Automatic Permeability Testing: The Challenges and Solutions Author: Alyce Hartvigsen, PBI-Dansensor A/S, Ringsted, Denmark

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1 Automatic Permeability Testing: The Challenges and Solutions Author: Alyce Hartvigsen, PBI-Dansensor A/S, Ringsted, Denmark In recent years, numerous product developments and major changes in the distribution systems within key industries such as food and pharmaceutical products have in part been made possible by the development of enhanced packaging materials. The proliferation of new ready-to-eat foods on supermarket shelves and over-the-counter medications is largely due to the convenient pouches and blister packs in which these products are supplied. As many of these products are very sensitive to chemical interactions with primarily moisture and oxygen, the barrier qualities of the packaging materials are of primary importance for assuring a sufficient shelf life of the products. One of the key indicators of the barrier properties of a packaging material is its permeability to gases and vapours. Consequently, permeability measurement is critical to the development and production of high-barrier materials, such as metallized films. However, the mechanism of permeation through polymeric materials and the quantification of this parameter can be complex and challenging. In the following paragraphs, the theory of permeation is presented, along with typical permeability measurement techniques. Finally, we present advantages of some of the employed measurement techniques and sensor technologies for automatic permeability testing of films and foils. Mechanism of Permeation Through Polymer Films Permeation of a gas or vapour through a polymer film can be represented by the following model, which is illustrated in Figure 1. The permeating gas or vapour (known as the penetrant ) dissolves or is absorbed into the surface of the film, leading to a build-up of the penetrant concentration at the film surface. The resulting concentration gradient in the film results in the diffusion of the penetrant from one side of the film to the other. This diffusion reaction is expressed mathematically using Fick s law. Once the penetrant reaches the opposite surface of the film, desorption occurs, and the gas or vapour is evolved into the atmosphere on the opposite side of the film. Figure 1- Model of Permeability Phenomenon Gas or vapour transfer through a package wall Phase 1 Package Wall Phase 2 F F p 2 c 2 c 1 p1 Sorption Diffusion (Fick s Law) Desorption The solubility and diffusion reactions that make up permeation are dependent on a number of variables, which can be grouped into Compositional variables and Environmental/geometric factors.

2 Compositional variables include chemical and physical properties of both the penetrant and the polymer, such as Chemical composition, Morphology, Penetrant concentration, and in some cases the presence of a Co-permeant. Environmental and geometric factors include Temperature, Relative humidity and packaging geometry (relationship of surface area to volume). Because of the sometimes complex interaction between these variables and difficulty in defining a general mathematical relationship to describe the permeability of a material at a given set of conditions, the best course of action is often the direct measurement of the permeation through the material at the desired conditions. An important factor in the correct measurement of permeability is the assurance that the measurement is performed under steady-state conditions (equilibrium) in the sample. Figure 2 illustrates a typical variation of penetrant flow through the film with time. Initially, the penetrant flow changes with time, and the system is at an unsteady state. Eventually, the penetrant flow becomes linear as a function of time. At this point, the permeation reaction has reached steady state, and the measurement of penetrant flow over time will be constant. The amount of time required for the attainment of steady state is dependent upon many of the same variables as the permeation rate itself: temperature, composition of polymer and penetrant, thickness of polymer, and more. As a result, the best way to ensure that steady state has been achieved during permeability measurement is to measure repeatedly and observe the trend in the results. Once the variation in the results decreases below a specified level of repeatability, the measurement is deemed complete. Q (ml at STP) Figure 2 Typical Permeation and Time Lag Curve B Steady State Unsteady State A L Time t Challenges of Permeability Measurement As mentioned earlier, the ability to measure permeability through polymer films is a key factor in the successful design of today s high-tech packages. However, there are many challenges associated with the measurement of permeability. Since permeation rates are dependent on external parameters such as temperature, relative humidity and driving force (difference in penetrant concentration across the film), the precise control of these parameters is vital to the measurement process. Because of the necessity of attaining and ensuring that the measurements are made under steady-state conditions, the permeability measurement process itself can be very time-consuming, particularly for high barrier materials. Once a permeability measurement is actually made, it is difficult to verify that the results are correct, since the limited published data for permeability measurements that exists generally applies only to common, monolayer polymers, while most packaging films consist of laminates, composites, co-extrusions, and other combinations of materials. Even if published data can be found, it is necessary to consider the

