TECHNOLOGICAL ASPECTS OF TEREPHTHALIC ACID FORMATION FROM PET WASTE BOTTLES IN

SUPERCRITICAL WATER

G.P. Panasyuk*, Mishal Khaddaj , V.I. Privalov, I.L. Voroshilov, G.P. Boudova

N. S. Kurnakov Institute of General and Inorganic Chemistry Russian Academy of Sciences (IONH RAN)

Leninsky Prospect 31, Moscow, GSP-1, 117907, Russia Panasyuk@igic.ras.ru Fax +7 (095) 954 1279

The utilization of the used polyethyleneterephthalate (PET) bottles is considered to be

an actual problem that faces the majority of countries. The quantities of these wastes are quite large (tens of millions of tons/year), and the tendency here is permanently towards the increasing of these amounts in accordance with the growth of PET bottles’ production all over the world. The recycling of such products can be divided into two directions [1,2]: 1) Manufacturing of secondary grits for consequent use in obtaining significant products

(mostly building materials). 2) Decomposition of PET to:

- initial components - terephthalic acid (TA) and ethylene glycol (EG), being marketable products,

- other substances with consequent deriving of useful materials.

It is commonly accepted that the efficiency of application of different methods utilization of PET wastes is faced by difficulties, related to the specific behavior of the polymer itself. So the presence of even small amounts of secondary substances (e.g. PVC, PE) may cause a different running of the decomposition leading, generally, either to unpredictable results or to the formation of undesirable compounds. That is why despite of fact that a multiple of technological schemes of utilization of PET have been suggested up till now, an ideal method for this sake has not been reached yet.

The present work is devoted to the investigation of decomposition of PET waste bottles of different modifications in the atmosphere of water vapor at 180-200 0C and neutral pH.

The choice of the object of investigation is related to the wide expansion of the initial material (PET bottles), the temperature range have been choose in accordance with the possibility of further using of big industrial autoclaves. MATERIALS AND METHODS OF INVESTIGATION

For the experimental part various PET bottles (colorless, colored, clean or with various by-products - screw-tops, labels) were used. Cut strips of PET bottles were treated in laboratory autoclaves of volumes 15 and 2000 ml at constant temperature in the range 180-200 0C. Strips were placed in containers where an amount of water was added. This amount

should be sufficient to maintain during the further treatment a saturated atmosphere of water vapor close to supercritical state. Hermetically closed autoclaves were put in a furnace at constant temperature during the necessary time, after which the autoclaves were removed from the furnace, cooled, opened and the resultant sample was token out of the container. The product was investigated:

a- directly, in its initial stage after autoclave treatment; b- after a treatment by a solution of ammonium hydroxide till the dissolving of the main

components of the product with a further filtration of the solution and addition of mineral acid to the filtrate till we reach an acidic pH. In this case white crystals are precipitated, then filtrated, washed and dried.

X-ray analysis was carried on diffractometer DRON-3M (Cu, Ka). Thermogravimetric

analysis (TG) and differential thermal analysis (DTA)were carried on derivatograph MOM, with a heating rate of 6 deg./min. NMR 13C spectra were obtained using Bruker AC-200. All chemical shifts in NMR 13C spectra are introduced with respect to the signal of tetramethylsilane (external standard). Samples were dissolved in dimethylsulfoxide (DMSO). The standard DMSO signal is considered to be at 39.5 ppm. RESULTS AND DISCUSSION

The results of NMR 13C of various samples after autoclave treatment of PET at 200 0C for 36 hours are illustrated in Fig. 1-3. From the NMR 13C we can distinguish 3 triplets in the range 58-66 ppm (Fig. 1a,2), the middle one of which corresponds to –CH2 group in EG [3]. This fact was confirmed by adding pure EG to the sample NMR 13C of which has been determined. In this case we notice a significant increase of the intensity of the middle triplet with respect to the surrounding ones (Fig. 1c). After dissolving these samples in NH4OH and adding mineral acid till the complete precipitation, the NMR 13C of the precipitant shows no signals of –CH2 groups belonging to EG. These signals (at 62 ppm) disappear completely while the other two neighboring –CH2 group signals remain present (Fig. 3).

The other part of the spectra consists of a doublet at 128-129 ppm (-CH in the benzyl ring) and singlet at 132-143 belonging to benzyl carbon atom related to the carboxyl group and an another singlet of the –COOH group itself (Fig. 2). This part of the spectrum is similar to that of TA. Nevertheless we notice that the character of the

Fig.1 NMR 13C of –CH2 signals in PET autoclaving treatment product (180 0C, 6.5 hrs.):

a) without decoupling b) with decoupling c) with decoupling + EG

(c)

(b)

(a)

70 60 ppm 50

0

10

20

30

40

50

60

70

5075100125150175 PPM

%

c

b

a

signal belonging to the carbon of the benzyl ring related with the carboxyl group is a quite complex one, and in the neighborhood of the intensive traditional –COOH signal (166 ppm) we see another, less intensive signal at 164.5 ppm. (Fig. 2). From these data we can conclude that in all cases we still have, probably, certain amounts of ester groups due to incomplete depolymerization of PET. This conclusion correlates with the

literature data [3,4].

