NZ714795B2 - Improved phase change compositions - Google Patents

Abstract

There are herein described phase change materials containing sodium acetate trihydrate having improved homogeneity, a process for the preparation of said materials, and their utility in phase change systems. More particularly, the present invention relates to the use of phase change compositions comprising sodium acetate trihydrate, at least one alkali soluble polymer for inhibition of sodium acetate anhydrous crystal formation in sodium acetate trihydrate containing phase change materials, and at least one sodium acetate trihydrate nucleation promoter, and, if a lower phase change temperature is required, at least one melting point depressing agent. prising sodium acetate trihydrate, at least one alkali soluble polymer for inhibition of sodium acetate anhydrous crystal formation in sodium acetate trihydrate containing phase change materials, and at least one sodium acetate trihydrate nucleation promoter, and, if a lower phase change temperature is required, at least one melting point depressing agent.

Description

IMPROVED PHASE CHANGE COMPOSITIONS Field of the Invention The present invention relates to phase change materials containing sodium acetate trihydrate having improved homogeneity, a s for the preparation of said materials, and their utility in phase change systems. More particularly, the t invention relates to the use of phase change compositions comprising sodium acetate trihydrate, at least one alkali soluble polymer for inhibition of sodium e anhydrous crystal formation in sodium acetate trihydrate containing phase change materials, and at least one sodium acetate trihydrate nucleation promoter.

Background of the Invention There are many heating and cooling s on the market and many of these are reliant on fossil fuels. With the ever increasing demand for more environmentally ly systems various alternative systems based on sunlight or water have been proposed such as for e oltaics, solar thermal electricity generators, hydroelectricity, wave power and bio-fuels.

An issue common to all solar-driven renewable energy conversion s, some hydro-driven devices, and wind turbines is that they cannot operate "on demand", as the sun does not always shine, the seas are not always high and the wind does not always blow.

This means that at some times these so-called intermittent renewable sources will generate electricity which cannot be easily integrated into their corresponding local icity grids, and as such there have been a number of storage solutions proposed.

The thermal energy e system, proposed in WC 200911 38771 converts surplus electrical energy from intermittent renewable sources into heat or cool when available, store the so-converted heat or cool in a thermal store, and then make it available as useful heat or cool on demand using phase change materials (PCMs) to effect the energy conversion via their inherent solid-liquid phase changing ties.

For practical application in domestic situations phase change materials capable of supplying warm, or even hot, water that is just above the comfort level temperature of the individual requiring the heated water are needed. In addition, phase change materials suitable for such practical utility should achieve rates of heat transfer into and out of, their phase changes, which are commensurate with domestic use, as well as deliver acceptable levels of thermodynamic ity (efficiency).

Sodium acetate trihydrate, (SAT), has a solid-liquid phase change within the desired temperature range for domestic utility. However, the practical applications of SAT as a phase change material (PCM) are limited by the unique and incongruous manner in which it melts, going from solid SAT, to a mixture of liquid SAT and a solid (sodium acetate/SA) at a set temperature, 58°C. This inability to provide a fully liquid solution at 58°C is an issue which is reflected in the thermodynamic stability of aqueous solutions upon initial heating, as well as the thermodynamic stability of, re-formed, aqueous solutions provided following re—heating after cooling in accordance with the normal g / cooling cycles in phase change systems.

This solid-formation on melting is a m for use as SAT as a PCM because this solid SA, once formed, would ordinarily be retained throughout the lifetime of the PCM in a phase change system.

Previous attempts to overcome this issue via the utility of three dimensional cross- linked polymers, such as cellulose based polymers and super absorbent polymers, to act as solid supports have been unsuccessful because, ndent of any nt initial reduction of solid formation, the underlying m of solid sodium e formation is unresolved, and over time solid sodium acetate will still precipitate out of solution, and collect at the base of the PCM storage vessel irreversibly. For y as a PCM, no solution which has a finite lifetime is acceptable.

It is an object of at least one aspect of the present invention to obviate or mitigate at least one or more of the aforementioned problems in relation to the utility of sodium acetate trihydrate as a potential PCM for use in aqueous phase change s.

It is an object of at least one aspect of the t invention to e improved phase change materials containing sodium acetate trihydrate having desirable neity; resistance to SA formation, which are suitable for use in phase change systems.

It is an object of at least one aspect of the present invention to provide improved phase change materials containing sodium acetate trihydrate which can be heated, cooled and ted in repeated cycles with retention of thermodynamic stability.

The Applicant has developed novel and inventive aqueous compositions for use as phase change materials comprising: sodium acetate trihydrate; at least one alkali soluble polymer for inhibition of sodium acetate anhydrous crystal formation in sodium acetate trihydrate containing phase change materials; and at least one sodium acetate trihydrate nucleation promoter. The Applicant has also developed a s for the preparation of said ed phase change materials.

