 |
 |
| From this fact, we can see that, when
expressing gas transmission quantity, the same pressure
must be used for comparison. In other words, the
quantity of gas transmitted must be divided by the
pressure to derive a gas transmission quantity per
unit pressure. In the case of a mixture of gases,
the partial pressure differential for each gas is
used. |
| Next, let us examine the area of gas
transmission. As shown in Figure 2-1 (3), when the
area of the film is larger than (1), naturally the
transmission quantity also increases. Accordingly,
the transmission quantity must also be shown in
terms of unit area. |
It is also clear that the quantity
of gas transmitted will be greater for a period
of two days than for a single day, greater for a
period of three days than two days, and so on (Figure
2-1 (4)). Accordingly, the gas transmission quantity
must also be divided by time to express the volume
per unit of time.
Finally, as shown in Figure 2-1 (5), if the film
is thick, the quantity of gas transmitted will decrease. |
|
|
| Figure 2-2 shows the relationship
between the thickness of ethylene vinyl
alcohol copolymer and the oxygen permeability.
These values are inversely proportional:
when the thickness is doubled, the quantity
of gas transmitted is halved, and when
the thickness is tripled, the transmission
quantity is reduced to one-third. |
|
| Accordingly, when comparing gas transmission
quantities, the film thicknesses must
also be the same. |
|
|
Figure
2-2 Relationship between thickness
and oxygen permeability |
|
|
|
| |
|
| From the above, it can be seen that
the quantities for gas transmission quantity (the
gas transmission rate) must have the same gas pressure
differential, gas transmission area, film thickness,
and duration. To accomplish this, the transmission
quantity should be divided by the gas pressure differential,
the gas transmission area, and the time. With regard
to film thickness, there is an inversely proportional
relationship, so the value must be multiplied by
the thickness. |
Accordingly, the gas transmission
rate can be expressed by the following equation:
Gas transmission rate = gas transmission quantity
(volume) x film thickness / (pressure differential
x transmission area x time) |
There is also a value known
as "gas permeability" for which thickness
conversion is not conducted. Gas permeability
is expressed as follows:
Gas permeability = gas transmission quantity (volume)
/ (pressure differential x transmission area x
time) |
| The gas transmission rate is specific
to each material. Whereas the gas transmission rate
is convenient when comparing the gas barrier properties
of different materials, it is more advantageous
to use gas permeability when comparing the gas barrier
properties of specific test specimens. |
cc 20 ƒÊm / (m224 hrsatm)
is frequently used as a unit for gas transmission
rate. This unit is derived by converting the film
thickness to 20 ƒÊm and expresses the
quantity of gas transmitted per day (24 hours),
based on a film area of 1 meter and a pressure of
1 atm (= one atmosphere).
Other values used as gas transmission rate units
are cm3 cm / (m224 hrsatm) and cm3 cm / (cm2scmHg).
In the latter value, "s" indicates per
second and cmHg indicates that pressure uses centimetres
of mercury as a standard. |
| |
|
| In some countries, the unit in3 mil
/ (100 in2dayatm) is also used, with "in"
indicating inches (1 inch = 2.54 cm) and "mil"
indicating 1/1000 inch. In academic papers, cm3
(STP) cm / (cm2sPa) and cm3 (STP) cm / (cm2scmHg)
are used. |
Here "(STP)" indicates that,
since the volume of the gas will change depending
on temperature and pressure, it has been converted
to standard temperature and pressure. "Standard
temperature and pressure" refers to absolute
temperature 273.15 K (Kelvin) and pressure 1.01325
x 105 Pa (pascal) -- in other words, 0degC and 1
atmosphere.
Pa is a unit of pressure that may be unfamiliar
to some people.
However, they have probably heard the term "hectopascal"
(1 hPa = 100 Pa) when listening to a weather report,
so they should be aware that it is a pressure unit
that is closely related to our daily lives. |
| In addition to the values presented
here, various other gas transmission rate units
are also used. For this reason, we have provided
a conversion chart for transmission rate units.1)
In this series, the unit cm3 (STP) cm / (cm2sPa)
that is in conformance with the Polymer Handbook
will be used. |
In addition, there is also an SI
unit (the unit of the International System of
Units, the system of scientific units used worldwide)
for oxygen permeability: fm / (sPa).1) 1 fm /
(sPa) = 8.752 cm3 / (m224 hrsatm). "fm"
is read "femtometer" and 1 fm = 10-15
m. |
| |
|
Table
2-1 Conversion factor of gas permeability
unit |
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|
