THE SCIENCE AND ENGINEERING REVIEW OF DOSHISHA UNIVERSITY, VOL. 52, NO.

2 July 2011

Alkaline Hydrolysis of Polyethylene Terephthalate

at Lower Reaction Temperature

Masakazu YAMASHITA* and Hideyuki MUKAI*

(Received April 5, 2011)

Alkaline hydrolysis of polyethylene terephthalate (PET) resin in water-ethanol mixed solvent was examined and found to
produce good results for the product, terephthalic acid, after protonation. The reaction proceeded smoothly at lower temperatures
than previous methods. The highest yield (89%) was obtained when PET was heated at 110qC for 0.5 hr in 10% NaOH solution of
water-ethanol mixed solvent (ratio 20:80 by weight). However, even this reaction condition is still unsatisfactory for the saving of
petroleum resources by recycling and is short of affirming the significance of recycling itself.

-G[YQTFU Hydrolysis, Alkali, Polyethylene terephthalate, PET bottle, Chemical recycle

 +PVTQFWEVKQP the ester group can be cleaved by various reagents,

including water, alcohols and so on. The cleavage of an
Polyethylene terephthalate (PET) is a plastic ester linkage by water is called hydrolysis and there is
widely used to manufacture bottles for soft drinks, fibers, growing interest in hydrolysis because it is the only
and films. PET production is growing rapidly, and the method which produces the monomers from which PET
1)
world's annual consumption amounts to 13 million tons. is produced.
On the other hand, the widespread application and Many types of hydrolysis of PET have been
non-biodegradability of PET create a huge amount of reported, but these reactions need a high reaction
waste to dispose of, causing a serious problem. temperature, implying much usage of energy; for
Therefore, from the awareness and concern for example, a supercritical methanol method2,3) and a
environmental pollution, recycling and reprocessing of supercritical water method4) require high reaction
waste PET have developed. Thus, the effective use of temperatures over 700qC. Glycolysis by ethylene
PET waste is an important problem for glycol also gave good results but required high reaction
environment-protection researchers. temperatures over 200qC.5,6) As for alkaline hydrolysis,
Various recycling methods for PET waste have many reports have been published, but heating to about
been proposed. Among them, chemical recycling has 200qC was still necessary in the reaction of aqueous
attracted attention, by which PET bottles are chemically ammonia solution.7)
degraded and purified into raw materials to In order to develop a more convenient hydrolytic
remanufacture PET resin. method for PET at lower reaction temperatures, we
PET is a polyester with functional ester groups and

*Department of Environmental Systems Science, Faculty of Science and Engineering, Doshisha University, Kyo-tanabe, Kyoto
610-0394. Telephone and Fax: +81-774-65-6578, E-mail: myamashi@mail.doshisha.ac.jp

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144 Masakazu YAMASHITA and Hideyuki MUKAI

examined the reaction in several solvents and found that ethanol and water (20:80) in a 300 ml flask, 2g PET resin
ethanol is a good co-solvent with water. Here, we was added and the flask was heated to 110qC for 0.5 hr.
report a simple terephthalic acid (TPA) recovery reaction Unreacted residue was filtered and the filtrate was
using ethanol as a solvent. (Scheme 1) acidified by 20% hydrochloric acid to give a solution
 with white solids. White solids separated by filtration
'ZRGTKOGPVCN were washed with water, dried, and identified by IR and
2.1. General melting point measurements as terephthalic acid (TPA).
The solvents and reagents were all commercial The yield of TPA was 89%.
products and used without further purification.
Commercially available PET bottles were used as PET 4GUWNVUCPF&KUEWUUKQP
resin after cutting into 3-4 mm square pieces, washed
with water and dried. Infrared(IR) spectra were PET resin was decomposed to disodium
measured on a Horiba FT-700 spectrometer. terephthalate (1) (Scheme 2) by the reaction with NaOH,
2.2. Hydrolysis of PET resin in NaOH solution which was then protonated to terephthalic acid (TPA) (2)
The typical reaction procedure was as follows. (Scheme 3) by hydrochloric acid. As the protonation of
To a 10% NaOH solution in 100 ml mixed solvent of

