Engineering Chemistry S Datta

330 ENGINEERING CHEMISTRY
3. Some adhesives when subjected to heavy static stress for long periods show a tendency
to creep even at room temperature.
4. Adhesives are generally susceptible to higher humidities.
5. Bonding strength is low compared to other joining methods like rivetting, welding
etc.
6. There is not available any single adhesive for joining all types of materials and hence
judicial selection is of great importance in bonding process.
7. Adhesives require appreciable time for developing their full strength bonding for
better performance.
Adhesion Theories
Several different theories have been developed that explain the mechanism of adhesion.
Mechanical theory of adhesion is based on the anchorage of the adhesive in the porous
rough surface of the adherends and hold the surface by interlocking action.
Electrostatic theory explains that by contact of the adherend and the adhesive, transfer
potentials are build up which sets up an electrical double layer and the corresponding coulombic
attraction forces develop between the two components.
Adsorption theory regards adhesion as essentially a special property of phase interfaces.
The forces that are responsible for adhesion are the so called secondary valence or van der
Waal’s forces.
Diffusion theory explains that adhesion is obtained by the mutual penetration of adhesive
and substrate. None of the above theories explain all the aspects of bonding. Latest theories
consider that bonding of adherend and adhesives takes place due to the chemical or physical
forces of attraction. By wetting the surface of the adherend with the adhesive, contact in
molecular dimensions takes place; when the interfacial boundary energy is lower than the
sum of the surface energies of the adhesive and the adherend, permanent adhesion occurs.
Adhesive Strength Development
1. By chemical reaction: The reactive low molecular wt. mono and/or oligomeric
ingredients of thermosetting adhesives are converted by chemical reaction into high mol. wt.,
crosslinked three-dimensional polymers with high bond strength.
2. By solvent evaporation: The adhesive is dissolved or dispersed in a solvent and applied
to the surfaces to be bonded and they are brought in contact with each other. The solvent gets
evaporated and thereby gelling and finally hardening takes place leading to strong bonding
between the surfaces.
3. By pressure application: Tacky substances in solvent-free form adhere spontaneously
to the surfaces of most materials, only under pressure.
4. By cooling: Thermoplastic materials are applied to the adherend surfaces in a molten
and hot condition which on cooling solidifies developing bonding strength.
Physical factors influencing adhesive strength:
1. Smoothness of the adherend surfaces: For wood surfaces smoothness of the surfaces
adds to the bonding strength of adhesives. In the case of other porous materials like paper,
leather etc., the large number of capillaries preferentially enclose the liquid portion of the
adhesive and disturb the equilibrium distribution of the adhesive on the adherend surface
leading to weak joint formation.
2. Surface tension: The wetting characteristics of an adhesive depend on its (a) viscosity
and (b) surface tension. The bonding between the surfaces is better when the surfaces can be