Materials, thin: Water vapour permeability (NT BUILD 265)

Abstract

1. SCOPE

This NORDTEST method specifies an accurate method to determine permeability coefficients for materials in sheet form. This method takes into account inherent cup effects in permeation experiments. This is necessary in order to properly interpret data for acrylic paints, wood, paper and other relatively permeable materials.

2. FIELD OF APPLICATION

This method is applicable to permeable materials in general, but it is especially suitable when more accurate permeability coefficients for relatively permeable samples are to be measured. The procedure described measures an intrinsic property of the material which may be modified if the material (e.q. a coating) is applied to a substrate. The permeability coefficient depends on the relative humidity (RH) on both sides of the material and on the temperature. For this reason, identical conditions of RH and temperature should always be used in comparative measurements. For hygroscopic materials the dependence on the RH is greater and for these materials it is recommended to perform measurements both at a low and at a high RH. RH levels at which the permeability is very sensitive to small changes in RH should be avoided, as this may introduce errors. This method is generally not suitable in such cases as when the RH exceeds 90 per cent on both sides of the layer. Neither is it applicable to materials that are deformed during the test conditions. This method requires samples of the test material in a number of different thicknesses. It should be noted that all these samples have to be identical in composition. (This may for instance not be the case for coatings in different thicknesses dried equally long times, due to differences in solvent retention.)