Abstract:
The present invention discloses a heat retentive food server for maintaining food at an elevated temperature, especially important in food service operations. The server is a container having an upper shell and lower shell and cavity defined therebetween containing a phase change material. The phase change material is preferably an ethylene acrylate copolymer, which absorbs energy upon heating, and then keeps freshly prepared hot foods placed in the server warm as the phase change material slowly cools and releases heat.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority under 35 U.S.C. § 120 to U.S. Provisional Application No. 60/652,130, filed on Feb. 11, 2005, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention The present invention relates to a heat retentive food server for maintaining food at an elevated temperature. The food server uses a phase change material as a heat storage medium. The invention also relates specifically to a thermoplastic, moldable, non-exuding phase change material for use in such food servers.  
         [0003]     2. Description of Related Art Several patents and publications are cited in this description in order to more fully describe the state of the art to which this invention pertains. The entire disclosure of each of these patents and publications is incorporated by reference herein.  
         [0004]     Phase change materials may be repeatedly converted between solid and liquid phases. Their latent heat of fusion can be used to absorb, store and release heat. These latent heats of fusion are greater than the sensible heat capacities of the materials. For example, in phase change materials, the amount of energy absorbed upon melting or released upon freezing is much greater than the amount of energy absorbed or released upon increasing or decreasing the temperature of the material over an increment of 10° C. for example, in the absence of a phase change.  
         [0005]     Stated alternatively, upon melting and freezing, a phase change material absorbs and releases substantially more energy per unit weight than a sensible heat storage material that is heated or cooled over the same temperature range. The ability of a phase change material to absorb and release a large quantity of energy in the vicinity of its melting/freezing point is the key to its usefulness in heat retention uses, e.g., for food servers.  
         [0006]     Various food service operations encounter the problem of keeping food warm after preparation and for a period of time until consumption. These include hospitals, nursing homes, hotels, airlines, among others. Numerous devices have been developed to help alleviate the problem of food getting cold. The simplest approach may be a cover or dome with a layer of insulating material disposed therein which can be place over a plate containing the warm food. Insulated bases that can hold a plate or container can be used in conjunction with such covers or domes. Other devices include servers which can be heated to an elevated temperature.  
         [0007]     Many servers which can be heated to an elevated temperature comprise a hollow shell with a cavity disposed therein. A heat-retentive medium is placed in the cavity so that when the container is heated, the heat-retentive medium will store heat and then release heat to maintain food placed adjacent to the server at an elevated temperature. The heat-retentive medium typically has been a wax or wax mixture, though this requires an additional enclosure within the server to seal the wax so that leakage through the server does not occur. Construction of such servers is time-consuming and expensive.  
         [0008]     Phase change materials have also been used as heat-retentive media with varying degrees of success. For example, U.S. Pat. No. 5,565,132 discloses a multi-component composite for use as a phase change material to alleviate problems with “oozing” or exuding that are typical of alkyl hydrocarbon phase change materials, and to lower the overall cost of the phase change material. The composite comprises a mixture of an alkyl hydrocarbon phase change material, a polyolefin resin, an ethylene copolymer and silica particles. A microwave absorbing additive can also be added. The ethylene copolymer component is disclosed to comprise about 8-12% by weight of the mixture.  
         [0009]     U.S. Pat. No. 5,520,103 discloses a phase change material that, when used as a heat storage medium, eliminates the problem of leakage common to servers using wax. The phase change temperature of the heat storage medium is preferably from 190° F. to 230° F. The only heat storage medium disclosed is ethylene vinyl acetate (EVA). Various food server configurations using a phase change material as the heat-retentive medium are also disclosed.  
         [0010]     One problem with the use of EVA as a heat-retentive medium in food servers is that over time and with repeated heatings, the EVA degrades and eventually cannot be re-used. This makes the life cycle for such food servers relatively short, thereby increasing the cost of use.  
