Quantitative Determination of Fluorine Content in Polylactide (PLA)-Talc Blends using Near Infrared Spectroscopy

Polylactide or polylactic acid (PLA) belongs to the family of aliphatic polyesters commonly made from –hydroxy acids and are considered biodegradable and compostable. Conventional inorganic fillers, such as talc, are still widely used as reinforcing fillers and nucleating agents in commodity polymers as PLA, because it has the potential to provide significant and cost-effective improvements in PLA formulations. Talc is a hydrated magnesium silicate with the general formula Mg3SiO10(OH)2. However, talc structure also contains several other ions in smaller quantity or traces, among which fluorine (F) deserves particular attention due to its high toxicity that can affect the overall compostability of PLA manufacts. For this reason, the current regulations on composting and biodegradability fixes strict limits of fluorine content in biopolymers materials, making mandatory analytical controls on products destined to the market. The objective of this study was to develop a NIRS method for the quantitative determination of fluorine content in polylactide (PLA)-talc blends. A blending profile was obtained by mixing different amounts of PLA granules and talc powder. FT-NIR spectra were collected using NIRFLex N-500 (Büchi, Switzerland) and all chemometric analysis were performed using NIRCal 5.0 (Buchi, Switzerland). The calibration model was built correlating wet chemical data (alkali digestion method) and NIR spectra. By means of a FT (Fourier Transform)-NIR instrument, Partial Least Squares (PLS) regression model was developed, spanning a concentration range of pure PLA (0 ppm of fluorine) to pure talc (800 ppm of fluorine). Fluorine content prediction (R2cal = 0.9498; SEC = 34.77; SECV = 46.94) was then externally validated by means of further 15 independent samples (R2EX.V. = 0.8955; SEP = 61,08). A positive relationship between an inorganic component as fluorine and NIR signal has been evidenced, and used to obtain quantitative analytical information from the spectra.