Polyvinyl chloride

Alan Hawk <alan.hawk [at] us__army__mil> writes

>Polyvinyl chloride has an abysmal reputation in the museum world,
>mostly due to vinyl film holders that quickly deteriorate and
>offgas.  However, in industrial applications, PVC has a good
>reputation ...

PVC was the subject of my PhD thesis completed in 2001 at the
National Museum of Denmark and Danish Technical University.

Hard or unplasticized PVC used in building applications, undergoes a
very different degradation pathway to that of the plasticized
version used in photograph pockets, children's toys, raincoats etc.
Degradation of PVC plasticized with phthalate esters (the most
commonly used plasticizers since the 1950s) is manifested by initial
migration of plasticizer from between the PVC polymer chains to the
surfaces of the material. A tacky surface forms which traps dirt.
With time the plasticizer evaporates and the plastic shrinks.
Phthalate plasticizers are bound to polymer chains only by weak
van der Waals and other physical forces and, therefore, are mobilized
readily with change of temperature or mechanical force.

Because phthalate plasticizers have a dual role, functioning both as
plasticizers and as anti-ageing additives in PVC, their loss leaves
the PVC polymer vulnerable to its own degradation reaction, namely
dehydrochlorination.  Dehydrochlorination is generally initiated by
ultraviolet light or high temperatures and results in the formation
of hydrogen chloride gas which readily dissolves in the moisture in
air to form hydrochloric acid.  In addition, the single
carbon-carbon bonds in the polymer backbone reorganize into
conjugated structures (alternate double bond, single bond). This
change in bonding structure is observed as darkening and
discolouration of the plastic because the increasing number of
conjugated bonds absorbs wavelengths of light at increasingly longer
wavelengths.

In summary, plasticized PVC primarily degrades because of the
migration of plasticizer manifested by stickiness and shrinkage.
Unplasticized PVC degrades primarily by the chemical reaction
dehydrochlorination on contact with sunlight or heat to form an acid
which corrodes metals and degrades organic materials in the
vicinity. When unplasticized PVC is used as a building material
which will be exposed to sunlight, rather than underground as pipes,
it requires large quantities of anti-ageing additives. When these
are exhausted, it degrades, as shown by thermal ageing of hard PVC
at 70 deg. C for one month days.

Yvonne Shashoua
Senior Researcher
Dept. of Conservation
National Museum of Denmark


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                  Conservation DistList Instance 26:24
                Distributed: Saturday, November 3, 2012
                       Message Id: cdl-26-24-001
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