Page 83 - Handbook of Adhesives and Sealants

Theories of Adhesion 51

TABLE 2.1 Forces at the Interface or Within the Bulk of a Material
Bond
energy
Type of force Source of force (KJ/mol) Description
Primary or Covalent forces 60–700 Diamond or cross-linked
Short Range polymers. Highly directional.
Forces
Ionic or electrostatic 600–1000 Crystals. Less directional than
covalent.
Metallic 100–350 Forces in welded joints.
Secondary or Dispersion 0.1–40 Arise from interactions between
van der Waals temporary dipoles. Accounts for
Forces 75–100% of molecular cohesion.
Forces fall off as the 6th power of
the distance.
Polar 4–20 Arise from the interactions of
permanent dipoles. Decrease with
the 3rd power of the distance.
Hydrogen bonding Up to 40 Results from sharing of proton
between two atoms possessing
loan pairs of electrons. Longer
range than most polar and
dispersion bonds.

ondary or van der Waals forces. The exact nature of these forces and
their influence on adhesive or cohesive strength are difficult to accu-
rately determine. However, a general awareness of their origin and
characteristics assists in understanding why strong bonds form and
why they fail.

2.2.2 The concept of surface energy
The forces holding an adhesive to a substrate or maintaining the co-
hesive integrity of a solid can be measured as the work necessary to
separate two surfaces beyond the range of the forces holding them
together. In one case, the surfaces are the adhesive and substrate; in
the other, they are like-molecules in the bulk of the material. This
force is dependent on the intermolecular forces that exist in the ma-
terial and upon the intermolecular spacing. It is sometimes referred
to as the surface energy, (gamma).
The certainty that liquids have a surface energy is easily demon-
strated by the fact that a finely divided liquid, when suspended in
another medium, assumes a spherical shape. In the absence of grav-
itational distortion of shape (i.e., the energy associated with having a