WO2000028300A1

WO2000028300A1 - Method and device for determining the permeability of a container or material to a gaseous substance

Info

Publication number: WO2000028300A1
Application number: PCT/FI1999/000920
Authority: WO
Inventors: Eero Hurme
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 1998-11-06
Filing date: 1999-11-04
Publication date: 2000-05-18
Also published as: FI982414A0; AU1163800A; FI982414A

Abstract

The invention relates to a method and a device for determining the permeability of a material and/or container. In the method, a gaseous substance is brought to the first side of a material or container wall placed in a test chamber and the permeation of the gaseous substance through the material or container is detected on the second side of the material or container wall. The test gas is an inexplosive mixture of hydrogen and another gas. The permeation of hydrogen through the material or container wall is detected by measuring the hydrogen concentration in the upper part of the test chamber or container. The permeability of the material or container wall to the other gaseous substance is determined indirectly on the basis of the hydrogen permeability measurement and a known correlation between hydrogen permeability and permeability to the other gaseous substance. The device comprises a test chamber (1), inside which the material (2) or container (11) is placed; means (5, 7) for supplying a test gas to the first side (A) of the material or container wall; and measuring means (8, 9) for measuring the concentration of hydrogen having permeated the material on the second side (B) of the material or container wall.

Description

METHOD AND DEVICE FOR DETERMINING THE PERMEABILITY OF A CONTAINER OR MATERIAL TO A GASEOUS SUBSTANCE

The present invention relates to a method as defined in the preamble of claim 1. Moreover, the in- vention relates to a device as defined in the preamble of claim 11.

Different materials and equipment, such as foils, packing materials and containers, are required to meet certain standards regarding their gas perme- ability properties. For instance, for packing materials for foodstuffs, certain permeability properties have been defined for each product. The permeability to oxygen is often the most important property to be determined for a material. Permeability to water vapor is also generally determined. Permeability to carbon dioxide is usually estimated in relation to oxygen permeability. The most common method used to determine oxygen permeability is the isostatic method, in which an increase in oxygen concentration is measured, used e.g. in a permeability measuring apparatus manufactured by the company MOCON Inc. (formerly Modern Controls Inc. (Mocon) ) , Minneapolis, USA. The method involves the use of two gases, a carrier gas and a test gas. On opposite sides of a piece of foil to be tested, a partial pressure difference of test gas is created while the total pressure remains equal on both sides of the foil. The increase in concentration can be determined in many ways, in which oxygen is detected e.g. by gas chromatographic or paramagnetic means or by using a coulometric sensor. For instance, in the Mocon Ox-Tran apparatus, oxygen permeability is determined in accordance with the ASTM D 3985 standard by stretching a foil sample across a test chamber so that the sample divides the chamber into two compart- ments. Oxygen flows at a constant rate on one side of the foil while a carrier gas is flowing on the other side. The carrier gas is mainly nitrogen mixed with a small amount of hydrogen. The function of hydrogen in the carrier gas is only to remove the residual oxygen present as an impurity before the carrier gas is ad- mitted into the measuring cell. The carrier gas conveys the oxygen that has penetrated the foil sample to a coulometric sensor, which generates an electric current of a magnitude directly proportional to the quantity of oxygen molecules reaching the sensor. From this electric current, software provided in the apparatus computes the rate of oxygen permeation of the foil.

A problem with the prior-art method is that the measurement of oxygen permeability is a slow and time-consuming operation, the measuring time being usually 12 - 24 hours. A further problem are the high investment and maintenance costs of the apparatus.

The object of the invention is to eliminate the drawbacks referred to above. A specific object of the invention is to disclose a method that enables faster and simpler determination of permeability than before. A further object of the invention is to disclose a method that allows the acquisition costs of the apparatus used for the determination to be reduced as compared with the prior-art apparatus.

A further object of the invention is to disclose a method that is applicable for use in quality control and product development in the production of foils, packing materials and packages.

The method of the invention is characterized by what is presented in claim 1. The apparatus of the invention is characterized by what is presented in claim 11. According to the invention, the test gas used in the method is an inexplosive mixture of hydrogen and some other gas. The permeation of hydrogen through a material or the wall of a container is detected by measuring the hydrogen content in the upper part of a test chamber or container. The permeability to the other gaseous substance is then determined indirectly on the basis of the hydrogen permeability measurement and a known correlation between hydrogen permeability and permeability to the other gaseous substance.

