PORTABLE FLAMELESS HEAT PACK

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefits of and priority to U.S. Provisional Patent Application Ser. No. 60/854,478, filed on October 25, 2006, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

This invention relates to a portable heat pack and, more particularly, to portable flameless heat pack that includes a single-use chemical heater. BACKGROUND

Heat packs with single-use, flameless chemical heaters generally include two reactants that can be mixed together to create an exothermic reaction. Typically, the reaction generates heat until at least one of the reactants is consumed. The reaction may be, for example, a hydrolysis reaction, an oxidation-reduction reaction, an acid- base reaction or may involve exothermic dissolution of a solid or liquid in a liquid.

The increase in temperature that such heat packs experience during exothermic reactions is about the same regardless of the heat pack's ambient temperature. Therefore, a heater whose temperature rises from about 10 ⁰ C to about 75°C will generally rise in temperature to over 100 ⁰ C in an ambient temperatures over 35 ⁰ C. Excessively high operating temperatures may cause the generation of steam in the heat pack. Steam generation can increase the pressure in a heat pack and possibly lead to structural failure as well as subject the object being heated to excessively high temperatures.

SUMMARY A portable flameless heat pack is disclosed that utilizes an exothermic chemical reaction to create heat. A heat sink made of phase-change material is provided in the heat pack. The phase-change material generally helps to reduce the maximum operating temperature of the heat pack. The phase-change material also

helps to prolong the heat pack's useful heating time, helping to maintain the heat pack's operating temperature near its maximum operating temperature. The heat pack is generally useful in places where electricity is not available, or where electric or flame heaters would not be safe, for instance, where flammable fumes may be present. The heat pack is generally portable and is generally shaped like an approximately flat pad. The heat pack also is flexible and, therefore, can conform to the contours of a surface to be heated. The heat pack generally includes at least one of each of three types of chambers: a chamber for the first of two chemical reactants, a chamber for the second of the two chemical reactants, and a chamber for a solid-to- liquid phase-change material. It is preferable that each chamber be formed by placing two flexible plastic sheets atop one another as the pack is manufactured. The flexible sheets are sealed (e.g., by heat sealing or sealing with adhesive) to each other at peripheries of the chambers. The chambers for the two reactants are separated by a manually destructible seal or seals, such as a frangible membrane that can be ruptured by a user to permit mixing of the first and second reactants, thereby initiating the exothermic reaction. The reactant chambers may be placed one atop the other, one within the other, or side-be-side as an initial single layer. A preferred arranged is side-be-side.

In some implementations, the phase-change material chamber is formed near one major side of the heat pack and extends substantially across that major side in all directions. In some implementations, the phase-change material is a Master™ Superflex™ sheet of wax available from The Kindt-Collins Company of Cleveland, Ohio. The sheet of wax may have a thickness between about 1/16" (i.e., approximately 0.16 centimeters) and about 1/8" (i.e., approximately 0.31 centimeters). More preferably, the sheet of wax has a thickness of about 3/32" (i.e., approximately 0.2 centimeters). The specific dimensions of the phase-change material may vary depending on the requirements of each application.

Heat may be provided by an exothermic chemical reaction, such as an oxidation-reduction reaction, a hydrolysis reaction, or an acid-base reaction. Heat also may be provided by the heat of solution between a solid and a liquid, or between

two liquids. An oxidation-reduction reaction between a permanganate and an organic compound containing hydroxyl groups is preferred. Heat packs utilizing oxidation/reduction reactions are disclosed in U.S. Pat. No. 5,035,230 (the '"230 patent") and U.S. Patent No. 6, 116,231, which are incorporated by reference herein in their entireties. Most preferred is a reaction between an aqueous sodium permanganate solution and an aqueous glycerol solution. The amount of heat produced can be selected by controlling the concentrations and amounts of the solutions. The two solutions tend to react rapidly without producing a gas by-product. It is preferred that the reaction produce little or no gaseous product, particularly little to no flammable gaseous product.

