Vacuum pyrolysis of commingled plastics containing PVC I. Kinetic study

Vacuum pyrolysis of commingled plastics containing PVCI. Kinetic study

Rosa Miranda a, Jin Yang b, Christian Roy a,b,*, Cornelia Vasile c

aChemical Engineering Department, Universite Laval, Sainte-Foy, Quebec, Canada G1K 7P4bInstitut Pyrovac Inc., Parc technologique du Quebec metropolitain, 333, rue Franquet, Sainte-Foy, Quebec, Canada G1P 4C7

cPetru Poni Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41 A, RO 6600 Iasi, Romania

Received 27 October 2000; accepted 19 January 2001

Abstract

Thermal decomposition of commingled plastics comprising high and low density polyethylene, polystyrene, polypropylene andpoly(vinyl chloride) was investigated under vacuum conditions by dynamic thermogravimetric analysis in the temperature range of25–600�C. The mixtures were representative of the major polymeric materials found in a municipal plastic waste (MPW) stream.

The study focused on the role of each polymer in the stabilization or destabilization of other interacting plastics, as well as the fateof chlorine produced from PVC. First, the decomposition of each single plastic was investigated. Then the binary combination of allabove polymers and finally the multicomponent mixture similar to a MPW sample were studied. Two approaches have been applied

to study the interaction between components in the commingled plastics. The first approach involved a comparison of the experi-mental curve of the decomposition of the mixture with a calculated non-interacting component decomposition curve based on thebehavior of the individual polymers. The second approach involved a comparison of the kinetic parameters of each plastic in themixture as determined by two different methods: (1) fitting the single polymer decomposition curve and (2) fitting the peak corre-

sponding to the pyrolysis of one particular polymer on the mixture pyrolysis curve. The results obtained indicate that some inter-actions occurred during MPW pyrolysis, mainly at high temperatures (>375�C). PS and PVC appear to be the plastics responsiblefor the interactions through their intermediate pyrolysis products. The chlorine from PVC in the mixture is released almost com-

pletely before 375�C, when the conversion of the MPW to pyrolysis products has reached ca. 13%. # 2001 Elsevier Science Ltd. Allrights reserved.

Keywords: Commingled plastics; PVC; Thermal degradation; Kinetics; Vacuum; Pyrolysis

1. Introduction

Thermal degradation of mixed plastics has recentlyreceived much interest because the disposal of municipalsolid waste (MSW) has become a growing and costlyproblem as space available for landfilling decreases.Pyrolysis of waste plastics is of interest because plasticrefuses represent an alternative source of energy andchemical raw materials.In North America, the average composition of the

MPW stream is 46 wt.% high and low density poly-ethylene (HDPE and LDPE), 16 wt.% polypropylene

(PP), 16 wt.% polystyrene (PS), 7 wt.% polyvinyl chlo-ride (PVC), 5 wt.% polyethylene terephthalate (PET), 5wt.% acrylonitrile-butadiene-styrene copolymer (ABS)and 5 wt.% of other polymeric materials [1]. Throughpyrolysis, some polymers such as PS can be readilydepolymerized, leading to the recovery of a significantproportion of the original monomer. The reported yieldof monomer from polystyrene varies between 40 and 60wt.% [2]. Polyethylene, polypropylene and poly(vinylchloride) do not yield significant amounts of monomers.However they are transformed into low molecularweight fragments and compounds which can be thesource of valuable liquid fuels and new chemicals [3].In order to recover high yields of useful products from

MPW pyrolysis and in order to control any potentiallyharmful products released during the pyrolysis reactions,the decomposition behavior of the MPW must be

0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(01 )00048-9

Polymer Degradation and Stability 72 (2001) 469–491

www.elsevier.nl/locate/polydegstab

* Corresponding author. Tel.: +1-418-656-7406; fax: +1-418-656-

2091.

E-mail addresses: [email protected] (C. Roy), cvasile@ichpp.

tuiasi.ro (C. Vasile).

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known. It is particularly desirable to understand thedecomposition behavior of each plastic component inthe MPW stream, the interactions involved, and espe-cially the release of chlorine during PVC pyrolysis. Theparticular problem caused by PVC decomposition is theformation of HCl and chlorinated hydrocarbons. Theformer can rapidly corrode the pyrolysis equipment.The latter may contaminate valuable pyrolysis products.In previous studies, the thermal decomposition beha-

vior of individual polymers such as PE [4–10], PP [11–15], PS [5,6,9,15–20] and PVC [21–24] has been exten-sively studied. Decomposition mechanisms have beenproposed and kinetic parameters were determined underdifferent sets of conditions. The degradation behavior ofa number of binary blends has also been studied byseveral researchers, with the purpose of modifying thethermal and thermo-oxidative stability or some physicalproperties of the plastic in a desired manner. Severalauthors have studied the degradation behavior of thefollowing binary blends: PVC–polyacrylonitrile [25],PVC–poly(n-butyl methacrylate) [25], PVC–poly-acrylamide [25], PVC–poly-(N-butyl methacrylamide)[25], PVC–(methyl acrylate) [25], PVC–poly(methylmethacrylate) (PMMA) [25,26], PVC–poly-a-methylstyrene(PAMS) [27], PVC–poly(tetramethylene sebacate) (PTMS)[28], PVC–poly(vinyl acetate) (PVA) [29], PVC–chlori-nated rubber [30], PVC–PS [31], PS–PMMA [32], PS–PVA [33], PS–polyisoprene [34], PS–polybutadiene [35]and PE/PS [31]. Other workers have studied the degra-dation of PP–PMMA [37,38], PS–PAMS [39], PS–poly-oxyethyleneglycol [40], PP in the presence of vinylpolymers [41], PVC–ABS [42–44], PVC–(polyester elas-tomer) (PETE) [45], PVC–polyurethane (PU) [43,44,46],PVC–(a-methylstyrene-acrylonitrile-methyl-methacry-late) (MS–AN) [47], PVC–PP [21,43,44], PVC–PMMA[48], PVC–PE [21], PVC–PS [21], PVC–polyamide 6(PA6) [21], PVC–PET [49], PP–ABS [43,44], PP–PU[43,44], PP–PE [38,50–52], PP–PS [50], PS–PE [50,53],PS–poly(2,6-dimethyl-1,4-phenylene oxide) (PPU) [54],PS–PMMA [48], PE-PMMA [38,48] and PS-Rubber(NR) [55].Knumann and Bockhorn [21] have found no obser-

vable interaction between components of the PE/PSbinary blends during degradation, while Koo and Kim[56,57] and Mccaffrey et al. [53] for PE/PS mixtures,Csomorova et al. [38] for a PE/PP blend, Day et al.[43,44] for PP/PVC mixtures, and Dodson and McNeill[31] who studied PVC/PS, have found significant inter-actions. Goulet and Prud’Homme [58] working withmixtures of PVC–poly(caprolactone) (PCL) and PVC–polyethylene adipate (PEA) and Jachowicz et al. [54]who studied PS–PPU blends using a non-isothermalthermogravimetric method demonstrated that in binarymixtures, the TG curve of the polymer mixture is not asimple superposition of the degradation curves of thecomponent polymers of the blends, due to the interactions

between the polymers. Csomorova et al. [38] also con-cluded that the thermal behavior of polymer blends isrelated to the miscibility of the respective componentsof a blend and to their interactions, where immiscibleblends show better stability than miscible blends.McNeil et al. [25–36] found that the nature of the inter-action between different polymers strongly depends onthe physical state of the system (the nature of the poly-mer, the miscibility of the polymer composition or thedegree of phase dispersion). In a heterogeneous blend,interactions occur in the bulk of one or both domainsand in the phase boundaries. In homogeneous samples,the degradation products of one polymer are directly incontact with the other polymers so that their combinedeffect on the thermal degradation is greater. McNeil etal. [25–36] suggest that the interaction between degra-dation products of one polymer with others also occuras a result of the diffusion of small mobile molecules orradicals from one to the other through the interfaciallayer. In that case, the degradation products of onepolymer may stabilize or destabilize the other polymersin the blend.Despite the fact that MPW is considered to be a source

of energy and chemical raw materials, little informationhas been published until recently on the decompositionbehavior of commingled plastics containing more thantwo polymers. Murata and Akimoto [50] studied binaryand tertiary polymer mixtures (PE, PP, PS) by TG andGC analysis. The results indicated that when PS waspresent in the mixture, the other polymers were desta-bilized. The authors suggested that the mixing of poly-mers influence the decomposition rate but not the intra-and inter-molecular reactions. Based on their results, themixing effect on the rate of decomposition was inter-preted in terms of an intermolecular radical transferbetween different polymer radicals. A similar result wasreported by Dodson and McNeill [31] who mentionedthat some radicals produced during PS degradationattack and thus destabilize the PE and PP polymers.Bockhorn et al. [59] studied complex plastic mixturessuch as PVC/PS/PE, PS/PA6/PE and PVC/PA6/PS/PE.The authors have calculated the conversion degree ofthe mixtures, based on the assumption of no interactionbetween the polymers. Wu et al. [22] reported a kineticmodel for a HDPE/LDPE/PP/PS/ABS/PVC mixture,assuming a summation of the degradation curves ofeach polymer in the mixture. After comparing the cal-culated results with those from the TG measurements,Wu et al. [22] concluded there was no observable inter-action between the components during degradation.The release of chlorine during the pyrolysis of pure

