Abstract:
An ink transfer system including an ink transfer decal that is transferable to a final substrate, such as a bare metal or pre-painted autobody part, without use of a separate adhesive. The transferred ink decal is capable of withstanding temperature elevations common during automotive paint baking without significant thermal degradation, discoloration, shrinkage or pealing.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to methods of transferring paint or ink to solid surfaces and more particularly to a method of transferring a substantially solid ink layer to a surface without the use of a separately applied adhesive. 
     In the vehicle manufacturing industry, painted or inked designs are typically applied through application of liquid paint to the surface of a body element, such as a fender or hood. An example of this type of application is pinstriping. Manual application of stripes or designs requires artistic skill, even if the process is augmented through use of stencils or tape. Airbrushed designs are very labor intensive and require considerable skill. 
     One prior art method of automating the application of designs or stripes is the use of adhesive decals. A decal is created by applying ink to a plastic film bearing adhesive on the opposite surface. The adhesive side of the decal may then be applied to the desired surface. Further protection may be achieved by applying a clear, protective, thermoset or photoset coating over the surface to seal the decal against the elements. 
     These prior art decals have very limited elongation properties and are difficult to apply to highly curved, edged or contoured surfaces. Further problems with prior art decals include separation of the decal from the substrate surface due to failure of the adhesive, and shrinkage, peeling and cracking of the decal itself over time. In addition, the decal typically presents a raised surface that can be felt and observed even after application of a clear coat. The raised decal detracts from the overall appearance and makes the decal prone to increased abrasion, chipping and peeling. Edges of prior art decals are prone to curl after exposure to heat during baking of a thermoset clear coat. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention presents an ink transfer system including a decal that may be subjected to high temperatures and UV radiation. The decal is provided as an alternative to hand painted detail trim for vehicles and accessories such as automobiles, motorcycles, boats, boat motors and helmets. The decal may be applied to various metallic, plastic or painted surfaces and is particularly advantageous when applied to pre-painted metal structures that are subjected to baking in order to cure overlying thermoset clear coats. 
     An ink transfer system according to the present invention may include one or more layers of temperature and UV resistant ink deposited upon a backing sheet by screening or other currently known or later developed techniques. If a waterslide decal is desired, the ink may be deposited upon waterslide decal paper having a water soluble coating. A transfer medium is deposited over the ink to aid in transfer of the ink to a final receiving substrate. Because the ink is typically deposited upon the backing sheet to form a thin film, the transfer medium aids transfer of the decal from the backing sheet to the substrate without wrinkling or damaging the decal. The transfer medium may be applied over the ink layer through screen printing and it becomes an integral part of the transfer system. Because the transfer medium is designed not to bond to the ink, it is removed prior to application of a subsequent coating such as a clear coat. 
     The decal exhibits high flexibility and elongation and may be applied over contoured surfaces. Alternatively, the decal may be applied to a flat surface that is later bent, stamped or otherwise shaped. 
     Antioxidants and UV absorbers/light stabilizers are added to the base resin to enhance heat and light stability and to increase tackiness and plasticity of the ink, particularly during heat curing of an overlying clear coat material as is often the case in automotive applications. Although the decal is solid once formed, the ink comprising the decal adheres particularly well to metal and most painted and plastic surfaces due to the low molecular weight polymers comprising the base resin (polymer vehicle). The addition of the antioxidants tends to suppress polymerization enhancing the ability of the polymer vehicle to migrate into the adjacent substrate and form a tight bond. The decal therefore provides a means for transferring solid ink to a substrate without the need for a separate, intervening adhesive layer. When applied to a pre-painted surface, the decal adheres to and binds with the underlying paint to form a uniform coating. Due to the thin layers of ink used to form the decal, the margins of the decal do not noticeably project above the substrate surface and are therefore unlikely to chip or peel, particularly if covered with a clear coating. 
     Other advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings, wherein is set forth by way of illustration and example, embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatical cross sectional view of a decal formed in the manner described herein. 
         FIG. 2  is a diagrammatical cross sectional view of a decal formed in the manner described herein. 
         FIG. 3 . is a top perspective view of a base coat applied to a backing sheet. 
         FIG. 4  is a top perspective view of an inked design applied over a portion of the base coat. 
         FIG. 5  is a top perspective view of a layer of transfer medium applied to, and extending past the margins of, the inked design and base layer. 
         FIG. 6  is a top perspective view of a waterslide decal formed in the manner described herein, the edge of the backing sheet of the decal being positioned against a selected substrate prior to decal transfer. 
         FIG. 7  is a top perspective view of the decal of  FIG. 6  in which the decal has partially slid off of the backing sheet and onto the substrate surface. 
         FIG. 8  is a top perspective view of the decal of  FIGS. 6 and 7  in which the decal has been fully removed from the backing sheet and has been oriented in final position upon the substrate. 
         FIG. 9  is a top perspective view of the decal of  FIGS. 6-8  in which the transfer medium has been partially peeled away from the decal and substrate surfaces. 
         FIG. 10  is a top perspective view of the decal of  FIGS. 6-9  in which the transfer medium has been fully removed and the decal and substrate are ready for application of a clear over coat. 
         FIG. 11  is a top perspective view of the decal of  FIGS. 6-10  after application of the clear over coat. 
         FIG. 12  is a diagrammatical cross sectional view of a decal formed in the manner described herein. 
     
