Page 12 - Handbook of Adhesion Promoters

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Mechanisms of Adhesion

There many methods which help in adhesion improvement. Technical literature usually
contains a discussion of roles of interlocking (mechanical adhesion), diffusion, adsorption,
surface reaction (chemical bonding), wetting, and electrostatic forces in the fostering
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adhesion in various material pairs.
There are several other important mechanisms in play which are to be considered in
order to form the full picture of combinations of principles for the overall adhesion in a
given product. In the case of diffusion, the effect of chain entanglement has to be included
which emphasized the mechanics of the chain effect on adhesion.
Hydrogen bonding is a mechanism by which adhesion properties can be reversed
depending on the physical state of matter. This differs substantially from the surface reac-
tion which is based on covalent bonding (a specific reaction between two substrates).
Properties of polymer blends are known to depend on the state of the interface
between the blend components. This in blends is frequently created by the use of compati-
bilizers, meaning a separate phase which acts by connecting two incompatible phases
using an intermediate layer of material which is able to agree with both partners. This
mechanism can be used as an example because it also applies to the pairs of materials
other than the components of the polymer blends.
Reversible hydrolysis is known to be a part of many adhesives and sealants. It per-
mits them to recover lost adhesion when water is removed from the interface between
adhesive and substrate.
Some crystalline properties of materials (especially in the case of various reinforce-
ments) contribute to increased adhesion between materials (e.g., fiber and polymer
matrix). Effects of heteroepitaxy and transcrystallinity immediately come to mind as
essential influences on adhesion.
There are now many connections between polymers and living organisms which are
influenced by adhesion, for instance in colonization of polymeric surfaces by microorgan-
isms leading to their biodeterioration or affecting the durability of implants. Organisms
such as barnacles or octopus have uncommon means of adhering to surfaces. Understand-
ing these mechanisms is crucial for the development of various additives included, for
example, in paints.
Finally, the difference between, for example, human tissue and polymeric material
used for prosthetics and other similar purposes is intentionally blurred by various means in
order to make these materials compatible. Here, we arrive at the importance of cellular
adhesion which is very important for the application of various man-made materials in
medicine and as such it also requires an explanation.