D. Spaseska,
M. Civkaroska
Journal of the University of Chemical
Technology
and Metallurgy, 45, 4, 2010, 379-384
ALKALINE HYDROLYSIS OF POLY(ETHYLENE TEREPHTHALATE) RECYCLED
FROM THE POSTCONSUMER SOFT-DRINK BOTTLES
D. Spaseska, M. Civkaroska
Faculty of Technology and Metallurgy
“SS Cyril and Methodius” University of Skopje
Republic of Macedonia
Received 30 June 2010
Accepted 28 September 2010
ABSTRACT
The post consumer Poly (ethylene terephthalate)PET) recycling science was initiated by the fact that this polymer
was non-degradable in nature, since its molecules were too large to decompose. There has been a growing need, however,
for a chemical recycling process as one of the most successful method for post-consumer PET transformation into
monomers. The solution of this problem seems to be in the creation of remunerative processes, for which post-consumer
PET is used as a source material.
This paper presents a review of works that cover PET waste recycling with alkaline hydrolysis of post-consumer
bottles for their depolymerization to monomers (terephthalic acid). The process is carried out with sodium hydroxide and
trioctyl methyl ammonium bromide (TOMAB) as catalyst within a relatively short time and temperature.
A mathematical model of PET recycling has been derived according to the definition of a 23 factorial experimental
design. The influence of the main factors on the alkaline hydrolysis conversion of the PET, as expressed by the yield of
terepthalic acid (TPA), in ascending order, includes the process temperature and the amount of catalyst, but the hold-up
time is negligible.
Keywords: PET recycling, hydrolysis conversion, terephthalic acid, factorial experimental design, mathematical
model of the process, hydrolysis time, amount of catalyst, hydrolysis temperature.
INTRODUCTION
Plastics make up a high proportion of waste the
volume and range of which increases dramatically. Although plastics make up between 5 mass % and 15% mass
of municipal solid waste it comprises 20-30 % of the
volume [1]. Most plastics are non-degradable and take
a long time to decompose, possibly up to hundreds of
years – although no one knows for certain as plastics
have not existed for long enough, since they started to
be land filled.
PET is one of the versatile engineering plastics
that are widely used in manufacturing of high strength
fibers, audio and video tapes and various types of packaging, mainly soft drink bottles and jars [2-4]. The
majority of the world PET production is for synthetic
fibers (in excess of 60 %) with bottle production accounting for around 30 % of global demand. One of the
main reasons for the widespread use of PET is the pos-
sibility for producing a number of different grades over
a broad range of molecular masses in a single
multiproduct polymerization plant [5]. A very important feature of PET, decisive in the choice of its wide
application in the manufacture of packaging for the food
industry is that it does not have any side effects on the
human beings. It should be pointed out, that PET does
not create a direct hazard to the environment but, due
to its substantial fraction by volume in the waste stream
and its high resistance to the atmospheric and biological agents, it is seen as a noxious material [2]. Therefore, the recycling of PET does not only serve as a partial solution to the solid waste problem but also contributes to the conservation of raw petrochemical products and energy.
The huge amounts of PET products cause serious environmental pollution, because commonly PET
content reaches about 12 % in municipal plastic waste
showing a slow rate of natural decomposition [1,6]. PET
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Journal of the University of Chemical Technology and Metallurgy, 45, 4, 2010
recycling is very important for at least two main reasons: firstly, to reduce the increasing volumes of plastic
waste and secondly - to generate value-added materials
from low cost sources by converting them into valuable
materials. The demand to expand grade uses of recycled
PET led to the research into alternative processing methods in order to produce higher value products. Numerous methods for recycling of disposable beverage bottles
and other containers made of PET or blends of PET
with other materials have been widely reported [7-12].
One of the most applicable methods for PET recycling is chemical recycling defined as the process leading to total depolymerization of PET to the monomers, or
partial depolymerization to oligomers and other chemical
substances. This means that the process results in the formation of the raw materials (monomers) from which the
polymer is made of, and there is no need for extra resources (monomers) for PET production.
Glycolysis, methanolysis, hydrolysis are the main
processes included in the chemical recycling processes
for PET [2, 3].
Hydrolysis is a method of PET waste chemical
recycling which attracts a growing interest related to the
development of factories for PET synthesis directly from
TPA and EG. Commercially, hydrolysis is not widely
used to produce food-grade recycled PET, because of the
cost associated with purification of the recycled TPA.
Hydrolysis as the method of PET waste recycling by the
reaction of PET with water in an acid, alkaline or neutral
environment, leads to total depolymerization to its monomers - terephthalic acid and ethylene glycol. The reaction time ranges from a few to 30 minutes at high temperature and under high pressure, and requires no additives, such as catalysts or neutralizers [13-20].
