WO2001022779A1

WO2001022779A1 - Microwavable gel containing microencapsulated phase change material

Info

Publication number: WO2001022779A1
Application number: PCT/US2000/040974
Authority: WO
Inventors: Matthew T. Maguire
Original assignee: Frisby Technologies, Inc.
Priority date: 1999-09-22
Filing date: 2000-09-22
Publication date: 2001-03-29
Also published as: AU1366401A

Abstract

A thermal storage device useful for heating or cooling, including a fluid-tight container containing gelled material, a hydroxy-containing material, and a phase change material. The phase change material can be microencapsulated, providing extended release or absorption of heat. The thermal storage device can be heated by a microwave source.

Description

MICROWAVABLE GEL CONTAINING MICROENCAPSULATED PHASE CHANGE MATERIAL

Background of the Invention The invention relates to heating and cooling storage devices which can be activated or charged for heating or cooling use by exposure to an external hot or cold source.

Summary of the Invention

The invention provides a thermal storage device which includes a gel material and which is useful for heating or cooling applications. Applications include for example, physical therapy, recreational use, for heating food or drink, and for similar uses. The gel contains phase change materials, particularly microencapsulated phase change materials (MicroPCM), which extend the time of useful heating or cooling. The repeatability of heating or cooling is superior to previously available devices.

The inventive thermal storage device is adapted to be heated, for example, by a microwave source, or alternatively, a conventional convection oven.

In one aspect, the invention provides a microwavable thermal storage device for heating or cooling. The device includes a sealed container having a gel- based material within it. The gel-based material includes a gelled material (which can be either an organic or inorganic gelled material), a hydroxy- or polyhydroxy- containing material that absorbs microwave energy, and a phase change material, which can be microencapsulated, and can also undergo a phase transition at a temperature of, for example, from about 7°C to about 93°C. The phase change material can also include a paraffmic hydrocarbon. The microencapsulated phase change material can be distributed or uniformly distributed, throughout the gelled material or the gel-based material.

Inorganic gelled materials include silicate and aluminate, silicones, silicon oxides, zinc oxides, tin oxides, titanium oxides, zirconium oxides, zinc alkoxides, tin alkoxides, titanium alkoxides, zirconium alkoxides, or alkali metal salts of these oxides and alkoxides. Polyhydroxy-containing materials can be hydrated salts, glycerine, ethylene glycol, or propylene glycol.

The microwave source can be a household-type microwave oven, or a convection oven. The gel-based material can be contained in a flexible fluid-tight container, including those made of a polymeric material. The device can be incorporated into an article of clothing, or in such an article as a bicycle seat. The device can also be incorporated into a thermal wrap useful for medical purposes, or also into packaging for food or drink. In another aspect, the invention provides a method of heating an object.

The method includes exposing a thermal storage device having a gelled material, a hydroxy-containing material, and a microencapsulated phase change material to a microwave source for a time long enough to warm the device.

In another aspect, the invention provides a method of cooling an object. The method includes exposing a thermal storage device having a gelled material, a hydroxy-containing material, and a microencapsulated phase change material to a refrigeration or freezing source for a time long enough to cool the device.

The devices of this invention provide several advantages. Thermal storage devices of the invention to be used as heat sources can be heated effectively by a microwave source, such as a conventional microwave oven. Devices according to this invention have temperature stability over a wide temperature range, including at relatively high temperatures (for instance, above 100°C), which can result in the desiccation of water-based gels which are included in previously available thermal storage devices. By "thermal storage device" is meant a device that is charged by heating it and used to heat an object or to protect it from a cold environment, a device that is charged by cooling it and used to cool an object or to protect it from a hot environment, or a device that protects an object from both heating and cooling. Although technically cold is merely the absence of heat, it is convenient to refer to a cold device that is used for cooling as a thermal storage device that stores cold. The presence of a gelled material in the thermal storage devices of the invention provides increased efficiency in the application of heat or cold to objects or parts of the body. The heat-conveying or heat absorbing (which may be thought of as cold-retaining) ingredients of the thermal storage devices, described below, do not fall to the bottom of their container, which would require continual adjustment for efficient heat transfer to or from the thermal storage device.

