Page 83 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.23

The only structural difference between PBT and PET is the substitution in PBT of four
methylene repeat units rather than two present in PET. This feature imparts additional flex-
ibility to the backbone and reduces the polarity of the molecule resulting in similar me-
chanical properties to PET (high strength, stiffness, and hardness). PBT growth is at least
ten percent annually, in large part due to automotive exterior and under-the-hood applica-
tions such as electronic stability control and housings that are made out of a PBT/ASA
(acrylonitrile/styrene/acrylic ester) blend. PBT/ASA blends are sold by BASF and GE
Plastics Europe. Another development involving the use of PBT is coextrusion of PBT and
a copolyester thermoplastic elastomer. This can then be blow molded into under-the-hood
applications to minimize noise vibration. Highly filled PBTs are also making inroads into
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the kitchen and bathroom tile industries. As with PET, PBT is also often glass fiber
filled so as to increase its flexural modulus, creep resistance, and impact strength. PBT is
suitable for applications requiring dimensional stability, particularly in water, and resis-
187
tance to hydrocarbon oils without stress-cracking. Hence, PBT is used in pump hous-
ings, distributors, impellers, bearing bushings, and gear wheels.
To improve PBT’s poor notched impact strength, copolymerization with 5 percent eth-
ylene and vinyl acetate onto the polyester backbone improves its toughness. PBT is also
blended with PMMA, PET, PC, and polybutadiene to provide enhanced properties tailored
to specific applications.
2.2.12.3 Polycyclohexylenedimethylene Terephthalate (PCT). Another polyalkylene
terephthalate polyester of significant commercial importance is PCT—a condensation
product of the reaction between dimethyl terephthalate and 1,4-cyclohexylene glycol as
shown below in Fig. 2.17. This material is biaxially oriented into films and, while it is me-
chanically weaker than PET, it offers superior water resistance and weather resistance. 188
As seen in the introductory Table 2.4, PCT differentiates itself from PET and PBT with its
high heat distortion temperature. As with PET and PBT, PCT has low moisture absorption,
and its good chemical resistance to engine fluids and organic solvents lends it to under-
the-hood applications such as alternator armatures and pressure sensors. 189

FIGURE 2.17 Synthesis route of PCT.

Copolymers of PCT include PCTA, an acid-modified polyester, and PCTG, a glycol-
modified polyester. PCTA is used primarily for extruded film and sheet for packaging ap-
plications. PCTA has high clarity, tear strength, and chemical resistance, and when PCTA
is filled, it is used for dual ovenable cookware. PCTG is primarily injection molded, and
PCTG parts have notched Izod impact strengths similar to polycarbonate, against which it
often competes. It also competes with ABS, another clear polymer. It finds use in medical
and optical applications. 190

2.2.12.4 Poly(ethylene Terephthalate) (PET). There are tremendous commercial appli-
cations for PET: as an injection-molding-grade material, for blow-molded bottles, and for
oriented films. In 1998, the U.S. consumption of PET was 4,330 million lb, while domes-
191
tic consumption of PBT was 346 million lb. PET, also known as poly(oxyethylene oxy-
terephthaloyl), can be synthesized from dimethyl terephthalate and ethylene glycol by a
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two-step ester interchange process, as shown in Fig. 2.18. The first stage involves a so-

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