Polar and Non-Polar Molecules
The adhesive forces are caused by intermolecular interactions. These interactions depend on whether the molecules are polar on non-polar.
Polar molecules are molecules with an uneven charge distribution. In covalent bonds, the shared pair of electrons will sometimes be held slightly closer to some atoms than others. This is due to the electronegativity of the atoms. This uneven distribution of negatively charged electrons within a molecule will cause one end of the molecule to become slightly negatively charged and the other to become slightly positively charged. These types of molecules are polar molecules called dipoles. The slightly negatively charged atoms in the dipole will be attracted to the slightly positively charged atoms in neighbouring dipoles. These intermolecular interactions can only occur at very small distances (less than 0.5 nm) and so the adhesive material must be able to make close contact with the surface.
Molecules are non-polar when the shared electrons are evenly distributed between the atoms. For example, electrons are held at an equal distance between hydrogen and carbon atoms. This means that hydrocarbon rubbers such as Natural rubber, SBR, BR, EPDM and IIR are nonpolar. The forces between non-polar molecules are weaker than dipole-dipole forces and are known as London dispersion forces. These occur due to the random movement of electrons which can lead to the formation of a temporary dipole within the molecule. This uneven charged molecule will induce dipoles in neighbouring molecules, by repulsing like charges.
Polar molecules will be attracted to other polar molecules via dipole-dipole forces. Non-polar molecules will be attracted to other non-polar molecules via London Dispersion forces. Polar and non-polar molecules, however, will not be attracted to each other. It is therefore important to ensure that the chosen adhesive is compatible with the substrate material.