Engineering Chemistry S Datta

ADHESIVES 331
uniformly wetted by the adhesive sol which is guided by the interfacial tension between the
two. When this tension becomes minimum, the surface is better wetted.
3. Thickness of the adhesive film: Very viscous adhesives produce thick film but do not
wet the surfaces properly leaving many voids and hence the bonding becomes weak. This bond
strength can be improved by applying pressure. The bonding strength can also be increased by
adding solvent and/or plasticizer to decrease the viscosity.
4. Physical characters of the adhesive film: The bonding strength of the adhesive film is
influenced by the following physical characteristics of the film.
(a) Tensile strength, shear strength and compressive strength of the film: These three
characteristics influence the bonding strength in a positive way.
(b) Creep rate—Highly plasticized adhesives have higher creep rates under stress conditions
than the rigid thermosetting materials. Creep rate under stressed condition
should be lower for better adhesives.
(c) Difference in thermal coefficients of expansion of adherends and adhesive—When
this difference is high the bonding strength weakens under temperature variations.
Substance can be added to the adhesive to alter the thermal coefficient of expansion
to match with that of the adherend and hence increasing the bond strength.
5. Application modes of adhesives: The mode of application of adhesive on the surface to
be joined also influences the bonding strength. Pressure, temperature and time are the factors
that affect this strength after the application of the adhesive. Sufficient time is to be allowed
so that the curing time be enough to establish a strong bonding. Similarly, temperature also
affects the process of bonding by allowing the solvent to evaporate. Less pressure is required
in case the adherend surfaces are smooth but in the case of porous surfaces, pressure should be
applied to join the surfaces together.
Chemical Factors Affecting the Adhesive Strength
1. Degree of polymerization: Bond strength of the adhesive is affected by the extent of
polymerization of the monomers. Degree of polymerization best suited for adhesive action is
different for different adhesives e.g., for cellulose derivatives partially degraded low molecular
weight products give higher adhesive action.
2. Polarity of adhesives: For organic contacting surfaces polar-group containing adhesives
provide better bond strength than non-polar adhesives. Thus, addition of polar molecules to
the non-polar adhesives increases the adhesive power.
3. Complexity of the adhesive molecule: Complexity and chain length influence the
adhesive action. In the case of phenol-aldehyde resin the higher mol. wt. products have greater
adhesive action. Similarly, in the case of cellulose esters the optimum chain length of fatty
acid is from 6 to 14 for better bond strength.
4. pH: Strong acids and strong alkalies affect the bond strength of adhesives negatively.
But the effects of pH are different in the case of different type of adhesives, e.g., slightly alkaline
pH gives better bond strength in the case of Glue which are protein in nature, whereas acidic
pH increases the bond strength of vulcanized rubber.
Technique of Bonding
Different type of adherend surfaces, different type of adhesives, differing bonding
processes give rise to different types of bonding. There are certain general steps involved in
common bonding processes.