Work of adhesion

Work of Adhesion

Work of adhesion offers a method to predict adhesion from a thermodynamic point of view. Adhesion is important in all applications where two different materials are bonded together. Wettability is required for good adhesion, and because of this contact angle, and surface free energy measurements are often used to evaluate the surface before the bonding process. 

Areas covered in this article

Basics of adhesion

Adhesion is the attraction between two dissimilar phases. There is no single theory to explain adhesion, but it is commonly divided into physical and chemical bonding and mechanical interlocking. Most typically adhesion occurs because of the combination of these different mechanisms.Adhesion mechanisms

Physical bonding refers to Van der Waals interactions between the materials. Van der Waals forces are very weak compared to those responsible for chemical bonding. The effective distance of Van der Waals forces is very short which means that the two materials have to make intimate contact for binding to occur.

Chemical bonding includes covalent, ionic, and metallic bonds. Chemical bonds are much stronger than physical bonds. Chemical bonds are typically responsible for cohesive forces inside the material. For example, most of the polymers have covalent bonds, and metallic bonds are responsible for the high thermal and electrical conductivity of metals. Chemical bonding between two dissimilar materials is not so common, although some of the pre-treatment methods such as plasma treatment can be used to increase the available chemical binding sites.

Mechanical interlocking happens when one material penetrates another. This requires some type of roughness of the substrate. One common example of this kind of adhesion is gluing. Adhesive, i.e. the glue, will flow to the cavities of the surface and fix the two pieces mechanically together. For the adhesive to properly work, it will need to have a suitable viscosity and surface tension to wet the substrate. The surface on the other hand has to have a suitable surface free energy to allow the adhesive to spread.

Adhesion failure mechanisms

To understand why bonding fails, we need to first define the ways the failure can occur. These can be divided into three categories:

  • Adhesive failure
  • Cohesive failure
  • Substrate failure

Bonding failure

Adhesive failure, or delamination, is one of the most common types of failure mechanisms. There the two dissimilar materials detach from each other. The failure can happen between a paint and a substrate or between the adhesive and either of the two substrates it is bonding together. Cohesive failure happens in the adhesive itself or inside the layer of a coating. Substrate failure is not related to the bonding process itself as it is a problem in the substrate.

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What is work of adhesion?

Work of adhesion (WoA) is a concept presented in a nineteenth-century physical chemistry text that discussed the wetting of solid by liquids. Due to its clear connection to wetting of solid by adhesive, it has been part of many discussions of adhesion.

When a material is cut in half, two new surfaces are formed. The work required to separate the material in two has to overcome the cohesion forces inside the material. Thus the work required is called work of cohesion and is written as

work of cohesion

This can be understood as energy added into the system as two new surfaces are formed when the material is cut in half.

Similarly, it is possible to look at the situation where two dissimilar materials are first in contact. In this case, however, there is interfacial energy in between the two materials to start with. As the materials are being separated, two new surfaces are formed but at the same time, the interface between them is destroyed. The work of adhesion can then be written as

work of adhesion

Work of cohesion and adhesion

Work of adhesion vs. practical adhesion

As the work of adhesion is based on thermodynamics which deals with ideal systems, there has been a lot of discussions what is the relationship between this so-called fundamental adhesion and practical adhesion.

Practical adhesion is concerned with the magnitude of mechanical force which has to be applied to break an adhesive bond. Practical adhesion thus is extremely important and can be measured with different laboratory measurements. However, practical adhesion can only be evaluated after the bond has been formed, whereas work of adhesion tries to do the same proactively before the bonding is done. Although fundamental adhesion is a prerequisite for the existence of practical adhesion, some authors have claimed that there is no relation between the two in practice.

In principle, the higher the value of WA, the more difficult it is to separate the two phases, at least from the thermodynamic point of view. This seems to apply to some systems but there is no clear consensus on what are the initial requirements for the work of adhesion to correlate with practical adhesion.

Work of adhesion measurement

Work of adhesion cannot be directly measured. If you look at the equation for the work of adhesion 

work of adhesion

, you would need to know the surface energy of both materials as well as the interfacial tension between them. If we would consider both of the phases being solids, the surface energies can be calculated through the contact angle measurements by utilizing some of the surface free energy theories. Similarly, the interfacial tension between the solids could be modelled based on the same theories. If we would utilize the OWRK method which is one of the most commonly used surface free energy theory, we could write the work of adhesion as

work of adhesion_OWRK2

, where d and p are dispersive and polar components of the surface energies, respectively.

Much more common is, however, to calculate work of adhesion for solid-liquid systems.

If the phases in contact are solid and liquid the equation for work of adhesion can be written as

WoA solid-liquid

If combined with Young’s equation

Youngs equation2

We get Young – Duprée equation

Young dupre

, where (γl) is the surface tension of the coating formulation and Θ contact angle between the coating formulation and the substrate.

The most typical way to determine the work of adhesion is through contact angle and surface tension measurements. The measurements are straightforward and can be done with the optical tensiometer. The surface tension of the coating formulation is first measured with pendant drop measurement. Then the contact angle is measured with the sessile drop measurement after which the work of adhesion can be calculated. 

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