Moisture Permeability Test Methods and Influencing Factors

Definition of Water Vapour Transmission (WVT)

The mass of water vapour that passes, vertically, through a unit area of a specimen in a set time at a specified temperature and humidity on both sides. In grams per square metre per hour (g/m2.h) or grams per square metre per 24 hours (g/m2.24h)

Definition of Water Vapour Permeability (WVP)

The mass of water vapour that passes through a unit area of a specimen, for a specified time, at a maintained temperature and humidity on both sides, per unit of water vapour pressure difference. In grams per square metre Pascal hour (g/m2.Pa.h)

What is moisture permeability performance?

The test checks a fabric’s ability to pass moisture at various temps and humidity levels. It simulates activities of a sweating human body. The test measures how well the fabric can transfer the body’s water vapor to the outside. Moisture permeability is how well water vapor passes through the fabric. It measures the flow of water vapour. The human body emits water vapour in motion. It is also affected by the external environment’s water vapour.

Characterisation indicators of moisture permeability

01 Moisture permeability (WVT) is the mass, in grams, of water vapor that passes through a unit area of the specimen in a set time at a fixed temperature and humidity. Researchers measure it in g/(m²-h) or g/(m²-24 h).

02 The testers evaluate the moisture permeability (WVP) of the specimen on both sides. This maintains the specified temperature and humidity. WVP is the mass of water vapor, in grams, that passes through a unit area of the specimen in a given time. The unit is grams per square metre of Pascal hours (g/(m²-Pa-h)).

03 The moisture permeability coefficient in the specimen on both sides. It must maintain the specified temperature and humidity. And it is the mass, in grams, of water vapour. It is the amount that, in cm per second, passes through a unit area of the specimen with a unit thickness. The driving force is a unit water vapour pressure difference. The unit is g-cm/(cm²-s-Pa).

Several ways of moisture permeability test

Method 1: Moisture absorption (desiccant) method

We place the desiccant (anhydrous calcium chloride) particles (0.63 to 2.5 mm) in an oven at 160°C for 3h to keep them 100% dry. Then, someone placed about 35g of the cooled desiccant in a test cup. They shook it to form a plane, with its surface about 4mm below the specimen. The specimen was then placed on the test cup, test side up. They placed a gasket press and tightened the nut. Then, use vinyl tape to seal the specimen gasket and pressure ring from the side. This will form the specimen assembly. The specimen is a combination of positive cups placed in the test instrument. After 1 hour of testing and humidity adjustment, remove the cups. Place them in a desiccator for half an hour to equilibrate and weigh. Then, according to the standard or the agreement, test the cups in the instrument for a specified time. Finally, weigh them again. A formula uses the weight difference from the two weighings. It gives the sample’s moisture permeability.

Commonly used standards

ASTM E96 Method A \C\E、JIS L 1099 A-1

Method 2: Evaporation (Positive Cup of Water) Method

Someone uses a measuring cylinder to fill it with water. It must be at the same temperature as the test conditions. The amount of water must meet each standard’s requirements. Load the test sample on the test cup. Place the positive cup in the test apparatus. After a time, weigh it to get the initial weight. Then, weigh it again after testing for another period. The formula uses the mass difference between the two second measurements. It finds the sample’s moisture permeability. The national standards describe the above method.

The experimenter fills a measuring cylinder with water at the test conditions. The amount of water must meet each standard’s requirements. Load the test sample onto a test cup. Place the cup in the test apparatus. After a period of balance, weigh the cup to get the initial weight. Then, weigh it again after a further test period. A formula uses the difference in mass from the two weighings. It finds the sample’s moisture permeability. The main standards are:

Commonly used standards

ASTM E96 Method B\E、JIS L1099 A-2、BS 7209

Method 3: Evaporation (pouring cup of water) method

Someone fills a measuring cylinder with water at the test conditions. The amount of water must meet each standard’s requirements. The technician loads the test sample on the test cup. The experimenter places the inverted cup in the test apparatus. After a time, someone weighs it to get the initial weight. Then, after testing for another period, the team weighs it again. The formula uses the mass difference of the two second measurements. It finds the sample’s moisture permeability.

Commonly used standards

ASTM E96 Method BW

Method 4: Potassium Acetate Method

Fill the test cup with a saturated potassium acetate solution to about 2/3 of its height. Seal the specimen in the cup and place it, upside down, in the test sink. We weigh the total mass of the test cup before the test and the total mass of the test cup after 15 minutes. The document describes the above method in accordance with JIS L1099.

