A kinetic study of the isothermal degradation process of Lexan® using the conventional and Weibull kinetic analysis

The degradation process of commercial grade Lexan® was investigated by thermogravimetric technique under isothermal experimental conditions at four different operating temperatures: 375 °C, 387.5 °C, 400 °C and 425 °C. The kinetic triplet (E _a , A, f(α)) was determined using conventional and Weibull kinetic analysis. The applied kinetic procedure shows that the investigated degradation process can be described by two-parameter autocatalytic Šesták–Berggren (SB) reaction model. It was established that the degradation process of Lexan® can be described by the following kinetic triplet: E _a = 158.3 kJ mol⁻¹, A = 8.80 × 10⁹ min⁻¹ and f(α) = α ^0.33 (1 − α)^1.62. It was established that the operating temperature has an influence on the values of SB reaction orders (m and n) (0.27 < m < 0.36 and 1.52 < n < 1.75). It was concluded that the apparent activation energy value of 158.3 kJ mol⁻¹, represent the composite value from a complex degradation reaction and can not compare with the dissociation energy of the weak bonds in bisphenol-A polycarbonate. Also, it was concluded that the Weibull shape parameter (β) shows that the considered process occurs under the same reaction mechanism, independently on operating temperature (T), i.e. the change of rate-limiting step does not occur (β < 1). In addition, it was established that the operating temperature has an influence on the basic characteristics of theoretically calculated density distribution functions (ddf) of apparent activation energies for considered degradation process. On the other hand, it was shown that the experimentally evaluated density distribution function of apparent activation energies represents the intermediate case between the calculated density distribution functions at 375 °C and 425 °C.