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WD-40
Name: Richard
Status: educator
Age: 60s
Location: N/A
Country: N/A
Date: 2000
Question:
What is the approximate composition of WD-40, the
commercial liquid sold in a spray can that is advertised to displace
water and prevent rust?
Specifically, I'm curious about its ability to displace water. Is it
simply a modified oil whose density is greater than that of water?
Assistance will be appreciated. If you can point the way, I'm willing to
do the searching.
Many thanks.
Richard
A retired chemistry teacher who is still curious.
Replies:
The exact formula of the magical WD-40 is a proprietary secret, so unless
some employee has loose lips we can't know exactly, but we can deduce some
components from its behavior:
1. It contains some volatile aliphatic hydrocarbons -- from the odor and
it's got to be relatively cheap in the scheme of chemical products.
Something around C8 -C12. There is probably a good fraction of branched
isomers because they tend to be lower melting which is what you want for a
"penetrating oil".
2. It has a much lower viscosity than standard "light weight" machine oil
[34.2 cps.] you get at the hardware store, so the hydrocarbons are probably
more toward the C8 range [ C8= 0.54 poise].
3. Its surface tension must be low, which is also consistent with
hydrocarbons, C8 for example has a surface tension of [ 21.80 dyne-cm, I
prefer the equivalent units ergs/cm^2]. That is a typical value for
hydrocarbons. But WD-40, in addition, may contain some tertiary alkyl amines
whose surface tension is remarkably low. Triethyl amine's surface tension at
only -4 C. is only [17.3 ergs/cm^2]. Such a component would also provide
some degree of low temperature lubrication.
4. But what is also very important is the interfacial tension between the
WD-40 and water, because this is a measure of how fast the product "chases"
water out of the nooks and crannies of a rusty screw thread. This is
achieved with surfactants.
The alcohol n-octanol has an interfacial tension of 8.5 ergs/cm^2 and
n-heptanoic acid has an interfacial tension with water of only 7.0
ergs/cm^2.
5. I would not be surprised if some metal ion chelating agents are also
in the gmisch.
Contrary to your question the density of WD-40 is less than that of water,
so there's no gravity at work.
Vince Calder
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Update: February 2012
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