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Definition
Relevance to performance
What
affects a film's tensile properties
Test
principles
Related
Terminology
Definition
Tensile
testers, such as an Instron, measure a film's
resistance to being pulled apart at a constant
rate of speed. ExxonMobil uses this test to
report three significant properties.
-
Ultimate
tensile strength is
the maximum force of resistance divided by the
film's initial cross-sectional area. Values
are expressed in Ibf/in2
(psi) in US standard
units and N/mm2 in metric (SI) units.
-
Tensile
modulus is
a stress-strain ratio calculated from any
point on the initial straight line portion of the
load-extension curve. It is measured just as
the material begins to experience tension. This value
is an indicator of the film's stiffness and
resistance to elongation in use. The units
are Ibf/in2 (psi) and
N/mm2.
-
Elongation is
the percent change in length of the material
under stress from start to break. Elongation "at
yield" is measured from start to yield
point. The units are %.
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Relevance to
performance
Tensile properties
are an important and common way to compare
physical properties of diverse materials, from
steel to plastic. In the narrower realm of
flexible films, these tests provide
measurement of attributes we can see and feel:
strength, stiffness, and resistance to
stretching. Some materials are stronger than
others, and some, like polypropylene, can
dramatically improve their strength through
orientation as shown in Table 3.
| Tensile
Property |
| Cellophane |
Oriented
PET |
Blown
LDPE |
OPP |
| Ultimate
strength (kpsi) |
7-18 |
20-40 |
1.5-4 |
15-40 |
| Elongation
(%) |
10-50 |
60-165 |
100-700 |
35-475 |
Table
3: Typical tensile values for common films*
*Although
there is a wide range of property values for
each material (because of the many choices
available in resin formula, processing, and
direction of testing), Table 3 shows the
characteristic performance range for each type
of film. The effect of orientation is
reflected in the property differences between
cast PP (unoriented polypropylene) and OPP (biaxially
oriented polypropylene).
Within
the arena of OPP films, tensiles are generally
not critical and rarely require discussion and
specification between supplier and customer.
This is so because oriented polypropylenes
provide a dependable range of tensile values.
Other properties are usually more important to
successful performance. There are two notable
exceptions worthy of explanation.
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Orientation
method: blown or tentered
The orientation method causes
characteristic differences in tensile
properties. Blown films are
"balanced," having similar
strength and elongation in the machine and
transverse film directions. Tentered films
(as are all ExxonMobil films) have higher
strength and lower elongation in the
transverse direction than in the machine
direction. Most OPP manufacturers produce
tenter-oriented films, which work well in
many diverse applications.
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Modulus
and web tension
Modulus, because it is a measure of
strength characteristics in the film's
elastic region, provides valuable insight
into stiffness and how extensible the film
is under normal use tensions. When
comparing two films of identical
thickness, the one with a higher modulus
will be stiffer and stretch less under the
same tension force.
NOTE: |
High
temperature modulus testing and
empirical trials on converting
equipment have yielded an industry
rule of thumb: OPP web tensions should
be controlled to .50lbf per
inch of film width, or less, for good
registration and no permanent
deformation (elongation, neck-in,
gauge bands). From a filmmaker's
perspective, the lowest controllable
web tension is best. Thinner films and
higher converting temperatures make
this more critical.
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What affects
a film's tensile properties
Resin selection and
orientation method are the primary variables
that influence tensile values. Therefore,
tensile properties are almost entirely defined
by product design itself. Small variations in
tensiles will inevitably result due to normal
process variation, but the performance effect
is insignificant.
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Test principles
Each end
of a film specimen, of specific width and
measured thickness, is held by a clamp or
grip. One grip is stationary, while the other
is pulled away from the first at a
pre-selected velocity. The machine
continuously measures the changing distance
between the grips and the force exerted on
them as they pull the film apart. The test is
completed when the sample breaks.
Today,
most machines run automatically after the
operator selects settings, loads the specimen,
and initiates the test. Most are also equipped
with microprocessors that perform the
calculations and automatically display all the
resulting values. But, the test concepts are
best understood by studying load-extension
curves like the ones shown in Graph 1. These
are actual machine direction (MD) and
transverse direction (TD) load-extension
curves for a typical, tenter-oriented, 75
gauge OPP. In Graph 1, load is plotted as a
function of extension, and the tensile tester
software calculates the following properties
that are noted in Table 4.