Summary of the Invention The Applicant has developed novel and inventive phase change itions containing sodium acetate trihydrate as a phase change material.

In a first aspect of the invention, there is provided a s for the preparation of a composition ning crystalline sodium acetate trihydrate as a phase change material, said process comprising: (a) mixing an aqueous solution comprising sodium acetate anhydrous with at least one suitable alkali soluble polymer for inhibition of sodium e anhydrous l formation in crystalline sodium acetate trihydrate ning phase change materials; and wherein said one or more alkali soluble polymers is independently selected from: the copolymer of methacrylic acid and methyl methacrylate, poly(methacrylic acid co-methyl methacrylate) having a molecular weight of from about 500,000 to about 1,000,000, or a sodium, potassium or zinc salt thereof; poly(methacrylic acid) having an average molecular weight of about 10,000, or the sodium salt thereof; or a e thereof; and at least one nucleation promoter; (b) heating the resultant mixture to provide a 58 °C phase change material containing lline sodium acetate trihydrate; and wherein said aqueous solution may optionally contain an agent for modifying the melting point of sodium acetate; and wherein the e obtained in step (a) comprises: from 40% to 60% of sodium acetate anhydrous; from 0.1% to 10% of at least one suitable alkali soluble polymer; from 0.1% to 5% of at least one suitable nucleation promoter; from 1% to 25% of an optional melting point sing agent; and water to balance.

Accordingly the present invention provides compositions containing sodium acetate trihydrate as a phase change material comprising: (26915652_1):AXG (a) sodium acetate trihydrate or sodium acetate anhydrous; (b) at least one suitable alkali soluble r; (c) at least one suitable nucleation promoter; and (d) water.

As demonstrated in the Examples hereinafter the Applicant has surprisingly found that the compositions of the present invention which contain sodium e trihydrate as a phase change material display unprecedented improvements in homogeneity and thermodynamic stability ties than previously achievable using sodium acetate trihydrate as a PCM. In particular the Applicant has found that the compositions of the t invention which contain sodium acetate rate as a phase change material are resistant to the nucleation of crystalline sodium e on heating and cooling.

According to a further aspect the present invention provides for the use of the compositions according to the invention as phase change materials suitable for use in phase change systems.

The Applicant has also developed a novel process for the preparation of the compositions according to the invention. According to a further aspect the present invention provides a process for the preparation of compositions containing sodium acetate trihydrate as a phase change material which comprises: (a) mixing an aqueous solution comprising sodium e ous with at least one suitable alkali soluble polymer; and at least one nucleation promoter; (b) heating the resultant mixture to provide a 58°C phase change material containing sodium acetate trihydrate.

When sodium acetate trihydrate is used, the following steps are followed: (a) heating the sodium acetate trihydrate to a temperature above 58°C; and (b) mixing the sodium acetate trihydrate with at least one suitable alkali soluble polymer; and at least one nucleation er.

BRIEF PTION OF THE DRAWINGS Embodiments of the present ion will now be described, by way of example only, with reference to the accompanying drawings: (26598650_1):AXG Figure 1 is a phase diagram of sodium e and water, showing the solubility limit of sodium acetate in water, at about 58°C, is at about 58.0%, whereas the corresponding (26598650_1):AXG sodium acetate trihydrate, which melts at 58°C, is made up of 60.28 % of SA and 39.72 % of water, a value noticeable higher than the solubility limit at 58°C, as indicated by point 2; Figure 2 is a proposed mechanism where the polymer interacts with the surface of highly metastable sub-critical clusters of sodium acetate les to prevent, or block, their further growth into crystallites, and hence itated material in due course, via a blocking mechanism; and Figure 3 is a representation of crystallisation atures obtained by afding varying levels of acetamide.

Detailed Description The novel compositions according to the present invention are s compositions containing sodium e trihydrate, NaOAc·3H2O, also known as SAT, as a phase change material. Any form of sodium acetate anhydrous, also known as SA, may be used in the preparation the novel compositions containing SAT as a PCM in accordance with the invention. For the avoidance of doubt this means that as all solid forms of NaOAc are crystalline, and therefore any crystalline form may be used.

The level of sodium acetate anhydrous used in the preparation of the aqueous compositions according to the invention is between about 40% to about 60% by weight of the total composition. This corresponds to a relative amount of from about 66% to about 100% of sodium acetate trihydrate in the aqueous compositions of the invention. For the nce of doubt the compositions according to the present invention are substantially free-from, and are more preferably free-from solid anhydrous sodium acetate.