         [0011]     A need exists to improve the heat storage medium for heat-retentive food servers, and more specifically to increase the life cycle of such servers.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     The present invention overcomes the problems associated with the prior art by improving the durability and usefulness of heat retentive food servers.  
         [0013]     In accordance with this concept, the invention herein provides a heat-retentive food server comprising: 
    (a) an upper shell;     (b) a lower shell;     said upper shell and said lower shell being joined together and having a cavity defined therebetween, said upper and lower shells being comprised of a thermoplastic material, and     (c) a heat storage material disposed in said cavity, said heat storage material comprising a phase change material substantially filling said cavity and being unrestrained therein, whereby said food server may be heated and placed adjacent to food for maintaining food at an elevated temperature,     wherein said phase change material comprises an ethylene acrylate copolymer.    
 
         [0019]     The invention also provides a heat-retentive food server comprising: 
    (a) an upper shell;     (b) a lower shell;     said upper shell and said lower shell being joined together and having a cavity defined therebetween, said upper and lower shells being comprised of a thermoplastic material, and     (c) a heat storage material disposed in said cavity, said heat storage material comprising a phase change material substantially filling said cavity and being unrestrained therein, whereby said food server may be heated and placed adjacent to food for maintaining food at an elevated temperature,     wherein said phase change material consists essentially of an ethylene acrylate copolymer.    
 
         [0025]     Moreover, in another embodiment, the invention provides a material for thermal energy storage comprising an ethylene acrylate copolymer.  
         [0026]     Furthermore, the invention provides a method for making a re-heatable food-warming device comprising the steps of: 
    (a) forming a container from a thermoplastic material wherein said container has an upper shell and a lower shell;     (b) injection molding a phase change material comprising an ethylene acrylate copolymer into an internal cavity formed between the upper and lower shells such that the phase change material substantially fills the cavity; and     (c) sealing the upper and lower shells with a polymeric sealant at their periphery to contain the phase change material within the internal cavity.   
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIG. 1  shows a top view of a heat retentive food server of the present invention.  
         [0031]      FIG. 2  shows a view taken through section  2 - 2  of  FIG. 1 .  
         [0032]      FIG. 3  shows a bottom view of the heat retentive food server of the present invention.  
         [0033]      FIG. 4  shows the heat retentive food server of  FIG. 1 , in combination with an underlying food plate.  
         [0034]      FIG. 5  shows a top view of an additional embodiment of a food server of the present invention.  
         [0035]      FIG. 6  shows a view taken through section  6 - 6  of  FIG. 5 .  
         [0036]      FIG. 7  shows a view taken through section  7 - 7  of  FIG. 5 .  
         [0037]      FIG. 8  shows a section view of an additional embodiment of the present invention.  
         [0038]      FIG. 9  is a graph depicting the thermal stability of certain ethylene copolymers under nitrogen. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0000]     Definitions  
         [0039]     The following definitions apply to the terms as used throughout this specification, unless otherwise limited in specific instances.  
         [0040]     As used herein, the term “about” means that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art. In general, an amount, size, formulation, parameter or other quantity or characteristic is “about” or “approximate” whether or not expressly stated to be such.  
         [0041]     In this disclosure, the term “copolymer” refers to a polymer polymerized from two or more monomers, and includes terpolymers. The more specific description ‘ethylene acrylate copolymer’, ‘ethylene-methyl acrylate (EMA) copolymer’, and the like, is meant to include copolymers which may also have a third monomer present.  
         [0042]     Copolymers of ethylene and an unsaturated C 1 -C 8  alkyl acrylate are well known. “Ethylene acrylate copolymers” may also be referred to as ethylene-acrylic acid ester copolymers. They can be manufactured from two high-pressure free radical processes: tubular processes or autoclave processes. The difference in ethylene acrylate copolymers made from the two processes is described in, e.g., “High flexibility EMA made from high pressure tubular process.” Annual Technical Conference—Society of Plastics Engineers (2002), 60 th  (Vol. 2), 1832-1836.  
