Patent Publication Number: US-6986435-B2

Title: Self-heating food and beverage container made from a thermally conductive polymer composition

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. provisional application No. 60/314,370 having a filing date of Aug. 23, 2001. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to a self-heating container for heating consumable items such as food and beverages. The container includes a heating medium that produces an exothermic reaction to warm the food and/or beverage. More particularly, the container is made from a thermally conductive polymer composition that can transfer heat effectively from the heating medium to the food/beverage. 
   Today, many people wish to pursue outdoor activities in environments where modern conveniences such as stoves and microwave ovens are not readily available. Activities such as mountain climbing, ice fishing, snowmobiling, and cross-country skiing are becoming more popular. These activities often take place in harsh climates where it is infeasible to prepare conventional hot meals. Still, many participants want to enjoy a hot meal or beverage while engaging in such pursuits. The food and beverage industry has developed self-heating food/beverage containers to meet this demand. Typically, these containers include two compartments. One compartment holds lime and the other compartment holds a sealed bag of water. A utensil can be used to pierce the bag and release the water. The layer of lime absorbs the flowing water and an exothermic reaction occurs. The reaction of the lime and water generates a sufficient amount of heat to warm the food/beverage. 
   Variations of self-heating cans are known in the prior art. These prior art systems have several drawbacks. First, the partition separating the fuel-containing chamber from the food/beverage chamber has a relatively small surface area for transferring heat. Second, this partition is made from plastic, such as polypropylene or polyethylene, which are heat-insulating materials. These non-conductive compositions limit the amount of heat that can be transferred across the partition. Third, the container has only a single outer wall, and heat can escape through this wall making the container hot-to-touch. Fourth, the poor heating mechanism of these devices means that the food/beverage must be heated for a longer period of time. 
   In view of the foregoing deficiencies among others, there is a need for an improved self-heating food/beverage container. The improved container should have a structure and design that allows for the effective transfer of heat from the heating medium to the chamber containing the food/beverage. The container should also have a structure and design that prevents heat from escaping through its outer walls. The present invention provides such containers. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a self-heating container for heating consumable items such as food and beverages. The container assembly includes a combination of an inner and outer container. Particularly, the container assembly includes a star-shaped inner container for holding media, such as lime and water, that will produce an exothermic reaction. This inner container is made of a thermally conductive polymer composition. The composition includes a polymer matrix and thermally conductive filler material. Preferably, the inner container has a thermal conductivity of greater than 3 W/m°K and more preferably greater than 22 W/m°K. An outer container surrounds the inner container so that a chamber is formed between the two containers. The chamber between the inner container and outer container holds consumable items, such as food and beverages, that will be heated. The outer container is made of a heat-insulating material such as polystyrene. 
   The thermally conductive composition used to make the inner container can include a thermoplastic or thermosetting polymer matrix. For example, a thermoplastic polymer selected from the group consisting of polyethylene, acrylics, vinyls, and fluorocarbons can be used to form the matrix. Preferably, a liquid crystal polymer is used. Alternatively, a thermosetting polymer selected from the group consisting of elastomers, such as polysiloxanes and polyurethanes, epoxies, polyesters, polyimides, and acrylonitriles can be used. 
   Suitable thermally conductive filler materials include aluminum, alumina, copper, magnesium, brass, carbon, silicon nitride, aluminum nitride, boron nitride, and zinc oxide. 
   In a preferred embodiment, the outer container has a peripheral wall with a double-wall structure. Particularly, an outer wall segment completely surrounds an inner wall segment such that a gap exists between the two segments. The gap can be filled with air or other heat-insulating materials. The inner and outer wall segments are made of a heat-insulating material such as polystyrene. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features that are characteristic of the present invention are set forth in the appended claims. However, the preferred embodiments of the invention, together with further objects and attendant advantages, are best understood by reference to the following detailed description taken in connection with the accompanying drawings in which: 
       FIG. 1  is a front perspective view of a self-heating container assembly of the present invention; 
       FIG. 2  is a cross-sectional view of a self-heating container assembly of the present invention through line  2 — 2  of  FIG. 1 ; and 
       FIG. 3  is a cross-sectional view of an alternative embodiment of the self-heating container assembly of the present invention showing the outer container having a peripheral double-wall construction. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention relates to a self-heating container assembly for heating consumable items such as food and beverages  19 . The container assembly includes in combination a first (inner)  12  and second (outer) container  14 . 
     FIG. 1  shows a front perspective view of the self-heating can  10  of the present invention having an outer container  14  with a spout  15  for access to the edible material within the can  10 . A heating core, generally referred to as  17 , resides within the outer container  14  to heat the edible material as described in detail below. 
   Referring to  FIG. 2 , a cross-sectional view through line  2 — 2  of  FIG. 1 , the container assembly  10  includes a first (inner) container  12  having a cross-sectional shape of a star. A thermally conductive polymer composition is used to make the star-shaped container  12 . The polymer composition contains a base polymer matrix and thermally conductive filler material. Thermoplastic polymers such as polyethylene, acrylics, vinyls, and fluorocarbons can be used as the matrix. Alternatively, thermosetting polymers such as elastomers, epoxies, polyesters, polyimides, and acrylonitriles can be used as the matrix. Suitable elastomers include, for example, polysiloxanes (silicones) and polyurethanes. Liquid crystal polymers are preferred due to their highly crystalline nature and ability to provide a good matrix for the filler material. Preferably, the polymer matrix constitutes about 30 to 60% by volume of the polymer composition. 
