Patent Publication Number: US-2007095241-A1

Title: Polymer-coated metal substrate

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to a coating to a metal substrate, particularly a polymer coating for a metal sheet that is used to produce a bullet jacket or ammunition cartridge. Even more particularly, the present invention relates to a polymer coating that is applied directly to a base metal in a sheet or strip form by methods that include coil-coating, reverse roll-coating, film lamination, and co-extrusion that results in performance benefits including reduced fouling in the rifling, reduced barrel wear, improved ballistics and lower costs.  
      Conventional bullet jackets are produced using copper/zinc alloys such as 95/5, which is known as gilding metal, 90/10, which is known as commercial bronze, 70/30, which is known as cartridge brass, and bronze-clad steel. The materials chosen today are typically used for their forming characteristics and functionality in the rifle or pistol barrel. The finished bullet jacket must be ductile enough to “groove” into the rifling. This “groove” provides the bullet with the required spin in order to maintain accuracy. Typically, copper/zinc alloy bullet jackets must be stress-relief annealed between two consecutive drawing (and ironing) steps, which adds time and expense to the manufacturing process.  
      A conventional bullet jacket is produced from 95/5 gilding metal or 90/10 commercial bronze by deep drawing. A sheet of the metal is blanked, cupped and annealed. After annealing, the cup is then pickled in acid, washed and drawn into the final form. The jacket is then trimmed prior to being filled with lead. Another technology used to produce jacketed bullets is electroplating, whereby a swagged lead bullet may be copper plated with an extremely heavy layer of copper (ie. 0.025 inches (0.635 mm)). This process is slow and expensive because the electroplating of individual parts is more difficult to control.  
      It is known in the art to coat a bullet jacket to improve the ballistic performance of the bullet, to reduce wear in firearms, or both. For example, a projectile having a metal or ceramic core surrounded by a plastic jacket is known in the art. The jacket of the projectile is made from a flexible, resilient material such as polytetrafluoroethylene (PTFE, also known as TEFLON®) so that the jacket takes most or essentially all of the deformation of the projectile and barrel when the projectile is fired. The jacket of the projectile thus protects the lead or ceramic core from deformation or damage and reduces wear on the barrel. In addition, when the projectile leaves the barrel, the ridges on the projectile formed by the rifling grooves of the barrel either reduce in size or disappear altogether. The projectile consequently has a smoother, more aerodynamically efficient surface during flight and has a more accurately predictable flight path.  
      In such a projectile construction, the core can be cast or swagged by a press having a relatively small capacity. The core then has the plastic jacket injection molded around it, preferably using a thermosetting resin, although a castable urethane plastic can also be used. The core of the projectile can be made of various weights, centers of gravity or shapes without changing the overall configuration of the projectile.  
      However, the thickness of the polymer coating in such a projectile requires the expensive step of individually molding or casting the polymer onto the previously-formed shells. Besides being cost prohibitive, individually molded parts have a mold parting line, which is caused by the basic function of injection molding in a split cavity mold. This “parting line” is not only cosmetically unacceptable but may also serve to negatively affect the accuracy of the bullet jacket. It is more desirable to apply the polymer coating before forming the shell. However, a polymer that could maintain its physical integrity during the drawing process for producing a bullet jacket was not previously known.  
      It is also known in the art to coat a bullet jacket with a low friction material such as molybdenum disulfide, nylon, polyurethane or polytetrafluoroethylene, for example. Such coatings however, have been applied to the bullet jacket after the bullet is cast or swagged.  
      Polymer coating compositions for application to primed metal substrates, as a topcoat that is useful on the interior of metal closures for vacuum-packed food products, for example, are also known. Such products, however, do not teach or suggest that the technology would be readily adaptable to methods of forming a bullet jacket. In particular, there is no teaching or suggestion that a polymer coating on a metal substrate would remain adhered to the substrate through the drawing process.  
      Therefore, there is a need for a method of coating at least one surface of a metal substrate with a polymeric material, the coating having sufficient thickness, ductility and durability to supplement or replace the conventional copper/zinc jacketing layer, but with the coating having an appropriate amount of flexibility and adhesion to the substrate to allow the substrate to be formed into the bullet jacket after the polymer coating is applied. Such a technology should also facilitate the use of a steel-based metal substrate instead of the conventional copper/zinc alloys.  
     SUMMARY OF THE INVENTION  
      In general, the present invention provides a method of producing a polymer-coated bullet jacket. The method comprises applying a polymer to a substrate and drawing the substrate to form a polymer-coated bullet jacket.  
