What is moisture permeability?

The moisture permeability test of textile fabrics is mainly to test the moisture permeability of fabrics at different temperatures and humidity. It simulates the ability of textile fabrics to transfer the body's natural water vapor to the external environment after sweating under different activity states. Moisture permeability is the quality or efficiency of water vapor transmission on both sides of the fabric; the assessment is the flow of water vapor, which comes from the water vapor emitted by the human body during exercise, and is also affected by the water vapor in the external environment.

Characterization index of moisture permeability

1 Moisture Vapor Rate (WVT)

Under the condition of keeping the specified temperature and humidity on both sides of the sample, the mass of water vapor passing through the unit area of the sample vertically within the specified time, in grams per square meter hour (g/(m²·h)) or grams per square meter 24 hours (g /(m²·24h) is the unit.

2 Moisture permeability (WVP)

Under the condition of maintaining the specified temperature and humidity on both sides of the sample, under the unit water vapor pressure difference, the mass of water vapor passing through the unit area sample vertically within the specified time, in grams per square meter Pascal hour (g/(m²·Pa·h) ) as the unit.

3 Moisture permeability coefficient

Under the condition of maintaining the specified temperature and humidity on both sides of the sample, and under the unit water vapor pressure difference, the mass of water vapor vertically passing through the unit thickness and unit area of the sample per unit time is measured in grams centimeters per square centimeter second Pascal (g cm / (cm²·s·Pa)) as the unit.

Common test methods for moisture permeability at home and abroad

1 Hygroscopic (desiccant) method

Place the desiccant (anhydrous calcium chloride) particles (0.63-2.5) mm in a 160°C oven for 3 hours to keep the desiccant 100% dry. Then put about 35 g of the cooled desiccant in the test cup, and shake it evenly, so that the desiccant is flat and its surface is about 4 mm lower than the sample. Then place the test side of the sample on the test cup, place the washer press piece, and tighten the nut. Then seal the sample gasket and the pressure ring from the side with vinyl tape to form a sample assembly. Place the positive cup of the sample assembly in the test instrument; after 1 hour of test humidity adjustment time, take it out, put it in the desiccator to balance for half an hour, weigh it, and then put it into the instrument for a period of testing according to the standard or the agreed test time. After the time, weigh again. The weight difference between the two weighings is used in the formula to obtain 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.

2 Evaporation (positive cup water method) method

Use a measuring cylinder to inject water at the same temperature as the test conditions; the amount of water is based on the requirements of each standard. Put the test sample on the test cup; place the cup in the test instrument; weigh it after equilibrating for a period of time to obtain the initial weight, and test it for a period of time before weighing it again. The mass difference of the second weighing is used in the formula to obtain the moisture permeability of the sample. The main standards are: GB/T 12704.2 Method A, ASTM E96 Method B/E, JIS L1099 A-2, BS 7209.

3 Evaporation (pour glass method) method

Use a measuring cylinder to inject water at the same temperature as the test conditions; the amount of water is based on the requirements of each standard. Put the test sample on the test cup; pour the cup and place it in the test instrument; weigh it after equilibrating for a period of time to obtain the initial weight; after testing for a period of time, weigh again. After calculation, the moisture permeability of the sample is obtained. The main standards are: GB/T 12704.2 Method B, ASTM E96 Method BW.

4 Potassium acetate method

Inject saturated potassium acetate solution into the test cup, about 2/3 of the height of the cup; then seal the sample at the test cup and place it upside down in the test water tank. 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.

The main factors influencing the test results

When doing the moisture permeability test, it often occurs that the same batch of fabrics, fabrics and garments are tested in different inspection institutions or at different times in the same institution, and the results are very different.

At present, there are many methods for testing the moisture permeability of textile fabrics at home and abroad. In daily life, there are indicators of waterproof and moisture permeability on brand sportswear, but generally no test methods are specified. However, the results of the various test methods that are currently common are not relevant and comparable. Therefore, to evaluate the moisture permeability of clothing, the first thing to determine is the test method. After determining the test method, attention should be paid to the various influencing factors mentioned in this article to ensure the repeatability of the test results.

