ALL ABOUT WATERPROOF BREATHABLE FABRICS
In less than ideal conditions, protection against wind and precipitation is required. Thus, in bad weather, the outerwear must be wind- and waterproof. A waterproof material is expected to fully protect from rain, snow, and wind and entirely prevent the penetration and absorption of liquid water. Usually, waterproof textiles act as a barrier between your body and the humidity in the environment. Indeed, waterproof fabrics may give you excellent protection from the elements, however, they are not able to transport the perspiration through the clothing to the outside causing a person to get wet from inside the clothing. One possible solution to this problem is given by the so-called waterproof breathable fabrics (WBFs). The technology for manufacturing waterproof breathable textiles has been constantly developing and improving since the introduction of Gore-Tex some 40 years ago. It is also quite diverse with plenty of new fabrics introduced through a steady blend of innovation and evolution. Garments made of waterproof breathable textiles are used by sports and outdoor enthusiasts and professionals in any conditions. A waterproof and breathable fabric incorporates two distinct functions of waterproofness and breathability. It should basically provide protection from the rain, wind and cold but also maintain comfort just below the fabric layer thanks to allowing internal water vapor to escape.
The classification is based on two criteria:
- The structure of the waterproof breathable layer.
- The technique used for developing WBF.
According to their structure, WBFs are basically: tightly woven made of long fiber cotton fabric, microporous (laminated or coated) with very small gaps throughout allowing water vapor to escape to the outside but now allowing water to permeate into the fabric from the outside, and finally the third kind is composed of nonporous structure and has been developed as hydrophilic films in which water vapor is absorbed on one side of the film and re-evaporates from the other side whilst preventing liquid water penetration. Any other types are just various combinations of these.
The main principle behind microporous membranes and coatings is the following: water drops cannot penetrate because the pores are smaller than a raindrop, while perspiration evaporates through because the pores are larger than vapor molecules. The diameter of a water vapor molecule is 40x10⁻⁹mm and the diameter of various drop types is much larger as you can see in Table 1.
Microporous membranes and coatings function well when the size of the pores on the outer surface of the fabric is no bigger than 0.0002-0.0003mm which is equivalent to 2-3μm. At the same time, the average pore size in microporous membranes is between 0.0001 and 0.005mm (1-50μm). Generally, water vapor transmission depends on the size and thickness of pores as the decrease in pore size and thickness increases the water vapor transmission.
Table 1: Diameters of various types of drops
Source: Waterproof and Water Repellent Textiles*, p.373
Major types of waterproof breathable fabrics
The woven fabric is dense with a minimum quantity of pores. The very first type of this structure is called Ventile, it’s made of quality long staple cotton fibers, and was introduced in England during the WW2. How does it work? The main principle is simple. When the fabric is dry, it’s highly breathable since the cotton fibers retain an open breathable structure. At this time, the spacing between warp and weft is about 10 μm. However, in contact with water, the cotton fibers absorb water and expand. As a result, the gaps shrink to about 3-4 μm thus making the fabric practically impermeable to water except for the cases under very high water pressure. For better results, the densely woven fabric is impregnated with a durable water repellent (DWR) to make sure that the fabric will not be exposed to further penetration.
Materials which have a solid polymer-based film or coating (with a high resistance to water penetration) are also used. Thin hydrophilic films are utilized because water vapor permeability rates through such kind of films depend on their thickness as thicker hydrophilic films are more permeable to liquids.
The transmission of moisture vapor is achieved thanks to molecular diffusion as the moisture is absorbed by the hydrophilic membrane or coating, diffused through the film, and released on the other side. The process is illustrated in the figure below. Sympatex is among the representatives of this type of WBF.
The mechanism of moisture transmission at hydrophilic structures
Source: Waterproof and Water Repellent Textiles, p.55
Membranes are thin films made from polymers. There are basically two types of membranes - microporous membranes (they’re hydrophobic) and hydrophilic membranes.
The most widely used polymers for microporous fabrics are expanded PTFE (ePTFE) and polyurethanes (PU). Microporous films have about 1.2-1.4 billion tiny holes per square centimeter. These pores are much smaller than the smallest raindrops and they usually range between 0.1 and 10μm (1mm=1000μm). At the same time, they’re larger than water vapor molecules so that they can pass through the pores of the membrane. The process is illustrated in the figure below.
