Accepted Manuscript

Title: Chemical recycling of plastics using sub-, supercriticalfluids

Author: Motonobu Goto

PII: S0896-8446(08)00345-8DOI: doi:10.1016/j.supflu.2008.10.011Reference: SUPFLU 1663

To appear in: J. of Supercritical Fluids

Received date: 1-9-2008Revised date: 1-10-2008Accepted date: 1-10-2008

Please cite this article as: M. Goto, Chemical recycling of plastics usingsub-, supercritical fluids, The Journal of Supercritical Fluids (2008),doi:10.1016/j.supflu.2008.10.011

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

Page 1 of 29

Accep

ted

Man

uscr

ipt

Recovered glass fiber (left) and carbon fiber (right) by the treatment in subcritical benzyl alcohol.

* Graphical Abstract

Page 2 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

1

Chemical recycling of plastics using sub- and supercritical fluids

Motonobu Goto

Bioelectrics Research Center, Kumamoto University, Kumamoto 860-8555, Japan

E-mail: mgoto@kumamoto-u.ac.jp; Phone: +81-96-342-3664

Abstract

The development of chemical recycling of waste plastics by decomposition reactions in sub- and

supercritical fluids is reviewed. Decomposition reactions proceed rapidly and selectively using

supercritical fluids compared to conventional processes. Condensation polymerization plastics such as

PET, nylon, and polyurethane, are relatively easily depolymerized to their monomers in supercritical

water or alcohols. The monomer components are recovered in high yield. Addition polymerization

plastics such as phenol resin, epoxy resin, and polyethylene, are also decomposed to monomer

components with or without catalysts. Pilot scale or commercial scale plants have been developed and

are operating with sub- and supercritical fluids.

Key-words: Plastics, Depolymerization, Decomposition, Supercritical fluid, Recycling

* Manuscript

Page 3 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

2

1. Introduction

Production of plastics in the world was 168 million tons in 1999, and is estimated to be 210 million

tons in 2010. Since the treatment of plastic wastes has become a serious problem, the development of

more effective recycling processes has been desired. There are three types of recycling for plastic wastes:

material recycling, thermal recycling (energy recovery), and chemical recycling (feedstock recycling).

For the recycling of plastics, chemical recycling is the most desirable process because plastics are

converted to their monomers, which can then be re-used.

Sub- and supercritical fluids such as water and alcohol are excellent reaction media for

depolymerization or decomposition of plastics. By using sub- and supercritical fluids, decomposition of

polymers proceeds rapidly and selectively. Condensation polymerization plastics are relatively easily

depolymerized into their monomers without catalysts in water or alcohol which act as reactant as well as

solvent. Addition polymerization plastics can also be decomposed with or without catalysts in sub- and

supercritical fluids. Composite plastics such as fiber-reinforced plastics are decomposed into smaller

molecular components and fiber materials.

A number of researchers have devoted their time in the development of recycling technology from

the fundamental to the practical scale. Pilot and commercial scale plants have been constructed for

chemical recycling of plastics using sub- and supercritical fluids. In this paper, research and

development on chemical recycling of plastics using sub- and supercritical fluids are reviewed.

2. Condensation polymerization plastics

Condensation polymers with ether, ester, or acid amide linkages can be depolymerized by solvolysis.

The depolymerization reaction may be hydrolysis in water or alcoholysis in alcohol. When solvolysis

proceeds selectively, a polymer can be depolymerized into its monomers. Condensation polymers are

easily decomposed to their monomers by hydrolysis or alcoholysis in near-critical water or alcohol.

Page 4 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

3

Polyethylene terephthalate (PET) is a typical condensation polymer abundantly used. Various chemical

recycling methods such as methanolysis in liquid methanol, glycolysis in liquid ethylene glycol, ester

exchange, and hydrolysis using alkali, have been developed.

PET was depolymerized to its monomers, terephthalic acid (TPA) and ethylene glycol (EG), in sub-

and supercritical water [1]. The yield of terephthalic acid was close to 100% under the conditions of 673

K, 40 MPa and a reaction time of 30 min. The yield of ethylene glycol was lower because of further

decomposition catalyzed by the terephthalic acid produced. Yamamoto et al. [2] showed the possibility

of depolymerization of PET in sub- and supercritical water. Secondary products observed were benzoic

acid, diethylene glycol, 1,4-dioxane, acetaldehyde, and crotonic acid.

Sako et al. [3] reported that methanolysis in supercritical methanol produced both monomers,

dimethyl terephthalate (DMT) and EG with almost 100% yield in 30 min without catalyst. De Castro et

al. [4] depolymerized PET in ethanol and ethanol/water under supercritical conditions. PET was

completely depolymerized into monomers in about 5 h at 528 K and the maximum recovery yield of

diethyl terephthalate was 98.5%. We have investigated the reaction mechanism and kinetics of the

depolymerization of PET to its monomers in supercritical methanol [5-8]. As the reaction time was

increased, the molecular weight of the polymer decreased. PET with a weight-average molecular weight

of about 47,000 (polymerization degree: n = 240 to 250) was decomposed to oligomers of 3,000 MW

(polymerization degree: n = 15) in 300 s and to 1,000 MW (polymerization degree: n = 5) in 600 s in

supercritical methanol. After a reaction time of 1200 s, PET decomposed to monomer sized components

and DMT increased gradually.

