Abstract:
A can body is formed from a sheet of metal which has a matte coating composition applied to the portion of the sheet which will become the outside surface of a reentrant convex bottom of the can body.

Description:
BACKGROUND 
     Two-piece cans for beer and other pressurized beverages generally have a reentrant convex shape for their bottom. This permits the use of thinner metal while resisting distortion of the can by the pressure of the beverage. 
     Coatings are generally used on the inside of beverage cans to prevent corrosion and to protect the flavor of the beverage. Coatings are also used on the outside of beverage cans to facilitate decoration and identification of the can and its contents. However, if the can is made of adequately corrosion resistant material such as aluminum or tin-plated steel of an appropriate quality, it is generally not necessary to apply any coatings to the outside of the bottom of the can. 
     Although it probably is unlikely, some think it conceivable that a concave can bottom of uncoated shiney metal, especially bright aluminum, could act as a reflector to concentrate the sun&#39;s rays and possibly start a fire in dry grass or underbrush if a used can is discarded by the roadside or in the woods. 
     Exterior coatings are generally applied to two-piece cans after the can body has been formed. If they were applied before forming, they could be abraided or damaged by the can forming process. And it is difficult to spray a matte coating on the bottom of a formed can without overspraying onto the sides. 
     SUMMARY OF THE INVENTION 
     The present invention provides a process for making a cylindrical can body with one end closed from a sheet of metal by a deformation process, said closed end having a reentrant convex shape, wherein first only the area of said sheet which will become the outside surface of said closed end is coated with a flexible coating composition which gives a matte finish, minimizing specular reflectance of light from said area, then the can body is formed from said sheet by a deformation process. 
     Generally two-piece cans are formed by a drawing and ironing process wherein a ram pushes the center of a flat sheet of metal into a female mold, often involving more than one ram and mold or more than one step. The initial forming steps are known as drawing. The final step or steps in which the precise can shape is formed is known as ironing. The ironing ram and mold typically will be shaped to form a reentrant convex shape at the bottom of the can. 
     Such a can is called a two-piece can to distinguish it from a three-piece can. A two-piece can includes a cup-shaped can body and one lid or end piece. A three-piece can includes a cylinder open at both ends and two lids. 
     Preferably, the coating composition is applied to flat sheet metal before any of the deformation is done to form the can body. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a plan view of part of an uncoiled sheet of metal indicating the location of can blanks which are to be coated according to the invention and cut from the sheet. 
     FIG. 2 is a perspective view of a partially formed cup shape drawn from a blank of FIG. 1. 
     FIG. 3 is a perspective view of a formed can body drawn and ironed from a blank of FIG. 1. 
     FIG. 4 is a cross-sectional view along the section line 4--4 in FIG. 3. 
    
    
     DETAILED DESCRIPTION 
     Suitable coating compositions are known in the art which are quite flexible and which dry to give matte finishes. By minimizing specular reflectance, any potential for discarded cans to accidentally cause fires in dry grass can be minimized. Matte finishes can be obtained by pigmentation, rough surfaces, or combinations thereof, such as by using the compositions disclosed herein. 
     The flexibility of the coating composition permits applying it to flat sheet and subsequently forming the can body. This avoids the difficulties of applying a paint to the reentrant bottom of a preformed can body. If the reentrant bottom of a preformed can is painted by spraying, it can be desirable to mask the sides of the can to prevent overspray. It is easier to confine the spray to a desired area of a flat sheet. 
     The reentrant convex bottom of the can body can have a variety of shapes. Generally the shapes will include some portions of smooth curves, although part of the bottom can be flat. The advantages of the invention are available in a wide variety of applications which will be apparent from the above to those skilled in the art of can making and coating. 
     Turning now to the drawing, FIG. 1 illustrates a portion of an uncoiled sheet 1 of metal, such as thin aluminum or steel, showing the placement of can blanks 2 to be punched from the metal and the location of the matte finish 3 of the invention on the blanks 2. The matte finish 3 can be coated onto the can blanks 2 before or after they are cut from sheet 1 but preferably before. 
     FIG. 2 is a perspective view of a can blank which has been partially formed by drawing a flat blank 2 into a cup shape. FIG. 2 illustrates vertical wall 4 and flat bottom 5 with the matte finish 3 in the middle of flat bottom 5. The partially formed can body is inverted in FIG. 2 with open end 6 shown at the bottom. Normally, open end 6 is considered to be the top of the can. FIG. 3 is a perspective view of a formed can body drawn and ironed from the partially formed blank of FIG. 2. The side wall 7 has been lengthened and flat bottom 5 has been decreased in diameter to produce angular shoulder 8 and reentrant convex shape 9. This convex shape 9 is the part of the can which is coated with matte finish 3. 
     FIG. 4 is a cross-sectional view taken along cut plane 4--4 of FIG. 3. FIG. 4 more completely illustrates the nature and relationship of convex shape 9 with its matte finish 3 to angular shoulder 8, wall 7 and open end 6. 
     Of course, the precise contour of a convex shape 9 and the use of angular shoulder 8 are matters of choice. Other contours, shapes and designs can be used in the spirit of the invention. 
     In the following Example, parts, percentages and proportions are by weight except as indicated otherwise. 
     EXAMPLE 
     Acrylic Resin A is prepared, then mixed with the other ingredients as indicated below to make a Can Coating Composition. A Wax Dispersion combined with an Antiblocking Agent is added as a lubricant for can forming, and the gloss level is adjusted as desired by additions of the Gloss Adjusting Clear. The Can Coating Composition is a low gloss blend of an acrylic resin, an epoxy resin and a melamine cross linker. 
     