    
    
     DETAILED DESCRIPTION 
     As required, a detailed embodiment of the present invention is disclosed herein; however, it is to be understood that the disclosed embodiment is merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. 
     An ink transfer system according to the present invention provides for the application of liquid, polyurethane based inks onto a backing sheet to form an ink transfer decal. The decal is coated with a removable transfer medium. The resultant decal is solidified, typically through evaporation of diluents after application of each layer of liquid constituent. It may then be removed from the backing sheet and applied to a final substrate. 
     Through the use of low molecular weight aliphatic polymers and antioxidants for polymerization suppression, an ink is provided that upon heating (during baking of a subsequently applied clear coat) becomes plasticized and tacky thereby obviating the need for application or use of a separate adhesive between the decal and the substrate surface. Due to the migration of low molecular weight polymers into the substrate surface, the decal binds to the substrate surface. In the case of pre-painted surfaces, particularly those painted with polyurethane based paints, the decal essentially becomes part of the painted coating. 
     The provision of a decal comprising thin films of high opacity ink is highly beneficial as it enables rapid application of complex designs to substrate surfaces without the inefficiencies of hand painting while also avoiding problems related to prior art adhesive decals such as peeling from the substrate due to failure of the adhesive; degradation upon subjection to elevated temperatures during baking of an overcoat such as a clear coat or finish; peeling, cracking or curling due to shrinkage; and increased subjection to abrasion or separation from the substrate due to projection of the prior art decal above the substrate surface. The ink transfer system of the present invention avoids these disadvantages by providing a thin film decal that self-bonds or adheres to the substrate and does not perceptibly project above the substrate surface. Indeed, upon proper application of the decal and subsequent covering with a clear coat finish, no transition can typically be felt by touch or observed by the naked eye due to location of a decal of the present ink transfer system upon a typical substrate surface. 
     Decal Preparation 
     Referring now to the drawings,  FIG. 1  illustrates a diagrammatical cross sectional view of a ink transfer system  100  formed in accordance with the present invention for application of solidified ink to a substrate  101  (see  FIGS. 6-11 ), such as a painted automobile, motorcycle or boat body panel or similar surface. The thickness of the various components are exaggerated in  FIGS. 1 and 2  to aid in illustration. 
     The ink transfer system  100  includes a backing sheet  102  and transfer medium  104  enclosing transparent base coat  106  and pigmented ink  108  layers, the latter two components comprising the ink transfer decal  109 . The transfer medium  104  may also be referred to as a premask. The base coat  106  and ink  108  may be referred to herein collectively as ink, inks or ink layers since the base coat  106  is formulated with similar constituents as an ink and may be formulated generally as an unpigmented ink. In addition, the base coat  106  may comprise a pigmented ink if enhanced opacity is required. The primary constituents and formation of the ink transfer system  100  are as follows. The backing sheet  102  has an upper surface adapted for receiving solvent-based polymer inks  108  and base coats  106 . An appropriate backing sheet  102  is formed of material that will accept the base coat  106  and ink  108  as they are screened upon the sheet  102  without reacting with the inks or softening under the influence of the ink or ink diluents such as organic solvents. The upper surface of the backing sheet  102  is typically provided with a thin coating of water soluble material that allows the decal  109  to be later transferred to a substrate  101  using a waterslide technique. 
     Typically, although not necessarily, a base coat  106  is screen printed onto the backing sheet  102  to form an initial layer or coat that helps hold subsequent layers of pigmented ink  108  in proper position to one another. If layers of ink are physically separated from one another, such as with spaced bands  108   a ,  108   b  and  108   c  of pinstriping as shown in  FIG. 2 , the base coat  106  provides a means for the individual bands  108   a - 108   c  to be joined to one another to form a single ink transfer decal  109 . 
     After the base layer  106  is applied it is dried to evaporate diluents, typically organic solvents. Next, one or more layers of pigmented ink  108  are typically screened onto portions of the base layer  102 . Typically, the pigmented ink  108  is applied so that a portion of the base coat  106  extends beyond the margins of the pigmented ink  108 . If the substrate  101  to which the resulting decal  100  is to be transferred is dark in color, the base coat  106  may contain light-colored (such as white) pigment to provide an opaque, light background to enhance the brightness of overlying layers of colored ink  108 . 
