Patent Grant
8691071
Permeation instruments are used to measure the transmission rate of a target analyte, such as oxygen, carbon dioxide or water vapor, through a film of interest. Typical films subjected to permeation testing are polymeric packaging films such as those constructed from low density polyethylene (LDPE), high density polyethylene (HDPE), oriented polypropylene (OPP), polyethylene terepthalate (PET), polyvinylidene chrloride (PVTDC), etc. Typically, the film to be tested is positioned within a test chamber to sealing separate the chamber into first and second cells. The first cell (commonly referenced as the sensing cell) is flushed with an inert gas to remove any target analyte from the cell and the second cell (commonly referenced as the analyte cell) filled with a gas containing a known concentration of the target analyte. A sensor for the target analyte detects the presence of target analyte that has migrated into the first cell from the second cell through the film.
Permeation instruments typically employ a flow-through method or an accumulation method for sensing the presence of target analyte in the first cell. Briefly, the flow-through method uses an inert flushing gas to continuously pick up any target analyte that has migrated into the first cell and deliver it to a remote sensor. The accumulation method allows target analyte to build up in the first cell for an accumulation period, with the sensor either positioned within the first cell or the first cell flushed with a flushing gas after the accumulation period for delivery of accumulated target analyte to a remote sensor.
The flow through method allows virtually all sensor types to be used, but are expensive and complex systems. The accumulation method, while permitting the use of less sensitive inexpensive sensors to accurately measure permeation of a target analyte through a film even at very low transmission rates, suffers from significantly longer test times.
Coulometric sensors are sensors that follow Faraday's Law, and are generally preferred for use in permeation instruments as they provide a number of advantages, including (i) extreme accuracy, (ii) elimination of any need to calibrate, (iii) ultra-high sensitivity to analyte, (iv) high specificity for a single analyte, (v) lack of temperature sensitivity, (vi) lack of pressure sensitivity, (vii) minimal sensitivity to flow, and (viii) low cost.
Most coulometric sensors are electrochemical. Unfortunately, electrochemical sensors are susceptible to the rapid loss of electrolyte to the surrounding environment, resulting in a rapid decline in sensitivity and a short useful life. The traditional method to solve this problem is to limit environmental access to the electrolyte by covering the sensor with a selective membrane that allows essentially unrestricted passage of the target analyte while limiting the passage of water molecules, or permitting access to the electrolyte only through a limited number of capillary columns. While effective for reducing the loss of electrolyte and thereby increasing the useful life of the sensor, such covered sensors suffer from a 100 to 1000 times reduction in sensitivity and a concomitant loss of all the coulometric sensor benefits described above, with the single exception of low cost.
Such covered electrochemical sensors are not widely used with permeation instruments employing the flow-through method as they do not possess the necessary sensitivity, and are not widely used with permeation instruments employing the accumulation method because they consume some of the target analyte during the measurement process, requiring complex corrective calculations in an imperfect effort to “correct” the sensed data.
It is possible to design and construct a coulometric sensor that doesn't suffer from the rapid lose electrolyte or sensitivity, See, U.S. Pat. Nos. 4,973,395 and 5,053,116, but such sensors are prohibitively expensive for use in low-cost permeation testing instruments.
Accordingly, a substantial need exists for a permeation instrument that enjoys the benefits achievable by using a coulometric sensor without suffering from the limited useful life inherent with electrochemical sensors.
The invention is directed to a method for measuring the transmission rate of an analyte through a film. The method includes the steps of (i) separating a chamber into a first cell and a second cell with a known area of a film, (ii) flushing the first cell with an inert gas to remove any target analyte from the first cell, (iii) introducing a gas containing a known concentration of an analyte into the second cell, (iv) sealing the first cell to gas flow through the first cell, and (v) sensing any analyte in the first cell with a sensor that consumes the analyte at a rate greater than the rate at which the analyte is passing through the film, until a steady state rate of analyte consumption is measured by the sensor.
