IMPROVED DURABILITY AND METHODS OF MAKING SAME

GOVERNMENT RIGHTS

[0001] This invention was made with Government support under Contract Nos. N68335-10-C-0523 (2765.01) and N68335-12-C-0117 (2675.02) awarded by the Naval Air Warfare Center. The Government has certain rights to the invention.

FIELD

[0002] The disclosed embodiments herein relate generally to coating materials, especially coating materials that are transparent and

hydrophobic. The coating materials according to the embodiments disclosed herein exhibit improved durability against abrasion and aggressive environments.

BACKGROUND

[0003] Watershedding on surfaces, such as automobile or aircraft windshields, affects the ability of the driver or pilot to see during operation under inclement conditions. A common way to increase the watershed capability of a surface is by applying a hydrophobic surface treatment that lowers the surface energy of the windshield, thereby inducing water to bead up on the windshield surface and roll off. Various types of surface treatments, such as waxes, hydrophobic silicone polymers (e.g., RAIN-X ^® water repellent) or superhydrophobic coatings with very high contact angles and extremely low roll off angles have been developed to improve the hydrophobicity of glass, metal or polymeric surfaces. However, these known surface treatments are not durable. [0Q04] US Patent No. 8,338,351 to Kanagasabapathy et al ¹ describes a coating composition and process for generating transparent, near- transparent and semi-transparent superhydrophobic coatings on surfaces having a contact angle of greater than 165 degrees that comprise hydrophobic nanoparticles of silsesquioxanes containing adhesion promoter groups and low surface energy groups.

[0005] US Patent No. 8, 187,707 to Van Benthem et al describes a superhydrophobic coating exhibiting a contact angle of greater than 140 degrees comprising raspberry shaped particles comprising primary particles and secondary particles adhering to the surface of the primary particles.

[0006] US Patent No. 6,340,502 to Azzopardi et al describes a composition for a hydrophobic/oleophobic coating containing an alkoxysilane and a halosilane each having a perfluorinated group at least at one end of the molecule and a method for applying the coating to a transparent substrate.

[0007] Examples of agents that are known to impart

hydrophobic/oleophobic property are fluorinated alkylsilanes as described in EP 0 492 417, EP 0 492 545 and EP 0 672 779. According to these publications, the hydrophobic layer is obtained from a solution containing fluorinated organosilanes (alkyl-trilaho- or trialkoxysilanes whose alkyl group includes at least one end group that is perfluorinated) in a nonaqueous organic solvent is applied to the surface of a substrate. One of the problems associated with these coatings is that of the coating's durability and erosion resistance, as evidenced by the progressive removal of these coatings, particularly by the action of a car's windshield wiper which makes reapplication of the hydrophobic coating necessary.

The entirety of this and all other publications cited herein is expressly incorporated hereinto by reference. [0008] US Patent No. 6,072,018 to Wilkes et al describes the formation of abrasion resistant inorganic/organic hybrid material formed by sol-gel condensation of at least one metal alkoxide and a low molecular silane- functionalized organic compound such as a di- or tri-amine, an aliphatic diol, an aromatic diol or a triol. The disclosed materials according to this publication functionalize a low molecular weight compound with an isocyanate-functionalized silane and are reported to be abrasion resistant but not hydrophobic.

[0009] There remains a need in the art for transparent hydrophobic coatings to improve the watershed capabilities of glass, polymeric and metallic substrates that are durable in abrasive or aggressive

environments, such as after exposure to acids, bases or organic solvents, rain or ultraviolet light. These needed coatings would be useful in numerous applications including as transparent protective coatings for windshields, solar panels, paints, electronics, and the like. It is towards providing such needed coatings that the embodiments of the present invention disclosed herein are directed.

SUMMARY

[0010] In general, the coating materials as disclosed herein exhibit improved durability, abrasion resistance, solvent resistance (e.g., water, oil, hydrocarbons, jet fuel, solvents, dilute acids, dilute bases), rain resistance, salt water resistance, UV light resistance, flexibility and adhesion to a variety of substrates, including metal, glass, plastic and painted surfaces. The new transparent hydrophobic coating materials of the embodiments disclosed herein thus exhibit improved durability in abrasive or aggressive environments, environmental resistance and chemical resistance that increases watershedding on glass, polymeric and metallic substrates. [001 1] The durable, transparent, inorganic-organic hybrid hydrophobic coating materials for glass, metal or plastic substrates according to the embodiments described herein is generally an acid catalyzed

condensation reaction product comprised of an organic polymeric silane, an inorganic metal alkoxide and a fluorinated silane.