      Polymers, according to the present invention, include but are not limited to, fluoropolymers such as polymers of perfluoropropyl vinyl ether (PPVE), tetrafluoroethylene (TFE), hexafluoropropylene (HFP), vinylidene fluoride (VDF) and mixtures thereof. Polymers may also include polypropylene (PP), polyethylene (PE), polyethyleneterephthalate (PET), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinyl-fluoride (PVF), ethylene-chlorotrifluoroethylene (ECTFE), ethylene-tetrafluoroethylene-copolymer (ETFE), fluoroethylene-propylene (FEP), perfluoralkoxy (PFA), polychloro-trifluoroethylene (PCTFE) and mixtures thereof. The polymer may also be a polymer having a melting temperature greater than about 200° C. Suitable polymers include non-fluorinated polymers such as polyesters, as exemplified by polyethylene terephthalate (PET). These polymers provide the benefit of fluorinated polymers in that they reduce barrel wear and fouling, but are not believed to impart the ability to defeat anti-ballistic body armor, as TEFLON coatings on bullet jackets do. In this way, the polyester coated substrate provides a bullet jacket that provides a more commonly acceptable ammunition round, that is, one that will not defeat body armor worn by law enforcement personnel. Polymers with an abrasion and wear resistance less than 20 mg per 250 revolutions as described by the Taber Abraser Test (ASTM D4060) are also envisioned as being suitable in the present invention. Surface roughness of the bullet jacket is also important and polymers with roughness values on the order of 25 microinches Ra or less as determined by Surface Roughness/Texture Test (DIN 4762 or ASME B46.1) may also be particularly suitable. It is also envisioned that the polymer may also include coloring to enhance the esthetics of the coated substrate. For example, a copper colored polymer may be used on a substrate to imitate the appearance of copper-plated substrate.  
      As mentioned above, bullet jackets may typically be produced using copper/zinc alloys such as gilding metal or commercial bronze, or bronze-clad steel. In the present invention, these may also be used as the substrate for the polymer coating, among others. For example, copper-plated steel, or non-plated steel, such as low carbon steel, may also be used. Interstitial-free steel, ultra-low carbon steel, high strength low alloy steel, multiphase steel, ultra-low carbon boron steel and stainless steel are all envisioned as being acceptable. A normalized or strand-annealed steel base metal are likewise envisioned as being suitable in the present invention because of the small grain size and equiaxed grain shape produced by these annealing methods. The grain size and shape are one of the keys to minimizing the exterior surface roughness of the final drawn part.  
      A suitable substrate may range in thickness from about 0.005 inch (0.127 mm) to about 0.50 inch (1.27 cm). A frequently used thickness is about 0.022 inch (0.56 cm). A plating layer included on a substrate may range in thickness from about 0.000020 inch (0.508 μm) to about 0.0010 inch (0.0254 mm). A frequently used thickness is about 0.000050 inch (0.00127 mm).  
      The polymer may be applied by any of a variety of methods, including coil-coating, reverse roll-coating, film lamination, and extrusion. Coil-coating and roll-coating are methods for applying liquified polymers and/or paints to the surface of steel in strip form. In these cases, the appropriate solvent is used to solubilize the polymer, which is then applied to the steel through the use of an applicator roll. The wet film is then dried and cured in a oven. In the film lamination process, a continuous self-supported sheet or film of the polymer is first produced in a secondary operation. The self-supported film, which may have a thickness ranging from about 0.0003 inch to about 0.005 inch (0.00762 mm to 0.127 mm), is then laminated onto the surface of the steel strip under the proper conditions of temperature and roll pressure. Extrusion techniques involve applying a layer of molten polymer directly to the steel strip while using the proper conditions of temperature and rolling pressure to produce a smooth layer of the polymer on the steel strip. In all the aforementioned techniques, the key is to produce a steel/polymer combination that will form the bullet jacket while remaining mutually attached and adherent. The mechanical properties of each material must be well matched in terms of strength and elongation. The polymer layer on the substrate may range in thickness from about 0.0003 inch (7.62 μm) to about 0.005 inch (0.127 mm). It is envisioned that a thickness of about 0.001 inch (0.0254 mm) may be particularly desirable.  
