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Common Test Methods and Comparison of Water Vapour Permeability
Table of Contents
- What is water vapour permeability?
- Characterisation indicators of water vapour permeability
- Common domestic and international test methods for water vapour permeability
- Comparison of Water Vapour Permeability Test Methods by Country
- Main influencing factors of test results
What is water vapour permeability?
The water vapour permeability test measures how well fabrics let moisture through. It does this at different temperatures and humidities. It simulates how well fabrics move water vapor. Bodies release it when they sweat. They release it to the outside. Moisture permeability is the quality of water vapor transfer on both sides of the fabric. It measures the flow of water vapor. The moving human body and the external environment emit water vapor.
Characterisation indicators of water vapour permeability
01 Water Vapour Transmission Rate (WVT)
The researchers hold the specimen at the specified temperature and humidity. We measure the mass of water vapor in grams per square meter per hour (g/(m²-h)) or grams per square meter per 24 hours (g/(m²-24 h)).
02 Water Vapour Permeability (WVP)
The specimen must meet the conditions on both sides. They are for the specified temperature and humidity. The conditions specify the mass of water vapor that passes through a unit area of the specimen in a given time. This happens under a certain difference in water vapor pressure. The unit of measure for this mass is grams per square meter Pascal hour (g/(m²-Pa-h)).
03 water permeability coefficient
The specimen is on both sides. It is to keep the temperature and humidity the same. There are water vapor pressure, time, area, and mass. They are in grams per square centimeter per second Pascal (g-cm/(cm²-s-Pa)).
Common domestic and international test methods for water vapour permeability
01 Moisture absorption (desiccant) method
The desiccant is anhydrous calcium chloride. The particles are (0.63-2.5) mm. Place them in an oven at 160°C for 3 hours to ensure the desiccant stays 100% dry. Then, I placed 35g of the cooled desiccant in a test cup. It shook evenly to form a plane with the desiccant. Its surface was about 4mm below the specimen. The specimen was then placed on the test cup with the test side up. Someone placed a press with a gasket and tightened the nut securely. Then use vinyl tape to seal the specimen gasket and pressure ring. Do this from the side to form the specimen assembly. The water vapor permeability tester had a mix of positive cups. They spent 1 hour in it. After that, they were put in a desiccator for 30 minutes. The instrument kept them for a duration set by the standard or the test agreement. After that, they weighed them again. The formula subtracts the weight difference of the second weighing. This gives the moisture permeability of the sample. The main standards are GB/T 12704.1, ASTM E96 method A/C/E, JIS L 1099 A-1.
02 Evaporation (positive cup water method) method
A measuring cylinder injects water at the test’s temperature. The amount is set by each standard. Load the test sample onto the test cup. The water vapor permeability tester places the positive cup in the test apparatus. After equilibrating, weigh the object to obtain its initial weight. Then, testers test and weigh it again. The formula uses the mass difference from the second weighing. It uses the difference to find the sample’s moisture permeability. The main standards are: GB/T 12704.2 Method A, ASTM E96 Method B/E, JIS L1099 A-2, BS 7209.
03 Evaporation (inverted cup water method) method
To fill the measuring cylinder with water at the test’s temperature, use it. The amount of water is per each standard’s requirements. Load the test sample into the test cup. The cup is then inverted and placed in the test apparatus. After equilibrating, we weigh it to get the initial weight. Then we weigh the item again after water vapor transmission rate testing. After calculating, we obtain the moisture permeability of the sample. The main standards are: GB/T 12704.2 method B, ASTM E96 method BW.
04 Potassium acetate method
Fill the test cup with saturated potassium acetate solution. Fill it about 2/3 full. Then, seal the specimen in the cup at the product, upside-down in the sink. Weigh the total mass of the test cup before the test and the total mass of the test cup after 15 minutes. The main standards are: JIS L1099 Method B-1, JIS L1099 Method B-2, ISO 14956.
Comparison of Water Vapour Permeability Test Methods by Country
Europe, the US, and Japan use moisture permeability standards. They use the absorption and evaporation methods. But, the test conditions in these standards vary, so the results vary too. So, how to choose?
The product’s destination sets the standard. But, the US leads this industry and its market is large. Testing data is also common there. So, the ASTM standard is the most used. Most of our customers now use this method to do their water vapour permeability testing. Finally, the hygroscopic method or evaporation method is then chosen. The researchers base the choice on the sample’s characteristics.
