CD jewel cases

Karl Gillies <karl.gillies [at] southlandmuseum__co__nz> writes

>Mention was made of jewel cases gassing off degrading compounds--I
>thought most jewel cases were usually made of stable polycarbonate,
>or am I wrong? ...
> ...
>So, is there an inert 'archival' type of plastic jewel case
>available?

The majority of standard jewel case inserts are in fact made from a
mixture of adenozine, polybutadiene and styrene. This mixture has
high impact strength, but low flexibility. Consequently, the mixture
is regularly plasticised. Generally, the unplasticised varieties
have a short life due to the CD locking crown breaking down through
forced flexing. Therefore the most commonly available jewel cases
are plasticised because of the longer life of the locking crown. The
plasticisers are organic acids and volatilise directly to the
internal atmosphere of a closed case. I have monitored a drop to pH
2 in as little as three days.

Most people would like to conveniently blame the paper inserts for
off-gassing, however this is generally not the case. The so-called
laser rot that happened on PDO discs illustrated this point. The
damage was actually to the lacquer on the label side of the disc,
and in normal jewel cases, the bronzing always appeared denser at
the outside edge of the disc, gradually fading towards the disc's
normal colour in the centre. Had the damage been due to the paper
insert as PDO intimated, the discolouration would have been even
over the whole surface of the disc because the paper insert is held
in the same plane, and therefore the exposure would have been even.
PDO discs which were marketed in machine ground wood pulp cardboard
sleeves showed an even discolouration, which demonstrates this. In
fact, in the jewel case, the discolouration was from the off-gassing
of the  plasticisers from the jewel case insert which effectively
surrounds the CD on all sides. The deleterious vapours flowed across
the label side from their emission point at the edge, reacting with
the lacquer (which was nitrocellulose based) and decreasing in
concentration as it flowed further across the disc. This produced
the classic edge bronzing of the PDO discs.

In effect, the rate at which the PDO discs decayed varied due to the
presence alkali buffers in the paper inserts having some
neutralising effect on the off-gassing. Paper inserts which were
found to have high residual buffers, were always associated with
discs that had minor discolouration, whereas paper inserts that had
a low, or no buffering were always associated with intense
discolouring. Therefore in contradiction to PDO, the paper inserts
were, on occasions, having a beneficial effect.

It is possible to purchase a pollutant scavenging insert to replace
the offending part of the jewel case, they are retailed through
conservation by design:

    <URL:http://www.conservation-by-design.co.uk/
        sundries/corincep1.html>

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

I have tested this product extensively, and have found it to work
extremely well.

To date, I have only found one DVD keep-case which has been
manufactured using an adenozine polybutadiene and styrene mix. It is
made in the USA, and from what I have seen is quite common on
commercially released DVDs. When I last went to Chicago, I had some
fun looking through the Virgin megastore and purchasing DVDs not
available in the UK (mostly Hammer Horror movies). I ended up doing
an impromptu survey of the stock looking for the tell tale signs of
that keep case. Of the 400 film titles I examined, the keep case was
'protecting' 250 titles. Regards,

Jonathan Farley


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                  Conservation DistList Instance 17:40
                 Distributed: Friday, October 31, 2003
                       Message Id: cdl-17-40-001
                                  ***