Technical requirements of the film
Tensile properties.19,20 A tensile test is conducted by applying stress to the sample (i.e., elongating it) and measuring the resulting strength.
Raw data consist of a stress–strain curve. From this raw data, mechanical properties can be defined. The most useful tensile
tests for plastic films are the ultimate tensile strength test, which demonstrates the maximum stress a material can withstand,
and the elongation at break test. Elongation at break is the elongation recorded at the moment of specimen rupture, often
expressed as a percentage of the original length. Materials with a high elongation at break withstand a high degree of deformation
before rupturing. A high elongation at break means a high level of flexibility.
Toughness is a material's resistance to fracture when stressed. It is defined as the amount of energy that a material can
absorb before rupturing, and can be found by calculating the area underneath the stress–strain curve. A material can be strong
but not tough; in that case it is said to be brittle.
Elastic modulus or secant modulus at 2%
: The elastic modulus (also called Young's Modulus) is a measure of a material's stiffness. This can be experimentally determined
from the slope of the stress–strain curve. For some materials, however, the elastic portion of the stress–strain curve may
not be straight enough to measure the elastic modulus. In such cases, the elastic modulus can be approximated from the secant
modulus at 2% (stress at 2% divided by 0.02). The higher the modulus, the higher the rigidity of the material. The lower the
modulus, the higher the ductility, and therefore, the greater the flexibility of the material.
Puncture resistance.21,22 Puncture resistance testing predicts the durability of a film while in use, especially its resistance to damage when impacted
by another object. Films with high puncture resistance are made of materials that can absorb the energy of an impact by both
resistance to deformation and increased elongation. Puncture resistance, measured in energy units, evaluates the film's strength
and extensibility properties. Puncture resistance is similar to tensile toughness.
Tear resistance: Tear resistance is a complex result of other basic properties, such as modulus and tensile strength. It is a measure of
the film's ability to resist tearing. Various standard methods are available for determining the tear resistance of plastic
films. Certain tests also can determine tear resistance at low rates of loading. Other tests measure the force required to
propagate a pre-cut slit across a sheet specimen.23–26
Flex durability.27 In the Gelbo Flex test, failure is determined by measuring the pinholes formed in a barrier structure after it has been
subjected to the test. This test combines a twisting motion with a horizontal pushing motion, thus repeatedly twisting and
crushing the film. The duration of the tests (number of cycles) and the amplitude of the deformation must be defined according
to the application.