WO2009118552A1

WO2009118552A1 - Superhydrophobic coatings and method for making the same

Info

Publication number: WO2009118552A1
Application number: PCT/GB2009/050277
Authority: WO
Inventors: Kevin David Sanderson; Ian Heaton Smith; Qian Feng Xu; Jian Nong Wang
Original assignee: Pilkington Group Limited
Priority date: 2008-03-28
Filing date: 2009-03-25
Publication date: 2009-10-01
Also published as: CN101544476A

Abstract

The present invention relates to a superhydrophobic coating, having a plurality of pores distributed thereon with an average size of about 100 nm to about 500 nm, characterized in that a majority of the pores are isolated from each other, and surfaces defining the said pores and the continuous part of the coating are composed of silica nano-particles, and in that said coating is surface modified by a monomolecular layer of perfluoroalkysilane. After the surface modification with perfluoroalkysilane, the coatings showed superhydrophobicity with a contact angle of 160° and a sliding angle approaching zero. The coatings also exhibit high transmittance in most of the UV-VIS wavelength range. The invention further provides a method for the production of these coatings in which a substrate is coated with a silica polymer sol gel comprising polymer nanospheres and these nanospheres are subsequently removed from the coating by decomposing or volatilizing them.

Description

SUPERHYDROPHOBIC COATINGS AND METHOD FOR MAKING THE SAME

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention is directed to substrates having a superhydrophobic coating upon at least one major surface and in particular to transparent substrates having a transparent coating and to methods for depositing said coatings on said substrates by a sol-gel method using polymer nano-spheres as a removable template.

2. Description of the Related Art

[0002] Lotus leaves, legs of water strider, and wings of cicada are typical examples for excellent water-repellent surfaces in nature. Since the mid 1990s such so-called superhydrophobic surfaces have aroused extensive interests due to their potential applications in various industrial fields, such as water-repellant and self-cleaning coatings for automobile glasses, optics, outdoor antenna and water transport tubes; water repelling and self-cleaning coatings for fabrics and textiles; coatings that impart wrinkle resistance to fabric; anti-soiling coatings for metal surfaces, transparent substrates and painted surfaces; anti-corrosion coatings; and coatings for biomedical applications, and other field where water is not welcome.

[0003] Generally, a surface with a contact angle (CA) of larger than 150° and a sliding angle (SA) of less than 10° can be called "superhydrophobic" (see Sun TL, et al. Bio-inspired surfaces with special wettability. Ace. Chem. Res. 2005, 38, 644-652, which is incorporated by reference in its entirety). Water droplets cannot stay on a superhydrophobic surface steadily, and roll off easily. During the rolling process, they may carry off dusts due to surface tension. As a result of these features, a superhydrophobic surface tends to stay clean and dry (see McHaIe G, et al. Analysis of droplet evaporation on a superhydrophobic surface. Langmuir 2005, 21 , 11053-11060). [0004] It has been shown that a low surface energy and a nano/ micro-structure may be the two critical factors for the preparation of a superhydrophobic surface (see A. Nakajima, et al. Recent studies on super- hydrophobic films. Monatsh Chem. 2001 , 132, 31 -41 ). A flat surface with a low surface energy can have a CA of not more than 120° (see Blossey R. Self-cleaning surfaces-virtual realities. Nature Mater. 2003, 2, 1476-1122). While the surface energy may be controlled by introducing groups of -CF₃, -CH₃ and -CH₂- (see Ma ML, et al. Superhydrophobic surfaces. Curr. Opin. Colloid Interf. Sci. 2006, 11 , 193-202, which is incorporated by reference in its entirety), the nano/micro structure should be precisely fabricated to have a high roughness.

[0005] Currently, excellent superhydrophobic surfaces with an elaborate nano/micro-structure are achieved by smartly designed methods, such as binary colloidal assembly (see Zhang G, et al. Mohwald H. Fabrication of superhydrophobic surfaces from binary colloidal assembly. Langmuir. 2005, 21 , 9143-9148), porous AI₂O₃ template method (see Lee W, et al. Nanostructuring of a polymeric substrate with well-defined nanometer-scale topography and tailored surface wettability. Langmuir. 2004, 20, 7665-7669), nano-lithography (see Pacifico J, et al. Superhydrophobic effects of self-assembled monolayers on micropatterned surfaces: 3-D arrays mimicking the lotus leaf. Langmuir. 2006, 22, 11072-11076; and Zhai L, et al. Patterned superhydrophobic surfaces: Toward a synthetic mimic of the namib desert beetle. Nano Letters. 2006, 6, 1213-1217), phase separation (see Zhao N, et al. Superhydrophobic surface from vapor-induced phase separation of copolymer micellar solution. Macromolecules. 2005, 38, 8996-8999), layer-by-layer assembly (see Zhai L, et al. Stable superhydrophobic coatings from polyelectrolyte multilayers. Nano Letters. 2004, 4, 1349-1353; U.S. patent application publication No. 2006/0029808), electrospinning, chemical vapor deposition of crossed nanofibres and electroless galvanic deposition on a rough surface.

