Page 60 - Handbook of Adhesives and Sealants

28 Chapter One

be ultrasonically welded, thereby, allowing efficient, fast, high volume
assembly operations.
At times, adhesive bonding may be more expensive than other fas-
tening methods. However, the overall cost of the final part may be less
through reduced material requirements, weight savings, elimination
of other operations such as drilling, countersinking, welding, etc., and
simplified assembly. Using associated production processes such as a
paint-drying oven to cure the adhesive may also save costs.

1.4.5 Other advantages
Adhesives are useful for providing secondary functions as well as the
primary function of holding the substrates together. Many designers
feel that one of the most valuable characteristics of adhesive bonding
is their multi-functional nature. In addition to performing a mechan-
ical fastening operation, an adhesive may also be used as a sealant,
vibration damper, insulator, and gap filler—all in the same applica-
tion.
Because adhesives are viscoelastic materials, they can act as vibra-
tion dampers to reduce the noise and oscillation encountered in some
assemblies. Adhesives can also perform sealing functions, offering a
barrier to the passage of fluids and gases. Another property of adhe-
sives that is often advantageous is their ability to function as electrical
and thermal insulators in a joint. The degree of insulation can be
varied with different adhesive formulations and fillers. Adhesives can
even be made electrically and thermally conductive with silver and
boron nitride fillers, respectively. Since adhesives usually do not con-
duct electricity, they prevent galvanic corrosion when dissimilar met-
als are bonded.

1.4.6 Mechanical limitations
The most serious limitation on the use of polymeric adhesives is their
time-dependent strength in degrading environments such as moisture,
high temperatures, or chemicals. For example, organic adhesives per-
form well between 60 and 350 F, but only a few adhesives can with-
stand operating temperatures outside that range. Chemical environ-
ments and outdoor weathering also degrade adhesives. The rate of
strength degradation may be accelerated by continuous stress or ele-
vated temperatures.
The combination of continuous stress along with high moisture con-
ditions is of special concern. Certain adhesives will only survive in
this environment if their service stress is significantly less than their
ultimate strength (e.g., less than 10% of ultimate strength).