HIGH EMISSIVITY COATING COMPOSITIONS, AND PROCESSES FOR PREPARING AND APPLYING THE SAME

Abstract

To provide a high emissivity coating composition capable of exhibiting a higher emissivity at a low elevated temperature and substantially reduced formation micro craze when coated upon a substrate, and enabling a simplified application process, a high emissivity coating composition, comprising a powder mixture for providing emissivity; a binder for providing adhesion; and a co-binder for promoting adhesion and film-forming, characterized in that the powder mixture comprises at least three metal compounds of formula A.sub.(y−3)B.sub.y/2O.sub.y, wherein y is 4; A is selectable from a group of Ni and Co; B is selectable from a group of Fe and Cr; and O is oxygen; and the co-binder is an aqueous solution comprising silica in a compound of Formula (1), wherein R.sub.1 is H—Si—(CH.sub.3).sub.2; and a compound of Formula (2), wherein R.sub.2 is CH.sub.3, is disclosed herein.

Claims

1. A high emissivity coating composition, comprising a powder mixture for providing emissivity; a binder for providing adhesion; and a co-binder for promoting adhesion and film-forming, wherein the powder mixture comprises at least three metal compounds of formula A.sub.(y−3)B.sub.y/2O.sub.y, wherein y is 4; A is selectable from a group of Ni and Co; B is selectable from a group of Fe and Cr; and O is oxygen; and the co-binder is an aqueous solution comprising silica in a compound of Formula (1): ##STR00003## wherein R.sub.1 is H—Si—(CH.sub.3).sub.2; and a compound of Formula (2): ##STR00004## wherein R.sub.2 is CH.sub.3.

2. The high emissivity coating composition of claim 1, wherein the three metal compounds of formula A.sub.(y−3)B.sub.y/2O.sub.y are NiFe.sub.2O.sub.4; NiCr.sub.2O.sub.4; and CoCr.sub.2O.sub.4.

3. The high emissivity coating composition of claim 1, wherein the powder mixture further comprises cobalt oxide (CoO), titanium diboride (TiB.sub.2), chromium diboride (CrB.sub.2), magnetite (Fe.sub.3O.sub.4), kaolin (Al.sub.4(OH).sub.8Si.sub.4O.sub.10), and silica (SiO.sub.2) in quartz form.

4. The high emissivity coating composition of claim 1, wherein the powder mixture comprises 20-50% by weight of a combination of NiFe.sub.2O.sub.4 and NiCr.sub.2O.sub.4; 5-20% by weight of CoCr.sub.2O.sub.4; 5-15% by weight of CoO; 5-10.6% by weight of TiB.sub.2; 1-5% by weight of CrB.sub.2; 1-15% by weight of Fe.sub.3O.sub.4; 5-15% by weight of Al.sub.4(OH).sub.8Si.sub.4O.sub.10; and 8-30% by weight of SiO.sub.2 in quartz form.

5. The high emissivity coating composition of claim 4, wherein the powder mixture comprises 10-30% by weight of NiFe.sub.2O.sub.4.

6. The high emissivity coating composition of claim 4, wherein the powder mixture comprises 10-20% by weight of NiCr.sub.2O.sub.4.

7. The high emissivity coating composition of claim 1, wherein the binder is an aqueous solution comprising potassium silicate.

8. (canceled)

9. The high emissivity coating composition of claim 1, wherein the compound of Formula (1) is dimethyl siloxy silsesquioxane, and the compound of Formula (2) is octamethyl cyclotetrasiloxane.

10. The high emissivity coating composition of claim 1, wherein the co-binder comprises 45-55% by weight in dry basis of a combination of silica, the compound of Formula (1), and the compound of Formula (2).

11. The high emissivity coating composition of claim 1, wherein the weight ratio of the powder mixture, the binder, and the co-binder is within a range of 2.5-3.5:2.3-3.2:1.

