Depolymerization of Nylons

Because of the higher value of nylon resin in comparison with other polymers used in carpet, nylon carpet has been looked at as a resource for making virgin nylon via depolymerization. The majority of polyamides used commercially are nylon 6,6 or nylon 6, and the largest supply of waste for recycling of nylons is obtained from used carpets. The waste carpets are collected, sorted, and then subjected to a mechanical shredding process before depolymerization.

Hydrolysis of Nylon 6 A process for depolymerizing nylon 6 scrap using high-pressure steam was patented by Allied Chemical Corporation in 1965 [16], and subsequent patents by AlliedSignal, Inc. were obtained [17, 18]. Ground scrap was dissolved in high-pressure steam at 125-130 psig (963-997 kPa) and 175-180oC for 0.5 h in a batch process and then continuously hydrolyzed with superheated steam at 350° C and 100 psig (790 kPa) to form e-caprolactam at an overall recovery efficiency of 98%. The recovered monomer could be repolymer-ized without additional purification. Braun et al. [13] reported the depolymerization of nylon 6 carpet in a small laboratory apparatus with steam at 340°C and 1500 kPa (200 psig) for 3 h to obtain a 95% yield of crude e-caprolactam of purity 94.4%. Recently, patents were issued to AlliedSignal for the depolymerization of polyamide-containing carpet [19, 20].

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].

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% [26].

Catalytic Pyrolysis Catalytic pyrolysis has been studied as a hybrid process for recovering caprolactam from nylon 6 followed by high-temperature pyrolysis of the polypropylene into a synthetic natural gas. Czernik et al. [27] investigated the catalysis of the thermal degradation of nylon 6 with an a-alumina supported KOH catalyst in a fluidized-bed reactor. In the temperature range of 330-360°C the yield of caprolactam exceeded 85%.

Bockhorn et al. [28] used a liquid catalyst composed of a eutectic mixture of 60 mol% NaOH and 40 mol% KOH, which melts at 185°C. At 290°C the caprolactam yield exceeded 95%. At this temperature the polypropylene is not degraded significantly. Based on a preliminary feasibility study, this process could be economically viable [29].

Recovery of Caprolactam Approximately 10-12% by weight of oligomers is formed in the synthesis of polycaprolactam (nylon 6). These oligomers are removed by extraction with water or by distillation under vacuum. In the process, two types of liquid wastes are formed: (1) a 4-5% aqueous solution of low-molecular-weight compounds, consisting of ca. 75% by weight of capro-lactam and ca. 25% by weight of a mixture of cyclic and linear caprolactam oligomers and (2) a caprolactam -oligomer melt, containing up to 98% caprolac-tam and small amounts of dimer, water, and organic contaminants. The recycle of caprolactam involves two different stages: depolymerization of polymeric waste and purification of the caprolactam and oligomers obtained.

A general recovery of caprolactam from liquid waste generates 20-25% oligomers along with organic and inorganic compounds as impurities. The distillation of caprolactam under reduced pressure produces a residue that consists of inorganic substances such as permanganates, potassium hydrogen sulfate, potassium sulfate, sodium hydrogen phosphate, and sodium phosphate. The larger portion of the residue contains cyclic and linear chain oligomers plus 8-10% of caprolactam. The types and exact amounts of impurities depends on the method used for the purification and distillation of caprolactam.

The cyclic oligomers are only slightly soluble in water and dilute solutions of caprolactam. They tend to separate out from the extracted waste during the process of concentration and chemical purification of the caprolactam. The cyclic oligomers tend to form on the walls of the equipment used in the process equipment. 6-Aminocaproic acid or sodium 6-aminocaproate may also be found in the oligomeric waste, especially if sodium hydroxide is used to initiate the caprolactam polymerization.

Many impurities are present in commercial caprolactam that pass into the liquid wastes from polycaprolactam (PCA) manufacture from which caprolactam monomer may be recovered. Also, the products of the thermal degradation of PCA, dyes, lubricants, and other PCA fillers may be contained in the regenerated caprolactam. Identification of the contaminants by infrared (IR) spectroscopy has led to the detection of lower carboxylic acids, secondary amines, ketones, and esters. Aldehydes and hydroperoxides have been identified by polarography and thin-layer chromatography.

