Pyrolysis of polyesters

Figure 11.7 Fourier transform infrared spectra of the oils/waxes derived from the pyrolysis of polyester resin (a), phenolic resin (b) and epoxy resin (c)...

Thermoset plastics have also been pyrolysed with a view to obtain chemicals for recycling into the petrochemical industry. Pyrolysis of a polyester/styrene copolymer resin composite produced a wax which consisted of 96 wt% of phthalic anhydride and an oil composed of 26 wt% styrene. The phthalic anhydride is used as a modifying agent in polyester resin manufacture and can also be used as a cross-linking agent for epoxy resins. Phthalic anhydride is a characteristic early degradation product of unsaturated thermoset polyesters derived from orf/io-phthalic acid [56, 57]. Kaminsky et al. [9] investigated the pyrolysis of polyester at 768°C in a fiuidized-bed reactor and reported 18.1 wt% conversion to benzene. 

Xylenes. The main appHcation of xylene isomers, primarily p- and 0-xylenes, is in the manufacture of plasticizers and polyester fibers and resins. Demands for xylene isomers and other aromatics such as benzene have steadily been increasing over the last two decades. The major source of xylenes is the catalytic reforming of naphtha and the pyrolysis of naphtha and gas oils. A significant amount of toluene and Cg aromatics, which have lower petrochemical value, is also produced by these processes. More valuable p- or 0-xylene isomers can be manufactured from these low value aromatics in a process complex consisting of transalkylation, eg, the Tatoray process and Mobil s toluene disproportionation (M lDP) and selective toluene disproportionation (MSTDP) processes isomerization, eg, the UOP Isomar process (88) and Mobil s high temperature isomerization (MHTI), low pressure isomerization (MLPI), and vapor-phase isomerization (MVPI) processes (89) and xylene isomer separation, eg, the UOP Parex process (90). 

In addition to the pathways depicted above, a 4-center concerted mechanism yielding ketenes has been reported during die vacuum pyrolysis of aliphatic polyesters (Scheme 2.4).89,90... 

A pyrolysis technique was investigated as a method for the chemical recycling of glass fibre-reinforced unsaturated polyester SMC composites. The proeess yielded liquid products and gases and also a solid residue formed in the pyrolysis of glass fibres and fillers. The solid residue was used as a reinforeement/filler in unsaturated polyester BMC composites, and the influenee on mechanical properties was studied in comparison with BMC prepared entirely from virgin materials. 

Y. Ishida, H. Othani and S. Tsuge, Effects of solvents and inorganic salts on the reactive pyrolysis of aromatic polyester in the presence of tetramethylammonium hydroxide studied hy pyrolysis gas chromatography/mass spectrometry, J. Anal. Appl. Pyrol., 33, 167 180 (1995). 

The /3-lactone dimer of dimethylketene can be prepared by pyrolysis of its polyester, which is formed by the base-catalyzed polymerization of dimethylketene.3"6 In addition to the rearrangement of the normal dimer described above,6 the direct dimerization of dimethylketene in the presence of aluminum chloride3 or trialkyl phosphites7 leads to the /3-lactone dimer. 

The color of the polymer can also be affected by inappropriate reaction conditions in the polymerization process, such as temperature, residence time, deposits of degraded polymer or the presence of oxygen. Degradation of polyesters and the generation of chromophores are thermally effected [29b, 29c, 39], The mechanism of thermal decomposition is based on the pyrolysis of esters and the formation of unsaturated compounds, which can then polymerize into colored products. It can be assumed that the discoloration takes place via polymerization of the vinyl ester end groups or by further reaction of AA to polyene aldehydes. 

In conclusion, we would like to mention that, in addition to this new direction, a large consumer of metal alkoxides (initially aluminium and titanium) is by tradition the technology of materials, where the alkoxides are used for hy-drophobization and for cross-linking of the polyhydroxocompounds, epoxides and polyester resins, and organosilicon polymers. The products of the partial hydrolysis and pyrolysis of alkoxides — polyorganometalloxanes — are applied as components of the thermally stable coatings [48J. 

Because of the ease of formation of these flammable pyrolysis products, polyesters have LOI values of 20-22 vol% (see Table 2.4), and hence, burn readily and because of the styrene content, give heavy soot formation. As these resins are cured at room temperature, bromine-containing flame retardants, which would decompose in melt-processed, thermoplastic polymers, may be effectively used. 

Research on the pyrolysis of thermoset plastics is less common than thermoplastic pyrolysis research. Thermosets are most often used in composite materials which contain many different components, mainly fibre reinforcement, fillers and the thermoset or polymer, which is the matrix or continuous phase. There has been interest in the application of the technology of pyrolysis to recycle composite plastics [25, 26]. Product yields of gas, oil/wax and char are complicated and misleading because of the wide variety of formulations used in the production of the composite. For example, a high amount of filler and fibre reinforcement results in a high solid residue and inevitably a reduced gas and oiFwax yield. Similarly, in many cases, the polymeric resin is a mixture of different thermosets and thermoplastics and for real-world samples, the formulation is proprietary information. Table 11.4 shows the product yield for the pyrolysis of polyurethane, polyester, polyamide and polycarbonate in a fluidized-bed pyrolysis reactor [9]. 