[0012] Certain embodiments will employ a silane functionalized polyol or polyamine as the organic polymeric silane, for example, a polyurethane silane or a polyurea silane. The silane functionalized polyol may, for example, be a polycaprolactone polyol having 2 to 4 hydroxyl groups or a polyamine having at least two primary or secondary amine groups, e.g., diethylenetriamine reacted with an isocyanate-terminated silane. The organic polymeric silane will preferably have a weight average molecular weight of between 50 and 10,000 g/mol.

[0013] According to some embodiments, the metal alkoxide will comprise at least one hydrolyzable compound having at least one silane group represented by the formula Si(R ¹ ) _x (R ² ) _4- x P ^er molecule, wherein R ¹ represents a Ci-C ₈ alkyl group, an epoxide group, a vinyl group, an acrylic group, R ² represents a hydrolyzable aikoxy group or halide group, and x is 0, 1 , 2 or 3. Preferred are tetramethoxysilane and tetraethoxysilane.

[0014] The fluorinated silane may be a compound having the formula Rf ¹ Si(R ¹ )x(R ² ) _3-x where Rf ¹ represents a fully or partially perfluorinated segment, R ¹ represents an alkyl group, a hydrolyzable aikoxy group or halide group, and x is 0, 1 or 2. The fluorinated silane may be a bis-silane terminated polyfluoropolyether or a fluoro-terminated silane, for example, (3,3,3-trifluoropropyl)trimethoxysilane or nonafluorohexyltrimethoxysilane.

[0015] Certain embodiments will have the organic polymeric silane and inorganic metal alkoxide present in a weight ratio of between about 1 :10 to about 10:1. The fluorinated silane may be present in an amount between about 0.0001 to 1 wt.%. [0016] The coating materials may also optionally contain metal oxide nanoparticles to increase abrasion resistance and/or a UV stabilizer to increase UV resistance, corrosion inhibitors for corrosion resistance.

[0017] The coating material may be applied as a coating onto a substrate, e.g., glass, polymeric or metal, and cured to form an optically transparent hydrophobic cured coating thereon.

[0018] The coating material may be made by reacting under acid- catalyzed hydrolysis condensation reaction conditions a reaction mixture comprising an organic polymeric siiane, an inorganic metal alkoxide, and a fluorinated siiane. The aqueous acid catalyst is typically present in an amount sufficient to achieve a pH of the reaction mixture of below 5, e.g., between about 2 to about 4. The acid catalyst may be a mineral acid or an organic acid, e.g., hydrochloric acid, sulfuric acid, nitric acid and acetic acid.

[0019] These and other aspects of the present invention will become more clear after careful consideration is given to the following detailed description of a presently preferred exemplary embodiment thereof.

DETAILED DESCRIPTION

[0020] As noted above the durable, transparent, inorganic-organic hybrid hydrophobic coating materials for glass, metal or plastic substrates according to the embodiments described herein is generally an acid catalyzed condensation reaction product comprised of an organic polymeric siiane, an inorganic metal alkoxide and a fluorinated siiane.

A. Organic Polymeric Siiane

[0021] The organic polymeric siiane component of the coating material will necessarily include either a polyol (including but not limited to diols, triols, tetraols, pentols, and the like) which is siiane functionalized with a metal alkoxide (e.g., an isocyanate terminated silane) through a urethane linkage or a polyamine which is silane functionalized with a metal alkoxide through a urea linkage. The reaction between the polyol and the isocyanate-terminated silane may be catalyzed using a tin catalyst such as dibutyltindilaurate. Other polyols, such as those derived from polyester, polyether, polycarbonate, and the like may also be used.

[0022] In preferred embodiments, the polyol or polyamine is silane- functionalized with isocyanatopropyltrimethoxysilane or

isocyanatopropyltriethoxysilane through urethane or urea linkages, respectively.

[0023] The polyurethane silane can be produced from a wide range of molecular, oligomeric, or polymeric polyether or polyester based polyols possessing at least 2 hydroxyls, preferably 3 or 4 hydroxyls. Polyols with molecular weights between 50 and 10,000 g/mol may be used, preferably 1000-2000 g/mol. For example, CAPA™ brand polyester polyols available commercially from Perstorp Corporation, or ARCOL ^® brand polyether based polyols commercially available from Bayer Material Science may be used.

[0024] Representative polyester and polyether polyols include polycaprolactone diols or triols, polyethyleneoxide diols or triols, polypropylene diols and triols with weight average molecular weights within the ranges noted above may satisfactorially be employed.

Preferably, a polycaprolactone triol with the structure:

where m+n+p =7-16 may be used.