For the nce of doubt, where the actual or relative amounts of water in any particular composition according to the invention described herein are not specified it should be understood that the actual or relative amount of water required will be that sufficient to reach either 100% of composition, either by weight or relative volume. Water may be used either in ed, or distilled form, or from regular supply.

As indicated hereinbefore, despite having a phase change within the desired temperature range for domestic heating purposes, the utility of sodium e trihydrate as a potential PCM to-date has been hampered due to inherent melting incongruences. During the melting process sodium acetate trihydrate s from solid SAT, to a e of liquid SAT and solid at a set temperature, 58°C. This is due to the formation of sodium acetate anhydrous, NaOAc, or SA. For the avoidance of doubt, where the term sodium acetate, or SA, is utilised herein it means sodium acetate anhydrous, as opposed to any hydrated form, or the rate, SAT in particular. As indicated by point 1, in Figure 1, a phase diagram of sodium acetate and water, the solubility limit of sodium acetate in water, at about 58°C, is at about 58.0%, s the corresponding sodium acetate trihydrate, which melts at 58°C, is made up of 60.28 % of SA and 39.72 % of water, a value noticeable higher than the solubility limit at 58°C, as indicated by point 2, in Figure 1. When SA is formed during the melting of aqueous SAT, a fully liquid state may be re-instated via the addition of more water, to alter the composition to the composition of 58% SA, and solubilise SA anhydrous, thereby providing a fully liquid material at 58°C. The so-produced solution is thermodynamically stable, i.e. it is neither in the metastable zone, nor in the supersaturated zone, thus no further solid material (SA) should crystallise out.

As explained hereinbefore, such a re-instated liquid solution sing water and sodium acetate trihydrate is not le for use as a phase change agent because, it is not capable of being cooled and reheated to provide a thermodynamically stable homogeneous liquid. As demonstrated in the Examples hereinafter, the Applicant has found when such a solution is cooled down and seeded with some sodium acetate trihydrate, a solid sample of sodium acetate trihydrate is . ant has also found that when such a cooled solution is heated up to 58°C again, a solution with some solid sodium acetate anhydrous, SA, is formed i.e. the solution is not in fact in thermodynamic equilibrium.

Mixing and/or agitation would return the composition to its thermodynamic equilibrium, a homogenous solution; however without such mechanism, as in this invention, a homogenous on does not l.

As illustrated in Figure 1, crystalline sodium acetate trihydrate melts to form sodium acetate ous and a trated solution of aqueous sodium acetate. In simple terms, and in accordance with general chemical practise, the addition of more water added would be expected to dissolve this additional sodium acetate (the anhydrous solid). More accurately, this extra water would be expected to dilute the concentrated solution comprising sodium acetate anhydrous and water, which in turn would enable the solid sodium acetate anhydrous to dissolve into it. Most surprisingly, the Applicant has demonstrated that this does not occur in practise, and further that the ance to dissolution of the anhydrous solid is in fact unaltered due to the additional water level.

Without wishing to be bound to any particular theory it is postulated herein that, during the melting process, les of sodium acetate trihydrate dissociate incongruently, and also that there is rapid exchange between the sodium e in the liquid phase and the sodium acetate in the solid phase. The solid sodium acetate anhydrous molecules are thought to initially be present in small clusters. Such small clusters are t to ly n hundreds of molecules of SA, and are not considered to be crystalline. It is further proposed herein that as any one of these clusters increases in size it can eventually attain the critical cluster size, or threshold size, ed to become a crystallite. It is further proposed herein that such crystallites, grouping together, form the dense crystals of sodium acetate that are observed as the unwanted solid precipitated material in the aforementioned Example. The ant has observed that the formation of solid itate, during the melting process, occurs too quickly for the dissolution of additional water to have any significant impact.

The Applicant has resolved this issue of non-homogeneous liquid formation in aqueous phase change materials containing sodium acetate trihydrate via the ion of PCMs containing 3H2O which are resistant to the formation of NaOAc crystallites via the utility of one or more particular alkali soluble polymers.

Without being bound to any particular theory it is thought that the particular polymers utilised in the PCMs containing sodium acetate trihydrate according to the present invention composition provide resistance to the formation of precipitated NaOAc in aqueous NaOAc·3H2O solutions via a combination of s including: viscosity effects; crystal habit modifying behaviour; 3D-lattice effects. As such, the one or more alkali soluble polymers suitable for use herein may also be referred to as SA crystal inhibitors, or inhibitors of the formation of crystalline SA from aqueous solutions.