1 Influence of desiccant

①The uniformity of particle size distribution of calcium chloride

The specification of calcium chloride will directly affect its moisture absorption effect. The national standard stipulates that the particle size of calcium chloride is: (0.63 ~ 2.5) mm, and it is stipulated that after the environment is balanced, the calcium chloride should be shaken up and down. The purpose is to Prevents the effect of deliquescence of calcium chloride on test results.

Calcium chloride has a strong hygroscopic effect. The finer the particles, the larger the specific surface area and the faster the moisture absorption speed. However, after its moisture absorption and deliquescence, a protective layer of calcium chloride hexahydrate will be formed on the surface. It is easier to form a protective layer in the wet stage, and it is easier to prevent the hygroscopicity of the internal calcium chloride in the subsequent formal test stage, thus affecting the test results.

Before the test, the particle size of the purchased desiccant must be controlled with the specified screen, and the uniformity of the particle distribution should be maintained as much as possible during the filling process.

② Dosage of calcium chloride

The national standard clearly stipulates the dosage of calcium chloride, about 35g, and the distance between the surface of the sample and the desiccant should be about 4mm, and the surface of the desiccant should be flat. This can effectively control the consistency of the air layer between the sample and the 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 layer in the sample assembly to make it 100% dry, forming a pressure difference between the water vapor and the outside of the sample. . The height of the air layer determines the height and path of water vapor transmission. If the desiccant is not a flat surface, the sample will be damp during the test, and there will be a sagging phenomenon. The desiccant is in direct contact with the fabric, and the water vapor will form a Direct passage, entering the experimental assembly, has a greater impact on the test results.

During the test, the amount of desiccant and the level of its surface in the sample cup should be strictly controlled.

2 Influence of conditioning time in evaporation method

In the same environment, after the test is adjusted for 0.5h and 1h, and the test time is 2h, the test results will be different due to the style of different fabrics. Therefore, it is suggested that the test adjustment time should be 1h.

3 The effect of wind speed

①The difference of surface wind speed of different test cups with the same equipment

At present, there are many manufacturers of moisture permeability testers, and different instruments can hold different numbers of sample cups at the same time. Some instruments can accommodate eight at the same time. If the sample cup in the box cannot move horizontally at a certain speed, it is difficult to keep the wind speed on the surface of the sample cup consistent, which will directly affect the test results of parallel test samples and cannot be parallel. Such test results are not acceptable, and the instrument needs to be repaired and calibrated to ensure the consistency of the test results of the parallel samples.

② Differences in air supply methods between different devices

At present, some of the moisture permeability testers on the market use a horizontal impeller parallel air supply design; some use a fan to supply air. The horizontal air supply system makes the wind on the same horizontal plane, so that the surface wind speed of each sample can be consistent. For the fan air supply system, it is difficult to achieve the same wind speed on the surface of each sample.

4 still air layer

In the positive cup method 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 has a greater impact on the moisture permeability test results. Therefore, GB/T 12704.2 Appendix B specifies the method of eliminating the influence of the still air layer and correcting the test results. Then during the test, we need to use the test cup that is consistent with the standard and put in the test water volume specified by the standard. In order to keep the still air layer in the cup consistent, so as to ensure the consistency of the test results of parallel samples.

5 The tightness of the sample and the test cup

In testing the pour-over method, the tightness of the specimen is very important. Poor sealing, the moisture in the cup will leak from the edge, or wet the surface of the sample from the edge, resulting in evaporative loss. Such test results are not acceptable and should be avoided.

6 Validity of test samples

The sample should be representative, the sample should not have creases and holes, and the coated fabric should not have obvious uneven thickness, blisters, etc. The test surface of the specimen should be oriented correctly.

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