Abstract:
Water based silicone coating compositions are provided offer superior coating properties, durability and resistance to high temperatures and to the effects of corrosive environments. These coating compositions are especially useful for coating the external and intricate internal surfaces of high temperature aluminum heat exchangers.

Description:
This is a division of application Ser. No. 08/145,078 filed Oct. 29, 1993 now U.S. Pat. No. 5,421,865. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to coating compositions, and more particularly to water based silicone coating compositions that offer superior coating properties, resistance to high temperatures and to the effects of corrosive environments and durability. The present invention provides water based silicone coating compositions, methods for producing such compositions, as well as high temperature heat exchangers whose surfaces are coated with such water based silicone coating compositions. 
     Silicone resins are known to demonstrate endurance towards environmental conditions such as weathering and extreme heat and cold. For this reason these resins have been found to be particularly useful in the paint industry. 
     Previously, silicone resins were made available to formulators in organic solutions. In particular, the resin consisted of so many parts by weight of silicone solids in an organic solvent such as xylene or toluene. However, due to increased concerns regarding: the suspected health hazards to persons exposed to such solvents; environmental considerations and the mandatory use of costly and burdensome pollution abatement procedures and equipment; and escalating costs for organic materials, the use of such organic solvents has been discouraged. As a result, suppliers of these silicone resins have worked towards developing silicone resin systems that are water based and therefore not dependent upon organic solvents. 
     Yet, such silicone resins which have been found to be particularly useful in the paint industry have often been found to be immiscible or otherwise incompatible with aqueous coating compositions. For those silicone resins which can be made part of water based emulsions which can then form the basis of a paint or coating composition, the ability of these coatings to generally match the performance (e.g., coating properties) of other temperature-resistant protective coatings is oftentimes not realized. Moreover, such compositions do not serve to adequately protect the underlying substrate from the effects of corrosive environments. 
     For devices such as high temperature aluminum heat exchangers which have an intricate network of passages therethrough and which are oftentimes subjected to corrosive environments, the deficiencies of prior art water based coating compositions take on significant proportions. 
     It is, therefore, an object of the present invention to provide water based silicone coating compositions that offer superior coating properties and resistance to high temperatures. 
     It is another object of the present invention to provide novel coating compositions that exhibit durability and resistance to the effects of a corrosive environment. 
     It is yet another object to provide a process for producing such water based silicone coating compositions. 
     It is still another object of the present invention to provide a high temperature heat exchanger whose surfaces are coated with such water based silicone coating compositions. 
     SUMMARY OF THE INVENTION 
     The present invention therefore relates to a high temperature coating composition. The coating composition is comprised of: a silicone resin emulsion; a non-water reactive filler material having a laminar structure; a water soluble nonionic surfactant; and water. 
     The present invention also relates to a process for producing a high temperature coating composition as described above. 
     The present invention further relates to a high temperature heat exchanger whose external and intricate internal surfaces are coated with a coating composition as described above. 
     The foregoing and other features and advantages of the present invention will become more apparent from the following description. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The coating compositions of the present invention are useful in a wide variety of applications, including use as high temperature protective coatings for aluminum parts used in the transportation industry, such as high temperature aluminum heat exchangers. 
     The inventive coating compositions generally will have a total weight percent solids content of approximately 30.4 to 55.7%; a pH range of 6.0 to 8.0; a viscosity of approximately 20 to 40 seconds as measured by a No. 1 Zahn Cup; a density of approximately 1.08 to 1.18 grams per cubic centimeter (g/cm 3 ); a volatile organic compounds content of less than 200 grams/liter; and a filler material (e.g. pigment) to resin solids (e.g. binder) ratio of approximately 1.0 to 2.2 through 1.0 to 4.2. The filler material to resin solids ratio indicates the quantity by weight of resin used with a specified amount of a particular filler material. 
     In general, the coating compositions of the present invention contain a silicone resin emulsion; a non-water reactive filler material having a laminar structure; a water soluble nonionic surfactant; and water. 
