Page 193 - Engineering Plastics Handbook

166 Engineering Plastics

of crystalline and amorphous regions). The amorphous resins also have
better dimensional stability.
Crystalline resins like PEEK will not deform until they reach the melt-
ing point of their crystals. However, once they are above their T , they can
g
only support a low load. PEI resins, however, will deform at lower tem-
perature, since they have no high-melting crystals. Nevertheless, by virtue
of the higher T , they can support a greater load and at higher tempera-
g
ture than the crystalline resin. In general, PEI resins are best used where
crystalline resins are unable to meet application requirements due to
issues of dimensional stability (such as warpage), flame retardance, or
high-temperature end-use conditions.
End-use conditions will dictate which resin is best suited for which
applications. Part testing at the conditions of the final application is always
recommended when one is selecting a thermoplastic resin. There are many
performance features to balance. No single material fills all needs, which
is why so many polymers coexist in the marketplace.
Figure 8.2 shows the viscosity versus shear rate curves for a BPADA-
MPD polymer at 355, 370, and 385°C (671, 698, and 725°F). Under simi-
lar conditions, more rigid polyimides, with only one flexible linkage, will
show no flow. The comparatively easy processing of PEI resins allows melt
extrusion of thin sheet and film [24], as well as numerous complex-shape
parts. Melt processing also allows a number of polymer blends to be made
by simple melt mixing.
Note that PEI resins should not be melt-processed by using nitride-
treated steel since in some cases corrosion may ensue.
PEI resins are fairly polar and will absorb moisture. They are best
dried prior to melt processing. Drying can be done at 150°C (302°F) for

1000

Viscosity (Pa⋅s) 100 355°C

370°C
385°C
10
100 1000 10000 100000
Shear rate (1/s)
Figure 8.2 Viscosity versus shear rate for PEI based on BPADA-MPD intermediates
at three temperatures.