The permeability of a material to hydrogen gas correlates with its permeability to other gaseous substances, which is why hydrogen permeability can be used indirectly for the determination of the level of permeability to other gaseous substances of a higher molecular weight, allowing significantly faster determination of permeability to another gaseous substance, e.g. oxygen, carbon dioxide, water vapor or other organic gaseous compounds. By using hydrogen instead of the above-mentioned substances in the measurement, the result of the determination of permeability can be obtained considerably faster than in previously known methods. The result of a hydrogen permeability test can be obtained in 2 - 4 hours, which is a clearly shorter time than e.g. the measuring time required for determining the oxygen permeability of the same foil. The rate of permeation of hydrogen gas through a foil is higher than the corresponding rate for any other substance because the molecular weight and size of hy- drogen are lower than those of other substances. A further advantage in the use of hydrogen is that hydrogen is inexpensive to use in the measurement. Furthermore, the measuring apparatus used in the measurement of hydrogen can ^'be made cheaper and simpler than the prior-art oxygen permeability measuring apparatus because hydrogen detectors are relatively cheap and in other respects, too, simpler solutions can be used. Since hydrogen is very light, the gas molecules will drift upward after penetrating the foil without a forced gas flow, so there is no need to use any special equipment, such as pumps or fans, in order to generate a gas flow in the other chamber compartment. When a test gas consisting of an inexplosive mixture of hydrogen and another gas is used as provided by the invention, the measurement can also be safely performed. In an embodiment of the method, the mixture of hydrogen and another gas used as test gas may be a hydrogen/nitrogen mixture or a hydrogen/air mixture. In an inexplosive mixture, the proportion of hydrogen is at most 5 %. in an embodiment of the method, the other gaseous substance for which the permeability of the material is determined indirectly via a hydrogen permeability measurement is oxygen (0₂) , carbon dioxide (C0₂) , water vapor or an organic volatile compound. in an embodiment of the method, the material is gas-tightly fastened in the test chamber so that the material divides the test chamber into a first chamber compartment lying on the first side of the material, i.e. below the material, and a second chamber compartment lying on the second side of the material, above the first chamber compartment. Next, a first gaseous substance is admitted into the first chamber compartment. The concentration of the first gaseous substance having permeated the material is measured in the upper part of the second chamber compartment, where hydrogen rises automatically without any forced flow.

In an embodiment of the method, a container whose wall delimits an interior space inside it is placed in the test chamber. Test gas is passed into the space inside the container. The concentration of hydrogen having permeated the wall of the container is measured outside the container in the upper part of the test chamber, where hydrogen rises automatically without forced flow.

In an embodiment of the method, a container whose wall delimits an interior spaced inside it is placed in the test chamber. Test gas is passed into the space outside the container. Next, the concentra- tion of hydrogen having permeated the wall is measured in the upper part of the interior space of the container, where hydrogen rises automatically without a forced flow. On the other side of the material or container wall, where the concentration of gas having permeated the material or container wall is measured, air can be used as carrier gas.

In an embodiment of the method, the material or container is made of plastic, such as plastic foil or laminate. In an embodiment of the method, the material or container is made of cardboard or cardboard laminate .

The device of the invention for implementing the above-described method comprises a test chamber inside which the material or a container with an interior space delimited by its wall is placed; means for supplying a test gas consisting of an inexplosive mixture of hydrogen and some other gas to the first side of the material or container wall; and means for meas- uring the concentration of hydrogen having permeated the material on the other side of the material or container wall.

In an embodiment of the device, the test gas is an inexplosive mixture of hydrogen and another gas, such as nitrogen or air. In the inexplosive test gas mixture, the proportion of hydrogen is at most 5 %.

In an embodiment of the device, the material is a film-like or sheet-like material. The test cham- ber comprises means for fastening the material so that the material divides the test chamber gas-tightly into a first chamber compartment lying on the first side of the material and a second chamber compartment lying on the second side of the material. In an embodiment of the device, the measuring means comprise a hydrogen detector and a sensor.

In an embodiment of the device, the second chamber compartment lies above the first chamber compartment. In this case, the sensor is placed in a tube opening into the upper part of the second chamber compartment.

In an embodiment of the device, the first side of the container wall where the test gas is supplied is the interior side of the container, and the second side of the container wall, where the measuring means have been arranged to measure the concentration of hydrogen having permeated the container wall, is the exterior side of the container. The sensor is preferably placed in a tube opening into the upper part of the test chamber.