         ______________________________________Acrylic Resin ASolvesso 100 hydrocarbon                   14.88solvent-ExxonEthylene glycol mono-   9.54ethylether acetaten-butanol               5.42Isopropanol             0.96Add in order - heat to reflux 118° C.Methylmethacrylate      18.72Ethylacrylate           27.36Methacrylic acid        1.92Di-tert butyl peroxide  0.78Load in premix tank.Add to reactor over 2 hours period.Hold at reflux for 3 hours.Solvesso 100            10.60Ethylene glycol mono-   10.60butyletherTotal                   100.780Loss                    0.780Yield                   100.00MMA/EA/MAA weight ratio               = 39/57/4Solids by weight    48% (90 min. at 150° C.)Acid number         25-30Viscosity           Z2-Z4 (Gardner Holdt)Liter weight        995 gAnti-blocking SolutionAcrylic Resin A         16.84Solvesso 100            29.90Syloid 74 silica pigment -                   18.48W. R. GraceEthylene glycol mono-   31.84butyletherSolvesso 100            2.94Grind in sand mill to fineness of &lt;1 μm                   100.00particle size.Adjust solids with 50/50 ethylene glycolmono-butylether and Solvesso 100. -   Solids by weight    26.563%Liter weight            1005 gFineness                &lt;1 μmWax DispersionSolvesso 150 hydrocarbon                   40solvent-ExxonPolyethylene wax        10AC 405-Allied ChemicalHeat to 100° C. and mix about 30 min. -Solvesso 150                   50Add as fast as possible with rapid                   100agitation and mix until temperature isbelow 30° C.solids by weight    10%Liter weight        884 gGloss Adjustment ClearAcrylic Resin A         84.96Epikote 828 epoxy       3.73resin-Shell ChemicalCymel 303 highly        2.54methylated melamineEthylene glycol mono-   2.00butyletherSolvesso 100            2.00Wax Dispersion          1.85Mix 1 hour.Ethylene glycol mono-   1.46butyletherSoya bean oil           1.46                   100.00Solids by weight    48.7%Density             0.9943Can Coating CompositionAcrylic Resin A         65.85Epikote 828             2.89Cymel 303               1.97Ethylene glycol mono-   4.00butyletherSolvesso 100            4.00Add in order with mixing.Antiblocking Solution   15.600Wax dispersion          1.40Mix 1 hour - sample to lab for glossadjustment.Anti-blocking solution  traceGloss Adjustment Clear  traceSolvesso 100            2.130Ethylene glycol mono-   2.130butylether                   100.00Solids by weight    40.7%Gardner Color       2 maxGardner Gloss       80 min at 85° angleViscosity           DIN 4 85-95Liter weight        985.9 gSolids by volume    33.400%Solids ProportionsResinsCymel 303           1.970   5.22Epikote 828         2.890   7.66Acrylic Resin A     32.868  87.12                       100.00PigmentFlatting agent     Syloid 74                       2.88______________________________________