Page 202 - Engineering Plastics Handbook

Thermoplastic Polyetherimide (PEI) 175

In the case of melt-processable BPADA-based polyetherimides, many of
these concerns have been met with great success. However, some of the
end-use features have been traded for the ease of processability. An exam-
ple is that the flexible isopropylidene group introduces somewhat weaker
carbon-hydrogen bonds, which can be attacked by oxygen [44–46]. A poly-
imide with no aliphatic hydrogens may not have this weakness, but may
not be melt-processable. In the PEIS resin, the flexibilizing sulfone link-
ages in the polyetherimide sulfone resin may not be as photo-stable as
other functional groups. So in areas of extreme performance, such as parts
for external aerospace applications, the melt-processable, BPADA-based
polyetherimides may not meet all application requirements. However, in
applications requiring a large volume of parts, and where the time needed
in making the part is an important factor, the melt-processable resins are
very valuable.
Not all resins meet all application needs, which is why so many mate-
rials coexist in the polyimide marketplace, each filling a special set of
consumer needs. Traditional, thermoset, and melt-processable poly-
imides each have their place.
Thermoplastic PEI resins provide a good balance of mechanical, phys-
ical, and rheological properties. Molded parts can handle short-term
thermal excursions to about 240°C (464°F) and extended heat contact
for as high as 170°C (338°F), while also providing high strength and stiff-
ness, good dimensional stability, transparency, moderate solvent resist-
ance, ignition resistance, and low smoke.

Photochemistry
PEI resins absorb ultraviolet (UV) as well as visible light. The UV-vis-
ible spectra of a typical BPADA-MPD polymer are shown in Fig. 8.7 at
both 0.8- and 1.6-mm (0.032- and 0.063-in) thickness. Short exposure to
light or UV radiation usually causes no significant changes in thermo-
plastic PEI.
Prolonged exposure to intense UV light causes a color change in PEI
resins and may lead to degradation. However, as in many systems, the pho-
tochemistry is often dominated by the presence of pigments. In most cases,
PEI resins exposed to high-intensity UV light show rapid color change and
loss of gloss, which then levels out after short exposure times. Some glass-
fiber-filled PEI resins have an Underwriters Laboratory (UL) listing of F1,
which indicates good retention of mechanical and flame-retardant prop-
erties after exposure to moisture and light. Polyetherimides also have
excellent resistance to ionizing radiation, such as gamma rays.
In applications where the thermoplastic PEI resins are exposed to
light or radiation, molded parts should be tested as close as possible to
actual end-use conditions to determine fitness for a specific application.