HiBarSens: Measurement of barrier properties by tunable diode laser absorption spectroscopy (TDLAS)

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1 HiBarSens: Measurement of barrier properties by tunable diode laser absorption spectroscopy (TDLAS) H. Beese, W. Grählert, S. Kaskel, J. Grübler *, J. Koch *, K. Pietsch * Fraunhofer IWS, Winterbergstrasse 28, 1277 Dresden, Germany * SEMPA SYSTEMS GmbH Zur Wetterwarte, 119 Dresden, Germany Abstract Organic electronic products require ultra-high barrier materials and an adequate metrology for barrier quality guarantee with water vapor transmission rates (WVTR) better than 1-4. Tunable diode laser absorption spectroscopy is a selective, highly sensitive non-invasive moisture sensor that has been applied to meet these demands. Two different methods the isostatic mode (carrier gas) and the quasiisostatic mode (accumulation) are compared in one setup and the results are discussed. Thus different interference effects have been identified and there influence on ultra-low permeation measurement was reviewed. As a result a new technology with diffusion controlled transport of the permeate and consequently higher moisture concentrations at the sensor site is shown for a reliable detection of ultra-low water vapor transmission rates. Introduction Beside the mechanical protection, packaging materials for food and pharmaceuticals have another important function: They have to protect the products against atmospheric gases. Especially water vapor and oxygen are critical features concerning the quality and durability of these products. Foils with barrier coatings are applied in order to suppress the gas permeation (the gas transport through a solid object). To ensure their long-time stability electronic and optoelectronic components have the highest demands on the barrier effect of encapsulation layers. The introduction and application of new technologies like OLEDdisplays, organic solar cells or vacuum insulation panels require barrier properties which are below the detection limit of today s commercial permeation measurement techniques (Zervos 29). Tunable diode laser absorption spectroscopy (TDLAS) is a very sensitive spectroscopic technique to detect trace level of gases. Apart from the possibility to reliably detect water vapor within the ppb (parts per billion) range neither a drift of the sensor nor a hysteresis of the sensor appears due to the optical measuring system. The sensor is applicable to typical operation conditions for permeation measurement e.g. pressure, temperature and shows a long-term stable calibration. Because of these advantages TDLAS appears well suited for testing permeation properties if carefully connected to a permeation measurement system. Beside the demand on ultra-sensitive instruments permeation measurement at ultra-barrier levels require enhanced temperature stability, pure purge gas and well-conditioned setups. A major challenge is the simple and reliable sealing of the sample and test cell without damaging it. A pure adapting of existing sealing technologies won t be appropriate. Furthermore there is a lack of knowledge concerning the measurement process and the interpretation of measured results. For the correct interpretation of the test results the measurement must be understood as well as the fact that the used method is comparable to the application at all. Upcoming techniques like calcium mirror test, gas chromatographic or mass spectroscopic sensor based measurements, pressure rise methods and radiometric methods claim to reach ultra-low detection limits, but do not show consistent results. The most significant reason is that the omnipresent substance water is hard to measure at low concentrations. Due to its high stickiness the removal of water from metal surfaces or polymer material is a very time consuming process that will in consequence determine the time of permeation measurements.

