Page 377 - Handbook of Adhesives and Sealants

Adhesive Composition and Formulation 333

as strong at high temperatures as the rubberized epoxy systems de-
scribed above. However, they offer low-cost formulations that can be
applied as hot melts. They crosslink by reacting with moisture in the
air to provide quick handling strength and latent thermosetting ca-
pability. They have a very high degree of elongation and resist hy-
drolysis unlike other resins with high elongation. Tensile elongation
of the films can vary from 15% to more than 100% compared to 2%
for most unmodified epoxy adhesives. 12
Reactive hybrid adhesives have also been found to bond well to oily
cold rolled steel as well as many other substrates. Apparently the elas-
tomeric phase adsorbs much of the oil that is on the surface of the
substrate without deterioration to the adhesive or cohesive strength
of the system.

9.3.4 Lowering the coefficient of thermal
expansion
Depending on the substrate, the curing temperature, and the service
temperatures that are expected, the adhesive formulator may want to
adjust the coefficient of thermal expansion of the adhesive system.
This will lessen internal stresses that occur due to differences in ther-
mal expansion between the substrate and the adhesive. These stresses
degrade the joint strength. There are several common occasions when
the difference in coefficient of thermal expansion between the sub-
strate and adhesive will result in internal stresses in the joint. One
such occurrence is when the cured joint is taken to a temperature that
is different from its curing temperature.
Ideally, the coefficient of thermal expansion should be lowered (or
raised) to match that of the material being bonded. With two different
substrate materials, the adhesive’s coefficient of thermal expansion
should be adjusted to a value between those of the two substrates.
This is usually done by using fillers as shown in Fig. 9.4. It is usually
not possible to employ sufficiently large filler loadings to accomplish
the degree of thermal expansion modification required. High loading
volumes increase viscosity to the point where the adhesive cannot be
easily applied or wet the substrate. For some applications and some
fillers, loading volumes up to 200 phr may be employed, but optimum
cohesive strength values are usually obtained with lesser amounts.
When temperature limits permit, it is useful to compensate for dif-
ferences in the coefficient of thermal expansion by the use of flexibil-
izers to absorb the internal stresses during thermal cycling. When
bonding materials with greatly differing rates of thermal expansion,
severe warping may be encountered at temperature extremes in the
absence of holding fixtures. To survive this type of stress, a very flex-
ible adhesive may be required rather than one that is highly filled.