WO1996039295A1

WO1996039295A1 - Thermal storage sheet

Info

Publication number: WO1996039295A1
Application number: PCT/US1996/009350
Authority: WO
Inventors: Gideon Salee
Original assignee: Battelle Memorial Institute
Priority date: 1995-06-06
Filing date: 1996-06-06
Publication date: 1996-12-12
Also published as: AU6096996A

Abstract

The invention is a thermal storage sheet element (1) and a method for producing it. The element comprises a porous carrier (2) defined by pore walls (3) throughout the structure and a heat absorbing material within the open pores (4). A protective carrier (5) may be used to encase the porous carrier (2). The method for making the thermal storage sheet element involves infiltrating a porous carrier (2) with a liquid activator which is contained in the wall surface of the carrier pores. Excess activator is removed leaving the open pores (4) to be filled with the heat absorbing material.

Description

THERMAL STORAGE SHEET

Field of the Invention

It is well known that certain materials are microwave sensitive, meaning that their structure is such that they convert microwave energy into heat. These materials may also be referred to as microwave susceptors, microwave couplers, microwave absorbers or microwave transducers or simply as activators. It is also known that there are classes of materials which are much better heat storage materials (for a variety of reasons) than are the microwave-sensitive materials, but they are general I \ non-polar materials which are not sensitive to microwaves. Consequently, microwave-sensitive materials and the heat storage materials may be mixed together such that, in a microwave field, the microwave energy is captured and converted to heat by the sensitive material and transferred to the heat storage material for later us Conventionally, the mixture (or solution) of heat storage material and microwave-sensitive material is generally contained on a substrate or carrier or in a reservoir or other container within the product to be heated. It is often desirable that the heat storage material be rather continuous throughout the container and that the microwave-sensitive material be rather uniform throughout the heat storage material. This provides a uniform heating of the storage material and a uniform distribution of heat to the user. When the heat storage material is a liquid and of lower density than the microwave-sensitive material, settling may occur resulting in higher concentration regions of the heat storage material near the bottom of the carrier or container. This produces poor heat uniformity and, ultimately, dangerous conditions due to hot spots during subsequent heating and use. US Patent 5,070,223 (Colasante) discloses microwave-reheatable clothing wherein a reservoir of microwave-sensitive material is sewn into the clothing. The patent disclosure refers to adhering a microwave-transducing material (e.g.. carbon) to a matrix network of spongy material so that it would be dispersed throughout the heat absorbing component (e.g.. oil). This is said to prevent settling out of the microwave-transducing component. The disclosure refers primarily to solid particle microwave-transducing materials, although buffer fat and other lipids are also mentioned. These could be solid or liquid depending on the specific material and the temperature, but the disclosure about the prevention of settling seems to suggest a solid transducing material.

Summary of the Invention

The invention is a thermal storage element and a method of making the element, wherein heat is produced within the element with microwave energy. The element is conveniently inserted (temporarily or permanently) into a particular finished product of the type which is exposed to microwaves to add heat to a heat storage material and then taken to a cold environment where heat is released gradually over an extended period of time. The thermal storage element is particularly useful because it may exhibit a very uniform distribution of heat across the element, even after many use cycles. This is both more comfortable and safer for the user. The inventive method is economically attractive because the element can be made in a continuous sheet form and then cut into appropriate shapes and sizes for a variety of thermal storage products.

The thermal storage element comprises a porous carrier having an open-pore structure, a heat absorbing material within the open pores, and a microwave sensitive liquid activator in a surface layer of the wall of the open pores of the carrier for absorbing microwave energy, converting it to heat and transferring the heat to the heat absorbing material. A protective cover may be used to encase the porous carrier and prevent the heat absorbing material from escaping from the porous carrier. The porous carrier may preferably be made of a material with large, chemically polar molecules so that the activator is more easily and permanently adsorbed into the surface layer of the pores. The carrier is ideally compressible (with minimal force) so that the heat absorbing material and the activator may be easily infiltrated into the structure. A preferred carrier material is polyetherurethane foam. The activator is a polar liquid which is generally substantially insoluble and immiscible in the liquid phase of the heat absorbing material and is non- volatile at the manufacturing and use temperatures. Water and glycols, including propylene glycol, dipropylene glycol and higher oligomers, are particularly preferred activators.

The heat absorbing material is generally a solid/liquid or solid/solid phase change material within the temperature range of use. This is useful in the amount of heat which can be stored and also in the manufacturing process wherein the liquid phase is easier to infiltrate into the carrier. Preferred materials for the heat absorbing material are waxes, particularly paraffin wax.

