Page 200 - Engineering Plastics Handbook

Thermoplastic Polyetherimide (PEI) 173

be taken to select colorants that can withstand the resin’s high process-
ing temperature [>350°C (662°F)] without shifting color or degrading the
polymer. Due to the natural deep-yellow color of many PEI resins, rela-
tively high levels of pigment (5 to 10 wt%) may be needed to achieve light
colors. Sometimes this high pigment loading can affect other mechanical
properties, such as impact. In glass fiber (GF) blends, the hardness of
metal oxides, such as TiO , can cause fiber breakage, which results in
2
lower strength and impact, although the modulus tends to be retained.
Often there can be a 10 to 20% drop in strength and impact properties
when one is comparing the same GF-PEI grade in white versus natural
or black-colored versions. This effect of metal oxide colorants is generally
true of all GF thermoplastics, not just polyetherimides. The property dif-
ferences between natural and white colors in non-glass-reinforced grades
will be less than in fiberglass-filled blends.
Note that due to their high melt-processing temperature, PEI resins are
not compatible with many standard polymer additives. Often additives
that work well in other, lower-temperature polymers will decompose or
volatilize in the hot PEI melt. In general, like most amorphous polymers,
PEI resins should be processed at least 150°C (270°F) above the polymer’s
T [340 to 400°C (664 to 752°F) for most polyetherimides]. Selection of
g
appropriate additives to blend with PEI is best handled by the resin man-
ufacturer or those with significant experience in the selection of suitable
blending materials.
The presence of water can have several types of effects on PEI depend-
ing on the exact nature of exposure. For example, if undried resins are
melt-processed, water (steam) will evolve and may cause degradation of
the polymer chains. Loss of molecular weight, if excessive, may lead to a
drop in mechanical properties. Additionally, molding of undried polymer
usually results in surface defects such as splay or silver streaking. PEI
should be dried prior to processing. At lower temperatures, below the soft-
ening point of the polymers, adsorbed water can lead to slight changes in
mechanical properties (e.g., a slight loss of stiffness) as well as changes in
the electrical properties of the molded parts [41–43].
In most cases, thermoplastic polyimides have very good retention of
properties when in contact with water. However, in some extreme cases,
the polymer chain could be cleaved by the action of water. Such hydroly-
sis would depend on many factors, including the nature of water contact
(immersion, intermittent exposure, condensation, freshly renewed or stag-
nant water, salt content, etc.). Temperature will obviously be important
as well as pH. Polyimides are more reactive at high pH than at lower pH.
In some cases, short contact with caustic solutions (high pH) is used to etch
polyimide surfaces to improve adhesion. Additionally, part design and
molded-in stress will influence hydrolysis and retention of properties on
exposure to hydrolytic conditions. Once again, due to the wide range of