PVC has been carefully studied by the authors in a pre-vious study [23,60]. That work revealed that 99.84 wt.%of the original chlorine is released between 200 and360�C, which is lower than the decomposition tempera-ture of most polymers found in the MPW stream. When

470 R. Miranda et al. / Polymer Degradation and Stability 72 (2001) 469–491

PVC is mixed with other polymers, the PVC pyrolysisand the release of chlorine may be advanced or delayed,due to interactions with the other plastics. In order toisolate the chlorine released and to control the amountof chlorine in the pyrolysis products [61], the chlorineevolution pattern must be clarified.Thus the objective of this work is to study the thermal

decomposition behavior of commingled plastics withspecial attention given to the fate of the chlorinereleased during PVC decomposition. The study has twointerconnecting parts: the study of the pyrolysis kinetics(Part I) and the analysis of the pyrolysis products (PartII). The objective of Part I is to study the interactions ofpolymers during the pyrolysis of a MPW sample, payingspecial attention to the influence of potential interac-tions and chlorine release during the PVC pyrolysis.Five principal plastic materials present in MSW,HDPE, LDPE, PP, PS and PVC were studied undervacuum and dynamic nitrogen conditions. These poly-mers are representative of MPW found in North Amer-ica [1]. This work includes the following steps:

1. The study of the thermal decomposition behaviorand determination of the kinetic parameters of thesingle polymers involved.

2. The study of the thermal decomposition behaviorand determination of the overall kinetic para-meters related to binary and multicomponentmixtures without PVC.

3. The study of the thermal decomposition behaviorand determination of the overall kinetic para-meters related to PVC-containing binary and mul-ticomponent mixtures.

The thermal decomposition behavior was studied bythermogravimetry (TGA) measurement. The kineticparameters, including the apparent activation energies,the pre-exponential factors and the reaction orders weredetermined by DTG curve fitting, using an optimizationprocedure. In order to evaluate the polymer interactionsin MPW, the experimental TG/DTG curves of the pyr-olysis of a mixture were first simulated by a sum of thesingle decomposition curves, assuming that each plasticthermally decomposes independently and no interactiontakes place. Secondly, kinetic parameters obtained byfitting the DTG peaks of the individual polymers werecompared with those estimated by fitting the peak onthe commingled plastic-related curve corresponding tothat particular plastic.

2. Experimental

2.1. Plastics investigated

The samples investigated were pure polymers withoutfillers, stabilizers and colorants including high density

polyethylene (HDPE), low density polyethylene (LDPE),polypropylene (PP), polystyrene (PS) and polyvinylchloride (PVC). The plastic samples used in this studywere commercial grade materials representative of plas-tics commonly found in MSW. The manufacturers sup-plied the typical properties of the tested materials andtheir elemental composition, which are presented inTable 1. To improve the miscibility of the components,the granules were ground to powder with a particle sizeless than 2 mm using a Wiley laboratory mill, Model 4.Liquid nitrogen was used for size reduction. Mixtureswere then homogenized in the mill. For binary mixtures,each plastic had an equal weight fraction (1:1). Themulticomponent mixtures studied had two differentcompositions, which are shown in Table 2. For moredetails concerning the analytical methods, see Mirandaet al. [23,60].

2.2. Thermogravimetric study

The thermal analysis experiments were performedwith a Seiko 220 microprocessor-controlled TG/DTAsystem, connected to a Seiko Model 5200 Data Station.This apparatus uses a horizontal differential balancemechanism. The sample mass was 10.0�0.1 mg for eachexperiment. The runs were carried out in two differentenvironments, nitrogen flow (60 ml/min) and vacuum(total pressure of 0.6 kPa) at four different heating rates:1, 5, 10 and 20�C/min (here the focus has been put onthe results obtained at 10�C/min). Each sample wasplaced in an open aluminum sample pan. The usualprocedure for the test under vacuum was to introduce anitrogen flow into the TG/DTA for 30 min at roomtemperature, load the sample into the pan, wait another60 min under nitrogen flow, close the system and turnon the vacuum pump. When the total pressure waslower than 1 kPa, the heating cycle was started. Theplastic sample was heated up to a final temperature of600�C, when complete decomposition of the polymerwas achieved. The thermogravimetric weight loss curve(TG, wt.%) and the weight loss derivative curve of the(DTG, %wt./min) were recorded as a function of timeand temperature.

3. Results and discussion

3.1. Kinetic study of individual polymers

The mechanism of single plastic pyrolysis is the basisfor the understanding of the pyrolysis of commingledplastics. Most previous studies concerning single plasticpyrolysis were carried out under either nitrogen atmo-sphere or isothermal conditions. Here, investigation ofsingle plastic pyrolysis under both vacuum and nitrogenatmosphere and dynamic conditions was carried out, as

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the main interest of this work is ultimately the pyrolysisof MPW in industrial vacuum pyrolysis continuous feedreactors.

3.1.1. Thermal decomposition of plastics under vacuumand dynamic nitrogen atmosphere conditionsFig. 1 shows the experimental results obtained at

10�C/min under vacuum for all the individual polymersinvestigated in this study. It is obvious that the order ofthermal stability of the studied plastics is HDPE>LD-PE>PP>PS>PVC, as evidenced by T0, the tempera-ture of 0.1% weight loss and Tp, the peak temperature(Table 3). The sample weight was recorded over a largetemperature range in the test runs using powder and pelletmaterials. The influence of the sample form and weight (2–20 mg) on the reaction extent was found to be negligible.In the literature [22], the thermal decomposition of

HDPE, LDPE, PS and PP has been categorized to belongto one group with one-stage reaction. The experimentalresults of this work also indicate that HDPE and PP

thermal decomposition can be well represented by asingle reaction model. For LDPE and PS, the singlereaction model provides an acceptable representation ofthe decomposition, with a poor simulation however ofthe first 10% of weight loss. Thus a two-reaction modelis proposed in this work for the decomposition ofLDPE and PS. McNeill [2] showed that there are twokinds of bonds in LDPE and PS: weak bonds and nor-mal bonds. The weak bonds (branches or double bondsas structural defects) are points where the degradationreaction is initiated. As the temperature increases, degra-dation of normal bonds also takes place. A thermogravi-metric study of PVC decomposition [23] revealed thatthree stages of weight loss are involved during thedecomposition. The first stage occurs in the temperaturerange of 200–320�C with a peak at 280�C; the secondstage occurs at 250–375�C with a peak at 320�C; thethird stage ranges from 375 to 520�C with a peak at440�C.The apparent activation energies (Ea) and pre-expo-

nential factors (A) as well as the orders of reaction (n)were calculated by fitting the TG/DTG experimentaldata to a kinetic model using an optimization proce-dure. Table 4 presents the models, the differential equa-tions involved and the kinetic parameters obtained foreach polymer. The systems of differential equationscorresponding to each model were solved using a 4thorder Runge Kutta algorithm. A simplex algorithm ofoptimization was used to minimize the least square cri-teria defined in Eq. (1).

Error ¼X

ðYexperimental � YsimulatedÞ2

ð1Þ

Table 1

Characteristics of the plastics investigated

Characteristic Test method LDPE HDPE PP PS PVC

Manufacturer Novacor Solvay Polymer Himont Dow Chemical Synergistics Inc.

Location Alberta Texas Ontario Montreal Texas

Grade Novapol LF0219A Fortiflex A60-70-119 PC366 Styron 678C SE-95OEG

Appearance Pellets Pellets Powder Pellets Powder

Density, g/cm3 D1505 0.91–0.95 0.903 1.4

D4883 0.962

D792 1.04

Melting point (�C) 115–130 125–130 165–170

Melt index (g/10 min) D1238 0.75 10.0

Vicat Softening point (�C) D1525 112–132 132.0 93.0 >75

Deflection temperature (�C) D648 84.0 91.1

C (%) 84.93 84.63 84.62 91.63 37.83

H (%) 14.18 15.23 15.23 8.31 4.75

N (%) 0.08 0.14 0.14 0.06 0.05

Cl (%) NDa ND ND ND 57.10

S (%) 0.81 ND 0.01 ND 0.07

O (%) NIL NIL NIL NIL 0.20

Gross calorific value (MJ/kg) 43.34 47.39 47.56 42.95 21.0

a ND, not detected.

