Background of the Invention

The present invention relates to a method for pre¬ paring aluminum and aluminum alloy surfaces in which the treated surface is particularly well-suited for painting or adhesive bonding. More particularly, the present invention relates to a method for preparing aluminum and aluminum alloy surfaces which are capable of forming environmentally stable bonds with either paint or adhesives and the product produced thereby.

Aluminum and its various alloys are widely used in sheet and panel form and in a multitude of different structur- al forms, many of these requiring a paint finish with others requiring that they be bonded to another object. In many instances the same aluminum component, such as a sheet or panel, may need to be adhesively bonded on one surface to aluminum honeycomb or core, for example, and to have the other surface painted. There are a number of methods in commercial use for preparing the surfaces of aluminum and its alloys so that they may be painted and there are also commercial methods for preparing aluminum surfaces so that they may be adhesively bonded. However, there is a need for a single pretreatment method that will serve to prepare the surfaces of aluminum and its alloys to prov.ide an excellent paint pretreatment and also to provide a reliably strong environmentally stable adhesive bond. While some methods have been used to accomplish these two distinctly different requirements for a single pretreat- meat process, improved painting quality, durability and con¬ sistency and the ability of such a single process to provide adhesive bonds with improved strength and environmental sta¬ bility continues to be sought. Such a single pretreatment

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method would avoid the substantial additional expense of pro¬ viding a protective layer on one surface of aluminum that is to be painted, for example, when the surfaces are run through a pretreatment process to permit effective adhesive bonding, the protective coating being removed and a further but differ¬ ent pretreatment process carried out to provide the proper pretreatment for subsequent painting of that surface. Also, there may be instances where it may be desired to stock pre- treated aluminum material in coil or other forms where the end use of the material is not known in advance. A single pre-? treatment process which would provide for either painting or adhesive bonding would be advantageous and economical.

Several pretreatment methods have been used to pro¬ vide bond joints of aluminum metal and aluminum alloys in adhesively joined structures. Typically, these methods com¬ prise the following steps: vapor degreasing the aluminum, cleaning the aluminum with an alkaline cleaner, rinsing, deoxidizing the aluminum, rinsing, subjecting the aluminum to acid anodization, rinsing, and then drying the aluminum before adhesively bonding the material. For instance, see U.S. patents Nos. 4,025,681; 4,085,012- and 4,127,451.

Likewise, various pretreatment methods have also been developed and are in commercial use for providing painted aluminum or aluminum alloy surfaces. Typically, these methods comprise the steps of vapor degreasing the aluminum, cleaning the aluminum with an alkaline cleaner, rinsing, deoxidizing the aluminum, rinsing, forming a chemical conversion coating on the aluminum to which the organic coating will adhere and then rinsing and drying before painting. See, for example, "Standard Recommended Practices for 'Preparation of Aluminum and Aluminum-Alloy Surfaces for Painting' ⁿ , ASTM D 1730, and "Voluntary Specification for High Performance Organic Coatings on Architectural Extrusions and Panels", Architectural Aluminum Manufacturers Assoc, Publication No. AAMA 605.2.

A single pretreatment method that was used commer¬ cially to prepare the surfaces of aluminum and its alloys for either adhesive bonding or painting comprised the steps of alkaline cleaning the aluminum, rinsing with hot water, acid cleaning with a non-aggressive acid, rinsing, caustic etching, rinsing, desmutting, rinsing, forming a chrornate-phosphate conversion coat, and then rinsing and drying before painting or adhesively bonding. While this method provided commercial¬ ly usable adhesive bonds and painted surfaces, efforts have continued to improve and upgrade the processing and to develop a single pretreatment method for preparing aluminum and alumi¬ num alloy surfaces which are capable of forming long-term environmentally stable bonds with either a paint or an adhesive, even under harsh conditions.

Summary of the Invention

Among the several objects of this invention may be noted the provision of methods for preparing aluminum and aluminum alloy surfaces so that these surfaces are capable of forming environmentally stable bonds with either paint or adhesives; the provision of such methods in which the adhesive bonds thus formed have advantageous long-term durability, particularly in aqueous environments, and adhesive failures at the aluminum/adhesive interface-are minimized; the provision of such methods in which the adhesive bonds formed rival those achieved by aircraft technology; the provision of such methods which serve as an excellent paint pretreatment and satisfy the requirements of Architectural Aluminum Manufacturers Associa¬ tion Publication No. AAMA 605.2; the provision of a body hav¬ ing a surface of aluminum or an alloy thereof which will form such advantageous environmentally stable bonds with either a paint or an adhesive.

