TM 5-805-6
movement, resistance to temperature extremes,
(5) Neutral curing silicone sealants. (Note: See
and resistance to weather and ultra violet light.
following paragraph 9 for general curing proper-
Quality silicone sealants have a life expectancy up
ties of sealants.) These sealants also cure by
to 50 years and are often specified in place of
reacting with moisture from the air, but the
polysulfide or polyurethane sealants. Silicones
resulting by-product may be an alcohol or ammo-
should not be exposed to abuse or traffic; they are
nia, depending on the nature of the polymer.
generally less resistant to abrasion and penetra-
Neutral-curing sealants are compatible with most
tion than polyurethane or epoxy sealants.
building materials and other types of silicone
(a) Silicone sealants are vulnerable to at-
sealants.
h. Structural silicone sealants. These sealants
tack by solvents, petroleum products, and some
oxidizing chemicals. Prolonged immersion in water
have minimum tensile, rheological, and aging-
degrades silicone sealants, and chlorine, as in
resistance values which exceed those required for
swimming pool water, accelerates the degradation.
silicones classed as elastomeric sealants. The
(b) Physical properties of silicone vary from
structural silicones are available in either one- or
high-strength low-elongation to low-strength high-
two-component systems and are used to support
elongation; the stronger materials are used for
and retain glazing or other types of panels, some-
structural bonding of glass and thin stone while
times with no structural support visible on the
the high-elongation silicones can withstand joint
outside of the building. Design of such structures
movement from plus 100 to minus 50 percent.
requires a rigorous structural analysis beyond the
Properties of two-component and single-component
scope of this manual. Due to their premium costs,
silicones are similar except that two-component
structural silicone sealants are usually limited to
systems provide an adjustable pot life (10 minutes
applications where high strength is required. How-
to 3 hours) and cure time (4 hours to 7 days). The
ever, they can generally be used in place of any of
two components may be two polymers or, more
the sealants described above, except that, like
commonly, a polymer and a curing agent which
other silicones, they will not accept paint and are
reacts with the polymer or acts as a catalyst to
more easily damaged than polyurethane sealants.
promote polymerization. Curing is accelerated by
i. Preformed sealants. Preformed sealants de-
higher temperature but is not usually affected by
pend on continuous, positive compression rather
ambient relative humidity. Additionally, some
than adhesion for their sealing action. They are
two-component silicones are not compatible with
generally designed for specific uses and fixed
some acetoxy single-component silicones as dis-
geometry joints in contrast to the more versatile
cussed in following paragraph 8.g.(4).
field-formed sealants. Preformed sealants are
(c) Special silicone sealants are available for
available as beads, tape, or special shapes for
solar collectors, sanitary sealing, and glazing.
specific tasks, and they may be either plain or
(d) Silicone sealants usually have high sol-
reinforced. Preformed sealants should be used in
ids content (low shrinkage), are available in colors,
joints that are subject to little or no movement.
and will not accept paint unless formulated to do
The service temperature ranges of common pre-
so, e.g., silicone-polyether copolymers will accept
formed sealants are: from -30 to +70 C (-20 to
+160 F) for vinyl chloride; from -40 to + 107 C
paint.
(-40 to +225 F) for polychloroprene; from -35 to
(4) Acid-curing silicone sealants. (Note: See
+148 C (-30 to +300 F) for isobutylene-isoprene
following paragraph 9 for general curing proper-
copolymers (butyl) or polyurethane; from -30 to +
ties of sealants.) In acid or "acetoxy" curing
70 C (-20 to +160 F) for polyethylene or
sealants, moisture from the air reacts with the
styrene-butadiene (SBR); and from -54 to +204 C
polymer to produce acetic acid; as a result, the
(-65 to +400 F) for silicone sealants. Some
curing process is accelerated by increased humid-
compounds based on polytetrafluoroethylene will
ity. The acetic acid byproduct can migrate into,
survive extended service at degrees 260 C (500 F).
and react with, porous substrates or it will eventu-
Figure 8 shows an application of a preformed
ally evaporate into the surrounding air. Some
sealant.
acid-curing silicones have high strength, but they
(1) Non-resilient tapes and beads. These seal-
are not recommended for use with concrete, ma-
ants are essentially preformed strips of non-curing
sonry, or insulating glass and may be imcompat-
ible with some two-component silicone sealants.
sealant. They are generally made from uncured
butyl or polybutene mixed with fillers such as
This incompatibility can cause a structural failure,
e.g., a field-applied single component silicone can
clay, calcium carbonate, and other fillers or pig-
ments which help to hold the shape of the tape
damage a two-component structural silicone seal-
before it is applied. Non-resilient tapes are really
ant in a factory-made window assembly.
8
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