10-18 PERMEABILITY

10-18 PERMEABILITY

Closely related to the chemical resistance of polymers is their permeability toward gases and liquids. The passage of gas may take place by simple diffusion through very fine channels between rigid molecular units, as in the case of crystalline, glassy, or highly crosslinked polymers, or by diffusion through viscous materials, as in the case of a rubbery polymer. Diffusion can be enhanced by a solution of gases or liquids in some component of the polymer structure, such as plasticizers. A higher degree of crystallinity, which results in a higher density and also a higher degree of crosslinking, lowers the rates of diffusion of both gases and liquids

through the polymer, thereby also improving their chemical resistance. It appears that CO 2 has a rate of permeability through most polymers considerably greater than that of oxygen and still greater than that of nitrogen. This is perhaps due to its ability to be absorbed by materials that

permit high rates of water transfer. The average ratio rates between N 2 :O 2 : CO 2 is 1:4:14. The absorption of water and its passage through polymer follow general principles: polymers

having polar groups show much a higher permeation or absorption rate than nonpolar polymers. Water itself may act as a plasticizer, contributing to the swelling of the polymer and the loosening of its network. The permeability of other solvent molecules under conditions of insolubility of the polymer resembles the mechanism for the permeability of gases, with the added complication of polar effects, as in the case of water transfer. Permeation or permeability rate is the product of the diffusion term and the solubility constant of the gas-liquid in the polymer network,

Permeability rate is a function of many parameters, but the gas permeability coefficient is a basic property of a material independent of specimen geometry.

There are certain molecular structures that lead to good barrier properties in polymers. A high- barrier polymer exhibits a high resistance to molecular flow of a permeating agent through the polymer matrix. This includes resistance to gas flow, resistance to liquid, and resistance to absorption and diffusion of organic vapors through the polymer. Frequently the low permeability

of a polymer toward oxygen or carbon dioxide P CO 2 , can make the material highly permeable toward water and water vapor. For example, polyvinyl alcohol has excellent resistance to gas

flow (permeation) but it is a very poor water barrier. In addition, it becomes a poor gas barrier when plasticized with water which causes swelling of the polymer, thereby loosening its structure. Conversely, polyethylene has very good water barrier properties but it is a poor gas barrier. To be a truly good barrier polymer toward the gases the material must have some degree of polarity as provided by nitrile, ester, chlorine, fluorine, or acrylic groups, it should have high chain stiffness, close chain-to-chain packing, crystallinity, and orientation. Note that polyethylene and polypropylene are poor gas barriers but excellent water barriers, whereas polyacrylonitrile is an excellent gas barrier but a poor water barrier. Dimethyl silicone rubber because of its open structure shows extremely high permeability to gases and water. It is used as flow (permeation) but it is a very poor water barrier. In addition, it becomes a poor gas barrier when plasticized with water which causes swelling of the polymer, thereby loosening its structure. Conversely, polyethylene has very good water barrier properties but it is a poor gas barrier. To be a truly good barrier polymer toward the gases the material must have some degree of polarity as provided by nitrile, ester, chlorine, fluorine, or acrylic groups, it should have high chain stiffness, close chain-to-chain packing, crystallinity, and orientation. Note that polyethylene and polypropylene are poor gas barriers but excellent water barriers, whereas polyacrylonitrile is an excellent gas barrier but a poor water barrier. Dimethyl silicone rubber because of its open structure shows extremely high permeability to gases and water. It is used as

The rate of transmission of liquid water through glass/polyester laminates and polyester resin castings is a function of resin structure and it always increases with temperature. The effect is a reduction in tensile stress, in flexural strength, and in modulus of elasticity with time when exposed to water for a long time. An additional requirement of high-barrier polymers used in food packaging and beverages is that the taste and odor of the pack aging cannot be affected in any way by interaction with the polymer wall.