Abstract:
Method and apparatus for testing ultralow moisture permeation through a sample such as a thin barrier film by exposing one surface of a sample to be tested for moisture permeation to a predetermined humidity of HTO. The HTO permeating therethrough is collected in a stream of dry gas, preferably methane, at a known flow rate, and monitored for its radioactivity content. Monitoring over a period of time and appropriate conversion allows accurate assessment of even permeation rates measured as very low fractions of a gram of water per square meter per day.

Description:
The invention relates to methods and apparatus which allow the measurement of extremely low rates of permeation of water, and more particularly to methods and apparatus for measuring an ultralow moisture permeation rate through objects such as plastic films and the like. 
     BACKGROUND OF THE INVENTION 
     With the development of better and better barrier materials, generally from plastic films, it has become desirable to be able to precisely measure the rate of permeation through such barrier materials in order to properly evaluate them. As barrier materials have improved in their resistance to moisture permeation, it has become necessary to be able to accurately measure lower and lower rates of permeation. 
     Gas permeability measuring devices have generally been known in the art, and some of these have been directed to serve the garment industry where fabrics that are highly resistant to water permeation are often desired. However, more recently, with the development of LCD&#39;s, LED&#39;s and OLED&#39;s, it has become important to develop barrier materials that have an extremely high resistance to moisture permeation and oxygen permeation. It has been shown scientifically that there is a relationship between the permeation of moisture and the permeation of oxygen through a barrier; the proportion is that, if there is a permeation of water equal to 1×10 −4  grams per unit time, for the same unit time, there will be a permeation of oxygen of about 1×10 −3 . Accordingly by measuring the moisture permeation rate, an adequate assessment can be obtained for the resistance of a particular object, such as a barrier film, to the permeation of both moisture and oxygen. 
     Because many present day products have been found to be highly sensitive to oxygen and moisture, often resulting in significant deterioration of the product, there has been a recent emphasis on developing better barrier materials. Products in the electronics fields, such as OLED&#39;s and LCD&#39;s, and certain pharmaceuticals are among products for which it is most important to resist such deterioration. The barrier materials that have been developed to protect such materials generally include multilayer composites of plastic films and thin layer inorganic materials, and the search has gone on for providing increasingly better multilayer, thin film barrier materials for this purpose. For example U.S. Pat. No. 6,413,645 entitled “Ultrabarrier Substrates” describes the problem and the search for more permeation-resistant materials. 
     In order to be able to effectively evaluate the performance of these new materials, adequate test equipment is required for detecting moisture permeation at these extremely low levels. Efforts have been made to use the amount of change in weight of a suitable desiccant in a closed container where the object closing the container has its opposite face exposed to a humid atmosphere as a measure of moisture permeation; however, the accuracy of such an apparatus has been frequently called into question. U.S. Pat. No. 4,663,969, issued May 12, 1987, discloses apparatus for testing water vapor transmission which employs a heated water bath; a solution containing a solute is employed along with an electric conductivity measuring device to measure the change in concentration, which will be indicative of moisture permeation. However, it is felt that such an apparatus is not suitable to measure extremely low rates of moisture permeation. U.S. Pat. No. 6,119,506 discloses an apparatus that is designed to allow measurement of mass transport. The flux of water vapor through a film or other object being measured is calculated by measuring results for exposure to a dry gas atmosphere, to a water-saturated atmosphere, and to atmospheres of different relative humidities; with a computer program being used to determine transmission rate for the object being tested. Humidity probes are used to provide outlet signals that are indicative of the water vapor concentration in nitrogen streams that are being caused to flow through a cell where such testing is occurring. In addition to being somewhat complicated, the apparatus is not felt to be well-suited to measuring extremely low moisture diffusion rates. 
     As a result, more accurate apparatus and methods have been sought for measurement of such ultralow permeation rates. 
