**8. Chemical recycling of nylon**

### **8.1 Introduction**

48 Material Recycling – Trends and Perspectives

by the diffused EG. Random scissions of PC take place to lower the average molecular weight until the resulting oligomer can be dissolved in the bulk EG solution but retains solid state. The solid oligomer dissolves in EG solution, and the size of the PC particle shrinks as the dissolution proceeds, which is a heterogeneous reaction. The dissolved oligomer continues to be depolymerized with EG in the bulk solution to produce its monomer, BPA,

Achilias et al., 2009, investigated pyrolysis of PC and PC based Waste Electric and Electronic Equipment as a means of chemical recycling of this polymer. A laboratory-scale fixed bed reactor was used and the appropriate pyrolysis temperature was selected after measuring the thermal degradation of model PC by Thermogravimetric analysis. After pyrolysis a large amount of oil was measured, together with a smaller amount of gaseous product, leaving also a solid residue. For both samples (model PC and a compact disc based on PC), the gaseous fraction consisted mainly of CO2 and CO, whereas in the liquid fraction a large amount of different phenolic compounds, including the monomer bisphenol A, was measured. It seems that recycling of used CDs by pyrolysis is a very promising technique having the potential of producing useful high-value chemicals, which may find applications

Recycled polycarbonate is usually less resilient, have decreased impact resistance when compared with newly manufactured polycarbonate. The addition of fillers and pigments can also decrease the plastic's resilience. This problem can be addressed by the use of chemicals to modify impact resistance in recycled polycarbonate. Up to 15% recycled material can safely be added to the virgin resin without significantly altering properties of

The nature of the compact disc (CD) does not allow it to be easily recycled. The disc is a multi-layer product consisting of PC substrate and three coatings. These coatings, aluminium, lacquer and printing, respectively make up only a small portion of the entire disc. These materials should be separated or recovered in order to recycle the polycarbonate. There are a variety of methods for the removal of paint or plating from engineering plastics, ranging from the chemical to physico-mechanical procedures. Such techniques include chemical stripping or chemical recovery (high-temperature alkaline treatment), melt filtration, mechanical abrasion, hydrolysis, liquid cyclone, compressed vibration, cryogenic

The disadvantages of PC include high melt viscosity and notch sensitivity. Used PC usually suffered from crazing caused by light, radiation and chemicals present in the service

As with other thermoplastics, the level of mechanical and physical properties of polycarbonate depends on the molecular weight. However, production waste, recyclates etc. frequently do not, or no longer, possess the required molecular weights. Direct material

environment, which make the problem of notch sensitivity even worse.

which is a homogeneous reaction.

in the petrochemical industry.

**7.4 Problems of PC recycling** 

grinding, dry crushing and roller pressing.

the virgin material.

**7.3 Pyrolysis of PC based polymers** 

Nylon is one of the early polymers developed by Wallace Carothers in 1935, at DuPont's research facility. Today, nylon is one of the most commonly used polymers. Nylons, also known as polyamides, can be produced by the reaction of a diamine with a dicarboxylic acid, condensation of the appropriate amino acid, ring opening of a lactam, reaction of a diamine with a diacid chloride, and reaction of a diisocyanate with a dicarboxylic acid. Nylon is a crystalline polymer with high modulus, strength, impact properties, low coefficient of friction, and resistance to abrasion. A variety of commercial nylons are available including nylon 6, nylon 11, nylon 12, nylon 6,6, nylon 6,10, and nylon 6,12. The most widely used nylons are nylon 6,6 and nylon 6. Polyamides are used most often in the form of fibers, primarily nylon 6,6 and nylon 6, although engineering applications are also of importance. Nylon 6,6 is prepared from the polymerization of adipic acid and hexamethylenediamine, while nylon 6 is prepared from caprolactam.

Nylon recycling has increased substantially in the last several years. Most recycling efforts have focused on recovery of carpet. According to the U.S. Department of Energy, about 3.5 billion lb of waste carpet are discarded each year in the United States, with about 30% of them made from nylon 6.

