Page 516 - Handbook of Adhesives and Sealants

446 Chapter Eleven

regarding the environmental resistance of various adhesive classifi-
cations appears in Chapter 17.

11.7.1 Service stress
The adhesive should be chosen with respect to the type of stress that
will be encountered. Peel or cleavage types of stress dictate a tough,
somewhat flexible adhesive to accommodate uneven stress distribu-
tion. Shear or tensile types of stress will result in more uniform stress
distribution and will allow the selection of rigid adhesives that have
higher cohesive strength. The chosen adhesive should have strength
great enough to resist the maximum stress experienced at any time
in service with reasonable safety factors. Over-specifying may result
in certain adhesives being overlooked which can do the job at lower
cost and with less demanding curing conditions. The adhesive must
also be resistant to lower stresses that will occur during long contin-
uous periods. Thermosetting adhesive systems are more likely to re-
sist these creep stresses than are elastomeric or thermoplastic adhe-
sives.
Adhesive types display wide differences in response to different
stresses and strain rates. Thermoplastic and many elastomeric ad-
hesives are not suitable for structural applications because they have
a tendency to distort and creep under moderate load. They also soften
on heating at moderate temperatures. Thermoplastic adhesives are
unable to withstand vibratory stresses for long periods although they
may exhibit greater strengths than thermosetting types for short du-
ration tests. Thermoplastic and elastomeric types usually possess high
peel strengths but relatively low tensile or shear strengths.
The thermosetting resins, on the other hand, are often used as the
base material in structural adhesives. They provide rigid bonds that
retain much of their strength at elevated temperatures. Tough ther-
mosetting and hybrid adhesives also perform well under vibratory
loads and have moderately good impact loading characteristics. Amine
cured epoxies have a room temperature lap shear strength of 2,000
psi; the strength is approximately 500 psi greater when an anhydride
or a polyamide curing agent is specified. Epoxy-phenolics have lap
shear strengths of nearly 3,500 psi, and the epoxy-nylons reach lap
shear strengths of 6,000 psi. Phenolic resins combined with polyvinyl
butyral result in adhesives with excellent shear strength. Room tem-
perature strength may be as high as 5,000 psi. The neoprene modified
phenolics have lower shear strengths than most modified phenolics.

11.7.2 Service environment
Another important factor to be considered is the temperature range
over which the adhesive will be used. At elevated temperatures, all