Abstract:
The present invention is directed to systems and methods which utilize a cavity ringdown spectroscopy (CRDS) technique for measuring vapor transmission rate through a barrier film by first allowing the vapor to accumulate over a period of time after a steady state condition has been achieved. In this manner, in one embodiment, water permeation through a plastic film, even at very low permeability rates, can be accumulated over time. The accumulated water vapor is then measured and a calculation made as to permeation per unit of time. One main advantage of the accumulated method is that minimum vapor detection is limited only by the ability to produce a good seal around the edges of the plastic film in the sample cell.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of commonly owned U.S. Provisional Application Ser. No. 61/381,827, filed on Sep. 10, 2010 and entitled “SYSTEMS AND METHODS FOR PERMEABILITY TESTING OF BARRIER FILMS USING OPTICAL CAVITY LIGHT DECAY TIMES,” the disclosure of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to measuring systems and more specifically to systems and methods for permeation rate testing of barrier films. Even more specifically, this disclosure relates to water vapor permeation rate testing of plastic barrier films with the testing occurring after a steady state has been achieved. 
       BACKGROUND OF THE INVENTION 
       [0003]    One important characteristic of certain materials, such as plastic barrier films, is the degree to which certain substances, such as gas and vapors, permeate the materials. In certain applications, such as used in photovoltaic devices, low permeability to water vapor results in longer lifetimes and thus contributes to lower cost and better reliability. Water vapor transmission rate (WVTR) is a widely used measurement for determining the barrier properties of a plastic film. It is a measure of the amount of water vapor that can permeate through a certain area of a film over a certain period of time. WVTR is one of the key properties in protective films for photovoltaic, organic light emitting diodes (OLED) and other electronic devices. 
         [0004]    In the above-identified provisional patent application there is discloses a system and method that uses cavity ringdown spectroscopy (CRDS) to measure water vapor permeation rate of plastic barrier films. In operation, the cavity ringdown system is arranged on a “flow” detection configuration in which high purity carrier gas is used to transport the water vapor that has permeated through a plastic film towards the CRDS detection system. One advantage of CRDS detection resides in the very long interaction optical path length that allows the detection of very small amounts of water vapor by means of optical absorption. 
         [0005]    In essence, the CRDS system will probe the water content present in a given gas flow by measuring the decay time in an optical cavity that uses a laser operating around 1392.5 nm (others resonant wavelengths can also be used, for instance mid-infrared lasers can increase the sensitivity of the CRDS since water exhibits a higher absorption strength in this spectral region) which is resonant to one of the water peak absorption bands. For a given permeation rate, the amount of water detected by the CRDS is a function of both pressure and flow rate. Pressure affects the reading due to the so called Dicke broadening in which the carrying gas will broaden the water absorption peak due to collisional interaction. Flow rate affects the CRDS reading by changing the water content of the carrying gas, the higher the flow rate the lower the water content and therefore the lower the CRDS reading. 
         [0006]    Water vapor transmission is usually reported in g/(m 2 -day) and the CRDS reports the permeation in terms of part per billion per volume (ppb v ). Two approaches can be followed to convert ppb v  to g/(m 2 -day): the first approach is a mathematical description that relates ppb v  and g/(m 2 -day). The second approach relies on measuring a group of samples with known permeation rates in g/(m 2 -day) and building a calibration curve using the readings from the CRDS unit. 
         [0007]    Under the best operational conditions, the flow method discussed above is limited by the lowest stable reading in the CRDS system which is around 0.4 ppb v  for one particular commercial instrument (other detection limits are possible with a long cavity or with other design changes). At a flow rate of 10 sccm, and a pressure of 10 psi the flow method can comfortably detect down to 5×10 −5  g/(m 2 -day) of water vapor. However, plastic films used in some current and future applications (for instance in organic light emitting diodes (OLEDs)) must exhibit permeation rate lower than 1×10 −6  g/(m 2 -day). 