In some implementations, one of the chambers includes an oxidizing agent, and, where necessary or desirable, a solvent. The second chamber contains liquid comprising a fuel, and, where necessary or desirable, a solvent. Many oxidizing agents are capable of generating suitable energies upon reaction with a corresponding fuel. Typical oxidizing agents include those comprising the alkali metal salts of the oxides of manganese and chromium. These include such compounds as potassium permanganate, and potassium chromate. Other suitable oxidizing agents are pyridinium dichromate, ruthenium tetroxide and chromic acid, as well as a host of other oxidizing agents known to those skilled in the art. Preferably, the oxidizing agent for use in the heat packs of the present invention comprises alkali metal salts of permanganate.

The corresponding fuels which are found suitable for the exothermic chemical reactions utilized in the devices and methods of the present invention are organic compounds. Particularly well suited organic compounds are alcohols. Alcohols are easily oxidized to carbonyl-containing compounds by the oxidizing agents described herein. Useful alcohols include primary alcohols, and preferably polyols which contain at least two hydroxyl groups. Such polyols are also readily oxidized to aldehydes and carboxylic acids. The oxidation of polyols and simultaneous reduction of the oxidizing agent are accompanied by the release of significant amounts of heat

energy. A preferred fuel for use in one of the preferred embodiments of the heat packs of the present invention is glycerine.

Oxidizing agents, fuels and solvents suitable for use in heat packs to be employed in the present device are described in U.S. Pat. No. 6,116,231, entitled "Liquid Heat Pack," and U.S. Pat. No. 5,984,953 entitled "Self-Regulating Heat Pack" each of which is hereby incorporated by reference in its entirety. For example, in particular embodiments, heat packs can employ oxidizing agents including these comprising the alkali metal salts of the oxides of manganese or chromium such as potassium permanganate, potassium chromate and others. The fuels for use in heat packs useful with the claimed invention should be complementary with the oxidizing agent. That is, the combination of oxidizing agent and fuel should provide a desired heat output, meet government safety standards, and be compact. In one of the preferred embodiments, the fuel comprises organic alcohols, such as polyhydroxy containing compounds. These include glycerine and similar polyols.

To avoid undesirably rapid heat evolution, the reactants can be diluted in a solvent, such as water, in some preferred embodiments, the oxidizing agent is potassium permanganate, the fuel is glycerine, and the solvent is water.

In some implementations, the phase-change material is a solid-to-liquid phase- change material and forms an effective sheet extending across one major side of the pack. The material may be a continuous flexible sheet, for example, a wax sheet that is manually conformable to the surface to be heated. Alternatively, the material may comprise multiple, discrete solid units, for example, rods, sticks or granules, that are spread across the pack and held in place, such that the chamber and the material together form an effective sheet.

If the chamber consists of two edge-sealed sheets, discrete solid units may be spread across one sheet and held in place within the chamber mechanically by adhesive, or most preferably, by a vacuum pulled between the sheets during pack assembly. Alternatively, the chamber for discrete solid units may be subdivided into small compartments by joining the sheets together in, for example, a grid pattern by

adhesive seals or heat seals. If discrete units are utilized to form an effective sheet of phase-change material, the units themselves may be rigid in the temperature range of use, the needed flexibility being provided by the chamber. If a continuous sheet is used, the sheet is sufficiently flexible to be mechanically conformed to the surface being heated. Phase-change heat sinks reduce the maximum temperature the heat pack achieves and prolong its useful heating time. The reaction may go rapidly to completion, providing a rapid, undamped initial temperature rise, and the excess energy can be captured by the heat sink. The energy stored in the heat sink is then slowly released back to the pack. Phase-change heat sinks absorb more heat per weight than a non-phase-change heat sink. The melting point of the phase-change material is preferably lower than the maximum desired temperature. A mixture of phase-change materials with different melting points may be used. The phase-change material may be supplemented with an additional heat sink, preferably a liquid that does not react with the heat-generating reactants. To accommodate a wide range of operating temperatures, different concentrations of chemicals may be chosen to provide the desired amount of heat for use in cold weather or in hot weather. The useful operating temperatures of any pack are limited by the ability to achieve sufficient heating for the intended heating task at its low limit, and swelling from steam generation or other solvent evaporation at its high limit.

