Development of a New, Highly Sensitive Sensor for Permeation Measurement

Optical Sensors Used for Permeation Testing of High Barrier Materials

A new measurement system for testing synthetic materials on oxygen permeability was developed. Unlike the currently used systems with electrochemical or barometrical sensors it was designed for optical measurement. The Fibox 4 trace together with chemical optical sensors was applied to detect trace oxygen concentrations and validate the new measurement system. Apart from an already available sensor type different other types of sensor material were tested to develop an oxygen sensor with an improved limit of detection for high barrier permeation measurements. This sensor type - PSt9 - together with the measurement cells seems to be a promising solution for easy, cost effective permeation measurements in technical applications.

A series of 16 measurement cells with different volumes were used in these experiments. The materials that should be tested for their barrier properties - film samples - were fixed between the upper and lower chamber of the measurement cell. Each chamber had two gas connectors, so first both chambers could be flushed with nitrogen (zero value). Then the lower chamber was flushed with oxygen. A chemical optical sensor spot (PreSens, Germany) was applied in the upper chamber to conduct permeation measurements. The sensor type PSt6 consists of an oxygen-sensitive dye embedded in a silicone matrix. The lower detection limit of a PSt6 sensor is 0.0025 % (25 ppm) oxygen in gas phase. This value is within the same range or even a level above commonly used measurement systems. Still, to conduct high barrier permeation measurements an even lower detection limit is needed. Therefore, different polymers with high oxygen permeation rate and several oxygen indicator dyes were tested to get a new and improved chemical optical sensor membrane. For validation of the measurement system the 100 µm PET film HOSTAPHAN® RN 100 was used to perform reference measurements.

Reference measurements were performed with the 100 µm PET film HOSTAPHAN® RN 100 using the PSt6 sensor type. From over 100 single measurements the oxygen permeability of the film is known to be in a range of 12.26 to 14.10 (cm³ (STP) 100 µm)/ (m² d bar) at 23 °C. All permeability values measured with the newly developed system were in this range (see Table 1). The accurate performance of the newly designed system is indicated by the low zero value and the correct measuring values of the reference films.

Measurement Range of the System

With carrier gas permeation methods it is not possible to measure films with very high permeability, since the absolute pressure difference between the upper and lower cells has to be zero. By checking permeability with optical sensors on the other hand it is possible to measure perforated and high permeable films. The highest measured permeability of a film was 2 x 10⁷ cm³ (STP)/ (m² d bar) for 30 µm PE film filled with CaCO₃.
The lowest oxygen permeation value measured with the standard PSt6 sensor was 0.01 cm³ (STP)/ (m² d bar) for a film with the layer composition PETmet / PETmet / PETmet / PE-LD with a thickness of 12 µm + 12  µm + 12 µm + 50 µm = 86 µm, usually used for encapsulation of vacuum insulation panels. In this special case it took about 4 weeks to measure a value of 0.01 cm³ (STP)/ (m² d bar). Long measurement periods are necessary for high barrier material.
Testing different sensor materials the very sensitive oxygen membrane PSt9 seemed to be the most promising to expand the measurement range of the system. The oxygen indicator dye, a transition metal complex, was integrated in a special polymer matrix displaying a high oxygen permeation rate. The PSt9 sensor has a dynamic range of 0 - 1000 ppm and a limit of detection of 0.5 ppm gaseous oxygen.