At one level by harnessing the ability of these particular polymers to se the viscosity of solutions it is believed that at least some of the sodium acetate anhydrous formed during the melting process may remain suspended long enough to dissolve and y reduce the potential for SA precipitation from solution, and subsequent collection within the phase change system equipment during use. It is also proposed herein that use of these particular polymers would e solid sodium acetate having a significantly increased effective surface area in solution, versus that previously possible (for SA only aqueous systems) when the solid is amassed at the base and only the top of this solid layer is in contact with the solution. It is further proposed herein that even dispersal of the solid sodium acetate in this more s solution for an extended period of time, may increase the dissolution rate and y reduce the rate at which the anhydrous form of sodium acetate could form, and potentially may prevent formation altogether.

Polymers suitable for use in the novel compositions herein containing SAT as PCMs in accordance with the first or further aspects of the invention as detailed herein are soluble in aqueous alkaline solution. As d herein polymers suitable for use herein are soluble in aqueous alkaline ons having a pH in excess of about pH 8. More particularly polymers le for use herein are soluble in highly alkaline s solution having a pH of about pH 9, such as aqueous sodium acetate solution. The compositions herein include one or more polymers, as defined herein, wherein each polymer may be independently present at levels of from about 0.1 % to about 10%, from about 0.2% to about 4%, from about 0.5% to about 2% are utilised in the s compositions according to the present invention.

In addition, polymers suitable for use herein have one or more carboxylic acid groups and may be utilised as acids, or as acid salts. For the avoidance of doubt the term polymer as used herein includes both polymers of repeated singular monomeric units, and co- polymers comprised of mixed monomeric units having varying repeating patterns.

A group of preferred polymers for use herein are polymers having repeating units of general formula I: -{[X]n-[Y]m}z- and salts thereof wherein z is 10 to 1,000; n = 1 to 10to 1,000; and wherein m = 0 to 1,000 wherein the ratio of n : m is in the range of from about X: Y and wherein the lar weight of the polymer is in the range of from about P to about Q, X is ndeme selected from groups based on the following monomers: ethylene, acrylic acid, methyl methacrylate, acrylamide, ethyl methacrylate, ethacrylic acid, ethyl oxide, diallyldimethylammonium de, vinylpyrrolidone, N-isopropylacrylamide, styrene, maleic acid and mixtures thereof, Y is independently selected from groups based on the following monomers: ethylene, acrylic acid, methyl rylate, acrylamide, ethyl methacrylate, ethacrylic acid, ethyl oxide, diallyldimethylammonium de, vinylpyrrolidone, N-isopropylacrylamide, styrene, maleic acid and mixtures thereof, A group of preferred polymers according to l formula I for use herein have monomeric repeating units of general formula II. and salts thereof wherein n, m and z are as defined before wherein the ratio of n : m is about 1 : 2 and wherein the molecular weight is in the range of from about 1,000 to 1,000,000.

Preferred polymers of general formula II for use herein are either acids or acid salts, more particularly either acids or metal acid salts, and especially acids or sodium, potassium or zinc acid salts. The copolymer of methacrylic acid and methyl methacrylate, poly(methacry|ic acid co-methyl methacrylate) having a molecular weight of from about 500,000 to about 1,000,000, and the , potassium or z'nc salts thereof are polymers within general formulae I, II and Ill, and are ularly suited for use herein.

Another group of preferred polymers for use herein according to general formula I for use herein have repeating units of general formula III: \I \ Na+ and metal salts f selected from sodium or potassium, and wherein z and n are as defined herein before.

A preferred polymer of formula III for use herein is poly(methacry|ic acid), particularly the sodum salt, preferably in 30 or 40% aqueous solution wherein the r has an average molecular weight of about 10,000 or 4-6,000 Thus, the present invention es compositions as defined hereinbefore wherein the one or more alkaline soluble polymers are selected from l formulae II or III and mixtures thereof each independently present at a level of from about at levels of from about 0.1% to about 10%, from about 0.2% to about 4%, from about 0.5% to about 2%. In addition, there are provided compositions as defined before wherein the polymer is: the copolymer of methacrylic acid and methyl methacrylate, poly(methacry|ic acid co-methyl rylate) having a molecular weight of from about 500,000 to about 003, or a sodum, ium or zinc salts f; the poly(methacry|ic acid), or the sodium salt having an average molecular weight of about 10,000, and wherein either polymer may be independently present at a level of from about at levels of from about 0.1% to about 10%, from about 0.2% to about 4%, from about 0.5% to about 2%. According to a further aspect the present invention provides a composition having either one or the above two polymers at the levels defined above.

The improved PCM compositions according to the present invention containing SAT as a PCM have been demonstrated to form homogeneous liquids without the need to add excessive onal water, which is in stark contrast to the precipitated systems observed without rs. However it is the demonstrated improvements in both homogeneity and thermodynamic stability following cooling and re-heating, between the SAT containing PCM systems of the invention with polymers, when compared to the behaviour displayed by sodium acetate rate without polymers which are truly revolutionary.

