Page 325 - Handbook of Adhesives and Sealants

286 Chapter Eight

resin blends provided resilience and toughness due to the elastomeric
ingredients in the formulation. A limitation to these systems was that
the elastomeric component usually lowered the glass transition tem-
perature and degraded the elevated temperature and chemical resis-
tance that was characteristic of the more rigid resin. These alloy
blends are still in wide use today especially in the aerospace and
transportation adhesive markets. They are commonly available in sol-
vent solutions and as supported or unsupported film. Some single and
two part liquid systems are also available.
More recently, advanced hybrid adhesive systems have been devel-
oped in an attempt to improve peel strength and toughness of ther-
mosetting resins without reducing high temperature properties. These
systems consist of:

1. Reactive hybrids, where two liquid components are reacted to-
gether rather than being merely blended together
2. Dispersed phase hybrids, where the flexibilizing medium is incor-
porated as discrete particles in a resin matrix

In reactive hybrid systems, the flexibilizing resin is reacted with the
base resin to provide flexibility and toughness without significant re-
duction of other properties. A typical example of this system is the
epoxy-urethane adhesive. Another method of providing toughness is
by introducing a specific microstructure within the adhesive. This mi-
crostructure consists of a physically discrete but chemically inter-
linked elastomeric phase as shown in Fig. 8.2. Vinyl and carboxy ter-
minated acrylonitrile butadiene (CTBN) polymers can be dispersed in
either epoxy or acrylic resin phases.
These hybrid adhesive systems possess high peel, impact, and shear
strengths without sacrificing elevated temperature or chemical resis-
tance properties. They also have a tendency to bond well to oily sub-
strates. It is believed that the oil on the substrate is adsorbed into the
formulation and acts as another flexibilizer in the adhesive system.

8.2.3 Method of reaction
Another distinction that can be made between adhesives is the man-
ner in which they react or solidify. There are several methods by which
adhesives can solidify:

By chemical reaction (including reaction with a hardener or reaction
with an outside energy source such as heat, radiation, surface cat-
alyst, etc.)