WO2003029786A1

WO2003029786A1 - Detection and measurement of oxygen permeation across a film

Info

Publication number: WO2003029786A1
Application number: PCT/AU2001/001244
Authority: WO
Inventors: Minh Huu Nguyen; Robert Holland; Anh Kim Thi Phan
Original assignee: University Of Western Sydney
Priority date: 2001-10-03
Filing date: 2001-10-03
Publication date: 2003-04-10

Abstract

A device for detecting the permeation of oxygen across a film comprising detector means for detecting oxygen and adhesive means for adhering the device to the film, wherein the detector means are arranged for permitting the detector means to detect the permeation of oxygen across the film.

Description

DETECTION AND MEASUREMENT OF OXYGEN PERMEATION ACROSS A

FILM

TECHNICAL FIELD

The invention relates to a device for detecting and measuring the permeation of oxygen across a film, particularly, but not exclusively, a film for packaging foods, food stuffs, pharmaceuticals and horticultural products.

BACKGROUND OF THE INVENTION

The exposure of many products, including, for example, foods, food stuffs, pharmaceuticals and horticultural products, to oxygen may substantially reduce the shelf life of the product. Films for packaging such products may limit oxygen exposure; however, nearly all films are permeable to oxygen to some extent.

The oxygen permeability of a film is typically measured by sealing the film as a barrier between two compartments in a specially designed flow cell, under a partial pressure gradient of oxygen. The permeation of oxygen across the film and into the compartment of lower oxygen pressure is measured by sampling the compartment of lower oxygen pressure and measuring sampled oxygen, for example by gas chromatography .

Another method for measuring oxygen permeability is that used by OX-TRAN 2/60 Oxygen Transmission Rate System

(MOCON) . In this method, the film is sealed as a barrier between a first and second compartment. A flow of oxygen is established through the first compartment and a flow of a mixture of nitrogen and hydrogen, treated to deplete oxygen, (the "carrier gas") is established through the second compartment. Oxygen that permeates across the film is received by the carrier gas and carried to an oxygen sensor, which is a coulometric fuel cell that produces an electrical current when exposed to oxygen.

A limitation of the above referred processes and apparatus is that the oxygen permeability can only be tested in the conditions provided in the apparatus. This is a problem because the permeabilities of a film to oxygen are dependent on a number of environmental factors including temperature and humidity. Although oxygen permeabilities in many films under specified conditions (typically, conditions provided by the apparatus) are known, it is difficult to precisely predict or extrapolate the barrier properties of these films in conditions of temperature and humidity that are different from the specified conditions. Apparatus such as OX-TRAN 2/60 are not designed to work at low temperatures at which some consumable products are stored and long conditioning times may be required for measuring oxygen permeabilities at particular humidities.

An alternative process measures oxygen permeability using an apparatus comprising ethyl cellulose film and containing 9, 10-dimethylanthracene (DMA) and erythrosin which is sealed between two pieces of test film in a cell, or sealed in a pouch made of the test film [Holland et al . 1980 Die Angewandte Makromolekulare Chemie 88: 209-221]. Oxygen which permeates across the test film is converted by erythrosin, when exposed to light, to singlet oxygen which then reacts with the DMA. The reaction of the singlet oxygen with the DMA causes a change in the UV absorbance which can be measured spectrophotometrically . A problem with the apparatus is that oxygen is frequently trapped with the apparatus at the time that the cell or pouch is sealed. This trapped oxygen must be removed before measurement of oxygen permeation across the test film, by illumination with white light. This illumination diminishes the quantity of the DMA and consequently limits the number of measurements that can be made using the apparatus. Further, the requirement of removing trapped oxygen is inconvenient. Finally, when used in a spectrophotometer, the apparatus must be mounted in a specially machined frame. Accordingly, limitations apply to the use of this process and apparatus.

There is a need for an improved device for detecting and measuring oxygen permeability across a film.

DESCRIPTION OF THE INVENTION

The invention seeks to minimise the above identified limitations and in a first aspect provides a device for detecting the permeation of oxygen across a film. The device comprises detector means for detecting oxygen and adhesive means for adhering the device to the film. The device is characterised in that the detector means are arranged for permitting the detector means to detect the permeation of oxygen across the film.

