Amyl versus Isopropyl Alcohol
Can you tell me the difference between Amyl Alcohol and
Isopropyl Alcohol and why the amyl is recommended for cleaning
plastics over Isopropyl? Does the Isopropyl dry things out where the
Amyl does not? Please clarify.
Both isopropyl and amyl alcohols are indeed alcohols (having a polar OH
functionality). The difference is that isopropyl alcohol has three carbons
and the OH is attached to the middle carbon. Amyl alcohol, on the other hand,
is a class of alcohols having five carbons. These carbons can be attached
together in a straight line with the OH at the end (n-pentyl alcohol) or the
carbons could be arranged in all sorts of manner with the OH attached in many
different carbon positions. As such, commercial amyl alcohol is most likely a
mixture of different 5-carbon alcohols.
I am not sure as to why amyl alcohol is the preferred cleanser for plastics. I
can only guess that some plastics are more soluble in isopropyl alcohol rather
than the amyl alcohol mixture. As such, isopropyl alcohol may cause some damage
to the surface of the plastic (the plastic will lose its sheen).
Greg (Roberto Gregorius)
Be aware that there are several different "amyl" alcohols. The general
but the carbons can be put together in a number of configurations.
It has a lower vapor pressure than isopropanol, and is more hydrophobic than
isopropanol. I am unaware of any general preference of one over the other.
It depends upon what "plastic" is being cleaned and what is being removed
from the surface of the plastic.
I found your question a little puzzling. As an engineer with years of
experience with a wide range of plastics, I have never once run across
any recommendation to use amyl alcohol for cleaning purposes. For many
plastics such as nylon, polyester, ABS, and especially solvent-
sensitive polycarbonate (Lexan), isopropyl alcohol is recommended for
cleaning purposes. In fact, because of polycarbonate's high solvent
sensitivity, the only universally recommended cleaning solvent for
this plastic is isopropyl alcohol.
The main difference between n-amyl and isopropyl alcohols (in this
context) is the length of the hydrocarbon "backbone". N-amyl alcohol's
carbon chain is 5 carbon atoms long, whereas isopropyl alcohol's
carbon chain is only 3 carbons atoms long. The result is that
isopropyl alcohol is more volatile and therefore evaporates faster
after it is used for cleaning.
Neither of these alcohols "dries out" plastic, since there is no water
(or any other substance) in plastic to "dry out" in the first place.
Both of these alcohols act as both polar and non-aggressive non-polar
solvents, which is very advantageous for cleaning. Acting as polar
solvents, they are able to dissolve polar contaminants such as salts,
and other water soluble contaminants.
Both of these alcohols are also able to dissolve non-polar
contaminants such as oils and greases, with isopropyl alcohol acting
as a somewhat more aggressive non-polar solvent because of its shorter
carbon "backbone" and therefore lower molecular weight. Unlike many
hydrocarbon solvents, neither of the two alcohols are damaging to any
To summarize, when comparing the two, isopropyl alcohol is the
cleaning solvent of choice for most plastics. Unlike normal-amyl
alcohol, which is not easily available, 99% isopropyl alcohol is
available at any drugstore. Isopropyl is better at removing oily
deposits, and dries faster after use.
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Update: June 2012