Non-isothermal Thermogravimetric Degradation Kinetics, Reaction Models and Thermodynamic Parameters of Vinylidene Fluoride Based Fluorinated Polymers

Abstract

Commercial fluororubber SKF 32, fluoroplastic F-32L, fluoroelastomer Kel-F, fluoroplastic FK 800 (labeled as SKF 32, F-32L, Kel-F and FK 800) and an in-house prepared poly(vinylidene fluoride-chlorotrifluoroethylene) (FKM) copolymer were investigated in terms of their thermal degradation kinetics, reaction models and corresponding thermodynamic parameters. All samples underwent a single step thermal degradation using thermogravimetric analysis (TGA) at different heating rates under nitrogen atmosphere. The kinetic parameters were determined through the Kissinger method and three isoconversional methods; viz. the Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS) and Starink. The activation energies of the SKF 32, F-32L, Kel-F, FK 800 and FKM obtained using the Kissinger method were 206, 200, 185, 221 and 243 kJ mol‒1, respectively. The activation energy values for the degradation obtained using the KAS method were: 180–225 kJ mol−1 for SKF 32, 192–209 kJ mol−1 for F-32L, 163–185 kJ mol−1 for Kel-F, 213–227 kJ mol−1 for FK 800 and 187–269 kJ mol−1 for FKM with the extents of conversion (α) = 0.1–0.9. These values of the activation energies obtained from the KAS method were in good agreement with those obtained using the FWO and Starink methods. In addition, the appropriate degradation reaction models were determined by means of the Coats-Redfern and Criado methods. The thermodynamic parameters, such as activation Gibb free energy, ΔG*, activation enthalpy, ΔH*, and activation entropy, ΔS*, for formation of the activated complexes during the thermal degradation were also determined and discussed. The positive values of the Ea, ΔG*, ΔH*, and ΔS* for SKF 32, F-32L, FK 800 and FKM indicated a non-spontaneous process, while the positive values of the Ea, ΔG* and ΔH*, and negative value of ΔS* for Kel-F meant that the formation of the activated complex was accompanied by a smaller decrease of entropy than for the other polymers.