Without being bound to any ular theory it is proposed herein that phenomenal abilities observed for the various different SAT / polymer systems tested for the delivery unprecedented inhibition of nucleation in aqueous sodium acetate trihydrate systems is due to a consistent, underlying technical effect common to them all. Whilst the precise mechanism of crystal nucleation inhibition in these s is unknown, it is proposed herein that the r interacts with the surface of highly metastable sub-critical clusters of sodium acetate molecules to prevent, or block, their further growth into crystallites, and hence precipitated material in due course, via a blocking mechanism. Figure 2 provides a proposed illustration of this mechanism.

The Applicant has found that even when the polymeric s herein are seeded with SA, to in effect force the formation of solid material, the so-formed solid material is very different in both appearance and behaviour to that observed in the corresponding solid material formed in non-polymeric systems. It is in this impact upon the behaviour of the resultant solid, which suggests that the solid may exist in a different structural habit, and that the polymers act as crystal habit modifiers.

Whilst these modified polymer containing aqueous solutions of sodium acetate are indeed most singly and desirably resistant to SA formation, they are not suitable for utility as PCMs because, as has been most clearly demonstrated both by the experiments and results sed herein and as the SA phase diagram of Figure 1, sodium acetate trihydrate does not readily nucleate, even in supersaturated solutions. This means that a nucleating agent is required to promote the nucleation of the sodium e rate from the aqueous solution.

Nucleating agents, as defined herein are also known as nucleators, or nucleation promoters. In some cases, the effective g of a particular material with a particular nucleating agent can be the result of iso-structural similarities between the nucleator and the salt hydrate in their crystalline forms, and indeed this is the case with many pairs.

For example strontium chloride hexahydrate acts as a nucleator for calcium chloride hexahydrate, and sodium tetraborate drate acts as a nucleator for sodium sulfate decahydrate and in each pairing they have very similar molecular packing in their respective crystalline forms.

For sodium acetate trihydrate identification of a suitable nucleator is a more challenging matter. Whilst disodium hydrogen phosphate (DSP) and tetrasodium pyrophosphate (TSPP) have been identified as potential nucleators for SAT their ism of action remains unknown. (T Wada and R Yamamoto "Studies on salt hydrates for latent heat storage. 1. l tion of sodium acetate trihydrate catalyzed by tetrasodium pyrophosphate decahydrate, Bulletin of the Chemical Society of Japan. Volume 55, page 3603, 1982; T Wada, R Yamamoto and Y Matsuo "Heat storage capacity of sodium acetate trihydrate during thermal cycling", Solar Energy. Volume 33, pages 373 to 375, 1984; and H Kimura, ating agents for sodium acetate trihydrate", Journal of the Japanese Association of Crystal Growth. Volume 9, issue 3, page 73, 1982.) Furthermore there is an ledged so-called deactivation of these nucleators at high temperatures which undermines their ial for consideration as potential pairs for SAT PCMs for use in phase change systems because by their nature PCMs are intended for long-term use, and require on-demand, reliable tion throughout the anticipated heating/cooling/re-heating cycles therefor. singly the Applicant has fied a particular hydrate of disodium hydrogen phosphate, the ate, to be the active nucleating species for SAT, and has also demonstrated that utility of this hydrate in aqueous polymeric solutions containing SAT as detailed hereinbefore provides itions highly suited for use as PCMs in phase change systems. The compositions according to the present invention lly contain one or more nucleation promoters each independently present at a level of from about 0.1 % to about 5%, from about 0.2% to about 3%, from about 0.5% to about 2%.

Thus the present invention additionally provides compositions containing sodium acetate trihydrate as a phase change material comprising: (a) from about 48 to about 60% of sodium acetate anhydrous; (b) from about 0.1 % to about 10% of at least one suitable alkali soluble polymer; (c) from about 0.1 % to about 5% of at least one suitable nucleation promoter; and (d) water to balance.

Alternatively, when sodium e trihydrate is used, the composition above changes to: (a) from about 80% to about 100% of sodium acetate trihydrate; (b) from about 0.1 % to about 10% of at least one suitable alkali e polymer; (c) from about 0.1 % to about 5% of at least one suitable nucleation promoter; and (d) water to balance.

