Page 528 - Handbook of Adhesives and Sealants

456 Chapter Twelve

capability in certain applications. These materials are available in
many forms and are characterized by a ‘‘mastic’’ consistency. Soft set-
ting natural resins and oil-based sealers are examples of non-
hardening sealants. They are available in drying or non-drying types.
The drying types remain soft inside the seal by developing a protective
skin. The non-drying types remain tacky to the touch and are uni-
formly soft. Some non-hardening sealants can be formed into various
shapes and applied as continuous beads or tape. They are usually
supplied with a coated paper or plastic film release lining that is re-
moved during application so that the sealant can be pressed into place
by hand. Table 12.3 lists representative non-hardening sealants.

12.2.2 Cure type
Sealants are sometimes characterized by their method of cure. Curing
parameters generally involve catalytic agents, time, temperature, or
humidity. As with adhesives, curing characteristics can vary signifi-
cantly within the various families and formulations. Depending on
their curing method, sealants are more popularly classified as: two-
part systems, single component sealants, or solvent or water release
sealants.

12.2.2.1 Two-part systems. Two-part systems are usually faster re-
acting than the other types of curing sealants. Faster cures make it
possible to handle prefabricated units and reduce the overall cycle
time required for the sealing operation. Faster cures are also impor-
tant when the sealant is used in outdoor applications. When applied
outdoors, the time from application of the sealant until its first expo-
sure to abrasion, weather conditions, or thermal cycle may be critical.
Similar to two-part adhesives, the two-part sealant is prepared and
packaged as two or more components. One component contains the
base resin component and compatible fillers, extenders, etc. Another
component contains the catalyst or curing agent and appropriate ad-
ditives. Perhaps a third component will contain a catalyst or acceler-
ator. These components must be metered to the proper mixing pro-
portions and mixed thoroughly prior to application. Some sealant
systems, such as the polysulfides, are known as oxidatively or cata-
lytically cured sealants. These are cured by the addition of an oxidiz-
ing agent such as a peroxide. Other two-part systems, such as the
silicones, urethanes, and epoxy systems, are cured by chemical reac-
tion.
Two-part sealants may cure at either ambient or elevated temper-
atures. Generally, sealants are applied and cured at either room or
plant conditions or at outdoor ambient conditions. When sealants are