SYSTEMS FOR IMPARTING NON-STICK AND NON-WETTING PROPERTIES TO SURFACES

Abstract: A material system and method for imparting non-stick and non-wetting properties to an inorganic article having hydroxyl groups on its surface, the system comprising a silane having the formula RnSiX4-n, wherein n is 1-2 and X is a hydrolyzable group, a metal alkoxide having at least three hydrolyzable branches and a formula R MX3 or MX4, wherein M is a metal and X is a hydrolyzable branch, and an acidified aqueous mixture with a lower alkanol.

Description: SYSTEMS FOR IMPARTING NON-STICK AND NON- WETTING PROPERTIES TO SURFACES RELATED APPLICATION An application entitled SYSTEMS FOR IMPARTING NON-STICK AND NON- WETTING PROPERTIES TO SURFACES, filed under S N 60/007,704 in the names of A. Carre, M Prassas and J Wakυ and assigned to the same assignee as this application, is directed to a material system and method for imparting non-stick and non-wetting properties to an inorganic article having hydroxyl groups on its surface, the system comprising a silane having the formula R„MX 4 . n wherein n is 1 or 2 and X is a hydrolyzable group, a colloidal metal oxide sol, and an acidified aqueous mixture with a lower alkanol The present application claims the benefit of French Application 95 1 1373, filed September 28, 1995 (28-09-1995), and of U S Provisional Application No 60/007,705, express mailed November 29, 1995, entitled SYSTEMS FOR IMPARTING NON¬ STICK AND NON- WETTING PROPERTIES TO SURFACES, by A Carre, M Prassas and J Waku FLELD OF THE INVENTION The field is a material system and a process for coating articles, particularly glass and glass-ceramic articles, to impart durable, non-stick and non-wetting properties BACKGROUND OF THE INVENTION Considerable effort has been directed at producing an effective, non-stick and non- wetting coating on the surface of an inorganic article, in particular a utensil for preparing and/or serving food The term "non-stick" signifies a surface that resists adherence of foreign substances, such as food. This property permits easy cleaning of dishes in which foods are cooked or baked. The term "non-wetting" indicates a surface that repels liquids such as water. The property is evidenced by a large contact angle between a drop of water and the surface on which the drop rests An advancing angle of at least 90° is considered as representative of a non-wettable surface. Non-stick, non-wetting articles are commonly produced by coating article surfaces with a polytetrafluoroethylene (PTFE) layer. For example, United States Patent No. 4,683, 168 (Hares et al ) describes a process for coating glass or glass-ceramic articles with such a layer to produce non-stick cooking utensils. However, PTFE coatings exhibit the disadvantage of being opaque. Also, they require a thermal sintering process at 350°- 400°C for their production, are relatively expensive to produce, and do not provide a desired degree of abrasion resistance. French Patent No 2674862 (Carre) discloses an alternative material system and process for producing durable, non-stick and water repellent coatings on glass and glass- ceramic surfaces. The process involves applying a mixture of a selected silane with a lower alkanol and water which may be acidified. The silane has the formula R^SiX^, where R is an alkyl radical and X is a hydrolyzable alkoxy or chloro group The mixture is applied to a surface containing hydroxyl groups, the silane is hydrolyzed and the alkanol-water mixture eliminated to form a polysiloxane coating adhering to the surface Numerous other processes and material systems have been proposed for rendering inorganic surfaces, particularly vitreous surfaces, non-sticking and water repellent The desired properties can be obtained initially. However, there has persisted a problem with respect to wear during use, in particular, scratching by cleaning agents. It is, then, a purpose ofthe present invention to improve on the Carre process by imparting a greater abrasion resistance to the coating there disclosed SUMMARY OF THE INVENTION The present invention resides in part in a material system for imparting non-stick and non-wetting properties to an inorganic article having hydroxyl groups on its suiface, the system comprising a silane having the formula R^SiX^,, wherein n is 1-2 and X is a hydrolyzable group, a metal alkoxide having at least three hydrolyzable branches and a formula R MX 3 or MX 4 wherein M is a metal and X is a hydrolyzable branch, and an acidified aqueous mixture with a lower alkanol. The invention further resides in a method of imparting non-stick and non-wetting properties to an inorganic article having hydroxyl groups on its surface which comprises