Table 2

Composition of the commingled plastics tested in this study

Polymer Percentage (wt.%)

Mixture Aa Mixture Bb

LDPE 30 30

HDPE 30 30

PP 20 15

PS 20 15

PVC 10

a Represents 79.2 wt.% of the plastics found in MSW [1].b Represents 90.6 wt.% of the plastics found in MSW [1].


where Y is a TG or DTG value, depending on the curvethat must be simulated.

3.1.2. Effect of pressureIn order to compare the results of this work which

were obtained under vacuum pyrolysis, with those fromthe literature, which were mostly obtained under atmo-spheric nitrogen flow, the influence of the pressure dur-ing the pyrolysis of the different polymers was studied.Polymer decomposition under both vacuum and N2 hasbeen carried out in this work. A comparison of the poly-mer decomposition characteristics under vacuum and N2

is presented in Table 3, for heating rates of 10�C/min. Ingeneral, the TG curves obtained under vacuum are verysimilar in shape to those obtained under nitrogen,although most plastics decompose a little earlier undervacuum than under nitrogen atmosphere. Therefore, itis believed that the plastic pyrolysis behavior undervacuum is in general comparable with that in nitrogenatmosphere.As shown in Table 3, the final temperature (Tf) and

the peak temperatures (Tp), measured under vacuum arereached at a lower temperature than in nitrogen atmo-sphere for all the plastics investigated except PVC.More details about PVC thermal decomposition undervacuum and nitrogen atmosphere have been presentedin the previous work of Miranda et al. [23]. In addition,vacuum pyrolysis of PP, PS and PVC produces lessfinal solid residue (SR) product than the reaction under

Fig. 1. TG and DTG curves of single plastics recorded under vacuum at a heating rate of 10�C/min.

Table 3

Characteristic temperatures and weight losses for thermal decomposi-

tion of the individual polymers under vacuum and nitrogen atmo-

sphere at 10�C/mina

Polymer Vacuum Nitrogen

To

(�C)

Tf

(�C)

Tpb

(�C)

SRc

(wt.%)

To

(�C)

Tf

(�C)

Tp

(�C)

SR

(wt.%)

LDPE 358 496 380 0.10 361 500 380 0.10

465 467

HDPE 375 500 468 0.10 376 500 472 0.11

PP 296 465 435 0.01 305 470 437 0.07

PS 265 446 335 0.02 268 450 340 0.05

410 416

PVC 220 512 287 5.70 218 535 287 10.50

322 322

439 456

a To, temperature corresponding to 0.1 wt% of weight loss; Tf, final

degradation temperature; Tp, peak temperature corresponding to a

maximum weight loss rate.b The decomposition of LDPE as well as PS occurs in two steps

whereas that of PVC takes place in three steps under the conditions

used in this study.c SR, solid residue (carbonaceous or undecomposed polymer).


nitrogen atmosphere. These differences may beexplained by an accelerated release of the volatile com-pounds formed during the thermal decomposition undervacuum, as the secondary condensation and aromatiza-tion reactions are avoided. Vacuum pyrolysis allows agreater quantity of hydrocarbons to be formed asopposed to pyrolysis in a nitrogen atmosphere.

3.1.3. Literature dataThe thermal decomposition of single polymeric mate-

rials has been widely investigated earlier. Table 5 pre-sents a summary of the kinetic parameters from theliterature, obtained under vacuum and nitrogen atmo-sphere. In the literature, the kinetic studies of the polymerdecomposition mostly were focused on the evaluation ofactivation energy without paying much attention to thepre-exponential factor or the reaction order.The Ea values reported in the literature widely vary

which suggest that each model has limited application.Some researchers indicated that Ea is dependent uponthe polymer purity and molecular weight [62]. Differ-ences can be attributed to the effect of experimental con-ditions, the method of kinetic parameter estimation, thekinetic model used, and the characteristics of the mate-rial as well as the range of temperatures investigated.

3.1.3.1. Polyethylene. Jellinek [5] and Madorsky [6]reported a dependence of the activation energy on theinitial molecular weight of the polymer for PE degrada-tion under vacuum. Wall et al. [8] suggested that theappropriate variable is the number and length of thebranches on the PE polymer backbone rather than the

molecular weight per se. Mucha [63] used thermo-gravimetry to study branched and linear PE thermallydegrading in N2 atmosphere. The apparent activationenergy was found to decrease from 330 kJ/mol for aninitial linear PE with a molecular weight of 707 000 to247 kJ/mol for initial molecular weights of 39 000 andless. For branched samples, activation energies between230 and 163 kJ/mol were reported. However Ea valuesfrom linear (271 kJ/mol) and branched samples (269 kJ/mol) reported by Dickens [10] were not significantlydifferent.Jellinek [5] reported that PE degradation follows the

kinetics of a zero reaction order. They tested three dif-ferent PE samples with an average molecular weight of23 000, 16 000 and 11 000 and they obtained three dif-ferent Ea values (276.3, 219.8 and 192.3 kJ/mol, respec-tively), which decreased with a decrease in molecularweight. Madorsky [6] obtained values of 263.3 kJ/molfor a slightly branched PE (MW= 20 000) and 267.5 forbranched PE following first order kinetics. Madorsky [6]and Anderson and Freeman [9] reported that the initialpart (3%) of the degradation process of PE follows zeroorder kinetics. Anderson and Freeman [9] reportedthree regions, the first of which was zero order; for thesecond region, between 15 and 35%, degradationappeared to involve a transition from zero order to firstorder kinetics while the final reaction occurring above35% degradation followed a first order kinetic law. Theoverall Ea in the three regions was 201, 255 and 280 kJ/mol, respectively. More recently, Conesa et al. [64]reported that the HDPE thermal decomposition followsthree zero order reactions; the first and the third regions

Table 4

Kinetic models used for single plastic thermal decompositiona

Kinetic model Differential equations Ea (kJ/mol) n A (min�1) Yield coefficient

HDPE ��!k1

VþRd½HDPE

dt¼ �A1e

�Ea1RT ½HDPEn1 250 0.65 1.71�1017

�1LDPE1 ��!k1

V1 þR1d½LDPE1

dt¼ �A1e

�Ea1RT ½LDPE1

n1 Ea1 ¼ 120 n1=1.40 A1=1.34�109 �1 ¼ 0:10

�2LDPE2 ��!k2

V2 þR2d½LDPE2

dt¼ �A2e

�Ea2RT ½LDPE2

n2 Ea2 ¼ 220 n2=0.60 A2=1.47�1015 �2 ¼ 0:90

PP ��!k1

VþRd½PP

dt¼ �A1e

�Ea1RT ½PPn1 125 0.40 2.04�108

�1PS1 ��!k1

V1 þR1d½PS1

dt¼ �A1e

�Ea1RT ½PS1

n1 Ea1 ¼ 120 n1=1.60 A1=1.06�108 �1 ¼ 0:10

�2PS2 ��!k2

V2 þR2d½PS2

dt¼ �A2e

�Ea2RT ½PS2

n2 Ea2 ¼ 185 n2=0.76 A2=2.32�1013 �2 ¼ 0:90

PVC ��!k1

aHClþ bId½PVC

dt¼ �A1e

�Ea1RT ½PVCn1 Ea1 ¼ 198 n1=1.04 A1=3.57�1018 b=0.52

bI ��!k2

cV1 þ eR1d½I

dt¼ bðA1e

�Ea1RT ½PVCn1 � A2e

�Ea2RT ½In2 Þ Ea2 ¼ 143 n2=1.15 A2=9.95�1010 e=0.36

eR1 ��!k3

fV2 þ gR2d½R1

dt¼ eðA2e

�Ea2RT ½In2 � A3e

�Ea3RT ½SR1

n3 Þ Ea3 ¼ 243 n3=1.58 A3=5.77�1016 g=0.06

d½R2

dt¼ gðA3e

�Ea3RT ½SR1

n3 Þ

a V, volatiles; I, intermediate; SR1, residue generated in the first stage; SR2, residue generated in the second stage.


involved surface reactions; the second zone involves abulk phase reaction. The three reactions have Ea valuesof 185, 271.1 and 221.5 kJ/mol, respectively.

