Patent Application
20080090010
Embodiments of the present invention will now be described in further detail.
There are two major factors which determine the degree of hydrophobicity of a surface: chemical composition and surface roughness. By lowering the surface energy it is possible to increase the hydrophobicity of a surface. An increase in surface roughness directly results in an increase in hydrophobicity.
When a coating composition of the present invention is applied to a surface and cured, a coating is formed comprising nanoparticles and microparticles such that the coated surface has a roughness in nanoscales and microscales.
The nanoscale and microscale roughness is illustrated in the following schematic representation:
In the schematic representation, the larger particles are the microparticles. The smaller particles are nanoparticles.
A “sol” is defined as solution of colloidal particles.
In a preferred embodiment of the present invention, the coating composition comprises a mixture of a sol solution prepared by the hydrolysis and condensation of a tri-functionalised alkylsilane, microparticles and an organic solvent.
In another preferred embodiment of the present invention, the coating composition comprises a mixture of a tri-functionalised alkylsilane, a catalyst for initiating the formation of a sol solution, an organic solvent, and microparticles. When such a coating composition is exposed to air during the curing process (e.g. on application to a surface), the catalyst initiates the hydrolysis and condensation of the tri-functionalised alkylsilane to form a sol solution.
A sol solution may be prepared by the hydrolysis and condensation of a tri-functionalised alkylsilane. Tri-functionalised alkylsilanes are compounds having a silicon atom bonded to an alkyl group and three functional groups capable of undergoing hydrolysis and condensation reactions. The functional groups may be any group capable of undergoing hydrolysis and condensation. Typically, the tri-functionalised alkylsilane is a trialkoxyalkylsilane.
A sol solution may also be prepared by the hydrolysis and condensation of the one or more compounds of the formula (A), optionally together with one or more additional compounds selected from the group consisting of compounds of the formula (B) and compounds of the formula (C), wherein the compounds of the formula (A), (B) and (C) are as described above.
The hydrolysis and condensation reaction forms hydrophobic covalently-linked networks. These networks form hydrophobic particles. These hydrophobic particles are nanoparticles or capable of reacting with further alkylsilane (or further compounds of the formula (A), (B) or (C)) to form hydrophobic nanoparticles. As the sol solution begins to dry, e.g. during curing, a covalently-linked network of hydrophobic nanoparticles is typically formed.
The hydrolysis and condensation reaction which results in the production of the hydrophobic nanoparticles is a modified sol-gel reaction. Typically, the sol solution is prepared by the hydrolysis and condensation of a tri-functionalised alkylsilane, typically, a trialkoxyalkylsilane of the formula R1Si(OR)3 wherein R1 is an alkyl group, typically a C1-30 alkyl, and each R is independently selected and is an alkyl group, typically a C1-3 alkyl. The modified sol-gel reaction is described below by reference to the reaction of a trialkoxyalkylsilane. The modified sol-gel reaction comprises two main reactions which usually occur concurrently:
Examples of the covalently-linked networks that may be formed by such reactions include the silsesquioxane or the amorphous polysilsesquioxane, or “ormosil”, shown below. Ormosil is an acronym for organically modified sols.
Typically, the sol solution is prepared by mixing one or more tri-functionalised alkylsilanes and a catalyst in an organic solvent. The catalyst initiates the formation of the sol solution, and the organic solvent facilitates dispersion and/or solubilisation of the reactants. The catalyst may be selected from the group consisting of acidified water, alkaline water, a tin catalyst and a zinc catalyst, e.g. dibutyltin dilaurate, tin octoate or zinc octoate. The organic solvent may be selected from the group consisting of an alcohol (e.g. methanol, ethanol, isopropanol and butanol), ethyl acetate, butyl acetate, toluene, hexane, light petroleum, diethylether, methylethylketone, tetrahydrofuran, and xylene. Preferably, the organic solvent is an alcohol.
As mentioned above, in some embodiments of the invention, the coating composition comprises a mixture of a sol solution (which has been prepared by the hydrolysis and condensation of a tri-functionalised alkylsilane), microparticles and an organic solvent. In further embodiments, a di- or tri-functionalised alkylsilane is added to the sol solution before the sol solution is mixed with microparticles. The tri-functionalised alkylsilane may be the same or different to the tri-functionalised alkylsilane used in the formation of the sol solution. The additional di- or tri-functionalised alkylsilane reacts with itself and with the hydrophobic nanoparticles to form a covalently-linked network of hydrophobic nanoparticles during the curing of the coating composition. Further, the di- or tri-functionalised alkylsilane may also react with functional groups on the microparticles and functional groups on the surface to facilitate binding of the hydrophobic nanoparticles to the microparticles and to the surface.