     SUMMARY OF THE INVENTION 
     It has now been found that an apparatus for measuring ultralow water permeation through an object, such as a thin film, can be effectively created by utilizing tritiated water vapor (HTO). By suitably mounting the object to provide controlled access to opposite surfaces and by supplying tritiated water vapor to the upstream surface, vapor at the downstream surface can be collected and monitored to precisely determine even extremely low permeation rates through the object. The method particularly lends itself to execution by carefully controlling the humidity at the upstream surface and by creating a controlled flow of dry gas, such as nitrogen or methane, past the downstream surface. The flowing stream will collect the radioactive permeated HTO, and by causing it to flow past a radiation monitor, the moisture permeation rate can be quickly and accurately calculated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing showing apparatus for measuring ultralow water permeation through a thin film, which apparatus embodies various features of the invention; 
     FIG. 2 is a sectional view through the mounting device shown in FIG. 1; and 
     FIG. 2 a  is an enlarged view of a portion of the device of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention provides a method and apparatus for accurately measuring ultralow moisture permeation through an object, such as a thin film having a high resistance to moisture penetration. As earlier mentioned, there has been substantial development of new materials which provide high moisture and oxygen resistance for use as barriers for LCD&#39;s, LED&#39;s and OLED&#39;s which require such barrier protection to assure long term performance, particularly for the cathode components thereof, which are frequently manufactured of calcium and are particularly susceptible to degradation from moisture. 
     The apparatus shown in FIG. 1 includes a central mounting device  11  where the object for which the permeation of moisture is to be measured is appropriately mounted so that a precise surface area of it, at opposite upstream and downstream surfaces thereof, will be exposed so as to facilitate monitoring moisture permeation therethrough. Although the device  11  is designed to mount and measure permeation through a thin film, it can be understood that similar mounting devices would be constructed to handle objects of different shape and/or thickness. 
     Associated with the mounting device  11  is a system  13  for supplying controlled atmospheres to the upstream and to the downstream surfaces of the object being measured, and a system  15  for both monitoring the radioactivity of a gaseous stream exiting the device  11  and interpreting the information to calculate the moisture permeation rate through the object. 
     The illustrated mounting device  11  includes upper and lower halves or parts  17 ,  19 . These two parts interface with each other to provide a central receptacle or chamber  21 , which in the illustrated device is a shallow circular region designed to centrally mount a flat thin sample  23 , the moisture permeation of which is to be determined. Accordingly, the central receptacle  21  is formed by a pair of facing shallow circular cavities  25   a ,  25   b  provided in the under surface of the upper part  17  and the top surface of the lower part  19 . Surrounding each of these cavities is a circular groove  27  of preferably rectangular or square cross section, which groove accommodates a sealing ring  29  of resilient material that extends outward past the respective surface. Preferably sealing- or O-ring  29  of square or rectangular cross section are placed in each of these grooves  27  so that, when the upper and lower parts  17 ,  19  are clamped or otherwise pressed together, the O-rings  29  seal against a thin film object  23  the permeation of which is to be measured. As a result, the flat film then essentially splits the central receptacle  21  horizontally into a lower upstream subchamber  31  and an upper downstream subchamber  33 . 
     To align the upper and lower parts  17 ,  19  of the mounting device, one of the parts is provided with a protrusion and the opposite part is provided with a complementary receptacle to receive the protrusion. In the illustrated embodiment, the upper part  17  is provided with a circular ridge  35  which is received in a circular groove  37  provided in the upper face of the lower part  19 . The ridge  35  is preferably chamfered so as to accommodate an O-ring  39  to assure a tight fit and seal is obtained between the two halves once they are aligned as shown in FIG.  2 . One or more locator pins  40  may also be provided. The halves are pressed tightly together by the employment of a number of carriage bolts and nuts  41 , which bolts are received in a plurality of vertical passageways located about the periphery of the mounting device  11 . Other suitable methods of pressing or clamping the two halves together so that the sample  23  being tested is sealed between the facing O-rings  29  could alternatively be used. 
     The gas supply system  13  includes a subsystem  4  for supplying an HTO atmosphere to the upstream subchamber  31  and a vapor collection subsystem  45  that includes an arrangement for supplying a flow of dry gas through the downstream subchamber  33  to collect the HTO that permeates through the sample  23  being tested. 
     The HTO supply subsystem  43  includes an HTO reservoir  47  connected through a ball valve  49  to the upstream subchamber  31  that is formed in the lower half  19  of the mounting device. The ball valve  49  is bolted or otherwise suitably secured to the lower half, and the HTO reservoir  47  is connected, as by bolts  50 , in turn to the ball valve. The ball valve  49  includes the usual spherical valve member  51  and a handle  53  for rotating the spherical ball member 90 degrees from the open position shown in FIG. 2 to the closed position where communication between the HTO reservoir  47  and the upstream subchamber  31  is totally broken. 
