Non invasive techniques for identification and characterization of polymers in contemporary artworks.

Contemporary art collections include a huge variety of polymeric materials - briefly identified as “plastics” - that react differently to the same environmental conditions and can undergo dramatically diverse degradation processes. Hence, a fundamental starting point in any plan of preservation of plastics artworks is the identification of types of polymers constituting the objects and, when possible, the assessment of the degradation stage for each polymer identified. At present, tests available for plastics identification are borrowed from industrial field: in most cases they require sampling and are destructive or, at least, micro-destructive. This fact limits their applicability in the museums context, where the integrity of the artwork is a fundamental value to be taken into account. Nowadays, methodologies based on evaluation of visual appearance and odors remain the most practiced alternative for a simple, non destructive classification of materials in plastic artworks. Thus, non invasive methods for rapid identification of materials and diagnosis of degradation are highly required for preservation of contemporary art collections. The aim of this work is to study applicability and potentialities of two non-invasive spectroscopic techniques, namely FORS (Fiber Optic Reflectance Spectroscopy) and Dielectric Spectroscopy, as measurement principles of portable systems for the quick identification of plastics and characterization of most frequent degradation phenomena. FORS is an optical technique based on acquisition of the reflectance spectrum in the UV-Vis-NIR spectral region (300-2100nm). By means of a portable spectrophotometer equipped with optical fibers probe-head, the spectrum is recorded in-situ in a totally non-invasive way, and, thanks to the versatility of optical fiber accessories, any point of the object can be measured without constraints due to size or shape of the object. The spectral data on the extended UV-Vis-NIR are exploitable for different purposes: the NIR spectrum can provide indications about the occurrence of chemical species characteristics of a given polymeric class. Instead, the spectral reflectance in the visible region is used for colorimetric analysis and is a powerful tool for monitoring changes in appearance that are symptoms of several degradation phenomena. Dielectric Spectroscopy consists in the measurement of the dielectric permittivity of materials using sensors operating in a very broad frequency region of the electromagnetic spectrum (from a few Hz up to tens of GHz). Dielectric permittivity is a physical parameter that can be related to the capability of polarization of the material in presence of an applied electrical field, and hence its measurement provides information about the tendency of molecular dipoles to follow the orientation of the applied field. This is in turn related to chemical and physical properties of materials. In this context dielectric spectroscopy can be useful for distinguishing between given classes of polymers (e.g. nitrate and acetate of cellulose), as well as for assessing structural molecular alterations (chain length changes) and monitoring ageing phenomena. Both these techniques are already used in conservation field for non-invasive diagnostic on artworks. Nevertheless, so far they have been employed and optimized for characterization of antique artifacts and ‘traditional’ artistic materials: the idea is now to apply these approaches to contemporary artworks, and in particular to polymeric materials. To this purpose, the first unavoidable step is to build specific spectral references databases of polymers of interests in the museum context. Therefore, a set of certified standards (Resinkit®) of the most common polymers has been characterized using both FORS and dielectric spectroscopy, and preliminary results are discussed here. This work is carried out in the framework of the on-going EC Research Project “Popart” (Preservation Of Plastic ARTefacts in museum collections). The research leading to these results has received funding from the European Community’s Seventh Framework Programme FP7/2007-2013 under grant agreement n° 212218