Adhesion is the tendency of certain dissimilar molecules to cling together due to
Mechanisms of adhesion
Five mechanisms have been proposed to explain why one material sticks to another:
Adhesive materials fill the voids or pores of the surfaces and hold surfaces together by interlocking. Sewing forms a large scale mechanical bond,
velcroforms one on a medium scale, and some textile adhesives form one at a small scale. This is similar to surface tension.
Two materials may form a compound at the join. The strongest joins are where atoms of the two materials swap (
ionic bonding) or share ( covalent bonding) outer electrons. A weaker bond is formed if oxygen, nitrogenor fluorineatoms of the two materials share a hydrogennucleus ( hydrogen bonding). Dispersive adhesion
dispersive adhesion, also known as adsorption, two materials are held together by van der Waals forces: the attraction between two molecules, each of which has a regions of positive and negative charge. In the simple case, such molecules are therefore polar with respect to average charge density, although in larger or more complex molecules, there may be multiple "poles" or regions of greater positive or negative charge. These positive and negative poles may be a permanent property of a molecule ( Keesom forces) or a transient effect which can occur in any molecule, as the random movement of electrons within the molecules may result in a temporary concentration of electrons in one region ( London forces).
Some conducting materials may pass
electronsto form a difference in electrical chargeat the join. This results in a structure similar to a capacitorand creates an attractive electrostaticforce between the materials.
Some materials may merge at the joint by diffusion. This may occur when the molecules of both materials are mobile and
solublein each other. This would be particularly effective with polymerchains where one end of the molecule diffuses into the other material. It is also the mechanism involved in sintering. When metalor ceramicpowders are pressed together and heated, atoms diffuse from one particle to the next. This joins the particles into one.
What makes an adhesive bond strong?
The strength of the adhesion between two materials depends on which of the above mechanisms occur between the two materials, and the surface area over which the two materials contact. Materials that wet against each other tend to have a larger contact area than those that don't. Wetting depends on the
surface energyof the materials. Well-known examples of adhesion are tape, glue, stickers, and rubbing dirt on golf balls.
Scratch testerMethod of testing the adhesion of coatings to substrates
* John Comyn, "Adhesion Science", Royal Society of Chemistry Paperbacks, 1997
* A.J. Kinloch, "Adhesion and Adhesives: Science and Technology", Chapman and Hall, 1987
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