WO2023152501A1

WO2023152501A1 - Process for manufacturing a phase change material for a temperature-controlled shipping package

Info

Publication number: WO2023152501A1
Application number: PCT/GB2023/050297
Authority: WO
Inventors: Tay CHU
Original assignee: Hydropac Limited
Priority date: 2022-02-11
Filing date: 2023-02-09
Publication date: 2023-08-17
Also published as: GB2615569A; GB202201805D0

Abstract

A process for manufacturing a phase change material for use in a shipping package comprises mixing together a quantity of a salt, such as sodium chloride, and a quantity of a gelling agent, such as xanthan gum, while the salt and the gelling agent are in granular form, and adding the mixed salt and gelling agent to water and mixing to provide the phase change material as an aqueous solution.

Description

PROCESS FOR MANUFACTURING A PHASE CHANGE MATERIAL FOR A TEMPERATURE-CONTROLLED SHIPPING PACKAGE

FIELD OF THE INVENTION

The present invention relates to a process for manufacturing a phase change material. In particular, the invention relates to a process for manufacturing a phase change material for use in a shipping package, particularly for the transportation of frozen products.

BACKGROUND TO THE INVENTION

Frozen products such as foods, animal feeds and pharmaceutical products require strict temperature control during transportation and storage in order to maintain the quality and viability of the product, and in particular to prevent the product thawing before delivery to its final destination. In many cases, the maximum acceptable temperature for a product during transportation is -12°C.

In order to transport frozen products to an end user, it is necessary to provide a suitable temperature-controlled distribution system (commonly referred to as a cold chain distribution system).

Cold chain distribution systems may incorporate active systems, passive systems, or a combination of both for maintaining a product at a target temperature. Active systems typically use a continuous external power source to regulate the temperature in an insulated enclosure. Examples of active systems include refrigerated containers, trailers, vans and so on.

Passive systems require no external power source, and instead typically use a precooled phase change material, together with suitable insulation, to maintain a target temperature range in an enclosure for sufficient time to complete the transportation of the product to its destination. This approach enables suppliers of temperaturesensitive products to transport small quantities of product in an economic manner. In particular, passive systems can be incorporated into parcels or shipping packages so that an enclosure inside the package is maintained at the appropriate temperature. Conveniently, such packages can be transported and stored in vehicles and environments that are not themselves temperature-controlled.

Shipping packages with passive cooling are commonly insulated using expanded polystyrene (EPS), rigid polyurethane foam (PUR), rigid polyisocyanurate foam (PIR) or vacuum insulated panels (VIP). Natural materials such as wool, cork, or starch-based foams can also be used. Typically, the insulating material is arranged within an outer carton to define a cavity within the package that is surrounded on all sides by the insulating material.

The product can be placed within the cavity together with the phase change material. The phase change material controls the temperature inside the cavity by absorption of heat energy in the environment as latent heat during the phase transition from solid to liquid. To maintain temperatures below the -12°C threshold, phase change materials with phase transition temperatures that are below the threshold may be selected.

One common approach is to use dry ice (solid CO2) as the phase change material, which sublimates at -78°C. While dry ice can be effective at keeping frozen products well below the threshold temperature, the very low phase transition temperature can result in cracking and/or shrinkage of product packaging, causing failure in particular of glass and plastic packaging. The evolving carbon dioxide can also potentially contaminate products, particularly if the packaging is damaged. Dry ice is also a potentially hazardous product, and is therefore subject to safety regulations for handling, transport and storage, increasing the costs involved. In some cases, the quantity of dry ice that can be used in a package is limited by regulation or carrier policy, which in turn limits the duration of transport unless the dry ice can be replenished during the journey. The availability of dry ice can be vulnerable to supply fluctuations, and the storage of dry ice for prolonged periods requires specialist refrigeration equipment. To avoid the challenges associated with dry ice, another approach is to use a phase change material that is a liquid or gel at room temperature, and that has a melting point lower than the threshold temperature. Such materials are usually contained in a rigid or flexible pack, commonly referred to as a phase change material pack or a refrigerant pack, freezer pack, cold pack and so on. It is known, for example, to use a salt such as sodium chloride solution as the phase change material, with a concentration of between 10% and 25% and a typical melting point of approximately -21 °C. Packs of this type can be frozen by placing the packs into a suitable freezer before use. In use, such packs are able to maintain the contents of a package below the target temperature for as long as the pack remains frozen.

