication is the longest.
Yet, these conventional techniques do not prevent contamination, such as dust, accumulation on coating surfaces; thus, these serve only to manage the problem, and do not provide satisfactory performance in heavy contamination environments.
Insulators are used with transmission and distribution systems, including power transmission lines, for example at locations where the lines are suspended, and typically have voltages, AC and DC, from 5 kV to 800 kV. The term insulator as used herein includes typical distribution line insulators, for example, from low-voltage (LV) lines to extra-high voltage (EHV) lines. More specifically, the term includes LV lines, medium-voltage (MV) lines, wherein the voltage is usually between 2.4 kV and 69 kV, high-voltage (HV) lines, wherein the voltage is usually below 230 kV, and EHV, being lines operating at voltages of up to 800 kV, and stretching as long as 1000 km. High-tension direct lines are also included in this group.
Known insulators include ceramics, glass and polymeric materials. Ceramic and glass insulators have been used for over 100 years. The widespread use of polymeric insulators began in North America during the 1970s. A currently popular line of insulators are RTV silicone rubber high voltage insulator coatings.
Ceramic insulators generally include clay ceramics, glasses, porcelains, and steatites. The ceramic is produced from the starting materials kaolin, quartz, clay, alumina and/or feldspar by mixing the same while adding various substances in a subsequent firing or sintering operation. Polymeric materials include, for example, filled and unfilled composites, such as ethylene propylene diene monomer (EPDM) rubber and silicone rubbers/elastomers, and can include resins, such as epoxy, polyester and polyolefin based polymers, and copolymers).
A wide variety of manufacturing techniques can be employed to construct insulators of the desired shape. Some of the processes that are most often used include machining, molding, extrusion, casting, rolling, pressing, melting, painting, vapor deposition, plating, and other free-forming techniques, such as dipping a conductor in a liquid dielectric or filling with dielectric fluid. The selection process must take into account how one or both of the electrodes made from conductive material will be attached or adjoined to the insulator.
In long-term use, an insulator is subject to superficial soiling depending on the location at which it is used, which can considerably impair the original insulating characteristics of the originally clean insulator. Such soiling is caused, for example, by the depositing of industrial dust or salts, or the separating out of dissolved particles during the evaporation of moisture precipitated on the surface.
One problem afflicting high voltage insulators used with transmission and distribution systems includes the environmental degradation of the insulators. Insulators are exposed to environment pollutants from various sources. Pollutants that become conducting when moistened are of particular concern. Two major sources of environmental pollution include coastal pollution and industrial pollution.
Coastal pollution, including salt spray from the sea or wind-driven salt-laden solid material such as sand, can collect on the insulator surface. These layers become conducting during periods of high humidity and fog. Sodium chloride is a main constituent of this type of pollution.
Industrial pollution occurs when substations and power lines are located near industrial complexes. The power lines are then subject to the stack emissions from the nearby plants. These materials are usually dry when deposited, and then may become conducting when wetted. The materials will absorb moisture to different degrees. Apart from salts, acids are also deposited on the insulator.
High voltage lines can be exposed to both sources of pollution. For example, if a substation is situated near the coast, it will be exposed to a high saline atmosphere together with any industrial and chemical pollution from other plants situated in close proximity.
The presence of a conducting layer on the surface of an insulator can lead to pollution flashover. In particular, sufficient wetting of the dry salts on the insulator surface is required to form a conducting electrolyte. The ability of a surface to become wet is described by its hydrophobicity. Ceramic materials and some polymeric materials such as EPDM rubber are hydrophilic, that is, water films out easily on its surface. In the case of some shed materials for high voltage insulator application, such as silicone rubber, water forms beads on the surface due to the low surface energy.
When new, the hydrophobic properties of silicone rubber are excellent; however, it is known that severe environmental and electrical stressing may erode the beneficial hydrophobicity properties.
Current remediation techniques for environmental degradation of a high voltage insulator include washing, greasing, and coatings, among others. Substation or line insulators can be washed when de-energized or when energized. Cleaning with water, dry abrasive cleaner, or dry ice can effectively remove loose contamination from insulator, but it is expensive and labor intensive. It is not uncommon that washings involve shutting down the power once every two weeks in the winter, and once per week in the summer when doing this kind of maintenance. These common occurrences of de-energization simply are not preferable.
Mobile protective coatings, including oils, grease and pastes surface treatment, can prevent flashover, but have damaging results to the insulator during dry band arcing. While a thin layer of silicone grease when applied to ceramic insulators increases the hydrophobicity of the surface, pollution particles that are deposited on the insulator surface are also encapsulated by the grease and protected from moisture. Another disadvantage of greasing is that the spent grease must be removed and new grease applied, typically annually. Grease-like silicone coating components, usually compounded with ATH, provide a non-wettable surface and maintain high surface resistance. Thus, while greasing can greatly reduce maintenance costs when viewed against washings, substation personnel have to remove the old grease compounds from the equipment, and then re-apply the new grease compound annually.
Fluorourethane and silicone RTV coatings are also known. Room temperature cured silicone rubber coatings are available to be used on ceramic or glass substation insulators. These coatings have good hydrophobic properties when new. Silicone coatings provide a virtually maintenance-free system to prevent excessive leakage current, tracking, and flashover. Silicone is not affected by ultraviolet light, temperature, or corrosion, and can provide a smooth finish with good tracking resistance.
Silicon coatings are used to eliminate or reduce regular insulator cleaning, periodic re-application of greases, and replacement of components damaged by flashover. They appear to be effective in many types of conditions, from salt-fog to fly ash. They are also useful to restore burned, cracked, or chipped insulators.
SYLGARD is one type of silicone coatings, and is marketed to restrict the rise in leakage currents and protect the insulators against pollution induced flashovers. The cured SYLGARD coating has a high hydrophobicity. In addition, there are a certain percentage of polymer molecules that exist within the cured rubber as low molecular weight free fluid. These molecules are known as “cyclics”. The free fluids are easily able to migrate to the surface of the coating and, as pollutants fall on the surface, they in turn are encapsulated and rendered non conductive and somewhat hydrophobic.
If leakage currents are controlled, there will be no arcing. If there is an extreme weather event then it may be that, for a time, the SYLGARD coating cannot control the surface leakage currents. In this case SYLGARD also provides a high degree of surface arc resistance. Incorporated into the formulation is an ATH filler, which releases H2O when it becomes hot, and consequently resists the degradative effects of high temperatures resulting from exposure of the coating to arcing.
Thus, none of the conventional techniques to limit contamination, such as dust accumulation on coating surfaces, provides satisfactory performance in heavy contamination environments. A need yet exists for a superior product that can minimize the maintenance necessary with conventional coatings. An HVIC that is self-cleaning and has a longer life than conventional coatings would be beneficial.
As previously discussed, there are other applications where superhydrophobic properties are desirable, beyond that of the high voltage industry. It is known that stiction is one of the major factors that limit the widespread use and reliability of micro-electromechanical systems (MEMS). The fundamental mechanism to prevent stiction is either increasing the surface roughness, or coating MEMS surfaces with hydrophobic materials.
In another application, it would also be beneficial to prepare superhydrophobic silicone/polytetrafluoroethylene (PTFE) films for biocompatibility in encapsulation of implantable microelectronics devices. At present, a variety of superhydrophobic surfaces have been fabricated from materials ranging from organic polymers (e.g., polystyrene, fluorinated polyelectrolytes, polypropylene) to inorganic materials (e.g., silica and alumina). Polydimethylsiloxane (PDMS) and polytetrafluoroethylene (PTFE) both have very low surface energies (˜21 mJ/cm2 for PDMS and 18.5 mJ/cm2 for PTFE). They have well established implant history due to their relatively inert and biocompatible properties. Silicone rubbers are the most widely used polymers in medical applications because of the strong Si—O—Si (siloxane) backbone, which provides enhanced chemical inertness and exceptional flexibility.
PTFE is highly hydrophobic, with undetectable water absorption. It would be beneficial to incorporate these two low surface energy biocompatible materials, i.e., PTFE nanoparticles in curable PDMS matrix, to create superhydrophobic films on a silicon wafer that was in the Cassie regime with the water contact angles around 160°. Such a film would suitable as a biocompatible coating for implantable microelectronic devices. Such a surface would be biocompatible not only due to the surface hydrophobicity, but also due to the surface structure (roughness), as such, biomaterials and bio-cells like protein or macrophage are not easy to adsorb on the surface. This is discussed in the following, herein fully incorporated by reference; Xiu, Y., et al. Superhydrophobic Silicone/PTFE Films for Biocompatible Applications in Encapsulation of Implantable Microelectronics Devices. in 56th Electronic Components and Technology Conference; 2006. San Diego, Calif.
Returning to the prospect of providing superhydrophobic coatings as a preferable surface treatment generally, surfaces with a combination of microstructure and low surface energy are known to exhibit interesting properties. A suitable combination of structure and hydrophobicity renders it possible that even slight amounts of moving water can entrain dirt particles adhering to the surface and completely clean the surface. It is known that if effective self-cleaning is to be obtained on an industrial surface, the surface must not only be very hydrophobic (Young's contact angle of over 90°, see Equation 2 hereinafter), but also have a certain roughness. Such surfaces are disclosed in, for example, WO 96/04123 and U.S. Pat. No. 3,354,022.
European Pat. No. 0 933 380 discloses that an aspect ratio of >1 and a surface energy of less than 20 mN/m are required for such self-cleaning surfaces. The aspect ratio is defined to be a quotient of a height of a structure to a width of the structure.
