Page 569 - Handbook of Adhesives and Sealants

490 Chapter Thirteen

nents is critical and must be carefully controlled at the job site. Sur-
faces must be very clean to achieve the best adhesion.
Single component polysulfides are an advantage in that no metering
or mixing is required. The formulation is similar to two part systems
except for the curing agent, an alkali oxide, which reacts with mois-
ture to initiate the polymer chain extension and crosslinking. Single
component polysulfides skin rapidly at high humidities. Cure is slow
reaching only about 50% of the ultimate properties in 7 days depend-
ing on the relative humidity.
Polysulfide sealants have a movement capability of 25%. Hard-
ness can be varied from that of soft rubber (Shore A of 20) to hard
rubbers (Shore A of 50) for applications that must withstand pene-
tration. Properly formulated, polysulfide sealants can have a service
life of 20 years or more. Most of the polysulfide sealants have excellent
resistance to solvents and fuels along with good weatherability.
Polysulfide sealants have been widely used in such applications as
potting and molding of electrical connectors, glazing of windows, and
sealing of windshields of automobiles. They have also been used with
asphalt or coal tar as sealants for roadways, bridges, and airport run-
ways.
Table 13.6 lists some of the advantages and disadvantages with poly-
sulfide sealants over other common sealants. The principle advantages
of polysulfide sealants are ease of application, good adhesion, good
resistance to weathering and solvents, negligible shrinkage, and good
moisture vapor transmission. 9

13.4.3 Polyethers
Silyl terminated polyether sealants, known as MS sealants, have be-
come the most used type of construction sealant in Japan since their
introduction some 20 years ago. In the U.S., MS sealants have been
primarily used for industrial application; construction use is still in
the developmental stages. 10,11 The MS sealants have high performance
capabilities with many of the same characteristics as a urethane seal-
ant. However, the one-part MS sealants cure much faster than a
one-part urethane sealant and find use in the construction industry
where property development speed is important.
One component and two component MS sealant formulations are
available. The MS polymer is silyl terminated polyether prepared from
high molecular weight polypropylene oxide. It is end capped with allyl
groups, followed by hydrosilylation to produce a polyether end capped
with methyldimethoxysilane groups. In the presence of appropriate
catalyst, the methoxysilane group can be cured by moisture. The water