WO2023144541A1

WO2023144541A1 - Phase change material for a temperature-controlled shipping package

Info

Publication number: WO2023144541A1
Application number: PCT/GB2023/050174
Authority: WO
Inventors: Tay CHU
Original assignee: Hydropac Limited
Priority date: 2022-01-26
Filing date: 2023-01-26
Publication date: 2023-08-03
Also published as: GB2615086A; GB202201008D0

Abstract

A phase change material for use in a shipping package for a frozen product is disclosed. The phase change material is an aqueous solution consisting of sodium formate having a concentration in the solution of at least 10% and, optionally, a gelling agent, such as xanthan gum, having a concentration in the solution of up to 1%. The remainder of the solution is water.

Description

PHASE CHANGE MATERIAL FOR A TEMPERATURE-CONTROLLED SHIPPING PACKAGE

FIELD OF THE INVENTION

The present invention relates to 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 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. Other proprietary phase change materials, with similar melting points, are also available, although such materials are often expensive and may have toxic or other undesirable characteristics. 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.

One significant disadvantage of phase change material packs of this type is that a powerful, industrial-type freezer is required to freeze the phase change material. To freeze a pack with a melting point of -21 °C, a freezer temperature of -30°C or less is typically recommended to achieve reasonable freezing times. This can require substantial investment in freezer hardware and associated electrical supplies, and results in significant energy consumption and operating costs.

Against that background, it would be desirable to provide phase change materials for use in shipping packages that have a phase transition temperature that avoids the need for specialist freezing equipment yet are still capable of maintaining a product temperature below the -12°C threshold temperature. It would also be desirable to provide phase change materials that are inexpensive and safe to use.

SUMMARY OF THE INVENTION

From a first aspect, the present invention provides a phase change material for use in a shipping package for a frozen product. The phase change material is an aqueous solution consisting of sodium formate having a concentration in the solution of at least 10% w/w and, optionally, a gelling agent having a concentration in the solution of up to 1 %. The remainder of the solution is water.

Accordingly, in embodiments, the phase change material consists only of sodium formate in water, with no additional constituents, or only of sodium formate, a gelling agent, and water, with no additional components. Said another way, the phase change material may be an aqueous solution consisting of at least 10% w/w sodium formate and between 0% and 1 % w/w of a gelling agent.

It has been found that a phase change material (PCM) with this composition has a phase transition (freezing/melting) temperature of approximately -16°C, allowing the PCM of the invention to be frozen using a standard freezer. This avoids the problems associated with the low-temperature freezing requirements of prior art PCMs. Furthermore, the PCM of the invention is able to maintain a temperature below a threshold temperature of -12°C for an adequate duration, is cost-effective and easy to manufacture, is non-toxic and does not present any significant safety problems.

Preferably, the concentration of sodium formate in the solution is less than or equal to 30%. More preferably, the concentration of sodium formate in the solution is between 15% and 25%. To optimise the cooling duration, a concentration between 20% and 25% is preferred. Most preferably, the concentration of sodium formate in the solution is 20% ± 1 %.

The gelling agent may comprise xanthan gum. Preferably, the concentration of gelling agent in the solution is between 0.5% and 0.8%.

The solution may have a phase change temperature of -16°C ± 1 °C. The addition of other components in the composition is not preferred. Accordingly, the PCM may consist substantially entirely of sodium formate in water, with the optional addition of a thickening agent (preferably xanthan gum). In this context, “substantially entirely” means that no or only trace amounts of other components may be present. The addition of functional quantities of other components may undesirably change the phase transition temperature.

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

From a second aspect, the invention resides in a phase change material pack for a shipping package, comprising a phase change material according to the first aspect of the invention, encapsulated in a container. The container may be rigid or flexible.

From a third aspect, the invention provides a shipping package, comprising at least one phase change material pack according to the second aspect of the invention.

The invention also extends, in a fourth aspect, to the use of a sodium formate solution as a phase change material for a shipping package, wherein the sodium formate has a concentration in the solution of at least 10%.

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 cross-sectional view of a shipping package including a phase change material pack according to the invention;

Figures 2 and 3 are plots of temperature against time during freezing and melting, respectively, for samples of phase change materials according to the invention with different sodium formate concentrations, together with comparative examples; and

Figures 4 and 5 are plots of temperature against time during freezing and melting, respectively, for a sample of phase change material according to the invention, together with further comparative examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention provide a phase change material (PCM) and a phase change material pack (PCM pack) for use in a shipping package.

Figure 1 shows a shipping package 50 in which the present invention can be used. The shipping package 50 comprises an insulated box 52 and removable insulated lid 53, which together enclose a cavity 54. The box 52 and lid 53 are of a suitable insulating material, such as an expanded polystyrene foam, and may be housed in an outer carton (not shown).

In this example, a plurality of vials 56, containing a frozen product, are placed in a receiving region 58 at the bottom of the cavity 54. Although not illustrated, the vials 56 may be held in place by recesses in the bottom wall of the cavity 54, by a shaped insert, or by other suitable means. A pre-frozen PCM pack 60, filled with the phase change material of the present invention, is placed on top of the vials 56. In the arrangement of Figure 1 , the PCM pack 60 keeps the product in the vials 56 frozen and below a target temperature of -12°C for as long as the PCM pack 60 itself remains frozen.

According to the present invention, the PCM comprises an aqueous solution of sodium formate (HCOONa) in water, optionally with a thickening or gelling agent, but with substantially no additional constituents.

It has been found that, at suitable concentrations, sodium formate solutions exhibit a solid/liquid phase transition temperature of approximately -16°C. Conveniently, therefore, the PCM can be frozen in a domestic-type refrigerator (typically at -18°C). Furthermore, the PCM is able to maintain a product below the target temperature of -12°C for a useful time period.

