Contact angle measurement or test inks?

A comparative study from our laboratory on test inks

The quality when bonding, coating or printing material surfaces is substantially determined by the wettability, which in turn is closely associated with the surface free energy (SFE). The determination of the SFE is therefore an important task in quality assurance. In this regard, contact angle measurement is increasingly replacing surface testing with test inks as a measuring method. There are good reasons for this, as Dr. Thomas Willers, Manager of the KRÜSS Training Center, and his team have been able to demonstrate based on a comprehensive study. They used both methods to investigate a wealth of materials such as plastics, glass, silicon, mica and aluminum. The scientists also compared untreated and plasma-treated surfaces. Dr. Willers first presented the results at the 10th European Adhesion Conference EURADH (22nd to 25th April 2014) in Alicante.

The two sets of test inks under investigation each comprised a range of liquids with set surface tension. The ink test is based on the assumption that the SFE of the solid is equal to the surface tension of that ink which just forms a stable film on the sample. Contact angle measurements with several liquids characterize the surface more detailed by splitting the SFE into polar and dispersive interaction components. For contact angle measurements, the standard test liquids water and diiodomethane were used as part of the study. The method comparison gave different results for many of the materials tested which are understandable when the interactions are taken into account. According to the OWRK model, incomplete wetting can occur when the values of surface tension and surface energy are equal – contrary to the assumption in the ink test. Wetting is only complete when liquid and solid are also the same with regard to their polar and dispersive interaction fractions.

Knowledge of the polar fraction of the SFE is particularly important for activating surfaces using plasma or flame treatment. These polarize the surface and prepare it for contact with polar liquids. The adhesion and the interfacial tension, which can be associated with the long-term stability of a bond, can also be calculated based on the two-component model.

Our new innovative Mobile Surface Analyzer – MSA measures surface free energy with two liquids, fully automatically and mobile using our “One-Click SFE” method. The MSA doses two parallel drops with one click, followed by the direct analysis of the contact angles and the derived results of the SFE. All steps are automated and happen within a second. This makes SFE measurement with the MSA not only more meaningful, but in most cases considerably faster than an ink test. Unlike the ink test, measurements can even be carried out on vertical surfaces and overhead as well as on convex surfaces.

More information: Application report on test inks