The reaction scheme of PET decomposition in supercritical methanol is shown in Fig. 1. The main

products in PET depolymerization were DMT and EG. Some amount of methyl 2-hydroxyethyl

terephthalate (MHET), bis-hydroxyethyl terephthalate (BHET), terephthalic acid monomethyl ester

(TAMME), diethylene glycol (DEG) and 2-methoxyethanol (ME) were also detected. TAMME, DEG and

Page 5 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

4

ME may be produced by the following side reactions. Dimerization of EG might produce DEG. ME might

be produced by the reaction of EG and methanol. TAMME might be produced from polymer, oligomer,

or MHET in the presence of water.

PET is degraded by random scission to polymers of smaller MW. It is then continuously

depolymerized to yield MHET, DMT, and EG by end scission. Produced MHET reacts further with

methanol to produce DMT and EG. We have developed a continuous mixture kinetics model to analyze

the depolymerization of PET [5]. The estimated molecular weight distribution (MWD) was compared

with experimental MWD obtained by SEC. The monomer yield changes were also calculated and

compared as a function of reaction time.

To improve the precision of the reaction kinetics model, we further investigated the PET

depolymerization mechanism in supercritical methanol. BHET, which is a compound of terephthalic

acid and two ethylene glycols combined with an ester linkage and is a structural unit of PET, and PET

oligomer (trimer) were used as model compounds for PET. Figure 2 shows the yield of DMT, MHET,

and BHET as a function of reaction time at 543 K and 14.7 MPa. The yield of MHET increased initially

and then decreased. However, the yield of DMT was low at short reaction times, but increased with the

decrease in the yield of MHET. These results suggested the existence of MHET as a reaction intermediate

in PET depolymerization in supercritical methanol. The results also suggested that the depolymerization

of PET would apparently occur in a successful manner.

Mitsubishi Heavy Industries, Ltd., Japan (MHI) has developed a chemical recycling process using

supercritical methanol for depolymerizing post-consumer PET bottles into monomers for use as feed

stocks for manufacturing PET resin [9, 10]. The pilot plant process consists of mainly 4 sections: the PET

bottle shredding section, the depolymerization with supercritical methanol section, the separation and

purification section and the hydrolysis section. In this process, post-consumer PET bottles are recycled

Page 6 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

5

into monomers as pure TPA and EG. MHI recovered high purity monomers whose qualities are

equivalent to those of virgin monomers using this pilot plant.

Nylon 6, which is a polymer synthesized by ring-opening polymerization of ε-caprolactam, was

depolymerized by hydrolysis in sub- and supercritical water [11]. ε-Caprolactam and ε-aminocaproic

acid were detected in the product liquid phase. The total yield of these monomers was about 100% for

reactions at 573 K in 60 min and at 603 K in 30 min. The yield of ε-aminocaproic acid decreased

rapidly as the reaction time increased. Thus, nylon 6 was decomposed by hydrolysis to ε-aminocaproic

acid followed by cyclodehydration to ε-caprolactam or further decomposition to smaller molecules as

shown in Fig. 3. This indicates that the cyclodehydration reaction proceeds in water near the critical

temperature.

Based on the results from nylon 6, 3-aminocaprolactam was synthesized from L-lysine by

cyclodehydration in subcritical water [12]. As the reaction time increased, the reaction product became

a more deeply colored yellow. At higher temperature, a yellow or green colored oil phase was observed.

Lysine was cyclodehydrolyzed to 3-aminocaprolactam and then further decomposed to smaller molecules

in subcritical water. Lysine was completely reacted in 30 min at 633 K. The highest yield of

3-aminocaprolactam was 51% in 20 min at 603 K. For longer reaction times, the yield of

3-aminocaprolactam decreased due to further decomposition. Therefore, sub- and supercritical water were

found to be excellent reaction media for cyclodehydration.

Polycarbonates, one of which is synthesized from bisphenol A and phosgene, are widely used as

commodity plastics and engineering plastics because of features such as temperature resistance, impact

resistance, and optical properties. Several depolymerization processes using alkali catalysts have been

reported to produce bisphenol A. Ikeda et al. [13] studied the decomposition of polycarbonate in water

at 403-573 K. The main products were phenol, bisphenol A, and p-isopropenylphenol. The reaction

was accelerated by the addition of Na2CO3, and the yield of identified products reached 68% in the

Page 7 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

6

reaction at 523 K for 1 h. As shown in Fig. 4, the decomposition mechanism was considered to produce

bisphenol A from polycarbonate initially, then p-isopropenylphenol and phenol were produced from

bisphenol A, and then phenol was produced by the decomposition of various intermediate products. The

presence of p-tert-butylphenol, which was contained in the polycarbonate resin as a molecular weight

adjustment regulator, in the products suggested that it was melted in the decomposition reaction of

polycarbonate. Pinero et al. [14] studied alkali-catalyzed depolymerization of polycarbonate by

alcoholysis in supercritical or near critical conditions to recover bisphenol A and dimethyl carbonate.