The thermal storage devices of this invention can be used for physical therapy and medical uses, such as treating sprained or strained joints or damaged muscle tissue by providing a heating or cooling source which is operative for an extended time. Certain preferred embodiments having flexible containers and flexible gels are easily manipulated to conform to body parts such as the neck, back, shoulder, elbow, wrist, hip, knee or ankle of the afflicted individual. The subject of physical therapy or medical applications of the thermal storage devices of the invention can be a human or animal subject. The thermal storage devices of this invention can also be used for recreational uses, such as for warming or cooling various sporting accessories before use. For example, the thermal storage device can be adapted for use as a bicycle seat cover or cover liner, and prewarmed or precooled before a ride, providing increased comfort for the rider in immoderate climactic conditions by protecting against a hot or cold environment.

The thermal storage devices of the invention can also be used for heating or cooling food or drink products. A further example of the use of the inventive thermal storage device in this area could be as a temperature control accessory for vendors of hot or cold food or drink that is, to be prewarmed or precooled and then included in an insulative container with food or drink that is to be kept hot or cold. This allows for convenient transportation of such food or drink without the necessity of an external energy source, such as an electrical supply, for heating or refrigeration.

Devices of this invention can also be used for thermal management of electronic components, for example, for use as a potting compound. The gelled material can be an electrically insulating material, and the microencapsulated phase change material (MicroPCM) included in the gelled material can efficiently absorb excess heat from electronic components.

The thermal storage devices can also be used in transportation applications, such as the extended warming or cooling of products in vehicles. Such products can be a very wide variety of products which are best transported under constant temperature conditions, including medical products, such as medicines, precious bodily fluids, or organs.

As used in the specification and appended claims, "gel" or "gelled material" refers to a substance having a continuous molecular skeleton enclosing a continuous liquid phase. The gel has elastic properties. The skeleton can be, for example, a polymeric material, such as a homopolymer, a fractal polymer, or a block or random copolymer. The resulting gel retains the rheological characteristics described herein.

As used in the specification and appended claims, "gel-based material" refers to a substance which includes a gelled material, and derives its structural properties, including its rheology, from those of the gelled material, but which contains additional ingredients, giving the gel-based material useful properties which the gel does not possess. The reader is referred to Brinker & Scherer's Sol-Gel Science, Academic Press, San Diego (1990), particularly chapters 1, 5 and 7. As used in the specification and appended claims, "microencapsulated" refers to containment of a material within relatively small (from about 1.0 to 50 microns in diameter), roughly spherical, oblate spherical, or cylindrical capsules. The reader is referred to some references for an explanation on how to fabricate microcapsules: Vandergaer, J.E., Ed.: Microencapsulation: Processes and Applications, Plenum Press, New York, 1974. Sparks, R.E.: Microencapsulation. Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 15, 3rd Edition, John Wiley and Sons, Inc., 1981.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.

Brief Description of the Drawings

Fig. 1 is a graph of the temperatures of the surface and core of a prior art microwavable thermal storage device, along with the temperatures of heated air and freezer, versus time over 37 minutes.

Fig. 2 is a graph of the temperatures of the surface and core of a particular embodiment of a microwavable gel-based thermal storage device according to the invention, along with the temperatures of heated air and freezer, versus time over 37 minutes.

Fig. 3 is a graph of the difference in surface temperatures between a prior art thermal storage device and a particular embodiment of the thermal storage device according to the invention.

Description of the Preferred Embodiments

The present invention provides a device which stores heat or cold and can be used for heating or cooling. The storage device is charged, that is pre-heated or pre-cooled by an external source of heat or cold to allow extended heating or cooling of objects, parts of the body, and the like.

The device includes a sealed container which contains a gel-based material capable of being heated by a microwave source without damage to the container or its contents. The device can be pre-cooled by any cold source, including a standard refrigerator, a freezer, ice water or any other source of cold.

The gel-based material which is included in the container of the device comprises a gel, a hydroxy-containing material, and a phase change material. The ingredients of the gel-based material are distributed homogeneously. This allows for the most effective and rapid heat transfer between various components of the thermal storage device.

