Detailed Introduction to Material Permeability Indexes

JINAN, China - Nov. 18, 2013 - PRLog -- 1. Material permeability

Any material has certain permeability; and the only difference is that some material has high permeability while others have low permeability. With low permeability, polymer can be used to prevent the permeation of oxygen and vapors from the air, which maintains the specific gas composition in the package and prolongs the guarantee period obviously.

Usually, the mostly concerned when using polymer and relevant material made of polymer for packaging are the permeability of oxygen, carbon dioxide, nitrogen and other common gases as well as water vapors; it can be described with two indexes: permeability and permeance. The former is the characteristic of material and would not vary with material thickness and area. While the latter is the property of finished products and changes with material thickness and structure transformation.

2. Gas Permeability Coefficient and Permeation Mass

Gas permeability of certain material is usually assessed in terms of gas permeability (gas permeability coefficient) and gas permeation mass. Inorganic gas permeability is directly determined by solubility (S) and diffusion coefficient (D). Therefore, gas permeability coefficient, permeation mass, solubility and diffusion coefficient should be assessed comprehensively.

Permeability coefficient (P) refers to the amount of gas volume, which penetrates through unit thickness and unit area of specimen per unit time, under constant temperature and unit pressure difference when permeation is stable, and is expressed in cm3·cm/cm2·s·Pa. While permeation mass (Q) refers to the gas volume which permeates through unit area of specimen per unit time, under constant temperature and unit pressure difference when the permeation is stable. Its unit is cm3/m2·d·Pa.

Units must be unified before calculation. For instance, the unit of permeability coefficient is cm3·cm/cm2·s·Pa and gas permeation mass cm3/m2·d·Pa. Testing time difference between the two units is 86400 times and area difference is 10000 times. Therefore, a coefficient of 1.1574×10-9 is provided for the unifying of units in GB 1038.

At present, definitions of gas permeability index are in chaos. For instance, Gas Transmission Rate (GTR) in ISO standard (ISO 2556, ISO 15105-1) refers to gas volume which permeates through unit area of specimen per unit time, under constant temperature and pressure difference when permeation is stables (the same with GB 1038 standard). Its unit is cm3/m2·d·atm. While in ASTM D1434 standard, GTR refers to amount of gas, by volume that penetrates unit area of specimen per unit time in testing condition, expressed in ml(STP)/m2·d. Suggestion is that users should firstly ascertain data unit and then convert all comparing data to the same units before comparing. ASTM D1434 standard lists some unit conversions between gas permeability units.

3. Water Vapor Permeability Coefficient and Transmission Rate

Material water vapor permeability is assessed in terms of water vapor permeability (water vapor permeability coefficient) and water vapor transmission quantity; and is accessed by some, in terms of water vapor permeance. However, the most commonly used is water vapor transmission.

Water vapor permeability coefficient (PV) refers to water vapor volume that permeates through unit thickness and area of specimen per unit time, under specified temperature, relative humidity and unit vapor pressure difference, expressed in g·cm/cm2·s·Pa. Water vapor transmission (WVTR in ISO 2528, ASTM F1249) refers to the amount of water vapor, by volume that penetrates one square meter and specified thickness of specimen within 24 hours, under specified temperature and relative humidity and water vapor pressure difference. The unit is g/m2·24.

There is no mention of water vapor permeance (P) in GB 1037 standard. But it is described in ASTM standard as follows: water vapor permeance that permeates through unit area of specimen pre unit time, under specified temperature, humidity and water vapor pressure difference between two sides of specimen. The unit is g/m2·s·Pa.

 Definitions of water vapor permeability indexes are very clear.

4. Notices

Firstly, in testing data comparison, testing conditions including environment temperature and humidity should be considered. Owing to the obvious influence of ambient environment on the permeability testing, permeability index comparisons should be the testing results of the same testing condition. Otherwise, bare data comparison is meaningless.

Secondly, material permeability testing (including gas permeability testing and water vapor permeability testing) is not significant for all materials. It has practical significance only in testing uniform and monolayer material. However, for multi-layer and composite material or package film for finished product, the calculation of material permeability is not significant, while the permeance quantity of material is of practical value. Of course, the confirmation of the permeability of monolayer homogeneous material can not be determined by a single specimen, great quantities of the same materials with different thicknesses should be carried out so as to guarantee the accuracy of the permeability.

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