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Definition
Relevance to performance
What
affects dimensional stability
Test
principles
Definition
Dimensional stability
is the change in length of an unrestrained
film sample subjected to a specific elevated
temperature. ExxonMobil typically measures
this property at 275°F (135°C). Machine
direction (MD) and transverse direction (TD)
values are evaluated and reported separately.
Units are reported as % change from the
original dimension.
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Relevance to
performance
In general, materials
expand when subjected to elevated
temperatures. Oriented films, on the other
hand, are likely to shrink because the polymer
has "memory" and tries to return to
its unoriented dimensions. Dimensional
stability values for different films (measured
at the same temperature) provide a relative
comparison of how much film distortion will
occur in heated processes, like oven drying or
package sealing. Acceptable temperatures for
processing a film vary and are dependent on
the film properties, the type of process
(contact with heated air or heated metal), the
dwell time, and whether the film is restrained
or not.
OPP films are
typically designed for minimal shrinkage. Most
ExxonMobil OPP films can be used in very
high-speed, low-dwell heat seal applications
with actual crimp jaw temperatures of up to
approximately 355°F (180°C) without causing
unsightly seal distortion. However, longer
dwell exposures (> ½ second) require that
temperatures do not exceed about 300°F
(149°C) to prevent severe shrinkage. OPP is
commonly used at temperatures between 220°F
and 300°F (104.5°C to 149°C), where it can
have a slight dimensional change. Test values
at 275°F (135°C) for tenter-oriented OPP are
typically -2% to -8% in both the machine and
transverse directions.
Some films are
designed to shrink in a predictable way, like
the new, developmental ExxonMobil film Bicor
TYTE. TYTE provides a crisp, tight overwrap
for products like CD jewel cases, video tapes,
and food or pharmaceutical boxes. Its
dimensional stability values are -9% in the MD
and -14% in the TD.
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What affects
dimensional stability
Dimensional stability
is mostly determined by the OPP film's
residence time and temperature in the
annealing section of the orientation process.
The annealing section involves the last zones
of the transverse direction orienter and is
where the oriented film continues to be held
at the edges with tenter clips while
surrounded by temperature-controlled oven air.
This relieves residual stresses and creates
"heat set" OPP. Without proper
annealing, OPP is prone to greater shrinkage
in heated environments.
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Test principles
ASTM D 1204 and ExxonMobil procedure #438 follow the
same principles, but differ in some specific
protocols. Both tests involve placing a film
sample of known original dimensions into a
temperature-controlled convection oven for a
certain period of time and measuring the
length of the sample after conditioning.
Results are reported as % change. Negative
numbers indicate shrinkage, while positive
numbers indicate expansion. Pertinent details
of the two procedures are summarized in Table
6.
| Test
Procedure |
Test
Conditions |
Specimen
Size |
Oven
Temperature |
Time
in Oven |
Pre/Post
Conditioning |
Precision |
Reporting |
| ExxonMobil
#438 |
1"
x 7" cut in MD, and one in TD |
Convection
oven controlled to target, typically
275°F (135°C) |
7
min |
None |
Nearest
.02 inches |
%
change |
| ASTM
D 1204 |
10"
x 10" |
Convection
oven controlled to target ± 1°C |
As
appropriate depending on film testing |
Yes,
as standard laboratory temperature and
humidity |
Nearest
.01 inches |
%
change |
Table
6: Comparison of dimensional stability test
conditions between ExxonMobil and ASTM
procedures
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