Introduction of Water Vapour Permeability & Transport Testing Methods and Analysis of its Influencing Factors

This paper focuses on the various methods to test moisture permeability and the analysis of the influencing factors of test results. Currently, outdoor sports are relatively active, and the outdoor clothing needed will be paid more attention. A good outdoor sportswear should have its own unique characteristics. Water vapor permeability is an important basic one.

Definition of WVT – Water Vapor Transport

WVT is the water vapor quality passing through the unit area of the sample vertically within the specified time at the speed of g/m²·h or g/m²·24h under the condition that the sample is keeping the specified temperature and humidity on both sides.

Definition of WVP – Water Vapor Permeability

The vapor quality passing through the unit area sample vertically in the prescribed time at the speed of g/m²·Pa·h under the condition that the sample is keeping the specified temperature and humidity on both sides, and under the unit vapor pressure difference.

The common water vapour permeability tester

Four methods to test the water vapour permeability

Method 1: method of moisture absorption (desiccant)

First, expose the desiccant (anhydrous calcium chloride) particles (0.63~2.5 mm) to an oven at 160℃ for 3 hours to make them completely dry. The, place the dried desiccant particles in the test cup but careful that keep them 4mm lower than 4 mm of the sample, and place the sample cup in the test instrument. After about bout 1 hour of humidity testing and controlling, begin to weigh; weigh again after a period of time for testing. The moisture permeability of the sample is obtained by applying the weight difference between the two weighings to the formula.

Common standards:
GB/T 12704.1
ASTM E96 METHOD A\C\E
JIS L 1099 A-1

Method 2: evaporation method (method of a positive cup)

First, inject water at the same temperature conditioned by the test condition into the measuring cylinder, and limit the amount of water according to the requirements of each standard. Then, place the test sample on the test cup, and place the cup straight into the test instrument. After a period of time for balancing, begin to weigh, and after testing for a while, weigh again. The moisture permeability of the sample is obtained by applying the weight difference between two weighings to the formula.

Common standards:
GB/T 12704.2 METHOD A
ASTM E96 METHOD B\E
JIS L 1099 A-2
BS 7209

Method 3: evaporation method (method of an inverted cup)

First, inject water at the same temperature conditioned by the test condition into the measuring cylinder, and limit the amount of water according to the requirements of each standard. Then, place the test sample on the test cup, and place the cup in reverse into the test instrument. After a period of time for balancing, begin to weigh, and after testing for a while, weigh again. The moisture permeability of the sample is obtained by applying the weight difference between two weighings to the formula.

Common standards:
GB/T 12704.2 METHOD B
ASTM E96 METHOD BW

Mode 4: Potassium acetate method

First, inject a solution of saturated potassium acetate the test cup at the level of about 2/3 of the cup. Then seal the sample in the test cup and reversely place it in the test tank. Finally, weigh respectively the total mass of the test cup before testing and after 15 minutes.
The above method is described according to JIS L1099 standard.

Common standards:
JIS L1099 METHOD B-1
JIS L1099 METHOD B-2
ISO 14956

Analysis of differences between moisture permeability test results

It is not strange that the same batch of fabrics or fabric testing will have a great difference with ready-made clothing testing when they are sent for moisture permeability test. For producers, these differences are impossible; but in my years of experience, it is impossible to conduct a test without differences; the key is whether these differences are within a reasonable range. Here are some factors that I think affect the results of the moisture permeability test.

1 Equipment: The main parameters of permeability test are temperature, humidity and wind speed.

A. Wind Speed–Difference of Wind Speed on the Surface of Sample Cup
The number of sample cups varies greatly with the moisture permeability testers provided by various manufacturers on the market. Sample cup manufacturers even have eight, so how to ensure the uniformity of wind speed on the surface of these eight sample cups has become a difficulty in equipment design. Wind speed is the main parameter that directly affects the test results. Like the three test specimens of a sample, if wind speeds on the surface of the cups don’t agree with each other, it will directly cause the failure of the parallel test results to be parallel. Any devices with too much parallelism, I believe, cannot be used in the test.

