Isothermal fast pyrolysis kinetics of synthetic polymers using analytical Pyroprobe

摘要

In this study, the global kinetics of fast pyrolysis of three synthetic polymers, viz., poly(methyl methacrylate) (PMMA), polystyrene (PS) and ultra-high molecular weight polyethylene (UHMWPE), were studied in an analytical Pyroprobe (R) by collecting isothermal mass loss data at short timescales of 2-150 seconds. The apparent activation energy and pre-exponential factor were determined using integral reaction models. PMMA fast pyrolysis was best described by the contracting cylinder model, while sigmoidal Avrami-Erofeev model was used to describe the fast pyrolysis of PS and UHMWPE. The apparent activation energies (in kJ mol(-1)) of fast pyrolytic decomposition of PMMA, PS and UHMWPE were 43.7, 52.6 and 48.4, respectively. The low values indicate that fast pyrolysis is a diffusion-limited process in the Pyroprobe (R) reactor. Statistical kinetic compensation effect, i.e. ln(A) = a*E-a+ b, was established for the three polymers by considering a large number of experimental data from the literature. In order to understand the time evolution of pyrolysate functional groups at short timescales, Pyroprobe (R) was coupled with an in situ Fourier transform infrared spectrometer (FTIR). The time taken for maximum evolution of vapors was 12-18 s and 35-45 s for PMMA and PS fast pyrolysis, respectively, at 500 degrees C, while it was 20-22 s for UHMWPE at 600 degrees C. The Pyroprobe (R) was also interfaced with gas chromatograph/mass spectrometer (GC/MS), and the major pyrolysates at 500 degrees C from PMMA, PS and UHMWPE were identified as methyl methacrylate, styrene and a range of C5-C21 alkanes and alkenes, respectively.