Abstract:
A method of forming composite polymeric coatings on a metal surface of a condensing heat exchanger substrate which includes providing an acidic starting solution which has at least one chemically distinct monomer and at least one solvent and where the monomer is polymerizable upon contact with the metal substrate. The monomer solution is applied to the substrate which will initiate spontaneous polymerization of the monomer on the surface of the metal substrate in the absence of any other catalyst or catalysts. The solution contacts the metal substrate for a time sufficient to form a polymeric coating on the substrate. The monomer further contains latent polymerization sites that do not react during spontaneous polymerization and are available as reaction sites for bonding with a later applied overcoating. A second protective polymeric overcoat is applied to said first polymer coating whereby the latent polymerization sites react with said polymeric overcoat to enhance bonding and adhesion between the two coatings.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to heat exchangers, and more specifically to a primer adhesive which is used as a protective coating for heat exchanger parts.  
         BACKGROUND OF THE INVENTION  
         [0002]    The current state of the art for manufacturing protective laminates for heat exchangers for condensing furnaces uses both a primer coat and an adhesive coat both which are high in VOCs. These coating/laminates are also expensive and difficult to process and result in a high rejection rate due to the complex processing required in their manufacture.  
           [0003]    It has long been an objective in the field to lay down a continuous coating on a metal that will have maximum adhesion to the metal substrate and also provide protection against corrosion induced by flue gas which travel through the adhered film. While the prior art teaches the technology of spontaneous polymerization, the use of these single coatings is not entirely suitable for protective coatings for furnaces, yet these is a desire for the processing convenience offered by such technology which is set forth in U.S. Pat. No. 5,807,612.  
           [0004]    It is therefore an object of the present invention to provide a primer/adhesive coating for heat exchanger components which overcomes the problems of the prior art described above.  
           [0005]    It is a further object of the present invention to provide an improved protective coating for condensing heat exchangers.  
           [0006]    It is yet another object of the present invention to provide a corrosion resistant coating which is receptive to chemical bonding with a protective overlayer.  
           [0007]    It is yet another object of the present invention to provide a composite coating system for condensing heat exchanger surfaces which is low in VOC generation, and which can be efficiently and economically produced.  
           [0008]    It is a further object of the present invention to provide a coating solution which is capable of undergoing spontaneous polymerization upon contact with a metal substrate which further contains latent polymerization sites that are capable for later reacting with an overcoating layer.  
           [0009]    It is yet another object of the present invention to provide a spontaneous coating system for forming an initial protective coating on a metal surface which contains unreacted sites which are later reacted to cause polymerization with a second applied coating.  
         SUMMARY OF THE INVENTION  
         [0010]    In the present invention a continuous organic coating is applied to a metal substrate by spontaneous polymerization on all locations where it adheres to the substrate. The coating is made from a monomer or a blend of monomers containing different polymerization sites so that some of the monomers sites do not react during the spontaneous polymerization and are left over or latent for reacting at a later time, but are still part of the coating. At a later time when a second polymeric overcoating is coated over the initial coating for further protection, the above described unreacted sites are polymerized and function to adhere the two coatings together. The polymerization of the first coating appears through spontaneous or auto-polymerization (S-poly process) and is more fully described by U.S. Pat. No. 5,807,612 which is incorporated herein by reference. The second or top coating is an over laminate which is used to increase the protection of the underlying metal which comprises the unreacted moieties or sites to improve adhesion between the two coatings.  
           [0011]    The composition of the coating solution comprises an acidic solution of organic monomer, capable of undergoing spontaneous polymerization upon contact with the metal substrate such as steel, copper, aluminum and the like, thereby forming an initial polymeric coating on the metal substrate. Spontaneous polymerization refers to the spontaneous polymerization of monomers upon exposure of the monomer solution to the metal substrate in an acidic solution such that electron transfer occurs, initiating polymerization of unsaturated sites on the monomers. The polymerization ordinarily proceeds at room temperature. Heat, however, maybe applied to the solution in order to increase the rate of polymerization or to effect the variation in the properties of the final coating. The preferred monomer solution is at least a two component solution, however, a one component monomer solution is also usable. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    For a further understanding of these and objects of the invention, reference will be made to the following detailed description of the invention which is to be read in connection with the accompanying drawing, wherein:  
         [0013]    [0013]FIG. 1 is a side sectional view of a composite protective coating for use on heat exchanger parts.  
