Page 544 - Handbook of Adhesives and Sealants

Sealant Classification and Composition 469

12.3.6 Other additives
Many other additives, similar to adhesive system additives, are used
for sealant formulations to provide special performance, application,
or storage properties. For example, antibacterial agents and freeze-
thaw cycle suppressants are commonly used in water-based sealant
formulations as well as with adhesives. Many sealant additives are
necessary because of the unique characteristics required of a sealant
such as long term exposure outdoors or color matching. Sealant for-
mulations may also include UV absorbers and ozone inhibitors to pre-
vent any undesirable sensitivity to outdoor environments. The linear
hydrocarbon rubbers are especially sensitive to sunlight and ozone
and are generally used only for indoor applications.

12.4 Sealant Formulation
Minor formulation changes in sealants will often result in major
changes in application and performance properties. Because of this
and because of the variety of base polymers available, it is difficult to
provide an organized guide to sealant formulation. A basic under-
standing of how some of the more important properties are controlled
through formulation will help in determining a correct sealant choice.
Prior to formulating a sealant, the desired property profile needs to
be defined. Table 12.9 presents a list of properties to be considered
when defining the performance requirements of a sealant in both its
cured and uncured states.

12.4.1 Application properties
The speed at which a sealant cures is critically important in some
industries. Catalytic primers, two component systems, or controlled
environments are used to increase the rate of cure. Depth of cure and
the rate at which cure is achieved throughout the joint are other im-
portant considerations for a sealant. Single component sealants such
as silicones, urethanes, and polysulfides, which depend on the diffu-
sion of atmospheric moisture or oxygen into the sealant for curing, can
take days or even weeks to cure entirely throughout the sealant bead.
These sealants will develop a skin that will then inhibit the diffusion
of water vapor to the center regions of the seal. In fact, some of these
systems are limited to very small cross-sectional beads or else they
will never fully cure throughout.
Shrinkage on curing can be a critical factor for sealant systems as
well as adhesives. Excessive shrinkage can result in internal stresses
and voids in the joint. Low shrinkage products are those with very