Flexible phase change material

Abstract

The present invention relates to a composition comprising 50-99 wt.% Na2SO4·10H2O and 0.1-5.0 wt.% alginate. Since the composition (and a bag comprising the composition) remains flexible during the phase change, the composition can be advantageously used as a phase change material. Its performance remains the same when subjected to a large number of subsequent heating and cooling cycles.

Description

Technical Field

[0001] The present invention relates to phase change material compositions, containers containing such compositions, and cooling garments containing such containers. Background Art

[0002] Latent heat storage has become increasingly important in recent years. Phase change materials (PCMs) play an important role here, as they are able to store thermal energy efficiently. They have the advantages of high energy storage density and isothermal storage process.

[0003] PCMs are advantageously used in, for example, buildings, industry, spacecraft and clothing. They can be used for space heating and cooling, off-peak energy storage, heat exchanger improvements, cooling or heating the human or animal body, etc.

[0004] However, the application of PCMs has begun to be limited by disadvantages associated with the phase changes exhibited by PCMs. For example, in PCMs that exhibit a solid-liquid phase change, this change is accompanied by changes in mechanical properties such as brittleness / plasticity, hardness, viscosity, and surface roughness. This is a disadvantage in applications where the flexibility of the PCM is important and where users prefer a good look and feel to the product containing the PCM.

[0005] This is particularly problematic in clothing. For example, once the PCM inside the garment changes from a liquid to a crystal, a flexible garment becomes stiff and brittle, which can be uncomfortable. Depending on how the PCM is incorporated into the garment, an undesirable increase in the garment's surface roughness may also occur.

[0006] The compound Na2SO4·10H2O is relatively harmless and has a sharp phase transition at 32.4°C, which can be used as a PCM in clothing applications. Effective application of this compound requires retaining its ten equivalents of water. For this reason, it is usually packaged in a watertight container (such as a pouch or pocket) to prevent water evaporation. The phase transition that Na2SO4·10H2O relies on for heat storage is a liquid-to-solid phase transition, which creates the above-mentioned problem when it is necessary to apply a flexible bag of Na2SO4·10H2O to clothing. The crystals formed during the phase transition cause the solid and non-flexible bag to have a rough surface. When a force is applied to the bag, the sharp crystals inside the bag will press against the inner surface of the bag and may damage it (e.g. puncture) as a result.

[0007] However, there is currently no satisfactory Na2SO4·10H2O-based PCM that maintains its flexibility during phase transition. More importantly, any improvement of Na2SO4·10H2O must not have the side effect of reducing the thermal storage capacity of the PCM and / or the distribution of the phase transition over the entire temperature trajectory. Summary of the invention

[0008] It is therefore an object of the present invention to provide a Na2SO4·10H2O based PCM which maintains its flexibility during the phase transition. It is also an object to provide a Na2SO4·10H2O based PCM which does not form sharp (crystalline) materials during the phase transition which could damage a closed container, for example. It is also an object of the present invention to have the phase transition occur at a single temperature, or at least to have the temperature trajectory of the phase transition as short as possible.

[0009] It has now been found that one or more of these objectives can be achieved by using specific additives to Na2SO4.10H2O.

[0010] Therefore, the present invention relates to a phase change material composition comprising:

[0011] -50.0-99.9wt.% Na2SO4·10H2O; and

[0012] - 0.1–5.0 wt.% alginate.

[0013] The composition of the present invention comprises 50.0-99.9wt.% of Na2SO4·10H2O, which is the actual phase change material in the composition. The content of Na2SO4·10H2O is usually in the range of 50.0-99.0wt.%. It can also be in the range of 70-98wt.%, 80-98wt.%, or 90-98wt.%. It can also be in the range of 75–95wt.%.

[0014] The higher the content of Na2SO4·10H2O in the composition, the more efficient the composition is in terms of energy that can be absorbed per kilogram of total composition. Therefore, the content of Na2SO4·10H2O is preferably as high as possible. On the other hand, the addition of additives reduces the content of Na2SO4·10H2O. However, additives are preferred for a number of reasons.

[0015] The melting point of pure Na2SO4·10H2O is 32.4°C. This melting point can be affected by certain additives. For example, it is known that the presence of NaCl lowers the melting point. In the composition of the present invention, a NaCl content of up to 20 wt.% may be present to adjust the melting point. The NaCl content in the composition containing Na2SO4·10H2O may also be 15 wt.% or less, 10 wt.% or less, 8 wt.% or less or 5 wt.% or less. It may also be in the range of 0.1-15 wt.%, in particular in the range of 0.5-10 wt.%.

[0016] Borax (Na2B4O5(OH)4·8H2O) can also be used to change the melting point of Na2SO4·10H2O. For this purpose, its content can be up to 10 wt.%, for example, in the range of 0.1-8.0 wt.%, in the range of 2.0-6.0 wt.%, or in the range of 0.5-4.0 wt.%.