     After solvents or diluents have substantially evaporated from the ink layer or layers  108 , a release coat or layer  110  may be applied to cover and extend beyond the margins of the ink  108  and base  106  layers. As with the base coat  106 , the release coat  110  may be formulated generally as an upigmented ink  108 . In addition, a release agent may be added to the release coat  110  formulation to aid in the later separation of the transfer medium  104  from the decal  109 . 
     After solvents or diluents have substantially evaporated from each previous layer or coat, the transfer medium  104  may be applied over the base coat  106 , ink layers  108  and release coat or layer  110  (see  FIGS. 1 ,  2  and  12 ). The transfer medium  104  is typically applied so that it extends beyond the margins of the base layer  106 , ink layer  108  and release coat  110 . The transfer medium  104  is typically a clear, flexible coating that protects the decal  109  during storage and greatly aids the dimensional stability of the decal  109  during transfer to a substrate  101  while still allowing sufficient flexion and elongation of the decal  109  for application over, and conformation to, irregularly shaped substrate surfaces. As with the previously applied ink  108 , base  106  and release  110  layers, the transfer medium  104  may be applied by screen printing, rolling, spraying or other currently known or later developed techniques. A coarser mesh may be used when screen printing the transfer medium  104  in order to deposit a somewhat thicker layer than was deposited when screen printing the base coat  106  and ink layer  108 .  FIGS. 1 and 2  illustrate the transfer medium  104  applied directly over the ink  108  and base  106  layers.  FIG. 12  illustrates a release layer  110  disposed between the ink layer  108  and transfer medium  104 . It should be appreciated that  FIGS. 1 ,  2  and  12  are diagrammatical to show succession of component layers and do not reflect relative dimensions of components, particularly vertical dimensions or thickness. In addition, it should be appreciated that the component layers are applied successively as liquids and therefore conform closely to the underlying surfaces. For example, voids shown between spaced bands of ink  108   a ,  108   b  and  108   c  would in fact be filled by the transfer medium  104  and the portions of the transfer medium  104  extending beyond the ink  108  and base coat  106  layers would conform to exposed portions of the base coat  106  and backing sheet  102 , respectively. 
     After screen printing, the transfer medium  104  is dried to evaporate solvents after which point the ink transfer system  100  has been formed and may be stored until required for transfer of the decal  109  to a substrate  101 . At each stage of solvent evaporation discussed above, the decal  109  and transfer medium  104  may be dried, for example, by passing the system  100  through a wicket drier or by inserting it into a drying rack. Drying for one to two hours is typically optimal but decals  109  may be produced with reduced dry times particularly if the decal  109  is exposed to increased air flow and mild heat such as with a blow dryer held approximately six inches from the decal surface. 
     Any of the ink or base coat layers  108  and  106  may be transparent, translucent or opaque, however, the base coat  106  is typically transparent unless subsequent coats of ink  108  require an opaque underlying layer to allow the pigment of the ink  108  to more readily show against the color of the substrate. For example, if the decal  109  is to be transferred to a dark colored substrate, a white or otherwise light-colored base coat  106  may be desired so that the coloration of subsequent coats of pigmented ink  108  may be more apparent. 
     Decal Formulations 
     The following examples are provided to further disclose the invention. In these examples as well as throughout the specification and in the claims, unless otherwise indicated, all parts and percentages are by weight. The constituents of the ink  108  and base coat  106  formulas are selected to provide improvements on the prior art such as heat resistance, chemical resistance, resistance to degradation due to exposure to light (particularly ultraviolet light), high flexibility and elongation characteristics, high tack (particularly during baking after transfer to a substrate) and self-adhesion. 
     In the first column in each table the components are identified. The next column provides the percent by weight of each component as a constituent of the example formulation. The third column provides approximate percent ranges for each component. These percent ranges do not represent limits for each component but are included as indicative of preferable amounts. Should one or more components be substituted with an equivalent compound it is understood that the relative quantity of the other components may need to be adjusted to optimize the formulation for a particular application or to enhance selected film characteristics. It should be appreciated that evaporation of diluents and/or solvents includes evaporation under ambient conditions as well as through application of heat and/or forced air. 
     By way of example, an initial vehicle (also referred to as a clear varnish) may be prepared as a constituent of the base layer  106 , as well as of the pigmented ink layers  108 , according to the following formulation scheme: 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Vehicle Formulation 
               
