US 3909387 A
An aluminum surface is specially anodized to provide a polymer-active oxide layer, following which a film-forming polymeric organic material is applied to the treated surface and bonded to it by means of the polymer-active oxide layer.
Description (OCR text may contain errors)
United States Patent Kolic et al.
1451 Sept. 30, 1975 APPARATUS FOR PRODUCING Primary Examiner lohn H. Mack POLYMER-COATED ALUMINUM Assistant E.\'aminerW. I. Solomon PRODUCTS Attorney, Agent, or FirnzHopgood, Calimafde, Kalil  Inventors: Edwin S. Kolic, Gahanna, Ohio;
Sigmund Bereday, Bayamon, PR.  ABSTRACT  Assign gm B r y, New York, NY An aluminum surface is specially anodized to provide Filed, Oct 30 1973 a polymer-active oxide layer, following which a filmforming polymeric organic material is applied to the PP N031 411,083 treated surface and bonded to it by means of the Related US. Application Data polymcr'activc Oxide layer  Division of Sci. N0. 130,310. April 1, 1970, Par. NO. The apparatus Comprises means for Continuously transporting a length of aluminum metal in and through a treatment zone, a hydraulic circuit including 52 us. 01. 204/206; 204/28; 204/207; Wetting head adjacent and generally directed at the 204 239 metal as it passes through the zone, the circuit 151 1 1m. 01. C25D 17 00 including means for Continuously pp y electrolyte  Field of Search 204/28, 206, 207, 239, to the Wetting head to discharge the same at the 204 3g E passing metal length, an electric-supply means including means for anodieally effecting an electrical  References Cited coupling with said metal length, and an electrical UNITED STATES PATENTS connection to a cathode in contact with the 7 7 7 electrolyte in the hydraulic circuit, such that in the gfanlliiuscr presence of a continuously sprayed liquid stream of 6] ES g "56 6 X electrolyte from the head to the metal, an E' b (W962 wagncrnm'w 264/206 electrochemical circuit is completed via the stream for 3,692,640 9 1972 Hamahe Ct 111.... 204/2s Carrying out the foregoing mcthod- 3,726,783 41973 H a n1 204 28 X 0 L l g 13 Claims, 11 Drawing Figures WATER AIR POLYMER RINSE DC Y APPLICA T/ON /5 Z 6 :p I
. OVEN I ////1|l11 i L 47 I E'N L I l L J P l U.S. Patent Sept. 30,1975 Sheet 1 Of6 3,909,387
AIR POLYMER DRY APPL ICA T/ON OVE'N .SUPPL Y (9 ac. HDWER EXl/Al/S 7' VENT FIG.3
27/ FILTER #54 TER SUPPI. Y 477 US. Patent Sept. 30,1975 Sheet 2 of6 3,909,387
US. Patent Sept. 30,1975 Sheet3of6 3,909,387
ii! 15 E2235 muuaiu w QE 80 8Q Obs 80 8M 00? 00m 00m 02 CURRENT DENSITY, AMP/Ff-Z US. Patent Sept. 30,1975 Sheet 5 of6 3,909,387
10,000 I I I I ZOO PREFERRED CURRENT DENSITY FOR \PROOUCING rum CLEAR POLYMER- ACTIVE FILMS 0N AA3003 IN 20 VOL. H P0 MINIMUM CURRENT DENSITY FOR X PRODUCING POLYMER-ACTIVE nms o- AA 3003 av 20 vo:..% H3 P04 '60 70 80 90100110 lea/30140150160170 I80 TEMPERATURE 1-- FIG. IO
US. Patent Sept. 30,1975 Sheet 6 of6 3,909,387
wkuwkwq Ewe ufikwmi R S Q & S E a 3 ms APPARATUS FOR PRODUCING POLYMER-COATED ALUMINUM PRODUCTS This is a division of copending application Ser. No. 130,310 filed Apr. 1, 1971- now US. Pat. No. 3,799,848.
This invention relates to the surface treatment of aluminum and, in particular, to a method and apparatus for producing a polymer-active surface thereon and for firmly bonding thereto as a protective coating a filmforming organic material. The invention also relates to products produced by the method.
STATE OF THE ART Many methods have been proposed for bonding filmforming polymers to aluminum surfaces but, generally speaking, the bond between the polymer film and aluminum has not been too satisfactory. A particularly difficult polymer to bond to aluminum is polytetrafluoroethylene (known by the trademark Teflon).
One method proposed has been to interpose an intermediate primer layer between the metal surface and the polymer layer. This method requires criticalhandling of the materials, is rather costly and does not assure a strongly adhering coating. One of the main problems with such methods in which the bond is generally of the physical type is the intrinsic non-adhesive characteristics of polytetrafluoroethylene.
A method proposed in US. Pat. No. 2,944,917 comprises providing an aluminum surface having anchoring cavities characterized by undercuts etched therein of average size of'less than 30 microns. On the surface is deposited an aqueous dispersion of polytetrafluoroethylene as a succession of elementary layers, each of the layers being dried and fused before the next layer is deposited. This type of bonding is mechanical in nature.
In still another method (U.S. Pat. No. 3,304,211), polytetrafluoroethylene is applied to a roughened substrate (e.g. aluminum) in the form of a powder of at least 75 microns in size upon which is then superimposed a continuous film of the same material at a temperature at least about 650F to cause the formation of a mechanically bonded laminate.
In a method of adhesive bonding, referred to as the *Fokker process, an aluminum substrate is prepared for coating by anodizing the aluminum in a chromic acid solution at a temperature of about 104F following which an organic adhesive is applied to the surface. The bond obtained compared to the same coating produced on the substrate having a pickled only surface showed that the peel strength of the bond on the anodized surface was substantially the same as the bond obtained with the pickled only surface, except that the bond data obtained using the anodizing treatment showed less scatter and appeared more uniform.
R. Hovwink and G. Salomon; ADHESION AND ADHESIVES, Vol. 2, second edition, Elsevier Publishing Co., Amsterdam-London-New York, 1967 In a recent patent, US. No. 3,533,920 (Oct. 13, 1970) a method is disclosed of bonding a polymeric fluorocarbon (e.g. polytetrafluoroethylene) to an aluminum surface previously anodized to provide an oxide film thickness of at least 1 mil (1000 microinches), the bath employed comprising 4% to 6% by volume of 66 Baume sulfuric acid, 0.5 to 6% oxalic acid and 5 to 25 grams per gallon of tannic acid. The aluminum is anodized at a current density of 20 to 150 amps/sq.ft until a coating of at least 1 mil thick is obtained. The purpose of the thick coating is to provide a porous layer. Dispersed particles of the fluorocarbon material are then impregnated into the pores by immersion of the anodized aluminum in an aqueous dispersion of the particles so that the particles enter the interstices and the pores of the oxide layer to provide mechanical bonding of the polymer to the aluminum after baking and sintering at an elevated temperature.
Generally, the use of anodized films in the prior art as a base on aluminum for receiving organic coatings has been predicated on employing relatively thick oxide films (up to 1 mil or more) characterized by a relatively high degree of porosity at the surface. It has been reported that, as a general rule, factors which increase the porosity of an anodic oxide film will improve the adhesion of a paint or organic coating. To assure the desired degree of porosity, relatively thick anodic films are produced since it is in the production of such films at fairly long treatment times and/or high current density that porosity is obtained at the surface of the oxide film. Such techniques are economically unattractive due to the cost of forming thick oxide films and, moreover, the oxide films so produced do not always provide good bonding characteristics with film-forming organic polymers.
It would be desirable to provide a method of anodically treating aluminum surfaces with the object of making such surfaces polymer-active to a broad spectrum of polymers without requiring the formation of thick oxide coatings.
OBJECTS OF THE INVENTION It is thus an object of the invention to provide an im- I proved method for bonding a film forming polymer to an aluminum surface.
Another object is to provide a superior polymercoated aluminum product characterized by enhanced strength of bond, particularly under environmental conditions which tend to attack the unprotected aluminum substrates.
It is a further object of the invention to provide an economic method for treating an aluminum member on a continuous basis, wherein the aluminum is succes- .sively cleaned, anodically treated in a special manner and then coated with an adherent layer of a filmforming organic polymer, for example halogen substi tuted hydrocarbons, such as polyvinylidene chloride, polytetrafluoroethylene, and the like.
