Cross-reference to Related Applications

[0001] This application claims priority to U.S. Provisional Patent Application No. 63/415,871 filed on October 13, 2022, which is incorporated fully herein by reference.

Technical Field

[0002] This disclosure relates to multi-layer coatings for aluminum heat exchangers that include a waterborne top coat.

Introduction

[0003] Methods for providing protective coatings on aluminum heat exchangers that are also environmentally friendly can be beneficial for both companies and the public at large.

SUMMARY

[0004] In one aspect, provided are methods for coating an aluminum alloy heat exchanger, the method comprising: cleaning a surface of a heat exchanger, the heat exchanger comprising an aluminum alloy; contacting the surface of the heat exchanger with a first mixture, the first mixture comprising a trivalent chromium salt, to provide a passivation layer derived from the trivalent chromium salt on the surface of the heat exchanger; contacting the passivation layer with a second mixture, the second mixture comprising a paste and a first resin in a bath, and applying a positive charge to the bath to provide an electro-coating derived from the paste and the first resin on a surface of the passivation layer; and contacting the electro-coating with a third mixture, the third mixture comprising water and a second resin having anti-corrosion and ultraviolet protection properties, to provide a top coating comprising the second resin on a surface of the electro-coating.

Brief Description of the Drawings

[0005] FIG. 1 shows a schematic of mount locations from condenser sections of the inlet side. [0006] FIG. 2 shows images of nose tube coating thickness for different samples. [0007] FIG. 3 shows an image of top coat penetration for different samples.

DETAILED DESCRIPTION

1. Definitions

[0008] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. In case of conflict, the present document, including definitions, will control. The materials, methods, and examples disclosed herein are illustrative only and not intended to be limiting. Methods and materials similar or equivalent to those described herein can be used in practice or testing of the disclosed invention. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety.

[0009] The terms “comprise(s),” “include(s),” “having,” “has,” “can,” “contain(s),” and variants thereof, as used herein, are intended to be open-ended transitional phrases, terms, or words that do not preclude the possibility of additional acts or structures. The singular forms “a,” “and” and “the” include plural references unless the context clearly dictates otherwise. The present disclosure also contemplates other embodiments “comprising,” “consisting of’ and “consisting essentially of,” the embodiments or elements presented herein, whether explicitly set forth or not.

[0010] The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (for example, it includes at least the degree of error associated with the measurement of the particular quantity). The modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression “from about 2 to about 4” also discloses the range “from 2 to 4.” The term “about” may refer to plus or minus 10% of the indicated number. For example, “about 10%” may indicate a range of 9% to 11%, and “about 1 ” may mean from 0.9- 1.1. Other meanings of “about” may be apparent from the context, such as rounding off, so, for example “about 1” may also mean from 0.5 to 1.4.

[0011] For the recitation of numeric ranges herein, each intervening number there between with the same degree of precision is explicitly contemplated. For example, for the range of 6-9, the numbers 7 and 8 are contemplated in addition to 6 and 9, and for the range 6.0-7.0, the numbers 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, and 7.0 are contemplated, and for the range 1.5-2, the numbers 1.5, 1.6, 1.7, 1.8, 1.9, and 2 are contemplated.

2. Methods

[0012] Disclosed herein are methods of providing a multi-layered coating on an aluminum heat exchanger, where each layer can have protective properties, such as corrosion inhibition and/or resistance. The method of coating the aluminum heat exchanger can include at least three different steps that result in at least three different and distinct layers on a surface of the aluminum heat exchanger.

[0013] The heat exchanger includes aluminum. Any suitable aluminum alloy can be included in the heat exchanger. Examples include, but are not limited to, 1000, 3000, 5000, 6000, and 7000 series aluminum alloys. In some embodiments, the heat exchanger includes a 1000 series aluminum alloy, a 3000 series aluminum alloy, a 5000 series aluminum alloy, a 6000 series aluminum alloy, a 7000 series aluminum alloy, or a combination thereof. In some embodiments, the heat exchanger includes a 1000 series aluminum alloy, a 3000 series aluminum alloy, a 5000 series aluminum alloy, a 6000 series aluminum alloy, or a 7000 series aluminum alloy. In addition, a surface, e.g., to be coated, of the heat exchanger can be any suitable area of the heat exchanger including, but not limited to, fins, tubes, plates, and combinations thereof.

