Polypropylene depolymerization

Upon thermal destruction of polyethylene the chain transfer reactions are predominant, but depolymerization proceeds to a much lesser extent. As a result, the products of destruction represent the polymeric chain fragments of different length, and monomeric ethylene is formed to the extent of 1-3% by mass of polyethylene. C—C bonds in polypropylene are less strong than in polyethylene because of the fact that each second carbon atom in the main chain is the tertiary one. 

Boric acid esters provide for thermal stabilization of low-pressure polyethylene to a variable degree (Table 7). The difference in efficiency derives from the nature of polyester. Boric acid esters of aliphatic diols and triols are less efficient than the aromatic ones. Among polyesters of aromatic diols and triols, polyesters of boric acid and pyrocatechol exhibit the highest efficiency. Boric acid polyesters provide inhibition of polyethylene thermal destruction following the radical-chain mechanism, are unsuitable for inhibition of polystyrene depolymerization following the molecular pattern and have little effect as inhibitors of polypropylene thermal destruction following the hydrogen-transfer mechanism. 

Earlier transition metals, as zirconium and hafnium, are still more active in hydrogenolysis, which allows zirconium hydrides to be used in depolymerization reactions (hydrogenolysis of polyethylene and polypropylene) [89], In this case, the zirconium hydride was supported on silica-alumina. Aluminum hydrides close to [(=SiO)3ZrH] sites would increase their electrophilicity and, thus, their catalytic activity. A catalyst prepared in this way was able to convert low-density polyethylene (MW 125000) into saturated oligomers (after 5h) or lower alkanes at 150°C (100% conversion). It was also able to cleave commercial isotactic polypropylene (MW 250000) under hydrogen at about 190 °C (40% of the starting polypropylene was converted into lower alkanes after 15 h of reaction). 

When the substituent R stabilizes radicals as in (A) and (C), chain scission is more likely than termination by coupling. Radicals (C) then propagate the depolymerization process with volatilization of polypropylene and polystyrene at a temperature at which these polymers would not give significant amounts of volatile products when heated alone. Moreover, unsaturated chain ends such as (B) would also initiate the volatilization process because of the thermal instability of carbon-carbon bonds in P position to a double bond (Equation 4.23). 

Reduction of bismuth compounds could take place by reaction with polymer radicals propagating the depolymerization of polypropylene, either by electron transfer or ligand transfer which are typical redox reactions between alkyl radicals and metal compounds 59... 

While condensation polymers such as PET and polyamides can be broken down into their monomer nnits by thermal depolymerization processes, vinyl (addition) polymers snch as polyethylene and polypropylene are very difficnlt to decompose to monomers. This is becanse of random scission of the carbon-carbon bonds of the polymer chains during thermal degradation, which prodnces a broad prodnct range. 

Early work on polyethylene and polypropylene has been reviewed by Madorsky[38] and Winslow and Hawkins [39]. Initiation by random scission or at weak links has been proposed. Since little monomer is evolved, chain depolymerization seems to be of minimal importance. The low molecular weight products formed are the result of inter- or intramolecular free radical transfer. 

Thermal processes are mainly used for the feedstock recycling of addition polymers whereas, as stated in Chapter 2, condensation polymers are preferably depolymerized by reaction with certain chemical agents. The present chapter will deal with the thermal decomposition of polyethylene, polypropylene, polystyrene and polyvinyl chloride, which are the main components of the plastic waste stream (see Chapter 1). Nevertheless, the thermal degradation of some condensation polymers will also be mentioned, because they can appear mixed with polyolefins and other addition polymers in the plastic waste stream. Both the thermal decomposition of individual plastics and of plastic mixtures will be discussed. Likewise, the thermal coprocessing of plastic wastes with other materials (e.g. coal and biomass) will be considered in this chapter. Finally, the thermal degradation of rubber wastes will also be reviewed because in recent years much research effort has been devoted to the recovery of valuable products by the pyrolysis of used tyres. 

It was observed that isotactic polypropylene decomposes thermally by a mechanism that varies at different temperatures and conditions [455]. Thus, at 340°C the major volatile product is propane, while at 380°C it is n-pentane, and at 420°C it is propylene. The propane is believed to originate from some weak spot in the polymeric chain. Formation of n-pentane involves a radical abstraction and a six-membered ring formation in a backbiting process. Propylene may come from a free-radical depolymerization process or a cyclic six-membered ring formation involving a terminal double bond [455]. 

Unlike polycondensation polymers, polymers of addition polymerization such as polyethylene and polypropylene when depolymerized in inert atmosphere (39) or in supercritical water (37) do not convert to just the monomer, but a homologous series of oligomers (alkanes and alkenes). Compared to pyrolysis in argon, for polyethylene, the portion of the lighter products increases in supercritical water depolymerizations conducted at 693 K and water densities of 0.13 and 0.42 g/cm. The 1-alkene to n-alkane ratio also increases in supercritical water and with density. These are shown in Figure 11. These results are attributed to the fact that in argon pyrolysis, the reaction proceed in the molten state of the polymer, whereas in supercritical water, some of degradation products... 

The origin of the talc ore used in polypropylene will have a significant influence on the color and long-term oven aging performance of the composite. Numerous tests have been performed to determine the variables in talc that contribute to this difference. It appears that talc surface chemistry can lead to talc-polymer and talc-stabilizer interactions that can cause discoloration and depolymerization under long-term heat (150-160°C) aging. High iron content in the talc can also adversely influence the level of performance. 

Irritant and allergic contact dermatitides from polyethylene and polypropylene are rare. Incompletely cured resins may cause contact dermatitis. It is most likely to be caused by added ingredients, such as catalysts and initiators. When sawing and grinding polyolefins, the heat may cause depolymerization and release chemicals, e.g., aldehydes, ketones, and acids, which might cause airborne contact dermatitis. Itching caused by the irritancy of heat-decomposed polyethylene plastics has been reported (Thestrup-Pedersen et al. 1989). 

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]. 

So far, various bio-based polymers have been developed, e.g., cellulose acetate, poly(alkylene succinate)s, starch-based blends, poly(3-hydroxy alkanoate)s (PHA) [1], poly(lactic acid) (PLA) [2], etc. Nowadays, some typical commodity plastics have also been S5mthesized from biomass, for example, polyethylene, polypropylene, poly(methyl methacrylate) [3], polyamide-4 [4], and polycarbonate [5]. If plastic materials are synthesized from renewable resources and circularly utilized with precise control of their depolymerization, an ideal recycling system could be constructed for plastic products, in which the resources and production energy could be minimized. Thus, the development of bio-based recyclable polymers is significant. In Scheme 9.1, t5 ical... 

In catalyst for depolymerizing polycaprolactone esters (2587), for esterification ( 4-58, 1766), for modifying polypropylene (269), for curing polyurethan rubber (539a), for synthesis of polyurethan (979). 

Recently, there have been great efforts to find catalysts which would lead to specific depolymerization. In this regard, polyethylene was depolymerized in the presence of NO, O2, and N2 (275 kPa NO, 690 kPa O2, and 3170kPa N2) to a mixture of benzoic acid, 4-nitrobenzoic acid, and 3-nitrobenzoic acid [32]. In an alternate work [33], zirconium hydride supported on silica alumina catalyst has been reported, which, in presence of hydrogen, cleaves the C—C bonds of polyethylene and polypropylene. The end products of the hydrogenolysis of these polymers have been diesel and lower alkanes and is still a subject of vigorous research. 

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