Features of Permeability Test for High Barrier Films
Permeability of films can be classified into high barrier, middle and low barrier according to the gas transmission rate of film at 25µm thick. To high barrier film, the gas transmission rate is less than 5 cm3/m2·24h·0.1MPa; to barrier film, it is between 5~200 cm3/m2·24h·0.1MPa; and to low barrier film it is greater than 200 cm3/m2·24h·0.1MPa. In the last few years, low barrier films has become a key developing point to the plastic packaging industry, and they are used widely in various industries such as food, pharmacy, cigarette, chemical, electronics, military and so on.
(PRWEB) July 8, 2005 -- Permeability of films can be classified into high
barrier, middle and low barrier according to the gas transmission rate of film
at 25µm thick. To high barrier film, the gas transmission rate is less than 5
cm3/m2·24h·0.1MPa; to barrier film, it is between 5~200 cm3/m2·24h·0.1MPa; and
to low barrier film it is greater than 200 cm3/m2·24h·0.1MPa. In the last few
years, low barrier films has become a key developing point to the plastic
packaging industry, and they are used widely in various industries such as food,
pharmacy, cigarette, chemical, electronics, military and so on.
Since
for most raw materials of package, they can be used in packaging industry only
after the reprocessing (such as coextrusion process and lamination process). The
reprocessing process could cause the alteration to both physical and chemical
properties of raw materials, and result in the change in their permeability. It
is necessary to test the permeability of the “finished product” to control the
quality of package. This is fully proved by tests of various specimens in the
Labthink Permeability Laboratory. Among these specimens more than 70% are high
barrier films including aluminum foil, aluminum coating film, K-coated film and
laminated film containing PVDC, EVOH, etc.
High barrier films have the
below features in permeability testing (Take the permeability testing using
differential method for an example):
1.Prerequisite
A high barrier
specimen has strict requirements on the test condition for permeability test. To
obtain scientific and accurate data, you must pay attention to the factors as
following:
1.1 Test method
Test results vary with different test methods
employment. For the permeability tester using vacuum-differential pressure
method, the degree of vacuum in test chamber is the most important parameter. In
the procedure V of ASTM D 1434-82(2003)the degree of vacuum in the lower
pressure side(test chamber) should be limited to below 26Pa; For ISO
2556:2001and GB/T 1038-2000, it should be not less than 27 Pa. Since the degree
of vacuum is directly interrelated to the state of specimen, whether the
specified degree according to standards of vacuum can be reached in the test
chamber is a key factor affecting the test results. The precision of the vacuum
gauge is also specified too. In ISO 15105-1 the minimum precision is 5Pa; in
GB/T 1038-2000 the precision of the vacuum gauge shall be not lower than
6Pa.
1.2 Equipment
Suitable range of the vacuum gauge must be
selected. Because the precision of the vacuum gauge is specified in the
standards, so this limits the range of the vacuum gauge in fact. To most of the
high quality vacuum gauge, its precision is generally equal to 0.1% of its full
range. For examplem, if the resolution of the vacuum gauge you choose is 6 Pa,
the full scale of it will be less than 6kPa. The vacuum gauge whose resolution
exceeds 0.1% of its full range is seldom seen in the market. It should be
specially indicate that the display resolution and the real resolution of the
vacuum gauge are totally different. The display resolution depends on AD
transducer and the resolution of the vacuum gauge is the actual limit of
detection. For example, if the full range of one vacuum gauge is 6kPa, the
instrument employing this degree of vacuum can have a display below 6Pa or even
below 1Pa; but the error between the display value and the actual value is an
times value which multiple the resolution of vacuum gauge.
1.3 Test
temperature
Test gas in the test chamber is as rarefied as near to the Ideal
Gas Model and follows the Ideal Gas Equation:
Where: p——gas
pressure
V——gas volume
n——moles of gas
R——gas
constant
T——temperature of gas
If the volume
of gas in the test chamber is invariable, we can get, and this reflects that gas
pressure changes with temperature obviously. Fluctuation of temperature could
affects the permeability test obviously, According to Arrhenius Law, this could
cause the alteration of P, S and D. If the stability of temperature cannot be
maintained during the entire test period, the equipment itself must have a
function to resist the interference of temperature fluctuation.
The best
way is to put the test chamber into a heat-insulated cell with a temperature
self-control function. Practice has proved that the temperature-controll
performance of VAC-V1 gas permeability tester can reduce the requirements of
test environment effectively, and is beneficial to resist the interference of
temperature fluctuation due to an excellent design in temperature insulation
even if the temperature control function is not being used.
2. Features
of testing Procedure
There are two obvious features in high barrier testing
procedure: comparing with other specimens (barrier films and high barrier
films), the pressure in test chamber changes more little and the test duration
time is longer. This feature is shown in permeability test using not only
differential method but also equal method.
The features of high barrier
films determine the little change values of pressure and the long test time. If
the permeability of the tested specimen is low, the quantity of gas permeating
through in the whole test process is little in total, so the pressure at the
side of low concentration to test gas changes very little. We need quite a long
time to collect enough gas pressure for the vacuum gauge detection. The wider
the range of a vacuum gauge is, the bigger the value of resolution is, and the
longer test time we need. A group of actual test results of high barrier are
shown in Table 1. For each film, the maximum pressure change is not bigger than
20Pa
Well, the most important difference between tests of high barrier
specimens and other specimens is that when you do permeability test to high
barrier film, strict requirements are demanded on test condition and equipment,
and needs a long test time. So when you select testing instruments please pay
attention to following technical parameters: range and resolution of vacuum
gauge, function of temperature compensation, and how to deal with the specimen
clamping, sample sealing and so on.
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Source : http://www.prweb.com/releases/2005/7/prweb258751.htm