The gelled materials to be used in the invention provide a structural framework capable of supporting themselves and their contents thereby allowing more even heat transfer between the device and the object to or from which heat is to be transferred. Without the integrity, and continuing support and structural rigidity, the contents of the device would, under the influence of gravity, accumulate in lower portions of the device, necessitating continual readjustment of the device for optimal performance. A variety of gelled materials are suitable for use in the thermal storage device of the present invention. The gelled material retains its physical properties over the temperature range to which the thermal storage device will be heated or cooled. Desirably, the gelled material should be non-toxic, so that disposal of the thermal storage device will not create an environmental hazard. The gels useful in the present invention are either organic or inorganic; both types are useful in the present invention. Useful gels can be formed from organic compounds such as carbohydrates including carbohydrates such as starch or cellulose; polyacrylic acid, polyacrylate, poiyacrylamide; polyols such as pentaerythritol; proteinaceous materials such as gelatin, or polymers and copolymers in solvents such as water, acetone, or alcohols, including dimethoxytetraglycol.

In certain embodiments organic-based gels can be used. It has been found that polyhydroxy-containing organic polymer-based gels, including polysaccharide, can work well for cooling or moderate heating in devices and methods of the present invention. These polysaccharide gels include those based on starches, celluloses, and derivatives of such materials. The derivatives can be based on nitrogen, phosphorus, halogen, hydroxy, carboxyl and alkyl substitution on the parent backbone or side chains.

In some embodiments of the invention, starch gels can be useful. Starch comprises a mixture of linear (amylose) and branched (amylopectin) polymers of α- D-glucopyranosyl units. Amylose is a linear polymer of D-glucopyranosyl units linked to each other by (l- 4) α-glucosidic links. Amylopectin is a highly branched polymer of α-D-glucopyranosyl units which are chiefly (l-»4) links, but also containing (1— »6) α-glucosidic links located at branch points. It is believed that (1— 6) α-glucosidic links are more heat resistant than (1— >4) α-glucosidic links. Other noncarbohydrate materials isolable from starch include fatty acids, proteins, enzymes, and inorganic materials, which are generally present in small amounts. Starch may be isolated from many sources, including the seeds of corn, waxy corn, wheat, rye, barley, sorghum, or rice, or the roots of such plants as tapioca, potato, or arrowroot, or from the pith of the sago palm tree. Starches are generally characterized by their gelatinization temperatures, which are the temperatures at which initially thin, opaque starch suspensions become viscous, semiopaque, and finally transparent. Amylose content ranges from almost zero to about 85%, with the majority of the remainder consisting of amylopectin. The thickening of some starch pastes is caused by association of the linear molecules of amylose. Corn starch forms a rigid gel. Potato starch, tapioca, and sago have less tendency to gel. Waxy starches (with unusually low or no amylose) do not gel in dilute dispersions, but at high concentrations (30%) form reversible gels, which redisperse at 50-60°C.

Starches may be modified by crosslinking, to increase shear resistance, heat resistance, and resistance to extremely high or low hydrogen-ion concentrations. Starches may be partially oxidized to yield improved stability. Starches can be derivatized by inorganic esterification with nitrates, sulfates, phosphates or xanthanates, or by organic esterification through treatment with carboxylic acids, acid anhydrides, acid chlorides, or vinyl esters. Starch ethers can also be formed for use in the present invention. Some preferred starches for use in some embodiments of the thermal storage devices of the invention are cold water hydrating starches, which are resistant to acidic conditions and temperatures up to 100°C. Especially preferred are those starches which have viscosities of at least 300 Brabender Units (BU) at 95°C initially, and at least 400 BU at 95°C after 15 minutes at that temperature. More preferably, the starches have viscosities of at least 350 BU initially at 95°C, and at least 450 BU after 15 minutes at that temperature. Suitable starches are available as MIRA-THIK® 468 starch (A.E. Staley Mfg. Co., Decatur, IL).

In high-temperature thermal storage devices of the invention, inorganic gelled materials are preferred. These embodiments are those in which the temperature stability of an organic gelled material is less reliable. Gels can contain water, but if the thermal storage devices of the invention are to be used at temperatures much higher than the boiling point of water, these gels can become desiccated and lose their advantageous structural properties. From a safety standpoint, the likelihood of device rupture increases as more water is driven into the vapor phase in a closed container. Thermal storage devices including gelled materials not containing water can be more stable to temperatures well above 100°C.

