Page 102 - Handbook of Plastics Technologies

THERMOPLASTICS

2.42 CHAPTER 2

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should have a moisture content of 0.15 percent or less for processing. Because mold
temperature is important to surface finish, higher mold temperatures may be required for
some applications.
Both crystalline and amorphous grades are available under the trade name Amodel
(Amoco); amorphous grades are available under the names Zytel (Dupont) and Trogamid
(Dynamit Nobel). Crystalline grades are available under the trade name Arlen (Mitsui). 291
Polyphthalamides are used in automotive applications where their chemical resistance
and temperature stability are important. 292 Examples include sensor housings, fuel line
components, headlamp reflectors, electrical components, and structural components. Elec-
trical components attached by infrared and vapor phase soldering are applications utilizing
PPA’s high-temperature stability. Switching devices, connectors, and motor brackets are
often made from PPA. Mineral-filled grades are used in applications that require plating,
such as decorative hardware and plumbing. Impact modified grades of unreinforced PPA
are used in sporting goods, oil field parts, and military applications.

2.2.24 Polypropylene (PP)
Polypropylene is a versatile polymer used in applications from films to fibers, with a
worldwide demand of over 21 million lb. 293 It is similar to polyethylene in structure ex-
cept for the substitution of one hydrogen group with a methyl group on every other car-
bon. On the surface, this change would appear trivial, but this one replacement changes the
symmetry of the polymer chain. This allows for the preparation of different stereoisomers,
namely, syndiotactic, isotactic, and atactic chains. These configurations are shown in the
introduction.
Polypropylene (PP) is synthesized by the polymerization of propylene, a monomer de-
rived from petroleum products through the reaction shown in Fig. 2.34. It was not until
Ziegler-Natta catalysts became available that polypropylene could be polymerized into a
commercially viable product. These catalysts allowed the control of stereochemistry dur-
ing polymerization to form polypropylene in the isotactic and syndiotactic forms, both ca-
294
pable of crystallizing into a more rigid, useful polymeric material. The first commercial
method for the production of polypropylene was a suspension process. Current methods of
295
production include a gas phase process and a liquid slurry process. New grades of
296
polypropylene are now being polymerized using metallocene catalysts. The range of
molecular weights for PP is M = 38,000 to 60,000 and M = 220,000 to 700,000. The
w
n
molecular weight distribution (M /M ) can range from 2 to about 11. 297
n
w

FIGURE 2.34 The reaction to prepare polypropylene.

Different behavior can be found for each of the three stereoisomers. Isotactic and syn-
diotactic polypropylene can pack into a regular crystalline array, giving a polymer with
more rigidity. Both materials are crystalline. However, syndiotactic polypropylene has a
298
lower T than the isotactic polymer. The isotactic polymer is the most commercially
m
used form, with a melting point of 165°C. Atactic polypropylene has a very small amount
of crystallinity (5 to 10 percent), because its irregular structure prevents crystallization;

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