A still further object is to provide an apparatus capable of continuously cleaning and coating aluminum products with a film-forming organic material.
These and other objects will more clearly appear when considered in the light of the following disclosure and the accompanying drawings, wherein:
FIG. 1 is a production-flow diagram schematically showing a succession of steps in carrying out the method of the invention;
FIG. 2 is a simplified view in perspective, and partly broken-away, showing apparatus of the invention at one of the locations of FIG. 1;
FIG. 3 is a sectional view, taken generally at the vertical plane 33 of FIG. 2, with partly broken-away elements;
FIG. 4 is a simplified view in perspective to illustrate alternative structure for part of the apparatus of FIG.
FIG. 5 is a sectional view of one of plural elements of FIG. 4, taken generally in the plane 55 of FIG. 4;
FIG. 6 is a simplified, fragmentary view in side elevation, to illustrate operation of the apparatus of FIG. 4;
FIG. 7 is a simplified, fragmentary view in perspective to illustrate a further modification of the invention;
FIG. 8 is a graph showing that the limiting oxide thickness is proportional to the applied current density and that the polymer-active oxide structure is achieved when dissolution of the oxidefilm occurs;
FIGS. 9 and 10 depict a log-linear plot of current density vs. temperature in degrees F illustrating the conditions for producing polymer-active oxide structures using anodizing solutions of sulfuric acid and phosphoric acid, respectively; and
FIG. 11 depicts'a corrosion-rating chart for evaluat ing the corrosion resistance of treated aluminum surfaces.'
I STATEMENT OF THE INVENTION Stating broadly, the method aspect of the invention comprises, providing a clean aluminum member, electrolytically subjecting the member to anodic treatment in an aqueous anodizing electrolyte maintained at a predetermined temperature to provide oxygen ions at the surface of the member for forming an oxide layer thereon of finite thickness, continuing the anodic-treatment at a current density predetermined to form said oxide layer for a time at least sufficient to reach said finite thickness at said predetermined temperature and effect partial dissolution of said oxide layer during the anodic formation of the oxide and thereby producing a polymer-active oxide surface, and then applying a film-forming polymeric organic material to the treated surface.
It is preferred for optimum results that the anodic treatment be carried out for a time at least sufficient to produce an oxide layer of limiting ormaximum film thickness determined by the selected-current density and the temperature of the electrolyte, the thickness of the limiting film approximating the ratio of the selected predetermined current density to the minimum current density at which the oxide layer can be formed. The limiting film thickness of'a particular current density results when a steady state of substantially an equilibrium is achieved between the formation rate and dissolution rate of the oxide layer, where by the surface of the oxide becomes polymer-active. Thus, where the selected predetermined current density exceeds the minimum current density, the ratio of the limiting film thickness of the two current densities is greater than one.
The limiting film thickness for a particular current density varies with the electrolyte and the operating conditions, especially the temperature, which affects the dissolution velocity.
An advantage of the foregoing method in producing polymer-active oxide layers is that thick porous oxide layers are not required to promote bonding between a film-forming polymer and an aluminum surface, although thick oxide layers can be used. On the contrary, the polymer-active layer can be very thin e.g. below 250 microinches, or even below 100 microinches, or, advantageously, range up to about 30 microinches and still provide a very strong bond with the applied polymer. Because very thin oxide layers provide good results, very short anodic treatment times can be employed which make the method economically attractive.
The polymer-active or polymer-receptive oxide structure remaining on the surface is characterized by being electrically conductive.
The foregoing method is particularly applicable to a continuous process and can be mechanized using an apparatus having means for continuously transporting a length of aluminum metal in and through a treatment zone, a hydraulic circuit including a wetting head adjacent and generally directed at the metal as it passes through the zone, the circuit including means for continuously supplying electrolyte to the wetting head to discharge the same at the passing metal length, and electric-supply means, including means for anodically effecting an electrical coupling with said metal length and an electrical connection to a cathode in contact with the electrolyte in the hydraulic circuit, such that in the presence of a continuous sprayed liquid stream of electrolyte from the head to the metal, an electrochemical circuit is completed via the stream. The electrical coupling with the metal length may be by direct electrical contact or contact through the electrolyte as may be obtained by using a bi-polar arrangement of the electrodes.
In the situation where the aluminum member already has an oxide layer prior to treatment, it is cleaned by electrolytically stripping it in situ and the treatment continued at a predetermined current density and temperature to provide preferably a balance between the formation rate and the dissolution rate so as to produce the polymer-active oxide surface.
The principal variables which are considered in carrying out the method of the invention on an aluminum member using an anodizing electrolyte are current density, temperatureand time. The electrolyte is preferably selected from the group consisting of phosphoric acid, chromic acid, sulfuric acid, oxalic acid, and mixtures of one or more of these acids, and alkali metal carbonates.
The polymers which produce the desired coatings on aluminum are those referred to as film-forming organic polymers. Examples of such film-forming polymers are:
I. acrylic resins, such as copolymers of esters of acrylic and methacrylic acids, a specific resin being a copolymer of methyl acrylate and methyl methacrylate;
2. epoxy resins, such as glycidyl ether of bis-phenol cured with triethylene tetramine;
3. silicone resins;
4. halogen substituted hydrocarbons, such as polyvinylidene chloride, polychlorotrifluoroethylene, polytetrafluoroethylene and the like;
5. polycarbonate resins, such as poly(bis phenol A carbonate);
6. polyimide resins;
7. hydrocarbon polymers, such as polypropylene and polyethylene; and
8. polyurethane resins.
DETAILS OF THE INVENTION In carrying out the preferred method of the invention, the predetermined current density and the temperature of the anodizing electrolyte should be at least sufficient after a sufficient time of anodic treatment to achieve substantially an equilibrium between the formation rate and the dissolution rate of the oxide film. When this occurs, a limiting or maximum film thickness of the oxide results after a given time period beyond which there is no further increase in the oxide film. At longer treatment periods, after the limiting film thickness has formed, it may tend to decrease in thickness. The expression substantially in equilibrium includes the situation where there is a dropping off in limiting film thickness due to a slight increase in the dissolution rate relative to the formation rate. It should be noted that the limiting film thickness varies with the particular aluminum alloy being anodized, other things being equal.
Generally, prior to anodizing, aluminum is cleaned with an alkaline cleaner which usually contains an inhibitor to protect the surface of the aluminum from croscope with a 40X objective and a 6X micrometer eyepiece.