Useful inorganic gelled materials include gels formed from metal oxides including silicone, metal alkoxides, or alkali metal salts of metal oxides. These include zinc oxide, tin oxide, titanium oxide, zirconium oxide, and silicates and aluminates in solvents such as water and alcohols. In some embodiments of the thermal storage devices of the present invention, silicone gels, derived from silicones, are useful, as are aluminum oxide and alumina trihydrate.

Silicones are synthetic polymers having a linear, repeating silicon-oxygen backbone with organic groups attached to the silicon atoms by carbon-silicon bonds. The general chemical formula is represented by

R_nSiO(_4-n/₂m) where R is an organic group, n is 1-3 and m. 1. The silicon has a repeating silicon-oxygen backbone, and has organic groups R attached to a significant portion of the silicon atoms. The R groups can include methyl, but can also be longer alkyl, fluoroalkyl, phenyl, vinyl, and the like. R can also be hydrogen, halogen, alkoxy, acyloxy or alkylamino and the like.

These types of compounds are referred to as polyorganosiloxanes. The silicon-oxygen backbones can be crosslinked, typically by conversion of chlorosilane monomers (that is, molecules of silicon with chloride substituents) to polymeric products. Variation in the type and extent of these crosslinkages results in a variety of products which can be manufactured from silicones. Silicone fluid, silicone gels, and silicone elastomers (rubbers) are all used widely in industrial products.

Silicone fluids (or oils) have linear polydimethylsiloxane (PDMS) chains of varying length. The chains can also be cyclized. Silicone gels have moderate crosslinking between polysiloxane backbones forming a network which is interspersed with non-covalently bound PDMS fluid. These materials are stable and maintain their performance at temperatures from -55°C to 200°C, in some cases up to 250°C.

The silicone gels which are suitable for use in the present device are commercially available. For example, SylGard gels (Dow Corning), including SylGard 170, 182, 184, and 527 or members of the RTV6100 series of silicone gels such as RTV6126-D1, RTV6136-D1, RTV6156, RTV6166, RTV6186, and RTV6196 (GE Silicones, Waterford, New York).

These materials are typically available as two components which are mixed, cured and set into a gel. The first component (Component A) is referred to as a base, and the second component (Component B) is a curing agent. These are typically mixed together, according to the manufacturers directions. Some silicone gels should be mixed in a 1 : 1 ratio of Component A to Component B. In these cases, slight variation .( about 10%) in the A:B ratio can result in variations in gel stiffness. Preferred ratios of base to curing agents can range from about 1 :1.1 to about 1.1 :1. The mixture of components undergoes curing at temperatures which can be between approximately room temperature and about 150°C. The curing time will vary inversely with the curing temperature. Preferred curing temperatures range from about 15°C to about 50°C, resulting in a curing time of about 3 minutes to about 60 minutes.

Gelled materials are present in the gel-based material of the invention in an amount of from about 30 to about 90% by weight, based on the weight of the gel- based material. In certain embodiments, the gelled material is present from about 40 to about 75%o by weight. This includes the total of components of a multi-component gelled material.

Hydroxy-containing materials are included in the present invention to act as microwave absorbers. Microwave energy is found in the radio frequency region of the electromagnetic spectrum between where radio waves and infrared radiation are found. For example, the microwave energy range used by household microwave ovens falls between about 915 and about 2450 megaHertz (2.45 gigaHertz). Expressed in another way, the wavelength of microwaves are between about 1 millimeter and 1 meter. The wavelength of microwaves from a typical household microwave oven are about 12.2 centimeters.

The presence of hydroxy (-OH) functional groups on molecules facilitates the conversion of microwave energy to thermal energy. This occurs by the absorption of the microwave energy by the -OH functional group. The molecule containing an - OH group will be aligned and realigned with the microwave energy field which fluctuates 2.45 billion times per second. This motion is transferred by friction with other molecules in the device, into thermal energy of the gel-based material of the invention.

