Thermal energy storage material comprising hydrated compound and water-swollen cross-linked polymer

Page 1bibliographic recordscan →

Page 2scan →

ethylene oxide (see the Day Chahroudi paper already

THERMAL ENERGY STORAGE MATERIAL referred to).

Examples of proposed inorganic thickening agents

Comprising hydrated compound and

WATER-SWOLLEN CROSS-LINKED POLYMER are clays or diatomaceous earth (for the first see U.S. 5 Pat. No. 3,986,969 and for both, see the Day Chahroudi

This is a continuation in part of U.S. application Ser. paper), or fibrous materials, such as fibrous silicon oxide No. 88,514 filed Oct. 26, 1979, now abandoned. or magnesium oxide (see British 'Specification No. The present invention is concerned with thermal 1,298,674) or zeolites (see U.S. Pat. No. 3,720, 198). energy storage materials and heat-exchange devices Of the above-mentioned proposed thickening agents, containing such materials. O the natural polymers are unstable to hydrolysis and Thermal energy storage materials may store thermal biodegradable, and these effects will considerably energy as specific heat and/or as latent heat. It is often shorten the life of a material containing such a polymer. desirable to use materials which store thermal energy as The above-mentioned synthetic polymers and inorganic latent heat, since this enables the volume occupied by thickening agents are more stable, but it appears that the storage material to be minimised. This is advanta 15 prior thermal energy storage materials containing such geous, for example, in materials operating in the temper thickening agents can only be used in shallow depths ature range 10 C. to 100 C. for the storage of solar (for example, about one inch) so that this dimensional energy or of heat extracted during refrigeration. limitation is imposed on heat exchange devices contain Materials which are useful for the storage of thermal ing such storage materials and the device must be dis energy as latent heat undergo reversible transition from 20 posed with its major dimensions horizontal. one form to another on heating to a characteristic tran It has been proposed in European Patent Application sition temperature. This transition may be from solid No. 99 a thermal energy storage material in which a phase to liquid phase (fusion) or from one crystal form hydrated compound having a transition temperature to to another (this latter transition also being referred to as 25 the anhydrous or a less hydrated form in the range 10 100 C. is dispersed and suspended in a hydrogel fusion).

A number of hydrated compounds, such as inorganic formed from a water-soluble synthetic polymer having salts, are known which undergo transition to the anhy a pendant carboxylic or sulphonic acid groups cross drous or a less hydrated form at a characteristic temper linked with cations of a polyvalent metal. ature on heating and revert to the more hydrated form Such a material need only contain a relatively minor on cooling. 30 amount of the polymer, preferably from 0.5 to 10% A potential drawback in the use of many of these based on the weight of the material.

hydrated compounds is incongruency of the phase tran It is an object of the invention to provide a thermal sition, that is, the transformation of the low-temperature energy storage material comprising a hydrated com solid phase to a two-phase condition where a solid and pound which material can be used in vertical columns liquid coexist. In the two-phase condition, the differ 35 of substantial height.

ence in densities of the two phases causes segregation It is a further object of the invention to provide a thereof, which limits their ability to recombine and thermal energy storage material which is stable to hy form the low-temperature single solid phase. Conse drolysis and biological breakdown.

quently the amount of heat recoverable on cooling is It is still a further object of the invention to provide a reduced. thermal energy storage material which, in comparison Attempts can be made to avoid the formation of two with the storage material described in the above-men phases above the transition point by controlling the tioned European patent application No. 99, can be pre initial composition of the material, but even for materi pared in a more controlled manner, is even more stable als with a congruent phase transition, there remains the at the upper end of the operating temperature range, problem that the solid phase tends to settle out in time. 45 and does not necessarily involve the use of heavy metal This limits both the kinetics of transformation and the compounds in its preparation.

uniformity of energy storage density within a container, These and other objects are achieved according to and results in deterioration of the material on repeated the invention by the provision of a thermal energy stor heating/cooling cycles. age material comprising a hydrated compound which Thermal energy storage materials have been pro 50 has a reversible transition to the anhydrous or a less posed which consist of a suspension of a hydrated inor hydrated form, preferably at a temperature in the range ganic salt in an aqueous gel or thickened aqueous me 0' to 100 C, and a minor amount, such as 0.5 to 10% dium, the gelling or thickening agent being a natural (based on the weight of the material), of a hydrogel organic polymer (or derivative thereof), a synthetic throughout which the compound is dispersed, the hy polymer or an inorganic thickening agent. 55 drogel comprising a water-swollen cross-linked poly Examples of proposed natural polymer thickening mer formed by cross-linking a water-soluble or water agents are gelatine (see British Specification No. dispersible synthetic hydrophilic polymer by a covalent 1,543,336), starch, cross-linked starch or cellulose poly cross-linking mechanism.

