Abstract:
A method of treating a surface of a metal foil or sheet to make the surface wettable. The method comprises obtaining a metal foil or sheet having an oil-coated surface to be treated, applying a solution of phosphoric acid in a polar, non-aqueous, water-free solvent evenly to the surface at a rate of application in a range of at least 10 mg/ft 2 , the phosphoric acid being contained in the solution at a concentration such that the phosphoric acid contacts the surface in an amount of 0.5 to 2.0 mg/ft 2 ; and removing the solvent by either drying the surface or heating the foil or sheet at an elevated temperature above ambient to evaporate the oil from the surface to be treated with the proviso that the elevated temperature does not fall in the range between 130 to 240° C. The resulting foil has a wettable surface and can be used, for example, for coating with a layer of another material (e.g. polymer or adhesive) and achieve high peel strength.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the priority right of our prior co-pending provisional application Ser. No. 60/703,125 filed Jul. 27, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     I. Field of the Invention  
         [0003]     This invention relates to the production of metal sheet articles (and particularly foils) having a hydrophilic surface (i.e. a surface that is wettable by a polar liquid, e.g. water). More particularly, the invention relates to methods of imparting wettability to surfaces of sheet articles made of aluminum and aluminum alloys.  
         [0004]     II. Background Art  
         [0005]     Aluminum sheet articles, and particularly aluminum foils (flexible sheets having a thickness in the range of 0.0002 to 0.004 inch), are often required to have fully wettable surfaces for compatibility with various user applications. For example, finstock sheet (which normally has a thickness of 0.004 inch or more) must have a fully wettable surface to allow for proper water drainage in condensers. Sheets having a thickness of 0.002 inch or more are often required to have wettable surfaces to avoid staining in water. Furthermore, metal foils are often intended to be directly printed with inks or directly painted, and this requires wettable surfaces.  
         [0006]     Even more importantly, metal foils may used with other materials to form the basis of multi-layer sheets by direct application of coating materials or by lamination (e.g. lamination with paper, polyester, vinyl or other plastics). Foils of this type are often referred to as “converter foils” because they are destined to be “converted” by customers to a multi-layer sheet or composite (e.g. after passing through a laminator where the foil is sandwiched with another supporting material). The main applications for converter foils are as flexible packaging and sheets required in industrial applications. Converter foils are often made from 1000-series aluminum alloys, e.g. AA1145 and AA1100, and are usually supplied in gauges from 0.0002 to 0.005 inches, so they are consequently quite thin and easy to damage. The surfaces of converter foils should be wettable by aqueous and polar materials, and should be of good appearance, especially when the foils are to be used for packaging purposes. The wettability of the surfaces determines the peel strength of the composites and is therefore a characteristic that requires strict control. The users of the foil normally require full wettability of the surface (so-called “A-wettable” surface quality). However, this is difficult to achieve. Surface wettability is adversely affected by the presence of lubricating oils that are always present on the surfaces of the foils in the condition in which they are obtained from the rolling mill (oils are used in the metal rolling process). To improve adhesion, it is necessary to remove oils from the surface before the foils are supplied to customers of the metal fabricator for coating or lamination procedures.  
         [0007]     A conventional way of removing surface oils and improving surface wettability is to subject sheet article to a prolonged heat treatment (heat anneal) in a furnace at a temperature above 280° for a period of 24-48 hours. Such extended annealing cycles are needed to fully evaporate the oil, but they cause recrystallization of the metal and necessarily convert them to the O-temper (alloys such as AA1145 and AA1100 are completely recrystallized at these temperatures). Foils in the O-temper are quite soft, and are therefore unsuitable, or less desirable, in some applications. Lesser degrees of crystallization require lower temperatures and/or shorter treatment times, but then complete removal of the oil cannot be assured. Thus, the practical result is that the requirement for wettability has limited the application of converter foils, for example, predominantly to O-temper material. It has also resulted in anneal cycles that are significantly longer than those used in other applications, such as the production of plate, fins or containers. If stronger foils or sheets were available, it would reduce damage, and open up the possibility of reducing gauges for many converter applications.  