The inventors have found that adhesive means for adhering the device to a film advantageously at least reduces and typically, substantially eliminates, trapping of oxygen with the device at the time that the device is adhered to the film and sealed from atmospheric oxygen. Further, as described herein, the inventors have found that the adhesive means, for example, an adhesive such as those commonly used on adhesive tape, is sufficiently permeable to oxygen to permit oxygen permeation across a film and into the adhesive means, for detection by the detector means. Consequently, the predominant, and typically, sole, source of oxygen which is detected by the device in use is that which has permeated across the film. Accordingly, the device is improved for detecting oxygen permeability across a film.

The device of the first aspect of the invention has a number of advantages further to the improved detection of oxygen permeability across a film. First, the device is sufficient for permitting determination of oxygen permeability of film, prior to use of the film in packaging, or when the film is in use, for example when used as package. Apparatus such as OX-TRAN 2/60 cannot detect oxygen permeability of a film when the film is used as package. Further, the device allows detection of oxygen permeability at the temperature and humidity conditions at which a product would be stored. Further, the adhesive means provides for an application of the device to a film which is simpler than the means for applying the device described in Holland et al . 1980.

Typically, the detector means are arranged within the adhesive means. A process for arranging the detector means within the adhesive means is described further herein.

To prevent the detection of atmospheric oxygen, that is, oxygen which has not permeated through the film, it is important that in use, the device is sealed from the atmospheric oxygen. The device may be sealed from the atmospheric oxygen, for example by folding film, for example, packaging film, about the device after adherence of the device to the film, and then, if desired, heat sealing the device within the folded film. The device is typically structured to permit the device, in use, to be sealed by the film to limit the function of the detecting means to detection of oxygen permeation across the film. Any other film or barrier may be used with the packaging film, provided that that film has an oxygen permeability which is the same as, or greater than the oxygen permeability of the packaging film.

The device may further comprise support means for supporting the device against the film. Where the device comprises support means, the detector means may be arranged in the support means , or support means and adhesive means, for detecting permeation of oxygen across a film. Examples of support means are those comprising ethyl cellulose or a compound comprising ethyl cellulose, cellulose esters or cellulose.

Alternatively, where the detector means is arranged with the adhesive means, the device may consist of the adhesive means and the detector means. For example, it is contemplated that the device may consist in an adhesive comprising the detector means. The adhesive comprising the detector means could be applied to a film as a spray- on formulation, or it could be applied by painting the adhesive comprising the detector means onto the film.

The adhesive means are permeable to oxygen and sufficient for adhering the device to film for packaging foods, food stuffs, pharmaceuticals or horticultural products. Accordingly, examples of adhesive means include those capable of adhering to film which comprises low density polyethylene, high density polyethylene, polyvinylchloride, ethylvinylalcohol or cast polypropylene, oriented PET, nylon 20, polyvinyl dichloride/cellophane, oriented PET/ethylvinyl alcohol or a mixture thereof. Such adhesive means typically comprises acrylate or a derivative thereof. Another example of an adhesive means is one comprising methylacrylate .

Typically, the detector means comprises a dye for detecting oxygen. Dyes which are contemplated are those capable of changing colour when contacted with oxygen and exposed to light. Rubrene is an example of such a dye. Rubrene is a coloured compound, however, the exposure of rubrene to light produces an activated form of rubrene which is colourless. This colour change can be visualised by naked eye. Contact of the activated form of rubrene with oxygen de-activates rubrene, which results in a decolouration of rubrene. Pentacene and naphthacene are other examples of dyes that could be used. The device may further comprise sensitising means for improving the sensitivity of the dye for detecting oxygen across a range of wavelengths. The sensitising means function in the device by permitting the dye to absorb light of a wavelength that otherwise would not be absorbed by the dye in the absence of the sensitising means. This increases the rate of the reaction for the production of activated dye, i.e. the activated form of rubrene, which in turn increases the reaction rate for activation of oxygen and consequence de-activation and decolouration of rubrene. This is important for very permeable films. Examples of sensitising means include tetraphenylporphine, rhodamine, methylene blue and erythrosin.

