Abstract:
A decal is suitable for application to an aircraft structure that is covered with an incompletely cured base coat of paint. The decal has a polyester film with an adhesive applied to a first side. A release layer is applied to the polyester film and comprises a pattern engaging the adhesive to form a plurality of grooves in the adhesive layer. When the decal is applied to an uncured base coat of paint, offgasses formed during curing and drying of the base coat escape from beneath the decal through the grooves.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 62/031,609, filed Jul. 31, 2014, the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    Primer and paint are often applied to aircraft structure to provide a protective coating, as well as an aesthetically pleasing decorative finish. The primer and paint layers are typically applied to metal or composite structure. In addition to enhancing the ornamental appearance of the aircraft, decals applied to the paint can also provide information, such as warnings, instructions, etc. using symbols and other indicia. 
         [0003]    Decals typically include an adhesive layer to secure the decal to the aircraft structure. When a decal is applied, gasses can form under the decal due to air entrapment between decal and painted surface, the slow curing of the paint underneath, the chemical interaction of the adhesive with the incompletely cured paint surface to which it is applied, or various other chemical reactions, depending upon the chemical properties of the materials utilized. The gasses can in turn form bubbles under the decals that are aesthetically unpleasing and can impact the durability of the decals. Accordingly, there is a need for decals that, when applied to an aircraft structure, minimize or eliminate the formation of bubbles between the decal and the aircraft surface. 
       SUMMARY 
       [0004]    A disclosed embodiment of decal is applied to an aircraft structure that is covered with an incompletely cured base coat of paint. The decal includes a polyester film with an adhesive applied to a first side. A release layer is applied to the polyester film and comprises a pattern engaging the adhesive to form a plurality of grooves in the adhesive layer. When the decal is applied to an incompletely cured base coat of paint, offgasses formed during curing and drying of the base coat escape from beneath the decal through the grooves. 
         [0005]    Also disclosed is an exemplary method of applying a decal to an aircraft structure that is covered with an incompletely cured base coat of paint. The method includes the step of removing a release layer from an adhesive surface of the decal to expose at least one groove formed in the adhesive layer by the release layer. The method further includes the step of applying the decal to the aircraft structure so that the adhesive adheres to the incompletely cured base coat of paint, wherein offgasses under the decal travel through the at least one groove to an edge of the decal. With the decal applied to the structure, a paper or polymer film application mask is removed from the decal, and a sprayed on 2 part urethane clear coat is applied. 
         [0006]    This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0008]      FIG. 1  shows a cross-sectional view of a decal in accordance with one exemplary embodiment of the present disclosure; and 
           [0009]      FIG. 2  shows an isometric view of a release coat of the decal of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIG. 1  shows a cross-sectional view of a decal  10  suitable for application to the outer surface of an aircraft structure  40 . In the illustrated embodiment, the aircraft structure includes an aluminum skin, typically a 6061-T6 or 2024-T3. It will be appreciated that the presently disclosed decals  10  are not limited for use on aluminum skins. In this regard, the decals  10  can be used on any suitable structure made from any generally known material, and the use of the decals with such structures and materials should be considered within the scope of the present disclosure. 
         [0011]    A coat of primer  44  is applied to the cleaned and treated aluminum skin  42 , and a base coat  46  of polyurethane paint is applied over the primer. The primer  44 , which is preferably a 2-part epoxy, inhibits corrosion and also provides a surface to which the base coat  46  of the paint will better adhere. The base coat is preferably a 2-part polyurethane paint, although it will be appreciated that any number of suitable paints and coatings may be utilized. 
         [0012]    The decal  10  is formed on a plastic film  12 , such as, for example, a polyester film. The polyester film  12  acts as the base of the decal  10 . A pressure sensitive adhesive  14  is disposed on one side of the polyester film  12 , and a release coat  16  is applied to the adhesive  14  so that the adhesive is sandwiched between the polyester film  12  and the release coat. 
         [0013]    The release coat  16  is preferably made from silicone or other suitable material to allow for easy removal from the adhesive layer  14 . The release coat  16  has a raised pattern  18  formed on the surface. The pattern  18  includes a series of raised ridges preferably in a honeycomb or diamond shaped pattern when viewed from above; however, it will be appreciated that the pattern is not limited to these shapes and can include any other suitable shapes or combinations thereof.  FIG. 2  shows an exemplary embodiment of a release coat  16  with an embossed honeycomb pattern  18 . 
         [0014]    The pattern  18  on the release coat  16  engages the adhesive layer  14  so that when the release coat  16  is removed from the adhesive layer, indentations from the pattern remain in the adhesive layer. That is, a negative of the pattern  18  is formed on the adhesive layer  14 . The negative of the pattern  18  is defined by a series of grooves, at least some of which extend to the edge of the adhesive layer  14 . 
