Abstract:
A non-destructive method for testing sealed packages for leakage, particularly packages having a gas-permeable portion and a non-gas-permeable portion, by applying a temporary barrier over the gas-permeable portion, wherein the temporary barrier has an aperture, and connecting the aperture to a pressure monitor, and recording the pressure drop in the sealed package, via the aperture, for a predetermined time.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for measuring the amount of gas that leaks through sealed packages. More specifically, the invention relates to gas leakage through packages that have been sealed by a cover of porous material fabricated as a mat of polyethylene fibers. This material acts as a permeable membrane to gases, but an impermeable membrane to bacteria. The membrane comprises a layer having pores which provide a tortuous path to the passage of bacteria; the material is commonly sold under the trademark designation “TYVEC.” The packages which use this material are typically semi-rigid plastic cases which protect medical devices and appliances after manufacture and before actual use. 
     The invention relates to U.S. Pat. No. 5,939,619, issued Aug. 17, 1999, entitled “Method and Apparatus for Detecting Leaks in Packaging,” and U.S. Pat. No. 6,050,133, issued Apr. 18, 2000, entitled “Method and Apparatus for Detecting Leaks in Packaging.” Both of these patents are owned by the assignee of the present invention. The present invention also relates to co-pending application entitled “Method for Measuring Gas Leakage From Sealed Packages,” Ser. No. 09/676,621, filed Oct. 2, 2000, and owned by the assignee of the present invention. The present invention permits a measurement of leakage that is wholly non-destructive to the package. 
     Gas sterilization is widely used for medical devices that must be sterile at the time of use, but cannot be subjected to sterilization by the application of high temperatures. Examples of such medical devices include cardiac pacemakers and catheter-based monitoring devices such as blood pressure probes. Typically, the medical device is sealed within a package that is permeable to gases but impermeable to bacteria. The package is then placed in a gas sterilization chamber, and a sterilizing gas such as ethylene oxide is introduced into the gas-permeable package to achieve sterilization. The sterilizing gas is then removed from the package, leaving the interior of the package sterile and non-toxic. 
     In a typical design, the medical device is placed within a thermoformed rigid plastic tray equipped with a flat sealing flange. A sheet of gas-permeable membrane, such as DuPont TYVEK® 1073-B (medical grade) brand membrane, which is available from E. I. duPont de Nemours &amp; Co., is then sealed to the sealing flange, typically by using an adhesive. The integrity of the seal is critically important to maintaining sterility. Leaks can result from incorrect setting of parameters in the automated sealing process, or from physical defects such as burrs on the face of the sealing equipment. 
     According to the known practice described in the prior art patents listed herein, a temporary barrier is formed over the gas-permeable layer, wherein the temporary barrier has an aperture with the gas-permeable layer to temporarily seal the gas-permeable layer except where the aperture is located. A tracer gas is applied under low pressure through the aperture so that it can enter into the interior chamber of the package. The entire package is placed into a larger sealed second chamber, and the concentration of tracer gas in the second chamber is measured, outside the package, to thereby measure the amount of tracer gas which has leaked through the package, presumably via leaks in the sealing flange, although leakage can also occur through pinhole defects in the plastic tray itself. 
     The methods described in the foregoing patents provide very accurate measurements and evaluations for sealed packages, under controlled conditions. However, in many applications, it is not necessary to achieve a high degree of accuracy in the leakage measurement, but is desirable to provide a quick evaluation of leakage as a production line test, to determine whether packages are leaking excessively. Excessive leakage is defined in terms of leakage beyond a predetermined range of acceptability, as a pass/fail parameter, and the precise degree of leakage does not need to by quantified. Methods for making this type of determination should produce results more quickly and at a lesser cost. 