In another aspect, the invention provides a film for packaging foods, food stuffs, pharmaceuticals or horticultural products, the film comprising a device according to the first aspect of the invention. Examples of such films include those which comprise low density polyethylene, high density polyethylene, polyvinylchloride, ethylvinylalcohol or cast polypropylene, oriented PET, nylon 20, polyvinyl dichloride/cellophane, oriented PET/ethylvinyl alcohol or a mixture thereof .

The inventors have also observed that the detector means for detection of oxygen can be incorporated into the adhesive means so as to permit the detector means to detect oxygen that has permeated across the film and into the adhesive means. More specifically, in the examples of the invention described further herein, the inventors have observed that an oxygen permeable adhesive, for example, an adhesive such as those typically used on adhesive tape, can be treated to permit diffusion of a dye into the adhesive in a manner which preserves the adhesive quality of the adhesive.

Thus in another aspect there is provided a process for producing a device for detecting the permeation of oxygen across a film. The process comprises contacting an adhesive with a solvent comprising detector means for detecting the permeation of oxygen across a film, to form an adhesive diffused with the solvent and drying the adhesive diffused with the solvent to remove the solvent, to produce the device.

The solvent for use in the process is typically one capable of diffusing through an adhesive comprising an acrylate or acrylate derivative, to form the adhesive diffused with the solvent. Examples of such solvents are toluene, tetralin or a derivative or mixture thereof. Solvents which can be evaporated at room temperature are preferred. Another example of a solvent is xylene. The solvent must be capable of dissolving the detector means but not the adhesive means.

Typically, the detector means comprised in the solvent is a dye for detecting oxygen. Dyes which are contemplated are those described above, i.e. those capable of changing colour when contacted with oxygen and exposed to light. Rubrene is an example of such a dye. When the solvent is dried in the process, the dye remains in the adhesive. The device may further comprise sensitising means for improving the sensitivity of the dye for detecting oxygen across a range of wavelengths. Sensitising means which are contemplated are those described above, i.e., an example being tetraphenylporphine.

The process may comprise a further step of fixing support means to the device, the support means for supporting the device against a film.

In another aspect, the invention provides a device produced by the above described process.

As described herein, the inventors have observed that the device of the first aspect of the invention can be used for detecting whether oxygen has permeated across a film, for detecting the permeation of oxygen across a film, and for detecting the rate of permeation of oxygen through film (known as the oxygen transfer rate or OTR) .

In one aspect, there is provided a method of detecting the permeation of oxygen across a film. The method comprises contacting the adhesive means of a device of the first aspect of the invention to a film to adhere the device to the film and ascertaining from the detector means for detecting oxygen whether oxygen has permeated across the film.

As noted above, typically the detector means is a dye, more particularly, a light sensitive dye and the detection of oxygen permeation and/or measurement of OTR is determined by measuring the change in colour of the dye, for example, the disappearance of the dye, spectrophotometrically .

Advantageously, as described herein, the device is sufficiently sensitive to provide by visual examination of the device, a quantitative estimate of the rate or amount of oxygen permeation across a film that is reasonably accurate, by comparing the amount of disappearance of the dye against a standard which correlates colour change of the dye against amount of oxygen permeation across the film.

In another aspect, the invention provides a device for detecting the permeation of oxygen across a film comprising a dye capable of changing colour when contacted with oxygen and exposed to light, the dye for detecting oxygen, and an adhesive for adhering the device to the film, wherein the dye is arranged within the adhesive for permitting the dye to detect the permeation of oxygen across the film.