mixing a silane having a formula R^SiX^,, with a metal alkoxide having a formula R MX 3 , or MX 4 , adding an acidified aqueous mixture of a lower alkanol, applying a coating ofthe mixture to a surface on the inorganic article, thermally processing the coating at a temperature of at least 100°C to form a solid, transparent film on the article surface DESCRIPTION OF THE INVENTION The present invention arose from studies directed at improving the polysiloxane, non-stick coating disclosed in French Patent No. 2674862 (Carre) The non-stick properties of he Carre coating are adequate However the coating, like other known non-stick coatings, is prone to damage and removal. This is particularly true when the coating is subjected to the abrasion occasioned by normal scrubbing Accordingly, efforts were directed at improving the abrasion-resistance ofthe known coating while either retaining, or improving on, its non-stick characteristics We have now found that the desired improvement can be achieved by incorporating a selected metal alkoxide with the prior silane component to achieve a hybrid coating. The coating retains the good non-stick and non-wetting characteristics of the prior polysiloxane coating It adds substantially better thermal stability and resistance to detergent and abrasion treatment. The new hybrid coating material, like the earlier silane coating material, is effective when applied to an inorganic article having hydroxyl groups on its surface, in particular, a glass or glass-ceramic article. The silane and the metal alkoxide are hydrolyzed to produce hydroxyl groups These groups, produced during hydrolysis, link with the hydroxyl groups on the article surface to create a surface exhibiting hydrophobic properties The coating material represents a mixture of at least three components (a) a silane having the formula R„SiX 4 . n , (b) a metal alkoxide having the formula R MX 3 or MX , and (c) a mixture of a lower alkanol and acidified water. The silane having the formula R^SiX^. is a silane wherein each R is chosen independently from among the methyl, ethyl, and propyl radicals, unsubstituted or partially or totally substituted by fluorine atoms, X is a hydrolyzable group chosen from among the methoxy, ethoxy, and chloro groups, and n = 1 or 2 A mixture of a lower alkanol and water is added to the silane constituent as a solvent The water is preferably acidified, at least where X is a methoxy or ethoxy group. Among the usable silanes encompassed within the above formula, it is preferred that R be a non-substituted lower alkyl radical, the methyl group being the most preferred The non-substituted silanes are preferred from a practical point of view They are more economical than silanes containing fluorinated radicals The silane component can be formed exclusively of a silane having the above formula where n = 2 or 1. It may also be a mixture of silanes having the above formula where n = 2 and where n = 1. In a mixture, the silane having the above formula where n = 2 preferably constitutes at least 20 mole percent ofthe silane mixture The most preferred silane ofthe above formula where n = 2 is dimethyldiethoxysilane The most preferred silane ofthe above formula where n = 1 is methyltriethoxysilane The alkanol employed in the alkanol-water mixture can be a lower alkanol such as methanol, ethanol, or a propanol such as isopropanol Ethanol or propanol is preferred, the choice depending on the method of application The role ofthe alkanol is simply to permit the solution and/or the dispersion ofthe silane in the aqueous mixture, the silane not being soluble in water Where a non-chlorinated silane constituent is used, the alkanol-water mixture can conveniently contain 5-93% by volume water, preferably 5-60% by volume In contrast, where a chlorinated silane component is employed, the proportion of water in the alkanol- water mixture can be reduced to a very small amount, i e , to the order of trace amounts The water is acidified with a mineral or organic acid, for example, to reduce the pH below approximately 4 where an alkoxylated silane constituent is used Examples of operable acids include hydrochloric acid, nitric acid, sulfuric acid, and acetic acid When a chlorinated silane constituent is utilized, it is not always necessary to pre-acidify the water It is acidified automatically as soon as the silane hydrolysis commences as a result ofthe formation of HCl. The proportion of silane in the composition is not very critical. For example, it may be about 0.06-1 mole/liter, preferably about 0 1-0.6 mole/liter, ofthe mixture The metal alkoxide component ofthe coating mixture has a formula R MX 3 or MX 4 This provides an alkoxide with at least 