Page 428 - Handbook of Adhesives and Sealants

370 Chapter Ten

Another toughened epoxy formulation comes from mixtures of
epoxies and urethane oligomers with pendant epoxy groups. 14 Curing
of the pendant epoxy groups unites the urethane and non-urethane
components through conventional epoxy reactions to give tough, du-
rable films. These adhesives are generally referred to as ‘‘reactive’’
epoxy urethanes. This approach, however, does not provide high
temperature properties as does the CTBN particle approach. These
reactive hybrid epoxy-urethane adhesives were developed initially for
bonding to oily cold rolled steel, but they have given good results on
other substrates as well. Table 10.12 shows lap shear values obtained
after curing 20 mins at 350 F on various untreated substrates.
A third approach is to use surface activated pre-cured rubber par-
ticles directly in the adhesive formulation. This process does not in-
volve separating out a rubbery phase during cure. Elastomers that
have typically been used in this approach have been urethanes and
15
reclaimed tire products. These materials are specially treated so that
they provide good adhesion to the epoxy resin matrix. The main ben-
efits of these formulations are low cost and moderate improvement in
toughness and flexibility.
Although the urethane oligomer and pre-cured rubber particle for-
mulations provide good peel strength and toughness, they do not
maintain the high thermal properties inherent in the epoxy matrix as
does the discrete CTBN or ATBN phase approach to toughening.
It also should be noted that soft, flexible epoxy resins have been
developed through modification 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 flexibil-
ized epoxies are found in applications having moderate temperature
requirements. They are primarily used in flexible adhesives, sealants,

TABLE 10.12 Cured Lap Shear Strength of Uncleaned
Substrates Bonded with an Epoxy Urethane Reactive
Hybrid System 14
Substrate
thickness Lap shear
Substrate (mils) (psi)
Oily steel 58 3,400
Galvanized steel 75 2,770
Polyester-painted steel 18 1,440
Painted steel/galvanized steel 18/75 1,760
Electrogalvanized steel 28 1,930
Organic-coated steel 33 2,160
SMC (polyester-glass) 100 600*
R 303 aluminum 65 3,340

*Substrate breaks