Intelligent Measurement of Water Vapor Transmission Rate of Exterior Building Materials

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Good waterproof is a basic property of exterior wall coatings for buildings. When there is a humidity difference between the two sides of the wall, water vapor will permeate through the wall from high humidity side to low humidity side, resulting in the moisture transfer in the wall. In addition, if the permeability of building materials is poor, the rain or snow water leak into the interior of the wall through the cracks, while the moisture in the wall cannot permeate outward, it will lead to the increase of water content in the wall, erode the wall and the insulation layer structure, and affect the durability, safety and insulation function of the wall. At the same time, water vapor in the wall may condense and freeze which will affect the strength of the material and cause a certain extrusion effect on the material, resulting in wall hollowing, shedding, cracking, mildew and other quality problems. Therefore, the external wall coating should be able to discharge the water vapor in the wall in time.

Test sample

We take an exterior building coating as test sample in this study. The tests were done following the procedure outlined in ASTM E96 – Standard Test Methods for Water Vapor Transmission of Materials. The instrument we used is Labthink newly launched C360M Water Vapor Transmission Rate Test System.

Summary of Test Methods

The method described in ASTM E96 is cup method, also named as gravimetric method. The water vapor transmission can be determined by calculating the dishes weight change from initial to time of weighing. Two basic methods, the Desiccant Method and the Water Method, are provided for the measurement of permeance. In the Desiccant Method the test specimen is sealed to the open mouth of a test dish containing a desiccant, and the assembly placed in a controlled atmosphere. Periodic weighings determine the rate of water vapor movement through the specimen into the desiccant. In the Water Method, the dish contains distilled water, and the weighings determine the rate of vapor movement through the specimen from the water to the controlled atmosphere. The vapor pressure difference is nominally the same in both methods except in the variation, with extremes of humidity on opposite sides.

The instrument we used in this paper C360M Water Vapor Transmission Rate Test System can be applied both the methods. But what we used in our paper is the water method.

Test procedure

1. Prepare the specimen according to specific standard. The test specimen should be of uniform thickness and be free of wrinkles, creases or other visible imperfections. Identify the permeating direction of the test and mark the permeating direction on the test specimen. Generally the water vapor permeates from the outside in.
2. Then mount the specimen in specific order and the test specimen should be flat and clamped within the test dish. The test dish should be kept in horizontal level and add water to 2/3 of the whole test dish. Do not splash water drop on the screws. The test dish and the O-ring should be dry to avoid large deviation of test results. Pay attention to the direction of the permeation for the specimen when mounting test specimen.
3. Place the test dishes into the holes of the tray and then close the door.
4. Set up test parameters for corresponding test dish, tick and check the corresponding test dish and start the test.
5. View the test results and reports. If necessary, repeat the above procedures to perform another round of tests.

Test Results

The average water vapor rate of the coatings tested in this paper is 137.8 g/(m2 24h) from six samples.

Conclusion

Water vapor transmission rate is an important performance index of exterior building wall coatings. We used an intelligent way to determine the water vapor transmission of the samples. The weighing and permeation process of the test are automatically carried out by the instrument in the same test chamber. The instrument automatically controls the permeation balance and judge the end of the test based on it. It does not need the frequent intervention by the test operator, avoids human interference, improves the efficiency and reduce labor intensity. The test temperature, humidity and air velocity in the test chamber can be adjusted and controlled automatically, the accuracy and stability of the test is high.

Labthink Instruments Co., Ltd, is dedicated to providing customers with packaging testing solutions and helping the manufacturers to reach their goals of quality and safety assurance through improvement of packaging testing techniques and development of high-end packaging testing instruments. The newly introduced C series barrier testing systems adopting self-sensing technology revolutionarily improved the testing accuracy, efficiency and automation, realized the paradigm shift of packaging barrier testing instruments. For more information please visit en.labthink.com