--"Try spreading a film of DOT 3, 4, or 5.1 brake fluid on a non-porous surface
and then wipe most of it off, leaving a very thin film. Come back in a couple
of days. It will be gone, unless you are near 100 percent humidity or the
temperature is extremely cold."--
It would have been more ACCURATE to say that if the surface is in a "warm and
dry location," after a couple of days, a "large percentage" of the volatiles
will have evaporated, though some longer-lasting residue will still remain.
For those who can handle some information overload, keep reading. Here is some
additional stuff that I looked up:
The newer DOT glycol ether-based fluids (DOT 4 and 5.1) have higher boiling
points and lower evaporation rates than the older fluids (DOT 3 and earlier). I
looked up the evaporation rates of about ten different glycol ethers to see
what the range was. The slowest evaporating glycol ether of them all had an
evaporation rate of 0.01 which can be compared to other fluids, as follows:
Butyl Acetate (BuAc) evaporates extremely rapidly and it has a standardized
evaporation rate of 1. The evaporation rate of acetone is 0.33, the evaporation
rate of stoddard solvent is 0.12 and the slowest evaporating glycol ether is
0.01
So, acetone evaporates about 33 times faster than the glycol ether in DOT 5.1
and stoddard solvent evaporates about 12 times faster than the glycol ether.
Just for comparison, when I clean parts in stoddard fluid, it usually takes
about 10 or 15 minutes (or longer) for them to dry, depending on conditions.
The residue that is left behind when the glycol ether (or polyglycol) brake
fluid evaporates includes a combination of lubricating oils and corrosion
inhibitors that are added to brake fluids. This residue may make it appear that
brake fluid does not evaporate, but it actually represents the less volatile
components of the fluid.
In addition, after brake fluid absorbs moisture, the boiling point goes way
down, and I suspect that this may raise the evaporation rate. However, I'll
have to do some more checking to be sure.
You can speed up the evaporation process considerably with more heat and air,
such as would be encountered in a hot engine compartment, or when a
dark-colored vehicle is parked in the sun.
When I got home this evening, I looked inside a coffee can containing some
wheel cylinder parts that I dumped in a few months ago. There was still some
kind of oily residue at the bottom of the can, but it was apparent that most of
the fluid had evaporated. It's been stored in a warm garage all summer.
I apologize for being so long-winded again, but maybe a few of you will find
this of some interest.
Alan
(Salt Lake City, Utah)
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