Page 76 - Handbook of Plastics Technologies

THERMOPLASTICS

2.16 CHAPTER 2

Polyamide-imide can be processed by injection molding, but special screws are needed
due to the reactivity of the polymer under molding conditions. Low-compression-ratio
139
screws are recommended. The parts should be annealed after molding at gradually in-
140
creased temperatures. For injection molding, the melt temperature should be near
355°C, with mold temperatures of 230°C. PAI can also be processed by compression
molding or used in solution form. For compression molding, preheating at 280°C, fol-
lowed by molding between 330 and 340°C with a pressure of 30 MPa, is generally
141
used.
Polyamide-imide polymers find application in hydraulic bushings and seals, mechani-
142
cal parts for electronics, and engine components. The polymer in solution has applica-
tion as a laminating resin for spacecraft, a decorative finish for kitchen equipment, and as
143
wire enamel. Low coefficient of friction materials may be prepared by blending PAI
144
with polytetrafluoroethylene and graphite.
2.2.8 Polyarylate
Polyarylates are amorphous, aromatic polyesters. Polyarylates are polyesters prepared
from dicarboxylic acids and bis-phenols. 145 Bis-phenol A is commonly used along with
aromatic dicarboxylic acids, such as mixtures of isophthalic acid and terephthalic acid.
The use of two different acids results in an amorphous polymer. However, the presence of
the aromatic rings gives the polymer a high T and good temperature resistance. The tem-
g
perature resistance of polyarylates lies between polysulfone and polycarbonate. The poly-
mer is flame retardant and shows good toughness and UV resistance. 146 Polyarylates are
transparent and have good electrical properties. The abrasion resistance of polyarylates is
superior to that of polycarbonate. In addition, the polymers show very high recovery from
deformation.
Polarylates are processed by most of the conventional methods. Injection molding
should be performed with a melt temperature of 260 to 382°C, with mold temperatures of
65 to 150°C. Extrusion and blow molding grades are also available. Polyarylates can react
with water at processing temperatures, and they should be dried prior to use. 147
Polyarylates are used in automotive applications such as door handles, brackets, and
headlamp and mirror housings. Polyarylates are also used in electrical applications for
connectors and fuses. The polymer can be used in circuit board applications, because its
high temperature resistance allows the part to survive exposure to the temperatures gener-
ated during soldering. 148 The excellent UV resistance of these polymers allows them to be
used as a coating for other thermoplastics for improved UV resistance of the part. The
good heat resistance of polyarylates allows them to be used in applications such as fire
helmets and shields. 149

2.2.9 Polybenzimidazole (PBI)
Polybenzimidazoles (PBIs) are high-temperature-resistant polymers. They are prepared
from aromatic tetramines (for example, tetra amino-biphenol) and aromatic dicarboxylic
acids (diphenylisophthalate). 150 The reactants are heated to form a soluble prepolymer
that is converted to the insoluble polymer by heating at temperatures above 300°C. 151 The
general structure of PBI is shown in Fig. 2.12.
The resulting polymer has high temperature stability, good chemical resistance, and
nonflammability. The polymer releases very little toxic gas and does not melt when ex-
posed to pyrolysis conditions. The polymer can be formed into fibers by dry-spinning pro-
152
cesses. Polybenzimidazole is usually amorphous, with a T near 430°C. Under certain
g

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.