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Definition
Relevance to performance
What
determines a film's seal properties
Test
principles
Related
Terminology
Definition
Crimp seals, which
are produced at controlled conditions of jaw
design, temperature, pressure, and dwell, are
measured in terms of force required to open
the seam. The following properties are
calculated and reported.
-
Crimp seal
strength is the maximum force to
peel open a seal made at specified
conditions. Units are gf/in or
gf/2.5 cm.
-
Crimp
minimum sealing temperature (Crimp
MST) for a specified pressure and dwell,
is the temperature (°F or °C) required
to achieve a certain minimum strength
seal. ExxonMobil has standardized with 200
g/in as the seal strength for minimum
acceptable performance. Other values may
be more appropriate for particular
applications.
-
Seal range
is the difference between the maximum and
minimum temperatures that will produce an
adequate seal at a specified pressure and
dwell. Units are of or °C.
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Relevance to performance
The strength of a
seal is important to package integrity, and
many end-users specify a minimum
requirement.
Crimp MST and seal
range are important, because they predict
packaging machine efficiency and productivity.
A lower MST corresponds to a wider seal range
and, therefore, a wider operating window. This
means that the packaging line will produce
acceptably sealed packages even when the speed
is ramping up and down, or when there is
variation in jaw temperature. On the other
hand, a higher MST and narrower seal range
forces the packager to control these variables
better, or else packages can be rejected for
weak, open seals, or distorted seals.
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What
determines a film's seal properties
A film's sealing
surface is a coated or coextruded layer. The
sealing properties can be impacted by several
factors:
-
Formulation of
sealant layer (resin and additive recipe)
-
Thickness of
sealant layer
-
Total thickness
of the film (With thicker films, it takes
longer for heat to transfer from the
sealing jaws to the sealing surfaces than
it does with thinner films. Therefore,
both the effective sealing temperature and
the resulting seal strength are lower.)
-
Bond strength of
sealant layer to adjacent layer
-
Contamination
(An otherwise good-sealing surface may not
seal well if additives from a laminating
web transfer onto it, or if dust/fines are
trapped in the seal area.)
-
Surface
treatment, including backside treatment,
can damage sealability
Seal properties are a
key design criteria when developing a sealable
film. In production, these properties are
regularly measured to make sure that the
formulation or process has not shifted.
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Test principles
In order to compare
sealing properties among different films,
ExxonMobil uses a set of standard conditions
for producing heat seals. They are summarized
in Table 12.
| Condition |
Description |
| Sealing
device |
Wrap-Ade
Crimp Sealer Model J or K, modified
with new PID temperature controllers |
| Jaw
design |
Vertically
serrated crimps |
| Temperature |
Both
jaws are heated and desired set point
is controlled to ± 2ºF (1ºC) |
| Pressure |
20
psi (1.4 bars) |
| Dwell |
¾
sec |
Table
12: ExxonMobil conditions for producing test
crimp seals
After a
test seal has been produced, its strength can
be measured with a tensile tester or a Suter
tester. ExxonMobil typically uses the simpler
Suter tester, which pulls the seal apart at 12
in/min and records peak force.
ASTM
and ExxonMobil test procedures have
similarities and differences. Unlike
ExxonMobil, the scope of ASTM test procedure F
88 is only the measuring of the force to open
the seal, not the making of the seal at a set
of standard conditions. ASTM specifies the use
of a tensile tester, while ExxonMobil uses a
Suter tester. The 12 in/min pull rate of
ExxonMobil is within the ASTM-defined a rate
of 10 to 12 in/min.
It is
often desirable to produce seals at a series
of temperatures, measure the seal strengths,
and develop a seal curve that looks like in
Graph 3.
Graph
3: Typical seal curve
Crimp
Seal Strength is defined at a particular
temperature, or as the average for a range of
temperatures. For the seal curve represented
in Graph 3, the following properties can be
derived.
Seal
Strength at 260°F (SS260F) = 800
g/in
SS260
- 300F = 790 g/in
Minimum
Sealing Temperature for a 200 g/in seal (MST200g)
= 200°F (93°C)
MST500g
= 217°F (103°C)
Seal
Range (SR) = Maximum - Minimum Sealing
Temperature
SR200g
= 300 - 200 = 100°F (56°C)
SR500g
= 300 - 217 = 83°F (46°C)
The
horizontal red line denotes a minimum
acceptable seal strength of 200 g/in. It
intersects the seal curve at 200°F (93°C),
so this is the MST200g. If you know
that the minimum acceptable seal strength is
another value, the same curve may be used to
determine the corresponding MST and seal
range. For example, 500 g/in seals occur at
217°F (103°C), so this is the MST500g.
Most
OPPs will distort at temperatures above 300°F
(149°C), if dwell time is in excess of ½
second or so. Therefore, this value is used as
the maximum sealing temperature. The
difference between the maximum and minimum
sealing temperatures is equal to the seal
range.
CAUTION: |
Seal
data cannot be compared when the seals
are produced on different equipment or
at different conditions.
|
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Related terminology
ASKCO
sealer |
The ASKCO sealer is a
laboratory heat sealer that ExxonMobil
uses for process control of coated
films. It has nine separate temperature
controlled, one-side heated,
low-pressure, flat-sealing stations. The
multiple stations allow a whole seal
curve to be generated from one sealed
film strip. |
Sentinel
sealer |
The Sentinel sealer is a
common laboratory heat sealer in the
flexible packaging industry. Like the
ASKCO sealer, it is a one-side heated,
flat sealer. Unlike the ASKCO, there are
no multiple stations, so it can only
make a seal at one temperature at a
time. ExxonMobil does not use this
sealer. Data generated with the Sentinel
sealer, by other companies, can not be
compared with ExxonMobil data. |
Failure
mode |
Failure mode refers to
how the seal fails as it is being pulled
apart. A "peel" failure means
that the films separated without
tearing. A "delamination"
failure is a type of peel failure where
the material separates between two
adjacent layers, rather than at the
sealing interface. (For example, a
printed extrusion lamination can fail
between the ink and poly layers, because
they formed a weak bond.) Finally, a
"tear" failure means that the
film tears as the seal is being pulled
apart. This suggests that the seal and
the bonding between layers are stronger
than the film. |
Suter
tester |
Suter tester is a device
specifically designed to measure peak
seal strengths. It is simpler and
speedier than using a tensile tester.
Suter testers are not commercially
available. |
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