Accepted Manuscript

Application of waste poly(ethylene terephthalate) in the synthesis of new oligomericplasticizers

Ewa Langer, Sylwia Wakiewicz, Marta Lenartowicz-Klik, Krzysztof Bortel

PII: S0141-3910(15)00168-8

DOI: 10.1016/j.polymdegradstab.2015.04.031

Reference: PDST 7645

To appear in: Polymer Degradation and Stability

Received Date: 27 January 2015

Revised Date: 24 April 2015

Accepted Date: 29 April 2015

Please cite this article as: Langer E, Wakiewicz S, Lenartowicz-Klik M, Bortel K, Application of wastepoly(ethylene terephthalate) in the synthesis of new oligomeric plasticizers, Polymer Degradation andStability (2015), doi: 10.1016/j.polymdegradstab.2015.04.031.

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Application of waste poly(ethylene terephthalate) in the synthesis of new

oligomeric plasticizers

Ewa Langera, Sylwia Wakiewiczb, Marta Lenartowicz-Klika, Krzysztof Bortela

aInstitute for Engineering of Polymer Materials and Dyes, 87-100 Toru, ul. M.

Skodowskiej-Curie 55, Poland, e-mail: e.langer@impib.pl

bSilesian University of Technology, Faculty of Chemistry, 44-100 Gliwice, ul. Strzody 9,

Poland

Keywords: chemical recycling, poly(ethylene terephthalate) waste, oligomeric plasticizer

ABSTRACT

New method of trensesterification of waste poly(ethylene terephthalate) (PET) with aliphatic

oligoesters was developed. The structures of obtained oligoesters were identified by NMR,

ESI-MS and SEC methods, and then correlated with physical properties determined by DSC

and TGA analyses. Physico-chemical properties of synthesized plasticizers were compared

with monomeric and polymeric commercial products. Products of the reaction of PET with

oligoesters based on azelaic acid with 1.4-butanediol and adipic acid with triethylene glycol

occurred to be remarkable substitutes of commercial plasticizers. They possessed lower

volatility and much higher thermal stability. Insertion of glycerol unit into aliphatic oligoester

and using it for the process of PET depolymerization resulted in obtaining of plasticizers of

branching structure with glycerol unit as a core. They possessed lower viscosity and higher

molecular mass in comparison with their linear equivalents.

1. Introduction

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Poly(ethylene terephthalate) (PET) is one of the most commonly used engineering plastics

which owes its popularity to its mechanical properties, chemical resistance, clarity, low O2

and H2O permeability, and good rigidity/weight ratio. The use of poly(ethylene terephthalate)

has increased significantly in recent years since its introduction as a material for the

production of beverage packaging. Moreover, it is widely used in the textile industry, high

strength fibres and photographic films. PET itself is not directly hazardous for the natural

environment but it does make up a considerable volume of all the municipal waste ending up

in landfills. It does not erode due to its high resistance to weathering and biological agents.

PET is a non-degradable plastic in normal conditions, as there is no known organism that can

consume its relatively large molecules [1]. However PET is one of the most extensively

recycled polymeric materials.

There are three distinct approaches to the recycling of post-consumer plastic packaging

materials. The Environmental Protection Agency (EPA) has adopted a new extensive

nomenclature that refers to physical reprocessing as secondary recycling (2) and chemical

processing as tertiary recycling (3). The EPA primary recycling (1) refers to the use of pre-

consumer industrial scrap and salvage to form new packaging, a common product in industry

[2].

There are numerous chemical ways of recycling PET, which include: hydrolysis, alcoholysis,

aminolysis, acidolysis, glycolysis and transesterification. PET is chemically re-processed by

its total depolymerization into monomers or partial depolymerization into oligomers and other

products [3]. Table 1 presents the chemical methods of chemical recycling of PET, the main

reactants and the products obtained [4-8].