As mentioned above, in other embodiments of the invention, the coating composition comprises a mixture of a tri-functionalised alkylsilane, a catalyst for initiating the formation of a sol solution, an organic solvent, and microparticles. When such a coating composition is exposed to air to begin the curing process (e.g. on application to a surface), the catalyst initiates the hydrolysis and condensation of the tri-functionalised alkylsilane to form a sol solution.
Suitable tri-functionalised alkylsilanes are alkylsilanes having three functional groups and one alkyl group that are capable of undergoing a modified sol-gel reaction. The three functional groups may be the same or different. The functional groups may, for example, be acetoxy, enoxy, oxime, alkoxy and amine. The tri-functionalised alkylsilane may, for example, be selected from the group consisting of trialkoxyalkylsilanes, triacetoxyalkylsilanes, trienoxyalkylsilanes, triaminoalkylsilanes, trioximealkylsilanes and mixtures thereof. In some embodiments, the tri-functionalised alkylsilane is a trialkoxyalkylsilane, e.g. a trialkoxyalkylsilane selected from the group consisting of methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, and mixtures thereof. A mixture of different tri-functionalised alkylsilanes may be used.
The alkyl group on the tri-functionalised alkylsilane may be, for example, methyl, ethyl, propyl, butyl or octyl.
The tri-functionalised alkylsilane may be copolymerised with a hydrophobic polymer. Advantageously, the hydrophobic polymer increases the hydrophobicity of the resultant coating and also enhances the binding of the nanoparticles to each other. The hydrophobic polymer may also react with functional groups on the microparticles (e.g. the hydrophobic polymer may react with hydroxyl groups on the microparticles), and with functional groups present on the surface of the substrate, to enhance the binding of the nanoparticles with the microparticles and the surface. The enhancement in binding results in improved durability and elasticity of the resultant coating.
The hydrophobic polymer may be a hydroxy-terminated polysiloxane. Examples of suitable hydroxy-terminated polysiloxanes include hydroxy-terminated polydimethylsiloxanes (PDMS), hydroxy-terminated polydimethylsiloxane-co-polyphenylmethylsiloxanes, hydroxy-terminated polydiphenylsiloxanes, hydroxy-terminated vinylsiloxane polymers, hydroxy-terminated polyphenylmethylsiloxanes, hydroxy-terminated vinylmethoxysiloxane homopolymers, silanol-terminated polytrifluoropropylmethylsiloxanes, silanol-terminated vinylmethylsiloxane-co-dimethylsiloxanes, and mixtures thereof.
Preferably, the microparticles have surfaces bearing hydroxyl groups. Such hydroxyl groups are able to react with other reactive groups in the various components of the coating composition (such as the hydroxy-terminated polysiloxane) during the curing of the coating composition to strengthen the binding of the microparticles to the nanoparticles and to the surface of the substrate.
Typically, the microparticles are particles of a cementitious material (such as Portland cement and gypsum), an inorganic oxide (which may also be a colorant) or a fibreglass material. Other materials that may be used include clay and fumed silica. Because cementitious materials impart better durability to the coating than many other microparticles, the microparticles are preferably particles of a cementitious material. The inorganic oxide may be particles of any inorganic oxide such as iron oxide red, iron oxide black, iron oxide yellow, iron oxide brown, iron oxide green, titanium(IV) oxide, chromium oxide green, and mixtures thereof.
Any inert volatile organic solvent may be used. For example, the organic solvent may be selected from the group consisting of an alcohol (e.g. methanol, ethanol, isopropanol and butanol), ethyl acetate, butyl acetate, toluene, hexane, light petroleum, diethylether, methylethylketone, tetrahydrofuran, and xylene.
In addition, other additives may be included in the coating composition of the present invention. For example, a colorant, sand or a cementitious material may be added to the coating composition. Colorants (e.g. a pigment or dye) may be added to provide colour to the coating composition. The addition of sand reduces the amount of cement that is required and improves the hardness and strength of the coated surface. Cementitious materials may be added to increase the durability of the coated surface.
