Polypropylene degradability

Polypropylene differs from polyethylene in its chemical reactivity because of the presence of tertiary carbon atoms occurring alternately on the chain backbone. Of particular significance is the susceptibility of the polymer to oxidation at elevated temperatures. Some estimate of the difference between the two polymers can be obtained from Figure 1J.7, which compares- the rates of oxygen uptake of eaeh polymer at 93°C. Substantial improvements can be made by the inclusion of antioxidants and such additives are used in all commercial compounds. Whereas polyethylene cross-links on oxidation, polypropylene degrades to form lower molecular weight products. Similar effects are noted... 

The catalytic degradation of polypropylene was carried out over ferrierite catalyst using a thermogravimetric analyzer as well as a fixed bed batch reactor. The activation of reaction was lowered by adding ferrierite catalyst, which was similar with that from ZSM-5. Ferrierite produced less gaseous products than HZSM-5, where the yields of i-butene and olefin over ferrierite were higher than that over HZSM-5. In the case of liquid product, main product over ferrierite is C5 hydrocarbon, while products were distributed over mainly C7-C9 over HZSM-5. Ferrierite showed excellent catalytic stability for polypropylene degradation. 

A simplification of stabilizer systems for polypropylene would be desirable. The ideal situation, of course, would be to have one inexpensive additive which could protect polypropylene to such a degree that a single resin could be used across the board, regardless of processing conditions or final application. This does not seem likely, however, because of the complexity of polypropylene degradation mechanisms which take place under various exposure conditions. 

Most reactive extrusion has dealt with polymers that can be difficult to handle by other techniques.68 Polymers can be cross-linked, grafted with other monomers, and so on. Extrusion cross-linking of polyethylene wire coating raises the maximum temperature of use. Polypropylene degrades in this process, but can be cross-linked successfully if mul-... 

Fig. 5 Polypropylene degradation during dissolution at high temperature, (a) Shift of MMD and (b) weight-average molecular mass (M ) versus dissolution time in TCB with 300 ppm BHT, with and without nitrogen purge...

How does commercial polypropylene degrade photo-oxidatively ... 

Canevarolo SV. Chain scission distribution function for polypropylene degradation during multiple extrusions. Polym Degrad Stab 2000 70 (l) 71-76. 

Pabedinkas, A. Cluett, W.R. Balke, S.T. (1989). Process control for polypropylene degradation during reactive extrusion. Polymer Engineering and Science, Vol.29, No.l5, (August 1989), pp. 993-1003, ISSN 1548-2634... 

Figure 5.14 shows the formation of carbonyl groups during irradiation of polyethylene and polypropylene films with cut-off filters (320 nm, 360 nm, 395 nm). Polypropylene degrades more rapidly than polyethylene because of its tertiary carbon atoms and the common mechanism of hydrogen peroxide formation [38]. 

Krupa I, Luyt AS. Thermal properties of isotactic polypropylene degraded with gamma irradiation. Polym Degrad Stab 2001 72 505-8. 

Pabedinskas, A., Cluett, W. R., Balke, S. T. Modeling of polypropylene degradation during reactive extrusion with implications for process control. Polym. Eng. Set. (1994) 34, pp. 598-612... 

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