3 hydrolyzable branches to increase cohesion and bonding ofthe components. This provides a thickened layer and increases the links with the inorganic surface to enhance the coating In the alkoxide formula, M is a metal, preferably selected from the group consisting of silicon, titanium, zirconium and aluminum R, if present, may be a methyl. ethyl, propyl, or butyl group In preparing a coating material, the mixture of alkanol and water is added to the silane as a solvent therefore If needed to allow the silane to undergo hydrolysis, acid is then added The mixture is continuously stirred to provide a homogeneous mixture The metal alkoxide is then added while maintaining an inert, e g N 2 , atmosphere This is necessary because most alkoxides are quite moisture sensitive, even at ambient, and tend to undergo hydrolysis The coating material is now applied to the surface to be protected The hydrolyzed X groups on the silane and alkoxide components can react by condensation with the hydroxyl groups present at an inorganic suiface to form stable bonds with that surface The overall result of these reactions is the formation of a thin, hybrid coating This coating is durably bonded to the surface ofthe treated inorganic article and confers non-stick and water repellent properties to that suiface The inventive compositions for treating inorganic surfaces exhibit limited stability and, therefore, must be used within a reasonably short time of their preparation The reaction ofthe hydrolyzed groups with the hydroxyl groups on an inorganic surface can be carried out at ambient temperature However, that reaction is rather slow (about 24 hours) The time required may be substantially shortened by a heat treatment, for example, about 80°C To complete densification, the coated article may be heated to a temperature of about 100°-400°C for a period of time This can be, for example, about 5-30 minutes, depending upon the particular composition used for the treatment and upon the temperature of heating One method of forming the protective coating comprises briefly dipping the article to be treated, while it is at an elevated temperature, e.g , 100°-300°C, into a bath ofthe inventive composition for a few seconds. The article is then permitted to cool naturally The evaporation ofthe alkanol-water mixture and the condensation reactions take place during that cooling This mode of forming the desired coating is especially economical and convenient in operation for glass and glass-ceramic articles The manufacturing of such articles customarily involves heating processes, e.g , in forming the article, or in annealing, tempering, ceramrning, or other forms of thermal treatment With such articles, the inventive process can be implemented in a production line The article will be at a temperature sufficiently high to be suitable for carrying out the thermal treatment necessary for the formation ofthe coating Other methods of forming the protective coating involve painting, spinning, or spraying the coating material onto an unheated surface The material is then heated at 100-300° C for 5-30 minutes to remove volatiles and set the coating The coating obtained from the inventive compositions imparts non-stick, hydrophobic, and water repellent properties to the coated surface These propeπies are suφrisingly superior to those that can be obtained from prior compositions based upon silanes alone In paπicular, the present coatings are much more resistant to detrimental effects from abrasives and detergents The nature ofthe protective coating, and its production, may be seen in the following projected formulations a A glass or glass-ceramic surface containing hydroxyl groups. OH OH OH Si Si Si b Hydrolysis of a hydrolyzable silane CH 3 CH 3 I ! (OR)-Si- (OR) + 2H 3 O * → OH— Si-OH ^ 2ROH + 2H * I I CH 3 CH 3 c Hydrolysis of a metal alkoxide (OR) OH (OR)— Zr- (OR) + 4H 3 O " → OH— Zr-OH + 4ROH + 4H ~ (OR) OH d The coated, hydrophobic surface O CH, (Zr-O-) n - (Si-O)„ O CH 3 Si The hydroxyl groups produced by hydrolysis in (b) and (c) link with those on the aπicle surface (a) to form a hydrophobic coating (d) This impaπs hydrophobic propeπies to the surface The presence of acid in the coating material is necessary to permit hydrolysis of the silane and alkoxide To this end, the ratio of acid to the sum of silane plus alkoxide should be at least 0 4, and preferably in the range of 0 6 to 5 2 Silanes and metal alkoxides have different hydrolysis rates A small amount of a known complexant molecule may be employed to control these rates This achieves a desirable structure in the hydrolyzed coating and renders it clear and transparent Diketonates, p-diketonates and carboxylic acids are among