A chemical procedure for reprocessing of poly(ethylene terephthalate) waste

Article abstract of DOI:10.1134/S1070427207060274

A procedure was developed for degradation of poly(ethylene terephthalate) waste with water to the starting monomers. The possibility of preparing diethyl terephthalate without a catalyst in a rocking cylindrical pressure vessel with the rocking axis passing through the geometric center was examined.

Full text of DOI:10.1134/S1070427207060274

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2007, Vol. 80, No. 6, pp. 992 994.
Pleiades Publishing, Ltd., 2007.
Original Russian Text
pp. 1018 1020.
R.Yu. Mitrofanov, P.N. Bochkarev, V.P. Sevodin, 2007, published in Zhurnal Prikladnoi Khimii, 2007, Vol. 80, No. 6,
MACROMOLECULAR CHEMISTRY
AND POLYMERIC MATERIALS
A Chemical Procedure for Reprocessing
of Poly(Ethylene Terephthalate) Waste
R. Yu. Mitrofanov, P. N. Bochkarev, and V. P. Sevodin
Biisk Technological Institute, Branch of Polzunov Altai State Technical University, Biisk, Altai krai, Russia
Received October 10, 2006; in final form, March 2007
Abstract A procedure was developed for degradation of poly(ethylene terephthalate) waste with water
to the starting monomers. The possibility of preparing diethyl terephthalate without a catalyst in a rocking
cylindrical pressure vessel with the rocking axis passing through the geometric center was examined.
DOI: 10.1134/S1070427207060274
Poly(ethylene terephthalate) (PET) patented in
1941 was initially intended for production of fibers;
however, thanks to its unique properties, it is also
widely used today as a structural material. As the
volume of PET production grows, the amount of
industrial and domestic PET waste increases also;
only in Russia it amounts to 300 400 thousand tons
annually.
Methods of types 2 and 3 have been fairly well
studied. These methods are mainly applicable to large-
tonnage process waste. The final products obtained
using methods of type 1 are TPA + EG or DMT +
EG.
Degradation of PET waste into TPA can be per-
formed in three modes: pressure hydrolysis, acid
hydrolysis in the presence of strong mineral acids,
and alkaline saponification. In all the cases, almost
theoretical yields are attained. However, acid and base
hydrolysis of PET waste exclude the possibility of EG
regeneration and require use of aggressive chemi-
cals, which causes drastic increase in the capital in-
vestments required for auxiliary reagents and effluent
treatment.
The existing mechanical procedures for utilization
of spent PET containers allow preparation of only a
secondary raw material for production of finished
items. However, the demand for items prepared in this
way is low. Furthermore, the use of secondary raw
materials in Russia and EU countries for packaging
of foodstuffs is prohibited by the legislation.
An alternative solution is chemical reprocessing of
spent PET containers and articles. The existing chem-
ical methods for PET reprocessing can be subdivided
into three types: (1) reprocessing to obtain the primary
monomers, namely, ethylene glycol (EG) and tere-
phthalic acid (TPA) or dimethyl terephthalate (DMT);
(2) glycolysis of PET with ethylene glycol to obtain
bis(hydroxyethyl) terephthalate and other oligomers,
which are then either subjected to polycondensation
to obtain PET or brought into reactions with dicar-
boxylic acids to obtain polyester polyols used in
production of polyurethane foam or elastomers; (3) de-
polymerization of PET with propylene glycol, fol-
lowed by the reaction of the decomposition product
with maleic anhydride to obtain unsaturated poly-
esters used in production of reinforced fibers.
In the 1960 1970s, a procedure for recycling of
substandard PET [1] formed in the course of produc-
tion was developed and commercially implemented.
The process involves depolymerization of the waste
in excess methanol in the presence of a catalyst at
2
a pressure of 20 25 kgf cm and 180 C for 3 5 h.
The process is intended for large-scale use; it is so-
phisticated, involves a large number of operations,
and requires regular supply of waste of the preset
quality. Therefore, this process is inapplicable to
small volumes of domestic wastes which are, as a rule,
strongly contaminated with mineral and organic im-
purities.
Furthermore, there are fragmentary published data
[2] that PET can be decomposed under appropriate
992