[0014] Prior to providing a layer or coating on the heat exchanger, the heat exchanger can be cleaned. For example, the method can include cleaning a surface of the heat exchanger. The surface that is cleaned can be the surface that is intended to be layered or coated with the passivation layer, electro-coating, and/or top coating. Cleaning the surface of the heat exchanger can remove contaminants, such as dirt and oils, from the surface. In addition, cleaning can prepare the surface for being contacted with the first mixture by e.g., removing oxide and/or hydroxide formation on the surface. In some embodiments, the cleaning includes contacting the surface with an alkaline mixture.

A. Passivation Layer

[0015] The method includes a step of providing a passivation layer on a surface of the heat exchanger. For example, the method includes contacting the surface (e.g., cleaned surface) of the heat exchanger with a first mixture to provide a passivation layer on the surface of the heat exchanger. The first mixture can include a trivalent chromium salt. An example trivalent chromium salt includes, but is not limited to, trivalent chromium sulfate. The first mixture can further include a salt of hexafluorozirconic acid, an adhesion promoter, or a combination thereof. In some embodiments, the first mixture further includes a salt of hexafluorozirconic acid.

[0016] The passivation layer is provided by an autocatalytic conversion process where the cleaned heat exchanger is placed within a bath, e.g., a conversion bath, including the first mixture, which can include the trivalent chromium salt. Alternatively, the autocatalytic conversion process can be carried out via spray coating or flow coating the first mixture onto the surface of the heat exchanger to provide the passivation layer. During this process the passivation layer is provided on the surface of the heat exchanger, where the surface of the heat exchanger is chemically modified to provide the passivation layer (e.g., the passivation layer is derived from the trivalent chromium salt). The provided passivation layer can inhibit and/or prevent corrosion on the surface of the heat exchanger. In addition, the passivation layer can aid in the adhesion of the electro-coating. The passivation layer can also be referred to as a corrosion inhibitive conversion coating.

[0017] The surface of the heat exchanger can be contacted with the first mixture for a varying amount of time. For example, the surface of the heat exchanger can be contacted with the first mixture for less than or equal to 12 minutes, less than or equal to 11 minutes, less than or equal to 10 minutes, less than or equal to 9 minutes, or less than or equal to 8 minutes. In some embodiments, the surface of the heat exchanger is contacted with the first mixture for greater than or equal to 10 seconds, greater than or equal to 30 seconds, greater than or equal to 1 minute, greater than or equal to 2 minutes, or greater than or equal to 3 minutes. In some embodiments, the surface of the heat exchanger is contacted with the first mixture for about 10 seconds to about 12 minutes, such as about 1 minute to about 5 minutes, about 30 seconds to about 10 minutes, or about 2 minutes to about 12 minutes.

[0018] The passivation layer can have a varying weight depending on how much time the surface of the heat exchanger is in contact with the first mixture. For example, the passivation layer can have a weight of at least 0.05 g/m ² , at least 0.1 g/m ² , at least 0.2 g/m ² , at least 0.3 g/m ² , at least 0.4 g/m ² , or at least 0.5 g/m ² . In some embodiments, the passivation layer has a weight of less than 1 g/m ² , less than 0.9 g/m ² , less than 0.8 g/m ² , less than 0.7 g/m ² , less than 0.6 g/m ² , or less than 0.5 g/m ² . In some embodiments, the passivation layer has a weight of about 0.05 g/m ² to about 1 g/m ² , such as about 0.5 g/m ² to about 1 g/m ² , about 0.1 g/m ² to about 1 g/m ² , or about 0.05 g/m ² to about 0.5 g/m ² .