[0017] Another optional additive is sodium carboxymethylcellulose, which may be present in an amount up to 10 wt.%. Typically, when present, it is present in an amount of 5 wt.% or less. For example, it is in the range of 0.5-4.0 wt.%. The role of sodium carboxymethylcellulose is to prevent water from separating from the composition.

[0018] In the composition of the present invention, water may be present in addition to the water provided by Na2SO4·10H2O. This is useful for the following reasons. The effective application of Na2SO4·10H2O as a phase change material requires the retention of its ten equivalents of water. For this reason, Na2SO4·10H2O is typically packaged in a watertight container (e.g., a soft bag or pouch) to prevent evaporation of water. A small amount of excess water may be present in such a container to allow some of the water to eventually escape from the container. This excess has little effect on the heat storage capacity and phase change temperature of the composition, while any lack of water (i.e., when the total water content of the composition is less than ten equivalents) will have a strong negative impact on the action of the composition in terms of heat storage capacity and phase change temperature. When a product contains excess water during the manufacturing process, this improves the long-term stability of the product.

[0019] For example, the content of any additional water (i.e., water in excess of ten water equivalents in Na2SO4·10H2O) in the compositions of the present invention may be up to 4.0 wt.% or up to 6.0 wt.%, for example, within the range of 0.5-5.0 wt.%, or within the range of 1.0-3.0 wt.%.

[0020] Alternatively, the content of the additional water may be in the range of 0.10-10.0 mol% of the amount of Na2SO4·10H2O (i.e., the molar ratio of the additional water to Na2SO4·10H2O is in the range of 0.10:1.0 to 10.0:1.0). The content of the additional water may be specifically in the range of 0.20:1.0 to 6.0:1.0, more specifically in the range of 0.40:1.0 to 4.0:1.0.

[0021] The composition of the present invention comprises alginate. Alginic acid is a hydrophilic linear polysaccharide copolymer having mannuronic acid and guluronic acid residues. The residues may be alternating, but the polymer may also comprise residue blocks in which a plurality of identical residues are linked together. In different alginates, mannuronic acid and guluronic acid residues may be present in different molar ratios, for example in the range of 25:75 to 75:25.

[0022] In the composition of the invention, the molar mass of the alginate is generally in the range of 5,000-500,000 g / mol. It may also be in the range of 10,000-250,000 g / mol or 30,000-400,000 g / mol. In particular, it is in the range of 20,000-200,000 g / mol, more particularly in the range of 40,000-120,000 g / mol.

[0023] The alginate is preferably a metal salt. For example, it is selected from the group consisting of sodium alginate, potassium alginate, calcium alginate and magnesium alginate. It can also exist as another salt, such as an ammonium salt.

[0024] The composition of the present invention may contain a fatty acid salt, i.e. a salt of a fatty acid. A fatty acid is a carboxylic acid with a long fatty chain (e.g. 6-30 carbon atoms), which may be saturated or unsaturated. The fatty acid salt in the composition of the present invention may in principle be any fatty acid salt, as long as it has a water solubility of 75 mg / L or less at 25° C. Preferably, the fatty acid salt is a saturated fatty acid salt, i.e. it contains a saturated alkyl chain.

[0025] Typically, the fatty acid salt comprises 10-30 carbon atoms, in particular it comprises a chain of 10-30 carbon atoms (i.e. it is a salt of a C10-C30 fatty acid). This chain may be branched, for example with one or more methyl groups. However, typically this chain is linear (i.e. unbranched).

[0026] Preferably, the fatty acid salt comprises a fatty acid carboxylate selected from the group consisting of caprylate, caprate, laurate, myristate, palmitate, stearate, arachidate, behenate and lignocerate.

[0027] The fatty acid salt in the composition of the present invention is usually a metal salt (i.e., it is a metal carboxylate). The metal is usually a divalent metal, in particular one selected from the group consisting of magnesium, calcium, manganese, zinc, tin and lead. Therefore, such a divalent metal carboxylate usually has a molecular formula M(C x H 2x+1 COO)2, wherein M is a divalent metal. In the molecular formula, x is usually in the range of 9-29, particularly in the range of 7-25, and more particularly in the range of 9-13.

[0028] Preferably, the fatty acid salt in the mixture of the present invention is selected from the group consisting of magnesium caprylate, magnesium caprate, magnesium laurate, magnesium myristate, magnesium palmitate, magnesium stearate, magnesium arachidate, magnesium behenate and magnesium lignocerate.

[0029] The composition of the invention may comprise more than one fatty acid salt, for example two, three or four. In particular, they are independently selected from the group consisting of magnesium caprylate, magnesium caprate, magnesium laurate, magnesium myristate, magnesium palmitate, magnesium stearate, magnesium arachidate, magnesium behenate and magnesium lignocerate.