Other prior art references include PCT/EP00/02424, which discloses that it is technically possible to render surfaces of objects artificially self-cleaning. The surface structures, composed of protuberances and depressions, required for the self-cleaning purpose, have a spacing between the protuberances of the surface structures in the range of 0.1 to 200 μm and a height of the protuberances in the range from 0.1 to 100 μm. The materials disclosed therein include hydrophobic polymers or a durably hydrophobized material. Detergents must be prevented from dissolving the supporting matrix. As in the documents previously described, no information is given either on the geometrical shape or radii of curvature of the structures used.
European Pat. No. 0 909 747 teaches a process for producing a self-cleaning surface. The surface has hydrophobic elevations from 5 to 200 μm. A surface of this type is produced by applying a dispersion of powder particles and of an inert material in a siloxane solution, followed by curing. The structure-forming particles are therefore secured to the substrate by an auxiliary medium.
Methods for producing these structured surfaces are likewise known. For example, U.S. Pat. No. 5,599,489 utilizes an adhesion-promoting layer between particles and the bulk material. Processes suitable for developing the structures are etching and coating processes for adhesive application of the structure-forming powders, and also shaping processes using appropriately structured negative molds.
However, it is common to these methods that the self-cleaning behavior of the surfaces is described by a particular surface roughness. The roughness may be defined by a number of metrics, such as the Wenzel Roughness (Equation 1).
Plasma technologies are widely utilized for processing of polymers, such as deposition, surface treatment and etching of thin polymer films. The advantages of using plasma techniques to prepare the Lotus Effect coating include that plasma technologies have been extensively employed in surface treatment processes in the electronic industry. Fabricating the Lotus Effect coating on various surfaces with plasma can be easily transferred from research to scale up production. Further, plasma-based methods can be developed into a standard continuous/batch process with low cost, highly uniform surface properties, high reproducibility and high productivity.
For example, U.S. Ser. No. 10/966,963, the disclosure of which is incorporated herein by reference, discloses plasma technologies, including superhydrophobic coatings, and methods of applying Lotus Effect materials as a superhydrophobic protective coating for external electrical insulation system applications, as well as the method of fabricating/preparing Lotus Effect coatings. However, plasma technologies have been found disadvantageous in certain applications, with its attendant relatively high cost, requirement of special equipments, etc.
Another limitation of the known superhydrophobic art include that the surfaces are uni-modal, in that the size distributions (height or diameter), do not vary beyond a relatively small tolerance. It would be beneficial to provide a multi-modal surface structure for improved superhydrophobicity, and such a surface that does not require the use of experimentally cumbersome low pressure plasma post-treatment.
Further, known superhydrophobic surfaces are typically constructed with one chemical species. It would be beneficial to provide a multi-species system, having two or more widely different chemical species for improved long lifetime (decomposition of organic contamination that can not be easily cleaned by water droplets on the surface).
As yet another advantage, if the superhydrophobic effect could be provided at or near ambient temperatures and pressures, it would be desirable.
It can be seen that a need yet exists for a superior coating and method that ultimately provides a surface exhibiting Lotus Effect properties, including superhydrophobicity and self-cleaning.
The present invention comprises superhydrophobic surface coatings, and methods for forming same. The outer surface of a device can play a critical role in determining the reliability of the device. This is true for a wide range of applications, whether the application is a transmission line for power delivery, or used in MEMS, or for biocompatible application in encapsulation of implantable microelectronics devices.
A preferred surface for such devices should have one or more of the following properties: (i) water repellence—hydrophobicity; (ii) self cleaning or de fouling, (iii) chemical/physical inertness, (iv) longevity (under single and multi-factor ageing conditions), (v) beneficial adhesion to the substrate; and, (vi) mechanical robustness. It is convenient to describe the water repellency of surfaces in terms of the liquid contact angle that they make to the surface. The contact angle is a specific and fundamental property of the surface and the liquid in contact. The parameters used to describe this property include the equilibrium (after the normal “recovery effects” known in practical devices, as it is well known that on many practical insulators, the hydrophobicity is lower directly after multifactor exposure, and then increases (recovers) with time after removal from the multifactor environment) (i) contact angle—the angle an isolated drop makes with the surface; and, (ii) hysteresis—the difference in contact angle between the advancing and receding fronts of a moving isolated drop.
An alternate metric for water repellency is the time, at constant high AC voltage, that an inclined dielectric surface can sustain the high voltage in the presence of a continual flow of fluid, most usually water. The approach is the focus of work within Working Group WG D1-14 of CIGRE. The usual Weibull scale parameter (probability for 63.3% of the samples to fail) for the endurance tests for elastomeric (silicone) dielectric plaques is within the range 1-4 minutes. Surfaces with improved hydrophobicity (superhydrophobicity and higher hydrophobicity than the base) will display higher values of Weibull scale parameter than the untreated versions. These values might be expected to be a factor of two (2) or more higher than the base cases. The improved performance is also evidenced by increases in the Weibull scale parameter. Optimal water repellency is given by a combination of high contact angle and low hysteresis. See TABLE 1.
Superhydrophobic surfaces occur when structures of a defined size (height and diameter) are created at the correct surface density and distribution. The liquid drops on such a surface are constrained to the tops of these “pillar” by the specific surface energies of the pillar tops. Conventionally, such surfaces have been designed to have a single distribution of sizes (height and diameter), and can thus be described as uni-modal superhydrophobic surfaces. Additionally, conventional surfaces are constructed using but a single chemical species.
Furthermore, to attain the correct surface energy for the coating, it conventionally has been necessary for the coatings to be treated in a well-defined, low pressure plasma. This requirement is difficult to achieve within normal manufacturing operations. Although superhydrophobic surfaces can be created on polymers following aggressive plasma treatments, it has been found these surfaces rapidly degrade under the combined effect of water vapor condensation, elevated temperature and UV irradiation.
In a preferred embodiment, the present invention comprises an inorganic surface of improved hydrophobicity (typically superhydrophobic in nature) which is stable under harsh multi-factor ageing environments such as salt, moisture, and high temperature.
In another preferred embodiment, the present invention comprises an inorganic, stable and inert superhydrophobic surface that possess beneficial longevity (is stable over time). As is known, since UV initiates the aging process, the present invention can also be described as a UV-stable superhydrophobic surface.
For example, the present invention can comprise an inorganic, stable superhydrophobic surface, wherein stable is defined as the surface maintaining a contact angle of greater than 150 degrees after 1,000 hours of multi factor ageing tests. The surface is preferably UV-stable, wherein UV-stable is defined as the surface maintaining a contact angle of at least 150 degrees after 1,000 hours of a UV weathering test according to ASTM D 4329. A preferred surface of the present invention can maintain a contact angle of greater than 150 degrees after 5,500 hours of multi factor ageing tests, and more preferably, maintain a contact angle of greater than 162 degrees after such ageing tests. Such surfaces can be upon, for example, a dielectric substrate, or an insulating substrate.
The inorganic, multi-factor stable superhydrophobic surface can include a superhydrophobic surface with two or more widely separated size distributions (height or diameter), and can thus be described as a multi-modal superhydrophobic surface, including, for example, bi-modal and tri-modal. In addition, a sufficiently high contact angle is attained, such that the water repellence matches that of the conventional art, without recourse to the experimentally cumbersome low pressure plasma post-treatment.
Alternatively, or in combination, the inorganic, multi-factor stable superhydrophobic surface can include a superhydrophobic surface with two or more widely different chemical species. The size distribution of the species can cover a wide range. Such size distribution can cover a range defined by the ratio of the mean of the largest distribution divided by the mean of the smallest distribution, for example, from 0.05 to 50. Such superhydrophobic surfaces with two or more widely different chemical species can also be multi-modal superhydrophobic surfaces.
Preferably, coating species are electrically insulating, and include at least two or more of the following, among others: SiO2, TiO2, TeO2, CeO2, Al2O3, calcium carbonate, barium sulfate, calcium phosphate and hydroxyapatite.
In another preferred embodiment, the present invention comprises methods of forming an inorganic, stable superhydrophobic surface. For example, the present invention can comprise a method of forming an inorganic, stable superhydrophobic surface comprising mixing one or more precursors and a solvent to form a first solution, reacting over time a mixed solution to form a reacted solution, applying the reacted solution to a clean substrate, and gelling the reacted solution on the substrate to form the inorganic, stable superhydrophobic surface, wherein the mixed solution is the first solution. This process can include another preferred step after the step of mixing one or more precursors and a solvent to form a first solution, wherein the solution of mixed one or more precursors and a solvent is then mixed with an acid and water, which such solution is then processed through the above reacting, applying and gelling steps.
In various preferred embodiments of these methods of forming an inorganic, stable superhydrophobic surface, eutectic liquids can be used, the one or more precursors are organometallic, the solvent is an alcohol, the step of mixing the one or more precursors and the solvent to form the first solution is run at a temperature of between 10-80° C., the acid is one of hydrochloric acid, sulfuric acid, phosphoric acid, chromic acid, oxalic acid, formic acid, and acetic acid, the step of mixing the acid and water in the first solution to form the second solution is run at a temperature of between 10-40° C., the step of reacting over time the mixed solution to form the reacted solution runs between 30 minutes and 8 hours, and the step of applying the reacted solution to the clean substrate is by one or more of dipcoating, spincoating and spray coating.
