CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to United States Provisional Application No.

61/457,727, filed May 20, 2011, and incorporated herein by reference in its entirety for all purposes.

FIELD OF THE INVENTION

The invention pertains to multi-layer coatings useful for providing a decorative and durable mirror-like finish on various types of metal substrates.

Background of the Invention

One conventional metai surface treatment method is electroplating, which supplies the performances of anti-corrosion, hardness, durability and mirror-like gfoss together with other accessoria! treatment. However, electroplating is potentially harmful to the environment owing to the large quantity of toxic byproducts and wastes that is generated. The whole process therefore operates at high cost. This limits and blocks the further development of the electroplating industry. The development of alternative techniques for achieving functional mirror-like coating on target meta!s, including steel Q19S-A and zinc alloy #3, which are low cost and environmentally friendly is therefore desirable.

SUMMARY OF THE INVENTION

The present invention provides a coating system having at least three layers which is capable of achieving a multiple function mirror-like coating on metals such as steel and zinc alloy (zinc alloy #3 and steel Q 195-A). The three layers include a base coating of epoxy, which provides anti-corrosion protection for metal surfaces and enhances the adhesion of the mirror-like coating which is also applied to the substrate. The decorative mirror-like coating may be formed on the base coating by the reaction of silver salt solution and glucose solution. A surface coating is fabricated on the middle mirror- 1 ike coating layer, which protects the mirror-like coating from being oxidized and scratched . The entire coating system is low in cost and easily adaptable to mass production. The processes of coating mixture preparation and the coating layers are ail environmentally friendly.

In one embodiment, the invention furnishes a method of providing a substrate with a multi-!ayer coating, the method comprising the steps of: a) forming a base coating comprised of epoxy on a surface of the substrate; b) forming a decorative mirror-like coating on the base coating by reacting an amine complex of silver, a reducing sugar and an acety!enic compound; and c) forming a surface coating on the decorative mirror-like coating which is capable of protecting the decorative mirror-like coating against scratching.

Another aspect of the invention provides a substrate having a mufti-layer coating on at Seast one surface, the multi-layer coating comprising a base coating comprised of cured epoxy, a decorative mirror-like coating comprised of silver on top of the base coating, and a surface coating comprised of poiysilicone on top of the decorative mirror-like coating.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OF THE INVENTION

The process of the present invention is a method of providing multiple layers of coatings to display a mirror-iike performance, together with functions of anti -corrosion and anti-scratch. Detailed exemplary procedures for carrying out such method, including the preparation of coating mixtures and the coating of such mixtures on substrates, are described below in the Examples.

Suitable substrates for use in the present invention include, but are not limited to, metal substrates (e.g., steel, zinc alloys). Prior to application of the base coating, one or more initial preparation steps such as surface polishing, surface cleaning and/or surface pretreatment may be carried out. For example, the substrate surface may be polished using an abrasive method such the use of abrasive paper, vibrating grinder or drum grinder. The substrate surface may also be cleaned by contacting the surface with an aqueous surfactant solution while sonicating (i.e., exposing the surface to ultrasound for a period of time, e.g., at least 30 minutes), rinsing the surface with water, contacting the rinsed surface with a volatile organic solvent or mixture of volatile organic solvents (e.g., a mixture of acetone and ethano!) while sonicating (subjecting to ultrasound), rinsing the surface with additional volatile organic solvent, and drying. If so desired, the substrate may be subjected to a pretreatment wherein the surface is contacted with a silane such as gamma-aminopropyl triethoxysi!ane. The siiane or mixture of stlanes may be provided in the form of a solution (e.g., a solution in aqueous ethanol). The substrate may be ultrasonicated for a period of time (e.g., at least 10 minutes) while in contact with the siiane. The sifane -treated surface may then be dried (e.g., in an oven at 110°C for more than 20 minutes), then cooied to room temperature.

Following any initial preparation steps as described above, an epoxy base coating is formed on the substrate surface, Typicaliy, an epoxy resin is utilized to form the base coating on the substrate surface, wherein the epoxy resin is cured using a solidifier to provide a base coating comprised of cured epoxy.