The method of fabricating a thermal storage sheet element according to the invention comprises infiltrating the open pores of the porous carrier with the liquid activator, introducing the activator into the surface layer of the wall of the open pores of the carrier and removing excess activator from the pores, and infiltrating the pores with the heat absorbing material. The porous carrier may also be encased with a protective cover for preventing the heat absorbing material from escaping from the porous carrier. It is preferred that the heat absorbing material is a solid/liquid phase change material within the temperature range of use so that the open pores of the porous carrier can be infiltrated with the heat absorbing material in the liquid phase and then solidified.

It is also preferred that the carrier is made of a compressible material so that the open pores of the carrier can be infiltrated with the liquid activator and the liquid heat absorbing material by immersing the carrier into a bath of the liquid and successively compressing and releasing the carrier to take up the liquid. The excess liquid activator can also be removed by compressing the carrier. After infiltration with the heat absorbing material in liquid phase, the carrier can be removed from the bath to lower the temperature and solidify it.

Description of the Drawings

Figure 1 is an elevation view of a section of the inventive thermal storage sheet product. Figure 2 is a close-up sectional view of a cutaway of the inventive thermal storage sheet product.

Figure 3 is a schematic view of the inventive process for making the inventive thermal storage sheet product.

Detailed Description of the Preferred Embodiments

The invention relates to thermal storage elements which contain a mixture of microwave-sensitive materials and heat storage materials in a sealed reservoir. When placed in a microwave field, the microwave energy is captured and converted to heat by the sensitive material and transferred to the heat storage material. The element then remains warm for an extended period of time without adding additional energy. This type of element is useful because it can be inserted into a particular product and taken where there is no available source of energy, e.g.. to a football stadium where the element in the product seat cushion can emit heat for a good part of the game. Many other personal comfort products are envisioned, from headwear to footwear.

Medical devices such as joint wraps, back pads and mattress pads, among others, are contemplated. Food service warmers, toys and hair rollers are also obvious choices. One of the characteristics that can limit the use of the element is a condition which results in hot spots in the mixture of the microwave-sensitive and heat storage materials. This condition can cause catastrophic failure of the container surrounding the mixture and/or injury to a close user. Hot spots can result from concentration gradients of the microwave sensitive material in the heat storage material, either as a result of the manufacturing process or the movement of the microwave sensitive material during use. For example, when the a higher density, microwave sensitive material is dispersed in the liquid heat storage material, it can eventually settle out. In the present invention, the microwave sensitive material is not dispersed in the liquid heat storage material, but is instead bound to the carrier to prevent settling.

The present invention provides an element which retains a continuous and uniform distribution of the microwave sensitive material in the heat storage material. This provides uniform heating in the mixture and uniform heat release over the product surface. The invention also provides a method for making the element in a sheet form, whereby the manufacturing process can be continuous to reduce costs. The sheet can then be cut into appropriate shapes for the final product.

The inventive element is shown in cross section in Figures 1 and 2, the latter being of higher magnification. The thermal storage element 1 is shown in sheet form and comprises a porous carrier 2 having open pores defined by pore walls 3 throughout the structure, a heat absorbing material 7 within the open pores 4 for temporarily storing and then releasing heat, and a microwave sensitive activator 6 within a surface layer of the pore walls 3 of the carrier for absorbing microwave energy, converting it to heat and transferring the heat to the heat absorbing material 7.

We refer to the mechanism as being an adsorption of the microwave sensitive material within the surface layer of the pore wall. By this we mean that, due to the affinity of the materials, the activator is adsorbed into the polymer chain of the carrier material within the surface layer. However, we do not want to be limited to this mechanism, as long as the activator is held in the surface layer of the pores and is not dispersed freely in the heat absorbing material where it could settle.

The carrier preferably has a fairly continuous distribution of pores throughout. And because the activator is adsorbed into the surface of the pore walls, it is also rather continuous throughout the sheet element. The adsorbed activator is also held fairly tight in the pore wall surface, especially when both the activator and the carrier contain polar molecules, so that it will not settle out during cyclic use of the element. The amount of activator can also be controlled by controlling the depth of the adsorbed layer in the pore surface.

