Page 613 - Handbook of Thermal Analysis of Construction Materials

580 Chapter 14 - Sealants and Adhesives

combined effects of solvent evaporation, ozone attack, migration of plas-
ticizers, UV radiation, water immersion, etc.
To prevent failure and promote certain performance characteris-
tics, additives, such as adhesion promoters, fillers, pigments, plasticizers,
and thixotropic agents, can be introduced into the polymer. Typical fillers
are carbon black, calcium carbonate, talc, clays, and ground silica. Primers,
release agents, and backup materials are used to promote better perfor-
mance. Primers provide better adhesion of the sealant to the substrate. The
more elastic the sealant, the greater the need for a primer. [2]
Adhesive performance is determined not only by the degree to
which the adhesive forms intimate contact with both surfaces, but also by
its ability to resist separation from these surfaces. Therefore, the adhesive
must possess the rheological properties that promote bonding, but resists
debonding. The dual set of properties can be achieved by changing the states
of adhesives from wetting to setting or by manifesting its viscoelastic
properties. [3]
Since an adhesion joint is expected to perform satisfactorily under
the expected service for the lifetime of the bonded structure, it is important
to know or predict the changes in the properties of the materials due to
environment, fatigue, temperature, loading rate, and age. Therefore, knowl-
edge of the chemical and physical properties of both adhesive and adher-
ends is required. [4]
Testing of adhesives is necessary in order to determine the level of
performance and/or predicted durability. Some of the tests provide infor-
mation on the working properties of the adhesive, such as viscosity, which
affects mixing, application, and spreadability as well as wetting and
penetration of the substrate. Other test methods measure the amount of resin
present (solid content). This not only influences the viscosity of the
adhesive, but also the performance of the bonded assembly. [5]

2.0 TEST METHODS

Adhesives and sealants may undergo progressive deterioration
with time as a result of environmental factors. This may be the result either
of chemical changes, for example, thermal degradation, polymer chain
hydrolysis, or physical changes such as crazing and cracking caused by
[6]
diffusion of environmental species into the polymeric materials. All these