The definition of water vapor transmission rate (WVT)
Under the specified temperature and humidity conditions on both sides of the sample; the mass of water vapor vertically passing through the unit area of the sample within the specified time. In grams per square meter hour (g/m2.h) or grams per square meter 24 hours (g/m2.24h)
Definition of water vapor permeability (WVP)
Under the specified temperature and humidity conditions on both sides of the sample; under the unit water vapor pressure difference, the mass of water vapor vertically passing through the unit area of the sample within the specified time. Grams per square meter Pascal-hour (g/m2.Pa.h)
Several methods of moisture permeability testing
Method 1: Hygroscopic (desiccant) method
Place hygroscopic desiccant (anhydrous calcium chloride) particles (0.63~2.5mm) in an oven at 160°C for 3 hours to keep the desiccant 100% dry. Place the desiccant in the test cup so that it is about 4mm lower than the sample. Place the sample cup in the testing instrument; after about 1 hour of testing and humidity adjustment time, weigh it. After testing for a period of time, weigh it again. The weight difference between the two weighings is applied to the formula to obtain the moisture permeability of the sample.
The above methods are described according to national standards.
Method 2: Evaporation (cup of water method) method
Use a measuring cylinder to inject water at the same temperature as the test conditions; the amount of water used is based on the requirements of each standard. Put the test sample on the test cup; place the cup in the test instrument; balance it for a period of time and weigh it to get the initial weight. After testing for a period of time, weigh it again. The mass difference in quadratic seconds is applied to the formula to obtain the moisture permeability of the sample.
The above methods are described according to national standards.
Method 3: Evaporation (pour a cup of water method)
Use a measuring cylinder to inject water at the same temperature as the test conditions; the amount of water used is based on the requirements of each standard. Place the test sample on the test cup; invert the cup and place it in the test instrument; weigh it after balancing for a period of time to obtain the initial weight. After testing for a period of time, weigh again. The mass difference in quadratic seconds is applied to the formula to obtain the moisture permeability of the sample.
The above methods are described according to national standards.
Method 4: Potassium acetate method
Pour saturated potassium acetate solution into the test cup to about 2/3 of the height of the cup; then seal the sample on the test cup and place it upside down in the test water tank. Weigh the overall mass of the test cup before the test and the overall mass of the test cup after 15 minutes.
The above method is described according to the JISL1099 standard.
1. Equipment factors
The main parameters of the penetrability test are temperature, humidity, and wind speed.
A. Wind speed–difference in wind speed on the surface of the sample cup
The moisture permeability testers provided by various manufacturers on the market now have large differences in the number of sample cups. Some manufacturers can have as many as 8 sample cups. How to ensure that the wind speed on the surface of these 8 sample cups is uniform and consistent has become a difficulty in the design of the equipment. Wind speed is the main parameter that directly affects the test results. If the surface wind speed of three test samples of a sample cannot reach an agreement, it will directly affect the test results of parallel test samples and cannot be parallel. I personally think that equipment with too large parallel differences cannot perform this test.
B. Wind speed–the difference in wind speed settings between various devices
Different brands of equipment have different designs for wind speed. Some use a horizontal impeller parallel air supply design; some use a fan to provide air, and then design the box to circulate the air in the test box; let me call it fan air supply for the time being. Personally, I think that the horizontal air supply system should be on the same level as the horizontal air supply, so that the surface wind speed of each sample can be consistent. However, with the fan air supply system, the surface wind speed of each sample cannot be consistent depending on how the sample is placed, and the results of parallel samples tested in parallel will be worse.
C. Wind speed–the use and calibration of anemometer
Some equipment manufacturers will provide an anemometer for measuring wind speed along with the equipment; it is used to measure or calibrate whether the wind speed in the equipment meets the standard requirements. There are also many equipment manufacturers that directly set the wind speed, without the need for the experimenter to adjust the wind speed by himself. Since the measurement unit of the anemometer belongs to the meteorological department, it is rare to see laboratories calibrating wind speed by themselves. However, after the equipment has been used for a period of time or repaired, its set wind speed may change. In this way, the actual wind speed in the equipment will not be known, and the tester cannot know whether the wind speed of the equipment meets the standard requirements and what the deviation is from the standard. This is one of the reasons why some laboratory test results vary widely.
2. The influence of desiccant
A. Deliquescence of calcium chloride
Although calcium chloride is a powerful hygroscopic agent, after it absorbs moisture and deliquesces, it will form a protective layer of calcium chloride hexahydrate on the surface, preventing the hygroscopicity of the internal calcium chloride. Therefore, the size of calcium chloride particles will affect its hygroscopic effect. The national standard stipulates that the particle size of calcium chloride is: 0.63~2.5mm, and stipulates that after the environment is balanced, the calcium chloride should be shaken up and down to prevent chlorine calcium chlorideUnderstand the impact on test results.
B. Dosage of calcium chloride
The amount of calcium chloride used determines the size of the still air layer between it and the sample. When calcium chloride is working, the first thing you need to do is to absorb the moisture in the gap between it and the sample. Use it to achieve 100% dryness. This creates a pressure difference of water vapor in the outside world. The intervening air layer also has a huge impact on water vapor transfer.
C. Still air layer
In the straight cup test, the evaporation of the test water must first pass through the still air layer; the still air layer will have a certain moisture resistance, which will have a greater impact on the moisture permeability test results. Therefore, Appendix B of GB/T12704.2 stipulates a method to eliminate the influence of the still air layer and correct the test results. Then during the test process, we need to use a test cup that is consistent with the standard and put in the test water amount specified by the standard. To keep the still air layer inside the cup consistent. This ensures the parallelism of similar test results.
3. Sealing between sample and test cup
In testing the pouring cup of water method, the density of the sample is an indispensable and important content. For most samples, sealing is not a problem, but for a few composite fabrics there may be difficulty in sealing. Samples with poor sealing properties will exist and the fabric will be wetted and spread during the test. Water dripping formed on the edge of the test cup and wetted the surface fabric. After this phenomenon occurs, the wind speed will take away the moisture from the moistened part of the surface fabric. It is not the moisture permeability produced by the moisture passing through the moisture permeable coating or moisture permeable membrane. Such test results are often much larger than the actual moisture vapor permeability, and the test results vary greatly from time to time, making it impossible to find the true value of the test sample. This also results in the producer losing the purpose of testing.
4. Validity of test samples
The sample for testing moisture permeability must be representative, and the sample must not have creases, holes, or obvious thickness unevenness in the coated fabric. These are the main factors that can inadvertently affect the test results, especially the uneven thickness of the coated fabric and the bubbles generated during the coating process. These are often invisible to the naked eye and can easily be ignored by the tester.
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