By Bert Striegler, picked from George M. Aldrich's web site, now closed down.
Back in 1983 there was quite a controversy in Radio Control Modeler
magazine about the tests that were necessary to measure the
"lubricity" of various oils that might be useful in model engines.
Castor oil was used as the benchmark, but it was obvious no one knew why this
was so. They apparently got a lot of info on various industry tests of
lubricants, but these were really designed for other purposes. This was my
answer. I will remind you that I was a lubrication engineer and not a chemist,
but I drew my chemical info from Bob Durr, the most experienced lubricant
scientist in the labs at Conoco. Bob worked with my group on many product development
projects and I can tell you that he is one smart hombre! Small changes were
made in the text, but surprisingly very little has really changed since this
was originally written. Here goes with the answer.
"I thought I would answer
your plea for more information on castor oil and its "film strength",
which can be a very misleading term. I have never really seen a satisfactory
way to measure the film strength of an oil like castor oil. We routinely use
tests like the Falex test, the Timken test or the Shell 4-ball test, but these
are primarily designed to measure the effect of chemical extreme pressure
agents such as are used in gear oils. These "EP" agents have no
function in an IC engine, particularly the two-stroke model engine types.
You really
have to go back to the basics of lubrication to get a better handle on what
happens in a model engine. For any fluid to act as a lubricant, it must first
be "polar" enough to wet the moving surfaces. Next, it must have a
high resistance to surface boiling and vaporization at the temperatures
encountered. Ideally the fluid should have "oiliness", which is
difficult to measure but generally requires a rather large molecular structure.
Even water can be a good lubricant under the right conditions.
Castor oil
meets these rather simple requirements in an engine, with only one really
severe drawback in that it is thermally unstable. This unusual instability is
the thing that lets castor oil lubricate at temperatures well beyond those at
which most synthetics will work. Castor oil is roughly 87% triglyceride
ricinoleic acid, which is unique because there is a double bond in the 9th
position and a hydroxyl in the 11th position. As the temperature
goes up, it loses one molecule of water and becomes a "drying" oil.
Castor oil has excellent storage stability at room temperatures, but it
polymerizes rapidly as the temperature goes up. As it polymerizes, it forms
ever-heavier "oils" that are rich in esters. These esters do not even
begin to decompose until the temperature hits about 650 degrees F. Castor oil
forms huge molecular structures at these elevated temperatures - in other
words, as the temperature goes up, the castor oil exposed to these temperatures
responds by becoming an even better lubricant!
Unfortunately,
the end byproduct of this process is what we refer to as "varnish."
So, you can't have everything, but you can come close by running a mixture of
castor oil with polyalkylene glycol like Union Carbide's UCON, or their MA 731.
This mixture has some synergistic properties, or better properties than either
product had alone. As an interesting sidelight, castor oil can be stabilized to
a degree by the addition of Vitamin E (Tocopherol) in small quantities, but if
you make it too stable it would no longer offer the unusual high temperature
protection that it did before.
Castor oil
is not normally soluble in ordinary petroleum oils, but if you polymerize it
for several hours at 300 degrees F, the polymerized oil becomes soluble.
Hydrogenation achieves somewhat the same effect.
Castor oil
has other unique properties. It is highly polar and has a great affinity for
metal surfaces. It has a flash point of only 445 degrees F, but its fire point
is about 840 degrees F! This is very unusual behavior if you consider that
polyalkylene glycols flash at about 350-400 degrees F and have a fire point of
only about 550 degrees F, or slightly higher. Nearly all of the common
synthetics that we use burn in the combustion chamber if you get off too lean.
Castor oil does not, because it is busily forming more and more complex
polymers as the temperature goes up. Most synthetics boil on the cylinder walls
at temperatures slightly above their flash point. The same activity can take
place in the wrist pin area, depending on engine design.
Synthetics
also have another interesting feature - they would like to return to the
materials from which they were made, usually things like ethylene oxide,
complex alcohols, or other less suitable lubricants. This happens very rapidly
when a critical temperature is reached. We call this phenomena
"unzippering" for obvious reasons. So, you have a choice. Run the
engine too lean and it gets too hot. The synthetic burns or simply vaporizes,
but castor oil decomposes into a soft varnish and a series of ester groups that
still have powerful lubricity. Good reason for a mix of the two lubricants!
In spite
of all this, the synthetics are still excellent lubricants if you know their
limitations and work within those limits. Used properly, engine life will be
good with either product. Cooked on a lean run, castor oil will win every time.
A mix of the two can give the best of both worlds. Most glo engines can get by
with only a little castor oil in the oil mix, but diesels, with their higher
cooling loads and heavier wrist pin pressures, thrive on more castor oil in the
mix.
Like most
things in this old life, lubricants are always a compromise of good and bad
properties. We can and do get away with murder in our glo engines because they
are "alcohol cooled" to a large degree. Diesels, though, can really
stress the synthetics we use today and do better with a generous amount of
castor oil in the lubricant mix. Synthetics yield a clean engine, while castor
oil yields a dirty engine, but at least now you know why! "
Bert Striegler
Bert was the Sr. Research Eng'r.
(ret.) at Conoco Oil Co. He's a graduate in aeronautical eng'rg., and a long
time modeler. I never understood how he wound up in the oil research
business, but I guess it's because he's just very smart ! I deserve no credit,
Bert's the brain ! /George M. Aldrich