Easy clean coating applied on stainless steel metallic surfaces used in the manufacture of household appliances

Abstract

The present invention related to a silicon based coating suitable for being applied onto metallic surfaces of a household appliance. The coating of the present invention protects the metallic surfaces of the household appliance against the yellowing or changes in color, through temperatures, among other factors, and eases the cleaning of the same.

Claims

1. A method for the application of a coating comprising a silicon base compound reinforced with one or more compounds comprising tungsten oxide, calcium oxide, and boron oxide to metallic surfaces of a household appliance to provide protection against staining and yellowing and to facilitate cleaning of said surfaces, the method consisting of: a) cleaning the metallic surfaces to be coated by washing the surfaces with water and a detergent to degrease the metallic surfaces; b) rinsing the metallic surfaces with water; c) activating the metallic surfaces using an acid; d) rinsing the metallic surfaces with water; e) hydroxylating the metallic surfaces using a sodium hydroxide solution having a concentration from approximately 1% to approximately 20% by weight to generate OH groups on the metallic surface; f) after step e) applying the silicon-based coating directly to the metallic surfaces comprising hydroxyl (OH) groups; g) after step f) curing the coated surfaces in three continuous heating stages, the first heating step has a range from approximately 0 C. to approximately 120 C., with a temperature ascending to a velocity in a range from approximately 1 C. per minute to approximately 5 C. per minute, followed by an isothermal period ranging from 0 to 60 minutes; starting the second heating step reaching 320 C. with a temperature ascending to a velocity in a range from approximately 1 C. per minute to approximately 10 C. per minute followed by an isothermal period ranging from 0 to 60 minutes and starting the last heating step up to reaching 420 C. with a temperature ascending to a velocity in a range from approximately 1 C. per minute to 15 C. per minute followed by an isothermal period ranging from 0 to 60 minutes; and h) cooling the cured surfaces.

2. The method according to claim 1, wherein the acid used in the activating step is selected from one of a solution of sulfuric acid with a concentration ranging from approximately 1% by weight to approximately 15% by weight, and a solution of nitric acid with a concentration ranging from approximately 5% by weight to approximately 10% by weight.

3. The method according to claim 1, wherein the applying of the coating to the surfaces during step f) is performed by way of one of aspersion, immersion and a combination of both.

4. The method of claim 1, wherein the coating on the metallic surface has a thickness of approximately between 0.5 to 2.0 microns, a hardening of 9H, impermeable to oxygen, stable and maintain weight at temperatures under 500 C.

5. The method of claim 1, wherein the coating on the metallic surface is lipo-oleophobic and hydrophobic type.

6. The method of claim 1, wherein the time of exposure to the acid during step c) ranges from approximately 0 to 20 minutes.

7. The method of claim 1, wherein metallic surfaces have a permanence time during step e) between approximately 0 to 20 minutes.

8. The method of claim 1, wherein metallic surfaces have a residence time during step f) between approximately 0 to 2 minutes.

9. The method of claim 1, wherein the coated surface during step h) is cooled at room temperature.

10. The method of claim 1, wherein the metallic surfaces are selected from brass, aluminum, copper, steel and stainless steel.

11. The method of claim 1, wherein the rinsing step b) is performed at approximately 40 C. and with agitation to aid in the cleaning of the metallic surfaces.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1Presents the thermal gravimetric analysis (TGA) and its first derivation, wherein the behavior of the coating with respect to temperature of the present invention can be appreciated. In this Figure, one can clearly see the thermal stability of the coating withstanding temperatures higher than those operational in an oven (room temperatures 400 C.).

(2) FIG. 2Is a graph which shows resistance to changes in color of the coating of the present invention with respect to the operation cycles of a cover of a stove.

(3) FIG. 3Is a graph which illustrates resistance to the changes in color of the coating of the present invention applied to the cover of a conventional oven plotted against the passage of years.

(4) FIG. 4Is a graph showing the easiness of cleaning of the coating of the present invention through a period of use of time of three years.

DETAILED DESCRIPTION OF THE INVENTION

(5) The following definitions are provided to allow a better understanding of the invention:

(6) The use of the term approximately provides an additional determined range. The term is defined in the following manner. The additional range provided by the term is that of approximately 10%. By way of example, but not in a imitative manner, if it states approximately 40 cm, the exact range is between 36 to 44 centimeters.

(7) The color change (E) is measured by means of a color spectrophotometer, which is compared against a standard (stainless steel without a coating).