2 Experimental setup The HiBarSens water vapor permeation test system uses the common permeation measurement setup, consisting of a permeation cell, divided into an upper and a lower compartment, which are separated by the barrier sample (Figure 1 right). In order to compare the isostatic (carrier gas) and the quasi isostatic (accumulation) measurement mode within one setup and to minimize errors of gas sampling, the sensor is directly placed into the lower compartment. This significantly shortens the transportation path-length of the permeate from the sample to the sensor and consequently reduces the exposed total surface area and volume which is important for the sensitivity in the quasi-isostatic mode (Beese et al.). VH1 VH2 gas inlet max. 3bar purifier mass flow controller VH3 feed gas compartment measurement cell laser laser exhaust gas VH4 permeate compartment Figure 1: Schema of the HiBarSens system for water vapor transmission rate measurement on ultra-barrier samples The lower compartment includes a White type multi path optic (White 1942) with a path length of 2.4 m within a chamber volume of 121 cm³. The laser diode spectrometer is attached to the lower compartment via a collimation optic whereas the detector is placed inside and connected via an electrical feed-through to the receiver unit. The detection principle is based on the measuring of the attenuation of the laser light intensity, caused by the excitation of the permeated water molecules. This attenuation is proportional to the moisture concentration, here the volume fraction. For the sensitive detection one of the strongest absorption lines ( = µm) in the NIR range has been chosen. To further enhance the sensitivity the laser diode spectrometer uses a wavelength modulation technique with second harmonic detection (Linnerud et al. 1998). The water vapor transmission rate in the isostatic mode can be calculated according to equation 1 with the water vapor volume fraction, the gas flow V *, the sample area A, the gas constant R, the temperature T and the molar mass of water M H2O. WVTR V p M H (1 ) A R T 2O iso In the quasi- isostatic mode the water vapor transmission rate can be calculated with the volume of the lower compartment V PC and the change of water vapor volume fraction within a certain time interval according equation 2. (1) WVTR qiso V t' PC p M A R T H2O (2) 2

3 Both equations are only valid in steady state conditions when the concentration gradient across the sample is fully applied. A further key feature of the permeation measurement system is the active sample sealing technology (Figure 1) that prevents leakages and unwanted permeation through the sample edges. Penetrating water vapor from ambient atmosphere or outgassed from the sample will be carried out by a purge gas flow in several cascades. The leak rate of the permeate compartment, regarding water vapor is comparable with metal seal performance or even better. In this manner no blank value, contributing a high level of uncertainty, needs to be considered and a previous determination of the moisture background level is not mandatorily needed for the calculation of the WVTR. The temperature of the setup can be adjusted by an external heating and cooling thermostat in a range from 1 C to C with an uncertainty of ±. K (Figure 2). temperature q / C temperature stability +/-,K : 36: 72: 18: 144: 18: time t / hh:mm Figure 2: Temperature stability of the permeation The upper compartment contains the feed gas. For a reliable supply the moisture at the feed side is generated by a two-pressure water vapor generator that is directly placed above the feed compartment. This water vapor generator provides a relative humidity between % and 9 % with an uncertainty of better ±2%. A 1 % relative humidity will be provided by a water soaked glass frit. Results and discussion The permeation measurement starts with a system purge with a pre-dried inert gas flow, by which the water of the initial gas atmosphere and the water that sticks to the walls or to the sample will be removed. Simultaneously the gradient of the moisture concentration inside the sample is formed. If the dilution is low and results in a steady-state water vapor concentration, significant above the lower detection limit, the isostatic water vapor 3

4 transmission rate can be calculated according to equation (1) and is shown in Figure 3. water vapor transmission rate WVTR / : 12: 24: 36: 48: 6: 72: time t / hh:mm Figure 3: Isostatic WVTR measurement of barrier substrate (Reuther, 38 C/9 RH%. To improve the sensitivity of the permeation measurement instrument, the purge or carrier gas flow can be reduced as shown in Figure 4. Since the water vapor concentration is not able to immediately equilibrate the step in the carrier gas flow results in a step of the calculated WVTR. In this case the WVTR approaches the steady state level from below. Water vapor transmission rate WVTR / Water vapor transmission rate of ultra barrier sample (POLO by Fraunhofer) (@ 38 C/ 9 % RH) Adjusting the gas flow 3. x 1-4 : 24: 48: 72: 96: 12: 144: Time t / hh:mm Figure 4: Isostatic WVTR measurement of ultra-barrier substrate (POLO, 38 C/9 RH% with reduction of carrier gas flow from 3 sccm to sccm at 68 h The lower limit of detection of the WVTR (LOD WVTR ) is determined by the carrier gas flow and the detection lower limit of the concentration sensor (TDLAS). The lower limit of detection of the concentration was determined with 1 ppb and is mainly caused by optical interference noise. Since independent low level permeation standards are hardly available, the calibration of the TDLAS based instrument was carried out according to a concentration measurement at the coulometric trace humidity generator (CSFG) of the German national metrology institute Physikalisch-Technischen Bundesanstalt (PTB). 4

