Investigating primary decomposition of polypropylene through detailed compositional analysis using two-dimensional gas chromatography and principal component analysis

Abstract

This work systematically assessed the effects of pyrolysis parameters, such as sample weight, particle size, carrier gas flow rate, and temperature, on product speciation and distribution in the pyrolysis of polypropylene (PP) for intrinsic kinetics. This study employed a Box-Behnken design (BBD), along with the analysis of BBD boundary conditions, as well as conditions beyond the BBD to validate the pyrolysis conditions in the primary decomposition of PP using a micropyrolyzer coupled to two-dimensional chromatography with flame ionization and time-of-flight spectrometer detectors (Py–GC×GC–FID/TOF–MS). Statistical differences and similarities in the product distribution of the experiments were determined using principal component analysis (PCA). Characteristic time analysis indicated that the sample weights employed in the study were within the isothermal kinetically controlled region, with film thickness ranging from 4.85 to 14.46 µm. The analysis of 600 pyrolysis products identified and quantified in the pyrolysis of PP for the condition combinations evaluated in BBD resulted in statistical differences at temperatures of 460, 530, and 600 oC, with C3 to C35 yields ranging from 39.88 ± 2.08 to 66.61 ± 3.26 wt%. However, no statistical differences were found in the pyrolysis product distribution within the ranges of sample weight (50 – 150 µg), particle size (53 – 125, 125 – 300, and >300 µm), and carrier gas flow rate (100 – 300 ml min-1) studied in the BBD experimental space. The analysis of the border conditions showed that a combination of the lowest carrier gas flow rate (100 ml min-1) and the highest sample weight (150 μg) resulted in statistically different product distribution at the studied pyrolysis temperatures compared to the combination suggested by BBD. Conditions beyond the BBD, including a sample weight of 800 µg and a carrier gas flow rate of 30 ml min-1, resulted in statistically different product distributions compared to the BBD. Notably, new products such as polycyclic aromatic hydrocarbons (PAHs) were identified. The findings from this work clearly show the importance of selecting appropriate pyrolysis parameters to obtain kinetically relevant experimental data, which in turn serves as a valuable guide for the development and validation of (micro-) kinetic models.