Polypropylene heterogeneous oxidation

G. George and M. Celina, Homogeneous and heterogeneous oxidation of polypropylene. In S. Halim Hamid and M. Dekker (Eds.), Handbook of Polymer Degradation, Second Ed., Inc., M. Dekker, New York, p. 277. 

It has been shown for non-stahilized polypropylene that oxidation takes place heterogeneously and spreads out beginning at easily oxidizable weak spots in the polymer. Figure 1.15. 

Figure 4.46 shows the influence of various processing temperatures on the heterogeneous oxidation of polypropylene. Caused by the higher mass temperature, more pronounced embrittlement than at a lower mass temperature is detected after four weeks of aging at 140 °C. 

Goss, B.G. S., Nakatani, H. et al. Catalyst residue effects on the heterogeneous oxidation of polypropylene. Polymer Degradation and Stability, 82 (2003), p. 119-126... 

The first report of the copolymerization of an epoxide, namely, ethylene oxide and C02 is contained in a patent by Stevens [6]. However, this process, when carried out in the presence of polyhydric phenols, provided polymers which were viscous liquids or waxes possessing copious polyether linkages with only a few incorporated C02 units. The earliest metal-catalyzed copolymerization of epoxides and C02 was reported in 1969 by Inoue and coworkers, who employed a heterogeneous catalyst system derived from a 1 1 mixture of diethylzinc and H20 [7, 8], Subsequently, Kuran and coworkers investigated a group of related catalysts prepared from diethylzinc and di- and triprotic sources such as pyrogallol, with a slight improvement over Inoue s system for the production of polypropylene carbonate) from PO and C02 [9],... 

M. Celina and G.A. George, Heterogeneous and homogeneous kinetic analysis of the thermal oxidation of polypropylene, Polym. Degrad. Stab. 1995, 50, 89-99. 

In spite of some imeertainties in the individual steps of the HAS meehanism in polymer stabilization due to the speerfie effeets of the polymer matrix and the environmental stress, the HAS-based nitroxides are eonsidered the key intermediate in the HAS reaetivity meehanism. Detection and quantification of the formed nitroxides using ESRI spectroscopic technique has been exploited for confirmation of the primary transformation step in HAS mechanism [15, 16, 20], as a consequence of interactions of HAS with oxygenated radical and molecular products of polyolefins (5). Monitoring of the nitroxide development enables tracing of the oxidation process within the polymer matrix. Consequently it is also a tool for marking the heterogeneity of the oxidative transformation of semicrystalline carbon chain polymers [polypropylene (PP), polyethylenes (PE)] or amorphous polymers [copolymers of ethylene with norbomene, polystyrene (PS), high impart polystyrene (HIPS), acrylonitrile-butadiene-styrene polymer (ABS)]. 

ESR techniques use nitroxyl radicals either dispersed in polymer matrix (spin probe) or covalently bonded to polymer chain (spin label) which are sensitive to the environment allowing molecular motion and microstructure of polymers to be identified from spectra (103). Quantitative methods of heterogeneous ESR spectra are divided into (J) outer hyperfine etrema, (2) signal intensities related to the relative concentration of the probe in different phases, and (3) simulation of the spectra. The presence of two well-separated outer maxima above the glass-transition temperature could be ascribed to two phases in natural rubber (104), miscible blends (105), immiscible blends (106), cross-linked polymers (107), and polyurethanes (108). ESR has used the measurement of the oxidation product to monitor the consumption of stabilizer in polypropylene (109). 

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