PETpla.net Insider 11 / 2016

INSPECTION 25 PET planet Insider Vol. 17 No. 11/16 www.petpla.net the measurement device. Measuring luminescence lifetime as the oxygen dependent parameter avoids common problems with intensity based meas- urements, particularly with the decay time which can depend on fluctuations in the intensity of the light source, or by signal loss and other factors involv- ing the indicator dye. Chemical optical sensor systems allow non-invasive and non-destruc- tive measurement in containers, as the sensor is integrated inside the vessel, while the opto-electronic device is placed outside. The sen- sors are read out through the trans- parent vessel wall or through an optical window without piercing or damaging the material. The systems perform through transparent materi- als up to a thickness of 10mm and even through turbid packaging. The sensors can measure both in liquid and in gaseous (headspace) phase. This way the interior oxygen con- centration of closed, filled packaging and permeability in parts per million to parts per billion ranges can be determined. Measuring oxygen permeability PreSens offers a complete system for permeation measurement in packaging materials like PET bot- tles. The set-up for oxygen ingress measurement with chemical opti- cal sensors is very simple (fig. 2). A trace oxygen sensor spot type PSt6 can be attached directly to the inside vessel wall, if the material is trans- parent or only slightly coloured, and the bottle is closed with a standard closure. For darker materials that can act like a filter for the excitation and emission light, special transparent, oxygen tight closures were designed which contain the sensor and can be attached to PET bottles with a safety screw. This safety screw is adapted to standard bottle threads. The sensor signal is read from the outside and transferred to a trace oxygen meter with a polymer optical fibre. Single-channel but also multi-channel oxygen meters are available that allow for simultaneous characterisa- tion of several bottles. It is recommended to perform oxygen ingress measurements under realistic conditions, so reliable results can be obtained. Ideally, PET bottles filled with the final product should be measured, but for products such as beer, oxygen-free carbonated water is used to ensure the barrier properties of the bottle are accurately investi- gated. For oxygen ingress measurements PET bottles are filled inside a nitro- gen-flushed glove box, so the oxygen content in the environment is <0.1% oxygen. Nitrogen-saturated water with a dissolved oxygen concentration of <100ppb is carbonised with a pres- sure tablet, which contains sodium carbonate, sodium hydrogen carbon- ate and citric acid. In addition silver nitrate (0.2mmol/L) is added to the water to avoid growth of bacteria that might consume oxygen and falsify the measurements. The bottles are filled and sealed with the standard closure or the oxygen sensitive cap with the integrated PSt6 sensor. After filling, the bottles are removed from the glove box, put into a climate chamber and shaken for half an hour. This way the temperature of the liquid inside the bottle can adjust to the measurement temperature. As oxygen permeation is dependent on temperature and humid- ity the permeation measurements are performed in a controlled environ- ment, in these experiments 30°C and 50% relative humidity. Continuous shaking of the bottles throughout the measurements guarantees equilibrium state of headspace and liquid. Permeation measurements were performed in different PET bottle types. Test 1: Outside coated PET bottles filled with oxygen-free water In a first experiment an externally coated bottle was compared to an identical non-coated bottle over a period of 30 days. Both bottles had a weight of 38g, a filling volume of 500ml and 20ml headspace. They were filled with nitrogen-saturated water that was not carbonised. During the first 48 hours both bottle types show a high and non-linear increase in oxygen concentration (fig. 3). There are two reasons for this strong increase: first, the matrix oxygen, dis- solved in the PET bottle wall migrates into the liquid inside the bottle. The second reason is the so called “steady state” permeation where oxygen molecules move from the high concentration side (atmosphere) to the low concentrations side (product inside the bottle) through the PET bottle wall. At the beginning of the Fig. 2: Illustration of non-invasive and non-destructive measurement of oxygen ingress into PET bottles Fig. 3: Oxygen ingress into an externally LC2 coated PET bottle compared to the respective non-coated reference bottle