Abstract:
A method includes applying a first powder to an aluminum article and heating the first powder to form a first layer on the aluminum article providing mechanical strength, corrosion durability and bonding potential. The method also includes applying a second powder to the aluminum article and heating the second powder to form a second layer on the aluminum article protecting the aluminum article from ultraviolet radiation. A coated article includes an aluminum substrate, an epoxy layer and a topcoat layer. The epoxy layer promotes adhesion, enhances corrosion durability and provides mechanical strength, and is formed by applying a first powder containing an epoxy to the aluminum substrate and curing the first powder. The topcoat layer provides resistance to ultraviolet radiation and environmental contaminants, and is formed by applying a second powder to the aluminum substrate and curing the second powder.

Description:
BACKGROUND 
       [0001]    In certain environments, aluminum articles are subjected to contaminants that cause corrosion or other undesired effects on the aluminum surface. Unprotected aluminum can become corroded by acids, salts and other reactive compounds to develop pits or holes on and through aluminum surfaces. Ultraviolet (UV) radiation can cause discoloring of aluminum surfaces. Aluminum articles, such as heat exchangers, are often coated to protect aluminum and aluminum alloy surfaces. Such coatings provide resistance to corrosion caused by environmental contaminants or ultraviolet (UV) radiation or increase mechanical strength. These coatings can be applied to aluminum surfaces in a number of ways. Coating methods include electroplating, dip coating, spray coating and electrostatic powder coating. Protective coatings include conversion coatings and paint coatings. 
         [0002]    Powder coating provides a less expensive way to coat aluminum articles. Powder coatings do not require special baths or large quantities of chemicals other than the powder coatings themselves. Powder coatings do not require solvents which can adversely impact air and water quality or can permanently damage aluminum articles. Traditional powder coatings have drawbacks, however. Prior to the present invention, powder coating formulations were generally optimized for one function (i.e. strength/bonding or UV resistance), but not both. Additionally, the traditional application of powder coatings did not provide the amount of control and uniformity that other coating processes possessed. Uniform levels of powder coatings are difficult to apply. In some cases, bare metal was left exposed following powder coating. This bare metal did not possess any of the protective characteristics that the powder coating provided. On the other hand, in some locations, the powder coating was excessively thick, which was detrimental for surface characteristics such as thermal and hydraulic properties. 
       SUMMARY 
       [0003]    A method according to the present invention includes applying a first powder to an aluminum article and heating the first powder to form a first layer on the aluminum article providing mechanical strength, corrosion durability and bonding potential. The method also includes applying a second powder to the aluminum article and heating the second powder to form a second layer on the aluminum article protecting the aluminum article from ultraviolet radiation. 
         [0004]    The present invention also provides a coated article having an aluminum substrate, an epoxy layer and a topcoat layer. The epoxy layer promotes adhesion, enhances corrosion durability and provides mechanical strength, and is formed by applying a first powder containing an epoxy to the aluminum substrate and curing the first powder. The topcoat layer provides resistance to ultraviolet radiation and environmental contaminants, and is formed by applying a second powder to the aluminum substrate and curing the second powder. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a side sectional view of a powder coating system for use on an aluminum article. 
           [0006]      FIG. 2  is a side sectional view of a powder coating system for use on an aluminum article having a conversion coating. 
           [0007]      FIG. 3  is a flow chart illustrating a method for coating an aluminum article. 
           [0008]      FIG. 4  is a flow chart illustrating another method for coating an aluminum article. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    The present invention describes a dual powder coating system which can provide improved bonding capabilities and improved environmental protection for aluminum articles such as heat exchangers. The dual powder coating system allows for the application of two powder-based coatings to an aluminum article: one to primarily enhance the mechanical strength, corrosion durability and/or adhesive bonding characteristics of the aluminum article and another one to primarily provide additional resistance to UV radiation. The dual powder coating system can work with any type of aluminum article and is particularly useful for aluminum heat exchangers, especially aluminum microchannel heat exchangers. While specific embodiments are described with reference to aluminum heat exchangers, the invention can also provide benefits to other aluminum articles. “Aluminum articles” refers to articles containing aluminum, aluminum alloys or a combination of the two. 