The maximum yield of bisphenol A was 90% (kg product/kg PC) at 423 K and 10 MPa using a solution of

methanol and NaOH (10 kg/m3). The yield of dimethyl carbonate, which has an optimum of 35%,

reached values of 8-20%. By using crystallization in water, bisphenol A crystals with a purity of 99.9

wt% were obtained. Pinero-Hernanz et al. [15] applied a shrinking particle model to analyze

nonstationary behavior for alkali-catalyzed methanolysis of polycarbonate. The model successfully

simulated the kinetic behavior of produced monomer components.

Polyurethanes are produced by the reaction of a polyisocyanate with a polyalcohol (polyol) in the

presence of a catalyst and other additives. By hydrolysis of polyurethane in water, it is possible to

obtain polyol and diamine corresponding to the starting isocyanate. Nagase et al. [16] found that the

optimum temperature and pressure were 523-573 K and 10-20 MPa. The decomposition of

polyurethane was almost 100% above the temperature of 543 K. Both polyol and diamine were

recovered almost completely. When the temperature was higher than 603 K, these recoveries decreased.

Dai et al. [17] decomposed polyurethane foam in subcritical water at 423-623 K. The product was a

two-phase liquid where the upper layer was an aqueous solution of tolylenediamine (TDA) and the lower

oil phase was polyol. The highest yield of TDA reached near 90%. Perfect liquid products were

obtained under economic conditions at 523 K for 30 min, and the yield of TDA reached 72%. Dai et al.

[18] also carried out the decomposition of polyol in supercritical water. The main products of

Page 8 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

7

decomposition of poly(propylene glycol) were hydroxyacetone and diols (propylene glycol, dipropylene

glycol, and tripropylene glycol). The yield of hydroxyacetone was 33.0 wt% at 703 K for 10 min and

the total yield of hydroxyacetone and diols was 56.7 wt% at 653 K for 60 min. Kobe Steel developed a

chemical recycling process in Mitsui-Takeda Chemicals in 1997, applying non-catalytic hydrolysis in

subcritical water [19]. Tolylenediisocyanate (TDI) distillation residue, which had been conventionally

incinerated, was successfully converted to TDA. The concept of the recycling process is shown in Fig. 5.

The commercial plant has been operating since 1998 with the capacity of 10 ton/day. The process

consists of a reaction process in subcritical water and a purification process. The recovered TDA was of

high quality with more than 99.5% purity. The expansion of the plant was completed in 2003.

3. Addition polymerization polymers

3.1. Phenol resin

The recycling of thermosetting resins, which are widely used for electronics, is important. Phenol

resin is one of these thermosetting resins and has a high thermal stability because aromatic units are

connected by methylene bonds. Prepolymers of phenol resin were decomposed into their monomers by

reactions at 523-703 K under an Ar atmosphere in sub- and supercritical water [20]. The total yield of

identified products depended on the kind of prepolymers, and the maximum yield reached 78% in the

reaction at 703 K for 0.5 h. The decomposition was accelerated by the addition of Na2CO3, and the yields

of identified monomers reached more than 90%. The molding material from phenol resin was also

decomposed mainly into phenol and cresols in supercritical water. Diphenylmethane was reacted in

supercritical water to confirm the oxygen supply reaction from supercritical water [21]. The production

of benzophenone from diphenylmethane suggested the reaction was supplied with oxygen from

supercritical water. This was confirmed by the reaction in H218O. The molding materials from phenol

resin were decomposed into their monomers by reaction in supercritical water. Addition of

Page 9 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

8

polycarbonate accelerated the decomposition of the molding material.

We applied sub- and supercritical water technology to the chemical recycling of printed circuit board

wastes into chemical resources. The circuit board was used as a reactant after removing the copper

coating. A batch reactor was used to evaluate the conversion and yields of monomers [22]. The reaction

products consisted of liquid, gas, and solid phases. The liquid phase was initially colorless and gradually

turned brown. The solid residue was black and covered by a tar-like material at higher reaction

temperature. The conversion was calculated from the mass of solid residue. In the decomposition in

supercritical water, about 80% of the feed was transformed into liquid phase or gases. About 20%

remained in the solid phase as residue. Higher conversions were obtained for longer reaction times and

at higher temperatures. Even in 20 minutes of the reaction, conversion was more than 60% at 723 K.

Elemental analysis of the feed sample gave the composition as H 6.82%, C 56.0%, and N 2.41%. The

ratios of H to C and N to C in the solid residue were lower than the feed sample. The H/C ratio was lower

at higher reaction temperature and longer reaction time. This indicates that the solid is carbonated in

supercritical water as the reaction proceeds. However, the N/C ratio was close to the feed value for longer

times and at higher temperature. According to GC-MS analysis, phenol, o-cresol, and p-cresol were found

as main components in the liquid phase.