[0006] But most of the prepared superhydrophobic surfaces are opaque, and only a few of them are translucent. Preparation of a superhydrophobic and also transparent surface is still a technical challenge. This is because light scattering increases as the roughness of a surface is increased to meet the requirement of superhydrophobicity. [0007] Thus, despite the foregoing efforts to manufacture a synthetic superhydrophobic surface coating, a need still exists for superhydrophobic coatings that have one or more of the following properties: may be applied or formed using relatively simple synthesis and working procedures; relatively free of undesirable impurities, finely controlled surface structures and compositions; optical transparency; and the ability to coat large-area surface.

BRIEF SUMMARY OF THE INVENTION

[0008] The present invention is directed to a substrate having a superhydrophobic coating upon at least one major surface said coating having a plurality of pores distributed thereon with an average size of about 100 nm to about 500 nm, characterized in that a majority of the pores are isolated from each other, that the surfaces defining the said pores and the continuous part of the coating are composed of silica nano-particles, and in that said coating is surface modified by a monomolecular layer of perfluoroalkysilane (FAS).

[0009] The superhydrophobic coating of the present invention can have a water contact angle (CA) of greater than about 150°, preferably greater than about 152°, more preferably greater than about 155°, and most preferably greater than about 160°. The coating can have a sliding angle (SA) of less than about 10°, and preferably about 0°.

[0010] In non-limiting embodiments of the present invention, the superhydrophobic coating can have a root mean square (RMS) roughness of less than about 100 nm, preferably less than about 70 nm, more preferably less than about 50 nm, even more preferably less than about 40 nm, so that the coating according to the present invention can minimize light scattering and achieve good transparency. In a preferred embodiment the substrate is a glass substrate and in particular a sheet of a soda lime float glass and the coated substrate has a visible light transmission of greater than 70% and in the more preferred embodiments of greater than 80%.

[0011] In another aspect of the present invention, the superhydrophobic coating of the present invention can be deposited upon the surface of a substrate by a unique sol-gel method, comprising the steps of: forming a silica-polymer sol-gel by combining a polymer nano-spheres dispersion with a silica precursor solution; applying the silica-polymer sol-gel onto the substrate; removing polymer nano-spheres by heat treatment; and modifying the applied coating by applying a layer of perfluoroalkysilane to the upper surface.

[0012] We have discovered that processes of this invention can be used to deposit a uniform coating having a desired surface roughness. The roughness is related to the size of the polymer nano-spheres, to the concentration of the polymer nanospheres in the sol gel and to the size of the silica particles in the sol gel.

[0013] The polymer(s) suitable for constituting the polymer nano-spheres can be any polymer that is able to volatilized and/or decomposed during the subsequent heat treatment, and is most preferably polystyrene (PS).

[0014] The polystyrene nano-spheres can be obtained by any conventional methods. In one embodiment of the present invention, the PS nano-spheres are formed by soap-free emulsion polymerization. In one embodiment of the present invention, the polystyrene can have a number average molecular weight of about 8 to 10 X 10⁴ Daltons.

[0015] In another aspect, the present invention provides a method of altering the wettability of a substrate surface, comprising providing on the substrate surface with a coating having a plurality of pores distributed thereon with an average size of about 100 nm to 500 nm, characterized in that a majority of the pores are isolated from each other , and surfaces defining the said pores and the continuous part of the coating are composed of silica nano-particles, and in that said coating is surface modified by a monomolecular layer of perfluoroalkysilane (FAS).

[0016] The method can include forming a silica-polymer sol-gel by combining polymer nano-spheres dispersion with silica precursor solution; applying the silica-polymer sol-gel onto a substrate; removing polymer nano-spheres by heat treatment; and applying a layer of perfluoroalkysilane thereto. The layer of perfluoroalkylsilane may be deposited using chemical vapor deposition (CVD) techniques.