Brief Comparison of Water Vapour Permeability Test Methods by Country
Method | Standard | Conditions | Temperature | Humidity | Wind speed | Test surface | Reagent volume | |
American standard | Drying method | ASTME96 (versions 95, 00, 05.10) | A | 23℃ | 50% | 0.02~03 | Drying method front facing CaCl₂ water cup method reverse facing H₂O | 6mm from sample |
Water cup method | B | 23℃ | 50% | 0.02~03 | 19±6mm | |||
Pouring cup method | BW | 23℃ | 50% | 0.02~03 | 19±6mm | |||
Drying method | C | 32.2℃ | 50% | 0.02~03 | 6mm from sample | |||
Cup method | D | 32.2℃ | 50% | 0.02~03 | 19±6mm | |||
Drying method | E | 37.8℃ | 50% | 0.02~03 | 6mm from sample | |||
Japanese standard | Calcium chloride method | JISL1099:2006 | A-1 | 40℃ | 90% | 0.8 | ||
Water method | A-2 | 40℃ | 50% | 0.8 | ||||
Potassium acetate method | B-1 | |||||||
Others | B-2 |
Main influencing factors of test results
In the water vapour permeability test, the same batch of fabrics and garments is often tested at different places. They test them at the same one at different times. The results show a large difference.
Now, there are many domestic and foreign methods for testing fabric moisture permeability. In daily life, sportswear brands specify waterproofness, but not its test method. The current common test methods’ results are not relevant or comparable. So, to evaluate clothing’s moisture permeability, the first thing is to pick a test method. After choosing the test method, also consider the factors mentioned here. They affect the test’s relevance.
01 Influence of desiccant
① Uniformity of particle size distribution of calcium chloride
The size of calcium chloride affects its moisture absorption. The national standard says its particle size is: (0.63 ~ 2.5) mm. After balancing, shake the calcium chloride up and down. This is to stop the calcium chloride from making the test results wrong.
Calcium chloride is very hygroscopic. The finer the particles, the more surface area they have and the faster they absorb water. But, their deliquescence forms a protective layer of calcium chloride hexahydrate. The finer the particles, the easier it is to form this layer. In the humidity adjustment stage, this layer stops the hygroscopicity. In the formal test, it stops the calcium chloride from absorbing water. This affects the test results.
Before the test, the bought desiccant must meet the particle spec. It must pass through the prescribed sieve. In filling, keep its particles as even as you can.
② Dosage of calcium chloride
The national standard on the calcium chloride dosage has clear rules. It’s about 35g. The sample and the desiccant should be 4mm apart. The desiccant’s surface should be flat. This can control well the air layer between the specimen and desiccant. The size of the air layer determines the total amount of moisture in the air layer. When the desiccant is working, it first dries the moisture in the air in the specimen. This dries it to 100% dryness and forms a pressure difference with the outside. The air layer’s height determines the path of water vapour. If the desiccant is not flat, the specimen gets damp in the water vapor transmission rate test. This causes sagging. The desiccant touches the fabric. Water vapour forms a direct channel from the fabric into the test. This has a big impact on the results.
Control the dosage of desiccant and its horizontality on the surface in the specimen cup. Do this strictly during the test.
02 The effect of adjustment time in the evaporation method
In the same environment, tests at 0.5h and 1h after the test time are 2h. The results will be due to different fabric styles. Therefore, we recommend a test conditioning time of 1 hour.
03 Influence of wind speed
① Difference in wind speed on the surface of different test cups with the same equipment
At present, many manufacturers make permeability testers. Each can hold a different number of specimen cups. Some water vapor permeability analyzers can fit eight at the same time. But, if the cups in the box can not move sideways at a certain speed, it is hard to keep the same wind speed on their surface. This directly affects the test results. The samples can not be parallel. Such test results are not acceptable. The instrument needs maintenance and calibration. This is to ensure the consistency of the results of parallel samples.
② Differences in air supply between different devices
Some testers on the market use a horizontal impeller with parallel air supply. Others use a fan for air supply. The air supply system is horizontal. This keeps the wind at the same level. It makes the wind speed the same over the small sample area. The fan air supply system is the issue. It’s hard to make the sample surface wind speed consistent.
04 Static air layer
In the cup method test, the test water first evaporates through the still air. The still air has some moisture resistance. This resistance has a big impact on the test results for moisture permeability. Therefore, GB/T 12704.2 Appendix B specifies how to remove the effect of the still air layer. It also corrects the test results. Then we need to use the test cup in testing. This is as the standard requires. We need to add the test water as the standard requires. To keep the air layer in the cup consistent, we must to keep the air layer still. This ensures that the water vapour permeability test results of parallel samples are consistent.
05 Sealing of sample and test cup
In testing the inverted cup water method, the sealing of the specimen is very important. Bad sealing will make water leak from the cup’s rim. It will wet the specimen’s surface and then evaporate. In a test, we should avoid this unacceptable water loss.
06 Validity of test samples
The sample should be typical. The specimen cannot have creases or holes. The coated fabric should not have uneven thickness or blistering. Correctly orient the test surface.obvious uneven thickness, blistering, etc.. The test surface of the specimen should be orientated correctly.
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