STAIN RESISTANT PARTICLES

Abstract

Methods of treating a cured porous inorganic; coating include contacting the coating with water vapour or a combination of water vapour and base, wherein the coating prior to curing includes a coating composition having particles with a core which is hollow or which comprises an organic polymer composition and a shell formed of an inorganic oxide. The shell has a thickness in the range from 2 to 75 nm and has at least one and no more than five enlarged pores, each enlarged pore having a diameter of between 5 nm and 300 nm.

Claims

1. A method for producing a coated substrate comprising the steps of: (a) applying to a substrate a coating composition to form a coating surface, the coating comprising particles comprised of: (i) a core which is hollow or comprises an organic polymer composition, and (ii) a shell having a thickness in the range of 2 to 75 nm which comprises an inorganic oxide, wherein a portion of the particles forms at least part of the coating surface; and (b) treating the coating surface with water vapour or a combination of water vapour and a base to form at least one and no more than five pores in the shell of the particles forming at least part of the coating surface, each pore communicating between the core and an outer surface of the shell and having a diameter of between 5 nm and 300 nm measured using an atomic force microscope, the at least one and no more than five pores being the largest pores of the particle.

2. The method according to claim 1, wherein the coating composition is cured before or after step (b).

3. The method according to claim 2, wherein the curing step performed at a temperature of at least 100 C. for at least 15 minutes.

4. The method according to claim 1, wherein the coating composition is cured before step (b).

5. The method according to claim 4, wherein the curing is carried out at about 150 C. or more.

6. The method according to claim 1, wherein step (b) is practiced by treating the coating surface with water vapour at a temperature of at least 100 C.

7. The method according to claim 1, wherein step (b) is practiced by treating the coating surface with water vapour at a temperature of no more than 600 C.

8. The method according to claim 1, wherein the coating composition comprises a binder which consists essentially of an inorganic material.

9. A method of producing a coated substrate comprising the steps of: (a) forming a coating on the substrate by applying to the substrate a coating composition comprising particles having a hollow core and a shell which comprises an inorganic oxide; (b) curing the coating composition to form a cured coating; and (c) treating the cured coating with water vapour or a combination of water vapour and base to thereby produce a coated substrate defined by the expressions:
R.sub.u2.8R.sub.0 and R.sub.r2.0R.sub.0 where, R.sub.0 is a specular reflection at 550 nm of the coating composition applied to a substrate to form a coating having an average thickness of between 100 and 120 nm and stored at 25 C. and 40% relative humidity under equilibrium conditions resulting in a coated substrate C.sub.0, R.sub.u is a specular reflection at 550 nm of the coated substrate C.sub.0 stored at 25 C. and 90% relative humidity for 400 minutes resulting in a coated substrate C.sub.1; and R.sub.r is a specular reflection at 550 nm of the coated substrate C1 after being stored at 25 C. and 40% relative humidity until equilibrium conditions are reached.

10. The method according to claim 9, wherein the curing of step (b) is carded out at a temperature of about 150 C. or more.

11. The method according to claim 9, wherein step (c) is practiced by treeing the cured coating with water vapour at a temperature of at least 100 C.

12. The method according to claim 9, wherein step (c) is practiced by treating the cured coating with water vapour at a temperature of no more than 600 C.

13. A method of treeing a cured porous inorganic coating by contacting the coating with water vapour or a combination of water vapour and base, wherein the coating prior to curing comprises a coating composition comprising particles comprised of a core which is hollow or which comprises an organic polymer composition and a shell comprising an inorganic oxide.

14. The method according to claim 13, wherein the water coating is contacted with water vapour at a temperature of at least 100 C.

15. The method of claim 13, wherein the water coating is contacted with water vapour at a temperature of no more than 600 C.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0063] FIG. 1 is a graph showing the ratio of the specular reflection at 550 nm (R) of a coating of the present invention stored under the conditions described in Test 2 relative to the specular reflection at 550 nm of the coatings stored at 25 C. and 40% relative humidity under equilibrium conditions (R(0))

[0064] FIG. 2 is an AFM image of a coating surface prior to steam treatment;

[0065] FIG. 3 is a two dimensional AFM image of the coating surface of a coating of the present invention.

[0066] FIG. 4 is a three dimensional AFM image of the coating surface of the coating of FIG. 3.

