Pyrolysis of polyamides

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). 

The products obtained in the pyrolysis of polyamides have been investigated by many authors. The experimental data are frequently contradictory, and hence the principal schemes of the reactions that occur, proposed by various researchers, also differ from one another. 

These reactions explain the formation of CO and CO2 in the pyrolysis of polyamides however, the amount of carbon dioxide obtained considerably exceeds the number of carboxyl groups in the initial polyamide. 

Hydrolysis of the amide bonds on account of the water formed as a result of the last reaction can explain the large amounts of CO2 and NH3 obtained in the pyrolysis of polyamides. 

At typical pyrolysis temperatures the isocyanate groups produced in pyrolysis of polyamides and polyurethanes are not stable and readily decompose to jdeld nitrile groups. 

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. 

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... 

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. 

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. 

Sthatheropoulos, M., Georgakopoulos, C. G., and Montaudo, G., A Method for the Interpretation of Pyrolysis Mass Spectra of Polyamides, /. Anal Appl Pyrolysis, 20, 15, 1992. 

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. 

In polyimides (PI) the imide bond is initially subject to thermal conversions. The presence in the chain not only of the imide bond but also of a definite number of amide bonds, the existence of which is caused by the kinetic arrest of the cyclodehydration reaction of polyamide acids in the solid phase, impedes the analysis of PI degradation [18]. Therefore, the role of each type of bond during the pyrolysis of PI should he estimated by thorough analysis of the composition and properties of the decomposition... 

Straka, R Nahunkova, J. Brokova, Z. Kinetic pyrolysis of coal with polyamide-6. J. Anal AppL Pyrol. 2004, 71, 213-219. 

A detailed analysis of the products of thermal destruction of poly-hexamethyleneadipamide and polycaproamide is given in [22]. The polyamides were heated at 300-305°C in a stream of dry nitrogen. The gaseous pyrolysis products contain large amounts of NH3, CO2, H2O, small amounts of n-hexylamine, n-pentylamine, and cyclopentanone. Analysis of the hydrolysis products of the residue obtained after pyrolysis of the polyamides permitted the authors to propose a scheme for the process. 

J. Zhang, M. Delichatsios, and S. Bourbigot, Experimental and numerical study of the effects of nanoparticles on pyrolysis of a polyamide 6 (PA6) nanocomposite in the cone calorimeter. Combustion and Flame, 156 (2009), 2056-62. 

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