Following anodic treatment, the panel was blown dry with air at 78F and a specimen was cut out of the cen- 5 ter area for oxide thickness measurements. A coating was then applied by dipping the panel into a tetrahydrofuran solution containing 60 g/l of vinylidene chloride polymer. The coating was air dried at 7580F. Coating adherance was based on a 180 bend and a pee] test with scotch tape after the panel was exposed to CASS environment for 17 continuous hours. Ratings were assigned as follows: very good, no coating pulled from surface in bent area; good, up to 25% coating pulled from the surface in bent area; fair, 2550% coatchemical attack by the alkaline environment. Thus, the ing pulled from Surfac n area; P more than 50% presence of such inhibitors can have a delaying effect of Coating Pu m Surface n nt ar a. on the dissolution of the oxide surface which is being The a st i a pt d tandard procedure for formed, in which case it i i ortant t carry out th evaluating the corrosion resistance of treated alumianodizing treatment at the predetermined current denhum Surfaces and the term CASS Stands or copper it d temperature f a i il h li i i fil accelerated acetic acid salt spray test identified as thickness is obtained, as this assures dissolution and ASTM Designation 36864T. subsequent formation of the polymer-active oxide sur- The procedure employed to evaluate corrosion resisface. Omitting inhibitors from the cleaning solution fatance is similar to the ASTM method detailed in the cilitates dissolution and subsequent formation of the 1953 Committee Report (Proc. Am. Soc. Testing Mat. polymer-active oxide surfaces when the aluminum The Corrosion pe nce at g iS member is treated in accordance with the invention. determined by calculating the weighted area which The importance of w ki t th li iti hi k is defective and reading the rating from the conversion of the oxide film in producing the polymer-active oxide plot illustrated in FIG. 11. surface will be apparent by referring to Example 1 and In making the evaluation, the percent defective area FIG, 8 which relate the limiting oxide thickness to the is determined by comparing test specimens with the applied current density. data of standard charts depicting unit CASS ratings ranging from 0 to 10. Referring to FIG. 11, it will be noted b wa of exam le that a CASS ratin of 7 corre- EXAMPLE 1 y y g sponds to a percent weighted defective area of 0.5, and The tests were carried out using a sulfuric aCld 316C- that a rating of 5 corresponds to a percent defectrolyte of about 12 vol% concentration at 72F and a tive area of 2.0. On the basis ofpercent weighted defecbright-buffed Aluminum Alloy l 100 2X6-inch panelstive area, a CASS rating of 7 indicates a fourfold supe- Each panel was pretreated by (1) Soak cleaning in a riority over a CASS rating of 5. Where longtime effec conventional commercial mild alkaline cleaning solutivoness f th ti i t b l ated, 17-ho i onta g inhibitors; Water g; p- 4O CASS ratings are determined, the 17-hour test being a ping in 50 vol.% HNO solution for 15 seconds; and (4) very i id l i Water timing The Panel Was Placed into, and removed Each of the aluminum panels was treated at various from, the Sulfuric acid electrolyte With current on d times in the sulfuric acid electrolyte and the oxide film electrical lead attached The rectifier was preset to the measured after each of the stated time intervals shown desired value prior to treatment. The bath was not agii T bl 1 at current d i i f 15, 10 d 5 tated during the anodic treatment. A Vari Tech model s/s ft respectively E h id thi k w VT 1 176A coulometer was used to measure the charge then coated with vinylidene chloride polymer, as dequantity of electricity passed- Th X d film Ihiekscribed hereinabove. The results are given in Table 1 nesses were measured on a polished section using a mias f ll TABLE 1 Treat- Oxide Experiment Charge Thick- Adhesion of Vinylide ment Time, Coulombs, Quantity ness, Chloride Coating After No. Min. ampsec ampmin/ft mils 17 Hours CASS Nominal Current Density 1S amp/ft 1 6 540 90 0.131 Poor 2 10 960 160 0175 Poor 3 35 3150 525 0.630 Poor 4 40 3860 643 0.788 Poor 5 4710 785 0.927 Poor 6 5370 895 1.050 4 Poor 7 66 6000 1000 1.190 Poor 8 70 6630 1 105 1.224 Very Good 9 6950 1 160 1.240 Very Good 10 8330 I385 1.225 Very Good 1 1 108 9600 I600 1.20% Very Good Nominal Current Density l0 amp/ft 12 30 1784 297 0.350 Poor 13 50 2904 484 0.552 Poor 14 60 3524 587 0.683 Poor TABLE 1 -Continued Treat- Oxide Experiment Charge Thiek- Adhesion o1 Vinylide ment Time, Coulomhs, Quantity ness, Chloride Coating After No. Min. amp-sec amp-min/ft mils 17 Hours CASS 15 70 4108 697 0.822 Poor 16 80 4717 787 0.875 Very Good 17 100 5989 1000 0.875 Very Good 18 120 6974 1 160 0.856 Very Good Nominal Current Density amp/ft 19 30 844 140 0.175 Poor 20 50 1402 234 0.285 Poor 21 63 1 800 300 0.332 Poor 22 90 2551 425 0.385 Very Good 23 120 3460 576 0.350 Very Good It will be noted from Table 1 and FIG. 8 based on Table 1 that when the limiting oxide film thickness is reached for a particular current density, very good bonding of the polymer vinylidene chloride coating is obtained. However, as for the values falling on the slope of the curves of FIG. 8, it will be noted that the oxide film is not polymer-active and does not become polymer-active until dissolution is achieved at the limiting film thickness. The very good CASS ratings obtained after 17-hour CASS environment were at least about 8 on the CASS rating chart of FIG. 11.
As will be apparent from FIG. 8, the limiting film thickness is dependent upon the current density used, other conditions being equal. Thus, by controlling the current density to any particular value, any desired limiting film thickness can be produced having polymeractive oxide structure or surface. In this connection, the invention is particularly advantageous in that a relatively broad range of conditions is possible over which very thin polymer-active films can be produced without requiring prolonged treatment times. To illustrate the manner in which the ultimate limiting film thickness can be controlled, based on current density and temperature, tests were conducted on Type 3003 aluminum alloy. The current densities at which polymeractive oxide films could be produced were determined by first establishing the logarithmic relationship between the temperature of the electrolyte and the minimum current density at which a polymer active surface is obtained of finite thickness as shown in FIGS. 9 and 10. Thereafter, in determining the optimum or permissible higher current density, the current density was increased from the minimum to that value of current density corresponding to the desired polymer-active limiting film thickness. Tests were conducted on brightbuffed Type 3003 aluminum alloy panels in 15 vol.% sulfuric acid aand vol.% phosphoric acid over a relatively broad temperature range. The panels were cleaned by: (1) scrubbing the surface thereof with a cotton wad saturated with a slurry of magnesium oxide and water and (2) water rinsing while wiping the surface with a clean piece of cotton to insure complete removal of any magnesium oxide from the aluminum surface. The minimum current density for producing a polymeractive film of limiting thickness on the cleaned aluminum surface was determined by varying the current density at a particular temperature from a lower current density to a high value until a polymer-active oxide film was obtained as characterized by very good adherence obtained with vinylidene chloride after 17 hour CASS, the polymer being applied in the manner described in Example 1. Thus, in FIGS. 9 and 10, a polymeractive surface could not be produced below the lines designated as the minimum current density. The results obtained are given in Tables 2 and 3 as follows.
