Page 324 - Handbook of Adhesives and Sealants

Adhesive Classifications 285

temperatures, these materials are not crosslinked and will tend to
creep under load at lower temperatures. This creep or long term de-
formation under load can occur at room temperature or even at lower
temperatures depending on the adhesive. Long term creep is often the
characteristic that prevents these adhesives from being used in struc-
tural applications. Thermoplastic adhesives also do not have general
resistance to solvents or chemicals as do the thermosetting adhesives.

8.2.2.3 Elastomeric adhesives. Because elastomeric adhesives have
unique rheological characteristics, they are given their own classifi-
cation. Elastomeric adhesives are based on synthetic or naturally oc-
curring elastomeric polymers having great toughness and elongation.
These adhesives are made from polymeric resins that are capable of
high degrees of extension and compression. They return rapidly to
their initial dimensions and shape after the load is removed. As a
result, elastomeric adhesives have great energy absorbing character-
istics and offer high strength in joint designs having nonuniform load-
ing. Elastomeric adhesives may be either thermosetting or thermo-
plastic. The thermosetting types can be used in certain structural
applications.
Elastomeric adhesives may be supplied as solvent solutions, water
dispersions, pressure-sensitive tapes, and single or multiple part sol-
ventless liquids or pastes. The form and curing requirements vary
with the type of elastomeric resin used in the adhesive formulation.
Elastomeric adhesives can be formulated for a wide variety of ap-
plications. Because elastomers are highly viscoelastic materials, they
are characterized by a high degree of elongation, low modulus, and
high toughness. This provides adhesives with high peel strength and
a high degree of flexibility to bond to substrates with different expan-
sion coefficients. Elastomers are also commonly used in adhesive for-
mulation for sealants, vibration dampers, and sound enclosures.

8.2.2.4 Hybrid adhesives. Adhesive hybrids are made by combining
thermosetting, thermoplastic, or elastomeric resins into a single ad-
hesive formulation. Hybrids have been developed to capitalize on the
most useful properties of each component. Generally high tempera-
ture, rigid resins are combined with flexible, tough elastomers or ther-
moplastics to provide improved peel strength and energy absorption.
However, early attempts at these combinations usually resulted in an
adhesive that was never better than its weakest constituent. The good
high temperature properties of the base resin were always sacrificed
by the addition of the flexibilizing additive.
The earliest approach to combat brittle failure was to develop ad-
hesive formulations by blending a flexibilizing resin into the body of
another resin. Thus, nitrile-phenolic, epoxy-polysulfide, and other