Page 13 - Handbook of Adhesion Promoters

6 Mechanisms of Adhesion

Based on the above short discussion, the mechanisms which are included in this
chapter will be presented in the following order:
• mechanical interlocking
• surface condition and shape
• diffusion and entanglement
• adsorption
• acid-base and electrostatic interactions
• surface free energy and wetting
• crystalline properties
• interphase formation
• chemical bonding
• hydrogen bonding
• reversible hydrolysis
• microbiological and biological adhesion
• cellular adhesion
There will be some inevitable similarities of adhesion principles between mecha-
nisms discussed in these groups because nature of things does not have clear borders.
Also, the understanding of some mechanisms will be reinforced by the information
included in the discussion of other principles of adhesion because they are inter-related.
This chapter is probably the most important part of this book because of its focus on prac-
tical principles by which adhesion can be improved as opposed to a focus on an explana-
tion of principles of physics and chemistry alone. For this purpose, practical observations
and examples will be included in the following discussion.

2.1 MECHANICAL INTERLOCKING
The surface of material, even when it appears smooth and shiny, is full of imperfections
such as voids, craters, pits, holes, pores, cracks, scratches, deep valleys, and other struc-
tures having many different names. In addition to the existing imperfections, an inten-
tional increase in the surface roughness is produced by a mechanical action (sandblasting
or sandpaper abrasion) and various physical (such as plasma treatment, flame, corona dis-
charge, laser, or microwave) or chemical processes (modification, wet etching, dissolution
of some components, etc.). These processes are discussed in Chapter 4.
Figure 2.1 shows that the use of increasingly rougher sandpaper results in the
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increased surface roughness of the aluminum sheets. The initial surface roughness of the
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aluminum sheet was R = 0.6 μm (as indicated in the figure caption). After using sandpa-
a
pers with grits of 80, 50, and 36, the surface roughness values changed to 3.75, 5.63, and
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8.31 μm, respectively. The bond strengths of samples roughened by these sandpapers
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were increased by 20, 50, and 30%, respectively. It can be noted that the surface that was
too rough (obtained with sandpaper having a grid of 36) resulted in a poor polymer pene-
trability, void formation, and stress concentration, all of which led to a lower bond
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strength increase. This means that the interlocking is guided by some principles related to
the shape and the size of imperfections on the substrate and most likely by the rheological
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properties of the material (adhesive) deposited on the surface for bonding.