ASTM D and ASTM E96 both address the determination of the water vapor transmission and permeance of coatings. ASTM E96/E96M. ASTM D Standard Test Methods for Water Vapor Transmission of Organic Coating Films. Buy ASTM D R TEST METHODS FOR WATER VAPOR TRANSMISSION OF ORGANIC COATING FILMS from SAI Global.
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The answer to the question posed is based on the substrate and the service environment. All paint and coatings are permeable to water, just to different degrees. Otherwise blistering and peeling will likely occur.
The Significance of the Permeability of Paints
These three terms are used to describe how quickly water vapor diffuses through a solid material. The Water Vapor Permeance is often astj call the permeance, and the water vapor permeability is often simply called the permeability. The Water Vapor Transmission WVTpermeance, and permeability are very closely related in that one is calculated from the other. First the WVT is determined. Then the permeance is calculated by dividing the WVT by the vapor pressure zstm the material under test.
The permeability is calculated by multiplying the permeance by the thickness of the material. Water vapor is the gaseous phase d11653 water. In the gas phase, water molecules move very rapidly and are not bound together. Water vapor can be produced by the evaporation of water or the sublimation of ice or snow.
It can also be produced by the boiling of water, but the steam that you can see coming out of your kettle technically is not aetm vapor. Water in the gas phase is invisible. The steam that you see consists of very small droplets of liquid water suspended in air. The fundamentals of the measurement of water vapor transmission in the laboratory are simple.
WVT is determined by sealing a free film of the test material to the top of a container that is filled with water left or desiccant right. If the container is filled with water, the method is called the wet cup method or the water method. If the container is filled with desiccant, the method is called the dry cup method or the desiccant method.
Essentially, the test determines how much vapor is lost through the film in the wet cup method or how much vapor the desiccant attracts or draws through the film for the dry cup method. The weight data is plotted versus time. The system may at first gain or lose weight more quickly, and then settle into a constant or steady-state rate of weight change after d163 couple of days, or longer, depending on the type of material being tested.
If the cup is filled d653 water, the dish loses water over the course of the test so the plot points downward. If the cup contains desiccant, the dish gains water weight over the course of the test, so the plot points upward.
In either case, the slope of the plot is the rate of weight change and its units are mass per time. For example, the units might be grams wstm hour or nanograms per 24 hours.
What are the formulas and units for WVT, permeance, and permeability? The WVT is determined by dividing the rate of weight change, which is the slope of the plot, by the area of the test specimen. The permeance and permeability may be calculated from the WVT. The scopes of the standards differ considerably.
ASTM D d163 the evaluations of films of coatings: The films may be aetm films or they may be applied to porous substrates. ASTM E96 covers many materials, including coatings. It can be used to test paper, plastic films, other sheet atm, fiberboards, gypsum and plaster products, wood products, and plastics. This correction can greatly change the results. ASTM D does not discuss this correction. See the slide below for more information about the correction.
What does the magnitude of the permeance indicate? Materials can be classified by the magnitude of their permeance as follows. How precise is the testing?
If a sample is tested twice by the dry cup method, and each test consists of three replicate trials, the difference between the permeance results from the two tests are expected to be less than a quarter of a perm if dd1653 permeance is less than 1 perm. For the wet cup method, two results, each the mean of triplicate trials, are expected to differ by less than The repeatability values reported by these two ASTM standards indicate the permeance values determined are ball park figures.
What should one consider when comparing the permeance of coatings? Finally, be cautious when comparing the results obtained by the dry cup method to the wet cup methods. D653 wet cup method can produce higher results than the dry cup method when the same coating is tested under the same air temperature and r1653 conditions.
ASTM D – 03 Standard Test Methods for Water Vapor Transmission of Organic Coating Films