1. Protection and restoration of the environment XI
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ENVIRONMENTALLY FRIENDLY CHEMICAL RECYCLING OF
POLYESTERS (PET, PPT) USING ALKALINE HYDROLYSIS UNDER
MICROWAVE IRRADIATION
A. Liarou, G.Z. Papageorgiou and D. S. Achilias*
Department of Chemistry, Aristotle University of Thessaloniki, 54124, Thessaloniki, Greece
*Corresponding author: E-mail: axilias@chem.auth.gr, Tel +30 2310 997822, Fax: +302310997769
ABSTRACT
Recently, a new polyester, namely poly(propylene terephthalate), PPT, has been put on the market
under the brand name Corterra™ to replace PET mainly in the production of fibers. This polymer
has extensive applications in carpeting, textiles and apparel, engineering thermoplastics, non-
wovens, films and monofilaments since it combines the properties of nylon and polyester. In this
study an environmentally friendly way to recycle PPT is proposed using alkaline hydrolysis under
microwave irradiation.
Microwave irradiation as a heating technique offers many advantages over the conventional heating
such as instantaneous and rapid heating with high specificity without contact with the material to be
heated. It is, therefore, a popular technique for heating and drying materials and is utilized in many
household and industrial applications. The main advantage of microwaves over conventional
heating sources is that the irradiation penetrates and simultaneously heats the bulk of the material.
Research efforts have thus lead to numerous applications in material processing techniques that
have resulted in shorter reaction times and greater convenience.
Recycling of different grades of poly(propylene terephthalate) as well as PET is examined here
using hydrolytic depolymerization in an alkaline solution, under microwave irradiation. The main
objective was to provide a recycling method for PPT, using an environmentally friendly way (i.e.
microwave irradiation instead of conventional heating) requiring thus lower reaction temperatures
and/or shorter reaction times with substantial energy saving. A final innovative part was the
introduction of a phase transfer catalyst during the depolymerization to facilitate further the
reaction.
The reaction was carried out in a sealed microwave reactor in which the pressure and temperature
were controlled. Experiments under constant temperature were carried out at several time intervals.
The main products were the monomers terephthalic acid (TPA) (obtained in pure form) and
propylene glycol, which were analyzed and identified. The depolymerised PPT residues were also
analyzed using DSC measurements. It was found that depolymerization is favoured by increasing
temperature, time and amorphous phase material.
The results of this study confirmed that PTT waste can be successfully converted into useful
products using an eco-friendly recycling technique
Keywords
Recycling; synthetic fibers; alkaline hydrolysis; poly(propylene terephthalate); microwaves.
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1. INTRODUCTION
As it is well-known, the production and consumption of polymer-based materials has recently
enormously increased. As a result, a large amount of polymers finds its way to wastes everyday. The
recovery of valuable products through the chemical recycling of polymers has been attracting
attention in recent years for both environmental and economic reasons. In particular, new methods
are developed for the quantitative recovery of monomers in a short time using environmental
friendly techniques. One of the major classes of polymers in the waste stream is that of polyesters
[such as poly(ethylene terephthalate), PET]. Chemical recycling processes for polyesteres are
divided as follows (Karayannidis and Achilias, 2007; Scheirs, 1998; Sinha et al., 2010): (i)
Glycolysis, (ii) Methanolysis, (iii) Hydrolysis and (iv) other processes: Glycolysis involves the
insertion of ethylene glycol units (or diethylene glycol and propylene glycol) in the polyester chains
to give bis (hydroxyalkyl) terephthalate (BHAT) which is a substrate for new polymer synthesis and
other oligomers. The production of secondary useful products, such as alkyd resins, has also been
proposed. Methanolysis actually is the degradation of polymers by methanol at high temperatures
and high pressures with main products: dimethyl terephthalate (DMT) and ethylene glycol (EG).
Hydrolysis of polyesters can be carried out in an acid, alkaline or neutral environment to produce
the monomers terephthalic acid (TPA) and ethylene glycol (EG). The growing interest in this
method is connected with the development of new factories for polyester (i.e. PET) synthesis
directly from TPA and EG. Neutral Hydrolysis is carried out with the use of hot water or steam.
Acid hydrolysis is performed most frequently using concentrated sulfuric, nitric or phosphoric acid.