A CHEMICAL PROCEDURE FOR REPROCESSING OF POLY(ETHYLENE TEREPHTHALATE)
993
conditions with alcohols, esters, and amines, with
the cleavage of the ester bond. These reactions are
considerably accelerated at elevated temperatures and
higher catalyst acidities. Aminolysis of PET yields
polyamido esters. The extent of aminolysis depends
on the amount and kind of the diamine.
The degree of filling was 0.6. The rocking rate was
20 min [8].
1
The purity of the compounds obtained was moni-
tored by TLC on Silufol UV-254 plates (Czechia),
eluent benzene acetone, 3 : 1. The melting points of
the products were measured with a PTP(M) capillary
device for melting point measurements and were not
corrected.
A possible alternative solution to the problem of
reprocessing of domestic PET waste into a valuable
raw material for chemical, paint-and-varnish, or phar-
maceutical industry is setting up small- or medium-
scale shops.
Terephthalic acid. The pressure vessel was charged
with 500 g of PET and 2000 ml of distilled water.
The vessel was hermetically sealed. Agitation was
switched on; the mixture was heated to 280 285 C,
kept at this temperature for 10 min, and cooled to
30 35 C. The vessel was opened, and the TPA ob-
tained was filtered off, washed with distilled water
(4 250 ml), and dried at 100 120 C. Yield of TPA
431.42 g (99.8%); light brown crystals subliming in
an open capillary without melting at approximately
300 C.
For example, procedures have been suggested for
preparing various paint-and-varnish materials, includ-
ing powders (hydroxyl- and carboxyl-containing com-
ponents for epoxy polyester compounds), insullacs,
alkyd oligomers, materials for road marking, etc.
[3 5]. Film-forming agents are prepared by ester inter-
change of PET with various polyols [3]. The reaction
occurs in two steps. The first step is destructive alco-
holysis to give unsymmetrical oligomers; the second
step, cross-linking, starts simultaneously, yielding
a 3D polymer. Depending on the reaction time and
PET : polyol ratio, it is possible to prepare oligomers
with the prescribed softening and dropping points
and with predictable solubility in organic lacquer
solvents.
Diethyl terephthalate (DET). The pressure vessel
was charged with 240 g of terephthalic acid and
2200 ml of 96% ethanol. The vessel was hermetically
sealed, agitation was switched off, and the contents
were heated to 230 240 C, kept at this temperature
for 6 h, and cooled to 25 30 C. Then the vessel was
opened, and the mixtrue was filtered and evaporated
to 1/3 of the initial volume. The residue was refluxed
for 30 min with addition of 2 g of activated carbon
and then filtered. The filtrate was cooled in an ice
bath with stirring; in so doing, the major fraction of
the diethyl ester crystallized. An additional amount of
the product was obtained by the filtrate evaporation.
Along with the symmetrical polymer, it is possible
to prepare polymers with various combinations of
comonomers (terephthalic acid, ethylene glycol,
p-hydroxybenzoic acid). Such polymers exhibit in-
creased shrinkage and are well dyed with dispersible
dyes without applying a pressure [6]. Polymers with
prescribed properties can be prepared by varying
the comonomer ratio.
The product was stirred with 1000 ml of 5% sodi-
um hydrogen carbonate at 10 20 C for 30 min, fil-
tered off on a Buchner funnel, and washed with dis-
tilled water (3 250 ml). The alkaline filtrate was
acidified on cooling with sulfuric acid; as a result,
a mixture of the monoester and TPA was obtained,
which was delivered to the esterification step with
addition of pure TPA.
Published data on the chemical processing methods
show that the most widely used, economically feasi-
ble, and environment friendly method is hydrolysis
of PET in a neutral medium to TPA and EG. The pro-
cess is performed in excess water at 180 220 C for
5 6 h; it is fairly simple and requires no addition-
al chemicals.
The crystalline diethyl ester obtained was dried at
30 35 C for 48 h. Yield 279.36 g (87.3%), mp 43
44 C.
EXPERIMENTAL
As a method of PET waste utilization, we examined
hydrolysis without a catalyst. It is known [9] that, at
temperatures exceeding 280 C, PET undergoes virtu-
ally instantaneous hydrolysis with cleavage of the
ester bonds. As the reaction time is varied from 2 to
120 min, the TPA yield varies insignificantly (95.5
96.9%), which is consistent with published data.
To prepare crude TPA, we used spent TPE contain-
ers; their pretreatment involved removal of labels and
adhesives, followed by shredding. No sorting by color
was performed.
The process was carried out in a rocking pressure
vessel [7] (a steel cylinder 1.1 m long, 0.065 m i.d.).
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994
MITROFANOV et al.
yield was 87.3%. With an increase in the reaction
time from 6 to 12 h, the yield increased by no more
than 4%; therefore, it is not appropriate to perform
the reaction for a time exceeding 6 h.
Advantages of the method are the high reaction
yield and the absence of a catalyst. The mixture ob-
tained after the reaction completion is a homogeneous
transparent solution which can be readily purified by
filtration to remove mechanical impurities and label
paper; the costs for pretreatment of PET waste can
thus be reduced.
Fig. 1. TPA yield Q vs. the PET : water ratio N.
CONCLUSIONS
(1) A convenient procedure was suggested for util-
ization of heavily contaminated domestic poly(ethyl-
ene terephthalate) waste, allowing its reprocessing on
small and medium scales using nonstandard equipment.
(2) The process involves formation of an insignif-
icant amount of waste.
Fig. 2. Yield of esterification products, Q, vs. reaction time
: (1) TPA, (2) ethyl hydrogen terephthalate, and (3) DET.
REFERENCES
1. US Patent 3403115.
2. Pilunov, A.G., Mikhitarova, Z.A., and Tseit-
The influence of the PET : water ratio on the TPA
yield is shown in Fig. 1.
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3. Kudyukov, Yu.P., Maslosh, V.Z., Savitskaya, A.V.,
and Litvinova, E.V., Lakokras. Mater. Ikh Primen.,
1993, no. 3, pp. 3 4.
The acid obtained is of low purity and cannot be
returned to the polymer production. Purification by
reprecipitation is labor-consuming and involves use
of alkalis and strong mineral acids. According to
published data [10, 11], TPA is a valuable raw mate-
rial for preparation of compounds used in fine organic
synthesis. One of such compounds is diethyl tere-
phthalate (DET).
4. Klosowska-Wolkewicz, Z., Wlock. Chem., 1982,
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Fibers), Moscow: Khimiya, 1976.
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in Rocking Autoclaves, Cand. Sci. Dissertation, Biisk,
1998.
9. Entsiklopediya polimerov (Polymer Encyclopedia),
Kabanov, V.A., Ed., Moscow: Sov. Entsiklopediya,
1977, vol. 3.
To prepare DET, we esterified crude TPA with
ethanol in the pressure vessel at 230 240 C. The de-
pendence of the yield of the esterification products
on the reaction time is shown in Fig. 2.
10. US Patent 4399303.
11. US Patent 3931210.
The esterification results demonstrated that at
a TPA : C2H5OH ratio of 1 : 9 the maximal DET
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