Page 193 - Handbook of Adhesives and Sealants

Standard Test Methods 163

Test Fixture (Fig. 4.18) is a device for applying stresses of various
magnitudes to lap shear specimens and then aging these stressed
specimens in different environments. A plot of stress to time of failure
can give important information about the loss of bond strength under
stressed conditions. This aging phenomenon and a typical stress-to-
failure plot are shown in Chapter 17.
There are several analytical tools that provide methods of extrapo-
lating rate and temperature test data. One of these tools is the Wil-
liams, Landel, Ferry (WLF) transformation. 18 This method uses the
principle that the work expended in deforming a flexible adhesive is
a major component of the overall practical work of adhesion. The ma-
terials used as flexible adhesives are usually viscoelastic polymers. As
such, the force of separation is highly dependent on their viscoelastic
properties and is, therefore, rate and temperature dependent. Test
data, taken as a function of rate and temperature, then can be ex-
pressed in the form of master curves obtained by WLF transformation.
This offers the possibility of studying adhesive behavior over a suffi-
cient range of temperature and rate for most practical applications.
High rates of strain may be simulated by testing at lower rates of
strain and lower temperatures.

4.5 Standard Test Methods for
Sealant Joints
Many of the standard adhesive tests described in the previous section
also pertain to sealants, especially if the need is to determine the ad-
hesion characteristics of the sealant. The fundamental property tests
used to measure consistency or working characteristics of the adhesive
before it is placed in a joint are equally relevant to sealants. However,
high performance sealants generally require test methods that are
different from those used with adhesives.
Many test methods have been developed specifically by the indus-
tries that utilize sealants, such as automotive and construction. There
are various industrial and professional organizations that are at-
tempting to standardize the large number of sealant tests that have
been developed over the years. The International Standards Organi-
zation (ISO) is writing test methods and specifications on a global
basis. As with adhesive joints, many of the most useful and popular
tests for sealant joints are defined in terms of ASTM test specifica-
tions. The more important properties and methods of testing sealants
are described below.
There are several important comprehensive specifications that are
used for sealant materials. These specifications describe test methods