Common Test Methods and Comparison of Water Vapour Permeability

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.