Processing of recyclables is necessary to transform the collected materials into raw materials for the manufacture of new products. In general there are two categories for nylon recycling, chemical and thermal recycling.


Recent Advances in the Chemical Recycling

of nylon 6.

**8.3.1 Catalytic pyrolysis** 

**8.3.2 Recovery of caprolactam** 

and oligomers obtained.

used to initiate the caprolactam polymerization.

**8.3.3 Applications of depolymerized nylon 6** 

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 51

the presence of base to recover pure ε-caprolactam. Boric acid (1%) may be used to depolymerize nylon 6 at 4000C under ambient pressure. A recovery of 93-95% ε-caprolactam was obtained by passing superheated steam through molten nylon 6 at 250-3500C. Sodium hydroxide has been used successfully as a catalyst for the base-catalyzed depolymerization

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*., 1998 investigated the catalysis of the thermal degradation of nylon 6 with an α-alumina supported KOH catalyst in a fluidized bed reactor. In the temperature range of 330°C to 360°C the yield of caprolactam exceeded 85%. Bockhorn *et al., 2001* use a liquid catalyst composed of a eutectic mixture of 60 mol% NaOH and 40 mol%

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 caprolactam and ca. 25% by weight of a mixture of cyclic and linear caprolactam oligomers and (2) a caprolactam-oligomer melt, containing up to 98% caprolactam 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

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 which 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 depend 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

Chemical recycling of nylon 6 carpet face fibers has been developed into a closed-loop recycling process for waste nylon carpet [Bajaj and Sharma, 1997; Brown, 2001]. The

KOH which melts at 185°C. At 290°C the caprolactam yield exceeded 95%.

high heat in the absence of sufficient oxygen for combustion. At these elevated temperatures, the polymeric structure breaks down.

#### **8.2 Depolymerization of nylons**

Due to the higher value of nylons in comparison with other polymers used in carpet, nylon carpet has been looked at as a resource for making virgin nylon via depolymerization. Most 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.

Fig. 17. Typical carpet construction.

Although there are many active recycling operations, they mostly focus their work in the recycling of nylon 6 and nylon 6,6 basically collected from carpet waste. It is obvious the need for further research to develop methods for recycling more commercially used nylons.

### **8.3 Hydrolysis of nylon 6**

A process for depolymerizing nylon 6 scrap using high pressure steam was patented by by AlliedSignal, Inc. (Sifniades et al., 1999]. Ground scrap was dissolved in high-pressure steam at 125-130 psig (963-997 kPa) and 175-1800C for 0.5 hour in a batch process and then continuously hydrolyzed with super-heated steam at 3500C and 100 psig (790 kPa) to form εcaprolactam at an overall recovery efficiency of 98%. The recovered monomer could be repolymerized without additional purification. Braun et al., 1999 reported the depolymerization of nylon 6 carpet in a small laboratory apparatus with steam at 3400C and 1500 kPa (200 psig) for 3 hours to obtain a 95% yield of crude ε-caprolactam of purity 94.4%. Recently, patents were issued to AlliedSignal for the depolymerization of polyamidecontaining carpet (Sifniades et al., 1999).

Acid hydrolysis of nylon 6 wastes [Chaupart, 1998] in the presence of superheated steam has been used to produce aminocaproic acid which under acid conditions is converted to εcaprolactam, and several patents have been obtained by BASF [Corbin et al., 1999]. Acids used for the depolymerization of nylon 6 include inorganic or organic acids such as nitric acid, formic acid, benzoic acid, and hydrochloric acid [Bajaj and Sharma, 1997]. Orthophosphoric acid and boric acid are typically used as catalysts at temperatures of 250- 3500C. In a typical process, superheated steam is passed through the molten nylon 6 waste at 250-3000C 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 ε-caprolactam. Boric acid (1%) may be used to depolymerize nylon 6 at 4000C under ambient pressure. A recovery of 93-95% ε-caprolactam was obtained by passing superheated steam through molten nylon 6 at 250-3500C. Sodium hydroxide has been used successfully as a catalyst for the base-catalyzed depolymerization of nylon 6.

