Waterproof fabric is coated with substances such as polyurethane, silicone, and wax. The term “waterproof” applies only to the fabric, not the entire garment. Breathability and durability are important factors in waterproof fabric. The garment must be rated for the amount of liquid it can withstand before the waterproofing fails. Once damaged, a fabric’s waterproof membrane cannot be repaired.
Waterproof fabric is a natural or synthetic fabric that has been coated with a substance to repel water. Common waterproofing substances include polyurethane, polyvinyl chloride, silicone, and wax. The term “waterproof” applies only to the fabric, not the entire garment. Even if a fabric is completely waterproof, seams must be sealed or taped and zippers must have storm flaps, or the garment is simply waterproof.
In most cases, waterproof fabric is subjected to laboratory testing and must meet industry standards. A waterproof garment will often be labeled with a rating indicating the amount of liquid the garment can be subjected to in 24 hours before the wearer becomes wet. For example, a “20K” rated rain jacket can withstand 20,000 millimeters of rain before the waterproofing fails. A 20K rating is considered excellent and is generally expensive. 5K and 10K classifications are more common, especially for inexpensive, non-specialized rain teams.
Woven fabrics with a waterproof coating have millions of tiny pores, about 1/20 the size of a raindrop but much larger than a water vapor molecule. These pores allow body heat and moisture to escape while keeping water liquid like rain. This breathability can also be classified, in grams of water vapor per square meter per 24 hours. For example, a breathability rating of 25K means that a garment can allow 25,000 grams of water vapor to pass through one square meter of the fabric in a 24-hour period.
Polyvinyl chloride and polyurethane can be made into clothing themselves, without a fabric backing. Such garments are waterproof, but do not allow the skin to breathe. This is often remedied by incorporating underarm vents or zippers into the garment design.
Silicone and Teflon are considered very effective coatings for waterproof fabrics. They form an effective moisture barrier without changing the look or feel of the original fabric, and also preserve the moisture wicking capabilities of the material. Rubber and wax are rarely used to coat waterproof fabrics because they change the appearance of the material, and exposure to the elements can cause them to crack and melt.
Once damaged, a fabric’s waterproof membrane cannot be repaired. Higher quality waterproof garments also have a “durable water repellent” (DWR) finish which tends to wear off long before the membrane breaks down, but there are sprays available that will restore the finish and allow water to seep through instead of soak. Crafters have also used sprays to waterproof material around the home with varying degrees of success.
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