mers (for first and third, see U.S. Pat. No. 3,986,969 and According to another aspect of the invention there is for second and third see the paper entitled "Suspension provided a method of preparing such a thermal energy Media for Heat Storage Materials' by Day Chahroudi storage material in which a water-soluble synthetic in Proceedings of the Workshop on Solar Energy Sub hydrophilic thermoplastic addition polymer having a systems for the Heating and Cooling of Buildings, Char substantially linear carbon-carbon backbone and active lottesville, Virginia, U.S.A., Apr. 16-18, 1975, pages hydrogen atoms in pendant groups thereof is reacted 56-59) and alginates (see U.S. Pat. No. 3,986,969 and 65 with a cross-linking agent therefor in an aqueous me British Specification No. 1,298,674). dium containing a hydrated compound which has a Examples of proposed synthetic polymer thickening transition temperature to the anhydrous or a less hy agents are polyvinyl alcohol, polyacrylic acid or poly drated form preferably in the range 0 to 100° C. (as 3 mentioned above), so as to form a hydrogel comprising The polymer preferably has active hydrogen atoms a water-swollen, covalently cross-linked polymer with (as determined by the Zerewitinoff method), such hy said hydrated compound dispersed therein. drogen atoms being present in, for example, hydroxy, In one embodiment of the invention, the polymer carboxy or amide: functional groups, as such groups may be cross-linked by an ionic cross-linking mecha help to make the polymer hydrophilic as well as provid nism in addition to the covalent cross-linking mecha ing sites for cross-linking. However, it is also possible nism. In this embodiment, ionic cross-linking is prefera for the hydrophilic groups and the cross-linking sites to bly effected by means of cations of a polyvalent metal, be different (for example, the polymer may have hydro as described in the above-mentioned European Patent philic functional groups as listed above and cross-link

Application 99, the disclosure of which is incorporated ing sites, such as carbonyl groups). herein by reference. The U.S. equivalent of this Euro In some cases, the polymer may have a cross-linking pean application is Ser. No. 913351; the disclosure of agent built into its structure (for example, when the this U.S. application is also incorporated herein. polymer is poly-N-methylol acrylamide or poly-N- There is disclosed in U.S. Pat. No. 4,003,426 a ther methylol methacrylamide). However, it is generally mal energy storage material which comprises a cross 5 preferred, so that the commencement of cross-linking linked polymeric resinous matrix (such as a matrix of a can be controlled, that a separate cross-linking agent be resinous polyester or "vinyl ester" polymer or an epoxy used, the cross-linking agent reacting with the appropri resin) having a plurality of substantially unconnected ate functional groups in the polymer to effect cross-link small closed cavities disposed therein and a hydrated 20 1ng.

salt encapsulated therein. The amount of polymer in The temperature conditions may be chosen such that cluded in such materials is disadvantageously high (that the cross-linking reaction proceeds at an acceptable is, at least 25% by weight, usually at least 35% by rate. For example, where access to the receptacle in weight). The presence of such a large amount of resin which the material is disposed in use is restricted, it is ous polymer in the material means that the material has possible (and preferable) to premix the polymer, the a disadvantageously low heat capacity per unit volume. 25 cross-linking agent, the hydrated compound and any The material according to the invention, which has a other desired materials (such as nucleating agents) in an structure similar to that of the material according to the aqueous medium in a convenient place, the premixed above-mentioned European Patent Application (this materials then being transferred to the receptacle by structure being totally different to that of the material 30 pumping or pouring, for example, before the cross-link described in the above-mentioned U.S. Pat. No. ing reaction has been completed. The temperature of 4,003,426) is generally formed in situ (that is, in an aque the aqueous medium during this premixing stage is pref ous medium containing the hydrated compound) by erably in the range of 40' to 70° C.