#### **8.3.1 Catalytic pyrolysis**

50 Material Recycling – Trends and Perspectives

Due to the higher value of nylons in comparison with other polymers used in carpet, nylon carpet has been looked at as a resource for making virgin nylon via depolymerization. Most 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

Although there are many active recycling operations, they mostly focus their work in the recycling of nylon 6 and nylon 6,6 basically collected from carpet waste. It is obvious the need for further research to develop methods for recycling more commercially used nylons.

A process for depolymerizing nylon 6 scrap using high pressure steam was patented by by AlliedSignal, Inc. (Sifniades et al., 1999]. Ground scrap was dissolved in high-pressure steam at 125-130 psig (963-997 kPa) and 175-1800C for 0.5 hour in a batch process and then continuously hydrolyzed with super-heated steam at 3500C and 100 psig (790 kPa) to form εcaprolactam at an overall recovery efficiency of 98%. The recovered monomer could be repolymerized without additional purification. Braun et al., 1999 reported the depolymerization of nylon 6 carpet in a small laboratory apparatus with steam at 3400C and 1500 kPa (200 psig) for 3 hours to obtain a 95% yield of crude ε-caprolactam of purity 94.4%. Recently, patents were issued to AlliedSignal for the depolymerization of polyamide-

Acid hydrolysis of nylon 6 wastes [Chaupart, 1998] in the presence of superheated steam has been used to produce aminocaproic acid which under acid conditions is converted to εcaprolactam, and several patents have been obtained by BASF [Corbin et al., 1999]. Acids used for the depolymerization of nylon 6 include inorganic or organic acids such as nitric acid, formic acid, benzoic acid, and hydrochloric acid [Bajaj and Sharma, 1997]. Orthophosphoric acid and boric acid are typically used as catalysts at temperatures of 250- 3500C. In a typical process, superheated steam is passed through the molten nylon 6 waste at 250-3000C in the presence of phosphoric acid. The resulting solution underwent a multistage chemical purification before concentration to 70% liquor, which was fractionally distilled in

and then subjected to a mechanical shredding process before depolymerization.

temperatures, the polymeric structure breaks down.

**8.2 Depolymerization of nylons** 

Fig. 17. Typical carpet construction.

containing carpet (Sifniades et al., 1999).

**8.3 Hydrolysis of nylon 6** 

high heat in the absence of sufficient oxygen for combustion. At these elevated

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*., 1998 investigated the catalysis of the thermal degradation of nylon 6 with an α-alumina supported KOH catalyst in a fluidized bed reactor. In the temperature range of 330°C to 360°C the yield of caprolactam exceeded 85%. Bockhorn *et al., 2001* use 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%.

#### **8.3.2 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 caprolactam and ca. 25% by weight of a mixture of cyclic and linear caprolactam oligomers and (2) a caprolactam-oligomer melt, containing up to 98% caprolactam 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 which 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 depend 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.

#### **8.3.3 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 [Bajaj and Sharma, 1997; Brown, 2001]. The

Recent Advances in the Chemical Recycling

oligomeric mixture was 1434 g/mole).

**8.4.1 Ammonolysis of nylon 6,6** 

for the ammonolysis of nylon mixtures.

conditions.

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 53

[BTEMB] as a phase transfer catalyst in the depolymerization of nylon 6,6. The product of the run with no phase transfer agent showed a 15.9% increase in weight compared to the weight of the original nylon 6,6. The calculated percent increase in weight for a 19-fold decrease in molecular weight (due to the addition of water) would be ca. 1%. Therefore, a large part of the increase must be due to leaching of silicates of the glass container (resin reaction kettle) by the strong alkali (50 wt%) at the temperature of the reaction (1300C) over 24 hours. The oligomer obtained had a viscosity average molecular weight of 1644 g/mole (the original nylon 6,6 had a molecular weight of 30,944 g/mole). The runs with phase transfer agent produced oligomers with decreases in weight of 40-50%. Although the occurrence of leaching of silicates from the glass container made quantitative assessment difficult, these results suggested that in the absence of phase transfer agent only oligomers are formed; however, soluble low molecular weight products are formed in the presence of phase transfer agent. The oligomers obtained were repolymerized in the solid state by heating at 2000C in a vacuum. The viscosity-average molecular weight of the solid state polymerized nylon 6,6 obtained was ca. 23,000 g/mole (the molecular weight of the

In order to isolate adipic acid, nylon 6,6 fibers were depolymerized under reflux with a 50% NaOH solution in the presence of catalytic amounts of benzyltrimethylammonium bromide. The oligomers formed in successive steps were depolymerized under similar conditions. However, hexamethylene diamine was not isolated. The overall yield of adipic acid was 59.6%.

