Determination of the proportion between hydrolysis and pyrolysis duringpoly(ethylene terephthalate) degradation in 18O-labeled steam
Y. Morohoshi1, S.Kumagai1,2, G. Grause1, T. Kameda1 and T. Yoshioka1*
1Graduate School of Environmental Studies, Tohoku University

2Research Fellow of the Japan Society for the Promotion of Sciencee-mail:yoshioka@env.che.tohoku.ac.jp TEL/FAX: +81-22-795-7212

AbstractPoly(ethylene terephthalate) (PET) was decomposed in 18O-labeled steam atmosphere in order to determine theproportions of pyrolysis and hydrolysis, producing labeled and unlabeled terephthalic acid(TPA). It was clear that 13% ofester bonds were hydrolyzed at 400 ºC at 25 vol% steam concentration, which fractions were improved with steam
concentration increase, resulting in 29% at 75 vol%. In addition, apparent activation energy was determined between 0vol% and 75 vol% steam concentrations from thermogravimetric analysis, which values decreased from 199 kJ mol-1 atpyrolysis condition to 132 kJ mol-1 at 75 vol%. The results implied that pyrolysis was dominant over hydrolysis at 400 ºC
since steam mainly reacted on the PET surface.
Keywords: PET, isotopic label, kinetics, proportion
were determined from distribution of labeled andunlabeled DMT.
In order to determine the apparent activation energy
for each steam concentration (0, 25, 50 and 75 vol%), a
10 mg PET particle was decomposed between 350 ºC
and 390 ºC using TGA.

1. Introduction

Thermal degradation is one method for conversion ofplastics into monomers, which is less affected byimpurities. Poly(ethylene terephthalate) (PET) is also
thermally decomposed into terephthalic acid (TPA),
which production is accelerated in teamatmosphere[1]. The recovered TPA is used as amonomer for PET synthesis or converted into benzene
in the presence of CaO [2]. However, PET isdecomposed in steam at higher temperatures by bothpyrolysis and hydrolysis. Therefore, the degradation
mechanism in steam atmosphere should be revealed inorder to estimate the exact yields or determine thesuitable operating conditions.
In this study, the proportions of pyrolysis and hydrolysis
in the steam degradation of PET using 18O-labeled water(H218O) were determined. In addition, apparent activationenergy for each condition was analyzed using
thermogravimetric analysis (TGA).

3. Result and discussion

The MS spectrum of labeled and unlabeled DMT
obtained from hydrolysis of H218O containing steam andion-exchanged water are summarized in Fig. 1. It can be
distinguished that the one (m/z =196) or two (m/z =198)
ester bonds were hydrolyzed in the presence of H218O
steam.
Abundance

m/z = 194
Scan 718 (32.026 min): PET 400 100% O18 1 111216.D (-716) (-)
195.1
196.1

4668

64643
194.1
1000

Abundance

1000

900
194.1

≈

52113

≈Scan 718 (32.024 min): PET 400 100% O16 1 111213 .D (-716) (-)
195.1

≈

900768196.0

700

2. Materials and Methods

800
600700500

Intensiry

Crushed PET bottles (200 mg, <250 μm) were filled
into a perforated quartz holder, which was suspended
outside of the heating zone in the tube reactor. The 10%
H218O steam mixed with ion-exchanged water wasgenerated from steam generator heated by an electricfurnace. When the experimental conditions wereachieved (400 ºC, total of helium and steam gas flow of300 ml min-1, 0, 25, 50, 75 or 100 vol% steamconcentration), the sample holder was introduced into theheating zone. The temperature was kept for 30 min.
Produced TPA was recovered and esterified withdiazomethane, forming dimethyl terephthalate (DMT)
without removing the 18O labeled isotope. The DMT wasanalyzed using gas chromatography-mass spectrometry(GC-MS), the proportions of pyrolysis and hydrolysis

m/z = 196

800

197.3

600m/z = 198

400

500
300

400

196
198.1

200

300
100

H218O

200
0
194.0

100

194.5

195.0

195.5

196.0

196.5
197.0

197.0

197.5m/z-->

H216O
194.0m/z-->

198.0194

194.5

195.0

195.5

196.0

196.5

197.0

196
Mass-to-charge ratio(m/z)

197.5

198.0

198

Fig.1 MS spectrum of DMT produced by steam
decomposition using ion-exchange water and 18Oisotope-labeled water.
The proportions of pyrolysis and hydrolysis of PET
degradation for each steam concentration were shown in
Fig. 2. The hydrolysis fraction was 13% at 25 vol% ofsteam, which was promoted with increasing steam
concentration, resulting in 29% at 75 vol%. However,

