Conserving plastic objects in museums: Emitted volatiles act as exhibit degradation markers

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  • Published: Oct 15, 2012
  • Author: Steve Down
  • Channels: Base Peak
thumbnail image: Conserving plastic objects in museums: Emitted volatiles act as exhibit degradation markers

Secreting plastics

Specific volatiles emitted from plastic objects in museums have been identified by SPME-GC/MS, giving a series of degradation markers and that will help with the preservation of valuable exhibits.

Like many substances, plastics release a stream of volatile compounds into the atmosphere which can lead to indoor pollution, such as plasticisers, residual monomers, dyes, fillers and flame retardants. But a team of scientists in France with responsibility for museums has turned this around by using the volatiles to signal which plastics are present in that location and trying to estimate their degree of degradation.

Agnès Lattuati-Derieux and colleagues from the Research Centre for Research on the Conservation of Collections (CRCC), Paris, and the 20th Century Contemporary Art Section of the Centre for Research and Restoration of Museums of France, Versailles, wanted to see if the emissions from plastics routinely found in museums formed characteristic signatures. These plastics would be used for exhibitions stands, displays and the like.

The findings from this study were then used to examine three relatively modern exhibits held in museums which contained plastic components, as part of the European POPART project which was designed to devise a strategy for preserving plastic objects in collections. The nature of the objects meant that the chosen analytical technique had to be non-destructive, so the team turned to headspace SPME-GC/MS to measure the volatiles.

Modern plastics and museum pieces

In the first instance, twelve plastics widely found in museums were examined. They belonged to seven classes, namely styrene polymers, polyalkenes, acrylics, polyamides, polyphenylene oxides, polyurethanes and cellulose esters. Small pieces of each were sealed in a vial and the SPME fibre was pushed through the seal to rest above the surface of the plastic. The vial was heated and the evolving volatiles were absorbed onto the fibre.

The first of the museum objects was a plastic comb dating from about 1910 that had broken into several pieces, indicating advanced degradation which was attributed to migration of the plasticiser. It was selected to assess the validity of the SPME-GC/MS technique by identifying the plasticiser and confirming the visual identification of the plastic.

The second object was a suitcase by Arman named La valise des nouveaux réalistes dated 1973. It had flexible polyester-based polyurethane foam inside that had aged badly, having a yellow discolouration and turning to powder. The third piece was a chair by Tallon dated 1965 that had a polyether-based polyurethane foam on the back. All of these exhibits were tested in situ by placing the fibre in their vicinity for 15 days.

All of the fibres were inserted into the injection port of a GC/MS instrument for electron ionisation mass spectrometry. The compounds were identified by direct interpretation of their mass spectra or by comparison of the spectra with those from a mass spectral library.

Degradation markers

More than 200 compounds were identified in total from the twelve standard plastics and they were placed into non-specific or specific categories. The non-specific set was described as compounds emitted from most of the plastics, or those which were detected only once. These included antioxidants, inhibitors and other additives and were exemplified by 2,4-di-tert-butylphenol, bisphenol A, low-molecular-weight linear carboxylic acids and several phthalate esters.

The specific compounds were of more interest because they represented a particular type of polymer and comprised mostly residual monomers. For instance, styrene, ethylbenzene and its oligomers like 1,3-diphenylpropane were detected for polystyrene. These oligomers were absent from acrylonitrile-butadiene-styrene polymers where styrene and ethylbenzene were dominant.

The polyalkenes emitted linear and branched alkanes as expected which could be used to identify this polymer class but not to distinguish polymers within the class. However, acrylic, polyamide and phenylene oxide polymers could be distinguished by the detection of volatile monomers like methyl methacrylate, caprolactam and phenol, respectively. Alkylene glycols and their monomethyl ethers were indicative of the polyurethanes and camphor isomers and other monoterpenes were emitted by celluloid.

The chromatogram of the old comb contained strong peaks for camphor and its isomers, strongly indicating that it is made of cellulose nitrate because camphor is the most common plasticiser used with this material.

The foam in the suitcase emitted some unidentified adipates as well as diethylene glycol and this was identified as poly(diethylene glycol adipate) polyurethane. The high levels of diethylene glycol are indicative of the high degree of degradation of the foam.

The foam backing in the chair exhibit emitted glycols and their low concentrations were consistent with good preservation. This foam was a polyether-based polyurethane.

So, the non-invasive nature of headspace SPME-GC/MS appears to be ideal for studying the nature and condition of plastics in museum artefacts where samples cannot be taken away for analysis. The technique will be useful for conservation personnel who need to estimate the degree of degradation and assess the risks of exposure of these and neighbouring exhibits to potentially damaging airborne compounds like acids.

Related Links

Journal of Cultural Heritage 2012 (Article in Press): "What do plastics emit? HS-SPME-GC/MS analyses of new standard plastics and plastic objects in museum collections"

Article by Steve Down

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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