Briefly, the method of the present invention com- ^" prises the steps of acid etching a clean surface of aluminum and aluminum alloys by contacting them with an acid etchant for a time sufficient to effect a loss of weight of the alumi- num and its alloys being treated in an amount in excess of 400 2 mg/ft , rinsing the acid-etched -surface and thereafter form-

■ing a chromate-phosphate conversion coating on the resulting

-rinsed and etched surface.

Two other aspects of thre present invention are methods of producing environmentally stable painted surfaces and environmentally stable adhesively bonded aluminum or aluminum alloy surfaces. The method for producing environ¬ mentally stable painted surfaces comprises the steps of pre¬ paring a surface of aluminum or aluminum alloy in accordance with the method of the present invention, painting the surface with the appropriate organic coating, and then curing the painted surface. Likewise, the method for producing environ¬ mentally stable adhesively bonded surfaces comprises the steps of preparing aluminum or aluminum alloy surfaces in accordance with the method of the present invention, applying a polymeric adhesive, positioning thereon a material to be adhesively bonded to the aluminum, and maintaining the material in con¬ tact with the adhesive until it is cured.

Two further aspects of the present invention are bodies having surfaces of aluminum or alloys of aluminum which will form an environmentally stable bond with either a paint or an adhesive. These bodies may be formed by preparing the surfaces of aluminum or aluminum alloys in accordance with the method of the present invention. Once painted or adhesively bonded, these bodies will exhibit long-term durability even under severe environmental conditions.

Other objects and features will be in part apparent and in part pointed out hereinafter.

Brief Description of the Drawing

Fig. 1 is a schematic flow diagram of a process of the present invention.

Description of Preferred Embodiments

In accordance with the present invention, it has now been found that aluminum and aluminum alloys can be prepared in such a manner that once prepared, the aluminum or aluminum alloy can be advantageously painted or adhesively bonded to other materials wherein the aluminum-paint and aluminum- adhesive bonds exhibit improved long-term durability while under harsh conditions.

Fig. 1 shows a flow diagram of a preferred embodi¬ ment of the present invention in which aluminum as received is subjected to a cleaning process in preparation for the surface treatment. The alkaline -cleaner preferred is a noπ- phosphated, non-siliσated mildly alkaline cleaner that either does not etch or only mildly etches the aluminum surface. One such suitable cleaner is sold by Turco Products, Inc., Carson, California, under the trade designation Turco 4215-NC-LT. The alkaline cleaner utilized is then removed in a hot water rinse. In some cases when the raw aluminum is particularly dirty, it may be desirable to vapor degrease the aluminum before the aluminum is alkaline cleaned. Experience has shown, however, that it is usually not necessary to perform this degreasing step.

After the aluminum is alkaline cleaned and rinsed, the surface of the aluminum is contacted with an acid etchant for a time sufficient to result in a loss in weight of the aluminum in an amount in excess of approximately 400 mg/ft . Preferably, the loss in weight of the aluminum is within the range of about 400 to about 1000 mg/ft and it is most pre¬ ferred to achieve a weight loss on the order of 700 mg/ft ² .

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A suitable acid etchant for this purpose is a solution con¬ taining sulfuric acid and flouride ions and a preferred acid etchant for this purpose is a solution containing about 8% to about 12% sulfuric acid by volume and about .005% to about 10% by weight flouride ions. The acid etchant is maintained at a temperature in the range of ambi'ent to about 150°F and is pre¬ ferred within a range of about 100° to about 120°F and in a most preferred embodiment it is maintained at a temperature of approximately 112°F. The acid-etenant and the aluminum are contacted for a time sufficient to achieve the desired loss of weight of the aluminum, this time varying with the strength of the particular acid-etchant employed. After the acid etching is completed, the etched aluminum is rinsed with water at ambient temperature. ^' In a preferred embodiment, after rinsing, the acid etched aluminum is desmutted with an' acidic agent that is oxi¬ dizing in nature. A suitable desmutter is one which is 10% of a product sold by Electrochem, St. Louis, Missouri, under the trade designation EA-578, and 10% nitric acid. After desmutting, the aluminum is once again rinsed before the chrornate-phosphate conversion coating is formed. The formation of this conversion coating may be achieved through the use of various commercially available products, preferably a product sold by Amchem Products, Inc., Ambler, Pennsylvania, under the trade designation Alodine 407-47, a mixture of chromic and phosphoric acid. In the preferred embodiment, the Amchem product is mixed to a concentration of 7 g./l. of hexavalent chromium ions and maintained at a temperature on the order of 80°F. To form the chromate- phosphate conversion coat, the aluminum is then contacted with this solution for a period sufficient to enable the aluminum to achieve a gain in weight preferably in the range of about

2 35 to about 100 mg/ft and most preferably on the order of

50 mg/ft ² .