Whilst any material capable of nucleating SAT is suitable for use herein, preferred materials which both nucleate SAT and retain their effectiveness at high temperatures are particularly suited for use in the compositions for use as PCMs according to the present invention. Such materials include: um hydrogen phosphate (DSP); tetrasodium pyrophosphate (TSPP); and hydrated forms thereof. Particular materials suitable for use herein are disodium hydrogen phosphate ate, and tetrasodium pyrophosphate decahydrate. Thus, the present invention provides compositions as defined hereinbefore wherein the nucleation promoters are disodium hydrogen phosphate (DSP); tetrasodium pyrophosphate (TSPP); and hydrated forms thereof each independently present at a level of from about 0.1 % to about 5%, from about 0.2% to about 3%, from about 0.5% to about 2%.

In addition, there are provided compositions as d hereinbefore wherein the nucleation promoters are disodium hydrogen ate dihydrate and tetrasodium osphate decahydrate and wherein the total level of these promoters is from about 0.5% to about 5%, from about 0.2% to about 2.5%, from about 0.5% to about 2%.

In addition to the r and the nucleation promoter the compositions according to the invention may additionally se a further agent to modify the melting point of SAT when in use as a PCM. Any suitable agent capable of providing a desirable melting point modification can be used, for the avoidance of doubt g point cation, means a lowering of the melting and crystallisation point. Such modifying agents may be utilised at a relative concentration level of from about 1 % to about 25%, 5% to about 25%, from about % to about 20%, from about 2% to about 10% of the total mass. Example agents for lowering the melting point of SAT in the compositions herein include: metal salts such as ithium acetate dihydrate; and organic compounds such as acetamide and trimethylolethane which could also include non-metal salts, e.g. ammonium acetate.

Thus the present invention additionally provides compositions containing sodium acetate trihydrate as a phase change material comprising: (a) from about 35% to about 60% of sodium acetate anhydrous; (b) from about 0.1 % to about 10% of at least one le alkali soluble polymer; (c) from about 0.1 % to about 5% of at least one suitable nucleation promoter; (d) from about 1 % to about 95% of an optional melting point depressing agent; and (e) water to balance.

Preferred herein are compositions wherein the lithium acetate dihydrate is ed as a melting point depressing agent at levels of from about 1 % to about 25%, from about 5% to about 25%, from about 10% to about 20%.

Lithium e dihydrate is 64.67% LiOAc and 35.33% water, values below copied from above but x by 64,67.

Preferred herein are compositions wherein the lithium acetate anhydrous is utilised as a melting point depressing agent at levels of from about 0.65% to about , from about 3.23% to about 16.17%, from about 6.47% to about 12.93%.

The melting point may also be depressed by on of acetamide and trimethylolethane.

The compositions of the invention containing SAT as a PCM can be made according to a process comprising: (a) mixing an aqueous on sing sodium acetate anhydrous with at least one suitable alkali soluble polymer; at least one nucleation promoter, and at least one melting point depressing agent; (b) heating the resultant mixture to provide a 58°C phase change material containing sodium acetate trihydrate.

According to a preferred process herein the from 35% to about 60% of sodium acetate anhydrous, from about 0.1% to about 10% in total of suitable alkali soluble polymer(s), from about 0.1% to about 5% in total of one or more nucleation promoter are utilised and optionally from about 1 % to about 25% of melting point sing agent may be included in the mixing stage. An example of this process is found in e 3 hereinafter.

According to a preferred process herein the from 35% to about 60% of sodium acetate anhydrous, from about 0.1% to about 10% in total of suitable alkali e polymer(s), from about 0.1% to about 5% in total of one or more nucleation promoter are utilised and optionally from about 1 % to about 95% of melting point depressing agent may be included in the mixing stage.

Alternatively the polymer may be formed in situ in the molten (heated) aqueous SAT mixture, in which instance the relevant monomers would be added into the s solution and polymerisation would be initiated. ore, the PCM can be made in either the heat battery ure before including the heat exchanger and g it or the PCM can be made in an external vessel and then poured into the enclosure containing heat exchanger.

The following non-limiting examples provided in the Experimental results hereinafter are representative of the PCM compositions according to the ion, as are the processes for their preparation.

Experimental Results e 1: Addition of polymer 1 to aqueous sodium acetate to prevent precipitation 1 % (2g) of a poly(methacrylic acid hyl rylate) 2: 1 copolymer having a molecular weight of 500,000- 1,000,000 (available from Fluka, as Polyacrylic acid, cas 250861 (labelled medium viscosity, mr ~500,000-1,000,000, copolymer of methacrylic acid and methyl methacralate) was added with stirring to a well-mixed 198g aqueous solution of sodium acetate (available from VWR International Ltd. (UK) as sodium acetate anhydrous 99%) at a concentration of 58.24% (17.002 mol dm-3) at from about 60 to about 70°C and then allowed to cool down to room temperature (RT).

