Application of Infrared Reflectance Spectroscopy on Plastics in Cultural Heritage Collections: A Comparative Assessment of Two Portable Mid-Fourier Transform Infrared Reflection Devices

Abstract

Plastics have been increasingly used to create modern and contemporary art and design, and nowadays, museum collections hold numerous objects completely or partially made of plastics. However, the preservation of these materials is still a challenging task in heritage conservation, especially because some plastics show signs of degradation shortly after their production. In addition, different degradation mechanisms can often take place depending on the plastic composition and appropriate environmental and packaging conditions should be adopted. Therefore, methods for in situ and rapid characterization of plastic artifacts’ composition are greatly needed to outline proper conservation strategies. Infrared (IR) spectroscopy, such as attenuated total reflection Fourier transform infrared spectroscopy (ATR–FTIR), is a well-established method for polymeric material analysis. However, ATR-FTIR requires an intimate contact with the object, which makes its application less appropriate for the in situ investigation of fragile or brittle degraded plastic objects. Mid-FTIR reflectance spectroscopy may represent a valid alternative as it allows in situ measurements with minimum or even no contact, and IR data can be acquired rapidly. On the other hand, spectral interpretation of reflectance spectra is usually difficult as IR bands may appear distorted with significant changes in band maximum, shape and relative intensity, depending on the optical properties and surface texture of the material analyzed. Presently, mid-FTIR reflection devices working in external reflection (ER-FTIR) and diffuse reflection (DRIFT) modes have been used in cultural heritage research studies. As the collected vibrational information depends on the optical layout of the measuring system, differences between ER-FTIR and DRIFT spectra are thus expected when the same polymer is analyzed. So far, ER-FTIR and DRIFT spectroscopy have been individually explored for the identification of plastic objects, but comparative studies between the application of two reflectance FTIR modes have not been presented yet. In this work, the use of two portable FTIR spectrometers equipped with ER-FTIR and DRIFT modes were compared for plastics identification purposes for the first time. Both references of polymeric materials and historical plastic objects (from a Portuguese private collection) were studied and the differences between ER-FTIR and DRIFT spectra were discussed. The spectra features were examined considering the two different optical geometries and analytes’ properties. This new insight can support a better understanding of both vibrational information acquired and practical aspects in the application of the ER-FTIR and DRIFT in plastic analysis.

Advantages of External Reflection and Transflection over ATR in the Rapid Material Characterization of Negatives and Films via FTIR Spectroscopy

“…With reflection techniques, the intensity of the radiation reflected by the sample surface is reported versus the frequency or wavenumber at the desired angle of detection. As previously described by several authors [15,17], there is an issue with the nomenclature used in reflection spectroscopy. The terms "external reflection" and "diffuse reflection" are used to indicate the different degree of penetration of the IR beam in the medium.…”

“…The R S gives rise to derivative-like spectral features and/or inverted bands (reststrahlen effect) for absorption bands with absorption k < 1, following the anomalous dispersion of n and for strong oscillators with absorption k >> 1 (strong absorbance coefficient), respectively [17,[21][22][23]. Thus, R S is responsible for the main spectral distortions that appear in the reflectance spectra according to Fresnel's law [15,18].…”

“…Identification of photographic material by established attenuated total reflection (ATR) spectroscopy has been extensively published, and various photographic techniques, such as albumen, collodion on paper or silver-gelatin have been examined in detail [8][9][10]. In addition, these instruments do not only provide the ATR mode, but also offer various external reflection (ER) techniques for contactless measurements [11][12][13][14][15].…”

“…To reach these deeper material layers, external reflection is a conceivable and truly non-invasive and contactless alternative measurement (Figure 1b). However, the penetration depth and intensity of reflection of the IR beam depends on various factors, and a part of the IR beam emerges on the transparent back of the sample without contributing to the resulting spectrum [15]. In search of a rapid and definitive technique for the material characterization of negatives and films via FTIR spectroscopy, transflection using an aluminum foil reflector was applied for the first time.…”

“…Identification of photographic material by established attenuated total reflection (ATR) spectroscopy has been extensively published, and various photographic techniques, such as albumen, collodion on paper or silver-gelatin have been examined in detail [8][9][10]. In addition, these instruments do not only provide the ATR mode, but also offer various external reflection (ER) techniques for contactless measurements [11][12][13][14][15]. While these techniques are adequate for the identification of historic polymers in most objects, photographic film represents a significant challenge for infrared spectroscopy due to both sides of the film support commonly being covered with layers of gelatin: silver halide emulsion on the front side and anti-curl layer on the back [16].…”