Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 198

A study of the kinetics of thermo-oxidative degradation may involve some
problems specific to the process of oxidation. For example, the initial stages of
thermo-oxidative degradation may be accompanied by a mass gain due to some ac-
cumulation of the oxidation products. This typically occurs when thermo-oxidative
degradation is studied at very slow heating rates and can be avoided by ramping
temperature at a faster rate [ 72 ]. Another problem is securing the saturation of the
polymer sample with oxygen, which is not difficult to accomplish by using very thin
polymer samples of low mass under large excess of oxygen [ 73 ].
Typically, the presence of oxygen accelerates the thermal degradation of poly-
mers. Relative to regular thermal degradation in an inert atmosphere, thermo-oxida-
tive degradation can start at 100-200 ﾰC lower temperature. The dramatic decrease
in thermal stability under oxidative conditions is reflected in a dramatic decrease
in the activation energy of the process. Examples of the application of an isocon-
versional method to thermo-oxidative degradation of several polymers are given
in Fig. 4.27 . The respective E ʱ dependencies show little variation with the extent
of conversion. For PS, PE, and PP, the mean E ʱ values fall within the range 80-
120 kJ mol − 1 . These values are consistent with activation energies of decomposition
of organic hydroperoxides. As discussed earlier (Sect. 4.3.1), this process usually
plays the role of the rate-limiting step in the thermo-oxidative degradation of many
polymers.
Of course, not all thermo-oxidative degradations have the same rate-limiting step.
For example, as seen in Fig. 4.27 , PMMA has markedly larger activation energy of
thermo-oxidative degradation than PP, PS, and PE. This is not surprising because
oxygen has a very different effect on PMMA than on other polymers. It actually
stabilizes PMMA. In the presence of oxygen, thermal degradation of PMMA starts
at temperature ~ 50 ﾰC higher than under inert atmosphere [ 71 ]. The decelerating ef-
fect of oxygen is illustrated in Fig. 4.28 that presents mass loss data measured while
periodically switching the gas atmosphere between air and nitrogen. It is seen that
a switch from air to nitrogen causes an increase in the slope of the mass loss curve

Fig. 4.27 Dependencies
of the activation energy
on conversion for thermo-
oxidative degradation of
several polymers. (Adapted
from Peterson et al. [ 65 ]
and Peterson et al. [ 71 ] with
respective permission of
Wiley and ACS)

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