The yield of phenol and cresols was higher at longer reaction time and higher temperature. The yield

reached 5% at 733 K in 80 min. Since the circuit board sample contains about 27.5% in weight of phenol

resin, the yield corresponds to 18% of the phenol resin. The yields of cresols were similar to phenol and

the highest yield was around 3%, which corresponds to 11% of the phenol resin. Therefore, about 30% of

phenol resin part was converted to phenol and cresols under this experimental condition. Total organic

carbon (TOC) was measured for liquid phase products. About 48% of carbon in the feed sample was

converted into small molecules dissolved in the liquid phase. The TOC yield was maximum at 673 K and

the yield at 723 K was lower than that at 673 K. This may be due to the progress of the conversion into

Page 10 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

9

the gas phase.

3.2 Polyethylene

Polyethylene is a thermoplastic commodity used extensively in consumer products and is produced at

a rate of over 60 million tons a year. The distribution of reaction product can be controlled for the

pyrolysis in supercritical water. Watanabe et al. [23] observed that pyrolysis in supercritical water was

different from that in argon. Higher yields of shorter chain hydrocarbons, higher 1-alkene/n-alkane ratio,

and higher conversion were obtained in supercritical water. The difference was explained by the

difference in the reaction phase. The enhancement of polyethylene decomposition by supercritical water

was believed to be due to dissolution of high molecular weight hydrocarbons into supercritical water and

diffusion of water into the molten polyethylene phase.

Moriya and Enomoto [24] compared polyethylene cracking in supercritical water with water-free

thermal cracking. The product from supercritical water cracking (698 K, 120 min) was a yellow-brown

grease. Oil conversion was 90.2 wt% and gas conversion was 6.5 wt%. The product from thermal

cracking was a blackish-brown liquid and oil was 71.8 wt% and gas was 12.0 wt%. The yield of oil

products for supercritical water was high and coke production was small compared to thermal cracking.

The main products were secondary alcohols of 2-propanol and 2-butanol, and ketones of 2-propanone and

2-butanone. Polyethylene was converted into 2-propanol and then oxidized into 2-propanone. This

suggests that the liberated hydrogen was donated to supercritical water and participated in polyethylene

cracking. From an experiment using D2O as a tracer, Moriya and Enomoto [24] showed that hydrogen

in supercritical water was captured into the product oil and the donation of hydrogen from the

supercritical water to the oil increased with a higher water fill rate for the reactor volume.

4 Fiber reinforced plastics

Page 11 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

10

Fiber reinforced plastics (FRP) have been widely used as a high strength material in recent years.

FRP is used in various industries such as petrochemistry, construction, automobiles, aircraft and

environmental businesses because of its heat-resistance, anti-corrosion properties, and mechanical

strength. Glass fiber containing FRP is called GFRP and carbon fiber containing FRP is called CFRP.

GFRP is a composite material manufactured by laminating unsaturated polyester resin with glass fiber

and filler. CFRP usually consists of carbon fiber and epoxy resin. It is known that the FRP is one of the

most difficult materials to fractionate into elemental components, namely fiber, filler and polymers,

unfortunately resulting in the incineration or landfill of used FRP materials without any attempt at

recycling.

Sugeta et al. [26] and Okajima et al. [27] studied the decomposition of FRP in sub- and supercritical

water using a batch-type reactor. FRP was almost completely decomposed at 653 K in 5 min.

Kamimura et al. [28] treated FRP containing glass fiber in supercritical methanol in the presence of a

catalytic amount of N, N-dimethylaminopyridine (DMAP). As a preliminary experiment, unsaturated

polyesters were almost completely decomposed in supercritical methanol. At the optimized condition of

548 K, 10 MPa and 5 h, the products were a MeOH-soluble oil (60%), a CHCl3-soluble solid (39%), and

residue (1%). Supercritical ethanol gave similar results. The results for roughly ground waste FRP in

methanol at 548 K, 11 MPa, and 6 h were a MeOH-soluble oil (28.2%), a CHCl3-soluble solid (16.2%),

and a residue (62.6%). Since the residue contained less than 3% of organics, the organic polymer phase

was almost completely decomposed. The rate and efficiency of the degradation depended on the amount

of DMAP.

Pinero-Hernanz et al. [29] used supercritical water to decompose epoxy resin containing carbon fiber.

