Paint applied to wood: Water permeability (NT BUILD 331)

  • Report #: NT BUILD 331
  • Approved: September 1987
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This NORDTEST method specifies an accurate method to determine the permeability coefficients for materials forming an attached coat when applied to a permeable substrate, e.g. wood. This method takes into account inherent cup effects in permeation experiments and eliminates the influence from the substrate during the test.


This method is universally applicable to permeable materials applied to a permeable substrate when it is desired to avoid the influence of the substrate during the test. 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.g. a coating) is not applied to a substrate. The permeability coefficient may be dependent on the relative humidity (RH) outside the cup and on the temperature. For this reason, identical conditions of RH at the outside of the cup and of 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 cases when the RH exceeds 90 per cent outside the cup. Neither is it applicable to materials that are deformed during the test conditions. This method requires samples of the test material applied to a substrate in a number of different thicknesses. It should be noted that all these samples must be identical in composition. (This may for instance not be the case for samples coated in different thicknesses which have had insufficient, although equal length of drying time, due to difference in the amount of retained solvent).

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