WO2004031319A1

WO2004031319A1 - Heat-generating articles

Info

Publication number: WO2004031319A1
Application number: PCT/GB2003/004366
Authority: WO
Inventors: Jonathan Webb; Padraig Gerard Troy
Original assignee: Craig Medical Products Ltd
Current assignee: Craig Medical Products Ltd
Priority date: 2002-10-04
Filing date: 2003-10-03
Publication date: 2004-04-15
Anticipated expiration: 2005-04-04
Also published as: GB2393732A; AU2003274311A1; GB0223069D0

Abstract

The present invention relates to a heat-generating formulation comprising iron powder which is oxidised in an exothermic reaction, wherein the formulation is formed into a coherent mass. The invention also relates to methods for forming the coherent mass and to articles comprising the heat-generating formulation as a coherent mass.

Description

Heat-Generating Articles

The present invention relates to heat-generating articles and, in particular, to articles wherein the heat-generating agent or formulation is in the form of a coherent mass. Such articles are capable of generating heat without the need for an external source of heat or power.

Heat-generating articles which do not require external heat or power supplies are known and they often use exothermic reactions as the heat source. Probably the most commonly used exothermic reaction in this context is the following:

4Fe + 3O, 2Fe₂0₃

The (heat) energy generated by this reaction (ΔH) is -1652 kj/mol. This means that the reaction is capable of generating a considerable amount of heat, if properly controlled and catalysed.

This simple chemical reaction has been used as a source of heat in a number of situations since the 1920's, for example in hand or foot warmers, where a local source of heat is used to combat the discomfort of cold hands or feet.

The European Patent Application No. EP 0 526 637A1 (Kiribai Chemical Industry Co. Ltd.) discloses an example of the type of heat-generating article which is known. The exothermic reaction used is that summarised above and the heat-generating formulation comprises iron in powder form along with various catalysts and other additives. The formulation is held in a gas-permeable, inner bag which is in turn kept in a gas -impermeable, outer bag until heat is to be generated. The exothermic reaction requires the presence of oxygen. Thus, until the heat-generating formulation is exposed to oxygen, the reaction will not take place and no heat will be generated. The article is made to generate heat by opening the outer bag and exposing the iron powder formulation in the inner bag to oxygen. Once the iron powder formulation is exposed to oxygen, the exothermic reaction begins and heat is generated. Whilst formulations of the type disclosed in EP 0 526 637A1 have been used for decades, the heat-generating formulations comprising the iron powder and additives have not previously been formed into a coherent mass. Rather, it has been preferred to use the formulations in powder form. The main reason for this is that it was previously thought that the use of loose powder was essential in order to ensure that the reaction rate is high enough to generate heat. The loose powder provides a large exposed surface area with which the oxygen may come into contact, allowing the exothermic reaction to take place.

Forming the formulation into a coherent mass was expected to substantially reduce the surface area of the formulation with which oxygen in the atmosphere can come into contact. This was expected to significandy slowing the reaction so as to render it useless for use in heat-generating articles, where a high temperature must be quickly reached and then maintained.

In US Patent No. 5,984,995 (Proctor & Gamble) the heat-generating formulation is a granulate form, with the dry powder formulation being agglomerated or compacted into the form of small pellets or tablets. This is done to overcome the problems associated with the dust which accompanies the powder formulations. However, the small size of the granules formed means that the heat-generating formulation still has a relatively high surface area:volume ratio.

In contrast to what was clearly previously believed by those knowledgeable in this field of technology, using a heat-generating formulation which has been formed into a coherent mass does not hamper the generation of heat. In fact, it has been surprisingly found that the use of the formulation in a coherent mass results in more uniform distribution of heat. Furthermore, the amount of heat generated by the formulation can be more accurately controlled; it is possible to set tighter tolerances than are possible when using a loose powder formulation. When the heat-generating formulation is in the form of a coherent mass, greater heat stability has also been observed, with increased heat retention and attendant increased efficiency, compared to equivalent loose powder or granular formulations.

