Thermo-oxidative degradation, reactions

The degradation and combustion behavior of polycarbonate/POSS hybrid system has been reported recently.48 Different loading contents of trisilanolphenyl-POSS (TPOSS) were melt blended with polycarbonate matrix (PC). The data shown in Table 8.4 indicate that no improvement in thermal stability parameters (i.e., onset decomposition temperature and peak decomposition temperature) was observed compared to the neat polycarbonate. The thermo-oxidative degradation process of the hybrid system proved to be a complicated process, which includes hydrolysis/alcoholysis of the carbonate linkage, free radical oxidative chain degradation, reformation, and branching and cross-linking reactions. 

Scheme 1. The major reactions of thermo oxidative degradation of P VC.

Table 1. Number average molecular weights (M ) of PVC as a function of reaction time in the absence and presence of BHT antioxidant during thermo-oxidative degradation (DOP, O2,200 C).

Degradation Reaction Mechanisms. A detailed discussion of a large number of reactions believed to take part in the thermo-oxidative degradation of PE at the tesq>eratures and oxygen concentrations encountered is given in ref. 2. In the following, only some main lines will be reviewed. 

FIG. 1. Scheme of the polymer (thermo-oxidative) degradation as a chain reaction. 

The thermo-oxidative degradation of the polyether chains is a classical chain reaction involving free radicals, characterised by the well known steps of initiation, propagation and termination [4, 9, 140] ... 

In the course of thermo-oxidative degradation of PAs stabilized with antioxidants, such as phenols and secondary aromatic amines and hindered nitroxy radicals chemiluminescence is observed. This effect is attributed to redox inhibition reactions. 

The two sequential processes that lead to the chemical degradation of a polymer due to oxidation are (1) oxygen diffusion and (2) chemical reaction [15], Researchers [16] have used the so-called unreacted core model to characterize and predict thermo-oxidative degradation in a composite laminate. According to this model, the composite weight loss due to oxidation, q can be expressed as a power law function of time ... 

This section deals not only with oxidative reactions of polymers at room temperature, but also with thermo-oxidative degradation at elevated temperatures. Although this type of degradation resembles photo-oxidative degradation, for the purpose of maintaining clarity, the latter is discussed separately in the next section. 

The electron beam irradiation is other type of energy that could produce degradation, in 2012 Tonny and Nemr reported the degradation of aciylonitrile/buta-diene/silica nanocomposite, the materials were electron beam irradiated at 25 and 100 kGy, finding that during the electron beam the thermal and thermo oxidative degradation are the principal reaction that occur confirmed by the fragments C-C of the polymer and volatiles as CO, CO2, CH4, H2O, etc. [69]. 

Polypropylene (PP) has wide acceptance for use in many application areas. However, low thermal resistance complicates its general practice. The new approach in thermal stabilization of PP is based on the synthesis of PP nanocomposites. This paper discusses new advances in the study of the thermo-oxidative degradation of PP nanocomposite. The observed results are interpreted by a proposed kinetic model, and the predominant role of the one-dimensional diffusion type reaction. According to the kinetic analysis, PP nanocomposites had superior thermal and fireproof behavior compared with neat PP. Evidently, the mechanism of nanocomposite flame retardancy is based on shielding role of high-performance carbonaceous-silicate char which insulates the underlying polymeric material and slows down the mass loss rate of decomposition products. 

A more sophisticated model (5), based on different reaction types, was chosen for PP-MAPP-Cloisite 20A thermo-oxidative degradation. The parameters are listed in Table 3. 

These results show that the second step in thermo-oxidative degradation of PP-MAPP-Cloisite 20A is described by one-dimensional diffusion (DO reaction type which is liable for the overall process of the carbonization in nanocomposite polypropylene structure. 

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