When dissimilar materials bond together we say that they exhibit the property of adhesion (for example, water on glass, paint on a wall, or frosting on a cake). When similar materials bond together we say that they exhibit the property of cohesion (for example, in clouds tiny water droplets join together to form rain, packing tape that wrinkles as you pull it off the roll sticks to itself and becomes hard to pull flat or wet sand grains stick together to let us build a sand castle).
Adhesives should have two properties - they should be "liquid like" so that they can flow across a surface to fill in tiny pores on the surface (see diagram below). Adhesives can be "wet", that is viscous, as in glue and paste or "dry", such as static cling or that used on the back of tapes.
Adhesives bond items together. There are natural adhesives made from plant matter such as starch and animal matter such as casein. Synthetic adhesives can be elastomers, thermoplastics, or thermosetting adhesives. Drying adhesives, such as white glue or rubber cement, consist of a mixture of polymeric materials in a solvent. When the solvent evaporates, the adhesive hardens. To be effective boding agents some adhesives require that the two surfaces be spread with the adhesive, which must dry for some time before the surfaces can be put together. This kind of adhesive is used in the making of shoes. The hot glue gun is an example of a hot adhesive a thermoplastic material that hardens upon cooling. Pressure-sensitive adhesives (tapes) can be either permanent (tapes used in construction) or removable (wound care dressings, athletic tape, price labels).
Mechanisms of adhesion:
- The adhesive works its way into the pores of the surface (substrate) in contact with the adhesive
- Electrostatic forces hold the substances together
- van der Waals forces (attractions between molecules that does not result in a chemical bond being formed - see diagram below) develop between molecules
- Moist adhesive diffuses into the pores of the substrate and then hardens.