COATING COMPOSITION

[0067] The present invention comprises applying a coating composition comprising core shell particles to a substrate, wherein the shell comprises an inorganic oxide and the core is hollow or comprises an organic polymeric composition.

[0068] Coating compositions herein typically comprise a binder. The primary function of the binder is to keep the integrity of the coating intact. Any suitable binder may be used but preferably the binder forms covalent bonds with itself upon curing and/or other components in the coating and/or the substrate. The binderbefore curingpreferably comprises inorganic compounds with alkyl or alkoxy groups. Further, the binder preferably polymerises itself to form a substantially continuous polymeric network. The binder is preferably structurally and/or chemical distinct from the shell.

[0069] In one embodiment of the invention the binder comprises an inorganic material. Preferably the binder consists substantially of an inorganic material. The binder preferably comprises compounds derived from one or more inorganic oxides. Preferably the binder comprises hydrolysable material such as inorganic alkoxides, inorganic halogenides, inorganic nitrates, inorganic acetates or a combination thereof. Preferred are inorganic alkoxides. Preferably the binder comprises alkoxy silanes, alkoxy zirconates, alkoxy aluminates, alkoxy titanates, alkyl silicates, aluminium nitrates, sodium silicates, or a combination thereof. Preferred are alkoxy silanes, preferably tri- and tetra-alkoxy silanes. Preferably, ethyl silicate, aluminate, zirconate, and/or titanate binders are used. Tetra alkoxy silane is most preferred.

[0070] The amount of binder in the coating composition is preferably 1% or more, more preferably 2% or more, by weight of the solid fraction. Preferably the amount of binder will be 40% or less, more preferably 25% or less, by weight of the solid fraction. The percentage is calculated as the amount of inorganic oxide in the binder relative to the amount of inorganic oxide in the rest of the coating.

[0071] The particles may comprise a mixture of different types, sizes, and shapes of particles. However, preferably the particles are substantially the same size and shape. The particle size distribution, as measured by its polydispersity index using Dynamic Light Scattering (DLS), is preferably less than 0.5, preferably less than 0.3, and most preferably less than 0.1.

[0072] In one embodiment the particles used herein are non-spherical such as, preferably, rod-like or worm-like particles. In another preferred embodiment the particles are substantially spherical.

[0073] Preferably the particles have an average specific size g where g=1/2(length+width) of about 500 nm or less, more preferably 300 nm or less and even more preferably 150 nm or less. The length is the maximum length possible, with the width being the maximum width measured at right angles to the line defining the length.

[0074] Preferably the particles have an average size of 1 nm or more. More preferably the particles have an average size of about 10 nm or more and even more preferably 50 nm or more. Particle size is measured by TEM.

[0075] Preferably the average specific diameter of the hollow core or void, when present, is 5 nm or more, more preferably 10 nm or more, even more preferably 20 nm or more. The average specific diameter of the void is preferably 500 nm or less, more preferably 100 nm or less, even more preferably 80 nm or less and yet even more preferably 70 nm or less. Preferably the shell is at least 1 nm thick, more preferably at least 2 nm, more preferably at least 5 nm, even more preferably at least 10 nm. The shell is 75 nm thick or less, preferably 50 nm or less, more preferably 25 nm or less and even more preferably 20 nm or less. Particles with a lower shell thickness have reduced mechanical properties while the formation of enlarged pores is more difficult in particles with a higher shell thickness.

[0076] In a preferred embodiment the void percentage, relative to the total volume of the particle (i.e. core and shell), is preferably from about 5% to about 90%, more preferably from about 10% to about 70%, even more preferably from about 25% to about 50%. The void precentage (x) may be calculated according to the following equation:

x=(4r.sub.a.sup.3/3)(4r.sub.b.sup.3/3)100

wherein r.sub.a is the radius of the core and r.sub.b is the radius of the outer shell.

[0077] The shell of the core shell particle comprises an inorganic oxide. Preferably the shell consist essentially of an inorganic oxide. Preferably the metal is selected from magnesium, calcium, strontium, barium, borium, aluminium, gallium, indium, tallium, silicon, germanium, tin, antimony, bismuth, lanthanoids, actinoids, scandium, yttrium, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, ruthenium, cobalt, nickel, copper, zinc, cadmium, and combinations thereof. Preferably the metal oxide is selected from titanium dioxide, zirconium oxide, antimony doped tin oxide, tin oxide, aluminium oxide, silicon dioxide, and combinations thereof. Preferably the shell comprises silica, more preferably at least 90%, by weight, silica. In a special embodiment, the particles consist of silica. Suitable shells, not containing the enlarged pores, are described in WO2008/028640 and WO2008/028641.