The heating method of this invention is initiated by eliminating the barrier between the reactant chambers and mixing the heat-producing chemical reactants. The pack is then placed on the surface to be heated. The chamber containing the phase-change material is placed either against the surface to be heated or oppositely, that is, away from the surface to be heated, as may be appropriate for a particular use. With the heat sink between the heating compartment and the surface to be heated, the heating rate at the surface is slower, and the maximum temperature of the surface is lower. With the heating compartment between the heat sink and the surface to be heated, the heating rate at the surface is faster, and the maximum temperature of the

surface is higher, and the wax or other phase-change material forms an outside insulating layer, reducing heat loss to the environment.

Any convenient reversible mechanical means may be utilized to conform the pack to the surface to be heated and to promote contact with the surface. Adhesive tape may be used for this purpose. Alternatively, a vacuum pad may be placed on the surface and over the heat pack, and a vacuum drawn. A vacuum pad is an air- impermeable pad having a peripheral gasket and an outlet that is connectable to a vacuum pump. When a vacuum is drawn, the vacuum pad presses the heat pack against the surface being heated. If the surface being heated is not horizontal, there should be a means to retain melted phase-change material across the surface being heated, and not have the material run to one side or one corner of the chamber. Vacuum sealing the material in the chamber may be sufficient for this purpose, as will subdividing the chamber into a grid of compartments. Tape applied in a crisscross fashion assists in this regard, and a vacuum pad is effective to prevent migration of liquefied phase-change material.

In one aspect, a user-activatable, flexible heat pack has two opposed major surfaces. The heat pack includes a first chamber containing a first reactant and a second chamber containing a second reactant. The first and second reactants are adapted to react exothermically with one another. A third chamber contains a solid- to-liquid phase change material. The first and second chambers are separated by a manually frangible seal. The phase-change material is located substantially along one of the opposed major surfaces.

In some implementations, the phase-change material is a flexible wax sheet. In some implementations, the phase-change material includes discrete solid units at fixed locations across one of the opposed major surfaces. According to certain embodiments, the first heat-producing reactant is sodium permanganate and the second heat-producing reactant is glycerol. In some embodiments, the heat pack has a non-reactive liquid heat sink in either the first or second chambers.

In another aspect, a method of heating a surface of an object includes activating a user-activatable, flexible heat pack, applying the heat pack to said surface and reversibly mechanically conforming the heat pack to the surface.

Yet another aspect includes a user-activatable heat pack with a first chamber containing a first reactant, a second chamber containing a second reactant and a frangible seal between the first and second chambers. Rupturing the frangible seal permits mixing of the first and second reactants to initiate an exothermic reaction. A third chamber contains a phase-change material. The phase-change material is adapted to at least partially change phase by absorbing at least some of the heat generated by the exothermic reaction, and to at least partially return to its original phase by releasing at least some heat to its surroundings.

In some embodiments, the phase-change material is adapted to change between a solid phase and a liquid phase during the exothermic chemical reaction. In certain implementations, the heat pack has two opposed major surfaces and the phase- change material is formed as a sheet that extends substantially across one of the two opposed major surfaces. According to certain embodiments, the phase-change material is wax.

In various implementations, the third chamber is substantially vacuum sealed and/or subdivided into a plurality of compartments, each compartment containing a discrete unit of the phase-change material. The phase-change material's melting temperature typically is lower than a desired maximum operating temperature of the heat pack.

Some embodiments include a non-reactive liquid heat sink in at least one of the first and second chambers. In some implementations, the first reactant is permanganate and the second reactant is glycerol.

According to another aspect, a method of heating a surface includes providing a user-activatable heat pack, rupturing a frangible seal in the heat pack to initiate an exothermic chemical reaction in the heat pack and placing the heat pack in contact with the surface to be heated.