[0025] Polyamines can alternatively be used in the same manner with reaction of the isocyanate-terminated siiane through an urea linkage. The polyurea siiane can be produced from a wide range of molecular, oligomeric, or polymeric polyamines possessing at least 2 primary or secondary amine groups, preferably 3 or 4 amines per molecule.

Polyamines with molecular weights between 50 and 10,000 g/mol may be used, preferably 100-1000 g/mol. For example, diethylenetriamine or JEFFAMINE ^® amines commercially available from Huntsman

Petrochemical Corporation, such as JEFFAMINE ^® T-403 polyether amine may be employed satisfactorily.

[0026] By the term, "polyamine" as used herein, it is meant any aliphatic or aromatic compound containing two or more primary or secondary amine functional groups. The polyamine compound may have any suitable backbone chain structure including saturated or unsaturated, and linear, branched, or cyclic. Representative polyamines include polyetheramines such as diamines with the structure: where x =2-70, preferably x is 2-7.

[0027] Alternatively the polyetheramine is a triamine with the structure:

where n=0-5, and x +y + z are 3-100. Preferably n=1 and x+y+z = 5-6. B. Inorganic Metal Alkoxide

[0028] The inorganic metal alkoxide component of the coating material comprises at least one metal alkoxide, such as those based on Si, Al, Ti, Zr, and the like. Preferred are silicon alkoxides. The silicon alkoxides may also comprise monofunctional organic moieties such as epoxide, alkyl, phenyl, vinyl, mercapto, methacrylate, and the like or be bis-silane terminated, such as bis-trimethoxysilylethane.

[0029] The preferred metal alkoxide comprises at least one

hydrolyzable compound having at least one silane group, Si(R ¹ ) _x (R ² )4 _-X) per molecule, wherein R ¹ represents an alkyl group (for example a C Cs, polymerizable group (e.g. epoxide, vinyl, acrylic), or other alkyls

terminated with another organic moiety (hydroxyl, isocyanate, amino, thiol, etc.), R ² represents a hydrolysable group (for example an alkoxy or halide group, preferably methoxy, ethoxy or chloro) and x is 0, 1 , 2, 3.

Preferably, the metal alkoxide is tetraethoxysilane or tetramethoxysilane.

C. Fluorinated Silane

[0030] Representative examples of fluorinated silane compounds include those having the formula Rf ¹ Si(R )x(R ² )3 _-x where Rf ¹ represents a fully or partially perfluorinated segment (for example a 3,3,3- trifluoropropyl, (perfluorobutyl)ethyl, (perfluorohexyl)ethyl,

(perfluorooctyl)ethyl, perfluorododecyl, perfluorotetradecyl,

heptadecafluoro-1 ,1 ,2,2-tetrahydrodecyl, nonafluorohexyl or tridecafluoro- 1 ,1 ,2,2-tetrahydrooctyl), R ¹ represents an alkyl group (for example a d- C ₈ , preferably Ci -C ₄ primary or secondary alkyl group) and R ² represents a hydrolysable group (for example an alkoxy or halide group, preferably methoxy, ethoxy or chloro) and x is 0, 1 , or 2.

[0031] Preferably, fluorinated silane compounds according to formula Rf ² [Q-C(R) ₂ - Si(R ¹ ) _x (R ² ) _3- x]z are used wherein Rf ² represents a multivalent poly(perfluorooxyalkyl) or poly(perfluoroxyalkylene) segments, Q represents an organic divalent linking group (examples include amide, ether, ester or urethane linking group), R ¹ represents an alkyl group (for example a C-i-Cs, preferably Ci -C ₄ primary or secondary alkyl group) and R ² represents a hydrolysable group and x is 0, 1 , or 2; R represents hydrogen or an alkyl group of 1 to 4 carbon atoms and the R groups may be the same or different. Preferably R is hydrogen.

[0032] The hydrolysable groups R ² may be the same or different and are generally capable of hydrolyzing under appropriate conditions, for example under acidic aqueous conditions, such that the fluorochemical silane compound can then undergo condensation reactions. Preferably, the hydrolysable groups upon hydrolysis yield groups capable of undergoing condensation reactions, such as silanol groups. [0033] Certain embodiments will employ a fluorinated silane

component which comprises either a bis-silane terminated

polyfluoropolyether or a fluoro-terminated silane, such as (3,3,3- trifluoropropyl)trimethoxysilane, nonafluorohexyltrimethoxysilane, and the like.

