Page 424 - Handbook of Adhesives and Sealants

Adhesives Families 367

Properties and curing characteristics of commercially available epoxy
hybrid adhesives are presented in Table 10.9.
The most common types of epoxy hybrid adhesives are epoxy resins
that are toughened with elastomeric resins and alloyed blends con-
sisting of epoxy-phenolic, epoxy-nylon, and epoxy-polysulfide adhe-
sives. These are described in the following sections.

10.3.2.1 Toughened epoxies. Because of the normally brittle nature of
the epoxy resin, epoxy adhesives have been toughened with many dif-
ferent resins during their history. These include thermoplastic parti-
cles, nylon, and various elastomers. The most successful of these are
epoxy adhesives that embody discrete nitrile rubber regions as the
elastomeric toughening component. Small rubber inclusions impart
toughness and high peel strength to structural epoxy adhesives. Both
DGEBA and DGEBF epoxy resins have been optimized for toughness
with additions of nitrile rubber. Dicyandiamide curing agents provide
one component toughened epoxy formulations. Toughened epoxy ad-
hesives are available as liquids, pastes, and films.
Many one component epoxies contain rubber toughening agents
such as carboxylic acid terminated butadiene nitrile rubber (CTBN). 6–
8
These elastomers are pre-reacted at the 5-15% level with the epoxy
resin at high temperature. During the final cure, the rubber must
precipitate out of solution into discrete particles within the epoxy resin
matrix. Ideally, a distinct phase will toughen the epoxy without sig-
nificantly reducing the high temperature characteristics or glass tran-
sition temperature (Tg) of the original epoxy resin. The small distinct
regions of elastomer within the epoxy matrix provide crack inhibitors
and internal stress relief mechanisms. These result in a general in-
crease in toughness and impact strength and moderate improvements
in peel strength without significant reduction of the glass transition
temperature of the adhesive. The high impact strength and elasticity
is retained at low temperatures.
Toughened epoxy adhesives are generally available as single com-
ponent, elevated temperature curing systems. Typical cure require-
ments are one hour at 250 F. CTBN modified epoxy adhesives tend to
be slower reacting and higher in viscosity than conventional epoxy
systems. Two component toughened epoxy adhesives have also been
developed to cure at ambient or mildly elevated temperatures. In these
systems, CTBN or amine terminated acrylonitrile butadiene (ATBN)
are used as the elastomeric phase and amine hardeners are used to
achieve cure. 9,10 Table 10.10 shows typical single and multiple com-
ponent epoxy adhesive formulations using CTBN toughening agents.
Table 10.11 also shows the bulk properties and peel strength of an
adhesive formulation with and without CTBN modification. Tensile
shear strength and temperature resistance of these adhesives are rel-