Conserving contemporary materials was first formally recognised as a discipline in 1991 marked by the conference ‘Saving the Twentieth Century’ organized by the Canadian Conservation Institute. Since mass production of plastics began in the 1940s, the global annual production has doubled every 15 years reaching 245 million tonnes in 2009. Plastics in new, recycled and upcycled forms have had a significant influence on industrial, domestic and cultural aspects of everyday life since the Second World War and therefore comprise an increasing proportion of museum collections. Since the start of the 21st Century, conservation research and practice for plastics have taken a preventive approach in which either the factors causing or accelerating degradation are removed in order to slow the major breakdown reactions. Gas adsorbents, also known as molecular traps or scavengers, are frequently used by museums to slow the rate of degradation of semi-synthetic plastics. Adsorbents are either installed in an active filter system in showcases and storage areas or simply placed in petri dishes or polyethylene bags inside storage boxes or enclosures.
Silica gel, activated carbon and zeolites are the most frequently used adsorbents in museum storage and display of all materials. They are introduced to create a microclimate by removing specific gases. Although widely employed to slow the degradation of semi-synthetic plastics, there has been little structured research into their effectiveness. Studies usually analyse the object before and after. However, it is difficult to detect degradation in plastics until it reaches an advanced stage.
In a recent research project, gases adsorbed from new model and degraded cellulose acetate (CA) sheets dating from the 1950s to 1990s were identified and quantified. CA has been used since 1910 to produce spectacle frames, Lego bricks, Gabo sculptures and Disney cels. CA hydrolyses to form acetic acid which results in the autocatalytic breakdown of the plastic if it is not removed. After exposure, silica gel, activated carbon, zeolite 4A, Corrosion Intercept and an archival card were desorbed using GC-MS and evolved gas analysis which both identified the adsorbed gases and how strongly they were bound. Silica gel, activated carbon, zeolite and the archival card were found to non-selectively adsorb gases. In addition to water, silica gel adsorbed acetic acid and phthalate plasticizer from CA. Zeolite 4A is frequently used to slow the onset of autocatalysis of CA by removing acetic acid and water from enclosures. However, results from the current research suggested that zeolite 4A also adsorbed the plasticizer from the CA film, causing it to shrink. Activated carbon adsorbed both the plasticizer and flame inhibitor triphenylphosphate. It was ineffective to include adsorbents with new, undegraded CA films to inhibit the onset of degradation. In the absence of the degradation product acetic acid, the phthalate plasticizer was adsorbed resulting in shrinkage.
In conclusion, storing cellulose acetate in an archival card box may be more effective at adsorbing acetic acid than using adsorbents. The non-selectivity of many of the general adsorbents used in conservation is of concern and suggests that they may be ineffective or even accelerate degradation. Low temperature storage may be a more effective approach to preventive conservation of plastics than adsorbents.
