When you imagine vulnerable objects in museum collections, often plastic is not the first material to come to your mind. However, recent surveys show plastic objects to be some of the most at risk for degradation in cultural collections. Rose King and colleagues review current research investigating plastic degradation due to the migration of plasticizer additives. They examine the various factors affecting migration as well as methods for qualitative and quantitative analysis of affected objects.
The idea is to provide conservators with the knowledge of intrinsic material properties and practical methods to use in their own collections. This direction helps prioritize vulnerable objects and reduce plasticizer migration, loss, and subsequent plastic degradation over time.
Plastics are made of polymers with additives such as colorants, heat stabilizers, inorganic fillers, and notably—plasticizers, which are necessary for facilitating manufacturing but are proving to compromise the final product long-term.
Plasticizer migration begins with breaking the interactions between the polymer matrix and weakly bound additives. The additives then diffuse through the bulk to the object’s surface, where they then evaporate. Research has shown the factors influencing this process can be complex, often depending on the type of plastic and additives, as well as their relative concentrations. Common environmental factors explored include the influence of temperature, humidity and ventilation. Additionally, possible “sink” materials, such as packing material or other plastic objects within the environment, can also enhance the additive emission rate from objects.
The loss of additives often causes noticeable changes to the objects such as distortion, embrittlement or loss of mass. Additionally, surface deposits are a common result of additive migration. These deposits—known as “bloom”—appear as oily, tacky or crystalline features on the surface, as shown in the photograph below. It comes from the polymer mixture undergoing a phase separation and the additives migrating to the surface.
Often, a good first step in tackling the problem of plastic degradation is to identify the particular additives present in objects. One method to do this is by first separating additives from the bulk polymer matrix. Often, this is done through destructive means, such as a solvent extraction, with subsequent identification via spectroscopy, chromatography or spectrometry. However, as analytical techniques used for heritage applications aim to be non-destructive and minimally-invasive, other means for identification are important topics for research.
If there is already bloom present on the surface, it has been shown this can be swabbed off and characterized via FT-IR or Raman spectroscopy, NMR, and chromatography-mass spectrometry. Alternatively, if there is no bloom present but degradation is suspected, environmental sampling can be employed as a non-invasive technique for additive identification. Because many of the species migrating to the surface readily evaporate, these volatile compounds can be sampled from the environment surrounding the object and characterized via gas chromatography-mass spectrometry.
In-situ techniques are also promising non-invasive options. FT-IR and Raman spectroscopy have proven successful in recent research, but caveats remain with distinguishing certain plastics and additives due to overlapping spectral bands. Additionally, if the additive concentrations in objects are very low, issues can arise in obtaining good enough signal-to-noise ratios for proper identification.
Current research on plasticizer migration is promising, and King and colleagues’ article provides an effective road map for identifying vulnerable objects. However, more work remains to be done identifying which protection methods are suitable for all the various plastics types and additives.