         [0015]    A liner  20  is attached to the release coat  16  opposite the adhesive layer  14 . The liner  20  is formed from paper, plastic film, or any other material suitable for providing protection to the release coat  16  during manufacture and handling of the decal  10  prior to application of the decal. 
         [0016]    The film  12 , adhesive layer  14 , release coat  16 , and liner  20 , can be assembled as part of the decal manufacturing process. Alternately, commercially available products can be used. Non-limiting examples of suitable products include 3M SCREEN PRINTABLE POLYESTER PERFORMANCE LABEL PRODUCTS 7218SA and 7220SA, both of which include a structured adhesive layer. 
         [0017]    On the surface of the polyester film  12 , a graphic layer  22  is formed. The graphic layer  22  is preferably formed from a polyester ink that has been combined with a crosslinking agent. The use of a crosslinking agent, such as isocyanate for example, improves adhesion as well as resistance to damage from abrasion, heat, and water. 
         [0018]    A polyurethane clear tie coat  24  is applied to the graphic layer  22 . The clear tie coat  24  serves as a tie layer between (1) the sprayed on 2-part polyurethane clear coat  50  (described later) applied to the aircraft surface and (2) the decal  10 , which has been applied to the base coat  46  of the aircraft structure. The tie layer function of the decal&#39;s clear tie coat allows good wet out and adhesion by multiple versions of sprayed on 2-part urethane clear coats  50 . The clear tie coat  24  also provides additional protection to the graphic layer of the decal, but is translucent or, preferably, transparent, to allow the graphic layer to be visible through the clear tie coat. A paper or polymer film application mask  26  is applied over the clear tie coat  24  to protect the decal prior to application and to facilitate application. 
         [0019]    Application of the decal  10  to the aircraft structure  40  will now be described. The decal  10  is applied while the base coat  46  is preferably dry to the touch, but still not completely dried and cured, i.e., the paint feels solid to the human touch, but there are still solvents present in the paint. Also, the chemical reaction between the polyol contained in the paint and the isocyanate mixed with the paint has not come to completion. The liner  20  and release layer  16  are removed from the back of the decal  10 , leaving the grooved adhesive layer  14  exposed. The decal is applied to the structure  40 , and a 2-part polyurethane clear coat  50  is applied over the decal  10  by a sprayer  52 . 
         [0020]    As the base coat  46  continues to dry and cure, paint solvents evaporate and outgas, and carbon dioxide evolves and outgases as a product of isocyanate hydrolysis. Specifically, the reaction of water (ever present except in a vacuum) and the isocyanate in the paint creates carbon dioxide. Additional carbon dioxide may be produced from small side reactions. For existing decals, the carbon dioxide and evaporating solvents can collect under the decal, forming undesirable bubbles. For the presently disclosed decal  10 , the grooves formed in the adhesive layer  14  provide a path to the edge of the decal. The carbon dioxide and evaporating solvents migrate to the edge of the decal through the grooves to minimize or prevent the formation of bubbles. 
         [0021]    In the disclosed embodiment, various trim procedures are optionally performed to facilitate application of the decal  10 . Referring to  FIG. 1 , after all of the layers except for the paper or polymer film application mask  26  have been assembled, the decal  10  is kiss-cut to remove excess material. That is, the layers from the clear tie coat  24  to the adhesive layer  14  are trimmed. The release coat  16  and liner  20  are left untrimmed or are trimmed so that the release coat and liner extend from the perimeter of the adhesive layer  14 . As a result, the pattern  18  on the release coat  16  is at least partially visible around the edges of the decal  10 . The paper or polymer film application mask  26  is then laminated over the decal  10  and kiss-cut to be slightly larger than the decal. 
         [0022]    When the decal  10  is kiss-cut as illustrated, a user can easily peel off the liner  20  (and release coat  16 ) while holding the paper or polymer film application mask  26 . The decal  10  can then be placed on the aircraft structure surface with the paper or polymer film application mask  26  on. The paper or polymer film application mask  26  is very thin and translucent, so the user can see the placement of the decal  10  through the mask. The paper or polymer film application mask  26  also allows a user to smooth out the decal without touching it or the clear tie coat  24 . Once user is satisfied with decal placement, the paper or polymer film application mask  26  is removed, and the 2-part polyurethane clear coat  50  can be applied by sprayer  52 . 
         [0023]    While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. In this regard, while exemplary embodiments are described with reference to decals applied to aircraft structure, it will be appreciated that the present disclosure is not limited to decals for aircraft structure but includes any other application of decals. Further, the chemical properties of the described decal layers and the surfaces to which they are applied are exemplary only and should not be considered limiting. In this regard, the chemical properties of the decal and application surface, and the chemical interaction therebetween, can vary and such variations should be considered within the scope of the present disclosure.