It has been observed that increasing the salt concentration in a phase change material is often beneficial in increasing the length of time that a phase change material pack can maintain a temperature below the threshold temperature. It has also been observed that the addition of a thickening or gelling agent also increases the length of time that the temperature can be maintained below the threshold temperature.

Accordingly, it would be desirable to produce phase change materials with a high salt concentration and a gelling agent. However, the production of high concentration solutions that include gelling agents on a commercial scale is not always straightforward. In particular, clumping can occur when mixing the components. One solution is to use a high-shear mixer or homogeniser to reduce or eliminate clumping, but such equipment can be expensive and have high power supply requirements, resulting in significant energy consumption. Another consideration is that the gelling agent is typically provided as a fine powder, which can present an explosion risk during processing.

Against that background, it would be desirable to provide a process suitable for producing a phase change material that includes a high salt concentration and a gelling agent that avoids or mitigates one or more of the problems associated with the prior art. SUMMARY OF THE INVENTION

From a first aspect, the invention provides a process for manufacturing a phase change material for use in a shipping package. The process comprises: mixing together a quantity of a salt and a quantity of a gelling agent while the salt and the gelling agent are in granular form; and adding the mixed salt and gelling agent to water and mixing to provide the phase change material as an aqueous solution.

It has been found that, by pre-mixing the salt and gelling agent in a dry state before adding the dry mixture to the water, a homogenous gelled solution with a high salt concentration can be produced without significant clumping, and without the need for a high-shear or other high-energy mixing process. Without wishing to be bound by theory, it is believed that by distributing the particles of gelling agent throughout the much larger volume of salt, the particles of gelling agent are spaced apart from one another upon initial contact with the water, reducing tendency of the gelling agent to form clumps and aiding dissolution.

The process may further comprise encapsulating the aqueous solution to provide the phase change material in a phase change material pack.

The process is particularly suited to the production of phase change materials with a high salt concentration. Accordingly, the quantity of salt may be such that the resulting aqueous solution has a salt concentration of at least 10% w/w or, more preferably, at least 15% w/w. In one example, the quantity of salt is such that the resulting aqueous solution has a salt concentration of between 20% and 25% w/w.

The quantity of gelling agent may be such that the resulting aqueous solution has a gelling agent concentration of less than 1 % w/w, for example between 0.5 and 0.8% w/w. The salt may be sodium chloride. In other cases, the salt may be selected from the group consisting of: sodium formate, sodium acetate and potassium chloride. The use of other organic and inorganic salts is also possible. It will be understood that the salt may be selected according to the phase transition temperature required for a particular application. The gelling agent may comprise xanthan gum.

Preferably, the water is sterilised. For example, the water may be sterilised by UV treatment and/or chlorination.

The process of the invention conveniently allows the salt and gelling agent to be supplied in a pre-mixed, dry form, which can then be stored and transported as required, and then subsequently mixed with water at a different time or location. Accordingly, the process may for example include packaging the blended salt and gelling agent after mixing together the salt and the gelling agent, and opening the resulting packages to add the blended salt and gelling agent to water.

Preferred and/or optional features of each aspect and embodiment of the invention may also be used, alone or in appropriate combination, in the other aspects and embodiments also.