In another aspect, the invention provides a device for detecting the permeation of oxygen across a film, the device comprising rubrene for detecting oxygen, an adhesive comprising acrylate or a derivative thereof, the adhesive for adhering the device to the film and a support comprising cellulose or a derivative thereof, the support for supporting the device against the film, wherein the rubrene is arranged within the adhesive for permitting the rubrene to detect the permeation of oxygen across the film. In another aspect, the invention provides a process for producing a device for detecting the permeation of oxygen across a film comprising contacting an adhesive comprising acrylate or a derivative thereof, with solvent selected from the group consisting of toluene, tetralin or a derivative or mixture thereof, the solvent comprising rubrene for detecting the permeation of oxygen across a film, to form an adhesive diffused with the solvent, and drying the adhesive diffused with the solvent to remove the solvent, to produce the device.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 Change of absorbance of device with time (three symbols refer to triplicates of one film) . Figure 2. Estimating of OTR by observance of colour of device with time against the visual scale. Figure 3 Visual scale used to distinguish films within a narrow range of OTR ( 3 sets A,B and C films in triplicates) .

EXAMPLE

Example 1. Preparation of device (I) .

A solution of toluene (Ajax Chemicals) comprising 0.5 g rubrene (Aldrich Chemicals) was prepared. A strip of about 12 cm in length of an adhesive tape (Scotch Tape, 3M, type 0910, product number 34-8505-8709-7) was soaked in 100 mis of the solution so that the tape was completely immersed in the solution for about 1 min. The strip was then dried in the dark until completely dry (usually about 1 hr at room temperature) to produce the device. The device was then stored in the dark until use. Example 2. Preparation of device (II) .

A solution of tetralin (Ajax Chemicals) comprising 0.5 g rubrene (Aldrich Chemicals) was prepared. A strip of about 12 cm in length of an adhesive tape (Scotch Tape, 3M, type 0910, product number 34-8505-8709-7) was soaked in 100 mis of the solution so that the tape was completely immersed in the solution for about 1 min. The strip was then dried in the dark until completely dry (usually about 1 hr at room temperature) to produce the device. The device was then stored in the dark until use.

Example 3. Preparation of device (III) .

A solution of toluene (Ajax Chemicals) comprising 0.5 g rubrene (Aldrich Chemicals) and 0.0096g of tetraphenylporphine (Aldrich Chemicals) was prepared. A strip of about 12 cm in length of an adhesive tape (Scotch Tape, 3M, type 0910, product number 34-8505-8709-7) was soaked in 100 mis of the solution so that the tape was completely immersed in the solution for about 1 min. The strip was then dried in the dark until completely dry

(usually about 1 hr at room temperature) to produce the device. The device was then stored in the dark until use.

Example 4. Application of device to a film. The process was performed in dim light to avoid activation of the detector means. The device was contacted with a film shown in Tables 1 and 2, to permit the adhesive means to adhere the device to the film. The device was adhered to the film taking care to avoid air bubbles and/or folds in the device or film. The device was then sealed from atmospheric oxygen using a further sheet of the film. Example 5. Detecting permeation of oxygen through a film. A device was applied to the film as described in Example 4 and was mounted onto a frame made of cardboard similar to a photographic slide.

The frame was then positioned inside a spectrophotometer (Varian model 634) at a position for illumination by a beam of light emitted by the spectrophotometer. An initial absorbance at 522 nm was measured.

The device and film, mounted on the frame, was then removed from the spectrophotometer and exposed to light using a 32 W diffused light viewing box.

Following illumination, the device and film, mounted on the frame, was returned to the spectrophotometer to measure a change in colour of the dye . The device and film, mounted on the frame was positioned in the spectrophotometer so that the region of the device that was measured was the same as that measured to obtain the initial measurement noted above.

A disappearance of the dye indicated that the film was permeated by oxygen. The size of the change of colour of the dye is a measure of the oxygen permeability of the film at the particular temperature and humidity conditions under which the method was performed. Example 6. Measuring the oxygen transmission rate of a film.

Materials and Method. The oxygen transmission rate (OTR) , or in other words, oxygen permeability, of a film, was measured essentially according to the method of Holland et al . 1980 and is described briefly below:

The device was applied to a film according to the method of Example 4 and a initial absorbance at 522 nm was measured as described in Example 5 in triplicate. The initial absorbance at 522 nm was about 2.2.

The device was then maintained in controlled temperature and humidity conditions for 5 minutes. The absorbance at 522 nm was then measured in triplicate. These steps were repeated 10 times. The measured absorbance values were then plotted against time as shown in Figure 1.