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After the chromate-phosphate conversion coat is formed, the aluminum is once again rinsed with water at ambient temperature, preferably rinsed twice and then dried in preparation for subsequent painting of adhesive bonding. Painting of aluminum prepared in accordance with the method of the present invention may be conveniently accom¬ plished with the assurance that' the painted surface will exhibit long-term durability even under harsh conditions. After the aluminum is pretreated- as set out above, it may be primed, painted and cured.

Adhesive bonding of aluminum prepared in accordance with the method of the present invention may also be performed with the assurance that the adhesive bond to the aluminum will exhibit long-term durability under even harsh conditions. After the aluminum is pretreated as set out above, it may be primed if necessary, have the adhesive applied and then bonded to another material. Several modified epoxy resins are read¬ ily available and are suitable for use in this invention including a product designated at FM123-2 manufactured and sold by the Bloomingdale Division of American Cyanamide; a product designated as AF126 modified epoxy resin having a 250 ^β F cure manufactured and sold by Minnesota Mining and Manu¬ facturing Corporation and the modified epoxy adhesive desig¬ nated as Hysol 9628 manufactured and sold by Hysol Division of Dexter Corporation. Many other resins are workable as adhe- sives for this invention.

The method of the present invention may be used to prepare aluminum and aluminum alloys whether it be in the form of sheets, coils, extrusions and some castings for painting or adhesive bonding. Moreover, this method may be performed in either a batch or continuous process (e.g., to treat the metal in coil form) in either a spray or immersion mode. Prefer¬ ably, the process is performed in an automated sequence from immersion tank to immersion tank.

The following examples will serve to further illus- - trate the present invention in accordance with a preferred embodiment.

Example 1

In accordance with a preferred embodiment of the present invention, sheets of aluminum alloy type 3003, 4 ft. X 9 ft. X .060 inches in size, were "prepared for subsequent painting or adhesive bonding by sequentially ^" immersing the sheets in a series of tanks each measuring 5 ft. wide by 34 ft. long and 11 ft. deep and containing about 13,600 working gallons of solution. The sheets are transported from tank to tank by programmed overhead carriers.

The initial tank was an alkaline cleaning tank that contained a solution made up of 5,099 lb. of Turco 4215-NC-LT, 17 gal. of Turco 4215-NC-LT additive and a balance of water. The ^' solution was maintained at 140°F and each sheet ws immersed in the tank for a period of about five minutes.

Each sheet was then removed from the alkaline clean¬ ing tank and immersed for one minute in an aqueous rinse tank that was maintained at 120°F. This rinse tank like those that follow were maintained at an overflow rate of 10 gal./minute. After rinsing, each sheet was immersed in the acid- etch tank which contained a solution made up of 1360 gal. of sulfuric acid, 350 lb. of ammonia bifluoride, 10 gal. of RS MX-81556, a surfactant sold by Amchem Co. of Ambler, Pa., and a balance of water. The tank was maintained at 112°F-and each sheet was immersed therein for a period of about four minutes.

After removal from the acid-etch tank, each sheet was immersed in an aqueous rinse tank, maintained at ambient temperature, for a period of one minute.

The sheets were then immersed for about three minutes in the desmutting tank which contained a solution made up of 1360 gal. of nitric acid, 1360 gal. of Electrochem EA-578 and the balance of water maintained at ambient tempera- ture.

After desmutting, each'sheet was immersed in an aqueous rinse tank maintained at 'ambient temperature for a .period of one minute.

The sheets were then immersed in the chromate- phosphate conversion coat tank which contained a solution made up of 600 gal. Alodine 407, 50 gal. Alodine 47 and a balance of water. The solution was monitored by a meter sold by Amchem Co. sold under the trade designation Lineguard 101 and maintained at 400 on its #1 scale and at 80°F and each sheet was immersed for a period of about 45 seconds.

Thereafter, each sheet was aqueous rinsed in each of two rinse tanks maintained at ambient temperature for a period of one minute.

The sheets were then forced air dried by heating with an air temperature of about 300°F.

The foregoing example was repeated using each of the following aluminum alloys:

3004 (sheet) 6063 (sheet and extrusion)

6005 (sheet) 6351 (sheet) 6061 (sheet and extrusion) 5005 (sheet) Almag 35 (castings)

Example 2

Panels of aluminum alloy type 3003 prepared as set forth in Example 1 were used to form a laminate with 0.003 inch perforated honeycomb core of aluminum alloy type 3003.

