The Science of Waterproofing: How Natural and Technical Fabrics Repel Water

At its core, a waterproof fabric is a barrier that prevents the penetration of water. This simple concept, however, encompasses a complex and evolving field of material science. The pursuit of staying dry has led to the development of two distinct approaches: harnessing the inherent water-resistant properties of natural materials and engineering sophisticated technical fabrics. This article delves into the science behind how both natural and synthetic textiles achieve water repellency, from the ancient wisdom of lanolin-rich wool to the advanced membrane technologies of modern outerwear.

The Intrinsic Water Resistance of Natural Fibers

Long before the invention of synthetic materials, humans relied on nature to provide protection from the elements. Certain natural fibers possess remarkable inherent water-resistant qualities, which have been utilized for centuries in clothing and textiles.

Wool and Lanolin: Nature’s Waterproofing

Wool, the fleece of sheep, is a natural performance fiber with a complex structure that contributes to its water-repellent properties. Each wool fiber is a complex structure composed of a protein called keratin, and is covered in a thin layer of lanolin, a waxy substance secreted by the sebaceous glands of sheep. Lanolin is a natural water repellent, and it is this coating that gives wool its initial resistance to moisture. When water comes into contact with wool, the lanolin causes it to bead up and roll off the surface, preventing it from immediately soaking into the fabric.

Beyond the lanolin, the physical structure of wool fibers also plays a crucial role. The outer layer of each fiber is made up of overlapping scales, similar to the tiles on a roof. This scaly structure helps to shed water, further enhancing the fabric's water resistance. Furthermore, the crimp of the wool fibers creates a lofty fabric with a high volume of air pockets. This not only provides excellent insulation but also slows down the penetration of water, giving the wearer more time to seek shelter before the fabric becomes saturated.

The Tradition of Waxed Cotton

Waxed cotton is another traditional material that has been used for centuries to create water-repellent garments. The practice of waxing fabric dates back to the 15th century, when sailors would treat their canvas sails with fish oils and grease to make them more efficient and resistant to the harsh marine environment. This practice was later adapted for clothing, with paraffin wax eventually replacing the original oils to create a more pleasant and effective waterproofing treatment.

The process of creating waxed cotton involves impregnating cotton fabric with a specialized wax formula. This can be done by hand or by machine, and the result is a fabric that is highly resistant to water. The wax fills the spaces between the cotton fibers, creating a barrier that prevents water from passing through. The result is a durable, breathable, and highly water-resistant material that develops a unique patina over time, telling the story of its wearer's adventures.

The Evolution of Technical Waterproofing

While natural materials offer a degree of water resistance, the demand for higher-performing and more reliable waterproof garments has driven the development of advanced technical fabrics. These materials utilize sophisticated technologies to provide superior protection from the elements, often combining a water-repellent finish with a waterproof membrane.

DWR Treatments: The First Line of Defense

A Durable Water Repellent (DWR) finish is a chemical treatment applied to the exterior of a fabric to make it water-resistant. It works by increasing the fabric's surface tension, causing water to form beads and roll off the surface rather than being absorbed. This is the first line of defense against precipitation, and it is crucial for maintaining the breathability of a waterproof garment. If the outer fabric becomes saturated with water, the waterproof membrane will be unable to breathe effectively, leading to a build-up of condensation on the inside of the garment.

Traditionally, DWR treatments have been based on fluorocarbons, also known as per- and polyfluoroalkyl substances (PFAS). These chemicals are highly effective at repelling water and oil, but they are also persistent in the environment and have been linked to a range of health concerns. As a result, there has been a significant shift towards the development and use of PFAS-free DWR treatments, which are typically based on silicone, wax, or plant-based polymers.

Waterproof Membranes: The Core of Modern Rainwear

The heart of most modern waterproof garments is a waterproof, breathable membrane. This is a thin layer of material that is laminated to the inside of the outer fabric. The membrane is engineered to be waterproof, meaning it prevents water from passing through from the outside, while also being breathable, allowing water vapor (i.e., sweat) to escape from the inside. This is achieved through two main types of membrane technology:

• Microporous Membranes: These membranes, such as the well-known GORE-TEX, are made from expanded polytetrafluoroethylene (ePTFE). The ePTFE is stretched to create a microporous structure with billions of tiny pores per square inch. These pores are approximately 20,000 times smaller than a water droplet, making the membrane completely waterproof. However, the pores are also about 700 times larger than a water vapor molecule, which allows sweat to pass through and escape, keeping the wearer dry and comfortable.

• Hydrophilic Membranes: These membranes, such as eVent, are made from a solid, non-porous layer of polyurethane (PU). Instead of relying on pores, these membranes move moisture through a process of absorption and diffusion. Water vapor from the inside of the garment is absorbed by the hydrophilic PU material and then transported to the outside, where it can evaporate. This process is driven by the temperature and humidity gradient between the inside and outside of the garment.

