Polyvinyl chloride

Mariko Kageyama <aspeciosus [at] yahoo__com> writes

>I am going to use a clay (Sculpey) for supporting osteological
>specimens during digital photography. The manufacturer said that the
>generic ingredients of clays were PVC, plasticizers, pigments and
>stabilizers. Is is not a good idea to apply PVC material to bones?

Polymeric clays such as Sculpey are prepared by physically mixing
PVC polymer, a stiff, unstable polymer with between 20-40%
plasticizers by weight. The most frequently used general purpose
plasticizers since the 1950s have been phthalates, particularly
di-(2-ethylhexyl) phthalate (DEHP). Because DEHP is only attached to
the PVC polymer chains by weak physical forces, such as Van der
Waals, and have no chemical bonds to hold them in place, they can
separate as a result of changes of temperature, pressing or folding
of the PVC clay, or in high relative humidity. When the plasticizer
relocates to surfaces, it either evaporates with time, is absorbed
by other materials in close contact including paper, polyethylene
and bone, or remains at surfaces imparting a tacky feel.

When the plasticizer leaves the PVC polymer chains, the material
becomes stiff and itself deteriorates. PVC degrades by
dehydrochlorination and shows a series of colour changes from white
to yellow to orange, red, brown and black. During this process,
hydrogen chloride is produced.  The acidic hydrogen chloride
provides a suitable environment for acid hydrolysis of the DEHP to
occur. The major degradation product is phthalic acid crystals which
are soluble in water.

The effects of plasticized PVCs degradation processes deem it a
non-archive quality material. Although contact between  your
osteological specimens and Sculpey would be brief, I consider wax or
polyethylene to be more suitable alternatives.

Yvonne Shashoua
Senior Researcher in Modern Materials
National Museum of Denmark


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                  Conservation DistList Instance 18:2
                   Distributed: Friday, June 18, 2004
                        Message Id: cdl-18-2-002
                                  ***