S. J. Evans, P. J. Haines and G. A. Skinner, Pyrolysis-gas-chromatographic study of a series of polyester thermosets. J. Anal. Appl. Pyrolysis, 55, 13-58, (2000). 

Among polyesters synthesized from 1,4-benzenedicarboxylic acid and aliphatic diols, poly (ethylene terephthalate) (PET) and poly (butylene terephthalate) (PBT) are the most frequently applied ones. Hydrolysis is evidently the easiest chemical recycling technique of polyesters, however they may be mixed with other waste plastics, thus it is useful to know the properties of their pyrolysis product. 

The fuel properties of the pyrolysis oil of an aliphatic polyether-segmented PU could be predicted by taking into account the linear and cyclic oxygenated saturated and olefinic hydrocarbon nature of the components. Elimination of the reactive and toxic diisocyanate from the pyrolysis oil of PU in a lower temperature pyrolysis step is easier in the case of polyether soft segment, because of the larger temperature difference of decomposition steps compared to that of polyester-segmented PU. 

The mechanism of formation of methyl esters from polyesters can be simpler, not involving the formation of an intermediate compound. Pyrolysis of ester type polymers in the presence of quaternary ammonium hydroxides such as TMAH may occur by the following simple steps a) the polymer when mixed with TMAH and heated to temperatures around 400° C undergoes hydrolysis because of the strong basic character of the reagent, b) TMAH salts of the hydrolyzed products are formed, c) the salts undergo thermal fragmentation to the methyl derivatives [40]. 

Another polyester with aromatic groups in its structure is poly(diallyl isophthalate), CAS 25035-78-3. This polymer has a crosslinked structure and its pyrolysis products were discussed in Section 3.1. The polymer used for the Py-GC/MS experiment described in Section 3.1 had M = 500,000, and the experimental conditions were similar to those for other examples described in this book (600° C pyrolysis in He and Carbowax column separation). The main pyrolysis product was found to be 1,4-benzendicarboxylic acid di-2-propenyl ester. This compound is the homolog of terephthalic acid dibutylene ester formed in the pyrolysis of PBT. 

Thermal decomposition of polyester copolymers is of considerable interest because of their common use in practice. A number of reports are available in literature describing either pyrolysis or slow thermal decomposition of polyester copolymers [57-60], etc. Some of these reports are summarized in Table 10.1.10. 

Studies on the bulk pyrolysis of polyhydroxybutyric acid from Bacilli and of bacterial polyalkanoates have shown the formation of 2,3-butenoic acid and 2,3-pentenoic acid (18) The presence of 2,3-butenoic acids and pentenoic acid in the pyrolysate of the particulate matter from sample 20C is interpreted as an indication of polyhydroxy-alkanoates in the sample. These mixed polyesters of hydroxy acids with 4, 5 and sometimes 6 carbon atoms are especially abundant in activated sludges (19). The occurrence of m/z 86 and 100 as abundant mass peaks in the spectra of the fluvial material and as very characteristic peaks in the discriminant function spectrum indicates that a significant amount of the mud fraction may consist of sewage debris. This impression was confirmed by identification of a number of other pyrolysis products in the data file. 

Kaminsky90,92 has reported the product distribution obtained in the fluidized bed pyrolysis of different condensation polymers (polyesters, polyurethanes, polyamides, etc.). Polyester degradation led to 51% of gases, with a high proportion of CO and C02, and 40% of oil rich in benzene, toluene and naphthalene, the formation of water also being detected. On the other hand, polyurethane and polyamide decomposition led to the formation of about 40% gases and 55% oil. In both cases, the gases obtained contained certain amounts of HCN. 

In a similar maimer, thermal degradation mechanisms of fully aromatic polyesters were investigated in detail by Py-GC/MS using a partially deuterated polymer sample. Moreover, the mechanisms of reactive pyrolysis of the same aromatic polyester in the presence of tetramethylammonium hydroxide (TMAH) was also studied by Py-GC/MS. In this case, the contribution of solvent (methanol) to the reaction was confirmed by use of a deuterated methanol solution of TMAH. 

Ishida, Y, Ohtani, H., and Tsuge, S., Effects of Solvents and Salts on the Reactive Pyrolysis of Aromatic Polyester in the Presence of Tetramethylammonium Hydroxide Studied by Pyrolysis-Gas Chromatography/Mass Sprectrometry /. Anal Appl. Pyrolysis, 33, 167,1995. 

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