Caprolactam is a thermally unstable compound that on distillation may form methyl-, ethyl-, propyl- and n-amylamines. Also, at high temperatures, caprolactam reacts with oxygen to form hydroperoxides that in the presence of iron or cobalt ions are converted into adipimide. N-alkoxy compounds are also formed by the reaction of caprolactam with aldehydes during storage.

Therefore, caprolactam and the depolymerized product from which capro-lactam is regenerated contain various impurities, which are present in widely fluctuating amounts depending on the processes involved. In particular, the presence of cyclohexanone, cyclohexanone oxime, octahydrophenazine, aniline, and other easily oxidized compounds affects the permanganate number. Also volatile bases such as aniline, cyclohexylamine, cyclohexanol, cyclohexanone, nitrocy-clohexanone, and aliphatic amines may be present in the caprolactam.

Caprolactam must be very carefully purified to exclude small concentrations of (1) ferric ions, which would catalyze the thermal oxidative degradation of polycaprolactam and (2) aldehydes and ketones, which would markedly increase the oxidizability of caprolactam. The impurities in caprolactam may retard the rate of caprolactam polymerization as well as having a harmful effect on the properties of the polymer and fiber. In the vacuum depolymerization of nylon 6, a catalyst must be used because in the absence of a catalyst, by-products such as cyclic olefins and nitrides may form, which affects the quality of the caprolactam obtained [3].

The caprolactam obtained must meet the specifications of permanganate number, volatile bases, hazen color, ultraviolet (UV) transmittance, solidification point, and turbidity, in order to be used alone or in combination with virgin capro-lactam [25]. Reported caprolactam purification methods include recrystallization, solvent extraction, and fractional distillation. One solvent extraction technique involves membrane solvent extraction. Ion-exchange resins have been shown to be effective in the purification of aqueous caprolactam solutions. In one such process, the oily impurities are removed by extraction with organic solvents, followed after treatment with carbon at 60-80°C. Cationic and anionic exchange resins are then used to complete the purification process. Ion-exchange resins remove all ionic impurities as well as colloidal and floating particles, that is, alkali metal salts formed in permanganate treatment are removed during ion-exchange treatment. Also the treatment of aqueous solutions of caprolactam with ion-exchange resins helps to remove the distillation residue. Treatment of caprolactam with activated carbon helps to remove anionic and cationic impurities.

Impurities in caprolactam have also been destroyed by oxidation with ozone followed by distillation. Ozonation treatment of waste caprolactam leaves no ionic impurities. However, the most commonly used oxidizing agents are potassium permanganate, perboric acid, perborate, and potassium bromate. Treatment of caprolactam with these oxidizing agents is carried out in a neutral medium at 40-60°C. Strongly alkaline or acidic conditions accelerate the oxidation of caprolactam to form isocyanates. The undesirable oxidation reaction is fast above pH 7 because of the reaction with isocyanate to form carbamic acid salts, which shifts the equilibrium to form additional isocyanate.

In a typical process, potassium permanganate is used to treat the cracked liquor exiting the depolymerization plant without any pH adjustment. The liquor is usually acidic because it contains some of the phosphoric acid depolymerization catalyst. The KMnO4 treatment is followed by treatment of the caprolactam aqueous solution with carbon followed by filtration. Next the filtered 20-30% caprolactam aqueous solution is concentrated to 70% and the pH is adjusted to 9-10 by addition of sodium hydroxide. The caprolactam alkaline concentrate is treated with KMnO4 followed by distillation under reduced pressure to remove water and low-boiling impurities.

Also the caprolactam aqueous solution may be hydrogenated at 60°C in the presence of 20% sodium hydroxide and 50% palladium absorbed on carbon to provide caprolactam of very high purity after distillation. Treatment with an ion-exchange resin before or after the oxidation or hydrogenation process also improves the quality of the caprolactam obtained after distillation. Caprolactam has also been purified by treatment with alkali and formaldehyde followed by fractional distillation to remove aromatic amines and other products.