      The present invention, therefore also provides a bullet jacket comprising a ferrous substrate and a polymer layer bound to the substrate that is essentially devoid of a metal layer on the ferrous substrate. The polymer may be a polyester or a fluoropolymer, for example. Suitable polymers include polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl-fluoride, ethylene-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene-copolymer, fluoroethylene-propylene copolymer, perfluoralkoxy polymer, polychloro-trifluoroethylene, polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), and mixtures thereof. In one particular example, a bullet jacket comprises a metal substrate, and a polyethylene terephthalate layer bound to the substrate. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In the present invention, the polymer-coated metal substrate is designed to be deep drawn into the form of a bullet jacket while maintaining its physical integrity. The polymer coating does not lose its adhesion or delaminate from the metal substrate during the drawing process. In the finished bullet jacket, the ductile polymer coating can “groove” into the rifling thus providing the bullet with a spin that is required for a proper trajectory. Concurrently, the polymer will inhibit direct contact between the metal substrate of the bullet jacket and the rifling, thus reducing or even eliminating the deposition of lead or other metals in the inner diameter of the barrel thereby reducing barrel fouling. The following examples should not be viewed as limiting the scope of the invention. The claims will serve to define the inventions.  
     EXAMPLE 1  
      A polymer-coated metal substrate for a bullet jacket was produced by coating both sides of a ferrous-based metal substrate with a urethane-based paint that is blended with a fluoropolymer. The formulation is described below: 
          Polyeurethane 56%     Water 24%     Co-solvent 12%     Surface modifier 1.2%     Teflon 2.0%     Bubble breaker 0.6%     Leveling agent 0.6% 
 
 The selected metal substrate was a 0.0245 inch (0.622 millimeter) thick sheet of copper-plated 1006 low carbon cold rolled steel. The cold rolled steel was first electroplated on both sides with 0.00008 inch (0.002032 millimeter) of cyanide copper. The painted coating was then applied at a thickness of 0.001 inch (0.0254 mm) using a roll-coating technique in the laboratory and thermally cured (425° F.) for 30-50 seconds. The polymer-coated metal substrate had the required physical integrity as evidenced by passing the Zero-T bend test. The Zero-T bend test is a simple test for paint and coating adhesion whereby the coated metal sample is bent 180 degrees onto itself and then pressed flat with the use of a rubber mallet. Scotch tape is then applied to the bend radius and quickly removed. Any poorly adhering coating will adhere to the tape. The clear tape is then applied to a sheet of white paper, which is used to show the presence of the loosely adhering coating. 
       

     EXAMPLE 2  
      A polymer-coated bullet jacket was produced using the polymer-coated metal substrate of Example 1. The polymer-coated bullet jackets were stamped from 4 inch×12 inch (10 cm×30.5 cm) sheets of the polymer-coated metal substrate. The sheets were hand-fed in a transfer press. A 9-mm caliber bullet jacket was then drawn and re-drawn to its final dimensions. The bullet jacket was then lead-filled to complete the bullet manufacturing process. The polymer-coated bullet was loaded into the chamber and tested. Several bullets were evaluated to verify the results. Ballistics firing tests revealed no metal-to-metal contact or sparking. The bullets maintained their spin and accuracy.  
     EXAMPLE 3  
      A polymer-coated metal substrate for a bullet jacket was produced by coating one side of a ferrous-based metal substrate with a 0.0008 inch (0.02 mm) thick film of a polyethylene terephthalate (PET). The selected metal substrate was a 0.0245 inch (0.622 millimeter) thick sheet of copper-plated 1006 low carbon cold rolled steel. The cold rolled steel was first electroplated on both sides with 0.00008 inch (0.002032 millimeter) of cyanide copper. The polymer film was applied by a lamination process whereby the substrate is first cleaned and degreased using a warm solution of sodium hydroxide. The substrate is then rinsed and dried prior to being heated to a temperature of 150° F. Once at temperature, the PET film is applied with the use of a roller-laminator. The polymer-coated metal substrate had the required physical integrity as evidenced by passing the Zero-T bend test.  
     EXAMPLE 4  
      A polymer-coated bullet jacket was produced using the polymer-coated metal substrate of Example 3. The polymer-coated bullet jackets were stamped from 4 inch×12 inch (10 cm×30.5 cm) sheets of the polymer-coated metal substrate. The sheets were hand-fed in a transfer press. A 9-mm caliber bullet jacket was then drawn and re-drawn to its final dimensions. The bullet jacket was then lead-filled to complete the bullet manufacturing process. The polymer coated bullet was loaded into the chamber and tested. The bullets were evaluated to verify the results. Ballistics firing tests revealed no metal-to-metal contact or sparking. The bullets maintained their spin and accuracy.  
      Based upon the foregoing disclosure, it should now be apparent that the method of the present invention will provide a polymer coated bullet jacket. It is, therefore, to be understood that any variations evident fall within the scope of the claimed invention and thus, the selection of specific component elements can be determined without departing from the spirit of the invention herein disclosed and described. In each of the foregoing examples, exact selection of the polymer or polymer blend to be used will be accessible to one of skill in the art.