         [0043]     The term “ethylene copolymer”, as used herein, refers to a polymer copolymerized from ethylene and one or more olefinic monomers, preferably one or more alpha-olefin monomers or vinyl acetate monomer. The copolymers of ethylene and alpha olefin can be made by processes employing Ziegler Natta catalysts or single site catalysts, for example, metallocene catalysts. Copolymers of ethylene and vinyl acetate are typically produced using free radical polymerization.  
         [0000]     Description  
         [0044]     In accordance with the present invention and referring now to the drawings, and more particularly to  FIG. 1 , there is shown an embodiment of a heat retentive food server.  FIG. 1  shows a food cover  1  which may be used with an underlying food holder  2 , such as the plate schematically shown in  FIG. 4 . The food cover  1  includes an outer shell assembly  10  having an outer surface  11  and inner surface  12 . The outer assembly  10  is comprised of an upper shell  15  and a lower shell  20 , and has a cavity  25  defined by the upper and lower shells. As shown in  FIG. 2 , the cavity  25  defined by the upper and lower shells is preferably a single cavity which is uninterrupted and undivided. In other words, there are no restrictions, partitions or additional enclosures located within the cavity.  
         [0045]     Lower shell  20  has an inner surface  30 , an outer surface  32 , and a top wall  34 . A side wall  38  extends downward from and merges with top wall  34 . Side wall  38  may also include a lip  42  at a peripheral portion  44 , which may also be referred to as the lower end  44  thereof. Lip  42  is comprised of a substantially horizontal portion  46  and a downwardly extending portion  48 . Lip  42  can engage the rim of a plate or other food holder placed therebelow as shown in  FIG. 4  to completely cover the plate and any food placed thereon.  
         [0046]     Upper shell  15  may include a top wall  50  with a side wall or side leg  52  extending downwardly therefrom. Upper shell  15  has an inner surface  54 , an outer surface  56 , and a lower end, or peripheral portion  62 . Thus, outer surface  11  of outer shell  10  is comprised of outer surfaces  32  and  56  of the lower and upper shells,  15  and  20  respectively. As shown in  FIG. 2 , the upper shell  15  surrounds the lower shell  20 , thereby defining the cavity  25 .  
         [0047]     The upper shell  15  may also include a central recess  58  and a handle  60 . The handle  60  may span the full diameter of the central recess  58 . The upper and lower shells  15  and  20  are joined at their peripheral portions at a joint  64 .  FIG. 1  shows a snap-ring configuration at joint  64 . However, numerous other configurations are acceptable. For instance, the upper and lower shells  15  and  20  may have flat surfaces at the ends thereof that can simply abut each other as depicted in  FIG. 2A . The joint  64  may be sealed with a polymeric sealant, for example. Any polymeric sealant capable of withstanding repeated heating to a temperature of at least about 230° F. without losing its material properties may be used. Examples of such sealants include polycarbonate sealants, e.g., 3M® No. 1838 which will withstand temperatures up to 350° F.  
         [0048]     A heat storage material  70  may be disposed in cavity  25 . The heat storage material  70  comprises, and more preferably is, a phase change material as described herein more fully below. The phase change material substantially fills the cavity and has a phase change temperature that is preferably in the range of about 190° F. to about 230° F. Generally, the phase change material is thermoplastic, moldable and non-exuding. Preferably, it is a solid at room temperature and changes to a viscous, gelatinous state when heated.  
         [0049]     The phase change material comprises an ethylene acrylate copolymer, and, preferably, consists essentially of an ethylene acrylate copolymer. The ethylene acrylate copolymer is preferably ethylene methyl acrylate, ethylene ethyl acrylate, ethylene n-butyl acrylate or a combination of one or more of ethylene methyl acrylate, ethylene ethyl acrylate, and ethylene n-butyl acrylate. More preferably, the ethylene acrylate copolymer comprises or consists essentially of ethylene methyl acrylate.  
