Polypropylene oxidation activation energy

The similar concept regarding the stability of various structured polypropylenes, the activation energies required for the oxidative degradation depends evidently on the nucleation of substrates [74, 75]. For three sorts of polypropylene homopolymer (h-PP), random-copolymer (rc-PP) and impact-copolymer (ic-PP), the activation energies of photodegradation present a relevant sequence which particularizes the involvement of molecular packaging in the polymer volume ... 

Ozonization of the polypropylene powder creates the peroxidic species in the polymer, as well. The activation energy [41] of the thermal decomposition of these peroxides is 100 kJ/mol. In the decomposition of peroxides more than one type of radicals was trapped. Moreover, the three exotherms (peak at 40,90, and 130 °C) were observed on DSC thermograms of ozonized sample which also indicates the presence of several types of peroxides. Besides the peroxidic bonds in polymer, selective thermal decomposition may occur also with such bonds in the polymer as, e.g., with end groups containing the initiator moieties [42], This, however, takes place at higher temperatures than it corresponds to usual temperatures at which the thermo-oxidation starts. 

FIG. 18.3 Activation energy of diffusion as a function of Tg for 21 different polymers from low to high temperatures, ( ) odd numbers (O) even numbers 1. Silicone rubber 2. Butadiene rubber 3. Hydropol (hydrogenated polybutadiene = amorphous polyethylene) 4. Styrene/butadiene rubber 5. Natural rubber 6. Butadiene/acrylonitrile rubber (80/20) 7. Butyl rubber 8. Ethylene/propylene rubber 9. Chloro-prene rubber (neoprene) 10. Poly(oxy methylene) 11. Butadiene/acrylonitrile rubber (60/40) 12. Polypropylene 13. Methyl rubber 14. Poly(viny[ acetate) 15. Nylon-11 16. Poly(ethyl methacrylate) 17. Polyethylene terephthalate) 18. Poly(vinyl chloride) 19. Polystyrene 20. Poly (bisphenol A carbonate) 21. Poly(2,6 dimethyl-p.phenylene oxide). 

Amorphous and semi-crystalline polypropylene samples were pyrolyzed in He from 388°-438°C and in air from 240°-289°C. A novel interfaced pyrolysis gas chromatographic peak identification system was used to analyze the products on-the-fly the chemical structures of the products were determined also by mass spectrometry. Pyrolysis of polypropylene in He has activation energies of 5-1-56 kcal mol 1 and a first-order rate constant of JO 3 sec 1 at 414°C. The olefinic products observed can be rationalized by a mechanism involving intramolecular chain transfer processes of primary and secondary alkyl radicals, the latter being of greater importance. Oxidative pyrolysis of polypropylene has an activation energy of about 16 kcal mol 1 the first-order rate constant is about 5 X JO 3 sec 1 at 264°C. The main products aside from C02, H20, acetaldehyde, and hydrocarbons are ketones. A simple mechanistic scheme has been proposed involving C-C scissions of tertiary alkoxy radical accompanied by H transfer, which can account for most of the observed products. Similar processes for secondary alkoxy radicals seem to lead mainly to formaldehyde. Differences in pyrolysis product distributions reported here and by other workers may be attributed to the rapid removal of the products by the carrier gas in our experiments. 

For polypropylene at 130 °C, y = 0.1mm. At the higher temperatures in melt processing, y decreases, because the activation energy for oxidation is higher than that for diffusion. Consequently, the inside wall of a polyolefin pipe, exposed to air while the melt cools, only oxidises to a depth of about 10 pm if insufficient antioxidant is present. 

Schwartz and co-workers [97] used isothermal differential thermal analysis to study the diffusion of Irganox 1330 (1,3,5 tris (3,5 di-tor -butyl-4-hydroxyl benzyl) mesitylene) in extruded sheets of isotactic polypropylene (iPP). Studies were conducted over the temperature range 80-120 °C. The measurements showed a clear relation between oxidation induction time and oxidation maximum time [both determined by isothermal dynamic thermal analysis (DTA)] and the concentration of stabiliser. It was possible to calculate the diffusion coefficients and the activation energy of diffusion of Irganox 1330 in iPP by measuring the oxidation maximum times across stacks of iPP sheets. 

The accumulation of peroxides in the oxidation of atactic polypropylene was studied in [48, 49], and the effective activation energy of the formation of some of its oxidation products was determined. 

