Page 85 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.25

199
terephthalic acid offer the elimination of the intermediate use of methanol. Generally,
PET resins made by direct esterification of terephthalic acid contain more diethylene gly-
col, which is generated by an intermolecular ether-forming reaction between β-hydroxy-
ethyl ester end groups. Oriented films produced from these resins have reduced
mechanical strength and melting points as well as decreased thermo-oxidative resistance
200
and poorer UV stability.
The degree of crystallization and direction of the crystallite axis govern all of the
resin’s physical properties. The percentage of structure existing in crystalline domains is
primarily determined through density measurements or by thermal means using a differen-
3
tial scanning calorimeter (DSC). The density of amorphous PET is 1.333 g/cm , while the
3 201
density of a PET crystal is 1.455 g/cm . Once the density is known, the fraction of crys-
talline material can be determined.
An alternate means of measuring crystallinity involves comparing the ratio of the heat
of cold crystallization, ∆H , of amorphous polymer to the heat of fusion, ∆H , of crystal-
cc
f
line polymer. This ratio is 0.61 for an amorphous PET and a fully crystalline PET sample
should yield a value close to zero.202 After the sample with its initial morphology has
been run once in the DSC, the heat of fusion determined in the next run can be considered
as ∆H . The lower the ∆H /∆H ratio, the more crystalline the original sample was.
f
cc
cc
In the absence of nucleating agents and plasticizers, PET crystallizes slowly, which is a
hindrance in injection molding applications, as either hot molds or costly extended cooling
times are required. In the case of films, however, where crystallinity can be mechanically
induced, PET resins combine rheological properties that lend themselves to melt extrusion
with a well defined melting point, making them ideally suited for biaxially oriented film
applications. The attachment of the ester linkage directly to the aromatic component of the
backbone means that these linear, regular PET chains have enough flexibility to form
stress-induced crystals and achieve enough molecular orientation to form strong, ther-
mally stable films. 203
Methods for producing oriented PET films have been well documented and will be
only briefly discussed here. The process as described in the Encyclopedia of Polymer Sci-
ence and Engineering usually involves a sequence of five steps which include 204
• Melt extrusion and slot casting
• Quenching
• Drawing in the longitudinal machine direction (MD)
• Drawing in the transverse direction (TD)
• Annealing

Dried, highly viscous polymer melt is extruded through a slot die with an adjustable gap
width onto a highly polished quenching drum. If very high output rates are required, a cas-
cade system of extruders can be set up to first melt and homogenize the PET granules, then
to use the next in-line extruder to meter the melt to the die. Molten resin is passed through
filter packs with average pore sizes of 5 to 30 µm. Quenching to nearly 100 percent amor-
phous morphology is critical to avoid embrittlement; films that have been allowed to form
spherulites are brittle and translucent, and are unable to be further processed.
The sheet is then heated to about 95°C (above the glass transition point of approxi-
mately 70°C), where thermal mobility allows the material to be stretched to three or four
times its original dimension in the MD. This uniaxially oriented film has stress-induced
crystals whose main axes are aligned in the machine direction. The benzene rings, how-
ever, are aligned parallel to the surface of the film in the <1,0,0> crystal plane. The film is
then again heated, generally to above 100°C, and stretched to three to four times its initial

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