3.1.3.2. Polypropylene. Madorsky and Straus [11] studiedthe thermal degradation of PP under vacuum at 400�C.

They found C5H10 and C5H12 to be the major products.The degradation activation energy was reported to be242.4 kJ/mol. Wall and Straus [12] reported an Ea valueof 246.6 kJ/mol under vacuum. Madorsky and Straus[11] as well as Kiang et al. [65] found the kinetics of PPpyrolysis to be first order because the rate determining

Table 5

Summary of literature data on the kinetic parameters for the thermal degradation of several plastics under nitrogen atmosphere or vacuum

Plastic (atmosphere) Temperature range (�C) Ea (kJ/mol)a n A (min �1) Reference

Polyethylene

(vacuum, isothermal) 251–293 [4]

(vacuum, isothermal) 276 0 [5]

(vacuum, isothermal) 300–475 285 1 [6]

(vacuum, isothermal) 268 [7]

(vacuum, isothermal) 400 285–301 1 [8]

(vacuum, isothermal) 246–480 280 1 [9]

(vacuum, isothermal) 410–475 274 [10]

(N2, dynamic) 20–600 HDPE 233 0.74 5.58�1015 [22]

LDPE 206 0.63 7.20�1013

Polypropylene

(vacuum, isothermal) 300–410 242.44 [11]

(vacuum, isothermal) 350–450 246.62 [12]



(vacuum, isothermal) 390–465 (IPP) 257 [14]

(N2, dynamic) 20–600 144 [43,44]

(N2, dynamic) 20–600 183 0.90 3.72�1012 [22]

(N2, dynamic) 20–600 12.5 (1) 1.0 4.8�10�2 [15]

230 (2) 0.5 1.2�1016

Polystyrene

(vacuum, isothermal) below 340 187 0 [5]

(vacuum, isothermal) 209 [16]

(vacuum, isothermal) 360–420 243 0 to 1 [6]

(vacuum, isothermal) 280–300 205 [17,18]

(vacuum, isothermal) 246–430 192, 0.0

251 1.0 [9]


(N2, dynamic) 20–600 172 0.5 3.0�1012 [22]

(N2, dynamic) 35–850 220 (1) 0.51 3.0�1012 [22]

276 (2) 1.02 3.67�1019 [20]

187(3) 0.80 8.58�1013

(N2, dynamic) 35–850 2.1 (1) 1.0 1.20�10�2 [15]

272 (2) 0.5 3.0�1020

Polyvinyl chloride

(N2, dynamic) 127–527 126 (1a) 1.5 9�1013 [22]

201 (1b) 0 9�1016

209 (2a) 1.5 2.88�1018

327 (2b) 1.5 3.36�1014

(N2, dynamic) 20-600 143.5 (1) 0.54 3.16�1012 [21]

234.1(2) 1.64 2.23�1016

(N2, dynamic) 35-850 136.8 (1a) 0.46 1.65�1012 [24]

146.5 (1b) 1.54 3.95�1012

239.1 (2) 1.62 4.70�1016

(N2, dynamic) 20-600 200 (1) 0.98 2.18�1018 [23]

153 (2) 1.10 1.26�1011

243 (3) 1.55 6.47�1016

(Vacuum, dynamic) 20-600 198 (1) 1.04 3.57�1018 [23]

143 (2) 1.15 9.95�1010

243 (3) 1.58 5.77�1016

a Numbers in parentheses indicate the decomposition steps of single plastics reported in literature.


step is the homolysis of the C–C bond, which can bedescribed by a well-defined rate constant. Moiseev et al.[64] found propylene to be a major product in theirinvestigation of the PP degradation. Kiang et al. [65]reported an Ea value of 230 kJ/mol in the range of 388–438�C. Vovelle and Mellottee [15] reported two reac-tions in the thermal degradation under nitrogen atmo-sphere of PP and PS. The first reaction exhibited a lowerEa corresponding to the first 10% of weight loss on theTG curves.

3.1.3.3. Polystyrene. Anderson and Freeman [9], Jelli-nek and Luh [66] and Kokta et al. [67] reported that thedegradation of polystyrene occurs via two reactionmechanisms. One mechanism is predominant at lowtemperatures up to 370�C, where the reaction is zeroorder (Ea ¼ 167 kJ/mol), while at temperatures above370�C the reaction is first order (Ea ¼ 250 kJ/mol).Molecular weight, tacticity and crosslinking [68] seem tohave a certain effect on the process [69].Marcilla and Beltran [20] applied a kinetic model inv-

olving the formation of an intermediate product duringthe thermal decomposition of PS, which exhibited agood correlation with the variation of the heating rate.The hypothesis of an activated intermediate was basedon the work of Richards and Salter [70] who suggestedthat two processes are important in the PS degradation:weak bond scission and intermolecular chain transfer.

3.1.3.4. Polyvinyl chloride. The thermal degradation ofPVC has been shown to proceed in two distinct stagesby Chang and Salovey [71] and Guyot and Bert [72].One stage is predominant up to around 350�C where thedegradation was reported to involve dehydrochlorina-tion to a polyene structure accompanied by the evolu-tion of a small quantity of benzene and traces of ethy-lene. Above 350�C, a backbone scission occurs, leadingto the formation and evolution of various aromaticcompounds like benzene, toluene, naphthalene, indene,anthracene, o-xylene, various chlorobenzenes, etc. [71].Marcilla and Beltran [24] proposed two different

models which considered two or three consecutive reac-tions occurring in two stages. Alternatively, a model offour reactions which involved two competitive (parallel)and two consecutive reactions was proposed by Wu etal. [22]. A thermogravimetric study performed by Mir-anda et al. [23] under vacuum revealed that three stagesof weight loss are involved during PVC decomposition.The first stage occurs in the temperature range of 200–320�C with a peak at 280�C; the second stage occurs at250–375�C with a peak at 320�C; the third stage rangesfrom 375 to 520�C with a peak at 440�C. A three appar-ent reaction kinetic model was proposed which assumedthat each weight loss stage on the TG curve corre-sponded to an apparent reaction. The authors appliedboth a series model and a parallel model. The kinetic

parameters and the conversion factors obtained fromboth models were quite similar: the activation energieswere found to be 198, 143, and 243 kJ/mol for the first,second and third apparent reactions, respectively.

3.2. Kinetic study of the decomposition of thecommingled plastics

Previous studies have shown that in some binarymixtures an interaction can occur between the compo-nents resulting in a change in the stability of each con-stituent polymer [25,34], or a change in peak temperaturewhich has been reported by Day et al. [43,44] for plas-tics derived from automobile shredder residues (ASR).In order to appreciate the interaction between the

most important plastics found in the MPW stream,binary plastic mixtures were first studied. Then multi-component polymer samples were studied. The binarymixtures investigated included mixtures of LDPE withother mentioned plastics found in MPW, mixtures ofPVC with each category of plastics, and a PP/PS mix-ture. In MPW, PE represents the largest quantity, with60% of the total load (see Table 2). PVC is consideredto be the most problematic polymer because it releasesHCl and chlorinated hydrocarbons during pyrolysis. Tostudy the potential interaction of PVC with other plas-tics, binary and multicomponent mixtures with no PVCwere studied first. Then mixtures containing PVC werestudied and compared with the results from mixtureswithout PVC.Two approaches were applied to study the polymer

interaction in the mixture. The first approach involved acomparison of the experimental TG/DTG curve with acalculated curve. A calculated curve is constructed basedon the assumption that each plastic in the mixture deco-mposed independently. Thus a simple summation wasperformed of the decomposition curves of each plasticwhich was obtained during the previous single polymerstudies (see Table 4). The differences between theexperimental curves and the calculated curves, includingthe differences in the shape and position of the peaks, isindicative of the intensity of the interaction. The secondapproach involved a comparison of the kinetic para-meters of each plastic in the mixture as determined bytwo different methods: (i) fitting the single polymerdecomposition curve with the kinetic parameters of thesingle polymer decomposition and (ii) fitting the peakcorresponding to the pyrolysis of that polymer, on thecommingled plastics pyrolysis curve. In Tables 6 and 8, thekinetic parameters from the individual plastic pyrolysiswere compared with those obtained from the com-mingled pyrolysis. The comparison of the kinetic para-meters can also illustrate the magnitude of theinteractions in a qualitative and quantitative manner.The following equations were used to calculate the

theoretical non-interaction DTG curve for a mixture


and to determine the kinetic parameters for each peakon a curve of a mixture.

dX

dt¼

XN

i¼1

�idXi

dti ¼ 1 to 5ð Þ ð2Þ

1 ¼XN

i¼1

�i ð3Þ

where dXdt is the instantaneous reaction rate of the poly-

mer mixture. Xi is the conversion degree of a specificpolymer, which is the ratio between the weight loss at time‘‘t’’, and the total weight loss of that polymer. �i is themass fraction of a certain plastic in the mixture. Con-sidering that five polymers are contained in the mixture,one obtains:

d Mix½

dt¼ �1

d½HDPE

dtþ �2

d½LDPE

dtþ �3

d½PP

dt

þ �4d½PS

dtþ �5

d½PVC

dtð4Þ

where

d LDPE½

dt¼ �21

d½LDPE1

dtþ �22

d½LDPE2

dtð5Þ

d PS½

dt¼ �41

d½PS1

dtþ �42

d½PS2

dtð6Þ

d PVC½

dt¼ �51

d½PVC

dtþ �52

d½I

dtþ �53

d½SR1

dt

þ �54d½SR2

dtð7Þ

�ij is the mass fraction of each independent reaction fora certain polymer. The decomposition of LDPE as wellas PS occurs in two steps (LDPE1, LDPE2, PS1, PS2)whereas that of PVC takes place in three steps under theconditions used in this study. A detailed description ofthe PVC thermal decomposition reactions involved canbe found in authors’ previous publication [23].