         [0008]     Chemical cleaning methods are also available to remove surface oil and thereby render the foil or sheet water-wettable. Such methods can be very expensive as they require the use of cleaning towers through which the foil or sheet is processed. Foil, being very thin, can be easily damaged while being processed through this kind of equipment, so maximum speed of travel and other restrictions are strictly enforced, with a consequent reduction of efficiency. Additionally, the chemical pre-treatments may contain chromates, phosphates and other chemicals that can cause the brightness of the foil or sheet to diminish, which is extremely detrimental in converter applications.  
         [0009]     U.S. Pat. No. 2,197,405, issued on Apr. 16, 1940 to Junius D. Edwards, discloses a method of treating aluminum surfaces prior to joining other materials thereto by means of adhesives. This involves the treatment of aluminum surfaces with phosphoric acid or a solution of phosphoric acid in water.  
         [0010]     However, there is a need for improved ways of making the surfaces of aluminum foils wettable without involving prolonged heating cycles.  
       SUMMARY OF THE INVENTION  
       [0011]     An exemplary form of the invention provides a method of treating a surface of a sheet of aluminum or an aluminum alloy to make the surface wettable, the method comprising: applying a solution of phosphoric acid in a solvent to the surface, the solvent being selected from the group consisting of (a) a polar, non-aqueous, water-free solvent, and (b) a mixture of a polar, non-aqueous, water-free solvent and a volatile polar hydrocarbon solvent; and removing the solvent by drying the surface at ambient temperature or by heating the surface at an elevated temperature above ambient, with the proviso that the elevated temperature does not fall in the range of 130 to 240° C.  
         [0012]     The phosphoric acid is preferably contained in the solution at a concentration such that the phosphoric acid contacts the surface in an amount of 0.5 to 2.0 mg/ft 2 , and is preferably applied to the surface in an amount of at least 10 mg/ft 2 .  
         [0013]     The invention also relates to a metal foil treated by the method above. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a schematic diagram showing steps in a preferred method according to the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     It will be appreciated that the term “aluminum” as used in the following is intended to include alloys of aluminum, particularly those used for the formation of converter foil, as well as pure aluminum itself.  
         [0016]     In the following description, reference is made to aluminum foil for the sake of convenience, but the description applies to other sheet articles made of aluminum (except where foil alone is clearly intended by the context).  
         [0017]     In the present invention, use is made of a solution containing phosphoric acid to impart substantially complete wettability to the surfaces of aluminum foils. The foils are generally not pre-treated or pre-cleaned before the application of the solution, and consequently they are treated in the form in which they are received directly from the rolling operation or rolling mill. The treatment solution consists of phosphoric acid dissolved in a water-free polar solvent, preferably isopropanol, and the concentration of phosphoric acid is made quite low. Ideally, the concentration of the solution is such that the amount of phosphoric acid applied to the surface (referred to as the “loading” of phosphoric acid on the surface) is within the range of 0.5 to 2.0 mg per square foot of treated surface, and more preferably 0.5 to 21.5 mg/ft 2 . If the loading is less than 0.5 mg/ft 2 , then uniform wettability over the entire surface of the foil cannot be guaranteed in all cases. If the loading is more than 2.0 mg/ft 2 , the resulting surface quality is poor in many cases. While the loading is, of course, dependent on the amount of solution applied to a surface, as well as to the concentration of the solution, the amount applied is normally constant because excess liquid drains from the surface, so the loading tends to be dependent mainly on concentration.  
         [0018]     The solution may be applied to the foil surfaces by any means, but is preferably applied by means that are compatible with high-speed processing and that leave the foil surface with an undamaged, high-quality finish. Ideally, the method employed is one that avoids direct mechanical contact of a solid article with the foil surface. The method may thus comprise dipping, spraying or coating. The most preferred method is spraying, ideally aerated spraying or ultrasonic spraying (i.e. ultrasonic liquid atomization in which high-frequency sound waves are used to produce a spray of atomized liquid). Ultrasonic spraying has the advantages that it can produce a soft (i.e. small droplet), low-velocity spray that virtually eliminates overspray and cannot damage the foil surface. Ultrasonic spraying also has the advantage that it employs a pressureless operation, and provides freedom from clogging of the spray jets.  
         [0019]     While the application of the solution is not intended to “wash-away” the oil present on a surface (e.g. by producing a significant run-off), the solution must be applied in an amount (and pattern of application) sufficient to ensure 100% coverage of the surface to be treated. This generally means that the rate of application of the solution must be at least 10 mg/ft 2  of treated surface. If it is less than this, full wettability cannot be guaranteed in all cases. However, the solution is preferably applied in an amount of no more than 30 mg/ft 2  to avoid run-off and over-utilization of materials.  