At RT a homogenous liquid was observed which is in stark contrast to the same solution but without any polymer where significant of precipitation of sodium acetate was observed. Further, cycling experiments were preformed throughout a temperature range of between about 25 to about 80°C which confirmed that the beneficial effects observed for polymer-assisted solution were consistent across the range tested. In addition, further experiments confirmed that sodium acetate could only be ted to form with seeding at Example 2: Addition of polymer 2 to aqueous sodium acetate to prevent precipitation (0.66% with respect to only polymer) 22.6g of a 30% aqueous solution of poly(methacrylic acid, sodium salt) having a molecular weight of 9,500 (available from Sigma , UK, as Poly(methacrylic acid, sodium salt) solution average Mn ~5,400, average Mw ~9,500 by GPC, 30 wt. % in H20, cas 541931) was added to a 59.16% (17.66 mol dm-3) aqueous solution of sodium acetate(1,000g) in accordance with the method of Example 1 and then allowed to cool down to room temperature (RT).

Example 3: ication that polymer systems provide homogeneous liquids at 58°C Test samples containing various levels of polymers 1 and 2, and varying initial aqueous concentrations of sodium acetate, were left undisturbed at RT for several weeks.

Where sodium acetate were observed, it appeared as a large white mass taking up the whole sample container. This mass is thought to be an intricate mixture of sodium acetate and water, and yet had the appearance of a solid. When the sample container was squeezed the mass was observed to be very soft. On careful stirring this mass was found to appear as fine needle-like crystals and furthermore, g such samples to 58°C ed in homogenous liquids, which were in agreement with the phase diagram of Figure 1, point 2, as discussed hereinbefore.

In addition test samples were carefully seeded with sodium acetate were also left undisturbed, and the resulting solid sodium e ed as a large white mass taking up the whole sample container. This mass is thought to be an intricate mixture of sodium acetate and water, and yet had the appearance of a solid. When the sample container was squeezed the mass was observed to be very soft. On l stirring this mass was also found to appear as fine needle-like ls, and furthermore, on heating to 58°C, these samples also provided in homogenous liquids, which were in agreement with the phase diagram of Figure 1, point 2.

This ability to form homogeneous liquids t the need to add excessive water is in stark contrast to the precipitated systems observed without polymers. However the ed improvements in both neity and thermodynamic stability following cooling and re-heating, between the systems of the invention with polymers and when compared to sodium acetate trihydrate without polymers is truly revolutionary.

Example 4: Identification of Active Nucleator for SAT in DSP Variable temperature powder X-ray diffraction was utilised to identify the dihydrate of disodium hydrogen phosphate as the active nucleator. Samples of sodium acetate trihydrate with um hydrogen phosphate dihydrate were cooled and crystallisation of sodium e rate took place. Heating the sample to 90°C resulted in the transition from the dihydrate to anhydrous disodium hydrogen phosphate. Subsequent cooling of this , now containing SAT and anhydrous DSP to 25°C, did not return the anhydrous DSP to the DSP dihydrate, and thus no crystallisation of sodium acetate trihydrate could take place.

Additional experiments were carried out to confirm that no deactivation of this system occurs at the temperatures required for y in compositions for use as PCMs, via either seeding with sodium acetate trihydrate or the active nucleators. The results of these experiments have confirmed that upon such seeding "reactivation" of the tors occurs and their effectiveness is fully restored. This is in agreement with previous observations but the Applicant is the first to tand and characterise the hitherto unexplained phenomena.

Example 5: Preparation of PCM Formulations containing SAT PCM 1 PCM 2 Material Weight % Weight % r 0.67% 0.67% Sodium acetate 56.20% 44.45% Water 40.31% 31.87% DSP·2H2O 1.20% 1.29% TSPP·10H2O 1.62% 1.73% LiOAc·2H2O - 20% Formulation 1 was prepared as follows sodium acetate anhydrous (134.1 kg, 1634.77 mol), water (94.6 kg, 5251.18 mol) and a thacrylic acid polymer (mol wt 9,500) as a % aqueous solution (5.3 kg), DSP (2.3 kg, 16.20 mol) and TSPP (2.3 kg, 8.65 mol) were mixed together and heated to about 70°C. This resultant 58°C PCM produced via this process has no anhydrous sodium acetate formation.

Formulation 2 was prepared as for Formulation 1. The ant 50°PCM produced also had no anhydrous sodium acetate formation.

Thus the PCMs according to the present invention are free from SA when viewed by the naked eye and experimental techniques such as X-ray powder diffraction.