Useful hydroxy-containing materials for use in the thermal storage device of the invention are water, glycerine, propylene glycol, ethylene glycol, and similar monomeric and polymeric species. If the thermal storage device is to be used for cooling or moderate heating applications, water is an abundant and suitable hydroxy- containing material. The high heat capacity of water can contribute to extended heating or cooling when the desired operating temperature is reached. Water is also a conveniently employed constituent of the thermal storage devices of the invention if the gelled material includes water. This is the case for some of the organic gelled materials mentioned above. Materials which have water associated with them can also be used in gels according to the invention. For example, hydrated materials such as salts which have coordinated water are useful components in some embodiments of the microwavable gels. Such hydrated salts include salts of both oganic and inorganic anions, including sodium, potassium, calcium, ammonium, or iron salts of acetate, silicate, chloride, nitrate, mono-, di- and tribasic phosphate, mono- and dibasic carbonate and mono- and dibasic sulphate. Exemplary materials include sodium citrate dihydrate, sodium acetate trihydrate, barium chloride dihydrate, dicalcium phosphate dihydrate, sodium carbonate hydrate. Multiply hydrated species are preferably used, such as dihydrate, trihydrate and higher hydrates.

Hydroxy-containing materials are used below their boiling points. If temperatures above the boiling point of a hydroxy-containing material are to be utilized, that hydroxy-containing material becomes unsuitable for the purposes of the invention. The thermal storage device container can become distended due to a water vapor pressure buildup if water is used as the hydroxy-containing material in a high temperature heating application. In such cases, a higher boiling point hydroxy- containing material is used. These include 1,3-propanediol (boiling point 214°C),

1,2-propanediol (187°C), 1 ,2-ethanediol (144-146°C), and glycerol (182°C at 20mm). Other hydroxy- and polyhydroxy-containing materials can be used, such as polyhydroxy-containing polymers.

The hydroxy-containing material is homogeneously distributed throughout one or more layers the gel-based material. In microwave heating applications, the heat generated by this material will have a relatively short conduction path to phase change material. This allows efficient heat transfer and storage.

The hydroxy-containing material can be present in the gel-based material of the invention in an amount of from about 2%> to about 40%> percent by -weight, based on the total weight of the gel-based material. In certain embodiments, including high temperature embodiments described in Examples, the hydroxy- containing material is present from about 4% to about 20% by weight, or from 4% to 10%.

Suitable temperature stabilizing means include phase change materials. Phase change materials are designed to utilize latent heat absorption associated with a reversible phase change transition, such as a solid-liquid transition. Certain phase change materials also absorb or emit heat upon solid-solid phase transitions. Thus, the material can be used as an absorber of heat to protect an object from additional heat, because a quantity of thermal energy will be absorbed by the phase change material before its temperature can rise. The phase change material can also be preheated and used as a barrier to cold, as a larger quantity of heat must be removed from the phase change material before its temperature can begin to drop. The phase change materials which are preferred for the present invention utilize a reversible solid-liquid transition. Phase change materials store thermal energy in the form of a physical change of state as the core material melts or freezes or undergoes a solid-solid transition. In order to maintain the ability of the phase change materials to recycle between solid and liquid phases, it is important to prevent dispersal of the phase change materials throughout the solvent (or carrier fluid) when they are in the liquid form. An approach which has found success is encapsulation of the phase change materials within a thin membrane or shell. Such thin membranes or shells should desirably not significantly impede heat transfer into or out of the capsules. The capsules can desirably also be small enough to present a relatively high surface area. This makes rapid heat transfer to and from the carrier fluid possible. Such capsules are known as microcapsule. Microcapsule range in size from about 10 to about 50 microns and are formed according to conventional methods well known to those with skill in the art. Heat transfer across the microcapsule material into its interior should be efficient for maximum utility in the present invention.

The composition of the phase change material is modified to obtain optimum thermal properties for a given temperature range. For example, the melting point for a series of paraffmic hydrocarbons (normal, straight chain hydrocarbons of formula C_nH_2n+2) is directly related to the number of carbon atoms as shown in the following table.

Table 1. Hydrocarbon Phase Transition Temperatures

In addition to the hydrocarbons listed here, other paraffmic hydrocarbons having a greater (or lesser) number of carbon atoms having a higher (or lower) melting point can also be employed in practicing the invention. Additionally, plastic crystals such as 2,2-dimefhyl- 1,3-propanediol (DMP) and 2-hydroxymethyl-2-methyl- 1,3-propanediol (HMP) and the like are also contemplated for use as the temperature stabilizing means. When plastic crystals absorb thermal energy, the molecular structure is modified without leaving the solid phase. Combinations of any phase change materials can also be utilized.

