WHAT EVERYBODY OUGHT TO KNOW ABOUT WATERPROOF AND WATER-REPELLENT FABRICS
Protection against environmental factors is the initial function of clothing. In a wet environment, the basic requirement for clothing is to keep the wearer dry. Therefore, the outer layer of your clothing needs to be waterproof or water-repellent. But what do these the two terms mean? Is there any difference between them?
In contact with water, water-repellent materials form beads on the outside that can be easily removed from the fabric surface. This means that water can’t easily penetrate the material; however, for longer contact with water or with a higher pressure difference, the material will absorb water. Water-repellent fabrics are often compact textile structures or common materials treated with hydrophobic chemicals. Waterproofing of a material is defined as the property of a material not to be penetrated by fluids, i.e. a waterproof material must be impervious to water.
There are two methods for measuring waterproofness of a fabric: one that simulates raining and another that subjects the fabric to hydrostatic pressure. The minimum value for the hydrostatic pressure without leaking at its surface, at which a fabric is considered rainproof is 5000 mm water column, while for waterproof materials the hydrostatic pressure can reach 10,000–15,000 mm water column. This means that if a water column was put over a small point, a piece of said fabric, it needs to reach 10-15 meters before water starts to leak through the fabric. The waterproof rating of materials designed for harsh conditions is usually between 15,000 and 30,000 mm water column. Such fabrics are completely waterproof even under very high pressure.
Fabric waterproof ratings
The main problem when using waterproof fabrics for garments is the comfort of the wearer. In the past, waterproof technologies relied mainly on covering and blocking the pores of the textile substratum. This way, the material acts as a barrier between the body and the humidity in the environment. However, the transfer of humidity must be analyzed in both directions from and towards the body.
Human skin sweats continuously, both at rest (insensible perspiration) and during activity (sensible perspiration). Therefore, the humidity produced through perspiration must be eliminated somehow. This problem is solved by the production of waterproof breathable textiles. They allow evaporating moisture to escape through the fabric. This reduces overheating and perspiration buildup and leads to more comfortable and drier skin.
First written attempts to obtain a water-repellent protective clothing are from the 15th century. At that time, sailors tried to impregnate their clothing with linseed oil, animal fat or wax. We can’t say if these attempts were successful or not, however, the first patented waterproof fabric was produced roughly 400 years later in England. Charles Macintosh patented it in 1823 and though this process would be improved over time, the product he got was far superior to any other similar product on the market.
By sandwiching a layer of liquid rubber (made with an oily liquid by-product of tar called naphtha) between two woven fabrics, Macintosh created a new material that would be resistant to water while also remaining flexible and wearable. Resistant to rainy weather and wet conditions, the new fabric was perfect for making coats. However, there were some problems with this fabric caused by the use of an unstable rubber. The problems were eliminated when a more stable textile material was produced thanks to the process of rubber vulcanization patented in November 1843 by Thomas Hancock in England and 8 weeks later by Charles Goodyear in the United States.
Despite the fact that rubber is a good material for waterproofing, there is a significant problem using rubberized textile fabrics for producing waterproof clothes. The downside of these garments is that they present a barrier to the evaporation of sweat and the very limited water vapor transfer leads to overheating of the wearer’s body. This way, sweat vapors condense in contact with the interior surface of the clothing and accumulate in the textile layers in direct contact with the skin. Thus, as far as the wearer's comfort is concerned, having a waterproof fabric is just half of the solution. The other half is called breathability or air permeability. Combining these two, the first waterproof breathable fabric was produced.
The first waterproof breathable fabric was produced in England during the WW2 (the mass production started in 1943). A densely woven breathable fabric, Ventile, is made from 100% cotton, utilizing quality long staple fibers. It was meant to replace flax in garments for military applications. So, how does this fabric function? When the fabric comes into contact with water, the cotton fibers swell, the size of the pores between the yarns decreases, and the interstices within the fabric close up, preventing the further passage of water. The water resistance properties of the material are enhanced further due to its impregnation with a DWR (durable water repellent). Ventile is now used for outdoor sportswear such as mountaineering, trekking, and nature watching because the fabric provides excellent protection against the wind, rain, snow, and cold. Moreover, Ventile is suitable for use in extreme and hostile environments. For example, it's used on Arctic, Antarctic, and Himalayan expeditions.