[0017] According to another aspect of the present invention, a method is provided for preparing an anti-reflective coating upon the surface of a substrate, comprising forming a silica polymer sol gel by combining a polymer nano-spheres dispersion with a silica precursor solution; applying the silica polymer sol gel onto a substrate; and removing the polymeric nanospheres by heat treatment.

[0018] In a non-limiting embodiment of the present invention, said anti- reflective coating comprises a silica coating having a refractive index in the range 1.25 to 1.40.

[0019] The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 is a TEM image of the prepared silica-PS sol-gel.

[0021] FIGS. 2(a) and 2(b) are SEM images of the coatings prepared from sol-gels without PS. FIGS. 2(c) and 2(d) are SEM images of the coatings prepared from sol-gels with 0.014% PS.

[0022] FIG. 3(a) is an AFM image of coating prepared from sol-gels without PS. FIG. 3(b) is an AFM image of coating prepared from sol-gels with 0.01 % PS.

[0023] FIGS. 4(a) to 4(d) are traces of the AFM tip when it drew a line on the surfaces of coatings prepared from sol-gels containing different additions of PS.

(a) No PS; (b) 0.002% PS; (c) 0.01 % PS. The corresponding roughness are 36.9 nm, 43.6 nm, 71.2 nm, respectively.

[0024] FIGS. 5(a) and 5(b) are XPS spectra of coatings before (a) and after

(b) FAS modification.

[0025] FIGS. 6(a) to 6(f) are contact angles of coatings prepared by silica-PS sol-gels containing different additions of PS: (a) 145.1 °, 0%; (b) 152.8°, 0.0002%; (c) 156.8°, 0.002%; (d) 155.8°, 0.01 %; (e) 160°, 0.014%; (f) 152.9°, 0.02%.

[0026] FIG. 7 is the photographs of the rolling processing a 4 μl_ water droplet on a coated glass placed almost horizontally. The photographing speed was 25 shots per second.

[0027] FIG. 8 is the transmittance of coated or uncoated glasses.

[0028] FIGS. 9(a) and 9 (b) are photographs of uncoated glasses. FIGS. 9(c) and 9 (d) are photographs of coated glasses.

[0029] FIG. 10 indicates the reaction of hydrolytic FAS with the silica coating.

[0030] FIG. 11 is a schematic illustration of the formation of a rough surface after the removal of big PS spheres which are originally embedded among small silica particles.

DETAILED DESCRIPTION OF THE INVENTION

[0031] For the purposes of this specification, unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0032] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical Value, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements. [0033] In the present invention, it has been discovered that superhydrophobic and transparent coatings prepared by a sol-gel method using polymer nano-spheres as a removable template can be applied to a wide variety of materials, including metals, ceramics, polymers, and fabrics, in particular glass.

According to one aspect of the present invention, a superhydrophobic coating is provided, having a plurality of pores distributed thereon with an average size of about 100 nm to 500 nm, characterized in that a majority of the pores are isolated from each other, and surfaces defining the said pores and the continuous part of the coating are composed of silica nano-particles, and in that said coating is surface modified by a monomolecular layer of perfluoroalkysilane (FAS).

[0035] As used herein, the term "a majority of the pores" refers to a percentage of the pores such as more than 50%, preferably more than 60%, more preferably 70%, 80%, or even 90% or higher. By "a majority of the pores are isolated from each other", it is meant that most of the pores in the coating surface don't connect to each other such that a structure collapse will not occur, as will be further described below.

[0036] In the context of this invention the term "nano-particles" is defined as particles of which the majority has a largest dimension of between about 1 nm and about 200 nm. For non-spherical particles the longest straight line that can be drawn from one side of a particle to the opposite side is denoted the primary axis. The length of the particle is to be measured along the primary axis, whereas the diameter of the particle is defined as the longest straight line that can be drawn at right angles to the primary axis. In a preferred embodiment, the majority of the nano-particles have a length of between about 1 nm and about 200 nm. In a further preferred embodiment, the majority of the nano-particles have a diameter of about 50 nm.

[0037] Similarly, the term "nano-spheres" is defined as polymer spheres of which have an average diameter of about 100nm to about 1000nm, preferably about 200 nm to 700 nm, and most preferably about 500 nm.