     The emulsion functions as a high heat resistant binder for the filler material and generally will have a total silicone resin solids content of approximately 38 to 82% by weight. The silicone resin emulsion is comprised of 100 parts by weight of (i) at least one silicone resin having one or more organic side groups attached such as phenyl, methyl, trifluoropropyl and/or vinyl moieties; (ii) an anionic surfactant effective for dispersing the resin in a water based emulsion and for binding the resin or particulate phase and the water or continuous phase; and (iii) an amount of water effective for providing a preselected silicone resin solids content by weight of the emulsion. The emulsion might also contain trace amounts of volatile organic compounds such as xylene and/or toluene present as a result of the manufacturing process. 
     Silicone resins, which may be used in the emulsion of the present inventive coating composition, include optionally crosslinked resins comprising units selected from the group consisting of R x  SiO y  where x is 3, 2 or 1 and y is 0.5, 1.0 or 1.5 respectively, and where the R groups are phenyl, methyl, trifluoropropyl and/or vinyl. A typical number average molecular weight and weight average molecular weight of such resins are 420 and 2190, respectively. Preferred silicone resins include lightly crosslinked or soft phenylmethyl silicone resins and moderately to highly crosslinked or medium-hard phenylmethyl silicone resins. The most preferred silicone resin is medium-hard phenylmethyl silicone resin where an increase in the hardness and fluid resistance of the cured inventive coating composition has been observed when this moderately to highly crosslinked resin is used. 
     Anionic surfactants suitable for use in the silicone resin emulsion of the present inventive coating include sodium lauryl sulfate, sodium linear alkyl benzene sulfonate, triethanol amine linear alkyl benzene sulfonate, sodium alpha olefin sulfonate, ammonium alkyl phenol ethoxylate sulfate, ammonium lauryl ether sulfate, ammonium alkyl ether sulfate, dialkyl ester of sodium sulfosuccinic acid, sodium cumene sulfonate, and ammonium xylene sulfonate. The preferred anionic surfactant is sodium lauryl sulfate. 
     It will be recognized by those skilled in the art that the water used in the silicone resin emulsion of the present inventive coating is preferably distilled or deionized water. 
     The preferred silicone resin emulsion is comprised of (i) from 48 to 72 wt. % soft or medium-hard phenylmethyl silicone resin; (ii) from 0.01 to 1.0 wt. % sodium lauryl sulfate; and (iii) from 52 to 27 wt. % water, based on the total weight of the emulsion. The average viscosity of the preferred silicone resin emulsion is from about 40 to about 180 centipoises as measured in accordance with Method B of ASTM D 1084 using a Brookfield LVF. The preferred silicone resin emulsions are available from Dow Corning Corporation, Midland, Mich. under the product designations Dow Corning® 1-0468 resin emulsion and Dow Corning® 1-0469 resin emulsion. 
     The non-water reactive filler material of the present inventive coating has a laminar structure and functions to form a barrier from corrosive elements thereby protecting the substrate and serves to improve coating properties. Filler materials contemplated by the present invention include pigments in the form of a paste containing flake metal or mineral and a dispersant. These pigments in addition to functioning as a barrier from corrosive elements and improving coating properties also impart color to the coating composition and include such substances as inhibited aluminum leafing pigment dispersed in a hydrocarbon distillate such as mineral spirits, inhibited aluminum leafing pigment dispersed in a hydrophilic media such as isopropanol, and leafing mica dispersed in water. The preferred filler material is comprised of from about 78 to about 82% by weight inhibited aluminum leafing pigment dispersed in mineral spirits and is available from Siberline Manufacturing Co., Inc., Lincoln Drive, Tamaqua, Pa., under the product name Aquapaste 205-5. The most preferred filler material is comprised of from about 66 to about 70% by weight inhibited aluminum leafing pigment dispersed in isopropanol and is available from Siberline Manufacturing Co. under the product designation L-1959-Pa. 
     The water soluble nonionic surfactant of the present inventive coating composition improves the wetting of the coating composition thereby helping filler material or pigment dispersion and inhibiting foam. These surfactants are also useful for reducing surface tension in the coating. Examples of suitable surfactants are acetylenic glycol, octylphenol ethoxylate, as well as many other surfactants which are well known in the coatings art. The preferred surfactant is a water soluble, liquid, nonionic octylphenol ethoxylate available from Union Carbide Chemicals and Plastics Company, Inc., Danbury, Conn. under the product name Triton® Nonionic Surfactant X-100. The most preferred surfactant is a water soluble nonionic acetylenic glycol available from Air Products and Chemicals, Inc., Performance Chemicals, 7201 Hamilton Boulevard, Allentown, Pa., under the product name Surfynol 465. 