reaction between the appropriate ingredients, generally When the polymer contains hydroxy functional the hydrophilic polymer and a separate cross-linking 35 groups, the cross-linking agent is preferably an alde agent therefor, as indicated below. hyde, a polyfunctional isocyanate (such as toluene diso One advantage of the material according to the in cyanate) or a polyfunctional carboxylic acid (such as vention is that there is minimal segregation in use (seg polyacrylic acid). Examples of suitable polymers con regation would be expected after solidification of the taining hydroxy groups include polyvinyl alcohol, a hydrate phase by solid sinking to the bottom of the hydroxyalkyl acrylate or methacrylate copolymer or receptacle in which the material is disposed in use). homopolymer (such as a polymer of hydroxyethyl acry There is no need to use the material according to the late or hydroxyethyl methacrylate) or an ethylene oxide invention in flat horizontal trays; the material can be polymer.

arranged in vertical columns of substantial height (for When the polymer contains amide functional groups, example, exceeding 50 cm). 45 it is preferably an acrylamide or methacrylamide poly A further advantage of the material according to the mer, such as a copolymer of acrylic or methacrylic acid invention is that the cross-linked polymer forming the with acrylamide or methacrylamide, partially hydro hydrogel can be prepared in situ by reaction between lysed polyacrylamide or polymethacrylamide, or an the appropriate ingredients as mentioned above. alkali metal or ammonium salt thereof. An example of a A still further advantage of the material according to 50 suitable cross-linking agent for such a polymer is an the invention is that, because the rate of cross-linking is aldehyde.

temperature-dependent, it is possible to control the Other suitable polymers containing amide groups are cross-linking reaction by the selection of appropriate N-vinyl pyrrolidone polymers; such polymers can be ingredients for cross-linking and the use of appropriate cross-linked (via the carbonyl group) using cross-link temperature conditions during cross-linking. 55 ing agents such as amines, hydroxyamines or hydrazine Yet a further advantage of the material according to derivatives.

the invention is that the proportion of polymer is low, When the cross-linking agent is an aldehyde, it is such as 0.1 to 10% more preferably from 2 to 3% (for preferably a lower aldehyde containing up to six carbon example, about 5%), based on the weight of the material atoms, such as glutaraldehyde or, most preferably, so that the material may have an advantageously high formaldehyde.

heat capacity per unit volume. When the polymer contains carboxy functional The water-soluble or water-dispersible hydrophilic groups, it may be, for example, an acrylic acid, meth polymer used to form the hydrogel is preferably ther acrylic acid, or itaconic acid polymer or a polymer of moplastic and preferably an addition polymer contain an itaconic acid half ester. Preferred polymers contain ing a carbon-carbon backbone. The polymer is prefera 65 ing carboxy groups are copolymers of acrylic or meth bly substantially linear, but it may be slightly branched acrylic acid with acrylamide or methacrylamide, or provided that the polymer remains at least water-dis partially hydrolysed polyacrylamide or polymethacryl persible, preferably water-soluble. amide, or an alkali metal or ammonium salt thereof, as 4 mentioned above with reference to acrylamide and sures efficient nucleation of the hydrate phase during methacrylamide polymers, cooling cycles, thereby inhibiting supercooling. Other suitable polymers containing carboxyl groups The thermal energy storage material according to the are produced by solubilizing a maleic anhydride co invention preferably contains a hydrated compound in polymer, such as a styrene-maleic anhydride copoly- 5 an amount of from 66 to 95% by weight and, optionally, e. a nucleating agent in an amount of from 1 to 10% based In some embodiments, the polymer preferably con onSubstantially the weight of the hydrated compound.

all the balance of the thermal energy tains 5 to 50% (for example 10 to 40%) carboxy groups, the percentage being based on the number of repeating storage material according to the invention is preferably units in the polymer backbone. In other embodiments, 10 water and, optionally, a dispersant which facilitates the polymer preferably contains 50 to 80% carboxy uniform and rapid solution of the polymer. Examples of groups on the same basis in order that the polymer such dispersants include certain organic liquids which are miscible with water. Particularly preferred such should be highly water-soluble. w

When such a polymer containing carboxy groups is organic liquids are lower aliphatic alcohols, such as cross-linked via the carboxy groups, the cross-linking 5 methanol or ethanol (for example, when the hydrated agent may be an amino resin (such as a ureaformalde iscompound is sodium sulphate decahydrate). The water hyde resin), a polyhydroxy compound (such as polyeth allpreferably present in an amount sufficient to hydrate the anhydrous form of the compound, and is prefera ylene glycol) or a polyamine.