The efficiency of resin removal reached 79.3 wt% at 673 K and 28 MPa with further improvement

through the use of potassium hydroxide as an alkali catalyst (up to 95.3 wt%). The tensile strength of the

reclaimed fibers was 90-98% of the virgin fibers. The overall rate of the degradation process was

Page 12 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

11

controlled by both surface reaction and mass transfer steps. Pinero-Hernanz et al. [30] used sub- and

supercritical alcohols to treat CFRP. Methanol, ethanol, 1-propanol, and acetone were employed in

batch and semi-continuous-type reactors at temperatures ranging from 473 to 723 K. Using a flow system

and an alkali catalyst improved the degradation process. Experiments performed in the semi-continuous

flow system enhanced the mass transfer steps and reduced temperature requirements. Elimination of

resin up to 98 wt% was achieved using at 623 K and 1.1 kg-alcohol/kg-fiber/min at the same solvent flow

rate at 548 K using 0.02 mol/L of KOH, 96.5 wt% was obtained.

Fukuzawa et al. [31, 32] of Hitachi Chemicals Ltd., Japan investigated the dissolution process of the

resin part of FRP by a process called “ambient pressure dissolving method” using various solvents with or

without catalysts in order to develop a recycling technology for FRP. They reported the influence of

catalyst and solvent on the depolymerization of FRP. The highest conversion was observed when K3PO4

was used as a catalyst and diethyleneglycol monomethylether (DGMM) as a solvent. The second highest

conversion was observed for benzyl alcohol (BZA).

We have extended the operation temperature under pressures higher than the vapor pressure [33].

Reactions were carried out with or without a catalyst (K3PO4) in DGMM and BZA under their subcritical

states at temperatures ranging from 463 - 623 K for 1 - 8 h in a batch reactor. The conversion of UP was

fast as the catalyst/solvent molar ratio increased and it was enhanced in the presence of K3PO4 catalyst in

subcritical BZA. The glass fiber recovered after the FRP treatment in subcritical BZA was relatively long,

while it became shorter and somewhat damaged at temperatures higher than 573 K. A similar trend was

observed when DGMM was used as a solvent. Carbon fiber was also recovered successfully by the

treatment in BZA with K3PO4. Figure 6 shows the fibers recovered by the treatment in BZA.

Matsushita Electric Works, Ltd., Japan has been developing FRP recycling technology using

hydrolysis in subcritical water since 2002 [34]. By their technology, thermosetting resin in FRP can be

recycled into basic materials with a material recycling rate of 70%. The concept of their subcritical

Page 13 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

12

water recycling process is shown in Fig. 7. After subcritical water hydrolysis of FRP, the resin was

dissolved into a reaction liquid. The UP components recovered, such as glycols and fumaric acid, were

separated from the aqueous reaction liquid and polymerized into polyester with new raw resin materials to

produce recycled UP resin. Styrene-fumaric acid copolymer (SFC) obtained in the aqueous phase was

also separated. The SFC has the potential to be a raw material for synthetic functional polymers,

because the molecular structure is similar to that of a commercial low-profile additive (LPA) for FRP

forming. Thus, the SFC can be recycled to LPA after modifying its carboxylic acid group to develop the

shrinkage control effect. A bench plat with a capacity of 40 kg FRP per operation has been developed

for the subcritical water hydrolysis at 503 K and 2.8 MPa. Inorganic materials are separated by a filter

press. The recovered inorganic materials are used as inorganic filler to produce FRP. A pilot plant with

a reactor capacity of 2.9 m3 was constructed in Tochigi, Japan.

5. Crosslinked polyethylene

Crosslinked polyethylene (XLPE) is a thermosetting resin which is difficult to recycle because of its

low fluidity and low moldability caused by crosslinking. XPLE is classified into three types based on the

cross-linking: peroxide-crosslinked polyethylene, radiation-crosslinked polyethylene, and

silane-crosslinked polyethylene [35]. If only the cross-linking points are decomposed in XLPE, the

polymer can be effectively recycled, which does not require decomposition into smaller molecules.

Since selective decrosslinking at crosslinking points is difficult in conventional processes, XLPE is

usually converted into oil by thermal and catalytic cracking. Watanabe et al. [36] observed that the

peroxide-crosslinked polyethylene was selectively decrosslinked without severe decomposition of

backbone chains by using supercritical water.

Material recycling of the insulation of crosslinked polyethylene cable was investigated by Hitachi

Cable Ltd., Japan [37-39]. A crosslinking element that consists of a siloxane bond in silane-crosslinked

Page 14 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

13

polyethylene (silane-XLPE) was selectively decomposed in supercritical alcohol or water. Gel fractions

characterizing the crosslinking density decreased with increases in reaction temperature and vanished

over 573 K in supercritical methanol as shown in Fig. 8. In water, a temperature over 643 K was

necessary to reduce the gel fraction to 0 wt%. The molecular weight started to decrease over 613 K in

water and methanol. In water, the molecular weight of the product was lower than that in methanol.

Therefore, the crosslinking element was completely decomposed without reduction of the molecular

weight for the reaction at 573-613 K in methanol. Supercritical methanol achieved selective

decomposition at crosslinking points, while supercritical water was less selective. The mechanical and

electrical properties of recycled polyethylene satisfied the requirement of the cable insulation. A

pilot-scale continuous process consisting of a reactor extruder (Ext-Chem) and a degasser extruder

(Ext-Degas) was developed. The process is divided into four sections: feeding, reactor, degasser, and

pelletization. Alcohol was fed by a high-pressure pump and heated to the supercritical state before it was

injected into the cylinder. A tube reactor was attached to the Ext-Chem to keep the state for 30 min.