The first microporous membrane ePTFE (expanded polytetrafluoroethylene) was created by Wilbert L. Gore and his son Robert W. Gore. In 1969, thanks to a happy accident (instead of slowly stretching the heated PTFE rod, he applied a sudden yank and it stretched 800%), Robert Gore created the new material. The first Gore-Tex materials appeared on the market in 1976, starting a revolution in the concept of waterproof breathable clothing. This membrane has approximately 1.4 billion pores per square cm (9 billion pores per square inch), with each pore 20,000 times smaller than a drop of water. The waterproof, breathable, microporous membrane introduced by Gore has an important place in the performance sportswear market.
Since the mass production of waterproof clothes with Gore-Tex material started some 50 years ago, there have been many developments in the so-called waterproof breathable fabrics.
Water-repellent fabrics are obtained using specific finishing hydrophobic treatments. Thanks to the impregnation with these hydrophobic treatments, water repellency offers light rain resistance.
The most frequent use of waterproof and/or water-repellent fabrics is in the clothing industry. Due to the complex requirements of the users (protection, comfort, functionality, etc.), waterproof fabrics must have a sum of properties that ensure the multifunctional characteristics of the garment. The level of performance of the waterproof and water-repellent materials used for clothing is determined by two groups of factors:
- Subjective variables related to the requirements and the level of comfort of the final user.
- Objective variables related to the environmental conditions, risk factors and specifics of the activities carried out by the user.
Waterproofness is a requirement imposed mainly by the environment (especially by the weather conditions) and is related to the behavior of textile materials towards the water. Having said that, textile materials can be divided into:
- Materials that absorb and retain water. They’re called hydrophilic materials.
- Materials that repel water. They’re called hydrophobic materials.
Conventional waterproofing treatments work the following way: by covering the pores with a layer of polymer or a membrane, fluids cannot pass through textile materials.
Considering the structure of the waterproof materials and/or the technology used, they are either:
- Inherent waterproof materials; or
- Textile materials with waterproofing finishing treatments.
Waterproof materials are generally obtained using surface finishing treatments. Covering is a general term referring to the application of one or more layers of adherent polymeric products on one or both sides of a textile material. This way, a film of polymeric material is formed on the surface of the textile.
There are two technologies used:
- Coating technology, where the polymer is applied by direct layering and impregnation. The polymer is usually in the form of a paste or a high viscosity liquid. Such coatings are extremely thin - in the range of 10-100 μm.
- Laminating technology that involves the formation of a laminating layer (membrane or foam) on the surface/surfaces of the textile material. The membrane is very thin (e.g. around 10 μm for PTFE) so the final thickness of the film remains in the range of 10-100 μm.
Coated waterproof fabrics
During impregnation, a solution or a low- or high-viscosity dispersion polymer is deposited uniformly on the entire textile surface using different processes. A general characteristic of the impregnated materials is that the components cannot be clearly separated because the polymer is dispersed among the structural elements of the textile surface. The finishing technology can cover either one side of the material or both sides of the material (this is called total impregnation).
Laminated waterproof fabrics
Laminated waterproof materials are multicomponent products (two or more layers, one of which is the textile fabric) requiring bonding by the use of:
- a special adhesive added to the polymer (solutions in organic solvents, powders, granules, fibers); and
- the adhesive properties of one or more component layers (membranes, foams, expanded foils).
Natural and synthetic polymers are suitable for laminating textiles. Rubber is the only suitable natural polymer, while the range of synthetic polymers is much wider. An analysis of the consumption of synthetic polymers shows that 90% of the synthetic polymers used for laminating textile materials are polyurethanes.
The morphological structure of the coated and laminated materials and the nature of the polymers are important, as they are key factors in obtaining a perfectly sealed waterproof product. The morphological structure of these materials includes:
- The number of layers that make the coated or laminated waterproof material and their relative position in the garment.
- The absence or the presence of pores (compact or porous layer) and the absence or the presence of other added substances.
- The structure of the textile substratum; woven, knitted or nonwoven fabrics that can have different finishing treatments.
Considering their position in the garment, the coated or laminated waterproof materials can be:
- With the covering layer towards the exterior - mainly materials covered with elastomers and some materials laminated with compact foils. These materials are recommended for chemical protection and protection against particles.
- With the covering layer towards the interior - used for wet weather protection. This variant is used for laminated materials (e.g. Gore-Tex) with a membrane or film with low mechanical strength. For increased durability, the polymeric film is covered with a hydrophilic polyurethane (PU) layer and/or a thin textile fabric.
Gore’s expanded PTFE membrane is often regarded as the starting point of commercially available high performance waterproof breathable membranes. Initially an ePTFE membrane claiming 90% void volume was laminated to a support fabric, however, the pores became contaminated by sweat or detergents thus reducing the overall waterproofness. To overcome this drawback, a thin hydrophilic polyurethane coating was applied to the body side of the membrane to prevent contamination (know as 2nd generation Gore-Tex).