So long as the rate of consumption of analyte by the sensor is greater than the transmission rate of analyte through the film, the sensor is able to essentially measure all analyte passing through the film into the first cell. Once equilibrium is reached, the analyte transmission rate measured by the sensor will remain substantially constant, with this steady-state current, which follows Faraday's Law, equating to the analyte transmission rate of the film. Since the transmission rate of analyte through most films of interest tends to be magnitudes lower than the rate at which analyte is consumed by typical covered electrochemical sensors, the instrument may employ standard, low cost, porous or nonporous membrane-covered electrochemical analyte sensors while achieving the functionality and benefits of a coulometric sensor.
1 is an enlarged cross-sectional side view of the encircled inlet area of the gap in the measurement unit shown in
2 is an enlarged cross-sectional side view of the encircled outlet area of the gap in the measurement unit shown in
3 is an enlarged cross-sectional side view of the encircled sensor passageway area of the gap in the measurement unit shown in
1 is an enlarged cross-sectional side view of the encircled inlet area of the gap in the measurement unit shown in
2 is an enlarged cross-sectional side view of the encircled outlet area of the gap in the measurement unit shown in
3 is an enlarged cross-sectional side view of the encircled sensor passageway area of the gap in the measurement unit shown in
1 is an enlarged cross-sectional side view of the encircled humidity control window in the measurement unit shown in
1 is an enlarged cross-sectional side view of the encircled humidity control window in the measurement unit shown in
Overview
Referring generally to
The film F can be a perforated or nonperforated film F, and can be porous or nonporous with respect to the target analyte A, so long as the analyte sensor 200 can consume the target analyte A at a rate greater than the rate at which the analyte A is passing through the film F. To ensure that the analyte sensor 200 is consuming all target analyte A that is passing through the film F, the analyte sensor 200 is preferably selected so that it consumes target analyte A at a rate that is at least ten times greater, preferably twenty times greater and most preferably 100 times greater, than the rate at which the target analyte A is likely to be transmitted through the film F.
Testing System
Construction
An exemplary embodiment of a testing system 10 capable of measuring the transmission rate of an analyte A through a film F in accordance with the present invention is depicted in
An analyte sensor 200 for the target analyte A is placed in fluid communication with the upper cell 1291 for sensing the presence of target anaylte A within the upper cell 1291. Typical target analytes include oxygen, carbon dioxide, carbon monoxide and water vapor. The analyte sensor 200 may be selected from any of the wide variety of commercially available consuming sensors capable of detecting and consuming the target analyte A, with electrochemical sensors generally preferred based upon the high sensitivity and low cost of such sensors and the fact that such sensors, when employed in the present invention, follow Faraday's Law—eliminating the need to calibrate the sensor.
The analyte sensor 200 communicates via electrical leads 80 with a suitable central processing unit 50 equipped with electronic memory (not shown), and optionally but preferably attached to a monitor 60 and/or printer 70 for storing and reporting analyte A concentrations detected by the analyte sensor 200.
Use
A film F to be tested is “loaded” into the testing chamber 129 so as to sealingly separate the testing chamber 129 into an upper cell 1291 and a lower cell 1292 with a known area of the film F exposed to both cells 1291 and 1292. Shutoff valves 31a and 31b are then opened to permit the flow of inert gas through the upper cell 1291 for flushing analyte A from the upper cell 1291. After flushing, the shutoff valves 31a and 31b are closed to seal-off the upper cell 1291 from the surrounding environment. Shutoff valves 32a and 32b are then opened to permit the flow of gas containing a known concentration of analyte A into the lower cell 1292. The presence of analyte A within the upper cell 1291 is then detected and recorded by the analyte sensor 200. By ensuring that the only route through which analyte A can enter into the upper cell 1291 is through the “exposed” area of the film F, and by selecting an analyte sensor 200 that consumes analyte A faster than the analyte A is transmitted through the film F, then the rate at which the analyte sensor 200 detects analyte A, once a steady state rate is attained, can be equated directly to the analyte transmission rate for the known “exposed” area of the film F.
