Gas Barriers: An Introduction Basic Reference 3
Fundamentals of gas transmission 1
Paper is porous because it is formed by intertwined cellulose fibers. Also, if aluminum foil is crumpled up, opened up, and then held up against the light, one can see that the foil now has many pinholes like stars in the night sky. The phenomenon of gas permeating through pinholes is called capillary flow mechanism. It is relatively easy to understand the phenomenon of gas permeating through something with holes.
The quantity of gas flow is proportional to the biquadrate of the pinhole size.It is also proportional to the number of pinholes and the pressure differential of both sides of the film. It is inversely proportional to the viscosity of the gas and thickness of the film. This may sound hard to understand, but one needs only to imagine a tank with a faucet (Fig. 3-1). When water runs out of a tank, more water flows out if the faucet is bigger and there are more water taps.
The size and number of the water tap is equivalent to the size and number of the pinholes. A large quantity of water flows out if there is a lot of water in the tank, whereas a smaller quantity of water flows out if there is less of it in the tank. The pressure differential of both sides of the film is equivalent to the amount of water. It is easy to understand from one’s own experience that it would be harder for something sticky, such as honey, to flow out of the tank than water.
Fig. 3-1 Quantity of water flow in various cases
The volume of water flowing out of the tanks in cases 2 and 3 is larger than that for 1. However, the water flow is less in case 4. Also, if honey is placed in the tank instead of water, as in case 5, the amount of honey flowing out will be less.
Plastic is used as package materials in jelly cups, mayonnaise bottles, and toothpaste tubes. At first glance, there are no holes in these plastic packages. Nevertheless, gas permeates through them even though there are no holes at first glance. The mechanism of gas transmission through a nonporous barrier is called activated diffusion mechanism, and it is currently the mainstream theory of gas transmission of plastic. Gas transmission by activated diffusion mechanism is affected by the type of plastic used, temperature, and humidity.
There are examples of gas transmission by activated diffusion mechanism all around us. Have you ever blown up a balloon and watch it deflate over time (Fig. 3-2)? The reason this happens is because the air inside the balloon escapes directly through the rubber.
Fig. 3-2 Changes in balloons over time
In this series, the mechanism of activated diffusion mechanism is explained, and the type and structure of plastic, which affect gas transmission, and the reasons why gas permeability changes depending on the temperature and humidity will be explained in due course.
The next article in this series will cover the mechanism of activated diffusion mechanism.
Reference
1.Gas baria-sei, hokousei Housouzairyo no Shintennkai, Toray Research Center (1977)
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