the complexants that may be used Choosing a ratio of silane to alkoxide in the coating material represents a compromise. The silane, as observed earlier, provides good non-stick propeπies These are evidenced by high contact angles However, the coating does not have good resistance to abrasion and detergent attack We have now found that the presence of ceπain metal alkoxides in a silane coating material markedly improves the abrasion and detergent resistance of a coating However, the metal alkoxide alone does not provide a coating having non-stick characteristics Accordingly, it is necessary that the silane content in a coating material be at least equal to the metal alkoxide content Preferably, the silane is in excess The effect is illustrated in FIGURE 1 ofthe accompanying drawing That FIGURE is a graphical illustration wherein contact angle in degrees is plotted on the veπical axis The mol percent of silane with respect to silane plus alkoxide is plotted on the horizontal axis The silane was diethoxy dimethyl silane (DEDMS) and the alkoxide was zirconium (IV) propoxide (Zr[pro] 4 ) in mixtures employed to obtain the plotted data It will be observed that the contact angle falls off rapidly as the silane content becomes less than 50% ofthe total silane plus alkoxide However, at least 5%, and preferably at least 10%, alkoxide is usually necessary to provide a substantial improvement in abrasion and detergent resistance. The presence of an acid is necessary to permit hydrolysis ofthe silane and alkoxide. To this end, the ratio of acid to the sum of silane plus alkoxide should be at least 0 4 and preferably at least 0.6 and still more preferably at least 1 2 This is illustrated in FIGURE 2 ofthe drawing Again, contact angle, in degrees, is plotted on the veπical axis, while the ratio of acid to the sum of silane plus alkoxide is plotted on the horizontal axis It will be observed that the contact angle becomes insufficient as the ratio falls below about 0.4, but is reasonably stable at higher ratios The acid is HNO^ or HCl, the alkoxide is zirconium propoxide, and the silane is diethoxy dimethyl silane SPECIFIC EMBODIMENTS TABLE I sets foπh a series of exemplary compositions that illustrate material systems in accordance with the invention. The silane employed in each composition was diethoxydimethyl silane (DEDMS). The alkanol selected was ethanol and the acids employed were 12M HCl and 15M HNO 3 . Both are shown in moles/liter (mol/1) The metal alkoxides were zirconium (IV) propoxide, shown as Zr(pro) 4 ; tetraisopropyl-oπhotitanate, shown as TIOT; and aluminum-sec-butoxide, shown as Al(but) 3 . Both the silane and alkoxide contents are shown in moles/liter (mol/1) The several compositions were applied to a glass-ceramic surface by spinning The coated samples were extensively tested for contact angle against water after various treatments Measurements were made on test pieces after (1) an initial thermal treatment at 120°C for one hour to set the coating (θ), (2) after being held at 360°C for 30 minutes (360 30); (3) after 30 minutes at 450°C (450730), (4) after exposure to a detergent, Super Soilax®, for 8 hours at 95°C (DT). TABLE I Component I 2 3 DEDMS (mol/1) 04 04 04 04 04 04 Zr(pro) (mol/1) 03 03 03 03 03 03 HCl (mol/1) 09 12 15 HNO 3 (mol/1) 12 15 19 ethanol (mol/1) 128 125 122 128 125 122 θ 96 100 91 103 99 90 360730 96 96 91 96 90 90 450730 96 94 91 86 80 70 DT 88 78 82 98 80 73 TABLE I (continued) Component 7 8 9 10 JJ. 12 DEDMS (mol/1) 04 04 04 04 04 04 Zr(pro) 4 (mol/1) ~ — 014 01 TIOT (mol/1) 02 02 02 02 02 0133 Al(but) 3 (mol/1) — ~ — 0133 HCl (mol/1) 10 15 HNO 3 (mol/l) — 12 15 19 19 ethanol (mol/l) 13 ( 132 129 129 θ 93 90 98 92 97 100 360730 86 90 75 65 80 97 450730 50 90 62 63 36 42 DT 86 90 90 70 83 85 Compositions 1, 4, 5 and 6 are prefeπed because of their stability, the constancy of their performance under various conditions, and ability to withstand thermal processing Composition 4 was subjected to various punishing treatments after being applied to commercial glass-ceramic aπicles having a lithium aluminosilicate (beta- spodumene) crystal phase Each coated aπicle was evaluated for food release using a scale of 0-5 with 5 indicating excellent release and 0 indicating no release The results were Treatment θ Food Release Uncoated 35 0 Coated 103 5 Detergent 98 4 8 hours/95 °C 4% Acetic acid 75 4 48 hours/95°C A coated aπicle, otherwise untreated, was subjected to a scrubbing action with a commercial cleansing pad After 100 rotations at a pressure of 0 25 Kg/cm 2 (3 5 lbs/inch 2 ), food release was considered good, after a thousand rotations, release was still considered adequate Example 13 The following material composition, also in mol/l, illustrates use of acetic acid as a complexant and of 2-propanol as a solvent Component Content DEDMS 0.64 Zr(pro) 4 0.44 HNO 3 2. 