Abstract:
An article of cookware has a copper core and is surrounded by relatively thinner outer aluminum layers. The outer aluminum layers are preferably anodized to provide a relatively inert hard and scratch resistant durable finish. This anodized finish also readily accepts non-stick finishes.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and the benefit of the U.S. Provisional Patent application of the same title, filed on 4 Sep. 2009, having application Ser. No. 61/239,869. 
    
    
     BACKGROUND OF INVENTION 
     The present invention relates to an improvement in the construction and fabrication of copper cookware. 
     While copper cookware is preferred for its superior heat transfer capabilities, the food contacting portions must be covered with an inert coating that prevents the leaching of copper and copper oxides into food, as well as the staining of the cooking surface. Traditionally, tin coatings have been used. However, these wear off over time. Further, tin is a soft metal which scratches easily. 
     Another approach is to clad copper with stainless steel so that after forming the food contacting surface is steel. While stainless steel is relatively durable and inert, it scratches easily. Far more significant, stainless steel is a poor conductor of heat, so the cooking performance is not as good as tinned copper, even when the stainless steel is relatively thin. While such clad metal constructions deploy some layers of aluminum between the copper and stainless steel, the steel is still a limitation to heat transfer. Further, such laminates are complicated to fabricate and expensive, having many layers that must be metallurgically bonded in the cladding process. For this reason, depending on the sheet cladding process, the copper core is not always continuous, and can be very thin, with holes or perforations though which the aluminum layers are extruded through to bond to each other encapsulating the copper. 
     It is therefore a first object of the present invention to provide copper cookware with improved durability for long life and easy maintenance without sacrificing the thermal performance. 
     It is also an object of the invention to provide copper cookware that requires less or no cooking oil to prevent food from sticking, as well as making cleaning and maintenance easier for the consumer. 
     SUMMARY OF INVENTION 
     In the present invention, the first object is achieved by providing a cookware article comprising a bottom having an upward facing first surface and an opposite the downward facing second surface, substantially upright wall surrounding said bottom and terminating at an upper rim to provide a fluid containing vessel, wherein the bottom and walls consist essentially of an inner core of copper metal and an outer cladding layers of aluminum surrounding both sides of the copper core. 
     A second aspect of the invention is characterized by the outer cladding layers of this cookware vessel being anodized to provide a scratch resistance alumina coating. 
     Another aspect of the invention is characterized by such a scratch resistant alumina coating on the interior of the fluid containing vessel being further protected by a non-stick coating. 
     The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a cross-sectional elevation of a first embodiment of the cooking vessel. 
         FIG. 1B  is a cross-sectional elevation of a second embodiment of the cooking vessel. 
         FIG. 2A  is a cross-section elevational of a third embodiment of the cooking vessel. 
         FIG. 2B  is an expanded cross-sectional elevation of the portion of the second embodiment indicated by the broken line oval in  FIG. 2A   
         FIG. 3A-3H  illustrate steps in the process of forming the vessels of  FIGS. 1 ,  2  and  3  from an aluminum clad copper sheet. 
         FIG. 4  is a cross sectional elevation of an alternative embodiment of the aluminum clad copper sheet that may be used to form the vessel of  FIGS. 1 ,  2  and  3 . 
         FIG. 5A-F  illustrate various optional configuration for the rim of the vessel of  FIGS. 1 ,  2  and  3 . 
         FIG. 6  is a graph comparing the theoretical thermal performance of the inventive pan with a stainless steel clad copper core pan. 
         FIG. 7  are diagrams comparing the theoretical thermal gradients across the pans compared in  FIG. 6  at two different time period. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 through 7 , wherein like reference numerals refer to like components in the various views, there is illustrated therein a new and improved anodized clad copper cookware, generally denominated  100  herein. 
     In accordance with the present invention the cookware article  100  has a continuous core of copper, or an alloy thereof,  110  that are substantially surrounded by aluminum, or an alloy thereof, cladding layers  121  and  122  on both sides, in which preferably at least on one side, and more preferably on both exterior sides of the aluminum have an anodized finish in which at least the food contacting layer is alumina or aluminum oxide (Al 2 O 3 )  131  and  132  (as shown in  FIGS. 2A and 2B ). Such an outer coating can be further coated with relatively durable non-stick coatings such as organic and inorganic non-stick coating  141 . Such coating typically comprise low surface energy organic polymers, as well as reinforcing filler and may be deposited as consecutive multiple layers of slightly different composition to improve adhesion and durability. The low surface energy polymers are typically fluoro-hydrocarbon polymers, and/or silicone containing polymers. Various US patents teach compositions of matter and methods of applying organic based and non-stick coatings to cookware vessels. These include U.S. Pat. No. 3,986,993 to Vassiliou (issued Oct. 19, 1976); U.S. Pat. No. 4,118,537 to Vary, et al. (issued Oct. 3, 1978); U.S. Pat. No. 4,321,177 to Wilkinson (issued Mar. 23, 1982); U.S. Pat. No. 5,691,067 to Patel (issued Oct. 25, 1997) and U.S. Pat. No. 6,133,359 to Bate, et al. (issued Oct. 17, 2000), all of which are incorporated herein by reference. The non-stick coating  141  protects the alumina layer  131  from degradation by acidic foods. 