Ext-Degas was mounted to the outlet of the mixture of polymer and alcohol gas to separate recycled

polyethylene and the gas. The recycled polyethylene was extruded as strands from the Ext-Degas. The

apparent viscosity of recycled polyethylene was close to that of raw polyethylene.

Hong et al. [40] studied decrosslinking of polyethylene in supercritical methanol. They confirmed

that the molecular weight of decrosslinked polyethylene was only slightly smaller than that of raw

polyethylene. The reaction temperature and gel content were dominant parameters affecting the rate of

the decrosslinking reaction.

6. The role of solvents

As described above, water and alcohols were mainly used as sub- and supercritical fluids in

depolymerization processes of plastics. In most of the case, role of sub- and supercritical fluids is both

Page 15 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

14

solvent as a reaction medium and reactant. In hydrolysis or alcoholysis reaction of condensation

polymerization plastics, water or alcohol is evidently one of the reactants and transformed into the

product molecules. In depolymerization of polyethylene, hydrogen was supplied from water [24], so

that, the role of water is not only a solvent but also a reactant.

The critical temperature of methanol is much lower than that of water. Properties of water such as

dielectric constant and ion product, change drastically around the critical point. Thus, catalytic effect of

water can be expected. When the depolymerized products are not enough stable in high temperature

solvent and supercritical condition is required, alcohols may be better solvent than water due to lower

critical temperature. During the depolymerization process, plastics phase is often solid or melt state.

In that case, dissolution of solvent into solid or melt phase is important to enhance the reaction, especially

initial stage of the depolymerization.

When we think of whole process of chemical recycling of plastics, separation and purification

process of depolymerization products is also important. Thus, we have to select the solvent which can

be efficiently used for separation and purification process as well as reaction process.

6. Conclusion

Reactions in sub- and supercritical fluids can be used to depolymerize spent plastics for the chemical

recycling process. Condensation polymerization polymers such as PET were monomerized by

solvolysis in supercritical water or alcohol. Addition polymerization polymers such as phenol resin

were also decomposed in sub- and supercritical fluids. For fiber reinforced plastics, fibers and chemicals

were recovered. Supercritical fluid was used for decrosslinking to recycle crosslinked polyethylene

without depolymerization of the polymers. Pilot to commercial scale plants have been constructed in

Japan for the chemical recycling of various plastics.

Page 16 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

15

References

[1] S. Saito (Ed.), Science and Technology of Supercritical Fluids, Sankyo Business, Tokyo (1996)

[2] S. Yamamoto, M. Aoki, M. Yamagata: Recovery of monomers from poly(ethylene terephthalate) by

hydrolysis under pressure. R-D Kobe Steel Engineering Reports, 46 (1996) 60-63

[3] T. Sako, T. Sugeta, K. Otake, N. Nakazawa, M. Sato, K. Namiki, M. Tsugumi: Depolymerization of

polyethylene terephthalate to monomers with supercritical methanol. J. Chem. Eng. Japan, 30 (1997)

342-346

[4] R. E. N. de Castro, G. J. Vidotti, A. F. Rubira, E. C. Muniz: Depolymerization of poly(ethylene

terephthalate) wastes using ethanol and ethanol/water in supercritical conditions. J. Appl. Poly. Sci., 101

(2006) 2009-2016

[5] M. Goto, H. Koyamoto, A. Kodama, T. Hirose, S. Nagaoka, B. J. McCoy: Degradation kinetics of

poly(ethylene terephthalate) in supercritical methanol. AIChE J., 48 (2002) 136-141

[6] M. Goto, H. Koyamoto, A. Kodama, T. Hirose, and S. Nagaoka: Depolymerization of polyethylene

terephthalate in supercritical methanol. J. Phys.: Condens. Matter, 14 (2002) 11427-11430

[7] M. Genta, T. Iwaya, M. Sasaki, M. Goto, T. Hirose: Depolymerization mechanism of poly(ethylene

terephthalate) in supercritical methanol. Ind. Eng. Chem. Res. 44 (2005) 3894-3900

[8] M. Genta, T. Iwaya, M. Sasaki, M., Goto: Supercritical methanol for polyethylene terephthalate

depolymerization: Observation using simulator. Waste Management 27 (2007) 1167-1177

[9] M. Genta, F. Yano, Y. Kondo, W. Matsubara, S. Oomoto: Development of chemical recycling process

for post-consumer PET bottle by methanolysis in supercritical methanol. Mitsubishi Heavy Industries,

Ltd. Technical Review, 40 (2003) 1-4

[10] M. Genta, R. Uehara, F. Yano, Y. Kondo, W. Matsubara, Development of chemical recycling process

for post-consumer PET bottle by methanolysis in supercritical methanol. Proc. 6th Int. Symp.

