PETpla.net Insider 09 / 2018

INSPECTION 36 PET planet Insider Vol. 19 No. 09/18 www.petpla.net Non-destructive testing of oxygen transmission in PET bottles The study sought to determine the feasibility of achieving a shelf-life of 180 days with a total oxygen concen- tration (dissolved and headspace) of less than 1 ppm. The non-invasive and non-destructive technique uses opti- cal-chemical sensors that can detect 1 ppb (part per billion) of dissolved oxygen. It measures oxygen ingress continuously, under real conditions and over a long period of time. Fig. 1 Factors affecting shelf life of products in PET bottles Experiment details Measurement is based on the effect of dynamic luminescence quenching by molecular oxygen (Figure 2). Fig. 2 Principle of dynamic quenching of luminescence by molecular oxygen (1) Luminescence process in absence of oxygen (2) Deactivation of the luminescent indi- cator molecule by molecular oxygen Collision between a luminophore in its excited state and oxygen results in collisional or dynamic quenching: radiationless deactiva- tion. Energy transfer takes place from the excited indicator molecule to oxygen. The indicator molecule’s total measurable luminescence signal decreases. The oxygen meter uses a blue- green light source for excitation. An optical polymer fibre is used as signal transducer and leads the emission light to a photodiode. The technique measures the luminescence lifetime as the oxy- gen-dependent parameter. Decay time does not depend on fluctua- tions in intensity of the light source and sensitivity of the detector. It is not influenced by signal loss caused by fibre bending or by intensity changes caused by changes in the geometry of the sensor and is, to a great extent, independent of the concentration of the indicator in the sensitive layer. Photobleaching and leaching of the indicator dye and variations in the optical properties of the sample have no influence. Optical chemical sensors meas- ure in both liquid and in gaseous (headspace) areas. They perform through transparent materials up to 10 mm thick and through turbid (cloudy) packaging. Oxygen levels can be detected in parts per mil- lion (ppm) to parts per billion (ppb) ranges. Dissolved oxygen Gaseous & dissolved oxygen Measurement range 0-2mg/l (ppm) 0-56.9 μmol/l 0-5% O 2 0-41.4hPa Limit of detection (LOD) 1ppb of dissolved oxygen 0.002% oxygen Resolution ± 0.0003mg/l at 0.001mg/l ± 0.0006mg/l at 0.09mg/l ± 0.010 μmol/l at 0.03 μmol/l ± 0.020 μmol/l at 2.8 μmol/l ± 0.0007% O 2 at 0.002% O 2 ± 0.0015% O 2 at 0.2% O 2 ± 0.007hPa at 0.023hPa ± 0.015hPa at 2.0hPa Accuracy (+20 °C) ± 1ppb or 3% of the respective concentration; whichever is higher Response time < 40 s < 6 s Calibration Conventional two-point calibration in oxygen-free environment (nitrogen); second calibration value, optimally between 1 and 2% oxygen Temperature range From 0 °C to +50 °C Long-term stability 100,000 data points without drift Table 1 Technical data of the PSt6 type sensor Six months shelf life for beer and CSD Oxygen causes chemical processes that impair shelf life, flavour and quality of beer and carbonated soft drinks (CSD). To protect the product, monolayer bottles can be coated with high–barrier material on either the inner or outer surface. One or more high-barrier layers can be incorporated in a multilayer structure, and oxygen scavenger materials can be incorporated in the bottle wall. Determining the barrier improvement factor (BIF) compared to non-treated bottles requires precise measurements of oxygen permeation under real conditions. Traditional analysing techniques do not operate in real conditions and/or require destructive measurement processes. Presens Precision Sensing GmbH undertook a study using an analysing method based on an oxygen-dependent measurement of luminescence decay time.