[0038] The term "silica precursor" refers to a silicon and oxygen-containing compound capable of forming silica. For example, it is well known that a range of silicon alkoxides of the formula Si(OR)₄, wherein R is independently -CH₃, -C2H5, or C3 to Cβ straight-chain or branched alkyl, can be hydrolyzed and condensed to form a silica network. A silica network is a known concept in the art and is described in Brinker, C. J. and G. W. Scherer, Sol-Gel Science (Academic Press, NY, 1990). Preferably R is methyl or ethyl. Such precursors include tetramethoxysilane (tetramethyl orthosilicate), tetraethoxysilane (tetraethyl orthosilicate), tetrapropoxysilane, tetrabutoxysilane. Also included as a silica precursor is silicon tetrachloride. Further silica precursors comprise organically modified silica, for example, CH₃Si(OCHs)₃, PhSi(OCH₃)₃ where Ph is phenyl, and (CH₃)₂Si(OCH₃)₂. Other silica precursors include metal silicates, such as potassium silicate, sodium silicate, and lithium silicate, as described in WO2007/050489, which is incorporated herein by reference in its entirety.

[0039] The present coatings showed high water contact angles. Although not willing to be bound by theory, it is believed that the first reason for this observation may be due to the formation of a porous structure by the present sol-gel method and the surface modification by FAS. FAS is usually used to lower the surface free energy because its long chain is composed of CH₂, CF₂ and CF₃ groups with an extremely low free energy. The reaction of hydrolytic FAS with the glass substrate is schematically shown in FIG. 10. XPS results

(FIG. 5) show that with FAS modification CF₂ and CF₃ groups have been successfully introduced on the surface of SiO₂ coating. Additional analysis of

Si and O elements with an atomic ratio of 1 :2 suggests that only monomolecular FAS was introduced and therefore the silica coating can also be detected by XPS.

[0040] The more important reason for the present observation of superhydrophobicity is related to the addition of PS nano-spheres. This addition (up to e.g. about 0.014%) led to an increase in CA from about 145.1 ° to about 160° and a concurrent decrease in SA from about 30° to a level approaching zero. Such drastic improvement in hydrophobicity could directly result from the development of a porous structure with a high roughness as the roughness has an important effect on CA and SA in terms of theoretical models. [0041] The relationship between the CA and roughness may be expressed by Wenzel's model as (see Wenzel RN. Surface roughness and contact angle. Ind. Eng. Chem. 1936, 28, 988-994, which is incorporated by reference in its entirety):

Wenzel's model:

cosθ_* = r cosθ

(1 )

where r is defined as the ratio of the actual area of a rough surface to the ideal flat surface, θ* and θ are the CAs on a rough surface and an ideal flat surface, respectively. The relationship may be represented by Cassie's model as (see Cassie ABD, et al. Wettability of porprous surface. Trans. Faraday Soc. 1944, 40, 546-551 , which is incorporated by reference in its entirety):

Cassie's model:

cosθ_* = f_y COsG₁ + /₂ cosθ₂

(2)

where fi and f2 denote the ratio of liquid-solid interface and liquid-air interface, respectively, with f? + /2= 1 , and θi and Θ2 are the theoretical CAs of the two interfaces. If the gravitation of water droplet is neglected, Θ2 is equal to 180°. In these two models, roughness is the only factor determining the ratio of the areas of the liquid-solid interface and the liquid-air interface. A rougher surface corresponds to a larger liquid-air interface, and thus a higher CA.

[0042] The present observations are consistent with these models. As shown in the non-limiting embodiments of the present invention, when no PS is added, the roughness of the coating is 36.9 nm, and the corresponding CA is only 145.1 ° and SA as high as 30°. With increasing the PS concentration to approximately 0.014%, the roughness increases. In this case, more air could be trapped in the holes left by PS spheres, and thus the CA increases to about 160.0°, and the SA decreases dramatically (FIG. 7).