Throughout this specification, concentrations expressed as percentages refer to the percentage of the substance in the final solution by weight (i.e. weight/weight or w/w).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which like reference signs are used for like features, and in which:

Figure 1 is a flowchart showing steps in a process according to the invention for producing a phase change material;

Figure 2 is a flowchart showing steps in another process according to the invention for producing a phase change material;

Figure 3 is a plot of temperature against time during melting for samples of phase change materials produced by the process of the invention; and

Figure 4 is a plot of temperature against time during melting for a sample of a phase change material produced by the process of the invention, together with a comparative example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention provide processes for manufacturing a phase change material for use in a shipping package. The phase change material is an aqueous solution comprising a salt and a gelling agent.

Referring to Figure 1 , in one embodiment of the process, in a first step 101 a suitable quantity of the salt and a suitable quantity of the gelling agent are mixed or blended together. During this first step 101 , both the salt and the gelling agent are in granular form. In this context, “granular form” means that the constituents are solid, dry and finely divided (e.g. in particulate form, such as granules, powders, flakes etc.). The gelling agent is preferably of a finer particle size than the salt. The first step 101 results in a quantity of blended salt and gelling agent, in which the particles of gelling agent are dispersed throughout the salt.

In step 102, the blended salt and gelling agent is added to water and mixed to dissolve the salt and gelling agent. The resulting aqueous solution forms the phase change material.

Optionally, in step 103, the aqueous solution is encapsulated in a container or package to provide a phase change material (PCM) pack.

Advantageously, in this process, none of the steps requires high shear or other high- energy mixing to avoid clumping, even for high-concentration formulations (e.g. salt concentrations of 15% or more, or even 20% or more). In step 101 , the salt and gelling agent can be mixed using any suitable mixer, including a paddle mixer, tumbler, helical blade mixer, drum mixer, and so on. In step 102, the blended salt and gelling agent can be mixed with the water using similar mixers, for example, or an IBC mixer.

Figure 2 shows another process according to the invention. In step 201 , the salt and gelling agent are blended while dry, as in step 101 above. In this case, however, in step 202 the blended salt and gelling agent are packaged, for example into bags. The packages can then be stored and later transported, in step 203, to another location (referred to here as a PCM facility). When required, in step 204, the packages are opened and mixed with water to form the aqueous solution. The PCM solution may then be encapsulated to form PCM packs, in step 205. The solution may be encapsulated in rigid or flexible containers, such as an LDPE pouch.

With this process, the salt and gelling agent can be conveniently blended at a facility equipped for handling powders in bulk, and then packaged into quantities suitable for addition to a known quantity of water at a different facility (such as a PCM manufacturing facility). In one example, the blended salt and gelling agent is packaged into 25kg bags, for addition to a quantity of water sufficient to produce 100 litres of the PCM aqueous solution.

After producing the PCM solution, the concentration of salt in the solution can be checked, for example by using a refractometer or other suitable analysis techniques.

In a preferred embodiment, the salt comprises sodium chloride. Preferably, the salt is a food-grade pure dried vacuum (PDV) salt, which results in a PCM with a solid- to-liquid phase transition temperature of approximately -21 °C. However, it is possible to use other salts, for example where different phase transition temperatures are preferred. For example, testing has shown that PCMs can be successfully produced using the process of Figure 1 at a salt concentration of 20% using sodium formate (approximate transition temperature -16°C), potassium chloride (approximate transition temperature -10°C) and sodium acetate (approximate transition temperature -18°C).

The addition of a small quantity (1 % or less, preferably between about 0.5% and 0.75%) of a thickening or gelling agent has been found to extend the time over which the PCM is able to maintain a temperature lower than the target temperature by about 25%. Xanthan gum has been found to be a suitable thickening or gelling agent. The addition of a thickening or gelling agent at concentrations higher than 1 % has been found to increase the viscosity of the PCM to a level that causes problems when filling PCM packs with the PCM. Alternative possible gelling agents include carboxymethyl cellulose, agar agar, and gelatin.