12. A process for preparing a high emissivity coating composition, comprising steps of providing a powder mixture comprising at least three metal compounds of formula A.sub.(y−3)B.sub.y/2O.sub.y, wherein y is 4; A is selectable from a group of Ni and Co; B is selectable from a group of Fe and Cr; and O is oxygen; providing a first aqueous solution comprising potassium silicate; providing a second aqueous solution comprising silica in a compound of Formula (1): ##STR00005## wherein R.sub.1 is H—Si—(CH.sub.3).sub.2; and a compound of Formula (2): ##STR00006## wherein R.sub.2 is CH.sub.3; and mixing the powder mixture with the first aqueous solution and the second aqueous solution.

13. The process of claim 12, wherein the mixing is for forming a uniform suspension.

14. The process of claim 12, wherein the three metal compounds of formula A.sub.(y−3)B.sub.y/2O.sub.y are NiFe.sub.2O.sub.4; NiCr.sub.2O.sub.4; and CoCr.sub.2O.sub.4.

15.-21. (Canceled)

22. A process for applying a high emissivity coating composition to a substrate, comprising steps of providing a high emissivity coating composition comprising a powder mixture and an aqueous solution; spraying the high emissivity coating composition at the surface of the substrate; drying the substrate; and heating the substrate, wherein the powder mixture comprises at least three metal compounds of formula A.sub.(y−3)B.sub.y/2O.sub.y, wherein y is 4; A is selectable from a group of Ni and Co; B is selectable from a group of Fe and Cr; and O is oxygen; and the aqueous solution comprises silica in a compound of Formula (1): ##STR00007## wherein R.sub.1 is H—Si—(CH.sub.3).sub.2; and a compound of Formula (2): ##STR00008## wherein R.sub.2 is CH.sub.3.

23. The process of claim 22, further comprising a step of applying an abrasive to the surface of the substrate.

24. The process of claim 22, wherein applying the abrasive is carried out by way of sandblasting.

25. The process of claim 22, wherein the spraying is for forming a film coating upon the substrate, said film coating having a wet thickness of 100-152 micrometers.

26. The process of claim 22, wherein the spraying is for forming a film coating upon the substrate, said film coating having a dry thickness of 25-75 micrometers.

27.-28. (canceled)

29. The process of claim 22, wherein the heating is carried out at 260-650° C.

30.-38. (canceled)

39. The process of claim 22, wherein the aqueous solution is a mixture of a first intermediate aqueous solution with a second intermediate aqueous solution, wherein said first intermediate aqueous solution comprises potassium silicate, and said second intermediate aqueous solution comprises silica, the compound of Formula (1), and the compound of Formula (2).

40.-41. (canceled)

Description

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0054] Now, mixture systems of metal compounds in the form of A.sub.(y−3)B.sub.(y/2)O.sub.y will be disclosed as high emissivity agents, wherein A can be nickel and cobalt, B can be iron and chromium, and y is 4. As is well known by a skilled people in the relevant technical field, much effort is required to develop a combination of components. For example, much trial is required to find the suitable mixture for appropriate solution to suitably spray using spray gun. In addition, much trial is also required to find a suitable mixture exhibiting both high emissivity and good film forming property. Surprisingly, a high emissivity can be achieved at a lower elevated temperature ranging from 400° C. to 650° C. including good film forming of a coating per embodiments in accordance with the present invention.

