Toughening Agents. Because of the normally brittle nature of the epoxy resin, epoxy
adhesives have been toughened with many different resins during their history. These
include thermoplastic particles, 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 adhesives are available
as liquids, pastes, and films.
Many one-component epoxies contain rubber-toughening agents such as carboxylic
acid terminated butadiene nitrile rubber (CTBN).6,7,8 These elastomers are generally pre-
reacted at the 5 to 15 percent 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 significantly reducing
the high temperature characteristics or 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 increase in toughness and impact strength and
moderate improvements in peel strength without significant effect on the glass transition
temperature of the adhesive. The high impact strength and elasticity are retained at low
temperatures.
Toughened epoxy adhesives are generally available as single-component, elevated temperature
curing systems that employ dicyandiamide as the curing agent. Typical cure requirements
are 1 hr at 125°C. 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 13.5 shows typical
single- and multiple-component epoxy adhesive formulations using CTBN toughening agents.
Table 13.6 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 relatively high. Peel strength is also high and does not decrease
rapidly with lower temperatures. The durability of CTBN modified epoxies are satisfactory
as measured by most long-term moisture tests, but it does not match the durability
of the vinyl-phenolic or nitrile-phenolic types.12 Nitrile-epoxy adhesive systems should
not be used in marine environments or under continuous immersion in water.13
Another toughened epoxy formulation comes from mixtures of epoxies and urethane
oligomers with pendant epoxy groups. Curing of the pendant epoxy groups unites the urethane
and nonurethane components through conventional epoxy reactions to give tough,
durable films. This approach, however, does not provide high temperature properties as
does the CTBN two-phase approach. Reactive hybrid epoxy-urethane adhesives were
developed initially for bonding to oiled steel substrates, but they also provide good results
on other substrates as well. (See Sec. 13.3, Epoxy Urethane.) Table 13.7 shows lap shear
values obtained after curing 20 min at 177°C on various untreated substrates.
Another approach to toughening is to use surface activated precured rubber particles
directly in the adhesive formulation. This process does not involve separating out a rubbery
phase during cure. Elastomers that have typically been used in this approach have been urethanes
and reclaimed tire16 products. These materials are specially treated so that they provide
good adhesion to the epoxy resin matrix. The main benefits of these formulations are
low cost and moderate improvement in toughness and flexibility.
Although the urethane oligomer and precured rubber particle formulations provide good
peel strength and toughness to the adhesive system, they do not maintain the high thermal
properties inherent in the epoxy matrix as does the discrete CTBN or ATBN two-phase
approach to toughening.
It also should be noted that soft, flexible epoxy resins have also been developed through
selective chemistry of the epoxy molecule. These systems do not have high temperature
properties, but they are marketed for their cost effectiveness and extremely low durometer.
These materials find themselves in applications having moderate temperature requirements.
They are primarily used in flexible adhesives, sealants, caulking compounds, encapsulants,
potting compounds, conformal coatings, and molding compounds for rubbery parts.17
Several other approaches have been developed for improving the toughness and flexibility
of epoxy adhesives. Modification of epoxy resin with alkyl resins is reported as a
means of flexibilizing epoxies for adhesive applications.18 Modification of epoxy resins
through the direct addition of engineering thermoplastic, such as polysulfone, has also been
proposed to enhance the toughness of epoxy adhesives.19 It is likely that the search for
toughened epoxy adhesives will continue in the future.
Other Components. As shown in Table 13.8, the epoxy resin and curing agent are rarely
used alone as an adhesive system. Rather, they are compounded with various modifiers and
additives in order to improve properties, such as strength, flow, and heat resistance, and to
add or advance other properties that are demanded by the specific application.
Secondary ingredients in epoxy adhesives include reactive diluents to adjust viscosity;
mineral fillers to lower cost, adjust viscosity, or modify coefficient of thermal expansion;
and fibrous fillers to improve thixotropy and cohesive strength. Epoxy resins are often modified
with other resins to enhance certain properties that are necessary for the application.
Often these modifications take the form of additions of elastomeric resins to improve
toughness or peel strength.
Toughening Agents. Because of the normally brittle nature of the epoxy resin, epoxy
adhesives have been toughened with many different resins during their history. These
include thermoplastic particles, 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 adhesives are available
as liquids, pastes, and films.
Many one-component epoxies contain rubber-toughening agents such as carboxylic
acid terminated butadiene nitrile rubber (CTBN).6,7,8 These elastomers are generally pre-
reacted at the 5 to 15 percent 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 significantly reducing
the high temperature characteristics or...
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