These methods of forming an inorganic, stable superhydrophobic surface can further include a step of cleaning the substrate prior to the step of applying the reacted solution to a clean substrate, wherein the step of cleaning the substrate includes one or more of Piranha solution cleaning, alkali/H2O2 cleaning, UV/ozone cleaning, and mechanical abrasion of the substrate. Further additional steps can include one or more of the fine-tuning the strength of the resultant inorganic, stable superhydrophobic surface by adjusting the ratio of the precursors if more than one precursor is used, the firing of the surface to strengthen the surface structure, and the post-treatment of the structured surface for improved hydrophobicity.
In yet another preferred embodiment, the present invention is a surface and method of forming a surface, wherein upon a coating, the surface achieves superhydrophobicity without any further surface treatment, herein termed “autophobicity”. For example, when using a suitable surface (such as that from the sol-gel process) to test coat the surface of silicone parts, the surfaces achieved superhydrophobicity without any further surface treatment. After the silica particles were dipcoated or painted on the silicone surface, and after a certain period of time, the surface changes from hydrophilicity, to hydrophobicity, and finally to superhydrophobicity.
The present invention further includes a surface and method of forming a surface that has the beneficial characteristic of the recovery of hydrophobicity.
In another preferred embodiment, the present invention improves the superhydrophobic effect (higher contact angle and lower hysteresis) through the post-treatment of a surface that displays a contact angle greater than 150°, with a coupling agent. The effectiveness of the coupling agent is enhanced if the agent contains chemical elements that are known to be hydrophobic in nature. Silanes such as trichloro or tri(m)ethoxyl silanes are preferred. The present treatment overcomes the drawbacks of conventional techniques, in that it may be accomplished at or near ambient temperatures and pressures. The ambient nature of the present treatment means that many geometries or sizes of device/insulator can be treated, thereby removing significant limitations of prior art techniques. The present treatment is compatible with one or more chemical species, and uni- or multi-modal superhydrophobic surfaces.
In a preferred form, the present invention comprises a method to prepare a superhydrophobic coating as a (super) protective coating for a wide range of devices. Coatings of this type can have a wide range of uses, and the substrate to which the same is applied can be varied, including polymers, ceramics, metals and glass. In particular, although not necessarily exclusive, by coating and etching polymer coating materials, the present invention provides a method to prepare superhydrophobic coatings, and prevent the problems of conventional coating systems.
In a preferred embodiment of the present invention, the dipcoating process can be used to coat the surface. The effects of the present invention are additive for subsequent coatings, it is believed due to the in-filling of the nanoparticles on the coatings structures. It has been found that repeated coatings develop an enhanced surface. For example, more than two (2) coatings, and more beneficially more than four (4) coatings, have been found preferred. In the repeat coatings, different methods of application (casting, dip coating, doctor blading, painting, spraying, etc.) may, if desired, be employed from those first employed.
The present invention preferably uses a sol-gel process to synthesize multi-modal, multi-species superhydrophobic surfaces in situ on preferably a dielectric surface.
Examples of dielectric surfaces include, among others: polymeric (filled and unfilled, thermoset and thermoplastic), glassy, ceramic, fresh (unexposed), aged (exposed such that the original water repellent properties have been degraded), and retreated to recover some water repellent properties.
These and other objects, features and advantages of the present invention will become more apparent upon reading the following specification in conjunction with the accompanying drawing figures.
The present invention comprises superhydrophobic surface coatings, and methods for forming same.
The present invention comprises an inorganic, stable under multi-factor ageing conditions, superhydrophobic surface. The surface preferably has a contact angle greater than approximately 150°, and a hysteresis below approximately 10°. The inorganic material can be selected from a group including, for example, germanium, gallium arsenide, silicon (oxide), titanium (oxide), cerium oxide, aluminum (oxide), copper (oxide), zinc oxide, indium tin oxide (derivatives). In preferred embodiments, the inorganic materials can be Si, Al or copper. The surface is multi-factor stable in that, multi-factor UV weathering tests according to ASTM D 4329 showed that the resulting surface retained a contact angle greater than approximately 150°, and more preferably 162°, after approximately 1,000 test hours. The surface preferable can maintain the over 150° contact angle even after 5,500 test hours, and a hysteresis below 10°.
The present invention can comprise a first method of forming an inorganic, multi-factor stable superhydrophobic surface with the following steps: mixing two precursors and a solvent to form a first solution; mixing an acid and water in the first solution to form a second solution; reacting over time the second solution to form a reacted second solution; applying the reacted second solution to a clean subsurface; and, gelling the reacted second solution on the subsurface to form the inorganic, multi-factor stable superhydrophobic surface. The strength of the resultant inorganic, multi-factor stable superhydrophobic surface preferably can be fine-tuned by adjusting the ratio of the precursors.
The precursors can be selected from the group including, for example, organometallic, and more preferably, for example, tetra organometallic and tri organometallic. The solvent is preferably from the group of, for example, alcohol, more preferably ethanol. The two precursors and the solvent are preferably mixed in a temperature range of 10-80° C.
The acid used in this method can be selected from the group including, for example, hydrochloric acid, sulfuric acid, phosphoric acid, chromic acid, oxalic acid, formic acid, and acetic acid. The acid and the water are preferably mixed in a temperature range of 10-40° C.
The reaction time of this method is preferably in the range of 30 minutes to eight (8) hours.
The reacted second solution can be applied to the clean subsurface by, for example, dipcoating, spincoating, painting, or spray coating.
The subsurface can be cleaned by one or more of the following steps, including piranha solution cleaning and alkali/H2O2 cleaning, UV/ozone cleaning, or via mechanical abrasion of the surface.
The reacted second solution can be gelled to the subsurface by, for example, base catalyzed reactions.
The present method can further include a firing process to strengthen the surface structure, and/or a post-treatment of the structured surface for improved hydrophobicity.
The present invention can comprise a second method of forming an inorganic, multi-factor stable superhydrophobic surface, utilizing nanoparticles as sacrificial templating agents. This method can comprise many of the steps of the first method, or more preferably the following steps: preparation of silica sol by typical acid catalyzed hydrolysis of tetraethoxy silane (TEOS) or tetramethoxy silane (TMOS); preparation of polymer nanoparticles from typical water emulsion polymerization, e.g. (poly(methyl methacrylate) (PMMA) or polystyrene (PS)); mixing the two preparations together or mixing the second preparation with a commercially available silica sol or metal oxide sol; dipcoating or spincoating to form films on substrates; gelling the solution on the surface to form the composite films; and, firing at an elevated temperature to decompose the polymer nanoparticles to form porous thin film. After these steps and preferably after post-treatment by coupling agents, an inorganic, multi-factor stable superhydrophobic surface results.
The precursors, solvents, and temperature ranges of this second method can be similar to the first method. Further, the firing process to strengthen the surface structure preferably is run at between 300-700° C. This second method preferably includes the post-treatment of the structured surface for improved hydrophobicity, and the sequential dipcoating in the as derived sol and polymer nanoparticle emulsion can also be employed to generate the surface structures.
The present invention can further comprise a third method using low vapor pressure liquids (a eutectic liquid mixture). Eutectic liquids are a group of liquids that show extremely low melting points upon mixing of two or more different high melting point agents. One group includes metal halides/substituted quaternary ammonium salt mixtures, for example, Li, Be, Na, Mg, Al, K, Ca, Ti(IV), V, Mn, Co, Ni, Ga(III), Y(III), Zr(IV), Mo(V), Ag, Cd(II), In(III), Sb(III), Hf(IV), W(IV), Au(III), Hg(II), Pb(II), Bi(III), Tin(II), Fe(III), Zinc(II), chromium(III), chloride, and bromide; urea(ethylene glycol)/quaternary ammonium salt (e.g., choline chloride) mixtures. The third method uses eutectic liquids as solvent and templating agents for the creation of surface structures by ambient spincoating, dipcoating or a spray coating method using sol-gel technology.
The sol-gel process temperature preferably is in the range of 10-100° C., and the reaction time preferably in the range of ten (10) minutes to one (1) week. After gelation, the liquid can be removed by, for example, solvent extraction, thermal decomposition, oxidation or evaporation at elevated temperatures. An ultra thin film will be formed, and a nanorough surface results due to the exposure of the structured gel on the surface. In one embodiment, the gelation time is in the range of less than one (1) hour to one (1) week. Post-treatment of the surface can be conducted to form superhydrophobic surfaces by implantation of self-assembled monolayers (SAMs).
In another preferred embodiment, the present invention comprises the manufacture of a superhydrophobic surface having a packing of particles defined by surface roughness, which can be uni-modal, or multi-modal. A method to prepare such a surface can be the sol-gel process. In a preferred form, this process does not necessarily require plasma treatment on polymeric insulations. Precursors used in the preparation of the surface can be selected from the follow groups, for example, organosilicate, organotitanate, organoaluminate, alkoxides of boron, and alkoxides of cerium. The sol-gel process involves the formation of inorganic nanoparticles under certain reaction conditions. A preferably temperature range is 10-80° C. The solvent can be alcohol.
After applying the coating on the surfaces, rough surface structures will be formed by the nanoparticles on the surface. The solvent evaporation needs to be controlled to prevent too densely packed nanoparticles (and resultantly not superhydrophobic). After a post-treatment, the surface will become superhydrophobic, self-cleaning and multi-factor stable.
A preferable particle size ranges from 30 nm to 5 μm. On some specially-designed surfaces, a post-treatment may not be necessary, and after coating of the nanoparticles, the surface can itself gain the superhydrophobicity from the underlying layers. After the damage or depletion of the topmost layer by the action of multi-factor ageing, the surface can recover to superhydrophobicity from the underlying layers.