TABLE 2 Aluminum Alloy 3003 and 15" lo H SO Current Charge vinylidene Chloride Temp. Density* Treatment Quantity Color of Adherence N0. "F. Amp/ft Time Min. Amp-Min/ft Oxide Film 17-hour CASS A. Minimum Current Density Tests 24 78:1 0.45 15.0 68 Clear Very Good 25 78:1 0.40 15.0 6.0 Clear Very Good 26 78:1 0.35 15.0 5.3 Clear Very Good 27 78:1 0.30 20.0 6.0 Clear Very Good 28 78:1 0.28 20.0 5.6 Clear Poor 29 78:1 026 20.0 4.0 Clear Poor 30 106:1 3.8 5.0 19.0 Clear Very Good 31 106:1 2.2 7.0 15.4 Clear Very Good 32 106:1 1.5 10.0 15.0 Clear Very Good 33 106:1 1.2 10.0 12.0 Clear Very Good 34 106:1 1.0 12.0 12.0 Clear Very Good 35 106:1 0.8 15.0 12.0 Clear Poor 36 130:] 4.0 5.0 20.0 Clear very Good 37 130:1 3.5 6.0 22.0 Clear Very Good 38 130:1 3.2 8.0 25.6 Clear Very Good 39 130:1 3.1 8.0 24.8 Clear Very Good 40 130:1 3.0 7 0 21.0 Clear Poor 41 130:1 2.8 8.0 224 Clear Poor B. Maximum Current Density 42 78:1 8.0 88.0 Clear Very Good 43 106:1 39.0 2.0 78.0 Clear Very Good 44 :1 118.0 1.0 118.0 Clear Very Good C. Preferred Current Density For Thin Films 45 78:1 1.6 15.0 24.0 Iridescent Very Good 46 78:1 1.1 1 1.0 12.1 Iridescent Very Good Slightly 47 78:1 1.0 6.0 6.0 Iridescent Very Good TABLE 2 -Continued Current Charge Vinylidene Chloride Temp. Density* Treatment Quantity Color of Adherence No. F. Amp/ft Time Min. Amp-Min/ft Oxide Film 17hour CASS 48 78:1 0.9 10.0 9.0 Clear Very Good 49 78:1 0.85 10.0 8.5 Clear Very Good 50 78:1 0.45 20.0 10.0 Clear Very Good 51 78:1 0.42 8.0 3.4 Clear Poor 52 106:1 7.0 2.5 17.5 Iridescent Very Good 53 106:1 5.0 3.5 17.5 Iridescent Very Good Slightly 54 106:1 3.5 1.8 17.5 Iridescent Very Good 55 106:1 2.2 5.0 11.0 Clear Very Good 56 106:1 1.9 3.0 5.7 Clear Very Good 57 130:1 3 I .5 0.8 23.6 Iridescent Poor Slightly 58 130:1 105 1.8 19.0, Iridescent Very Good 59 130:1 10.0 1.7 17.0 Clear Very Good Steady State Current Density TABLE 3 Aluminum Alloy 3003 and 20'- lo Phosphoric Acid Current Charge Vinylidene Chloride Temp. Density* Treatment Quantity Color of Adherence No. F. Amp/ft Time-Min. Amp-Min/ft Oxide Film l7-Hour CASS A. Minimum Current Density Tests 60 82:1 2.4 10.0 24.0 Clear Very Good 61 82:1 2.2 10.0 22.0 Clear Good 62 82:1 2.1 10.0 21.0 Clear Poor 63 82:1 2.0 10.0 20.0 Clear Poor 64 97:1 5.4 4.0 21.6 Clear Very Good 65 97:1 5.2 4.0 20.8 Clear Good 66 97:1 5.0 4.0 20.0 Clear Fair 67 97:1 4.8 4.0 19.2 clear Poor 68 97:1 4.6 5.0 23.0 Clear Poor 69 97:1 4.4 5.0 22.0 Clear Poor 70 127:1 28.0 0.8 224 Clear Very Good 71 127:1 27.0 0.8 21.6 Clear Very Good 72 127:1 26.0 0.8 20.8 Clear Very Good 73 127:1 25.0 1.5 37.5 Clear Poor 74 127:1 24.0 0.8 19.2 Clear Poor 75 127:1 22.0 0.9 19.8 Clear Poor B. Maximum Current Density 76 82:1 82 1.5 Clear Very Good 77 97:1 185 1.0 185 Clear Very Good 78 127:1 920 0.75 690 Clear Very Good C. Preferred Current Density for Thin Films 79 82:1 5.5 22.0 Iridescent Very Good 80 82:1 4.9 4.5 22.0 Iridescent Very Good Slightly 81 82:1 3.75 5.0 18.8 Iridescent Very Good 82 82:1 3.7 5.0 18.5 Clear Very Good 83 82:1 3.5 5.0 17.5 Clear Very Good 84 97:1 14.0 2.0 28.0 Iridescent Very Good 85 97:1 1 1.2 2.0 22.4 Iridescent Very Good Slightly 86 97:1 8.55 3.0 25.6 Iridescent Very Good 87 97:1 8.1 3.0 24.3 Clear Very Good 88 97:1 4.5 5.0 22.5 Clear Very Good 89 127:1 58 0.3 17.4 Iridescent Very Good Slightly 90 127:1 52 0.4 20.8 Iridescent Very Good Slightly 91 127:1 46 0.4 18.4 Iridescent Very Good 92 127:1 44 0.4 17.6 Clear Very Good "Steady State Current Density It will be noted from FIGS. 9 (sulfuric acid) and 10 (phosphoric acid) that the current density for producing a polymer-active oxide surface of limiting film thickness varies logarithmically withtemperature, the higher the temperature, the higher the minimum current density necessary to produce a polymer-active surface of limiting film thickness. For Aluminum Alloy 3003, the minimum in FIG. 9 for sulfuric acid is repre sented by line AB of the chart, while the-desirable working maximum is represented by line DC, the values on line DC for a selected temperature being about 37 times the minimum values along line AB. The preferred range is encompassed by area ABI-IG, thevalues along line GH for a selected temperature being about 7 times the minimum represented by line AB. A preferred current density for producing very thin clear polymer-active oxide films on Aluminum Alloy 3003 is that encompassed by area ABFE, the values along line EF for a selected temperature being about 3.5 times the minimum current density represented by line AB. It should be noted that in FIG. 9, the preferred current density is only one-tenth the current density normally used in conventional anodizing at 68F and the maximum current density is about one-third.
about 7 times the minimum along line I]. A preferred current density for producing very thin clear polymeractive films on AA 3003 is that encompassed by area IJNM, the values along line MN for a selected temperature being about 1.7 times the minimum represented by line I]. The preferred concentration of phosphoric acid regarding the above may range from about l to 30 vol.%.
The minimum current may vary according to the particular aluminum alloy. However, whatever the minimum current density, the operable current density may range broadly up to 37 times the minimum and the preferred current density may range up to 7 times the minimum.
An advantage of working with thin clear polymeractive films is that specularity can be maintained by applying clear film-forming polymers to the surface thereof while obtaining very good adherence.
MECHANISM OF FORMATION OF THE POLYMER-ACTIVE OXIDE LAYER To understand the mechanism of formation of the polymer-active layer, it would be helpful to define the terms limiting or maximum film thickness, oxide formation rate" and oxide dissolution rate.
The limiting film thickness is the maximum oxide thickness that can be attained on aluminum during anodizing for a particular applied current density. For all conditions of anodizing, films can reach a maximum limiting thickness at some point in time and may thereafter begin to decrease in thickness. According to some observers, this decrease in film thickness is a result of an increase in film resistance of thicker oxide layers and the generation of heat at the electrolyte-oxide interface which accelerates the dissolution of the oxide film to the extent that the dissolution rate exceeds the rate of formation of the oxide.
The oxide formation rate is the electrochemical rate of growth of the oxide layer at the aluminum-aluminum oxide interface. At a current efficiency of 100%, the oxide formation rate can be calculated by Faradays law of electrolysis which states that 96,500 coulombs (ampere-seconds) are required to form one electrochemical equivalent weight of oxide.
With regard to the oxide dissolution rate, this is defined as the rate of dissolution of the oxide film at the oxide-electrolyte interface. This parameter is determined experimentally since it is dependent on several variables.
Thin limiting films can be produced by changing operating conditions as follows:
Increase in solution agitation With regard to the production of polymer-active grade films, it is believed that the oxide dissolution which occurs during limiting film formation. collapses the oxide cells which, while being polarized with an anodic potential, results in the formation of a defective crystal lattice structure having charged centers. The oxide structure obtained is polymerreceptive and electrically conductive.
The electrical conductivity of the polymer-active oxide film, especially thin films of thickness ranging up to about 30 microinches, is determined by placing a 1- inch diameter polished copper rod weighing 347 grams on the surface and impressing a 20-volt potential across the anodic oxide film. A drop of only 40 to 200 millivolts at 650 milliamps has been measured for an oxide film shown to be polymer receptive. For substantially nonconductive films, less than 10 milliamps of current have been observed at an impressed voltage of 20 volts.
The oxide dissolution rate, which is determined experimentally, is affected by the type and concentration of the electrolyte, temperature of the solution, and also whether or not the solution is agitated. Thus, while dissolution rate may vary with change in electrolyte composition, as stated above, this parameter can be determined easily by experiment for each composition depending upon the aluminum alloy being anodized.
As stated hereinbefore, it is preferred in carrying out the invention that the anodizing be effective to produce the limiting film thickness since presence limiting film thickness is not substantially affected by the pressence of inhibitors in the anodizing solution.
Since the formation of the polymer-active surface at the limiting film thickness is achieved by providing substantially a steady state or an equilibrium between the formation rate and the dissolution rate of the oxide, it is desirable not to have the dissolution rate during the surface treatment exceed the formation rate to such an extent that there is a complete stripping off of any oxide layer initially formed on the surface. This could occur in a continuous flow system if the temperature and/or agitation at the anode surface are too excessive. Considerable agitation of the solution relative to the surface being treated could, through excessive erosion, prevent any formation of the oxide. Thus, the relative velocity between the solution and the aluminum element should preferably be as close to zero as possible, although this is not essential so long as erosion of the oxide surface is avoided.
As illustrative of various embodiments of the invention, the following additional examples are given.
EXAMPLE 2 Sulfuric acid is an example of a conventional acid anodizing electrolyte when used in concentrations of about 5 to 30 percent by volume, e.g. about 10% to 25% by volume, over current densities of 10 to 25 amps/sq.ft. at moderately low temperatures of 65 to F. The foregoing conditions in conventional practice generally produce an oxide which is dense and hard. This oxide, however, is not polymer-active, although some mechanical bonding is possible with it.
