Page 441 - Handbook of Adhesives and Sealants

Adhesives Families 381

BMI adhesives offer significant improvement in processability over
polyimides. They are more like epoxy systems than aromatic polyim-
ide systems with respect to their curing requirements. BMI resins cure
by addition reactions, and so no volatiles are given-off during cure.
These systems are usually cured for several hours at 350 F under
pressure and then post cured at or near the maximum service tem-
perature without pressure.

10.3.7.3 Polybenzimidazole. The polybenzimidazole (PBI) adhesive
has shear strength on steel of 3,000 psi at room temperature and 2,500
psi at 700 F. In comparison polyimide adhesives offer a shear strength
of approximately 3,000 psi at room temperature, but polyimide ad-
hesives do not have the excellent strength at 700–1000 F that is char-
acteristic of PBI. Generally for high temperature service, polybenzim-
idazoles are chosen when the maximum short term resistance to the
temperatures are required. Polyimides are chosen when long term re-
sistance to high temperatures are required. Polybenzimidazole adhe-
sives retain good mechanical properties at temperatures as low as
300 F. Therefore, they represent one of only a few adhesives that
exhibit good adhesion characteristics at high temperatures as well as
in the cryogenic temperature range
Volatiles are released during cure of polybenzimidazole adhesives
that contribute to a porous, brittle bond line with relatively low peel
strength. Venting or curing under partial vacuum helps alleviate such
problems. Curing requirements consist of high temperatures (approx-
imately 550–600 F) and pressures (200 psi). Post cure is generally
required in an inert environment. As a result, large area bonding is
very difficult.
Polybenzimidazole adhesive was originally developed in the early
1960s for the aerospace industry as a supported film. It was thought
to be an ideal adhesive for spacecraft application where temperatures
could range from very high to very low depending on the craft’s solar
orientation. As a result of their adverse processing requirements, po-
lybenzimidazole adhesives are only used in special applications and
their commercial use is limited. Today, polyimides and BMI adhesives
have replaced polybenzimidazole adhesives in most high temperature
applications.

10.3.8 Polyesters
Polyesters are a large class of synthetic resins having widely varying
properties. Polyester resin-based adhesives are relatively low cost for-
mulations that have found niche applications. They may be divided