In a multi-modal embodiment of this particle packing invention, the precursors used in the preparation of the surface can be selected from the follow groups, for example, organosilicate, organotitanate, organoaluminate, alkoxides of boron, and alkoxides of cerium. The process involves the formation of inorganic nanoparticles under certain reaction conditions, and therefore the size of nanoparticles can be well controlled. Preferably temperature ranges include 10-80° C., and the process utilizes an alcohol solvent is alcohol.
Mixing different (>2 particle size distribution) nanoparticles together form the multi-modal nanoparticles. Thus, after applying the coating on surfaces, rough surface structures will be formed by the nanoparticles on the surface. After a post-treatment, the surface will become superhydrophobic, self-cleaning and multi-factor stable.
In another preferred embodiment, the present invention comprises the manufacture of a superhydrophobic coating comprising multi-species particles. The manufacture of surfaces using multi-species can use the sol-gel process. In a preferred form, this process does not necessarily require plasma treatment on polymeric insulations.
Precursors used to form such a coating can be selected from the following group, for example, organosilicate, organotitanate, organoaluminate, alkoxides of boron, and alkoxides of cerium. This process involves the formation of inorganic nanoparticles under certain reaction conditions, and the temperature ranges and solvents are similar as previously disclosed.
Preparation of Nanoparticles is Under Controlled Conditions, for Example, Base, Salt, temperature and water. The preparation of the core-shell structure can be from seeded nanoparticle growth. Preparation of raspberry structures (multi-species) is through the addition of precursors from different species by controlling the time of addition of the second precursor, or the addition of surface functional groups for bonding between two different nanoparticles. Alternatively, different nanoparticles can be mixed together to form nanoparticles of multi-species. Preferably particle sizes range from 30 nm to 20 μm.
In another preferred embodiment, the present invention comprises a near-ambient temperature surface treatment using a coupling agent applied to a surface to increase the contact angle and decrease the hysteresis. This embodiment can utilize the sol-gel process, and can be uni-modal or multi-modal, and uni-species or multi-species. The process includes the use of a coupling agent to increase the contact angle and decrease hysteresis. This process is especially useful for glass and ceramic surfaces. The surfaces have original contact angles>40°, more preferably >90°, and more preferably >140°.
In this embodiment, near ambient preferably means temperatures of <200° C. and ambient pressure. The coupling agent can be selected from the group, for example, of (fluoro)alkyl silane or phosphate ester or (fluoro) alkyl carboxylic acid, and more specifically, the coupling agent is fluoroalkyl silane (trichloro-(1H,1H,2H,2H-perfluorooctyl) silane (PFOS)).
As is evident, the present invention comprises technologies that can create a surface/coating that does not require aggressive processing, and is stable. Such techniques include: the sol-gel process, providing a relatively simple and practical chemistry that can be used to create superhydrophobic surfaces, and not requiring a separate treatment to make them superhydrophobic; templating, which shows the promise of the sol-gel process, with the benefits of post-treatment to modify the surface energy by low surface energy moiety in the coupling agents, and the inherent stability of oxide surfaces; multi-modal (size) and multi-species embodiments that were seen to significantly increase the superhydrophobicity; and, post-treatment, which showed improved superhydrophobicity on polymer surfaces, and beneficially activates the sol-gel surfaces on glassy substrates.
A preferred superhydrophobic surface coating according to the present invention includes many beneficial attributes, including that it: is stable under harsh environmental conditions; has good compatibility with the surface upon which it is applied, for example, silicone and porcelain—glassy; remains adhered to the surface after expansion and contraction of the underlying device; is capable of application in a simple and practical way; does not degrade in the environment it is used in; and, brings practical benefits to the operation of the underlying device. In addition, the coating preferably both inhibits the formation of a continuous film of water, and removes dust with the high water flow on the surface.
The present invention can comprise a sol-gel process to manufacture superhydrophobic surfaces in situ. In a preferred form, this process uses a plasma treatment to enhance the surface compatibility for reliable superhydrophobicity.
The present invention can further comprise utilizing a surfactant (cationic, such as cetyl trimethylammonium chloride (CTAB), anionic such as sodium dodecyl sulfate (SDS), or nonionic such as pluronic (PO)x(EO)y(PO)z).
As described, the present invention can comprise many innovative methods to form the surface, including utilizing a low vapor pressure liquid (eutectic liquid mixture formed by the mixing, either as a single step within the formulation or as a pre mix, of two or more solid species with high melting points that leads to a dramatically reduced melting point of the mixture), as a solvent/solvent co agent and templating agent, wherein polymeric silica sols can be synthesized by acid/base catalyzed reaction.
During the film formation process, the solvent stays in place without appreciable loss. It is then gelled under a base (ammonia hydroxide) environment to form a strong film. The density of the film can be reduced after extraction of the eutectic liquid.
Correspondingly, the index of reflection can be tuned to form an anti-reflective layer for enhanced transparency. The low pressure liquid can also act as a template to form mesopores in the film that further reduce the density, dielectric constant, and refractive index.
After the eutectic liquid is extracted with ethanol, hexane and dried under controlled conditions, the surface can be treated with silanes, and superhydrophobic self-cleaning and multi-factor resistant films are thereafter produced. Alternatively, the eutectic liquid can be burned out at elevated temperature to form porous silica films.
In yet another preferred embodiment, the present invention comprises achieving superhydrophobocity by incorporating hydrophobic groups, such as hydrocarbon and fluorocarbon into/on inorganic material during or after a two step acid/base catalyzed sol-gel processing method, which is applied to make microstructure surfaces.
In a first method, a sacrificial polymer emulsion with specific surfactants is used to template and control the two scale distributed pores in the silica. The polymer particle size can be well controlled over the range of, for example, 50-500 nm during the emulsion polymerization. The formation of mesospores between 5-30 nm can then be controlled by self assembly of the surfactants.
The polymer and surfactants are then decomposed at elevated temperatures to generate the desired pores. This superhydrophobic coating can easily be created by dipcoating the object in the sol/polymer solution, firing the resulting structure, and subsequently undergoing a post-treatment to form SAMs. The film thinness can be controlled by the dipping rate and the concentration.
In a second method, a coprecursor sol-gel solution is applied to the substrate, where at least one precursor has hydrophobic groups (for example, a hydrocarbon chain or a fluorocarbon chain) attached to the Si atom. A specific reaction procedure is used in order to form well-ordered particle structures.
The two scale roughness can be created by the silica nanoparticle formed and the pores due to phase separation, because of the presence of the hydrophobic groups. For this method, the dipcoating environment needs to be relatively tightly controlled in order to control the solvent evaporation rate. The film strength can be fine-tuned by adjusting the ratio of the precursors and the dipcoating conditions.
For both of these methods, the firing process can be applied in order to further strengthen the film. After the firing process, a SAM treatment can be applied. Generally, the SAM can be fluoro/hydrocarbon trichlorosilane, fluoro/hydrocarbon trimethoxysilane or fluoro/hydrocarbon triethoxysilane. The fluorocarbon or hydrocarbon chain has 1-18 carbons, more preferably 10-18 carbons.
A specific nonpolar solvent can be selected, and a post two-stage thermal treatment applied, in order to form strong SAMs and make low surface energy coatings.
The wetting of a solid with water, with air as the surrounding medium, depends on the relation between the interfacial tensions water/air, water/solid and solid/air. The ratio between these tensions determines the contact angle of a water droplet on a given surface, and is described by Young's equation (Equation 2). If a droplet is applied to a solid surface, it will wet the surface to a certain degree. At equilibrium, the energy of the system is minimized, which can be described by the Young's Equation:
where γSL, γSV, and γLV are the interfacial free energy per unit area of the solid-liquid (SL), solid-vapor (SV), and liquid-vapor (LV) interfaces, respectively. θ is the contact angle for a smooth surface.
Young's Equation 2 can only be applied to a flat, smooth surface, yet such a surface rarely existing for solids. As previously discussed, when a water drop is placed on a lotus plant surface, the air entrapped in the nanosurface structures prevents the total wetting of the surface, and only a small part of the surface, such as the tip of the nanostructures, can contact with the water drop. Air is enclosed between the wax crystalloids, forming a composite surface. This enlarges the water/air interface while the solid/water interface is minimized. Therefore, the water gains very little energy through adsorption to compensate for any enlargement of its surface. In this situation, spreading does not occur, the water forms a spherical droplet, and the contact angle of the droplet depends almost entirely on the surface tension of the water.
As previously discussed, the contact angle at a heterogeneous surface, and thus the one that is measured in practice can be described by Cassie's equation, Equation 1, where f is the remaining area fraction of the liquid-solid interface, and r is the Wenzel roughness ratio (or ratio of the real surface to the projected surface) of the wet area. Due to the different growing/treatment mechanism, the f and r can be very different for the Lotus Effect surfaces, leading to difference in water contact angle even if the surface chemistry is similar.
Besides the water contact angle, the hysteresis should also be considered in determining the surface hydrophobicity. The sliding angle and driving force needed to start a drop moving over a solid surface can be described via Equations 3 and 4, respectively:
In Equation 3, α is the sliding angle, m is the weight of the water droplet, w is the width of the droplet, γLV is the surface tension of the liquid, and the θA and θR are the advancing and retreating contact angles, respectively. In Equation 4, F is the critical line force per unit length of the drop perimeter. These equations indicate how the difference between the contact angles on a sloping surface (the hysteresis) affect the water repellence (hydrophobicity).