The oxygen transmission rate was calculated as previously published by Holland et al (1980) . The following equation was used: OTR = (A₀ - A_t) 1066 / T where: OTR is cubic centimeter of oxygen per m² of film per 24 hours per atmosphere of pressure difference; A₀ is the original absorbance A is the absorbance at time T (measured in hours) .

1066 is the conversion factor, corrected for 24 hours per day, the volume of oxygen in air being 20.9%, and the extinction coefficient of rubrene being 12,070 at 522 nm wavelength.

Results and Discussion. A typical measurement is shown in Figure 1, where the absorbance values for cast polypropylene are plotted in triplicates .The slope of the straight section is A/T = (2-0.5) / (37 - 5)/60 = 2.815 absorbance units/hour. OTR = 2.815 x 1066 = 2998 cc/m²/day/atm.

We have measured the OTR of a variety of films and compared them with published results and the measurements by the OXTRAN instrument (Tables 1 and 2) . Table 1 shows that the device of the invention works for low barrier films where OXTRAN is not readily usable. In this table, "OTR from literature" figures were likely to have been determined by the manufacturer by gas chromatographical methods. The OXTRAN measurements were carried out by us, only specific ones (marked with **) were done in duplicates, as it was time consuming to carry them out. The measurements using the device of the invention were done in triplicates as the process was simple and the samples were small .

For medium and high barrier films (Table 2) , the device of the invention shows consistently a lower rate of transmission than that found by OXTRAN. This may be due to some system errors in either method. The error is less likely with the device of the invention due to the high conversion of oxygen diffused into the test area. On the other hand, in the OXTRAN instrument, the Coulox sensor being a fuel cell, has problems such as sensitivity of the probe or test conditions. Example 7. Preparation of a visual scale of measurement of OTR.

When a spectrophotometer is not available, OTR can still be estimated by using a graded colour scale for visual comparison with the fading colour of device of the invention.

We found that a Visual Color Scale using the colour of the device at 5 absorbance levels, being 2.2, 1.7, 1.2, 0.7 and 0.2 for rubrene, can provide an approximate value of OTR for a film. Figure 2 shows the number of days over the range of visual colour change for different values of OTR.

As an example, a device showing the change of colour from 2.2 absorbance to 1.7 in one day should have its OTR as about 20 mL/m² day at . This OTR is confirmed when the device fades to 1.2 absorbance value after 2 days.

It is possible to construct a table covering the whole range of changes of colour in minutes to many days, which give estimates of OTR for all low barrier and high barrier films .

Figure 3 shows that the visual colour scale can be used to differentiate films within a close range of OTR. Three films in triplicates are measured and shown as A, B and C sets of curves. Starting with the device around 2.2 absorbance, after 15 minutes, the set A curves show average absorbance of 1.5, corresponding to an OTR of around 3000ml/m²/day/atm. At the same time, the set B curves show an average absorbance value near to 1.1, corresponding to OTR of 3060 and the set C curves show an average absorbance value near 0.6 (equivalent to OTR around 5000) .

The results from using such a visual colour scale are considered quite adequate for most purpose because the OTR of a film can vary significantly with temperature and with some films, humidity. Accordingly, the visual colour scale is useful in commercial applications for determining whether a polymer film is a low or high barrier film.

Table 1. OTR measurement ( cc/m² .day .atm) by ADF - Low barrier films

Table 2. OTR measurement by ADF & OXTRAN 2/60 Medium and high barrier films (cc/m2. day. atm)

Claims

1. A device for detecting the permeation of oxygen across a film comprising detector means for detecting oxygen and adhesive means for adhering the device to the film, wherein the detector means are arranged for permitting the detector means to detect the permeation of oxygen across the film.

2. A device according to claim 1 wherein the detector means are arranged within the adhesive means.

3. A device according to claim 1 wherein the device is structured to permit the device, in use, to be sealed by the film to limit the function of the detecting means to detection of oxygen permeation across the film.