Here you see a comparison between the old and the new version of GORE-TEX Pro. On top is the face fabric in red color, in gray/white is the ePTFE membrane, and the grey strands just below the membrane depict the lining. In the new version, the white PU layer has been replaced by the thin gray ePTFE "sandwich".
Water-repellent fabrics, unlike waterproof fabrics, have open pores making them permeable to air and water vapor. However, at high hydrostatic pressure, liquid water can pass through these fabrics. In order to obtain a water-repellent fabric, a hydrophobic material is applied to the fiber surface. As a result of this procedure, the fabric remains porous allowing air and water vapor to pass through. A downside is that in extreme weather conditions (during a heavy rain, for example) the fabric leaks.
The advantage of hydrophobic textiles is the enhanced breathability, however, they offer less protection against water. Water-repellent fabrics are used mainly in the production of conventional clothing or as an exterior layer of waterproof clothing. The hydrophobicity can be either permanent (due to the application of durable water repellents, DWR) or temporary.
There are two groups of water-repellent textiles:
- Inherent water-repellent textile materials.
- Textile materials finished with water repellent.
Water repellency is specific to compact textile structures. Thus, inherent water-repellent materials are (a) high density woven fabrics, made of very fine yarns and filaments and (b) nonwoven materials.
The hydrophobization of the textile materials is achieved by using different chemicals. These substances orient their hydrophobic groups towards the textile fibers thus forming a protective brush against water. The water hydrophobization agent applied on the surface of the garment allows the water drops to maintain their shape without spreading onto the fibers. In general, the main limitations of the water-repellent treatments refer to the extended surface porosity.
Hydrophobization can be achieved through the use of several technologies and/or materials. The main types are the following:
- Hydrophobization with additives (aluminum organic salts, aluminum soaps, paraffin emulsions with aluminum salts).
- Hydrophobization with resin type reactive agents.
- Hydrophobization through chemical modification of the fibers.
- Textile finishing with nanoparticles. Oleophobization techniques give textile materials the property of repelling oils and thus creating a protection against dirt and smudges while increasing the hydrophobization effect. Fluorocarbon resins are often used as oleophobization agents.
- Plasma treatment of the textile materials, plasma polymerization or plasma depositing of organic-silicone polymers can give a hydrophobic character to materials that are typically not hydrophobic (like cotton, for example).
The traditional technology used for producing water-repellent fabrics requires a huge amount of water. This is related to two problems: 1) water pollution, and 2) high energy consumption. In contrast, plasma treatment does not require these large volumes of water or wet chemicals, or a large amount of energy required to dry the fabric.
Textile materials are valuable thanks to properties like strength, flexibility, light weight etc. Moreover, other properties adding more functionality such as hydrophobicity, oleophobicity, or antibacterial activity further increase their value. However, few textiles are inherently water repellent and none are oil repellent. Traditionally, water repellency of a textile is achieved by the application of functional chemicals, followed by careful drying. Some of these chemicals include metal salts, pyridinium-based finishes, silicone finishes and fluorocarbon. Among them, only fluorocarbon finishes can repel both oil and water.
The table below illustrates the main features of water-repellent, waterproof, and waterproof breathable fabrics.
The main advantage of water-repellent fabrics is their inherent breathability making them suitable for manufacturing outdoor clothing that ensures comfort to the wearer. Additionally, they are cheaper than waterproof and waterproof breathable fabrics.
Waterproof fabrics are more resistant to wetting than water-repellent fabrics and cheaper than waterproof breathable fabrics; however, these are their only advantages. Apparently, they have many more disadvantages in comparison to the other two materials and that’s why they have been replaced by waterproof breathable fabrics in the production of outdoor clothing.
Waterproof breathable materials are waterproof, windproof and relatively breathable. They ensure comfort even during activities in extreme weather conditions. Their biggest disadvantage is the higher price.
Features of waterproof, waterproof breathable, and water-repellent materials
In addition to protection from weather conditions like rain and wind, breathability is expected from sportswear. That’s why water-repellent or waterproof breathable fabrics are preferred.
Water-repellent fabrics are coated with a finish (such as DWR) that is resistant but not impervious to penetration by water; whereas waterproof breathable fabrics help regulate heat and release moisture, and are impervious to water. They prevent the penetration and absorption of liquid water from outside while at the same time allowing water vapor to be transmitted to the outside of the fabric. Additionally, waterproof, windproof, and breathable fabrics provide good protection in a cold environment and better thermal comfort.