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013

hydrolysis fractions between 75 vol% and 100 vol% weresimilar, probably as a result of mass transportphenomena, well known in combustion processes. As thereactive gas approached the substrate, gas product wasformed, which moved in a countercurrent with thereactive gas and limited its presence at the surface of thesubstrate. As a consequence, the hydrolysis rate waslimited by the access of steam, which was controlled by
the gas product evolution. Pyrolysis therefore remained
dominant at all steam concentrations at this temperature.
The exchange of 18O-labeled hydroxyl groups between
TPA and steam, and vice versa, was not observed whenunlabeled TPA was exposed to 18O-labeled steam.
Exchange of hydroxyl groups during contact with thecondensed water phase can also be excluded becauseof the low reactivity of the acid under the conditions
used[3].

Reaction ratio [% ]13238040hydrolysis(surface)OH

R'
H 18OOOOROR'

H O-OORO H HOOOR

18O+HHO

R'18

H218O
18

H2 O

PET10060

hence PET was pyrolyzed anywhere. In addition, it wasconfirmed that TPA did not decompose under theconditions used.

2926TPA100

hydrolysis

pyrolysis

8777

pyrolysis(surface and inside)71

74

2000255075
Steam concentration [vol% ]100

Fig.2 Selectivity of hydrolysis and pyrolysis in steam
decomposition of PET at 400 °C and various steam
concentrations.
The apparent activation energy for each steam
concentration was determined from the Arrhenius plot
(eq. 2). The highest correlated reaction model g(α) wasselected from Table 2 in Grause et al. [4] and reaction
constant kapp, calculated using eq.1.
dα(1) k app  gα dt

 E app  RT k app  A  exp(2)

where, :decomposition rate [-], t: time [min], kapp:apparent reaction constant [min-1], g():reaction model,
A: frequency factor [min-1], Eapp: apparent activationenergy [kJ mol-1], R: gas constant(8.314 J mol-1 K-1), T:
temperature [K]Activation energy was decreased with steam
concentration increase, resulting in 199, 190, 137 and132 kJ mol-1 at 0, 25, 50 and 75 vol% steam
concentration, respectively. Eapp decreased drasticallybetween 25 and 50 vol%. The same tendency wasobserved for the hydrolysis ratio shown in Figure 2.
The reaction mechanism of pyrolysis and hydrolysis insteam decomposition of PET is shown in Fig.3. PET washydrolyzed by contacting steam, therefore hydrolysis
might have progressed only at the PET surface. Incontrast, the driving force of pyrolysis was only heat,

R'OH
OO
O

R

R'O
OHOOR

Fig.3 Reaction mechanisum of hydrolysis(surface) andpyrolysis(surface and bulk) in steam decomposition ofPET using 18O- isotope-labeled water.

4. Conclusion
In this study, the proportions of pyrolysis andhydrolysis during steam decomposition of PET weredetermined using 18O-labeled water. Additionally,apparent activation energies for each steam
concentration were also determined. Both approaches,18O-labeling and TGA, displayed similar results. Higher
steam concentrations caused little changes in theactivation energy and hydrolysis rate, supporting theassumption that hydrolysis occurred preferentially at thePET surface. The combination of TGA and atom labelinghas proven to be a useful approach in kinetic studies andmight be applicable to other condensation polymers.

Acknowledgment
This work was partially supported by the Ministry ofEducation, Culture, Sports, Science and Technology(MEXT), Grant-in-Aid for Scientific Research (A),
21241018, and by the Japan Society for the Promotion ofScience (JSPS), Grant-in-Aid for JSPS Fellows, 24-4996.

References[1] G. Grause, W. Kaminsky and G. Fahrbach. Polym.
Degrad. Stab., 85 (2004), 571-575.
[2] S. Kumagai, G. Grause, T. Kameda, T. Takano, H.
Horiuchi and T. Yoshioka. Ind. Eng. Chem. Res., 50(2011), 1831-1836.
[3] K. W. Wedeking and J. M. Hayes. Chem. Geol, 41(1983), 357-370.
[4] G. Grause, M. Furusawa, A. Okuwaki and T.
Yoshioka. Chemosphere, 71 (2008), 872-8.

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013