D. Metal Oxide Particles

[0034] Metal oxide particles may optionally be used in the coating formulation to impart desired properties, such as abrasion resistance, electrical or optical properties. For example, metal oxide particles of silica, titania, zirconia, and/or alumina may be used. Silica (S1O2) is preferred. For optical transparency, it is preferred that the particles are less than about 100 nm, e.g. between about 1 nm to about 100 nm. The preferred particle size is 1-10 nm diameter spherical nanoparticles. If present, the particles can be included in the formulation up to about 50 wt.%, preferably between about 20 wt.% to about 30 wt.%, based on total formulation weight.

D. Composition and Properties

[0035] The composition of the formulation can vary depending on the desired final properties for flexibility, hardness, abrasion resistance, transparency, or other desired physical properties. Generally the weight ratio of the polymeric silane to the metal alkoxide or organic functional metal alkoxide in the formulation may be between about 1 :10 to about 10:1 , preferably about 3:1. The weight percentage of the fluorosilane in the formulation could be used in a range of from about 0.0001 to 1 weight %, preferably between about 0.0005 to about 0.001 weight %.

[0036] The coating materials may be produced by mixing the inorganic and organic components in a suitable solvent, such as isopropanol, with water and an aqueous acid catalyst. The aqueous acid catalyst is added to initiate the hydrolysis of the hydrolyzable silane groups. Preferred acid catalysts include mineral acid such as hydrochloric acid, sulfuric acid and nitric acid, or an organic acid, such as acetic acid. Sufficient acid catalyst is added to reduce the pH of the reaction mixture to below 5, preferably a pH of between about 2 to about 4.

[0037] The fluorinated silane component may then be added directly to the solution of inorganic and organic components. Alternatively, the fluoro component may be prehydrolyzed with acidified water in a suitable solvent with or without the aid of a fluoro-functionalized surfactant prior to addition to the coating solution. The coating formulation is produced by hydrolysis and condensation of the organic, inorganic and fluoro silane components, leading to a fluoro-functionalized organic-inorganic network through Si-O- Si bonds.

[0038] The thus obtained coating formulation may be mixed in a solvent, or alternatively without a solvent. If used, the solvent may be an alcohol (methanol, ethanol, propanol, isopropanol, butanol, or the like) or other water miscible solvents, such as acetone. The concentration of the solids in the formulation will depend on the desired thickness for the end application, or application methods. Typically however, the formulation will have between about 5 wt.% to about 100 wt.% solids, with a preferred solids concentration being between about 15 wt.% to about 25 wt.%.

[0039] A solution of the coating material may be applied to the substrate using any convenient coating method including dip, brush, flow coat, spray, and the like. The curing of the coating can be accomplished at a wide range of temperatures depending upon the desired properties, for example abrasion resistance, flexibility, etc., or thermal limitations for the coated substrate. The coating may be cured at temperatures ranging from about 25 °C to about 150 °C, preferably about 75°C. The

temperature of curing may be modified for compatibility with the substrate. [0040] The thickness of the cured coating may range from about 0.5 micron to about 20 microns, preferably from about 1 micron to about 5 microns.

[0041] The cured hydrophobic coatings as described herein will exhibit optical transparency. Specifically, the cured hydrophobic coatings will exhibit a transparency to visible light of at least about 99.5%, preferably at least about 100%. The coatings will also exhibit a change in haze to visible light less than 1% and preferable less than 0.1%.

[0042] The hydrophobic coatings described herein are suitable for coating a variety of substrate materials to provide increased chemical resistance, oil repellency, water repellency, liquid/gas barrier, abrasion resistance, corrosion resistance, and watershed capability to the substrate. Suitable substrates include, but are not limited to, glass, metals such as aluminum and steel, plastics such as polycarbonate and acrylic, hardened cement, concrete, or grout, wood and painted surfaces.

[0043] The present invention will be further understood by reference to the following non-limiting examples thereof.

EXAMPLE 1

[0044] A coating material comprising the following formulation was applied as a coating of about 1-5 micron thickness onto a 3/16 inch thick polycarbonate substrate. The coating was cured at a temperature of about 90 °C. The coated substrate was thereafter tested for contact angle, watershed angle and visual appearance including transparency, haze and clarity characteristics and compared to an uncoated

polycarbonate substrate. Coating Material Formulation:

Synthesis of Silane Functional Polyol:

[0045] Polycaprolactone polyol is measured into cleaned and thoroughly dried glassware. In a separate cleaned and thoroughly dried piece of glassware, the correct molar ratio of isocyanate silane is measured (e.g. a polycaprolactone diol would require twice the molar amount of isocyanate to caprolactone). The isocyanate glassware is covered with a nitrogen blanket and sealed with a rubber septum. The catalyst (i.e. dibutyltin dilaurate) is measured into the polycaprolactone polyol. It is to be 0.1%, by weight, in relation to the combined

measurements of isocyanate and polycaprolactone polyol. This bottle is then also covered with a nitrogen blanket and sealed with a rubber septum.

