Abstract:
A method of treating a metal material to increase the metal&#39;s ability to adhere to other objects is provided. The metal material is prepared to receive a sol-gel solution coating. A sol-gel solution is prepared and the sol-gel solution is applied to the metal material. Subsequently, an epoxy-based adhesive coating is applied over the sol-gel solution, thereby creating an adhesive layer on the metal material.

Description:
RELATED PATENTS  
       [0001]    The related patents are U.S. Pat. No. 5,814,137 issued Sep. 29, 1998, U.S. Pat. No. 5,849,110 issued Dec. 15, 1998, U.S. Pat. No. 5,869,141 issued Feb. 9, 1999, U.S. Pat. No. 5,939,197 issued Aug. 17, 1999, U.S. Pat. No. 5,958,578, and U.S. Pat. No. 6,037,060 issued Mar. 14, 2000, all of which are incorporated by reference herein.  
       RELATED APPLICATION  
       [0002]    Additionally, this application incorporates by reference application titled “Fiber-Metal Laminate Interphase Coating” invented by Matthew S. Tillman et al.; attorney docket BOEI-1-1039. 
     
    
     
       FIELD OF THE INVENTION  
         [0003]    This invention relates generally to metal surface treatments and, more specifically, to a method of applying an adhesive layer to a titanium foil surface.  
         BACKGROUND OF THE INVENTION  
         [0004]    Metal treatment prior to bonding is a key factor for both the initial adhesion of a bonded joint and its long-term environmental durability. Current metal prebond surface preparations are either inconvenient or complex to use, contain hazardous materials (strong acids, hexavalent chromium, volatile organic compounds), and/or do not provide the performance necessary for successful long-term durable bonds. Past bond failures, primarily due to inadequate surface preparation, have been a limiting factor in the current use of bonded hardware, especially for primary structure.  
           [0005]    The bonding of titanium using standard surface preparation techniques has not always been an easy or reproducible process. The very passive nature of titanium and the difficulty involved in chemical processing of titanium alloys have minimized the use of bonded titanium parts for primary or secondary structure. Several programs have used titanium bonding successfully; however, the surface preparation techniques employed are often arduous and involve hazardous chemicals and processes.  
           [0006]    Therefore, there is an unmet need in the art for an economical and environmentally sound method of preparing metallic materials for bonding with other materials.  
         SUMMARY OF THE INVENTION  
         [0007]    The invention provides an improved method of adhering metallic materials and non-metallic materials with other metallic and non-metallic materials. A method of treating a metal surface to increase the metal&#39;s ability to adhere to other objects is provided. The metal surface is prepared to receive a sol-gel solution coating. A sol-gel solution is prepared and the sol-gel solution is applied to the metal surface. Subsequently, an epoxy-based adhesive is applied over the sol-gel coating, thereby creating an epoxy compatible adhesive layer on the metal surface. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0009]    [0009]FIG. 1 is a flowchart of a metal laminate process according to the invention; and  
         [0010]    [0010]FIG. 2 is an exploded view of a metal laminate. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    The present invention provides a method of metal surface treatment, by either batch or continuous roll process, for durably securing an adhesive layer to a metallic material. By way of overview and with reference to FIGS. 1 and 2, one presently preferred embodiment of the instant invention includes a metal-to-adhesive bond process  20  including a metal surface preparation process  22 , a sol-gel preparation and application process  24  with a subsequent drying step  42 , and an epoxy coating step  26  with subsequent drying step  46 . Specific details of the metal-to-adhesive bond process  20  are described with more detail below.  
         [0012]    The metallic material  54  is preferably titanium or a titanium alloy, and is more preferably a foil Ti-15V-3Cr-3Al-3Sn alloy, and is suitably less than about 0.015 inches thick. However, other metallic materials, such as, without limitation, aluminum, are within the scope of this invention. Additionally, it is anticipated that the present invention is employable with metallic material of any thickness. The process of the instant invention is preferably utilized to form titanium/graphite laminate structures. However, any other fiber/metal or metal/metal laminate such as, without limitation, a titanium/titanium laminate structure is also considered within the scope of this invention.  
         [0013]    Referring now to FIGS. 1 and 2, a presently preferred metal-to-adhesive bond process  20  is shown in FIG. 1. It will be appreciated that many of the steps of the metal surface preparation process  22  are well known in the art. As a result, a detailed explanation of each of the steps in the metal surface preparation process  22  is not necessary for understanding this invention.  
         [0014]    The metal surface cleaning process  22  preferably begins with an alkaline cleaning or aqueous degreasing block  28  to remove surface contamination. Block  28  is generally concerned with initial removal of lubricating oils that may be found on the surface of the metal. It is to be understood that this block  28  is an optional step for a continuous roll method of production depending upon the condition of the metal. Alternately, the block  28  may otherwise be omitted if the metal is not greasy or oily. Then at block  32 , the metal  54  receives an adequate rinse.  
         [0015]    A block  32  deoxidizes the surface of the metal  54 . More specifically, at the block  32  any metal oxide that may be formed on the surface of the metal  54  is removed. Either a chemical deoxidation process or a mechanical deoxidation process is suitably performed at the block  32 . Further, either deoxidation process is suitably employable with both the batch and continuous roll process. However, it will be appreciated that the mechanical deoxidation process may be better suited to the continuous roll process than to the batch process due to the reduced use of hazardous chemicals.  
         [0016]    Chemical deoxidation is preferably performed with an industry standard Hydrofluoric acid or Nitric acid mixture, HF and HNO 3  respectively, wherein the metal  54  is immersed in the HF/HNO 3  mixture for approximately two minutes. Conversely, mechanical deoxidation is suitably performed by either a dry-grit blast abrasion process or a wet-grit blast abrasion process. An additional rinse subsequently occurs at a block  33 .  
         [0017]    The metal  54  then receives a surface conditioning at a block  34 . The metal  54  is immersed in a heated alkaline solution for approximately 5 minutes. The solution temperature range is preferably about 140 degrees Fahrenheit to about 210 degrees Fahrenheit, with a temperature of about 190 degrees Fahrenheit being optimal. In a presently preferred embodiment, the alkaline solution is a dilution of Turco 5578, resulting in an optimal 5%-50% caustic range. The table below is an example of a suitable makeup of the Turco 5578 solution employed in the present invention. At a block  35 , another rinse follows the surface conditioning at the block  34 .  
                                                 TURCO 5578 solution makeup per 100 gallons                    Recommended                   Makeup Volume               Component   (approx. gallons)   Control                       Water   30   —           Turco 5578L   50   25-35 oz./gal.           Water   Balance   —           Temperature   —   175° F.-205° F.                      
 
