Surface treatment in general
Contact angle and surface tension in the optimization of wetting and coating
The nature of material surfaces is as equally important for processing and final use as the volumetric properties. The cleanliness, surface free energy and roughness are decisive factors for adhesion when bonding, printing or coating. The wettability and adhesion behavior in the presence of dirt and water is also relevant for numerous materials and applications. Many steps in surface preparation and enhancement can be optimized by means of surface chemical methods with the help of our measuring instruments.
Process stations and baths for surface cleaning are often water-based, wherein surfactants are used to improve wetting and mobilize hydrophobic substances. Surfactants improve wetting by reducing the high surface tension of water. Quality control can be carried out based on accurate measurements of the surface tension using our semi-automatic or fully automatic Force Tensiometers – K11, K20 or K100.
During the cleaning process, hydrophobic substances are mobilized by incorporating them into surfactant molecule clusters, so-called micelles. Our Force Tensiometer – K100 measures the surfactant concentration at which micelles are formed, and in this way characterizes the efficiency of surfactants with regard to the cleaning effect.
The surfactant content of cleaning baths can be determined based on the concentration-dependent, dynamic surface tension using our bubble pressure tensiometers. For this purpose, a calibration curve is created in the laboratory using a stationary instrument such as the Bubble Pressure Tensiometer – BP100. Compliance with specified limits can then be checked on site using our mobile instrument, the Bubble Pressure Tensiometer – BP50.
Investigating wettability with the help of contact angle measurements indicates the success of surface cleaning. With a homogeneously cleaned, uniform surface, the contact angle is the same everywhere. Hydrophobically contaminated areas can be detected by an increased contact angle. Our Mobile Surface Analyzer – MSA mobile measuring instrument checks the quality on site, non-destructively and on samples of any size.
The higher the surface free energy of a solid, the better the adhesion and wetting. Materials with low surface free energy, in particular plastics, are often subjected to activating pre-treatment before bonding, coating or printing. Common treatment methods include plasma treatment, corona treatment and flame treatment as well as the chemical action of oxidizing gases such as fluorine and ozone.
Our contact angle measurement instruments determine the surface free energy, and quantify the success of the treatment process. An important partial result of these measurements is the polar fraction of the surface free energy. This describes the extent to which the plastic surface has approached the polar liquid, water, from a surface chemical point of view. As a result, the polar fraction characterizes the affinity with water-based paints and coating substances.
Additionally measuring the surface tension of a coating substance enables the work of adhesion, as a measure of the bonding, and the interfacial tension, as a parameter for the long-term stability, to be calculated. The coating quality can be specifically optimized based on the results.
In many cases, wetting and adhesion can be improved by roughening the surface. The effect of the increased roughness on wetting can be measured based on the difference between the contact angle when wetting (advancing angle) and de-wetting (receding angle). Our software-controlled dosing units for our contact angle measuring instruments enable the dispensed drop to be increased and decreased in size in a controlled manner. Alternatively, the advancing and receding angles are measured using our Force Tensiometer – K100 by immersing and removing the sample (Wilhelmy method).
Surfactants are dissolved in galvanizing baths to improve wetting. In doing so, the improved wetting is achieved by reducing the surface tension. The effect of surfactants can therefore be determined by measuring the surface tension. The surfactant content of a bath can be measured by measuring the dynamic surface tension with our Bubble Pressure Tensiometers BP100 and BP50.
Frequently, surfaces are expected to exhibit not good but poor wettability. This applies whenever surfaces are to remain clean and are not to form layers of adsorbed substances after coming into contact with other substances. Examples include the walls of buildings, painted vehicle bodies, spectacle lenses and window panes, cooking utensils and medical instruments. Such surfaces are frequently given a hydrophobic coating.
Whether a coating is optimum or not can be determined by means of the contact angle. Hydrophobic surfaces have a high water contact angle. Low adhesion is indicated by a low surface free energy of the solid.
Ultrahydrophobic coatings promote a self-cleaning effect, with which drops of water roll off the surface taking dirt particles with them (Lotus effect). This behavior can be quantified with the help of tilting devices for our optical Drop Shape Analyzers – DSA30 and DSA100 based on the roll-off angle in order to optimize such coatings. The roll-off angle is the angle of inclination of the surface at which a dispensed drop rolls or slides off the surface.
AR280: Optimizing flame treatment of polymer surfaces
A supplier for the automotive industry was confronted with a very high reject rate when applying a decorative foil on flame-activated polypropylene. After unsuccessful efforts with test inks, non-
destructive on-site measurements of the surface free energy showed were the problem came from.
AR272: Why test inks cannot tell the full truth about surface free energy
The SFE is determined for 16 materials and plasma treated polymers. The differences – which are quite large in some cases – are explainable considering that ink tests do not take the polar part of SFE into account.
AR264: Light metal, but difficult to bond
Aluminum alloys, which in vehicle manufacturing are prepared for bonding in conversion baths, are investigated. With different alloys, dwell time and temperature have different effects on the measured surface free energy.
AR256: How plastics lose their hydrophobia
The increase in surface polarity due to ozone treatment is demonstrated on POM and PBT plastics based on contact angle measurements. The results also show a different effect of duration on the surface activation.
AR253: Correlation of receding water contact angle data with moisture vapor transition rate on corona treated polypropylene packaging film
The increase in surface free energy of a PP film due to corona treatment is quantified by means of contact angle measurements. The undesirable increase in moisture permeability with excessive treatment duration correlates well with the measured receding angle.