In some implementations, the heat pack has at least one major surface and the phase-change material is a sheet that is proximate the at least one major surface. In those instances, placing the heat pack in contact with the surface to be heated can include placing the at least one major surface that the phase-change material is proximate against the surface to be heated. Alternatively, placing the heat pack in contact with the surface to be heated can include placing the major surface that is not proximate the phase-change material against the object. Typically, the method includes reversibly mechanically conforming the heat pack to the surface to be heated.

In still another aspect, a user-activatable heat pack has two opposed major surfaces and includes a first chamber containing permanganate, a second chamber containing glycerol and a frangible seal between the first and second chambers. Rupturing the frangible seal permits mixing of the permanganate and glycerol to initiate an exothermic reaction. A third chamber contains a phase-change wax material. A non-reactive liquid heat sink may be in at least one of the first and second chambers. The phase-change wax material is adapted to at least partially change phase by absorbing at least some of the heat generated by the exothermic reaction. Moreover, the phase-change wax material is adapted to at least partially return to its original phase by releasing at least some heat to its surroundings. Additionally, the phase-change wax material's melting temperature is lower than a desired maximum operating temperature of the heat pack.

In some implementations, one or more of the following advantages may be present.

For example, a heat pack may be created that has a controllable maximum temperature. In some instances, that maximum temperature is predictable to within a range of less than 25°C, preferably predictable to within a range of 20°C and even more preferably predictable to within a range of 15 ⁰ C over a relatively wide range of ambient operating temperatures. Moreover, the heat pack may operate within a range of temperatures that is relatively predictable for extended times without an external energy source, regardless of ambient temperature. Such a heat pack may be

particularly useful in applications, such as curing cement or other adhesives, medical applications, or preventing ice formation.

Additionally, a method of controlling the maximum operating temperature of a heat pack regardless of ambient operating temperatures may be realized. A desirable operating temperature may be achieved and maintained for a relatively extended period of time.

Heat packs as disclosed herein may be used to heat a variety of products and/or surfaces including, for example, food items (such as meals-ready-to-eat), drinks and injured and non-injured body parts. Other features and advantages will become apparent from the description, claims and the drawings.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a side view of a heat pack.

FIG. 2 is a partially exploded plan view of another implementation of a heat pack.

FIG. 3 is a side, partial cutaway view of yet another implementation of a heat pack.

FIG. 4 is a side, partial cutaway view of still another implementation of a heat pack. FIG. 5 is a graph of temperature versus time after activation for the heat pack described in Examples 1 and 2 and tested as described in those examples.

FIG. 6 is a graph of temperature versus time after activation of the heat pack described in Examples 3 and 4, and tested as described in those examples.

FIG. 7 is a graph of temperature versus time after activation of the heat pack described in Examples 5, 6 and 7, and tested as described in those examples.

FIG. 8 is a graph of temperature versus time after activation for the heat pack described in Examples 8, 9, 10 and 11, and tested as described in those examples.

Like reference characters refer to like elements. The mechanical drawings are not drawn to scale.

DETAILED DESCRIPTION

FIG. 1 is a side view of a substantially flat heat pack 100 that includes two opposing major faces 102, 104. The heat pack 100 includes a first chamber 106 containing a first chemical reactant, a second chamber 108 containing a second chemical reactant and a third chamber 110 containing a phase-change material (not shown). The first and second chambers 106, 108 are arranged side-by-side relative to one another and separated by a frangible seal 112 that can be manually ruptured or broken by a user. Rupturing the seal 112 enables the first and second chemical reactants to contact each other thereby initiating an exothermic chemical reaction in the heat pack 100.

The third chamber 110 extends the entire length of the heat pack 100 and is in contact with both the first 106 and second 108 chambers. In some implementations, the phase-change material in the third chamber 110 is flexible and forms a substantially continuous sheet that extends substantially across the entirety of major face 102.

In some implementations, the heat pack 100 may be formed by sealing three flexible plastic sheets together at their peripheries and then creating the seal 112 between the first 106 and second 108 chambers.

