Page 23 - Handbook of Adhesion Promoters

16 Mechanisms of Adhesion

Chain entanglements are very important for obtaining high adhesion. This statement
can be verified based on the typical maximum strength which is in the range of 100 N/m
for entanglements compared with around 0.04 N/m for adhesive strength caused by van
der Waals forces in fully wetted applications.
2.4 ADSORPTION/INTERACTION

The adsorption theory was introduced more than 50 years ago and it is the most accepted
theory of adhesion in spite of the fact that it is a combination of actions of many different
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physical principles. The theory states that materials adhere because of action of inter-
atomic and intermolecular forces between adhesive and substrate if they are in the inti-
mate contact. In our discussion, this theory is split into the primary physical principles and
discussed under separate sections below, such as acid-base, electrostatic, ionic, coulombic,
wetting, and surface energy. The theory of adsorption is also known under the name of the
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thermodynamic theory.
2.5 ACID-BASE AND ELECTROSTATIC INTERACTIONS

2.5.1 ACID-BASE INTERACTIONS
Acid-base interactions are the major factors among short range (<0.2 nm) intermolecular
forces which help to keep two molecules in proximity until other mechanisms of adhesion
could make this interaction more permanent.
Drago’s equation was proposed to predict exothermic enthalpy of acid-base interac-
tion, as follows

– ΔH AB = E E + C C B [2.4]
A
B
A
where:
susceptibility of components to undergo electrostatic inteaction
E A , E B
susceptibility of components to undergo covalent bonding
C A , C B
A, B acidic and basic component, respectively
This equation can be used to predict endothermic enthalpy of polymers adsorbed on
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inorganic substrates. More frequently this enthalpy is predicted based on Gutman’s
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donor and acceptor numbers, as follows
DN × AN
– ΔH AB = ------------------------ [2.5]
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where:
DN donor number (basicity)
AN acceptor number (acidity)
Acid-base interactions and many important scales characterizing interactions of solvents
have been broadly discussed elsewhere and the discussion and data can be used in other
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areas including adhesion of polymers.
XPS chemical shift and acid-base interaction correlate well and can be used for
quantification of polymer susceptibility using standard probes to establish a calibration
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curve. Then, the magnitude of −ΔH of the interaction between solvent and polymer
AB
for more complex polymeric systems, such as a fully formulated coating or adhesive, can