Page 152 - Handbook of Adhesives and Sealants

Joint Design 121

free to deform. If the bottom of the joint is bonded, the sealant may
rupture in order to deform.
Sealants can be installed at any temperature, and outdoor sealants
can be installed at any season of the year. The ideal situation is when
the sealant is installed during moderate temperatures so that it will
be subjected to both compression and tension during a normal tem-
perature cycle.

3.6.1.2 Deformable sealants. The rules described in the preceding sec-
tion hold true for elastic sealants that are characterized by their re-
covery. Another broad class of sealants is deformable sealants that
show some degree of instantaneous elasticity or recovery under short-
term load, but they creep under longer-term loading. The deformable
or low recovery sealants show a great deal of stress relaxation. When
held in a deformed state, these materials will relax into equilibrium
so that a new shape is formed in this unstressed state.
The maximum width of these sealants should never be more than
3 ⁄4 in. since the sealant could begin to show some permanent change
with several cycles. Another limitation for this class of sealant is that
they are limited to areas of application in which the movements do
not exceed 12.5%. By limiting this group of sealants to applications
requiring less than 12.5% movement, the sealant will not become
distorted with repeated temperature cycles. Another property of most
sealants in this group is that with the loss of solvent, some sealants
become quite tough and can get harder in time. This age hardening
is not conducive to large joint movements.

3.6.2 Shear joints
The most typical types of sealant shear joints occur in glazing sealants
and sealants placed between glass panels. Strains in shear joints are
less than in butt joints for a given thickness. However, the movement
14
in shear can cause catastrophic failures under certain circumstances .
The strain on the sealant when the shear joint moves 50% is only
1
1
25%. The proper joint shape is a square, such as ⁄2 in. ⁄2 in., if a
great amount of movement is expected. If a sealant with 50% joint
movement is used, the surface could be displaced 112% without ex-
tending the sealant more than 50% in either direction. With use of
low modulus silicone sealant with 100% extensibility, the surfaces
could be displaced up to 175% without extending the sealant more
than 100%. In each case the sealant gets no compression.
Figure 3.27 illustrates the movement in lap shear joints. If greater
movement is anticipated, then it is necessary only to widen the dis-
tance between the moving surfaces.