Ammonolysis currently is the preferred route currently in use at the DuPont Company for the depolymerization of nylon 6,6 carpet waste [Kassera, 1998]. McKinney, 1994, has described the reaction of nylon 6,6 and nylon 6,6/nylon 6 mixtures with ammonia at temperatures between 300 and 3500C and a pressure of about 68 atmospheres in the presence of an ammonium phosphate catalyst to yield a mixture of the following monomeric products: HMDA, adiponitrile, and 5-cyanovaleramide from nylon 6,6 and εcaprolactam, 6-aminocapronitrile, and 6-aminocaproamide from nylon 6. The equilibrium is shifted toward products by continuous removal of water formed. Most of the monomers may be transformed into HMDA by hydrogenation. Kalfas, 1998 has developed a mechanism for the depolymerization of nylon-6,6 and nylon-6 mixtures by the ammonolysis process. The mechanism includes the amide bond breakage and amide end dehydration (nitrilation) reactions, plus the ring addition and ring opening reactions for cyclic lactams present in nylon 6. On the basis of the proposed mechanism, a kinetic model was developed

Bordrero, *et al., 1999* utilized a two step ami/ammonolysis process to depolymerize nylon 6,6. The first step is based on an amminolysis treatment of nylon 6.6 by n-butylamine at a temperature of 300°C and a pressure of 45 atm. Free HMDA and NN'-dibutyladipamide are generated. The second step is ammonolysis of NN'-dibutyladipamide at a temperature of 285°C and a pressure of 50 atm. The end product is adiponitrile (ADN). It is estimated that the yields could be about 48% for ADN and about 100% for HMDA at optimized reaction

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 [BASF, 2001].

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 super-heated steam to form caprolactam. The program has diverted over one hundred thousand tons of post consumer carpet from the landfill to produced virgin-quality caprolactam [Brown, 2001].

#### **8.4 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. Polk et al., 1999 reported the depolymerization of nylon 6,6 and nylon 4,6 in aqueous sodium hydroxide solutions containing a phase transfer catalyst. Benzyltrimethylammonium 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 (% 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. Nylon 4,6 fibers (Mv = 41,400 g/mole) did not undergo depolymerization on exposure to 100 mL of 25 wt% sodium hydroxide solution at 1650C. 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 in 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, solid state polymerization was performed on nylon 4,6 oligomers formed via alkaline hydrolysis with 50 wt% NaOH at 1650C for 24 hours. Solid state polymerization of the nylon 4,6 oligomers resulted in an increase in intrinsic viscosity from 0.141 to 0.740 dl/g. That corresponds to an increase in viscosity average molecular weight from 1846 g/mole to 16,343 g/mole.

The product of the depolymerization of nylon 6,6 with 50% aqueous sodium hydroxide solution was relatively low molecular weight oligomers. A series of experiments were run in order to examine the applicability and efficiency of benzyltrimethylammonium bromide

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

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

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 super-heated steam to form caprolactam. The program has diverted over one hundred thousand tons of post consumer carpet from the landfill to

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. Polk et al., 1999 reported the depolymerization of nylon 6,6 and nylon 4,6 in aqueous sodium hydroxide solutions containing a phase transfer catalyst. Benzyltrimethylammonium 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 (% 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. Nylon 4,6 fibers (Mv = 41,400 g/mole) did not undergo depolymerization on exposure to 100 mL of 25 wt% sodium hydroxide solution at 1650C. 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 in 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, solid state polymerization was performed on nylon 4,6 oligomers formed via alkaline hydrolysis with 50 wt% NaOH at 1650C for 24 hours. Solid state polymerization of the nylon 4,6 oligomers resulted in an increase in intrinsic viscosity from 0.141 to 0.740 dl/g. That corresponds to an increase in viscosity average molecular