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.

It has been determined that solutions with sodium formate concentrations of between about 10% and about 30% are able to maintain a temperature of less than -12°C for an acceptable time, comparable to an equivalent quantity of a prior art 22% sodium chloride solution. The length of time below the threshold temperature of -12°C was found to be maximised for sodium formate concentrations of between about 20% and 25%.

The balance of the solution is 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 formate were prepared by mixing food-grade sodium formate powder (Perstorp Holding AB, Malmo, Sweden) with xanthan gum (80 mesh, Aqua-Sol Ingredients Ltd, Billingshurst, UK) and water (sterilised by UV treatment and chlorinated with 5 ppm chlorine) using a high-shear mixer. The concentration of xanthan gum in the final solutions was 0.5%. Six sodium formate solutions were prepared in this way, with sodium formate concentrations in the final solutions of 5%, 10%, 15%, 20%, 25% and 30%.

As a comparative example, an aqueous solution of sodium chloride and xanthan gum was also prepared, in which the concentration of xanthan gum was 0.5% and the concentration of sodium chloride was 22%. As a control example, a sample of the sterilised water without addition was also prepared.

160g 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 1 shows the freezing curves of the samples. It can be seen that each of the sodium formate samples freezes at a higher temperature than the sodium chloride sample. The freezing point of the sodium formate samples is approximately -16°C, compared with approximately -21 °C for the sodium chloride sample (ignoring supercooling effects). The freezing time generally increases with sodium formate concentration, although there was little difference in freezing time between the 20%, 25% and 30% compositions. All of the sodium formate samples froze fully before the sodium chloride sample.

The results of Figure 1 illustrate that sodium formate solutions are suitable for freezing in a domestic-type refrigerator, with a minimum temperature of around -18°C.

Figure 2 shows the melting curves of the samples after removal from the refrigerator. The temperature threshold of -12°C is indicated in Figure 2. All sodium formate samples with a concentration of 10% and above were able to maintain a temperature lower than the threshold temperature for an acceptable time period. On this basis, a minimum concentration of 10% sodium formate is preferred. Samples with a sodium formate concentration of 15% and above maintained a temperature lower than the threshold temperature for the same time or longer than the sodium chloride sample.

The 20% and 25% sodium formate sample remained below the threshold temperature for the longest times. Increasing the sodium formate concentration further to 30% resulted in a reduction in the time maintained below the target temperature. Accordingly, a sodium formate concentration of between about 20% and about 25% is considered to offer optimum performance. A sodium formate concentration of about 20% is particularly preferred, to maximise the longevity of the cooling ability whilst minimising the cost of constituents.

Comparison with other salt solutions

A 20% sodium formate solution with 0.5% xanthan gum was prepared as described in the examples above. For comparison, the following aqueous solutions were also prepared, using the same method: 20% potassium formate with 0.5% xanthan gum

20% sodium acetate with 0.5% xanthan gum

20% potassium chloride with 0.5% xanthan gum

22% sodium chloride with 0.5% xanthan gum

Water (control)

Again, 160g 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 cooled to the freezer temperature. The samples were then removed and placed at room temperature. The temperature of each sample was monitored until all samples were fully melted. The results are shown in Figures 4 and 5.

The potassium formate sample did not exhibit phase change material behaviour: it did not fully freeze over the duration of the freezing time, and did not hold at a transition temperature during melting. Accordingly, potassium formate is not suitable as a PCM.

The sodium acetate sample exhibited a phase transition temperature of -18°C, which is too low to be reliably freezable in a domestic-type freezer at -18°C. In addition, sodium acetate is basic, and the resulting solution has a high pH and can cause skin irritation. Sodium acetate is therefore not preferred as a PCM.

The potassium chloride sample exhibited a phase transition temperature of -10°C. Accordingly, a potassium chloride solution is not suitable for maintaining a target temperature of less than -12°C.

The sodium chloride sample exhibited a phase transition temperature of -22°C, an increased freezing time and an inferior longevity compared to the sodium formate sample. Accordingly, only the sodium formate solutions of the present invention provide a suitable phase transition temperature for freezing in a domestic-type freezer, combined with an ability to maintain a temperature of less than -12°C for a useful time period.

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 phase change material for use in a shipping package for a frozen product, the phase change material being an aqueous solution consisting of:

- sodium formate having a concentration in the solution of at least 10% weight/weight; and

- optionally, a gelling agent having a concentration in the solution of up to 1 % weight/weight; the remainder of the solution being water.

2. A phase change material according to Claim 1 , wherein the concentration of sodium formate in the solution is less than or equal to 30%.

3. A phase change material according to Claim 2, wherein the concentration of sodium formate in the solution is between 15% and 25%.

4. A phase change material according to Claim 3, wherein the concentration of sodium formate in the solution is 20% ± 1 %.

5. A phase change material according to any preceding claim, wherein the gelling agent comprises xanthan gum.

6. A phase change material according to any preceding claim, wherein the concentration of gelling agent in the solution is between 0.5% and 0.8%.

7. A phase change material according to any preceding claim, wherein the solution has a phase change temperature of -16°C ± 1 °C.

8. A phase change material according to any preceding claim, wherein the water is sterilised.

9. A phase change material according to Claim 8, wherein the water is sterilised by UV treatment and/or chlorination. A phase change material pack for a shipping package, comprising a phase change material according to any preceding claim encapsulated in a container. A shipping package, comprising at least one phase change material pack according to Claim 10. Use of a sodium formate solution as a phase change material for a shipping package, wherein the sodium formate has a concentration in the solution of at least 10% weight/weight, the solution optionally includes a gelling agent having a concentration in the solution of up to 1% weight/weight, and the remainder of the solution is water.