Any of the epoxy resins known in the art may be utilized in the present invention, Epoxy resins are characterized by the presence of two or more three- membered cyclic ether groups (epoxy groups) per molecule. Particularly useful are diglycidyl ethers of bisphenol A, derived from bisphenol A and epichlorohydrin. Other suitable epoxy resins may include epoxy cresol novo!ac resins and polynuclear phenol - gSyeidy! ether-derived resins. Reactive diluents containing a single epoxy group per molecule, such as glycidyl 2-methylphenyl ether, may be used in combination with the epoxy resin if so desired. Any of the known soiidifiers (curing agents) for epoxy resins may be employed, including both catalytic curing agents and coreactive curing agents. Examples of catalytic curing agents include Lewis acid catalysts (e.g., complexes of boron trifluoride with amines or ethers and boron trichloride complexes) and Lewsis base catalysts (e.g., tertiary amines or poiyamines converted into tertiary amines upon reaction with epoxide groups), Coreactive curing agents generally contain active hydrogen atoms and include, but are not limited to, poiyamines (which may contain aliphatic, aromatic, cyc!oaliphatic and/or heterocyclic groups, for example), polyaminoamides, polyphenols, polymeric thiols, polycarboxylic acids, and anhydrides,

One or more leveling agents may be used in the base coating composition. Suitable leveling agents include, for example, polyacrylates and silicone-acrylates. The base coating composition may also contain an air release additive (defoaming agent) such as BYK-A530 silicone polymer air release additive and/or a siiane coupling agent.

One or more organic solvents may be utilized in the base coating composition as well to serve as a vehicle for the other components. That is, the epoxy resin, solidifier, leveling agent and so forth may be dissolved in an organic solvent or mixture of organic solvents such as alcohols and ketones (e.g ., C1-C6 aliphatic alcohols and C3-C6 aliphatic ketones such as acetone, ethano!, butanoi, 4-hydroxy-4- methyl-2-pentanone). Preferably, the solvent is relatively volatile (e.g., normal boiiing point not more than 100°C) to facilitate its removal from the base coating composition once the base coating composition has been applied to the substrate surface,

In one embodiment of the invention, the base coating is formed as two iayers (a !ower layer and an upper layer). The lower layer may be first formed on the substrate surface by applying a lower layer composition comprised of ieveiing agent, soiidifier and solvent (e.g., 8 ketone and/or alcohol). Before applying to the substrate surface, the lower layer composition may be degassed, by ultrasonication for example, Any suitable coating method may be used, such as brushing, dipping or spraying. The applied coating may then be heated under conditions effective to remove solvent (e.g., 50°C for at feast 6 hours) and provide a dried lower layer. An upper layer composition is then applied to the dried lower layer, using any of the same coating techniques. The upper layer composition may be comprised of epoxy resin, soiidifier, leveling agent, reactive diiuent and solvent (e.g., alcohol and/or ketone) and may be degassed by uitrasonication prior to application. Following application of the upper layer composition, the coated substrate may again be heated (e.g. 80°C for at !east 4 hours), under conditions effective to both remove solvent and induce curing of the epoxy resin by reaction with the soiidifier.

A decorative mirror-fike coating is then formed on the base coating by the following procedures. The surface of the base coating may be first cleaned or roughened by contacting the surface with an organic solvent or mixture of organic solvents (e.g., C1-C6 alcohois, C3-C6 ketones, in particular an acetone/ethano! mixture) and then water, suitably by rinsing. The cleaned substrates may then be dried under conditions effective to remove residua! volatiles (e.g., in an oven at 90°C, with the substrate thereafter being cooied to room temperature before the next step), A sensitization step to further prepare the coated substrate for application of the mirror-iike coating may then be carried out by contacting the base coating surface with an acidic solution containing stannous ions (prepared, for example, by dissolving stannous chloride into aqueous HCl). The coated substrate may, for example, be dipped into the acidic solution, rinsed with water, and then dried (e.g ., in an oven at 90°C). The decorative mirror-like coating results from the reaction of a soluble amine complex of si!ver such as [Ag(NH ₃ ) ₂ 3 ⁺ , a reducing sugar such as g!ucose _r and an acetylenic compound such as butynedio!. As a result of such reaction, the silver complex is converted, at least in part, to metallic siiver, which plates onto the substrate surface. The decorative mirror-like coating thus formed is comprised of silver. A solution of the soluble silver complex may be prepared, for example, by dissolving a silver salt such as siiver nitrate into water, adding caustic such as sodium hydroxide to form a precipitate, and then adding an amount of ammonium hydroxide effective to dissolve the precipitate (the resulting solution will contain [Ag(NH ₃ ) ₂ ]+). This solution may then be combined with a solution of reducing sugar (e.g., glucose) which also contains an acidic catalyst such as a carboxyiic acid (e.g., citric acid, lactic acid) and a solution of the acetylenic compound (e.g., butynedio!). The acetylenic compound solution may be first combined with the sugar solution and the resulting solution then combined with the silver amine complex solution to provide a mirror-like coating composition. The mirror-like coating composition is contacted with the coated substrate by, for example, immersing the coated substrate in the mirror-like coating composition, preferably immediately after such composition has been prepared.

