Page 172 - Handbook of Adhesives and Sealants

142 Chapter Four

Cure rate is an important factor when the expense of jigs and fix-
turing equipment is high or fast production rates are critical. It is also
used as a quality control test to determine if the adhesive’s or sealant’s
curing mechanism has changed from lot to lot or if it may have been
spoiled by storage, moisture contamination, etc. ASTM D 1144 pro-
vides a recommended practice for determining the rate of bond
strength development for either tensile or lap shear specimens.
There are also several methods of determining cure rate on the bulk
material. These are generally analytical procedures that are common
in most polymeric material laboratories. With these methods, funda-
mental properties, such as dielectric loss, mechanical damping, and
exotherm, are measured as a function of time and temperature. Sev-
eral of these test methods are described in the next chapter.
Cure time is very important for sealants as well as adhesives. Often
the sealant will be required to function as a barrier or resist the move-
ment of substrates very soon after it is applied. With construction
sealants, for example, it may not be possible to delay the environmen-
tal conditions until after the adhesive cures. Thus, curing time be-
comes a critical parameter in selection of the sealant. ASTM C 679 is
a method for determining the time that a mechanic can work the seal-
ant into the joint before the sealant starts to skin or solidify.

4.3.1.6 Hardness. Hardness of the adhesive or sealant may be used
as an indication of cure. It may also be used as a quality control check
on certain substrates. Hardness may be determined in several ways:
resistance to indentation, rebound efficiency, and resistance to scratch-
ing or abrasion. The indentation method is the most commonly used
technique.
There are several ways of measuring indentation, but they only dif-
fer in the type of equipment used. Basically, they all measure the size
of indentation produced by a hardened steel or diamond tool under a
defined pressure. A durometer is an instrument for measuring hard-
ness by pressing a needle-like instrument into the specimen. Durom-
eters are available in several scales for measuring hard materials to
soft elastomers. The two types appropriate for most cured adhesives
and sealants are the Shore Type A and Shore Type D. ASTM C 661
offers a method for measuring indentation hardness of elastomeric
type sealants.
Lower hardness readings than expected may be an indication of un-
der-cure or of a formulation change in the product. It may also be an
indication of entrapped air in the adhesive or sealant or an unwanted
chemical reaction with the environment. Higher hardness readings
than expected may be an indication of over-cure.