B. Electro-Coating

[0019] The method includes a step of providing an electro-coating on a surface of the passivation layer. For example, the method includes contacting the passivation layer with a second mixture in a bath and applying a positive charge to the bath to provide an electro-coating on a surface of the passivation layer. The second mixture can include a paste and a first resin (e.g., polymer resin). In other words, the second mixture can be a two-component system. The paste and first resin combination can have anti-corrosion properties. In some embodiments, the first resin includes an epoxy resin. An example paste and resin combination includes, but is not limited to, PPG POWERCRON® epoxy coating. The second mixture can further include water. In some embodiments, the second mixture includes about 75% water and about 25% of paste, resin, and other components used for the coating process.

[0020] In order to provide the electro-coating, electrically charged particles in the second mixture can carry a positive charge and can be attracted to a negative metal surface of the heat exchanger following a positive charge being applied to the bath. This can be referred to as a cathodic system - where a positively charged resin and paste are deposited onto a negatively charged part (e.g., the heat exchanger). Accordingly, in some embodiments, the heat exchanger acts as a cathode when the positive charge is being applied to the bath. And, as the paste and resin are deposited on the surface of the heat exchanger (e.g., the surface of the passivation layer), the electro-coating is derived from the paste and the first resin. The electro-coating, being derived from the paste and the first resin, can inhibit and/or prevent corrosion on the surface of the heat exchanger. The electro-coating can also be referred to as an e-coating.

[0021] Contacting the passivation layer with the second mixture can take place in a bath or tank. The bath or tank can include the second mixture. In addition, contacting the passivation layer with the second mixture can be done for a varying amount of time. For example, time can vary based on the surface area needed to be coated with the electro-coating. It is within the knowledge of the skilled artisan to select a suitable amount of time based on the surface area needed to be coated.

[0022] The electro-coating can have a varying thickness. For example, the electro-coating can have a thickness of less than 50 pm, less than 45 pm, less than 40 pm, less than 35 pm, or less than 30 pm. In some embodiments, the electro-coating has a thickness of greater than 5 pm, greater than 10 pm, greater than 15 pm, greater than 20 pm, or greater than 25 gm. In some embodiments, the electro-coating has a thickness of about 5 pm to about 50 gm, such as about 10 gm to about 50 gm, about 5 gm to about 30 gm, or about 15 gm to about 50 gm.

[0023] Following application of the electro-coating, the heat exchanger can be further processed. For example, the heat exchanger can be washed (e.g., rinsed), heated (e.g., at temperatures that can cure the electro-coating), or both after applying the electro-coating. Washing the electro-coating is distinct from the cleaning step of the heat exchanger. In some embodiments, washing the electro-coating includes rinsing the electro-coating with a permeate. The curing process can create a smooth, hard shell that can resist or mitigate corrosion, pitting, and/or flaking. In addition, curing can crosslink the electro-coating, which can provide a high- quality finish without runs, drips, and/or sags. In some embodiments, after applying the electrocoating, the heat exchanger is washed prior to being heated. In some embodiments, after applying the electro-coating, the heat exchanger is washed twice, in two different steps, prior to heating.

C. Top Coating

[0024] The method further includes a step of providing a top coating on a surface of the electro-coating. For example, the method includes contacting the electro-coating with a third mixture to provide a top coating on a surface of the electro-coating. The third mixture can include water and a second resin. The third mixture can be a single component system (e.g., not a two-component system that requires a component in addition to the resin). The second resin can have anti-corrosion and ultraviolet (UV) protection properties. In some embodiments, the second resin includes an acrylic resin with corrosion and UV protection properties, a polyurethane resin with corrosion and UV protection properties, or a combination thereof. In addition, the third mixture can include adhesion promoter(s). In some embodiments, the third mixture further includes embedded stainless steel.