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 wt-% (Carta et al., 2003). 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). The main advantage of this method is that
it can tolerate highly contaminated post-consumer PET such as magnetic recording tape, metallized
PET film, or photographic (X-ray) film.
Concerning the chemical recycling of PET, a number of studies have been published (Sinha et al.,
2010; Carta et al., 2003; Karayannidis and Achilias, 2007). However, little work has been
performed on the depolymerization of a new polyester, poly(propylene terephthalate) (PPT) aimed
at replacing PET in fibers’ production. PPT is an aromatic polyester made from the
polycondensation of 1,3-propanediol (1,3-PDO) with either terephthalic acid or dimethyl
terephthalate. It was first synthesized, just like poly(ethylene terephthalate) (PET) and poly(butylene
terephthalate) (PBT), in 1941 by Winfield and Dickson. However, despite its excellent properties, it
became commercially available only recently because one of its raw materials (1,3-PDO) was very
expensive and was available only as a small volume fine chemical. A recent breakthrough in the
synthesis of 1,3-PDO by Shell Chemical Co. at a much lower price via the hydroformylation of
ethylene oxide gave also a boost in the production of PPT. PPT has an odd number of methylene
units between the terephthalate moieties in its chemical structure in comparison with two common
homologous polyesters, poly(ethylene terephthalate) and poly(1,4-butylene terephthalate) (PBT),
and its molecule takes on an extended zigzag shape. Because of this special structure, PPT has
outstanding resiliency, chemical resistance, and good thermal properties for fibers (mainly carpet
fabrics) and engineering thermoplastics. PPT fibers exhibit high elasticity, excellent recovery rate,
dye ability and stain resistance, high UV stability, low water absorption and low electrostatic
charging. Global commercial interest in PTT will expand capacity and end uses. An increase in the
uses of PPT products will result in a greater amount of waste materials.
The repeating unit of this macromolecule has the chemical structure:
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O CH2 CH2 CH2 O C C
O O n
Microwave-assisted organic synthesis has revolutionized chemical research (Adam, 2003; Lidstrom
et al., 2001). Microwave irradiation, as a heating technique, offers many advantages over
conventional heating, such as instantaneous and rapid heating with high specificity, without contact
with the material to be heated. It is, therefore, a popular technique for heating and drying materials
and is utilized in many household and industrial applications. The main advantage of microwaves
over conventional heating sources is that the radiation penetrates and simultaneously heats the bulk
of the material. Research efforts have thus led to numerous applications in material processing
techniques that have resulted in shorter reaction times and greater convenience.
Although the use of microwave irradiation in chemical reactions is a rather well-established
technique, the papers published on the recycling of polymers are very limited. Some papers have
been published on the recycling of PET (Nikje and Nazari, 2006; Liu et al., 2005; Li et al., 2008;
Krzan, 1998) and none on the use of microwave irradiation in the recycling of PPT. During the past
few years hydrolysis of waste PET was investigated in our laboratory as potential method for the
chemical recycling of soft drink bottles (Kosmidis et al., 2001). In addition, PET recycling under
microwave irradiation was examined using hydrolysis, glycolysis and aminolysis (Achilias et al.,
2010; 2011; Siddiqui et al., 2010).
In this study, depolymerization of PPT, taken from a commercial product (i.e. Corterra™ from Shell
Chemicals), was subjected to alkaline hydrolysis in a lab-scale microwave reactor, in order to study
the effect of microwave irradiation on its degradation. This polymer has extensive applications in
carpeting, textiles and apparel, engineering thermoplastics, non-wovens, films and monofilaments
since it combines the properties of nylon and polyester. The reaction was carried out in a sealed
microwave reactor, in which pressure and temperature were controlled and recorded. The main
products were the monomers terephthalic acid (obtained in pure form) and propylene glycol, which
were analyzed and identified. The effect of several process parameters, including the degree of
crystallinity of the original polymer on the amount of PPT depolymerized and TPA recovery, was
investigated. The main objective was to provide a recycling method for PPT, using an
environmentally friendly way (i.e. microwave irradiation instead of conventional heating), thus
requiring lower reaction temperatures and/or shorter reaction times with substantial energy saving.
