Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 62 of 707

the beginning of the preparation the surface tension value of water can be seen first (72.8 mN/m at a bath temperature of 20°C). When adding sur- factants, the surface ten- sion decreases. Bubble pressure ten- Figure. 6a: Measurement of parameter bubble lifetime. siometers measure the sur- face tension on the basis of the differential pressure method. In this process, air is pumped through a cap- illary in the liquid to be analyzed (Fig. 5). A special sensor measures the internal pressure of the developed bubble on the capil-lary peak. The dynamic surface ten- sion is calculated by making up the difference of the maximum and minimum bubble pressure and the usage of the calibration factor k: = k • p, with p = pmax pmin The bubble pressure tensiometers are calibrated in pure water, free of surfac- tants. The calibration factor k is determined automatically by the measuring device independent of the bubble lifetime. The differential pressure method consider- ably simplifies the operation with the tensiometers because the measurement does not depend on the depth of immersion of the capillary into the liquid.2 In contrast to static methods of measuring surface tension, the dynamic measuring method allows measurement of the surfactant concentration above the critical micelle concentration (CMC), which is the standard concentration range used in water-based cleaning processes. The precondition for the mea- surement is the quantification of the relation between the process value surfac- tant concentration and the surface tension. The dynamic surface tension depends on the age of a newly created surface. The Figure 6b: Reference curve for analyzing surfactant concentration. quantitative differences of dynamic surface tension values of different surfactant concentrations depend on the surface age (bubble lifetime, see Fig. 5). The one- time determination of the optimal bubble lifetime can be processed in a laborato- ry by the following proce- dure. The AUTO-mode of the tensiometer is used to measure the surface tension of particular reference sam- ples with a known concen- tration over the complete bubble lifetime range. The optimal bubble lifetime val- ue can be found at the highest difference of the 61

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