The heat pack 100 preferably comprises a flexible material which is not deleteriously affected by either the oxidizing agent or the fuel/gelling agent or any solvent which is chosen for the individual chambers, and which is resistant to the temperature to be achieved. Such materials include polyethylene, polypropylene, polyester (such as MYLAR™ film obtainable from DuPont), aluminum, aluminized polymer film, and other conventional plastic or other packaging materials suitable for containing heated liquids such as rubber, vinyl, vinyl-coated fabric and polyethylene. A thickness of about 0.02 mm to about 0.1 mm has been found to be satisfactory using clear vinyl.

FIG. 2 is a partially exploded plan view of another implementation of a heat pack 200.

The illustrated heat pack 200 includes a first chamber 206 containing a first chemical reactant, a second chamber 208 containing a second chemical reactant and a third chamber 210 containing a phase-change material (not shown). The second 208 and third 210 chambers are constructed from respective pairs of flexible plastic sheets that are sealed to one another at peripheral edges thereof. The first chamber 206 is formed in a similar manner, except the peripheral seal of the first chamber 206 includes a frangible portion 212 that can be at least partially broken by a user manipulating the heat pack 200.

The first chamber 206 is positioned inside chamber 208. Accordingly, when the frangible seal 212 of the first chamber 206 is broken, the first chemical reactant inside the first chamber 206 and the second chemical reactant inside the second chamber 208 can come into contact with one another thereby initiating an exothermic chemical reaction.

In some implementations, the third chamber 210 includes a sheet or an effective sheet of solid-to-liquid phase-change material. The third chamber 210 is placed within the second chamber 208 during final assembly of the heat pack 200 (i.e., before the second chamber 208 is peripherally sealed). In FIG. 2, the third chamber 210 is shown in its constructed form prior to its placement within second chamber 208. It is generally desirable that the third chamber 210 (and the phase- change material therein) span substantially across one of the major surfaces of the heat pack 200. For that reason, in some implementations, the third chamber 210 is adhesively affixed to the interior of the associated major surface of the third chamber 210 or affixed thereto by spot welding. In such an implementation, after activation, the reactants might reside substantially along an opposite major surface of the heat pack 200.

FIG. 3 is a side, partial cutaway view of another implementation of a heat pack 300.

The heat pack 300 is substantially flat and includes two opposing major faces 302, 304. The heat pack 300 includes a first chamber 306 containing a first chemical reactant, a second chamber 308 containing a second chemical reactant and a third chamber 310 containing a phase-change material 350. The first and second chambers 306, 308 are arranged side-by-side relative to one another and separated by a seal 312 {e.g., a frangible seal) that can be manually ruptured by a user. Rupturing the seal 312 enables the first and second chemical reactants to contact each other thereby initiating an exothermic chemical reaction in the heat pack 300.

The third chamber 310 extends substantially the entire length of the heat pack 300 and is in contact with both the first 306 and second 308 chambers. In the illustrated implementation, the phase-change material 350 in the third chamber 310 is flexible and forms a substantially continuous sheet that extends substantially across the entirety of major face 304.

In the illustrated implementation, the heat pack 300 may be formed by sealing three flexible plastic sheets together at their peripheries and then creating the seal 312 between the first 306 and second 308 chambers.

FIG. 4 is a side, partial cutaway view of another implementation of a heat pack 400. Except for the features discussed below, the illustrated heat pack is similar to the heat pack of FIG. 3. In the illustrated heat pack 400, the third chamber 410 includes a number of compartments 411, many (in some implementations, all) of which contain a small amount of phase-change material 450. In some implementations, each piece of phase- change material 450 is formed as a rod, a stick or a granule. The illustrated third chamber 410 is subdivided into small compartments 411 by joining the sheets together in a grid pattern by adhesive seals or heat seals. In some implementations, the phase-change material 450 itself is substantially rigid in the temperature range of use. However, the overall heat pack 400 enjoys a degree of flexibility by virtue of the spaces provided between each piece of phase-change material 450.