[0046] The polycaprolactone polyol and catalyst are set to stir in an ice bath. The isocyanate is slowly added dropwise into the stirring

caprolactone-catalyst mixture, using a positive nitrogen flow to control the addition rate. The ice bath should be maintained during the addition step; the reaction generates heat and the ice bath decreases the chances of side reactions. Once all of the isocyanate is added, the reaction is allowed to come to room temperature as the ice bath melts. The reaction is stirred for at least six hours at ambient temperature. The reduction of the isocyanate peak (~2270 cm ^"1 ) can be measured via FTIR. Once the isocyanate is fully reacted and the peak removed, the polyurethane silane should be bottled and covered with nitrogen. The bottle should be amber glass, or other dark glass, to reduce the exposure to light, as this may cause discoloration of the remaining catalyst.

Standard Coating Synthesis:

[0047] The silanes are mixed first. A molar ratio of 0.4 moles organic silane and 0.6 moles of inorganic silane is typical. A small amount of a fluorinated silane, not to exceed 0.5% of the total coating formulation, is added. To this mixture, a fluorosurfactant is added in a comparable amount to the fluorosilane. This will allow for better mixing of the fluorosilane in the solvent-based system.

[0048] Next, the solvent is added to the silane mixture. The solvent volume should be approximately 50-80% of the total coating solution, but can be up to 90% depending on the desired coating thickness. Typical solvents include isopropanol, ethanol, or 1-propanol.

[0049] Following full dispersal of the silanes into the solvent, acidified water is added. The water should be acidified to a molarity of 0.05-0.1M, depending on the rate of hydrolysis desired. Hydrochloric acid or nitric acid can be used to decrease the pH of the water solution. The acidified water is added in the molar ratio sufficient to hydrolyze the alkoxy groups on the silanes.

[0050] The coating solution will be mixed for 1-2 hours depending on hydrolysis completion. Once the silanes are hydrolyzed, the coating is filtered through a 1 μΐη filter prior to coating. Coating application can be performed using a flow, brush, spray or dip coat method for best results. Once coated, allow excess coating to roll off the substrate before curing. The coating is tack free in approximately 5 minutes at ambient temperature. The coating may alternatively be cured at 75-90°C for at least 30-60 minutes for increased hardness and toughness.

Table 1. S ecific coatin formulation

[0051] Table 2 shows increased water contact angle and decreased watershedding angle for selected coated substrates.

Table 2. Water contact angle and watershedding angle for coated and upcoated substrates

[0052] Table 3 shows excellent optical properties for coated substrates, with no loss of transparency, haze, or clarity.

Table 3. Optical properties for coated and uncoated substrates

[0053] Table 4 shows that the coatings provide improved abrasion resistance to the treated substrate as measured by lower haze values. Table 4. Effect of Taber Abrasion on light transmission properties for coated and upcoated substrates (ASTM D4060 - 500g/500cycles; CS10F wheels)

[0054] The addition of colloidal silica (Nissan Chemical Snowtex IPA- ST) has the ability to improve abrasion resistance as shown in Table 5.

Table 5. Effect of abrasion (Reciprocal steel wool) with addition of colloidal silica: Coating A vs Coating B

EXAMPLE 2

Synthesis of Silane Functional Polyamine:

[0055] The polyamine (e.g. diethylenetriamine) was measured into cleaned and thoroughly dried glassware. In a separate cleaned and thoroughly dried piece of glassware, the correct molar ratio of isocyanate silane was measured (e.g. diethylenetriamine requires three times the molar amount of isocyanate). The isocyanate glassware was covered with a nitrogen blanket and sealed with a rubber septum.

[0056] The polyamine was set to stir in an ice bath. The isocyanate was slowly added dropwise into the stirring amine mixture, using a positive nitrogen flow to control the addition rate. The ice bath was maintained during the addition step since the reaction generates heat and the ice bath decreases the chances of side reactions. Once all of the isocyanate was added, the reaction was allowed to come to room temperature as the ice bath melts. The reaction was stirred for at least six hours at ambient temperature. The reduction of the isocyanate peak (~2270 cm-1) was measured via FTIR. Once the isocyanate was fully reacted and the peak removed, the polyurea silane was bottled and covered with nitrogen.

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[0057] While the invention has been described in connection with what is presently considered to be the most practical and preferred

embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope thereof.