Adhesion
Adhesion refers to the bonding between two different types of liquid or solid surfaces, caused by physical attractive forces – interactions between molecules at the interface – as well as by covalent chemical bonds.
Mechanical interlocking, for example due to roughness, or entanglement of macromolecules can also cause adhesion. This article primarily deals with adhesion caused by physical attraction.
What is the difference between adhesion and cohesion?
Adhesion refers to the bonding at the interface between different substances/phases, while cohesion describes the internal bonding of a substance.
In which areas does adhesion play a role?
Adhesion occurs to varying degrees at every interface between different liquid or solid, immiscible/insoluble phases and also plays a role in many biological or medically relevant processes. A prominent example is the gecko, which can even move upside down on glass panes thanks to adhesion.
Technically, adhesion is relevant wherever surfaces of different materials are temporarily or permanently bonded – especially in gluing, but also in coating or printing. The required strength of adhesion depends on the application: particularly strong and mechanically stable, for example, in aircraft or vehicle construction; moderate wherever the bond is to be reversible, for example, in price stickers or adhesive strips.
However, the lowest possible adhesion is often required, for example, in non-stick and easy-to-clean surfaces. Optimizing the adhesion required for the respective purpose is one of the tasks of interface analysis.
▶ Read our use cases: Adhesion on polymers, Adhesion on metals, Adhesion on glass
How is adhesion measured?
Most measurement methods for adhesion directly or indirectly determine the force required to separate an adhesive bond. Examples include the peel adhesion test for adhesive tapes according to ASTM D3330 or the tensile shear test according to ISO 4587/EN 1465, in which the force required to separate two bonded plates is measured.
The crosshatch test according to ASTM D 3359/ISO 2409 is established for the adhesion of coatings. In this test, a grid pattern is cut into the coating and, when an adhesive tape is pulled off, the proportion of grid squares pulled off with the tape is checked.
Mechanical tests are destructive, sometimes cumbersome, often manual, and frequently subjective in their evaluation of results. Therefore, it is advantageous to validate adhesion by addressing its physical cause: molecular interactions at the interfaces. Interfacial chemical methods such as measurements of surface tension and the wetting angle (contact angle) make it possible to assess adhesion even before the liquid adhesive or coating material comes into contact with the substrate. Although mechanical tests cannot usually be completely avoided, they can be significantly reduced.
How are adhesion and surface tension/surface energy related?
A conceptual model helps to explain the relationship between adhesion and surface tension: Surface tension is the work that must be expended per unit area to form a surface. If a column of liquid were to be divided in cross-section, two surfaces of equal size would be created. In doing so, cohesion – the attraction between similar molecules – must be overcome. Accordingly, the work of cohesion WC corresponds to twice the surface tension σ (small Greek sigma; equation 1):
Accordingly, when the two quantities of liquid are combined, the two surfaces would disappear and the analogous cohesion energy would be released.
If, instead, two different, immiscible liquids are brought together in the same way, two surfaces also disappear, but an interface is formed. Work must also be done to form this interface, which is expressed in the interfacial tension. The work of adhesion WA between the two phases is therefore the sum of the two surface tensions σ1 and σ2 ,reduced by the interfacial tension σ12 (Equation 2):
This relationship was first formulated by Dupré and applies to both liquid-liquid and liquid-solid interfaces. σ is usually referred to as surface free energy (SFE) for solids and less commonly as surface tension. For liquid-solid interfaces, the contact angle is required to determine the work of adhesion.
Why do pretreatments such as plasma or flame activation improve adhesion?
Various pretreatment methods improve wetting and adhesion prior to bonding, coating, or printing. Plastics in particular usually have a low surface free energy (SFE) initially. Accordingly, the contribution of the substrate surface to the work of adhesion WA is low (see Equation 2). Moreover, when in contact with aqueous systems, plastics surfaces are chemically incompatible and the interfacial tension σ12 becomes high, which reduces the value for WA.
The various treatment methods increase the SFE. The treatment generates chemical groups that make the substrate surface more "water-like," which is why the interfacial tension decreases (details below).
The success of such pretreatments can best be quantified using contact angle measurements.
How are adhesion and contact angle related?
The contact angle describes the "roundness" of a drop on a surface and is a measure of its wettability. The smaller the contact angle, the better the wetting. It is intuitively understandable that wetting and contact angle are related to adhesion: for adhesion to occur at all, the liquid must first wet the solid and must not bead up.
