During delivery of food and other products, insulated boxes and thermal bags are often used to keep the product warm while it is being transported from one place to another. These bags and boxes typically also have a heat reflective material on the inner surface, and they reduce the rate at which the product loses heat. Once the product is removed from the cooking device, its temperature begins to decline even though it is placed in an insulated box or bag.

Phase change materials have been found to be particularly useful for food warming, especially during transportation. Various food warming devices using phase change materials have been proposed, such as in United States Patent Nos.

4,246,884,4,982,722,5,254,380, and 5,520,103.

A device having an electric heating element sandwiched between two cross- linked high density polyethylene (HDPE) sheets is available from Vesture Corporation (the Vesture Heat Storage Board). In some situations, a plastic such as HDPE, can be considered to be a phase change material. However, the maximum latent heat of such a product occurs at about 270°F, a temperature that is too high for serving hot food since it could burn or char the food or burn the people handling the food. At a temperature of 140°F which is desired for serving hot food, the Vesture Heat Storage Board will only store and release the sensible heat of the cross-linked HDPE which produces only about 6 calories over a temperature range of 18°F. This value is very low, and is available for only a short period of time (determined by the insulation) because cross-linked HDPE has little capacity for thermal energy storage below its crystalline melting temperature of about 270°F.

For these reasons, it appears that the cross-linked HDPE plates or sheets are simply being used in the Vesture Heat Storage Board as a cover to protect the electric heating element which is the source of the energy for food warming purposes.

Another heating device is disclosed in U. S. Patent No. 5,750,962, assigned to Vesture Corporation. This device uses a paraffin-based phase change material and includes an electrically resistive heating element which operates to control heating of the phase change material.

Another heating device is disclosed in U. S. Patent No. 5,884,006. The heating device comprises a container and a rechargeable phase change material unit removably positioned in the container. The rechargeable phase change material units inclues a first shell portion and a second shell portion forming a cavity with a phase change component including a phase change material disposed in the cavity. It includes a heating element which controls heating of the phase change material. The rechargeable phase change material units is typically placed in an insulated bag such as those used to deliver pizzas. When the pizza is delivered to the customer, the pizza is removed from the insulated bag and given to the customer. The heating device is returned to the store and reused.

However, when a customer picks up a pizza, hamburger, or other food to take home, heating devices such as are described above cannot be used to keep the food warm during transportation. The heating devices are too expensive to be given away to the consumer, and there is no way to ensure that the heating devices would be returned to the store.

Therefore, there is a need for a food warming device which can be used in situations where the consumer picks up food to take it home. There is a need for a food warming device which is inexpensive enough to be sold in retail stores for general consumer use and which for some applications can even be discarded after it has been used. There is also a need for a food warming device which can be made in a variety of sizes and shapes to be used with many different kinds of take- out food.

The present invention provides a solution to the needs existing in the art by providing an inexpensive food heating device which emits sufficient heat to keep food hot while it is transported to the consumer's home.

According to one aspect of the invention, a heat storage board for heating food and the like is provided. The heat storage board comprises a porous support

containing a phase change material. The porous support is encapsulated in a film covering. The phase change material is capable of being charged with heat energy by a heat source.

The phase change material can be selected from materials including, but not limited to, linear crystalline alkyl hydrocarbons, alkyl hydrocarbon waxes, crystalline fatty acids, linear crystalline primary alcohols, ethylene copolymers, polyethylene, polyethylene glycol, polyethylene oxide, and acetamide.

In a preferred embodiment of the invention, the porous support is a ceiling tile type material, i. e. a porous mineral fiber tile. When the heat storage board is to be charged at lower temperatures, from about 90°F to about 200°F, the porous support can contain an organic binder. When the heat storage board is to be charged at temperatures above about 350°F, the porous support should be substantially free of organic binder.

In another embodiment of the invention, an outer plastic or metal shell encloses the porous support. The porous support may or may not be wrapped in the film covering prior to application of the outer shell.

The heat source includes, but is not limited to, conduction, convection, radiant heat, electrical resistance, electrical inductance, microwave heat, radio frequency, or combinations thereof.

When the heat source comprises microwave heat (such as from a microwave oven), a microwave absorbing additive is preferably added to the phase change material, the film covering, or the outer shell.

Preferred film coverings include plastic film, metal film, metalized plastic films, and polymeric and silicone coating films.

According to another aspect of the invention, a method of producing a heat storage board is provided. The method comprises providing a porous support, placing the porous support in a bath of phase change material, allowing the porous support to absorb phase change material, and encapsulating the porous support in a film covering.

Fig. 1 presents a cross-section of a one embodiment of the heat storage board of the present invention.

Fig. 2 presents a cross-section of a second embodiment of the heat storage board of the present invention.

Figs. 3 and 4 are graphs showing the effect of using a heat storage board on the temperature of a hamburger over time.

Fig. 1 shows a cross-section of the heat storage board of the present invention. The heat storage board 10 includes a porous support 12 containing a phase change material. There is a film covering 14 encapsulating the porous support 12. The film covering 14 protects the porous support 12 and prevents the phase change material from coming into contact with the food.