In an embodiment of the device, the first side of the container wall where the test gas is supplied is the exterior side of the container, and the second side of the container wall, where the measuring means have been arranged to measure the concentration of hydrogen having permeated the container wall, is the interior side of the container. The sensor is preferably placed in a tube opening into the upper part of the interior space of the container. In an embodiment of the device, the material or container is made of plastic, such as plastic foil or plastic laminate.

In an embodiment of the device, the material or container is made of cardboard or cardboard laminate.

A particularly appropriate field of application of the quick measuring method and device of the invention is in quality control and product develop- ment in the production of packing materials. The method is primarily applicable for quality control in the production of a known material. The method of the invention can be used in industrial production of packing materials, in establishments investigating packing materials and in product development in food industry. The method makes it possible to quickly determine the degree of permeability of an unknown material/package.

In the following, the invention will be de- scribed in detail by the aid of a few examples with reference to the drawing, wherein

Fig. 1 presents a skeleton diagram of a first embodiment of the measuring apparatus designed for implementing the method of the invention, Fig. 2 is a diagram representing the relationship between hydrogen permeability results measured using the measuring apparatus in Fig. 1 and oxygen permeability,

Fig. 3 presents a skeleton diagram of a sec- ond embodiment of the measuring apparatus of the invention designed for implementing the method of the invention, and

Fig. 4 presents a skeleton diagram of a third embodiment of the measuring apparatus of the invention designed for implementing the method of the invention. Fig. 1 shows a diagram of a measuring apparatus which can be used to determine the hydrogen permeability of a foil sample, e.g. a packing material sample, in order that the oxygen permeability of the foil sample can be determined indirectly from the hydrogen permeability, the correlation between hydrogen permeability and oxygen permeability being known.

A corresponding test apparatus was used in the experiments described below. The measuring appara- tus comprises a test chamber 1, in which a foil sample 2 is fastened in a gas-tight manner by its edges with elements 14 so that it divides the test chamber 1 into two chamber compartments 3, a lower first chamber compartment 3 and an upper second chamber compartment 4. The first chamber compartment 3 comprises an inlet connection 5, through which a hydrogen/nitrogen mixture used as a test gas, humidified to a desired relative humidity level if desired, can be supplied via a valve 6 into the first chamber compartment 3 on the first side A of the foil sample, and an outlet connection 7, through which the test gas can exit from the first chamber compartment 3. On the second side B of the foil sample, in the second chamber compartment 4, air is used as a carrier gas. Furthermore, the appara- tus comprises a hydrogen detector 8, whose hydrogen sensor 9 is mounted in a gas-tight manner in a tube 10 which opens into the second chamber compartment 4. The apparatus is simpler than the aforementioned Mocon Ox- Tran apparatus because for the measurement of hydrogen no forced gas flow is needed in the upper second chamber compartment 4 and therefore no means for generating a gas flow are needed.

Using test equipment as illustrated in Fig. 1, three different foil samples were tested to estab- lish their hydrogen permeability and its correlation with the oxygen permeability of the foil samples. In the tests, three foil samples were used which, prior to the measurement of hydrogen permeability, had been stored at room temperature at a relative humidity of rH 40 - 50%.

The foil samples were packing plastic foils manufactured by Wihuri Oy Wipak.

1. The first foil sample was a foil with the type marking ESB 65 FP. Its oxygen permeability is 2 cm³0₂/m²d, 20°C, rH 50%.

2. The second foil sample was a foil with the type marking OPAE 65 HT . Oxygen permeability 45 cm³

0₂/m²d, 20°C, rH 50%

3. The third foil sample was a foil with the type marking ESE 80 HT . Oxygen permeability 95 cm³0₂/m²d, 20°C, rH 50%.

Oxygen permeability levels as found in the foil samples, 2 - 95 cm³ 0₂/m²d, 20°C, rH 50%, are in general use e.g. in foils for foodstuffs and raw meat.

A circular foil sample having an area of 8.5 cm² was mounted in a horizontal position in the test chamber 1. The volume of the chamber compartments 3 and 4 was 8.5 cm³. Test gas consisting of an inexplosive gas mixture containing 5% hydrogen and 95% nitrogen was supplied into the lower first chamber compart- ment 3 at a rate of 10 cm³/min. The upper second chamber compartment 4 contained air as a carrier gas. Having permeated the foil sample, the hydrogen molecules ascended to the sensor 9 of the hydrogen detector 8. The hydrogen detector 8 used was Sensistor 8505 TM, manufactured by Sensistor Ab, Sweden. The sensor 9 was only kept in the tube during the measurement (for 10 - 20 s). The measurements were carried out at room temperature (20°C, rH 50%) . Measurement was ended when the measurement result deviated by less than 1% from the previous result, obtained 20 minutes before. The results of the determination are presented in Table 1 below. The correlation of hydrogen permeability with the oxygen permeability of the same foil samples is presented in Fig. 2.

