ES -12 is the same as Duracool 12a ,OZ -12a, HC-12a.
It is as was suggested, a blend of hydrocarbons, loosely put, propane and
butane.
It is illegal for use in automobiles in some states but not all.  I believe
it is quite legal in Canada.   Duracool comes from Edmonton, Alberta.
There seem to be conflicting tales.
On one hand it is said that using hydrocarbons as an automobile refrigerant
is illegal.
On the other hand it is said that replacing a first line ozone depleting
refrigerant with a second line non ozone depleting refrigerant is quite
legal.
A recent Idaho court case granted a patent to OZ for it.
In many countries it is used extensively as a vehicle refrigerant, including
Germany , China and many others.  It is highly touted as a refrigerant in
Australia and supported by the "greenies" everywhere.
It has been shown that in refined form it is less auto-igniting than R-134a.
It is certainly a much better refrigerant than most others and mixes with any
of the a/c oils in common use.    Only a small amount is necessary and it
requires no licence to use.   However a licensed technician must recover any
R-12 before it can be used.
I put 1 can of Duracool in my sons Jetta the other day and that was enough
for it to cool amazingly well.  The pressures are off but it cools well
nevertheless.  35 psi low side,150 psi high side.
1 can of Duracool is 6 ounces of propane.   This is only a fraction of the
hydrocarbon gas in a coleman cylinder and probably about the amount in a can
of WD-40 or hair spray.       Many people carry things like that in their van
routinely.
With quantities that small I think the safety hazards are overstated.
It is on sale at NAPA parts in Myrtle Beach SC among many other places.

Steve Cotsford