Page 197 - Engineering Plastics Handbook

170 Engineering Plastics

to increase both thermal and electrical conductivity. Electrical proper-
ties will vary with filler loading as well as part design, gating, and melt
processing. Normally, PEI resins are good electrical insulators. Often a
resin-rich skin on a molded part can affect conductivity.

Polymer Blends
PEI resin may also be combined with other polymers. In general, to
produce a useful blend, the free energy of mixing ∆G [Eq. (8.11)] must
be favorable.
∆G =∆H − T(∆S)
Equation 8.11 Free energy of mixing.
Since the combinatorial entropy of mixing large-molecular-weight poly-
mers always gives an unfavorable ∆S term, formation of compatible blends
requires a favorable enthalpic interaction ∆H. The PEI resins have pro-
duced a surprising number of miscible (one-phase) and compatible blends.
Compatible blends are phase-separated mixtures having sufficient attrac-
tion between phases to provide some level of molecular adhesion, result-
ing in stable morphology and giving rise to good mechanical properties.
PEI forms miscible blends with polyesters such as polybutylene tereph-
thalate (PBT), polyethylene terephthalate (PET), and polyethylene naph-
thanoate (PEN) [30–32]. These blends have a single T between that of the
g
PEI and that of polyester. In blends with slower crystallizing polyesters
such as PET and PEN, crystallization is reduced and one-phase, trans-
parent compositions can be molded. Such blends have reduced thermal per-
formance versus the base PEI polymer, but improved melt flow, reduced
yellowness, and slightly better solvent resistance.
Blends of BPADA-based polyimides are also miscible with polyaryl
ether ketones such as PEEK [33, 34]. It appears that the isopropylidene
moiety is important in developing miscibility in these systems [35]. In this
case, the lower T (150°C) of PEEK acts to reduce the higher T of PEI.
g
g
When injection-molded, many PEEK-PEI blends are transparent. Such
miscible PEEK blends can be induced to crystallize with long molding
cycles or secondary heat treatment, yielding opaque systems due to PEEK
crystal formation. Care must be taken during such heat treatment
(annealing) to ensure that the accompanying volume change—in going
from amorphous to crystalline regions—does not distort the part. Films
of PEEK-PEI blends have also been prepared.
Phase-separated PEI blends have been investigated. Combinations of
PEI with polycarbonate (PC) or polycarbonate ester (PCE) copolymers
have a fine, laminar two-phase morphology [36]. Combinations of PEI with
polycarbonate or polyester carbonates yield a family of two-phase opaque
systems that have reduced heat capability versus PEI, but show improved
impact and better melt flow [37, 38].