Studies on Thermal Degradation of AromaticPolyesters by Pyrolysis-Gas Chromatography
Yoshihiro Sugimura and Shin Tsuge※, Department of Synthetic Chemistry, Faculty of Engineering,
Nagoya University, Nagoya 464, Japan
Abstract
Thermal degradation of aromatic polyesters(m=2,3,4,5, and 6) was studied by pyrolysis/gas Chromatography using a Curie-point pyrolyzer directly attached to a gas Chromatograph with a thermallystable polar separation column (Thernon-1000) to cover awide range of the degradation products. The characteristicpeaks appearing on the pyrograms were identified using amass spectrometer with a chemical ionization source which
was also directly attached to the gas Chromatograph. On thebasis of the observed pyrograms, the common degradation
mechanisms of the aromatic polyesters were discussed.

Introduction
Among the many aromatic polyesters, polyethyleneterephthalate (PET) and polybutyleneterephthalate (PBT) aremost widely used in the fields of fibers and thermoplastics.
The thermal behaviors of these polymers are usually investigated by thermogravimetry (TG), differential thermal analysis(DTA), and differential scanning calorimetry (DSC). Thesethermal methods provide very useful information about thebulk thermal characteristics of materials, such as the glasstransition temperature, the heat of crystalization, the melting
point, the weight loss during the thermal degradation, etc.
These data, however, are usually very difficult to interpret interms of the molecular structure of the polymers. On the other
hand, pyrolysis/gas chromatography (PGC) and pyrolysis/
mass spectrometry (PMS) supply very useful information
about the structures of the pyrolysis proddcts and the degradation mechanisms.
Recently, Luderwald, et al. (I-3) have reported extensive
studies on various polyesters by PMS to elucidate degradation
mechanisms. PGC, however, has been applied mostly for theidentification of polyesters since they mainly yield relatively
small fragments such as carbon monoxide, carbon dioxide,
olefins, and some benzene derivatives at elevated temperatures
between 500 and 1,000℃, which are necessary for the mash
pyrolysis of the thermally stable polyesters (4,5).
In this work, a series of aromatic polyesters were pyrolyzed
between 485 and 765℃ in a Curie-point pyrolyzer which wasdirectly attached to a gas chromatograph with a thermallystable polar separation column. The characteristic peaks
appearing on the pyrograms, which cover a fairly wide rangeof the degradation products were identified using a mass spec-trometer with a chemical ionization source which was also
directly attached to the gas chromatograph. On the basis of
the observed pyrograms, the degradation mechanisms of thearomatic polyesters were discussed.
Experimental

Materials. Five kinds of aromatic polyesters such as PET,

polypropyleneterephthalate (PPrt), PBT, polypenteneterephthalate (PPeT), and polyhexeneterephthalate (PHT) weresynthesized by an ordinary method using titanium-n-butoxide
as a catalyst, respectively, from dimethylterephthalate andvarious diols, HO(CH2)nOH(n = 2, 3, 4, 5, and 6).
Conditions for PGC. A Curie-point pyrolyzer (Japan Analytical Industry, Model JHP-2, Tokyo, Japan) was directly attached to a gas chromatograph with flame ionization detector(FID) (Shimadzu, Model GC-4CM, Kyoto, Japan). About 0.1
mg of the polymer sample was pyrolyzed for 5 see under a flowof carrier gas (35 ml/min of N2) using a small piece of ferromagnetic foil, of which the Curie-point was either 485, 590,690, or 765℃. A thermally stable and highly polar separation
column, 2 m long copper tubing (3 mmi.d.)packed with 5% ofThermon-1000(shimadzu,usable up to 270℃) on 80/100
mesh Chromosorb W (acid washed and DMCS treated) wasused. The column temperature was programmed from 50 to250oC at a rate of 6oC/min to separate the fragments with awide range of boiling points. The peak identification of thepyrograms was carried out using a quadrupole mass spectrometer with a chemical ionization source (JEOLCO, Model JMSQ10A, Tokyo, Japan), which was also directly attached to thegas chromatograph through a glass jet separator.
Results and Discussion
Previous pyrograms of polyesters have primarily dealt will
only relatively small fragments (4,5). In this work, however
relatively large fragments were separated using a thermally.
stable polyester column, Thermon- 1000, whose characteristics
are basically the same as those of PEG-20M. Figures la and b
show the typical pyrograms of PET and PBT at 590℃. On
these pyrograms clearly separated peaks of fairly large
fragments can be seen. Although the first cluster of peaks withshorter retention time than that of benzene were notsufficiently separated under the column conditions utilized,it
was expected that they were composed of olefins, some aldehydes and alcohols. Of course, considerable amounts of carbon dioxide and carbon monoxide should also have eluted'Author to whom correspondence should be addressed.
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JOURNAL OF CHROMATOGRAPHIC SCIENCE.VOL. 17'MAY 1979 269

'here. However. these components were not detected by FID.
The identified main peaks above benzene (peak 1) on the pyrogram of PET were vinylester of benzoic acid (peak 2), biphenyl (peak 3), divinylester of terephthalic acid (peak 4), and dibenzoate of ethyleneglycol (peak 5). On the pyrogram of PBT,
the peaks of the homologous esters can be seen.
Figure 2 illustrates comparative bar-pyrograms of the fivepolyesters at 590℃. where minor peaks whose intensity wasless than 1% among the total were neglected. It is worth
noting that the homologous series of the polyesters shows theanalogous pyrograms corresponding to the chemical structures of the polymer chain. From these data it was possible todeduce the following common degradation mechanisms forthese homologous polyesters.
Among the thermally weak linkages, i.e.. the C-O bonds
along the polymer chain (6), those located at β-position fromC = O bonds were most likely subject to the following thermalcleavage (1 ,3,7):

However, at elevated temperatures used for mash pyrolysis,
the acid-form terminals were rapidly reduced into the more'stable phenyl terminals by elimination of carbon dioxide.
Therefore, the final degradation products appearing on thepyrograms were supposedly formed by taking either pathway
shown in Figure 3. Although the corresponding esters of benzoic acid (c) and terephthalic acid (e) can be seen on every pyrogram, the peak of dibenzoate (f) was observed only on thepyrogram of PET under the column conditions utilized.
Among the degradation products, biphenyl (peak d) was most
likely formed through the bimolecular reaction of the formed
terephthalic acid. This mechanism was supported by the factthat terephthalic acid yields biphenyl as well as benzene atelevated temperatures above 500℃. It is interesting to noticethat the formation of both benzene and biphenyl decreased as
the number of methylene groups in the polymer chain rose.
Figure 4 shows a series of the bar-pyrograms of PET atvarious pyrolysis temperatures, 485, 590. 690, and 764℃. Themain degradation products were basically the same regardlessof the pyrolysis temperatures utilized. However, at the pyrolysis temperature of 485℃, the formation of biphenyl became
negligibly small, while the relative yield of benzene increased
with the rise of the pyrolysis temperature. Basically, the sametendency was observed for the other polyesters.
Since the peaks (c), (e), and (f) were quite characteristic of
the chemical structures of the original polymers, the identification of the polyesters and the quantitative estimation of thecomponents in a given mixture system of the polyesters caneasily be don} using the pyrograms observed under the same
pyrolysis conditions.

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