Instrumented Impact Strength
The test method used to test the impact strength of films. This test method gives an index of the material's dynamic strength and approximates what will occur when the package is dropped. An impacting head on the end of a pendulum is swung through an arc into and through a film sample. The tester has a means of measuring the difference between potential energy of the pendulum at maximum height in free swing and the potential energy of the pendulum after rupture of the sample. This difference in energy is defined as impact strength and is reported in units of pounds. It is useful in predicting resistance of a material to breakage from dropping or other quick blows. The higher the resulting measuring number, the greater the impact strength.
Coefficient of Friction
The coefficient of friction is related to the slip properties of the film. It is the ratio of the frictional force between two surfaces of film (inside or outside surfaces together) and the force pressing the two surfaces together. In this test, a sheet of film is wrapped around a weight or sled 2-1/2" square, 1/4" thick, and weighing 200 t 5 grams. The film is wrapped in
such a manner that there are no wrinkles or other contaminants on the surface to be tested. The sled is then placed on a second sheet of film which is securely attached to a polished metal surface. The sled is then pulled horizontally by a device such as an Instron which records the force required to start movement and the force required to keep the sled moving.
Elongation
Elongation is the amount of stretching (%) that has occurred when the sample breaks as
compared to its original length. A large value for elongation is an index of toughness, since it
indicates a material will absorb large amount of ener before breaking.
Free or Unrestrained Shrink
The amount of unrestrained shrink (%) that occurs at a specified temperature. In this test a 10 cm square of film is immersed in hot oil for 8 seconds and measured after removal. Because of the influence of orientation, free shrink in both the longitudinal and transverse directions is reported. Free shrink may be an indicator of the ability of the film to shrink against a package, removing unsightly package blemishes like "dog ears."
Gloss
The brightness or luster of a film. As a visual sensation dependent on an individual's perception, gloss cannot be measured, however, it is possible to measure the amount of light that reflects from a film's surface from a 45 degree angle. The amount of gloss that a film has is to some extent dependent on its thickness, for this reason, the gauge of the test sample accompanies the value. The higher the value, the more gloss a film has. Gloss is measured on a glossmeter. This instrument has an incandescent light source and a photosensitive receptor and responds to visible light.
Haze
Haze is a film's light scattering property which gives it a cloudy or milky appearance. Haze
values are expressed as a percentage; the higher the value, the hazier the film. Since the
thickness of a material can affect these values, the gauge of the sample tested accompanies the haze value. Haze is measured by a special hazemeter which as an incandescent light source an geometrically arranged photocells that measure the transmitted and scattered light.
Modulus of Elasticity-Stiffness
The stiffness or resistance to bending of a packaging film. This modulus provides an indication of whether a film is soft or stiff. The higher the modulus value, the stiffer the material. Modulus is an important property in the proper operation of high speed form/fill/seal equipment. A relatively stiff film is normally more likely to machine better on a high speed machine than a soft film.
Moisture Vapor Transmission Rate
The amount of water vapor (grams) which will pass through 100 square inches of film in 24 hours at 100 degree F and 100% relative humidity. The water vapor transmission rate is gauge dependent.
Oxygen Transmission Rate
Specially constructed cells are used to measure oxygen transmission rate. After the film sample has been clamped into the cell, oxygen is flushed through chambers on both sides of the sample. Oxygen is admitted to one side of the sample; the test chamber on the other side is evacuated and oxygen is allowed to permeate through the film chamber for a measured length of time. The amount of oxygen (cc's) which will pass through a square meter of film in 24 hours at 73 degrees F is measured. The thickness (gauge) of the film has a direct relationship on the oxygen transmission rate.
Seal Strength
The seal strength is the amount of force necessary to break a trim seal. A 1" wide strip
of film containing a seal is placed in clamps or jaws of a testing device; the jaws are then moved apart at a controlled speed until the seal breaks. The results of this test are recorded as pounds per linear inch. When comparing the seal strengths of two films, the higher the value, the stronger the seal.
Shrink Tension
Shrink tension is the force (psi) that is exerted on a package by the film as it is
shrunk. Depending on the application, a film with high or low shrink tension may be desirable.
For example, in overwrapping thin, flexible items such as magazines or greeting cards, a film
with high shrink tension values would be undesirable since the product may be distorted when the film is shrunk. Because of the influence of the orientation, shrink tension values for both the longitudinal and transverse directions are provided.
Tear Resistance
The film's ability to resist zippering or propagation of a tear.
Tear Propagation
The force required to propagate, or spread, a tear. In this test a section of film is
held between two jaws and an initial cut is made on the end of the sample. The jaws are moved apart until it tears from the initial cut. The force, in grams, is recorded that is required to spread and tear. Tear propagation is heavily dependent on the thickness of the sample. For this reason, the actual gauge of the sample accompanies the tear propagation value.
Tensile Strength
The testing machine consists of clamps to hold the sample, some means of gradually
increasing the load on the 1" wide strip of film until it breaks and indicators which show the load and the amount of elongation. Tensile strength is reported in pounds per square inch of original cross sectional area. Tensile strength is quite literally the amount of force necessary to pull a material apart. The elongation is the amount a material will stretch before breaking. To perform the test, measured, gauged specimens of film 1" wide are clamped into the testing machine and stretched until they break.
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