A final innovative part was the introduction of a phase transfer catalyst during the depolymerization
to facilitate further the reaction.
2. EXPERIMENTAL
2.1 Materials
PPT used was supplied by Shell Co., Houston, TX under the Trade name CorterraTM. The chemicals
used were reagent grade. Amorphous PPT films were prepared by melt-pressing with an Otto Weber
PW 30 hydraulic press at 250°C and under a load of 6 kN on a ram of 110 mm, followed by
quenching in cold water. In addition, PET flakes were prepared from used clear PET bottles, from
which the labels and glue had been removed. The bottles were cut and fed to a rotary cutter
producing flakes with a maximum size of 6 mm. The phase transfer catalyst Hexadecyl TriMethyl
Ammonium Bromide (HDTMAB) was obtained from Aldrich. The chemical structure of the
catalysts is:
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CH3
H3C N+ C16H33 Br-
CH3
2.2 Hydrolytic depolymerization
PPT and PET decomposition reaction was conducted in a microwave reactor (model Discover from
CEM corporation), equipped with a digital temperature control system and pressure sensors inserted
directly into the 10 mL PTFE reaction tube. Pellets of sodium hydroxide (10 g) were dissolved in
100 mL of distilled water and the resultant NaOH solution (2.5 M, 10%w/v) was used for the
experiments. Polyester flakes (0.5 g) together with 5 mL of NaOH solution were added into the
reactor, sealed under inert atmosphere (N2) and the heat-up period to the desired set-point started. In
most experiments 0.01 g of HDTMAB was also fed into the reactor. When the set temperature was
achieved the reaction time began and the polymer decomposition was followed for a specified time
period. After that time period, the reaction vessel was automatically cooled and the reaction mixture
was filtered to remove the unreacted polyester residues. The final unreacted polymer was measured
upon filtration of the final mixture through a G3 glass filter, washing with water, drying in a
vacuum oven at 40oC and weighing. The experiments were repeated using a second PPT sample
obtained in amorphous condition by initially melting the original commercial PPT sample followed
by quenching.
When PET is hydrolyzed in sodium hydroxide the disodium salt and ethylene glycol are produced,
according to the following chemical reaction:
O O
H O CH2CH2 O C C n OH
2n NaOH
O O
n NaO C C ONa + n HOCH2CH2OH + H2O
The same reaction hold for PPT except that propylene glycol is produced instead of EG.
The TPA-Na2 salt was continuously acidified with sulfuric acid, H2SO4 (10%) to a pH of 2.5 to
precipitate the TPA monomer. Finally, the mixture was filtered and washed with absolute ethanol.
The solid TPA produced was dried in a vacuum oven at 40oC and weighed.
O O O O
NaO C C ONa + H2SO4 HO C C OH + Na2SO4
2.3 Analysis of the Results
The % yield in TPA was calculated using the formula:
NTPA
TPA Yield (%)  100 (1)
NTPA,0
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where, NTPA and NTPA,0 refer to the number of moles weighed and the theoretical number of TPA
moles that will be produced upon complete decomposition of PET, respectively.
The percent degradation of PPT (or similarly of PET) was calculated using the following equation:
W PPT ,0  W PPT , f
PPT Degradation (%)  100 (2)
WPPT ,0
where, WPPT,0 and WPPT,f refer to the initial and final weight of PPT, respectively.
2.4 Product characterization
The determination of purity of terephthalic acid was performed by titration with 0.5 N NaOH
solution. About 1 g of TPA is weighed to the nearest milligram into a 250 mL conical flask. To
dissolve the sample 25 mL of analytical grade pyridine is added by pipette and the suspension is
heated with a reflux condenser until a clear solution is obtained. The condenser is then washed out
by the addition of about 5 mL of pure pyridine through the top and the content of the flask is titrated
with approximately 0.5 N standard sodium hydroxide solution to the phenolphthalein endpoint.
The chemical structure of the TPA separated, was confirmed by recording its IR spectra. The
instrument used was an FTIR spectrophotometer of Perkin-Elmer, Spectrum One. The resolution of
the equipment was 4 cm-1. The recorded wavenumber range was from 450 to 4000 cm-1 and 32
spectra were averaged to reduce the noise. A commercial software Spectrum v5.0.1 (Perkin Elmer
LLC 1500F2429) was used to process and calculate all the data from the spectra. The KBr pellet
technique was used.
