WO1991000316A1

WO1991000316A1 - Humidity buffer formulation

Info

Publication number: WO1991000316A1
Application number: PCT/AU1990/000262
Authority: WO
Inventors: Brian David Patterson
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 1989-06-28
Filing date: 1990-06-19
Publication date: 1991-01-10
Also published as: DD296216A5; DD296216C5

Abstract

The present invention provides a humidity buffer formulation comprising a water-swellable, water-insoluble polymer and a mixture of (1) a non-volatile hydrophilic liquid and (2) water. By adjusting the relative proportions of (1) and (2) in the mixture, the relative humidity maintained by the humidity buffer formulation in a sealed container can be varied. The invention also relates to methods of maintaining the relative humidity in a sealed container at a predetermined level and to a container containing the humidity buffer formulation of the present invention. In preferred forms the humidity buffer formulation includes a hydrophilic fibrous material, which enables the humidity buffer formulation to be produced as free-flowing granules. In a further preferred form the humidity buffer formulation is prepared in the form of a sheet.

Description

HUMIDITY BUFFER FORMULATION

FIELD OF THE INVENTION

The present invention relates to a humidity buffer formulation, a method for maintaining the relative humidity in a sealed container within narrow limits despite temperature fluctuations and a container including the humidity buffer formulation. BACKGROUND ART

If air is sealed within a container at a given relative humidity and temperature, the relative humidity will change greatly with changes in temperature. For instance, if air of 50% relative humidity is sealed within a container at 25°, its relative humidity will increase to 100% when the temperature is decreased to 14° at constant pressure. Below 14 moisture will condense out in the container. Conversely the relative humidity will fall within the container if the temperature is increased above 25°.

In the prior art, a number of materials are known which absorb large quantities of water (see for example EP 0268498). A number of these compounds will absorb 200 to more than 1000 grams of water per gram of material. Most of these super-absorbers are copol mers of acrylamide or acrylonitrile or salts of acrylic acid which exist as parallel chains of atoms cross-linked at intervals to form a ladder-like structure. The copolymer may also be reacted with starch to form graft copolymers of starch and acrylonitrile. Examples of such products are sold under the trade names "Agrosoke", Igetagel", "Terra-Sorb", "Super-Sorb", "Super Slurper" and "Alcosorb".

The present inventor has found that by using such a water swellable polymer it is possible to formulate a humidity buffer. DISCLOSURE OF THE INVENTION Accordingly, in a first aspect the present invention 316

- 2 - consists in a humidity buffer formulation comprising a water-swellable, water-insoluble polymer and a mixture of (1) a non-volatile hydrophilic liquid and (2) water; the polymer being swellable by the mixture and the relative proportions of (1) and (2) in the mixture being so selected as to provide a predetermined relative humidity in a sealed container.

In a second aspect the present invention consists in a method of maintaining the relative humidity in a sealed container at a predetermined level comprising providing within the container a humidity buffer formulation comprising a water-swellable, water-insoluble polymer and a mixture of (1) a non-volatile hydrophilic liquid and (2) water; the polymer being swellable by the mixture; the relative proportions of (1) and (2) being such as to maintain the relative humidity at the predetermined level.

In a third aspect the present invention consists in a sealed container in which the internal relative humidity is maintained at a predetermined level, the sealed container being characterised in that a humidity buffer formulation is provided within the container, the humidity buffer formulation comprising a water-swellable, water-insoluble polymer and a mixture of (1) a non-volatile hydrophilic liquid and (2) water; the polymer being swellable by the mixture and the relative proportions of (1) and (2) in the mixture being so selected as to provide the predetermined level of relative humidity in the sealed container.

In preferred embodiments of the present invention the humidity buffer formulation of the first, second and third aspects of the present invention includes a hydrophilic fibrous material.

The hydrophilic fibrous material may be any of a number of such compounds known in the art which are capable of forming a layer on the outside of the polymer when swelled by the mixture. A number of natural and artificial fibres can be used, including cellulose and synthetic polymer fibres treated so that their surface is hydrophilic. It is presently preferred that the hydrophilic fibrous material is polyethylene pulp QP3850 made by DuPont.