Finally, the use of the heat-generating formulation in a coherent mass also overcomes problems associated with the known use of loose powder or granular formulations, namely lumping or uneven distribution of material, which tend to result in uneven temperature distribution and potential discomfort to the user.

The formation of a coherent mass also has the further advantage that it allows the heat-generating formulation to be made into a number of convenient forms, including laminates, strips, pads and tablets. These forms can be easily and cleanly incorporated into articles.

According to the first aspect of the invention, a heat-generating formulation is provided which is formed into a coherent mass. The formulation comprises iron, in the form of iron powder. The formulation may also comprise an aqueous solution.

Preferably, the heat-generating formulation is formed into a non-particulate heating element, where "non-particulate" means that the heating element is not in the form of a plurality of small granules, pellets or tablets.

In one of the preferred embodiments, the formulation further comprises a metal salt, which acts as a catalyst of the reaction. The metal salt is preferably sodium chloride and may be added to the heat-generating formulation in the form of an aqueous solution (brine).

Preferably, the formulation also comprises an absorbent material, which controls the release of the aqueous solution and therefore helps to control the rate of the exothermic reaction. A wide variety of suitable absorbent materials are known. Amongst the most effective materials are absorbent gelling materials, such as superabsorbents. The absorbent gelling material can be natural, synthetic or modified natural polymers and materials. The absorbent gelling materials can be inorganic materials, such as liquid gels, or organic compounds, such as cross-linked polymers, or alginates, reticular carboxymethylcelluloses, grafted starches, natural modified polysaccharides or synthetic derivatives of acrylamides, acrylonitriles or polyacrylates. Silica gel and water-absorbing resins are examples of preferred absorbent materials.

In another preferred embodiment, the formulation includes carbon, preferably activated carbon, which serves as a catalyst for the exothermic reaction. The carbon also aids in the dispersion of the heat generated by the reaction. When included in the heat-generating formulation, activated carbon also has water-absorbing and water-holding properties.

In a further preferred embodiment, the formulation may also comprise one or more binders which assist in the formation of the coherent mass and which ensure that the coherent mass remains intact. Materials commonly used as binders include starch, gelatin and sugars such as glucose, sucrose, dextrose, lactose and molasses. Natural and synthetic gums which may be used include acacia, sodium alginate, carboxymethylcellulose, methylcellulose and polyvinylpyrrolidone. The binders may be used as a solution and/or in a dry form. Therefore, polymer latexes, also known as emulsion polymers, which are colloidal dispersions of submicroscopic polymer particles in a continuous medium (the best known example is the natural latex produced by the rubber tree evea brasiliensis which is still an important source of one of the world's most widely used polymers) and man-made latexes, comprising dispersions of polymer (e.g. acrylic, styrene acrylic, PVdC, styrene butadiene and acetate) particles in water may also be used.

In one embodiment of the invention, the formulation comprises 10-90% iron powder, 5-60% water, 0-70% metal salt, 2-25% carbon, 1-50% absorbent material, 0-20% silica gel, and 0-50% binder.

According to a second aspect of the present invention, methods for forming the coherent mass heat-generating formulations according to the first aspect of the invention are provided. The coherent mass of the heat-generating formulation is preferably formed by first mixing all of the components of the formulation (including the water) together, and then forming a coherent mass from this wet mixture, using one or more of a variety of known methods.

One such known method is by compression. Where the formulation is compressed, it preferably includes a binder, as mentioned above. This will assist in forming the coherent mass and then in maintaining a coherent mass following compression, especially when the coherent mass is packaged and transported, and later during use.

In a preferred embodiment, the degree of compression is sufficient for the coherent mass to be maintained, as discussed above, whilst allowing oxygen to gradually permeate throughout the mass, allowing all of the formulation to react and produce heat. The skilled person would have no difficulty in adjusting the compression forces to achieve this result, and no special or sophisticated tests are required to test, these properties of the coherent mass.

