Abstract: water vapor permeability, Transmission Rate, Permeance and Permeability
With the popularization of barrier property testing, relevant parameters become a major concern in property evaluation of materials. But due to the different definitions of some parameters in domestic and international standards, it is difficult for operators and researchers to distinguish definitions and applications of these parameters, which not only affects data transmission, but also results in fault property evaluation of materials. This article introduces the definition, application, differences and conversion relations of three water vapor permeable parameters -Water Vapor Transmission Rate, Water Vapor Permeance and Water Vapor Permeability. It also explains the things to be noted during actual application.
According to the mass transfer theory, gas molecules or water vapor molecules can transmit through plastic films under the driving of chemical potential energy inside polymer materials. Because chemical energy depends on chemical activity and chemical activity is in proportion to gas concentration, molecule concentration Ci and partial pressure pi conforms to the formula below:
Where k is a constant value, i.e., the gas flow transmited through film depends on the partial pressure on two sides of film. If the gas flow through area A during time t is Q, permeability P can be calculated with the formula below:
Where p1、p2 represent the partial pressures on two sides of specimen. Partial pressure difference△p = p1－p2, . l refers to film thickness.
2.Description of Water Vapor Permeable Parameters in International Standards
The following are basic standards commonly used for water vapor permeable property testing:
2.1 ASTM E96-05
In ASTM E96-05, the three parameters below are used to describe water vapor permeable property testing:
1. Water Vapor Transmission Rate: The steady water vapor flow in unit time through unit area of a body, normal to specific parallel surfaces, under specific conditions of temperature and humidity at each surface.
The abbreviation of Water Vapor Transmission Rate is WVT. The metric unit of WVT is g/h·m2.
2. Water Vapor Permeance: The time rate of water vapor transmission through unit area of flat material or construction induced by unit vapor pressure difference between two specific surfaces, under specified temperature and humidity conditions.
Therefore, water vapor permeance is the ratio of WVT to water vapor partial pressure between two sides of specimen. The commonly used unit is g/ Pa·s·m2
3. Water Vapor Permeability: The time rate of water vapor transmission through unit area of flat material of unit thickness induced by unit vapor pressure difference between two specific surfaces, under specified temperature and humidity conditions.
Water vapor permeability is the product of water vapor permeance and film thickness. The commonly used unit is g·cm/ Pa·s·cm2.
2.2 ISO 2528:1995
In ISO 2528:1995, there is only one parameter describing water vapor permeable property:
Water vapour transmission rate (WVTR): Mass of water vapour transmitted through a unit area in a unit time under specified conditions of temperature and humidity. It is expressed in grams per square meter per 24h [g / (m2·d)].
2.3 GB/T 1037-1988
In GB/T 1037-1988, there are two parameters describing water vapor permeable property :
1. Water Vapor Transmission Rate（WVT）： The steady water vapor flow transmitted in 24 hours through 1m2 area of a body, under specific conditions of temperature and humidity. Unit: g /m2·24h.
2. Water Vapor Permeability（PV）：the time rate of water vapor transmission through unit area of specimen of unit thickness induced by unit vapor pressure difference, under specified temperature and humidity conditions.
Note： GB/T 1037-1988 is issued referring to ASTM E 96-80 Standard Test Method for Water Vapor Transmission Rate of Materials Using Cup Method.
2.4 Summary
According the units of these terms, we can see that WVT in GB/T 1037-1988 is identical with the term Water Vapor Transmission Rate defined by ASTM E96-05 (WVT for abbreviation) and ISO 2528:1995 (WVTR for abbreviation), all of which can be expressed in g/m2·24h. Water vapor permeability defined in GB/T 1037-1988 and ASTM E96-05 uses the same unit. While this definition is not mentioned in ISO 2528:1995. The term water vapor permeance is not mentioned either in GB/T 1037-1988 or ISO 2528:1995.
3.The Application of Water Vapor Permeable Parameters
Water vapor transmission rate is the Mass of water vapour transmitted through a unit area in a unit time, which is decided by two factors: barrier property of material and concentration difference of water vapor between two sides of specimen. WVT correlates with test environment and specimen property. Water vapor permeance is only in connection with specimen itself. Water vapor permeability also depends on the material of specimen only. These three parameters conform to the formulas below:
water vapor permeance= water vapor transmission rate/△p， water vapor permeability= water vapor permeance×l （3）
where，△p is the partial pressure between two sides of specimen，l is film thickness
Generally, we are only concerned with WVT (Water Vapor Transmission Rate，unit：g/m2·24h)and water vapor permeability(Water Vapor Permeability，unit：g·cm/cm2·s·Pa). Neglecting the affection caused by different definitions in standards, water vapor permeance is rarely used, which mainly caused by application habits. However, where water vapor permeability is concerned, there is mistake conception. The permeability is meaningful only for homogeneous materials, in which case it is a property characteristic of bulk material. his parameter only makes sense to some homogenous monolayer materials. Moreover, the data consistency of materials with different thickness must be confirmed before application. Although Water vapor permeability of several kinds of monolayer materials can be applied to structure designing of laminated materials, there is no significance testing water vapor permeability of multiplayer laminated materials.
4.Conclusion
The parties concerned should attach great importance to the inconsistent definitions of water vapor permeable parameters. Users cannot pay attention to the name of permeable parameters only. To avoid chaotic application of different definitions and to provide conveniences for communications and exchanges, users should compare units of these parameters and clarify the meaning mentioned in related standards.