Supercritical Fluids (2003) 1381-1386

Page 17 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

16

[11] M. Goto, M. Umeda, A. Kodama, T. Hirose, S. Nagaoka: Monomerization of nylon 6 in sub-and

supercritical water. Kobunshi Ronbunshu, 58, (2001) 548-551

[12] M. Goto, M. Umeda, A. Kodama, T. Hirose, S. Nagaoka, S. Matsuda, S. Masuhara, J. Hiraki:

Synthesis of 2-amino-ε-caprolactam by cyclodehydration of lysine in subcritical water. J. Chem. Eng.

Japan, 37 (2004) 353-356

[13] A. Ikeda, K. Katoh, H. Tagaya: Monomer recovery of waste plastics by liquid phase decomposition

and polymer synthesis. J. Materials Science 43 (2008) 2437-2441

[14] R. Pinero, J. Garcia, M. J. Cocero: Chemical recycling of polycarbonate in a semi-continuous

lab-plant. A green route with methanol and methanol-water mixtures. Green Chem., 7 (2005) 380-387

[15] R. Pinero-Hernanz, J. Garcia-Serna, M. J. Cocero: Nonstationary model of the semicontinuous

depolymerization of polycarbonate. AIChE J., 52 (2006) 4186-4199

[16] Y. Nagase, M. Yamagata, T. Matsuda, T. Naito, K. Kodama, R. Fukuzato: Chemical recycling process

for scrap polyurethane plastics using supercritical water. Proc. 5th Meeting on Supercritical Fluids, (1998)

127-132

[17] Z. Dai, B. Hatano, J. Kadokawa, H. Tagaya: Effect of diaminotoluene on the decomposition of

polyurethane foam waste in superheated water. PolymDegrad. Stab. 76 (2002) 179-184

[18] Z. Dai, B. Hatano, H. Tagaya: Decomposition of a polyol in supercritical water. Polym. Degrad. Stab.,

80 (2003) 353-356

[19] R. Fukuzato: Current status of supercritical fluid technology in the East Asia. Proc. 6th Int. Symp.

Supercritical Fluids, Tome 1 (2003) 1-10

[20] Y. Suzuki, H. Tagaya, T. Asou, J. Kadokawa, K. Chiba: Decomposition of prepolymers and molding

materials of phenol resin in subcritical and supercritical water under an Ar atmosphere. Ind. Eng. Chem.

Res., 38 (1999) 1391-1395

[21] H. Tagaya, Y. Suzuki, T. Asou, J. Kadokawa, K. Chiba: Reaction of model compounds of phenol

Page 18 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

17

resin and molding materials of phenol resin in supercritical water for chemical recycling of polymer waste.

Chem. Lett. (1998) 937-938

[22] M. Goto, M. Kitamura, T. Hirose, K. Shibata: Depolymerization of printed circuit board in

near-critical water. Hydrothermal Reactions and Techniques, World Scientific, New Jersey (2003)

201-208

[23] M. Watanabe, T. Adschiri, K. Arai: Polyethylene decomposition via pyrolysis and partial oxidation in

supercritical water. Kobunshi Ronbunshu, 58 (2001) 631-641

[24] T. Moriya, H. Enomoto: Characteristics of polyethylene cracking in supercritical water compared to

thermal cracking. Polym. Degrad. Stab., 65 (1999) 373-386

[25] T. Moriya, H. Enomoto: Conversion of polyethylene to oil using supercritical water and donation of

hydrogen in supercritical water. Kobunshi Ronbunshu, 58 (2001) 661-673

[26] T. Sugeta, S. Nagaoka, K. Otake, T. Sako: Decompositon of fiber reinforced plastics using fluid at

high temperature and pressure. Kobunshi Ronbunshu, 58 (2001) 557-563

[27] I. Okajima, K. Yamada, T. Sugeta, T. Sako: Decomposition of epoxy resin and recycling of CFRP

with sub- and supercritical water. Kagaku Kogaku Ronbunshu, 28 (2002) 553-558

[28] A. Kamimura, K. Yamada, T. Kuratani, Y. Taguchi, F. Tomonaga: Effective depolymerization waste

FRPs by treatment with DMAP and supercritical alcohol. Chem. Lett., 35 (2006) 586-587

[29] R. Pinero-Hernanz, C. Dodds, J. Hyde, J. Garcia-Serna, M. Poliakoff, E. Lester, M. J. Cocero, S.

Kingman, S. Pickering, K. H. Wong: Chemical recycling of carbon fibre reinforced composites in

nearcritical and supercritical water. Composites: Part A 39 (2008) 454-461

[30] R. Pinero-Hernanz, J. Garcia-Serna, C. Dodds, J. Hyde, M. Poliakoff, M. J. Cocero, S. Kingman, S.

