Damage limitation - museum conservation of plastics

Materials World magazine
,
2 Mar 2011

Plastics are ubiquitous in our everyday lives. But while some quarters are focusing on reusing and recycling plastic products, others are desperately trying to keep them as they are. Dr Brenda Keneghan, Polymer Scientist at the Victoria and Albert museum in London, UK, discusses museum conservation.

Initially, the first semi-synthetic plastics were used to imitate more valuable natural materials such as amber, ivory, coral and tortoiseshell. As their properties have developed, plastics have entered into collections as materials in their own right.

A common misconception, however, is that plastics last forever, but those working in museums and galleries have been battling against their deterioration for over a quarter of a century – one of the most famous incidents being the spectacular disintegration of a valuable cellulose acetate sculpture at the Philadelphia Museum of Art when its glass display case was opened.

Whereas the ageing and degradation of a plastic artefact in non-museum circumstances can be tackled through replacement, degradation in a heritage context takes with it the importance of originality and cultural significance.

Chemical and physical

Polymer degradation can be attributed to chemical and/or physical factors. Chemical factors can cause changes to the base polymer or additives, which may result in physical changes or even complete disintegration. Physical factors, such as the migration of additives or interaction with the environment, can also cause physical changes, such as warping or brittleness.

Chemical degradation is caused by the interaction of the plastic with oxygen, water, ozone, atmospheric pollutants and radiation (light and heat). Different reactions can occur depending on the chemical nature of the polymer involved. Chain scission is a common reaction, where selected chemical bonds are broken, leading to a decrease in molecular weight and the loss of physical properties. Other common reactions include the formation of crosslinks and/or chromophores, resulting in increased brittleness and darkening of colourless materials.

Condition surveys of artefacts in museums worldwide have indicated that there are several materials highly susceptible to chemical degradation. These include the early semi-synthetic materials – cellulose nitrate and cellulose acetate – and the fully synthetic polymers – polyurethane and polyvinyl chloride (PVC).

Other classes of materials that experience less dramatic chemical degradation, include nylon, polystyrene, acrylics and epoxies, which generally yellow and/or become brittle rather than experience catastrophic failure.

Cellulose nitrate and acetate can degrade by either oxidation or hydrolysis, often resulting in the complete disintegration of the object. In addition, the hydrolysis reactions produce acidic by-products, which can cause severe damage to neighbouring objects.

Because of their extremely high surface area, polyurethane foams are particularly susceptible to oxidation. Objects made from rubber are also prone to oxidation, while flexible PVC is highly susceptible to both chemical (crosslinking and chromophore creation) and physical degradation due to the migration of its plasticiser.

Conservation efforts

Once initiated, plastic degradation cannot be prevented, reversed or stopped, but sometimes it may be slowed down. This is achieved by ‘preventive or inhibitive conservation’ whereby contributing factors – light, heat, humidity, oxygen, acids – are removed, or their level reduced from the vicinity of the objects.

In the case of oxidation or autocatalysis, gas scavengers, such as activated carbon or zeolites, can help by adsorbing the oxygen or acidic gaseous by-products that cause or continue the chemical reactions responsible for degradation.

Practical conservation treatments are generally undertaken on heritage objects to stabilise degraded materials, preserving the object’s original significance and value. Such treatments include cleaning, adhesion, consolidation and filling. The high sensitivity of many plastic materials (especially when degraded) to organic liquids and even water, means that such treatments are considered risky and are often shied away from.

Now the EU is supporting a project that aims to improve the preservation. The Preservation of Plastic Artefacts in Museum Collections (POPART) is being funded with 2.1 million euros under the Seventh Framework Programme's Environment Theme. This involves 11 key partners, including the Centre National de la Recherche Scientifique (CNRS) in Paris, France, The National Museum of Denmark (Natmus), Istituto di Fisica Applicata Nello Carrara (IFAC-CNR), Italy, Instituut Collectie Nederland (ICN), The Netherlands, and the Polymer Institute, Slovak Academy of Sciences (PISAS), Slovakia.

The project, which began in October 2008, involves five key steps – identification of polymer artefacts, collection surveys, polymer degradation assessment, conservation treatments and dissemination.

To aid identification, the project is building a reference sample collection of plastics representing new and degraded materials commonly found in museum objects.

Several of the project partners are employing analytical techniques ranging from fourier transform infrared spectroscopy and near infrared spectroscopy to pyrolysis gas chromatography/mass spectrometry (Py-GC-MS). To keep the project focussed on real collections, condition surveys are being undertaken in museum collections in the United Kingdom, France and The Netherlands.

Polymer degradation assessment involves application of chemiluminescence, Py-GC-MS, fibre optic reflectance spectroscopy and dielectric spectroscopy. This has resulted in the rate constants being determined for chain scission reactions by chemiluminescence on aged and un-aged sample materials.

Chemiluminescence is also being used to estimate the remaining service life of polymeric materials which are not yet showing signs of degradation. Experiments have shown that the low frequency region of 0.01-1Hz is the most promising interval to obtain useful information on the degradation of polymers using dielectric spectroscopy.

Cleaning hazards

Interventive conservation treatments, such as cleaning and consolidation, are being investigated rigorously. Prior to this project, the only references to the cleaning of plastics came from industrial or technological sources, which are generally not appropriate for heritage objects. For example, a polish that cleans acrylics by abrasion is not acceptable in a museum environment as it removes part of the surface and may also cause stress and shorten the object’s life. It was, therefore, necessary to evaluate cleaning practice in-depth.

Each participating partner chose a plastic to investigate. The Victoria and Albert museum chose polystyrene as there are a lot of objects made from this in the various collections. Standard samples of plastics were obtained from manufacturers and cleaning techniques were subdivided into mechanical, aqueous and solvent. The effects on the materials were examined, such as changes in gloss and contact angle, before and after cleaning.

Using this methodology, several materials and techniques commonly used in the conservation of more traditional have been eliminated.

Although the cleaning of plastic objects might not appear to be of great general significance, this is not the case for heritage collections. It is hoped that the POPART project will be a turning point in the conservation of objects made from synthetic polymers in museum and gallery collections.

Further information

Dr Brenda Keneghan, Conservation Department, Victoria & Albert Museum, South Kensington, London, SW7 2RL, UK. Tel: +44 20 7942 2119. Email: B.Keneghan@vam.ac.uk Also see http://popart.mnhn.fr for more information on the POPART: an international project on the preservation of plastic artefacts in museums.