(1) The surface is superhydrophobic so that water drops have very large contact angle and small sliding angle; and
(2) The adhesion between the water drop and dust particles is greater than the adhesion between the surface and dust particles.
Adhesion of two components, such as adhesion of dust or dirt to a surface, is generally the result of surface-energy-related parameters representing the interaction of the two surfaces which are in contact. In general, the two contacted components attempt to reduce their free surface energy.
Strong adhesion is characterized by a large reduction in free surface energy of two adhered surfaces. On the other hand, if the reduction in surface free energy between two components is intrinsically very low, it can generally be assumed that there will only be weak adhesion between the two components. Thus, the relative reduction in free surface energy characterizes the strength of adhesion. And it was described by the Laplace-Dupre equation with work of adhesion (wa)
w
a=γ(1+cos θ) Equation 5
where γ is the surface tension of liquid that is in contact with the surface and θ is the Young's contact angle.
Usually dust particles include materials having higher surface energies than the surface materials, they are generally larger than the surface microstructure, and they just contact with the tips of these microstructures. This reduced contact area minimizes the adhesion between the lotus leaf surface and dust particles, so the particles can be picked up and removed from the leaf surface by the water droplet, which is contacting the whole area of the particle surface. Therefore, it is likely that hydrophobic particles are less likely be removed by water droplets than hydrophilic dust particles on a lotus leaf, and small particles, which have a size close to or even smaller than the microstructures, possibly will be pinched in the microstructures, instead of being removed by water droplet.
Several methods for the preparation of Lotus Effect surfaces are disclosed, including templating, sol-gel, multi-modal, multi-species, pre-treatment, and post-treatment.
Templating Technology
Templating in the context of these surfaces means using one material (nanoparticle or surface micron/nanostructure) as a template to generate microstructures in another material. Two methods include:
(1) sacrificial polymer (polymethyl methacrylate, ((PMMA)) nanoparticle templating; and
(2) rough surface imprinting of a silicone surface.
In templating by polymer emulsion suspension, focus is drawn on a PMMA/surfactant emulsion particle mixture that is coated on the insulator surface. Here, templating refers to using sacrificial materials (PMMA) to generate porous materials (silica). The PMMA is treated with extreme conditions to leave a porous substrate. This process is shown schematically in
A first step is to prepare the nano PMMA particles. This can be done with emulsion polymerization conducted by the control of the reaction conditions and formulation. The synthesis of silica sol, accomplished using acid catalyzed hydrolysis and condensation, can be employed in order to first make linear and branched silica without gelation (for longer shelf life of the mixture). Then, three components (nanoparticle emulsion, the silica sol and a surfactant) are mixed. The mixture could be applied by either dipcoating or painting a thin film formed on the insulator surface (a microscope glass slide can be used) by evaporation of solvent and water.
The preparation of the PMMA polymer emulsion can use methyl methacrylate (MMA) monomer, initiator potassium persulfate (KPS), surfactant, and sodium dodecyl sulfate (SDS). Triton X can be added in order to generate a mesoporous structure. The structures of KPS, SDS, and Triton X reagents are shown in
The pores give the required roughness, thus to understand how this technology operates, it is important to measure the pore structure. In order to characterize the porous film, the Brunauer-Emmett-Teller (BET) surface area measurement method was conducted using a Gemini 2375, Micrometrics device. In the measurement, the film on glass slides was measured without getting the films off the surface. The BET equation is:
where p is the vapor pressure, p0 is the saturation vapor pressure, n is the adsorbed molecules in mole, nm is the monolayer capacity in mole, c is a constant that is temperature dependent. Through the Kelvin equation, the following is provided:
where r is the curvature of pores, Vm is the molar volume of adsorbate (nitrogen in this example), and γ is the surface tension of the nitrogen at the temperature. When the vapor pressure p is known, the pore radius can be calculated according to the Kelvin equation, and thus the pore diameter and its distribution can be measured.
The results are shown in
When PFOS treatment is applied, the results in TABLE 3 are seen, illustrating the effect of pore size on the contact angle of PFOS-treated polymer templates on a glass substrate.
Templating can be run by polymer emulsion suspension, or by individual dipcoating of individual polymer suspensions. In templating by polymer emulsion suspension, the components are combined in a single liquid. A very similar effect can be obtained by avoiding the mixing of the chemical components, and dipcoating the insulator in each component separately. Such an approach would have practical interest as it would preserve the components in a production environment.
Templating can also be run by imprinting from rough surfaces. This approach takes a partly cured polymer surface and then impresses a hard surface with a prefabricated rough surface. In one embodiment, the polymer system can be silicone (polydimethyl siloxane), for example, curable SYLGARD 184 (from Dow Corning), which includes a base polymer and curing agent. Before curing, it is liquid so it is relatively easy to process structures of various sizes and shapes. After curing, the silicone becomes solid, and the surface structure can be maintained.
The process description preferably is as follows: after mixing, the base mix (silicone resin/catalyst with curing agent) is spun coated at 4000 rpm to make a thin (10-20 μm) film on a pre-cleaned glass slide. Prior to the spin coating, the glass surface can be treated with ally triethoxysilane (ATES) to ensure good adhesion of the silicone to the slide. The molding rough surface, which is pretreated with PFOS, is then imprinted onto the silicone surface to make a replica rough surface on the silicone film.
The glass slide, silicone, template sandwich is placed in a vacuum at room temperature for two (2) hours. This step can be important to ensure that all of the trapped air is removed from the rough template surface. After the vacuum treatment the silicone is cured at 100° C. for 45 minutes, the template is then peeled off to reveal the imprinted surface. As long as the surface of the template is not damaged during the un bonding, then it may be reused. In preferred embodiments, two templates can be used, for example, commercially available copper foil for FR-4 board, and an etched and oxidized aluminum (to give aluminum oxide) film (AAO).
Copper foil with a controlled roughness surface (
One method for generating rough silicone surface (replica molding) can include using copper foils with rough surfaces as the template for castable/mouldable dielectric materials (epoxy, polyester resins and silicone elastomers (filled and unfilled). The copper surface was first treated with fluoroalkyl silane (PFOS) as a mold release agent. In this case, curable silicone SYLGARD 184, was used as the castable/mouldable material. The SYLGARD was employed with a resin to curing agent ratio of 10:1. Silicone can be applied to the template (copper sheet) by many convenient methods, such as painting, spin coating, dipcoating and molding.
The material is transformed into a thermoset state, and in the case of the silicone elastomer, it was cured at 100° C. under vacuum for between 45 min-2 hrs. Then, the silicone surface was released from the rough copper foil surface and the replicate surface was formed. The surface showed a contact angle of over 150°, and a hysteresis of below 10° (TABLE 4).
In common with many other elastomeric materials, the surface would be expected to benefit from diffusion driven autophobicity—the time dependent post production improvement in hydrophobicity.
A rough surface can also be created in situ on a mould tool. The most appropriate starting material for the mould is aluminum, which can be readily converted to a porous alumina surface (
Anodic porous alumina has a packed array of columnar hexagonal cells with central, cylindrical, uniformly sized holes ranging from 4 to 200 nm in diameter.
Through the anodic oxidation of aluminum in sulfuric acid, the surface was changed from aluminum to aluminum oxide, and surface roughness was created as shown in
AFM studies (
TABLE 5 shows the multi-factor ageing performance of templated surfaces. An interesting result is that for the alumina surface, the basic structure is likely to be relatively immune to the effects of UV component of the multi-factor exposure. However, the hydrophobicity comes from the PFOS treatment, and thus the fall in performance must be ascribed to the change in surface chemistry, i.e. loss of the hydrophobic groups, rather than the change in the surface itself.
This is an interesting comparison with plasma modified surfaces (
Although the loss of water repellency after 1000 hours precludes this approach from practical use, it is worth while noting that earlier experiments with polymer surfaces had inferior initial properties, and showed a drop to these levels in 100 to 200 hours.
Sol-Gel Technology
In prior experiments, superhydrophobic coatings were developed on polybutadiene surfaces using a SF6 or CF4 plasma treatment. However, one of the more serious problems for a polybutadiene surface is that it is very sensitive to UV irradiation.
As can be seen from the SEM image (
The templating experiments with alumina and the polymer template showed that it was possible to create an inorganic surface. Furthermore, these surfaces can be treated to give the correct chemistry. Thus, a method that grew an inorganic coating on the surface of the insulator was pursued. The technology selected for this was the sol-gel process.
The sol-gel process is an established process for making glass/ceramic materials. The sol-gel process involves the transition of a system from a liquid (the colloidal “sol”) into a solid (the “gel”) phase. The sol-gel process allows the fabrication of materials with a large variety of properties: ultra-fine powders, inorganic membranes and thin film coatings.
The sol is made of solid particles of a diameter of few hundred nanometer, usually inorganic metal salts, suspended in a liquid phase. In a typical sol-gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, then the particles condense in a new phase, the gel, in which a solid macromolecule is immersed in a solvent.
There are numerous application areas for sol-gel technologies. One of the largest application areas is thin films, which can be produced on a piece of substrate by spin-coating or dip-coating. Other methods include casting, painting, spraying, electrophoresis, inkjet printing or roll coating. Optical coatings, protective and decorative coatings, and electro-optic components can be applied to glass, metal and other types of substrates with these methods.