         [0050]     The heat storage material  70  may contain additional components to enhance or improve certain attributes, or to reduce cost. For example, a microwave absorbing additive could be included to facilitate heating the phase change material using a microwave oven. Possible additional components include other polymers, e.g., polyethylene and/or higher melting polyethylenes such as high density polyethylene or low density polyethylene, as well as other ethylene copolymers. Generally, the heat storage material  70 , and preferably the phase change material, should contain at least about 12% by weight of an ethylene acrylate copolymer.  
         [0051]     Referring again to  FIGS. 1 and 2 , the upper and lower shells  15  and  20  are preferably made from a thermoplastic material capable of withstanding repeated prolonged temperature increases to at least about 230° F. without a loss of material properties. Any type of thermoplastic material capable of withstanding great temperatures can certainly be used. The heat storage material  70  is used to fill the cavity  25 .  
         [0052]     The fact that the phase change material is a solid at room temperature facilitates construction of the food cover  1  containing the phase change material in cavity  25 . The phase change material may be formed as a block in substantially the same shape as the cavity  25 . The block can be formed by injection molding or other suitable means to the desired configuration. The upper and lower shells  15  and  20  can then simply be joined together around the phase change material and sealed with sealant (such as a polycarbonate sealant discussed above) at joint  64 .  
         [0053]     Alternatively, the upper and lower shells  15  and  20  may be partially sealed together. The phase change material may be injection molded into the cavity  25 , and then the seal between the upper and lower shells  15  and  20  may be completed to contain the phase change material.  
         [0054]     An advantage of the phase change material disclosed herein is that leakage problems are eliminated without the need for other artificial seals, enclosures or restraints in the cavity as required with many prior art servers. The food server may be made of a simple two-piece construction.  
         [0055]     When the food server is heated so that the phase change material in cavity  25  reaches its phase change temperature (i.e., it changes from a solid to a gelatinous state), heat is stored therein. The phase change material will typically change phase when heated above its melting temperature for a sufficient time to ensure the phase change is complete. The exact time will depend on the particular material and the design of the server.  
         [0056]     The heat stored in the material  70  is released at a relatively slow rate as the material changes from a gelatinous state back to a solid. Also, the temperature of the material  70  remains relatively constant during the phase change. Thus, when the food server  1  is placed over a plate containing food, heat is directed through the outer shell  10  and will maintain food placed thereunder at an elevated temperature for an extended period of time.  
         [0057]     The underlying food holder may be of virtually any material, e.g., ceramic or thermoplastic. Typically, food will be placed under the cover shortly after it is removed from a cooking apparatus, for example an oven or a steamer, and will be at a temperature in the range of about 170° to 200° F. and more preferably from about 185° to 195° F. The food cover  1  will generally maintain such food at a temperature of at least about 145° F. for at least 60 minutes. Thus, when it reaches its ultimate destination, the food is still at an acceptable and desirable serving temperature.  
         [0058]     The food server  1  may also include an insulator  80 . In the embodiment shown in  FIG. 2 , the insulator  80  is interposed between the phase change material and the upper shell  15 . The insulator  80  will prevent heat stored in the phase change material from dissipating rapidly through the upper shell  15 . In this way, the heat stored is transferred more efficiently to the food to maintain the food at an elevated temperature rather than dissipating through the upper shell  15  and into the environment. The insulator  80  can be any type of insulating material capable of withstanding the temperatures described herein. One acceptable insulating material  80  is a high strength composite paper made from a combination of ceramic fiber, inert fillers and reinforcing fiberglass One such paper is commercially available from the Unifrax Corporation of Niagara Falls, N.Y. (formerly the Carborundum Company) and is known as Fiber Frax® grade 440 ceramic fiber paper.  