Commonly used standards

JIS L1099 Method B-1、JIS L1099 Method B-2、ISO 14956

Comparison of Moisture Permeability Test Methods by Country

Common standards for moisture permeability are the European, American, and Japanese. They use the moisture absorption and evaporation methods, respectively. But, their test conditions are inconsistent. So, the results are also inconsistent. So, how to choose?

The product must meet the testing standards for its target area. The U.S. leads this industry and has a large market. So, the ASTM standard is the most used testing method. Most of our customers now use this method to do their testing. Finally, select the moisture absorption or evaporation method based on the sample’s traits.

Brief Comparison of Moisture Permeability Test Methods

Methods		Standard	Condition	Temperature	Humidity	Air velocity	Test surface	Reagent quantity
American standard	Drying method	ASTM E96 (versions 95, 00, 05, 10)	A	23℃	50%	0.02~0.3	Drying method with front side facing CaCl: Water cup method with reverse side facing H20	Distance from sample бmm
	Water Cup Method		B	23℃	50%	0.02~0.3		19±6mm
	Pouring Cup Method		BW	23℃	50%	0.02~0.3		19±6mm
	Drying method		C	32.2℃	50%	0.02~0.3		Distance from sample бmm
	Cup method		D	32.2℃	50%	0.02~0.3		19±6mm
	Drying method		E	37.8℃	50%	0.02~0.3		Distance from sample бmm
Japanese standard	Calcium chloride method	JISL 1099:2006	A-1	40℃	90%	0.8
	Water method		A-2	40℃	50%	0.8
	Potassium acetate method		B-1
	Others		B-2

Moisture Permeability Test Procedure

The specimen, medium, and moisture cup are in a controlled environment. Weighers weigh it periodically. The weight change shows the required moisture permeability rate and value. The above testing principles divide the test into four steps:

First: Prepare the assemblage. Use the standard to select the right amount of media to form a unit assembly.

Second: Pre-wet the assemblage and record M1. Equilibrate the assemblage by placing it under selected temperature and humidity. Then, after the equilibrium time, weigh M1.

Third: Test assemblage and record M2. After weighing, return the sample to the chosen temp and humidity. Then, weigh M2 after the testing time has elapsed.

Fourth: Record and report the average result of the three test samples.

Moisture Permeability Test Results

Moisture transmission rate (WVT) is a measure of moisture permeability. Researchers express it in g/(m2-h) or g/(m2-24h).

Requirements for moisture permeability test

1. The test sample should not be too thick, within 10mm, to reduce the edge’s moisture effect on the results.

2. National standards have moisture permeability cups of different sizes. This leads to a variation in the sampling amount. At least leave a sample of A4 paper size that can meet the test requirements.

3. Before testing, confirm the test surface, the test standard, and the test conditions: temperature, humidity, and the desiccant/water method.

Analysis of differences in moisture permeability test results

When testing moisture permeability, results often differ greatly. This is due to testing the same batch of fabrics and garments in different labs, or at different times in the same lab.

There are many tests for textile fabric moisture permeability. In daily life, testers evaluate brand sportswear for waterproofing and moisture permeability. But, they usually do not specify the test method. The results of various common test methods lack relevance and comparability. To test clothing’s moisture permeability, we must first choose a method. Choose the test method. Also, consider the factors in this article. They affect test result repeatability.

The delivery test for moisture permeability will encounter some issues. Tests on the same batch of fabrics or garments will yield big differences. The producer sees no difference as impossible. But, the test allows for some difference. The key is whether it’s within a reasonable range. Some factors affect the results of moisture permeability testing.

1.Equipment Factors

The main parameters of the permeability test are temperature, humidity and wind speed.

A.Wind speed — sample cup surface wind speed differences

Various manufacturers now sell moisture permeability testers. They differ greatly in the number of specimen cups. Some manufacturers’ sample cups can reach a wind speed of 8. The challenge is to ensure uniformity at that speed. So, designing the equipment has become difficult. Wind speed is the main factor affecting the test results. The three test samples have different surface wind speeds. This will affect the parallel test results. Parallel difference is too large equipment can not perform this test.

Many companies make moisture permeability testers. The number of specimen cups they can hold varies by instrument. Some instruments can hold eight at the same time. If the specimen cups in the box can’t move at a certain speed, it’s hard to maintain the same wind speed on their surfaces. This harms the test results of parallel test samples, which cannot be parallel. Such test results are not accepted. We must maintain and calibrate the instrument to ensure consistent results. This is for parallel samples.