         [0014]    [0014]FIG. 2 is a schematic flow diagram illustrating one embodiment of a method of coating heat exchanger parts.  
         [0015]    [0015]FIG. 3 is a schematic flow diagram illustrating a second embodiment of a method of coating heat exchanger parts. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    A continuous organic coating is applied to a metal substrate by spontaneous polymerization, where it adheres. This coating is made from (a) monomer(s) containing differing polymerization sites so that some monomers, or sites, do not react during the spontaneous polymerization and so are “left over” for reacting later, but are still a part of the coating. If needed or desired, mixed in with the coating is an agent which will cause the unreacted site, or monomer, to react when either heat or a chemical accelerator used to cause polymerization of the second monomer or reactive site is applied.  
         [0017]    At a chosen time, when a ‘plastic’ film is over-laminated to increase protection of the metal, the laminate, is applied and the heretofore unreacted moieties polymerized, using an accelerator if desired, which adheres the coating to the laminated film. FIG. 1 is a schematic illustration of a composite laminate which illustrates the present invention. The composite  10  comprise a metal substrate  12 , a galvanized layer (for steel)  14 , a primer coating  16  and a thermally and chemically resistant coating  18 .  
         [0018]    The coating that can provide reaction sites for bonding an applied film to the metal with an adhesive will be accomplished by the use of spontaneous polymerization with a mixed monomer system, or one monomer with two reaction sites, where the functionality of the monomers, or of the two sites on the same molecule, is similar, differing only in activation energy so that only the monomer with the lower activation energy forms a polymer during spontaneous polymerization, leaving the other monomer for a later reaction that causes the applied film to adhere. Also, the reactive sites could differ in reaction mechanism such that a coating of a polymer would be formed by reaction among the one type of moiety only, leaving the other free to react later by a different mechanism. The former could be accomplished by, for instance but not limited to, the use of vinyl and allyl moieties, either on separate but mutually reactive monomers or as part of a single monomer, for instance but not limited to, the ester adduct of allyl alcohol and acrylic acid, or a mixture of such monomers, with a peroxide chain initiator of a type to induce reaction of the allyl groups not reacted during spontaneous polymerization admixed during spontaneous polymerization. The method of forming a coating having two moieties differing in reaction mechanism, could be achieved by, for instance but not limited to, an epoxy as one reactive moiety and a vinyl or allyl as the other.  
         [0019]    The thermoplastic coating, that, when melted, can induce adhesion to an over laminated film may be made, for instance but not limited to, by using such monomers as styrene and an alpha-olefin. One that contains thermoplastic polymeric blocks that can induce adhesion to an over laminated film may be made similarly, by forming a block copolymer where one monomer forms a block with significantly higher melt and softening points than the block formed by the other monomer so that the coating retains much of its integrity when heated sufficiently that the block with the lower melting and softening points becomes soft enough that it will wet an over laminated film when the film is applied. In order to form blocks, a terpolymer may be required, wherein two monomers react preferentially to form a block, with one of these two monomers in excess, which monomer has been selected as appropriate for reaction with a third monomer, to form a second block with lower melting and softening points so that it preferentially becomes soft when the coating is heated to effect lamination. Alternatively, an initial coating with the desired thermal and/or chemical properties is formed on the metal in one bath, then a second coating, made using (a) monomer(s) that will react with (an) uncured monomer(s) may be formed in a second bath. Transition from the first bath to the second shall be quick enough that little or no drying takes place in order that the monomer(s) in the second bath can diffuse into the uncured, wet coating formed in the first bath to react later, thereby achieving a gentle gradient of properties in the polymer and assuring maximal intercoat adhesion. The composition of the coating solution comprises an acidic solution of organic monomer, capable of undergoing spontaneous polymerization upon contact with a metal substrate, thereby forming a polymeric coating on the metal substrate.  
         [0020]    “Spontaneous polymerization” refers to the spontaneous polymerization of monomers upon exposure of the monomer solution to the metal substrate in an acidic solution such that electron transfer occurs, initiating polymerization of unsaturated sites on the monomers. Polymerization ordinarily proceeds at room temperature. However, heat may be applied to the solution in order to increase the rate of polymerization, or to effect a variation in the properties of the final coating.  