In some implementations, if the chamber consists of two edge-sealed sheets, discrete solid units of phase-change material 450 may be spread across one sheet and

held in place within the chamber mechanically by adhesive, or by a vacuum pulled between the sheets during heat pack assembly.

EXAMPLES Twenty five (25) cm x twenty three (23) cm heating pads were made for the examples. These had three compartments formed from three approximately parallel sheets of plastic, which were sealed around their edges. Two of the sheets were sealed down the middle with a breakable seal. This formed the two reactant storage compartments. The third sheet formed the third compartment, into which a flat piece of wax (phase-change material) would be placed. When the seal between the two chemical compartments was broken, the chemicals were mixed by shaking the pack back-and-forth for several seconds.

A flat fiberglass on nonmetallic honeycomb panel with four imbedded thermocouples was used. This material was similar to the wall of an aircraft, to simulate the curing of a patch. The activated heating pad was placed on the surface over the thermocouples with the heat sink side away from the surface. To simulate temperatures other than ambient, the heat pack was placed into a refrigerator or an oven. To adjust the temperature of the panel, a water bottle of the appropriate temperature was placed on the panel. When the desired starting temperature was achieved, the water bottle was removed, and the heating pack was activated and placed on the panel with the heat sink away from the panel. The panel temperatures were then recorded in a computer using Lab VIEW™ software, available from National Instruments Corporation, headquartered in Austin, Texas.

Examples 1 and 2 are substantially identical except that the heat pack of Example 1 included a sheet of wax {see curve 61 in FIG. 5) while the heat pack of Example 2 did not include a sheet of wax {see curve 62 in FIG. 5). Comparatively, the heat pack with the sheet of wax produced a lower maximum temperature and a slower temperature drop-off rate.

Example 1

The first chemical compartment was filled with 350 ml of 9% by weight sodium permanganate in water solution. The second chemical compartment was filled with 233 ml of 25% by volume glycerol in water solution. A 76.9 g sheet of wax with a melting point of 83 ⁰ C was placed inside the third compartment. The heating pad was activated by rupturing the seal between the two chemical compartments and shaking the pack to mix the chemicals. Line 61 of FIG. 5 shows the average temperature over time of the four embedded thermocouples inside the panel. After reaching its maximum temperature of 76 ⁰ C in about 6 minutes, the panel was held to within 13°C for 54 minutes.

Example 2

The first chemical compartment was filled with 350 ml of 9% by weight sodium permanganate in water solution. The second chemical compartment was filled with 233 ml of 25% by volume glycerol in water solution. No sheet of wax was placed inside the third compartment. The heating pad was activated by rupturing the seal between the two chemical compartments and shaking the pack to mix the chemicals. Line 62 of FIG. 5 shows the average temperature over time of the four embedded thermocouples inside the panel. After reaching its maximum temperature of 81 ⁰ C in about 7 minutes, the panel was held to within 2O ⁰ C for 53 minutes. This pack was identical to the one in Example 1 except for the lack of a phase-change heat sink. Over the course of one hour the temperature range after reaching the maximum temperature in Example 2 was 54% larger than for Example 1.

Examples 3-11 illustrate the temperature performance of various heat packs that include a sheet of wax. In general, those heat packs provided a fast, relatively undamped initial rate of temperature rise, a capped maximum temperature, a difference in operating temperature across varying ambient temperatures that is

narrower than the variance in ambient temperatures, and a long heating time over which the temperature drops quite slowly.

Examples 3 and 4 are substantially identical except the heat pack and the panel in Example 3 were initially cooled to 8°C (see curve 31 in FIG. 6) while the heat pack and panel in Example 4 were not initially cooled (see curve 32 in FIG. 6).