Thermal characteristics of the original PPT samples and those obtained after degradation were
obtained using a Differential Scanning Calorimeter (Perkin-Elmer, Pyris Diamond DSC). The
instrument was calibrated using high purity Indium and Zinc standards. Samples of about 5 mg were
used. The samples sealed in aluminum pans were initially heated from 0 to 270oC at a rate of
10oC/min. Subsequently cooled to 0oC at a rate of 20oC/min and reheated to 270oC. Tests were
performed under a nitrogen atmosphere.
3. RESULTS AND DISCUSSION
3.1 Degradation kinetics
Depolymerization experiments were carried out using either amorphous or crystalline PPT and PET
for comparison. Three temperatures, i.e. 120, 150 and 180 oC were selected for isothermal tests.
The effect of the type of polymer used on the alkaline hydrolysis at different depolymerization time
periods is shown in Figure 1a - c for the experiments carried out at 120, 150 and 180 oC,
respectively. It was observed that at all experimental conditions, the amorphous material lead to
higher degradation values compared to the crystalline. Crystallinity, since it reflects to condensed
material, poses some resistance during degradation due to reduction of diffusion rates of the alkali
solution into the bulk of the polyester. In addition, at low temperatures degradation was more
pronounced in PPT due to its lower Tg compared to PET (Tg of PPT is 47oC compared to 80oC for
PET) which means an increased polymer chain mobility and permeability. This observation was
transverse at the highest temperature used (i.e. 180 oC) as PPT crystallizes much more than PET at
such temperatures during the experiments. Moreover, as expected, an increase in the reaction
temperature leads to an augmentation in the decomposition of both PPT and PET. Temperature is a
very crucial factor, since as it can be observed from this Figure the degradation of amorphous PPT
at 180oC is almost 90% in only 30 min, while at 120oC even after 60 min the polymer degradation is
less than 80%.
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100
o
120 C
PET
80 PPT-crystaline
Reacted polymer (%)
PPT-amorphous
60
40
20
0
0 20 40 60
Irradiation Time (min)
(a)
100
80
Reacted polymer (%)
60
40
o
150 C
20
PET
PPT-crystaline
PPT-amorphous
0
0 20 40 60
Irradiation Time (min)
(b)
100
80
Reacted polymer (%)
60
40
o
180 C
20
PET
PPT-crystaline
PPT-amorphous
0
0 10 20 30 40 50 60 70
Irradiation Time (min)
(c)
Figure 1. Amount of polymer reacted versus irradiation time during alkaline hydrolysis of PET,
crystalline PPT and amorphous PPT at 120 (a), 150 (b) and 180 oC (c).
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On comparing the present results on PET degradation with corresponding values obtained without
the use of microwave irradiation (Kosmidis et al., 2001), it can be postulated that at all temperatures
investigated, the time required for the degradation of PET at a certain level has been considerably
shortened when using microwave irradiation. Specifically, the time required to achieve 98 wt-%
TPA yield at 180oC has been shortened from 1 h to 0.5 h. Correspondingly at 150oC, the time
required to achieve 76 wt-% TPA yield has been shortened from 5 h to 1h and at 120oC the time
required for 33 % TPA yield from 7 h to 1.5 h. This difference is even greater if we take into
account also the pre-heating period, which in conventional depolymerization is between 20 to 40
min, while under microwave irradiation only 2 min. Moreover, after 1h hydrolytic depolymerization
at 150oC the conversion to TPA using microwave irradiation has increased from 35% to 76% and at
120oC has almost been tripled from 6% to 20%.
3.2. Product characterization
Subsequently the polymer recovered after filtration was characterized using differential scanning
calorimetry. Results comparing the polymer recovered after degradation of crystalline PPT at 120 or
150 oC at 30 min are compared to corresponding of the original material in Figure 2. As it can be
seen the original material presents a high endothermal peak at 230 oC which is the melting point of
the polymer. The solid recovered after degradation at 120 oC exhibits almost the same melting point,
meaning that besides the mass loss it is the same material that has been recovered. However, at the
degradation experiment at higher temperatures (i.e. 150 oC or 180 oC) the endothermal peak
(melting point of the polymer) is shifted to lower values (i.e.140 oC) and even a bimodal peak
appears. This means that the material recovered is no longer a polymer like the original one but
rather an oligomer or a mixture of oligomers. Almost the same phenomena have been observed
when amorphous PPT was used (shown in Figure 3). The original melting point of almost 227 oC is
shifted to 145 oC after degradation at 150 oC for 30 min.
