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Mechanical Properties of Plastic Films.
We mostly use American standard ASTM test methods.
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Measures strength, elasticity,
toughness etc.
Strips of film are clamped in
the jaws of a computer-controlled tensile tester
and stretched at a constant speed until they break.
Separate tests are done in the film’s MD
and TD (machine direction and transverse direction,
i.e. lengthways and widthways). The force and
extension are graphed and a number of properties
calculated. These include strength, elongation,
yield point, elastic modulus, break energy etc
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Tear Resistance - Rectangular strips of film 25 mm wide and 75
mm long are pre-slit down the middle for 50
mm of their length, leaving 25 mm intact. The
slit strip resembles a pair of trousers.
The “legs” are clamped in the jaws
of a tensile tester and pulled apart, so that
it tears in the line started by the slit. The
test result is the maximum force registered
while tearing. Separate tests are done in the
film’s MD and TD.
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Puncture
Resistance - The film is held
in a clamp with a 100 mm diameter opening. A probe
(attached to the load cell of a tensile tester
used in compression mode) is driven into the film
until it ruptures.
The standard probe tip diameter
is 19 mm, but non-standard probe sizes are also
available: 12.7 mm diameter to simulate fingers,
and 1 mm to simulate semi-sharp objects. Puncture
force, energy and probe displacement are recorded.
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The
film is held in a clamp with a 127 mm diameter
opening. A “dart” with a rounded tip
of 38 mm diameter is loaded with weights and dropped
onto the clamped film. Many drops are done, each
on a fresh film area, with the dart weight increased
or decreased each time according to whether or
not the film ruptures.
We calculate the “Wf”
or “F50” result, the theoretical dart
weight at which exactly 50% of drops would result
in film rupture.
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Specialised modulus tests, seal strength, cling force,
bond strength and other tests are also available.
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