Fig. 2 shows another embodiment of the heat storage board 16. In this embodiment, an outer shell 18 surrounds the film covering 14. The film covering 14 prevents the phase change material from coming into contact with the outer shell 18.

This avoids any possible materials incompatibility problem between the phase change material and the outer shell 18.

The porous support 12 is desirably formed into a discrete shape, such as a circle, square, or rectangle, although it can have any shape or size needed. By forming the porous support 12 into a discrete shape, the heat storage board can be easily manufactured. The porous support 12 can be formed to the desired shape using any suitable means, such as cutting, stamping or slitting.

The phase change material is preferably selected from linear crystalline alkyl hydrocarbons, alkyl hydrocarbon waxes, crystalline fatty acids, linear crystalline primary alcools, ethylene copolymers, polyethylene, polyethylene glycol, polyethylene oxide, and acetamide. Preferred phase change materials for use with the invention have a melting/freezing temperature of from about 90°F to about 220°F. If the heat storage board is used to heat meat products for delivery purposes, then the desired phase change material has a minimum melting/freezing temperature of at least about 140°F.

Suitable phase change materials for use in the present invention include, but are not limited to, commercially available products such as Paraflint C80 available from Schümann Sasol of Hamburg, Germany, Shellwax 300 available from the Shell Oil Company, and Petrolite X-1151 available from the Baker Chemical Company.

The phase change material can be charged with heat energy by a heat source which can include conduction, convection, radiant heat, electrical resistance, electrical inductance, microwave heat, and radio frequency.

When the phase change material is charged using microwave energy, a microwave absorber, such as glycerin or carbon black is preferably included as an additive to the phase change material. Alternatively, a microwave susceptor coating can be added to the film covering or the outer plastic shell.

An antioxidant is preferably added to the phase change material to minimize thermal oxidative decomposition during repeated thermal cycling. The antioxidant is preferably present in an amount in the range of about 0.1 to 5% of the phase change material. Suitable antioxidants include, but are not limited to BHT (butylated hydroxy toluene) available from Eastman Chemical Co. and Santowhite powder available from the Monsanto Co.

The porous support is designed to contain the phase change material. It can be made from products including, but not limited to, particle board, plasterboard, wallboard, fiberboard, insulation, foams, floor panels, ceiling board and the like.

Ceiling board is an inexpensive, readily-available porous support.

For many applications where the temperature of the oven used to charge the heat storage board is in the range of 200°F to 250°F, ceiling tile material such as Item No. 280 from Armstrong World Industries, Lancaster, Pennsylvania, should work well (either unpainted or with the paint coating removed). However, this product includes an organic binder in addition to mineral fiber and fibrous glass which will not withstand high temperatures for long periods of time. Therefore, in cases where the temperature of the charging oven is on the order of, for example, 400°F, it is preferable to use a product which does not include an organic binder.

An example of a product which meets this requirement is Ceramaguard from Armstrong World Industries. It contains only ceramic and mineral fiber materials and can withstand temperatures of several hundred degrees Fahrenheit for extended periods of time.

Examples of suitable film coverings include, but are not limited to, plastic film, metal film, metalized plastic films, and polymeric and silicone coating films. Plastic

films, such as DuPont Mylar (polyethylene terephthalate), DuPont Teflon FEP, DuPont TeflonT" PFA, and DuPont KaptonT" polyimide, can be used. Film made from heat stabilized nylon produced by Gemini Plastic Enterprises, Inc., Maywood, California may also be used. Metal films, such as aluminum foil, can also be used as the film covering. Likewise, metalized plastic films such as metalized Mylar can be used. Films formed from polymeric and silicone coatings could also be used.

The heat storage board can be produced by placing a piece of porous support into a bath containing phase change material in the liquid state. The porous support is left in the bath long enough to absorb the phase change material, preferably until the porous support is saturated with the phase change material.

The phase change material used in this invention will normally have a melting point above room temperature. Consequently, it will have to be heated to a temperature above its melting point to convert it into a liquid in order to soak the porous support in it.

After the porous support has been soaked in the phase change material for a period of time, it is removed from the bath. Any excess phase change material can be drained and/or wiped off the porous support. A substantial amount of phase change material is retained in the porous support.

The porous support is then encapsulated in the film covering. If a plastic film or a metalized plastic film is used, it can be sealed with, for example, a heat seal.

Plastic films may be shrink-wrap sealed. A metal film could be sealed with a crimp- seal or a weld-seal. Films formed from polymeric or silicone coatings can also be used to encapsulate the porous support. In this case, the coating could be applied by dipping the board into a bath of the liquid coating material. Alternatively, the coating could be brushed, rolled, or sprayed onto the board. After it is applied, the liquid coating is cured to a stiff film layer. A silicone coating, such as a silicone conforma coating, number 1-2577 available from Dow Corning Corporation, Midland, Michigan, could be used for this type of film covering.