Example 3

The first chemical compartment was filled with 350 ml of 10% by weight sodium permanganate in water solution. The second chemical compartment was filled with 233 ml of 25% by volume glycerol in water solution. A 76.9 g sheet of wax with a melting point of 83 °C was placed inside the third compartment. The pack was placed in a refrigerator until it was cooled to 8 ⁰ C. The test panel was cooled to 8°C using a bottle of cold water. The heating pad was activated by rupturing the seal between the two chemical compartments and shaking the pack to mix the chemicals. Line 31 of FIG. 6 shows the average temperature over time of the four embedded thermocouples inside the panel. After reaching its maximum temperature of 70 ⁰ C in about 7 minutes, the panel was held to within 12°C for 53 minutes.

Example 4

The first chemical compartment was filled with 350 ml of 10% by weight sodium permanganate in water solution. The second chemical compartment was filled with 233 ml of 25% by volume glycerol in water solution. A 76.9 g sheet of wax with a melting point of 83 ⁰ C was placed inside the third compartment. The heating pad was activated by rupturing the seal between the two chemical compartments and shaking the pack to mix the chemicals. Line 32 of FIG. 6 shows the average temperature of the four embedded thermocouples inside the panel. After

reaching its maximum temperature of 81 ⁰ C in about 5 minutes, the panel was held to within 16 ⁰ C for 55 minutes.

Examples 5-7 are substantially identical except the heat pack and the panel in Example 5 were initially cooled to 10°C (see curve 41 in FIG. 7), the heat pack and the panel in Example 6 were initially cooled to 16°C (see curve 42 in FIG.7) and the heat pack and panel in Example 7 were not initially cooled (see curve 43 in FIG. 7).

Example 5 The first chemical compartment was filled with 350 ml of 8% by weight sodium permanganate in water solution. The second chemical compartment was filled with 233 ml of 25% by volume glycerol in water solution. A 76.9 g sheet of wax with a melting point of 83 ⁰ C was placed inside the third compartment. The pack was placed in a refrigerator until it was cooled to 1O ⁰ C. The test panel was cooled to 10 ⁰ C using a bottle of cold water. The heating pad was activated by rupturing the seal between the two chemical compartments and shaking the pack to mix the chemicals. Line 41 of FIG. 7 shows the average temperature of the four embedded thermocouples inside the panel. After reaching its maximum temperature of 61 ⁰ C in about 8 minutes, the panel was held to within 8°C for 52 minutes.

Example 6

The first chemical compartment was filled with 350 ml of 8% by weight sodium permanganate in water solution. The second chemical compartment was filled with 233 ml of 25% by volume glycerol in water solution. A 76.9 g sheet of wax with a melting point of 83 ⁰ C was placed inside the third compartment. The pack was placed in a refrigerator until it was cooled to 16°C. The test panel was cooled to 16°C using a bottle of cold water. The heating pad was activated by rupturing the seal between the two chemical compartments and shaking the pack to mix the chemicals.

Line 42 of FIG. 7 shows the average temperature of the four embedded thermocouples inside the panel. After reaching its maximum temperature of 68°C in about 9 minutes, the panel was held to within 11 ⁰ C for 51 minutes.

Example 7

The first chemical compartment was filled with 350 ml of 8% by weight sodium permanganate in water solution. The second chemical compartment was filled with 233 ml of 25% by volume glycerol in water solution. A 76.9 g sheet of wax with a melting point of 83 °C was placed inside the third compartment. The heating pad was activated by rupturing the seal between the two chemical compartments and shaking the pack to mix the chemicals. Line 43 of FIG. 7 shows the average temperature of the four embedded thermocouples inside the panel. After reaching its maximum temperature of 72°C in about 7 minutes, the panel was held to within 14°C for 53 minutes.

Examples 8-11 are substantially identical except the heat pack and the panel in Example 8 were initially activated at 22°C {see curve 51 in FIG. 8), the heat pack and the panel in Example 9 were initially heated to 28°C {see curve 52 in FIG. 8), the heat pack and panel in Example 10 were initially heated to 33 ⁰ C {see curve 53 in FIG. 8) and the heat pack and panel in Example 11 were initially heated to 53°C {see curve 54 in FIG. 8).