[0025] The top coating can be referred to as a waterborne top coating. For example, the third mixture (used to provide the top coating) can include greater than 50% water by weight of the third mixture, greater than 55% water by weight of the third mixture, greater than 60% water by weight of the third mixture, greater than 61% water by weight of the third mixture, or greater than 62% water by weight of the third mixture. In some embodiments, the third mixture includes less than 90% water by weight of the third mixture, less than 85% water by weight of the third mixture, less than 80% water by weight of the third mixture, less than 75% water by weight of the third mixture, or less than 70% water by weight of the third mixture. In some embodiments, the third mixture includes about 50% to about 90% water by weight of the third mixture, such as about 55% to about 85% water by weight of the third mixture, about 55% to about 75% water by weight of the third mixture, or about 60% to about 80% water by weight of the third mixture. [0026] The third mixture can have a varying solids content. For example, the third mixture can include a solids content of less than 35% by weight of the third mixture, a solids content of less than 34% by weight of the third mixture, a solids content of less than 33% by weight of the third mixture, a solids content of less than 32% by weight of the third mixture, a solids content of less than 31 % by weight of the third mixture, a solids content of less than 30% by weight of the third mixture, a solids content of less than 25% by weight of the third mixture, or a solids content of less than 20% by weight of the third mixture. In some embodiments, the third mixture includes a solids content of greater than 10% by weight of the third mixture, a solids content of greater than 11% by weight of the third mixture, a solids content of greater than 12% by weight of the third mixture, a solids content of greater than 13% by weight of the third mixture, a solids content of greater than 14% by weight of the third mixture, a solids content of greater than 15% by weight of the third mixture, a solids content of greater than 20% by weight of the third mixture, or a solids content of greater than 25% by weight of the third mixture. In some embodiments, the third mixture includes a solids content of about 10% to about 35% by weight of the third mixture, such as about 15% to about 35% by weight of the third mixture, about 20% to about 35% by weight of the third mixture, or about 25% to about 35% by weight of the third mixture. The solids content can correspond to the resin solids content.

[0027] The top coating can be applied as a spray coating. For example, the third mixture can be sprayed onto a surface of the electro-coating. Following spray coating, the top coating may not require a further heating step. For example, in some embodiments, the top coating does not require a heat or chemical cure, e.g., to crosslink or further crosslink the resin. The top coating may be self-healing.

[0028] The waterborne top coating can provide advantages compared to solvent borne counterparts. For example, the waterborne top coating can penetrate (e.g., penetrating the depths and recesses of the heat exchanger, not penetrating into the aluminum alloy itself) further into the heat exchanger, which can provide more protection. Penetration of the top coating can be improved by using waterborne methods, which can allow the top coating to flow over the electro-coating in such a way that can offer improved penetration. In some embodiments, the top coating penetrates the heat exchanger at least 0.5 mm, at least 0.75 mm, at least 1 mm, at least

1.25 mm, at least 1.5 mm, at least 2 mm, at least 2.25 mm, at least 2.5 mm, at least 3 mm, at least

3.25 mm, at least 3.5 mm, at least 4 mm, at least 4.25 mm, at least 4.5 mm, at least 5 mm, at least

5.25 mm, at least 5.5 mm, at least 6 mm, at least 6.5 mm, or at least 7 mm. In some embodiments, the top coating penetrates the heat exchanger less than 10 mm, less than 9.8 mm, less than 9.7 mm, less than 9.6 mm, less than 9.5 mm, less than 9.4 mm, less than 9.3 mm, less than 9.2 mm, or less than 9.1 mm.

[0029] In some embodiments, the top coating penetrates the heat exchanger about 0.5 mm to about 10 mm, such as about 1 mm to about 10 mm, about 2 mm to about 9.5 mm, about 3 mm to to about 9.5 mm, about 4 mm to about 10 mm, about 4.1 mm to about 9.5 mm, about 4.2 mm to about 9.2 mm, or about 4.3 mm to about 9 mm.