     It should be further appreciated that, in contrast to copper cookware clad with stainless steel, it is difficult to add a non-stick coating, without adding further intermediate layers than degrade thermal conductivity further, and would not improve the durability of the non-stick coating like the hard anodized alumina layer of the instant invention. 
     The cookware article of  FIGS. 1 and 2  is formed as shown in  FIG. 3A-3F . First, a substantially planar copper sheeting  10  that has been clad with aluminum metal, or an aluminum alloy,  20  and  21  on both sides to produce clad sheet  30 , is placed in the mating die set used for deep drawing. Such cladding processes to produce sheet  30  generally require some degree of cold rolling; however the rolling can also be hot, deploy multiple passes and/or deploy additional layers to promote adhesion between the copper and aluminum layers. In  FIG. 3B , the aluminum clad copper sheet  30  is deep drawn between mating dies to form the general shape of vessel  100 . 
     It has been discovered that unlike conventional 5-ply cladding of steel/aluminum/copper/aluminum/steel, the construction of  FIG. 3A  is difficult to form into cookware by traditional deep drawing processes. It is believed that steel clad constructions of the softer copper and aluminum are easier to form because of the strength of the steel layer, which is usually as thick as the aluminum and copper layers in the clad construction. 
     Accordingly, another other aspect of the invention is improvements in the drawing process, in combination with the selection of materials and layer thicknesses to enable the consistent production of cookware of  FIG. 1  and  FIG. 2 . Generally, it is preferred that the outer aluminum layers  20  and  21  together have a total thickness that is from about 80% to 200% of the copper thickness in sheet  30 . More, preferably, the aluminum layers  20  and  21  should have an equal thickness. However, the optimum with in this range may vary considerably, depending on the workability of the aluminum alloy deployed after cold and/or hot rolling and annealing. Currently, it is believed that purer aluminum alloys are more preferred for their workability in forming the vessel  100 . 
     As it is important that the final cookware have a good surface appearance, it is desirable that the drawing process shown in  FIG. 3B , or ironing if used to form the vessel in  FIG. 1B , produce minimum surface roughness that cannot be economically finished with further polishing operations. Such polishing operation ( FIG. 3D ) is preferably performed after trimming in step ( FIG. 3C ) to form the rim of the vessel  140 . The step shown in  FIG. 3C  may also include other rim finishing processes shown in  FIG. 5B-F , as discussed below. 
     It is also important that the drawing process result in a consistent product that has a low defect level and that requires a consistent level of final surface finishing, such as polishing, to minimize the reject and rework required to provide efficient production capability. 
     It has been discovered that the above objectives are more readily achieved when the clad material shown in  FIG. 3  has a consistent thickness and is either annealed during the drawing process, such as in hot working, or after, as well as just before forming in cookware by deep drawing or a combination of drawing and ironing processes. If the clad sheet  30  has not been annealed previously it is desirable that such annealing occur for about 15 to 30 minutes at temperatures that range from about 250° C., with correspondingly lower times for higher temperatures, as for about 5 minutes at about 430° C. However, it is also important that the annealing not be excessive, as this can lead to an “orange peel” like surface roughness appearance after forming the cookware, which is likely due to too large a grain size in the aluminum before forming. Thus, depending on the cladding process and the nature of the cookware forming process, the annealing process, if any, would be adjusted accordingly to obtain the right balance of mechanical properties in the copper core and outer aluminum layers. 
     The need for annealing the clad metal sheet  30  before forming is somewhat dependent on the drawing conditions and the shape of the pan, that is drawing at higher rates or to higher local strain ratios, generally requires some annealing to reform the grain structure in the aluminum alloy that is formed either in the rolling or cladding process. Generally, for clad material  30  in which the aluminum alloys is 3003 grade annealing for about 15 minutes at between about 260 to 300° C. is helpful, but for 1050 grade aluminum the annealing temperature is preferably at least about 280° C. 
     A currently preferred construction for the clad material  30  used to form the vessel  100  has a copper core  10  that is about 1 mm thick and surrounding aluminum layers  20  and  21  that are each at least about 0.4 mm thick. It may be preferable that these aluminum layers  20  and  21  are slightly thicker, that is at least about 0.5 mm thick, so that the total thickness of the aluminum from both sides of the clad metal is about the same or greater than the copper core thickness. 
     Further, the vessel formed by deep drawing can be ironed to increase the wall height, while thinning the copper and or aluminum layers therein, leaving a thicker copper layer in the bottom, as shown in the vessel  100  in  FIG. 1B . Such ironing is optional and represented by the step shown in  FIG. 3C . In ironing, a series of mating internal and external dies of increasing smaller gap are inserted around the vessel wall to draw in upward to reduce the thickness. 