[0055] To form a high emissivity property at a temperature ranging from 400° C. to 650° C., a combination of nickel spinel and cobalt spinel is present. Nickel spinel is obtained from approximately 20% to 50% based on total weight of powder mixture. Nickel spinel can be obtained from nickel ferrite (NiFe.sub.2O.sub.4), and nickel chromite (NiCr.sub.2O.sub.4). Nickel ferrite and nickel chromite present from approximately 10 to 30%, and from approximately 10 to 20% based on the total weight of the powder mixture, respectively. Both nickel spinel can be obtained from Alfa Aesa. Cobalt spinel is cochrmite (CoCr.sub.2O.sub.4), obtained from Alfa Aesa, and generally presents from approximately 5% to 20%. The following fillers can be obtained to fulfill the other coating properties of the coating film. Cobalt oxide (CoO), obtained from Alfa Aesa, can be selected from approximately 5% to 15% based on total weight of powder mixture. It gives hardness which can be employed from titanium diboride (TiB.sub.2) and chromium diboride (CrB.sub.2), obtained from Alfa Aesa. Generally, titanium diboride (TiB.sub.2) and chromium diboride (CrB.sub.2) are present from approximately 5 to 10.6%, and from approximately 1 to 5% based on the total weight of the powder mixture, respectively. Magnetite (Fe.sub.3O.sub.4), kaolin (Al.sub.4(OH).sub.8Si.sub.4O.sub.10), and 8-30% silica (SiO.sub.2) in quartz form can be obtained from Nanostructured & Amorphous Materials, Inc. Magnetite is selected from approximately 1% to 5% based on the total weight of powder mixture. Kaolin, obtained from Sigma-Aldrich, is selected in an amount of 5% to 15%. Silica, obtained from Nanostructured & Amorphous Materials, Inc., is generally present in amount of 20% to 40%. These % mentioned filler is based on the total weight of powder mixture.

[0056] Emissivity, hardness, filler component, and metal substrate are bonded together when silicate binder and co-binder are dried to create film forming.

[0057] An aqueous solution of silicate binder, potassium silicate (K.sub.2SiO.sub.3), is obtained by adding 20-40% potassium silicate powder, obtained from Alfa-Aesa, and 60-80% of deionized water based on the total weight of aqueous solution. Optionally, commercially available aqueous solution of potassium silicate can be obtained from Aremco, Ceramabind 643-1.

[0058] Co-binder is an aqueous solution of silica in the mixture between silsesquioxane and cyclotetrasiloxane. Preferably, the co-binder comprises 45-55% by weight in dry basis of a combination of silica, silsesquioxane, and cyclotetrasiloxane. Silsesquioxane and cyclotetrasiloxane may be selected from dimethyl siloxy silsesquioxane and octamethyl cyclotetrasiloxane, respectively.

[0059] The weight ratio of powder mixture, the aqueous potassium silicate, and silica in the mixture between dimethyl siloxy silsesquioxane and octamethyl cyclotetrasiloxane is 1-5:0.5-4:1, or more preferably 2.5-3.5:2.3-3.2:1. Then, the mixture is mixed and stirred to form a uniform suspension.

[0060] A metal substrate may be selected from carbon steel, metal alloy such as 5Cr, 9Cr, 12Cr metal, and stainless steel such as 18Cr metal. The substrate is blasted using Garnet blast media, obtained from Australia, to clean the surface to NACE 1, and to generate a surface profile of 13-75 micrometers.

[0061] The uniform suspension is applied on the metal substrate using an appropriate method such as spray gun, and painting, most preferably spray gun having a nozzle of 1-2 mm. Wet film thickness is controlled to be 100-152 micrometers. The wet film was measured using wet film thickness gauge. Then, the dry film is obtained of 25-75 micrometers. The dry film thickness was ensured using Elcometer. The coating thus applied is dried in room temperature for an hour. The coating is then cured by way of sintering from room temperature to a target temperature of 260-650° C. at an appropriate heating rate of 5-10° C./minute.

[0062] Emissivity of the coating is measured using industrial practice, Pyrometer, Flir GF 309. The emissivity experiment is conducted at temperature of 400° C. and 650° C. Temperature of substrates are measured using contact temperature such as surface thermocouple or skin temperature or contact temperature. Then emissivity of the pyrometer is adjusted until temperature on the Pyrometer displays the same temperature with contact temperature.

[0063] Now, this disclosure will be illustrated in more specific details per following Examples. However, the scope of the disclosure is not limited to these Examples.

EXAMPLES

Example 1

[0064] The powder mixture was mixed together using the following composition.