Using Young's equation, the contact angle θ (small Greek letter theta) is linked to the surface tension σlg of the liquid (liquid/gas), the free surface energy of the solid σsg (solid/gas) and the interfacial tension σsl (solid/liquid) (Equation 3):
Together with Equation 2, this results in the relationship according to Young-Dupré (Equation 4) for the work of adhesion WA:
The work of adhesion can therefore be determined by measuring the surface tension of the liquid and the contact angle with the solid.
Drop Shape Analyzers (DSA) from KRÜSS not only measure the contact angle, but also feature the precise surface tension methods Constrained Sessile Drop and pendant drop, allowing the work of adhesion to be determined using a single measuring instrument.
Which molecular forces cause adhesion?
There are a number of models that describe the intermolecular forces at an interface and their contribution to the work of adhesion. Most of these approaches split the surface tension of the liquid and the free surface energy of the solid into polar ("water-like") and disperse ("oil-like") interactions. The most popular of these models is that of Owens, Wendt, Rabel & Kaelble (OWRK).
To simplify matters, it is assumed that adhesion between two phases is caused by interactions of the same type, i.e., polar interactions only occur if both phases have polar components. The more similar the two phases are in terms of their polarity, the greater the adhesion (and the lower the destabilizing interfacial tension).
▶ More detailed information on the interaction models can be found in the glossary article on surface free energy.
The surface pretreatment methods mentioned above primarily increase the polar component of the free surface energy by oxidizing the surface and thus creating polar chemical groups. However, this can also lead to overactivation: if the substrate is proportionally more polar than the coating after treatment, the two phases are not completely compatible, and the resulting interfacial tension reduces adhesion and long-term stability.
Using contact angles and surface tension data, it is possible to determine optimal polarity target values for good adhesion, predict these values, and optimize the surface tension/energy profiles.
▶ Learn more about coating optimization in our application report AR296 ("Predicting Coatability") about a collaboration project between BYK and KRÜSS.
What is the significance of the interfacial interaction model for adhesion through covalent chemical bonds?
In some coating and bonding processes, adhesion is based on covalent bonds, e.g., when bonding glass. In such cases, physical interaction models do not allow direct calculation of adhesion. However, since the formation of covalent bonds requires close interfacial contact, good wetting and initial adhesion are necessary, and it is still useful to determine the work of adhesion and interfacial tension.
Does the model of polar and disperse interactions work universally for the optimization of adhesion?
The calculation of work of adhesion based on polar and disperse interactions has proven itself many times in practice. Nevertheless, there are many systems in which the calculated values correlate poorly with the adhesion of cured coatings and bonds. For example, the conventional advancing angle or recently advanced contact angle (RACA) hardly changes in some systems, even though there are significant differences in the mechanically measured adhesion. It has been shown that in many cases, the receding angle measured during dewetting correlates more strongly with the final mechanical adhesion than the advancing angle for wetting.
How can adhesion be characterized based on dewetting?
Intuitively, there is an analogy between the detachment of a coating or the pulling apart of an adhesive bond and the withdrawal of a wetting liquid from a surface. Although this is not a scientific explanation, the receding angle, which characterizes the dewetting behavior, often correlates very well with the results of mechanical adhesion measurements. An example of this good correlation is the crosshatch test for coatings, as KRÜSS, together with its cooperation partners BYK and Plasmatreat, was able to demonstrate on a broad data basis.
▶ Read our application report AR301 for the results of this study.
The Stood-up Drop method is available for measuring dewetting, which determines dewetting based on the recently receded contact angle (RRCA). The measurement is reproducible, takes only seconds, and is performed safely with pure water, making the method well suited for quality control prior to bonding and coating processes.
Literature
- A. M. Dupré, P. Dupré. Théorie mécanique de la chaleur. 1869.
- D. H. Kaelble, Dispersion-Polar Surface Tension Properties of Organic Solids. In: J. Adhesion 2 (1970), pp. 66-81.
- D. Owens; R. Wendt, Estimation of the Surface Free Energy of Polymers. In: J. Appl. Polym. Sci 13 (1969), pp. 1741-1747.
- W. Rabel, Some aspects of wetting theory and its application to the investigation and modification of the surface properties of polymers. In: Farbe und Lack 77,10 (1971), pp. 997-1005.
- T. Young, An Essay on the Cohesion of Fluids. Philosophical Transactions of the Royal Society of London, The Royal Society, London 1805, Vol. 95, pp. 65-87.