In producing a polymer-active surface using sulfuric acid, a buffed aluminum panel (2 inches wide by 6 inches long and 0.063 inch thick, identified as Alloy 3003), is precleaned by l vapor degreasing in trichloroethylene; (2) soaking in an alkaline cleaning solution (inhibited alkaline cleaner containing basic alkaline salts, surfactants, and emulsifying agents) for seconds at F; (3) water rinsing for 60 seconds at l20F; (4) acid dipping in a solution containing 50 volume percent nitric acid for 15 seconds; and (5) water rinsing for 30 seconds at approximately 78F. The
cleaned panel is then immersed in a 15 volume percent sulfuric acid solution at 130F and treated anodically at a current density of 10.5 amps/sq.ft. for 1.8 minutes at a direct voltage of 2.4. Thereafter, the panel is removed, water rinsed at 76F and forced-air dried at 78F. As will be noted from FIG. 9, the current density employed is in the preferred range.
The foregoing treatment produced a polymer-active surface which was also characterized as being substantially electrically conductive.
Following the treatment, the panel with the polymeractive film or layer was dipped in a solution of vinylidene chloride and tetrahydrofuran containing 60 gpl (grams per liter) of the vinylidene chloride for about 3 seconds to provide a 0.2 mil layer coating which was air dried at 78F. The coated area without baking exhibited very good adhesion when the panel was bent upon itself at 180 and subjected to a peeling test by pulling on the bent edge with adhesive or Scotch tape applied to the surface of the coating. To show that the adhesion was more than a mere mechanical adhesion, the panel was subjected to a CASS test for 17 hours, after which the bend and peel tests were conducted on otherportions of the panel. The panel, following the CASS test, exhibited very good adhesion and the CASS rating was at least about 8.
Tests have shown that the behavior of electrolytes are affected by various parameters. For example, electrolytes behave differently from each other at various temperatures and current densities. For instance, sulfuric acid provides very good adhesion after the CASS test where the polymer-active layer is produced at l68F at a treatment time of 7.5 minutes and a current density of 60 amps/sq.ft. (at 188 amps-sec/sq.inch or 450 amp-min/sq.ft.). However, when the aluminum panel was treated at l20F for the same time of treatment (7.5 minutes) at a current density of 60 amps/sq.ft. (450 amp-min/sq.ft.), the adherence of the vinylidene chloride coating was poor after the CASS test, although the adherence appeared to be very good before the CASS test. By dropping the temperature down to 120F, the treatment time was not sufficient to produce the limiting film thickness and, with it, the polymer-active surface. However, the treatment time of 7.5 minutes was sufficient at 168F.
Phosphoric acid appears to give good results over a relatively broad range of temperature, e.g. 61 to 1 F and higher, and over a range of 5 to more than 20 volume percent. A volume percent solution of phosphoric acid gave excellent results on Alloy 3003 (cleaned as in Example 1) panel when coated with vinylidene chloride both before and after a 17-hour CASS test.
An electrolyte solution containing 3.8% by weight of sodium carbonate gave very good results at a temperature of 104F and 18 amps/sq.ft. (450 amp-min/sq.ft.) with vinylidene chloride (25 minutes treatment time). However, poor results were obtained at 81F and 10 amps/sq.ft. at 45 minutes of treatment time (450 ampmin/sq.ft), both before and after the CASS test due to the fact that the time of treatment was not sufficient to form the limiting film thickness and, with it, the polymer-active surface.
In the case of an electrolyte solution containing 10% by weight of oxalic acid, very good adhesion of vinylidene chloride was obtained at a temperature ranging from about 130 to 141F, a current density of about 27 to 34 amps/sq.ft. (450 amp-min/sq.ft.) while poor adhesion was obtained at 50 amps/sq.ft. (450 ampmin/sq.ft.) at 77F and 35 amps/ft (450 amp-min.ft. at 115F temperature.
Similarly, very good results have been obtained with chromic acid solutions (e.g. 5% by weight) at temperatures of l50F to 170F, at 38 to amps/sq.ft. (450 amps-min/sq.ft.) before and after a l7-hour CASS test with vinylidene chloride, while poor results were obtained at 4 amps/ft (450 amp-min/ft at 80F.
The foregoing poor results were due to the fact that the time of anodizing was not sufficient to reach limiting film thickness and the accompanying polymeractive surface.
As short time treatments and thin films are particularly desirable in carrying out a continuous process commercially, an electrolyte containing phosphoric acid is preferred.
EXAMPLE 3 As stated hereinbefore, when the dissolution rate of the aluminum oxide exceeds the formation rate, a polymer-active oxide surface is not obtained. This is illustrated by the following.
A buffed 2X6 inch aluminum panel (Alloy 3003) was cleansed as described in Example 2 and thereafter treated anodically in a 20 volume percent solution of phosphoric acid electrolyte at 127F and 24 amps/sq.ft. for 0.8 minutes. A vinylidene chloride coating was then applied as in Example 2, the panel being similarly evaluated before and after the l7-h0ur CASS test. Due to a rather high dissolution rate, a polymer-active surface was not obtained, as a result of which the applied vinylidene chloride coating showed poor bonding before and after the CASS test. However, at 28 amps/ft for 0.8 minutes, very good adherence was obtained after l7-hour CASS.
Similarly, the same alloy was tested at 82F, one being anodized in 20% phosphoric acid for 10 minutes, at 2 amps/ft the other for 10 minutes at 2.4 amps/ft? The latter had a polymer-active surface as evidenced by very good adherence of vinylidene chloride after 17- hour CASS. The one with the lower current density had an excessive dissolution rate and did not produce an adherent coating.
The data of Tables 2 and 3 illustrate how the electrolyte system of, for example, sulfuric acid or phosphoric acid, can be handled by adjusting current density and temperature to avoid an excessive dissolution rate and assure reaching the limiting film thickness at which substantially a balance between the formation rate and the dissolution rate occurs in forming polymer-active surface EXAMPLE 4 This example illustrates how a conventionally anodized aluminum member can be converted to a polymeractive surface.
In converting the surface of a conventionally oxidized member to one which is polymer-active, a buffed 4X6 aluminum panel (Alloy 5557) was cleaned as described in Example 2 and conventionally anodized in a 15 volume percent sulfuric acid electrolyte for 15 minutes at 16 amps/sq.ft. and F. This treatment produced a dense hard oxide layer of high specularity. After water rinsing and forced air drying at 78F, the panel was cut to provide two specimens, 2X6 inches in size. One of the specimens with the hard dense oxide layer was coated with vinylidene chloride as in Example 2, but the coating did not adhere and could be easily peeled off.