The laminate was formed by applying a film of adhesive sold by American Cyanamid Co. of Havre de Grace, Md., under the trade

designation Cybond 3500, onto the aluminum panel, positioning the perforated honeycomb core on the adhesive film, applying a second layer of the same adhesive film onto the honeycomb core, and laying a second panel of aluminum onto the adhesive to form a composite. The composite was then moved into a lam¬ inating press which was operatedrat a temperature of about

295°F and a pressure of 10 psi for a period of about 17 minutes to form the laminate. The laminate was then removed from the press and allowed -to-coo- A laminate panel prepared in this manner was cut into two inch square samples and divided into three groups: one which was subjected to condensing humidity at 140°F for 1000 hours, a second which was subjected to condensing humidi¬ ty at 140°F for 3000 hours and a third which served as the control maintained at 70°F. The laminate samples were there¬ after subjected to tensile tests in accordance with ASTM Flat¬ wise Tensile Test C297-67. The load at which the laminate separated was recorded in pounds and the mode of failure was visually determined and classified as either adhesion, cohesion or core, and by relative percentages of each.

The results of these tests are listed in the follow¬ ing table and are grouped according to the sheet from which the laminate samples were cut.

Control 1340 335 10% core; 90% cohesion Control 1560 390 50% core; 50% cohesion Control 1540 385 30% core; 70% cohesion Control 1380 345 50% core; 50% cohesion 1000 hours 1560 390 60% core; 40% cohesion 1000 hours 1460 365 20% core; 75% cohesion; 5% cohesion/adhes 1000 hours 1460 365 60% core; 40% cohesion 1000 hours 1000 250 10% core; 90% cohesion 3000 hours 1500 375 50% core; 50% cohesion 3000 hours 1300 325 15% core; 85% cohesion 3000 hours 1540 385 10% core; 90% cohesion 3000 hours 1520 380 30% core; 70% cohesion Control 1440 360 20% core, 80% cohesion Control 1300 325 15% core; 85% cohesion

TABLE (Continued)

Test Test Dura- Panel tion (Hrs.) Load (Lbs.) Load (psi) Failure Mode

1000 1440 360 25% core; .75% cohesion (1 full cell lost adhesion)

1000 1320 330 25% core; 75% cohesion 1000 1560 ^• 390 60% core; 40% cohesion (90% of 1 cell & 10% of another cell lost adhesion)

1000 1640 410 50% core; Sθ% cohesion 3000 1480 370 40% core; 60% cohesion (1 full cell & 5 partia cells lost adhesion)

I

3000 1740 435 60% core 40% cohesion H t I

3000 1600 400 40% core 60% cohesion

3000 1400 285 10% core 90% cohesion (2 full cells lost)

Control 1500 375 40% core 60% cohesion

Control 1680 420 60% core 20% cohesion; 20% cohesion/adhesion

Control 1640 410 40% core 10% cohesion; 50% cohesion/adhesion

Control 1600 400 60% core 40% cohesion

1000 1740 435 60% core 40% cohesion

TABLE (Continued)

Test Test Dura¬ Panel tion (Hrs.) Load (Lbs.) Load (psi) Failure Mode

1000 1000 250 40% core; 40% cohesion; 20% adhesion (2 ful 1 partial cell lost)

1000 ^• 1380 345 40% core; 5% cohesion; 55% cohesion/adhesio

4 Control 1660 415 60% core; 40% cohesion

Control 1560 390 50% core; 50% cohesion

Control 1680 420 60% core; 40% cohesion

Control 1300 325 adhesive to block failed, but core was starting to tear

1000 1700 425 60% ^' core; 40% cohesion

1000 1660 415 60% core; 40% cohesion

1000 1800 450 50% core; 50% cohesion

1000 1740 435 60% core; 40% cohesion

Example 3

Panels of aluminum alloy type 3003 prepared as set forth in Example 1 were thereafter painted with two paints sold by PPG Industries of Pittsburgh, Pennsylvania, under the trade designations PPG-Duranar and PPG-Duracron. These painted surfaces were found to meet the standards of the Architectural Aluminum Manufacturers Association, "Voluntary Specification for High Performance Organic Coatings on Archi¬ tectural Extrusions and Panels", Publication No. AAMA 605.2. In view of the above, it will be seen that the several objects of the invention are achieved and other advan¬ tageous results attained.

As various changes could be made in the above methods and products without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.