3 influence of ambient conditions such as atmospheric pressure on the results, as well as how exactly the results are expressed (for example permeability vs. permeance vs. penetrant transmission rate). All in all, it is often difficult to know for sure the correct or true permeability of a given material at a given set of conditions. This lack of documentation of results further emphasizes the importance of direct measurement of permeability in packaging development. For permeability measurement in most packaging applications, the principal penetrants of concern are normally water vapour and oxygen gas. The permeability measurement techniques for each of these components will now be covered in turn. Water Vapour Permeability Testing The permeability of water vapour is a key parameter for many film and membrane packaging applications, including food, pharmaceuticals, electronics, construction, and hygiene products. Consequently, the testing of water vapour permeability is widespread throughout the packaging and other industries. We will now focus on several methods of water vapour permeability testing. Gravimetric Testing Traditionally, the measurement of water vapour permeability across a film has been performed using the gravimetric, or cup method. In this method, a film is placed in a cylindrical holder and sealed carefully around the perimeter to prevent leakage of water vapour around the film perimeter. Consequently, any mass transfer of water vapour into or out of the receptacle can only occur via permeation through the film itself. The sealed receptacle is placed into a climate chamber, in which the temperature and relative humidity are carefully controlled at the desired levels. Depending on the application, gravimetric measurement is generally performed either using the Absorption method ( dry cup ) or Desorption method ( wet cup ). These two methods are illustrated in Figure 3. In the Absorption method, the cup is filled with a water-absorbing material (for example calcium chloride or molecular sieve), and the relative humidity outside the cup is maintained at some high level (for example, 90% RH). All water vapour that permeates through the film is absorbed into the material in the cup, resulting in a net weight gain of the cup system. The cup is weighed at regular intervals on a precision balance, and the weight is plotted as a function of time. Once the permeation rate reaches steady state, the relation between weight and time should approach linearity. From the slope of the graph, the water vapour transmission rate (WVTR) can be calculated. The Desorption method employs the same concept, except that the high humidity atmosphere is located inside the receptacle (either as liquid water or as a salt solution, depending on the relative humidity level desired). In this test, water vapour permeates through the film from the interior of the cup, leading to a net weight loss over time. The determination of WVTR is performed in the same manner as for the Absorption method. Figure 3. Gravimetric WVTR Measurement Methods Cup method Absorption ( Dry Cup ) High RH (climate chamber) Absorbent material Low RH (climate chamber) Cup method Desorption ( Wet Cup ) Water or salt solution

