The torque-time curves of polypropylene (PP) powder treated under various thermooxidative degradation conditions were obtained through processing in the mixing chamber of a rheometer. Meanwhile, the Fourier transform infrared (FTIR) spectra of the corresponding samples were determined, and the quantitative analysis of the carbonyl indices of the FTIR spectra of the samples of the PP powder was carried out to provide evidence for the rheological characterization. PP granules, to which an antideteriorant was added before they were commercially supplied so good antidegradation could be achieved, was investigated for the sake of contrast. The analysis of the experimental results showed that the height of the torquetime curve of the PP powder and the corresponding value of the equilibrium torque could be used to characterize or evaluate the variations of the thermooxidative degradation of the PP powder. Under the same processing conditions, the heights of the torque-time curves of the PP powder and the corresponding values of the equilibrium torque decreased with the enhancement of the thermooxidative degradation treatment before mixing; on the contrary, the heights and areas of the characteristic bands of the carbonyl groups in the FTIR spectra of the PP powder and the corresponding values of the carbonyl index increased. The quantitative analysis of the FTIR spectra provided evidence for the conclusion that the heights of the torquetime curves of the PP powder and the corresponding values of the equilibrium torque could be used to characterize or evaluate the thermooxidative degradation of the PP powder. If the treatment under thermooxidative degradation conditions weakened or the degradation of the PP powder just began (i.e., in the viscosity range for processability), the evaluation method using the heights of the torque-time curves of the PP powder or the corresponding values of the equilibrium torque could provide more sensitivity than the method using the values of the carbonyl index. Consequently, the method using the heights of the torque-time curves to evaluate the thermooxidative degradation of the PP powder had its advantages. The application of the torque-time curves could be used to evaluate not only the variation of the thermooxidative degradation of the PP powder treated under aging conditions before mixing but also the variation of the degradation, including the mechanochemical degradation, of the PP powder during the period of mixing. The dependence of the variation of the degradation of the polymer on the processing time during mixing could be evaluated by the study of the variation of the torque-time curves. It can be concluded that the application of torque-time curves to the evaluation of degradation of PP powder has the advantages of being convenient, real-time, in situ, online, and production-oriented.

A real-time study to the evolution of the mechanisms of degradation of both PP powder and PP granular during the mixing process was achieved. A quantitative analysis of combination of the rheological torque-time curves obtained by a Haake rheometer with FTIR spectra was made. The construction of a series of the characteristic functions of the torque-time curve and the FTIR characteristic functions for polypropylene (PP) made the real time and quantitative analysis relatively profound. By means of these characteristic functions, the characters of the mechanisms of thermooxidative degradation, and the mechanism of mechanochemical degradation for PP were investigated. Two criterions for the mechanism of thermooxidative degradation and the mechanism of mechanochemical degradation were obtained. In the thermooxidative treatment process and the mixing process, the necessary condition for the significant occurrence of thermooxidative degradation of PP is Dh r (i) > 0.05, whereas the sufficient condition for the significant occurrence of mechanochemical degradation is Dh r (i) 0.05 (D l (i) < 0, and i > 1). A detailed description of the evolutions of the degradations for both the PP granules in which an antideterionant was added and the PP powder without any antideterionant was made. The roles and the evolutions of both the mechanism of thermooxidative degradation and the mechanism of mechanochemical degradation were discussed.