Table Continued Comparative Surface Bonding Characteristics The other specimen with the hard oxide layer was 5 Surface Prepared as Pracliced i in the Invention (b) treated anodically in a 15 volume percent phosphoric Polymer acid solution for 2 rnir utes at 20 amps/sq.ft. at a terri- Thickness, Adhesion perature of 1 F and its oxide modified by partial dis- Test Adhesion Prior h r solution to produce a polymer-active surface. After CASS Exposure CASS Exposure water rinsing and forced-air drying the panel at 78F, 10 I3A i Very good Very good a vinylidene chloride coating was applied as in Example 1 Very Very good A l Very good Very good 2. The polymer coating exhibited excellent adhesion before and after a I7-h0l1l' exposure test. (b) Aluminum Alloy 3003 panels 2X6 inches in size cleaned as noted in Example l and ariodically treated using l5 volume percent phosphori aci I '1 t l t f A polymer coating for the purposes of this invention 5 minutes m and a Cumm density of IX p q t r t tt r0 y t or is generally considered good when it exhibits a 17-hour l5 CASS rating of at least 7 The results of Tables 4 and 5 demonstrate the allaround capability of the polymer-active surface produced by the invention of providing excellent adher- EXAMPLE 5 ence with a broad spectrum of polymers even after 5 The purpose of this example is to illustrate the broad hours of CASS testing, It will be noted that even tetraspectrum f polymeric materials hi h can b dh fluoroethylene, the most difficult polymeric material to ently bonded to aluminum i accordance i h the i bond to a substrate, exhibited excellent adherence to vention. Aluminum panels (Alloy 3003) of 2X6 inch in the polymer-active Surfacesize were cleaned as in Exam le 2 and anodicall p y EXAMPLE 6 treated in 15 volume percent phosphoric acid solution for 5 minutes at 110F and a current density of 18 The object of this example is to demonstrate that the amps/sq.ft. to form an oxide of limiting thickness charinvention is applicable to a broad range of aluminum acterized by the polymer-active surface. The materials alloy products. tested and the results obtained are given in Tables 4 Buffed panels of a wide range of alloys shown in and 5 as follows. Table 6 were cleaned as in Example 2 and then anodi- TABLE 4 Test Polymer Application No Polymer Solvent Concentration Method Drying Conditions 1A Polyvinylidene Chloride Tetrahydrofuran 6 w/o Dip Coated Baked 5 min 280F 2A Polyvinylidene Chloride Tetrahydrofuran 6 w/o Dip Coated Air Dried 78F 3A Polyvinylidene Chloride Aqueous Dispersion 20 w/o Dip Coated Baked 5 min. 280F 4A Polyvinylidene Fluoride Dimethylformamidc 6.7 w/o Dip Coated Air Dried 78F Baked 1 hr. 302F 5A Epoxy Resin MlBK/xylene/toluene (a) 27 w/o Dip Coated Baked [.5 hr 2l2F 6A Silicone Resin Toluene/IPA/xylene I25 w/o Dip Coated Air dried 78F 7A Acrylic Resin Toluene 2O w/o Dip Coated Baked 1 hr I49F 8A Acrylic Resin Toluene 20 w/o Dip Coated Air dried 78F 9A Polyurethane Tetrahydrofuran l0 w/o Dip Coated Baked 1 hr 149F IOA Polyurethane Tetrahydrofuran l0 w/o Dip Coated Air dried 78F 1 IA Polyurethane Cellosolve acetate/ toluene 18.6 w/o Dip Coated Baked 1 hr 2l2F 12A Polycarbonate Methylene chloride 10 W/o Dip Coated Baked 1 hr I49? 13A Polycarbonate Methylene chloride 1 1 w/o Dip Coated Air dried 78F 14A Tetrafluoroethylene Aqueous dispersion 30 w/o Dip Coated Baker I5 min 9U0F 15A Polyimide Dimethylformamide 20 w/o Dip Coated Baked 30 min 300F 2 min 600F (a) MIBK designates methylisobutyl ketone TABLE 5 cally treated in 15 volume percent phosphoric acid at l lOF and 20 amps/sqfi. for about 5 minutes. Thereaf- Comparative Surface Bonding ter, a vinylidene chloride coating was applied in the charactersms manner described in Example 2 and each of the coated Surface Prepared as Practiced in the invention panels evaluated after 17 hours exposure to the CASS Polymer Adh Af environment. Very good adhesion was observed for all Thickness, esion ter Test No mil Adhesion Prior 56-hours of the alloys tested to CASS Exposure CASS Exposure 6() TABLE 6 1A 0.15 Very good Very good 2A 0.15 Very good Very good SURFACE BONDING CHARACTERISTICS OF TREATED 3A Q6 Very good Very good PURE ALUMINUM AND ALUMINUM ALLOYSW 4A 0.6 Very good Very good Aluminum Specimen Adhesion of Vinylidene Chloride 5A ().l Very good Very good Alloy Size Coating After l7-Hours 6A 0.] Very good Very good No. Inches CASS Exposure 7A 1 Very good y good 8A l Very good e y gO Commercially l X 6 Very Good 9A l Very good Very good pure lOA l Very good Very good I 2 X 6 Very Good 1 1 l Very good e y go d 2014 3 X 4 Very Good 12A 1 Very good Very good 2025 2 X 0 Very Good TABLE 6-Continued Aluminum Specimen Adhesion of Vinylidene Chloride Alloy Size Coating After l7-Hours No. Inches CASS Exposure 3003 2 X 6 Very Good 5005 2 X 6 Very Good 5255 3 X 4 Very Good 5456 3 X 4 Very Good 5557 2 X 6 Very Good 5657 3 X 4 Very Good 6061 2 X 6 Very Good 6063 2 X 6 Very Good 7075 2 X 6 Very Good EXAMPLE 7 Tests have indicated that the polymer-active surface is heat stable at rather high'temperature without substantially losing its characteristic. A 4X6 inch panel (Alloy 3003) was pretreated as in Example 2 and then provided with a polymer-active surface by anodically treating the panel in a 15 volume percent phosphoric acid solution at 1 10F and 16 amps/sq.ft. for minutes. After water rinsing and forced-air drying at 78F, the polymer-active surface was tested for electrical conductivity by placing the end face of a 1 inch diameter polished copper rod weighing 347 grams on the surface and impressing a 20 volt potential across the polymeractive surface layer. A 70 millivolt drop was measured at 650 milliamps, thus indicating that the polymeractive surface was substantially electrically conductive.
Thereafter, the panel was heated to 1000F in air for 2 hours. After cooling, it was found that the electrical conductivity was unchanged. The panel was then coated with vinylidene chloride as in Example 2 and exhibited excellent adherence to the aluminum substrate after 17 hours of CASS testing.
A cleaning solution and method found particularly preferable in preparing the surface for electrolytic anodizing to produce the polymer-active structure or surface are as follows.
The panel which may or may not be vapor degreased is dipped in a solution containing the following ingredients:
The panel is dipped in the above solution for l to 3 minutes at 170F to l75F. The panel is thereafter rinsed for about 60 seconds at 120F and then subjected to the following anodic treatment in, for example, 20 volume percent phosphoric acid solution. The preferred current density is just below that which produces an iridescent oxide film with a charge quantity of about 20 amperc-minutes/ft". The electrolyte is maintained at 1 F and the time of treatment is about to seconds. Following the anodizing treatment, the panel is removed, water rinsed at about 78F and forced-air dried at about 78F preliminary to applying a polymer coating.
Another method of cleaning the aluminum panel is to employ a slurry of magnesium oxide. As illustrative of this method in the production of polytcrafluoroethy lene coating (Teflon) the following example is given.
EXAMPLE 8 A bright-buffed aluminum panel 2 inches wide by 6 inches long and 0.065 inch thick (AA 1 is cleaned by: (1 scrubbing the surface thereof with a cotton wad saturated with a slurry of magnesium oxide and water and (2) water rinsing while wiping the surface with a clean piece of cotton to insure complete removal of any magnesium oxide from the aluminum surface. Follow-.
ing cleaning, the panel is treated anodically in a 12 volume percent sulfuric acid solution at about 2 amps/ft for a period of 10 minutes (20 amp-min/ft the temperature of the electrolyte being maintained at about F. This treatment is calculated to give a polymeractive oxide film of about 20 micro-inches in thickness. After the anodic treatment, the panel is removed, water rinsed and forced-air dried at 78F. The polytetrafluoroethylene coating is applied to the surface by l dipping the treated panel in a 45% tetrafluoroethylene aqueous dispersion for about 15 seconds and slowly withdrawing to provide a thin wet film, (2) drying the film at F in air, (3) baking the dried film at 375F for 3 minutes to volatilize any of the wetting agents present, and (4) then fusing the film by baking at 730F for 3 minutes to provide a strongly bonded coating of Teflon.
The adhesion of the coating is tested by scoring the coated surface with a series of crossed lines 0.047 inch apart running through the thickness of the coating and then pulling on the scored area following the application of Scotch tape thereto. The coating does not pull away which demonstrates that it is strongly bonded to the aluminum surface via the polymer-active oxide surface.
As examples of electrolyte acid bath compositions (aqueous), the following are given:
A. phosphoric acid: 5/o to 30"/o; e.g. l0/o to 25 70;
B. sulfuric acid: 5"/o to 30/o; e.g'. lO/o to 25 /o;
C. oxalic acid: 1% by weight to saturation by weight;
D. chromic acid: 1% by weight to 25% by weight.
With regard to the foregoing electrolytes, generally speaking, the higher the temperature, the greater is the current density required to effect formation of the polymer-active oxide structures and the shorter the time of treatment. In any event, for a particular alloy, the time employed for a particular electrolyte, composition, etc., should be sufficient to effect a partial dissolution of the anodic oxide.
The invention is readily adaptable to a continuous process whereby the solution or the member being treated move relative to one another by employing an apparatus system for carrying out two operations in situ, to wit: (1) removing the natural oxide layer from the aluminum member and thereby providing a clear aluminum surface; and (2) forming a layer of the polymer-active oxide. The continuous process is advantageous in that a controlled balance can be achieved and maintained by selecting the current density corresponding to a thin film that is polymer-active in minimum time.
