Page 86 - Handbook of Plastics Technologies

THERMOPLASTICS

2.26 CHAPTER 2

dimension in the TD. This induces further crystallization, bringing the degree of crystal-
linity to 25 to 40 percent, and creates a film, which has isotropic tensile strength and elon-
gation properties in the machine and transverse directions. The film at this point is
thermally unstable above 100°C and must be annealed in the tenter frame to partially re-
lieve the stress.
The annealing involves heating to 180 to 220°C for several seconds to allow amor-
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phous chain relaxation, partial melting, recrystallization, and crystal growth to occur.
The resultant film is approximately 50 percent crystalline and possesses good mechanical
strength, a smooth surface that readily accepts a wide variety of coatings, and good wind-
ing and handling characteristics. PET films are produced from 1.5 µm thick as capacitor
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films to 350 µm thick for use as electrical insulation in motors and generators.
Due to the chemically inert nature of PET, films that are used in coatings applications
are often treated with a variety of surface modifiers. Organic and inorganic fillers are often
incorporated in relatively thick films to improve handling characteristics by roughening
the surface slightly. For thin films, however, many applications require transparency,
which would be marred by the incorporation of fillers. Therefore, an in-line coating step of
either aqueous or solvent-based coatings is set up between the MD and TD drawing sta-
tions. The drawing of the film after the coating has been applied helps to achieve very thin
coatings.
2.2.12.5 Polytrimethylene Terephthalate (PTT). PTT has been produced and marketed
as three grades by Shell Chemicals under the trade name Corterra since the late 1990s,
when Shell was able to develop a low-cost method of producing the starting raw material
1,3-propanediol. Corterra is used in the textile and carpet industries, which take advantage
of its stain resistance, wearability as a result of high resilience and elastic recovery, color
fastness, and soft hand.

2.2.12.6 Polyethylene Napthalate (PEN). Polyethylene napthalate (PEN) gained com-
mercial importance in the late 1980s. Compared to PET, it has higher thermal resistance
and tensile strength as well as better barrier properties and UV resistance. This is a result
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of the napthenic ring structures. Both the T (124°C) and T (270 to 273°C) of PEN are
m
g
higher compared to PET, while the crystallization rate of PEN is slower than PET. Cur-
rently, PEN is more expensive than PET, leading to the development of copolymers. Ap-
plications for PEN include fibers, films, and blow-molded products. Due to cost
considerations, the markets for PEN blow molded products are generally in the medical
arena.
2.2.13 Polyetherimide (PEI)
Polyetherimides (PEI) are a newer class of amorphous thermoplastics with high-tempera-
ture resistance, impact strength, creep resistance, and rigidity. They are transparent with an
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amber color. The polymer is sold under the trade name of Ultem (General Electric) and
has the structure shown in Fig. 2.20. It is prepared from the condensation polymerization
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of diamines and dianhydrides.
The material can be melt processed because of the ether linkages present in the back-
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bone of the polymer, but it still maintains properties similar to the polyimides. The
high-temperature resistance of the polymer allows it to compete with the polyketones,
polysulfones, and poly(phenylene sulfides). The glass transition temperature of PEI is
215°C. The polymer has very high tensile strength, a UL temperature index of 170°C,
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flame resistance, and low smoke emission. The polymer is resistant to alcohols, acids,
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and hydrocarbon solvents but will dissolve in partially halogenated solvents. Both
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glass- and carbon-fiber-reinforced grades are available.

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