Microencapsulated phase change material (MicroPCM) is desirably distributed homogeneously throughout the gel-based material of the thermal storage device. This can be accomplished by thorough mixing of microencapsulated phase change material with the gelled material before the curing stage. Heterogeneous distribution of microencapsulated phase change material after curing can be accomplished by preparing gels with different MicroPCM loadings and combining the gels in a layer within a given container without appreciable mixing between the layers. This can be useful when heat gain or loss is expected to be substantially from only one side of the container. In such cases, it can also be useful to adjust the stiffness of the gel so that minimal mixing between layers occurs.

In some preferred embodiments, the microcapsules of phase change material are individually surroundingly encapsulated and embedded within the gelled material. In some more preferred embodiments, the microcapsules of phase change material are substantially spaced apart from one another, with other ingredients of the gel-based material filling the space between the microcapsules. In microwave heating applications, in which a hydroxy-containing compound absorbs microwave energy, the heat generated by these compounds will most quickly find its way to phase change material.

The specific application will dictate the temperature range over which the phase change material is designed to operate. For example, in some embodiments the device of the invention will be heated to about 70°C. In order to most effectively maintain this temperature, the phase change material should undergo a phase transition at a temperature which depends on the intended use of the product. Phase transition temperatures in the range of about -30-0°C are generally appropriate for storage of frozen foods or biological samples; 7-15 °C are generally appropriate for medical cold therapy; 37-49°C, for warm therapeutic medical uses; and about 60- 93 °C for food heating uses.

The thermal storage device of the invention can also be adapted to have application in both heat and cold. A low temperature-melting phase change material and a higher temperature-melting phase change material could be simultaneously present in the gel-based material. A single phase change material can be used to both heat and cool an object. For example, a thermal storage device having a phase change material selected to melt at higher temperature can be thermally charged by simply being present in a room proximate a heat source. Thereafter, when in use in cooler ambient conditions, the object sought to be warmed will remain warm for an extended period, until the phase change material completely solidifies. Similarly, the same thermal storage device can be charged by storing it in a cool environment. Thereafter, when in use in warmer ambient conditions, the object sought to be cooled will remain cool for an extended period until the phase change material fully melts. Preferred methods involve preheating or precooling the thermal storage device with external means such as a conventional oven, a microwave oven, or a refrigerator. Additionally, the thermal storage device can be fabricated with microcapsules containing two discrete types of phase change material, one suitable for assisting the maintenance of objects or body parts at each of two desired temperatures.

The phase change material is present in the gel-based material of the invention from about 10% to about 60% by weight, based on the weight of the gel- based material. In some embodiments, the phase change material is present from about 20%) to about 40% by weight.

The gel-based material described above is contained within a fluid tight container, which readily transfers heat into and out of the gel-based material. The container must be fluid tight in order to reliably retain its contents. The container must be resistant to erosion or rupture which can be caused temperatures ranging from about -40°C to about 250°C.

Container materials can be chosen from a wide variety of substances, including polymeric materials such as polyethylene, polypropylene, and mylar, or metallic type materials such as aluminum or aluminized polymer film, or other materials suitable for containing fluids such as rubber, vinyl, and vinyl-coated fabric. Other materials not listed here are also suitable, as long as thermal transfer is efficient, and the material is reasonably flexible. The thickness of the material can vary according to convenience, but should not be so thin as to undergo rupture upon heating, and should not be so thick as to inhibit heat transfer. It has been found that in the case of high density polyethylene, the thickness can be about 0.10 mm, although the thickness of the material constituting the container can be slightly higher or lower depending on how rigid and flexible the container needs to be.

The container can be constructed of thin, flexible, thermally conductive material comprising an upper layer and a lower layer. The edges of the material can be bonded together in such a way as to provide a leak-tight fluid barrier, and this can be accomplished by such methods as soldering, heat sealing, ultrasonic welding, solvent welding, fold sealing, and the use of adhesives. The container generally has a single chamber, but containers with multiple chambers, each containing gel-based material, can also be employed.

The shape of the thermal storage device of the invention can be any of a large variety of shapes which are useful for warming and cooling various objects and body parts. For example, a square or rectangular shape can be employed. Round thermal storage devices can also be used. In other embodiments, toroidal shapes could be employed. The size of the thermal storage device is not at all critical, and it is envisioned that almost any size which is practical to construct could be used.

