Pyrolysis of polypropylene

L. Ballice and R. Reimerta, Classification of volatile products from the temperature-programmed pyrolysis of polypropylene (PP), atactic-polypropylene (APP) and thermogravimetrically derived kinetics of pyrolysis. Chem. Eng. Process., 41(4), 289-296 (2002). 

A. Marcilla, A. Gomez, J. A. Reyes-Labarta and A. Giner, Catalytic pyrolysis of polypropylene using MCM-41 kinetic model. Polymer Degradation and Stability, 80, 233-240 (2003). 

The carbonization of polypropylene is similar to that of polyethylene. During slow pyrolysis of polypropylene and for a temperature increase from 400 to 700°C, the yield in the liquid phase remains higher than 80% with a very small increase in the yield of gas phase (less than 20%). On the other hand, in flash pyrolysis of polypropylene, an increase of temperature from 550 to 700°C leads to a decrease of the yield in the liquid phase down to 40% with an increase of the gas phase up to 60%. As presented for PE (see Section 2.1), Sawagushi et al. [21] present the results of steam gasification of PP in a fixed bed reactor (Figure 10.8). 

D. Seth and A. Sarkar, Thermal pyrolysis of polypropylene effect of reflux-condenser on the molecular weight distribution of products. Chemical Engineering Science, 59, 2433 (2004). 

The pyrolysis of polypropylene gives similar results to the pyrolysis of polyethylene (Table 17.3). The amount of methane and oil is slightly higher, the amount of aliphatics is lower. The feedstock recycling of polyolefins is easy. Up to 50 wt% can be obtained as aromatics if the pyrolysis gas is cycled and used as fluidizing gas. The other 50% are gas components. Benzene and toluene reach 25 wt%. 

Table 17.3 Pyrolysis of polypropylene (PP) and mixtures of PP and PE in the pilot plant (TWSl) using pyrolysis gas as fluidized gas. Products in wt%...

Figure 6.1.15. The yield of fragment molecules with a specific number of atoms as a function of equilibrium temperature (Teq) during pyrolysis of polypropylene.

Since the (4R,6R) tetramer and (4S,6S) tetramer are enantiomers, they are not separated by conventional chromatographic columns. The same is true for the (4S,6R) and (4R,6S) stereoisomers. Based on the formation of these stereoisomers, analytical pyrolysis of polypropylene is able to differentiate between isotactic and syndiotactic polymers. One fragment molecule that can be used for this purpose is the tetramer. However, other fragments from the two polymers also are diastereoisomers and, for this reason, the pyrograms of isotactic and syndiotactic polypropylene show differences. 

The results of pyrolysis of polypropylene in air depends on the pyrolysis heating rate because the pyrolysis process competes with the oxidation [108], By heating between 120° C and 280° C in air, polypropylene is reported to generate ethene, ethane, propene, propane, isobutene, butane, isobutane, pentadiene, 2-methyl-1-pentene, 2,4-dimethyl-1-pentene, 5-methyl-1-heptene, dimethylbenzene, methanol, ethanol, 2-methyl-2-propene-1-ol, 2-methylfuran, 2,5-dimethylfuran, formaldehyde, acetaldehyde, acrolein, propanal, methacrolein, 2-methylpropanal, butanal, 2-vinylcrotonaldehyde, 3-methylpentanal, 3-methylhexanal, octanal, nonanal, decanal, ethenone, acetone, 3-buten-2-one, 2-butanone, 1-hydroxy-2-propanone, 1-cyclopropylethanone, 3-methyl-2-buten-2-one, 3-penten-2-one, 2-pentanone, 2,3-butanedione [109]. 

Pyrolysis of polyethylene and polypropylene for 1 s at 750°C gives mixtures of ethylene, propylene, and butenes in 91-92% yields.194 Pyrolysis of polypropylene over HY zeolite gives a mixture of C4-C9 hydrocarbons, mostly... 

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. 

Pyrolysis of Polypropylene. For the measurement of rates of pyrolysis, the temperature range is limited to that of conveniently measurable rates. Figure 2 shows the thermograms of APP and IPP which provides the choice of temperature for pyrolysis. 

Pyrolysis of Polypropylene. There have been several published studies of pyrolysis of polypropylene. They are more in discord than in... 

Even though the pyrolysis of polypropylene is mechanistically complicated (vide infra), the kinetics is first order because the rate determining step is the homolysis of the C—C bond describable by a well-defined rate constant. We found the activation energy for pyrolysis to be 51 and 56 kcal mol"1, respectively, for IPP and APP. Other literature values are 60 kcal mol"1 measured at 350°-400°C by Wall and Straus (10), 58 kcal mol 1 in the range 336°-366°C reported by Madorsky and Straus (12) and 55 kcal mol"1 found by Moissev et al. (13) in the temperature range 320°-420°C. 

Table V. Kinetic Results for Oxidative Pyrolysis of Polypropylene...

In order to reconcile to some degree the results cited here, the mechanism for pyrolysis of polypropylene needs to be considered. The mechanism commonly accepted is based on those proposed for the gas-phase degradation of simple hydrocarbons (19, 20). 

For polymers with low ceiling temperature, such as poly (a-methylsty-rene) and poly (methylmethacrylate), unzipping is the predominant degradative process. However, this is unimportant for pyrolysis of polypropylene at low temperature. Unzipping becomes more important above 800°C but still is not the dominant reaction. 

We are not aware of any significant study of autoxidation of polypropylene at elevated temperatures, i.e. oxidative pyrolysis. On the other hand, much is known about gas-phase oxidation of hydrocarbons at high temperatures and the cool-flame limit (25,26,27,28). The reactions are recognized as free radical chain reactions propagated by peroxy radicals and hydroperoxides which was essentially a development of Backstrom s scheme for the oxidation of aldehydes (29). These mechanisms may be adapted to the oxidative pyrolysis of polypropylene. 

Fig. 13.3 Comparison of gas chromatograms of oil obtained with AICI3 (1) or TiCVAlClj as catalysts for the pyrolysis of polypropylene. Retention time is in minutes [31]...

Figure 15.30. Thermal decomposition and heat of pyrolysis of polypropylene in nitrogen.

Zhang JH, Li J, Cao J, Qian YT (2008) Synthesis and characterization of larger diameter carbon nanotubes from catalytic pyrolysis of polypropylene. Mat Lett 62(12-13) 1839-1842... 

Thermogravimetric data were obtained for the pyrolysis of polypropylene, and several previously published... 

The isobutane and especially the methane peaks are rather small, indicating that this reaction is not very frequent for the primary radical. For this radical, hydrogen transfer with the sixth carbon atom might he more important, as the 2-methylpentane peak could be explained in this way. Another possibility is that this peak has to be ascribed to intramolecular hydrogen transfer reactions (i.e., formation of 2-methylpentadiene). Table 6.1 lists the products expected from intramolecular hydrogen transfer during the pyrolysis of polypropylene. 

Table 6.1 Products expected from intramolecular hydrogen transfer during pyrolysis of polypropylene (main peaks found in program are underlined)...

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