Suitable hydrated compounds for use in the material bly present in a small excess. The material may contain according to the present invention include, for example, weight.in When water an amount of, for example, from 25 to 75% by a water-miscible organic liquid is in calcium chloride hexahydrate (the fusion point of cluded which is 29 C); sodium sulphate decahydrate (the water-soluble, but onlywhen (for example the polymer is not highly sparingly water-soluble or wa fusion point of which is 32 C.); disodium hydrogen ter-dispersible), it is preferably present in a relatively phosphate dodecahydrate (the fusion point of which is 25 minor amount, compared with water, for example from 35.5° C); sodium thiosulphate pentahydrate (the fusion 5 to 25% based on the weight of water. point of which is 50° C); sodium acetate trihydrate (the The material according to the invention is preferably fusion point of which is 58 C.); barium hydroxide octa used in a method of heat exchange which comprises hydrate (the fusion point of which is 75° C); zinc nitrate alternately heating the material to a temperature above hexahydrate (the fusion point of which is 35° C); potas- 30 the transition temperature of the hydated compound, sium fluoride tetrahydrate (the fusion point of which is and extracting heat from the material by passing a fluid 18.5° C); sodium carbonate decahydrate (the fusion at a temperature below the above-mentioned transition point of which is 35° C); lithium chlorate trihydrate temperature in heat-exchange relationship therewith. (the fusion point of which is 8 C.) and eutectic mixtures The alternate heating and cooling of the material can be of inorganic salts, 35 repeated fro many cycles. In use, the material is prefera For the storage of solar energy, the hydrated com bly retained in a receptacle of a gas-or vapour-bar pound preferably has a fusion point in the range 10' to rier material.

100° C., more preferably 20' to 90° C. and is preferably The present invention also comprises a heat exchange nontoxic, non-corrosive and readily available at low device, which comprises a receptacle of a gas-or va cost. Preferred hydrated compounds meeting some or 40 pour-barrier material containing the thermal energy all of the above requirements are cetain hydrated inor storage material according to the invention and means ganic salts, such as, sodium sulphate decahydrate, diso for supplying a cooling fluid in heat-exchange relation dium hydrogen phosphate dodecahydrate, sodium thio ship with the thermal energy storage material. sulphate pentahydrate, sodium carbonate decahydrate In order that the invention may be more fully under and calcium chloride hexahydrate. 45 stood, the following Examples are given by way of Some of the above-mentioned hydrated compounds, illustration only.

when cooled below the fusion point thereof, tend to EXAMPLE 1 undergo supercooling (that is, they do not transform back to the hydrated form until the temperature is 80 g. of anhydrous sodium sulphate Na2SO4, 8 g, of below the theoretical fusion point). This may result in 50 borax Na2B4O7.10H2O and 100 cm3 of water were thor less hydrated forms of the compound being formed, oughly mixed at above 40 C. to ensure that all undis with consequent reduction in the amount of energy solved sodium sulphate remained in the anhydrous released. In order to avoid supercooling, the material state.

may be nucleated, for example, by a heat-transfer To the mixture were added 10g of the sodium salt of method as disclosed in U.S. Pat. No. 2,677,243, by care- 55 a water-soluble acrylamide polymer containing acrylic ful control of the proportions of the ingredients of the acid units. The polymer, which had an average molecu composition, or by addition of an insoluble nucleating lar weight of about 7.5 million and had a ratio of car agent. Sometimes the polymer forming the hydrogel boxyl: amide radicals of about 1:9, was a material com may act as the nucleating agent. A preferred nucleating mercially available from Allied Colloids Ltd. as WN23. agent for sodium sulphate decahydrate is borax, as pro- 60 To the resulting mixture were added 5 cm3 of forma posed in U.S. Pat. No. 2,677,664. Other suitable nucleat lin (an aqueous solution containing approximately 40% ing agents are disclosed in British Specification Nos. by weight formaldehyde and 14% by weight methanol), 1,500,245, 1,543,336 and 2,001,096 and German Offen with stirring, followed by 15 cm of methanol, with legungsschrift No. 2,550,106. rapid stirring.

When a nucleating agent is present, this agent, like 65 The viscosity of the mixture increased progressively the hydrated compound, is dispersed and suspended in to a uniformly thick but smooth consistency, and finally the hydrogel and effectively immobilized therein. This to a firm dry gel having a density of about 1.4 g/cm. wide dispersion of immobilized nucleating agent en The mixture contained no excess of water over that 5 required to completely hydrate all the sodium sulphate; EXAMPLE5 on cooling it fully transformed to a solid.

