Adhesion is the bonding of one material to another, i.e., an adhesive to a substrate, owing to a variety of possible interactions. Cohesion is the internal strength of an adhesive as a result of a variety of interactions within the adhesive.
The adhesive has a modified molecular structure in the adhesion zone because of bonding to the substrate surface. The phenomenon of adhesion is caused by molecular interactions between the substrate surface and the adhesive. A distinction can be made between weak intermolecular interactions and strong chemical bonds. However, chemical bonds only form for very few substrate/adhesive combinations. For some of these bonded joints it has been demonstrated that chemical bonds account for up to 50% of all the interactions. The long-term stability of these bonds relies directly on their resistance to moisture. In addition to the intermolecular and chemical adhesion forces, the bonding mechanism occasionally referred to as “micro-mechanical adhesion” can play a role, depending on the morphology of the substrate surface. “Micro-mechanical adhesion” is generally only considered to be of secondary importance. But if there are regular undercuts in the substrate (even introduced by design) which the adhesive flows around, this can increase the strength of the bonded joint.
The transition zone in which chemical, mechanical and optical properties of the adhesive are altered varies in thickness, from a few nanometers up to the millimeter range. The thickness depends on the nature of the substrate surface, the adhesive and the curing conditions. Where there are thick transition zones or thin bonded joints, the behavior of the entire bonded joint may be determined by the properties of the transition zone, because in this case there is no cohesion zone.
In the cohesion zone, the adhesive possesses its nominal properties, as indicated on the data sheets. These properties are determined by four molecular forces: a. The chemical bonds within the adhesive polymers; b. The chemical bonds resulting from crosslinking of the polymer; c. The intermolecular interactions between the molecules in the adhesive; d. The mechanical adhesion between various molecules in the adhesive. The four cohesive forces affect the properties of the non-cured adhesive and determine the viscosity of the adhesive. The curing of the adhesive chiefly involves solidification of the adhesive via bonds between the molecules in the adhesive. This involves new bonds being formed and existing bonds being strengthened.
Both adhesion (including the transition zone) and cohesion play their part in maximizing the strength of a bond. Just as with a chain, the weakest link in a bonded joint determines what loads the joint can be subjected to. The cohesive properties of an adhesive are already determined by the producer. It is up to users to try and attain these by curing under optimum conditions. Moreover, the adhesive strength must be sufficiently high. This is the case when the natural strength of the adhesive, and not the adhesion, is the limiting factor in strength tests.
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This article is reproduced from Adhesives.org/Sealants.org.