Measurement Unit
Construction
An exemplary embodiment of a measurement unit 100 capable of quickly and accurately measuring the transmission rate of an analyte A through a film F in accordance with the present invention is depicted in
The measurement unit 100 includes (i) a housing 110, (ii) mounting plates 120, (iii) an actuator 130, (iv) a valve 140 for controlling fluid communication with an analyte sensor 200, (v) channels 150 in the housing 110 and mounting plates 120 for directing test gas (not shown) into a lower cell 1292 in the mounting plates 120, and (vi) a flow control system (not collectively numbered) including flow control channels 170 and o-ring seals 180 for selectively opening and sealing closing an upper cell 1291 in the mounting plates 120 to fluid flow. The measurement unit 100 optionally, but preferably, also includes a humidity control system 190.
The housing 110 includes an upper section 111 and a lower section 112 that cooperatively define a retention chamber 119.
Referring to
It is generally preferred to configure the testing chamber 129 to provide an upper cell 1291 of about 1 cm3 to about 3 cm3. An upper cell 1291 larger than about 3 cm3 is too slow to respond as molecules of analyte A within the upper cell 1291 can be consumed and detected by the analyte sensor 200 only when the molecules enter the analyte sensor 200 and the upper cell 1291 relies solely upon diffusion to move molecules within the upper cell 1291. An upper cell 1291 smaller than about 1 cm3 tends to cause areas of the film F to contact with the upper surface (not numbered) of the upper mounting plate 121 during the testing period, thereby introducing error into the test results as analyte A cannot readily pass through the film F into the upper cell 1291 through these “covered” areas.
Referring to
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In order to extend the useful life of the analyte sensor 200, especially when an electrochemical sensor is employed, the passageway 173a should be closed at all times except during testing periods (i.e., only after the upper cell 1291 has been flushed with an inert gas and sealed so that the only analyte A in the upper cell 1291 is analyte A that has permeated through a test film F). Referring to
The transmission rate of analyte A through most plastic films F is sensitive to humidity, with an increase in humidity tending to result in an increase in the transmission rate. Most analyte sensors 200 are also somewhat sensitive to humidity, especially if permitted to “dry out”. Hence, in order to obtain consistent and comparable test results it is important to maintain a constant relative humidity within the testing chamber 129, especially within the closed upper cell 1291. To maintain a constant humidity within the upper cell 1291, a humidity control system 190 can be provided. A suitable humidity control system 190 is shown in
Use
The mounting plates 120 are removed from the retention chamber 129 by activating the actuator 130 to lower the actuator shaft 131 into a removal position (not shown) where the o-ring seals 180 within the gap 160 no longer contact the upper section 111 of the housing 110, and sliding the mounting plates 120 out through an open side (not numbered) of the lower section 112 of the housing 110.
The upper mounting plate 121 is then separated from the lower mounting plate 122, and a sample of the film F to be tested placed atop the lower mounting plate 122 over the test chamber 129 so as to fully engage the entire periphery of the o-ring 125 encircling the test chamber 129.
The upper mounting plate 121 is then placed back atop the lower mounting plate 122 and secured to the lower mounting plate 122 so as to sealingly clamp the film F between the plates 121 and 122, thereby sealingly separating the testing chamber 129 into an upper cell 1291 and a lower cell 1292 with a known area of the film F exposed to both cells 1291 and 1292. The “loaded” mounting plates 120 are then slid back into the retention chamber 119.