Table 1. Methods of chemical recycling of PET

Method Reactant Reaction products

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Hydrolysis Water Terephthalic acid and ethylene glycol

Alcoholysis Methanol Dimethyl terephthalate and ethylene glycol (1.2-ethane diol)

Aminolysis Amine Terephthalamide Glycolysis Ethylene glycol Bishydroxyethyl terephthalate

and ethylene glycol

The utilization of PET waste generates value-added products such as unsaturated polyester

resins, oligo- or polyester plasticizers, acrylate/methacrylate terminated oligoesters and raw

materials for polyurethanes [5-8]. Our study focused on oligoester plasticizers obtained by

means of the chemical recycling of PET.

The most commonly used plasticizers are monomeric plasticizers, such as: phthalates,

adipates and benzoates. Their disadvantages include lower resistance of the bond line to heat

and possible migration. However, the use of phthalic plasticizers has been on the decrease due

to their toxicity and tendency to sweat out. Some companies and sectors have looked for safer

materials as alternatives to certain phthalates. While oligomeric plasticizers have smaller

plasticizing ability compared to monomeric plasticizers, they exhibit limited volatility and

migration and do not undergo extraction, which is essential in many applications [9].

There are many publications in the literature on obtaining dioctyl terephthalate (DOTP) in the

process of PET alcoholysis. There are few studies, however, that describe the synthesis of

oligomeric plasticizers based on the products of PET waste depolymerisation. For instance,

Dupont et al. [10] reported on the alcoholysis of PET scrap using 2-ethyl-1-hexanol (EH) at

reflux temperature for the purpose of synthesizing DOTP plasticizers for flexible poly(vinyl

chloride) (PVC). The DOTP produced by this method was equivalent to commercial grades in

terms of its plasticization efficiency for PVC.

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Dutt and Soni, on the other hand, synthesized an oligomeric plasticizer with an average

molecular mass in the range of 450-900 g/mol from PET waste also through alcoholysis using

EH. They used it in nitrile rubber and nitryle-PVC blends [11].

The boiling point of EH at atmospheric pressure is about 180 C and the efficiency of

alcoholysis is very low. Three methods for improving the efficiency of alcoholysis are known:

use of sub- and supercritical EH, use of transesterification catalyst and/or addition of some

cosolvents [12].

The use of a cosolvent is a new way to accelerate the chemical reaction and it also improves

reaction efficiency. The imidazole ionic liquid assisted in the process of PET alcoholysis with

EH, as a cosolvent in obtaining DOTP. This process was catalyzed by the addition of 1.2%

(w/w) of zinc acetate. The yield of DOTP reached 93% at reflux temperature and a reaction

time of 5 h, while the weight ratio of the ionic liquid:EH:PET was 2:2:1. The reaction time of

traditional reflux temperature alcoholysis of PET without ionic liquid as a cosolvent should be

at least 10 h.

Oligoester plasticizers, with hydroxyl end-groups and an average molecular mass of 2500

g/mol, were also obtained by the degradation of PET waste by polyethylene glycol 400 and

adipic acid in the presence of a transesterification catalyst. These compounds were tested as

plasticizers in a poly(vinyl acetate) dispersion adhesives for flooring applications. The

samples containing synthesized plasticizers were more flexible and had a higher thermal

stability in comparison to commercial plasticizer 1.2.3-triacetoxypropane [13].

In recent years, the following compounds have been used as catalysts for the glycolysis or

transesterification of PET: metal acetates [14-15], phosphates [16], solid super-acid, metal

oxide [17-18], carbonate [19], sulfate [20] and ionic liquids.

In chemical synthesis organotin compounds are used expecially in the esterification and

transesterification reactions of mono- and polyesters. Organotin compounds such as

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butylstannoic acid are used as catalyst to reduce the formation of unwanted by-products and

also provide the required colour properties [21].

The aim of the presented work was the full replacement of the low molecular weight toxic

phthalate-based plasticizers for PVC, which have been used so far, with environmentally

friendly materials. The paper presents a new, previously unexplored method for obtaining

oligomeric plasticisers in a reaction of PET waste transesterification by means of oligoesters.

