Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 61 of 707

There is an optimization Figure 5: Differential pressure method for measuring surface tension. potential regarding eco- nomical and ecological points of view. Regulation of the essential process parameters for cleaning quality (i.e., the cleaning agent concentration and a consumption-oriented dos- ing) only occurs in single cases. The reasons for these problems are evident in the high volume of different yet interrelated parameters in the cleaning process and a missing systematic prepa- ration and generalization of several individual expe- riences in order to realize high process reliability. These problems can be solved by monitoring the process conditions with the help of simple process parame- ters. Therefore, it is re-quired to have a practicable measuring technology that can be used for the analysis as well as for continuous or discrete monitoring of the processes. The surfactant concentration's influence on cleaning power is measured with the help of the dynamic surface tension. For monitoring and dosing the builders it is possible to control their concentration by measuring the conductivity or by using acid–base titration. A continuous monitoring of the cleaning agent con- centration allows a consumption-oriented dosing and, thus, an economic and eco- logical optimization of the cleaning processes. Optimization contains: • a decrease in costs of using cleaning agents • avoiding accumulation of cleaning agent components in subsequent rinsing baths • an extension of bath life • a decrease in costs of water and wastewater management Monitoring the surfactant concentration and dosing according to con- sumption are possible due to the parameter surface tension. Therefore, bubble pressure tensiometers are used to measure the concentration of free surface active agents, as shown in Figure 3. Free surfactants that are not bound to oil, con- tamination, and the metal surface attach to the developed bubble at the capillary peak. In cleaning processes, only these free surfactants are available to remove con- tamination and oil from the metal surface. The current cleaning power can be con- trolled using this method. Monitoring the surfactant concentration by measuring the surface tension of process liquids in cleaning baths allows a proper use of the surfactant com- ponents (wetting agent). Required extra dosages can be recognized using the resulting measurements. In order to visualize the monitoring procedure, Figure 4 shows the result of continuous measurement in a cleaning bath. At 60

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