[0043] The roughness of the coatings prepared from silica-PS sol-gels are obviously higher than that of the coating prepared from pure silica sol-gel according to the roughness data and 3-D morphology of the coatings determined by AFM (FIGS. 3 and 4). Generally, the roughness of a coating is determined by the pore structure, which is in term related to the concentration of pores in the coating. It is evident from the present result (FIG. 2) that the addition of PS spheres changes the pore structure. This change may be a consequence of the volatilization and decomposition of the added PS spheres during heat treatment. And the process of removal of the PS spheres is schematically shown in FIG. 11. Thus, it is possible to directly control the pore structure and the roughness of a coating by adjusting the concentration of PS spheres in the sol-gel. [0044] However, as shown in one non-limiting embodiment of the present invention, it should be noted that when the addition of PS reaches the highest studied level of about 0.02%, the CA decreases to about 152.9°. This may be a result of structure collapse. That is, when the PS concentration is kept to be less than about 0.014%, most of the holes in the coating don't connect to each other, and thus the number of holes and the roughness increase with increasing PS concentration. But, when the PS concentration is too high, some of the holes may begin to link with each other, leading to a collapsed structure in the coating. Such structural collapse may result in a decrease in roughness and thus CA. The concentration of PS nanospheres should be less than that which results in a coating which is not superhydrophobic. The optimum concentration in a particular sol gel may be determined empirically. Typically it will be less than 0.02% by weight.

[0045] The good control of the pore structure by addition of PS spheres is also related to the good stability of the present silica-PS sol. In the sol-gel, PS spheres and silica particles might have the same negative charge, and their electric double layer would play an important role in preventing the particles from aggregation and settling. The average size of silica particles is roughly equal to each other typically being in the range 1 to 100nm and more prefereably in the range 25 to 75nm. The average size of the silica particles , shown in FIGS. 1 and 2 being a little more or less than 50 nm. This observation indicates that the 500 nm PS spheres had little effect on the silica particle size during the preparation of sol-gels and subsequent coatings. However some agglomeration of the particles does occur as a result of the hydrolysation and polymerization of the silica particles. We have discovered that the sol gel may be stabilized by contol of its pH value. The addition of ammonia to adjust the pH of the dispersion of PS nanospheres results in a sol gel having a relatively alkaline pH value.We have discovered that stirring the sol-gel allows the ammonia to volatilize and reduces its pH. The resulting sol-gel is more stable and can be stored for longer periods. This is potentially useful in a production environment.

However the use of these more stable sol gels in the processes of this invention has been discovered to result in coatings which have a lower CA. We have discovered that the hydrophobicity of these coatings may be increased by increasing the proportion of PS nanospheres in the sol gel. Preferably the PS nanospheres comprise from 0.1 to 5.0% by weight of the stabilized sol gel formulations.

[0046] While increasing roughness is favorable for superhydrophobicity, such increase is detrimental to transparency as light scattering is severe on a rough surface. It is because of this reason that the preparation of transparent superhydrophobic coatings has been a technical challenge. It is reported that the surface roughness should be controlled to be less than 100 nm in order to minimize light scattering and achieve good transparency. Otherwise, the coating would become opaque or translucent (see, e.g. Nakajima A, et al. Preparation of transparent superhydrophobic boehmite and silica films by sublimation of aluminum acetylacetonate. Adv. Mater. 1999, 11 , 1365-1368, which is incorporated by reference in its entirety).

[0047] In contrast to single-phase coatings which roughness is difficult to control, the roughness of the present coating depends on: 1 ) the sizes of silica particles and PS spheres, 2) the concentration of PS spheres in the sol-gel, and 3) the removal of PS spheres during heat treatment. The average size of silica particles is about 50 nm and the light scattering of these fine particles may be neglected. The size of PS spheres is 500 nm, which may lead to big pores exceeding 100 nm (FIGS. 2(c) and 2(d)) on the surface of the coating (FIG. 4(c)). But the RMS roughness is affected by not only the size of pores but also the spacing between the adjacent pores. The PS nanospheres will typically have an average size which is in the range 1 to 500nm, preferably in the range 1 to 50nm. We have discovered that it may be necessary to prepare the smaller nanospheres in the presence of an emulsifier in order to avoid agglomeration of the sol-gel. The present experiments show that with the addition of even a small amount of PS spheres, the coatings prepared from two-phase sol-gels could be well controlled to be smaller than 100 nm to meet the requirement for transparency.

[0048] The sol gel may be applied to the surface of the substrate using conventional techniques such as spraying or dipping. One than one application may be necessary in order to deposit the desired quantity of the sol on the surface of the substrate. The resulting coatings will generally have a thickness in the range of from 50A to 5000A more preferably in the range 100A to 10OOA

The PS nanospheres may be removed to heating the coating to a temperature at which they are decomposed or volatilized. It is necessary to first remove any excess solvent by heating the coated substrate relatively slowly to a temperature which is typically in the range 250⁰C to 300⁰C. When the excess solvent has evaporated the coating may be heated to a higher temperature typically of the order of 650°C relatively quickly and maintained at that temperature until the nanospheres have been volatilised and pore formation is complete.

