FUNCTION / Waterproof Breathable
Waterproof Breathable Textiles

Water-Vapor Permeable & Liquid-Water Impermeable Textiles allow vapor to pass through but not liquid.  Water-vapor permeability means the vapor emitted from the surface of an exercising body can transfer to outside the textile, so the temperature decreases and the wearer maintains ultimate comfortableness and dryness. Liquid-water Impermeability means the textile resists water penetration. Most commonly, the surface of this type of textile is coated with PU, PTFE, TPU, TPE or other materials. The same effect can also be achieved by laminating fabric with a water-impermeable films.  The membranes in textiles are designed to protect its wearer against climate agents such as wind and water ( rain or snow) . It depends on the benefits that are sought and the final application of fabric , or other textile membrane is used. Broadly speaking , there are two types of membranes : microporous and hydrophilic.

Hydrophilic Membrane Waterproof Breathable Fabrics

A hydrophilic membrane means that there is no hole in the surface of the waterproof membrane. The hydrophilic membrane utilizes a hydrophilic functional group on the molecular chain to achieve a moisture permeability effect. The moisture-permeable process of the hydrophilic material is that the inner layer absorbs moisture, the middle layer diffuses, and the outer layer discharges moisture.  The finishing method can complete the hydrophilic membrane fabric by direct coating or lamination membrane bonding. Its moisture permeability is lower than that of the microporous hydrophobic membrane fabric, but its waterproof and the washing durability are usually higher than the microporous hydrophobic membrane fabric. This kind of fabric is without air permeability because there are no holes in the surface of the fabric.

Microporous Membrane Waterproof Breathable Fabrics

The surface of the microporous membrane has billions of micropores per square inch. These holes are around 20,000 times smaller than water droplets and about 700 times larger than water vapor molecules. thus the sweat vapor can easy pass through the fabric. Because these holes are smaller than water droplets and can also prevent external water from entering. Therefore, the microporous waterproof and moisture permeable membrane has water resistance and moisture permeability.

The Difference of Test Methods for Water Vapor Permeability

Upright Cup Test (A method)
Also known as JIS L 1099, JIS Z 0208, ISO 2528, Desiccant Method of ASTM E96, JIS K 6328 (JIS is short for Japanese Industry Standards). The A-1 method uses Calcium Chloride solution to simulate sweat while A-2 method uses just water. A desiccant, calcium chloride, is put into a cup. A piece of fabric is then secured over the cup and placed in a controlled environment. Then after period of time the cup is weighed to see how much water has been absorbed into the cup through the fabric. The weight is then extrapolated to show the number of grams per 24 hours per square meter (g/m2‧24hrs) of sweat that will pass through the fabric.
Typical maximum with current technologies are in 15,000 g/m2‧24hrs. High-performance fabrics may get A1 test scores in 10,000 to 15,000 g/m2‧24hrs range and usually shows that a fabric has a fairly quick release of moisture.
           A-1 method                                      A-2 method

Inverted Cup Test (B method)
Also known as JIS L 1099 is similar to the ASTM E96-BW test method. A desiccant, Potassium Acetate, is put into a cup and sealed with a piece of ePTFE (Teflon/Gore-Tex film). The fabric to be tested is then placed over the cup with the fabric side to the cup.
The cup is then inverted into a pan of water. Then after period of time the cup is weighed to see how much water has been absorbed into the cup through the fabric. The weight is then extrapolated to show the number of grams per 24hours per square meter of fabric that will pass through the fabric.
The B-1 variant of test method puts membrane in direct contact with water while B-2 variant adds an ePTFE film between the water and the fabric.
Current upper range is 30,000 g/m2‧24hrs.

    ASTM E96-BW method

                                                             JIS L1099 B-1 method 

 Sweating Hot Plate Test (Ret test) 
Also known as ISO 11092 named Ret test. In this test, fabric is placed above a porous metal plate. The plate is heated and water is channeled into the metal plate, simulating perspiration. The plate is then kept at a constant temperature. As water vapor passes through the plate and the fabric, it causes Evaporative Heat Loss and therefore more energy is needed to keep the plate at a constant temperature. Ret is the measurement of the resistance to evaporative heat loss, its unit is Pa‧m2/W. The lower the Ret value is, the less resistance to moisture transfer and therefore higher breathability.
When the moisture resistance is measured, it is required to cover the porous electric test board breathable waterproof membrane, and the water entering the electric heating plate evaporates and passes through the membrane in the form of water vapor, so that no liquid water is in contact with the sample. After placing the sample on the membrane, it is determined that the heat flow required to keep the test plate constant at a certain water evaporation rate, and the moisture resistance of the test sample is calculated from the vapor pressure of the sample.

               Sweating Guarded Hot Plate

Comparison of Test Methods

Ret, A1, B1 and B2 test results do not correlate with each other. It all depends on the type of coating or lamination and how it moves moisture. Typically hydrophobic coatings perform better than hydrophilic laminations on the Ret and A1 Test. And vice versa, the hydrophilic laminations will perform better on the B1 test.