         [0059]     An additional embodiment of the present invention is shown in  FIGS. 5 through 7 . Referring now to  FIG. 5 , a food server  100 , which comprises a food holder, or food pan, is shown therein. As shown in  FIG. 6 , the pan includes an outer shell assembly  102  having an inner surface  104  and outer surface  106 . The outer shell assembly  102  is comprised of a lower shell  108  and an upper shell  110 . The lower shell  108  can be described as surrounding the upper shell  110 . The upper shell  110  has an inner surface  112  and an outer surface  114 . The lower shell  108  has an inner surface  116  and an outer surface  118 . Thus, inner surface  104  is comprised of inner surface  112  and inner surface  116  of the upper and lower shells,  110  and  108  respectively. A cavity  120  is defined by the outer shell assembly  102 . The cavity is defined by the upper and lower shells  110  and  108  and is preferably a single, undivided and uninterrupted cavity as previously described with respect to the embodiment shown in  FIGS. 1 and 2 . While the particular embodiment shown in  FIG. 5  is rectangular in shape, the server can be of any shape that is desired, including, but not limited to, shapes such as circular, oval and square.  
         [0060]     The upper shell  110  has a substantially flat base portion for placing food thereon, i.e., a bottom wall or base  122  and upwardly extending side wall  124 . The bottom wall  122  and upwardly extending side wall  124  thereby form a trough or open space for holding food. The upper shell  110  may further include an outwardly extending lip  126  at the upper end or peripheral portion  128  of the upper shell. The lip  126  circumscribes the entire periphery  128  of the upper shell  110 .  
         [0061]     The lower shell  108  includes a substantially flat support portion, i.e., a bottom wall or support portion  130 , and upwardly extending side walls  132 . The lower shell  108  surrounds the upper shell  110  as previously described, and may include an outwardly extending lip  135  at the upper end, or peripheral portion  134  thereof. The upper and lower shells  110  and  108  are joined together at their peripheral portions  134  at a joint  138 , thereby defining cavity  120 . The joint  138  is sealed with a sealant such as a polycarbonate sealant, discussed above.  
         [0062]     A phase change material  136  is disposed in cavity  120 . The phase change material  136  substantially fills the cavity  120  and is unrestrained therein as described with respect to the food cover embodiment. The phase change material  136  is as described above, comprising an ethylene acrylate copolymer. Outer shell  102  may likewise be made of the materials described with respect to the embodiment shown in  FIGS. 1 and 2 .  
         [0063]     When the server  100  is heated so that the phase change material  136  changes phase to a gelatinous state, food which is typically at a temperature in the range of about 170° F. to 200° F. or more preferably, from about 185° to 195° F. is placed on the food holder  100  after being removed from an oven or other heating device, such as boiling water or a steamer. The phase change material  136  will transfer heat through the upper shell  110  to the food so that it is maintained at a temperature of at least about 145° F. for at least about 60 minutes. Because the food is in direct contact with the shell  110 , the temperature of the food will most likely stay above about 145° F. for a period generally longer than an hour. An insulator  140  may be disposed between the phase change material  136  and lower shell  108  so that less or no heat is lost to the environment and is instead directed through upper shell  110  to the food placed-thereon. The insulator  140  may be made from any suitable material, including any material that is described above with respect to the insulator  80 .  
         [0064]     An additional embodiment of an upwardly opening food holder is shown in  FIG. 8 . The configuration shown there, designated by the numeral  200 , is similar to a commonly-known dinner plate and is comprised of an outer shell assembly  202  having a cavity  204  disposed therein. Outer shell assembly  202  is comprised of a lower shell  206  and an upper shell  208 . The two shells are joined at joint  210  and sealed with a sealant such as a polycarbonate sealant. As described elsewhere herein, the cavity  204  will be substantially filled with a phase change material  211  and can be heated so that it will maintain food at an elevated temperature. An insulator  212  may be disposed between the lower shell  206  and the phase change material  211 .  
         [0065]     As is clear from the foregoing, the phase change material can be formed, by injection molding or otherwise, to match the configuration of virtually any cavity shape. A two-piece construction can then be placed around the formed phase change material and sealed to construct the server. Alternatively, the phase change material may be injected directly into the two-piece construction.  