     Any suitable source of HTO can be placed in the reservoir  47  before it is installed on the mounting device. Preferably, a crystalline salt  54  that forms a compound containing water of crystallization is used; more preferably, one that has a vapor pressure at ambient temperature which will provide a desired water partial pressure in the closed chamber comprising the reservoir, ball valve and upstream subchamber  31  is employed. Potassium chloride (KCl) is preferred, and anhydrous potassium chloride will form KCL.2HTO when exposed to HTO. For example, 0.8 g KCl and 1 cc of HTO will provide such a crystalline supply of HTO that will likely provide sufficient HTO for test purposes for as long as one year under normal test conditions and usage. 
     The upper part of  17  of the mounting device  11  contains a gas flow inlet passageway  55  leading into the upper subchamber  33  and a gas exit passageway  57  leaving an opposite region of the downstream subchamber. It is of course understood that the size of the mounting device and the subchambers can be varied as desired; however, it has been found that using a device that exposes about 100 sq. cm of a thin film sample  23  to a controlled atmosphere provides satisfactory test results and sufficient surface area so as to provide a representative test reading for thin film material designed to serve as barrier layers as well known in this art. 
     The overall gas supply system  13  includes a tank  61  of gas under pressure and the usual pressure regulator  63  to supply the gas to the mounting device at the appropriate pressure. Although various dry gases might be used, including argon, nitrogen and dry air, it has been found that methane is preferred because the molecular weight of methane is very close to the molecular weight of water, as a result of which any potential stratification in the downstream subchamber at low flow of gas therethrough is positively avoided. A test device such as this utilizing HTO, for general safety considerations, would normally be operated under a standard laboratory hood, and if methane is employed, the tank would normally also be located under the hood. If desired, a second cylinder of argon or the like might be also provided, with a 2-way valve to allow selection of either one for a particular test. 
     For example, ultradry methane at a tank pressure of 2500 psi may be fed through the pressure regulator  63  to reduce its pressure, to a suitable pressure for the testing/monitoring purposes of this invention. It is preferably passed first through a desiccant dryer  65  to remove any possible moisture that might be present. 
     The flow of methane leaving the desiccant dryer  65  enters a 4-way crossover connector  67  with one leg  69  leading through a small ball valve  71  to the gas inlet passageway  55 . A similar exit conduit  73  containing a ball valve  75  is connected to the gas exit passageway  57  and leads to a 4-way connector  77  that contains a ball valve, which ball valve always allows flow out of the 4-way connector as depicted in FIG. 1 by the arrowhead, with rotation of the valve connecting the exit to one of 3 inlets for purposes to be explained hereinafter. During normal testing, the inlet stream from conduit  73  is directed through the valve. Both of the ball valves  71 ,  75  are connected to the 4-way connectors by flexible tubing so as to allow the upper part  17  of the mounting device  11  to be removed to facilitate the removal and replacement of the sample film  23  being tested, as explained hereinafter. 
     Normally, the ball valve connector  77  will allow flow from the exit conduit  73  horizontally straight through to a monitoring chamber  79 , which is a commercial piece of equipment that is associated with a tritium monitor for monitoring the amount of radioactivity present in the permeated tritium, which emits beta particles. An outlet  81  from the radiation monitoring chamber  79  passes through a conduit network  83  that includes a desiccant dryer  85  which will remove and accumulate all HTO that was collected in the flow through the mounting device  11 . Then the methane, stripped of all its radioactivity in the desiccant dryer  85 , is vented through a suitable vent line  87 . The overall gas supply network  13  also includes a second conduit  89  leading from the pressure regulator  63  to the vent line  87  through a check valve which serves as a safety bypass should, for some unknown reason, undesired high pressure reach the downstream side of regulator. 
     Lower conduits  91   a  and  91   b  lead from the first crossover  67  and to the second crossover  77 , respectively. They are connected to a pair of ports  93   a ,  93   b  that connect to the upstream subchamber  31 . These are provided for purging the upstream subchamber of any residual HTO preparatory to changing the sample  23  that is to be tested. To prepare to change the sample film, the ball valve  49  is closed, and then a purge flow of gas is caused to sequentially flow through the upstream and downstream subchambers until the radiation monitor  79  indicates there is no longer any radioactivity present in either of these two gas streams that are exiting from the mounting device. When the ball valve connector  77  is rotated generally counterclockwise, as depicted, the methane flow from the pressure regulator  63  becomes a purge flow through the conduits  91   a, b  and the upstream subchamber  31 . Preferably after the radiation monitor  79  indicates the upstream chamber  31  has been purged, the downstream chamber  33  is purged. 
