Page 407 - Handbook of Adhesives and Sealants

354 Chapter Ten

having tensile shear strength greater than 1000 psi and properties
that do not change significantly with moderate aging. Non-structural
adhesives are those with less than 1000 psi shear strength and du-
rability that requires a sheltered or relatively mild environment.
Several adhesives are difficult to classify in this manner. For ex-
ample, is a hot melt textile adhesive used in clothing manufacture a
structural or non-structural adhesive? It has moderate strength and
sufficient heat and chemical resistance to withstand washings and
perhaps dry-cleaning. The clothing manufacturer may consider a hot
melt adhesive to be as ‘‘structural’’ a fastener as thread or stitching.
Hot melts are perfectly adequate for the application. However, hot
melts are not generally considered structural adhesives in that they
are not used as a primary fastener in a structural application requir-
ing long lasting exposure to a variety of environments. On the other
hand, cyanoacrylate adhesives, more commonly known as superglues,
are usually classified as structural adhesives. However, their resis-
tance to heat and moisture is relatively weak. Thus, classification of
adhesives as either structural or non-structural should only be used
to broadly define adhesive families and not to judge functional differ-
ences.
Polymeric adhesive families can also be classified by their chemical/
physical nature as being either thermoplastic, thermosetting, elasto-
meric, or a hybrid modification depending on the polymer chemistry
of the main constituent resins. Thermoplastic adhesives are not cross-
linked and revert to a fluid state when heated. Thermosetting adhe-
sives result in crosslinked molecular structures and do not soften with
heat or solvents. Elastomeric adhesives are formulated from natural
or synthetic rubbers; they are characterized by high elongation and
usually low tensile strength. Elastomeric adhesives can be either ther-
mosetting or thermoplastic. Because of their relative molecular mo-
bility, thermosetting elastomeric adhesives soften with heat but do not
become fluid as do the thermoplastics. Therefore, they are not consid-
ered in the same classification as thermosetting adhesives. Hybrid ad-
hesives are a class of adhesives compounded from a mixture of poly-
mers for specific property adjustment. They generally consist of two
or more resin constituents each of which has characteristics that are
valued in the application but are difficult to obtain with a single resin
(e.g., high temperature resistance and toughness). Usually, hybrids
are thermosetting in character and are considered to be structural
adhesives. The adhesive families represented by these four broad clas-
sifications are described in Tables 10.2 through 10.5.
The types of polymeric resins that are listed under thermosetting
and hybrid adhesives are noted for high strength, creep resistance,
and resistance to environments such as heat, moisture, solvents, and
oils. Their physical properties are well suited for structural adhesive
applications.