TABLE-US-00001 Composition Weight (%) Nickel ferrite 15.0 Nickel chromite 13.0 Cochromite 13.3 Cobalt oxide 10.3 Titanium diboride 10.6 Chromium diboride 2.3 Magnetite 3.0 Kaolin 9.9 Silica (quartz form) 22.6 Total 100

[0065] The binder was prepared by making an aqueous solution of 30% by weight of potassium silicate powder and 70% by weight of deionized water. The powder mixture, the binder, and a co-binder comprising silica in a mixture between dimethyl siloxy silsesquioxane and octamethyl cyclotetrasiloxane, were mixed together at a weight ratio of 2.5:2.3:1, forming a uniform suspension. Specifically, the co-binder comprised 45-55% by weight in dry basis of a combination of silica, dimethyl siloxy silsesquioxane, and octamethyl cyclotetrasiloxane. The 18Cr stainless steel substrate, having a size of 5×5 cm and thickness of 3 mm, was blasted using Garnet blast media, obtained from Australia, to generate a surface profile of 13-50 micrometers, and the surface cleanness was equivalent to NACE 1. The suspension was coated on the blasted substrate using a spray gun of 1-2 mm nozzle. The wet film thickness was controlled to be 100-152 micrometers by controlling the spraying speed. The coated sample was left at room temperature for an hour to dry out bulk water. The dried sample was cured at a heating rate of 5° C./minute from room temperature to a target temperature of 650° C. The final dry thickness after curing was 25-75 micrometers. Micro craze of the coating film was not found using an Optical Microscope. Adhesive ability of the coating film on the substrate was measured per ASTM D4541 and its adhesion was 14 MPa. Emissivity of the coated substrate was measured using Pyrometer, Flir GF309. Emissivity of the coated substrate was 0.81 and 0.93 at 400° C. and 650° C., respectively.

Example 2

[0066] The powder mixture was mixed together using the following composition.

TABLE-US-00002 Composition Weight (%) Nickel ferrite 15.0 Nickel chromite 13.0 Cochromite 13.3 Cobalt oxide 10.3 Titanium diboride 10.6 Chromium diboride 2.3 Magnetite 3.0 Kaolin 9.9 Silica (quartz form) 22.6 Total 100

[0067] The binder was obtained from Aremco, Ceramabind 643-1, 30% by weight of potassium silicate. The powder mixture, the binder, and a co-binder comprising silica in the mixture between dimethyl siloxy silsesquioxane and octamethyl cyclotetrasiloxane, were mixed together at a weight ratio of 2.5:2.3:1, forming a uniform suspension. Specifically, the co-binder comprised 45-55% by weight in dry basis of a combination of silica, dimethyl siloxy silsesquioxane, and octamethyl cyclotetrasiloxane. The 18Cr stainless steel substrate, having a size of 5×5 cm and thickness of 3 mm, was blasted using Garnet blast media, obtained from Australia, to generate a surface profile of 13-50 micrometers, and the surface cleanness was equivalent to NACE 1. The suspension was coated on the blasted substrate using a spray gun of 1-2 mm nozzle. The wet film thickness was controlled to be 100-152 micrometers by controlling the spraying speed. The coated sample was left at room temperature for an hour to dry out bulk water. The dried sample was cured at a heating rate of 5° C./minute from room temperature to a target temperature of 650° C. The final dry thickness after curing was 25-75 micrometers. Micro craze of the coating film was not found using an Optical Microscope. Moreover, adhesive ability of the coating film on the substrate was measured per ASTM D4541 and its adhesion was 15 MPa. To ensure that the coating film can resist a thermal shock, the samples were heated from room temperature to 650° C. within 3 minutes, then they were cooled to 300° C. within 6 minutes. The thermal shock tests were conducted in 21 cycles. Adhesion of the 21 thermal shock cycles was 14 MPa. On the other hand, adhesion of high emissivity coating obtained from a commercially available coating composition is in the range of 8 to 13.5 MPa.

[0068] Moreover, the performance of the coating compositions prepared in accordance with Example 2 was also compared with the performance of a commercially available coating composition at different elevated temperatures (400° C. and 650° C.) by the following experiment.

[0069] A 18 Cr stainless steel substrate having a size of 5×5 cm was provided. Half of the substrate was then coated with the coating composition of Example 2; the other half was coated with the commercially available coating composition. Steps and parameters of the coating process in accordance with Example 2 were applied to both halves of the substrate. Next, the entire substrate was heated at an elevated temperature of 400° C., the emissivity of both halves of the substrate were separately measured at such elevated temperature.

