Page 146 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polycondensation Polymers 109

of polyester resin can be achieved through heating to about 190°C followed by slow cooling. Rapid
cooling, called quenching, produces a more amorphous material from which transparent fi lm is
made. Film strength is increased through application of heat and pulling of the film. Biaxially ori-

ented PET film is one of the strongest films available. Thicker film, because of its low carbon diox-
ide permeability, is used in the manufacture of containers for carbonated drinks through injection

molding. It is also used as magnetic film in X-ray and other photographic applications. Thinner fi lm
is used in such food applications as food packaging, including the boil-in-a-bag foods.

PET is difficult to mold because of its high melting, T 240°C, point. PET also crystallizes rela-
m
tively slowly so that extra care must be exercised to insure that the PET molded products are fully
crystallized or the partial crystallized portions will be preferred sites for cracking, crazing, shrink-
age, and so on. Thus, nucleating agents and crystallization accelerators have been used to improve
the crystallization rate. Postannealing has been used where appropriate.
Poly(butylene terephthalate) (PBT), because of the addition of two methylene units in the
diol-derived portion, has lower melting point than PET with a T of about 170°C (Equation 4.54).
g
Moldability of aryl polyesters has been improved through the use of PBT instead of PET or by
use of blends of PET and PBT. These aryl polyesters are used for bicycle wheels, springs, and
blow-molded containers.

O
O
O O R O
HO O + H 2 O
+ (4.54)
HO OH
OH
R
Poly(butylene terephthalate), PBT

By comparison to PET, PBT melts at a lower temperature and crystallizes more rapidly and is
often employed as a molding compound. PBT offers a balance of properties between nylons and
acetals with low moisture absorption, good fatigue resistance, good solvent resistance, extremely
good self-lubrication, and good maintenance of physical properties even at relatively high use tem-
peratures. Fiber-reinforced PBT molding compound is sold under the trade name Celanex. A PBT

molding compound was first sold under the trade name Valox. Today, there are many PBT molding
compounds available.
Table 4.3 contains selected physical properties of PET and PBT.

TABLE 4.3
General Physical Properties of PET and PBT
PET PBT
o

Heat-deflection temperature (1,820 kPa; C) 100 65
Maximum resistance to continuous heat ( C) 100 60
o
Crystalline melting point ( C)
o
–5
Coefficient of linear expansion (cm/cm- C, 10 ) 6.5 7.0
o

Compressive strength (kPa) 8.6 × 10 4 7.5 × 10 4
Flexural strength (kPa) 1.1 × 10 5 9.6 × 10 4
Impact strength (Izod: cm-N/cm of notch) 26 53
Tensile strength (kPa) 6.2 × 10 4 5.5 × 10 4
Ultimate elongation (%) 100 100
Density (g/mL) 1.35 1.35

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