The product of the depolymerization of nylon 6,6 with 50% aqueous sodium hydroxide solution was relatively low molecular weight oligomers. A series of experiments were run in order to examine the applicability and efficiency of benzyltrimethylammonium bromide

are very similar in physical properties to those obtained from virgin caprolactam.

monomer, and repolymerization of caprolactam into nylon polymer [BASF, 2001].

produced virgin-quality caprolactam [Brown, 2001].

**8.4 Hydrolysis of nylon 6,6 and nylon 4,6** 

weight from 1846 g/mole to 16,343 g/mole.

[BTEMB] as a phase transfer catalyst in the depolymerization of nylon 6,6. The product of the run with no phase transfer agent showed a 15.9% increase in weight compared to the weight of the original nylon 6,6. The calculated percent increase in weight for a 19-fold decrease in molecular weight (due to the addition of water) would be ca. 1%. Therefore, a large part of the increase must be due to leaching of silicates of the glass container (resin reaction kettle) by the strong alkali (50 wt%) at the temperature of the reaction (1300C) over 24 hours. The oligomer obtained had a viscosity average molecular weight of 1644 g/mole (the original nylon 6,6 had a molecular weight of 30,944 g/mole). The runs with phase transfer agent produced oligomers with decreases in weight of 40-50%. Although the occurrence of leaching of silicates from the glass container made quantitative assessment difficult, these results suggested that in the absence of phase transfer agent only oligomers are formed; however, soluble low molecular weight products are formed in the presence of phase transfer agent. The oligomers obtained were repolymerized in the solid state by heating at 2000C in a vacuum. The viscosity-average molecular weight of the solid state polymerized nylon 6,6 obtained was ca. 23,000 g/mole (the molecular weight of the oligomeric mixture was 1434 g/mole).

In order to isolate adipic acid, nylon 6,6 fibers were depolymerized under reflux with a 50% NaOH solution in the presence of catalytic amounts of benzyltrimethylammonium bromide. The oligomers formed in successive steps were depolymerized under similar conditions. However, hexamethylene diamine was not isolated. The overall yield of adipic acid was 59.6%.

### **8.4.1 Ammonolysis of nylon 6,6**

Ammonolysis currently is the preferred route currently in use at the DuPont Company for the depolymerization of nylon 6,6 carpet waste [Kassera, 1998]. McKinney, 1994, has described the reaction of nylon 6,6 and nylon 6,6/nylon 6 mixtures with ammonia at temperatures between 300 and 3500C and a pressure of about 68 atmospheres in the presence of an ammonium phosphate catalyst to yield a mixture of the following monomeric products: HMDA, adiponitrile, and 5-cyanovaleramide from nylon 6,6 and εcaprolactam, 6-aminocapronitrile, and 6-aminocaproamide from nylon 6. The equilibrium is shifted toward products by continuous removal of water formed. Most of the monomers may be transformed into HMDA by hydrogenation. Kalfas, 1998 has developed a mechanism for the depolymerization of nylon-6,6 and nylon-6 mixtures by the ammonolysis process. The mechanism includes the amide bond breakage and amide end dehydration (nitrilation) reactions, plus the ring addition and ring opening reactions for cyclic lactams present in nylon 6. On the basis of the proposed mechanism, a kinetic model was developed for the ammonolysis of nylon mixtures.

Bordrero, *et al., 1999* utilized a two step ami/ammonolysis process to depolymerize nylon 6,6. The first step is based on an amminolysis treatment of nylon 6.6 by n-butylamine at a temperature of 300°C and a pressure of 45 atm. Free HMDA and NN'-dibutyladipamide are generated. The second step is ammonolysis of NN'-dibutyladipamide at a temperature of 285°C and a pressure of 50 atm. The end product is adiponitrile (ADN). It is estimated that the yields could be about 48% for ADN and about 100% for HMDA at optimized reaction conditions.

