Abstract:
A method to inhibit separation of an extruded thermoplastic polymer coating from a metallic alloy container body due to body forming operations. The container body and the extruded thermoplastic polymer coating are inductively heated causing the polymer to flow into the microsurface irregularities on the body interior surface which adheres the polymer to the body.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to processes for making containers for food and beverages. More specifically, the invention relates to a method of inhibiting delamination of thermoplastic polymeric coatings from container bodies during container fabrication.  
         BACKGROUND OF THE INVENTION  
         [0002]    Metallic alloy containers such as aluminum alloy containers used for food and beverage storage are commercially shaped by a process in which blanks of aluminum alloy are extrusion coated with thermoplastic polymer, cupped, wall ironed, bottom domed, necked, and then flanged to form a cylindrical container body having an internal polymer lining with an opening at the top of the cylinder.  
           [0003]    An aluminum alloy lid or “end” is then attached to the cylindrical body, covering the opening in the cylindrical body, to seal the container. Ends for beverage containers are discs that have been stamped from aluminum alloy sheets coated with polymer. The discs have contoured shapes to facilitate attachment to container bodies and opening means such as pull tabs to allow the container contents to be emptied from the assembled containers.  
           [0004]    The adhesion of the extruded thermoplastic polymer coating to the container body surface can be reduced by the drawing and ironing stages of body formation. The loss of adhesion can result in delamination of the polymer from the container body thus rendering the body unsuitable for use as a food and beverage container. High delamination rates waste materials, time and effort, thereby increasing costs to the container manufacturer.  
           [0005]    Thus a need exists for a method to inhibit delamination of extruded thermoplastic polymers from container bodies during container body formation.  
           [0006]    One of the objectives of the this invention is to provide a method to inhibit delamination of extruded thermoplastic polymers from container bodies during container body formation.  
           [0007]    Additional objectives and advantages of our invention will become apparent to persons skilled in the art from the following detailed description of the invention.  
         SUMMARY OF THE INVENTION  
         [0008]    The invention provides a method to inhibit delamination of an extruded thermoplastic polymer coating from a container body. The method comprises inductively heating the open end of a container body, prior to affixing the can end to the body, to adhere the polymer to the container.  
           [0009]    The invention also provides a method of making a container body that comprises forming a cylindrical body having an exterior surface, an interior surface and an edge defining an orifice. The body interior surface is coated with a polymeric liner and the body exterior surface may optionally be decorated. The container body edge near the orifice is inductively heated and an end is joined to the body to forma completed container.  
           [0010]    The invention inhibits delamination of extruded thermoplastic polymer coatings from container bodies as a result of container formation operations. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0011]    The invention provides a method for making a container suitable for use as a beverage and food container. The container body may comprise an aluminum alloy, steel, aluminum alloy-coated steel, or aluminum-coated steel. Aluminum alloy is particularly preferred for use in this invention. Aluminum alloys suitable for shaping into container bodies include aluminum-manganese alloys of the AA3000 series and aluminum-magnesium alloys of the AA5000 series.  
         [0012]    To form a container body, aluminum alloy blanks are preferably extrusion coated with a thermoplastic polymer for example, by the methods described in U.S. Pat. Nos. 5,407,702 and 5,919,517. Suitable polymer coatings for use in this invention are polyesters such as polyethylene terephthalate, polyethylene terpthalate copolymers, blends of polyethylene terphthalate homopolymers with polyethylene terphthalate copolymers, and polyolefins such polypropylene, polyethylene, and in particular polypropylene copolymers. Polyamides (nylon), polyimides, polycarbonates and polyvinyl chlorides are also suitable for use in this invention. The preferred thermoplastic polymers are blends of polyethylene terphthalate homopolymers with polyethylene terphthalate copolymers. Suitable blends of polyethylene terphthalate homopolymers with polyethylene terphthalate copolymers. These polyester resins are available commercially as Melinar 5122C and Crystar 4462 from duPont Inc. of Wilmington Del.  
         [0013]    Polymeric interior coatings are applied to container bodies to improve preservation and taste characteristics of the food and beverages that are stored in the containers. The polymer coating may also optionally contain additives such as dyes, pigment particles, anticorrosion agents, antioxidants, adhesion promoters, light stabilizers, lubricants, and mixtures thereof.  
         [0014]    The coated blanks are cupped, wall ironed, bottom domed, necked, and then flanged to form a cylindrical container body having an internal polymer lining with an opening at the top of the cylinder.  
         [0015]    During the body forming process, the polymer coating is stretched along the body interior surface and can separate from the body. Such delamination typically occurs near the opening at the top of the cylindrical body.  
         [0016]    To inhibit the extruded polymer form separating from the container body the inventors have discovered that heating of the container near the open end adheres the polymer to the container, thus reducing the occurrence of delamination resulting from body formation.  
         [0017]    However, heating of an entire container body is undesirable due to reduction in the metallurgical properties of the metal alloy body. The preferred means of heating the container open end is by induction. Induction heating is performed by placing the open end of the container body within an inductor coil and passing alternating electric currents through the coil. The energized coil causes the container, and the thermoplastic polymer on the container body, to experience eddy currents which in turn cause the metal and polymer to heat. Heating causes the polymer to flow into the microsurface imperfections in the can body interior surface, adhering the polymer to body.  
         [0018]    Heating by induction has several advantages. The heating operation is relatively fast, since heat is induced directly within the metal and polymer within the coil as opposed to heating by radiant energy. Induction heating is also localized to only the portion of the container body and the polymer adjacent to the coil since only the areas of the container body within the induction coils&#39; electromagnetic field experience heating. Preventing the entire container body from heating is important to prevent undesirable tempering and polymer delaminating effects within the metal portions of the container.  
         [0019]    After induction heating, the container may be filled with a product and an end may be attached to the body to seal the container.  
         [0020]    The invention thus provides a method to inhibit delamination of an extruded thermoplastic polymer coating from a container body. The method does not cause undesirable tempering and polymer delaminating effects within the metal portions of the container and reduces wasted materials, time and effort, caused by high delamination rates due to container body manufacturing processes.  
         [0021]    The foregoing disclosure of our invention has been made with reference to some particularly preferred embodiments. Persons skilled in the art will understand that numerous changes and modifications can be made without departing from the spirit and scope of the following claims.