         [0010]    Heat exchangers are used in a variety of environments, including marine, industrial and urban environments. Often, heat exchanger surfaces are aluminum and subject to the corrosion and discoloring described above. Heat exchangers can contain inlet and outlet manifolds; heat exchange tubes, coils or channels; fins and other structures that are made of aluminum or aluminum alloys. All of these surfaces need to be protected in order to prevent or reduce corrosion and other undesired effects. 
         [0011]    Multiple coatings can be applied to the surfaces of a heat exchanger. Various coatings can be applied to an aluminum heat exchanger by spraying, dipping, painting or brushing, anodization, electroplating and other methods. In order for multiple coatings to effectively adhere to the surface of a heat exchanger, the surface chemistry of the heat exchanger must sometimes be changed. The surface chemistry is modified to provide improved bonding potential between the surface of the heat exchanger and subsequent coating layers. An adhesion promoting coating can be used to modify the heat exchanger surface so that later applied coatings bond to the surface strongly. Epoxy-based coatings are one type of coating that improves bonding and adhesion between an aluminum surface and later applied coatings. Epoxy-based coatings also provide additional mechanical strength and improved corrosion durability of the aluminum surface. 
         [0012]    Epoxy-based coatings can deteriorate when exposed to UV radiation. As such, a second coating can be applied to surfaces of a heat exchanger to provide UV protection. Acrylics, polyester-based thermoplastic polyurethanes and polyester triglycidyl isocyanurate (TGIC) are types of coatings that provide resistance to UV radiation. The second coating also adds a mechanical barrier to protect any areas of the heat exchanger that were missed when the first coating was applied. 
         [0013]    Some heat exchangers (i.e. aluminum microchannel heat exchangers) often have complex geometries with sharp corners and edges and small spaces that make coating the heat exchanger surfaces difficult. Powder coatings typically do not provide as much control and self-leveling as some of the other coating technologies might (e.g., electrophoretic painting). Nonetheless, powder coatings make up for some of these deficiencies with other advantages (no toxic solvents, easier to apply, no need for rinsing and drying operations, etc.). A single application of a powder coating can leave areas of the heat exchanger uncoated where bare aluminum is exposed. Applying a second powder coating over a first powder coating minimizes the impact of uncoated and exposed aluminum. The second coating is able to infiltrate gaps left by the first coating so that the aluminum receives some level of additional protection. While the second coating primarily provides protection from UV radiation, the second coating also provides at least some increase in mechanical strength and corrosion protection. 
         [0014]      FIG. 1  illustrates a side sectional view of dual powder coating system  10  and aluminum article  12 . Dual powder coating system  10  includes first layer  14  and second layer  16 . First layer  14  is formed on surface  18  of aluminum article  12  using a first powder. First layer  14  improves the mechanical strength, corrosion durability and bonding capabilities of surface  18 . Second layer  16  is formed over first layer  14  on surface  18  using a second powder. Second layer  16  improves the UV resistance of surface  18 . 
         [0015]    The first powder and the second powder are applied to surface  18  in different steps. The first powder can be applied following cleaning of surface  18 . Where surface  18  was previously contacted with brazing flux material and brazed, any residual flux may need to be removed so that first layer  14  can bond strongly with surface  18 . A method for removing residual flux is provided in International Application No. PCT/US09/42552, filed May 1, 2009, which is incorporated by reference. 
         [0016]    First layer  14  is formed by applying the first powder to surface  18 . The first powder is selected to provide additional mechanical strength and improve corrosion durability of surface  18  and/or promote adhesion and bonding between surface  18  and later applied coatings. Suitable first powders include epoxy-based powder coatings such as epoxies, polyester epoxies, acrylic epoxies, fusion-bond epoxy powder coatings and combinations thereof. Specific examples of suitable first powders include epoxy powder coatings based on Bisphenol A or Bisphenol F resins. 