A protective cover 5 may be used to encase the porous carrier 2. This cover is microwave transparent, meaning that sufficient microwave energy can readily pass through the cover to reach and be absorbed by the activator. Typical cover materials include nylon/high density polyethylene (HDPE)/linear low density polyethylene (LLDPE) copolymer, or a polypropylene/nylon (coated with polyvinylidene chloride) copolymer. The material composition is not critical to the invention so long as it is relatively microwave transparent and helps prevent the heat absorbing material from escaping the porous carrier.

One method for producing the inventive element is shown schematically in Figure 3. A roll 31 of the porous carrier material 30 is immersed in a liquid 32 which includes the activator. The liquid 32 is contained in a reservoir 33 and the carrier 30 is guided through the liquid by means of a roll 34. The liquid 32 including the activator infiltrates the open pores of the porous carrier 32 and then is adsorbed into the surface layer of the open pore walls of the carrier. Excess activator liquid is removed from the pores by compressing the carrier in pinch rolls 35. The time during which the liquid 32 is held in the pores before being compressed out of the pores may be used in certain cases to control the amount of activator adsorbed into the surface of the pores. The liquid 32 may consist entirely of activator or may also include other liquids whose purpose may be helpful in the infiltration of the carrier. Any other liquids preferably evaporate before the protective cover sheet 43 is applied. After the excess liquid activator is removed from the pores, the carrier is guided over roll 36 into a reservoir 38 containing heat absorbing material 37 in liquid form. The heat absorbing material is normally solid at room temperature and is held at an elevated temperature in the reservoir 38. Below the surface of the liquid, the carrier is again compressed to dispel air from the pores, and then is allowed to expand and take up the liquid heat absorbing material 37 to fill the pores in the place of the dispelled air. The carrier is guided out of the second bath by means of guide rolls 40 and 41 as the heat absorbing material solidifies in the carrier pores. A protective cover sheet 43 can be applied to each side of the carrier 30 from the dispensers 42 prior to rolling the element on roll 44. In the alternative, the carrier sheet may be cut into finished size and shape and then encased in a protective cover. The protective cover sheets 43 can be applied in solid form to the carrier or they can be initially applied in liquid form and be allowed to solidify prior to rolling.

Other methods can be used to produce the thermal storage element, but this method allows a continuous sheet of the element to be produced. This is a very efficient way of producing the element. For use in the final products, the sheet carrier can be cut into appropriate shapes and sizes, encased in a protective cover and inserted into the product. The final product can be produced continuously in one production line.

The substrate or carrier is preferably a high porosity, open-celled polymer foam. It is also preferably chemically polar, which allows the activator to more readily be adsorbed into the cell walls. Suitable compositions are urethanes, cellulosics, silicones, vinyls and modified olefins such as maleic anhydride-grafted olefins. Foams based on polar elastomers such as nitrile rubber or chloroprene-based rubbers, or carbon black-containing rubbers are also suitable. Polyether urethanes are particularly preferred because they are compatible with many of the preferred activators. The foam materials are conveniently dense enough to provide for any needed rigidity in the ultimate use of the product, but also are porous enough to provide sufficient surface area for the necessary activator and sufficient pore volume for the heat storage material. The activators which are useful in the invention are chemically polar liquids at use temperatures. The selected activator should have a chemical affinity for the carrier such that it can be adsorbed into the carrier surface. Though it is not necessary, this also means that the carrier will preferably also be chemically polar to more tightly bind the activator. Activators which are solids are not useful in the present invention if they cannot be adsorbed in a liquid state into the surface of pore walls of the carrier or compounded into the elastomer prior to the foaming process. It is possible that an activator which is solid at the normal use temperature (for example, 45-60° C), but has a moderately higher melting point (for example, 100° C), could be useful in the inventive thermal storage element if it can be adsorbed into the carrier as a liquid. Generally, activators which are liquid over the use temperatures are preferred.

Though again not necessary for the invention to be operable, it is preferable for maximizing the effectiveness of the heat storage material that the activator should also be insoluble and immiscible in the heat storage material and generally non- volatile at the manufacturing temperatures and at the use temperatures. Water or water mixtures (such as water/alcohol, water/glycol or water/glycerol) are useful activators, water is desirable because its cost and toxicity are low. Other activators are glycols, glycol oligomers and glycerine.

Any heat storage materials can be used in the invention, but the preferred materials are the phase change materials (PCMs). Sensible heat storage materials release heat with a drop in temperature, so are not useful in many applications. Moreover, one must raise the temperature in order to store more heat, and elevated temperatures can be dangerous to the container materials or the user. Consequently, the amount of heat one can store is fairly limited. However, if the intended range of temperatures is large, then sensible heat storage materials may be useful.