The inclusion of hydrophilic fibrous material in the humidity buffer formulation enables the humidity buffer- formulation to be produced as free-flowing granules. This also increases the speed and ease with which the humidity buffer formulation can be prepared. In addition, the efficiency of the buffering effect of the humidity buffer formulation is increased due to the consequent increase in the ratio of the surface area of the humidity buffer formulation to its volume.

In a fourth aspect the present invention consists in a humidity buffer formulation in the form of a sheet comprising a solid support and a water-swellable, water-insoluble polymer and a mixture of (1) a non-volatile hydrophilic liquid and (2) water; the polymer being swelled by the mixture and the relative proportions of (1) and (2) in the mixture being so selected so as to provide a predetermined relative humidity in an enclosed air space; the formulation being affixed to the solid support by applying a water-soluble polymer to the solid support coated with the water-swellable, water-insoluble polymer and the mixture.

The support on which the sheet of humidity buffer is formed, or between which it is sandwiched, may be any of a number of cloth or gauze-like materials. A number of natural and artificial fibres can be used as either woven or non-woven materials. It is presently preferred that the cloth used is the non-woven polyethylene cloth Tyvec 1622 E, made by DuPont. By formulating the humidity buffer as a sheet whose 0316

- 4 - tensile strength and dimensions are not affected by uptake or loss of water, a more robust humidity buffer formulation is produced.

As used herein, the term "predetermined level of relative humidity" is intended to convey the desired level of relative humidity for any application. As would be readily recognised by persons skilled in the art the desired level of relative humidity will vary from application to application. For example, in the packaging of live vegetable or other produce, a high relative humidity, above 90% is desirable, whereas in the conservation of ancient metal objects the desired relative humidity may be in the range of 40 - 45%.

The water-swellable, water-insoluble polymer used in the humidity buffer formulation of the present invention may be any of a large number of super absorbent polymers well known in the art, such as the products known under the trade names "Terra-Sorb", "Agrosoke", "Igetagel", "Super-Sorb", "Super Slurper" and "Alcosorb". However, it is presently preferred that the water-insoluble, water-swellable polymer is an acrylonitrile-sodium acrylate copolymer and most preferably is the compound known under the trade name of "Alcosorb AB3S".

The non-volatile hydrophilic liquid may be any of a number of such compounds known in the art which are capable of causing swelling of the water-insoluble water-swellable polymer. However, it is presently preferred that this non-volatile hydrophilic liquid is selected from the group consisting of ethylene glycol, glycerol, digol, dimethylsulphoxide and dimethylformamide. It is particularly preferred that the non-volatile hydrophilic liquid is glycerol.

The relative humidity level maintained by the humidity buffer formulation of the present invention may be adjusted over a wide range by varying the proportions of water and the non-volatile hydrophilic liquid used. In order that the nature of the present invention, and the manner in which this adjustability can be achieved, may be better understood, preferred forms of the present invention will now be described with reference to the following examples.

EXAMPLE 1

HUMIDITY BUFFER FORMULATION FOR 30% RELATIVE HUMIDITY

Alcosorb AB3S (Allied Colloids Wyong NSW) is dried in an oven at 110°C to constant weight. To 26 gram of this material is added 65.2 grams of anhydrous glycerol and 8.8 grams of distilled water. The components are then mixed rapidly to a smooth paste and placed in an hermetically-sealed vessel able to withstand pressure. The vessel and the contents are then heated to 60°C for 24 hours. The vessel is then cooled slowly (typically over 24 hours) to the operating temperature of the humidity buffer by placing it in an insulated container.

EXAMPLE 2 HUMIDITY BUFFER FORMULATION 70% RELATIVE HUMIDITY

The same procedure as that in Example 1 is followed with the exception that the weights of the components are as follows:-

Polymer 26 grams Glycerol 39.7 grams Water 34.3 grams

EXAMPLE 3

HUMIDITY BUFFER FORMULATION 80% RELATIVE HUMIDITY

Polymer 26 grams

Glycerol 29.0 grams Water 45.0 grams The relatively strong gels which result may be broken up into particles and sealed within pouches of a non-woven material or other material which is freely permeable to water vapour.