         [0066]     Thus, regardless of the exact configuration of the food server, the servers of the present invention are of simple construction, eliminate problems of leakage associated with prior art servers, and effectively transfer the heat required to maintain food at a desirable temperature for an extended period of time. The inner chambers and other restraints required in previously known servers to eliminate leakage are eliminated.  
         [0067]     The invention herein also provides a method for making a re-heatable food warming device. The first step comprises forming a container by thermoforming or preferably, injection molding, a shell from a thermoplastic material. The container preferably has an upper shell and a lower shell and is similar to the food servers described for  FIGS. 1-7  herein. The upper and lower shells each have an inner surface. When sealed together, the upper and lower shells define a specifically configured cavity. Alternatively, the shell may be molded in one piece.  
         [0068]     In the second step, a phase change material comprising an ethylene acrylate copolymer is formed to match the size, shape and configuration of the cavity formed between the upper and lower shells. Thus, the phase change material will substantially fill the cavity formed when the upper and lower shells are sealed together. The phase change material is preferably injection molded into the cavity. The upper and lower shells are sealed together, or the seal of a one-piece shell may be completed, in any suitable manner. The seal may be formed or completed at any suitable time before, during, or after the injection of the phase change material. One particularly suitable sealing method is to use a polymeric sealant such as a polycarbonate sealant.  
         [0069]     Alternatively, the phase change material may be formed separately, for example when the shell has been molded in two pieces that allow a molded solid phase change material to be placed between them. The upper and lower shells are then sealed together in any suitable manner. One particularly suitable method is to use a polymeric sealant such as a polycarbonate sealant.  
         [0070]     The resulting food warming device may be heated and reheated and is useful for holding, or placing adjacent to, freshly prepared hot foods to maintain the food at an elevated temperature for an extended period of time.  
         [0071]     The present invention can be illustrated by the following examples, which are intended to be exemplary only and are not meant to limit the scope of the invention.  
       EXAMPLES  
     Examples 1-3, Comparative Example 1  
       [0072]     The TGA (Thermogravimetric Analysis) test was run by starting samples at 40° C. and raising temperatures at 10° C. per minute and measuring the weight loss of the polymers as they degraded. In this way, ageing of the materials was accelerated and their relative thermal stability determined.  
         [0073]     After 30 minutes the samples were at 340° C. and after 37 minutes the samples were at 410° C.  
         [0074]     The data demonstrate the fact that EVA (ethylene/18 wt % vinyl acetate, line  1  of  FIG. 9 ) begins to degrade before the more stable E/nBA (ethylene/27 wt % n-butyl acrylate, line  2  of  FIG. 9 ), E/EA (ethylene/18 wt % ethyl acrylate, line  3  of  FIG. 9 ), or even better E/MA (ethylene/24 wt % methyl acrylate, line  4  of  FIG. 9 ) copolymers. Without wishing to be held to any theory, it is believed that the greater thermal stability of the ethylene acrylate copolymers provides improved durability and usefulness as a heat storage material in a food server, as described herein.  
         [0075]     The data are tabulated in Table I below.  
                                     TABLE I                           Thermal Stability of Ethylene Copolymers under Nitrogen            Time of                       Ramp@10° C./m       in   Example 1   Example 2   Example 3   Comparative       (start at   E/24 wt %   E/18 wt %   E/27 wt %   Example 1       40° C.)   MA   EA   nBA   E/18 wt % VA                0 minutes    100%    100%    100%    100%       (40° C.)   remaining   remaining   remaining   remaining       30 minutes   99.8%   99.7%   99.5%   96.5%       (340° C.)   remaining   remaining   remaining   remaining       37 minutes   96.6%   93.2%   91.2%   86.4%       (410° C.)   remaining   remaining   remaining   remaining                  
 
         [0076]     Although preferred embodiments of the invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous modifications without departing from the scope and spirit of the invention as defined by the appended claims.