     Those skilled in the art will recognize that the water used as the solvent in the present inventive coating composition is preferably distilled or deionized water. 
     The preferred coating composition of the present invention comprises (i) from 39.6 to 67.3% by weight of a phenylmethyl silicone resin emulsion having a total weight percent silicone resin solids content of approximately 48 to 72%; (ii) from 8.3 to 19.2% by weight of inhibited aluminum leafing pigment dispersed in either mineral spirits or isopropanol; (iii) from 0.05 to 3.0% by weight of water soluble nonionic acetylenic glycol; and (iv) from 10.5 to 52.0% by weight water, and has a total weight percent solids content of approximately 34.4 to 50.0%. 
     The most preferred coating composition comprises (i) from 53.4 to 59.0% by weight of a phenylmethyl silicone resin emulsion having a total weight percent silicone resin solids content of approximately 58 to 62%; (ii) from 14.6 to 16.2% by weight of inhibited aluminum leafing pigment dispersed in either mineral spirits or isopropanol; (iii) from 0.3 to 0.5% by weight of water soluble nonionic acetylenic glycol; and (iv) from 24.3 to 31.7% by weight water, and has a total weight percent solids content of approximately 43.7 to 48.3%. 
     In addition to the above components, the present inventive coating composition may advantageously contain some or all of the following ingredients, including antifoams, pH buffers, anti-rust agents, biocides, fungicides, antifreeze agents, etc. However, some such additives may have an adverse effect on the coating&#39;s durability, corrosion resistance, coating properties and/or resistance to high temperatures. 
     In preparing the coating composition of the present invention, water and surfactant are blended together and the surfactant is allowed to dissolve for at least thirty (30) minutes. A filler material is then added to the resulting water solution. Dispersion of the filler material in the water solution is achieved by gentle agitation of the solution with a paddle to break apart the large agglomerates. This is followed by ensuring that the filler material is completely covered by the water solution and allowing the filler material to fully disperse by soaking for approximately 24 to 72 hours. During this period, the dispersion is agitated at least twice per day. A letdown is then prepared by blending a silicone resin emulsion and water for approximately 15 to 20 minutes. The letdown is allowed to fully dissolve for approximately 24 to 72 hours. The dispersion is then added to the letdown and thoroughly blended. The resulting admixture is then allowed to fully disperse for at least 24 hours. As can be well understood by those skilled in the art, excessive stirring during preparation of the coating composition of the present invention is to be avoided. 
     In addition to the above description, the coating compositions of the present invention are further developed by reference to the illustrative, but not limiting, examples set forth below. 
     SPECIFIC EMBODIMENT 
     In the working examples set forth below, the following components were used: 
     Emulsion - an anionic water emulsion of a soft phenylmethyl silicone resin obtained from 
     Dow Corning® Corporation and having a product designation Dow Corning® 1-0468 resin emulsion. 
     Filler - a water dispersible, inhibited aluminum leafing pigment having a fine particle size obtained from Siberline Manufacturing Co. and having a product name Aquapaste 205-5. 
     Surfactant - a water soluble nonionic acetylenic glycol with inherent low foaming characteristics obtained from Air Products and Chemicals, Inc. and having a product name Surfynol 465. 
     Water - deionized water. 
     COATING COMPOSITION PREPARATION 
     The inventive coating composition was prepared according to the following method: 76.7 grams (g.) of Water and 2.2 g. of Surfactant were blended and the Surfactant allowed to dissolve for about 30 minutes to form a water solution. 76.7 g. of Filler was then added to the water solution and the resulting dispersion gently agitated with a spatula to break apart the large agglomerates. Care was taken to ensure that the Filler was completely covered by the water solution and then the Filler was allowed to fully disperse by allowing it to soak for 24 hours, minimum. During this period, the dispersion was agitated twice per day. 281.5 g. of Emulsion and 63 g. of Water were then blended together for about 15 to 20 minutes to form the letdown. The dispersion was then added to the letdown, thoroughly blended, and allowed to fully disperse for approximately 24 hours. 