4 While gravimetric measurement of WVTR is generally recognised as the true method in theory, providing absolute measurements of water vapour transmission rates through films, there are a number of disadvantages of using this method in practice. Sealing of the cups must be perfect and often is done with wax. A climate chamber is needed to attain controlled temperature and humidity. Salt solutions, used to generation of humidity levels less than 100%, can be messy and corrosive. The reproducibility of gravimetric testing can be poor. Preparation of the samples can be time-consuming and labourintensive. Most importantly, gravimetric testing can take many days or even weeks to obtain a result, particularly when high-barrier materials are tested. The time delay in obtaining results makes this type of testing impractical for production quality control, where prompt feedback to allow process adjustments is the key to minimising product variation and off-specification production. Automated WVTR Testing Many of the disadvantages of the gravimetric WVTR test method can be overcome using automated permeability testing. There are several types on automatic WVTR testers commercially available today, and some companies develop their own tester design. We focus here on testing using Dynamic Relative Humidity Measurement, as described in the test standard ASTM E Figure 4 illustrates the principle of this test method. A film sample is inserted into the test chamber, where a system of O-rings seals the film at the perimeter and prevents leakage around the sample. The lower test chamber has a saturated atmosphere maintained by a small water reservoir, while the upper chamber contains a sensitive, fast-responding relative humidity sensor. The upper chamber is first dried to a defined humidity level using dry air. When the drying is complete, the air-flow is stopped and the valves closed. From that point, the chamber is a closed system, in which transmission of water vapour through the sample causes an increase in relative humidity in the upper chamber. The instrument measures the time required for the upper chamber humidity to increase from a pre-defined lower limit to a pre-defined upper limit. The measured time interval is compared to the time obtained during calibration with a standard film of known permeability, and the result is expressed as the water vapour transmission rate in g/m 2 /24 hr. The test cycle is repeated until the variation in the results reaches a defined level of repeatability, indicating that the sample has reached equilibrium. Figure 4 Automatic WVTR Testing by Dynamic Relative Humidity Measurement Sensor and Electronics dry Upper Chamber Lower Chamber: saturated atmosphere Measuring Principle : Compare time required for incremental RH increase in Upper Chamber (i.e > 10.5 % RH)

5 Advantages of Automated WVTR Tester There are a number of advantages of the Dynamic Relative Humidity Measurement method of automatic WVTR testing compared to both other automated test methods and traditional gravimetric methods. The location of the relative humidity sensor directly in the measuring chamber provides an excellent correlation to results obtained using gravimetric test methods. Testing is far less labourintensive, with fast and easy sample preparation and completely automated operation once the tester is started. Satisfactory test results can be obtained in a matter of hours, instead of days or weeks as is the case with gravimetric test methods. The tester can measure over a wide range of water vapour transmission rates, from high barrier materials to breathable membranes. The tester is self-contained, with no computer required for operation, and requires very little regular maintenance or service. As a result, the Dynamic Relative Humidity Measurement method offers a rapid, easy and dependable alternative to traditional gravimetric test methods. Oxygen Permeability Testing The permeability of polymer films to oxygen is in many applications even more critical than that of water vapour. A large percentage of food spoilage and degradation processes are either a direct result of reaction with oxygen or occur most readily in the presence of oxygen (e.g. many bacterial spoilage mechanisms). Exposure to oxygen can also cause degradation of many pharmaceutical products, electronic components, and other delicate materials. As a result, low permeability to oxygen is an important criterion for packaging in a great number of applications, and the testing of oxygen permeability is a key requirement for many package material producers as well as end-users. We will now review several methods for determining the oxygen permeability, or oxygen transmission rate (OTR) across a film. OTR - Differential Pressure Method There are two principle methods for the testing of gas permeability e.g. oxygen) across a film. In the differential pressure, or manometric method, permeation across a film is driven by a difference in absolute pressure across the film. The sample is placed in a chamber connected to a vacuum pump. The pump is used to evacuate one or both sides of the chamber. The film is exposed to a flow of oxygen gas at constant pressure (normally atmospheric pressure) on one side of the chamber. The difference in oxygen partial pressure across the film is the driving force for the permeation reaction. As the oxygen permeates through the film, the pressure in the other side of the chamber increases. By measuring the change in pressure over time and finding the slope of the change once it becomes linear, the oxygen transmission rate across the film can be determined. A typical configuration of the differential pressure testing method is shown in Figure 5. Figure 5 Gas Permeability Testing Differential Pressure Method Key to Components: 1: Transmission Cell 2: Film Sample 3: Filter Paper 4: Cell volume-control device 5: Pressure Sensor 6: Gas Feeder 7: Vacuum Pump 8: Gas Supply 9: Stop Valves

