Glue Adhesion And Cohesion
Is the difference between cohesion and adhesion?
Occasionally, we are asked ‘what is the difference between adhesion and cohesion’? The words sound very similar, however, from an adhesive point of view, they are totally distinct parameters.
Adhesion is the bonding of one material to another, namely an adhesive to a substrate. The strength of the bond between the adhesive and the substrate, is referred to as adhesive strength. For an adhesive to stick to any material, it must have adhesion.
Cohesion is a totally different thing. Cohesion is defined as the internal strength of an adhesive as a result of a variety of interactions within the adhesive. In other words, cohesion is how well the adhesive sticks to itself.
To explain the two properties, if we apply a layer of adhesive onto a piece of steel, and after the adhesive has cured, we are able to rub off a surface layer of adhesive, the adhesive has low cohesive strength. If a layer of adhesive remains on the steel and is difficult to remove, the adhesive has high adhesive strength. There you have this is the core difference between adhesion and cohesion.
Within a glued joint, there are three distinct zones, the adhesion zone, the transition zone and the cohesive zone. We shall look at these individually.
- The Adhesion Zone. Due to the bond of the adhesive to the substrate, the structure of the adhesive is modified within the adhesion zone. Adhesion itself, is due to the molecular interactions between both the adhesive, and the surface onto which the adhesive is applied. These interactions occur on two levels.
First we have intermolecular bonds, which are weak, followed by chemical bonds, which are tremendously strong. Chemical bonds are the least common type. An example of a chemical bond would be the bond between a methacrylate adhesive and a metal, or a UV adhesive on glass or even an epoxy resin on metal. These types of adhesives form a chemical bond with the surface onto which they are applied.
- The Transition Zone is where the mechanical properties of the adhesive are altered, depending on the thickness of the glue line. Some adhesives, will cater well with a glue line thickness of 0.1 mm and below. Methacrylate, epoxy and polyurethane adhesives prefer a minimum of 0.5 mm and often 1.0 mm. The thickness of the glue line will depend primarily on both on the flow properties of the adhesive, and the pressure applied to the substrates after they have been consolidated. The nature of the surface and the adhesive cure conditions, will also effect the mechanical properties of the adhesive, within the Transition Zone.
- The Cohesion Zone, is where the adhesive possesses its nominal properties, as indicated on the data sheets. These properties are determined by the following molecular forces…
- The chemical bonds within between the polymer particles within the adhesive.
- The chemical bonds resulting from the use of a crosslinking agent reacting with the polymer particles within the adhesive (i.e. adding an isocyanate to a polychloroprene adhesive)
- The intermolecular interactions between the polymer particles within the adhesive.
- The mechanical adhesion between the polymer particles within the adhesive.
In the above figure, we can see the strong adhesion forces between the adhesive and the substrate and the strong cohesion forces between the polymer particles within the adhesive.
The four molecular forces detailed above, affect the properties of the non-cured adhesive, and determine it’s supply properties. The curing of the adhesive chiefly involves the solidification of the molecules within the adhesive. The cure of the adhesive may take place due to the evaporation of a carrier solvent, the reaction of two components, the application of heat or indeed the evaporation of water. It all depends on the nature of the adhesive.
When we manufacture adhesives, the cohesive properties of the adhesive are already within the product, it is only with the correct end use customer application , that the optimum adhesive performance characteristics can be achieved.
The chemistry, technology and industrial application of both adhesives and sealants, is highly technical. At W.I Polymers, we are the only indigenous research based manufacturer and distributor of industrial adhesives, to offer a Tier 3 level of technical support. This is the highest available in Ireland and upon troubleshooting any technical issue, our scientists and technicians will find the solution to your gluing problems.