Quantification of PLA contamination in PET during injection moulding by in-line NIR spectroscopy

In-line spectroscopy is a useful tool for monitoring processing conditions and polymer quality in the melt state in processes such as extrusion and injection moulding. This study extends the scope of this technique to the quantitative detection of poly(lactic acid) (PLA), which is a contaminant of concern in recycled poly(ethylene terephthalate) (PET). The near-infrared spectra of thermally treated PLA were studied for evidence of spectral changes which could be used as markers for PLA in PET, and these are discussed in detail. Several chemometric models for quantifying PLA in PET were developed and the influence of chemical or physical changes on the performance of the models is discussed. The selection of the final model was based on criteria aimed at eliminating interference due to spectral changes resulting from physical processes, to give a robust model based specifically on the chemistry of the polymers studied. Finally, the models were tested against a new test mixture of known composition and the outputs are discussed.