         [0008]    Therefore, there exists a need for a gas permeation measurement technique having the ability to analyze for a specific molecule such as water at permeation rates in the range of 1×10 −6  g/(m 2 -day). 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention is directed to systems and methods which utilize a wavelength-tuned cavity ringdown spectroscopy (CRDS) technique for measuring vapor transmission rate through a barrier film by first allowing the vapor to accumulate over a period of time after a steady state condition has been achieved. In this manner, in one embodiment, water permeation through a plastic film, even at very low permeation rates, can be accumulated over time. The accumulated water vapor is then measured and a calculation made as to permeation per unit of time. One main advantage of the accumulated method is that minimum vapor detection is limited only by the ability to produce a good seal around the edges of the plastic film in the sample cell. 
         [0010]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
           [0012]      FIG. 1  shows one embodiment of a diagram of a permeation cell with CRDS apparatus; 
           [0013]      FIG. 2A  shows another embodiment of the concepts discussed with respect to  FIG. 1 ; 
           [0014]      FIG. 2B  shows the decay curve using the apparatus of  FIG. 2A ; 
           [0015]      FIG. 3  shows the effects of both pressure and flow rate on CRDS measurements; 
           [0016]      FIG. 4  shows a calibration curve using films with known permeation; 
           [0017]      FIG. 5  shows one embodiment of a schematic diagram of an accumulation system; and 
           [0018]      FIG. 6  shows one embodiment of a method of operation in accordance with aspects of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1  shows one embodiment  10  of a diagram of a permeation cell with CRDS apparatus. The operation of  FIG. 1  is explained in more detail in the above-identified provisional application in that drying agent  11  (in this example high purity gas N 2  or any other carrier gas) along with dryer  12  serves to dry the system. Water (or another substance such as oxygen or carbon dioxide, or other vapors) may be passed to the wet side of film  18  or a volume of moisture may be contained on the wet side of the chamber in chamber  13 - 2 . This can be accomplished, for example, by using a wet sponge. Dry air (or other transport mechanism) passes through chamber  13 - 1  and then the vapor or other analyte that permeates through film  18  from wet side  13 - 2  to dry side  13 - 1  is passed into CRDS  14  and the amount of vapor is measured. Flow meters  15 ,  16  and  17  keep track of the respective vapor flows. 
         [0020]    As discussed, the advantages of using the CRDS for WVTR measurement resides in the very long interaction path lengths through the water vapor volume which enhance sensitivity compared to non-dispersive spectroscopy techniques such as Fourier Transform Infrared Spectroscopy. Compared to a system which measures light intensity, the CRDS measures decay time with a very long interaction optical path. 
         [0021]      FIG. 2A  shows another embodiment  20  of the concepts discussed herein. In this embodiment, the light wavelength is tuned to match the water vapor absorption (around 1392.5 nm). Light within the chamber forms an optical cavity by use of high reflective minors  27  and  27 ′ in the well-known manner. The system uses fast electronics to measure the decay time within optical cavity  23  as the light leaving the chamber impacts upon detector  26 . This then allows for the calculation of the water contents in ppb v  using the set of equations presented in table 1.  FIG. 2B  shows a representative decay curve that may be obtained from the system of  FIG. 2A . 
         [0022]    Note that in some situations, as discussed above, different analyst (vapors) may be present and the laser (or other collimated energy source) can be frequency tuned to resonate with a selected analyte. This tuning can be changed from time to time (even during the measurement of a given sample) to allow the system to provide measurements for different vapors, if desired. 
         [0023]    Useful equations used to determine water vapor content through CRDS: 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Optical equations in Cavity Ringdown Spectroscopy 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 First Measurement: 
                 