B.Wind speed — the wind speed setting differences between the equipment

Different brands of equipment, the design of the wind speed are not the same. Some use a horizontal impeller parallel wind design. Some use a fan to send the wind. Then, the box design circulates the wind in the test box. I’ll call it fan surface wind it. I think the horizontal air supply system will be at the same level. So, the wind speed on the small samples will be the same. A fan air system can’t sample how to place. The wind speed on the surface of small samples is not the same. Parallel testing of parallel samples will yield worse results.

C.Wind speed — the use of anemometers and calibration

Some equipment manufacturers include an anemometer to measure wind speed. We use it to check if the wind speed meets the required standards. Many equipment manufacturers will set the wind speed directly. The tester won’t need to adjust it. The meteorological department owns the anemometer’s measurement unit. So, it’s rare to see the lab calibrate the wind speed. However, after someone uses the equipment for a period of time or repairs it, the set wind speed may change. Thus, the actual wind speed in the equipment will be unknown. The tester will not know if it meets the standard requirements or how much it deviates from them. This is also one of the reasons for the large difference in test results in some test laboratories.

2. Influence of desiccant

A. The deliquescence of calcium chloride

Calcium chloride is a strong hygroscopic agent. Its deliquescence forms a protective layer of calcium chloride hexahydrate on the surface. This layer prevents the hygroscopicity of the calcium chloride inside. Calcium chloride particle size affects moisture absorption. The national standard specifies a size of 0.63 to 2.5 mm. The environment needs, after shaking, that the average size meet this standard. This is to prevent deliquescence from affecting the test results.

B. Dosage of calcium chloride

The national standard specifies a calcium chloride dosage of about 35g. The sample and desiccant must be about 4mm apart. The desiccant’s surface must be flat. This can effectively control the consistency of air layer between specimen and desiccant. The size of the air layer determines the total amount of moisture in the air layer. When the desiccant works, it dries the moisture in the air layer in the specimen. This makes it 100% dry. It then creates a pressure difference of water vapour with the outside of the specimen. The height of the air layer determines the path of water vapour transmission. If the desiccant is not a plane, the specimen will sag during the test. The desiccant is in direct contact with the fabric. The water vapour will create a direct channel into the experiment. It will reach the assembly through the contact surface. This will greatly impact the test results. The test must control the desiccant’s dosage and its position in the cup.

The dosage of calcium chloride controls the size of the static air layer between it and the specimen. When calcium chloride works, it must first suck out the water from the air space between it and the specimen. It must reach 100% dryness and create a vapor pressure difference with the outside world. The air layer in between also has a huge impact on the water vapour transmission.

C.The uniformity of particle size distribution of calcium chloride

Calcium chloride specifications will affect its moisture absorption. The national standard for its particle size is 0.63 to 2.5 mm. The environmental balance limits the calcium chloride’s dust. The average of shaking it prevents deliquescence on test results.

Calcium chloride is very hygroscopic. The finer the particles, the larger their surface area. This increases the speed of hygroscopicity. But, it deliquesces. It forms a protective layer of calcium chloride hexahydrate on the surface. If the surface layer is very fine, it is easier to form a protective layer when adjusting the test’s humidity. It makes it harder for the internal calcium chloride to absorb moisture in the formal test stage. This affects the results.

Before the test, control the purchased desiccant with the prescribed sieve. It must meet the particle specification. Also, keep its particle distribution uniform when filling it.

3.Static air layer

In the positive cup method test, the test water evaporates. It first passes through the stationary air layer. This layer has a certain wet resistance. It greatly affects the results of the moisture permeability test. The standard specifies a method to remove the static air layer and correct the test results. Then we need to use test cups that meet the standard. We must add the amount of test water specified in it. In order to keep the static air layer in the cup consistent. So as to ensure the parallelism of the test results of this parallel sample.

4.The sealing of the sample and the test cup

In the test inverted cup water method, the density of the sample is not less important content. For most samples, sealing is not a problem. But, a few composite fabrics may have sealing issues. Poorly sealed samples will have fabrics that will wet and spread during the test. Dripping and wetting of the surface fabric can occur at the edge of the test cup. When this occurs, the wind speed carries away the water from the wetted portion of the surface fabric. It is not the amount of water that passes through the permeable coating or film that does so. Such tests often give results much higher than the true moisture permeability. The results vary so much between tests that it’s impossible to find the true value of the test samples. This also results in producers also lose its test purpose.

5.The validity of the test samples

The moisture permeability test samples must be representative. They must have no creases, holes, or obvious uneven thickness in the coated fabric. The main factors that will affect the test results are: the fabric’s uneven thickness and tiny bubbles from the coating. Testers often overlook these bubbles, which are likely to be invisible.

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