(8) The coating of the present invention comprises a silicon based compound coating reinforced with one or more compounds comprising tungsten oxide, calcium oxide, and boric oxide. Preferably, the coating of the present invention is obtained from a solution of or a mixture of alkali metal silicates (sodium, potassium and/or lithium), in addition to a silicon oxide, aluminum oxide and titanium oxide nanometric densifier, as well as zirconium oxide, and additionally reinforced with tungstates, molibdates, nickelates and borates.

(9) The coating of the present invention is deposited onto the stainless steel surfaces by means of immersion and/or aspersion techniques, same which have been used for the application of the coating of the invention.

(10) The stainless steels, used according to the present invention are of the ferritic, austenitic, martensitic and Duplex (austenitic-ferritic) type.

(11) The coating of the present invention is applied onto metallic surfaces by means of the immersion and/or aspersion process which can be used widely on surfaces which come into direct contact with high temperatures. As a result, metallic surfaces such as, for example, brass, aluminum, copper, steels and preferably stainless steels, such as, for example, the cover of a stove which comprises knobs, heating pan, steel discs, burners; ovens, microwave ovens, etc. which can be subject to temperatures which oscillate between room temperature up to 400 C. can be coated with the coating of the present invention.

(12) The high resistance to the temperature of the coating which is shown in the thermal gravimetric analysis (FIG. 1) allows for having a safety area under 500 C. (approximately 297.8 C.), in which, the coating is stable and does not present weight loss. FIG. 1 shows said safety area, wherein additionally, the operational temperature of the stove is shown as being much lower than said safety area. This stability allows it to be used on any part of the stove, given that the operational temperature is found to be lower than that of this safety area.

(13) Other properties of the nano-structured coating of the present invention, which has a thickness approximately between 0.5 to 2.0 microns, is its resistance to thermal shock, to vapor exposure, it has a hardening of 9H, it inhibits oxygen permeation, it does not change the appearance of the metallic surface (transparent), in addition to conferring it with preservation properties (its appearance is preserved through time and use avoiding the change in color through oxidation on the surface).

(14) The safety zone and the properties of the coating prevent the metallic surface of the household appliance from staining, or from changing color or appearance, etc. Additionally, the cleaning of the parts which contain the coating eases the actual cleaning and creates a grease repellant. On this point, without wishing to limit ourselves by the theory, the coating of the invention is one of a lipo-oleophobic and hydrophobic type, which is grease repellant, oil repellant and aqueous solution repellant, and which does not allow foods to become anchored onto the surface, thereby avoiding a high degree of soiling, grease adherence and easing in the cleaning of the surface.

(15) FIGS. 2 and 3 illustrate the resistance to color changes (yellowing), given the repeated exposure to temperature cycles (room to 315 C.). The analysis of these figures shows that the coating of the present invention grants protection to the surface of the covers of household appliance stoves.

(16) Preparation and Coating of Metallic Surfaces Method

(17) The coating of the present invention is prepared using the sol-gel method, which has been adapted for the present invention.

(18) Potential advantages which this method presents versus the traditional techniques are summarized as follows: High degree of homogeneity and purity of the materials High thermal stability Ease of addition of elements within the network of materials. An excellent degree of hydroxylation of the material. With stable OH groups up to temperatures nearing 400 C. Allows the design of structural, texture, optic properties, varying the synthesis parameters.

(19) To generate oleo-phobic type effects, the coatings must provide the substrates a low-energy surface.

(20) The process for the preparation and application of the coating of the present invention comprises the following steps: a) Cleaning of the piecesthey are washed with water and a degreasing agent, for example a detergent. b) Rinsingrinse with water at a 40 C. temperature approximately and with a physical action by means of agitation which aids in the cleaning of the surface. c) Activationthe surface is activated using acids, such as, for example, sulfuric acid in concentrations from to 1% up to 15% by weight or nitric acid in concentrations from to 5% up to 10% by weight. The time of exposure to the acid is in a range of approximately 0 to 20 minutes. d) Rinsingthe surface is rinsed with water. e) Hydroxylationthe surface is hydroxylized using basic solutions, preferably sodium hydroxide, with a concentration range between 1% to 20% by weight, and a permanence time is used between approximately 0 to 20 minutes. f) Coatingthe depositing of the silicon based compound coating reinforced with other oxides comprising, tungsten oxide, calcium oxide, and boric oxide is applied by means of immersion controlling the entrance and exiting velocities of the piece, the residence time of the coating solution is approximately 0 to 2 minutes. g) Curing treatmentthe coating is cured in three steps or heating areas. The first heating area has a range of approximately 0 to 120 C., with a temperature ascending to a velocity with a range of approximately 1 C. to 5 C. per minute, the second step reaches approximately 320 C. with a temperature ascending to a velocity with a range of approximately 1 C. to 10 C. per minute, the last step reaches approximately 420 C. with a temperature ascending to a velocity with a range of approximately 1 C. to 15 C. per minute. Upon finalizing each temperature increment, isothermal periods are undergone (0 to 60 minutes). h) Coolingthe piece is cooled at room temperature and remains ready to be assembled into the household appliance.