When a pouch holding a particular formulation is placed within a container whose humidity it is desired to control, the relative humidity of the enclosed air can be maintained within 10% of the desired relative humidity over a range of temperatures between 0 and 30 C. EXAMPLE 4 Humidity Buffer Formulation for 50% Relative Humidity Glycerol is diluted with water to give a solution whose refractive index at 25°C is 1.4365. A weight of 675 grams of this solution is homogenised with 275 grams of the resin "ALCOSORB AB3S" (Allied Colloids Ltd.) determined by dry weight to have a water content of 10%. The mixture is cured overnight at 70 C in a sealed container, and then homogenised with 50 grams of polyethylene pulp QP3850 (Du Pont) . The resulting humidity buffer formulation consists of free-flowing white granules that will buffer an enclosed airspace to a relative humidity of 50%.

EXAMPLE 5

Humidity Buffer Formulation for 90% Relative Humidity Glycerol is diluted with water to give a solution whose refractive index at 25°C is 1.3643. A weight of 675 grams of this solution is homogenised with 275 grams of the resin "ALCOSORB AB3S" (Allied Colloids Ltd.), determined by dry weight to have a water content of 10%, and 50 grams of polyethylene pulp QP3850 (Du Pont) . The resulting humidity buffer consists of free flowing white granules that will buffer an enclosed airspace to a relative humidity of 90%.

The free flowing granules may be sealed within pouches of non-woven material or other material which is freely permeable to water vapour. When such a pouch is placed within a container whose humidity it is desired to control, _e relative humidity of the enclosed air space can be c αtained within 10% of the desired relative humidity over a range of temperatures between 0 and 30°C. EXAMPLE 6

Humidity Buffer in Sheet Form for 60% Relative Humidity

Glycerol is diluted with water to give a solution whose refractive index at 25°C is 1.4562 (about 10% water) . A weight of 530 grams of this solution is mixed with 289 grams of the resin "ALCOSORB AB3S" (Allied

Colloids) determined by dry weight to have a water content of 10%, to give a smooth paste. The paste is spread in a uniform layer over 2000 square cm of non-woven, fibrous, hydrophilic polyethylene (Tyvec 1622E, Du Pont), and covered by the same material. The sandwich of humidity buffer between the two layers of the non-woven material is cured for 2 h at 70 C in a sealed vessel to give a flexible sheet. A solution of polyvinyl alcohol (average M.W. 14,000, British Drug Houses), made by dissolving 27 grams in 180 grams water, is spread evenly over both sides of the sheet of humidity buffer. The water is removed by the gel from the polyvinyl alcohol, so that the gel particles become bound to each other and to the sandwiching Tyvec sheet by the polyvinyl alcohol. This process is completed by curing the sheet overnight at room temperature in a sealed container. After curing, the sheet of humidity buffer is dry to the touch and will buffer an enclosed space at 60% relative humidity. EXAMPLE 7 In order to demonstrate the efficacy of the humidity buffer formulation of the present invention, a range of formulations for a variety of relative humidities were produced, and the humidity produced in a sealed container containing these formulations at 20°C was assessed. Eight formulations each including 20.34g of Alcosorb AB3S with a water content of 10.5% and 49.6g of glycerol/water were produced in which the ratio of glycerol to water was varied. The desired relative humidity, the refractive index of the glycerol/water mixture and the percentage of water in the glycerol/water mixture are set out in Table 1.

TABLE 1

% Desired Relative Refractive Index Percentage of weight Humidity of glycerol/water water in glycerol/ mixture water mixture

20% 1.4660 4.3

30% 1.4579 9.2 40% 1.4484 6.4

50% 1.4368 23.3

60% 1.4230 32.8

70% 1.4064 43.9

80% 1.3868 58.3 90% 1.3646 72.4

The results of these experiments are shown in the accompanying Figure where the points represent the actual % relative humidity obtained whilst the line shows the ideal situation. As can be seen the results demonstrate that the humidity buffer was able to provide a relative humidity within 5% of the desired relative humidity.

Humidity buffer formulations controlling the relative humidity between 10 and 95% relative humidity may be made in analogous ways by increasing or decreasing the proportion of glycerol relative to that of water in the formulation, while keeping the proportion of the water-swellable, water-insoluble polymer constant. As is clear from the Examples above it is possible to use water-swellable, water-insoluble polymers which have not been oven dried and glycerol which is not anhydrous, however, in these cases, their water content must be determined and allowed for in the formulation. INDUSTRIAL APPLICABILITY

The humidity buffer formulation of the present invention has a number of industrial applications, including:- 1. In the packaging of living produce at high humidities. The invention enables packaged flowers, fruits and vegetables to be kept at relatively high humidities (above 90%) without the risk of condensation occurring when the temperature falls. Such condensation can often lead to damage of these products.

