Polypropylene storage boxes

Tim Edwards <sadete<-a t->hantsnet< . >hants< . >gov< . >uk> writes

>Does anyone have any views on polypropylene boxes for volume
>storage? ... I believe the material itself is sound
>but am concerned with its performance in a disaster situation eg.
>would it be likely to melt at temperatures at which paperboard
>enclosures would only char?

Last year I did an MA looking at the problems associated with the
conservation of paper that has come into contact with plastic during
a fire. Polypropylene (PP) was one of the plastics I concentrated
on--although in the form of document wallets rather than boxes. From
the very basic tests I was able to carry out I would be dubious
about using PP for archival storage.

PP is rarely used in a pure form and is often found as a copolymer
of polyethylene (PE). It is therefore difficult to give precise
figures for the melting temperature, which can be anything in the
wide range of 108 - 212 C. (In a typical office fire involving
paper, the temperatures reached may be in the region of 1000 deg.
C.)  I heated test samples to around 250 deg. C.  The papers charred
slightly, but the plastic melted completely, leaving a sticky waxy
stain which proved impossible to remove.

I also looked at material from a real fire situation.  There was a
variety of papers and volumes all encased in the plastic from an
office letter tray. Within the tray there were some documents inside
PP wallets and some inside paperboard wallets. The PP wallets had
melted, the paperboard wallets were severely charred. Most of the
documents inside were charred around the edges, but otherwise in
good condition. However those documents that were in direct contact
with the PP had a 'coating' of plastic that had melted into the
paper itself.  Although the text was mostly legible through this
layer, this was lucky: it had obviously been very sticky and had
attracted a lot of dirt and soot, which could not easily be removed.

It would appear from this material that the paperboard enclosures
provided as much protection from the heat as the plastic wallets,
without the complications of leaving sticky residues. However these
results are not conclusive, as the relative position of the paper
and plastic wallets within the outer plastic tray may have had an
effect. There is also the question as to the relative merits of the
behaviour of paperboard and plastics when wet to consider. But at
this stage I would certainly be inclined to stick to traditional
phase boxes.

I am hoping to do some more research in this area, including the
behaviour of polyesters such as Melinex/Mylar and would welcome any
comments or experiences people may have.

Catherine Hoskins
Conservation Department
Public Record Office
Kew
Richmond
Surrey TW9 4DU

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                  Conservation DistList Instance 11:42
                Distributed: Wednesday, November 5, 1997
                       Message Id: cdl-11-42-001
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