             
          
           
               
                 Component 
                 % by weight 
                 range 
               
               
                   
               
               
                 Thermoplastic polyester polyurethane based resin 
                 25 
                 20-30 
               
               
                 such as resins comprising diphenylmethane 
               
               
                 diisocyanate. 
               
               
                 Solvent system comprising: 
               
               
                 Ethyl 3 ethoxypropionate 
                 39 
                 35-45 
               
               
                 Cyclohexanone 
                 19 
                 15-25 
               
               
                 Diacetone alcohol 
                 17 
                 15-25 
               
               
                   
               
             
          
         
       
     
     To prepare the vehicle, the solvent system is first prepared by mixing the above solvents in an appropriate container. The resin is added to the solvents and the contents of the container are heated to the melting point of the resin, typically 100° F. to 140° F. The solvents and resin are stirred until the resin has melted and mixed thoroughly with the solvent. The sides of the container may be scraped while mixing to insure a homogeneous formulation. 
     The formulation of a clear base layer  106  may be prepared according to the following formulation scheme. Note that the base layer  106  formulation may be prepared essentially as unpigmented ink. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Base Layer Formulation 
               
             
          
           
               
                 Component 
                 % by weight 
                 range 
               
               
                   
               
             
          
           
               
                 Vehicle (see above) 
                 92 
                 85-95 
               
               
                 Resin flow modifier such as Modaflow produced 
                 2 
                 1.5-2.5 
               
               
                 by Solutia Inc. 
               
               
                 UV absorbers such as substituted hydroxyphenyl- 
                 3 
                 1-4 
               
               
                 benzotriazoles, hydroxyphenyl-s-triazines, 
               
               
                 hydroxy-benzophenones and oxalic anilides, for 
               
               
                 example Tinuvin 1130 or Tinuvin 400. 
               
               
                 Hindered amine light stabilizers such as those 
                 1 
                 0.5-1.5 
               
               
                 derived from 2,2,6,6-tetraalkyl piperidine, 
               
               
                 substituted piperizinedione, and/or decanedioic 
               
               
                 acid, for example Tinuvin 123. 
               
               
                 A high molecular weight sterically hindered 
               
               
                 phenolic antioxidant such as: 
               
               
                 Irganox 1010 
                 1 
                 0.5-1.5 
               
               
                 Irganox 1035 
                 1 
                 0.5-1.5 
               
               
                   
               
               
                 Note: 
               
               
                 Tinuvin 1130, Tinuvin 400, Tinuvin 123, Irganox 1010, and Irganox 1035 are produced by Ciba Specialty Chemicals Corporation. 
               
             
          
         
       
     
     Alternatively, a clear base layer  106  may be prepared without UV stabilizers and antioxidants as follows. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Alternative Base Layer Formulation 
               
             
          
           
               
                   
                 Component 
                 % by weight 
                 range 
               
               
                   
                   
               
             
          
           
               
                   
                 Vehicle (see above) 
                 98.5 
                 97.5-100  
               
               
                   
                 Resin flow modifier 
                 1.5 
                 1.0-2.5 
               
               
                   
                   
               
             
          
         
       
     
     As an example of a pigmented ink  108 , a black pigmented ink may be prepared according to the formula set forth in Table 4. For other colors, the percent of the selected pigment added may be modified as required to obtain the desired color and to increase or decrease opacity of the resultant ink. 
                                                 TABLE 4                   Pigmented Ink Formulation            Component   % by weight   range                    Vehicle (see above)   76.3   50-90       Dibasic ester (solvent)   5   3-8       Black pigment   7.2   5-8       Resin flow modifier   1.5   1.5-2.5       Vehicle (see above) added as required up to   10    0-10       approximately 10% by weight to optimize       viscosity and/or opacity.                    
The solvent and pigment may be premixed with a smaller portion of the vehicle, such as 23% by weight, to aid in dispersion and solubilization or suspension of the pigment. Typically, the premix is transferred for grinding and dispersion of pigment in a three-roll mill. After the premix has been ground in the mill to form a relatively homogeneous dispersion with particulates in the range of 0 to 3 microns, the premix is removed from the mill and mixed with the remaining clear varnish.
 