Recent Advances in the Chemical Recycling

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 55

Western Europe alone approximately 327 000 tones of PMMA are consumed each year with an increasing percentage of approximately 4% per year. In contrast with condensation polymers (e.g. PET), addition polymers, like PMMA, cannot be easily recycled to monomer by simple chemical methods. Instead, thermo-chemical recycling techniques like pyrolysis are usually applied. Thus, various processes for the depolymerization of PMMA have been described in literature. Among them the most prominent ones are the molten metal bath process and the fluidized bed pyrolysis [Kaminsky et al., 2004; Smolders and Baeyens, 2004; Sasse and Emig, 1998]. The first one although widely used in several countries exhibits several serious disadvantages including that the raw condensate MMA may be contaminated by the metal used (usually lead) or other by-products [Sasse and Emig, 1998]. The effect of temperature, addition of filler and amount of feed on the amount and distribution of pyrolytic products was investigated by Kaminsky and co-workers in a fluidized bed reactor [Kaminsky et al., 1991; 2001; 2004, Grause et al., 2006]. Furthermore, PMMA thermal pyrolysis results in a close to 97% recovery of the monomer methyl methacrylate (MMA) at relatively low temperatures (400–500 oC) [Smolders and Baeyens, 2004]. It has been reported that the liquid pyrolysis product was so pure that it could be

Achilias, 2006 and 2007 investigated the chemical recycling of PMMA using pyrolysis, aiming at the recovery of pure monomer able to be re-polymerized back to polymer. Conventional (thermal) pyrolysis was carried out using either model polymer or a commercial product as feedstock. The experiments were carried out in a laboratory fixed bed reactor at 450 oC, which was found in literature to be the optimum temperature for the maximization of MMA monomer amount [Kaminsky et al., 2001]. The liquid product obtained from both the model and the commercial samples was very high, 99% and 98%, respectively. The monomer recovery was higher by feeding pure PMMA (98.3 wt.%) compared to the commercial sample (94.9 wt.%). In both experiments the gas fraction was very small. Only 0.6 or 1.5 wt.% of gases were produced. Also the residue obtained was very low 0.1 and 0.4 wt.%. Furthermore, the gas composition of both samples was approximately the same with large amounts of CO2 followed by CO and methane. The former are degradation products of PMMA and MMA due to the existence of oxygen in the macromolecular chain. The liquid fraction mostly consists of the monomer MMA in a large amount (99 and 97 wt.% for the model polymer and the commercial product, respectively)

The potential use of the liquid pyrolysis fraction as a raw material for the reproduction of

Achilias, D.S. Chemical Recycling of Poly(methyl methacrylate), *WSEAS Trans on Environm* 

Achilias, D.S., Chemical recycling of poly(methyl methacrylate) by pyrolysis. Potential use

Achilias, D.S.; A. Giannoulis, G.Z. Papageorgiou, Recycling of polymers from plastic

of the liquid fraction as a raw material for the reproduction of the polymer, *Eur* 

packaging materials using the dissolution/ reprecipitation technique, *Polym. Bull.*

polymerized again without any further treatment [Kaminsky et al., 1991].

with a small percentage of some other organic compounds mainly esters.

PMMA by polymerization was also investigated (Achilias, 2007).

**10. References** 

*Develop* 2(2), 85-91 (2006)

63(3), 449-465 (2009).

*Polym J,* 43 (6), 2564-2575 (2007)

#### **8.4.2 Recovery of nylon 6,6 monomers**

Adipic acid and hexamethylene diamine (HMDA) are obtained from nylon 6,6 by the hydrolysis of the polymer in concentrated sulfuric acid (Figure 18). The adipic acid is purified by recrystallization and the HMDA is recovered by distillation after neutralizing the acid. This process is inefficient for treating large amounts of waste because of the required recrystallization of adipic acid after repeated batch hydrolyses of nylon 6,6 waste. In a continuous process, nylon 6,6 waste is hydrolyzed with an aqueous mineral acid of 30- 70% concentration and the resulting hydrolysate is fed to a crystallization zone. The adipic acid crystallizes and the crystals are continuously removed from the hydrolysate. Calcium hydroxide is added to neutralize the mother liquor and liberate the HMDA for subsequent distillation.