A sample of the solid was sealed in a circular polyeth Example 4 was repeated except that disodium hydro ylene tube of 4 cm diameter and 10 cm in length. The gen phosphate was replaced by 292 g of anhydrous ends of the tube were sealed. sodium carbonate.

The tube was disposed vertically and alternately Similar results were obtained.

heated to about 60 C. (the heating time being about one EXAMPLE 6 hour) and cooled to about 20° C. by heat-exchanging with water circulating outside the tube (the cooling Example 4 was repeated, except that the polymer time being three to four hours). Reproducible thermal 10 WN 33 was replaced by 60 g of polyvinyl alcohol, arrests, with no detectable segregation of the constitu available from Monsanto under the Trade Mark Gel ents, were obtained for more than 500 cycles of heating wato 1-90 G.

and cooling. Similar results were obtained. Another sample of the solid was held at 45 C. for a Gelvatol 1-90 G contains less than 4% acetate groups prolonged period; no segregation was detectable after 15 and has a molecular weight of about 115,000. over 500 hours at this temperature. EXAMPLE 7

Example 2

11.76 kg. of anhydrous sodium sulphate were dis

Example 1 was repeated, except that the polymer was solved in 24.0 kg. of water at 70° C. in a PVC container, replaced by the same amount of a non-ionic water-solu 20 and then 0.47 liter of formalin was added to the solution. ble acrylamide polymer which had an average molecu A pre-mixed powder containing 7.06 kg. of anhydrous lar weight of 13 million and was commercially available sodium sulphate, 2.35 kg. borax and 4.17 kg. of the from Allied Colloids Ltd. as W25. polymer identified in Example 4 as WN 33 were added In the thermal cycling test, reproducible thermal gradually to the hot solution with rapid stirring using a arrests were obtained for more than 500 cycles of heat 25 motor-driven two-bladed impeller rotating at 1000 ing and cooling. In the test in which the material was r.p.m. Stirring was continued for 60 seconds until the stored for a prolonged period at 45 C., no segregation viscosity of the slurry increased sufficiently to prevent was detected after over 500 hours. settling of suspended solids.

EXAMPLE 3 The resulting material was transferred to a strength 30 ened tank, lined with welded polypropylene sheet, in 80 g of anhydrous sodium sulphate Na2SO4, 8 g. of which was located a parallel plate heat exchanger. The borax Na2B4O7.10H2O, 1.5 g. of aluminium sulphate top of the tank was closed with a gasket/lid assembly Al2(SO4)3-14H2O and 100 cm of water were thoroughly fitted with a tube connected to the space above the mixed at above 40 C. to ensure that all undissolved storage material, to allow for expansion and contraction sodium sulphate remained in the anhydrous state. 35 of the material while maintaining ambient pressure in To the mixture were added 10 g. of the same polymer the tank.

as used in Example 1. Reproducible thermal arrests were obtained for over To the resulting mixture were added 5 cm of forma 1000 cycles. - lin, with stirring, followed by 17.5 cm of methanol, EXAMPLE 8 with rapid stirring.

The resulting mixture, on cooling, fully transformed 350 g of calcium chloride, 5 g of barium carbonate to a solid. A sample of the solid was sealed in a tube as and 30 g of a linear polymer of molecular weight about in Example 1 and subjected to a thermal cycling test as 100,000 (prepared by polymerisation of hydroxyethyl in Example 1. methacrylate in an aqueous medium using ammonium Reproducible thermal arrests were obtained for more 45 persulphate as polymerisation initiator) were thor than 500 cycles of heating and cooling. oughly mixed while adding 70 ml. of ethanol. Another sample of the solid was held at 45° C. for a 500 ml. of water (at 40° C) were then added with prolonged period; no segregation was detectable after vigorous agitation, followed by 50 ml. of formalin. over 500 hours at this temperature. The resulting gel, when used in the thermal cycling 50 test described in Example 1, gave reproducible thermal

Example 4

arrests for many cycles.

