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Gas Barriers: An Introduction Basic Reference 5

Types of polymer and gas permeability coefficient

In Introduction – Basic Reference 4, we explained that gas transmission goes through the processes of dissolution, diffusion, and desorption. We also explained that many monomers are joined to form a polymer, that plastic is made up of polymers, and that gas permeates through the amorphous part of the polymer.

Incidentally in the case of a polymer, various polymers may be synthesized by changing monomer types. Fig. 5-1 shows the oxygen transmission coefficients of various polymers.

As you can see from the graph, depending on the polymer type, the oxygen transmission coefficient changes one million-fold. Ethylene-vinylalcohol copolymer, which can be seen in the graph, has a thickness of about 10µm and is used for mayonnaise bottles. A thickness of 37cm is needed to obtain the same oxygen barrier properties with polyethylene.

However it will not be possible to squeeze mayonnaise out of the bottle if it is as thick as this. This example alone explains how good the ethylene-vinylalcohol copolymer is as a gas barrier material. Polymers can be classified by their chemical constitution, such as the polyamide and polyester series. Another example is vinyl system polymer, which is synthesized from monomers with a chemical formula of CH2=CXY. Hydrogen (H), chlorine (Cl), the hydroxyl group (OH), the methyl group (CH3), and the phenyl group () come into X and Y. If Y is a hydroxyl group, it cannot be synthesized directly. In this case, polyvinyl acetate is synthesized first, and then the acetic acid group is changed into the hydroxyl group. Atoms of X and Y or types of functional groups and polymer names are listed in Table 5-1. Fig. 5-2 is a graph of the oxygen transmission coefficients of vinyl group polymers.

The Y of ethylene-vinylalcohol copolymer being OH or H means that there is an OH group or H atom randomly in one molecule. Of the atoms or functional groups described here, OH or CN groups, F and Cl are not electrically neutral. They are polarized into positive and negative, and this is called a polar radical. When the oxygen transmission coefficient is seen from the perspective of a polar radical, you can see that the more polar radicals there are, the higher the oxygen transmission will be.

Electric charge in the molecule is biased for polymers with a polar radical. This means that the positive and negative charges attract each other, making the intermolecular forces stronger. The energy needed to separate the molecules is called the cohesive energy density and the density of such molecules will be higher.

Fig. 5-3 shows the relationship between the cohesive energy density and the oxygen transmission coefficient5). As is shown, the higher the cohesive energy density, the more the oxygen transmission coefficient will decrease. This is because when the intermolecular forces are stronger, it becomes difficult for a gas molecule to push open the polymer chains and this lowers the gas transmission.

Incidentally, what each polymer is used for is more or less determined by the cohesive energy density. A polymer with a lower density is used for rubber, medium density is used for plastic, and high density is used for fiber. Polyester, polyamide, acrylonitrile and polyvinyl alcohol shown in Fig. 5-3, which have high cohesive energy densities, are all used for fiber. Instead of these names, if you hear the words polyester fiber, nylon fiber, acrylic fiber and vinylon fiber, you can see that they are familiar fibers for industrial use or for making cloth.

Although gas barrier materials and fibers are used for totally different purposes, they have something surprising in common from the perspective of cohesive energy density. In Introduction – Basic Reference 3, we explained about the deflating rubber balloon. Now you know the reason why there are no rubber balloons with excellent gas barrier properties.

The next article in this series will focus on free volume, which is closely related to the gas permeability.

1) Kobunshi to Mizu, Kyoritsu Shuppan Co., Ltd. (1995)
2,3) Polymer handbook 4th Edition,John Wiley & Sons,Inc. (1999)
5) Gas baria-sei, houkousei Housouzairyo no Shintennkai, Toray Research Center (1997)