Pickering, E. Lester: Chemical recycling of carbon fibre composites using alcohols under subcritical and

supercritical conditions. J. Supercritical Fluids, 46 (2008) 83-92

[31] H. Fukuzawa, K. Shibata, H. Izawa, Proc. 13th conference of the Japan Society of Waste

Page 19 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

18

Management Experts, (2002) I: 428

[32] H. Fukuzawa, K. Shibata, H. Izawa, A. Matsuo, Proc. Int. Symp. Feedstock Recycling 2002 (2002)

A30

[33] T. Iwaya, S, Tokuno, M. Sasaki, M. Goto, K. Shibata: Recycling of fiber reinforced plastics using

depolymerization by solvothermal reaction with catalyst. J. Mat. Sci. 43 (2008) 2452-2456

[34] T. Nakagawa: FRP recycling technology using sub-critical water hydrolysis. JEC Composites

Magazine, March-April (2008) 40-43

[35] S. Venkatraman, L. Kleiner: Properties of three types of cross-linked polyethylene. Adv. Polym.

Tech., 9 (1989) 265-270

[36] S. Watanabe, K. Komura, S. Nagaya, H. Morita, T. Nakamoto, S. Hirai, F. Aida: Development of

cross-linked polymer material recycling technology by supercritical water. Proc. Int. Conf. Properties and

Applications of Dielectric Materials, 7 (2003) 595-598

[37] T. Goto, T. Yamazaki: Recycling of silane-cross-linked polyethylene for insulation of cables using

supercritical alcohols. Hitachi Cable Review, 23 (2004) 24-27

[38] T. Goto, S. Ashihara, T. Yamazaki, K. Watanabe: Evaluation of recycling technology of insulation of

cross-linked polyethylene insulated cable using supercritical alcohol. IEEJ Trans. PE, 126 (2006) 400-406

[39] T. Goto, T. Yamazaki, T. Sugeta, I. Okajima, T. Sako: Selective decomposition of the siloxane bond

constituting the crosslinking element of silane-crosslinked polyethylene by supercritical alcohol. J. Appl.

Polym. Sci., 109 (2008) 144-151

[40] S. M. Hong, H. K. Cho, C. M. Koo, J. H. Lee, W. Y. Park, H. S. Lee, Y. W. Lee: Decross-linking of

cross-linked polyethylene using supercritical methanol. Korean Chem. Eng. Res. 46 (2008) 63-68

Page 20 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

19

List of figure captions

Figure 1 Reaction scheme for decomposition of PET in methanol

Figure 2 Relationship between the yields of products and the reaction time in decomposition of PET

Figure 3 Decomposition of nylon 6 in subcritical water

Figure 4 Decomposition reaction of polycarbonate.

Figure 5 The concept of chemical recycling of TDI to TDA

Figure 6 Recovered glass fiber (left) and carbon fiber (right) from treatment in BZA.

Figure 7 A concept of FRP recycling developed at Matsushita Electric Works, Ltd.

Figure 8 Gel fraction and molecular weight of the reaction products from silane-XLPE by sub- and

supercritical water or methanol

Page 21 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

20

Figure 1 Reaction scheme for decomposition of PET in methanol

PET

Oligomer

MHET

DMT EG TAMME

DEG OH2 ME

MeOH

MeOH

MeOH

MeOH OH2

C241

O

O

C244

O

OH CH3

OH CH2CH

2OH

C258

O

O

C261

O

OCH3

CH3

C282

O

O

C285

O

CH2CH

2O

C300

O

O

C303

O

O CH2CH

2OHCH

3MHET

DMT

TAMME

EG

ME

DEG

OH CH2CH

2O CH

3

OH CH2CH

2O CH

2CH

2OH

+ +

+ +

PETn

k2

k1

k4

k5 k6

k3

k7k8

Page 22 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

21

Figure.2 Relationship between the yields of products

and the reaction time in decomposition of

PET

0

20

40

60

80

100

0 10 20 30 40 50 60 70Reaction time (min)

Yie

ld (

% )

DMT

MHET

BHET

Page 23 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

22

NH

O

NH(CH2)5CO n

NH2(CH

2)5COOH

Secondary products

Nylon 6

-aminocaproic acid

-caprolactam

Fig. 3 Decomposition of nylon 6 in subcritical water.

Page 24 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

23

Fig. 4 Decomposition reaction of polycarbonate.

Page 25 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

24

Fig. 5 The concept of chemical recycling of TDI to TDA

Page 26 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

25

Fig. 6 Recovered glass fiber (left) and carbon fiber (right) from the treatment in BZA.

Page 27 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

26

Fig. 7 A concept of FRP recycling developed at Matsushita Electric Works, Ltd.

Page 28 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

27

Fig. 8 Gel fraction and molecular weight of the reaction products from silane-XLPE by sub-

and supercritical water or methanol

Page 29 of 29

Accep

ted

Man

uscr

ipt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

28

Table 1. Critical parameters of water and alcohols

Critical temperature [K] Critical pressure [MPa]

Water 647.10 22.064

Methanol 512.64 8.097

Ethanol 513.92 6.148

1-Propanol 536.78 5.175

Acetone 508.10 4.700

Benzyl alcohol 715.00 4.300