3.2.1. Free PVC commingled plasticsComparisons of the experimental curves with the

calculated non-interaction TG/DTG curves are pre-sented in Figs. 2–6. If no interaction occurs betweenthe polymers in the mixture, the calculated and the

Table 6

Comparison of apparent activation energy for the single polymers with their values in the mixtures without PVCa

Polymer Individual polymersb Mixture

P1/P2 temperature

(�C)

P1/P2

Peak

Temperature

(�C)

Ea

(kJ/mol)

Polymer Temperature

(�C)

Peak

temperature

(�C)

Ea

(kJ/mol)

Polymer

decomposition

step assignment

LDPE/HDPE To 340 375 380 100 LDPE1 To 340 379 100 LDPE1

50/50 Tf 496 500 465 220 LDPE2 Tf 500 467 220 LDPE2

% SR 0.10 0.10 468 250 HDPE % SR 0.095 467 248 HDPE

LDPE/PP To 340 296 380 100 LDPE1 To 316 380 98 LDPE1

50/50 Tf 496 465 435 125 PP Tf 492 445 125 PP

% SR 0.10 0.01 465 220 LDPE2 % SR 0.02 445 210 LDPE2

LDPE/PS To 340 265 335 80 PS1 To 285 332 78 PS

50/50 Tf 496 446 380 100 LDPE1 Tf 486 376 98 LDPE1

% SR 0.10 0.02 410 270 PS2 % SR 0.06 414 268 PS

465 220 LDPE2 458 210 LDPE2

PP/PS To 296 265 335 80 PS1 To 286 332 78 PS

50/50 Tf 465 446 410 270 PS2 Tf 450 410 270 PS

% SR 0.01 0.02 435 125 PP % SR 0.10 419 110 PP

HDPE/LDPE/

PP/PS

To 335 80 PS1 To 287 335 75 PS

P/PS Tf 380 100 LDPE1 Tf 486 365 85 LDPE1

30/30/20/20 %SR 410 270 PS2 %SR 0.06 420 245 PS

435 125 PP 420 110 PP

465 220 LDPE2 445 207 LDPE2

468 250 HDPE 455 243 HDPE

a P1, first polymer; P2, second polymer; To, temperature at 0.1% of weight loss; Tf, final degradation temperature; Tp, peak temperature; % SR,

wt.% solid residue.b Value at a heating rate of 10�C/min, under vacuum.


experimental curves should coincide. If differencesare found, this may indicate that some interactionis occurring. The kinetic parameters from fitting asingle polymer pyrolysis and from fitting that polymerpeak on the mixture curve are presented in Table 6.As has been mentioned before the decomposition ofLDPE as well as PS occurs in two steps; in Table 6the first reaction has been represented by LDPE1 andPS1 while the second reaction is indicated by LDPE2

and PS2.

3.2.1.1. HDPE/LDPE mixture. In the case of theHDPE/LDPE mixture (Fig. 2), the experimental curveis very similar to the calculated non-interaction curve.Both curves overlap. The activation energy (Ea) of themixture appears to be unaffected by the mixing. Itappears from these results that there seems to be nosignificant interaction in the HDPE/LDPE mixture.

3.2.1.2. LDPE/PP mixture. The LDPE/PP curve (Fig. 3)shows some difference between the DTG experimental

Table 7

Summary of the degradation behavior of the commingled plastics involving no PVC

Mixture Interaction effect Mechanism of the interactions Reference

LDPE/PP 50/50 vacuum �Slightly retarded PP weight loss

LDPE stabilized

Intermolecular radical transfer

by hydrogen abstraction

This study

�Reduced Ea for LDPE as an effect of interaction

PE/PP (mechanically mixed)

Nitrogen

�PP reduced PE stability and vice versa

as a function of composition

Intermolecular radcal transfer [38]

�Superposition of two single peaks

up to 60% of PP in the blend, above this

concentration one peak is predominant

PE/PP Nitrogen �The mixing of polymers influences

the decomposition rate

Intermolecular radical transfer [50]

�PE destabilized

HDPE/PP Nitrogen �Reduced Ea for PP as an effect of

interaction which does not depend

on the HDPE concentration

[51]

LDPE/PP 50/50 vacuum �Slightly retarded PS1 weight loss.

LDPE2 destabilized

Intermolecular radical transfer


This study

�Reduced Ea for LDPE2 as

an effect of interaction

PE/PS Nitrogen �Two peaks clearly identified Intermolecular radical transfer [50]

�PE destabilized

PE/PS Mixed powders �No effect on Tmax of polystyrene [31]

�PE destabilized

PS/PP 50/50 vacuum �Slightly retarded PS1 weight loss Intermolecular radical transfer


This study

�PP destabilized

�Reduced Ea for PP as an effect

of interaction

PP/PS extrusion conditions

graft copolymers

�Acceleration of PP degradation [41]

PP/PS �PP destabilized Intermolecular radical transfer [50]

�Only one peak

PP/PS Mixed powders �No effect on Tmax of polystyrene [31]

�PP destabilized

HDPE/LDPE/PS/PP

30/30/20/20 vacuum

�Three stages of weight loss identified Intermolecular radical transfer


This study

�Slightly retarded PS1 weight loss; PP as

well as LDPE and HDPE destabilized;

PS2 slightly stabilized

PE/PP/PS Nitrogen �The mixing of polymers influences

the decomposition rate

Intermolecular radical transfer [50]

�Destabilization of PP and PS


curves and the calculated curve. One peak appears inthe experimentally measured curve, while two peaks arefound on the theoretical curve. PP decomposition wasslightly retarded after the first 10% weight loss andLDPE was destabilized. The Ea of LDPE in the mixturewas lower compared with the Ea from single plastics(Table 6). The destabilization could be explained by anintermolecular transfer of a hydrogen atom from theless stable polymer to a free radical depropagating chainof the second component during decomposition. Theconsequence is an activation of the degradation processinvolving the less stable components by a destabilizationof the second component.Csomorova et al. [38] described the behavior in nitro-

gen atmosphere of PE/PP whose components are notvery miscible. They found that the less stable polymerusually causes destabilization of the more stable poly-mer. This behavior was displayed by the polymer

blends, which are the closest to each other on the degra-dation temperature scale. It was reported that in a PP/PE blend containing more PP than PE, PP causes areduction of the Ea of PE. The reduced activation energyfor PE was attributed to the initiation effect of PP radi-cals escaping from the bulk during PE decomposition[38]. A similar conclusion was reached by Dodson andMcNeill [30,31], and Murata and Akimoto [50]. Itappears that when the plastics have very close Ea values,the behavior of the blend could result in a decrease ofthe Ea of the more stable polymers due to a hydrogentransfer from the more stable polymer towards theradicals of the less stable polymers.

3.2.1.3. LDPE/PS mixture. The weight loss behavior forthe LDPE/PS mixture was found to be different from thecalculated non-interaction curve (Fig. 4). The firstdecomposition reaction involving PS (PS1) occurred at

Table 8

Comparison of apparent activation energy values of the thermal decomposition under vacuum of single polymers and their values in the mixture

containing PVCa

Polymer Individual polymersb Mixture

P1/P2 Temperature

(�C)

Peak

temperature

(�C)

Ea

(kJ/mol)

Polymer Temperature

(�C)

Peak

temperature

(�C)

Ea

(kJ/mol)

Polymer

decomposition

step assignment

P1/P2

PVC/HDPE To 220 375 287 198 PVC1 To 230 285 200 PVC1

50/50 T2 375 322 143 PVC2 T2 376 328 143 PVC2

Tf 512 500 439 243 PVC3 Tf 515 440 245 PVC3

%SR 5.7 0.10 468 250 HDPE % SR 5.0 482 258 HDPE

PVC/LDPE To 220 340 287 198 PVC1 To 231 288 198 PVC1

50/50 T2 375 322 143 PVC2 T2 370 320 143 PVC2

Tf 512 498 380 100 LDPE1 Tf 515 380 100 LDPE1

% SR 5.7 0.10 439 243 PVC3 % SR 4.8 440 240 PVC3

465 220 LDPE2 475 233 LDPE2

PVC/PP To 220 296 287 198 PVC1 To 234 295 202 PVC1

50/50 T2 375 322 143 PVC2 T2 368 323 143 PVC2

Tf 512 465 435 125 PP Tf 515 453 135 PP

% SR 5.7 0.01 439 243 PVC3 % SR 5.0 450 245 PVC3

PVC/PS To 220 265 287 198 PVC1 To 228 290 200 PVC1

50/50 T2 375 322 143 PVC2 T2 360 320 143 PVC2

Tf 512 446 335 80 PS1 Tf 515 337 80 PS1% SR 5.7 0.02 410 270 PS2 % SR 5.0 416 274 PS2

439 243 PVC3 436 243 PVC3

HD/LD/PP/PS/PVC To 287 198 PVC1 To 252 291 200 PVC1

30/30/15/15/10 T2 322 143 PVC2 T2 252 320 143 PVC2

Tf 335 80 PS1 Tf 512 337 80 PS1% SR 380 100 LDPE1 % SR 1.50 380 100 LDPE1

410 270 PS2 422 275 PS2435 125 PP 422 122 PP

439 243 PVC3 458 245 PVC3

465 220 LDPE2 465 218 LDPE2

468 250 HDPE 465 248 HDPE

a P1, first polymer; P2, second polymer; To, temperature at 0.1% of weight loss; T2, onset temperature for second stage; Tf, final degradation

temperature; Tp= peak temperature; % SR, wt.% solid residue.b Value at a heating rate of 10�C/min, under vacuum.