' An illustrative embodiment of the method and apparatus of the invention is shown in general outline in FIG. 1, for the continuous mass-production treatment of aluminum rod, strip, or sheet material 10, moving from left to right. The above-noted stripping and oxidizing steps take place at a single station 11, after which the thus-oxidized aluminum is water-rinsed at 12 and air-dried at 13, prior to polymenapplication at 14 and oven-treatment (if needed) at 15. In general, the aluminum processing at 11 comprises a continuously circulating electrolyte system involving a flowdischarge element or spray-head electrode 16, positioned above the aluminum l and flooding the same with the electrolyte; collector 17 receives the electrolyte after contact with the aluminum 10, and dashed line 18, including a pump 19, will be understood to suggest completion of the circulating system. Preferably, the electrode 16 is excited as the cathode, and the aluminum is the anode, as suggested by polarity legends at 16 and at an aluminum-contacting shoe element 20. The aluminum member may be stationary and the electrolyte directed to it at a relatively high rate of flow.
Processing means 11 is shown in greater detail in FIGS. 2, and 3, wherein aluminum bar material 10 is shown being continuously advanced in the right-to-left direction. At its passage through means 11, the aluminum 10 is guided on spaced supporting rolls 21-22 which may be continuously driven by means 23. The rolls 21-22 are fixedly journalled (by means not shown) and are preferably sized for constant partial immersion in the electrolyte at normal electrolyte level, in the collector l7. Collector 17 may be a rectangular pan, having a central drain 24 which includes orifice means 25 whereby a normal desired electrolyte level 26 can be maintained in pan 17; a filter 27 cleans the electrolyte before it returns to a reservoir or sump 28, from which the pump 19 recirculates clean electrolyte.
The spray electrode 16 is shown in FIG. 3 as a thin and generally rectangular and substantially closed box, having a width W overlapping and extending beyond the effective width dimension W of the aluminum 10 to be treated; its length dimension L (FIG. 2) extends substantially along the path of movement of the alumi num 10 and is shown centrally positioned between rolls 21-22. The bottom surface 29 of head 16 is foraminous or otherwise perforated, the effectively open area of surface 29 being preferably such, in relation to the flow rate and pressure of electrolyte supplied by pump 19, that continuous streams (rather than droplets) of electrolyte span the relatively short space to the top surface of aluminum 10, and such that these streams impinge continuously over and thus flood the entire exposed top surface of the aluminum 10. The head 16 may include an upstanding supply pipe 30, guided for vertical positioning adjustment (suggested by rack-and-pinion means 31) and forming part of the supply circuit 18, through flexible connection 32. A flexible wiper member 33, fixed to the exit end of head 16, serves to wipe most of the electrolyte from the aluminum 10 prior to passage beyond the collecting area of pan 17. In order to maintain a desired electrolyte temperature condition, at discharge to the aluminum 10, heater means 34 is shown at reservoir 28, supplied by means 35 having a control provision 36 which responds to the instantaneous output of a heat detector or probe 37 (such as a thermocouple, bead thermistor, or the like) carried by and exposed to the flooded interior of head 16. The rectangular prismatic phantom outline 38 over the pan 17 and over the spray electrode 16 will be understood to suggest splash limiting means which serves the further function of a venting hood, with exhaust means 39. In use, the pump 19 establishes plural liquid streams of electrolyte between head 16 and the aluminum, thus completing the electrical circuit for anodic treatment of the aluminum via aluminumcontacting shoe 20. The number of these streams and the flow rate assure fullarea liquid coverage of the top surface of the aluminum, for substantially the full extent of passage beneath head 16. The bottom Surface of the aluminum will have been prewetted (with electrolyte from pan 17) by the time the aluminum is exposed to the spray discharge; thus electrolyte that impinges on the top surface of the aluminum and flows over the side edges thereof will be automatically drawn underneath the aluminum, assuring full coverage of the aluminum during its exposure to head 16.
In a typical and successful employment of the invention it need not matter whether the aluminum to be treated is coated with a naturally formed aluminum oxide or whether the specimen is of a commercially available anodized variety. It also need not matter whether the specimen is or is not chemically clean; for example, a grease-laden specimen is very quickly stripped and cleaned so that the desired treatment takes place nonetheless. The electrolyte may comprise 5 to 30 volume percent solution of phosphoric acid. A bath containing 20 volume percent phosphoric acid and 4.5% by weight of oxalic acid has been found useful in producing polymer-coated aluminum products.
For the short exposure time stated hereinabove, the substrate thickness of the polymer-active layer may be in the order of about 0.001 mil l microinch) in thickness and range up to as high as 0.15 or 0.25 mil (150 to 250 microinches) and, more preferably, up to about 0.03 mil (3O microinches).
In the embodiment of FIGS. 4 to 6, the spray electrode 16 is replaced by an array of spaced, parallel, porous roller electrodes 41-42-43-44-45, internally supplied with electrolyte from manifold means 40 connected to supply 18, and having continuous wetting contact with the top surface of the aluminum specimen 10. These rollers are shown to comprise an inner nonrotatable tubular support element 46 (FIG. 5) fixed to its electrolyte supply pipe 46 and extending, in concentrically supporting relation within substantially the full axial length ofa porous outer tubular roller element 47; the porous member 47 may be a perforated rigid or semi-rigid basket with a cloth cover of material inert to the electrolyte. Suitable bearing means (not shown) supports the outer element for rotation about inner element 46, and a sprocket wheel 48 on one end of element 47 enables its driven rotation. Sprocket-chain means 49 is shown driving all sprocket wheels in unision, at a speed synchronized with the feed of the aluminum 10 so as to establish no-slip contact or nearcontact of the wetting means 47 with the top surface of the passing aluminum specimen. A cap 50 closes the undriven end of each roller 41, and this cap may be carried by either of the tubular members 46-47.
As shown in FIG. 5, the cylindrical body of the inner member 46 is locally apertured, as by slits or a window 51, of limited angular extent which may in general be up to about Such opening localizes the radially outward flow of electrolyte so as to flood the rotated outer porous member 47 just prior to its contact or near-contact relation with the aluminum 10, such that the velocity of the solution relative to member 10 is as close as possible to zero so as to minimize erosion of the oxide layer. In operation, FIG. 6 shows that a solid wall 52 of liquid electrolyte is established by each roll (41, 42 45) in flooding intimate contact with the entire top-surface extent of the passing aluminum strip this electrolyte spills over the edges of the aluminum, and a substantial fraction thereof runs over the prewetted bottom surface of the aluminum. If desired, bottom rolls 55-56, 57-58-59, generally similar to rolls 41-42-43-44-45, may be provided for similar contact and wetting action with the bottom surface of the aluminum. Such bottom rolls should preferably be driven in the opposite direction of the upper rolls, for similar no-slip" relation with the passing aluminum specimen, as suggested by arrows. The bottom rolls may also be internally supplied with fresh electrolyte (with internal radial-flow local flooding aperture 51 in the region just prior to contact with the aluminum), but we prefer to rely on the wetting action achievable through partial immersion of the botto-rolls in the collector-pan electrolyte, as shown in FIG. 6; in the latter event, there is, of course, no need to internally supply the lower rolls. Electrical contact is shown at shoe electrolyte flow was adjusted to 7 liters/minute at a dc. applied voltage of 30, the aluminum member (4 feet long, being moved at a rate not exceeding 3 feet per minute. Each extrusion was pulled through the reaction zone at the desired rate, with the temperature of the sprayed electrolyte controlled at about 122F and the current density ranging from about 12 to 27.5 amps/sq.ft. at retention times of 10 to 60 seconds. The treated extrusions were thereafter cold water rinsed at 75F and forced-air dried at 78F. The polymer bonding characteristics were evaluated with a vinylidene chloride coating which was applied to the test pieces by dipping them in a 60 gpl of polymer contained in a tetrahydrofuran solution and the coated pieces thereafter air dried at room temperature. As in previous examples, the adhesion was tested before and after 17 hours of CASS by bending the extrusion 90 (which caused the metal to crack) and then applying and pulling off Scotch tape from the bend zone (peel test). The results to the aluminum and at 53 to the if ld hi h 20 obtained are summarized in Table 7 as follows:
TABLE 7 SUMMARY OF EXPERIMENTAL WORK ILLUSTRATING FEASIBILITY OF SIMULTANEOUSLY CLEANING AND PRETREATING ALUMINUM MEMBERS TO PROVIDE A POLYMER-ACTIVE LAYER ON A CONTINUOUS BASIS Anodic Treatment Conditions Extrusion Bonding Characteristics Based on Equivalent Vinylidene Chloride Coating Retention Feed Adhesion Prior to Adhesion After l7-HRS Exp. Temp Current Density Time, Rate, CASS Exposure CASS Exposure" No. F amp/ft sec Ft/min Top Bottom Top Bottom 20/o Phosphoric Acid and 4.5"'/o Oxalic Acid l6A 122 27.5 1 Very good Very good Very good Very good 17A I22 27.5 20 1.5 Very good Very good Very good Very good 18A I22 27.5 15 2.0 Very good Very good Very good Very good 19A 122 27.5 10 3.0 Poor Poor Poor Poor 20A 122 27.5 15 2.0 Very good Very good Very good Very good 21A 122 27.5 l0 3.0 Poor Poor Poor Poor" "Extrusions. l.75-inch wide by 0.125-inch thick, were not cleaned prior to treating.
