Page 194 - Handbook of Adhesives and Sealants

164 Chapter Four

Figure 4.18 Alcoa stressing fixture for measuring simultaneous effect of stress
and environmental conditions. 18

and also provide minimum values for several different properties of
sealants. Unlike test methods for adhesives, which are separate doc-
uments, these specifications for sealants combine several test meth-
ods. They also provide minimum values for categories of sealants.
ASTM C 920 is such a comprehensive document. It describes the min-
imum acceptable properties required to meet the specification, as well
as movement capabilities of the sealant.

4.5.1 Movement capability
The movement capability of a sealant joint is perhaps one of its most
important characteristics. At a minimum, the sealant joint will move
with response to daily and seasonal temperature changes. This move-
ment capability is determined by the specimen geometry and the co-
efficient of thermal expansion of the substrates.
The movement capability is always stated as a percentage value
that indicates the amount of movement the sealant can take in exten-
sion and compression. Movement capability must consider the envi-
ronment and length of exposure time (i.e., prolonged elongation at low
temperatures, prolonged compression at high temperatures, and com-
binations of the two).
ASTM C 719 is a sealant test procedure that defines several con-
ditions for which the sealant must operate in both extension and com-
pression. The test involves the cycling of sealant joints at a movement