How Can Ozone and Relative Humidity Affect Artists' Alkyd Paints? A FT-IR and Py-GC/MS Systematic Study

Abstract

Knowledge of the chemical-physical reactions that determine the main degradation behaviour of artists' alkyd paints represents one of the main problems within the museum exhibitions. The collection and interpretation of these data on degradation phenomena, especially after ozone exposure at different relative humidity values, can be useful for their conservation needs. Therefore, a systematic investigation of these materials may help achieve this goal. Firstly, surface-level identification of the main functional groups of ad hoc created and aged alkyd paints was performed using attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR). Subsequently, these paints were investigated by pyrolysis-gas chromatography and mass spectrometry (Py-GC/MS), allowing for precise bulk identification of the organic compounds before and after accelerated ageing. A first successful attempt to provide quantitative Py-GC/MS data on alkyd-based paints is here presented and discussed. Comparing the results, it was possible to obtain new insights into the degradation behaviour of alkyd paints when exposed to ozone, allowing us to devise specific preventive and conservation strategies for these artistic materials.

Keywords: Py–GC/MS; alkyd paints; degradation behaviour; ozone; relative humidity.

Figure 1

Total ion current (TIC) pyrograms obtained from (a) Alk_ref, (b) Alk_50%RH, and (c) Alk_80%RH samples.

Figure A1

TIC pyrogram after TD analysis, highlighting the presence of orthophthalic acid and corresponding mass spectrum. The naturally aged alkyd resin was analysed by double shot Py–GC/MS (DS-Py–GC/MS) in two steps: (i) thermal desorption (TD) at 50 °C–20 °C/min–250 °C; and (ii) flash pyrolysis for 1 min.

Figure 2

Schematic representation of the polycondensation reaction of alkyd resin identified from PY–GC/MS analysis.

Figure 3

ATR-FTIR spectra of pure alkyd samples: comparison between fresh (green), pure alkyd reference (black), O₃ + 50%RH (red), and O₃ + 80%RH (blue) aged samples. The * marks all bands related to the phthalic-based compound, the + marks all bands related to phthalate, whereas the # indicates the oil bands.

Figure 4

The trend of FTIR normalized and integrated absorbance band at 1719 cm⁻¹ for the evaluation of alkyd resin stability when mixed with various inorganic pigments.