     The present invention provides a pass/fail test which can be quickly performed at considerably less cost in terms of test equipment and testing time, because it relies on measurement of internal package gas pressure, and specifically pressure drop, caused by leakage of gas from within the package. The method of the present invention can be performed in a short time, perhaps 30-60 seconds, using very much less expensive equipment than prior art methods. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a method for testing sealed packages of the type described herein for leakage, according to a more efficient and faster process than is known in the prior art. The method requires a barrier layer to be overlaid on the gas-permeable membrane of the package, the barrier layer having a single aperture to the gas-permeable membrane. A pressure-sensitive monitor is affixed over the aperture, to monitor the pressure inside the package, and more particularly, to monitor the relative drop in pressure caused by leakage from the package. The internal package gas pressure is initially incrementally increased as a step of the method, and the incremental drop in pressure is measured as a function of time, usually by determining whether pressure drops incrementally between two desired set points within a predetermined time increment. 
     It is a principal object and advantage of the present invention to provide a method for measuring leakage through packages under faster test conditions than have heretofore been known. 
     It is another object and advantage of the present invention to provide a leakage test method which does not destroy the package being tested. 
     Other and further objects and advantages of the invention will become apparent from the following specification and claims and with reference to the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross section view of an apparatus for practicing the method of the prior art; and 
     FIG. 2 shows a cross-section view of an apparatus for practicing the method of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawing figures, it should be understood that the test apparatus shown in each figure does not form a part of the present invention, and that the drawings are not-necessarily to scale. Reference to the drawings enables a better understanding of the methods of the prior art and of the present invention, and particularly to show the principles of the present invention and its advantages over the prior art method. In the figures, like reference characters refer to the same or functionally similar parts of the respective devices illustrated in each of the figures. 
     Referring first to FIG. 1, a prior art apparatus is shown which is useful for practicing the leakage testing method of the prior art, as described in U.S. Pat. No. 6,050,133. A leak detection apparatus  20  has confined therein a sealed package  10  which is positioned for leakage testing. The sealed package  10  includes a tray  11 , a sealing flange  14  with an adhesive sealant  16  applied thereto and a gas-permeable sheet or membrane  18  affixed to the sealing flange  14  by the sealant  16 , all enclosing an interior chamber  12 . The sealant  16  has an external edge  17  which forms a sealing bead around the perimeter of the flange  14  and the perimeter of the gas-permeable sheet or membrane  18 . As described hereinbefore, the gas-permeable membrane is a porous membrane formed of a thermoplastic or paper that allows passage through the membrane of a gas but not larger particles, such as dust, bacteria, etc. In one embodiment of a sealed package that the present method is usefully applied to, the gas-permeable membrane  18  is a mat of polyolefin fibers available from duPont under the trademark TYVEK. Typically, the gas-permeable membrane  18  has a thickness in the range of between about 0.127 and 0.254 millimeters. 
     The leak detection apparatus  20  has a housing  22  which encloses a chamber  24 , of sufficient size for holding the package. The housing  22  is made from metal or other non-gas-permeable material. The housing  22  has an inlet  26  which is connected to a source  28  of carrier gas and/or a source of purge gas. A suitable purge gas can be air, nitrogen, or any other gas which can purge tracer gas quantities from chamber  24 , and a suitable carrier gas can be any gas, such as air or nitrogen, which is suitable for carrying tracer gas to a detector without affecting the tracer gas measurement capabilities of the detector. The housing  22  also has an outlet  30  which is connected to a suitable detector  34 , via a conduit  32 . The detector  34  may be a coulox detector or any other type of detector for the tracer gas being used, and may include a mass spectrometer or infrared gas detector. The conduit  32  has a vent exit  41  for selectively removing purge gas from the testing apparatus. The housing  22  has a removable cover  36  which provides a seal  35  to enclose chamber  24 . The cover  36  has an aperture  38  which is connectable via conduit  42  to a source  40  of tracer gas. 
     Prior to inserting the package  10  into chamber  24 , a temporary barrier layer  44  is applied between the cover  36  and the membrane  18 , either by adhering the temporary barrier layer  44  to the underside of cover  36  or to the top surface of membrane  18 . In either event, the layer  44  has an aperture  46  which is aligned with aperture  38  of cover  36 , for delivery of tracer gas to the exposed portion  39  of the gas-permeable membrane  18 , and all of the remaining surface area of membrane  18  is covered by layer  44 . 
     One preferred material for barrier layer  44  is a gas-impermeable tape with a thin coating of adhesive, to bond to the gas-permeable membrane  18  to seal without voids or gaps. When the tape is removed after testing, the adhesive remains with the tape, leaving little or no residue on the surface of the gas-permeable sheet  18 . 