2. Maintenance of herbarium specimens. A relative humidity of 50 to 55% is required in display cases. This is difficult to achieve in many conditions, but the formulation of the present invention would allow this. In addition, when herbarium specimens are dried in the field, the formulation of the present invention would allow them to be maintained at the correct humidity under variable conditions of temperature.

3. In the horticultural trade. Bare-rooted plants, cuttings and scion material may be sent through the post in packages containing sachets of the humidity buffer formulation of the present invention in order to maintain the correct humidity.

4. Conservation of ancient metal objects. Ancient bronze objects may be kept with sachets of the humidity buffer formulation in order to maintain the ideal humidity of between 40 and 45% relative humidity. Salt solutions such as lithium chloride have been used to control humidity in such cases, but they introduce the risk of spillage and consequent corrosion. The humidity buffer formulation of the present invention would not suffer from these disadvantages.

5. Conservation of wooden, fabric and leather objects in museums. The problem of changes from the ideal storage humidity of 50 to 60% relative humidity arise during transit from one museum or exhibition site to another, or within display cases. As humidity changes, the objects swell or contract. This stresses the object and eventually causes visible deterioration. The humidity buffer formulation of the present invention would be ideal for minimising deterioration from this cause.

6. Conservation, of pictures, paper and textiles where the materials need to be kept at relative humidities between 50 and 55%.

7. Biotechnology applications including growth of microorganisms under defined conditions of water activity. For diagnostic purposes, bacteria and fungi are often isolated and grown on agar plates of defined water activity. For the water activity to remain stable, the relative humidity above the plate must be kept in balance, that is, at the same relative humidity in equilibrium with the water activity in the plate. Depending on the application, this may vary from 70 to 95% relative humidity (0.7 to 0.95 water activity). Saturated solutions of salts are often used for this purpose, but they are inconvenient to prepare and can spill easily. The humidity buffer formulation of the present invention can be formulated to provide the required water activities in the atmosphere without such problems. From the foregoing description, it should be apparent that the invention encompasses an advantageous advance or alternative over the prior ar- Further, it should be clear that the invention may ba embodied in other specific forms without departing from the spirit or the essential characteristics thereof. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS :-

1. A humidity buffer formulation comprising a water- swellable, water-insoluble polymer and a mixture of (1) a non-volatile hydrophilic liquid and (2) water; the polymer being swellable by the mixture and the relative proportions of (1) and (2) in the mixture being so selected as to provide a predetermined relative humidity in a sealed container.

2. A humidity buffer formulation in the form of a sheet comprising a solid support and a water-swellable, water- insoluble polymer and a mixture of (1) a non-volatile hydrophilic liquid and (2) water; the polymer being swellable by the mixture and the relative proportions of (1) and (2) in the mixture being so selected as to provide a predetermined relative humidity in an enclosed air space; the formulation being affixed to the solid support by applying a water-soluble polymer to the solid support coated with the water-swellable, water-insoluble polymer and the mixture.

3. A humidity buffer formulation as claimed in Claim 1 in which the humidity buffer formulation includes a hydrophilic fibrous material.

4. A humidity buffer formulation as claimed in Claim 3 in which the hydrophilic fibrous material is polyethylene pulp.

5. A humidity buffer formulation as claimed in any one of Claims 1 to 4 in which the water-swellable, water-insoluble polymer is an acrylonitrile-sodium acrylate copolymer.

6. A humidity buffer formulation as claimed in any one of Claims 1 to 5 in which the non-volatile hydrophilic liquid is selected from the group consisting of ethylene glycol, glycerol, digol, dimethylsulphoxide and dimethylformamide.

7. A humidity buffer formulation as claimed in Claim 6 in which the non-volatile hydrophilic liquid is glycerol.

8. A method of maintaining the relative humidity in a sealed container at a predetermined level comprising providing within the container a humidity buffer formulation as claimed in any one of Claims 1 to 7, the relative proportions of (1) and (2) being such as to maintain the relative humidity at the predetermined level.

9. A sealed container in which the internal relative humidity is maintained at a predetermined level, the sealed container being characterised in that a humidity buffer as claimed in any one of Claims 1 to 7 is provided within the container.