Depolymerization of nylon

Because of the higher value of nylon resin in comparison with other polymers used in carpet, nylon carpet has been looked at as raw material for making virgin nylon, via depolymerization. The majority of polyamides used commercially are nylon 

6 and nylon 6. The waste carpets are collected, sorted and then subjected to a mechanical shredding process before depolymerization. 

Nylon 6 is made by polymerizing a single monomer, caprolactam, and the process may be reversed. Chemical recycling of nylon 6 carpet face fibers has been developed into a closed-loop recycling process for waste nylon carpet. The recovered nylon 6 face fibers are sent to a depolymerization reactor and treated with superheated steam in the presence of a catalyst to produce a distillate containing caprolactam. The crude caprolactam is distilled and repolymerized to form nylon 6. The caprolactam obtained is comparable to virgin caprolactam in purity. The repolymerized nylon 6 is converted into yam and tufted into carpet. The carpets obtained from this process are very similar in physical properties to those obtained from virgin caprolactam. 

The 6ix Again program initiated at BASF, now part of Honeywell Nylon Inc., has been in operation since 1994. Its process involves collection of used nylon 6 carpet, shredding and separation of face fibers, pelletizing of face fiber for depolymerization, chanical distillation to obtain purified caprolactam monomer, and repolymerization of caprolactam into nylon polymer.  

Evergreen Nylon Recycling LLC, a joint venture between Honeywell International and DSM Chemicals, was in operation from 1999 to 2001. It used a two-stage selective pyrolysis process. The ground nylon carpet, without separation, was dissolved with high-pressure steam and then continuously hydrolyzed with super-heated steam to form caprolactam. With a 100 thousand tons per year capacity, the program diverted over one hundred thousand tons of post-consumer carpet from the landfill to produced virgin-quality caprolactam. -  

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Ammonolysis is the preferred route currently in use at the DuPont Company for the depolymerization of nylon-6,6 carpet waste. McKinney13 has described the reaction of nylon-6,6 and nylon-6 mixtures with ammonia at temperatures in the range of 300-350°C at a pressure of about 68 atm in the presence of an ammonium phosphate catalyst to form a mixture of nylon-6,6 and nylon-6 monomers (HMD A, A A, and s-caprolactam) and adiponitrile, 5-cyanovaleramide, 6-aminocapronitrile, and 6-aminocaproamide. 

CL must be very carefully purified to exclude small concentrations of (1) ferric ions which would catalyze die thermal oxidative degradation of polycaprolactam and (2) aldehydes and ketones which would markedly increase oxidizability of CL. The impurities in CL may retard die rate of CL polymerization as well as having a harmful effect on die properties of die polymer fiber. In die vacuum depolymerization of nylon-6, a catalyst must be used because in die absence of a catalyst by-products such as cyclic olefins and nitrides may form, which affects the quality of die CL obtained.1... 

Sodium hydroxide has been used successfully as a catalyst for the base-catalyzed depolymerization of nylon-6. At 250°C, a pressure of 400 Pa, and a sodium hydroxide content of 1%, the yield of e-caprolactam was 90.5%.49... 

The depolymerization of nylon-6,6 and nylon-4,6 involves hydrolysis of the amide linkages, which are vulnerable to both acid- and base-catalyzed hydrolysis. In a DuPont patent,9 waste nylon-6,6 was depolymerized at a temperature... 

Hie hydrolytic depolymerization of nylon-6 was followed by gel permeation chromatography (GPC), viscometry, and gravimetry. GPC determinations were performed on a Waters 150C chromatography system using benzyl alcohol as die eluant, two Plgel 10-p.m crosslinked polystyrene columns, and a differential refractometer detector. The flow rate was 1 mL/min. The concentration of the polymer solutions was 0.5 wt% and dissolution was accomplished at 130°C. 

Lem, K.W. Letton, A. Izod, T.P.J. Lupton, F.S. Bedwell, W.B. Composition Containing Caprolactam-Free Residue from Depolymerization of Nylon 6 Carpet and Use Thereof in Paving Asphalt, Plastic Lumber and Crack Sealants. US Patent 6,214,908, April, 10, 2001, USP 6,414,066, July 2, 2002. 

Scheme 1.63. Formation of s-caprolactam by depolymerization of nylon 6. After Levchik (1999).

CAN oxidation in ionic liquids is a useful development in view of the limitation in solvent systems for such a reagent. Depolymerization of nylon-6 to give caprolactam occurs when it is heated with DMAP in an ionic liquid at 300°. ... 

Acid hydrolysis of nylon 6 wastes [21, 22] in the presence of superheated steam has been used to produce aminocaproic acid, which under acid conditions is converted to e-caprolactam, and several patents have been obtained by BASF [23, 24]. Acids used for the depolymerization of nylon 6 include inorganic or organic acids such as nitric acid, formic acid, benzoic acid, and hydrochloric acid [23, 25]. Orthophosphoric acid [24] and boric acid are typically used as catalysts at temperatures of 250-350°C. In a typical process, superheated steam is passed through the molten nylon 6 waste at 250-300°C in the presence of phosphoric acid. The resulting solution underwent a multistage chemical purification before concentration to 70% liquor, which was fractionally distilled in the presence of base to recover pure e-caprolactam. Boric acid (1%) may be used to depolymerize nylon 6 at 400°C under ambient pressure. A recovery of 93-95% e-caprolactam was obtained by passing superheated steam through molten nylon 6 at 250-350°C [23]. 

Hydrolysis of Nylon 6,6 and Nylon 4,6 The depolymerization of nylon 6,6 and nylon 4,6 involves hydrolysis of the amide linkages, which are vulnerable to both acid- and base-catalyzed hydrolysis. In a patent granted to the DuPont Company in 1946, Myers [33] described the hydrolysis of nylon 6,6 with concentrated sulfuric acid, which led to the crystallization of adipic acid from the solution. Hexamethylene diamine (HMDA) was recovered from the neutralized solution by distillation. In a later patent assigned to the DuPont Company by Miller [34], a process was described for hydrolyzing nylon 6,6 waste with aqueous sodium hydroxide in isopropanol at 180°C and 2.2 MPa pressure. After distillation of the residue, HMDA was isolated and on acidification of the aqueous phase, adipic acid was obtained in 92% yield. Thorbum [35] depolymerized nylon 6,6 fibers in an inert atmosphere at what was reported to be a supeiatmospheric pressure of up to 1.5 MPa and at a temperature in the range of 160-220°C in an aqueous solution containing at least 20% excess equivalents of sodium hydroxide. 

Depolymerization of nylon 6,6 is more complicated than that of nylon 6 because... 

Marin L, Zabulica A, Sava M (2013) Symmetric liquid crystal dimmers containing a luminescent mesogen synthesis, mesomorphic behavior, and optical properties. Soft Mater 11 32-39 Shukla U, Rao KV, Rakshit AK (2003) Thermotropic liquid-crystalline polymers synthesis, characterization and properties of poly(azomethine esters). J Appl Polym Sci 88 153-160 Shukla SR, Harad AM, Mahato D (2006) Depolymerization of nylon 6 waste fibers. J Appl Polym Sci 100(1) 186-190... 

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