The sol-gel approach involves the creation of surface roughness (fractal surface) by the evaporation of solvent (ethanol) after the formation of silica gel skeleton (gelation through silanol condensation). The surface hydrophobicity was achieved by the incorporation of the hydrophobic groups. The individual components are termed precursors (e.g. tetraethoxysilane, tetramethoxysilane, isobutyl trimethoxysilane, etc.).
When two or more precursors are combined to make the process more practical, the process is termed a coprecursor process. The second precursor contains the hydrophobic hydrocarbon/fluorocarbon group, e.g., TMOS-IBTMOS, TFPS-TEOS, etc., and at least one of the precursors contributes hydrophobic groups, such as isopropyl, trifluoropropyl groups, etc.
TMOS (Precursor), IBTMOS (Coprecursor), and ethanol are first mixed together using the amounts given in TABLE 6 under “Material Example I.” HCl (0.1 M) is added to adjust the pH of the mixture to around 1.8-2.0. The reaction is started by heating to 60° C., and then holding for five (5) hours. After the reaction NH3H2O, 1.1M, a base, was added to the solution to initiate gelation of the polymer.
Before complete gelation, the solution may be cast onto a suitable substrate (microscope glass slide, elastomer, etc) to form a thin layer. The surface was covered to ensure slow evaporation of the ethanol and ammonia. After two (2) days, the film was completely gelled and the ethanol was completely evaporated. A thin silica layer was left on the substrate surface. Implementation on a glass microscope slide showed that the surface was superhydrophobic directly after coating due to the hydrophobic side chains present in IBTMOS coprecursor (
Physical characteristics of solutions resulting from different sol formulation:
Contact angle of films resulting from various formulations of bulk silica layers:
Although superhydrophobic surfaces may be attained (TABLES 7-8) using the correct formulations, the multi-factor ageing resistance of the IBTMOS version is only sufficient for indoor or protected environments. This limitation is most probably due to the existence of the tertiary carbon on the isopropyl chain. After the multi-factor ageing (QUV) test for 1000 hours, the superhydrophobicity was found to be lost (contact angle<<150° and hysteresis>>10°) and the water droplet was completely dispersed over the surface. The multi-factor ageing (QUV) performance was improved by the incorporation of a stable chain into the chain backbone.
TEOS (Precursor), TFPS (Coprecursor), and ethanol are first mixed together using the amounts given in TABLE 6 under “Material Example II.” HCl (1 M) is added to adjust the pH to around 1.8-2.0. The reaction was started by heating to 60° C., where the temperature was maintained for five (5) hours. After the reaction, 0.1 g ammonia hydroxide (29% wt) was added (1.1 M) to 2 g solution for gelation of the polymer.
Before complete gelation, the solution was cast onto a suitable substrate to form a thin layer. The surface was covered to ensure slow evaporation of the ethanol and ammonia. After two (2) days, the film was gelled and the ethanol was evaporated. A thin silica layer was left on the substrate, of which a suitable illustrative example is a glass microscope slide.
The surface is superhydrophobic due to the hydrophobic side chains present in TFPS coprecursor (
Initially, a eutectic liquid was formed by mixing choline chloride and urea together in a ratio of 2:1
A formulation is described in TABLE 6 under “Material Example III.” Tetraethoxysilane (TEOS—Precursor): 0.6 g, eutectic mixture (C—U): 1-6 g, ethanol: 1.5-3 g, 1M HCl aqueous solution: 0.3 g are all mixed together. Hydrolysis and condensation occurs after the addition of HCl to the mixture, and stirring for three (3) hours. The solution was coated onto a s suitable substrate. In this example the solution was then spin coated onto one (1) square inch glass microscope slides at 3000-6000 rpm to form uniform films. The coated glass slide was placed in a desiccator with a container of 1 ml ammonia (29%) at the bottom, to promote gelation. After two (2) days, the glass slide was removed from the desiccator and extracted with absolute ethanol for three (3) hours to remove the eutectic liquid in the film, and thus yield a porous thin film. The transmittance of the film coated glass slide from UV-Visible is comparable to the transmittance of glass slides or better than that of glass slides. The contact angle was 171.0° and hysteresis was <4°.
Initially, a eutectic liquid was formed by mixing choline chloride and urea together in a ratio of 2:1. An alternate method mixes the appropriate amounts of choline chloride and urea with the main formulation components.
Another formulation for the eutectic method (not disclosed in TABLE 6) was: tetromethoxysilane (TMOS—Precursor): 0.5 g, choline chloride-urea (C—U): 3 g, 1M HCl aqueous solution: 0.3 g. Hydrolysis and condensation occurs after the addition of HCl to the mixture, and stirring for three (3) hours. The solution was then spincoated onto one (1) square inch glass microscope slides at 3000-6000 rpm to form uniform films. The coated glass slide was placed in a desiccator with a container of 1 ml ammonia (29%) at the bottom, to promote gelation. After two (2) days, the glass slide was removed from the desiccator and extracted with absolute ethanol for three (3) hours to remove the eutectic liquid in the film, and thus yield a porous thin film. The transmittance of the film coated glass slide from UV-Visible is comparable to the transmittance of glass slides or better than that of glass slides. The contact angle was 170.8° and hysteresis was <4′. Similar hydrophobicity improvements would result for alternate substrates such as elastomers, plastics and resins.
Tyzor TPT (Precursor) was used for the reaction described in TABLE 6 under “Material Example IV.” Initially, the eutectic liquid (C—U) is mixed with an ethanol/water mixture (2.5 ml of water in 58.5 ml C—U with between 0.5 to 2.5 ml of ethanol), then TPT is slowly added TPT, after the final addition of the TPT, the mixture is continually stirred for two (2) hours. This will result in a solution that is suitable for the desired application process. The solution can be coated on both ceramic surfaces and polymeric surfaces. The surfaces may be either treated as received or after surface treatment by plasma or UV/ozone.
After the coating was made, it was then exposed to an ammonia atmosphere for four (4)-48 hours. Afterwards, the surface was rinsed with ethanol to remove the eutectic liquid, and a rough surface resulted. Then, the surface can be treated hydrophobic by plasma deposition, SAM treatment or other surface treatment methods that may make the surface hydrophobic. After the surface was treated, the contact angle on the surface was over 145°.
Multi-Modal Size Distributions
The Lotus leaf has a very complicated surface that is rough on the microscopic and nanometric scale. The manufacture of a multi-modal (in size) surface using the sol-gel process was investigated.
Synthesis of monodisperse silica spheres is accomplished using the Stöber-Fink-Bohn method that involves using base (ammonia) as catalyst. The production of a uni-modal distribution is first described.
TEOS at 60° C. undergoes hydrolysis to form silanol groups that are further catalyzed under ammonia to undergo condensation between each other to form siloxane polymers (branched or linear depending on the reaction conditions) dissolved in solvent. When the polymer particles are large enough, it can form aggregates which are separated from the solvent by the negative surface charges.
At the beginning, the surface charge is not large enough to repel the aggregates, so the aggregates undergo particle growth to form larger ones by the Ostwald ripening process with smaller particles dissolving and redepositing on large particles until uniform size was achieved. When more TEOS is added, the particle can further grow to form a larger one, with the reaction between newly added hydrolyzed TEOS and the silica sphere surface silanol groups (seed growth). The particle size was controlled by catalyst (pH), water concentration and TEOS concentration.
Multi-modal size distributions are achieved by growing two or more uni-modal distributions separately and then mixing them together. After the coating process (dipcoating or painting) on the desired surface, surface roughness can be created.
Controlling the amount of ammonia hydroxide, water, TEOS, and the reaction temperature, the particle size of silica can be controlled. Equation 8 gives the average diameter (d) in terms of the concentrations (mol/l) of water (H2O), Ammonia (NH3) and TEOS in room temperature, and thereby shows how the size distributions might be controlled.
d=((82−151NH3+1200NH32−366NH33)√{square root over (TEOS)})H2O2exp((0.128NH32−0.523NH32−1.05)√{square root over (H2O)}) Equation 8
The surface structure (morphology and surface roughness), has been analyzed mainly using scanning electron microscopy (SEM).
When using tri-modal or more particle sizes, the silica spheres will not form a densely arrayed structure on the surface. As can be seen in the figures, the surfaces are rougher than the densely packed ones. And as a result, the surface contact angle was higher (superhydrophobicity was achieved) than the uni-modal distribution particles.
TABLE 10 shows a range of potential size ratios:
The general results for the multi-modal approach can be summarized as shown in
The different size distributions are determined by control of the process conditions. The below variants (a), (b), (c) and (d) disclose the basic approaches using precursor sol gels.
(a) Following TABLE 6 under “Material Example V,” mix TEOS 6 ml, ammonia hydroxide (25% V/V) 15 ml, and absolute ethanol 200 ml at 60° C. and keep stirring for five (5) hours. A silica colloid resulted with a diameter of 80 nm with a polydispersity of 16%.
(b) Following TABLE 6 under “Material Example V,” mix TEOS 3.5 ml, ammonia hydroxide (25% V/V) 4 ml, and absolute ethanol 50 ml at 60° C. and keep stirring for five (5) hours. A silica colloid resulted with a diameter of 300 nm with a polydispersity of 10%.
(c) Following TABLE 6 under “Material Example V,” it is possible to use a seeded growth approach. Use the procedure (a) to prepare the seed solution. Then TEOS and H2O (7 ml/1.2 ml) are repeatedly added to the seed suspension after the seed suspension has stopped reacting at a five (5) hour interval. The final particle sizes after ten (10) additions was 197 nm. By changing the initial reaction condition and the repeat condition, a series of silica particles was prepared as shown in TABLE 10.