     According to the method of the prior art, the package  10  is placed into chamber  24  of the testing apparatus  20 , and the chamber  24  is then purged of all residue gases. The purge gas is then stopped and the tracer gas is applied into conduit  42  at a pressure of up to about a maximum of 1.0 psig, until the pressure in package chamber  12  is equalized with the tracer gas pressure. The package  10  is maintained within the testing apparatus chamber  24  for a predetermined length of time to allow the concentration of tracer gas to build up within the chamber  24  if any leaks are present in the package  10 . The concentration of tracer gas is then conveyed from chamber  24  to a detector  34  where it is measured, preferably by a trace gas analyzer using a mass spectrometer or other suitable instrument. In a preferred embodiment, the mass spectrometer is a quadrupole residual gas analyzer tuned for helium, as the tracer gas. The analyzer is usually linked to a microcomputer (not shown) with a human interface such as a display panel, data storage, and programs to compare test data with control samples. However, tracer gases other than helium can be used; for example, if carbon dioxide is used as a tracer gas an infrared analyzer can be a suitable alternative to the mass spectrometer. 
     Referring to FIG. 2, a testing apparatus  80  is shown which permits the practice of the method of the present invention. The leak detection apparatus  80  comprises a sealed package  10  which is positioned for leakage testing. The sealed package  10  includes a tray  11 , a sealing flange  14  with an adhesive sealant  16  applied thereto and a gas-permeable, sheet or membrane  18  affixed to the sealing flange  14  by the sealant  16 , all enclosing an interior chamber  12 . The sealant  16  has an external edge  17  which forms a sealing bead around the perimeter of the flange  14  and the perimeter of the gas-permeable sheet or membrane  18 . As described hereinbefore, the gas-permeable membrane is a porous membrane formed of a thermoplastic or paper that allows passage through the membrane of a gas but not larger particles, such as dust, bacteria, etc. In one embodiment of a sealed package that the present method is usefully applied to, the gas-permeable membrane  18  is a mat of polyolefin fibers available from duPont under the trademark TYVEK. Typically, the gas-permeable membrane  18  has a thickness in the range of between about 0.127 and 0.254 millimeters. 
     A temporary barrier layer  44  is applied over the membrane  18 , by adhering the temporary barrier layer  44  to the top surface of membrane  18 . The layer  44  has an aperture  46  which is aligned with an aperture  48  inside a conduit  47 . All of the remaining surface area of membrane  18  is covered.by layer  44 . A seal  49  is formed about the conduit  47  to provide a complete seal of the conduit against the top surface of barrier layer  44 . 
     One preferred material for barrier layer  44  is a gas-impermeable tape with a thin coating of adhesive, to bond to the gas-permeable membrane  18  to seal without voids or gaps. When the tape is removed after testing, the adhesive remains with the tape, leaving little or no residue on the surface of the gas-permeable sheet  18 . 
     The conduit  47  is connected to a pressure sensor  50  which is capable of detecting very small pressure variations. In addition thereto the pressure sensor  50  may also include a pressurizing device which can apply a very small, incremental, positive pressure to the interior chamber  12  via the aperture  46  and the exposed portion  39  of the membrane  18 . 
     The steps of the method include placing the temporary barrier layer  44  over the membrane  18  as shown. A slight positive pressure of up to about 1.0 psig. is applied to the interior  12  of the package  10  via the pressure sensor device  50 . Finally, the conduit  47 , connected to the pressure sensor  50  transmits the interior pressure variations to the sensor  50 , and the sensor  50  is monitored to detect any pressure drops-which might occur. The observed pressure drop is compared against known measurements previously made to sealed packages, and a measurement of relative leakage is therefore possible. The process provides a very low pressure stress internal to the package, while permitting a measurement of leakage. A major advantage of the method over the prior art is that it provides a leakage measurement at very low cost, and at a very rapid measurement time. 
     The present invention may be embodied in other forms without departing from the spirit or essential attributes thereof; and it is, therefore, desired that the present embodiment be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.