Page 109 - Handbook of Adhesives and Sealants

Theories of Adhesion 77

Creep can occur when enough force is applied to a mass of linear
molecules to cause them to disentangle or overcome their crystalline
order. Creep is a slow process. Creep is more pronounced at temper-
atures above the adhesive’s glass transition temperature, T . Polymers
g
with low T cannot be used where large loads must be carried at mod-
g
erate temperatures. Crosslinking reduces creep because the polymer
segments are immobilized by the network structure and cannot easily
slide by one another.

2.4.3.2 Long term effects. Potentially harmful external stress may be
of mechanical, thermal, or chemical origin. Temperature, humidity,
salt spray, fluids, gases, mechanical loads, radiation, and vacuum are
the most common extreme environments. They may be sufficiently
great to cause premature failure. The required environmental resis-
tance depends on the individual application. These requirements must
be established before the final joint configuration and material selec-
tions can be made. The effects of harsh environments on adhesives
and sealants are detailed in Chapter 17.
Sustained loads can cause premature failure in service even though
similar unloaded joints may exhibit adequate strength in the same
environment. The cumulative effect of different types of stress (e.g.,
mechanical and chemical) may often be an overlooked factor regarding
adhesive failure. Temperature variation combined with cyclic loading,
for instance, can greatly reduce durability when compared with simple
single parameter aging of the same sample.
There are times when a properly cured unstressed bond fails for no
apparent reason. It may exhibit apparent adhesive failure under low
or moderate load after extended aging. When behavior of this type is
encountered, the cause is often desorption, i.e., displacement of the
adhesive from the surface by a chemical from the environment or from
within the substrate (Fig. 2.15). Attachment to substrate sites involves
a dynamic equilibrium between the adhesive and other molecules that
may be present. Water, solvent, plasticizer, and various gases and ions
may all compete for the surface attachment sites. Moisture is by far
the most effective desorbing substance commonly available. Bonds are
able to resist desorption only if the low molecular weight substances
are unable to penetrate the joint or if the substrate-adhesive bond is
thermodynamically or kinetically favored over the substrate-desorber
interaction. Some widely used coupling agents form thin films between
the substrate and the adhesive. These have very high affinities for
each material. Such substances will be difficult to displace by itinerant
molecules.
An adhesive joint may contain as many as five different layers: first
adherend, first boundary layer, the adhesive, second boundary layer,
and second adherend. The life of a typical adhesive joint may be di-