$$\begin{array}{ccccc} \begin{bmatrix} \mathsf{H} & \mathsf{H} & \mathsf{O} & \mathsf{O} \\ \hline \\ \mathsf{N}(\mathsf{CH}\_{2})\_{6}\mathsf{N}-\mathsf{C}(\mathsf{CH}\_{2})\_{4}\mathsf{C} & \\ \\ \end{bmatrix}\_{\mathsf{D}} & \begin{array}{c} \begin{array}{c} \begin{array}{c} \\ \end{array} \\ \end{array} & \begin{array}{c} \begin{array}{c} \\ \end{array} \\ \end{array} & \begin{array}{c} \begin{array}{c} \begin{array}{c} \\ \end{array} \\ \end{array} \\ \end{array} & \begin{array}{c} \begin{array}{c} \begin{array}{c} \\ \end{array} \\ \end{array} & \begin{array}{c} \begin{array}{c} \\ \end{array} \\ \end{array} \\ \end{array} & \begin{array}{c} \begin{array}{c} \begin{array}{c} \\ \end{array} \\ \end{array} \\ \end{array} \end{array} \end{array} \end{array} \end{bmatrix}$$

Continuous recovery requires adipic acid crystals having an average diameter of ca. 40-50 nm. Such crystals are obtained by continuously introducing the hot hydrolysate containing 10-20% adipic acid into an agitated crystallization vessel while maintaining an average temperature of 20-300C. The slurry obtained from the crystallization vessel is filtered to collect the adipic acid crystals and the filtrate which contains the HMDA acid salt is continuously neutralized with calcium hydroxide. The calcium salt formed is removed by filtration and the HMDA in the filtrate is isolated by distillation.

In the case of nylon 6,6 waste recycled by ammonolysis, nylon is treated with ammonia in the presence of a phosphate catalyst. Reaction occurs at 3300C and 7 MPa. Distillation of the reaction mixture produces ammonia which is recycled and three fractions containing (a) caprolactam, (b) HMDA and aminocapronitrile and (c) adiponitrile. Aminocapronitrile and adiponitrile are hydrogenated to yield pure HMDA and the caprolactam is either converted to aminocapronitrile by further ammonolysis or distilled to produce pure caprolactam. The HMDA produced by this process is extremely pure (>99.8). The main impurities are aminomethylcyclopentylamine and tetrahydroazepine which are expected to be removed more effectively in the larger distillation columns employed in larger plants.

#### **9. Chemical recycling of poly(methyl methacrylate)**

Poly(methyl methacrylate) (PMMA) is a major type of thermoplastics used throughout the world in such applications as transparent all-weather sheets, electrical insulation, bathroom units, automotive parts, surface coating and ion exchange resins, etc. The plastics made from PMMA are widely used under the commercial trade names PLEXIGLAS or PERSPEX. In

Adipic acid and hexamethylene diamine (HMDA) are obtained from nylon 6,6 by the hydrolysis of the polymer in concentrated sulfuric acid (Figure 18). The adipic acid is purified by recrystallization and the HMDA is recovered by distillation after neutralizing the acid. This process is inefficient for treating large amounts of waste because of the required recrystallization of adipic acid after repeated batch hydrolyses of nylon 6,6 waste. In a continuous process, nylon 6,6 waste is hydrolyzed with an aqueous mineral acid of 30- 70% concentration and the resulting hydrolysate is fed to a crystallization zone. The adipic acid crystallizes and the crystals are continuously removed from the hydrolysate. Calcium hydroxide is added to neutralize the mother liquor and liberate the HMDA for subsequent

Continuous recovery requires adipic acid crystals having an average diameter of ca. 40-50 nm. Such crystals are obtained by continuously introducing the hot hydrolysate containing 10-20% adipic acid into an agitated crystallization vessel while maintaining an average temperature of 20-300C. The slurry obtained from the crystallization vessel is filtered to collect the adipic acid crystals and the filtrate which contains the HMDA acid salt is continuously neutralized with calcium hydroxide. The calcium salt formed is removed by

