Page 437 - Handbook of Adhesives and Sealants

378 Chapter Ten

when polymerized leads to a ‘‘ladder’’ type of polymeric structure. The
polyimide molecule shown in Fig. 10.5 is a good example of a polymer
with an aromatic ring or ladder structure. High temperatures must
cause at least two molecular chains to break before the molecule un-
dergoes chain scission and resulting degradation of physical proper-
ties.
In addition to their high temperature resistance, polyaromatic res-
ins are unfortunately also noted for their difficult processing condi-
tions. Many of these resins must be made with aggressive, high tem-
perature solvents that are difficult to remove from the final product.
Many aromatic resins harden via a condensation mechanism, produc-
ing a water byproduct as the resin crosslinks. Voids or bubbles in the
bond line can be commonly encountered either from the solvent or
from water liberated due to the condensation reaction. 19
Polyaromatic types of adhesives were developed for high tempera-
ture aerospace applications, but they have limited use due to their
high cost and difficult processing parameters. These adhesives are
generally provided as a supported film, although polyimide resins are
also available in solvent solution. During cure, temperatures of 550–
650 F and high pressures are required. Often a multi-step cure pro-
cess is recommended where slight pressure is initially applied until
the adhesive begins to crosslink and then higher pressures are applied
throughout the remainder of the cure cycle. Volatiles are released dur-
ing cure which contribute to a porous, brittle bond line with relatively
low peel strength. Often a vacuum is required during cure to eliminate
the volatiles that are formed. Table 10.14 lists physical properties and
curing characteristics of these high-temperature adhesive systems.
The toughening of these high temperature adhesives can provide a
difficult challenge, since the service temperatures usually exceed the
degradation point of most elastomeric toughening additives. Improve-
ments in the toughening of high temperature epoxies and other high
temperature thermosets have been accomplished through the incor-
poration of high temperature thermoplastics such as polyarylene ether
ketone (PEK) and polyaryl ether sulfone (PES).

10.3.7.1 Polyimide. Polyimides are well known resins that have been
developed into aerospace adhesive formulations mainly by NASA.
They have a glass transition temperature of at least 200 F greater

Figure 10.5 Aromatic polymeric structure (polyimide) provides
unique high temperature resistance.