[0030] In addition, because the top coating is waterborne, and not solvent borne, it and the third mixture can, independently, have decreased amounts of volatile organic compounds. For example, the top coating and the third mixture, independently, can include less than or equal to 75 g/L of volatile organic compounds, less than or equal to 70 g/L of volatile organic compounds, less than or equal to 65 g/L of volatile organic compounds, less than or equal to 60 g/L of volatile organic compounds, less than or equal to 55 g/L of volatile organic compounds, less than or equal to 50 g/L of volatile organic compounds, less than or equal to 45 g/L of volatile organic compounds, less than or equal to 40 g/L of volatile organic compounds, less than or equal to 35 g/L of volatile organic compounds, less than or equal to 30 g/L of volatile organic compounds, less than or equal to 25 g/L of volatile organic compounds, or less than or equal to 20 g/L of volatile organic compounds. In some embodiments, the top coating and the third mixture, independently, include greater than or equal to 0.5 g/L of volatile organic compounds, greater than or equal to 0.75 g/L of volatile organic compounds, greater than or equal to 1 g/L of volatile organic compounds, greater than or equal to 2 g/L of volatile organic compounds, greater than or equal to 3 g/L of volatile organic compounds, greater than or equal to 4 g/L of volatile organic compounds, greater than or equal to 5 g/L of volatile organic compounds, greater than or equal to 10 g/L of volatile organic compounds, or greater than or equal to 15 g/L of volatile organic compounds. In some embodiments, the top coating and the third mixture, independently, include about 0.5 g/L to about 75 g/L of volatile organic compounds, such as about 5 g/L to about 60 g/L of volatile organic compounds, about 10 g/L to about 50 g/L of volatile organic compounds, or about 5 g/L to about 45 g/L of volatile organic compounds.

[0031] The top coating can have a varying thickness. For example, the top coating can have a thickness of about 10 pm to about 100 pm, such as about 20 pm to about 90 pm, about 20 pm to about 80 pm, about 25 pm to about 80 pm, about 20 pm to about 75 pm, about 25 pm to about 75 pm, about 30 pm to about 50 pm, or about 30 pm to about 40 pm. In some embodiments, the top coating has a thickness of about 25 pm to about 75 pm. In some embodiments, the top coating has a thickness of less than 100 pm, less than 95 pm, less than 90 pm, less than 85 pm, or less than 80 pm. In some embodiments, the top coating has a thickness of greater than 10 pm, greater than 20 pm, greater than 25 pm, greater than 30 pm, or greater than 35 pm.

D. Coloring the Top Coating

[0032] The method can further include coloring the top coating. The top coating can be custom tinted or colored. The tinting or coloring can demonstrate that the entirety of the e-coat has been coated with the top coating. Any color preference can be used to color the top coating. In some embodiments, coloring the top coating includes adding a pigment dispersion. The pigment dispersion can be a liquid or a paste mixed with water to create the dispersion. The pigment dispersion can be inorganic or organic. In some embodiments, the pigment dispersion is inorganic. The pigment dispersion can be included in the third mixture.

[0033] The disclosed invention has multiple aspects, illustrated by the following non-limiting examples.

3. Examples

Example 1

Examination of Coated Heat Exchangers from Two-year Coastal Field Study

[0034] A field study of microchannel condensers coated with an e-coat were divided into three sections to evaluate various top coats (Table 1). The coated condenser was placed in a coastal environment for two years pressurized and with fans operational. After one year, the condenser was rinsed off with water and testing was continued. No leak was detected after two years of exposure, but fouling was present. At the end of the test, the coils were evaluated. The purpose of the field test was to assess waterborne top coatings as an alternative to solvent borne top coatings.

Table 1 : Sample Information

[0035] Examination'. Photo documentation of each section was taken before and after rinsing. The sections were rinsed with deionized (DI) water at a low pressure, dried with compressed air and placed into a drying oven overnight. After drying, a nylon brush was used to remove fouling for further examination of the core sections.

[0036] Samples of the condenser were cut from the locations indicated in FIG. 1 to determine the coating thickness at the tube nose and penetration of the top coat. Average tube nose coating thickness and coating penetrations are summarized in Table 2 and Table 3, respectively.