Fig. 2. TG and DTG curves of the binary 50/50 HDPE/LDPE mixture recorded under vacuum at a heating rate of 10�C/min: ( — ) experimental;

(....) calculated.

Fig. 3. TG and DTG curves of the binary 50/50 LDPE/PEP mixture recorded under vacuum at a heating rate of 10�C/min: ( — ) experimental; (....)

calculated.


10% weight loss and was slightly retarded, although theEa1 value from the mixture simulation is lower than thatfrom pure PS simulation. The second reaction (PS2) inthe mixture has the same temperature peak as for purePS. LDPE2 was destabilized and the Ea of LDPE2 wasreduced as a result of the interaction.

3.2.1.4. PS/PP mixture. In the case of PS/PP mixtures(Fig. 5), the experimental TG/DTG curve is very differ-ent with the calculated non-interaction curve. After thefirst 10% weight loss, the PP decomposition shifts to alower temperature compared with the pure PP decom-position. For PS decomposition however, both the PS1and PS2 temperature peaks seem to be unaltered. A simi-lar result was reported by Dodson and McNeill [30,31]who indicated that both PE and PP begin to degrade atlower temperatures in the presence of PS, suggesting anattack on these polymers by some radicals involved in PSdegradation. Murata and Akimoto [50] studied binaryand ternary polymer mixtures (PE, PP, PS) by TG andGC analysis. They found that when PS is present in themixture, the other polymers are destabilized. They sug-gested that mixing the polymers influenced the decom-position rate but not the intramolecular reaction. Themixing effect on the decomposition rate was interpretedin terms of an intermolecular radical transfer betweendifferent polymers. Since the polystyryl radicals are

stable, they can more probably participate in an inter-molecular transfer, thus explaining the variation inthermogravimetric and kinetic data.

3.2.1.5. HDPE/LDPE/PS/PP mixture. As shown inTable 2, the plastic in the multicomponent mixture hasthe same proportion as that found in the MPW of NorthAmerica. In order to isolate the PVC influence from theother interaction effects, the mixture without PVC wasfirst studied. This multicomponent mixture represented80 wt.% of the MPW.The experimental and the theoretical TG/DTG non-

interaction curves are presented in Fig. 6. The two curvesare different, especially with respect to the shape andposition of the high temperature peaks. Because of theinteraction, overlapping of the high temperature peakswas observed, which resulted in some difficulty in thepeak temperature determination. The experimental TG/DTG curves exhibited three main stages of weight loss.The first one between 200 and 375�C with 17.5% weightloss can be attributed mainly to PS1 and LDPE1 toge-ther with a lower portion of PP, HDPE and LDPE. Thisweight loss was found to be higher than that on thecalculated non-interaction curve obtained at the sametemperature range. The theoretic weight loss was 6.1%which consists of 14.6 wt.% from net PS, 12.3 wt.%from PP, 1.4 wt.% from LDPE, 1 wt.% from HDPE

Fig. 4. TG and DTG curves of the binary 50/50 LDPE/PS mixture recorded under vacuum at a heating rate of 10�C/min: ( — ) experimental; (....)

calculated.


(14.6�0.2+12.3�0.2+1.4�0.3+1�0.3=6.1%). Thesecond stage of weight loss with a temperature peak of420�C corresponded to the PS2 and PP peaks, wherePS2 was slightly retarded and PP was shifted to a lowertemperature. The last stage with a temperature peak of460�C was attributed to LDPE2 and HDPE, where bothpolyethylenes were destabilized as an effect of the mix-ing, which caused a reduction in the Ea values forLDPE2 and HDPE.The results of binary mixtures suggest that PE toge-

ther with PP or PS appear to be responsible for a shift inthe peak temperature of PE to lower temperatures whilePP or PS in the mixture are stabilized. However whenPP and PS are both present in a binary mixture, PP isdestabilized while PS is stabilized. In terms of mechan-ism, it is suggested that when PP undergoes degradationvia random scission plus extensive intermoleculartransfer involving tertiary H-abstractions from the samePP polymer by primary radical and H-abstraction fromsome PE radical, the effect is to accelerate the PEdegradation and decelerate the PP degradation. WhenPS is present in the mixture the reaction mechanism canbe explained as follows: between 200 and 300�C, PSgoes through degradation via random scission of thebackbone, producing an intermediate, but no volatileproducts are evolved. Then above 300�C, the PS degradesto give monomers via depropagation and b-scission [53].

Polystyryl radicals produce other radical intermediatesby intramolecular radical transfer reactions which arestabilized by H-abstraction from PS and/or PE mole-cules when PE is present in the system. The effect is tostabilize PP, and to accelerate PE degradation. WhenPS and PP are both in the mixture, PS promotes the PEdegradation by intermolecular H-abstraction.The behavior of the mixture containing both poly-

olefins and PS suggests another mechanism of degrada-tion. The intermediate product from the first PSdegradation reaction, PS1, appears to be responsible forthe early decomposition of LDPE1 and some portion ofthe PP because of the decomposition mechanism invol-ving an intermolecular H-abstraction. The intermediateproduced by intramolecular H-transfer at the chain endappears to cause a shift of the DTG curves of PP,LDPE2 and HDPE towards lower temperatures withPS2 being stabilized. It is presumed that PP as well as PScaused destabilization of the LDPE2 and HDPE viaintermolecular H-abstraction. The observed interactionsfor the five mixtures as well as the relevant literaturedata are summarized in Table 7.

3.2.2. PVC containing commingled plasticsExperimental and calculated TG/DTG curves for the

mixtures containing PVC are presented in Figs. 7–11.Table 8 shows the comparison of the kinetic parameters

Fig. 5. TG and DTG curves of the binary 50/50 PS/PP mixture recorded under vacuum at a heating rate of 10�C/min: ( — ) experimental; (....)

calculated.


obtained from the individual plastics and from the mix-ture pyrolysis. For all the binary and the multi-component mixtures, the measured initial decompositiontemperature was slightly retarded by more than 7�C(PVC/PP), and the residue quantity appears to be higherthan that from the calculated curves. As mentionedbefore, PVC1, PVC2 and PVC3 represent the three stagesof weight loss during PVC pyrolysis.

3.2.2.1. PVC/HDPE mixture. In the case of the PVC/HDPE mixture three stages of weight loss are observed(Fig. 7). The first two stages occur between 256 and376�C and exhibit a total weight loss of 32.5%, whichcorresponds to the weight loss of dehydrochlorination(PVC1) and the formation of other volatiles such as ben-zene (PVC2). In the first two stages, the weight loss of netPVC was 64% and that of net HDPE was 1.0%. No sig-nificant difference has been found between experimentaland calculated curves at the dehydrochlorination stage.The PVC2 temperature peak was slightly retarded (6�C).The third stage between 375 and 515�C with 63%weight loss was attributed to the overlapping of thebreakdown of the polyene chain (PVC3) and HDPEbackbone. The HDPE maximum rate of volatilizationwas shifted (14�C) towards a higher temperature due tointeraction effects. There is an increase of 8 kJ in the Ea

for HDPE. The amount of residue was found to be higherat 520�C than 50% compared with the single PVC residuecorresponding to the same decomposition temperature.

3.2.2.2. PVC/LDPE mixture. The behavior of the PVC/LDPE mixture presented in Fig. 8 is similar to that ofthe PVC/HDPE mixture described above. The experi-mental curve is quite similar to the theoretical curve upto 375�C with a weight loss of 32.7%, of which 32% canbe attributed to the PVC and 0.7% to LDPE. Theweight loss of each plastic in the mixture equals exactlyhalf of the weight loss of the pure plastic pyrolysis (PVChaving 64% and LDPE having 1.4% at 375�C). Recal-ling that each plastic in the mixture represents 50% ofthe total sample mass, the above results imply that eachplastic in the mixture contributes to its expected weightloss as if there was no interaction. Above 375�C, thethird stage of PVC decomposition (PVC3) overlaps withthe second decomposition step of LDPE (LDPE2). Atthis stage, the TG/DTG curve was shifted to a highertemperature (+10�C) as compared to the calculatedcurve. This phenomenon can be attributed to the factthat the LDPE was stabilized as an effect of interaction,causing a 13 kJ increase in the Ea. The residue yieldobtained at the end is higher than that expected basedon the behavior of the individual plastics.