Graph
1: Typical OPP load-extension curves
| Results |
Sample
Description |
Thickness
(mil) |
Ultimate
Strength (kspi) |
Modulus
(kpsi) |
Elongation
(%) |
| 1 |
MD
pull |
.75 |
18.9 |
343 |
174 |
| 2 |
TC
pull |
.75 |
39.3 |
687 |
45 |
Table
4: Tensile values from Graph 1
Table 4
values are software-generated results based on
the following equations.
| Ultimate
Tensile Strength (psi)
= |
Max
Load (lbf) |
= |
Max
Load (lbf) |
| Initial
cross-sectional area |
1
in x .00075 in |
Modulus
(psi) = At any point on the elastic region
tangent line (Stress ÷ Strain) =
| Load
(lbf) |
÷ |
Extension
(in) |
= |
Load
(lbf) |
÷ |
Extension
(in) |
| Initial
cross-sectional area |
Initial
grip separation |
.00075
in2 |
2
in |
| Elongation
(%) = |
Extension
at failure x 100 |
= |
Extension
at failure x 100 |
| Initial
grip separation |
2
in |
Test
conditions like pull speed, initial grip
separation, full-scale load, and sample width
will affect the results. ASTM test procedure D
882 describes a protocol for making choices
about these settings. For simplicity and
accuracy in comparing values, ExxonMobil uses
highly automated tensile testers and has
standardized to a particular set-up. These
conditions are summarized in Table 5.
| Test
Condition |
Machine
Direction (MD) |
Transverse
Direction (TD) |
Ultimate
Strength |
Modulus |
Elongation |
Ultimate
Strength |
Modulus |
Elongation |
Crosshead
speed (in/min),
i.e. pull velocity |
20 |
.5 |
20 |
20 |
.5 |
20 |
| Grip
separation (in) |
2 |
2 |
2 |
2 |
2 |
2 |
| Sample
width (in) |
1 |
1 |
1 |
1 |
1 |
1 |
Table
5: ExxonMobile standard conditions for tensile
testing
ExxonMobil
has two test procedures for tensile testing:
one for use with Instron equipment (#506) and
one for use with Sintech machines (#510).
With both tests, the specimen is subjected to
identical evaluation conditions.
NOTE: |
The
tensile properties for all coated and
metallized films are measured on base
sheet prior to coating or
metallization.
|
Tensile
properties can change with small changes in
temperature; so it is important to conduct
tests in a controlled environment. Standard
laboratory temperature is 72°F (22°C) ±
2°F (1°C).
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Related terminology
Young's
modulus
Modulus of elasticity
Tangent modulus |
Modulus of elasticity
Tangent modulus These are all terms
synonymous with tensile modulus, a
material property defined as the
stress-strain ratio of the initial
portion of a load-extension curve. A
higher modulus value suggests that this
material will be stiffer and have more
resistance to elongation in use (when
comparing films of the same thickness).
Standard units are Ibf/in2
(psi) and N/mm2. |
Secant
modulus |
Secant modulus is an
alternate approximation of modulus and
better predictor of performance, when
there is no straight line portion on the
load-elongation curve. It is the
stress-strain ratio at the point on the
curve that corresponds to a specific
designated extension. For example, a 1%
secant modulus would be the material's
stress/strain, in psi or N/mm2,
at the point of extension that is 1% of
initial sample length. This can be a
useful value, because with no early
straight line portion of the curve, the
traditional tangent calculation will
render a result that predicts the
material to be stiffer and less
extensible than it really is. |
Strain |
Strain is
the ratio of the change in length of the
sample (also called extension) to the
original length of the sample. Strain is
a unitless value and is converted to %
elongation by multiplying by 100. |
Stress |
Stress is
load (Ibf or N) divided by the original
cross-sectional area (in2 or
mm2) of the
specimen. By definition, this value is
normalized for gauge. |
Yield point |
Yield point
is the point during the test cycle when
the specimen continues to elongate, but
there is no increase in load. OPP films
typically do not have a clear yield
point. |
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