The coherent mass is preferably formed by compressing the wet mixture of the formulation components between one or more pairs of rollers. The formulation can be fed between the rollers with confining sheets to produce a relatively thin, sheetlike laminate product. Alternatively, the formulation may be compressed without confining sheets, to produce strips or tablets. In one embodiment, the compressed formulation includes cellulose fibres. These fibres give the coherent mass added structural integrity and help to prevent crumbHng of the mass during packaging, transport and use.

Where the formulation is compressed between confining sheets, the sheets are preferably non-woven material or material of a fibrous nature, and can be either man-made or natural. The sheets are preferably highly permeable to oxygen. In another embodiment of the invention, the degree of oxygen permeability of the confining sheets is selected to provide a further level of control of the rate at which the exothermic reaction occurs and, therefore control of the heat generated and the temperatures the heat-generating article reaches.

One preferred material for use as the confining sheets is cellulose tissue, and more preferably non-woven cellulose tissue. The sheets are preferably between 10 and 100 gsm, with an open pore structure. This affords structural integrity, whilst allowing high oxygen penetration. Where greater structural stability is required, thicker confining sheets may be used, although the thicker sheets will tend to have reduced oxygen permeability. Clearly, various other materials may be used which have the appropriate properties for putting the present invention into practice. Examples of alternative materials include plastic polymer films, woven or non- woven textiles and paper.

The above discussed compression method for forming the heat-generating formulation into a coherent mass is very simple and cheap. In particular, the absence of a separate, subsequent step to add water to the compressed article is highly advantageous as it greatly simplifies the manufacturing method.

In an embodiment of the invention, a heat-generating article is formed by compression, whereby the heat-generating formulation is compressed between two confining sheets, but the formulation is only compressed in specific areas, so that areas of coherent mass and areas of loose powder are formed. The loose powder is then removed and the areas of coherent mass are cut out and optionally sealed to form packet-like articles. This method is useful for large-scale manufacture of compressed heat-generating articles.

An alternative method of forming a coherent mass from the heat-generating formulation is to spray a thick slurry formed from the formulation onto a fabric base. In this method, it is necessary to carefully dry the sprayed product to lower the high water content.

In a yet further method, the formulation is formed into a coherent mass using a method based upon a process traditionally used to produce paper. The formulation is provided as a powder and is mixed with cellulose fibres and laid using air pressure variations on to a fibrous substrate. A latex binder solution is then added and suction applied to force the solution right through the substrate. This is then compressed through rollers and dried using hot rollers in order to lower the water content to an acceptable level for the present invention. Care must be taken when using this method that the iron powder does not cause an explosion.

Both of the latter methods involve the addition of large amounts of water to the formulation. It will therefore be necessary to control the methods carefully, to ensure that the exothermic reaction is not prematurely initiated.

All of the above discussed methods are clearly different to known tableting or compaction methods which apply greater forces to the formulations being tableted or compacted. Therefore, the coherent masses formed using the methods of the present invention will have different properties to pellets or tablets formed from the same or similar compositions but using those tableting or compaction methods.

According to a third aspect of the present invention, articles are provided which incorporate the coherent mass heat-generating formulations according to the first aspect of the invention.

In order for the coherent mass heat-generating formulations to be useful in heat- generating articles, it must be possible to control the start of the exothermic reaction. This is done by controlling the exposure of the formulation to oxygen.

To do this, the coherent mass of heat-generating formulation is preferably packaged in a gas-permeable inner wrapping. This wrapping may be made from gas permeable material or it can simply have holes in it. This inner wrapping is surrounded by a gas-impermeable outer wrapping which prevents oxygen coming into contact with the heat-generating formulation until the heat-generating article is to be used. Upon use, the outer wrapping is opened or removed, in order to allow oxygen to come into contact with the heat-generating formulation and the exothermic reaction is started. In an alternative embodiment, the coherent mass of heat-generating formulation may be packaged in a single gas-impermeable inner wrapping. When the exothermic reaction is to be started, the wrapping is rendered gas-permeable in one of a number of possible ways. For example, the gas-impermeable wrapping could comprise a section which is gas-permeable, but which is covered to render it gas-impermeable until the exothermic reaction is to be started. To start the reaction, the cover over this section is removed, thereby allowing oxygen into the wrapping.