                   
                     
                       
                         
                           τ 
                           zero 
                         
                         = 
                         
                           
                             d 
                             
                               c 
                                
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   R 
                                 
                                 ) 
                               
                             
                           
                            
                           No 
                            
                           
                               
                           
                            
                           gas 
                            
                           
                               
                           
                            
                           in 
                            
                           
                               
                           
                            
                           cavity 
                         
                       
                     
                   
                 
               
               
                   
               
               
                 Second Measurement: 
                 
                   
                     
                       
                         
                           τ 
                            
                           
                             ( 
                             v 
                             ) 
                           
                         
                         = 
                         
                           
                             d 
                             
                               c 
                                
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   R 
                                   + 
                                   
                                     
                                       σ 
                                        
                                       
                                         ( 
                                         v 
                                         ) 
                                       
                                     
                                      
                                     
                                         
                                     
                                      
                                     N 
                                      
                                     
                                         
                                     
                                      
                                     d 
                                   
                                 
                                 ) 
                               
                             
                           
                            
                           Gas 
                            
                           
                               
                           
                            
                           in 
                            
                           
                               
                           
                            
                           cavity 
                         
                       
                     
                   
                 
               
               
                   
               
               
                 Calculate Content: 
                 
                   
                     
                       
                         N 
                         = 
                         
                           
                             1 
                             
                               c 
                                
                               
                                   
                               
                                
                               
                                 σ 
                                  
                                 
                                   ( 
                                   v 
                                   ) 
                                 
                               
                             
                           
                            
                           
                             ( 
                             
                               
                                 1 
                                 
                                   τ 
                                    
                                   
                                     ( 
                                     v 
                                     ) 
                                   
                                 
                               
                               - 
                               
                                 1 
                                 
                                   τ 
                                   zero 
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                   
               
               
                 c—speed of light 
               
               
                 d—cell length 
               
               
                 R—reflectivity of mirror 
               
               
                 N—molecular density (content) 
               
               
                 σ—absorption cross section 
               
               
                 τ—ring-down time 
               
               
                 v—laser frequency 
               
             
          
         
       