         [0017]    First layer  14  is formed by applying the first powder to surface  18  and heating the first powder. The first powder can be applied to surface  18  by spraying, dipping, fluidized bed spraying, electrostatic deposition, electrostatic magnetic brush coating and combinations thereof. The first powder can be applied to surface  18  and then heated, applied to an already heated surface  18  or a combination of the two. Heat can be applied to surface  18  and/or the first powder by induction heating, oven heating, infra-red heating and combinations thereof. 
         [0018]    One method of applying the first powder to surface  18  and forming first layer  14  includes spraying the first powder onto surface  18  and then heating the first powder. A wide variety of spray guns or nozzles can be used, depending on the consistency of the first powder. The first powder is sprayed evenly across surface  18 . The first powder is then heated (cured) either directly or by heating surface  18 . Once the first powder melts, it forms a uniform film on surface  18 . The first powder and surface  18  are cooled to form first layer  14 . 
         [0019]    Application methods can also be combined to increase their effectiveness. For example, during fluidized bed spraying, the first powder is fluidized, suspended in a stream of air (or other gas). Often, the fluidized first powder is sprayed onto heated surface  18  using suitable spray guns or nozzles. Once the fluidized first powder contacts heated surface  18 , first powder melts into a liquid. The liquid is cooled, forming first layer  14 . Fluidized bed spraying can be combined with electrostatic deposition. The fluidized first powder is applied using an electrostatic spray gun. The electrostatic spray gun ionizes the first powder, imparting its particles with a positive electric charge. Heated surface  18  is grounded or imparted with a negative charge. The positively charged fluidized first powder uniformly deposits on heated surface  18  due to the powder&#39;s positive electrical charge and melts into a liquid form. The liquid is cooled, forming first layer  14 . In one exemplary embodiment, the first powder is electrostatically sprayed onto surface  18  and then heated to provide a uniform first layer  14 . 
         [0020]    Second layer  16  is formed by applying the second powder to surface  18 . The second powder is selected to provide additional UV resistance to first layer  14  and surface  18 . Epoxies can deteriorate following exposure to UV radiation. Where first layer  14  is epoxy-based, first layer  14  can deteriorate unless it is protected from UV radiation. Suitable second powders include thermoset powder coatings such as acrylics, polyester-based thermoplastic polyurethanes, polyester triglycidyl isocyanurate (TGIC) and combinations thereof. Specific examples of suitable second powders include acrylic clearcoats such as PCC10106 (available from PPG industries). 
         [0021]    Second layer  16  is formed in similar fashion to first layer  14 . Second layer  16  is formed by applying the second powder to surface  18  (already covered with first layer  14 ) and heating the second powder. The second powder can be applied to surface  18  by spraying, dipping, fluidized bed spraying, electrostatic deposition, electrostatic magnetic brush coating and combinations thereof. The second powder can be applied and then heated, applied to an already heated surface  18  or a combination of the two. Heat can be applied to surface  18  and/or the second powder by induction heating, oven heating, infra-red heating and combinations thereof. The methods and examples described above with respect to the first powder can also be used for the second powder. 
         [0022]    First layer  14  and second layer  16  can have varying thicknesses depending on various needs such as the heat exchanger&#39;s operating environment. Typically, first layer  14  and second layer  16  each have a thickness between about 15 microns and about 35 microns. This range of thickness is appropriate for most heat exchange applications. In an exemplary embodiment, first layer  14  and second layer  16  each have a thickness of about 25 microns. 