16 H 2 O 3.33 Acetic acid 0.03 2-propanol 2.20 A mixture of these components was prepared and applied to a glass-ceramic suiface by spraying. The sprayed aπicle was cured for 30 minutes at 250°C Surface finish, food release and durability were found to be excellent The non-stick propeπies were not affected by either heat treatment at 360°C for 30 minutes or by exposure to Super Soilax at 95°C for 8 hours. Example 14 The following material composition, expressed in mol/l, illustrates the use of dimethyl dichlorosilane Ingredient Content DMDCS* 0.33 Zr(pro) 6 0.06 H 2 O 0.33 Acetic acid 0.02 Alkanol 15.90 imethyl dichlorosilane A mixture of these ingredients has been prepared and coated on a glass-ceramic surface by spinning. After a curing of 250°C for 30 minutes, the contact angle was 1 16°

Claims: -15- WE CLAIM 1. A material system for impaπing non-stick and non-wetting propeπies to an inorganic aπicle having hydroxyl groups on its surface, the system comprising a silane having the formula R n SiX,,.,, , wherein n is 1-2 and X is a hydrolyzable group, a metal alkoxide having at least three hydrolyzable branches and a formula R MX 3 or MX 4 wherein M is a metal and X is a hydrolyzable branch, and an acidified aqueous mixture with a lower alkanol 2 A material system in accordance with claim 1 wherein M is selected from the group consisting of Zr, Si, Ti and AJ 3 A material system in accordance with claim 2 wherein the metal in the metal alkoxide is zirconium 4. A material system in accordance with claim 3 wherein the metal alkoxide is zirconium propoxide 5. A material system in accordance with claim 1 wherein the inorganic aπicle is a glass or a glass-ceramic aπicle 6. A material system in accordance with claim 1 wherein R in the silane is a methyl, ethyl, or propyl radical, optionally, paπially or totally substituted for by fluorine atoms 7. A material system in accordance with claim 1 wherein the hydrolyzable group x in the silane is chosen from a group consisting of methoxy, ethoxy and chloro groups 8. A material system in accordance with claim 1 wherein the silane is diethoxydimethyl silane -16- 9 A material system in accordance with claim 1 wherein the concentration of silane is 0 06-1 0 mole/liter 10 A material system in accordance with claim 9 wherein the concentration of silane is 0 1-0 6 moles/liter 1 1 A material system in accordance with claim 1 wherein the ratio of alkoxide concentration to alkoxide plus silane is 0 05-0 5 12 A material system in accordance with claim 1 wherein the lower alkanol is ethanol 13 A material system in accordance with claim 1 which additionally contains a complexant 14 A material system in accordance with claim 1 wherein the ratio of acid to the sum of silane plus alkoxide is at least 0 4 15 A material system in accordance with claim 14 wherein the ratio is in the range of 0 6-5 2 - 17- 16 A method of impaπing non-stick and non-wetting propeπies to an inorganic aπicle having hydroxyl groups on its surface which comprises mixing a silane having a formula R^SiX.,.,, with a metal alkoxide having a formula R MX 3 or MX , adding an acidified aqueous mixture of a lower alkanol, applying a coating ofthe mixture to a surface on the inorganic aπicle, thermally processing the coating at a temperature of at least 100°C to form a solid, transparent film on the aπicle surface 17 A method in accordance with claim 16 which comprises applying the mixture by dipping a heated aπicle in the mixture 18 A method in accordance with claim 16 which comprises thermally processing the coating at a temperature of 100-300°C 19 A protective coating for an inorganic surface which is the combined hydrolyzed product of a silane having the formula R„SiX . n .wherein n is 1-2 and X is a hydrolyzable group, and a metal alkoxide having at least three hydrolyzable branches and a formula R MX 3 or MX 4 wherein M is a metal and X is a hydrolyzable branch. 20 A film consisting essentially ofthe hydrolyzed product of a mixture of a silane having the formula R^SiX,,.. , wherein n is 1-2 and X is a hydrolyzable group, and a metal alkoxide having at least three hydrolyzable branches and a formula R MX 3 or MX 4 wherein M is a metal and X is a hydrolyzable branch

Inventors: CARRE ALAIN, PRASSAS MICHEL, WAKU-NSIMBA JEAN, CARRE, ALAIN, PRASSAS, MICHEL, WAKU-NSIMBA, JEAN
Applicants: CARRE ALAIN, CORNING INC, PRASSAS MICHEL, WAKU NSIMBA JEAN, CARRE, ALAIN, CORNING INCORPORATED, PRASSAS, MICHEL, WAKU-NSIMBA, JEAN
Application year: 1996
Main classification: C03C
IPC codes: C03C17, C04B41, C09D4, C09D183