The comparison of the signals of NMR 13C –{H} data for various samples prepared from different variations of PET bottles is illustrated in Fig 3. These data show a similar character of the NMR 13C spectra independent of

the variation of the initial properties of PET bottles. NMR 13C spectra for the precipitants obtained after dissolving products of autoclaving treatment of PET in NH4OH with consequent addition of mineral acid are the same as for all the signals as for the autoclaving treatment product samples, except the circumstances of disappearing of the signal belonging

165 130 ppm 60 Fig.2 NMR 13C of PET autoclaving treatment product (200 0C, 36 hrs.). Initial contents : various PET bottles (colorless, colored) , screw-tops, labels

Fig.3 Comparison of signals of NMR 13C –{1H}) of PET autoclaving treatment product (200 0C, 36 hrs.). Initial characteristics of material:

a- various PET bottles (colorless, colored) , screw-tops, labels;

b- colorless PET bottles;

c- colorless PET bottles, autoclave treatment product dissolved in NH4OH with consequent addition of mineral acid.

to –CH2 groups of EG at 62 ppm, as we mentioned above. These data show that after the mentioned above NH4OH/mineral acid treatment, of the

autoclave resultant product we predominantly obtain terephthalic acid, independent of the properties of the initial PET bottle, no nevertheless small amounts of byproducts still exist.

The destruction of initial polymer results in infringement of its thermal stability. After autoclave treatment of various PET samples the results DTA and TG analysis of the products are illustrated in Fig. 4. As we see, DTA and TG curves for all the investigated samples are similar. In all DTA curves we have only endothermal effects in the range 120-410 0C. The most intensive effect is noticed at 390-410 0C, and it is related to the sublimation of terephthalic acid. The mass loss (? m) here is more than 80%. The low temperature endothermic effects are, probably, related to the desorption of water and ethylene glycol from the particles obtained upon the depolymerization of PET. The similarity of the curves indicates the weak dependence of the resultant product of polymerization on the character of the initial material.

The X-ray patterns of our samples are shown in Fig.5. From these results we can conclude that terephthalic acid is present in all the patterns. Characteristic bands of TA in the range of 2? equal to 17.4, 25.2, 28.0, 29.8, 39.7 and 41.2 degrees are present, although we can find

Fig.4 DTA and TG (?m, mass loss in %) curves of autoclaving treatment product (200 0C, 36 hrs.). initial contents:

a- colorless PET bottles;

b- colorless PET bottles, autoclave treatment product dissolved in NH4OH with consequent addition of mineral acid;

c- various PET bottles (colorless, colored) , screw-tops, labels;

d- colorless and colored PET bottles.

(c) (d)

(a) (b)

some deviations, specially after dissolving these samples in NH4OH and adding mineral acid till the complete precipitation. This might be due to the fact that in this case we obtain very small crystals of a size less than 1 µ (Fig. 6c).

Electron microscopic photos illustrated in Fig. 6 show that the presence of wide range of crystals of the product of autoclave treatment of PET bottles independent of their variation (from 1 to 30 µ ). The crystals of TA have usually a less size (up to some microns) and a more narrow size distribution (Fig. 6b).

Fig. 5 X-ray diffraction patterns of autoclaving treatment product (200 0C, 36 hrs.) of:

a- colored bottles b- colorless bottles c- colorless PET

bottles, autoclave treatment product dissolved in NH4OH with consequent addition of mineral acid

(a)

(b)

(c)

60 50 40 30 20 2? /degrees (Cu, Ka )

REFERENCES [1]. S. Aslan, B. Immirzi, P. Laurienzo, M. Malinconico, E. Martuschelli, M. G. Volpe, M. Pelino, L. Savini, J. Materials Sci., 32, 1997, p. 2329. [2]. G.P. Panasyuk, Mishal Khaddaj, I.L. Voroshilov, G.P. Boudova, I.V. Miroshnichenko, 7-th Meeting on Supercritical Fluids, Proceeding, 2000, p. 505 [3]. G.P. Panasyuk, Mishal Khaddaj, V.I. Privalov, I.V. Miroshnichenko, Plasticheskie Massy, 2, 2002, p. 27 [4]. E. Breitmaief, W. Voeltef , 13C NMF Spectroscopy, Verlag Chemie, Weinheim, NY, 1978, 344 p.

(a) (b) (c) Fig.6 Electron microscopic photos of: curves of autoclaving treatment product (200 0C, 36 hrs.). initial contents :

a- Autoclave treatment product of PET bottles (200 0C, 36 hrs.) b- TA c- Autoclave treatment product dissolved in NH4OH with consequent addition

of mineral acid