5 water vapor volume fraction by TDLAS / ppm rel. error err / % water vapor volume fraction by CSFG / ppm 1 3 Figure : Volume fraction of water vapor measured by TDLAS resulting from the set values of the coulometric trace humidity generator (CSFG) of the national metrology institute (PTB) The lower detection limit of the isostatic TDLAS based method, using a carrier gas flow of 3 sccm or less, was determined at the 1 - level. Constant and reliable purge gas flows, much below this level, are difficult to achieve and back diffusion becomes a significant issue so that the quasi-isostatic mode was tested for these low detection limits. The quasi isostatic measurement starts after the system purge by closing the gas valves V3 and V4 (Figure 1) next to the permeate compartment. Incoming water vapor due to permeation will be accumulated inside the permeate compartment and an increasing concentration will be measured (Figure 6). A blank value of WVTR = x 1-6 can be determined using a dry feed gas compartment The subsequent increase in concentration after 78 hours results from added water vapor to the feed gas compartment. Based on this concentration increase a WVTR of 3 x 1-4 can be calculated under steady state conditions using equation (2). water vapor volume fraction / ppm system purge measurement of the blank value WVTR= x1-6. : 24: 48: 72: 96: 12: time t / hh:mm quasi-isostatic measurement WVTR= 3x1-4.x1-4 4.x1-4 3.x1-4 2.x1-4 1.x1-4 Figure 6: Typical chart of an quasi-isostatic WVTR measurement of an ultra-barrier 3 C/1 RH%. water vapor transmission rate WDD /

6 To compare the isostatic and the quasi-isostatic permeation measurement mode a PCTFE ultra-barrier film (Honeywell, TR8TR) was first measured in isostatic mode until it reached a steady state WVTR of 2.8 x 1-4 (Figure 7). Subsequently a quasi isostatic measurement of that well-conditioned sample was done. The resulting quasi-isostatic water vapor transmission rate shows a factor 4 to smaller and not constant value. Repeated measurements confirm this result. water vapor volume fraction / ppm WVTR iso WVTR q-iso WVTR iso WVTR q-iso WVTR iso WVTR q-iso WVTR iso : 24: 48: 72: 96: 12: 144: time t / hh:mm Figure 7: Consecutive WVTR-measurements in isostatic and quasi-isostatic C / 1 RH% The significant difference in the results can be explained by the adsorption of water vapor molecules at walls and within the sample. These absorbed molecules are not available to the gas atmosphere and hence for the gas concentration measurement. In consequence the quasi- isostatic WVTR is underestimated. The factor of that underestimation depends on the sample and the experimental setup as well as on the measured concentrations. This relationship can be expressed by an absorption isotherm for each test material. However this isotherm is not a priori known and therefore a correction of that effect is impossible. Figure 8 shows the underestimation factor for different samples. The essential role of that adsorption effects has been confirmed by applying a mass balance. The mass balance shows the same underestimated factor for the mass of water measured in gas phase related to the mass of water that was consequently outgassed water vapor transmission rate WVTR / 6