Once the heat-generating formulation is activated by allowing oxygen to come into contact with it, 80-90% of the full temperature can be reached within 2-3 minutes. The temperature then quickly plateaus and can be maintained for a number of hours.

When the heat-generating formulation is in a coherent mass, the exposure to oxygen is limited by the relatively small surface area of the coherent mass. The heat generation may be controlled even further by adjusting the rate at which the oxygen comes into contact with the surface area by adjusting the gas permeability of the inner wrapping. The slower the exposure of the heat-generating formulation to oxygen, the lower the temperature generated and the longer the period of which heat is generated. Thus, the use of a coherent mass allows an unprecedented degree of control over the heat generated upon activation of the heat-generating formulation.

The heat-generating formulation of the present invention may be incorporated into any number of articles to provide a heat source. Such articles include hand and foot warmers, of the type which are already commercially available.

One type of heat-generating article which is particularly useful is a heated blanket. The heat-generating blankets according to the present invention preferably comprise one or more flat, thin slabs of heat-generating formulation in a coherent mass. The slabs preferably do not cover the entire area of the blanket. Rather, they are positioned between two sheets arranged to spread the heat generated by the heat- generating formulation over the whole area of the blanket.

Insulation is preferably provided to prevent the formation of a localised area of intense heat where the heat-generating formulation is positioned. This insulation also assists in the deflection and dispersion of the heat over the whole area of the blanket.

The size of the slab of heat-generating formulation will depend upon the size of the blanket, the temperature to be generated and the length of time over which the temperature is to be maintained. However, a blanket capable of covering an adult human can be heated by one or more flat slabs having a total size of approximately 1200 cm², for example, one slab of 40x30cm or two slabs of 20x30cm.

Preferably, the blanket will maintain a temperature of 36.5°C ± 1°C for 4 hours ± 1 hour.

The sheets are preferably made from a lightweight material. The material is preferably reflective, to help spread the heat generated locally by the heat-generating coherent mass. In a preferred embodiment, the sheets are made from polymer films, typically polyolefins, which may incorporate, for example, a foil or foam laminate as insulation.

The heat-generating coherent mass in the blanket is to be exposed to oxygen only when the blanket is to be used and heat is to be generated.

The heat-generating blankets of the present invention can be used, for example, in mountain rescues or the like, where people have been exposed to the elements and are suffering from hypothermia or at risk thereof. The blankets are completely portable and so would have the added advantage that they could be used immediately, without the need to first get the person to a power supply. Another important area where such blankets may be used is before, during and after surgery. It has long been recognised that it is very important to maintain a patient's body temperature at the normal temperature of 37°C (97°F). It has been found that there is a significant risk that the body temperature of patients undergoing surgery will drop below this. This is due to a number of factors, including cool operating theatres, open body cavities, intravenous fluids and anaesthesia, which impairs body temperature regulation. A drop in body temperature below 37°C stresses the cardiovascular system and can provoke irregular heartbeats and heart attacks. Furthermore, it has been found that a drop in body temperature during surgery not only results in patient discomfort, but also appears to extend the time the patient need to spend in intensive care after the operation.

At present, heated blankets such as Bair Hugger® blankets are used in operating theatres. These are rather like air mattresses which are placed over the patient during surgery and through which warm air is forced. The benefits of this so-called forced-air warming therapy are well documented. However, the blankets are bulky and they cannot be used when certain types of operation are being carried out, such as hip or knee operations. Infection is a particular problem associated with these types of operations.

The heat-generating blankets, incorporating using the coherent mass heat-generating formulations discussed above, have a number of advantages over the Bair Hugger® blankets.

Firstly, the heat-generating blankets are not as bulky as the Bair Hugger® blankets. They also allow the surgeons better access to the patient. Additionally, the heat- generating blankets may be easily shaped to accommodate surgery of any type and ensuring that as much of the patient's body is covered and warmed as possible.