   Thermally conductive filler materials are added to the polymer matrix. Suitable filler materials include, for example, aluminum, alumina, copper, magnesium, brass, carbon, silicon nitride, aluminum nitride, boron nitride, zinc oxide, and the like. Mixtures of such fillers are also suitable. The filler material preferably constitutes about 20 to about 70% by volume of the composition. More preferably, the filler material constitutes less than 60% of each composition. 
   The filler material may be in the form of granular powder, whiskers, fibers, or any other suitable form. The granules can have a variety of structures. For example, the grains can have flake, plate, rice, strand, hexagonal, or spherical-like shapes. The filler material may have a relatively high aspect (length to thickness) ratio of about 10:1 or greater. For example, PITCH-based carbon fiber having an aspect ratio of about 50:1 can be used. Alternatively, the filler material may have a relatively low aspect ratio of about 5:1 or less. For example, boron nitride grains having an aspect ratio of about 4:1 can be used. Preferably, both low aspect and high aspect ratio filler materials are added to the polymer matrices as described in McCullough, U.S. Pat. Nos. 6,251,978 B1 and 6,048,919, the disclosures of which are hereby incorporated by reference. 
   The filler material is intimately mixed with the non-conductive polymer matrix to prepare the thermally conductive composition using techniques known in the art. Conventional injection-molding, blow-molding, melt-extrusion, or other suitable method can be used to form the composition into the shape of the inner container  12 . 
   As shown in  FIG. 1 , a second (outer) container  14  completely surrounds the inner container  12  so as to create a chamber  16  therebetween. The outer container  14  can have any suitable shape but is preferably in the shape of a cylinder. A heat-insulating material such as polystyrene or plastic, such as polypropylene or polyethylene, is used to make the peripheral wall of the outer container. The peripheral outer wall  20  of the star-shaped inner container  12  serves to separate the inner container  12  from the outer container  14 . The food or beverage  19  that will be heated is placed in the chamber  16  located between the two containers  12  and  14 . The food or beverage  19  can be added directly to the chamber  16 , or a food/beverage-containing package such as a tin foil packet can be placed in the chamber  16 . 
   The first container  12  contains media that will produce an exothermic reaction when mixed together. Typically, the reactant media are water  28  and lime  26 , but other non-toxic materials can be used. The lime  26  and water  28  are stored in separate compartments  22  and  24  prior to use. For example, the lime  26  and water  28  can be stored in separate breakable capsules, or the lime  26  and water  28  can be stored in separate compartments with a thin membrane  30  therebetween. Other structures for keeping the water  26  and lime  28  separate for later mixing can be used. When a person wishes to heat the food or beverage, the lime  28  and water  26  are mixed together to produce an exothermic reaction. For example, the entire can  10  may be shaken to break the membrane  30  and mix the lime  28  and water  26 . The heat given off during the reaction is transferred along the peripheral edges of the thermally conductive first container  12  to warm the food/beverage  19  located in the chamber  16 . The star-shape design of the inner container  12  is important, because it provides a large surface area for radiating heat to the food/beverage  19  in chamber  16 . As used herein, the term, “star”, refers to the well known figure having five or more points. It should be understood that a “star’ configuration is preferred for the inner container  12 . However, other configurations, such as squares, triangles, and other geometric shapes, may be employed. 
   The self-heating containers of the present invention have several advantageous features over conventional containers. As discussed above, the unique cross-sectional star shape gives the first container  12  a large surface area for better heat conduction. Further, the first container  12  is made from a thermally conductive polymer composition that provides an optimum pathway for transferring heat from its interior to the food/beverage material  19  located in the chamber  16 . The thermally conductive composition preferably includes a crystalline polymer matrix and is anisotropic. Thus, thermal conductivity is higher along the planar surface of the inner container  12  than through the surface of the container  12 . 
   In an alternative embodiment, the outer container  14  has a peripheral wall with a double wall construction as shown in  FIG. 3 . The gap between the outer wall segment  26  and inner wall segment  28  is preferably filled with air but could be filled with other insulating media and materials. Both wall segments are made of a heat-insulating material such as polystyrene or plastic. For example, polypropylene or polyethylene can be used. This double wall construction prevents the loss of heat to the outside environment. As a result, more heat is retained within the chamber  16 , and the time needed to sufficiently warm the food/beverage material  19  is decreased. The heated food/beverage material  19  retains the heat for a longer period of time. Further, the double wall construction keeps the self-heating container  10  cool-to-touch allowing a person to safely handle the container. 
   It is appreciated by those skilled in the art that various changes and modifications can be made to the illustrated embodiments without departing from the spirit of the invention. All such modifications and changes are intended to be covered by the appended claims.