2. Experimental

2.1. Materials

A sample of PET flakes was acquired from Industrie Maurizio Peruzzo POLOWAT Sp. z o.o.

(average molecular weight 50000 g/mol). Anhydrous 2-ethylhexanol, adipic acid (Ad), azelaic

acid (Az), diethylene glycol (DE), dipropylene glycol (DP), 1.4-butandiol (BD), triethylene

glycol (TE) and glycerine (Gl) were purchased from Brenntag Polska. All reagents were used

as purchased without further purification. Fascat 4100, butylstannoic acid, was used as a

catalyst.

2.2. Transesterification of PET

Waste PET-based plasticiser synthesis was conducted in two stages. A 1000 ml glass reactor

equipped with an agitator, a splash-head, a thermometer and an azeotropic cap was filled with

dicarboxylic acid, glycol and monohydroxyl alcohol (Table 2). The reaction was carried out in

a temperature range of 140160 C under atmospheric pressure. The reaction was carried out

until an acid value of less than 10 mgKOH/g was achieved. PET waste and 0.06% w/w of the

Fascat 4100 esterification catalyst was added in situ to the oligoester obtained. The

temperature of the reaction was increased to 190-210 C. The total time of the synthesis was

10-12 hours.

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The following plasticisers were obtained in the oligoester degradation of PET waste:

azelaic acid with diethylene glycol and a 2-ethylhexanol end-group (designated as

PETDEAz);

adipic acid with dipropylene glycol and a 2-ethylhexanol end-group (designated as

PETDPAd);

azelaic acid with dipropylene glycol and a 2-ethylhexanol end-group (designated as

PETDPAz);

adipic acid with 1.4-butanediol and a 2-ethylhexanol end-group (designated as

PETBDAd);

azelaic acid with 1.4-butanediol and a 2-ethylhexanol end-group (designated as

PETBDAz);

adipic acid with glycerine and dipropylene glycol and a 2-ethylhexanol end-group

(designated as PETDPAdGl);

azelaic acid with glycerine and dipropylene glycol and a 2-ethylhexanol end-group

(designated as PETDPAzGl);

azelaic acid with glycerine and glycol and a 2-ethylhexanol end-group (designated as

PETDEAzGl);

The obtained products were characterized using NMR spectroscopy, ESI-MS and SEC

spectrometry techniques, thermal analyses (DSC, TGA) and were checked for volatility.

Table 2. Composition of synthesized plasticizers*

Symbol Composition

PETD

EAz

PETD

PAd PETD

PAz

PETB

DAd PETB

DAz

PETD

PAdGl PETD

PAzGl PETD

EAzGl

PET (g) 30.20 31.61 28.90 34.08 31.00 24.64 22.24 23.00 2-ethylhexanol (g) 13.70 14.27 13.10 15.39 13.99 16.69 15.07 15.58

Adipic acid (g) - 32.05 - 34.55 - 37.52 - - Azelaic acid (g) 39.40 - 37.80 - 40.47 - 43.59 45.05

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Glycerine (g) - - - - - 3.94 3.55 3.67 Dipropylene glycol

(g) - 22.06 20.20 - - 17.22 15.54 -

Diethylene glycol

(g) 16.70 - - - - - - 12.70

1.4-butanediol (g) - - - 15.98 14.53 - - - * the reactant amounts provided have been calculated per 100g of reactant load

2.3. Characterization of the transesterification products

Nuclear Magnetic Resonance (NMR) spectra were recorded using a UNITY/INOVA 300

MHz (Varian Associates Inc.) multinuclear NMR spectrometer. 1H and 13C NMR spectra

were run in deuterated chloroform (CDCl3) using tetramethylsilane (TMS) as an internal

standard.

Differential scanning calorimetry (DSC) analyses were carried out using a DSC 2010 Thermal

Analysis Calorimeter. Measurements and calibration were carried out at a heating rate of 10

C/min in a nitrogen atmosphere.