In the case of nylon 6,6 waste recycled by ammonolysis, nylon is treated with ammonia in the presence of a phosphate catalyst. Reaction occurs at 3300C and 7 MPa. Distillation of the reaction mixture produces ammonia which is recycled and three fractions containing (a) caprolactam, (b) HMDA and aminocapronitrile and (c) adiponitrile. Aminocapronitrile and adiponitrile are hydrogenated to yield pure HMDA and the caprolactam is either converted to aminocapronitrile by further ammonolysis or distilled to produce pure caprolactam. The HMDA produced by this process is extremely pure (>99.8). The main impurities are aminomethylcyclopentylamine and tetrahydroazepine which are expected to be removed

Poly(methyl methacrylate) (PMMA) is a major type of thermoplastics used throughout the world in such applications as transparent all-weather sheets, electrical insulation, bathroom units, automotive parts, surface coating and ion exchange resins, etc. The plastics made from PMMA are widely used under the commercial trade names PLEXIGLAS or PERSPEX. In

**8.4.2 Recovery of nylon 6,6 monomers** 

Fig. 18. Depolymerization of nylon 6,6 by hydrolysis.

filtration and the HMDA in the filtrate is isolated by distillation.

**9. Chemical recycling of poly(methyl methacrylate)** 

more effectively in the larger distillation columns employed in larger plants.

distillation.

Western Europe alone approximately 327 000 tones of PMMA are consumed each year with an increasing percentage of approximately 4% per year. In contrast with condensation polymers (e.g. PET), addition polymers, like PMMA, cannot be easily recycled to monomer by simple chemical methods. Instead, thermo-chemical recycling techniques like pyrolysis are usually applied. Thus, various processes for the depolymerization of PMMA have been described in literature. Among them the most prominent ones are the molten metal bath process and the fluidized bed pyrolysis [Kaminsky et al., 2004; Smolders and Baeyens, 2004; Sasse and Emig, 1998]. The first one although widely used in several countries exhibits several serious disadvantages including that the raw condensate MMA may be contaminated by the metal used (usually lead) or other by-products [Sasse and Emig, 1998]. The effect of temperature, addition of filler and amount of feed on the amount and distribution of pyrolytic products was investigated by Kaminsky and co-workers in a fluidized bed reactor [Kaminsky et al., 1991; 2001; 2004, Grause et al., 2006]. Furthermore, PMMA thermal pyrolysis results in a close to 97% recovery of the monomer methyl methacrylate (MMA) at relatively low temperatures (400–500 oC) [Smolders and Baeyens, 2004]. It has been reported that the liquid pyrolysis product was so pure that it could be polymerized again without any further treatment [Kaminsky et al., 1991].

Achilias, 2006 and 2007 investigated the chemical recycling of PMMA using pyrolysis, aiming at the recovery of pure monomer able to be re-polymerized back to polymer. Conventional (thermal) pyrolysis was carried out using either model polymer or a commercial product as feedstock. The experiments were carried out in a laboratory fixed bed reactor at 450 oC, which was found in literature to be the optimum temperature for the maximization of MMA monomer amount [Kaminsky et al., 2001]. The liquid product obtained from both the model and the commercial samples was very high, 99% and 98%, respectively. The monomer recovery was higher by feeding pure PMMA (98.3 wt.%) compared to the commercial sample (94.9 wt.%). In both experiments the gas fraction was very small. Only 0.6 or 1.5 wt.% of gases were produced. Also the residue obtained was very low 0.1 and 0.4 wt.%. Furthermore, the gas composition of both samples was approximately the same with large amounts of CO2 followed by CO and methane. The former are degradation products of PMMA and MMA due to the existence of oxygen in the macromolecular chain. The liquid fraction mostly consists of the monomer MMA in a large amount (99 and 97 wt.% for the model polymer and the commercial product, respectively) with a small percentage of some other organic compounds mainly esters.

The potential use of the liquid pyrolysis fraction as a raw material for the reproduction of PMMA by polymerization was also investigated (Achilias, 2007).
