Adhesion in general
Measurement of wetting and adhesion when bonding or coating
As one of the main concepts in interfacial chemistry, adhesion describes the bonding together of two phases which come into contact with one another at an interfacial layer. Interfacial chemical quantities such as surface free energy and work of adhesion describe the adhesion characteristics for processes such as bonding, coating and printing. Tensiometric analyses and contact angle measurements also contribute to ensuring the quality of preparatory cleaning steps and non-stick coatings.
Questions relating to adhesion
- Adhesion when bonding
- Stability of paint, varnish and other coatings on solid surfaces
- Printing and printability
- Quality testing of cleaning steps before bonding or coating
- Development and testing of non-stick coatings
- Adhesion of particles and fibers
Contamination due to grease and other substances with low surface free energy significantly affects adhesion. For this reason, bonding or coating is generally preceded by cleaning. The cleaning agents used are usually surfactant solutions which can be characterized by means of our tensiometers. The resulting surface tension value is a measure of the wetting properties of the solution. Good wetting is necessary to ensure good contact with the whole of the material surface.
The cleaning action of surfactants is based on the incorporation of hydrophobic substances in surfactant molecule clusters, so-called micelles. The concentration at which these occur is called the critical micelle concentration (CMC). It is a measure of surfactant efficiency. Our Force Tensiometer – K100 determines the CMC fully automatically and therefore provides information for a suitable choice and dosage of surfactant.
The surfactant content of cleaning baths can be checked using our bubble pressure tensiometers. For this purpose, a calibration curve can be produced with our Bubble Pressure Tensiometer – BP100 laboratory instrument and then used on site for carrying out rapid checks with our Bubble Pressure Tensiometer – BP50 mobile instrument.
Our contact angle measuring instruments help to check whether cleaning has been successful. As the contact angle reacts very sensitively to chemical differences at the surface, it can be used to indicate the homogeneity of cleaning when measured at different positions. Our Mobile Surface Analyzer – MSA mobile instrument even enables non-destructive checking to be carried out easily on site on surfaces of any size.
Wetting in the bonding or coating process can be measured directly based on the contact angle between the substances concerned. Temperature-regulated measurements are also possible, for example investigations into the wetting of a hot melt adhesive at the processing temperature.
The roughness of the solid also has a great effect on wettability. For this reason, roughening of the surface is often included in the pre-treatment process. Combined measurements of the contact angle when wetting (advancing angle) and when de-wetting (receding angle) characterize the effect of roughness on the wetting behavior.
The surface free energy is one of the most important variables for adhesion to a solid. Solids with high surface free energy, such as metals for example, are usually easier to coat or bond. In the case of materials with low surface free energy, particularly plastics, this is frequently increased by methods such as corona, plasma or flame treatment and chemical treatment.
The surface free energy can be determined based on contact angle measurements with several liquids. The work of adhesion can be calculated as a measure of the adhesion with the help of a separate characterization of the two phases. In this way, the two phases can be optimized to give the best adhesion or coating result. The success of pre-treatment is reflected particularly by the polar fraction of the surface free energy, which is frequently increased by introducing polar groups into the surface.
The interfacial tension between material and coating substance is a measure of the inherent stability of the bond. This value should be as low as possible; if the interfacial tension is high, the coating can detach (delaminate) more easily, for example if water penetrates through small cracks.
All the stated quantities can also be determined on powders or fibers. For example, the adhesion of fibers in composite materials or the bonding of dust particles to walls can be calculated using this method.
The lowest possible adhesion is required for such items as cooking utensils and sanitary ware. The quality of non-stick coatings, for example made of PTFE or silicone, can likewise be determined based on the contact angle or the surface free energy. A high water contact angle and a low surface free energy are required here.
The roll-off angle is the definitive factor for very hydrophobic, self-cleaning surfaces with minimal adhesion. This is measured with the help of tilting devices for our drop shape analysis instruments. The roll-off angle gives the surface inclination at which a dispensed drop rolls or slides off the surface.
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.
AR271: Wettability of carbon fibres using single-fibre contact angle measurements - a feasibility study
The very low wetting forces on a carbon fiber can be measured with the help of a K100SF single-fiber contact angle measuring instrument. Differences in mean value extend beyond the scatter band and thereby quantify the wettability of the fibers.
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.
AR262: The Effect of an Oxygen-Helium Atmospheric Plasma on the Surface Energy of Medical Plastics
Surface modification of polymers by means of plasma treatment significantly improves the wettability by water. As a result of this discovery, higher biocompatibility of the plastics in medical applications can be expected.
AR256: How plastics lose their hydrophobia
The increase in surface polarity by ozone treatment is proven with the help of contact angle measurement. The results also show the dependency of the degree of surface activation on the duration of the treatment.
AR207: Applications of Sessile-Drop and Pendant-Drop Techniques in Offset Printing Technology
Fountain solutions and paints as well as the printing and not printing areas of printing plates are characterized in respect to their surface tensions and surface free energies. The article shows how to use these values to evaluate the materials used and control printing quality.