The decomposition temperature of the plasticisers was determined on the basis of the TGA

(Thermogravimetry) analysis according to EN ISO 11358:2004Plastics -

Thermogravimetry (TG) of polymers - General principles. Samples were heated at a rate of

20 C/min from 25 C to 900 C under nitrogen atmosphere.

Electrospray ionisation mass spectrometry (ESI-MS) experiments were performed using an

AmaZon (Bruker-Daltonics, Brema, Germany) mass spectrometer equipped with an

electrospray ionisation source. Samples were dissolved in a solution of CHCl3/methanol ((v/v)

1:1). The mass spectrum was acquired over the range of m/z 503000 in the positive ion

mode.

Molecular weight was measured by means of size exclusion chromatography analysis (SEC)

using a Waters system equipped with refractive index detector. Two 300x7.5 mm (Polymer

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Laboratories VARIAN) Pl-gel m Mixed C columns were used and maintained at 40 C.

Fisher Chemicals tetrahydrofuran (THF) was used as eluent at a flow rate of 1 mL/min.

Polystyrene standards (Polymer Laboratories) were used to calibrate the system.

The viscosity of the plasticizers was determined in accordance with ISO 2555.

The volatility of the plasticizers was determined by the authors own method. Volatility was

determined by placing the samples of the tested plasticisers in petri dishes in a drier without

air circulation for 2 h at temperatures of 160 to 180 C, and the obtained values (expressed in

%) were calculated on the basis of the mass loss of the sample.

3. Results and discussion

Oligoester plasticizers were obtained in a 2-step reaction. The first stage consisted of the

synthesis of oligoesters from dicarboxylic acid (adipic or azelaic acid), glycol (diethylene,

dipropylene or 1.4-butanediol glycol) and 2-ethylhexanol. It was conducted until an acid

value of less than 10 mgKOH/g was achieved. The second stage, on the other hand, involved

the transesterification of waste PET using the previously synthesised oligoester using the

Fascat 4100 catalyst (Fig. 1) until a hydroxyl and acid value of less than 10 mg KOH/g was

achieved. This made it possible to assume that the oligoesters obtained had 2-ethylhexanol

ends on both sides.

Fascat 4100 (butylstannoic acid, BuSn(O)OH) is insoluble solid in a series of solvents and is

categorized as stable oligomeric structure at room temperature. However, on increasing the

temperature these particular arrangements can be destabilized, resulting in more active

molecular species [22]. One of the main advantages of this catalyst is lack of necessity of

neutralization or filtration at the end of reaction. Besides it provides energy savings with

lower reaction temperatures.

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According to literature data, transesterification mechanism using Lewis acid catalysts, the

acid site (in this case free tin orbital) is joining the oxygen of the carbonyl group, increasing

the electrophilicity of the adjoining carbon atom and making it more susceptible to nucleofilic

attack [23]. For this reason depolymerisation of PET chains via transesterification reaction

using oligoesters tipped with 2-ethylhexanol takes place.

HOCRCOHO O

+ 34 HOR'OH + 2 OH

3

OOCRCOCRCOR'OOO O

+

CO

COCH2CH2OO

n

Fascat 4100

products of transesterification

stage 1

stage 2

Fig. 1. Scheme of obtaining of linear oligoesters of PET

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HOCRCOHO O

+ 912 HOR'OH + 3 OH

CO

COCH2CH2OO

n

Fascat 4100

products of transesterification

stage 1

stage 2

HO OHOH

+

C ORC OOR'

C OR

OC O

O

OCRCO O O O

OR'OCRCOCRCOR'O

OOO

O OCRCOO

3 3

+

3

Fig. 2. Scheme of obtaining of branched oligoesters of PET

On the basis of an analysis of 1H NMR (Fig. 3-4) and 13C NMR (Fig. 5-6) spectra it is

confirmed that both the anticipated reactions, i.e. the synthesis of oligoesters and

transesterification, occurred. This is revealed by the fact that no free function groups, i.e.

