Page 107 - Handbook of Adhesives and Sealants

Theories of Adhesion 75

TABLE 2.3 Shrinkage of Common
Adhesives 19
Adhesive types % Shrinkage
Acrylics 5–10
Anaerobics 6–9
Epoxies 4–5
Urethane 3–5
Polyamide Hot Melts 1–2
Silicones (Curable) 1

2.4.3 Operation in service
Once the solidification mechanism is complete, the joint is generally
exposed to its service environment. We refer here to the ‘‘joint’’ rather
than the adhesive or sealant because now that the bond is created,
the joint is a single entity that has individual characteristics of its
own.
The service environment may include cyclic exposure to tempera-
ture, stress, chemicals, radiation, or a number of other environments
that are common to the application. It is important that the joint resist
the environmental conditions so that a practical working strength can
be maintained throughout the expected service life. The effect of ser-
vice conditions on the adhesive joint occurs mainly through localized
stress or environmental aging.

2.4.3.1 Short term effects. Localized stresses are mainly due to the
immediate effects of temperature and differences in thermal expan-
sion coefficient. The effect of differing thermal expansion coefficients
on internal stress generated during cure has been discussed in the
preceding sections. However, thermal stresses could easily occur dur-
ing the joint’s service life.
If the temperature is uniform throughout the bond, the approximate
stress on a thin rigid bond may be calculated from the following re-
lationship: 20

T (k k )
S 1 2
(1/E ) (1/E )
1
2
where S shear stress on the adhesive due to differential thermal
expansion rates of the adherends, without consideration
for adhesive strain
E , E Young’s moduli of the adherends
2
1
T temperature differential between zero stress temperature
and service temperature (zero stress condition usually ex-
ists at the cure temperature)
k , k thermal expansion coefficients of adherends
2
1