Page 64 - Handbook of Adhesives and Sealants

32 Chapter One

to protect a surface or an article. They can exclude noise and vibration,
improve appearance, and perform a joining function. Certain sealants,
like adhesives, can be used to assemble parts, and many adhesives
can be used to seal. Sealants can also be used as electrical or thermal
insulators, fire barriers, and as products for smoothing, filleting or
faying. No matter what the application, a sealant has three basic func-
tions:

1. It fills a gap between two or more substrates
2. It forms a barrier by the physical properties of the sealant itself
and by its adhesion to the substrate
3. It maintains its sealing property for the expected lifetime, service
conditions, and environments.

Unlike adhesives, there are not many functional alternatives to the
sealing process. Innovative product design can possibly accomplish the
same function as a sealant. Soldering or welding can be used instead
of a sealant in certain instances, depending on the substrates and the
relative movement that the substrates will see in service. However,
the simplicity and reliability offered by organic elastomers usually
make them the apparent choice for performing these functions. Many
sealants are designed for specific applications. Table 1.9 gives typical
applications for various classes of sealants.
The proper application of a sealant involves more than merely
choosing a material with the correct physical and chemical properties.
As with adhesives, the substrates to be sealed, the joint design, per-
formance expectations, production requirements, and economic costs
must all be considered. Table 1.10 is a partial list of considerations
that are often used to select sealants in the construction industry.

1.5.1 Mechanical considerations
Important mechanical properties of sealants include elongation, com-
pressibility, tensile strength, modulus of elasticity, tear resistance, and
fatigue resistance. Depending on the nature of the application, a seal-
ant may require very little strength or great strength. The sealant
must have sufficient mechanical characteristics to remain attached to
the substrates during service and to provide a barrier. The substrates
could move considerably, requiring that the sealant expand and con-
tract significantly without loosing adhesion from the surface. Defining
the sealant’s movement capability is a complex process. Temperature,
test rate, and joint configuration will influence the result.
While movement capability is very important, other consequential
mechanical properties are: unprimed adhesion strength to various