neither hydroxyl nor carboxyl groups, were present. In plasticisers, glycols can connect on

both sides with dicarboxylic acid, on both sides with terephthalic acid (TA) or simultaneously

with dicarboxylic and terephthalic acid. All these combinations were observed in 1H NMR

spectra in the range of 3.25-5.50 ppm and in 13C NMR spectra in the range of 60-75 ppm

which were characteristic for the protons and atoms of the carbons of groups -CH2-O-, >CH-

O- respectively in different chemical environments. In addition, the structures of the

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compounds found in the mixture created in the reaction were confirmed through an ESI-MS

analysis. The NMR analysis is an important tool in the study of the structure of the oligomer

chain but only an ESI-MS investigation can determine the subtle differences in the chemical

structure. These spectra clearly indicate that a statistically significant transesterification

reaction took place, where the PET chains have chemically degraded into fragments of

different lengths.

Fig. 3. 1H NMR (CDCl3, 300 MHz) spectra of a) oligoester DPAz and b) products of

transesterification of PET with DPAz (PETDPAz)

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Fig. 4. 1H NMR (CDCl3, 300 MHz) spectra of a) oligoester DEAzGl and b) products of

transesterification of PET with DEAzGl (PETDEAzGl)

77.16 Chloroform-d

Fig. 5. 13C NMR (CDCl3, 75 MHz) spectrum of products of transesterification of PET with

DPAz (PETDPAz)

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Fig. 6. 13C NMR (CDCl3, 75 MHz) spectrum of products of transesterification of PET with

DEAzGl (PETDEAzGl)

On the basis of an analysis of a sample ESI-MS spectrum of a PETDPAz specimen (Fig. 7), it

was found that the plasticizers had 2-ethylhexanol connected with azelaic acid at their ends

(m/z= 721.3, 961.5, 1100.7, 1261.1, 1699.6 and 1999.8) or longer fragments of oligoester (-

Az-DP-) obtained in the first stage of the synthesis (m/z = 1482.3, 2156.6 and 2306.4), where

a -TA-GE-TA- fragment constituted the core. The presence of DE (m/z = 1261.1 and 1699.6)

is the result of an ethylene glycol (EG) reaction taking place, an important side reaction in

PET synthesis [24].

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Fig. 7. ESI-MS spectra of PETDPAz

In the case of the introduction of glycerine, i.e. an additional reactant into the esterification

reaction, a regularity was observed, which differentiated these branched out plasticiser

molecules from the previously discussed linear molecules (Fig. 8). Namely, one glycerine unit

constituted the core of a plasticizer molecule, which connected directly with a unit of TA. The

further structure of the individual arms of the oligoester branching out from the glycerine

molecule is akin to forms created in the linear plasticisers.

Fig. 8. ESI-MS spectra of PETDEAzGl

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Table 3. The results of a gel chromatography analysis of the plasticizers obtained

Symbol Number average molecular weight

Mn (g/mol)

Weight average molecular weight

Mw (g/mol) Dispersity

PETDEAz 1430 2500 1.75 PETDPAd 1530 2780 1.82 PETDPAz 1420 2600 1.83 PETBDAd 1610 2730 1.70 PETBDAz 1900 3400 1.80

PETDPAdGl 1590 3830 2.41 PETDPAzGl 1650 3950 2.39 PETDEAzGl 1670 3920 2.34

PETTEAd 1520 2780 1.83 H-1 2270 4230 1.87

The SEC analysis also leads to the conclusion that the degradation of waste PET occurred by

transesterification reaction, leading to the formation of the anticipated products - oligoesters

with TA-EG- units embedded in their structure. In weight terms, the average molecular

weight in the case of linear oligoesters ranged from 2500 to 3400 g/mol, and their dispersity

ranged from 1.70 to 1.83. However, in the case of branched oligoesters, these values ranged

from 3830 to 3950 g/mol and 2.34 to 2.41 respectively. It was found that adding glycerine to

the synthesis as a reactant caused adverse effects as it led to a significant increase in the

dispersion of the product in each and every case.