         [0020]     The preferred minimum and maximum loading of phosphoric acid and the preferred minimum and maximum application of the solution define the concentration of phosphoric acid required in the solution employed. For example, if the phosphoric acid loading is to be the minimum of 0.5 mg/ft 2  and the solution application is to be a maximum of 30 mg/ft 2 , the concentration of phosphoric acid in the solution should be 1.66 wt %. If the phosphoric acid loading is to be the maximum of 2.0 mg/ft 2  and the solution is applied at the minimum of 10 mg/ft 2 , the concentration should be 20 wt %. An effective range of concentrations is therefore 1.66 to 20% (w/w), more preferably 1.66 to 5% (w/w).  
         [0021]     The solution is preferably applied at ambient temperature and the foil or sheet itself should preferably be held at low temperature (ambient to at most 50° C.). The solution should preferably be kept in contact with the foil surface at ambient temperature for a holding period of at least 30 seconds before a heating or drying step, described below, is carried out. During this time period, the foil may be coiled. There is no particular maximum time during which the solution may be left in this way in contact with the foil surface, but the heating or drying step is preferably commenced as quickly as possible, so the process should preferably be continued as soon as possible after the 30 second holding period.  
         [0022]     Without wishing to be bound by a particular theory of operation, it is believed that the surface is made wettable by this method for the following reasons. Phosphoric acid is soluble in polar solvents such as alcohols, ketones and a few other organic solvents. The solvents are able to penetrate through the layer of oil on the foil surface and reach the bare surface of aluminum where the phosphoric acid can then act to improve wettability. The acid reacts with the aluminum surface, particularly when the solvent escapes by evaporation (see later) leaving a concentrated layer of phosphoric acid, thereby producing a wettable phosphate layer that renders the surface hydrophilic. A similar effect is not present in water-based or water-containing solutions, as such solutions do not penetrate the oil layer and wet the foil surface. Further, if such solutions were made to wet the surface, e.g. by adding surfactants, they would not dry quickly enough to concentrate the acid in the available time. Water can also undesirably stain the surface of aluminum. While the method of the invention may leave residual oil on the foil surface, it is found that the residual oil is generally incorporated into any coating materials (particularly organic polymers) that are subsequently applied. Moreover, as the oil has been rendered less adherent to the foil surface by the polar solvent, it will evaporate at a lower temperature than otherwise required.  
         [0023]     The preferred polar solvents for use in the present invention are isopropanol, ethanol and methyl ethyl ketone (MEK) as they have suitable ability to penetrate the oil layer and a good rate of evaporation. As noted, the solutions employed are always water-free. Methanol and acetone are also effective, but they are toxic, so if they are used, special precautions have to be taken to avoid contamination of the environment, and this reduces the economic desirability of using these solvents in the process.  
         [0024]     It is also possible to replace part of the polar solvent with a non-polar solvent of low volatility that is totally miscible with the acid polar solvent mixture, such as, for example, a solvent sold under the trademark NORPAR® 13 (a carbon-hydrogen saturated hydrocarbon with an average of 13 carbon atoms and having an initial boiling point of about 222° C. and a dry point of 242° C.) marketed by ExxonMobil Chemical Company. In such a case, the polar solvent may be substituted with the non-polar solvent in the range of 0% to 80% by volume, more preferably 0 to 20% by volume. This increases the volatility of the solution, which is beneficial when applying the mixture onto the metal sheet. However, a disadvantage is greater solvent removal during annealing, so this alternative may not be desirable for all cases.  
         [0025]     Following the application of the solution and coiling (if coiling is desired), the foil is subjected to a heating or drying step. This may take one of two forms, or a combination of both. One form is a low temperature treatment that involves drying the foil at ambient temperature or heating the foil at a temperature above ambient but no higher than 130° C. Drying or heating at such a temperature causes evaporation of any residual polar solvent, but does not cause any annealing of the metal of the sheet or foil. A fully-hardened sheet or foil (i.e. one having a cold-rolled temper) may therefore be obtained by this form of heating or drying. The heating or drying time may be the minimum required to achieve full evaporation of the solution (which may be several hours if the foil is in coil form). Longer heating times are not required, but would not be harmful as the structure of the metal is not changed by the applied temperature. As an example, the sheet or foil may be kept at 100° C. for 4 to 5 hours, which is just adequate to evaporate all of the isopropanol when this material is used as the polar solvent and the foil is treated in the form of a coil having a weight of 2000 pounds.  
