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Name: Nathan
Status: Other
Grade: Other
Location: CO
Country: USA
Date: July 2006


Question:
When fluorine is used to repel water on fabrics such as DWR-Durable Water Repellency (a topical surface treatment used on garments), is it increasing or decreasing the surface tension on the fabric?



Replies:
It is INCREASING the surface tension of the water at the interface with the fluorocarbon. When a liquid has a low surface tension, it easily spreads out and there is little difficulty making a surface. With a high surface tension, there is a high energy cost to making a surface and the liquid will not spread out to fill nooks and crannies of whatever it is touching.There is not much energy cost for the interface between cotton and water, so water readily penetrates cotton. With the fluorocarbon coating, the interface with water is not as energetically favorable, so the surface area is minimized.

Fluorocarbons - not fluorine in its elemental form - are extremely "hydrophobic", that is, they have lower affinities for water than most other substances. Consequently, water does not cozy up to them very well.

Richard Barrans


Hi Nathan,

Fluorine (per se) is not (and because of its extreme reactivity and toxicity could never be) used to repel water on fabrics. I think you are thinking of a class of chemicals called "Fluorocarbons", which are compounds (often polymers) typically consisting of chains of carbon atoms with fluorine and other atoms, mainly hydrogen, attached.

Fluorocarbons, or more accurately, Perfluorinated Hydrocarbons, are extremely hydrophobic. That means they strongly repel water. When a fabric is treated with these materials, the very thin coating of these compounds on each fiber lowers the surface tension of the fabric in contact with water, causing the water to repel and bead up.

Recently there has recently been a lot of concern about the use of these compounds, not because they are inherently toxic (they are not), but because they are extremely stable, and do not break down in the environment. For example, until recently the company 3M had a family of remarkably effective Perfluorinated Hydrocarbon waterproofing products that they voluntarily stopped manufacturing a few years ago, because of fears they would never break down, and therefore continue to accumulate in the environment forever.

Regards,
Bob Wilson


Nathan,

I was unfamiliar with this particular coating so I looked it up in Wikipedia:

http://en.wikipedia.org/wiki/Durable_Water_Repellent

I imagine the polymer acts much the same way that Teflon does and decreases the surface activity on the substrate. The polymer gets physically (not chemically) bound to the surface, and presents the C-F bond, which, although polar, is a very short chemical bond and provides for a very low surface area for interaction with water. Thus, it is not so much that the surface tension is reduced, it is that the surface activity is reduced.

Greg (Roberto Gregorius)



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