[0049] In addition to roughness, anti-reflectivity is also influenced by the homogeneity of porous films. That is, homogeneous porous films have a lower refractive index than that for bulk materials, and only a thin film with a lower refractive index could reduce the reflection according to typical geometric optics. Thus, another reason for the observation of anti-reflectivity in most of the UV-VIS wavelength range (FIG. 8) may be due to the presence of homogeneous porous structures in the present coatings as shown in FIGS. 2 and 3.

[0050] The present superhydrophobicity and high transparency are achieved by a sol-gel approach, which is suitable for large-area coating. Compared with chemical vapor deposition and physical vapor deposition, the present approach doesn't require complex plasma activation of the substrate before coating, and the coating process is under atmospheric pressure without the requirement of vacuum and other expensive facilities, not to say the gas fluctuation which affects the uniformity of the coating. Prior to the present sol-gel approach, some other wet chemical methods have also been used for the preparation of superhydrophobic coatings, including the layer-by-layer method, hydrothermal method, and AI2O3 template method. Nevertheless, these methods involve long coating processes and/or limited by the dimension of the template used.

[0051] In summary, a simple sol-gel method is disclosed for the preparation of transparent superhydrophobic coatings. A stable two-phase sol-gel of silica and PS can be easily prepared by adding PS nano-spheres into the precursor solution. It is shown that using PS nano-spheres as a removable template is a practical way to controlling the roughness of the coatings. After the modification of FAS, the CA of 4 μl_ water droplet on the prepared coating can reach 160° while the corresponding sliding angle is close to 0°. Additionally, the transmittance of the coated superhydrophobic glass is even higher than the uncoated glass in the UV-VIS wavelength range of 440-900 nm. This may be due to the small roughness and homogeneous pore structure of the coating. The present method has advantages for easy concurrent achievements in superhydrophobicity and high transparency and being suitable for large-scale coating.

[0052] The following examples are included to demonstrate preferred embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples which follow represent techniques discovered by the inventor to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.

EXAMPLES

[0053] Unless indicated to the contrary, all parts and percentages are by weight.

Preparation of silica-PS sol-gel:

[0054] Two solutions (named A and B, respectively) were prepared first. For solution A, 500 nm PS spheres with a number average molecular weight of 9 X 10⁴ Daltons were prepared first via a method of emulsion polymerization without emulsifier. A certain amount of PS nano-spheres were then dispersed into 20 ml_ ethanol. 3 ml_ ammonia was used to adjust the pH value, and the suspension was stirred at 45 ⁰C for an hour. For solution B, 3 ml_ tetraethyl orthosilicate (TEOS) was dispersed into 25 ml_ ethanol by ultrasonic vibration for 10 minutes. To prepare a silica-PS sol-gel, solution B was mixed with solution A, and the mixed solution was stirred at 45 ⁰C for an hour.

Coating of silica-PS sol-gel:

[0055] A transparent flat-glass substrate was coated by dip-coating at the speed of 2.65 cm min^"1. The substrate was immersed into the sol for 5 minutes before the first dip-coating, and 5 seconds before the subsequent run. After each coating the substrate was dried at room temperature (25 ⁰C) for 5 minutes, and this step was repeated 5 times. With increasing coating time, some of the glass substrates became semi-transparent. And the coated glass substrates were heated at 500 ⁰C for 10 minutes to remove the residual solvent and PS nano-spheres. After this, the substrates became transparent again. The substrates were cleaned with an H₂SO₄/H₂O2 (50/50 wt.%) solution for one hour and ultrasonicated in acetone for 10 minutes, and then rinsed by a large mount of distilled water before coating.

Chemical vapor deposition (CVD) of perfluoroalkysilane (FAS):

[0056] FAS solution was used for lowering the surface free energy. This solution was prepared by adding 1 :100 (vol. %) FAS and triplicate water into methanol. The coated glass substrates were put into a sealed vessel containing 0.3 ml FAS solution. The distance between the glass substrates and the solution was 55 mm. And then the vessel was kept at 150 ⁰C for 3 hours to cover the coatings by a monomolecular layer of FAS.

