ADHESIVES, GLUES AND SEALANTS
What is the contact angle?
One of the requirements, but not enough to produce the phenomenon of adhesion between two materials is that the materials wet to each other, the adhesive needs to have a good wettability on the substrate.
The contact angle is defined as the angle formed by a drop of adhesive in contact with the surface of the substrate; the value of this angle will indicate the degree of wettability and therefore the adhesion of the adhesive on the substrate.
For contact angles greater than or equal to 90 º it is said that the adhesive does not wet the substrate, it does not generate any adhesion between the adhesive and the substrate; if the contact angle is below 90 degrees we say that the adhesive wets the substrate causing adhesion between both materials.
The use of specific devices as goniometers as well as the use of computers with specific software, allows determining the contact angle of an adhesive on any substrate.
The measurement of contact angles to measure the degree of adhesion means controversy today, because it is a system that depends on many factors and is not easily reproducible in all places.
Depending on the selected liquid and the surface roughness of the substrate, the resulting values of contact angle measurement can vary enormously, these limitations and the lack of reproducibility of these measurements, is what makes this type of measurement additional input information on the degree of adhesion, but certainly it does not ensure a proper adhesion in the union.
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