The week's editorial reviews the major theories of adhesion.
Unfortunately, there is no single, comprehensive theory to explain why
adhesives stick. Certain theories apply better to specific situations
than others. It is, therefore, crucial to be aware of the fundamentals
behind each theory.
The actual mechanism of adhesive attachment is not yet
very explicitly defined. Several theories attempt to describe the phenomenon
of adhesion. No single theory explains adhesion in a general, comprehensive
way. Some theories are more applicable for certain substrates and applications;
other theories are more appropriate for different circumstances.
Each theory has been subjected to much study, question, and controversy.
However, each contains certain concepts and information that are useful
in understanding the basic requirements for a good bond. The most common
theories of adhesion are based on (1) adsorption, (2) mechanical interlocking,
(3) electrostatics, (4) diffusion, and (5) weak boundary layers.
Adsorption Theory
The adsorption theory states that adhesion results from molecular contact
between two materials and the surface forces that develop, usually designated
as secondary or van der Walls forces. For these forces to develop, the
adhesive must make intimate, molecular contact with the substrate surface.
The process of establishing continuous contact between an adhesive and
the adherend is known as "wetting." Wetting can be determined
by contact angle ( ) measurements. Complete, spontaneous wetting occurs
when = 0 deg, or the material spreads uniformly over a substrate to form
a thin sheet.
Wetting is favored when the substrate's surface tension, better known
as the critical surface energy, C, is high and the surface tension of
the wetting liquid, , is low (i.e., C substrate > adhesive). Low-energy
polymers, therefore easily wet high-energy substrates such as metal and
glass. Conversely, substrates having low surface energies (e.g., polyethylene
and fluorocarbons) will not be readily wet by other materials and are
useful for applications requiring nonstick, passive surfaces.
After intimate contact is achieved between adhesive and adherend through
wetting, it is believed that permanent adhesion results primarily through
forces of molecular attraction. Four general types of chemical bonds are
recognized as being involved in adhesion and cohesion: electrostatic,
covalent, and metallic, which are referred to as primary bonds, and van
der Walls forces that are referred to as secondary bonds. In most practical
adhesive applications, secondary bonds are the predominant element that
contributes to adhesive strength.
Mechanical Theory
At one time, adhesion was thought to occur only by the adhesive flowing
and filling micro-cavities on the substrate. When the adhesive then hardens,
the substrates are held together mechanically. The surface of a substrate
is never truly smooth but consists of a maze of peaks and valleys. According
to the mechanical theory of adhesion, in order to function properly, the
adhesive must penetrate the cavities on the surface, displace the trapped
air at the interface, and lock-on mechanically to the substrate.
One way that surface roughness aids in adhesion is by increasing the total
contact area between the adhesive or sealant and the adherend. If interfacial
or intermolecular attraction is the basis for adhesion, increasing the
actual area of contact will increase the total energy of surface interaction
by a proportional amount. Thus, the mechanical theory generally teaches
that roughening of surfaces is beneficial because it (1) gives "teeth"
to the substrate (mechanical interlocking) and (2) increases the total
effective area over which the forces of adhesion can develop. However,
roughening is only effective if the adhesive wets the surface well.
Electrostatic and Diffusion Theories
The electrostatic and diffusion theories of adhesion are generally not
regarded as highly as the other theories in general bonding practice.
However, there are certain applications where these are very important
and help explain why bonds form.
The electrostatic theory states that electrostatic forces are formed at
the adhesive-adherend interface. These forces account for resistance to
separation. This theory gathers support from the fact that electrical
discharges have been noticed when an adhesive is peeled from a substrate.
Electrostatic adhesion is regarded as a dominant factor in biological
cell adhesion and particle adhesion.
The fundamental concept of the diffusion theory is that adhesion arises
through the inter-diffusion of molecules in the adhesive and adherend.
The diffusion theory is primarily applicable when both the adhesive and
adherend are polymeric, having compatible long-chain molecules capable
of movement. Solvent or heat welding of thermoplastic substrates is considered
to be due to diffusion of molecules.
Weak-Boundary-Layer Theory
According to the weak-boundary-layer theory, when bond failure seems
to be at the interface, usually a cohesive rupture of a weak boundary
layer is the real event. This theory largely suggests that true interfacial
failure seldom occurs. In most cases joint failure results from a cohesive
failure of a weak boundary layer. Weak boundary layers can originate from
the adhesive, the adherend, the environment, or a combination of any of
the three.
Weak boundary layers can occur on the adhesive or adherend if an impurity
concentrates near the bonding surface and forms a weak attachment to the
substrate. When failure occurs, it is the weak boundary layer that fails,
although failure may seem to occur at the adhesive-adherend interface.
In addition to external contamination, examples of weak boundary layers
are corrosion or oxide layers on metal surfaces and low molecular weight
constituents (e.g., release agents, plasticizers) on polymeric surfaces.
Weak boundary layers must be removed by physical or chemical means so
that there is no weak link in the adhesive joint that would contribute
to premature adhesive failure.
Should you have any comments or feedback, please contact
me.
If you'd like to attend a LIVE on-line training course on the subject,
click here!
Edward M. Petrie
|
SpecialChem4Adhesives Members Reactions