The balance of the solution is preferably water. Preferably, sterilised water is used. The water may for example be UV-sterilised and/or chlorinated. When chlorination is used, the amount of chlorine is typically 5 ppm or less.

The addition of further components to the PCM solution is not preferred, since an undesired effect on the phase transition temperature and/or melting time would be expected. Examples

Aqueous solutions of sodium chloride were prepared by first mixing food-grade PDV sodium chloride powder with xanthan gum (80 mesh (0.177 mm sieve size), AquaSol Ingredients Ltd, Billingshurst, UK), using a helical blade mixer, to produce a dry blended mixture. The mixed sodium chloride and xanthan gum was then mixed with water (sterilised by UV treatment and chlorinated with 5 ppm chlorine) using an IBC agitator mixer. The concentration of xanthan gum in the final solutions was 0.5%. Two sodium chloride solutions were prepared in this way, with sodium chloride concentrations in the final solutions of 9% and 22%.

200g samples of each solution were placed into a freezer at a temperature of approximately -32°C. The temperature of each sample was monitored and the samples were left in the freezer until all samples had fully frozen and cooled further to the freezer temperature. The samples were then removed and placed at room temperature. Again, the temperature of each sample was monitored until all samples were fully melted.

Figure 3 shows the melting curves of the samples after removal from the freezer. Both samples underwent a solid to liquid phase transition at approximately -21 °C. However, the 22% concentration sample took significantly longer than the 9% concentration sample to fully melt (around 3h 48m for the 9% sample, compared to around 9h 22m for the 22% sample, an increase of around 150%).

Accordingly, Figure 3 demonstrates a significant performance advantage of the higher-concentration composition.

To illustrate the effect of the gelling agent, a 22% sodium chloride solution without a gelling agent was prepared as a comparative example. Figure 4 shows the melting curve of a sample of this comparative example along with the same quantity of a sample of the 22% sodium chloride solution with 0.5% xanthan gum solution described above, with both samples pre-cooled to approximately -32°C. The sample with xanthan gum remained at the -21 °C transition temperature for 125% of the time of the sample without a gelling agent, showing that the presence of a gelling agent is also important in providing a performance advantage.

In further comparative examples, attempts were made to produce aqueous solutions with 22% sodium chloride and 0.5% xanthan gum using different processes, not according to the invention. The results are detailed in Table 1 . Table 1 : comparative examples

Further modifications and variations not explicitly described above may also be contemplated without departing from the scope of the invention as defined in the appended claims.

Claims

1. A process for manufacturing a phase change material for use in a shipping package, comprising: mixing together a quantity of a salt and a quantity of a gelling agent while the salt and the gelling agent are in granular form; and adding the mixed salt and gelling agent to water and mixing to provide the phase change material as an aqueous solution.

2. A process according to Claim 1 , further comprising encapsulating the aqueous solution to provide the phase change material in a phase change material pack.

3. A process according to Claim 1 or Claim 2, wherein the quantity of salt is such that the resulting aqueous solution has a salt concentration of at least 15% w/w.

4. A process according to Claim 3, wherein the quantity of salt is such that the resulting aqueous solution has a salt concentration of between 20% and 25% w/w.

5. A process according to any preceding claim, wherein the quantity of gelling agent is such that the resulting aqueous solution has a gelling agent concentration of between 0.5 and 0.8% w/w.

6. A process according to any preceding claim, wherein the salt is sodium chloride.

7. A process according to any of Claims 1 to 5, wherein the salt is selected from the group consisting of: sodium formate, sodium acetate and potassium chloride.

8. A process according to any preceding claim, wherein the gelling agent comprises xanthan gum.

9. A process according to any preceding claim, wherein the water is sterilised.

10. A process according to Claim 9, wherein the water is sterilised by UV treatment and/or chlorination.

11. A process according to any preceding claim, comprising packaging the blended salt and gelling agent after mixing together the salt and the gelling agent, and opening the resulting packages to add the blended salt and gelling agent to water.