Contacting is typically continued for 5 to 10 minutes at approximately room

temperature or such other conditions effective to form the desired mirror-like coating on the substrate, generally without stirring or vibration. Thereafter, the substrate may be rinsed with water and dried. If so desired, the mirror-fike coating layer may be treated to improve its oxidation resistance by, for example, contacting the mirror-like coating layer with a solution of a thiol such as hexadecanethio! and a surfactant such as an ethoxylated nonylphenol (typically, at 30-S0°C for i-iO minutes). Thereafter, the substrate may be rinsed with water and dried.

Following the formation of the mirror-like coating layer, a surface coating is formed on such layer to protect the mirror-fike coating layer (e.g., from oxidation and scratching). Such surface coating may suitably be based on organic silicon-containing compounds such as silanes and organosi!tcates. Thus, the protective surface coating may be comprised of poiysilicone (i.e., a composition comprised of silicon and oxygen). Organic silicon-containing compounds may be reacted, by hydrolysis and/or condensation reactions for example, to provide the protective coating. Illustrative organic silicon-containing compounds include, for example, silanes, tetraa!kyl silicates such as tetraethylsilicate, and alkyla!koxysilanes such as methy!tnmethoxysilane. An iliustrative example of a suitable silane is 3 ~glycidoxypropy!trimethoxysiiane although other epoxyfunctional siianes as well as other organosi!anes having alkoxy

substituents and/or substituents with functional groups other than epoxy groups may also be used. The organic silicon -containing compounds may be admixed, combined with water or other liquid medium such as an alcohol, and treated with an acid such as a carboxylic acid (e.g., acetic acid) to provide a surface coating composition. If desired or needed to adjust the appearance of the final coated substrate, a dye (in particular, a violet dye such as tri(4~(dimethylamino}phenyf)methytinium chloride) is also present in the surface coating composition, The substrate having the mirror- like coating thereon is contacted (by immersion, for example) with the surface coating composition for at least one minute, then subsequently heated {e.g., at 100°C for more than 18 hours) to yield the coated substrate in accordance with the invention.

Examples

1, Preparation of coating mixtures and coating layer with anti -corrosive function.

1) Preparation of pretreatment mixture: Mix Si!quest® AllOO (γ-aminopropy! triethoxysilane, product of Crompton) with DI water and ethanoi. Stir for more than 10 minutes to obtain a uniform solution.

2) Preparation of base coating mixture with anti -corrosive function;

(1) Lower layer mixture: The mixture used to prepare the lower layer of the base coating includes a group A component and a group B component. The group A component is obtained by mixing acetone, ethanoi and 8YK-361N leveling agent (product of Byk). The group B component is composed of ethanoi and TZ-550 solidifier (a cycloamine curing agent, supplied by Tzar industrial Co., Ltd. in China).

(2) Upper layer mixture: The mixture used to prepare the upper layer of the base coating also includes a group A component and a group B component. The group A component includes butan-1 -o!, 4-hydroxy-4- methyl-2-pentanone, acetone, CGE (glycidyl 2-methylpheny! ether), BYK-361N leveling agent and E-51 epoxy (liquid reaction product of epichlorohydrin and bisphenol-A; epoxide eq. ca. 190 g/eq). The ingredients are added in sequence and stirred until a uniform mixture is obtained. The group B component is the solidifier (curing agent) TZ- 550. 3) Fabrication of the coating layer with a nti -corrosive function (base coating). (1} Surface polishing of substrates: Steel Q195-A or zinc alioy #3 substrates are polished with abrasive papers of #240, #360 and #800, respectively; a vibrating grinder or drum grinder can also be used to polish the same substrates using suitabie grinding stones.

(2) Substrate cleaning; The polished substrates are immersed in a mixture of DI water and cleanser essence. The substrates are sonicated for more than 30 minutes. The samples are rinsed one by one using running DI water. The substrates are cleaned in 1 : 1 (vol.) acetone and ethanoi by ultrasonication again for more than 30 minutes. The substrates are rinsed with running ethanoi and then dried in air.

(3) Substrate pre-treatment: The cleaned and dried substrates are immersed in the pretreatment mixture, while ultrasonicating for more than 10 minutes. The samples are oven -dried at 110°C for more than 20 minutes, and then cooled down to room temperature.

(4) Base coating lower layer mixture: The group A and group B components of the base coating lower layer are mixed by stirring vigorously. The mixture is degassed by ultrasonication for more than 5 minutes. The base coating iower layer mixture is coated by brushing, dipping or spraying on substrates pre -treated as described in step (3). The coated samples are then heated at 50°C for more than 6 hours.

