Problems with plastic and how to spot PVC
While some plastics can play an important role in helping
us preserve valuable documents and artefacts, the use
of the wrong plastic can actually speed up the process
of deterioration. For example, conservators recommend
that you never store valuable photos, negatives or slides
(and important paper documents) in PVC plastic (PVC stands
for PolyVinyl Chloride).
PVC is remarkably versatile, long lasting and cheap
and has been used in a wide variety of applications.
Unfortunately, some of those applications include slide
holders, photo album protective sheets and negative
sleeves which bring PVC in close contact with photographic
film and prints. Over time, plastisers, which are added
to PVC to keep it flexible, leach out of the plastic
and form a damaging residue over the photographic film.
(PVC is also a major plastic used in car dashboards.
Over time you might have noticed a film build up on
the inside of the windscreen. This is the plastiser
in the PVC that evaporates out under the action of heat
and light.)
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If the plastic on the
hot wire burns
green, the plastic is PVC. |
PVC also forms acid compounds over time that speed
up the deterioration of the film. Conservators believe
that PVC can dramatically shorten the life of film and
photos. They recommend that you use other plastics like
polyester (mylar), polyethylene or polypropylene.
But how do you spot PVC from these other plastics.
It's literally impossible to tell just by looking. Well,
here's a simple method. (Adult supervision is recommended
for this test.)
Find a piece of copper wire (about 10cm long by 0.5-1.0mm
thick) and insert one end into a cork. Holding the cork,
insert the wire into the flame of a spirit burner or
bunsen burner. (If you don't have a burner you can improvise
with small cotton wool ball soaked in methylated spirits
placed in a ceramic dish. Be sure to have something
ready to snuff out the flame before the cotton wool
starts to char.)
Heat the copper wire till it glows red. Then, touch
the wire onto a sample of your plastic so that a small
amount of plastic is melted on to the wire. Now, immediately
return the wire to the flame. If the plastic is PVC
you'll see a momentary green or blue-green colour in
the flame being produced by the reaction between chlorine
(from the PVC) with the hot copper.
To check what the results look like, try the test
out first with a sample of plastic that you know is
PVC (ask your chemistry teacher at school). Compare
the results obtained with a plastic the you know is
not PVC (like freezer wrap or oven bags).
Illustrations by Alec Ellis.
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