Alkaline hydrolysis of PET is usually carried out
with the use of an aqueous alkaline solution of NaOH,
or KOH of a concentration of 4-20 mass % [2, 14]. The
reaction products are EG and the disodium terephthalate salt TPA-Na2. Pure TPA can be obtained by neutralization of the reaction mixture with a strong mineral acid (e.g. H2SO4). Apart from the aqueous alkaline
hydrolysis of PET, alkali decomposition in non-aqueous solutions has been reported [21]. The addition of
an ether (such as dioxane, or tetrahydrofuran) as a mixed
solvent with an alcohol (methanol, or ethanol) accelerated the reaction. The main advantage of this method is
that it can tolerate highly contaminated post-consumer
380
PET such as magnetic recording tape, metalized PET
film, or photographic film (X-ray film) [2] . The process is relatively simple and less costly than methanolysis.
Kosmidis et al. [22] have made a comparison between
the acid hydrolysis with catalyzed and non-catalyzed
alkaline hydrolysis. They point out that alkaline hydrolysis using special phase transfer catalysts seems to be
most promising for future industrial applications.
The process, named Solid State Shear Pulverization [23, 24] for production of PET powders requires
lower experimental temperature and pressure in comparison with using PET flakes. Solid-state shear pulverization is a continuous, one-step process that has been
initially conceived as a means to convert pelletized or
flaked polymer feedstock into powder. Under certain
process conditions, polymer chain scission accompanies powder production, as evidenced by reduction in
molecular mass [25] and the production of free radicals
[26]. The yield of terephthalic acid was as high as 98 %
by pulverization and depolymerization of PET [27].
The alkaline hydrolysis of PET with [22], or without [28] the use of a phase transfer catalyst, compared
with the acid hydrolysis, has been studied by D. S.
Achilias and G. P. Karayannidis [13]. Overall material
balances were carried out for the hydrolysis of PET.
Also, it was postulated that recycling according to the
scheme:
is the only one within the framework of sustainable development.
In the works of G. P. Karayannidis and D. S.
Achilias [28, 29] as well as the works of J.Das et al.
[30], hydrolysis in alkaline environment was employed
in order to recover pure terephthalic acid monomer that
could be repolymerized to form the polymer again. Thus,
recycling of PET does not only serve as a partial solution to the solid waste problem, but also contributes to
the conservation of raw petrochemical products and energy. A phase-transfer catalyst was introduced in the alkaline hydrolysis, so that the reaction takes place at atmospheric pressure and in mild experimental conditions.
R. López-Fonseca et al. [31] were focused on the
identification of the catalytic behavior, if any, of a series of quaternary phosphonium and ammonium salts
as phase transfer catalysts for the alkaline hydrolysis of
PET, and on the determination of the kinetics of the
D. Spaseska, M. Civkaroska
phase transfer catalyzed process. The use of selected
phosphonium quaternary salts exhibited a remarkably
positive effect on the experimental conditions under
which the depolymerisation of poly(ethylene terephthalate) by alkaline hydrolysis can be carried out, especially in terms of low operating temperature. The proposed kinetic model accounted for the unanalyzed and
catalyzed reactions and predicted for the reaction rate a
linear correlation with the concentration of the quaternary salt. The notable increase in the phase transfer catalyzed reaction rate was related mainly to the greater
value of the pre-exponential factor while the value of
the activation energy was hardly modified by the presence of the quaternary phosphonium salt, thereby suggesting a similar mechanism for the alkaline hydrolysis
with or without phase transfer catalyst.
The present study was undertaken with the aim
of PET depolymerization at milder conditions than those
used generally.
Mathematical design of the process is a significant domain of study in the investigated process, because it points out the correlation between the main
process parameters and the ultimate parameters (yield
or characteristics of the products) [32-34]. In this work
a mathematical model for the catalytic alkaline hydrolysis has been derived. The effects of various operating
parameters, temperature, hold-up time and the amount
of catalyst on the process, have been investigated. The
three-factor interaction effect is significantly related to
the hydrolysis conversion of the recycled PET bottles.
tor and an electric heating mantle. 1.5 L of the sodium
hydroxide solution (5-15 mass %), were added into the
reactor and heated to the desired reaction temperature,
80 and 120°C. Agitation was started in order to keep
the mixture homogeneous and the reflux condenser set.
The desired quantity of PET flakes (11.52 g) and the
catalyst (trioctyl methyl ammonium bromide, TOMAB)
were then added. The reaction time started and the mixture was allowed to react for 3 and 5 h. At the end of
the reaction, the reaction mass was neutralized to pH 7
with H2SO4 and filtered through a glass filter (G3). The
TPA in the mixture was precipitated by the addition of
H2SO4 down to pH 2.5-3. It was then removed by filtration with a G3 glass filter and washed with water. The
final solid TPA produced was dried in a vacuum oven
at 120°C and weighed. The final unreacted PET was also
measured by filtration of the final mixture through a G3
glass filter. The solid PET that remained was washed
with water, dried in a vacuum oven at 120°C and
weighed.