Alternatively, the coating can be applied using a molding process. In this process, the porous support is placed in a mold. It rests on one or more pins projecting from the surface of the mold, so that the majority of the surface of the

porous support does not contact the inside surface of the mold. The mold is then closed, and a coating material is injected into the mold, encapsulating the porous support in the coating. The coating material can be a polymeric material, including thermoplastic polymers and thermoset polymers, or a silicone material. After the coating hardens and the encapsulated heat storage board is removed from the mold, the small holes formed by the pins can be filled with a resin material. They could also be closed by melt sealing.

An outer shell can be applied around the porous support when a more durable heat storage board is needed. The outer shell can be applied around either the porous support itself, or the encapsulated porous support. The outer shell can be made of an upper and a lower shell portion which form a shell with a cavity. The porous support fits into the cavity in the outer shell. The outer shell could be made of molded plastic, such as nylon or SUPECT" polyphenylene sulfide available from the General Electric Company. It can be sealed, for example, using an adhesive or by melt sealing the plastic.

When the outer shell is used with a film covering encapsulating the porous support, the film covering prevents the phase change material from coming into contact with the outer shell. This eliminates any potential incompatibility problem between the phase change material and the outer shell.

Example 1 Two five inch square pieces of Armstrong ceiling tile, Item No. 280, one half inch thick, were prepared and heated in an oven at a temperature of 400°F for 30 minutes. With the exception of the painted surface of the tiles, the samples did not suffer any discoloration, melting, or other decomposition. The paint coating did turn from white to a dull yellow. It is anticipated that the base tile material will also undergo some thermal decomposition after extended exposure to 400°F temperatures due to the present of the organic binder in this particular ceiling tile.

The dry weight of sample 1 was 49 g, and the dry weight of sample 2 was 50 g.

The preheated samples were then immersed in a bath of Paraflint C-80 wax for 30 minutes. Air bubbles were still coming out of the sample after 15 minutes, but

after 30 minutes no bubbles were visible. The samples were removed from the bath, drained, and wiped with a paper towel to remove all excess surface wax. The weight of sample 1 was 142 g, and the weight of sample 2 was 144 g. Thus, the weight percent of paraffin in sample 1 was 65%, and it was 63% for sample 2.

The boards were then encapsulated in a film covering of aluminum foil.

The heat storage boards were tested to determine their effectiveness in keeping single, plain hamburgers hot. The hamburgers were reheated in a microwave, and the meat patty temperature was measured to be 165-170°F. The heat storage boards were heated in an oven at a temperature of 400°F for one hour.

The reheated hamburgers and the heated heat storage boards were then placed in three small insulated bags. One of the bags contained a hamburger. A second bag contained a hamburger and one of the heat storage boards. The third bag contained only a heat storage board. Thermocouples were placed in the meat patties to measure their temperature. In the bag containing only the heat storage board, a thermocouple was placed between the surface of the heat storage board and the inside surface of the bag.

Fig. 3 shows the results of this test. The minimum desirable serving temperature for hot food is 140°F. Fig. 3 shows that the hamburger in the bag containing the heat storage board maintains this temperature for at least 50 minutes longer than the hamburger alone.

Example 2 Two samples of Armstrong Ceramaguard 605 measuring 6"x 6"x 5/8"thick were prepared and heated in an oven at a temperature of 400°F for one hour. The weight of the dry boards was 149 g.

These samples were soaked in a bath of Paraflint C80 wax for 30 minutes.

The samples were drained, and the surfaces wiped dry with a paper towel. The weight of the soaked boards was 310 g. The weight percent of Paraflint C80 wax in the samples was 52%.

The samples were encapsulated in a film covering of aluminum foil.

The heat storage boards were tested to determine their effectiveness in keeping single plain hamburgers hot. The hamburgers were reheated in a microwave, and the meat patty temperature was measured to be about 170°F. The heat storage boards were heated in an oven at a temperature of 400°F for one hour.

The reheated hamburgers and the heated heat storage boards were then placed in two small insulated bags. One bag contained a hamburger with a thermocouple in the meat patty. In the second bag, a heat storage board was placed on the bottom of the bag, a hamburger with a thermocouple in the meat patty was placed over the heat storage board, and a second heat storage board was placed over the hamburger.

Fig. 4 shows the results of this test. The temperature of the hamburger with no heat storage board fell below 140°F after only about 20 minutes. In contrast, the hamburger in the bag with the heat storage boards actually increased in temperature during that period. The temperature of the hamburger remained above 160°F for three hours.

The heat storage boards of the present invention have many uses. They can be used to keep food hot in delivery bags while is it being transported from the restaurant or store to the place where it is to be consumed.

The heat storage boards could be used to keep food warm in institutional delivery carts. They could also be used as liners in bread baskets, or as warming trays, pads, or trivets on a serving table. The heat storage boards could also be used in lunch boxes or picnic baskets.

Since it is relatively inexpensive to make these heat storage boards, they can be sold for general consumer use, and in some cases, may even be marketed as single use, throw away items. For commercial applications where a more durable product is required, these heat storage boards can be made in a more substantial version, which is still less expensive than other products on the market.

While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the compositions and apparatus disclosed herein may be made without departing from the scope of the invention, which is defined in the appended claims.