     The percentages given for the black pigment are not necessarily representative for other pigment colors. For example, when preparing a white ink it may be advantageous to add white pigment in the range of 30 to 50% of solids. 
     A top release coating or layer  110  may be applied over the ink layer  108 . A release layer  110  formulation may be formulated according to the formula described in Table 3 for the alternative base layer to comprise 98.5% vehicle and 1.5% flow modifier. One or more additional components may be utilized to enhance release of the transfer medium, for example a release layer  110  formulation may comprise 95.55% weight vehicle as described in Table 1, 1.45% weight flow modifier (such as the Modaflow product described above) and 3.0% weight release and flow agent. An acceptable release and flow agent includes EFKA-7375 supplied by EFKA Additivies B.V., the Netherlands. 
     Once formulated, the release layer  110  may be applied over the ink layer or layers  108  and dried, or allowed to dry, to evaporate the diluent. The release layer  110  is typically applied via screen printing and is typically applied to extend beyond the margins of the ink  108  and base  106  layers. The release layer  100  increases the overall thickness of the decal  109  and enhances the handleability and dimensional stability of the decal  109 . The transfer medium  104  may then be applied over the release layer  110 , the release layer  110  remaining adhered to the ink layer  108  while enhancing release of the transfer medium  104  from the decal. 
     Descriptions of Vehicle, Base Layer and Ink Components 
     Polyurethane Resin 
     The above formulations include a thermoplastic polyester polyurethane as prepared in the vehicle formulation. Aliphatic polyurethanes such as polyester polyurethanes are advantageous as they exhibit greater stability in outdoor conditions because they tend to degrade less under exposure to weathering and UV light than aromatic polyurethanes. Linear polyurethanes such as those employing 4,4′ diphenylmethane diisocyanate, 4,2′ diphenylmethane diisocyanate and/or 2,2′ diphenylmethane diisocyanate can be formulated to yield a thermoplastic polyester polyurethane with high flexibility and elongation characteristics. An appropriate commercial product includes Estane 5715 manufactured by Noveon, Inc. 
     UV Stabilizers 
     UV light can cause degradation and discoloration in ink films. UV light is absorbed by constituent polymers causing bond cleavage and release of free radicals leading to depolymerization. In some cases, excessive crosslinking also occurs causing embrittlement. Therefore, UV stabilizers such as UV absorbers and hindered amine light stabilizers have been added to the disclosed ink and base coat formulations to increase longevity of the decals during exposure to outdoor light and the high intensity UV radiation that may be used to cure photoreactive over-coatings such as clear coats. UV absorbers such as substituted hydroxyphenyl-benzotriazoles, hydroxyphenyl-s-triazines, hydroxy-benzophenones and oxalic anilides may be added to the ink and base coat formulations. Examples of appropriate commercial products include Tinuvin 1130 and Tinuvin 400 produced by Ciba Specialty Chemicals Corporation. 
     Further protection from degradation due to UV radiation may be obtained through the addition of hindered amine light stabilizers (HALS) such as those derived from 2,2,6,6-tetraalkyl piperidine, substituted piperizinedione, and/or decanedioic acid. The HALS used with the above exemplary formulations comprises bis-(1-octyloxy-2,2,6,6-tetramethyl-4-piperidyl)-sebacate. Examples of appropriate commercial products include Tinuvin 123 produced by Ciba Specialty Chemicals Corporation. 
     Antioxidants 
     Antioxidants such phenolic based anti-oxidants are used to hinder oxidation of ink  108  and base coat  106  polymers and in particular are helpful to hinder thermally induced oxidation during exposure of the decal  109  to elevated temperatures. In addition to protecting the decal  109  during exposure to heat, the antioxidants suppress polymerization during curing at ambient temperatures thereby increasing the flexibility and elongation characteristics of the decal  109  as well as essentially eliminating shrinkage and associated problems in the prior art such as peeling and cracking. During baking of the substrate material, after transfer of the decal  109  and application of the clear over coat, the decal  109  softens due to its thermoplastic qualities and takes on properties of a resinous adhesive. At this time, the polymerization suppression of the antioxidants causes the ink  108  and base coat films  106  of the decal  109  to become tacky and to bind to one another, the substrate  101  and the clear coat. Once baking of the clear coat is completed and the substrate  101  is allowed to cool to ambient temperatures, the resultant finish comprises coatings that have substantially fused. Suitable phenolic antioxidants include 3,5-bis(1,1-dimethylethyl)-4-hydroxybenzene propanoic acid,2,2-bis[[3-[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]-1-oxopropoxy]methyl]1,3-propanediyl ester and benzenepropanoic acid, 3,5-bis(1,1-dimethylethyl)-4-hydroxy-,thiodi-2,1-ethanediyl ester. Suitable commercial antioxidant products for use in ink  108  and base coat  106  formulations include Irganox 1010 and Irganox 1035 produced by Ciba Specialty Chemicals Corporation. 
     Flow Agents/Modifiers 
     As indicated above, a flow modifier may be added to the base coat and ink formulations. Flow modifiers, such as Modaflow resin, enhance film properties by aiding in pigment dispersion, facilitating evaporation of solvents and release of entrained air, improving adhesion of the ink film  108  to the substrate  101  or prior ink coatings, including base coat  106 , and reducing surface imperfections of the cured decal films  109 . 
     Flow modifiers increase the surface wetting and flow characteristics of the ink as it is screened on to the substrate, enhancing the ink&#39;s ability to self level to the substrate surface and enabling the production of thinner decal films  109 . Flow modifiers are typically added in the range of 0.1% to 2% of total resin solids but may be adjusted to levels outside of this range to optimize flow characteristics for a given ink formulation. 
     A solvent system comprising ethyl 3 ethoxypropionate, cyclohexanone, and diacetone alcohol in the ratio disclosed in Table 1 may be used in formulating the vehicle. Dibasic ester may be added to enhance formulation of a pigmented ink. 
     Pigments selected to impart color to an ink should be selected for compatibility with other ink components, in particular the polymer resin, and for other advantageous characteristics such as opacity, gloss, bleed resistance and resistance to thermal and light degradation. Examples of appropriate pigments for use in the disclosed ink formulations include pigments for liquid inks supplied by Sun Chemical Corporation such as red pigment, product No. 228-0013, and violet pigment, product No. 288-0022 which are disclosed in product literature as being heat fast to at least 500°. 
     Description of Transfer Medium Formulation 
     The transfer medium may be prepared according to the following formulation scheme: 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Transfer Medium Formulation 
               