Waterproof garments are typically constructed in one of three ways: 2-layer, 2.5-layer, or 3-layer. In a 2-layer construction, the membrane is laminated to the outer fabric, and a separate, loose-hanging liner is added to the inside to protect the membrane. In a 3-layer construction, the membrane is sandwiched between the outer fabric and an inner lining fabric, creating a single, durable material. A 2.5-layer construction is a lighter-weight option where the membrane is laminated to the outer fabric and a thin, protective print or coating is applied to the inside.

The Balance Between Waterproofing and Breathability

The ideal waterproof garment would be completely impervious to water while also being highly breathable, allowing all sweat and moisture to escape from the inside. In reality, however, there is an inherent trade-off between these two properties. The more waterproof a fabric is, the less breathable it tends to be, and vice-versa. This is because the same properties that make a fabric effective at blocking water also tend to inhibit the passage of air and moisture.

The Inherent Trade-off

The challenge for fabric manufacturers is to find the right balance between waterproofness and breathability to create a garment that is both protective and comfortable. For high-intensity activities where the wearer is likely to be sweating a lot, breathability is paramount. A garment that is not breathable enough will trap moisture on the inside, leaving the wearer feeling clammy and cold, even if no external water has penetrated the fabric. On the other hand, for low-intensity activities in heavy rain, waterproofness is the primary concern.

To help consumers make informed decisions, the performance of waterproof fabrics is often rated using two key metrics: Hydrostatic Head (HH) and Moisture Vapor Transmission Rate (MVTR).

Measuring Performance: Hydrostatic Head and MVTR

Hydrostatic Head (HH) is a measure of how waterproof a fabric is. It is determined by applying a column of water to the surface of the fabric and measuring how high the column can get before water starts to seep through. The higher the HH rating, the more waterproof the fabric. A fabric is generally considered to be waterproof if it has an HH rating of 1,500mm or more.

Moisture Vapor Transmission Rate (MVTR) is a measure of how breathable a fabric is. It is determined by measuring how much water vapor can pass through a square meter of the fabric over a 24-hour period. The higher the MVTR rating, the more breathable the fabric. A good level of breathability for outdoor activities is generally considered to be 10,000g/m²/24hr or more.

Environmental Considerations and the Future of Waterproofing

The pursuit of high-performance waterproofing has come at a cost to the environment. Many of the chemicals used in traditional DWR treatments are persistent and have been linked to a range of health and environmental problems. This has led to a growing demand for more sustainable waterproofing solutions.

The Problem with PFAS

Per- and polyfluoroalkyl substances (PFAS) are a large group of synthetic chemicals that have been widely used in DWR treatments for decades. They are highly effective at repelling water and oil, but they are also incredibly persistent in the environment, earning them the nickname "forever chemicals." PFAS have been found in water, soil, and wildlife all over the world, and they have been linked to a range of health problems in humans, including cancer, thyroid disease, and developmental issues.

In recent years, there has been a growing awareness of the dangers of PFAS, and many countries and regions have introduced regulations to restrict their use. This has forced the textile industry to look for safer, more sustainable alternatives.

The Shift to Sustainable Alternatives

The good news is that there are now a number of PFAS-free DWR treatments available. These treatments are typically based on silicone, wax, or plant-based polymers, and they offer a good level of water repellency without the harmful environmental side effects of PFAS. While some of these alternatives may not be as durable or oil-repellent as their fluorocarbon-based counterparts, they are constantly being improved and refined.

The shift to sustainable waterproofing is not just about DWR treatments. There is also a growing interest in the use of natural and recycled materials in waterproof garments. By choosing products made from sustainable materials and treated with PFAS-free DWR, consumers can help to reduce the environmental impact of their outdoor gear.

Frequently Asked Questions

What is the difference between "waterproof" and "water-resistant"?

A water-resistant fabric will resist the penetration of water to some degree, but it will eventually become saturated if exposed to enough moisture. A waterproof fabric, on the other hand, is impervious to water, even under pressure.

How do I care for my waterproof garments to maintain their performance?

It is important to keep your waterproof garments clean, as dirt and oil can compromise the performance of the DWR treatment and the waterproof membrane. Most waterproof garments can be machine-washed, but it is important to follow the care instructions on the label. It is also a good idea to periodically re-apply a DWR treatment to maintain the garment's water repellency.

Are natural or synthetic waterproof fabrics better?

Both natural and synthetic waterproof fabrics have their own advantages and disadvantages. Natural fabrics like wool and waxed cotton are often more breathable and have a more traditional aesthetic, while synthetic fabrics with waterproof membranes offer a higher level of waterproofness and are often lighter in weight.

Why does my waterproof jacket feel clammy inside?

If your waterproof jacket feels clammy on the inside, it is likely due to a build-up of condensation. This can happen if the DWR treatment has worn off and the outer fabric has become saturated with water, or if you are working hard and your body is producing more moisture than the breathable membrane can handle.

Published by SELVANE Knowledge — Material intelligence for considered wardrobes.

References

[1] How Rain Gear Works | REI Expert Advice [2] The History of Waxed Cotton - Mariners, Steve McQueen ... [3] PFAS in textiles in Europe’s circular economy - EEA No Render

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