[0078] The organic polymer composition of the core comprises homopolymers, random co-polymers, block-copolymers, diblock-copolymers, triblock-copolymers, and combinations thereof.

[0079] Preferably the core comprises a polymer selected from polyesters, polyamides, polycarbonates, polyurethanes, vinyl polymers such as polystyrenes, poly(meth)acrylates and combinations thereof.

[0080] Other suitable polymers are listed in WO2008/028640 on page 5, line 31 to page 7, line 5 which is incorporated herein by reference.

[0081] In a preferred embodiment, the core material comprises a cationic polymer. The cationic group may be incorporated in the polymer or may be added in any other form such as, for example, by the addition of a cationic surfactant. Preferably the cationic groups are at least partially bound to the polymer. Preferably the cationic groups are incorporated into the polymer during polymerisation.

[0082] Preferably, the polymer comprises latex, such as NeoCryl XK-30*, available from DSM NeoResins B.V. As used herein, the term latex refers to stabilized suspension of polymeric particles. Preferably the suspension is an emulsion.

[0083] Preferably the latex comprises polymer and a cationic surfactant. Preferably, the surfactant comprises an ammonium surfactant.

[0084] Any suitable polymer may be used such as, for example, homopolymers, random co-polymers, block-copolymers, diblock-copolymers, triblock-copolymers, and combinations thereof.

[0085] The latex preferably comprises an aqueous cationic vinyl polymer.

[0086] Most preferably, the latex comprises a vinyl polymer obtainable from monomers selected from at least styrenic monomers, (meth)acrylic monomers, cationic functionalized monomers and potentially cationic monomers or combinations thereof.

[0087] The compositions herein may comprise a solvent. Preferred solvent include water, organic solvents, and combinations thereof. However, depending on the chemistry of the binder, many solvents are useful. Suitable solvents include, but are not limited to, water, non-protic organic solvents, alcohols, and combinations thereof. Examples of suitable solvents include, but are not limited to, isopropanol, ethanol, acetone, ethylcellosolve, methanol, propanol, butanol, ethyleneglycol, propyleneglycol, methyl-ethyl-ether, methyl-butyl-ether, toluene, methyl-ethylketone, and combinations thereof.

[0088] Generally, the coating composition comprises an amount of non-reactive solvent to adjust the viscosity of the particles and binder to such a value that thin layers can be applied to the substrate. Preferably, the viscosity will be about 2.0 mPa.Math.s or more, preferably 2.2 mPa.Math.s or more, even more preferably about 2.4 mPa.Math.s or more. Preferably, the viscosity is about 20 mPa.Math.s or less, preferably about 10 mPa.Math.s or less, more preferably about 6 mPa.Math.s or less, and even more preferably about 3 mPa.Math.s or less. The viscosity can be measured with an Ubbelohde PSL ASTM IP no 1 (type 27042).

[0089] Preferably, before curing, the amount of solids in the coating compositions herein is about 5% by weight or less, more preferably about 4%, by weight, or less, even more preferred about 3%, by weight, or less. Preferably the amount of solids is about 0.5%, by weight, or more, more preferably about 1%, by weight, or more, more preferably about 1.5%, by weight, or more.

[0090] The present compositions are suitable for forming optical coatings. As used herein, the term optical coatings refers to coatings with an optical function as major functionality. Examples of optical coatings include those designed for anti-reflective, anti-glare, anti-dazzle, anti-static, EM-control (e.g. UV-control, solar-control, IR-control, RF-control etc.) functionalities.

[0091] Preferably the present coatings are anti-reflective. More preferably the present coatings has a degree of anti-reflective properties such that, when measured for one coated side at a wavelength between 425 and 675 nm (the visible light region), the minimum reflection is about 2% or less, preferably about 1.5% or less, more preferably about 1% or less. The average reflection at one side, over the region of 425 to 675 nm will preferably be about 2.5% or less, more preferably about 2% or less, even more preferably about 1.5% or less, still more preferably about 1% or less. Generally, the minimum in the reflection will be at a wavelength between 425 and 650 nm, preferably at a wavelength of 450 nm or higher, and more preferably at 500 nm or higher. Preferably, minimum is at a wavelength of 600 nm or lower. The optimal wavelength for the human eye is a minimum reflection around 550 nm as this is the wavelength (colour) at which the human eye is most sensitive.