The main investigated parameters were as followed:
reaction temperature (x1) 80 and 120°C
catalyst concentration (x2): 0,1 and 2,0 mol/mol PET
reaction time (x3): 3 and 5 h.
The % yield of TPA was used as a reflection function
(y).The percent degradation of PET was calculated using the following equation:
PET degradation (%) = (W PET,0 – W PET,f) 100 / W PET,0
where, W PET,0 and W PET, f refer to the initial and final
mass of PET, respectively.
EXPERIMENTAL
RESULTS AND DISCUSSION
Materials
The postconsumer soft-drink PET bottles free
from caps and label were used as a starting material for
depolymerization. PET bottles were consequently cut
to the size of 6 mm.
Depolymerization of PET in alkaline solution
The alkaline depolymerization was carried out
in accordance with the method described in the work of
Kosmidis et al. [22]. In order to derive the mathematical relationship for the depolymerization process, the
experiments were carried out according to an experimental design [32]. The depolymerization reaction was
carried out in a three neck, 2 L round-bottom reactor
equipped with a reflux condenser, a mechanical agita-
PET was first hydrolyzed in sodium hydroxide
to yield the disodium salt, Na2TPA according to the
chemical reaction shown in Scheme 1. The salt Na2TPA
was then acidified with 98 % sulphuric acid to precipitate out TPA.
Initially the reaction mixture consists of the solid
organic phase of PET and the aqueous alkaline solution
(NaOH). The phase transfer catalyst treoctyl methyl
ammonium bromide (TOMAB) fulfills the requirement
of having enough organic character in order to avoid
steric hindrance. The cationic part of the catalyst does
not carry the hydroxide anion into the organic phase
(extraction mechanism) but on its surface (interfacial
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Journal of the University of Chemical Technology and Metallurgy, 45, 4, 2010
Scheme 1. Chemical reactions involved in the process.
mechanism). In this way the PET macromolecules on
the surface of the particles can be easily attacked by the
OH- and then depolymerized. The terephthalate anion
produced returns to the aqueous phase and forms the disodium terephthalate salt with Na+. The reaction proceeds
until complete depolymerization of PET to Na2TPA, while
the catalyst remains in the aqueous phase [30].
For the purpose of determination of the dependence of yield of TPA on process parameters, like temperature, catalyst concentration and hold-up time, the experiments have been carried out in accordance with an experimental design, resulting in a derived mathematical model
for the degradation process [32]. The derivation of the mathematical model of the process is enabled by a minimal number of the performed experiments. As the reflection function, the yield of TPA has been taken. The obtained results
are presented in Table 1. From the experimentally obtained
results, the derived equation with coded variables for the
produced TPA is given with the relationship:
Y = 55,75 + 3,5 X1 + 37,25 X2 + 3,5 X1X2 + 4 X2X3 –
(1)
5,75 X1X2X3
The equation with natural variables is given by
the following relationship:
y = 161,2105 – 1,2895 x1 – 11710,5263 x2 – 36,1974 x3
+ 139,4737x 1 x 2 + 0,3178 x 1x3 + 3447,3684 x 2x3 –
30,2632 x1x2x3
(2)
The dependence of the obtained TPA concentration on temperature and catalyst concentration, according to equation (2) are presented in Fig.1 and Fig.2,
respectively. According to the relationship (1) and Fig.1
Table 1. Yield of TPA in dependence on process parameters.
No.
X1
0
t/ C
1.
2.
3.
4.
5.
6.
7.
8.
382
80
120
80
120
80
120
80
120
X3
X2
Catalyst conc.
mol(x0,01)/mol
PET
0,1
0,1
2,0
2,0
0,1
0,1
2,0
2,0
Time /
Y
TPA / % w.
h
3
3
3
3
5
5
5
5
32
15
80
100
5
22
92
100
Fig. 1. Effect of temperature on the amount of TPA produced
during alkaline hydrolysis of PET with the amount of
catalyst and time fixed at the varied levels.
D. Spaseska, M. Civkaroska
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Fig. 2. Effect of the amount of the phase transfer catalyst
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neglecting effect. Taking into account the coefficients
before the individual parameters (equation 1), the most
expressed influence on the reflection function (Y) has
the parameter “catalyst concentration”.
CONCLUSIONS
PET depolymerization in alkaline medium and
catalyst presence results in TPA production.
Almost complete conversion of PET in relatively
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be achieved.
The alkaline hydrolysis with phase transfer catalysts is one of the most promising methods for PET
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The experimental design was successfully used
for the determination of the best conditions for carrying out the PET alkaline depolymerization process as
well as for the optimal yield of TPA.
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