[0070] The same experiment was replicated at an elevated temperature of 650° C.

[0071] Following the above experiments, it was found that, at the lower elevated temperature (400° C.), the substrate coated with the coating of Example 2 exhibited a substantially higher emissivity (ε=0.81) than the substrate coated with the commercially available coating composition (ε=0.70). Likewise, at the higher elevated temperature (650° C.), the substrate coated with the coating of Example 2 exhibited a significantly higher emissivity (ε=0.93) than the substrate coated with the commercially available coating composition (ε=0.92).

Example 3

[0072] The powder mixture was mixed together using the following composition.

TABLE-US-00003 Composition Weight (%) Nickel ferrite 15.0 Nickel chromite 13.0 Cochromite 13.3 Cobalt oxide 10.3 Titanium diboride 10.6 Chromium diboride 2.3 Magnetite 3.0 Kaolin 9.9 Silica (quartz form) 22.6 Total 100

[0073] The binder was obtained from Aremco, Ceramabind 643-1, 30% by weight of potassium silicate. The powder mixture, the binder, and a co-binder comprising silica in a mixture between dimethyl siloxy silsesquioxane and octamethyl cyclotetrasiloxane, were mixed together at a weight ratio of 2.5:2.3:1, forming a uniform suspension. Specifically, the co-binder comprised 45-55% by weight in dry basis of a combination of silica, dimethyl siloxy silsesquioxane, and octamethyl cyclotetrasiloxane. The carbon steel substrate, having a size of 5×5 cm and thickness of 3 mm, was blasted using Garnet blast media, obtained from Australia, to generate a surface profile of 13-50 micrometers, and the surface cleanness was equivalent to NACE 1. The suspension was coated on the blasted substrate using a spray gun of 1-2 mm nozzle. The wet film thickness was controlled to be 100-152 micrometers by controlling the spraying speed. The coated sample was left at room temperature for an hour to dry out bulk water. The dried sample was cured at a heating rate of 5° C./minute from room temperature to a target temperature of 650° C. The final dry thickness after curing was 25-75 micrometers. Micro craze of the coating film was not found using an Optical Microscope. Emissivity of the coated substrate was measured using Pyrometer, Flir GF309. Emissivity of the coated substrate was 0.81 and 0.93 at 400° C. and 650° C., respectively.

Example 4

[0074] The powder mixture was mixed together using the following composition.

TABLE-US-00004 Composition Weight (%) Nickel ferrite 10.0 Nickel chromite 10.0 Cochromite 5.0 Cobalt oxide 15.0 Titanium diboride 5.0 Chromium diboride 5.0 Magnetite 15.0 Kaolin 15 Silica (quartz) 30 Total 100

[0075] The binder was obtained from Aremco, Ceramabind 643-1, 30% by weight of potassium silicate. The powder mixture, the binder, and a co-binder comprising silica in a mixture between dimethyl siloxy silsesquioxane and octamethyl cyclotetrasiloxane, were mixed together at a weight ratio of 2.5:2.3:1, forming a uniform suspension. Specifically, the co-binder comprised 45-55% by weight in dry basis of a combination of silica, dimethyl siloxy silsesquioxane, and octamethyl cyclotetrasiloxane. The 12Cr alloy steel substrate, having a size of 5×5 cm and thickness of 3 mm, was blasted using Garnet blast media, obtained from Australia, to generate a surface profile of 13-50 micrometers, and the surface cleanness was equivalent to NACE 1. The suspension was coated on the blasted substrate using a spray gun of 1-2 mm nozzle. The wet film thickness was controlled to be 100-152 micrometers by controlling the spraying speed. The coated sample was left at room temperature for an hour to dry out bulk water. The dried sample was cured at a heating rate of 5° C./minute from room temperature to a target temperature of 650° C. The final dry thickness after curing was 25-75 micrometers. Micro craze of the coating film was not found using an Optical Microscope. Adhesive ability of the coating film on the substrate was measured per ASTM D4541 and its adhesion was 14.7 MPa. Emissivity of the coated substrate was measured using Pyrometer, Flir GF309. Emissivity of the coated substrate was 0.85 and 0.94 at 400° C. and 650° C., respectively.