         [0018]    If desired, an optional step of acidic desmutting at block  36  may be performed next for smut removal. In a presently preferred embodiment, acidic desmutting at the block  36  is not performed. However, depending upon the metal  54  employed, the block  36  may provide desired additional surface treatment. When the block  36  is performed, a rinse follows at block  37 .  
         [0019]    Following the metal surface preparation process  22  is the sol-gel preparation and application process  24 . The sol-gel preparation and application process  24  includes preparation of the sol-gel at a block  38  and application of an aqueous solution of a sol-gel to the metal  54  at a block  40 . In a presently preferred embodiment, the sol-gel is a mixture of a zirconium alkoxide such as zirconium n-propoxide, 3-glycidoxy-propyltrimethoxysilane, glacial acetic acid, and a surfactant. In a presently preferred embodiment, the surfactant is suitably Antarox BL-240 from Rhodia. However, other surfactants are considered within the scope of this invention, such as, without limitation, Tomadol 91-8.  
         [0020]    Application of sol-gel coating to the parts at the block  40  is preferably completed within about 8 hours of completion of surface preparation process  22 . At block  40 , the sol-gel solution is suitably applied to a part by spray-drenching the sol-gel solution onto the surface of the part. The sol-gel solution is preferably sprayed generously to the surface of the part. Excess sol-gel solution is allowed to run off the surface of the part. Preferably, part surfaces should not be allowed to dry and should be drenched with fresh sol-gel solution several times during the sol-gel solution application period. However, it will be appreciated that the surface may be dry. The coated surface is allowed to drain or excess solution is metered off with a suitable metering device, such as without limitation, matched-gap rubber rolls.  
         [0021]    In a presently preferred embodiment, sol-gel-coated metal  54  is preferably oven dried at a block  42  under elevated temperatures for about 3 to about 6 minutes. However, it will be appreciated that exact drying time depends on the configuration of the part. As such, the drying times may be above or below the preferred range.  
         [0022]    After the sol-gel coating is dry, the epoxy coating step  26  is performed. The epoxy coating step includes a block  44  at which an epoxy coating is applied over the sol-gel coating, preferably within 24 hours of completion of the sol-gel preparation and application  24 . In a presently preferred embodiment, the epoxy coating is an epoxy-based spray adhesive.  
         [0023]    In a presently preferred embodiment, the epoxy is applied to the sol-gel coated metal with a High Volume, Low Pressure (HVLP) spray gun. The epoxy is preferably continuous over the surface of the area to be bonded. Epoxy thickness within the bond area is preferably maintained above about 0.00065 inches. At block  45 , the coating is allowed to degass under ambient conditions, preferably for a minimum time of about 30 minutes.  
         [0024]    The metal-to-adhesive bond process  20  has been successfully tested in a batch mode and the metal-to-adhesive bond process  20  is scalable to continuous operation as well. As such, residence times obtained during batch process optimization can be used to determine processing tank size and mechanical processes formalized.  
         [0025]    This results in the following preferable ranges for each block in the process  20 :  
                                                                                 Temperature   Thickness       Block   Processing Agent   Time Range   Range   Range                                28   Degrease or   0.5-30 minutes   75-200° F.   N/A           Clean       32   Chemical   0.5-10 minutes   75-200° F.   N/A           Deoxidation           OR           N/A           180-320 grit wet/   0.1-10 minutes   N/A   N/A           dry blast       34   Alkaline   0.5-30 minutes   75-200° F.   N/A           conditioner       36   Desmut Solution   0.5-30 minutes   75-200° F.   N/A           (optional)       40   Epoxy based   N/A   N/A   0-0.001 inches           SolGel       44   Epoxy adhesive   N/A   N/A   0-0.005 inches           coating                  
 
         [0026]    Continuous processing of titanium foil can be achieved by using various combinations of the above steps. Continuous processing may include a roll-to-roll operation where the titanium foil runs through a cleaner, a deoxidizer (preferably a mechanical deoxidizer such as a wet grit blast), and a surface conditioner such as those described in blocks  28 ,  32 , and  34 , respectively. The titanium foil will then run through a sol-gel deposition step followed by drying of the sol-gel coating and application of the adhesive coating such as those described at the blocks  40 ,  42 , and  26 . The processed titanium foil would preferably be recoiled in a known manner for shipping to a laminate processor.  
         [0027]    [0027]FIG. 2 depicts a preferred embodiment of an epoxy-coated metal material  50  made by the metal-to-adhesive bond process  20 . As illustrated, the epoxy coating  52  is present on both longitudinal sides of the metal  54 . However, it will be appreciated that the epoxy coating  52  can be applied to any surface of the metal  54 . Further, it is to be understood that any number of alternating layers of metal  54  and epoxy coating  52  can be made to form laminate structures of any desired strength or size.  
         [0028]    While the preferred embodiment of the invention has been illustrated and described as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.