         [0026]     Alternatively, in a second form of the heating or drying procedure, which may be referred to as a high temperature treatment, the heating may be carried out at a temperature of 240° C. or above). At this temperature, not only is the polar solvent evaporated fully, but the metal may be partially or fully annealed. If a “fully hard” foil is still required, the heating time should be maintained for just long enough to remove the polar solvent but not long enough to influence the properties of the foil. This may require, for example, 3 to 4 hours at 240° C. Alternatively, if the foil is to be “fully soft”, then heating periods of 8 to 16 hours may be employed at temperatures above 240° C. This period of heating is more than adequate to evaporate the solvent, but is required to achieve the desired metallurgical properties.  
         [0027]     Unless a full O-temper is required, the treatment temperature should normally be kept below about 280° C. and the treatment time adjusted accordingly. The treatment time must again be the minimum time required to achieve full evaporation of the solvent, but more time may be required to achieve a desired metal temper. This makes it possible to obtain partially annealed tempers that can yield significantly stronger foil than the totally recrystallized metal.  
         [0028]     In the case of the high temperature treatment, the maximum temperature should generally not exceed 400° C. as this may cause deterioration of the shape of the coil and other damage to the foil itself.  
         [0029]     If desired, both the low temperature form of heating (drying) and the high temperature form may be carried out sequentially. In other words, heating at a temperature of 130° C. or less may be carried out to fully evaporate the polar solvent and to allow the phosphoric acid to work on the surface to make it wettable. The foil may then be heated at a temperature of 240° C. or more to produce a desired transformation of the metal temper. If appropriate, the foil may be stored for an indefinite period of time, ideally in the coiled condition, between these two forms of heating.  
         [0030]     Heating at a temperature in the intervening range of 130-240° C., i.e. between the temperatures employed in the low and high temperature forms, is to be avoided (except for the brief time the foil takes to heat up to 240° C.) because foil heated at temperatures within this range become hydrophobic and the use of such temperatures is thus counter-productive. Heating rates normally employed to raise the temperature to the range employed in the second form do not result in harmful residence times in the intervening range, so no special concern about heating rates is required, provided the temperature is allowed to rise without interruption through the intervening range.  
         [0031]      FIG. 1  of the accompanying drawings sets out the steps of the method in more graphic form. The FIGURE is believed to be self-explanatory, except that “ALTERNATIVE A” shows the option of heating (drying) at 130° C. or less, and “ALTERNATIVE B” shows the option of heating at 240° C. or above.  
         [0032]     The invention is further illustrated by the following Examples, which are not intended to be limiting.  
       COMPARATIVE EXAMPLE 1  
       [0033]     H19 aluminum foil of 0.002 inch gauge was immersed in isopropanol and the excess was removed by shaking. After one minute at ambient temperature in air, the foil was rendered dry. The wettability of the foil was tested by placing a drop of water on the surface. It was completely non-wettable.  
       COMPARATIVE EXAMPLE 2  
       [0034]     H19 aluminum foil was placed in an oven at 100° C. and was heated for 40 seconds. It was completely non-wettable.  
       EXAMPLE 1  
       [0035]     H19 foil was treated with 0.05% (w/w) solution of phosphoric acid in isopropanol. The aluminum sample was immersed in this solution, removed and the excess was then removed by shaking. The foil was then air-dried. The foil became A-wettable. The application of the solution to the surface was about 2 g per square foot. This means that about 0.4 mg per square foot of acid was left on the foil surface after evaporation of the solvent.  
       EXAMPLE 2  
       [0036]     The foil produced in Example 3 was heated at 100° C. for periods of 40 seconds to 2 hours. It remained wettable.  
       COMPARATIVE EXAMPLE 3  
       [0037]     The foil produced in Example 3 was heated at 200° C. The sample then became non-wettable.  
       EXAMPLE 3  
       [0038]     A solution of 44 vol % NORPAR®, 54 vol % water and 2 vol % phosphoric acid solution was sprayed onto a non-wettable aluminum foil. The foil became hydrophilic after one minute.