             
          
           
               
                 Component 
                 % by weight 
                 range 
               
               
                   
               
             
          
           
               
                 Propylene glycol methyl ether acetate (solvent) 
                 40 
                 25-50 
               
               
                 Propylene glycol methyl ether (solvent) 
                 17 
                 10-25 
               
               
                 Diacetone alcohol 
                 10 
                  5-15 
               
               
                 Resin flow modifier 
                 3 
                 1-6 
               
               
                 Nitrocellulose 
                 10 
                  5-15 
               
               
                 Polyvinyl acetate resin 
                 20 
                 10-30 
               
               
                   
               
             
          
         
       
     
     The transfer medium  104  is a film applied in liquid form over the base coat  106  and ink  108  layers. After drying, the transfer medium  104  enhances the handleability of the decal which would otherwise be quite thin and subject to folding over or wrinkling during transfer to the substrate  101 . The transfer medium  104  is typically applied to form a thicker film than the individual ink  108  or base coat  106  films. The transfer medium  104  disclosed in Table 5 is a film comprised of polyvinyl acetate and nitrocellulose that are formulated in a solvent system comprising propylene glycol methyl ether acetate, propylene glycol methyl ether, and diacetone alcohol. These solvents are selected to solubilize and compatibilize the nitrocellulose and polyvinyl acetate. A flow modifier (such as the Modaflow product described above) is added primarily to enhance surface wetting and flow characteristics of the transfer medium formulation. The transfer medium  104  typically does not contain additives to enhance UV or thermal stability because it is removed after application of the decal  109  to the final substrate  101  and discarded. 
     Polyvinyl acetate, such as polyvinyl acetate beads from McGean, product no. ASB-516, are added to the transfer medium  104  as a binder and for improved adhesion to metal and plastic surfaces. Other advantageous characteristics imparted or enhanced by the addition of polyvinyl acetate to the formulation include thermoplasticity, flexibility, solubility in selected solvents, resistance to yellowing, glossiness, transparency, and resistance to oil, grease, and abrasion. 
     Nitrocellulose is a film forming polymer and enhances the ability of the transfer medium  104  to become a thin, strong film upon curing. In general, polyvinyl acetate enhances the elasticity of the transfer medium film  104 , while nitrocellulose enhances the rigidity and strength of the transfer medium film  104 . 
     An alternative embodiment of the transfer medium  104  may comprise a release agent for enhanced release and separation of the transfer medium  104  from the ink layer  108  or layers. This alternative transfer medium  104  may be prepared according to the following formulation scheme: 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Alternative Transfer Medium Formulation 
               
             
          
           
               
                   
                 Component 
                 % by weight 
                 range 
               
               
                   
                   
               
             
          
           
               
                   
                 Acrylic methacrylate resin 
                 34.8 
                 25-40 
               
               
                   
                 Plasticizer 
                 10 
                  7-12 
               
               
                   
                 Release agent 
                 3 
                 1-5 
               
               
                   
                 Solvent system comprising: 
               
               
                   
                 Propylene glycol methyl ether acetate 
                 33 
                 25-35 
               
               
                   
                 Propylene glycol methyl ether 
                 12.2 
                 10-15 
               
               
                   
                 Diacetone alcohol 
                 7 
                  5-10 
               
               
                   
                   
               
             
          
         
       
     
     The alternative transfer medium formulation described in Table 6 is comprised of an acrylic methacrylate resin, such as BR-201 (a butyl methacrylate thermoplastic acrylic resin) supplied by Dianal America, Inc., formulated in a solvent system comprising propylene glycol methyl ether acetate, propylene glycol methyl ether and diacetone alcohol. Typically, the solvent system is first prepared by transferring specified solvents or equivalents to an appropriate mixing vessel. A plasticizer and a release agent may then be added. An acceptable commercial plasticizer includes Paraplex G-60 supplied by The C.P. Hall Company. An acceptable release agent includes Additol VXL 6383 supplied by Signet Chemical Company. The solution is then heated to approximately 140° F. or typically at least to the resin melting point (BR-210 typically melts at temperatures from 110 to 120° F.). The resin is then added while mixing the solution and the resulting formulation is stirred until components are in solution and thoroughly mixed. 
     A further embodiment of the transfer medium  104  may be prepared according to the following formulation scheme: 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Alternative Transfer Medium Formulation 
               
             
          
           
               
                   
                 Component 
                 % by weight 
                 range 
               
               
                   
                   
               
             
          
           
               
                   
                 Acrylic methacrylate resin 
                 28.48 
                   
               
               
                   
                 Plasticizer 
                 8 
               
               
                   
                 Release agent 
                 3 
               
               
                   
                 Solvent system comprising: 
               
               
                   
                 Propylene glycol methyl ether acetate 
                 37.38 
               
               
                   