""Six-inch lengths were coated as noted in Example 2. ""Cualing adhesion was based on a Ml-degree bend and peel test with Scotch Tape.
"Four-foot extrusions were pulled through the reaction Zone at the indicated feed rate to simulate a continuous feed system.
supplies electrolyte to the upper roller-head assembly.
The arrangement of FIG. 7 illustrates application of the invention to treatment of aluminum material of contoured section, as for example the L-shape section or angle of continuous element 10'. The material 10' is processed by upper and lower electrolyte-wetting rolls 60-61. The upper roll 60 is externally characterized by a convex surface of revolution to match, in contacting or near-contacting relation, the concave surfaces of the aluminum as it passes the treatment region. In like fashion, the lower roll 61 is externally characterized by a concave or V surface of revolution to match the convex surfaces of the passing aluminum specimen. Both rolls 60-61 may be of the internally flooded nature described in conjunction with FIGS. 4 to 6; however, in the form shown, a wetting head on cathode 62 having an apertured bottom concave surface delivers a continuous flow of electrolyte to the upper roll 60. Both rolls 60-61 may be cloth covered, for maximum entrainment of the electrolyte and to assure a solid wall of liquid from cathode (62) to anode (10).
In carrying out the continuous process, an apparatus similar to that shown in FIGS. 1 to 3 was employed in which an extrusion of aluminum Alloy 6063 of 1.75 inches wide and 0.125 inch thick was used. The aluminum extrusion or member was made anodic and the The aluminum extrusions which were treated in accordance with the invention contained natural oxide with grease spots and fingerprints thereon which were visible prior to the treatment. The original areas of surface contamination were marked with a scriber. Following treatment, the natural oxide had been completely removed along with the grease spots and fingerprints.
Under comparable conditions, the minimum treatment time required to provide a polymer-active surface was about the same for stationary or continuously moving extrusions.
The phosphoric-oxalic acid electrolyte exhibited good throwing power and it is expected that the electrolyte will be capable of providing a polymer-active surface on very complex shapes. As can be seen in Experiments 16A through 21A, a polymer-active surface was obtained on the underside of the extrusions without the aid of a cathode on that side.
It is believed, according to infrared analysis, that the excellent adhesion obtained with the yariety of different polymeric materials appears to be due to the combined effect of the crystallographic structure of the oxide, which apparently causes polymer films to undergo structural modification at the interface and the microscopically etched nature of the oxide surface. Thus, at the polymer-aluminum oxide interface, the polymer is less crystalline due to a disordering of the polymer. It
is believed that this relaxed conditions causes an iondipole interaction or the existence of like forces of similar magnitude approximating chemical bonds in energy.
Whatever the theory, the invention provides a superior polymer-coated aluminum member comprising a film-forming polymeric organic material bonded to the surface of the aluminum member via a polymer-active aluminum oxide layer integral with said surface, the polymer-active oxide layer being further characterized by being electrically conductive.
Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.
What is claimed is:
1. Apparatus for the continuous surface-processing ofa member of aluminum metal, comprising means for continuously transporting the metal member in and successively through a first electrolytic treatment zone for establishing a polymer-active member surface, a second washing and drying zone, and a third polymerapplying zone for coating the polymer-active surface; said apparatus in the electrolytic treatment zone comprising hydraulic-circuit means including a wetting head in adjacency above and generally directed at the metal member as it passes through said zone, said circuit means further including a reservoir for containing electrolyte and means for continuously supplying electrolyte from said reservoir to said head to discharge the same at the passing metal member, temperature sensing means for sensing the electrolyte temperature at the wetting head, heater means associated with said reservoir including temperature control means for controlling the temperature of the electrolyte in said reservoir in accordance with the temperature sensed at said wetting head, and said circuit further including electrolyterecirculation means including a collection pan for establishing an electrolyte-collection level spaced from and beneath the path of movement of the metal mem ber through the first zone, and electric-supply means including means for anodically effecting an electrical coupling with said metal member and a cathodic electrical connection to an electrically conducting member which is adapted to be in contact with the electrolyte in said hydraulic circuit, whereby in the presence of a continuously flowed electrolyte stream of electrolyte from said head to the metal member an electrochemical circuit may be completed via said stream; washing and drying means in the second zone; and liquid coating means in the third zone for applying to the member a liquid composition of film-forming polymeric organic compound.
2. The apparatus according to claim 1, wherein the wetting head is a spray head,
3. The apparatus according to claim 1, wherein the transport means for the metal member includes at least one roll in said first zone and having a surface at least in part exposed to liquid electrolyte spilled after discharge from said head to the metal member.
4. The apparatus according to claim 3, wherein said roll is adapted to be partly immersed in electrolyte collected in said pan.
5. The apparatus according to claim 1 wherein said collector pan has a drain, including means for restricting drain flow to maintain collector level.
6. Apparatus for the continuous surface-processing of a member of aluminum metal, comprising means for continuously transporting the metal member in and successively through a first electrolytic treatment zone for establishing a polymer-active member surface, a second washing and drying zone, and a third polymerapplying zone for coating the polymer-active surface; said apparatus in the electrolytic treatment zone comprising hydraulic circuit means including a wetting head in adjacency above and generally directed at the metal member as it passes through said zone, said circuit means further including means for supplying electrolyte to said head to discharge the same at the passing metal member, a collector pan disposed below said wetting head and below the metal member passing through said zone for catching said electrolyte, drain means cooperably associated with said pan including means for restricting drain flow so as to control electrolyte level in said collector to a level below the path of movement of the metal member through the first zone, r recirculating circuit coupling the electrolyte in said collector pan from the drain means to said hydraulic circuit means eeding said wetting head for recirculating said eleittrolyte to said hydraulic circuit, an electrolyte reservoir coupled to said recirculating circuit including means associated with said electrolyte reservoir for recirculating said electrolyte from the reservoir to and through said hydraulic circuit, temperature sensing means for sensing the electrolyte temperature at the wetting head, heater means associated with said reservoir including temperature control means for controlling the temperature of electrolyte in said reservoir in accordance with the temperature sensed at said wetting head, and electric-supply means including an anodic electrical contact to the metal member and a cathodic electrical connection to an electrically conducting member which is adapted to be in contact with the electrolyte in said hydraulic circuit, wherein in the presence of a continuously flowed electrolyte stream from said wetting head to the metal member an electrochemical circuit may be completed via said stream; washing and drying means in the second zone; and liquid-coating means in the third zone for applying to the member a liquid composition of film-forming polymeric organic compound.
7. The apparatus of claim 6, wherein the wetting head is vertically adjustable relative to the metal member passing through the treatment zone.
8. The apparatus of claim 6, wherein said wetting head is a spray head having a flat foraminated bottom, the effective area of the openings in the bottom being such in relation to the volume rate of continuously supplied electrolyte as to assure plural continuous electrolyte streams over an exposed area of the metal member.
9. The apparatus of claim 8, wherein the width of the spray head is adapted to overlap and extend beyond the width of the metal member.
10. The apparatus of claim 6, wherein the wetting head comprises at least one rotatable porous roll adapted to feed electrolyte to the moving metal mem ber.
11. The apparatus of claim 10, including a plurality of porous rolls adapted to be driven in synchronization with the feed rate of the metal member.
metal member and adapted to be in near-contact relation with the upper and lower surface of said metal member, said rolls being driven in synchronization with the feed rate of the metal member.