Assessment of historical polymers using attenuated total reflectance-Fourier transform infra-red spectroscopy with principal component analysis

Introduction: Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy with a diamond ATR crystal was used to examine 41 historical polymer fragments and a selection of polyurethane fragments subjected to accelerated ageing. The advantages and limitations of FTIR data for polymer assessment is discussed. Moreover the efficacy of the data interpretation tool principal component analysis (PCA) is debated for potential applications in polymer characterisation and polymer degradation. Results: Analysis of polymer fragments by ATR-FTIR spectra was shown, as expected, to be an ideal method for polymer classification. Curved surfaces could be tolerated when spectral data were carefully collected, similarly opaque samples could be analysed due to the reflective nature of the FTIR technique used. More importantly perhaps, these results reaffirm the necessity to examine individual spectra as further information can be obtained which allow a better understanding of the material’s stability. It was possible to identify potential degradation of cellulose nitrate and rubber, discriminate between the ether and ester-form of polyurethane, and discriminate between high and low density polyethylene. It was also shown that PCA could be used to unambiguously identify samples which contained cellulose acetate, cellulose nitrate, polycarbonate or polyurethane with a selection of known samples, but without the use of a spectral library. Conclusions: This study supports previous publication results indicating that ATR-FTIR is a useful tool for the examination of objects containing polymers. Here it was shown that polymers could be characterised in object fragments that were not specially prepared and without the use of a spectral library. PCA was shown to be a useful tool for the unambiguous identification of cellulose acetate, cellulose nitrate, polycarbonate or polyurethane polymers in historical plastics with different additives, plasticisers or age. More excitingly, even though spectral features were similar for new and aged samples of polyurethane, PCA was able to discriminate between samples of foam that had been treated by heat (50°C for 24 h or 144 h) or by exposure to light, although more results for other polymeric materials are required to support this proof of concept study.