                 Propylene glycol methyl ether 
                 14.24 
               
               
                   
                 Diacetone alcohol 
                 8.9 
               
               
                   
                   
               
             
          
         
       
     
     The alternative transfer medium formulation described in Table 7 is comprised of an acrylic methacrylate resin, such as BR-201 (a butyl methacrylate thermoplastic acrylic resin) supplied by Dianal America, Inc., formulated in a solvent system comprising propylene glycol methyl ether acetate, propylene glycol methyl ether and diacetone alcohol. Typically, the solvent system is first prepared by transferring specified solvents or equivalents to an appropriate mixing vessel. The release agent such as a silicone-free, fluorocarbon-modified polymer may then be added and the solution mixed. The solution is then heated to approximately 140° F. or typically at least to the resin melting point (BR-210 typically melts at temperatures from 110 to 120° F.). The resin is then added while mixing the solution, as is the plasticizer. The resulting formulation is stirred until components are in solution and thoroughly mixed. An acceptable commercial plasticizer includes Paraplex G-60 supplied by The C.P. Hall Company. An acceptable release and flow agent includes EFKA-7375 supplied by EFKA Additivies B.V., the Netherlands. 
     Yet a further embodiment of the transfer medium  104  may be prepared by adding 3% by weight of the release agent identified above as Additol VXL 6383 in accordance with the following formulation scheme: 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 Alternative Transfer Medium Formulation 
               
             
          
           
               
                   
                 Component 
                 % by weight 
                 range 
               
               
                   
                   
               
             
          
           
               
                   
                 Acrylic methacrylate resin 
                 27.52 
                   
               
               
                   
                 Plasticizer 
                 8 
               
               
                   
                 Release agent (EFKA-7375) 
                 3 
               
               
                   
                 Release agent (Additol VXL 6383) 
                 3 
               
               
                   
                 Solvent system comprising: 
               
               
                   
                 Propylene glycol methyl ether acetate 
                 36.12 
               
               
                   
                 Propylene glycol methyl ether 
                 13.76 
               
               
                   
                 Diacetone alcohol 
                 8.6 
               
               
                   
                   
               
             
          
         
       