     In addition, the conduit network  83  includes a bypass conduit  79  incorporating a suitable valve which allows a purge flow of dry gas to be directed through the network downstream of the radiation chamber  79  through the dryer  85 , and then back through the radiation chamber; such flow pattern may be used generally before replacement of conduits or components is undertaken in this region to assure that there is no residual radioactivity in the system itself or to ascertain that the dryer  85  is effectively sequestering all of the HTO. By rotating the valve in the connector  77  clockwise, as depicted in FIG. 1. a side flow conduit  98  containing a suitable off/on valve is interconnected to the exit so as to route the flow exiting the dryer  85  back to the tritium monitor  79 . By opening a valve in a second bypass conduit  99 , this flow path of dry gas from the conduit  97  to the vent  87  via sequential flow through the dryer  85  and then through the radiation monitor  79 , is completed. 
     The radiation monitor  79  is connected to a conversion unit  95  which can include a CPU that is programmed to make calculations from the signals received from the radioactivity monitor to determine the amount of HTO collected during a given period of time for a known flow of methane gas. From such readings and the knowledge of the amount of liters of gas flow and the length of time during which the test was carried out, the unit  95  is programmed to provide a readout in the form of the number of grams of water, i.e. HTO, which permeate through the sample  23  being tested, per square meter per day (or other desired unit of time) under ambient conditions. 
     As an example of the overall operation, an appropriate sized sample  23  of a barrier film to be tested is carefully installed in the mounting device  11  so that it rests upon the upper surface of the protruding square cross-section O-ring  29  in the lower half  19  of the device, as best seen in FIG. 2 a . The upper half  17  is then carefully set in place, and the carriage bolts and nuts  41  installed so as to clamp the film  23  securely between the mating O-rings  29  and to seal the central receptacle about its periphery by the O-ring  39 . The ball valve  49  is then opened so as to allow the upstream subchamber  31  to be filled with an HTO partial pressure at the surface of the sample  23  being tested. Generally, after the sample  23  is installed, a period of about an hour is allowed to pass to permit the HTO to generally saturate the film sample. During this time, a slow flow of dry methane is allowed to pass through the downstream subchamber  33  until it is noticed that some radiation is being detected. After a further short period of time, a timer in the conversion unit  95  is activated, and the actual test begun with a standard flow of methane, for example, about 1 liter per hour of dry methane, being caused to flow through the downstream subchamber  33  and then through the radiation monitor  79 . If desired a volumetric flow monitor (not shown) may be included to assure precision; however, such should not be necessary, as small variations can be tolerated. As previously indicated, the methane/HTO leaving the radiation monitor  79  passes through the final desiccant dryer  85 , which absorbs all the collected HTO exiting the radiation monitor and allows only totally dry, non-radioactive methane to flow out the vent  87 . The signals generated at the radiation monitor  79  for the time of the test are continuously fed to the conversion unit  95  which is programmed to calculate a moisture permeation rate in desired terms, as for example, grams of water per square meter of surface area per day. 
     As previously mentioned, when the test has been satisfactorily completed, the ball valve  49  is closed, and the upper and lower subchambers  33 ,  31  are totally purged of HTO by flowing dry methane through both chambers until no radiation is still being detected. Then the mounting device  11  is opened, and the sample  23  is removed and replaced with the next one to be tested. 
     Although the invention has been described with regard to certain preferred embodiments which constitute the best mode presently known to the inventors to carry out the invention, it should be understood that various changes and modifications as would be obvious to one having ordinary skill in this art can be made without departing from the scope of the invention which is defined by the claims that are appended hereto. Even though the working example is directed to testing improved barrier materials suitable for the formation of a flexible OLED or the like, it should be understood that other materials may alternatively be tested by appropriately altering the mounting device. Disclosures of all previously enumerated U.S. patents are expressly incorporated herein by reference. Particular features of the invention are enumerated in the claims appended hereto.