Example 8

The first chemical compartment was filled with 350 ml of 6.6% by weight sodium permanganate in water solution. The second chemical compartment was filled with 233 ml of 25% by volume glycerol in water solution. A 76.9 g sheet of wax with a melting point of 83°C was placed inside the third compartment. The heating pad was activated at 22°C by rupturing the seal between the two chemical

compartments and shaking the pack to mix the chemicals. Line 51 of FIG. 8 shows the average temperature of the four embedded thermocouples inside the panel. After reaching its maximum temperature of 64 ⁰ C in about 8 minutes, the panel was held to within 1O ⁰ C for 52 minutes.

Example 9

The first chemical compartment was filled with 350 ml of 6.6% by weight sodium permanganate in water solution. The second chemical compartment was filled with 233 ml of 25% by volume glycerol in water solution. A 76.9 g sheet of wax with a melting point of 83 ⁰ C was placed inside the third compartment. The pack was placed in an oven until it was heated to 28°C. The test panel was heated to 28°C using a bottle of hot water. The heating pad was activated by rupturing the seal between the two chemical compartments and shaking the pack to mix the chemicals. Line 52 of FIG. 8 shows the average temperature of the four embedded thermocouples inside the panel. After reaching its maximum temperature of 69°C in about 6 minutes, the panel was held to within 12 ⁰ C for 54 minutes.

Example 10

The first chemical compartment was filled with 350 ml of 6.6% by weight sodium permanganate in water solution. The second chemical compartment was filled with 233 ml of 25% by volume glycerol in water solution. A 76.9 g sheet of wax with a melting point of 83 ⁰ C was placed inside the third compartment. The pack was placed in an oven until it was heated to 33 ⁰ C. The test panel was heated to 33 ⁰ C using a bottle of hot water. The heating pad was activated by rupturing the seal between the two chemical compartments and shaking the pack to mix the chemicals. Line 53 of FIG. 8 shows the average temperature of the four embedded thermocouples inside the panel. After reaching its maximum temperature of 75 ⁰ C in about 7 minutes, the panel was held to within 14 ⁰ C for 53 minutes.

Example 11

The first chemical compartment was filled with 350 ml of 6.6% by weight sodium permanganate in water solution. The second chemical compartment was filled with 233 ml of 25% by volume glycerol in water solution. A 76.9 g sheet of wax with a melting point of 83°C was placed inside the third compartment. The pack was placed in an oven until it was heated to 53°C. The test panel was heated to 53 ⁰ C using a bottle of hot water. The heating pad was activated by rupturing the seal between the two chemical compartments and shaking the pack to mix the chemicals. Line 54 of FIG. 8 shows the average temperature of the four embedded thermocouples inside the panel. After reaching its maximum temperature of 82°C in about 4 minutes, the panel was held to within 16°C for 56 minutes.

A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention.

For example, the shape and configuration of chambers within a heat pack can be modified. The heat pack can include ventilation provisions to release excess vapor. The frangible seal between reactant chambers could be adapted to be broken in a number of ways. A variety of materials may be suitable for use as the phase-change material. The phase-change material may have dimensions that vary considerably.

Moreover, the heat pack can include provisions for suppressing the reaction. For example, the heat pack can contain a releasable reaction suppressant composition that, in response to a selected temperature occurring in the heat pack, automatically releases suppressant composition into the reaction chamber, thereby suppressing the exothermic reaction.

The heat pack can include complexing agent that complexes reversibly with at least a portion of the first reactant so as to progressively release the complexed first reactant over time as a concentration of uncomplexed first reactant decreases during an exothermic reaction with the second reactant.

The heat pack can include a second frangible separator that is responsive to an exothermic chemical reaction within the reaction chamber. The second frangible separator may be operable to provide fluid communication between the reaction chamber and another chamber that acts as a pressure relief chamber for the heat pack. During operation, an environmental parameter (e.g., pressure and/or temperature) associated with the exothermic chemical reaction may operate the second frangible separator.

Accordingly, other implementations are within the scope of the claims.