Page 170 - Handbook of Adhesives and Sealants

140 Chapter Four

The shelf life is generally defined as the time that an adhesive or
sealant can be stored, preferably under controlled conditions, and re-
main unchanged at least from a fundamental property point of view.
ASTM D 1337 provides a method for determining the shelf life of an
adhesive or sealant. The changes in consistency (viscosity) or bond
strength are measured after various storage periods at a specified tem-
perature.
All of the materials tested for shelf life are stored in unopened con-
tainers. Storage in containers that are once opened exposes the resin
to oxygen and humidity that, depending on the type of resin, could
drastically reduce the shelf life and affect final properties. Most poly-
meric adhesives and sealants have a shelf life greater than 6 months
at room temperature. However, some one-component adhesives need
to be stored at refrigerated conditions to have a practical shelf life.
Certain adhesives or sealants may also have limited life when stored
at cold temperatures or when exposed to repeated freeze/thaw cycles.
Generally, these are water based products, but freeze/thaw stability
problems are not restricted to them. The resistance to freeze/thaw
cycles or to low temperature storage is measured using procedures
similar to the shelf life measurement discussed above. Even though
an adhesive or sealant may have good low temperature storage prop-
erties, it should be brought to room temperature before mixing or ap-
plication.

4.3.1.3 Working life. The working life of an adhesive or sealant is the
time from when the product is ready for use (i.e., mixed and ready to
apply to a substrate) and the time when it is no longer usable because
the setting mechanism has progressed to such an extent that the prod-
uct is no longer workable. This characteristic is also known as the pot
life of the adhesive. ASTM D 1338 establishes two procedures for de-
termining working life. One method uses viscosity change, and the
other uses shear strength as the criteria for determining when the
effective working life has expired.
Working life is usually determined on a volume of adhesive or seal-
ant material that is practical and normally used in production. The
volume of the tested material must be defined in the test report be-
cause many adhesives and sealants have a working life that is depen-
dent on sample mass.

4.3.1.4 Tack. Tack is the property of an adhesive to adhere to another
surface on immediate contact. It is the ‘‘stickiness’’ of the adhesive
while in the fluid state. Sometimes tack is also referred to as ‘‘green
strength’’. Tack is an important property for many pressure sensitive
adhesives and preformed sealants.