     
         [0024]    The vapor to be measured is input to the chamber via inlet  24  and removed via outlet  25 . Light source  21  is a laser light tuned to the desired frequency. A portion (in the example, 99%) of the light is sent to test chamber  23 , while a portion is sent to reference cell  28  for detection by detector  29 . Accurate wavelength control is preferred to ensure that the wavelength of the light source matches the specific water absorption band for a resonant condition. Therefore, the emission wavelength of the light source needs to be measured constantly. For instance, changes in temperature of the laser diode that is used as light source, can shift the emission wavelength (by modifying the effective index of refraction of the laser structure) of the laser, detuning it from the resonant condition. One way to ensure constant operation at the resonant wavelength is to add a reference cell (containing water) with detector  29  as presented in  FIG. 2A . If the wavelength coming from the laser source matches the resonant absorption of the water contained in the reference cell, no light or very little light will reach the detector, and resonant operation will be ensured. 
         [0025]      FIG. 3  shows the effects of both pressure and flow rate on CRDS measurement readings when used to measure the water vapor transmission rate (WVTR) through a plastic barrier film with a WVTR of 10 −3  g/(m 2 -day). Water vapor transmission is usually reported in g/(m 2 -day) and the CRDS reports the permeation in terms of parts per billion per volume (ppb v ). Two approaches can be followed to convert ppb v  to g/(m 2 -day). The first approach is a mathematical description that relates ppb v  and g/(m 2 -day). The second approach relies on measuring a group of samples with known permeation rates in g/(m 2 -day) and building a calibration curve using the readings from the CRDS unit. 
         [0026]      FIG. 4  shows the development of a calibration curve using the CRDS readings from samples with known permeation. Note that in order to build the calibration curve, a given pressure and flow rate must be chosen. The “flow” detection configuration develops around the best operation conditions in terms of pressure, temperature and flow rate so as to ensure the highest accuracy and lowest detection limit. 
         [0027]    Accumulation Method 
         [0028]      FIG. 5 , along with  FIG. 6  shows one embodiment  50  of a schematic diagram of an accumulation system, together with graph  500 , in accordance with the concepts of the invention. The accumulation system and method shown and described with respect to  FIG. 5  is preferably used for permeation rates from 1×10 −6  g/(m 2 -day) and lower. In the accumulation method, the water permeation of a plastic film is determined by measuring the accumulated water vapor transmission per unit of time (accumulated) instead of instantaneous measurements of the water content in a flow of carrier gas as in the flow method. 
         [0029]    In operation, at time t&lt;t 0 , the system works as in the flow method with valves  101 ,  102  and  104  open and valves  103  and  105  closed. This step is used to prepare the CRDS for measurements by mainly flowing high purity gas (He or N 2 , for example) into the system. This gas should not contain water (or any other gas) that has an absorption band overlapping water vapor (or other gas) being monitored. This is controlled by process  601  of method  60  ( FIG. 6 ). Method  60  can be achieved using code-controlled applications running one or more processors, such as on processor  530 ,  FIG. 5  or can be actuated manually. Input gas flows through value  101  into permeation cell  51  and out through valve  102  to form an accumulation volume  52  which is essentially the piping of system  50 . Where the volume fills with a gas, the excess will escape via valve  104  after passing through CRDS  53 . In the embodiment shown, the accumulation volume is understood to be the entire circulation volume contained within the system between valve  101  and valve  104 , including the CRDS, the permeation cell and optional pump  54 . This allows for monitoring the initial value before the accumulation process begins. 
         [0030]    Process  602  ( FIG. 6 ) determines when t&lt;0 ends and when t=0 process  603  begins. This would typically be based upon evaluation of the CRDS reading during the continuous flow method and the value is below or close to the lower detection limit of the CRDS the accumulation phase will begin. 
         [0031]    At t=0, input valve  101  and output valve  104  are closed and feedback valve  105  is opened. This step prevents new gas from entering or accumulated gas from leaving the now closed loop. Moisture will accumulate in this closed loop as it permeates through sample film  520  in permeation cell  51 . Between t=t 0  and t=t 1 , under control of processes  604 , water vapor is permeating form the we side through film  520  and is accumulating around the dry side of permeation cell  51 . However, it is important that the water vapor diffuses towards the CRDS detector, this can be accomplished by simple diffusion from the permeation cell having high water vapor content to the CRDS detector where the initial water vapor content is lower or by the help of option pump  54 , that can help to reach a steady state content of water vapor in volume  52  faster. Processes  604  continues until process  605  determines when time t=t 1 . This decision would typically be based upon a predetermined fixed time for accumulation (t 1 −t 0 ) or may be determined based upon the CRDS reading a desired value at t 1 . 
         [0032]    At time t=t 1 , valve  103  opens and valves  102  and  105  close. Now, as controlled by process  606 , no more moisture will accumulate since permeation cell  51  is now isolated from accumulation volume  52 . While the total accumulation volume is reduced a small amount, due to the isolation of cell  51 , the effect is not considered significant to the measurement. Preferably, this excluded volume is kept to a minimum or accounted for by calibration curves from known samples. During the accumulation period beginning at time t=t 1 , the moisture in the accumulation volume and within the cavity of the CRDS will begin to equilibrate, resulting in a steady reading of moisture content. The time to reach equilibration will depend upon the volume and length of the accumulation volume, the gas diffusivity, and moisture adsorptivity of the accumulation volume wall materials. The time may be reduced by the use of option pump  54 . 
         [0033]    Processes  606  and  607  determine when time t=t 2 . This is typically based upon numerical evidence indicating that the CRDS reading of moisture content has reached a steady value, such as the reading changing less than 1% over a 10-minute period. In one embodiment, t 2  is at least 1 hour. 
         [0034]    Once the reading is steady, the water vapor transmission rate (WVTR) can be computed under control of process  608 . Theoretically, WVTR is equal to the product of the moisture content measured at time t 2  (X H2O ) and the accumulation volume (V) divided by the product of the test film area (A) and the time of accumulation (t 1 −t 0 ). This can be represented mathematically as in the following equation: 
         [0000]    
       
         
           
             
               WVTR 
                
               
                 [ 
                 
                   g 
                   
                     
                       m 
                       2 
                     
                     - 
                     day 
                   
                 
                 ] 
               
             
             = 
             
               
                 
                   
                     X 
                     
                       
                         H 
                         2 
                       
                        
                       O 
                     
                   
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                     [ 
                     
                       g 
                       
                         m 
                         3 
                       
                     
                     ] 
                   
                 
                 * 
                 
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                     [ 
                     
                       m 
                       3 
                     
                     ] 
                   
                 
               
               
                 
                   A 
                    
                   
                     [ 
                     
                       m 
                       2 
                     
                     ] 
                   
                 
                 * 
                 
                   
                     ( 
                     
                       
                         t 
                          
                         
                             
                         
                          
                         1 
                       
                       - 
                       
                         t 
                          
                         
                             
                         
                          
                         0 
                       
                     
                     ) 
                   
                    
                   
                     [ 
                     day 
                     ] 
                   