Phase change materials are intended to include both materials which release latent heat during a liquid to solid phase change as well as those materials which release heat during a structural transformation in the solid state. Latent heats of fusion can be much greater than the sensible heat capacities of materials, especially when temperature range of operation is narrow, so that much greater heat can be stored.

Also, PCMs absorb or release the large quantities of latent heat at roughly constant temperature during the liquid/solid phase change. Particularly preferred PCMs include polyols, glycols, polyethylene glycols, long chain organic acids, alkylparaffins, waxes and salt hydrates. Neopentyl glycol is an example of a material which can store heat with a solid/solid morphological change.

Waxes, particularly paraffin wax, are particularly preferred PCMs. Paraffins are generally chemically classed as saturated hydrocarbons. Lower weight members of saturated hydrocarbons are well known as fuels. At heavier weights are the oils which are liquid at room temperature and then the paraffin waxes which are solid at room temperature.

Paraffin wax is a mixture of solid hydrocarbons having the general formula C_nH_2n+2 and typically has a melting point somewhere within the range of 45-60° C. Particular waxes can be selected for preferred melting points. Paraffin waxes generally exhibit the desirable melting temperatures, are relatively non-toxic, are good heat storage materials and are relatively inexpensive. Derivatives of paraffin wax, such as alkyl paraffin wax derivatives, having similar melting points may also be used. The term paraffin wax is meant to include not only materials according to the general formula C_nH_2n+2, but also modified waxes such as alkyl paraffin wax derivatives of such materials and similar compounds. Examples include long chain alcohols (e.g.. stearyl alcohol and cetyl alcohol), and long chain saturated fatty acids

(e.g.. capric acid, lauric acid, myristic acid, palmitic acid and stearic acid)

Particular waxes which are useful in the invention are beeswax (mp. 62-65° C), candelilla wax (mp. 68-70° C), camauba wax (mp. 82-85.5° C), cotton wax, wool wax, monton wax (mp. 80-86° C), and mixtures of waxes. Mixtures of paraffins are also very useful to provide a PCM that is suitable for almost any temperature within the range of applications.

The PCM generally has a melting point of between about 30° C and 90° C, preferably between about 30° C and 65° C. Depending on the use, the range could certainly be expanded. If the product will be in contact with a human user, the ideal melting temperature may be more on the order of between about 30° C and 40° C, because higher temperatures may be uncomfortable.

Other materials may be included with the PCM material without varying from the inventive concept. For example, preservatives may be included to inhibit bacterial growth over the life of the thermal storage product. Dyes, antioxidants, flame retardants and the like can also be included if desired.

Examples of the Preferred Embodiments

Example 1. A low-density polyetherurethane foam measuring 10x10x0.5 cm and weighing 1.23 grams was soaked with dipropylene glycol (DPG). The soaked foam was rolled with a metal cylinder to remove excess DPG from the pores. 2.57 grams of DPG remained adsorbed into the foam.

A molten paraffin wax (Bolar 941) having a melting point of about 52° C was poured onto the foam to fill the pores. 34 grams of wax were taken up in the pores, creating a composite pad which was about 90% wax. The wax was allowed to solidify and the composite pad was encased in polymeric film laminate and sealed. The composite pad was placed in a 1 kilowatt microwave oven for 35 seconds after which the temperature of the composite was about 50° C. After a total of 55 seconds, the temperature was raised to about 65° C.

Example 2. A cellulosic carrier with a multiplicity of open cells can be soaked with water such that the water is adsorbed into the cells walls. Excess water can be removed by compressing or vibrating the carrier. When the excess water is removed from the cells, paraffin wax can be infiltrated into the cells by soaking the carrier in a hot bath of the wax. The carrier may then be removed from the bath and the wax allowed to solidify. To speed up the process, either or both of the water and the wax may be infiltrated into the carrier under elevated pressure and the excess water

(activator) may be removed under reduced pressure.

Claims

I CLAIM:

1. A thermal storage element, comprising:: a porous carrier having an open-pore structure including pore walls; a heat absorbing material within the open pores of the porous carrier; and an activator for absorbing microwave energy, converting it to heat and transferring the heat to the heat absorbing material, wherein the activator is held within a surface layer of the walls of the open pores of the carrier.