The first commercial microporous layer in textiles was introduced by W.L. Gore in the 70s and consisted of a microporous structure that was about 70% air. This membrane has around 1.4 billion pores per square centimeter, with each pore 20,000 times smaller than a drop of water, but 700 times larger a vapor molecule. The membrane is laminated to a conventional fabric to provide enough mechanical strength (the film itself is mechanically weak).
Working principles of microporous structure in water vapor transfer
Source: Waterproof and Water Repellent Textiles, p.49
Nowadays, Japan is a pioneer in the production of new microporous coatings and laminates. In the last 10 years, Japanese companies have developed a number of new microporous coatings and laminates. Among the main reasons for this are the vast home market and export capacity for synthetic-based wear. Europe is also a big player and a major market for membrane-based waterproof breathable textiles thanks to the high popularity of outdoor recreation and nature tourism.
Bi-component structures are a combination of microporous and hydrophilic membranes and coatings. Hydrophilic membranes are coated with a thin hydrophilic layer to increase waterproofness and to seal the surface pores, thus reducing the possibility of contamination of the microporous layer by dust particles, detergents, pesticides, etc. A hydrophilic finish (often chemically modified PU or polyester by incorporating poly(ethylene oxide) on a microporous structure is used to upgrade the water-resistance of microporous coatings. This ensures enhanced waterproofing capacity while allowing water vapor molecules to pass through at the same time.
Biomimetic and smart waterproof breathable textiles
Inspired by nature, biomimetic fabrics are designed to mimic the structure and function of naturally occurring materials and processes. The advent of many discoveries and inventions have stemmed from observing and imitating natural processes and mechanisms. The producers of biomimetic fabrics often draw inspiration from biology and try to find practical applications and/or develop products in the field of high-performance and functional textiles.
A breathable fabric with temperature-dependent response; adaptive insulation mimicking penguins in extreme cold conditions; water-repellent fabrics based on superhydrophobicity or a lotus leaf reaction towards water (it beads up and rolls off, cleaning the leaf’s surface in the process) are just a small part of all biomimetic projects related to high-performance and functional textiles.
Main properties of waterproof breathable fabrics
The main principle of how waterproof breathable textiles function (regardless of the fabric construction) depends on the relative humidity levels: if it’s more humid inside the jacket than outside, moisture will want to move outward to establish equilibrium and the opposite - if it’s more humid in the ambient environment than inside the outerwear, moisture will want to move inward. Therefore, the exterior face fabric of the jacket is critical for the passage of moisture meaning that the DWR applied must keep the face fabric drier than the inner side to allow the escape of interior moisture.
The most important features of waterproof breathable materials are two functions: waterproofness and breathability. The former refers to a material impervious to water, while the latter term refers to the ability of a fabric to allow water vapor to escape.
The most popular way to measure waterproofness of a fabric is by applying hydrostatic pressure measured in mm. Different manufacturers use different numbers to label a fabric “waterproof” so sometimes it can be really difficult to figure out if a material is truly waterproof or not. Now if the water column is at least 5000mm, a fabric is usually considered truly waterproof, although the minimum level can be as low as 1000mm according to some manufacturers.
Table 2: Fabric waterproof ratings
There are various methods for measuring the breathability of a WBF. One of them is through measuring moisture transmission - MVTR (moisture vapor transmission rate). The value represents the rate at which moisture passes through a square meter of the fabric in 24 hours. It is measured in g/m2 per day. Just as with the waterproofness ratings, most manufacturers apply various methodologies and tests to determine the breathability of a fabric so don’t just blindly accept what you’re told by the producers about the ratings of their products. Their main goal is to promote more sales so they have a serious reason to inflate the ratings of their products.
Table3: Fabric breathability ratings
WBFs are used mostly in the clothing industry, in particular in sportswear, workwear, and leisure wear. Functionality becomes a top priority, however, it must be combined with subjective and objective features such as style, quality, comfort, weight, and price. Depending on the particular outdoor activity, the importance of one or more of the aforementioned features can vary from the most important feature to a completely irrelevant one. Colors can also play a significant role since for forestry, fishing and hunting users often prefer camouflage patterns or muted colors, while professionals such as policemen, firefighters, construction workers rely on clothes in bright colors like yellow and orange. Waterproof breathable jackets, parkas, pants, hats, gloves, socks, and shoes are among the most popular products on the WBFs market. In addition, crew members of all kinds of vessels, including military and passenger ships use gear with WBFs. The primary function of the garment is in case of an emergency to delay the onset of hypothermia, thus extending the survival time and increasing the chance of escape or rescue.