PPT crystaline original
PPT crystaline-120oC-30min
PPT crystaline -150oC-30min
Heat flow endo Up (a.u)
0 25 50 75 100 125 150 175 200 225 250
o
Temperature ( C)
Figure 2. DSC traces of original crystalline PPT and the material remained after alkaline hydrolysis
under microwave irradiation at 120 and 150 oC for 30 min.
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8. Protection and restoration of the environment XI
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PPT amorphous original
PPT amorphous-150oC-30min
Heat flow endo Up (a.u)
0 25 50 75 100 125 150 175 200 225 250
o
Temperature ( C)
Figure 3. DSC traces of original amorphous PPT and the material remained after alkaline hydrolysis
under microwave irradiation at 150 oC for 30 min.
3.3 Characterization of the purity of TPA received
The purity of TPA received was investigated by titration as it is reported in the experimental part, as
well as by FTIR analysis in order to detect any PET oligomers. The purity of TPA based on
carboxyl content was found always to be greater than 99% and on average equal to 99.4%.
Furthermore, characteristic FTIR spectra of the product obtained from different experimental
conditions appear in Figure 4. From these spectra the following comments can be made.
 The main peak at 1689 cm-1, is due to the existence of a C=O stretching band in the carboxyl
group. If this absorption is in values greater than 1700 cm-1 then it denotes the existence of a
carbonyl group in an ester. However, (as in our case) if it is in less than 1700cm-1 it is
characteristic of carbonyl groups present in a carboxyl acid. Therefore, it seems that total
depolymerization to monomer TPA is achieved.
 The peak at 1285 cm-1 shows the existence of an C-O bond present in TPA.
 The absorption peaks at 1510 and 1575 cm-1 prove the existence of a benzene ring.
 The broad peak between 2500 and 3000 cm-1 is indicative of an –OH (hydroxyl group) in
terephthalic acid.
 The absorption peak at 783 cm-1 proves the para- position of the carboxyl groups in the
benzene ring.
 All spectra taken were similar which means that the same product is always produced.
Therefore, it was concluded that the solid produced was pure monomer terephthalic acid.
The suitability of TPA received for direct polymerization to PET was also investigated by
polymerizing it with ethylene glycol using tetrabutyl titanate as catalyst. This terephthalic acid when
esterified and polycondensed with ethylene glycol gave a pure white polymer which showed an
intrinsic viscosity, [ ] = 0.53 dL g-1.
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100
80
Transmittance (%)
60
40
20
PPT crystaline 120 C, 30 min
0 PPT crystaline 150 C, 30 min
PPT crystaline 180 C, 30 min
4000 3500 3000 2500 2000 1500 1000 500
-1
Wavenumber (cm )
Figure 4. FTIR spectra of the solid received after microwave depolymerization of crystalline PPT
with 10% NaOH at 180 oC, 30 min (a); 150oC, 30 min (b); 120oC, 30 min (c).
4. CONCLUSION
Hydrolytic depolymerization of crystalline and amorphous PPT in alkaline solution under
microwave irradiation was investigated as an effective technique for the chemical recycling of PPT
and recovery of its monomers TPA and PG. Results were compared to corresponding from PET
recycling. Microwave irradiation shortens very much the time needed to achieve a specific
degradation of polymer. Degradation is favored by increased temperature, irradiation time and use
of amorphous instead of crystalline material. High depolymerization degrees (near 90%) occurred in
30 min at 180oC. The solid material remained after degradation had characteristics similar to the
original polymer when degradation took place at low temperatures (120 C), while at higher
degradation temperatures rather a mixture of oligomers was received. Finally, the purity of the
monomer recovered was checked by three different methods.
Finally, it should be stressed that all this research has been carried out and proved valid for bench
scale experiments. Further scaled up experiments are needed if this method could be employed in
industrial scale, where tons of polymers need to be tackled.
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