ISO 7783:2018

ISO 7783:2018 pdf free.Paints and varnishes一Determination of water-vapour transmission properties一Cup method.
4 Principle
A test assembly consisting of a self-supporting coating, or a non-self-supporting coating on porous substrate, sealed to the rim of a cup Is placed in a test enclosure kept at a specified temperature (e.g. 23 °C) and relative humidity (e.g. 50 %). The relative humidity in the cup is maintained at a constant level — either at 93 % by means of a saturated salt solution (wet-cup method) or at 3 % by means of a desiccant (dry-cup method). Because of the difference between the partial pressure of the water vapour inside the test cup and the partial pressure of the water vapour in the test enclosure, water vapour diffuses through the coating under test. By weighing the test assembly at suitable time intervals, the change In mass of the test assembly Is followed. From the change in mass and the test area, the water- vapour transmission rate and the water-vapour diffusion-equivalent air layer thickness are calculated.
5 Apparatus and materials
5.1 Substrate for non-self-supporting coatings
Any homogenous, porous material which has a water-vapour transmission rate above 240 g/(m2.d) is suitable for use as the substrate for non-self-supporting coatings, for instance polyethylene frits, cellular-concrete discs, glass frits, unglazed ceramic tiles.
When using cellular-concrete substrates, the coating shall be applied on the smooth side.
lithe coating system under test does not include a primer and it is necessary to use one before applying the coating system under test, do so, but the transmission rate of the primed substrate will have to be determined separately.
5.2 Test cup
Test cups are made of glass, plastic or metal. The test cup used shall be resistant to corrosion under the conditions of the test.
NOTE For aluminium test cups. a wall thickness of 1 mm has been found to be satisfactory.
The exact surface area of the test piece exposed is defined by the design of the cup. The area of the exposed surface shall be at least 50 cm2 for non-self-supporting coatings and at least 10 cm2 for self- supporting coatings.
The cup shall he so designed that an efficient seal is made between it and the test piece, using sealing material (see £5), if necessary.
When the saturated solution () or desiccant (4) has been placed in the cup, the area of the surface of the saturated solution or desiccant shall be similar to that of the exposed surface of the test piece. The air gap between the test piece and the surface of the solution or desiccant shall he between 10 mm and 30 mm.
5.3 Ammonium dihydrogen phosphate (NH4H2PO4) solution for wet-cup method Prepare a saturated solution of ammonium dihydrogen phosphate (analytical grade) in contact with undissolved crystals, using water of at least grade 3 purity as defined in Iso 3696.
In the wet-cup method, which is the reference method, the relative humidity in a cup containing this saturated solution will be 93 %. The resulting water vapour pressure difference relative to the test enclosure, in which the relative humidity is maintained at 50 %, is 1 207 Pa at standard temperature (23 °C) and pressure (101 325 Pa).
5.4 Desiccant for dry-cup method
The desiccant shall be either dried silica gel in the form of granules passing a 4 mm sieve but retained on a 1,6 mm sieve, or anhydrous calcium chloride which has been dried at 200 °C.
It shall be possible to complete the test before the efficiency of the desiccant is reduced appreciably.
In the dry-cup method, the relative humidity in the cup shall be 3 %. The resulting water-vapour pressure difference relative to the test enclosure, in which the relative humidity is maintained at 50 %, is 1 400 Pa at standard temperature (23 °C) and pressure (101 325 Pa).
5.5 Sealing material
It shall be ensured that the test assembly is fully sealed, with the exception of the test area. The sealing material shall be impermeable and free from cracks. For sealing, mechanical clamps, wax or two-component sealing materials have been found suitable. The use of molten wax for sealing the test assembly is described in Annex B.ISO 7783 pdf download.

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