[0092] Preferably, the refractive index of the coating composition is between 1.20 and 1.40 and more preferably between 1.25 and 1.35.

[0093] The coating composition can be applied to a substrate. Any suitable substrate may be used. Preferred are substrates that may benefit from an optical coating especially those that would benefit from an anti-reflective coating. The substrate preferably has a high transparency. Preferably the transparency is about 94% or higher at 2 mm thickness and at wavelength between 425 and 675 nm, more preferably about 96% or higher, even more preferably about 97% or higher, even more preferably about 98% or higher.

[0094] The substrate herein may be organic. For example, the substrate may be an organic polymeric such as polyethylene naphthalate (PEN), polycarbonate or polymethylmethacrylate (PMMA), polyester, or polymeric material with similar optical properties. In this embodiment, it is preferred to use a coating that can be cured at temperatures sufficiently low that the organic substrate material remains substantially in its shape and does not suffer substantially due to thermal degradation. One preferred method is to use a catalyst as described in EP-A-1591804. Another preferred method of cure is described in WO 2005/049757.

[0095] The substrate herein may be inorganic. Preferred inorganic substrates include ceramics, cermets, glass, quartz, or combinations thereof. Preferred is float glass. Most preferred is low-iron glass, so-called white glass, of a transparency of 98% or higher.

[0096] Preferably the coating composition is applied to the article so that the resultant dry coating thickness is about 50 nm or greater, preferably about 70 nm or greater, more preferably about 90 nm or greater. Preferably the dry coating thickness is about 300 nm or less, more preferably about 200 nm or less, even more preferably about 160 nm or less, still more preferably about 140 nm or less.

[0097] Preferably the substrate is cleaned before the coating is applied. Small amounts of contaminants such as dust, grease and other organic compounds cause the coatings to show defects.

[0098] A number of methods are available to apply coatings on substrates. Any method of applying a wet coating composition suitable for obtaining the required thickness would be acceptable. Preferred methods include meniscus (kiss) coating, spray coating, roll coating, spin coating, and dip coating. Dip coating is preferred, as it provides a coating on all sides of the substrate that is immersed, and gives a repeatable and constant thickness. Spin coating can easily be used if smaller glass plates are used, such as ones with 20 cm or less in width or length. Meniscus, roll, and spray coating is useful for continuous processes.

[0099] Once applied to the substrate the coating may require curing. The curing may be carried out by any suitable means which is often determined by the type of binder material used. Examples of means of curing include heating, IR treatment, exposure to UV radiation, catalytic curing, and combinations thereof.

[0100] If a catalyst is used it is preferably an acid catalyst. Suitable catalysts include, but are not limited to, organic acids like acetic acid, formic acid, nitric acid, citric acid, tartaric acid, inorganic acids like phosphoric acid, hydrochloric acid, sulphuric acid, and mixtures thereof, although acid with buffer capacity are preferred.

[0101] In a preferred embodiment the curing is achieved by heating. Curing may be performed as low as room temperature (e.g. 20 C.) although it is generally carried out at about 150 C. or more, preferably about 200 C. or more. Preferably, the temperature will be about 700 C. or less, more preferably about 500 C. or less. Curing generally takes place in 30 seconds or more. Generally, curing is performed in 10 hours or less, preferably 4 hours or less.

[0102] In one embodiment, the coating composition is heat-curable and is applied to a glass plate before a tempering step of said plate. The tempering step is usually carried out at temperature of up to 600 C. In this case the curing and tempering process are thus carried out in one step.

[0103] In one embodiment, after curing the coating treated with water vapour or a combination of water vapour and base. In an alternative embodiment, the coating is treated with water vapour or a combination of water vapour and base prior to curing.

[0104] The water vapour (steam) may be applied to the coating by any suitable means. Preferably the water vapour is added at a temperature of at least 100 C., more preferably at least 150 C., even more preferably at least 200 C., yet even more preferably at least 300 C. and most preferably at least 400 C. Preferably the steam treatment temperature is no more than 600 C. and more preferably no more than 500 C. Conveniently the water vapour can be added after the optional curing step while the oven is still hot.