Example 6: Addition of acetamide to sodium acetate trihydrate to depress melting point Ratio of SAT to acetamide needed for mixes Mass (g) Molar sodium acetate Ratio ide trihydrate 9:1 19.0798 0.9202 8:2 1 1.9579 7:3 16.8629 3.1371 6:4 15.5112 4.4888 :5 13.9462 6.0538 4:6 12.1130 7.8870 3:7 9.93611 10.0639 2:8 7.3091 12.6909 1 :9 4.0760 15.9240 a) Starting from the trihydrate form of sodium acetate: 95.53 wt% sodium acetate trihydrate (CAS 0-4) with 2.17 wt% water with 2.30wt% of an aqueous solution of poly(methacrylic acid, sodium salt) (available from Sigma Aldrich UK CAS 541931) was made up, and heated to 60-70 °C, whilst stirring, to create a homogeneous mixture. b) Starting from the anhydrous sodium acetate: 57.85 wt% sodium e (available from VWR ational Ltd. as sodium acetate anhydrous 99% CAS 1273) with 39.84% water and 2.31 wt% aqueous solution of poly(methacrylic acid, sodium salt) was heated to 60-70 °C whilst stirring, to create a homogeneous mixture.

To s the melting and freezing temperatures, this on was added in varying amounts with acetamide, as shown in the table above. Temperatures were recorded for the melt and freeze (nucleated with a seed crystal) of the samples, and the range of depressions is shown below. Crystallisation temperatures from 58°C to 285°C can be gained by adding acetamide up to 70 molar %, whereupon the mix nears the eutectic point, after this point freezing temperature increases. This is shown in Figure 3.

Exam le 7: Addition of trimeth olethane to sodium e trih rate to de ress melting point Sodium acetate trihydrate solution was made as in the methods described in Example 1. To make the mixture samples the SAT solution was added to TME as described in the table below.

Mass (g) Molar sodium acetate Ratio TME tn'hydrate 9:1 182132 1.7868 8:2 16.3836 3.6164 7:3 14.5096 5.4904 6:4 12.5895 7.4105 The es can either be nucleated manually with a seed l, or 2wt% of a nucleator, disodium en phosphate dihydrate. By increasing the content of tn'methyolethane from 0 to 40 molar %, the freezing point of the material has a range from 42°C to 58°C.

The effect of increasing the amount of TME on the freezing temperature of the samples

Claims (9)

Claims

1. A process for the preparation of a composition containing crystalline sodium e trihydrate as a phase change material, said process comprising: (a) mixing an aqueous solution comprising sodium acetate ous with at least one suitable alkali soluble polymer for inhibition of sodium acetate anhydrous crystal formation in crystalline sodium acetate trihydrate containing phase change materials; and wherein said one or more alkali soluble polymers is independently selected from: the copolymer of methacrylic acid and methyl rylate, poly(methacrylic acid co-methyl methacrylate) having a molecular weight of from about 500,000 to about 1,000,000, or a sodium, potassium or zinc salt thereof; poly(methacrylic acid) having an average molecular weight of about 10,000, or the sodium salt thereof; or a mixture thereof; and at least one nucleation promoter; (b) heating the resultant mixture to provide a 58 °C phase change al ning crystalline sodium acetate trihydrate; and wherein said aqueous solution may optionally contain an agent for modifying the melting point of sodium e; and n the e obtained in step (a) comprises: from 40% to 60% of sodium acetate anhydrous; from 0.1% to 10% of at least one suitable alkali soluble polymer; from 0.1% to 5% of at least one suitable nucleation promoter; from 1% to 25% of an optional g point depressing agent; and water to balance.

2. A process according to claim 1, wherein the sodium acetate anhydrous is t at a level of from about 45% to about 60% by weight of the composition.

3. A process according to any preceding claim, wherein said one or more alkali soluble polymers are independently present at levels of from about 0.2% to about 4% by weight of the composition.

4. A process according to any preceding claim, wherein said one or more le nucleation promoters are each independently present at a level of from about 0.2% to about 3% by weight of the composition. (26915652_1):AXG

5. A process according to any preceding claim, wherein said one or more suitable nucleation promoters are independently selected from: disodium en phosphate (DSP) and hydrated forms thereof; tetrasodium pyrophosphate (TSPP) and hydrated forms thereof; and mixtures thereof.

6. A process according to any preceding claim, wherein said the tion promoters are disodium hydrogen phosphate dihydrate and tetrasodium pyrophosphate decahydrate present at a total combined level of from about 0.5% to about 5% by weight of the composition.

7. A process according to any preceding claim, additionally comprising an agent for modifying the g point of sodium acetate trihydrate at a level of from about 1% to about 25% by weight of the composition.

8. A process according to claim 7, wherein said agent is lithium e dihydrate.

9. A process according to claim 8, wherein said agent is lithium acetate ous. Sunamp Limited By the Attorneys for the Applicant SPRUSON & FERGUSON Per: (26915652_1):AXG