4. A device according to claim 1, further comprising support means for supporting the device against the film.

5. A device according to claim 4 wherein the detector means are arranged within the support means.

6. A device according to claim 1 wherein the adhesive means is capable of adhering the device to film for packaging foods, food stuffs, pharmaceuticals or horticultural products.

7. A device according to claim 6 wherein the film comprises low density polyethylene, high density polyethylene, polyvinylchloride, ethylvinylalcohol or cast polypropylene, oriented PET, nylon 20, polyvinyl dichloride/cellophane, oriented PET/ethylvinyl alcohol or a mixture thereof .

8. A device according to claim 6 wherein the adhesive means comprises an acrylate or derivative thereof.

9. A device according to claim 8 wherein the adhesive means comprises methylacrylate .

10. A device according to claim 1 wherein the detector means comprises a dye for detecting oxygen.

11. A device according to claim 10 wherein the dye is one capable of changing colour when contacted with oxygen and exposed to light.

12. A device according to claim 11 wherein the dye is rubrene .

13. A device according to claim 4 wherein the support means comprises ethyl cellulose or a compound comprising ethyl cellulose, cellulose esters or cellulose.

14. A device according to claim 11, further comprising sensitising means for improving the sensitivity of the dye for detecting oxygen across a range of wavelengths.

15. A device according to claim 14 wherein the sensitising means is tetraphenylporphine.

16. A device for detecting the permeation of oxygen across a film comprising a dye capable of changing colour when contacted with oxygen and exposed to light, the dye for detecting oxygen, and an adhesive for adhering the device to the film, wherein the dye is arranged within the adhesive for permitting the dye to detect the permeation of oxygen across the film.

17. A device for detecting the permeation of oxygen across a film, the device comprising rubrene for detecting oxygen, an adhesive comprising acrylate or a derivative thereof, the adhesive for adhering the device to the film and a support comprising cellulose or a derivative thereof, the support for supporting the device against the film, wherein the rubrene is arranged within the adhesive for permitting the rubrene to detect the permeation of oxygen across the film.

18. A film for packaging foods, food stuffs, pharmaceuticals or horticultural products, the film comprising a device according to claim 1.

19. A film according to claim 18, wherein the film comprises low density polyethylene, high density polyethylene, polyvinylchloride, ethylvinylalcohol or cast polypropylene, oriented PET, nylon 20, polyvinyl dichloride/cellophane, oriented PET/ethylvinyl alcohol or a mixture thereof .

20. A process for producing a device for detecting the permeation of oxygen across a film comprising contacting an adhesive with a solvent comprising detector means for detecting the permeation of oxygen across a film, to form an adhesive diffused with the solvent, and drying the adhesive diffused with the solvent to remove the solvent, to produce the device.

21. A process according to claim 20 wherein the solvent is one capable of diffusing through an adhesive comprising an acrylate or acrylate derivative, to form the adhesive diffused with the solvent.

22. A process according to claim 21 wherein the solvent comprises toluene, tetralin or a derivative or mixture thereof .

23. A process according to claim 20 wherein the detector means is a dye for detecting oxygen.

24. A process according to claim 23 wherein the dye is one capable of changing colour when contacted with oxygen and exposed to light.

25. A process according to claim 24 wherein the dye is rubrene .

26. A process according to claim 24 wherein the solvent further comprises sensitising means for improving the sensitivity of the dye across a range of wavelengths.

27. A process according to claim 26 wherein the sensitising means is tetraphenylporphine.

28. A process according to claim 20 comprising the further step of fixing support means to the device, the support means for supporting the device against a film.

29. A process for producing a device for detecting the permeation of oxygen across a film comprising contacting an adhesive comprising acrylate or a derivative thereof, with solvent selected from the group consisting of toluene, tetralin or a derivative or mixture thereof, the solvent comprising rubrene for detecting the permeation of oxygen across a film, to form an adhesive diffused with the solvent, and drying the adhesive diffused with the solvent to remove the solvent, to produce the device.

30. A device produced by the process of claim 20.

31. A method of detecting the permeation of oxygen across a film comprising contacting the adhesive means of a device according to claim 1 with a film to adhere the device to the film and ascertaining from the detector means for detecting oxygen whether oxygen has permeated across the film.