The work aims to develop a method for the rapid discrimination of virgin polypropylene (PP) and post-industrial PP regrind, and evaluate its applicability to PP cap products. An ultrasound-assisted solvent extraction method combined with ultraviolet-visible spectroscopy (UV-Vis) was adopted to acquire UV-Vis spectra of PP sample extracts, and the effects of extraction solvent, ultrasonic extraction time, and sample mass on the spectral responses were investigated and optimized. Principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) were further applied for sample discrimination. The optimized pretreatment conditions were determined as follows: dichloromethane as the extraction solvent, a sample mass of 1.0 g, a solvent volume of 5.0 mL, and ultrasonic extraction at room temperature for 120 min. Stable spectral differences between virgin PP and PP regrind extracts were observed in the wavelength ranges of 230-280 nm and 300-350 nm. OPLS-DA models established with different spectral preprocessing methods all exhibited good discrimination performance. After wavelength selection based on variable importance in projection (VIP) values, an OPLS-DA model was established with standard normal variate -transformed spectra in the wavelength range of 239-345 nm. The model achieved R2X, R2Y, and Q2 values of 0.999, 0.983, and 0.945, respectively, with a classification accuracy of 100%, and no evidence of overfitting was observed. However, the UV-Vis spectral differences of PP cap products containing low proportions (<20%) of PP regrind were not evident. Overall, UV-Vis spectroscopy combined with chemometric analysis can provide a rapid screening and discrimination method for virgin PP and post-industrial PP regrind, but it is not suitable for identifying low proportions of PP regrind (<20%) in PP products. Representative recycled PP samples will be further collected to enhance the model's generalizability.
Key words
polypropylene /
post-industrial recycled plastic /
ultraviolet-visible spectrum /
chemometrics
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