[0105] The water vapour treatment preferably continues for at least 1 minute, more preferably at least 15 minutes, even more preferably at least 45 minutes. The duration of the treatment is preferably controlled to achieve a desired enlarged pore size.

[0106] The base may be applied to the coating by any suitable means. In a preferred embodiment, the base is added in a gaseous form. In a second preferred embodiment, a pH neutral compound that can liberate a base at higher temperature is embedded in the coating. Any suitable base may be used. Preferred bases include ammonia, primary amines, secondary amines, tertiary amines, metal hydroxides, pyridine, metal amides, primary phosphines, secondary phosphines, tertiary phosphines, primary arsanes, secondary arsanes, tertiary arsanes or a combination thereof. The base may also be derived from any suitable pH neutral compound that can liberate a base, for example, when subjected to higher temperatures. Preferably, the pH neutral compound to be used in the present invention comprises a labile protecting group (P.sub.g) and a base (B) which is covalently linked.

[0107] Preferably, the labile protecting group (P.sub.g) is selected from carbobenzyloxy (Cbz), tert-butyloxycarbonyl (BOC), 9-fluorenylmethyloxycarbonyl (Fmoc), benzyl (Bn), p-methoxyphenyl (PMP), (,-dimethyl-3,5-dimethoxybenzyloxy)carbonyl (Ddz), (,-dimethyl-benzyloxy)carbonyl, phenyloxycarbonyl, p-nitrophenyloxycarbonyl, alkylboranes, alkylaryl boranes, arylboranes or any other suitable protecting group.

[0108] The base (B) to be used in the pH neutral compound can suitably be selected from primary, secondary or tertiary aryl- or alkylamino compounds, aryl or alkyl phosphino compounds, alkyl- or arylarsino compounds or any other suitable other compound.

[0109] During the steam treatment the concentration of water in the environment of the coating is preferably more than 1 gram per cubic meter, more preferably more than 5 gram per cubic meter, most preferably more than 10 gram per cubic meter. During the steam treatment, the concentration of water is preferably less than 1000 gram per cubic meter, more preferably less than 750 gram per cubic meter, most preferably less than 500 gram per cubic meter.

[0110] During the steam treatment with a combination of water vapour and base, the concentration of base is preferably more than 0.00001 gram per cubic meter, more preferably more than 0.0001 gram per cubic meter, most preferably more than 0.001 gram per cubic meter. During the steam treatment with a combination of water vapour and base, the concentration of base is preferably less than 1 gram per cubic meter, more preferably less than 0.1 gram per cubic meter, most preferably less than 0.01 gram per cubic meter.

[0111] The mechanism by which the water vapour and combined water vapour and base treatment improves the properties of the coating is not entirely understood. However, it does not appear that the base is acting as a curing catalyst and the effect is most evident after the coating is already cured. While not wishing to be bound by theory it is believed that the steam treatment causes a surface rearrangement of the coating which creates a small number of enlarged pores, which enables water more readily released, while still maintaining a barrier to solid contamination. This leads to a reduction of staining and aids cleanability.

[0112] It has been found that the coatings according to the present invention show good optical properties and cleanability.

[0113] The coated substrates of the present invention, after immersion in water, as described in test 1, have an increase in reflection of preferably no more than 40%, more preferably no more than 30% and even more preferably no more than 20% after 45 minutes drying under ambient conditions (i.e. 25 C. @ 40% relative humidity) after the coating is immersed in deionised water for 15 minutes at room temperature.

[0114] The invention will now be further illustrated, though without in any way limiting the scope of the disclosure, by reference to the following examples.