The thermal storage device can be incorporated into a variety of objects which are desirably heated or cooled. For example, articles of clothing such as scarves, socks, shoes, hats, mufflers, gloves, or similar clothes can be outfitted with the inventive thermal storage devices. Medical uses are envisioned, with the thermal storage device being used as a thermal wrap, for the treatment of injuries, aches and pains, in which case the device can be either heated or cooled. The device can also be used as a component of a microwavable bicycle seat, again either warm or cold, depending on the ambient temperature. The thermal storage device can be activated to a hot or warm state by exposure to a conventional microwave oven, for a period of time sufficient to bring the device to the desired temperature. The device can also be heated by a conventional convection oven. Alternatively, the device can be cooled by sufficient exposure to a refrigeration source. The invention will be further described in the following examples, which do not limit the scope of the invention described in the claims.

EXAMPLES

The following examples are illustrative of certain embodiments of the claimed invention. These examples show some of the useful methods of making and using the invention, and also include some of the performance characteristics of the invention.

Example 1. Method of Making Microwavable Gel-based Material The gelled material was a commercially available two-component silicone gel stable up to 200°C (SYLGARD 527, obtained from Dow Corning (Midland, MI). A phase-change material (116 g of Thermasorb® 122, a microencapsulated blend of hydrocarbon paraffmic compounds undergoing a phase transition at about 50°C, available from Frisby Technologies, Inc., Bay Shore, New York) was combined with 23.3 g of propylene glycol and 104.9 g each of a two-part silicone gel. The parts A and B of the Sylgard material comprised 60%) of the resulting mixture. The propylene glycol comprised 6.7%>, and the Thermasorb 122 comprised 33.3%.

Other mixtures were also compounded and tested. When 40%> of Thermasorb D 122 material was used with a 60%> Sylgard gel/propylene glycol mixture, gel formation was not acceptable. The level of MicroPCM was kept at less than 40%) in subsequent mixtures using this gel material.

The procedure for formulating microwavable gels was as follows. The gel parts A and B were mixed together according to the manufacturer's instructions. Propylene glycol was mixed into the gel components. MicroPCM was added mixed, and the entire mixture was introduced into a flexible bag. In other formulations, glycerin was used as a microwave absorber.

Example 2. Performance of Microwavable Gel-based Material Tests were performed to evaluate the benefits of incorporating phase- change material-containing microcapsules into a microwavable gel. A phase change material undergoing a phase transition temperature of approximately 122°F (50°C) (Thermasorb® 122, available from Frisby Technologies, Inc., Bay Shore, New York) was selected for inclusion in the test sample. The test sample was constructed to the following physical dimensions: a bag that was 2.25 inches wide, 14.25 inches long, and 0.5 inches thick was used to contain the gel. The mass of the gel inserted was 9.3 ounces compared to a pouch containing a previously available microwavable thermal storage device containing clay, which weighs 11.7 ounces. The gel pack according to one embodiment of the invention was only half the thickness of the clay-containing pouch. This makes the inventive gel pack approximately 20%) lighter and 50%> thinner compared to the one inch thick existing pouch.

The top of a freezer was fitted with a piece of closed cell styrofoam having slots into which the test samples fit tightly. A parabolic top with an electrical heater was placed over the cut out sections. The electrical heater kept the air temperature above the test samples at approximately 100°F to simulate the skin temperature of an individual. The lower ends of the test samples were exposed to the freezer, which was kept at a temperature between 10°F and 20°F. The samples were microwaved for 1 minute and 25 seconds using a 750 watt microwave oven and then equipped with precision thermocouples. The thermocouples were inserted into the center (core) of the pouch as well as on the outside surface in contact with the heated air. The test samples were then placed in the slots of the test fixture and the top closed off. Data was recorded once a minute for 37 minutes. The data is displayed graphically in Fig. 1 (for the prior art pouch) and Fig. 2 (for the pack according to an embodiment of the invention). When the surface temperature of the samples became warmer then the center temperature, this point was labelled the "cross-over point". At this point the heat starts flowing away from the warmer "skin" side to the insert. Only when the core temperature of the insert is warmer than the surface temperature, will the wearer feel the warming effect of the gel pouch.