(d) Following TABLE 6 under “Material Example V,” mix TEOS 6 ml, ammonia hydroxide (25% V/V) 15 ml, and absolute ethanol 200 ml at room temperature and keep stirring, at the same time increase the temperature from room temperature to 60° C. in 45 min, then keep stirring for four (4) hours. A silica colloid resulted with polydispersity that was shown in the following SEM image in
Preparation of Multimodal Colloidal Silica and Coating on Silicone Surfaces: Mix Two or more silica colloidal particles prepared using steps (a), (b), (c) and (d) above in the appropriate mixtures to deliver the appropriate different particle sizes. The resultant materials may then be dipcoated onto a suitable substrate; in this example, a silicone surface (cured SYLGARD 184 film) was chosen to be treated with the colloidal mixtures.
After some time (3-4 hours), the contact angle of the surface will change from superhydrophilicity (below 10°) to superhydrophobicity (>150°). This shows the autophobicity of the surface, which implies the self-recovery of the surface hydrophobicity at some circumstances was lost. This was confirmed by the multi-factor (QUV) aging test. The surface may show improved results if the silicone surface was pretreated by O2 plasma, UV/Ozone or some appropriate methods.
Multimodal surfaces can be prepared from mixing the appropriate amounts of the different variants detailed above ((a), (b), (c) and (d)). A selection of practical preparation details are provided below (1-iv):
(i) Dipcoat a glass slide with a colloidal silica mixture of 850 nm and 350 nm. After the solvent was dried, the contact angle was measured, and water droplets spread on the surface indicating hydrophilic (low contact angle performance). Then, the as-coated surface was post-treated with PFOS. This post-treatment resulted in a contact angle of 167.0° and a hysteresis of 4.5°, which showed a superhydrophobic surface.
(ii) Dipcoat a silicone surface with a colloidal silica mixture of 850 nm and 350 nm particle sizes. Immediately after the solvent removal, contact angle measurements demonstrated that water droplets spread on the surface indicating hydrophilic behavior (low contact angle performance). After four (4) hours at ambient conditions with no subsequent post-treatment, a contact angle of 165.6° and hysteresis of 4.7° was achieved, which indicated a superhydrophobic surface. The generation of hydrophobicity after production of the surface herein is termed “autophobicity”. Environmental ageing (multi and single factor) on these silicone substrate surfaces showed that the contact angles directly after removal are in the ranges of 158° to 170° (after 1000 hours), and 160° to 170° (after 2000 hours). Multi-factor (condensation, temperature, UV) ageing shows a higher degradation at 1000 hours (contact angle directly after removal of between 158° to 162°), whereas single (temperature) ageing shows lower degradation (contact angle directly after removal of between 165° to 170°). These silicone surface examples display the commonly observed recovery of hydrophobicity with time, wherein after multi-factor ageing for 1000 hours, the hysteresis falls from 12° to 8° after six (6) days of ambient recovery.
(iii) Dipcoat an EPDM (known to contain diffusible/mobile species such as low molecular weight species, oil, waxes, etc.) surface with a colloidal silica mixture of 850 nm and 350 nm. Immediately after the solvent removal, the contact angle was measured, and water droplets spread on the surface indicating hydrophilic behavior (low contact angle performance). After four (4) hours at ambient conditions with no subsequent post-treatment, a contact angle of >150° and hysteresis of <10° was achieved, which indicated a superhydrophobic surface. The hydrophobicity can be further enhanced through the use of the post-treatment approach disclosed in step (i) above.
(iv) Dipcoat an EPDM (without diffusible/mobile species) surface with the as prepared colloidal. After the solvent removal, the contact angle was measured and water droplets spread on the surface indicating hydrophilic behavior (low contact angle performance). In this situation, no improvement in hydrophobicity occurred with time. It is believed that the absence of autophobicity may be explained by the absence of diffusible or mobile species within the substrate matrix. Furthermore, it is likely that the surface will achieve superhydrophobicity using the post-treatment approach disclosed in step (i) above.
In some instances it is advantageous to functionalize the surface in situ during the coating manufacture. One advantage of such functionalization is that it promotes the interparticle adhesion. The preparation procedures for surface functionalized nanoparticles are shown below:
(i) Mix TEOS 6 ml, ammonia hydroxide (25% V/V) 15 ml, and absolute ethanol 200 ml at 60° C. and keep stirring for five (5) hours. To the reaction solution add a mixture of the aminopropyltriethoxysilane (APS) APS/ethanol (0.2 ml/10 ml) solution, then stir for 12 hours at 60° C. under an N2 atmosphere. The silica surface functionalized with amino groups is prepared directly when applied to the substrate. The autophobicity and post-treatment processes previously disclosed may not be essential to achieve improved hydrophobicity, but can serve to improve performance of the surface.
(ii) Mix TEOS 3.5 ml, ammonia hydroxide (25% V/V) 4 ml, and absolute ethanol 50 ml at 60° C. and stir for five (5) hours. A silica colloid resulted with a diameter of 300 nm and a polydispersity of 10%. The silica particles were centrifuged from the ethanol, and then subsequently washed with ethanol. The process was repeated, in this example four (4) times proved to be sufficient, and then the colloid was redispersed in toluene. The silica/toluene mix was then dissolved in a solution of glycidoxy propyl trimethoxysilane (GPS)/toluene (0.1 ml/5 ml), at room temperature, and continuously stirred for 24 hours under N2 atmosphere. The resulting particle surface was then functionalized with epoxy groups. The autophobicity and post-treatment processes previously disclosed may not be essential to achieve improved hydrophobicity but can serve to improve performance of the surface.
(iii) Mix TEOS 3.5 ml, ammonia hydroxide (25% V/V) 4 ml, and absolute ethanol 50 ml at 60° C. the mixture is kept stirring for five (5) hours. A silica colloid resulted with a diameter of 300 nm with a polydispersity of 10%. The reaction mixture was then added to a solution of allyl trimethoxysilane/ethanol (0.3 ml/10 ml). This was then stirred for 12 hours at 60° C. under N2 atmosphere. The silica surface was then functionalized with vinyl groups. The autophobicity and post-treatment processes previously disclosed may not be essential to achieve improved hydrophobicity, but can serve to improve performance of the surface.
The surface functionalized silica (amino-, epoxy-, vinyl-; steps i to iii above) can be applied in many variations to functionalize silicone films (epoxy-PDMS, amino-PDMS, silyl-PDMS). The resulting surface adhesion is markedly improved.
Porous Silica Particles For Silicone Superhydrophobic Coating
Autophobicity, post-treatment and recovery after multi-factor ageing are all important elements of a superhydrophobic surface and the attendant substrate. Thus, if the particulate surfaces resulting from the synthesis of inorganic surfaces can be engineered to maximize these features, hydrophobicity will be improved. One convenient method is to synthesize a porous structure for the inorganic particles. It is believed that such a structure will aid autophobicity, post-treatment and recovery after multi-factor ageing through the provision of reservoirs close to the surface.
It has been found convenient to use the Stöber method for the synthesis of silica particles. After adding templating agents like Pluronic series surfactant (e.g. P123), or Triton X 100, porous silica nanoparticles will be generated. The application of the particles on silicone surface by a method describe above produces a surface that will automatically convert to a superhydrophobic surface (contact angle over 150° and hysteresis below 10°), and maintain the superhydrophobicity for prolonged time (over 1000 hours).
Mix TEOS 3 ml, ammonium hydroxide 7.5 ml, absolute ethanol 100 ml and Pluronic P123 3 g at 55° C. and keep stirring for five (5) hours. A mesoporous silica nanoparticles colloid will result. The particles are then centrifuged out of the ethanol, and are then washed with absolute ethanol. The centrifuge and wash procedure is repeated, preferably 4-5 times, which was often sufficient. The particles are redispersed in absolute ethanol for application (dipcoating) on silicone rubber surfaces. The surface showed change from superhydrophilicity to superhydrophobicity after 3-6 hours—the autophobicity effect. Other surfactants, such as Triton X 100 and cetyl trimethyl ammonium bromide can also be used to prepare the mesoporous silica particles. The trimethyl benzene can be used to control the pore size.
Multi-Species
In order to achieve better adhesion between the coating and the substrate, better self-cleaning, and also incorporate more functionality into the surface coating, a second species was added to the conventional uni-species applications. Experiments were mainly focused on TiO2 by the sol-gel process to form TiO2 (core)/SiO2 (shell) or SiO2 (core)/TiO2 (shell) particles.
Monodisperse spherical TiO2 particles were first prepared by controlled hydrolysis of titanium tetraisopropoxide in ethanol. An ethanol volume of 100 ml was mixed with 0.4-0.6 ml of aqueous salt (NaCl), followed by addition of 2.0 ml of titanium tetraisopropyloxide at ambient temperature under inert gas atmosphere, using a magnetic stirrer. Reagents had to be mixed completely so that nucleation would occur uniformly throughout the solution. Depending on the concentration, visible particle formation started after several seconds or minutes, and gave a uniform suspension of TiO2 beads. After five (5) hours the reactions were finished, and the spheres were collected on a Millipore filter and washed with ethanol.