WBFs are indispensable to cold-weather expeditions. Especially useful for Arctic, Antarctic or high altitude expeditions, they ensure more comfort and better protection from the elements. In cold conditions, you need a waterproof insulated jacket and pants to keep you warm and dry as well as waterproof boots. They are especially useful for trails and easy snow routes and together with a pair of gaiters can help keep feet drier in wet conditions. Be warned, though, that no membrane can keep your feet dry in prolonged wet conditions so don’t rely too much on your waterproof boots. Additionally, the membrane can easily get clogged up with dirt and sweat and on such occasions, it will stop providing adequate protection. A waterproof pack can also be very useful because it will keep your gear safe and dry. The military also uses waterproof breathable materials for cool-to-cold and extreme conditions because they provide excellent protection not only from precipitation but also from the wind. And, as you probably know, cold and windy weather leads to hypothermia much faster than cold and windless weather does.
Other applications include:
- Agriculture - groundcovers for weed control, packaging for product transport, and tree shelters.
- Outdoor gear - tents, umbrellas, hats, gloves, skiwear, sleeping bag covers, fasteners, and sealings.
- Medical - bed sheets, pillow protectors, stretchers, hygiene products, orthopedic orthoses, modern wound dressings, wheelchair cushions, surgical drapes.
- Construction - used in roofs because of their lower weight, resistance to water, sound insulation capacity, UV resistance, and controlled translucence.
Table 4: Typical usage areas of waterproof breathable fabrics
Source: Waterproof and Water Repellent Textiles, p.35
It is a billion dollar business ($1.43 billion in 2014 as membrane products account for 71%) and market research and management consulting companies like Grand View Research Inc. and Global Market Insights Inc. predict that it will expand further with between 5 and 6% on average in the next several years. There are numerous reasons for this as the main ones include: growing market for active sportswear and rising demand for waterproof breathable clothing and gear (nearly 80% of the WBFs market share belongs to the production of active sportswear), increased awareness of the need for sustainable and environmentally friendly products from the consumer, and increased demand for military and medical applications.
Currently Europe has the biggest market share (about 30% of the market), however, Asia-Pacific is expected to have the highest growth rate thanks to the rapidly expanding market for waterproof breathable clothing and footwear in China. The major players in the waterproof breathable textiles industry come from the US, Europe, and Asia as the industry has changed a lot during the last two decades. While the production of WBFs was concentrated almost entirely in the US, Europe, and Japan, nowadays the biggest players manufacture predominantly in Asian countries like China, Taiwan, South Korea, and Thailand. Especially in China, there is a boom in the technical clothing manufacture, which has led to a global decrease in fabric and finished garment prices due to the lower costs and rising competition.
Market leaders include companies like Toray Industries Inc. (Japan), Teijin Limited (Japan), W. L. Gore & Associates Inc. (USA), Finetex EnE (South Korea), Stotz & Co. AG (Switzerland), AkzoNobel N.V. (Netherlands).
Finally, you can see a projected value of the US WBFs market between 2014 and 2024. The figure below shows that the overall value of the market is expected to rise from $277.4 million to more than 460 million (66.7%) as ePTFE, PU, and polyester are the most used materials.
US Waterproof breathable market (in million U.S. dollars)
Functional outerwear must be waterproof for the maintenance of user comfort. Garments made of waterproof breathable fabrics shield from rain and wind but allow for water vapor to be transferred from the inner to the outer environment thus keeping you warm and dry this way.
Performance composite fabrics prevail as the microporous membranes led by Gore-Tex dominate the market of waterproof breathable textiles today. The microporous film provides a barrier between your body and water droplets to give you protection from precipitation.
There are so many applications of waterproof breathable materials other than their traditional use in active sportswear. They’re used in military, hospitals, construction, agriculture, transportation, and many other industries.
Keep in mind that waterproof breathable fabrics are not perfect and they won’t work in all conditions. For example, in cold and wet weather, moisture vapor turns quickly to liquid water. This moisture builds up in clothing and compromises the breathability and insulation capacity of your layered clothing system. WBFs have some limitations leading to customer dissatisfaction on certain occasions, however, they are an essential part of any amateur or professional outdoorsman’s gear.
* In J. Williams (Ed.), Waterproof and Water Repellent Textiles and Clothing, 2018, Elsevier Ltd.