Table 1. Results of hydrogen permeability tests. Permeability was determined for two parallel foil samples.

In Fig. 2, the results of hydrogen measurement tests are presented in a coordinate system in relation to known oxygen permeability values of the foil samples. In the figure, the hydrogen permeability values are mean values obtained in two measurements. The figure shows oxygen permeability on the horizontal axis and hydrogen concentration on the vertical axis. Through the three points, a straight line can be drawn which describes the correlation between hydrogen permeability and oxygen permeability. On the basis of the tests, it can be stated that hydrogen permeability can be determined in 2-4 hours, which is clearly less than the time required for the determination- of oxygen permeability of corresponding foils, which is 12-24 hours. As can be seen from the figure, the oxygen permeability of the foil material tested bears a definite correlation with its hydrogen permeability. Therefore, the hydrogen permeability of the foil can be used indirectly for the determination of its oxygen permeability.

In addition to oxygen permeability, the indi- rect method of determination is also applicable in the determination of the permeability of materials to other gaseous substances, such as carbon dioxide, water vapor and volatile organic compounds.

Fig. 3 shows a diagram representing a measur- ing apparatus for determining the permeability of a closed container, package, e.g. a bottle, to hydrogen gas, so that it can be used for indirect determination of the permeability of the container to oxygen, carbon dioxide, water vapor or an organic volatile compound when the correlation between hydrogen permeability and permeability to oxygen, carbon dioxide, water vapor or an organic volatile compound is known.

The measuring apparatus comprises a test chamber 1 whose wall 12 delimits an interior space 13 inside it. The container has an inlet connection 5 opening into the interior space 13 and provided with a valve 6 for the supply of test gas into the interior space 13 of the container 11, and an outlet connection 7 for discharge of the test gas from the interior space. From the first side A of the wall 12 of the container 11, in this case from the interior side, the gas permeating the container wall comes to the second side of the container wall, i.e. to the outside of the container and into the test chamber 1, which contains air as a carrier gas. Moreover, the apparatus comprises a hydrogen detector 8. A sensor 9 is gas- tightly mounted in a tube 10 opening into the upper part of the test chamber 1. The hydrogen detector 8 measures the concentration of the gas having pene- trated the container wall. The measuring apparatus in Fig. 4 comprises a test chamber 1 in which a container 11 whose wall 12 delimits an interior space 13 inside it is mounted. The test chamber 1 has an inlet connection 5 opening into its interior and provided with a valve 6 for the supply of test gas into the test chamber outside the container 11, and an outlet connection 7 for discharge of the test gas from the test chamber. From the first side A of the wall 12 of the container 11, in this case from the exterior side, the gas permeating the container wall comes to the second side B of the container wall into the interior space 13 of the container, which contains air as a carrier gas. Moreover, the apparatus comprises a hydrogen detector 8. A sen- sor 9 is gas-tightly mounted in a tube 10 opening into the upper part of the interior space 13 of the container 11. The hydrogen detector 8 measures the concentration of the gas having penetrated the container wall. The invention is not restricted to the examples of its embodiments described above, but many variations are possible within the scope of the inventive idea defined in the claims.

Claims

1. Method for determining the permeability of a material and/or container, in which method a gaseous substance is brought to the first side of the material or container wall placed in a test chamber and the concentration of the said gaseous substance on the second side of the material or container wall is detected, characteri zed in that an inexplosive mixture of hydrogen and another gas is used as test gas; the permeation of hydrogen through the material or container wall is detected by measuring the hydrogen concentration in the upper part of the test chamber or container; and the permeability to the other gaseous substance is then determined indirectly on the basis of the hydrogen permeability measurement and a known correlation between hydrogen permeability and permeability to the other gaseous substance.

2. Method as defined in claim 1, characteri zed in that the mixture of hydrogen and another gas is a hydrogen/nitrogen mixture or a hydrogen/air mixture.

3. Method as defined in claim 2 or 3, characteri zed in that the hydrogen content in the mixture of hydrogen and another gas is at most 5%.