         [0023]    The time and temperature required for curing the first and second powders depends on the design of surface  18  (e.g., convoluted surfaces, flat surface, etc.), the chemistry of surface  18  (e.g., aluminum, aluminum alloy, etc.), the characteristics of the first and second powders selected, the thicknesses of first layer  14  and second layer  16  and the curing oven characteristics. For most surfaces  18  and first and second powders described herein, a curing temperature between about 190° C. (375° F.) and about 200° C. (390° F.) is typical. At these temperatures, curing times between about 10 minutes and about 15 minutes are appropriate. 
         [0024]    First layer  14  and second layer  16  can also contain additional corrosion-inhibiting compounds. These compounds can be incorporated into the first powder and/or the second powder so that they are incorporated into first layer  14 , second layer  16  or both layers  14  and  16 . Suitable additional corrosion-inhibiting compounds include corrosion inhibitive pigments, galvanically sacrificial metals (e.g., zinc, zinc alloys, magnesium), lanthanoids, molybdates, vanadates and tungstates. 
         [0025]    In one exemplary embodiment of the present invention, the first powder is applied to bare surface  18  of aluminum article  12  to form first layer  14  as described above and illustrated in  FIG. 1 . In another exemplary embodiment, a conversion coating is applied to surface  18  of aluminum article  12  before the first powder is applied. Conversion coatings typically offer adhesion promoting and/or corrosion inhibiting characteristics to surface  18 . 
         [0026]      FIG. 2  illustrates a side sectional view of dual powder coating system  10   a  and aluminum article  12  with conversion coating  20 . Conversion coating  20  is applied to surface  18  before the first powder is applied and before first layer  14  is formed. Conversion coating  20  can be applied by spraying, dipping, fluidized bed spraying, electrostatic deposition, electrostatic magnetic brush coating or any other suitable coating method. Examples of suitable conversion coatings include chromate and phosphate conversion coatings, coatings containing trivalent chromium, zinc phosphate, iron phosphate, or manganese phosphate and combinations thereof. 
         [0027]    Dual powder coating system  10   a  includes first layer  14  and second layer  16  as described above. First layer  14  is formed on aluminum article  12  over conversion coating  20  using a first powder. First layer  14  improves the mechanical strength, corrosion durability and bonding capabilities of aluminum article  12 . Second layer  16  is formed over first layer  14  on aluminum article  12  using a second powder. Second layer  16  improves the UV resistance of aluminum article  12 . 
         [0028]    Dual powder coating systems  10  and  10   a  provide a method for coating an aluminum article.  FIG. 3  is a flow chart illustrating a method of coating an aluminum article according to the present invention. Method  22  includes applying a first powder to an aluminum article (step  24 ), and heating the first powder to form a first layer on the aluminum article (step  26 ). The first layer provides increased mechanical strength, enhanced corrosion protection and improved bonding potential for the aluminum article. Method  22  also includes applying a second powder to the aluminum article (step  28 ), and heating the second powder to form a second layer on the aluminum article (step  30 ). The second layer protects the aluminum article from ultraviolet radiation. Application of each powder to the aluminum article and heating the powder can occur successively or contemporaneously. According to method  22 , the first powder is heated before the second powder is applied to the aluminum article. According to method  22 , the first powder can be applied directly to surface  18  of aluminum article  12  or to an aluminum article  12  having a conversion coating  20 . 
         [0029]      FIG. 4  is a flow chart illustrating another method of coating an aluminum article. Method  32  includes applying a first powder to an aluminum article (step  34 ), applying a second powder to the aluminum article (step  36 ) and heating the first and second powders to form first and second layers on the aluminum article (step  38 ). According to method  32 , the second powder is applied to the aluminum article before the first powder is heated to form the first layer. The first and second layers are formed contemporaneously. According to method  32 , the first powder can be applied directly to surface  18  of aluminum article  12  or to an aluminum article  12  having a conversion coating  20 . 
         [0030]    The dual powder coating system and method of the present invention provide an aluminum article with enhanced mechanical strength, improved bonding capability, enhanced corrosion durability and improved resistance to UV radiation. Aluminum articles can possess improved features in all of these areas rather than having to pick and choose from among them. 
         [0031]    While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.