Deterioration of plastic

Suzanne Kitto <suzanne.kitto [at] armouries__org__uk> writes

>We have a number of revolvers in our collection dating from 1950s
>that have deteriorating plastic grips.  According to the
>manufacturers the grips are Bakelite but I have never seen Bakelite
>deteriorate in this manner.  The grips have in places a white bloom
>which in areas has formed small white compact crystals. On some of
>the grips the plastic has laminated and cracked.  All the grips are
>giving out a strong unpleasant odour which can only be described as
>smelling of excrement.  My feeling is that the smell maybe from a
>degrading oil.  I would be grateful for any ideas on what could be
>happening or if any one else has come across anything similar.

I was wondering if the plastic handles might be some form of
cellulose-based plastic, as they can often be rather smelly as they
deteriorate, and not just nitric acid vapors or acetic acid vapors.
Her submission reminded me of a similar experience I had while
working at the Frederick Law Olmsted National Historic Site in
Brookline, Massachusetts.

In their collections we found some very early plastic triangular
drafting tools, they were in various stages of deterioration, and
smelled just awful.  The closest odor I could ascribe them to was a
strong ripe cheese, or vomit.  I conducted some research, and found
out that the earliest drafting tools were made of thin sheets of
wood.  Sometime in the late 19th or early 20th century, with large
production of early plastics, especially cellulose nitrate, clear
plastic drafting tools were introduced. These were of course very
popular because you could see through them to the paper, and they
were not easily broken.  However, within a few years, the drawbacks
of cellulose nitrate were discovered, namely flammability, and rapid
deterioration, subsequently releasing destructive nitric acid
vapors.

Since cellulose nitrate was made from cellulose and nitric acid, in
later years cellulose acetate was introduced which is less
flammable, less likely to degrade, and when it did degrade it was
less destructive because it released acetic acid.  Of course it also
had its drawbacks, including the well known "vinegar syndrome".

Subsequent to the introduction of cellulose acetate, perhaps into
the 1950's, other cellulose-based plastics were introduced,
including cellulose butyrate, which was made with butyric acid.
Apparently the thinking of the polymer chemists was that since butyl
compounds are highly branched, they might be less likely to degrade
and off gas.  However, they do off gas, and as a result release
butyric acid vapors.  Butyric acid is one of the highly
characteristic smells of vomit, and as a result, when these plastics
degrade they smell terribly.  Could this be the excrement-like odor
Suzanne was referring to?  It would be rather easy to test for
cellulose-based plastics, using a number of tests outlined in Dr.
Odegaard's recent book on microchemical spot testing, available
through Archetype.  I have also known cellulose-based plastics to
degrade in a fashion similar to her description; cracking and
forming lighter-colored zones of shattered plastic as it
deteriorates.  I hope this helps, Tom

Tom Braun
The Daniels Objects Conservation Laboratory
Minnesota Historical Society


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                  Conservation DistList Instance 15:43
                 Distributed: Monday, December 17, 2001
                       Message Id: cdl-15-43-010
                                  ***