The colorant may be any inorganic oxide, such as iron oxide red, iron oxide black, iron oxide yellow, iron oxide brown, iron oxide green, titanium(IV) oxide, chromium oxide green, and mixtures thereof.
Typically, the size of the nanoparticles ranges from about 1 nm to about 200 nm. Preferably, the size of the nanoparticles ranges from about 1 nm to about 50 nm, and more preferably from about 1 nm to about 20 nm.
Typically, the size of the microparticles ranges from about 1 μm to about 100 μm. Preferably, the size of the microparticles ranges from about 1 μm to about 50 μm, and more preferably from about 1 μm to about 20 μm.
In some embodiments, the composition of the present invention comprises the following components in the proportions indicated:
1MTMS = methyltrimethoxysilane
2PDMS = polydimethylsiloxane
3OTES = octyltriethoxysilane
When a coating composition of the present invention is applied to a surface and cured, a coating comprising hydrophobic nanoparticles and microparticles is formed, wherein the nanoparticles and microparticles give the coating a roughness in nanoscales and microscales. When the composition comprises, inter alia, a sol solution or reagents to form a sol solution, a covalently-linked network of nanoparticles is formed. Preferably, the hydrophobic nanoparticles are linked to the microparticles and to the surface. The arrangement of the hydrophobic nanoparticles and microparticles in the coating results in the coating having both nanoscale and microscale roughness. Both the hydrophobicity of the nanoparticles, together with the microscale and nanoscale roughness, contributes to the hydrophobicity of the coating.
Tests conducted by the inventors have shown that superhydrophobic surfaces with contact angles greater than 130° and contact angle hysteresis of less than 20° may be produced using such coating compositions.
The hydrophobic coating composition may be prepared by mixing nanoparticles, or precursors capable of forming nanoparticles, and microparticles and an organic solvent to form a slurry. The nanoparticles may be provided by a sol solution as described above.
In one form, the coating composition comprises a mixture of a sol solution prepared by the hydrolysis and condensation of a tri-functionalised alkylsilane, microparticles, and an organic solvent. Such a composition may be prepared by mixing the sol solution, microparticles, and the organic solvent and stirring until the composition is formed as a slurry. Stirring may be carried out at room temperature (e.g. about 15° C. to about 30° C.) or at a suitable elevated temperature (e.g. up to about 80° C.). Alternatively, the mixture may be sonicated in an ultrasonic bath. Typically, the mixture is stirred at room temperature for between about 1 min and about 1 hour, e.g. about 0.5 hour, or sonicated for about 5 min to 10 min at room temperature.
For example, an embodiment of the composition of the present invention may be prepared by mixing methyltrimethoxysilane (MTMS) (100 g), polymethylsiloxane (PDMS) (0-200 g) and ethyl acetate (50-150 mL), and then stirring the mixture for 3 to 6 hours at 60° C. to form a sol solution. The resulting sol solution may be blended with gypsum or cement (having a particle size of about 10 μm to 100 μm) in a ratio of 1:0.2-5 by weight to form a coating composition of the invention as a slurry. The slurry may then be applied to a surface of a substrate and cured. Typically, after curing at room temperature for 24 hours, the resulting surface has a water contact angle greater than 165°.
In another form, the coating composition comprises a mixture of a tri-functionalised alkylsilane, a catalyst for initiating the formation of a sol solution, an organic solvent, and microparticles. Such a composition may be prepared by mixing the tri-functionalised alkylsilane, the catalyst for initiating the formation of a sol solution, the organic solvent, and microparticles and stirring until the composition is formed as a slurry. Stirring may be carried out at room temperature or elevated temperature. Alternatively, the mixture may be sonicated in an ultrasonic bath. Typically, the mixture is stirred at room temperature for between about 1 min and about 1 hour, e.g. about 0.5 hour, or sonicated for about 5 min to 10 min at room temperature.
The composition may be applied to the surface of the substrate by any means known in the art for applying slurries to a surface. The coating composition may, for example, be applied by brushing, dip coating, rolling or spraying.
The composition of the present invention is ideally suited to be applied to substrates such as bricks, cement tiles, wall facades (render) and grout.
Curing may be carried out at room temperature (e.g. about 15° C. to about 30° C.) or at suitable elevated temperatures, e.g. up to about 80° C., in the presence of air. Typically, however, curing is carried out at room temperature. The duration of the curing process is typically between about 12 hours and 48 hours.
