Nylons pyrolysis

Ammonia is used in the fibers and plastic industry as the source of nitrogen for the production of caprolactam, the monomer for nylon 6. Oxidation of propylene with ammonia gives acrylonitrile (qv), used for the manufacture of acryHc fibers, resins, and elastomers. Hexamethylenetetramine (HMTA), produced from ammonia and formaldehyde, is used in the manufacture of phenoHc thermosetting resins (see Phenolic resins). Toluene 2,4-cHisocyanate (TDI), employed in the production of polyurethane foam, indirectly consumes ammonia because nitric acid is a raw material in the TDI manufacturing process (see Amines Isocyanates). Urea, which is produced from ammonia, is used in the manufacture of urea—formaldehyde synthetic resins (see Amino resins). Melamine is produced by polymerization of dicyanodiamine and high pressure, high temperature pyrolysis of urea, both in the presence of ammonia (see Cyanamides). 

The use is described of a process involving both hydrolysis and pyrolysis to recover caprolactam from nylon 6 used in carpet fibres. By means of precise temperature control and the use of a catalyst, nylon 6 can be isolated from the PP backing. The process has been developed by the National Renewable Resource Laboratory, and interest has been shown by AlliedSignal who are considering a cooperative research and development project. 

Levin, B.C., A summary of the NBS litterature Reviews on the chemical nature and toxicity of the pyrolysis and combustion from seven plastics acrylonitrite-butadien-styrenes (ABS), nylons, polyesters, polyetylenes, polysterenes, poly(vinyl-chlorides) and rigid polyurethane foams, KB SIR 85-3267, 1986... 

Conversion of polymers and biomass to chemical intermediates and monomers by using subcritical and supercritical water as the reaction solvent is probable. Reactions of cellulose in supercritical water are rapid (< 50 ms) and proceed to 100% conversion with no char formation. This shows a remarkable increase in hydrolysis products and lower pyrolysis products when compared with reactions in subcritical water. There is a jump in the reaction rate of cellulose at the critical temperature of water. If the methods used for cellulose are applied to synthetic polymers, such as PET, nylon or others, high liquid yields can be achieved although the reactions require about 10 min for complete conversion. The reason is the heterogeneous nature of the reaction system (Arai, 1998). 

Pyrolysis produces three principal products - pyrolytic gas, oil, and char. Char is a fine particulate composed of carbon black, ash, and other inorganic materials, such as zinc oxide, carbonates, and silicates. Other by-products of pyrolysis may include steel (from steel-belted radial tires), rayon, cotton, or nylon fibers from tire cords, depending on the type of tire used. 

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. 

The pyrolysis liquid of Nylon 6,6 contains alkadienes and cycloalkenes in addition to cyclopentanone in the gasoline boiling range, furthermore this fraction also involves hexanedinitrile and even alkylamines. The components of the diesel oil boiling range... 

Electrical and electronic devices are made utilizing several various types of plastic materials, thus when discarded their waste is difficult to recycle. The plastics employed in housing and other appliances are more or less homogeneous materials (among others PP, PVC, PS, HIPS, ABS, SAN, Nylon 6,6, the pyrolysis liquids of which have been discussed above). However, metals are embedded in printed circuit boards, switches, junctions and insulated wires, moreover these parts contain fire retardants in addition to support and filler materials. Pyrolysis is a suitable way to remove plastics smoothly from embedded metals in electrical and electronic waste (EEW), in addition the thermal decomposition products of the plastics may serve as feedstock or fuel. PVC, PBT, Nylon 6,6, polycarbonate (PC), polyphenylene ether (PPO), epoxy and phenolic resins occur in these metal-containing parts of EEW. 

M. Nielsen, P. Jurasek, J. Hayashi and E. Furimsky, Formation of toxic gases during pyrolysis of polyacrylonitrile and nylons, J. Anal Appl Pyrol, 35, 43-51 (1995). 

A typical analysis of L. fendleri seed oil showed the presence of 16 0 (1%), 18 0 (2%), 18 1 (15%), 18 2 (7%), 18 3 (14%), lesquerolic (54%), and auricolic (4%) acids. As lesquerolic acid is the C20 homologue of ricinoleic with the same p-hydroxy alkene unit, it undergoes similar chemical reactions but produces (some) different products. For example, pyrolysis should give heptanal and 13-tri-decenoic acid (in place of 11-undecenoic acid). This could be converted to 13-ami-notridecanoic acid, the monomer required to make nylon-13. Similarly, alkali-fusion will give 2-octanol and dodecanedioic acid in place of decanedioic (sebacic) acid. This C12 dibasic acid is aheady available from petrochemical products and has a number of applications. A recent account of the status of this oil is available (126). 

Besides p eliminations, 1,3 or 1, n eliminations also may take place, for example during pyrolysis with the formation of cycles. One reaction of this type takes place during the pyrolysis of nylon 6,6 ... 

Other materials in waste that is thermally processed also were studied by pyrolytic techniques, typically with the purpose of regenerating the monomers or of obtaining other useful small molecules. For example, pyrolytic studies were performed for the evaluation of the possibilities for re-utilization of nylon carpet waste [7], the recycling of thermoset polymeric composites [8], the recovery of methyl methacrylate from poly(methyl methacrylate) waste [9], as well as for other raw material recovery from pyrolysis of plastic waste [10]. The results of incineration of various other types of waste also were studied at model scale [11, 12). These studies were applied to specific waste materials associated with the manufacturing process or to municipal solid waste [13-15)... 

Figure 13.3.2. Result for a Py-GC/MS analysis of polycaprolactam or nylon 6. Pyrolysis done on 0.4 mg material at 60(f C in He, with the separation on a Carbowax type column.

Pyrolysis of nylon 6 generates a significant proportion of caprolactam, which can be considered the monomer. The cleavage of the peptide bond explains easily the formation of the monomer. The molecules of polymers have a spatial arrangement and do not have a linear form as typically indicated with planar chemical formulas (see also Section 1.3). The idealized spatial form for the molecule of nylon 6 is suggested below ... 

A similar behavior with that of nylon 6 can be seen during pyrolysis of nylon 12 or polylauryllactam, CAS 24937-16-4, which has the idealized formula [-NH(CH2)nCO-]n. The results for a Py-GC/MS analysis of a nylon 12 sample are shown in Figure 13.3.4. The pyrolysis was done in similar conditions as for other examples, at 600° C in He, and the separation was done on a Carbowax column (see Table 4.2.2). The peak identification was done using MS spectral library searches only and is given in Table 13.3.3. 

The number of fragment molecules obtained during pyrolysis of nylon 12 is higher than that for nylon 6. However, the similarity in the reaction types occurring during pyrolysis of the two polymers is obvious. Even formation of azacyclotridecan-2-one, which is the equivalent of caprolactam for nylon 6, takes place in the pyrolytic process of nylon 12. 

To a considerable extent, pyrolysis of nylons made from a diamine and a dibasic carboxylic acid generate the same type of compounds as poly(amino acids). It includes... 

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