4. Method as defined in any one of claims 1 -

3, characteri zed in that the other gaseous substance the permeability to which is determined indirectly via a hydrogen permeability measurement is oxygen (0₂) , carbon dioxide (C0₂) , water vapor or an organic volatile compound.

5. Method as. defined in any one of claims 1 -

4, characteri zed in that the material is gas-tightly mounted in the test chamber so that the material divides the test chamber into a first chamber compartment lying on the first side of the material below the material and a second chamber compartment lying on the second side of the material above the first chamber compartment; hydrogen is admitted into the first chamber compartment; and the concentration of hydrogen having permeated the material is measured in the second chamber compartment.

6. Method as defined in any one of claims 1 - 4, characteri zed in that a container whose wall delimits an interior space inside it is placed in the test chamber; test gas is passed into the interior space of the container; and the concentration of hydrogen having permeated the container wall is measured outside the container in the upper part of the test chamber.

7. Method as defined in any one of claims 1 - 4, characteri zed in that a container whose wall delimits an interior spaced inside it is placed in the test chamber; test gas is passed into the test chamber space outside the container; and the concentration of hydrogen having permeated the wall is measured in the upper part of the interior space of the container.

8. Method as defined in any one of claims 1 - 7, characteri zed in that air is used as a carrier gas on the second side of the material or container wall where the concentration of gas having permeated the material or container wall is measured.

9. Method as defined in any one of claims 1 - 8, characteri zed in that the material or container is made of plastic, such as plastic foil or laminate .

10. Method as defined in any one of claims 1 - 8, characteri zed in that the material or container is made of cardboard or cardboard laminate.

11. Device for determining the permeability of a material (2) and/or container (11) in accordance with any one of claims 1 - 10, charact eri zed in that the device comprises a test chamber (1) , inside which the material

(2) or a container (11) with an interior space (13) delimited by its wall (12) is placed; means (5, 7) for supplying a test gas consisting of an inexplosive mixture of hydrogen and an- other gas to the first side (A) of the material or container wall; and measuring means (8, 9) for measuring the concentration of hydrogen having permeated the material in the upper part of the test chamber on the second side (B) of the material or container wall.

12. Device as defined in claim 11, characteri zed in that the mixture of hydrogen and another gas is a hydrogen/nitrogen mixture or a hydrogen/air mixture.

13. Device as defined in claim 11 or 12, characteri zed in that the hydrogen content in the test gas mixture is at most 5 %.

14. Device as defined in any one of claims 11 - 13, characteri zed in that the measuring means comprise a hydrogen detector (8) and a sensor (9) .

15. Device as defined in any one of claims 11 - 14, characteri zed in that the material is of a film-like or sheet-like type; and that the test chamber (1) comprises elements for fastening the material so that the material divides the test chamber gas-tightly into a first chamber compartment (3) lying on the first side (A) of the material and second cham- ber compartment (4) lying on the second side of the material (B) .

16. Device as defined in claim 15, characteri zed in that the second chamber compart- ment (4) lies above the first chamber compartment (3) ; and that the sensor (9) is placed in a tube (10) which opens into the upper part of the second chamber compartment (4) .

17. Device as defined in any one of claims 11 - 14, characteri zed in that the first side

(A) of the container wall where the test gas is supplied is the interior side of the container, and the second side (B) of the container wall, where the measuring means (8, 9) have been arranged to measure the concentration of hydrogen having permeated the container wall, is the exterior side of the container.

18. Device as defined in claim 17, characteri zed in that the sensor (9) is placed in a tube (10) opening into the upper part of the test chamber (1) .

19. Device as defined in any one of claims 11 - 14, characteri zed in that the first side (A) of the container wall where the test gas is supplied is the exterior side of the container, and the second side (B) of the container wall, where the measuring means (8, 9) have been arranged to measure the concentration of hydrogen having permeated the container wall, is the interior side of the container.

20. Device as defined in claim 19, char- acte ri zed in that the sensor (9) is placed in a tube (10) opening into the upper part of the interior space (13) of the container.

21. Device as defined in any one of claims 11 - 20, characteri zed in that the material or container is made of plastic.

22. Device as defined in any one of claims 11 - 20, characteri zed in that the material or container is made of cardboard or cardboard laminate.

23. Use of the method as defined in any one of claims 1 - 10 and/or the device as defined in any one of claims 11 - 22 for determining the permeability of packing materials to a gaseous substance.