         [0021]    Although the preferred monomer solution is at least a two-component solution, a one-component monomer solution is also potentially usable. Both monomers and small polymers, e.g., small molecules with one or more repeating units soluble in the solvent also may find application. Useful monomers capable of reacting in an autopolymerization mode by electron withdrawing include carbonyl, carboxyl, carboxylate, carboxamide, nitrile groups and the like, thio analogues of the oxygen-containing functional groups, acrylic acid, acrylamide, acrylonitrile, and alkyl acrylates in which the alkyl moiety contains 1 to 40 carbon atoms, phenyl acrylate and p-tolyl acrylate, cycloalkyl acrylates such as cyclohexyl acrylate, methacrylic acid, methacrylamide, methacrylonitrile, alkyl methacrylates. Electron donating monomers are 1-alkenes such as ethylene, propylene and like unsaturated olefins, alpha-olephins containing aromatic substituents, especially styrene, acyclic or cyclic monoolefins, conjugated dienes such as butadiene, isoprene, propylene, 2,4-hexadiene. 1-Alkenes having substituents further removed from the double bond are also effective. In particular, allyl alcohol could provide the hydroxyl group in a primer for reaction with an over laminate.  
         [0022]    A third and even a fourth monomer (or more) may be introduced into the monomer solution.  
         [0023]    Process parameters which may be used to affect the final properties of the coatings include monomer concentration, metal surface pretreatment, polymerization time, and drying temperature.  
         [0024]    An acidic monomer solution is provided, the clean metal substrate is submerged (dipped) into the monomer solution for a prescribed period of time.  
         [0025]    A “dried-in-place” (reverse) roll coating method may also be used, and in a production situation may be preferred, especially if coils of metal are to be coated.  
         [0026]    No special surface treatments are necessary for the autopolymerization method. The only requirement is a clean surface, readily supplied by simple cleaning processes.  
         [0027]    The pH of the monomer solution may be adjusted using acids such as dilute sulfuric acid, hydrochloric acid, dilute nitric acid, acetic acid, phosphoric acid, and citric acid.  
         [0028]    While not required, an accelerator may be added to the monomer solution to increase the reaction rate.  
         [0029]    A number of economically important metals, including aluminum, iron, copper, steel, zinc, and alloys thereof may be coated.  
         [0030]    Chemical modification of monomers cited in the patent to provide moieties that will not react during the S-poly process but will react with an overcoat of adhesive or paint is the method of choice. The moieties cited below are best to be pendant from the monomer molecule reacting in the S-poly formation of the primer coat in order that they would better be available for reaction with the reactive groups in the overcoat of adhesive or paint.  
       1. Electron Accepting Monomers  
       [0031]    Useful pendant groups capable of reacting with suitable groups on an applied overcoat include carboxyl. The carboxyl groups could be made more available for reacting with the overcoat of adhesive or paint than those in the cited patent (e. g., (meth) acrylic acid) by first forming the half-ester with a (long-chain) di-alcohol such as butane diol or higher molecular weight (longer chain) homologs, then reacting the available hydroxyl with an acid chloride group formed on a difunctional acid (e. g., phthalic or succinic (butanedioic) acid). The pendant carboxyls can react with alcohol moieties found in the overcoat of adhesive or paint.  
         [0032]    Should the reaction immediately above be stopped at the point of esterification of one alcohol moiety on the difunctional alcohol, then the pendant hydroxyl group would be available for reaction with epoxy, carboxylic or isocyanate moieties.  
         [0033]    The pendant carboxyls can be reacted with a primary amine, to produce an amide, that can react with epoxy or urethane moieties found in the overcoat of adhesive or paint.  
         [0034]    The ratio of the above modified electron accepting monomers to electron donating monomers should be weighted in favor of the modified monomers. The literature cites a from a 95:5 to a 5:95 ratio of electron accepting monomers to electron donating monomers can be used. A ratio of modified electron accepting monomers to electron donating monomers giving the best properties would need to be ascertained, but it is likely that modified electron accepting monomers would be over 50% of the monomers, and likely more, to the limit of 95%.  