2. A thermal storage element as in Claim 1, wherein the carrier is compressible.

3. A thermal storage element as in Claim 1 , wherein the porous carrier is comprised of a material having chemically polar molecules.

4. A thermal storage element as in Claim 1, wherein the activator is a liquid.

5. A thermal storage element as in Claim 4, wherein the activator is chosen from the group consisting of glycols, glycol oligomers, water, glycerine and mixtures thereof.

6. A thermal storage element as in Claim 5, wherein the activator comprises water.

7. A thermal storage element as in Claim 5, wherein the activator comprises dipropylene glycol.

8. A thermal storage element as in Claim 1 , wherein the heat absorbing material is a solid/liquid phase change material within a predetermined temperature range of use.

9. A thermal storage element as in Claim 8, wherein the activator is a liquid.

10. A thermal storage element as in Claim 9, wherein the activator is substantially immiscible in the liquid phase of the heat absorbing material.

11. A thermal storage element as in Claim 9, wherein the heat absorbing material has a melting point within the range of 45-60° C.

12. A thermal storage element as in Claim 9, wherein the heat absorbing material is a wax.

13. A thermal storage element as in Claim 12, wherein the heat absorbing material is paraffin wax.

14. A thermal storage element as in Claim 12, wherein the activator is chosen from the group consisting of glycols, glycol oligomers, water, glycerine and mixtures thereof.

15. A thermal storage element as in Claim 1 , wherein the carrier is a polyetherurethane foam, the activator is dipropylene glycol, and the heat absorbing material is paraffin wax.

16. A thermal storage element for providing uniform heat across a surface of the element, comprising: a porous carrier having an uniform, open-pore structure including pore walls, the carrier comprised of a material having chemically polar molecules; a heat absorbing material within the open pores for temporarily storing and then releasing heat; a liquid activator for absorbing microwave energy, converting it to heat and transferring the heat to the heat absorbing material, wherein the activator is contained within a surface layer of the walls of the open pores of the carrier; and a protective cover encasing the porous carrier and preventing the heat absorbing material from escaping from the porous carrier.

17. A thermal storage element as in Claim 16, wherein the activator is chosen from the group consisting of glycols, glycol oligomers, water, glycerine and mixtures thereof.

18. A thermal storage element as in Claim 16, wherein the heat absorbing material is a solid/liquid phase change material within a predetermined temperature range of use.

19. A thermal storage element as in Claim 18, wherein the heat absorbing material has a melting point within the range of 45-60° C.

20. A thermal storage element as in Claim 19, wherein the heat absorbing material is a wax.

21. A thermal storage element as in Claim 18, wherein the carrier is a polyetherurethane foam, the activator is dipropylene glycol, and the heat absorbing material is paraffin wax.

22. The method of fabricating a thermal storage sheet element, comprising:: providing a porous carrier having a uniform, open-pore structure, the open pores defining pore walls; infiltrating the open pores of the porous carrier with a liquid activator capable of absorbing microwave energy and converting it to heat, such that at least a portion of the liquid activator enters into a surface layer of the walls of the open pores; removing any excess activator from within the open pores; and infiltrating the open pores with a heat absorbing material capable of temporarily storing and then releasing heat.

23. The method of fabricating a thermal storage sheet element according to Claim 22, further comprising:: encasing the porous carrier with a protective cover for preventing the heat absorbing material from escaping from the porous carrier.

24. The method of fabricating a thermal storage sheet element according to Claim 22, wherein the heat absorbing material is a solid/liquid phase change material comprising infiltrating the open pores of the porous carrier with the heat absorbing material in the liquid phase.

25. The method of fabricating a thermal storage sheet element, comprising:: providing a carrier comprised of a compressible material and having a uniform, open-pore structure, the open pores defining pore walls; infiltrating the open-pores of the carrier with a liquid activator capable of absorbing microwave energy and converting it to heat, such that the liquid activator enters into a surface layer of the walls of the open pores; compressing the carrier to remove excess activator from within the open pores of the carrier; providing a liquid bath of a heat absorbing material which is a solid/liquid phase change material within the temperature range of use of the sheet product; immersing the carrier in the bath and then compressing and releasing the carrier to take up the liquid heat absorbing material into the open pores of the carrier; and removing the carrier from the bath and causing the heat absorbing material to solidify.

26. The method of fabricating a thermal storage sheet element according to Claim 25, further comprising:: encasing the porous carrier with a protective cover for preventing the heat absorbing material from escaping from the porous carrier.

27. All novel disclosures and combinations thereof.