Table 2: Average Tube Nose Coating Thickness (pm)

[0037] Comments: All four core sections exhibited comparable macroscopic paint performance after rinsing and drying. No E-coat removal was seen in the cores (excluding shipping damage). The cross sections showed comparable average coating thickness for all three top coats, summarized in Table 2. All sections had an average top coat thickness that was within the range of 25 pm to 75pm. Top coats were detected in the microscopy cross-section, as shown in FIG. 2. Table 3: Average Top coat Penetration (mm)

[0038] The samples met the finpack penetration requirement of at least 2 mm as summarized in Table 3 and shown in FIG. 3. Tubes from Section A displayed the least top coat penetration, with an average of 2.4-mm. Cross-section of the cores in Section B illustrated notable separation of the E-Coat and top coat from both tubes throughout the entire length. Although the tubes exhibited signs of corrosion, the coating remained intact on top of the corroded areas. Tubes from section B displayed an average top coat penetration of 7.7-mm. Tubes from Section C showed signs of corrosion of the top coat and had an average top coat penetration of 5.3 -mm. [0039] It is understood that the foregoing detailed description and accompanying examples are merely illustrative and are not to be taken as limitations upon the scope of the invention.

[0040] Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications, including without limitation those relating to the chemical structures, coatings, steps, compositions, formulations, or methods of use of the invention, may be made without departing from the spirit and scope thereof.

[0041] For reasons of completeness, various aspects of the invention are set out in the following numbered clauses:

[0042] Clause 1. A method for coating an aluminum alloy heat exchanger, the method comprising: cleaning a surface of a heat exchanger, the heat exchanger comprising an aluminum alloy; contacting the surface of the heat exchanger with a first mixture, the first mixture comprising a trivalent chromium salt, to provide a passivation layer derived from the trivalent chromium salt on the surface of the heat exchanger; contacting the passivation layer with a second mixture in a bath, the second mixture comprising a paste and a first resin, and applying a positive charge to the bath to provide an electro-coating derived from the paste and the first resin on a surface of the passivation layer; and contacting the electro-coating with a third mixture, the third mixture comprising water and a second resin having anti-corrosion and ultraviolet protection properties, to provide a top coating comprising the second resin on a surface of the electro-coating.

[0043] Clause 2. The method of clause 1, wherein the cleaning comprises contacting the surface with an alkaline mixture.

[0044] Clause 3. The method of clause 1 or 2, wherein the first mixture further comprises a salt of hexafluorozirconic acid.

[0045] Clause 4. The method of any one of clauses 1-3, wherein the trivalent chromium salt comprises trivalent chromium sulfate.

[0046] Clause 5. The method of any one of clauses 1-4, wherein the passivation layer has a weight of at least 0.05 g/m ² .

[0047] Clause 6. The method of any one of clauses 1-5, wherein the surface of the heat exchanger is contacted with the first mixture for less than or equal to 12 minutes.

[0048] Clause 7. The method of any one of clauses 1-6, wherein the heat exchanger acts as a cathode when the positive charge is being applied.

[0049] Clause 8. The method of any one of clauses 1-7, wherein the electro-coating has a thickness of about 5 pm to about 50 pm.

[0050] Clause 9. The method of any one of clauses 1-8, wherein the heat exchanger is washed, heated, or both after applying the electro-coating.

[0051] Clause 10. The method of any one of clauses 1-9, wherein the third mixture comprises a solids content of less than 35% by weight of the third mixture.

[0052] Clause 11. The method of any one of clauses 1-10, wherein the second resin of the third mixture comprises an acrylic resin, a polyurethane resin, or a combination thereof.

[0053] Clause 12. The method of any one of clauses 1-11, wherein the first mixture, the third mixture, or both further comprise an adhesion promoter.

[0054] Clause 13. The method of any one of clauses 1-12, wherein the top coating penetrates the heat exchanger at least 0.5 mm.

[0055] Clause 14. The method of any one of clauses 1-13, wherein the third mixture is sprayed onto the electro-coating.

[0056] Clause 15. The method of any one of clauses 1-14, wherein the third mixture comprises less than or equal to 75 g/L of volatile organic compounds. [0057] Clause 16. The method of any one of clauses 1-15, wherein the top coating has a thickness of about 10 pm to about 100 pm.

[0058] Clause 17. The method of any one of clauses 1-16, wherein the method further comprises coloring the top coating.

[0059] Clause 18. The method of clause 17, wherein coloring comprises adding a pigment dispersion to the top coating.

[0060] Clause 19. The method of any one of clauses 1-18, wherein the top coating does not require a heat or chemical cure.