300 g of anhydrous disodium hydrogen phosphate EXAMPLES 9 AND 10 (Na2HPO4) were dissolved in 500 ml of water at a tem perature of about 50° C. The solution was vigorously Examples 1 and 6 were repeated, except that, in each stirred while 50 g of an acrylamide polymer, available 55 case, the formalin was replaced by a corresponding from Allied Colloids Ltd. as WN 33 was added. Stirring amount of glutaraldehyde.

was continued as the mixture thickened. Similar results were obtained. After about 5 minutes, 50 ml of warm formalin were Similar results to those described in the above Exam added to the mixture. The mixture gelled to a uniformly ples can be obtained using as the polymer: thick consistency and then transformed to a solid on (i) an acrylic or methacrylic acid polymer (such as cooling. those available from Allied Colloids Ltd. as Versicol E The polymer WN 33 differs from WN 23 used in or K), an itaconic acid (or half-ester) polymer, a methyl Example 1 in that it has a higher ratio of carboxyl: vinyl ether-maleic anhydride copolymer or a styrene amide radicals (the ratio is about 7:3 for WN 33) and a maleic anhydride copolymer (these polymers being lower molecular weight (about 4.5 million). 65 cross-linked using, for example, an amino resin, a poly The resulting gel, when used in the test described in hydroxy compound, or a polyamine); Example 1, gave reproducible thermal arrests for many (ii) a vinyl pyrrolidone polymer (cross-linked using, cycles. for example, amines, hydroxy amines or hydrazines); or

Page 6scan →

(iii) a polymer of N-methylol acrylamide or N the range 0 to 100 C, the improvement comprising methylol methacrylamide (the latter being self-gross that the hydrogel is formed by reacting a water-soluble linking via the methylol groups). synthetic hydrophilic thermoplastic addition polymer What is claimed is: having a substantially linear carbon-carbon backbone 1. In a thermal energy storage material which com and active hydrogen atoms in pendant groups thereof prises at least one hydrated compound which has a with a cross-linking agent therefor in an aqueous ne reversible transition to the anhydrous or a less hydrated dium containing said hydrated compound, such that the form and a minor amount of hydrogel throughout resulting hydrogel comprises a water-swollen, cova which said compound is dispersed, the improvement lently cross-linked polymer.

comprising that the hydrogel comprises a water-swol O 7. A method according to claim 6, wherein said active len cross-linked polymer formed by cross-linking a syn hydrogen atoms are present in amide groups and said thetic hydrophilic polymer by a covalent cross-linking cross-linking agent is formaldehyde. mechanism. 8. A method according to claim 6, wherein said poly 2. A thermal energy storage material according to mer has pendant acid groups and wherein the reaction is claim 1, wherein said synthetic hydrophilic polymer has 15 carried out in the presence of a water-soluble salt of a functional groups selected from the group consisting of polyvalent metal, such that the cross-linked polymer is hydroxy, carboxy and amide groups, said functional cross-linked by cations of said polyvalent metal as well groups being reacted with a cross-linking agent to form as by said cross-linking agent. said cross-linked polymer. 9. In a method of extracting heat by passing a fluid in 3. A thermal energy storage material according to 20 heat-exchange relationship with a thermal energy stor claim 1, wherein said synthetic hydrophilic polymer has age material comprising at least one hydrated inorganic groups which have been reacted with a cross-linking salt selected from the group consisting of sodium sul agent selected from the group consisting of an alde phate decahydrate, disodium hydrogen phosphate do hyde, a polyfunctional isocyanate and a polyfunctional decahydrate, sodium thiosulphate pentahydrate, sodium carboxylic acid. 25 carbonate decahydrate and calcium chloride hexahy 4. A thermal energy storage material according to drate and a minor amount of a hydrogel throughout claim 3, wherein said synthetic hydrophilic polymer is which said salt is dispersed, the improvement compris selected from the group consisting of polyvinyl alcohol, ing that the hydrogel comprises a water-swollen cross a hydroxyalkyl acrylate polymer, a methacrylamide linked polymer formed by cross-linking a water-soluble polymer, an acrylamide polymer and a hydroxyalkyl 30 linear synthetic thermoplastic polymer by a covalent methacrylate polymer. cross-linking mechanism.

5. A thermal energy storage material as set forth in 10. A method according to claim 9, wherein said claim 3 in which the groups which have been reacted polymer contains acrylamide repeating units and are hydroxy groups. acrylic acid repeating units. 6. In a method of preparing a thermal energy storage 35 11. A method according to claim 10, wherein said material which comprises a hydrogel having dispersed polymer has been cross-linked by means of formalde therein a hydrated compound which has a transition hyde.

temperature to the anhydrous or a less hydrated form in as