Fig. 6. TG and DTG curves of the multicomponent 30/30/20/20 HDPE/LDPE/PP/PS mixture recorded under vacuum at a heating rate of 10�C/

min: ( — ) experimental; (....) calculated.


3.2.2.3. PVC/PP mixture. The experimental weight losscurve was obtained for the PVC/PP mixture and wascompared with the calculated curve assuming no-inter-action (Fig. 9). The two curves do not coincide, indi-cating that some interaction is taking place. Theexperimental curve presents three maximum rates ofweight loss, two between 200 and 370�C and one between370 and 510�C. In the 200–370 �C temperature range, theposition for the maximum rate of HCl production wasshifted (8�C) to a higher temperature, but no significantdifference was found in the Ea. The weight loss at 370

�Cwas 38%, which corresponded to the sum of 50% weightloss of each individual polymer at the same temperature(pure PVC having 64% and PP having 12.25% weightloss).In the 370–515 �C temperature range, one peak was

observed which corresponded to the overlapping curvesof the polyene chain scission (PVC3) and PP decom-position. The PP temperature peak was shifted to highertemperature as an effect of mixing, but the third peak ofPVC remained unaltered. A similar retardation of PP inPVC/PP mixtures was observed by Day et al. [43,44]who compared peak temperatures and apparent activa-tion energies (Ea) of binary mixtures under nitrogenatmosphere. The difference between their work and thiswork is that they used two single reactions to fit thePVC thermal degradation.

3.2.2.4. PVC/PS mixture. In the case of the PVC/PSmixture (Fig. 10), HCl production is slightly retardedwithout a significant change in Ea. Between 200 and370�C, three main reactions take place; first the dehy-drochlorination (PVC1), then the formation of volatilessuch as benzene (PVC2) and thirdly a weak devolatili-zation due to PS chain-scission reactions (PS1). At370�C, the weight loss was 39.2%, which correspondsto the calculated results based upon the behavior ofthe individual polymer pyrolysis (pure PVC having 64%and PS having 14.6% weight loss at 370�C). Above370�C, PS breaks down and produces styrene mono-mers, dimers, trimers, etc. At this moment, the polyeneformed from the first stage of PVC degrades to mainlyform alkyl-aromatics. The PS maximum temperature(Tmax) was slightly increased while the third PVCdecomposition peak (PVC3) was shifted to lower tem-peratures.Dodson and McNeill [30,31] and McNeill et al. [34]

studied PVC/PS interaction effects during the degrada-tion of the blend by thermogravimetry (TG) in nitrogenatmosphere and by thermal volatilization analysis(TVA) under vacuum. The authors reported that, as ablend effect, the HCl production was slightly retardedand PS was considerably stabilized. The authors sug-gested that this effect resulted from an intermoleculartransfer from the Cl. radicals to both the PVC and the

Fig. 7. TG and DTG curves of the binary 50/50 PVC/HDPE mixture recorded under vacuum at a heating rate of 10�C/min: ( — ) experimental; (....)

calculated.


other constituent (PS); therefore, the Cl. radicals are lessavailable to initiate the dechlorination. The resulting PSmacroradical undergoes scission but not volatilizationbelow 350�C. The polyene structure of PVC formed byHCl loss, stabilizes PS at a higher temperature range byreacting with polystyryl radicals.A recent work by Knumann and Bockhorn [21]

reported no significant interaction between mixtures ofPVC with polyolefins compared with TG/DTG theore-tical curves based upon the behavior of individualpolymers. These authors, in agreement with Dodsonand McNeill [30,31], reported a discrepancy for a PVC/PS mixture. Another possible explanation of the poly-olefins and/or polystyrene stabilization in the mixturewith PVC could be the fact that the decomposition ofthese polymers takes place in the same temperatureregion with carbonaceous residue formation from PVC(T>380�C). This residue has an aromatic condensedstructure that appears as a result of condensation/dehy-drogenation reactions. It is possible that a high quantityof hydrogen is present in the system. The native hydro-gen could inhibit the radical chains of the polymerdecomposition.

3.2.2.5. HDPE/LDPE/PS/PP/PVC mixture. The mul-ticomponent mixture contains the five main plasticmaterials found in the MPW, which represents 90 wt.%

of MPW. Each plastic in the mixture has the same pro-portion as that found in MPW: HDPE/LDPE/PP/PS/PVC=30/30/15/15/10. Fig. 11 presents the measuredTG/DTG curves of the multicomponent mixture. Acomparison of the experimental and the calculated non-interaction curves suggests that an interaction appearsto be taking place during the decomposition reactions.In Fig. 11, some difficulty in peak temperature assign-ment was encountered. Three main weight losses can beobserved in the experimental curve (Fig. 11). Between200 and 375�C, the weight loss was 11.65%, which isquite close to the theoretic value, 11.15%, calculated bya summation of the individual polymer weight losses atthe same temperature (1%�30% HDPE, 1.4%�30%LDPE, 12.25%�15% PP, 14.6%�15% PS and 64%�

10% PVC). The HCl evolution in the mixture wasslightly retarded but not above 375�C beyond which therest (88.35%) of the mixture starts to decompose. Thisimplies that there is little contamination from chlorineassociated to the major decomposition products of theplastic mix, which occurs at 375–510�C, since chlorine isreleased almost completely before 375�C. In the indus-trial continuous feed reactor, however, contact shouldbe avoided between HCl and the pyrolytic organicvapors.Two maximum rates of decomposition are observed

at 375–510�C. The first, with a peak temperature of

Fig. 8. TG and DTG curves of the binary 50/50 PVC/LDPE mixture recorded under vacuum at a heating rate of 10�C/min: ( — ) experimental; (....)

calculated.


422�C, was attributed to PS2 and PP. The second with apeak temperature of 465 �C corresponds to the over-lapping peaks LDPE2, HDPE and PVC3. Between 375and 520�C, the second peak of polystyrene pyrolysis(PS2) is moved to a higher temperature (i.e. is stabi-lized), the conjugated residue of PVC dehydrochlorina-tion (PVC3) is also stabilized, while the peaks of the PPand HDPE pyrolysis were shifted to lower tempera-tures.The results indicated that the mixture containing PVC

and polyolefins or PS, present two kinds of interactioneffects. First, the polyolefins slightly stabilize the HClevolution. Chlorine radicals promote low temperaturedegradation of polyolefins which do not decompose tovapors below 360�C. H-abstraction by Cl. radicals takesplace and, therefore, the Cl. radicals are less available toinitiate the dechlorination. Second, the polyene chainstabilizes polyolefins. The conjugated double bondsresulting from PVC dehydrochlorination reacted withany of the radical intermediates produced by intramo-lecular transfer reactions (polystyryl radical in the caseof PS) of the second polymer by intermolecular radicaltransfer mechanism. The latter effect can be attributedto the formation of a char residue, a characteristic of thedegradation of PVC as reported by Day et al. [43,44].The residue quantity as well as the quantity of compoundswith high molecular weight increases due to the inter-

molecular chain transfer as was also found in the pyr-olysis study of the other binary and tertiary polymermixtures containing polystyrene [73]. The intermolecularchain transfer could be responsible for the slight altera-tion in the composition of the pyrolysis products,depending on the composition of the commingled plas-tics. This aspect will be the topic of another paper.In the case of the PVC/PP binary mixture, the effect

of the stabilization of HCl production is greater thanthe cases of PVC binary mixtures with other polymers.The reason could be that the PP degradation is attrib-uted to the random scissions plus extensive intermoleculartransfer involving tertiary H-abstraction from the poly-mer by the primary radical [2], where H-abstraction fromPP by Cl. radicals can be easier than H-abstractionfrom PVC.During the thermal degradation of the HDPE/LDPE/

PP/PS/PVC mixture, the degradation mechanism couldbe a dehydrochlorination stage occurring between 200and 300�C as well as the formation of an intermediatefrom PS without any volatilization. The HCl produc-tion is stabilized via H-abstraction by Cl radicals fromthe polymers that are nearest in the temperature scale.Then stabilization of PS intermediates occurs by H-abstraction from PP, LDPE2 and HDPE. In mixturescontaining both PVC and PS the intermolecular radicaltransfer reaction appears to have a lesser effect on the

Fig. 9. TG and DTG curves of the binary 50/50 PVC/PP mixture recorded under vacuum at a heating rate of 10�C/min: ( — ) experimental; (....)

calculated.


Fig. 10. TG and DTG curves of the binary 50/50 PVC/PS mixture recorded under vacuum at a heating rate of 10�C/min: ( — ) experimental; (....)

calculated.