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 Alkaline Hydrolysis of Polyethylene Terephthalate at Lower Reaction Temperature 145

disodium terephthalate (1) to TPA (2) is easily achieved To determine the water/ethanol ratio appropriate
by acidification of the solution, we investigated the for hydrolysis in this study, we conducted an experiment
favorable reaction conditions for the formation of the under the following conditions (heating temperature:
disodium salt (1). 110qC, reaction time: 0.5 hr, and 10% NaOH solution).
3.1. Alkaline hydrolysis of PET resin using ethanol as a The results are shown in Table 2.
solvent. Table 2 demonstrates that the water/ethanol ratio
First, several reactions were conducted using by weight is a critical parameter for this hydrolysis. The
ethanol to elucidate its efficiency as a solvent. TPA yield increased markedly from 4.9 to 89.0% with
3.1a. Effect of heating temperature changes in the ratio of water/ethanol from 100:0 to 20:80,
To examine the most effective heating temperature while the insoluble matter decreased markedly from 94.3
appropriate for the reaction in Scheme 2, we conducted to 0%. The TPA yield was highest (89.0%) at a
several reactions using water/ethanol solvent (ratio water/ethanol ratio of 20:80. Thus, in these reactions, it
15:85) and 10% NaOH for 0.5 hr. The results are was found that 20:80 was the optimal water/ethanol ratio
shown in Table 1. by weight.
Table 1 demonstrates that the amount of insoluble 3.1c. Examination of reaction time
matter that remained unreacted decreases as the To determine the appropriate reaction time for this
temperature rises. All PET pieces were decomposed hydrolysis we conducted an experiment under the
when heated at 110qC, leaving no residual matter, and following conditions: water/ethanol ratio: 20:80, heating
the yield of TPA (2) reached 87%. temperature: 110qC, and 10% NaOH solution. The
3.1b. Examination of water/ethanol ratio results are shown in Table 3.

Table 1. Effect of Temperature.

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146 Masakazu YAMASHITA and Hideyuki MUKAI

Table 2. Effect of Water/Ethanol ratio.

Table 3. Effect of Reaction Time.

Table 3 demonstrates that the degradation 3.1d. Concentration of NaOH in a solution

reaction of the resin proceeds in 0.5 hr. It seems that To determine the NaOH solution concentration
degradation reaches its highest state at 0.5 hr and a appropriate for this hydrolysis, we conducted an
prolonged reaction causes undesirable side reactions experiment under the following reaction conditions:
which reduce the yield of TPA. Actually, the yield water/ethanol ratio: 20:80, heating temperature: 110qC,
gradually decreased. Thus, this study demonstrated and reaction time: 0.5 hr. The results are shown in
that the appropriate reaction time was 0.5 hr. Table 4.

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 Alkaline Hydrolysis of Polyethylene Terephthalate at Lower Reaction Temperature 147

Table 4. Effect of NaOH Concentration.

Table 4 demonstrates that the yield was greatly ethanol, it is considered to be more poisonous in the
influenced by the NaOH concentration in the solution. case of ingestion, inhalation or percutaneous
From these results, the NaOH concentration was absorption. Ethanol is therefore recommended for
found to be a critical parameter in this hydrolysis. this hydrolysis from health and environmental aspects.
Entry 17-20 demonstrates that reactions with low 3.3. Hydrolysis of PET using m-cresol
concentration NaOH afforded undissolved PET As a representative of an aromatic hydroxyl
residues, however, with over 10% NaOH compound, m-cresol was investigated as a solvent in this
concentration, all PET pieces degraded (Entry 21 and hydrolysis reaction. In this reaction, the optimum yield
22). As the TPA yields were correlated with that of of 93.6% TPA was obtained when PET was heated in
the remains of insoluble matter, 10% concentration of 100ml water/m-cresol (20:80) with 8% NaOH at 100qC
NaOH (Entry 21) was found to be sufficient and for 1 hr; however, judging from its high boiling point and
optimal. human toxicity, m-cresol is not recommended as the
3.2. Hydrolysis of PET using methanol solvent for this purpose.
Methanol is usually a better solvent than ethanol
because it dissolves many inorganic salts; therefore, 4. Conclusion
we attempted this reaction using methanol as the
solvent instead of ethanol. As shown in Table 5, an Increased PET bottle waste is becoming a social
almost comparable yield of the product (TPA) was problem and several recycling methods have been found
obtained. Although methanol is less expensive than

Table 5. Hydrolysis in Water/Methanol solution.

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148 Masakazu YAMASHITA and Hideyuki MUKAI

that NaOH hydrolysis of PET in water-ethanol dimethyl terephthalate and ethylene glycol”, Polymer

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terephthalate)”, Polymer Degradation and Stability 87,

1). The Council for PET Bottle Recycling Home Page, 117-120 (2005).

http://www.petbottle-rec.gr.jp/top.html 6) Y. Yang, Y. Lu, H. Xiang, Y. Xu, Y. Li, “Study on

2) S.H. Mansour, N.E. Ikladious, “Depolymerization of methanolytic depolymerization of PET with supercritical

poly(ethylene terephthalate) wasting using 1,4-butanediol and methanol for chemical recycling”, Polymer Degradation and

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