Example 5

[0076] The powder mixture was mixed together using the following composition.

TABLE-US-00005 Composition Weight (%) Nickel ferrite 30.0 Nickel chromite 20.0 Cochromite 20.0 Cobalt oxide 5.0 Titanium diboride 10.0 Chromium diboride 1.0 Magnetite 1.0 Kaolin 5.0 Silica (quartz) 8.0 Total 100

[0077] The binder was obtained from Aremco, Ceramabind 643-1, 30% by weight of potassium silicate. The powder mixture, the binder, and a co-binder comprising silica in a mixture between dimethyl siloxy silsesquioxane and octamethyl cyclotetrasiloxane, were mixed together at a weight ratio of 2.5:2.3:1, forming a uniform suspension. Specifically, the co-binder comprised 45-55% by weight in dry basis of a combination of silica, dimethyl siloxy silsesquioxane, and octamethyl cyclotetrasiloxane. The 12Cr alloy steel substrate, having a size of 5×5 cm and thickness of 3 mm, was blasted using Garnet blast media, obtained from Australia, to generate a surface profile of 13-50 micrometers, and the surface cleanness was equivalent to NACE 1. The suspension was coated on the blasted substrate using a spray gun of 1-2 mm nozzle. The wet film thickness was controlled to be 100-152 micrometers by controlling the spraying speed. The coated sample was left at room temperature for an hour to dry out bulk water. The dried sample was cured at a heating rate of 5° C./minute from room temperature to a target temperature of 650° C. The final dry thickness after curing was 25-75 micrometers. Micro craze of the coating film was not found using an Optical Microscope. Adhesive ability of the coating film on the substrate was measured per ASTM D4541 and its adhesion was 14.0 MPa. Emissivity of the coated substrate was measured using Pyrometer, Flir GF309. Emissivity of the coated substrate was 0.80 and 0.92 at 400° C. and 650° C., respectively.

Example 6

[0078] The powder mixture was mixed together using the following composition.

TABLE-US-00006 Composition Weight (%) Nickel ferrite 15.0 Nickel chromite 13.0 Cochromite 13.3 Cobalt oxide 10.3 Titanium diboride 10.6 Chromium diboride 2.3 Magnetite 3.0 Kaolin 9.9 Silica (quartz) 22.6 Total 100

[0079] The binder was obtained from Aremco, Ceramabind 643-1, 30% by weight of potassium silicate. The powder mixture, the binder, and a co-binder comprising silica in a mixture between dimethyl siloxy silsesquioxane and octamethyl cyclotetrasiloxane, were mixed together at a weight ratio of 3.5:3.2:1, forming a uniform suspension. Specifically, the co-binder comprised 45-55% by weight in dry basis of a combination of silica, dimethyl siloxy silsesquioxane, and octamethyl cyclotetrasiloxane. Two 18Cr stainless steel substrates, having a size of 5×5 cm and thickness of 3 mm, were blasted using Garnet blast media, obtained from Australia, to generate a surface profile of 13-50 micrometers, and the surface cleanness was equivalent to NACE 1. The suspension was coated on the blasted substrates using a spray gun of 1-2 mm nozzle. The wet film thickness was controlled to be 100-152 micrometers for both substrates by controlling the spraying speed. The coated samples were left at room temperature for an hour to dry out bulk water. The dried samples were cured using heating rate of 5° C./minute from room temperature to different target temperatures 260° C. for one sample and 400° C. for another. The final dry thickness after curing was 25-75 micrometers for both samples. Micro craze of the coating film was not found in any sample using an Optical Microscope. Moreover, adhesive ability of the coating film on the substrates was measured per ASTM D4541 and the adhesion was 15.0 MPa for both samples. Emissivity of the coated substrates was measured using Pyrometer, Flir GF309. Emissivity of the coated substrate was 0.81 and 0.93 at 400° C. and 650° C., respectively, for both samples.