Page 463 - Handbook of Adhesives and Sealants

400 Chapter Ten

systems cure at room temperature, but do not reach full strength for
3–7 days. Since little or no solvents are used in the formulations,
shrinkage during setting is negligible. Hence, polysulfides are used in
a variety of sealing applications.
Because of their relatively low strength, creep tendency under low
loads, and low resistance to elevated temperatures, these materials
are primarily used as sealants that are applied with an extrusion gun.
Resistance to water, organic solvents, grease, oils, and salt water is
better than that of the other thermoplastic rubber adhesives. Low
temperature properties are excellent with retention of flexibility to
80 F. However, high temperature resistance is poor, and the material
usually softens at 160–200 F with very little strength over 250 F.
Some polysulfide elastomers have been used to reduce the rigidity
of two part thermosetting epoxies. These modified adhesives have been
used to bond aluminum and have very good durability although low
tensile strength relative to the unmodified epoxy adhesive.

10.4.1.11 Silicone. Silicone resins have low shear properties but ex-
cellent peel strength and heat resistance. Silicone adhesives are avail-
able as either solvent solutions for pressure sensitive adhesives or as
one or two part liquids.
Silicone adhesives are generally supplied as solvent solutions for
pressure-sensitive application. These systems cure via a condensation
and radical polymerization process. The resulting adhesives are very
tacky and exhibit only moderate peel strengths as a result of their
very poor cohesive strength. The adhesive reaches maximum physical
properties after being cured at elevated temperature with an organic
peroxide catalyst. A lesser degree of adhesion can also be developed
at room temperature.
Silicone adhesives retain their adhesive qualities over a wide tem-
perature range, and after extended exposure to elevated temperature.
They are very tacky materials that bond to a wide variety of sub-
strates. Because silicones have a relatively low surface energy, they
bond well to many low surface energy plastics such as polyethylene
and fluorocarbons. Table 10.19 shows typical adhesive-strength prop-
erties of pressure-sensitive tape prepared with aluminum-foil backing.
Pressure sensitive silicone adhesives are often used in the form of
pressure sensitive tapes. A large application is pressure sensitive tape
used in the electronics industry for various applications on printed
circuitry. They are also often used as adhesives on silicone rubber
backing for gasketing on ovens and other high temperature apparatus.
Room-temperature-vulcanizing (RTV) silicone-rubber adhesives and
sealants form flexible bonds with high peel strength to many sub-