Secondly, the coherent nature of the heat source and its packaging means that there is no risk of infection from the blankets. Indeed, these simple articles can easily be sterilised. Even if the blanket were to be damaged during use, they would not pose any danger and their use could be continued. This would clearly not be the case if the heat-generating formulation were in a loose powder or granular form rather than a coherent mass.

The high degree of control afforded by the heat-generating formulation in a coherent mass means that heat-generating articles such as blankets can be produced which reach a specified temperature for a predetermined length of time. Thus, blankets can be tailored to different situations or even different types of operations.

Thus, to summarise, the use of a heat-generating formulation in the form of a coherent mass presents a number of advantages over the known loose powder or granular formulations. Furthermore, forming a coherent mass will allow the heat- generating formulations to be used in articles for which they previously could not seriously be considered.

Aspects of the present invention are illustrated, by way of example only, in the accompanying drawings.

Figure 1 shows a schematic, cross-sectional view of a heat-generating blanket; and

Figure 2 is a schematic diagram showing the production of the heat-generating formulation in the form of a coherent mass by compression, according to one embodiment of this method.

The heat-generating formulation is in the form of a coherent mass, shaped as a flat slab 12. The heat-generating formulation is surrounded by an inner wrapping 13 which is gas permeable. The wrapped heat-generating formulation is positioned ♦ between two overlying sheets 21, 22 which help to spread the heat generated by the heat-generating formulation over their entire area. One of the sheets 21 will be adjacent to the patient or user when in use. This sheet includes an insulated area 23 adjacent to the heat-generating formulation. The other sheet 22 includes apertures 24 which are sealed by a removable cover sheet 25 before the blanket is to be heated. When the blanket is to be used, the cover sheet is removed and this reveals the apertures through which oxygen is able to enter the space between the sheets. This oxygen can then come into contact with the heat-generating formulation within the inner wrapping, thereby initiating the exothermic reaction.

As shown in Figure 2, the heat-generating formulation in its mixed (wet) powder form 31 is fed onto a travelhng supporting sheet 32 which is pulled from a supply roll 34. A second supporting sheet 33 is fed from a second supply roll 35 and these two sheets sandwich the heat-generating formulation between them. The sheets and formulation pass between a pair of rollers 36, 37, which compresses the powder between the sheets into a consolidated mass 12.

Once the heat-generating formulation has been formed into a coherent mass, it can optionally be cut into pieces of the desired shape and size, for incorporation into articles such as heating blankets.

Claims

1. A heat-generating formulation comprising iron powder, wherein the formulation is formed into a coherent mass.

2. A formulation as claimed in claim 1, wherein the formulation further comprises a salt solution.

3. A formulation as claimed in claim 2, wherein the formulation further comprises an absorbent material.

4. A formulation as claimed in any one of the preceding claims, wherein the formulation further comprises carbon.

5. A formulation as claimed in any one of the preceding claims, wherein the formulation further comprises a binder.

6. A method for forming the heat-generating formulation as claimed in any one of the preceding claims, wherein the formulation is formed into a coherent mass by compression.

7. A method as claimed in claim 6, wherein the formulation is compressed between one or more pairs of rollers.

8. A method as claimed in claim 7, wherein the formulation is compressed between two confining sheets to form a laminate product.

9. A method as claimed in claim 7, wherein the formulation is compressed into strips or tablets.

10. A heat-generating article comprising a heat-generating formulation as claimed in any one of claims 1-5.

11. An article as claimed in claim 10, wherein the article is a blanket.

12. An article as claimed in claim 11, wherein the heat-generating formulation is in the form of a flat, laminate product.

^'5

13. An article as claimed in either of claims 11 and 12, wherein the blanket comprises the heat-generating formulation positioned between two sheets.

14. An article as claimed in claim 13, wherein the formulation extends over only 10 a portion of the area of the blanket.

15. An article as claimed in any one of claims 11-14, wherein an insulated area is provided adjacent to the heat-generating formulation.

15 16. An article as claimed in any one of claims 10-15, wherein the article is disposable.

17. An article as claimed in any one of claims 10-16, wherein the article is sterile.