In Table 4, the viscosity designations of the synthesised plasticisers and commercial,

polymeric plasticizer (H1) are presented. The viscosity of plasticizers does not depend only

on the molecular weight, but to a large extent also on the structure of the compound obtained

and the raw materials used in the synthesis. Lower viscosity levels are observed in products

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having a branched structure, which were synthesised using glycerine. Longer aliphatic chains

produce a lower viscosity. By using the same glycols, a lower viscosity was observed in

plasticizers with azelaic acid than with adipic acid.

Table 4. Viscosities of the plasticizers obtained

Symbol Viscosity (mPas)

PETDEAz 13 000

PETDPAd 23 500 PETDPAz 10 700

PETBDAd 30 500 PETBDAz 27 500

PETDPAdGl 9 000 PETDPAzGl 9 500 PETDEAzGl 8 750

PETTEAd 9 600 H-1 6 750

The volatility of oligoesters at temperatures of 160, 170, 180 C was determined. In

characterising plasticisers, their volatility, particularly at higher temperatures, is an important

parameter. Taking into account the processing temperature of plasticised PVC compositions,

it is required that the plasticizers present as low a volatility as possible, due to the possible

loss of the plasticizer during the process as well as its escape into the environment. In

addition, the high volatility of the plasticiser makes plasticised products lose their properties

during use particularly in high temperature environments. For all the synthesised oligoesters,

the volatility values were lower compared to the volatility of the monomeric DEHP

plasticiser. At a temperature of 180 C in particular, the difference between the synthesised

products and a monomeric commercial product is significant. PETTEAd and PETBDAz

oligoesters have a volatility of 1% at this temperature. The higher volatility values of

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branched oligoesters containing glycerine correspond to higher dispersion values compared to

linear products. It is observed that the dispersion value also has an influence on the thermal

stability of oligoesters. This general tendency, along with an increase in dispersion, leads to a

decrease in thermal stability, which corresponds to a lower temperature at which a given mass

loss occurs (Fig. 9).

Fig. 9. Plasticiser mass loss versus temperature

Table 5. Decomposition temperature and glass transition temperature (Tg) of the obtained plasticisers

Composition Decomposition temperature (C) Tg (C) PETDEAz 431.0 -44.6 PETDPAd 444.1 -47.3 PETDPAz 381.2 -45.8 PETBDAd 423.6 -46.2 PETBDAz 421.1 -51.1

PETDPAdGl 418.6 -45.5 PETDPAzGl 410.3 -48.2

PETDEAzGl 445.8 -43.6 PETTEAd 398.9 -45.3

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H1 379.3 -49.4

The glass transition temperatures of the obtained plasticisers compared to the commercial

varieties are similar and range from -43.6 to -51.1 C.

When comparing the decomposition temperatures of the obtained oligoesters with those of the

commercial plasticiser it was found that these compounds have a much higher thermal

stability. Only PETDPAz has a decomposition temperature which is similar to that of the

polymeric commercial plasticiser. As shown in the diagram (Fig. 10) the mass loss curve is

also similar. The plasticiser samples containing glycerine revealed a much higher

decomposition temperature compared to its analogue varieties without glycerine.

Fig. 10. Volatility of synthesised oligoesters compared to that of commercial plasticisers

4. Conclusions

The research results proved that it is possible to use waste PET for the synthesis of new

oligoesters, which can be used as plasticisers. Taking into consideration the general

characteristics of each sample of the synthesised plasticiser and comparing them with

monomeric and polymeric commercial products, PETBDAz and PETTEAd seem to be the

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most promising. Mentioned plasticizers possessed lower volatility and much higher thermal

stability in comparison with commercial products.

It was proved that in spite of relatively high molecular mass of obtained plasticizers of

branching structure they possess lower viscosity in comparison with synthesized oligoester

products of linear structure. However the disadvantage of these products is their high

molecular-weight dispersity (above 2.3) what results in higher volatility.

The results of tests of the useability of the synthesised plasticisers in obtaining plasticised

PVC compositions will be presented in a later article.

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