In one embodiment, this above process is amended for multi-modal particles by:
(i) At the beginning of the TiO2 particle synthesis, add silica sphere/ethanol dispersion (˜5% wt, 10 ml) (150 nm), which was synthesized using the Stöber method—the TiO2/SiO2 (core) particles can be formed as shown in
(ii) After the synthesis reaction of TiO2 particles started for 30 minutes, silica sphere/ethanol dispersion (˜5% wt, 10-50 ml) was added to the reaction media for another four and one-half (4.5) hours. The particles SiO2 (shell)/TiO2 (core) can be formed as shown in
(i) Mix TEOS 4 ml, ammonia hydroxide (25% V/V) 4 ml, and absolute ethanol 50 ml and deionized water 3 ml at 60° C. and keep stirring for five (5) hours. In another reaction vessel, mix absolute ethanol 100 ml, sodium chloride (0.1 mol/L) 0.4 ml, and add into the solution 2 ml TPT within 10 min. Then add the prepared (TEOS-based) silica particle colloid to the TPT based solution, keep stirring at room temperature for 48 hours. The resultant mixture will provide a multi species TiO2/SiO2 particle solution for further application.
(ii) Mix TEOS 4 ml, ammonia hydroxide (25% V/V) 4 ml, and absolute ethanol 50 ml and deionized water 3 ml at 60° C. and keep stirring for five (5) hours. In another reaction vessel, mix absolute ethanol 100 ml, sodium chloride (0.1 mol/L) 0.4 ml, and add into the solution 2 ml TPT within 10 min, and then keep stirring for four (4) hours, TiO2 particle size is around 800 nm. Then, add the as prepared silica particle colloid, and keep stirring at room temperature for 48 hours. The SiO2/TiO2 particles were formed.
(iii) Dipcoat a glass slide with the as prepared colloidal. After solvent was dried, the contact angle was measured, and water droplets spread out on the surface, thus indicating a low contact angle and hydrophilic property. Then, the as coated surface was post-treated with PFOS. After this treatment, a contact angle of 169.5° and hysteresis of 2.0° was achieved, which showed a superhydrophobic surface.
When an elastomeric (silicone) surface was substituted for the glass slide described in step (iii) above, the same coating method was used. However, after the solvent was dried from the surface, the contact angle was immediately measured, and the water droplets spread on the surface. After four (4) hours at ambient conditions, the autophobic process caused the contact angle to increase to 168.2.6° and provided a hysteresis of 2.5°, which showed a superhydrophobic surface.
Similar results occurred with an EPDM (with oil) surface that gave an autophobic contact angle of 166.8° and hysteresis of 2.6°, which showed a superhydrophobic surface. An EPDM (without oil) surface gave a completely hydrophylic surface that did not show any autophobic benefit. Subsequent treatment with a silane would be expected to develop superhydrophobicity.
(i) Mix TEOS 6 ml, ammonia hydroxide (25% V/V) 15 ml, and absolute ethanol 200 ml at 60° C. and keep stirring for five (5) hours. A silica colloid resulted with a diameter of 80 nm. The APS/ethanol (0.3 ml/5 ml) solution was added to the original solution and then stirred for 12 hours at 60° C. under N2 atmosphere. The silica surface functionalized with amino groups was prepared.
(ii) In a second reaction vessel, mix absolute ethanol 100 ml, sodium chloride (0.1 mol/L) 0.4 ml, and add into the solution 2 ml TPT within 30 min, and then add glycidoxy propyl trimethoxysilane (GPS)/ethanol solution (0.25 g/5 g) into the mixture and stir for 24 hours. The TiO2 surface was then functionalized with epoxy groups. Then add the TiO2/ethanol mixture to the prepared silica/ethanol mixtures. A functionalized SiO2/TiO2 structure with different particle sizes was formed.
(iii) The as prepared mixtures can be directly coated on silicone surface by dipcoating (single or multiple dips), and after 3-4 hours, the surface changed from superhydrophilicity to superhydrophobicity. As the number of dipping processes is increased, it resulted in higher contact angles and lower hysteresis, which means improved superhydrophobicity. This phenomenon shows the autophobicity of the as surface. Multi-factor ageing (QUV) test showed that superhydrophobicity was maintained out to 1000 hour and 2000 hour aging periods. After ageing the surfaces showed self recovery as evidenced by changes in contact angle and hysteresis. Under quick evaporation of ethanol after dipcoating, even monodispersed silica coating can show initial superhydrophobicity with a contact angle of over 160° and a hysteresis of below 5°.
Spincoating, spray coating and painting on surfaces also are good choices for coating surfaces.
The surfaces were characterized with SEM, and TABLE 11 shows examples of different combination of different particles, wherein for silica particle preparation, the data is related to method example VI, and for titania particles, the data is related to method example X(ii)).
When using these multi-modal particles to coat the surface of silicone parts from a number of manufacturers, the surfaces achieved superhydrophobicity without any further surface treatment (this could be termed surface autophobicity). After the silica particles were dipcoated or painted on the silicone surface, after a certain period, the surface will change from hydrophilicity to hydrophobicity and finally to superhydrophobicity as shown in
The autophobicity (improvement in hydrophobicity with resting time) of silicone surfaces (the effect applies to all silicone surfaces) is shown in
When the superhydrophobic silicone surface was further treated with PFOS, the contact angle increases to 176° and hysteresis<1° as shown in
Pre-Treatments
The surfaces that are to be coated need to be sufficiently hydrophilic so that the liquids are retained on the surface. That is, if the surface is not hydrophilic enough, then uniform liquid films can not be formed. This effect is particularly important for glassy surfaces, like porcelains. The pre-treatment process makes the to-be-coated surface more receptive to surface coatings such as silica sol.
Through the pre-treatment, the surface will have more reactive sites, and after the coatings are applied, better chemical bonding is formed, and thus the adhesion between the substrate and the coatings is greatly enhanced by the chemical bonding.
In an example of the pre-treatment, microscope slides were cleaned in Piranha solution (70:30 (vol/vol) mixture of 96% sulfuric acid and 30% aqueous hydrogen peroxide), and subsequently rinsed extensively with de-ionized water and ethanol. The water droplet contact angle measurement changed from around 40° to below 15° due to the formation of hydroxyl groups on the slide surface (
This procedure can also be replaced with UV or ozone cleaning for five minutes followed by water and ethanol rinsing or oxygen plasma cleaning for two (2) minutes.
Post-Treatments
Early work showed that it is not sufficient to have only a rough surface to achieve superhydrophobicity; it is equally important to have the correct surface chemistry. This is shown by the hydrophobicity of the template surfaces that are enhanced by surface treatment. The surface modification is termed “post-treatment”. Post-treatment means surface modification by low surface energy materials through the formation of chemical bonding between the substrate and the surface modification agents. In experiments, fluorocarbon trichloro silanes was used.
This step is not necessary for silicone surfaces, but does improve the superhydrophobicity of the surfaces. On ceramic surfaces (those that do not have the mobile species that activate the surface in silicones), post-treatment is clearly beneficial, if not required.
Post-treatment to maintain long term multi-factor QUV stability of the superhydrophobicity is especially effective when using fluorosilanes (
Silica samples were placed in a PFOS/n-hexane solution (10 mM) for 30 minutes to permit adsorption of a PFOS layer on the SiO2 surface. Subsequently, the samples were rinsed and treated at 150° C. in air for 30 minutes to promote silane hydrolysis, and 220° C. for five (5) minutes to promote the condensation between silane molecules, thereby forming a stable silanated layer on the silica surface.
The practical aspects of post-treatment can be critical, as the effects are due to:
TABLE 12 illustrates the Rf chain length (
To demonstrate the practical applicability of the various embodiments described herein, sheets of insulation used to manufacture external insulators were coated. The deposited surfaces were created using the sol-gel process for multi-modal, multi-species.
The surface received post-treatment to elevate the superhydrophobicity. These surfaces were subjected to multi factor ageing using temperature, UV and water. The surfaces completed the ageing period (1000 hours) with a practically useful level of retained superhydrophobicity.
Recovery, Hysteresis, and Other Findings
Work related to the present invention has uncovered many novel, non-obvious and surprising results conventionally unknown in the superhydrophobic art, as clearly evident in TABLES 13-17, and
Table 13 is a Reduced Analysis of Variance (ANOVA) Table for Contact Angle (degrees):
Table 14 is a Reduced Analysis of Variance (ANOVA) Table for Hysteresis (degrees):
Multi-factor Condensation, Thermal and UV ageing on these samples continues out past 1,000 hours.
Further findings are shown below and
B—Multi factor—Condensation and Temperature and UV Cycles
Table 15 is an Analysis of Variance for Contact Angle (degrees), using Adjusted SS for Tests:
Table 16 is an Analysis of Variance for Hysteresis (degrees), using Adjusted SS for Tests:
Tables 15 and 16 illustrates the P statistic is the critical aspect—the lower the value the more significant the effect. Table 17 collates the information contained in the P statistic of Tables 15 and 16 and displays it as the significance of the individual factors.
As shown, the analysis shows that one needs to look at both contact angle and hysteresis, and why it is believed that the technique will work on a range of surfaces, and that recovery will be a feature of elastomers, but most probably not on ceramics/glass insulators.
While the invention has been disclosed in its preferred forms, it will be apparent to those skilled in the art that many modifications, additions, and deletions can be made therein without departing from the spirit and scope of the invention and its equivalents as set forth in the following claims.
Under the provisions of 35 U.S.C. § 119(e), this application claims the benefit of U.S. Provisional Application Nos. 60/786,305 filed 27 Mar. 2006, and 60/793,801 filed 23 Apr. 2006, both of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60793801 | Apr 2006 | US | |
| 60786305 | Mar 2006 | US |
