Storing plastic

Susanne Grieve <sgrieve [at] marinersmuseum__org> writes

>Colin Williamson <smileplas [at] aol__com> writes
>
>>We have several hundred plastic tokens dating from about 1900 to
>>1985. Typically these are transport, credit, shop or advertising
>>tokens about 25mm diameter and 2-3mm thick. They are variously made
>>from vulcanite, cellulose nitrate (celluloid), cellulose acetate,
>>urea-formaldehyde thermosetting material, or polystyrene. ...
>> ...
>> ... Does anyone have any
>>recommendations for an alternative storage solution whilst retaining
>>visual access to both sides of the tokens?
>
>I recommend storing the tokens in an oxygen free system such as the
>RP System by Mitsubishi. The process that most adversely affects the
>plastics and rubbers is oxidation. By removing the oxygen you can
>prevent the formation of acid which ultimately causes the demise of
>the rubber or plastic. ...

Use caution when storing plastics in sealed containers, such as is
required for anoxic storage.

In museums, plastics degrade predominantly by two
mechanisms--oxidation (reaction with oxygen) and hydrolysis
(reaction with water or moisture). Anoxic (oxygen free) storage, as
suggested by Grieve, is a good idea only if the main mechanism of
degradation is oxidation.  This is the case for Vulcanite.  However,
for cellulose nitrate (celluloid) and cellulose acetate, which are
esters of cellulose, and urea-formaldehyde, the degradation occurs
predominantly by hydrolysis.  Hydrolysis is slowed by removing
moisture, i.e., storage at low relative humidity.  Anoxic storage
without decreasing RH may have no benefit for plastics that degrade
by hydrolysis, and may cause increased degradation, as explained
below. Polystyrene does not hydrolyse, and is not susceptible to
oxidation in the absence of UV radiation at room temperature. Anoxic
storage would not have any significant effect on polystyrene.

Plastics that degrade by hydrolysis usually generate acidic volatile
degradation products.  For esters like cellulose acetate and
cellulose nitrate the acids are acetic acid and nitric acid.  These
acids are formed by reaction of the plastic with water, not by
reaction with oxygen.  If objects made of these plastics are sealed
into an impermeable container, such as is required for anoxic
storage, then, as hydrolytic degradation proceeds, the concentration
of the acids will increase, and this will cause even more rapid and
greater degradation of the plastic in the sealed container.
Cellulose acetate and cellulose nitrate must never be stored in
sealed containers, unless there is very careful planning of the
package to include chemical absorbers to remove the degradation
gases that inevitably will be produced, or the relative humidity is
kept very low (approaching 0% RH for long term storage).

The effects of micro-environments, including factors like
temperature, RH, zeolite acid-scavengers, and desiccants, on the
degradation of cellulose acetate photographic film is described in

    Jean-Louis Bigourdan, Peter Z. Adelstein and James M. Reilly.
    "Use of Micro-Environments for the Preservation of Cellulose
    Triacetate Photographic Film",in Journal of Imaging Science and
    Technology, Vol. 42, No. 2 (March/April 1998): 59-66, available
    at

        <URL:http://www.imagepermanenceinstitute.org/
            shtml_sub/microenvJIST.pdf>.

    **** Moderator's comments: The above URL has been wrapped for
    email. There should be no newline.

It should be noted that when low RH levels for photographic film are
discussed, it is usually recommended not to go below 20% RH because
the gelatin emulsion layer becomes too brittle below this value.
Such a lower limit restriction does not apply to cellulose acetate
and other plastic objects that do not have gelatin layers, such as
plastic tokens, the original subject of this query.

R. Scott Williams
Senior Conservation Scientist (Chemist)
Conservation Research Division
Canadian Conservation Institute
1030 Innes Road
Ottawa, Ontario, Canada K1A 0M5
613-998-3721
Fax: 613-998-4721


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                  Conservation DistList Instance 20:32
                 Distributed: Sunday, December 17, 2006
                       Message Id: cdl-20-32-003
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