Polystyrene

These are some observations I would like to make, (not necessarily
in a coherent order), after reading Mr Trock's reply to an earlier
request that I did not read as I set "no mail" while away.

I believe that you will find the material inside jewel boxes is not
polystyrene, but a mix of three plastics, Acrylonitrile, Butadiene
and Styrene (ABS), this appears to be the same for 3.5 in Floppy
disks, and many audio cassettes (though some audio cassettes are
purely styrene based).

ABS has a completely different set of characteristics to
polystyrene, and the quantity of plasticising agent may vary
dependant upon the percentage mix of the plastics. It is very
difficult to identify a heavily plasticised ABS from a lightly
plasticised one without sticking a red hot pin through it and
sniffing, and that is very subjective (not to mention dangerous!).

The plasticiser used in ABS is a subliming organic acid, (I forget
the name at the moment). So ABS does not appear to sweat as do other
plastics. Therefore unnoticed by us, the atmosphere of the
microclimate inside a jewel box can have a pH ranging from 7 down to
around 2 (that's the lowest I have measured), and you just can't
tell which ones are going to be the bad ones until they mess up the
CD, by which time of course, it is too late. Because of this it is
wise to view every ABS jewel box with suspicion.

Recently I have discovered what appears to be a complete
Poly-carbonate Jewel Box which might serve as a replacement for ABS
jewel boxes in an archival situation, but so far I haven't located
the manufacturer.

It is also possible obtain unplasticised polyethylene CD and
laserdisk cases which are also good in an archival situation, but as
they are not in the standard jewel box pattern, original CD paper
inserts can not be stored properly together with the disks.

The best method I have found is to remove the CDs from the jewel
boxes completely and store them in microchamber envelopes (supplied
by Conservation Resources), it is the only way to be absolutely
sure.

As I have said before, Polystyrene is a very broad ranged
description of a material. Its reactive behaviour and plasticity and
requirement for plasticiser is defined by the length of the
hydrocarbon chain without necessarily changing the name of the
plastic. Because of this diversity of makeup, photographic
conservators I am sure would not recommend the use of PS for storing
photographs, but would recommend a plastic of defined chain length.
Most conservators I know recommend a specific chain length material
such as unplasticised polyethylene, or Polyethylene terephthalate,
but certainly not polystyrene.

I must also point out that not all clear polystyrenes are
plasticiser free. Although clear polystyrenes are naturally more
pliable, this does not mean that plasticisers are not added in the
manufacturing process, be very careful!

Jonathan Farley

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                  Conservation DistList Instance 11:4
                  Distributed: Thursday, June 26, 1997
                        Message Id: cdl-11-4-002
                                  ***