EXAMPLES

Example 1

Composition of Formulation (in Weight-%)

[0115]

TABLE-US-00001 2-propanol 90.5 Water 5.0 SiO.sub.2(OH) 1.6 Ethanol 1.4 Methanol 0.7 NeoCryl XK-30* 0.5 Acetic acid 0.2 Nitric acid 0.1

[0116] Core shell particles were produced using latex (NeoCryl XK-30available from DSM NeoResins BV) and tetramethoxysilane according to the method disclosed in WO2009/030703 and in particular page 6, lines 8 to 29, with the resultant silica shell, latex core particles having the following properties:

TABLE-US-00002 pH after dilution with ethanol: 5.7 Particle size of latex in water (determined by DLS): 63 nm Particle size of core-shell particle in water (determined by 79 nm DLS): Particle size of core-shell particle in ethanol (determined by 108 nm DLS): Polydispersity: <0.1 Isoelectric point: 4 to 5 Particle size core-shell after drying (determined by TEM) 55 nm Shell thickness after drying (determined by TEM) 10 nm

[0117] Nitric acid was then added to a pH of 3.6. The particle size was stable at 84 nm for at least two weeks.

[0118] Coating process: The coatings were applied to 1010 cm.sup.2 glass plates (2 mm thickness, Guardian Extra Clear Plus) via dip-coating. 10 mm per second was chosen as appropriate dip speed using the coating formulation as described above. A coating thickness of 120 nm was achieved.

[0119] Curing process: The coated glass substrates were heated to 450 C. (heating rate of 900 C. per hour) then kept at 450 C. for 15 minutes. The oven was then cooled to room temperature to complete the curing process (cooling rate of 300 C. per hour).

[0120] Steam Treatment: [0121] (a) with water vapour: coated articles cured according to the procedure as described above were treated with water vapour at 450 C. for 60 minutes. Water vapour was pumped through the oven (V=0.018 m.sup.3) at an addition rate of water of 4 gram per minute. [0122] (b) with a combination of water vapour and ammonia: coated articles cured according to the procedure as described above were treated with water vapour and ammonia at 450 C. for 30 minutes. Water vapour was pumped through the oven (V=0.018 m.sup.3) at an addition rate of 4 gram per minute. Ammonia was pumped through the oven at an addition rate of 0.020 gram per minute.

Test 1: Immersion in Water at Room Temperature

[0123] The coated substrates were immersed in deionised water at room temperature. The specular reflection at 550 nm was measured before immersion and after 1 minute and 15 minutes of immersion time. After 15 minutes immersion, the coated substrates were allowed to dry under ambient conditions for a period of 45 minutes. After this drying time, the reflection of the coated substrates was determined again. Then, the substrates were heated to 100 C. for 5 minutes. The reflection was determined after the heating step. The results are depicted in Table 1.

TABLE-US-00003 TABLE 1 Immersion test results (reflection (R) minimum in %). R after R after R after R before 1 min 15 min 45 min R after immersion immersion immersion drying heating No post-cure 0.55 3.2 2.8 2.7 0.55 treatment Post-cure 0.52 0.63 0.70 0.66 0.50 treatment with water vapour Post-cure 0.48 0.56 0.52 0.52 0.48 treatment with water vapour and base

[0124] As illustrated in table 1, water uptake leads to an increase of refractive index of the coating and consequently to an increase in reflection. The results clearly show a reduction in specular reflection at 550 nm of the coating upon steam treatment with water vapour or with a combination of water vapour and base.

Test 2: Exposure to High Relative Humidity

[0125] The coated substrates were positioned in a climate chamber at 25 C. and 40% relative humidity. Then, the humidity level was increased to 90%. The coatings were left to equilibrate for about 400 minutes under these conditions. During this equilibration period, the reflection was measured. After equilibration, the humidity was decreased to 40%. The coated substrates were allowed to equilibrate for about 600 minutes in this atmosphere. At the end of this equilibration period, the reflection was measured. At the end of the experiment, the coated articles were heated to 100 C. for 5 minutes. The reflection was determined after the heating step. All reflection values are normed with the starting reflection at 40% humidity and 25 C.

[0126] The results illustrated in FIG. 1 clearly show that the treated coatings have a reduced specular reflection at 550 nm indicative of the coatings having a reduced water uptake and an increased water release.

[0127] Visual observations confirmed that coating comprising the particles of the present invention were free of water stains. In contrast, coating comprising particles without the enlarged pores were more prone to exhibiting water stains.

[0128] FIGS. 3 to 4 illustrates the surface of the coating of example 1 in which the particles of about 40 to 100 nm diameter, each comprises one enlarged pore of about 20 to 50 nm diameter. Prior to the steam treatment (FIG. 2), no visual pores were detected from the atomic force microscopy (AFM) image, indicating that the non-enlarged pores were less than 1 nm.