Fig. 11. TG and DTG curves of the multicomponent 30/30/15/15/10 HDPE/LDPE/PP/PS/PVC mixture recorded under vacuum at a heating rate of

10�C/min: ( — ) experimental; (....) calculated.


Table 9

Summary of the degradation behavior for the PVC-containing mixtures under study and that reported in literature


PVC/HDPE 50/50

vacuum

�Three stages of weight loss identified Intermolecular transfer by Cl. radical This study

�Overlapping DTG curves of PVC3

and HDPE

�Slight effect on

dehydrochlorination stage

�Slightly retarded PVC2

weight loss. HDPE stabilized. Increased

Ea for HDPE as a effect

of interaction

�Increased residue quantity

PVC/PE

(powder and melt)

Helium

�Decomposition rate was expressed as

the sum of the rates of the components

of the mixture

�No interaction [21]

�No significant interaction

�No reaction of HCl with

the second polymer

PVC/LDPE 50/50

vacuum

�Three stages of weight loss

identified

�Intermolecular transfer by Cl. radical This study

�Overlapping DTG curves

of PVC3 and LDPE2

�No effect on dehydrochlorination

stage

�LDPE2 stabilized. Increased Ea

for LDPE2 as an effect of interaction

PVC/PP 50/50

vacuum

�Three stages of weight loss identified �Intermolecular transfer by Cl. radical This study

�Overlapping DTG

curves of PVC3 and PP

�HCl evolution retarded

�PP were stabilized. Increased Ea for PP

as an effect of interaction

�Increased residue quantity

PVC/PP

(powder and melt)

Helium

�Decomposition rate was expressed

as the sum of the decomposition

rates of the components of the mixture

�Intermolecular radical transfer Cl. radical [21]

�No significant interaction

�No reaction of HCl with the

second polymer

PVC/PP Nitrogen �PP and PVC higher

temperature peak stabilized.

[43,44]

PVC/PS 50/50

vacuum

�Three stages of weight loss identified �Intermolecular transfer Cl. radical This study

�Overlapping DTG curves of

PVC2 and PS1�Overlapping DTG curves of

PVC3 and PS2�HCl evolution and PS2 were

slightly retarded

�PVC3 was slightly destabilized

PVC/PS

(powder and melt)

Helium

�Decomposition rate was expressed

as the sum of the decomposition

rates of the components of the mixture

�Intermolecular transfer by Cl. radical [21]

�Small discrepancy

�No reaction of HCl with the second

polymer

PVC/PS (film) �HCl production slightly retarded

(350�C)

�Intermolecular transfer by Cl. radical [31]

(continued on next page)


stability of the polymers in the mixture. Otherwise inmixtures containing PP and PS the effect on the polymerstability is higher, where the intermolecular hydrogentransfer is predominant.

3.2.3. Literature dataThe authors have found only one study dealing with

the thermal degradation of MPW-derived commingledplastics. Wu et al. [22] proposed a model for a mixtureof six plastics from MPW, based on the assumption thatthe degradation rate of the plastic mixture is the weighedsum of the rates of primary reacting species. Theyneglected the possible interactions between the plasticcomponents in the mixture subjected to pyrolysis.Studies on the plastic interaction in binary and multi-

component mixtures from this work as well as from theliterature review are summarized in Table 9. In all thecases involved, the dehydrochlorination seems to becomplete before the decomposition of the other poly-mers begins and it appears that there are no reactions ofhydrogen chloride with the second polymer. This meansthat kinetic models can be applied to understand thethermogravimetric data, which may enable the choice ofoptimum pyrolysis operating conditions. An analysis ofthe pyrolysis products obtained at the optimum operat-ing conditions has been carried out and will be pre-sented in Part II of this paper.From literature, reaction kinetics models for optimal

pyrolysis of the plastic waste mixtures typically found inMSW have not been sufficiently developed. Only a fewstudies have reported the Ea values of polymer mixturesconsidering the interaction effect between polymers.Csomorova et al. [38] reported a reduction of the Ea forPE as an effect of mixing in PE/PP blends in nitrogenatmosphere. The Ea was 249 and 171 kJ/mol, for a 50/

50 mixture of PE/PP. A reduction of the Ea for PE inmixtures containing HDPE/PP was reported by Albanoand De Freitas [51] as well, with Ea ¼ 201 kJ/mol for a50/50 mixture. Day et al. [43,44] reported no change inEa for a 50/50 PP/PVC mixture compared with indivi-dual polymer results. The reported Ea values were 133.1,124.4 and 153.7 kJ/mol for PVC1/PP/PVC2. All thekinetic parameters reported in the literature concerningcommingled plastic pyrolysis were obtained for thepyrolysis in N2 atmosphere. The results presented in thiswork (Tables 6 and 8) differ slightly with those from theliterature. Such a difference has been attributed to theeffect of experimental conditions, to the method ofkinetic parameter estimation, the kinetic model proposedand to the characteristics of the plastics used in the study.There is still a need to obtain fundamental informationon devolatilization and pyrolysis of plastic mixtures sincelimited data are currently available in the literature.

4. Conclusions

A kinetic study of the pyrolysis of the five main plas-tics contained in a typical MPW stream revealed thatsome interactions occur between the plastics during themixture pyrolysis. A comparison of the TG measuredand the calculated curves in the hypothesis of non-interaction, clearly shows that the decomposition rate ofmost polymers, represented as DTG peaks, was sig-nificantly altered in terms of peak shape and positions.However, a comparison of the kinetic parameters showsthat the activation energy of each polymer in the mix-ture changed only slightly (�13 kJ/mol) when com-pared with the single polymer pyrolysis. In the mixture,each plastic has a cerain interaction with the other

Table 9 (continued)


�PS chain scission reaction is accelerated

by PVC but no volatilization below 350�C

�Reaction of polystyrl radical with polythene

�PS stabilized at higher temperature range

(350–500�C). Tmax of PS is shifted to higher

temperatures in mixture and depending on

composition

HDPE/LDPE/PS/PP/

PVC

20/20/15/15/10

vacuum

�Three stages of weight loss identified �Intermolecular radical transfer This study

�HCl evolution as well as PS2 slightly retarded

�PP as well as HDPE destabilized

�PVC3 retarded

HDPE/LDPE/PS/PP/

PVC 35/35/10/10/10

HDPE/LDPE/PS/PP/

PVC/ABC

10/12/13/10/47/08

Nitrogegn

�Decomposition rate was expressed as the

weighting sum of individual components

�The interaction was considered negligible

�No interaction [22]


plastics and influences the overall rate of mixturedecomposition. The intensity of the interaction mainlydepends on the reactivity of the intermediate degrada-tion products released by that polymer. Conclusionsregarding the general nature of the interaction and itseffect on the stability are summarized below.

1. Intermolecular hydrogen transfer seems to be thepredominant reaction between components of thepolymer mixtures.

2. PS appears to be responsible for the shift of DTG/TG curves of polyolefins towards low decomposi-tion temperatures while PS itself in the mixture isstabilized.

3. The HCl evolution in the mixture was slightlyretarded but not above 375�C beyond which therest (88.35%) of the mixture starts to decompose.This implies that there is little contamination fromchlorine associated to the major decompositionproducts of the plastic blend, which occurs at 375–510�C, since chlorine is released almost completelybefore 375�C.

4. The results suggest that chlorine radicals pro-moted low temperature degradation of polyolefins,which do not decompose to pyrolytic vapors below360�C. H-abstraction by Cl. radicals takes placeand therefore, the Cl. radicals are less available toinitiate the dechlorination. Second, the polyenechain stabilizes polyolefins. The latter effect can beattributed to the formation of a char residue, acharacteristic of the degradation of PVC.

5. In mixtures containing both PVC and PS, theintermolecular radical transfer reactions appear tohave less effect on the stability of the polymers inthe mixture than in mixtures containing PP and PSwhere the effect on the polymer stability is higherand the intermolecular hydrogen transfer is moresignificant.

The results suggest that when the five MPW plasticsare pyrolysed together in order to recover valuable mate-rials, the pyrolysis can be separated in two steps: first at375�C at which point the weight loss is 11.65 wt.%, ofwhich 4.65 wt.% is attributed to HCl production and7.0 wt.% corresponds to various volatiles formed fromthe polymers present in the mixture. The second step isbetween 375 and 520�C where 86.85 wt.% of all theliquid product is obtained, while 1.5 wt.% of solid resi-due is left behind; little contamination of the organicliquid with HCl occurs during this step.

Acknowledgements

This paper has been carefully reviewed by Dr. Ann-ette Schwerdtfeger. The authors would like to thank the

Ministere de l’Education du Quebec for the financialsupport (Programme quebecois des bourses d’excel-lence) of one of the co-authors of this work, R.M.Thanks are due also to the polymer suppliers listed inTable 1 of this paper.

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Vacuum pyrolysis of commingled plastics containing PVC I. Kinetic study

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