                 
               
             
           
         
       
     
         [0000]    For example, if the measurement was conducted with the continuous flow method at pressure of 10 PSI, 10 sccm and temperature of 300K with a test film having a WVTR value of 1×10 −6  g/m 2 /day using a film area of 50 cm 2 , the test would result in a CRDS reading of only 0.3 ppb v  (or 3×10 −7  g/m 3 ) which is below the detection limit of the instrument. However, using the method (process  60 ) described above with an accumulation volume V=40 mL and an accumulation time (t 1 −t 0 )=2 hours, the resulting CRDS reading would be 10 ppb v  (or 1×10 −5  g/m 3 ) which is easily measured. Other accumulation volumes or times are possible depending upon the range of the WVTR to be measured. Note that the actual WVTR may vary slightly from the theoretical value if the walls of the accumulation volume have significant moisture adsorptivity. To help reduce the water adsorbed by the tubing walls of the accumulation volume, PTFE coatings or other highly hydrophobic materials can be used to coat the tubing walls. In such cases, the method can be calibrated by measuring films with known values of WVTR and developing a calibration curve. When time t=t 2 , measurements are completed and valves  101 ,  102  and  104  are opened and valve  103  is closed effectively returning the system to the continuous flow measurement condition. 
         [0035]    Gas flow to the wet side is not required during the accumulation measurement. The purpose of the gas flow to the wet side is to balance the pressure above and below the film during the continuous flow measurement and presumably, the pressures have already balanced above and below the test film during the continuous flow process (at time t 1 &lt;t 0 ). The gas flow to the wet side may continue or it may shut off at time t 0 . Either way does not affect the accumulation measurement so long as the pressure remains balance. 
         [0036]    Note that the above-described accumulation method can reduce the testing time of films by screening samples, for example, by measuring the slope ( 502 ,  FIG. 5 ) of permeation curve  500 , before steady state (t=t 2 ) has been reached. This then can be used to infer permeation properties of the films. 
         [0037]    Continuing in  FIG. 6 , processes  609  through  613  illustrate one embodiment of using a CRDS to pass/fail a particular film under test after the permeated gas has had time to accumulate. Note that once the vapor has accumulated, any system can be used to measure the permeability of the film to the applied substance. In this embodiment, process  60  uses a CRDS technique to measurement moisture content as described above. Process  609  determines if the WVTR is greater than a specification required for a particular application (Spec  1 ). If it is, then the film has failed the test. However, if the WVTR is not greater than the specification then process  610  determines that the film has passed. 
         [0038]    In a pass/no-pass system, the testing is finished. Optionally, even if the film fails for one purpose it might be acceptable for another purpose. In this regard, process  611  determines if the WVTR is above a second specification (Spec  2 ) required for other applications. If so, the film is discarded by process  613 . If not, process  612  allows the film to be used for other purposes. Multi-levels can be used to “grade” the film. 
         [0039]    In some embodiments, the light wavelength can be tuned to match one of the water vapor (or other measured gas) absorption bands (for instance 1392.5 nm). Light within the chamber forms an optical cavity by use of high reflective minors  27  and  27 ′ in the well-known manner. The system uses fast electronics to measure the decay time within the optical cavity as the light leaving the chamber impacts upon a detector. This then allows for the calculation of the water content in ppb v  using the set of equations presented in Table 1. 
         [0040]    Note that in some situations, different gas vapors may be measured for permeability through a substance and when this is done the laser (or other collimated energy source) can be frequency tuned to resonate with the selected vapor. This tuning can be changed from time to time (even during the measurement of a given sample) to allow the system to provide measurements for different vapors, if desired. For water vapor, tuning could be, for example, 1392.5 nm, 2900 nm, 1950 nm, and 1450 nm, other analyte vapors could be, for example, CO 2  and O 2 . For CO 2 , the tuning could be 4.3 um, 2.7 um, 2 um, 1.6 um, 1.4 um. For O 2 , the tuning could be 0.7596 um, 1.58 um, 1.27 um, 1.06 um, 0.69 um, 0.63 um. The carrier gas can be selected from the list of nitrogen, helium, argon, neon, xenon, krypton or air. 
         [0041]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.