Characterization:

[0057] The microstructure of the silica-PS sol-gel was examined by transmission electron microscope (TEM, JEM-2010, INCA OXFORD). The roughness and morphology of the surfaces were characterized by atomic force microscope (AFM, Multimode Nanoscope Ilia, U.S.A.). The structure of the coating was also studied by scanning electron microscope (SEM, JSM-7401 F, JEOL Ltd). The CA and SA were measured by a contact angle meter (OCA20, Germany). 4.0 μL pure water (18.2 MΩ-cm in resistivity) was used to measure the CA and SA. X-ray photoelectron spectroscopy (XPS, Monochromated Al Ka, Kratos Axis Ultra DLD) was used to detect the chemical compositions of the coating surfaces. The transmittance spectra of prepared superhydrophobic coatings were determined by UV-VIS spectrophotometer (Evolution 300, U.S.A.).

[0058] Figure 1 is the TEM image of the prepared silica-PS sol-gel. It can be seen that the small silica particles have an average size of 50 nm, and the big PS sphere 500 nm. The PS nano-sphere appears to be covered by silica nanoparticles, but there is no evidence for a tight combination between the large and small particles.

[0059] Figures 2(a) and 2(b) are the low- and high-magnification SEM images of coatings prepared from sol-gels containing no PS, while figures 2(c) and 2(d) are those for coatings prepared from sol-gels containing 0.014% PS. Both of the two coatings were heated at 500 ⁰C for 10 minutes to remove the residual solution or PS spheres. After heat treatment, the two coatings are made of silica particles. It can be seen from Figs. 2(c) and 2(d) that original PS spheres of -500 nm have been removed, leaving a lot of holes, and some of the holes are interconnected, which induces a collapsed structure. Comparison between the two different coatings shows that the coating without addition of PS spheres is denser than the one with addition of PS spheres.

[0060] AFM was used to examine the morphology and roughness of the coating. This was done in a 20x20 μm scanning area. Figure 3 shows the 3-D morphologies of the coatings prepared from sol-gels containing 0 and 0.01 %

PS, and Fig. 4 the traces of the AFM tip when it drew a line of 20 μm on the coatings with different additions of PS. It can be seen that the roughness generally increases with increasing PS addition. The root mean square (RMS) roughness of the coatings prepared from silica-PS sol-gels containing 0,

0.002%, 0.01 % PS are 36.9 nm, 43.6 nm, 71.2 nm, respectively.

[0061] XPS spectra of coatings before and after FAS modification is shown in Figs. 5(a) and 5(b), respectively. The characteristic range of X-ray photoelectrons is 3 to 8 atom layers (1 -3 nm). The surface of the coating before FAS modification is composed of SiO2 (Fig. 5(a)), and the atomic ratio of Si to O is 29.2:63.69 (= 1 :2). The surface of the coating after FAS modification contains F, C, Si, O (Fig. 5(b)). A strong fluorine peak at 687 eV is observed, and the atomic ratio of C to F is 14.33:32.38. This ratio, which is between 1 :2 and 1 :3, indicates that the surface is covered by CF₂ and CF₃ groups. Si and O elements are also detected by XPS and the atomic ratio is close to 1 :2.

[0062] After the modification of a rough porous coating with FAS, all of the coatings showed high CAs except for the one prepared from the sol-gel containing no PS. Figure 6 illustrates the morphologies of a water droplet of 4 μl_ on coatings containing different concentrations of PS. The CAs are 145.1 ±1 °, 152.8±1 °, 156.8±1 °, 155.8±1 °, 160.0±1 ° and 152.9±1 ° for the PS concentration of 0, 0.0002%, 0.002%, 0.01 %, 0.014% and 0.02%, respectively.

[0063] The water droplet was difficult to be attached on a horizontal superhydrophobic glass by a normal syringe, and if the water droplet happened to drop down from a very short distance, it could bounce up like a ball and rolled away quickly. A static water droplet could stay like a ball on the superhydrophobic glass. But when it was placed slowly on a slightly tilted superhydrophobic glass, it rolled down very quickly. Figure 7 is the rolling process of a 4 μl_ water droplet on a coating with a high CA angle, indicating a very low sliding angle (SA). However, on the coating with a small CA (without PS addition), the SA is larger than 30°. With increasing CA, SA decreased dramatically. Particularly, when the CA reached 160.0°, the water droplet started to roll off even when the substrate was placed horizontally (Figure 7). This observation suggests that the SA was extremely low (close to 0°) and there was very low resistance during the rolling process.