(21) The coating was subjected to the following tests to prove its adhesion, abrasion, water repellence, grease repellence, resistance to temperatures above 400 C., stability in ultraviolet light, to chemical agents and that it maintains its appearance.

(22) The performance of the coated pieces was evaluated subject to the following tests:

(23) Standard test method for measuring adhesion according to the ASTM D-3359B specificationThis method indicates the procedure of the adherence between the film and the substrate by applying an adhesive film over the cut area on the film. The coating of this invention meets with the 5B adherence.

(24) Film hardness by pencil test according to ASTM D-3363This method is used to evaluate the organic coatings over a substrate, the pencils are calibrated from graphite hardness ranging from 6B up to 9H. The coating of this invention meets with a 9H hardness.

(25) Abrasion test according to norm ASTM D4060This determines the ability which the coating has to mechanical degradation. The results are expressed in terms of weight loss in the coating which can be from 500 to 1000 revolutions. The coating has a weight loss of 0.0007 grams using one kilogram in weight load, during 1000 cycles and using a CS-17 type of rock to be used.

(26) Staining test according to norm ASTM D1308Various food products are placed on the coating for a pre-determined amount of time and temperature. The coating of this invention meets with the appearance standards (change of color, loss of shine, adhesion without bubbling etc.)

(27) Humidity test ASTM D 1735The coating meets with 720 hours in the humidity chamber without presenting a change in coloring, bubbling or detachment. The coating of this invention meets with the specification.

(28) Standard Method for Thermal shock test ASTM C385the coating of this invention meets with the specification.

(29) The coating of this invention meets with the properties which grant the metallic surfaces of household appliances protection, preferably those manufactured with stainless steel, as well as protection against staining, yellowing and aids in the cleaning of the same.

Example 1Measurement of the Color Change of the Coating of the Present Invention Throughout the Years

(30) A stainless steel first plate coated with the coating of the present invention was placed on the cover of a conventional stove, measuring 1010 cm (example 1) near the backburner of the cover and a second 1010 cm stainless steel plate also coated with the coating of the present invention was placed near the oval burner.

(31) The color change was measured at year one, at year two and at year three after the following use conditions: at one hour of exposure to 400 C. and at one hour of exposure to room temperature.

(32) The results are shown in FIG. 3.

(33) It was observed that the coating did not change color at the end of the three year term as opposed to the cover without the coating, which starting on the first year changed color with a yellowing tendency. It was concluded that the coating of the present invention prevents yellowing and maintains its appearance through time.

Example 2Measurement of Color Change of the Coating of the Present Invention Through Multi-Cycles

(34) The change in color of a cover with the coating and another cover without the coating was determined at multi-cycle cycles. The cover with coating and the cover without coating in a conventional stove and, particularly, the triple ring burner of each one of said covers, were subjected to heating for 250 cycles, wherein each cycle represents one hour turned on at a maximum capacity (approximately 270 C.) and one half hour turned off.

(35) Upon finalizing the 25 cycles the cover without the coating was found to present a color change of 33.22 (according to ASTM 02244-11), whereas the cover with the coating of the present invention only presented a color change of 5.99 (according to ASTM 02244-11). The results of said experiment are shown in FIG. 2.

(36) It is concluded that the coating of the present invention is effective against color changes and yellowing of the pieces subjected to temperatures in a household appliance.

Example 3Ease of Cleaning of the Coating of the Present Invention

(37) A conventional stove cover was coated with the coating of the present invention. Over said cover the following foods, which are frequently used by a user, were poured: tomato, lime, milk, oil, lard and lard/oil. It complies with the method ASTM D.

(38) The cover stained with the before mentioned foods was subjected to heating conditions, that is, the four burners and the oven were kept on for one hour, followed by three hours of being cooled down.

(39) The same experiment under the same heating conditions was carried out at year one, year two and year three of continuous use of the cover with the coating.

(40) The experiment allowed arriving at the conclusion that despite the passage of time and the conditions of use, the cover protected by the coating of the present invention is easily cleanable. The results are shown in FIG. 4.

(41) As will be apparent to a person skilled in the field of the invention, the present invention covers any variation or modification which does not depart from the scope of the present invention described.