Also, nylon 6 waste may be hydrolyzed in the presence of an aqueous alkali metal hydroxide or acid to produce an alkali metal or acid salt of 6-aminocaproic acid (ACA). The reaction of nylon 6 waste with dilute hydrochloric acid is very fast at 90-100°C. The reaction mixture is poured into water to form a dilute aqueous solution of the ACA salt. Filtration is used to remove undis-solved impurities such as pigments, additives, and fillers followed by treatment of the acid solution with a strong cation-exchange resin. A sulfonic acid cationic-exchanger absorbs ACA, and pure ACA is eluted with ammonium hydroxide to form a dilute aqueous solution. Pure ACA is obtained by crystallization of the solution.

Alternatively, nylon 6 waste may be hydrolyzed with aqueous sodium hydroxide, and the sodium salt of ACA converted into pure ACA by passing the aqueous solution through an anion-exchange resin.

Applications of Depolymerized Nylon 6 Chemical recycling of nylon 6 carpet face fibers has been developed into a closed-loop recycling process for waste nylon carpet [25, 30-32]. 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 yarn 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 of the BASF Corp. has been in operation since 1994. Its process involves collection of used nylon 6 carpet, shredding and separation of face fibers, pelletizing face fiber for depolymerization and chemical distillation to obtain a purified caprolactam monomer, and repolymerization of caprolactam into nylon polymer [32].

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 scrap is dissolved with high-pressure steam and then continuously hydrolyzed with superheated steam to form caprolactam. The program has diverted over 100,000 tons of postconsumer carpet from the landfill to produced virgin-quality caprolactam [30, 31].

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. Thorburn [35] depolymerized nylon 6,6 fibers in an inert atmosphere at what was reported to be a superatmospheric 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.

Polk et al. [36] reported the depolymerization of nylon 6,6 and nylon 4,6 in aqueous sodium hydroxide solutions containing a phase-transfer catalyst. Ben-zyltrimethylammonium bromide was discovered to be an effective phase-transfer catalyst in 50% sodium hydroxide solution for the conversion of nylon 4,6 to oligomers. The depolymerization efficiency (percent weight loss) and the molecular weight of the reclaimed oligomers were dependent on the amount and concentration of the aqueous sodium hydroxide and the reaction time. Table 16.4 exhibits the effects of experimental conditions on the depolymerization efficiency and the average molecular weight of the oligomers. The viscosity-average molecular weight was calculated from the Mark-Houwink equation: = KMva, where Mv is the viscosity-average molecular weight, K = 4.64 x 10-2 dL/g and a = 0.76 at 25°C in 88% formic acid. Nylon 4,6 fibers (Mv = 41,400 g/mol) did not undergo depolymerization on exposure to 100 mL of 25 wt% sodium hydroxide solution at 165°C. Out of 6.0 g of nylon fibers fed for depolymerization, 5.95 g were unaffected. When the concentration of sodium hydroxide was increased to 50 wt %, the depolymerization process resulted in the formation of low-molecular-weight oligomers. Hence, even in the presence of a phase-transfer agent, a critical sodium hydroxide concentration exists between 25 and 50 wt % which is required to initiate depolymerization under the conditions used. Soluble amine salts were also obtained.

In order to establish the feasibility of alkaline hydrolysis with respect to recycling of nylon 4,6, it was necessary to determine whether the recovered oligomers could be repolymerized to form nylon 4,6. For this purpose, solidstate polymerization was performed on nylon 4,6 oligomers formed via alkaline hydrolysis with 50 wt% NaOH at 165°C for 24 h. The solid-state polymerization process was carried out in a round-bottom flask at 210°C for 16 h under vacuum. Solid-state polymerization of the nylon 4,6 oligomers resulted in an

Table 16.4 Effects of Experimental Conditions on Depolymerization Efficiency and Average Molecular Weight of Oligomers [36]

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Responses

  • ralph reiniger
    Why naoh used in nylon 6 recovery?
    6 years ago
  • Huriyyah
    Why is sodium hydroxide used a catalyst in the reaction to form nylon 6,6?
    5 years ago

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