(5) Base coating upper iayer mixture: The group A and group 8 components of the base coating upper iayer are mixed by stirring vigorously. The mixture is degassed by ultrasonication for more than 10 minutes. The base coating upper layer mixture is coated onto the base coating iower layer by brush, dipping or spraying on substrates coated in step (3). The samples are heated at 80°C for more than 4 hours.

2. Preparation of mirror-like coating solutions and coating layer with metal decorative function and gloss:

1) Preparation of roughening solution: Mix acetone and ethanoi at a volume ratio of 1 : 1 at room temperature. Store the roughening solution in a bottle with a lid. 2} Preparation of sensitizing solution: Add stannous chloride into concentrated hydrochloric acid (36%) at room temperature until the stannous chloride is dissolved. Add DI water to make the concentrations of stannous ch!oride and hydrochloric acid in the solution 5g/L and 5g/L, respectively,

3) Preparation of mirror-like coating solutions : Mirror-like coating solutions include solution A, B and C, in which :

(1) Solution A is prepared as follows to get a solution of [Ag(NH ₃ ) ₂ ] ⁺ with a concentration of lOg/L: Dissolve stiver nitrate into 01 water. Dissolve sodium hydroxide into a small amount of DI water, and add the solution of sodium hydroxide into the above-mentioned silver nitrate solution while stirring vigorously, to get khaki -colored precipitation. Add a suitable amount of 10% of ammonium hydroxide into above mixture until the precipitation is dissolved. Add DI water into the solution until the concentration of [Ag(NH ₃ ) ₂ ] ⁺ in the solution is lOg/L.

(2) Solution B is a solution of glucose in water and ethanoi. Mix ethanoi and DI water to provide a mixed solvent. Dissolve glucose and citric acid into the aforementioned mixed solvent. Add DI water to make the concentration of glucose in the solution 8g/L.

(3) Solution C is a butynedio! solution in DI water with a concentration of 8.6 g/L

4) Preparation of anti-oxidant solution for mirror- 1 ike coating fayer: The use of this solution is optional. Dissolve 1-hexadecanethio! into iso-propanol. Stir until the solution is uniform (solution A). Add surfactant of polyoxyethylene (9) nonylphenyl ether (IGEPAL CO-630) into DI water to provide solution B. Mix solution A and B to obtain a uniform solution.

5) Fabrication of mirror-like coating layer.

(1) Sample cleaning : Rinse the substrates coated with anti -corrosive base coating as described previously with running 1: 1 (vol.) of acetone and ethanoi, and then DI water, respectively. Dry the samples in oven at 90°C. Cooi down the samples to room temperature in air.

(2) Sensitization of samples: Dip the cleaned and dried substrates coated with anti-corrosive coating into stannous chloride solution, and then rinse the samples with running DI water. Dry the samples in oven at 90°C, and then cool down to room temperature. (3) Fabrication of mirror-like coating: solution A and 8 used to obtain the mirror-like coating will be mixed at a volume ratio of A: B = 1; 1 to 1 : 2, The reaction will be faster for ratio of A: B - 1 : 2. At first, mix solution B and C while stirring, in which solution C is 5% of the total volume of A and B, Solution A is then added into above solution of B and C Immerse substrates pre-treated by roughening and sensitizing steps at room temperature (ca, 20-25 °C) into above mixture solution immediately for ca. 4-6 min. The reaction is carried out without stirring or vibration, and a shiny mirror-iike coating will be formed on the surface of the samp!es. Rinse the samples with DI water more than 3 times, and dry at room temperature.

(4) Anti-oxidation treatment of mirror-iike coating layer (optional step): Immerse the samples with the mirror-like coating layer deposited thereon into the anti-oxidation solution at 40 °C for 5 min. Rinse the sample with DI water for more than 3 times, and dry at room

temperature, aration of surface coating mixtures with anti-scratch function and coating iayer,

1) Mix siianes of 3-g!ycidoxypropy!tnmethoxysiiane (A187), tetraethy!silicate (TEOS) and methyltrimethoxystlane (MTMS) at room temperature. Sonicate the mixture in an ultrasonic bath for 3 minutes. Add DI water into the above siiane mixture at room temperature, and then sonicate for 3 minutes. Add acetic acid into the solution at room temperature, and treat at room temperature for more than 12 minutes. Butan-1 -o! is added to dilute the coating product.

2) Violet dye of tris(4-(dimethylamino)phenyil)lmietuhylium chloride is added into above anti-scratch coating mixture to adjust the appearance of the coated sample if necessary.

3) Fabrication of the coating layer with anti -scratch function. Immerse the samples coated with the mirror-iike coating Iayer in above anti-scratch coaling mixture for more than 1 minute. Heat the samples at 100 °C for more than 18 hours.