The gel pouch containing microencapsulated MicroPCM showed 100% improvement over a product available for microwave heating which contains a clay- based material which can trap water (available as a product called Thermcore®). This means that in 10°F conditions, persons wearing a garment incorporating a microwavable gel pouch according to the invention will feel the scarf keeping them warmer for over twice as long as previously available clay-based pouches (see Fig. 3).

The previously available clay-containing pouch felt moist after being microwaved whereas the sample prepared according to the invention was dry and soft to the touch. The bag that held the MicroPCM-containing gel also made this pouch easier to insert and remove so a garment containing such a pouch could be readily laundered.

This example demonstrates that by using the microwavable gel in place of previously available clay-containing products, the working time of the pouch could be increase by a factor of almost two with less weight and bulk than required by previously available products.

Example 3. Repeatability of Heating Using Microwave Oven The repeatability of heating using a microwave oven was tested, using a gel-based pouch as described in Example 2, heated for 1 minute and 20 seconds in a 750 W microwave oven. The same pouch was repeatedly heated on 34 separate occasions and the temperature measured with a thermocouple immediately after heating. The pouch was measured to be heated to a temperature of 64.2°C with a standard deviation of 4.8°C for the 34 separate experiments. The results indicate that the microwavable gelled thermal storage devices are robust and reliable means for absorbing microwave energy to be dissipated as heat.

OTHER EMBODIMENTS

It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.

Claims

What is claimed is:

1. A microwavable thermal storage device for heating or cooling comprising a sealed container having therein a gel-based material comprising: a) a gelled material; b) a hydroxy-containing material that absorbs microwave energy; and c) a microencapsulated phase change material.

2. The thermal storage device of claim 1, wherein the gelled material is an organic gelled material.

3. The thermal storage device of claim 1, wherein the gelled material is an inorganic gelled material.

4. The thermal storage device of claim 3, wherein the inorganic gelled material is selected from the group consisting of silicate and aluminate, silicones, silicon oxides, zinc oxides, tin oxides, titanium oxides, zirconium oxides, zinc alkoxides, tin alkoxides, titanium alkoxides, zirconium alkoxides, and alkali metal salts of these oxides and alkoxides.

5. The thermal storage device of claim 1, wherein the hydroxy-containing material is a polyhydroxy-containing material.

6. The thermal storage device of claim 5, wherein the polyhydroxy-containing material is selected from the group consisting of hydrated salts, glycerine, ethylene glycol, and propylene glycol.

7. The thermal storage device of claim 1, wherein the microencapsulated phase change material undergoes a phase transition at a temperature of from about - 32°C to about 93°C.

8. The thermal storage device of claim 7, wherein the phase change material comprises a paraffmic hydrocarbon.

9. The thermal storage device of claim 7, wherein the microencapsulated phase change material is substantially uniformly distributed throughout the gelled material.

10. The thermal storage device of claim 1, wherein the microencapsulated phase change material is non-uniformly distributed throughout the gel-based material.

11. The thermal storage device of claim 1 , wherein the microwave source is a household-type microwave oven.

12. The thermal storage device of claim 1, wherein the gel-based material is contained in a flexible fluid-tight container.

13. The thermal storage device of claim 12, wherein the flexible fluid-tight container is made of a polymeric material.

14. The thermal storage device of claim 1, wherein the device is incorporated into an article of clothing.

15. The thermal storage device of claim 1, wherein the device is incorporated into a bicycle seat.

16. The thermal storage device of claim 1, wherein the device is incorporated into a thermal wrap useful for medical purposes.

17. The thermal storage device of claim 1, wherein the device is incorporated into packaging for food or drink.

18. The thermal storage device of claim 1, wherein the gelled material comprises a silicone gel, the hydroxy-containing material comprises propylene glycol, and the microencapsulated phase change material undergoes a phase change at a temperature of from about 40°C to about 93 °C .

19. A method of heating an object, said method comprising exposing a thermal storage device comprising a gelled material, a hydroxy-containing material, and a microencapsulated phase change material to a microwave source for a time sufficient to raise the temperature of the device, and contacting the thermal storage device with an object to be heated.

20. A method of cooling an object, said method comprising exposing a thermal storage device comprising a gelled material, a hydroxy-containing material, and a microencapsulated phase change material to a refrigeration source for a time sufficient to lower the temperature of the device, and contacting the thermal storage device with an object to be cooled.