       2. Electron Donating Monomers  
       [0035]    An organic acid esterified with allyl alcohol (propen-1-ol) would produce a monomer with allyl (propenic) unsaturation, an electron donating group. Using a difunctional organic acid would allow chain extension and carboxy termination. The carboxy could be used without modification, or it could be amidized, or esterified with a diol ,to react with suitable groups on an applied overcoat.  
       3. Combination of Monomers  
       [0036]    Using an acrylic compound as the electron accepting monomer, and propene as the electron donating monomer in the monomer solution, with appropriate adjustment of the ratio of these compounds to one another, a thermoplastic coating with a desirable melting point, i. e., 165 to 200 degrees Celsius, and the appropriate amount of olefin to induce adhesion to an olefin film (e. g., ethylene-propylene copolymer) could be produced. Adhesion to the film would be achieved by heating the coating to its melt point and applying the olefin film with sufficient pressure that adhesion between the liquefied coating and the film is achieved.  
         [0037]    Since more than 2 monomers may be used in the monomer solution, choices among appropriate monomers to effect singular properties could be made.  
       4. Application  
       [0038]    Common commercial methods of application of the monomer solution to a moving metal strip could be used. One of particular appeal is “dried-in-place” application, wherein the solution is applied by a metered (reverse) roll, then rinsed, if desired/required, then dried/cured. Spray and other methods of application may be used, but since the concentration of monomers is low, metered roll application appears to be the most desirable method.  
         [0039]    In one embodiment of the present invention as illustrated in FIG. 2, a roll of metal is coated with a primer by the S-poly method, then over coated to make the desired product, a corrosion-resistant strip of metal with the desired esthetic properties.  
         [0040]    At Station  1 , a roll of metal of the desired alloy, thickness and width is unrolled and pulled along the path described.  
         [0041]    At Station  2 , the surface of the metal is cleaned, which likely would include an acid etch.  
         [0042]    At Station  3 , a controlled amount of the primer monomer solution, composed of selected electron donating and electron receiving monomers, is applied, here by the reverse roll method.  
         [0043]    The surface polymerization occurs between Stations  3  and  4 , depositing a film of primer polymer.  
         [0044]    If desired, the deposited primer film may be rinsed at Station  4 .  
         [0045]    If desired, the deposited primer film may be dried at Station  5 . It may even be cured, if so desired.  
         [0046]    At Station  6 , the overcoat is applied. It may be applied by any commercial method, or by a second S-poly process. In the latter event, Stations  4  and  5  would not exist.  
         [0047]    At Station  7 , the overcoat, and the primer coat if not previously cured, is/are cured.  
         [0048]    At Station  8 , the coated metal product is wound up.  
         [0049]    In a second embodiment of the present invention as illustrated in FIG. 3, a roll of metal is coated with a primer by the S-poly method, then over coated with an adhesive, then laminated to a thermoplastic film, to make the desired product, a corrosion-resistant strip of metal with the desired esthetic and mechanical properties.  
         [0050]    At Station  1 , a roll of metal of the desired alloy, thickness and width is unrolled and pulled along the path described.  
         [0051]    At Station  2 , the surface of the metal is cleaned, which likely would include an acid etch.  
         [0052]    At Station  3 , a controlled amount of the primer monomer solution, composed of selected electron donating and electron receiving monomers, is applied, here by the reverse roll method.  
         [0053]    The surface polymerization occurs between Stations  3  and  4 , depositing a film of primer polymer.  
         [0054]    If desired, the deposited primer film may be rinsed at Station  4 .  
         [0055]    If desired, the deposited primer film may be dried at Station  5 . It may even be cured, if so desired.  
         [0056]    At Station  6 , the adhesive overcoat is applied. It may be applied by any commercial method, or by a second S-poly process. In the latter event, Stations  4  and  5  would not exist.  
         [0057]    At Station  7 , the thermoplastic film is applied with a laminating roller.  
         [0058]    At Station  8 , the adhesive overcoat, and the primer coat if not previously cured, is/are cured.  
         [0059]    At Station  9 , the coated metal product is wound up.  
         [0060]    Note: should the approach of using a thermoplastic first (S-poly) coat and thermally laminating the applied film be used, step  6  would not be used, and steps  4  and  5  would be.  
         [0061]    While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.