Polyamides pyrolysis

For chemical recycling of plastics refuse, a cascade of cycled-spheres reactors was developed which combined separation and decomposition of polymer mixtures by stepwise pyrolysis at moderate temps. In low-temp, pyrolysis, mixtures of PVC, PS and PE or PS, polyamide-... 

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]. 

Among the several kinds of polyamides composed of the large variety of acyclic and aromatic amino carboxylic acids or diamines and dicarboxylic acids, two Nylons are the most extensively applied in many fields. Nylon 6 and Nylon 6,6 are found in various waste streams, they may be present in pyrolysis recycling feeds as well. 

Amide group scission (termed as cA-elimination mechanism) and intramolecular rearrangement of two amide groups, leading to cyclic compounds [20] are the main pyrolysis reactions in acyclic polyamides. The former reaction is outlined in Scheme 12.1b and the ester exchange drawn in Scheme 12.2 is analogous to the latter one. 

H. Bockhom, S. Donner, M. Gemsbeck, A. Homung and U. Homung, Pyrolysis of polyamide 6 under catalytic conditions and its application to reutilization of carpets, J. Anal Appl Pyrol, 58-59, 79-94 (2001). 

Recently the pyrolysis of polymer mixtures has become a focus of interest due to the increasing role of plastics recycling. Many researchers have investigated the thermal decomposition of various polymers in the presence of PVC. Kniimann and Bockhom [25] have studied the decomposition of common polymers and concluded that a separation of plastic mixtures by temperature-controlled pyrolysis in recycling processes is possible. Czegfny et al. [31] observed that the dehydrochlorination of PVC is promoted by the presence of polyamides and polyacrylonitrile however, other vinyl polymers or polyolefins have no effect on the dehydrochlorination. PVC generally affects the decomposition of other polymers due to the catalytic effect of HCI released. Even a few per cent PVC has an effect on the decomposition of polyethylene (PE) [32], HCI appears to promote the initial chain scission of PE. Day et al. [33] reported that PVC can influence the extent of degradation and the pyrolysis product distribution of plastics used in the... 

For shredder light fractions, a mixture of polyolefins and polyurethanes containing as well as polyamides, PVC, polystyrene and blends, Wanzl et al. [23, 24] and Basel [25] have shown for the treatment of the pyrolysis gases from rotary kiln pyrolysis that by using dolomite beds at 500°C and a residence time of 20 s, chlorine content can be reduced from 1000 ppm to below the detection limit of 1 ppm. 

Other copolymers of polyamides include poly(glycols) sequences. Examples from this group are nylon 12-b/ock-poly(tetramethylene glycol) with the idealized formula -[NH-(CH2)ii-C(0)]x [-0-(CH2)4-O-]y and poly[(ethylene glycol)-co-1,6-hexanediamine-co-(methylpentamethylene diamine)-co-1,4-benzenedicarboxylic acid]. Pyrolysis of these copolymers generates a mixture of compounds, some typical for amides such as nitriles and some typical for polyethers. 

In addition to comonomers, nylons are frequently used in blends. The pyrolysis of blends typically shows little interaction between the compounds generated from the individual blend components. However, a study on the co-pyrolysis of several polyamides in the presence of PVC showed interactions [40]. The study was done on nylon-12, nylon-6,6 and poly(1,4-phenylene terephthalamide) (Kevlar) in the presence of poly(vinyl chloride). Polyamide-PVC mixtures (typical mass ratio 1 1) were pyrolyzed at 700 and 900°C. It was found that the presence of PVC promoted the hydrol ic decomposition routes of amide groups and volatile nitrile formation from all examined polyamides due to the hydrogen chloride eliminated from PVC under pyrolysis. In the presence of PVC, an elevated yield of alkenenitriles was observed from nylon-12. For Kevlar in the presence of PVC, it was noticed the evolution of benzeneamine, benzoic acid, benzenenitrile and benzeneisocyanate. At 900°C in the presence of PVC, an enhanced evolution of HCN from nylon-12 and nylon-6,6 was noticed. 

The chemistry of synthetic jasmine materials was given an enormous boost in the 1930s when Nylon 66 was launched as a product. Nylon 66 is a polyamide prepared using adipoyl chloride and hexamethylenetetramine as monomers. The 66 in the name refers to the fact that there are 6 carbons in each type of unit that lies between the amide links in the polymer chain. Thus, adipic acid is the key feedstock for Nylon 66 and the introduction of the latter meant that the former became a basic chemical commodity. Pyrolysis of the calcium or barium salt of adipic acid produces cyclopentanone, and so the availability of large quantities of the acid meant that the ketone could also be prepared at low cost. 

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. 

Additional information can be obtained from certain special tests. Thus, the Gibbs indophenol test is positive with PEEK, PAR, and PEI (see Section 6.1.3). Using the color reaction with jo-dimethylamino benzaldehyde (Section 6.1.2), one can differentiate HT-polyamides, such as PA 6-3-T, from polymers that develop phenolic decomposition products during pyrolysis. While with polyamides, the red coloration obtained after the addition of concentrated hydrochloric acid remains, with polycarbonates it turns blue. 

The products from the thermal degradation of Noryl GTX poly(phen-ylene oxide)-polyamide in air and in nitrogen have been identified and quantified. Ecotoxicologic testing of the products of pyrolysis with aquatic organisms indicated that in a fire, no greater harm than burned beech wood is to be expected when the fire-fighting water reaches aquatic ecosystems. ... 

Schulten, H. R., Plage, B., Ohtani, H., and Tsuge, S., Studies on the Thermal Degradation of Polyamides by Pyrolysis-Field Ionization Mass Spectrometry and Pyrolysis-Gas Ghromatography, Angew. Makromol. Chem., 155,1,1987. 

Ballistreri, A., Garozzo, D., Giuffrida, M., and Montaudo, G., Analysis of Polymers by Mass Spectrometry. Metastable Mapping of Pyrolysis Products of an Aromatic Polyamide,. Anal Appl Pyrolysis, 12, 3, 1987. 

Most of the pyrolysis FI/FD studies in the literature have been done in the Py-Fl-MS mode. An alternative approach is to place the polymer (from solution) directly on the field emitter, and then heat the wire to induce pyrolysis. Some of the earlier studies from Schulten s laboratop used this method (Py-FD-MS). Polymers examined were polyamides and a polyester. Derrick et al. reported the Py-FD-MS analysis of a poly(olefin sulfone). ... 

Bahr, U., Luederwald, I., Mueller, R., and Schulten, H.-R., Pyrolysis field desorption mass spectrometry of polymers. III. Aliphatic polyamides, Angew. Makromol. Chem., 120, 163, 1984. 

Recently, the thermal decomposition processes of various aUphatic-aromatic polyamides were investigated by Py-GC/MS and Py-MS using both Cl and El modes. The thermal decomposition of the polyamides of aromatic-diamine and aliphatic-dicarboxylic acid was strongly influenced by the structure of the aliphatic subunits. The formation of compounds with succinimide and amine end groups was observed in the pyrolysis of the polyamides containing succinic subunits via an intramolecular exchange and a concomitant N-H hydrogen transfer. 

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