     The inventive coating composition prepared by the above procedure had the following approximate physical properties: 
     Filler to Resin Solids ratio: 1.0 to 2.7 
     Percent Solids: 46.0 
     pH at 25° C.: 6.8 
     Density: 1.13 g/cm 3   
     Viscosity (ASTM D 4212; No. 1 Zahn Cup): 32 sec. 
     EXAMPLES 1 TO 6 
     The coating composition prepared by the above procedure was then applied to test panels. The test panels were aluminum alloy panels having the commercial designations A1 Alloy 3003-0 and A1 Alloy 6061-T6, and measuring either 0.032×2.0×4.0 inch or 0.125×4.0×4.0 inch. The test panels were conversion treated prior to coating by dipping or immersing each test panel into a no rinse chromate conversion coating consisting of a combination of hexavalent and/or trivalent chromium, mineral acids, and organic or inorganic binding agents and available from Betz Metchem, Inc., 200A Precision Drive, Horsham, Pa. under the product name Permatreat 1900; blowing off the excess coating with an air jet; and curing the coating 2 hours, minimum at 121°±14° C. The inventive coating composition was then applied to the cured conversion coated test panels by dipping or immersing the panels in the coating composition; allowing the excess coating to drain from the panels; and curing the coating composition 45±10 minutes at 300°±25°° F., and then 60±10 minutes at 350°±25° F., and then 90±10 minutes at 550°±25° F. 
     The test panels were then subjected to a variety of tests, the identity of which and the results obtained therefrom, as set forth below in Table I. 
     
                                           TABLE I__________________________________________________________________________SUMMARY OF EXAMPLES 1 TO 6  A1 ALLOYEXAMPLE  TEST PANEL          PROPERTY                 TEST METHOD/DESCRIPTION                                      RESULTS__________________________________________________________________________1      3003-0* Thickness                 Total thickness less substrate                                      0.0002 to 0.0004 inch                 thickness          Hardness                 ASTM D 3363          Pencil hardness of F          Adhesion                 ASTM D 3359, Test Method A except                                      5A, i.e., no peeling or                 parallel and 1/4 inch apart. (Ref.                                      removal                 STD-141. Method 6301)2      3003-0* Fluid  Appearance after exposure to Skydrol                                      No wrinkling, roughening,          Resistance                 500B-4 and TT-S-735, Type III at room                                      blistering, checking or                 temperature for 168 hours                                      crazing3      3003-0* Corrosion                 ASTM B 117, for 168 hours (Ref. MIL-                                      No evidence of corrosion          Resistance                 STD-810, Method 509)4      3003-0* Thermal                 Appearance, weight loss, and                                      No evidence of flaking,          Stability                 flexibility after exposure to air                                      crazing, or cracking after                 450° and 600° F. for 168 hours (Ref                 ASTM                 exposure and 90° bend                 D 522, Test Method A, for resistance                                      weight loss = 5.7%                                      (450° F.)                 cracking)            weight loss = 17.2%                                      (600° F.)5      3003-0* Heat and                 Appearance after exposure to air at                                      No evidence of cracking,          Corrosion                 500° F. for 72 hours followed by                                      blistering, flaking,          Resistance                 to salt spray, per ASTM B 117, for                                      crazing, corrosion, or                 hours.               deterioration6      6061-T6**          Abrasion                 FED-STD-141, Method 6192, using 1000                                      Wear Index 27 mg/100          Resistance                 gram load and CS-10 abrasive wheel.                                      cycles__________________________________________________________________________ *Test panel measured 0.032 × 2.0 × 4.0 inch **Test panel measured 0.125 × 4.0 × 4.0 inch 
    
     The above-referenced Examples demonstrate that the coating composition of the present invention displays clearly acceptable properties for the intended application or uses thereof. In particular, the coating composition displays minimum thickness and viscosity so as to preclude core passage blockage of high temperature aluminum heat exchangers. Moreover, the composition displays abrasion resistance satisfactory to preclude sand and dust erosion and displays acceptable levels of heat and corrosion resistance. 
     Although this invention has been shown and described with respect to the specific embodiment thereof, it will be understood by those skilled in the art that various changes may be made without departing from the spirit and scope of the claimed invention.