[0030] The solubility of the fatty acid salt (if present) in water at 25°C is 75 mg / L or less. Typically it is 60 mg / L or less. It may also be 50 mg / L or less, 40 mg / L or less, or 30 mg / L or less. It is typically in the range of 1-50 mg / L. It may also be in the range of 5-40 mg / L.

[0031] The present invention also relates to a method for preparing a composition as described above, comprising mixing Na2SO4.10H2O with a fatty acid salt, in particular with a fatty acid salt such as magnesium stearate.

[0032] When the composition of the present invention is used as a phase change material, the composition is typically present in a container. For use in clothing, a flexible container, such as a pouch or pocket, is preferred. Such a container is preferably watertight to prevent the escape of water required for the phase change of the composition. Therefore, the present invention further relates to a container comprising a composition as described above, wherein the container is watertight.

[0033] Surprisingly, it was found that the composition of the present invention (e.g. when present in a bag) remained flexible when subjected to a large number of subsequent heating and cooling cycles. This indicates that the PCM inside it has no tendency to change from a liquid state to a crystalline state. Measuring the thermal properties of the composition in the container by monitoring the temperature during the cycle confirmed that the phase change occurred in each cycle.

[0034] The composition of the invention has an advantageous application in clothing. Therefore, the present invention also relates to a garment (such as a jacket, coat, trousers or hat) comprising a composition as described above, in particular a container comprising such a composition.

[0035] The invention also relates to a method for cooling a person or an object, comprising using a container as described above, or a garment comprising such a container, such as a jacket, trousers or a hat. DETAILED DESCRIPTION

[0036] Example

[0037] The composition was prepared as follows and placed into an aluminum foil bag which was then sealed to form a watertight container:

[0038] -85.9wt.%Na2SO4·10H2O

[0039] -1.6wt.% sodium carboxymethyl cellulose

[0040] -1.6wt.% borax

[0041] -2.1wt.% water

[0042] -8.3wt.% sodium chloride

[0043] -0.5wt.% sodium alginate

[0044] The bag was heated in a water bath at 50°C and left at this temperature for 15 minutes. The bag was then flexible and could be easily deformed manually. Thereafter, it was cooled in a refrigerator to 5°C and left at this temperature for 15 minutes. The bag was still flexible and no solid particles could be felt in the bag. The only difference from the heated bag was the ductility: the heated bag was more easily deformed than the cooled bag. This sequence of heating and cooling was considered a thermal cycle.

[0045] The bag was subjected to 420 thermal cycles and weighed before and after each cycle. The results showed that the weight of the bag did not change over time and the bag had the same flexibility after each cycle.

[0046] Temperature monitoring during the cycles indicated that a phase transition occurred at 24°C.

Claims

1. A composition comprising: - 50-99 wt.% Na2SO4·10H2O; and - 0.1-5.0 wt.% alginate; and - 0.1-5.0 wt.% of a fatty acid salt having a water solubility of 75 mg / L or less at 25°C.

2. The composition according to claim 1, wherein the alginate is selected from the group consisting of sodium alginate, potassium alginate, calcium alginate and magnesium alginate.

3. A composition according to claim 1 or 2, wherein the molar mass of the alginate is in the range of 5,000-500,000 g / mol.

4. The composition according to claim 1 or 2, wherein the molar mass of the alginate is in the range of 20,000-200,000 g / mol.

5. The composition according to claim 1 or 2, wherein the water solubility of the fatty acid salt at 25°C is in the range of 1-50 mg / L.

6. The composition according to claim 1 or 2, wherein the fatty acid salt is of the formula M(C x H 2x+1 COO)2, wherein x is in the range of 9-29.

7. The composition of claim 6, wherein the divalent metal is selected from the group consisting of magnesium, calcium, manganese, zinc, tin and lead.

8. The composition according to claim 1 or 2, wherein the fatty acid salt is selected from the group consisting of magnesium caprylate, magnesium caprate, magnesium laurate, magnesium myristate, magnesium palmitate, magnesium stearate, magnesium arachidate, magnesium behenate and magnesium lignocerate.

9. The composition according to claim 1 or 2, further comprising 0.5-4.0 wt.% of borax.

10. The composition according to claim 1 or 2, further comprising 0.5-4.0 wt.% of sodium carboxymethylcellulose.

11. The composition according to claim 1 or 2, further comprising 0.15-15.0 wt.% of NaCl.

12. The composition according to claim 1 or 2, further comprising 0.1-5.0 wt.% of water.

13. A container comprising the composition of any one of claims 1-12, wherein the container is water-impermeable.

14. A cooling garment comprising the composition of any one of claims 1 to 12 or comprising the container of claim 13.

15. A method for cooling a body or object comprising using the container of claim 13 or the cooling garment of claim 14.