6 The differential pressure testing method has several disadvantages for oxygen permeability testing. The method has relatively poor sensitivity to very low oxygen transmission rates. In addition, pressure gauges are blind to the composition of the gas exerting the pressure. Consequently, direct measurements using the manometric testing method must be performed using pure, dry oxygen. If humidified oxygen is used in the test, it is not possible to determine how much of the pressure increase was caused by oxygen and how much by water vapour. In theory, it is possible to then measure the WVTR of the material and subtract that value from the overall transmission rate to calculate the oxygen transmission rate, but such determinations are complicated and are rarely done in practice. As a result, determination of oxygen permeability using the manometric test method is relatively infrequent, particularly for high-barrier materials. OTR - Equal Pressure Method The other principal method for gas permeability testing is called the equal pressure method. In this method, the sample is inserted into a test chamber, forming a sealed barrier between the two sides of the chamber. For the test of oxygen permeability, an oxygen-rich test gas (typically pure oxygen or air) flows through the one side of the chamber, while a nitrogen carrier gas flows on the other side of the film. Unlike the differential pressure method, the absolute pressure on both sides of the sample is equivalent (normally atmospheric pressure). The driving force for permeation is the oxygen partial pressure difference in across the sample. Oxygen permeating through the sample is then swept by the carrier gas to an oxygen sensor, which measures the concentration of oxygen in the carrier gas. Higher rates of oxygen permeation will yield higher oxygen concentrations in the carrier gas. Knowing the flow rate of the carrier gas, it is then possible to calculate the volume of oxygen that has permeated the sample, which is then expressed as the oxygen transmission rate through the sample. As with all permeability measurements, it is important to ensure that the permeation reaction has reached steady state before the final measurements are recorded. In the equal pressure method, the sample is subject to a stabilisation or conditioning period at the desired measuring conditions, in order to allow the permeation reaction to approach or reach steady state before measurement begins. Measurement is done repeatedly, and the trend in the results is observed. Once the variation between the results has decreased to an accepted tolerance level, the test concludes and the results are reported. Oxygen permeability measurement using the equal pressure method provides several key advantages over the differential pressure method. Since the sensor is specific for oxygen, it is possible to measure using both dry and humid test gas. Moreover, the availability of extremely sensitive oxygen sensors allows the measurement of materials of very low oxygen permeability (high barriers), which are required for particularly oxygen-sensitive packaging applications. As a result, the majority of automated oxygen permeability testing performed in industry employs testers based on the equal pressure method. Several types of equal pressure method oxygen permeability testers are commercially available. The primary difference between the available testers concerns the type of oxygen sensor used. We focus here on the newest available technology, employing a ceramic solid state, Zirconium Oxide sensor.

7 Zirconium Oxide Sensor The Zirconium Oxide sensor is illustrated in Figure 6. This sensor measures the partial pressure of oxygen in a gas stream. The sensor functions according to the Nernst equation, in which the sensor signal in millivolts (mv) is proportional to the logarithm of the oxygen partial pressure in the gas. The sensor reference gas is atmospheric air (20,9% O 2 ). Thus, a measurement of atmospheric air is equivalent to a sensor signal of 0 mv. Table 1 provides the approximate sensor response in mv for a range of oxygen concentrations. As the oxygen concentration in the sample decreases, the measured sensor signal increases exponentially. Figure 6 Zirconium Oxide Sensor Gas sample output Insulation Gas sample input Thermo Heating element + - Sensor EMF Table 1: Oxygen Concentration vs. Zirconium Oxide Sensor Response Oxygen Concentration, ppm Approx. Sensor Signal, mv , , ,1 305 This relation of sensor signal to oxygen concentration enables the Zirconium Oxide sensor to accurately detect both very low and high concentrations of oxygen in the carrier gas. The reproducibility of the sensor is better than 10 parts per billion (ppb) of oxygen in the measured gas. At the same time, the sensor is capable of measuring very high oxygen concentrations (up to 100% O 2 ) without any degradation of sensor function. Unlike other commonly used sensor types, the Zirconium Oxide sensor is not subject to saturation by high oxygen concentrations as does not need to be regenerated. The lifetime of the sensor is typically 5 years or more under normal operating conditions. New Oxygen Permeability Tester Figure 7 illustrates the process flow diagram for the new Oxygen Permeability Tester. The sample is inserted into the test chamber, where a specially designed O-ring system ensures a tight seal around the sample and minimizes leakage of oxygen from the atmosphere. Test gas (oxygen) flows through the upper part of the chamber, while carrier gas flows through the lower chamber to the oxygen sensor. The flow rates of both the test and carrier gases are automatically controlled and maintained constant.

