Page 375 - Handbook of Adhesives and Sealants

Adhesive Composition and Formulation 331

A common method of toughening adhesives is blending the primary
resin with other polymers, including thermoplastics or elastomers.
However, this technique usually combines the good and bad charac-
teristics of each resin system. The epoxy-nylon adhesives provide a
major improvement in toughness over a pure epoxy formulation, but
they have sensitivity to moisture because of the nylon (polyamide)
constituent. Nitrile-phenolic adhesives have good high temperature
properties due to the phenolic constituent, and they also have good
peel strength due to the nitrile constituent. However, neither the peel
strength nor the temperature resistance is as good as it could have
been if the adhesives were formulated from the pure nitrile or phenolic
base resins respectively. In these conventional hybrid systems, the
added toughening resin reduces the overall glass transition tempera-
ture of the system, thereby, reducing elevated temperature perform-
ance and environmental resistance of the more brittle constituent.
However, newer adhesives systems have been developed with im-
proved toughness without sacrificing other properties. When cured,
these structural adhesives have discrete elastomeric particles embed-
ded in a resin matrix. This can be accomplished in a variety of ways.
One approach with epoxy and acrylic adhesives is by incorporating
small rubber inclusions into the adhesive formulation. The rubbery
polymer is dissolved in the resin binder, and when the adhesive cures,
the rubber precipitates as droplets of about one micron in diameter.
These rubber inclusions absorb energy and stop a crack from propa-
gating throughout the bond-line. This allows enhanced resistance to
fracture, impact, and thermal stress with minimal change in the gross
properties of the matrix resin.
The most common toughened hybrids using discrete elastomeric
particles are acrylic and epoxy adhesive systems. One such toughened
acrylic system is based on a crosslinked network of polymethyl meth-
acrylate with vinyl terminated acrylonitrile butadiene copolymer
rubber particles existing as the dispersed phase. Similarly, adhesive
systems containing epoxy resins with carboxyl-terminated butadiene
acrylonitrile (CTBN) liquid polymers are available. These systems pro-
vide a balance of shear, peel, and environmental performance prop-
erties. The CTBN epoxy resin modifications are used to toughen both
adhesives 10 and composites. 11
Figure 9.3 shows comparison of a control epoxy adhesive formula-
tion and one containing 15 phr CTBN. With the CTBN formulations,
lap shear strength is higher from 50 to 75 C( 60 to 167 F), and
peel strength is four times higher at room temperature. The glass
transition temperature for these systems are 135 C (275 F) for the
control and 129 C (264 F) for the CTBN modified adhesive.
Another class of reactive hybrid adhesive has been developed com-
bining resin mixtures of epoxy and urethane chemistry. These are not