the following definitions apply to the terms as used throughout this specification , unless otherwise limited in specific instances . 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 . 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 . 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 . 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 . in accordance with the present invention and referring now to the drawings , and more particularly to fig1 , there is shown an embodiment of a heat retentive food server . fig1 shows a food cover 1 which may be used with an underlying food holder 2 , such as the plate schematically shown in fig4 . 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 fig2 , 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 . 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 fig4 to completely cover the plate and any food placed thereon . 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 fig2 , the upper shell 15 surrounds the lower shell 20 , thereby defining the cavity 25 . 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 . fig1 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 fig2 a . 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 . 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 . 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 . 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 . referring again to fig1 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 . 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 . 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 . 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 . 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 . 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 . 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 . the food server 1 may also include an insulator 80 . in the embodiment shown in fig2 , 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 . an additional embodiment of the present invention is shown in fig5 through 7 . referring now to fig5 , a food server 100 , which comprises a food holder , or food pan , is shown therein . as shown in fig6 , 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 fig1 and 2 . while the particular embodiment shown in fig5 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 . 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 . 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 . 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 fig1 and 2 . 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 . an additional embodiment of an upwardly opening food holder is shown in fig8 . 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 . 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 . 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 . 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 fig1 - 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 . 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 . 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 . 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 . 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 . 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 . after 30 minutes the samples were at 340 ° c . and after 37 minutes the samples were at 410 ° c . the data demonstrate the fact that eva ( ethylene / 18 wt % vinyl acetate , line 1 of fig9 ) begins to degrade before the more stable e / nba ( ethylene / 27 wt % n - butyl acrylate , line 2 of fig9 ), e / ea ( ethylene / 18 wt % ethyl acrylate , line 3 of fig9 ), or even better e / ma ( ethylene / 24 wt % methyl acrylate , line 4 of fig9 ) 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 . 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 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 .
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 .