During curing, the organic solvent evaporates leaving a coating on the surface comprising nanoparticles and microparticles. A proportion of the nanoparticles are located on the surface of the microparticles thereby forming a coating having a roughness in microscales and nanoscales. If the coating composition comprises precursors capable of forming nanoparticles, then the precursors form the nanoparticles during curing.
When a coating composition comprising a sol solution prepared by the hydrolysis and condensation of a tri-functionalised alkylsilane, microparticles, and an organic solvent is cured, a coating is formed on the surface of the substrate.
When a coating composition of the present invention comprising a mixture of a tri-functionalised alkylsilane, a catalyst for initiating the formation of a sol solution, an organic solvent, and microparticles begins to cure, the catalyst initiates the hydrolysis and condensation of a tri-functionalised alkylsilane to form a sol solution. The hydrolysis and condensation of the tri-functionalised alkylsilane forms hydrophobic covalently-linked networks which form hydrophobic nanoparticles or are capable of reacting with further alkylsilane to form hydrophobic nanoparticles. On curing, the hydrophobic nanoparticles become linked to each other and, typically, also become linked to the microparticles and to the surface of the substrate to form a coating on the surface of the substrate.
When applied to a surface and cured, the coating composition of the present invention produces a hydrophobic coating on the surface. In preferred embodiments of the invention, the water contact angle is greater than 130°, i.e. the coated surface is superhydrophobic. Thus, a water droplet placed on the treated surface easily beads on the surface and rolls off at the slightest vibration. Typically, the contact angle hysteresis is less than 20°.
In more preferred embodiments, the water contact angle is greater than 150°.
In most preferred embodiments, the water contact angle is greater than 160°.
The coating composition of the present invention can be used to form a hydrophobic coating on the surface of a substrate to render the surface water-resistant. Such coatings can be used to reduce or inhibit fouling of the surface by biological organisms, dirt, ice or chemicals.
Preferred embodiments of the invention will now be described, by way of example only, with reference to the following Examples.
This example describes an embodiment of the hydrophobic coating composition. The components of the coating composition are set out in Table 1 below by parts per weight.
1MTMS = methyltrimethoxysilane
2PDMS = polydimethylsiloxane
3Tin catalyst = dibutyltin dilaurate
4OTES = octyltriethoxysilane
A mixture of methyltrimethoxysilane (MTMS) (100 g), polymethylsiloxane (PDMS) (10 g), tin catalyst (1 g) and toluene (200 g) was stirred at 60° C. for 3 hours. The resultant sol solution was then blended with octyltriethoxysilane (OTES) (10 g) and 3-aminopropyl-triethoxysilane (2 g). This mixture was then added to a cement/sand/pigment (50/15/10 wt/wt/wt) mixture to form a slurry.
The slurry was applied to a wet concrete surface by brushing, and then cured at 40° C. for 12 hours.
After curing at room temperature for 24 hours, the coated surface showed extreme water resistance and had a water contact angle larger than 165°.
The contact angle was measured on a Ramé-Hart goniometer in conjunction with RHI 2001 Imaging System software. Measurements were reproduced until five concordant results were obtained. All measurements were performed on a dry section of the sample, to negate any chemical changes due to wetting. A sessile drop was used to measure contact angles with the addition and subtraction of water from the drop facilitating the measurement of the advancing and receding contact angles.
This example describes three embodiments (A, B and C) of the hydrophobic coating composition. The components of the coating compositions are set out in Table 2.1 below by parts per weight.
1OTES = octyltriethoxysilane
2MTMS = methyltrimethoxysilane
3PDMS = polydimethylsiloxane
4Tin catalyst = dibutyltin dilaurate
The components were mixed together and sonicated for about 5 to 10 minutes to the form embodiments A, B and C as a slurry. Each of the resultant slurries was then deposited on the substrate by dip coating and allowed to air dry for about 10 to 30 minutes. The coated substrate was then placed in an oven at 60° C. for about 18 to 24 hours.
The water contact angles of the coated substrates are set out in Table 2.2. The water contact angles were measured as described above in Example 1.
Although the invention has been described with reference to particular examples, it will be appreciated by those skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. All such variations and/or modifications are to be considered within the scope of the present invention the nature of which is to be determined from the foregoing description. It will be appreciated by those skilled in the art that the invention may be embodied in many forms. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.
In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004900175 | Jan 2004 | AU | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AU05/00043 | 1/14/2005 | WO | 00 | 7/14/2006 |