Referring to
Referring to
Referring to
With the mounting plates 120 in the “closed” position, the lower cell 1292 is flushed with a test gas containing a known concentration of target analyte A and continuously supplied with “fresh” test gas throughout the testing period to ensure that the concentration of target analyte A within the lower cell 1291 remains constant. Test gas is introduced into the lower cell 1292 by placing the inlet channel 151a in the upper section 111 of the housing 110 in fluid communication with a pressurized source of test gas 22 and allowing the test gas to flow sequentially through the inlet channel 151a in the upper section 111 of the housing 110, through that portion of the gap 160 surrounded by the larger diameter o-ring 151w, through the inlet channel 151b in the upper mounting plate 121, through the inlet channel 151c in the lower mounting plate 122, through the lower cell 1292, through the outlet channel 152c in the lower mounting plate 122, through the outlet channel 152b in the upper mounting plate 121, through that portion of the gap 160 surrounded by the larger diameter o-ring 152w, and out from the measurement unit 100 through the outlet channel 152a in the upper section 111 of the housing 110.
Target analyte A will permeate through the film F as the analyte A seeks to diffuse through the film F from a region of higher concentration (i.e., the lower cell 1292) to a region of lower concentration (i.e., the upper cell 1291). Since test gas continuously flows through the lower cell 1292 the concentration of target analyte A in the region of higher concentration remains constant throughout the relevant test period. Similarly, since the analyte sensor 200 consumes target analyte A within the upper cell 1291 faster that the target analyte A permeates through the film F, the concentration of target analyte A in the region of lower concentration also remains constant at essentially zero throughout the relevant test period.
Eventually, the system will reach a steady state condition where the rate at which analyte A is detected in the upper cell 1291 by the analyte sensor 200 and reported by the central processing unit 50 remains constant. This steady state rate equates directly to the permeation rate for the film F for the “exposed” area of the film.
A 1.0 mil thick polyethylene terephthalate mylar film is placed between the mounting plates of the permeation testing system depicted in
Oxygen within the upper cell is continuously sensed with a high-sensitivity standard electrochemical oxygen sensor covered with a porous membrane. Utilizing a reporting cycle of five (5) minutes, the transmission rate of oxygen through the film (O2TR) is calculated from the amperes sensed by the sensor each reporting cycle utilizing EQUATION A. The O2TR calculated for each reporting cycle throughout the testing period is graphically depicted in
O2TR=Amperes/(Area)(k1)(k2)(k3) (EQUATION A)
Wherein:
| Number | Name | Date | Kind |
|---|---|---|---|
| 3498110 | Brun | Mar 1970 | A |
| 3524341 | Roy | Aug 1970 | A |
| 3590634 | Pasternak et al. | Jul 1971 | A |
| 3618361 | Stephens et al. | Nov 1971 | A |
| 4563249 | Hale | Jan 1986 | A |
| 4656865 | Callan | Apr 1987 | A |
| 4815316 | Tantram | Mar 1989 | A |
| 4973395 | Mayer et al. | Nov 1990 | A |
| 5053116 | Mayer | Oct 1991 | A |
| 5131261 | Tou et al. | Jul 1992 | A |
| 5265463 | Loebig | Nov 1993 | A |
| 5390539 | Mayer | Feb 1995 | A |
| 5513515 | Mayer | May 1996 | A |
| 5591898 | Mayer | Jan 1997 | A |
| 5939617 | Lim et al. | Aug 1999 | A |
| 6009743 | Mayer | Jan 2000 | A |
| 6360588 | Ross et al. | Mar 2002 | B1 |
| 6387329 | Lewis et al. | May 2002 | B1 |
| 6766682 | Engle et al. | Jul 2004 | B2 |
| Number | Date | Country |
|---|---|---|
| 62-119433 | May 1987 | JP |
| Entry |
|---|
| Orchard, G.A.J et al., Oxygen and Water-Vapor Diffusion Through Biaxially Oriented Poly (ethylene terephthalate), Journal of Polymer Science: Part B: Polymer Physics, vol. 28, 603-621 (1990). |
| Number | Date | Country | |
|---|---|---|---|
| 20100051481 A1 | Mar 2010 | US |
