Adhesion
Adhesion is the molecular attraction exerted between bodies in contact.
Mechanisms of Adhesion
Five mechanisms have been proposed to explain why one material sticks to another:
Mechanical Adhesion
Two materials may be mechanically interlocked. Sewing forms a large scale mechanical bond, velcro forms one on a medium scale, and some textile adhesives form one at a small scale.
Electrostatic Adhesion
Some conducting materials may pass electrons to form a difference in electrical charge at the join. This results in a structure similar to a capacitor and creates an attractive electrostatic force between the materials.
Diffusive Adhesion
Some materials may merge at the joint by diffusion. This may occur when the molecules of both materials are mobile and soluble in each other. This would be particularly effective with polymer chains where one end of the molecule diffuses into the other material. It is also the mechanism involved in sintering. When metal or ceramic powders 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 energy of the materials.
See also
References
- John Comyn, Adhesion Science, Royal Society of Chemistry Paperbacks, 1997
- A.J. Kinloch, Adhesion and Adhesives: Science and Technology, Chapman and Hall, 1987