7 3 3 * quotient m out / m gas quotient WVTR iso / max. WVTR q-iso quotient increasing WVTR #1 #2 #3 #4 sample number Figure 8: Quotient of the outgassed mass after the quasi-isostatic measurement to the accumulated mass in the gas phase in comparison to the quotient of quasi-isostatic and isostatic measured WVTRs; * outlier # #6 To overcome the interfering effect of adsorption, a constant concentration of the permeate like that in the isostatic carrier gas techniques is mandatory. To emphasize the essential role of a constant partial pressure at the permeate compartment for a reliable WVTR measurement, the condition is expressed as isocapnic mode. To reach this isocapnic conditions with a certain lower detection of the laser diode spectroscopic sensor, a well-defined low removal capacity is needed. Therefore a new method, using the diffusion controlled removal of the permeate is applied to the originally experimental setup. According to Figure 9 the lower compartment is connected via a capillary to a concentration sink. This concentration sink can be an absorber material as well feed gas compartment LDS D 1 b b 1 e x concentration sink c purge gas Figure 9: Setup of diffusion controlled removal of the permeate to increase the gas concentration in the permeate chamber as a purge gas stream to ensure a zero concentration, or at least a constant low concentration. Since there is no purge gas flow, the permeated water vapor diffuses through the capillary to the concentration sink. The cross section of the capillary A capillary as well as the length of the capillary dx, limit the permeate transport. The amount of removed water vapor e c x 7

8 as well as the WVTR of the barrier sample can be calculated using Fick s law by measuring the concentration gradient along the capillary and a known diffusion coefficient D. A WVTR D x A capillary sample (2) Figure 1 shows a measurement using both carrier gas method and the method of diffusion limited removal of water vapor. 6 water volume fraction / ppm a) WVTR iso = 3 x 1-4 b) WVTR diff = 2.2 x1-4 c) WVTR diff = 4.2 x 1-4 d) WVTR iso = 4.7 x temperature / C - Figure 1: Measured water vapor volume fraction during a carrier gas experiment and an experiment with diffusion controlled removal of the 9 RH% A second design of that diffusion limited removal uses a membrane with known permeability to limit the transportation of the permeate. A comparison of test results using the carrier gas method and the diffusion limited removal method with capillary and membrane is shown in Figure 11. ln (WVTR / ) % r.h. WVTR=7. x 1-4 WDD( T) 19.6 g m 1 d 1 e kJ mol RT diffusion controlled setup membrane diffusion controlled setup isostatic results 1 isostatic results 2 WVTR=1.4 x / (RT / J mol -1 ) Figure 11: Arrhenius graph of WVTRs measured with diffusion controlled setup, membrane diffusion controlled setup and carrier gas 8 RH% 8

9 For the comparison of the three different measuring setups the Arrhenius plot, using different temperatures, has been applied. The water vapor transmission rates, using the diffusion limited setup, agree well with the isostatic results and therefore approve the suitability of such a method and setup for ultra-barrier measurement. Conclusion The water vapor permeation measurement system HiBarSens was designed and tested to study the water vapor transmission rate of ultra-barrier films. The instrument uses tunable diode laser absorption spectroscopy (TDLAS) as a high sensitive and non-invasive sensor. The sensor position inside the permeation compartment enables the use of three different measuring modes. The isostatic carrier gas mode is well suited for high precise measurement of ultra-barrier samples with a WVTR down to the WVTR of 1 -. The quasi isostatic accumulation mode shows WVTR down to 1-6 but has been identified with a major drawback the interfering absorption of water molecules to the inner surfaces that is not easily correctable. To overcome that interfering issue the permeation measurement instrument needs to be isocapnic (constant partial pressure at the permeate compartment). The diffusion controlled removal of the permeate, as the third mode, combines isocapnic conditions and high water vapor concentration level to measure ultra-low but reliable WVTRs. Test results with that diffusion controlled mode are showed and verified by an isostatic measurement. References Beese, Harald; Grählert, Wulf; Hopfe, Volkmar: Device and method for determining the permeation rate of at least one permeate through an element that forms a diffusion barrier. WO28/1411. Linnerud, I.; Kaspersen, P.; Jaeger, T. (1998): Gas monitoring in the process industry using diode laser spectroscopy. In: Applied Physics B: Lasers and Optics 67 (3), S White, John U. (1942): Long Optical Paths of Large Aperture. In: J. Opt. Soc. Am 32 (), S. 28. Zervos, Harry (29): Barrier Films for Flexible Electronics. Hg. v. IDTechEx Ltd. Online 9

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