Durable water repellent
Durable water repellent, or DWR , is a coating added to fabrics at the factory to make them water-resistant (hydrophobic). Most factory-applied treatments are fluoropolymer based; these applications are quite thin and not always effective. Durable water repellents are commonly used in conjunction with waterproof breathable fabrics such as Gore-Tex to prevent the outer layer of fabric from becoming saturated with water. This saturation, called 'wetting out,' can reduce the garment's breathability (moisture transport through the breathable membrane) and let water through. As the DWR wears off over time, re-treatment is recommended when necessary. Many spray-on and wash-in products for treatment of non-waterproof garments and re-treatment of proofed garments losing their water-repellency are available.
Older methods for factory application of DWR treatments involve applying a solution of a chemical onto the surface of the fabric by spraying or dipping.
More recently the chemistry is applied in the vapor phase using chemical vapor deposition (CVD) machinery. The advantages of CVD include eliminating the use of hazardous and environmentally harmful solvents in the application process; requiring less chemical; and an extremely thin waterproof layer that has less effect on the natural look and feel of the fabric.
Later advances have eliminated perfluorinated acids, considered to be potentially hazardous to human health by the US Environmental Protection Agency and numerous research studies, from the application process.
Some researchers have suggested that the use of PFASs in water-repellent clothing is over-engineering, and comparable performance can be achieved using specific silicon- and hydrocarbon-based finishes.
Certain types of fabrics need to be re-treated to maintain water-repellency, as fluoropolymers decompose over time when exposed to water and chemicals. Washing the garment with harsh detergents usually accelerates DWR loss; in addition, soaps often leave a residue which attracts water and dirt. On the other hand, rain water or salt water affects DWRs less significantly. Affected garments can be treated with a 'spray-on' or 'wash-in' treatment to improve water-repellency. In some cases heat treatment can reactivate the factory applied repellent finish and aid in the repelling of water, and other liquids such as oils. On the other hand, some DWR products do not require heat treatment to be activated, and sometimes DWR treatments can be revitalized simply by washing the fabric with a suitable cleaner.
Cravenette was an old process to make cloths, water repellant. It was a performance finish that repels water.  Various U.S.-based suppliers, such as A. Murphy, W.G. Hitchcock, and H. Herrmann, were offering Cravenette treated cloths in the early 20th century. 
- Frequent Questions | PFOA and Fluorinated Telomers | OPPT | OPPTS| US EPA
- Philippa J. Hill, Mark Taylor, Parikshit Goswami, Richard S. Blackburn. "Substitution of PFAS chemistry in outdoor apparel and the impact on repellency performance." Chemosphere, 2017, 181, 500-507. doi:10.1016/j.chemosphere.2017.04.122
- Steffen Schellenberger, Philippa J. Hill, Oscar Levenstam, Phillip Gillgard, Ian T. Cousins, Mark Taylor, Richard S. Blackburn. "Highly fluorinated chemicals in functional textiles can be replaced by re-evaluating liquid repellency and end-user requirements." Journal of Cleaner Production, 2019, 217, 134-143. doi:10.1016/j.jclepro.2019.01.160
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