Application report
Application report:KRÜSS AR286

Industry:

CleaningAuthors:

Maria Mihhailova, Dr. Daniel FreseDate:

10/2018

Mobile Surface Analyzer – MSA

Method:Keywords:

cleaning bath, contamination, surface free energy, sessile drop

Determine how clean surfaces are: Quickly and on the go
Optimizing residence times in cleaning baths
Time is money, and that's also the case with industrial cleaning. Surfaces need to be cleaned until they have reached thenecessary degree of cleanliness–and not longer. But how can the cleanliness of a surface be measured reliably? Wecleaned steel plates of varying lengths in an ultrasonic bath and determined the respective surface free energies (SFE).
What can't be seen by the naked eye was able to be made visible in this way: After 120 seconds, the optimum degree ofcleaning was reached; any longer waiting time had no additional influence on the SFE of the steel plate samples. Once
optimized, the success of the cleaning process could be checked in an instant with one-click measurements with either an“OK” or “not OK” result.

Background
Making contamination measurable
Basic cleaning of surfaces is a requirement for a
multitude of processes, such as coating, painting, andbonding. If contamination remains on the surface, thisresults in faulty coating, visual paint defects and poor
adhesion.
There are various methods available to measure thecleanliness of a surface, which could for example
be based on fluorescence or even the use of X-rays.
The disadvantage of these methods include limiting
the verification of certain types of contamination,
the influence of the distance of the sensor on theresults or even simply the high costs of the instrumentscombined with laborious measurements costs.
One method that reacts with a high degree of sensitivity
even to minimal contamination is the contact angle
measurement. According to Young's Equation, the shapea drop will take on a surface is determined based on theKRÜSS GmbH | Borsteler Chaussee 85 | 22453 Hamburg | Germany | www.kruss-scientific.com

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surface tension of the liquid applied and the SFE of thesample surface (Fig. 1 A). Insoluble contamination(grease, oils, etc.) create a surface with low SFE, whichleads to a larger contact angle (Fig. 1 B). Contamination
active on the interface (surfactants, etc.) dissolve in drops
and lower the surface tension, which is expressed in atime-dependent lowering of the contact angle (Fig. 1 C).

Table 1: Test liquids used and their surface tension σ taking
dispersive (d) and polar parts (p) into account based onOWRK.

Test liquid

σd [mN/m]

σp [mN/m]

Water

21.8

51.0

Diiodmethane

50.8

0.0

Results
Fig. 2 shows example steel plate samples after increasingcleaning duration in the ultrasonic bath. Visually an effectcan only be seen after very long cleaning periods.

Fig. 2: Photo of steel plates after cleaning with increasingcleaning duration from left to right

In the samples examined, the water contact angle
dropped from the original 90° for the uncleaned samples
to down to 13° for the cleaned samples. This indicatessoiling that is not water-soluble.

Fig. 1: Schematic representation of the wetting of a solid
body using liquid drops and the influenceof contamination on the contact angle

This method has the advantage of providing you withposition-related information about the type of impurity(soluble/insoluble) and is easy and quick to apply. It is
non-invasive and can be performed anywhere with amobile device.

Fig. 3 shows the SFE results of the steel plate samples
examined. Here, even after only 10 seconds of cleaning, asignificant increase in the SFE was observed, particularly
for the polar part. After a cleaning time of 120 seconds,
the maximum SFE is reached, and an additional increaseof the cleaning duration has no additional effect. Basedon this, a time of 120 seconds can be defined as theoptimum cleaning duration.

Testing
Steel plates (see Fig. 2) were cleaned at various treatment
durations in an ultrasonic bath (made by Elma, Elmasonic
S10H, frequency 50/60 kHz) filled with a 3% Mucasol®
solution. After cleaning, the SFE of the steel plate wasdetermined according to the OWRK calculation method
using the Mobile Surface Analyzer – MSA [1], [2], [3].
Depending on the treatment duration, 4 steel plates wereexamined, and five drops of 2 µL per test liquid and plate
were applied and measured. Both the cleaning and themeasurements were performed at 30 °C. The liquidparameters are listed in Table 1.

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Fig. 3: SFE and its polar partand disperse part according
to OWRK based on thecleaning duration at 0, 10, 60,
120, 300 and 600 seconds forfour steel plate samples foreach cleaning duration. Theerror bars correspond to thestandard deviation of fiveapplied drops per test liquidand sample each.

Based on these results, a simple procedure can then be
defined for routine one-click measurement in quality
control which can be used as an OK criteria, for example
a polar part of the SFE >30 mN/mFig. 4: Automation program for the ADVANCE software for
the Mobile Surface Analyzer – MSA with a defined validity
limit of 30 mN/m for the polar part of the SFE

If this is lower, then the user will get a “Not OK” result
and can take appropriate measures.

Fig. 5: Software message from ADVANCE when the validity
limit is not reachedSummary
In the example of steel plate samples, it was shown howlightning-fast, simple and also mobile contact angle
measurements were able to optimize the treatment timein a cleaning bath. Contamination that is invisible to thenaked eye can be verified on the samples based on themeasurement parameters of the surface free energy andcan be identified as water-insoluble contamination (forexample, oils). After the cleaning duration has been
optimized, the success of the cleaning can easily bechecked with a simple click as a part of process control.

Bibliography[1]D. H. Kaelble, Dispersion-Polar Surface Tension
Properties of Organic Solids. In: J. Adhesion 2(1970), P. 66-81.

[2]

D. Owens; R. Wendt, Estimation of the Surface Free
Energy of Polymers. In: J. Appl. Polym. Sci. 13 (1969),
P. 1741-1747.

[3]

W. Rabel, Einige Aspekte der Benetzungstheorie undihre Anwendung auf die Untersuchung undVeränderung der Oberflächeneigenschaften vonPolymeren. In: Farbe und Lack 77, 10 (1971),
P. 997-1005.

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