B. Wind Speed–Differences in Wind Speed Settings among Equipment
Different brands of equipment have different designs for wind speed. Some use horizontal impeller parallel air supply design; some use fan air supply and ensure air circulation in the test box through the design of the box, which I call fan-supported air supply for the time being. Personally, I think that in the horizontal air supply system, the surface wind speeds of each small sample can reach the same level in the event of the horizontal air supply on the same level. However, in the fan-supported air supply system, no matter how the samples are placed, the surface wind speed of each sample cannot reach the same. Consequently, the results of parallel test will be worse.

C.Wind Speed–Use and Calibration of Anemometer
Some equipment manufacturers will give an anemometer to measure the wind speed along with the equipment, which can be used to measure or calibrate whether the wind speed in the equipment meets the standard requirements. There are also many equipment manufacturers that set the wind speed directly without the need for the experimenter to adjust the wind speed. Because the measuring unit of anemometer belongs to the meteorological department, it is seldom seen that the laboratory calibrates the wind speed by itself. However, the set wind speed of the equipment may change after a period of use or maintenance. In this way, the actual wind speed in the equipment will not be known, and thus the experimenter is unable to know whether the wind speed of the equipment meets the standard requirements, and the degree of deviation from the standard. That explains why the results of some laboratory tests are quite different.

2 The effect of desiccant

A. The deliquescence of calcium chloride
Calcium chloride is a powerful desicating agent, but it will form a protective layer of calcium chloride hexahydrate on the surface after moisture absorption and deliquescence, which prevents the hygroscopicity of internal calcium chloride. Therefore, the size of calcium chloride particles will affect its moisture absorption effect. The national standard stipulates that the size of calcium chloride particles is 0.63~2.5mm and that after environmental balance, calcium chloride needs to be shaken up and down, in order to prevent the effect of the deliquescence of calcium chloride on the test results.

B. Calcium chloride dosage
The amount of calcium chloride determines the size of the static air layer between calcium chloride and the sample. When calcium chloride works, the first thing is to absorb the interstitial water between calcium chloride and the sample for 100% dry, so as to form a pressure difference between water vapor and the outside of the sample. The influence of air layer between on water vapor transfer is also very great.

C. Static Air Layer
In the positive cup test, the evaporation of tested water first passes through the static air layer, while the static air layer will have a certain moisture resistance, which has a greater impact on the moisture permeability test results. Therefore, Appendix B of GB/T 12704.2 specifies the method of eliminating the influence of the static air layer and correcting the test results. So in the process of testing, we need to use the test cup in accordance with the standard regulations and pour into it the tested water in the amount that is prescribed by the standard to keep the static air layer in the cup consistent. This will ensure the parallelism of the parallel test results.

3 Sealability of sample and test cup

In the method of inverted cup test, the density of sample is an important part. Sealing is not a problem for most samples, but it may be difficult for a few composite fabrics. For samples with poor sealability, the fabrics will be wetted and diffused during the testing process and at the edge of the test cup, the water starts dripping to wet the surface fabrics. After this phenomenon occurs, the wind speed will take away the moisture of the wetting part of the surface material. The moisture permeability is not generated by the moisture permeable coating or film. Such test results are often much larger than their true moisture permeability, each test results varying greatly, and so it is impossible to find the true value of the test sample. This also results in the loss of testing purposes for producers.

4 Effectiveness of Test Samples

The samples tested for moisture permeability should be representative, and without creases, holes, or any obvious thickness unevenness of coated fabrics. These are the main factors that will inadvertently affect the test results, especially the uneven thickness of coated fabrics and the air bubbles generated by the coating, which can not be observed with the naked eye in many cases and is most easily ignored by the experimenters.