     After the vessel  100  is trimmed and/or finished at rim  140  it preferably undergoes a surface polishing in the step as represented by  FIG. 3E  before the optional anodizing in the step shown in  FIG. 3F . Additional polishing may be deployed after anodizing. Further, the optional non-stick coating  141  is applied in the step represented by  FIG. 3G , after which an additional coating ( FIG. 3H ) may be applied to external surfaces  132 , adding layer  142  shown in  FIG. 2B . Coating  142  can be a ceramic, enamel or lacquer coatings for appearance or to provide additional durability or chemical resistance to the alumina layer  132 . The order of steps shown by  FIGS. 3G and 3H  may be reversed. Coating  142  may extend around the entire exterior of the vessel  100  or just the surrounding walls  101 , leaving exterior bottom  102  coated with aluminum oxide  132  formed by anodizing the aluminum layer  122 . Ideally any coating on the exterior bottom  102  is relatively thin to avoid impairing the heat transfer from the flame or heating element to the copper core  110 . 
       FIG. 4  illustrates another embodiment where the aluminum layers  20  and  21  are at least about as thick as the copper layer  11  which has a plurality of spaced apart perforations  12  to allow the opposing aluminum layers to metallurgically bond at region  13  with each other during the cladding process. As the perforations  12  in the copper may be exposed when the rim  140  is trimmed, as shown in  FIG. 5A , it may be preferable to fold the rim  140  one or more times as shown in FIG.  5 BC-F to conceal the perforated nature of the copper layer  11 . This can be achieved by folding the rim  140  over on the outside wall  101  of the vessel  100  ( FIG. 5C-5F ) as well as reaming out a thin ring of copper at rim  140  and then sealing the surrounding aluminum layers  121  and  122  together to completely cover the copper core  11 , as shown in  FIG. 5B . Alternatively the copper core  10  or  11  can be hidden or protected by making a single fold with rounding of the aluminum as the end of the folded section ( FIG. 5D ) where the edge at the end of the fold is inserted into vessel wall ( FIG. 5E ). Alternatively, the double fold of the rim  140  shown in  FIG. 5F , triples the rim thickness and completely hides any perforation that might be visible in the copper layer  11 . Depending on the ductility of the finished vessel the same treatments could be used to increase the wall thickness at the rim  140  where the copper core  110  is continuous as shown in the embodiments of  FIG. 1-3 . 
     It has been discovered from both Finite Element Modeling and actual testing that the inventive pan provides unexpected advantages over the prior art stainless steel clad cookware having the layer structure: 300 series grade stainless steel (SS) (1.0 mm)/Al (3003 alloy) (0.4 mm)/Cu (1 mm)/Al (0.4 mm)/SS (1 mm) with a total thickness of 3.8 mm. 
     This was compared against the inventive construction: anodized aluminum (Alumina or aluminum oxide) 0.04 mm/Aluminum (3003 alloy) 0.4 mm/Cu (99.99%) 1.0 mm/Aluminum (3003 alloy) 0.4 mm/anodized aluminum (Alumina or aluminum oxide) 0.04 with a total thickness of about 1.808 mm. 
     In comparison to convention stainless steel clad cookware, it should be noted that even though the outer alumina layer would be expected to have a relatively low thermal conductivity than stainless steel, being both an oxide and a generally porous material, the actual performance compares rather favorably with a finite element model that is discussed below. 
     The FEM model simulated a hot flame with 2 heat sources, each ½ inch wide disposed 2″ from the center line of the pan (i.e. sources are symmetric about the center of the pan, spaced 4 inches apart) with a heat source/flame temperature 2,200K and a heat flux of 80,000 W/m 2 . It should be appreciated that in these theoretical models, the pans were dry for the simplicity of modeling accurately. It was also assume that Top surface of the pan had an emissivity 0.85 in radiating heat to the ambient atmosphere while the bottom surface facing the flame had a convection rate of 10 W/m 2  to an average ambient of 1400° K. 
       FIG. 6  compares the derived temperature at the center and edge of the inventive pan having the Al 2 O 3 /Al/Cu/Al/Al 2 O 3  construction against a pan having the construction Stainless steel/Al/Cu/Al/Stainless steel over a seven (7) minute period from the initiation of heating with heat source modeled as equivalent to a flame that impinges a narrow annular region about the geometric center of the pan. 
     It should be appreciate the inventive construction reached the cooking temperature of about 200° C. at the center in about 103 seconds, while it takes more than twice as much time (250 sec.) for the prior art pan. 
     Further, the difference in temperature between the center and edge of the inventive pan is only 88° C., when the 200 C is reached the center. In contrast, the prior art pan still has a temperature gradient of 113° C. when the center reaches 200° C. 
       FIG. 7  shows the same trend in which shading bands represent the average temperature though the thickness at 50 and 250 second 
     While the invention has been described in connection with various preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be within the spirit and scope of the invention as defined by the appended claims.