     
     Dispersion of solids in the above formulations may be evaluated using a grind block or smear test. Preferably, mixing is performed until solids are tested as being under approximately 3 microns in size. 
     Decal Application/Transference of the Ink to a Substrate 
     Once formed, an ink transfer system  100  may be stored until needed. The decal  109  may be transferred to a final substrate  101 , such as a pre-painted automotive body part, according to the method or steps set forth below. 
     First, the ink transfer system  100  is trimmed, if desired, to remove excess backing sheet  102  material. Generally, cutting or trimming of the transfer medium  104  should be avoided unless it extends sufficiently beyond the margins of the base coat  106  and ink layers  108  to allow removal of excess. Next, the ink transfer system  100  is soaked in water for several minutes, two or more minutes generally sufficing for relatively small decals. Extended periods of soaking should be avoided as the water soluble coating on the backing sheet  102  may dissolve sufficiently to cause the decal  109  and transfer medium  104  layers to fall away from the backing sheet  102 . 
       FIG. 6  is a top perspective view of the lower edge of the backing sheet  102  positioned against a final substrate  101  with the ink transfer system  100  facing the viewer. The decal  109  including the attached transfer medium  104  is then slid off of the backing sheet  102 . In  FIG. 7 , the decal  109  has been slid partially off of the backing sheet  102  and onto the surface of the substrate  101 . 
     At this point, the decal  109  is held together by the strong adhesion of the base coat  106 , ink layers  108 , and release coat  110  to one another and the weaker, though sufficient, adhesion of the transfer medium  104  to the release coat  110 . As indicated above, the transfer medium  104  is not composed of the same material as the ink  108  and, although flexible and capable of stretching, it is less so than the release coat  110 , ink  108  and base coat  106  layers since it is designed to provide dimensional stability during transfer of the decal  109 . For these purposes the transfer medium  104  is also typically screen printed or otherwise deposited to form a somewhat thicker layer. 
     The ink transfer decal  109  is applied to the substrate surface  101  with the transfer medium  104  on top and the base coat  106  contacting the substrate surface  101 . Due to the moisture on the decal  109  (the substrate may also be prewetted, for example by spraying with water from a spray bottle or hose, not shown) and the thinness and flexibility of the decal  109 , the surface tension of the water between the decal  109  and the substrate  101  causes the decal  109  to conform closely to the substrate surface  101  and to loosely adhere. At this point the decal  109  may be moved or adjusted to its final position, often determined by registration marks (not shown) on the substrate  101 , and any wrinkles or bubbles may be smoothed out using a squeegee, sponge or other appropriate device. 
     In  FIG. 8 , the decal  109  is shown fully removed from the backing sheet  102  and placed in final position upon the substrate surface  101 . In  FIGS. 6 through 7 , the dashed lines defining the ink  108  and base coat  106  layers indicate that these layers are covered by and viewed through the transfer medium  104 . 
     Because the ink  108  and base coats  106  are prepared from linear chain polyester polyurethanes, and polymerization is somewhat suppressed through use of antioxidants, the ink  108  and base coat  106  films of the decal  109  are extremely flexible and exhibit high elongation characteristics. Elongation of over 400% may be obtained when stretching the ink  108  and base coats  106 . Because of these properties, the decal  109  may be applied to molded surfaces having irregular shapes. The transfer medium  104  allows for stretching and other manipulation of the decal  109  during application to the substrate  101  while aiding in handling of the decal  109  which would otherwise be difficult due to the extreme thinness of the ink  108  and base coat  106  layers. In other applications, the decal  109  may be transferred to a relatively flat surface for ease of registering and then subjected to considerable stretching and elongation due to bending or stamping of the piece post-transfer. 
     The decal  109  is then allowed to dry through air drying or other means. After the decal  109  and transfer medium  104  dries and substantially all surface moisture has evaporated, typically 1 to 24 hours after transfer, the transfer medium  104  may be peeled away from the deposited decal  109 .  FIG. 9  provides a top perspective view of the decal  109  showing a portion of the transfer medium  104  peeled away from the decal  109  and substrate  101 . Because it is removed after the water between the decal  109  and the substrate  101  has evaporated, it is important that the transfer layer  104  not stick or bind to the decal  109  excessively.  FIG. 10  illustrates the decal  109  after removal of the transfer medium  104 . At this point in the transfer process a clear base coat  106  may still be slightly visible or discernable from the surrounding surface of the substrate  101 . 
     After 12 to 24 hours of air drying the water has typically evaporated from the decal  109  and substrate  101  completely and a clear coat may be applied. A commonly used, solvent-based, thermoset clear coat is typically applied by spraying. Such clear coatings often comprise thermoset polyurethanes that must go through a heat conversion to complete polymerization. Because of the extensive polymerization and crosslinking such coatings typically provide very good chemical resistance. As an alternative to a heat set system, a photoreactive (UV) coating may be applied. 
       FIG. 11  is a top perspective view of the decal  109  after application of the clear coat and baking. At this point in the ink transfer process, the decal  109  has fused to the substrate  101  and clear coat and, due to the thinness of the base coat  106  and ink  108  layers, presents a design that does not protrude or project above the adjacent finish. 
     The substrate  101  bearing the decal  109  and overlying clear coat is then baked in an oven or otherwise heated to temperatures typically in the range of 250 to 325 degrees Fahrenheit. Due to the presence of antioxidants, the ink  108  and base coat  106  films may readily withstand exposure to such temperatures without degradation, discoloration, shrinkage, peeling or other defects. During the baking cycle the decal  109  softens becoming fluid or semi-fluid. While heated the decal  109  further polymerizes and adheres to compatible underlying surfaces such as typical polyurethane-based paints and coatings used in the automotive industry. The softening and adherence characteristics of the decal  109  are typically achieved as the temperature during the baking cycle rises to 185° F. and continue to improve as the temperature increases to 265° F. The decal  109  may be subjected to temperatures in excess of 375° F. 
     The ink film forming mechanism employed in the above ink embodiments is partially that of a solvent evaporative coating yielding a thermoplastic ink film. Film formation occurs during evaporation of the diluents and the concomitant coalescence of the polymer chains. Polymerization to form linear, polyester-based polyurethane chains yields an ink film having thermoplastic characteristics while still exhibiting good resistance to solvents and other chemicals after curing. 
     Addition of Free Isocyanates to Enhance Polymerization and Cross Linking 
     If further chemical resistance, more complete polymerization and/or enhanced abrasion resistance of the base coat  106  and ink layers  108  is desired, free isocyanates may be added to the ink formulation. Free isocyanates with particular advantages include aliphatic polyisocyanate resins based on hexamethylene diisocyanate. Such compounds react well with polyester hydroxyl-functional coreactants such as are used to prepare the vehicle described in Table 1. Resultant coatings prepared using polyisocyanate compounds exhibit enhanced resistance to solvents and oxidizing chemicals as well as increased abrasion resistance. The coatings have enhanced light stability, resistance to heat degradation and resist yellowing or other discoloration. A commercial product appropriate for providing free isocyanates with the above advantages includes Desmodur N 100, produced by Bayer Polymers LLC. Typically, these products are added to the ink formulation at levels of 0.005 to 0.01% of resin solids. 
     As a further embodiment, a thermal set ink transfer system may be provided through the addition to the ink  108  and base coat  106  formulations of melamine and an acid catalyst. The melamine reacts with the acid catalyst to form crosslinkages that set the film, reducing or eliminating thermoplasticity but increasing chemical and heat resistance. 
     It is to be understood that while certain forms of this invention have been illustrated and described, it is not limited thereto except insofar as such limitations are included in the following claims and allowable equivalents thereof.