[0064] Figure 8 is the transmittance spectra of glasses coated or uncoated by superhydrophobic coatings in the UV-VIS wavelength range. All coated glasses show excellent transparency (Fig. 9). The transmittance of coated glasses even exceeds the uncoated one in 503-900 nm and 440-900 nm wavelength ranges for 0 and 0.14% PS, respectively. The transparency can be clearly verified when the coated glass is placed on a written paper. No difference in transparency can be visualized between the uncoated (Figs. 9(a) and 9(b)) and coated (Figs. 9(c) and 9(d)) glasses although they show a large difference in CA (hemi-sphere vs. complete sphere).

[0065] All of the methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure.

While the products and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A coated substrate having a superhydrophobic coating upon at least one major surface said coating comprising a plurality of pores distributed thereon with an average size of about 100 nm to about 500 nm, characterized in that a majority of the pores are isolated from each other, the surfaces defining the said pores and the continuous part of the coating are composed of silica nano-particles and in that said coating comprises a monomolecular layer of perfluoroalkysilane on its exterior surface.

2. A coated substrate according to claims 1 characterised in that the coated substrate is a glass, fabric, textile, metal or plastic substrate.

3. A coated substrate according to either of the preceding claims characterized in that the substrate is a glass substrate.

4. A coated substrate according to any of the preceding claims characterised in that the coating has a water contact angle of greater than 150°.

5. A coated substrate according to claim 4 characterised in that the coating has a water contact angle of greater than 155°

6. A coated substrate according to any of claims 1 to 5 characterised in that the coating has a sliding angle of less than 10°.

7. A coated substrate according to any of the preceding claims characterised in that the coating has a root mean square roughness of less than 100 nm.

8. A coated substrate according to any of the preceding claims characterised in that the coating is transparent in the UV-VIS wavelength range.

9. A coated substrate according to any of claims 3 to 8 characterised in that the coated substrate has a visible light transmission which is greater than 70%.

10. A coated substrate according to any of the preceding claims characterised in that the coating is from 50A to 5000A thick.

11. A method for preparing a coated substrate according to any of claims 1 to 10 which is characterized in that it comprises the steps of: forming a silica-polymer sol-gel by combining a polymer nano-spheres dispersion with a silica precursor solution; applying the silica-polymer sol-gel onto a surface of the substrate; removing polymer nano-spheres by heat treatment; and applying a layer of perfluoroalkysilane thereto.

12. A method according to claim 11 characterised in that the polymer nano spheres are polystyrene nanospheres.

13. A method according to claim 12 characterized in that the polystyrene has a number average molecular weight of 8 to 10 X 10⁴ Daltons.

14. A method according to any of claims 11 to 13 characterised in that the polystyrene concentration is less than that which results in a structural collapse of the coating.

15. A method according to claim 14 characterised in that the polystyrene concentration is in a range from 0.0002% to 0.02% by weight, based on the total weight of the silica-polymer sol-gel.

16. A method according to claim 15 characterised in that silica polymer sol gel is a gel which is not storage stable at room temperature.

17. A method according to any of claims 11 to 14 characterised in that the polystyrene concentration is in the range 0.1 % to 5.0% by weight.

18. A method according to claim 17 characterised in that the silica polymer sol gel has been stirred so as volatilize any ammonia present and reduce its pH value.

19. A method according to any of claims 11 to 19 characterised in that the polystyrene nanospheres have an average particle size of from 1 to 500nm.

20. A method according to claims 19 characterised in that the polystyrene nanospheres have an average particle size of from 1 to 50nm.

21. A method according to either of claims 19 or 20 charatehsed in that the polystyrene sol gel further comprises an emulsifier.

22. A method according to any of claims 11 to 21 characterised in that excess solvent is removed from the coated substrate by heating under controlled conditions.

23. A method according to any of claims 11 to 22 characterised in that the PS nanospheres are removed by heating the substrate to a higher temperature after any excess solvent has been removed.

24 A method according to any of claims 11 to 23 characterised in that the perfluoroalkylsilane layer is applied using a chemical vapour deposition process.

25. A method of altering the wettability of a substrate surface, comprising: providing on the substrate surface with a coating having a plurality of pores distributed thereon with an average size of 100 nm to 500 nm, characterized in that a majority of the pores are isolated from each other, and surfaces defining the said pores and the continuous part of the coating are composed of silica nano-particles, and in that said coating is surface modified by a monomolecular layer of perfluoroalkysilane.

26. The method according to claim 25, characterised in that said coating is applied using a method according to any of claims 11 to 24.