Page 199 - Handbook of Adhesives and Sealants

Standard Test Methods 169

Figure 4.22 Typical creep curve for sealant. 20

Ozone and ultraviolet (UV) radiation are factors in environmental
exposure of many sealants. Major cities now exhibit 50 pphm of ozone
in their atmospheres, which is the level set in standard ozone envi-
ronmental tests. Ozone at this level will cause surface cracking after
reasonable exposure. UV will similarly cause some hardening of cer-
tain sealants and will cause some to lose adhesion. UV is of special
concern for sealants that are used in glass applications. Resistance to
surface degradation after several hours in a standard weatherometer
is indicative of good performance for a sealant.
Resistance to solvents or chemicals is not a standard requirement
for sealants. However, the tests are easy to run, and most sealant
manufacturers have such data. The time for immersion varies consid-
erably, and it should be recognized that the absorption of chemical
into the sealant is generally not very fast.
The effect of heat aging at moderate temperatures on sealants is
generally through the loss of volatile plasticizers. Higher tempera-
tures result in increased crosslinking, oxidation, and pyrolysis of the
sealant. This, in turn, results in increased hardness and reduction of
movement capability. ASTM C 920 specification requires a maximum
percentage of weight loss of 10–12% after heat aging for two weeks at
158 F. The conditioning generally specified is the application of accu-
mulated time at temperature expected in service. For example, the
total hours of service expected at 200 F is estimated over the sealant’s
life and incorporated into the testing methodology. It is unwise to at-
tempt to accelerate testing by increasing temperature above the actual
service temperature without knowledge of the material characteristics
of the sealant. At higher temperatures, additional reaction mecha-
nisms may take place related to the thermal movement of molecules
within the sealant.