Barrier property testing has not been fully popularized in our country, but its importance and practicality are widely known. Moreover, the testing subjects of some researchers have come beyond the common transmission testing field and into the special gas transmission rate testing. This shows that domestic barrier property testing has developed to a certain level, and has become a positive testing demand. Meanwhile, barrier property testing has expanded to more application fields, including aeronautics, aviation, special gas storage, medical device, building and many other fields with great potentiality. As a new area in barrier property testing, how to test special gas transmission rate and how to select testing methods and instruments need to be summarized in actual testing. Labthink Lab has many experiences in special gas transmission rate testing and consulting services for her clients. Hence, in this article, the author, integrating gas characteristics and relevant testing information, gives an introduction to special gas transmission rate testing method and relevant testing suggestions.

Oxygen and water vapor/moisture are the most common testing gases in barrier property testing. Those gases are safe according to their characteristics. Though oxygen can help burning, it’s safe and breathe supportive. Water vapor is also safe and harmless. But if the test gases changes to special gases, the testing process would be quite complicated owing to the great differences in their characteristics. For example, H_e is inert gas with stable characteristics, flameless, safe and no corrosive; H₂ is safe, but explosive and flammable; CO can burn and poisonous; and CH4 is also explosive and flammable with a special note that it is an organic gas.

The flammability and toxicity of test gas have direct relationship to the life safety of the test operator; but the corrosiveness of the test gas and whether the test gas is organic gas or not will directly affect the instrument structure and testing. As to specimen, there are obvious different influences of organic gases and other gases. Though the testing demands are from different fields, most specimens for gas transmission rate testing are high polymers or complex formed by high polymer and metal, paper, fiber, etc. When organic gas permeates into the high polymer, high polymer specimen would occur swelling which would greatly change its appearance and barrier property with increasing gas transmission rate and more difficulties to achieve transmission balance. Therefore, there are obvious differences between inorganic gas transmission testing and organic gas transmission testing.

The author holds that special gases can be categorized into 3 kinds: the first kind is non-flammable and non-toxic, including CO₂, N₂, and inert gases; the second kind is flammable or toxic inorganic gases, including H₂, CO, NO, HCI, H₂S, SO₂ and CI₂, etc; the third kind is organic gas/vapor, such as methane and ethanol. The testing methods and instruments for the above three kinds are different and will be introduced in detail. One thing needs to be specially pointed out is that if the test gas is of strong corrosiveness, testing instruments should be customized and its feasibility of testing methods should be evaluated.
2.1 non-flammable and Non-toxic Inorganic Gas

The testing for such gases can be accomplished with differential pressure method gas transmission instruments with no instrument re-design and no accessories. The operator only needs to purchase testing gas and take care of the tail gas.

This kind of gases is the so-called dangerous gas with certain dangers when testing. Since they are toxic, flammable and explosive or both, dangerous gas quantity of the whole lab (including those inside the instruments) should be strictly controlled according to the characteristics of the gases. At the same time, the handling of tail gas should be strengthened so as to guarantee safety. The testing method for this kind of gases is also differential pressure method. But, proper adjustments are made in structural design according to different gas characteristics.

Only a few organic matters are gaseous under normal temperature, such as methane; most of them are liquid with volatility, such as ethanol. Therefore, the testing gas mentioned here are organic vapor.

Organic gases can lead to swelling of high polymer materials, and further affect their permeability obviously. Therefore, the testing methods of organic gases are quite different from that of inorganic gases. The biggest difference is the quite low gas concentration when testing. Considering the characteristics of organic gases, there should be an organic gas generating system to provide test gas of accurate concentration. For non-flammable and non-toxic inorganic gas, the high concentration side should be of 100% concentration of testing gas; for flammable or toxic inorganic gases, the high concentration side should be controlled within the safe concentration scope; as to organic gas test, only when high concentration side is no higher than 100ppm, can we regard specimen swelling by organic gas as being no influence to specimen permeability.

Organic gas transmission rate can be tested with isostatic method. Labthink has developed professional isostatic method organic gas transmission rate testing device - PERME^TM　OR2/410 Organic Gas Transmission Rate Testing System. This instrument is of patent design, and consists of transmission chamber, organic gas generator, timed sampling valve, isolation room and FID (flame ionization detector). It’s easy to operate, and it’s the organic gas transmission rate tester with highest automation. The drafting of relevant national standard of this method has begun.

PIC 1. Structural Drawing of Isostatic Method Film Organic Gas Transmission Rate Tester

 

Though isostatic method should be applied for organic gas transmission rate testing, some use differential pressure method (also widely applied in documentations) for some specific purposes. In fact, differential pressure method can be used for the organic gas transmission rate testing of certain organic gases. But, the testing instruments must be customized as that of flammable or toxic inorganic gases.