8 Testing temperature is controlled within the tester. For lower testing temperatures, the tester is connected to an external water thermostat. Humidifiers on the test and carrier gas streams allow the adjustment of gas humidity within a range of 20-90% RH within certain temperature ranges. Figure 7 Oxygen Permeability Tester Process Flow Diagram O 2 F H OM N 2 F H S F: Flow regulation H: Humidifier S: Sensor OM: Gas outlet manifold One key difference between the new tester and other commercial testers concerns the measurement of existing oxygen concentration in the carrier gas. Typical nitrogen carrier gases contain up to 3-4 ppm oxygen. In some testers, this oxygen is chemically removed from the carrier gas prior to entering the sample chamber. In the new tester, it is not necessary to remove the existing oxygen from the carrier gas. Instead, the original oxygen concentration of the carrier gas is measured and is deducted from the overall oxygen content in the gas from the chamber to determine oxygen transmission through the sample. Each measuring cycle therefore consists of two separate measurement steps. In the first step, the concentration of oxygen in the carrier gas itself is measured through a gas stream bypassing the test chamber. This first result is known as the reference measurement. In the second part of the cycle, the tester measures the oxygen concentration of the carrier gas exiting the sample chamber. This gas stream contains the original oxygen in the carrier gas plus the oxygen that has permeated through the sample from the test gas side. This measurement is known as the chamber or sample measurement. The reference measurement result is then subtracted from the chamber measurement result. The difference between these results represents the oxygen that has permeated through the sample. Leak Test (Individual Zero) For measurements of high-barrier samples, it is important to normalize the result against any intrinsic leak within the tester. This procedure is accomplished by performing a leak test, or individual zero, of the sample prior to measurement. In this step, nitrogen carrier gas is routed to both the upper and lower sides of the test chamber. With equivalent gas compositions on both sides of the sample, the

9 permeability should in theory be zero. In practice, a low permeability rate is normally observed, and is the typically the result of permeation of oxygen from the atmosphere through the tester components. This leak test result is saved in memory. Following the leak test, oxygen test gas is routed to the upper chamber, and the normal permeability measurement is performed. In the measurements, the leak test result is deducted from the overall permeability result, thereby normalizing the permeability measurement to the intrinsic leak rate of the tester. Advantages of New Oxygen Permeability Tester The new oxygen permeability tester offers a number of advantages to the user. The tester is capable of measuring a broad range of oxygen permeability rates, and can thus be utilized for the measurement of both high barrier materials and breathable films. The durable Zirconium Oxide sensor has a normal operating lifetime of 5 years or more, and the exponential increase of sensor signal with decreasing oxygen concentration allows the precise measurement of small changes in oxygen content of the carrier gas. The tester is equipped with state-of-the-art humidification systems, allowing precise control of the relative humidity of the test gas and carrier gas within the measurement ranges. It is a self-contained unit that is easy to master and does not require a computer for operation. The overall flexibility of the tester makes it suitable both for quality control testing and for advanced research applications. Summary The testing of water vapour and oxygen permeability rates is an important factor in the design, development and production of many of today s advanced packaging applications. However, the theory of permeation can be complex, and permeability testing can be time-consuming and sometimes difficult to execute. Developments in automated permeability testers enable permeability testing to be accomplished more quickly and easily, allowing this testing to be integrated into product quality control regimens. The automated tester technologies presented here provide fast, dependable testing of water vapour and oxygen permeability rates across flat films, and are suitable for use in production facilities, research laboratories and testing institutions.