The absorption of oxygen by polyolefins for one of the investigated temperatures under comparable conditions is presented in Fig. 48. From the figure it is distinctly evident that the oxidation of polypropylene proceeds incomparably more rapidly than the oxidation of polyethylene and the copolymer. The values of the activation energy of the oxidation process were calculated on the basis of the kinetic curves of the absorption of ojgrgen at various temperatures. The activation energy proved to be equal to 21.8 kcal/mole for polypropylene, 30.8 kcal/mole for the copolymer, 31.9 kcal/mole for low-pressure polyethylene, and 32.7 kcal/mole for high-pressure polyethylene. This means that the tendency of the polyolefins toward oxidation decreases in the indicated sequence. 

Fig. 11 Modification in the activation energy of oxidation for different stages of degradation of polypropylene. The data were taken from [99C1].

Fig. 18 Activation energy of p-relaxation in (a) low-molecular weight glasses and (b) linear polymers vs the cohesion energy or cohesion energy of Kuhn statistical segment, respectively [86, 88,103]. (a) (1) Pentanol (2) isopropylbenzene (3) 5-methyl-3-heptanol (4) decalin (5) 1,1-diphenylpropane (6) diethyl phthalate (7) glycerol (8) 6>-terphenyl (9) hexamethyl disolox-ane (10) tetra-a-methylstyrene (11) pentastyrene. (b) (1) Polyethylene (2) polyisoprene (3) poly(dimethylsiloxane) (4) poly(diethylsiloxane) (5) poly(phenylene oxide) (6) poly(ethylene terephthalate) (7) polytetrafluoroethylene (8) polycarbonate (9) polyamide (10) polypropylene (11) polymethacrylate (12) poly(vinyl fluoride) (13) poly(vinyl acetate) (14) poly(vinyl chloride) (15) poly(vinyl alcohol) (16) poly(methyl methacrylate) (17) poly(diphenyl oxypheny-lene) (18) poly(butyl methacrylate) (19) polystyrene (20) polyacrylonitrile (21) poly(a-methylstyrene) (22) poly(cyclohexyl methacrylate) (23) polyimide I (24) polyimide II (25) poly(metaphenylene isophthalamide) (26) polyisobutylene...

Polyolefin melts under excessive loads are subject to mechanically and oxidatively activated chain cleavage. Polyolefins do not depolymerize. Polyethylene and polypropylene are relatively insensitive to purely thermal degradation, but react easily with oxygen and radicals. Under mild conditions and the presence of oxygen, oxidation is preferred to thermal degradation due its lower level of activation energy (oxidation PE approx. 96 kj/mol vs. thermal degradation PE approx. 264 kj/mol) [20]. 

The thermal resistance of non-stabilized polypropylene is lower than that of polyethylene. Polypropylene also emits more volatile components at lower temperatures in the form of saturated and unsaturated aliphatic hydrocarbons. Actual activation energy for strictly thermal degradation is lower than expected the available data vary between 124 and 260 kj/mol. Activation energy for oxidative degradation ranges from 65 to 85 kJ/mol [755], [756]. 

Because the chemiluminescence method is a tool frequently used to qualify the stabilizing effect in polypropylene of various hindered amine, the analysis of CL kinetic parameters (oxidation induction time, oxidation rate, activation energy of oxidation, CL intensity) could provide reliable information related to the photodegradation state of polypropylene modified with different HALS compounds [76-78]. 

The activation energy corresponds to the slope of the plot of the ln(k) as a function of temperature. Though our experimental protocol does not simply measure a rate constant for the oxidation reaction, we can still use this form by recognizing that the rate constant is directly proportional to the rate of a reaction and that the rate of a reaction can be defined as the reciprocal of the time taken to reach a specified point in that reaction. We will use the onset of oxidation as the specified point in the reaction and so will use the value for the reciprocal of the time until onset as our k value in the Arrhenius plot. Figure 2 illustrates the Arrhenius plot for the oxidation of polypropylene. From this plot, the activation energy of the oxidation reaction of the polypropylene under study is... 

The catalytic effect of various metals, metal oxides, and metal stearates on the thermo-oxidative degradation has been examined in the solid phase and in solution. In the latter case activity decreased in the order Cu, Mn, Fe, Cr, Co, Ni, Ti, Al, Zn, and V. The chemiluminiscence observed during thermo-oxidation has been interpreted as arising from the decomposition of ctketone hydroperoxides followed by oxidation of the polymer chain by secondary peroxy radicals. U.v. and fluorescence microscopy together with energy-dispersive. Y-ray analysis have been used to develop a model for the distribution of partially degraded polymer in isotactic polypropylene. ... 

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