Fluorine, Water Tension, and Fabric

name         Nathan
status       other
grade        other
location     CO

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?
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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
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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
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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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