Thermal and Catalytic Degradation of Cmmingled Plastics, 1996

8/12/2019 Thermal and Catalytic Degradation of Cmmingled Plastics, 1996

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FUEL

ELSEVIER Fuel ProcessingTechnology49 ( 1996)65-73PROCESSINGTECHNOLOGY

Therm al and catalytic degradation of commingledplastics

Manjula M. Ibrahim Eric Hopkins Mohindar S. Seehra *Physics Department West Virginia Uniuersity Morgantown WV 26506-6315 USA

Received 2 1 September 1995;accepted28 February 1996

Abstract

Tberm ogravime try, in situ electron spin resonance (ESR ) spectrosco py and in situ and ex situX-ray diffraction (XRD) are employed to investigate the thermal and catalytic degradation of asample of commingled plastics (CP). XRD studies show CP to contain about 90% polyethylene(PE) and 10% polypropylene (PPE) and a smaller amount of TiO,. Analysis of the weight lossdata in argon to 550C yields an activation energy E = 38 kcal mol- I for the thermal decomposi-tion of CP. In XRD studies, a melting point of 1 35C is inferred w hereas the onset of irreversibledegradation begins only around 360C. The in situ ESR experiments of CP and CP loaded with10% Al,O, (nanoscale) and 10% sulfur, both under 500 psig of H,, show that for CP alone anESR signal indicative of degradation is first seen near 380C whereas for the loaded CP thisdegradation temperature is reduced to 280C. This enhanced catalytic degradation detected byESR is believed to be due to elemental sulfur.Keyw ords: Activation energy; Electron spin resonance; Plastics degradation ; X-ray diffraction

1 Introduction

The properties and thermal degradation of model polymers such as polymethylene(PM), polyethylene (PE) and polypropylene (PPE) are reasonably well known [1,2]. Thedegradation/depolymerization process is considered to be a free radical chain reactioninvolving initiation, propagation, transfer and termination with activation energy E = 45to 60 kcal mol- and significant degradation occurring only above 380C [ 1,2]. In this

* Corresponding author.0378-3820/%/ 15.00 Copyright 6 1996Elsevier Science B.V. All rights reserv ed.PII SO378-3820(96)01023-5


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66 M .M. Ibrahi m et al./Fuel Processing Technology 49 (1996) 65-73

paper, we report the results of our recent investigations of the composition and thermaland catalytic degradation of a sample of commingled plastics (CP) employing thetechniques of X-ray diffraction (XRD ), thermogravimetry and in situ high-temperature/high-pressure electron spin resonance (ESR) spectroscopy. This project is apart of a larger program of coprocessing waste plastics and coals into useful liquefactionproducts.

2. Experimental proceduresThe sample of CP used in these investigations was supplied by Dr. Larry Anderson of

the University of Utah who obtained it from the American Plastics Council. Theelemental analysis of the CP provided by Dr. Anderson is as follows: C, 83.3%; H,13.4%; N, 0.08%; S, 0.024%; ash, 0.98%; oxygen (by difference), 2.21%. The X RDinvestigations were carried qut with a Rigaku /DM ax diffractometer under both in situand ex situ heating conditions. Thermogravimetric investigations were done with aMettler 30 00 system. For in situ ESR investigations at 9 GHz, w e employed agold-plated TE ,a2 microwave cavity, internal modulation at 100 KHz, sapph ire sampletubes for high-pressure capabilities and an indirect sample heating system details ofwhich have been given elsewhere [3,4].

3. Results and discussionThe room temperature X-ray diffractogram of the CP sample is shown in Fig. 1,where we have identified the peaks due to PE and PPE. A crude estimate of the relative

concentrations of PE and PPE in the sample can be made from the relative areas underthe peaks, this estimate being 90 % PE and 10% PPE in the CP samp le. A more accuratedetermination should be possible by developing calibration curves using calibratedmixtures of PE and PPE. This w ork is now in progress. However, for the sake of theseinvestigations, it is clear that the CP sample contains primarily high-density PE.

The effects of heating the sample in air ex situ at different temperatures for 30 min,followed by furnace cooling to room temperature and X-ray diffractometry, are alsoshown in Fig. 1. For heating at 100, 200 and 300C followed by cooling to roomtemperature, PE in the sample appears to recrystallize since the prominent PE peaks arestill visible. For heating at 360C the intensity of the PE peaks diminishes considerablyand peaks due to TiO, become distinct. For heating at 42O C, the PE peaks disappea rcompletely suggesting irreversible degradation of PE.In Fig. 2, we show the results of a second experiment in which the CP sam ple washeated in situ during XRD measurements in nitrogen atmosphere using a specialhigh-temperature furnace. The weaker PPE peaks are no longer visible partly because ofthe attenuation caused by the X-ray window of the furnace, and the broad peak near28 = 10 is due to the Duco cement used for gluing the samp le. The significant resulthere is that the PE peaks disappear between 130 and 13 5C which coincides with the


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M .M. Ibr ahim et al./ Fuel Processing Technology 49 (1996) 65-73 6-l

Ti02 lines

unheated

100-c

360C

i420C

5 10 15 20 25 30 35

2-theta degrees)Fig. 1. Room temperature X-ray diffmctograms of the commingled plastics heated ex situ in air to thetemperatures indicated. PE and PPE respectively represent polyethylene and polypropylene and the numbers inparentheses show the Miller indices of the lines.

melting temperature of PE [l]. Above this temperature, PE loses its long-range crys-talline order resulting in disappearance of the Bragg peaks. The broad peak near28 5: 18 at higher temperatures is due to amorphous PE [5]. Com bining this result withthe results of Fig. 1, we conclude that irreversible degradation of CP begins only aro und360C whereas 135C is its melting tem perature. Our conclusion that thermal degrada-tion of CP begins near 360C is quite consistent with earlier experiments on pure PE [2].

Using the data of Fig. 2 and the procedure outlined by Aggarwal and Tilley [5]involving deconvolution and measuring the areas under the peaks after correcting theintensity for atomic scattering and diffraction angle, we show in Fig. 3 the relativeintensity of the amorphous and crystalline components of CP as a function of tempera-ture. At room temperature, the crystallinity of CP is about 50% . The crystallinecomponent disappears near 135 C, resulting in a net increase of the amorphouscomponent at this transition. With further increase in temperature, there is a systematicloss of the amorphous component, so that by 450C even the amorphous componentdisappears, most likely due to loss of material.Further confirmation of the above conclusions comes from our thermogravimetricexperiments on CP carried out at heating rates of 5, 10 and 20C min- in argon flow


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M .M. Ibrahi m et al./Fuel Processing Technology 49 (1996) 65-73

PE ua 1

PE (200)

5 10 15 20 25 30 35

24 C

25O C

a-theta degrees)Fig. 2. In situ X-my diffractograms of the heated commingled plastics to the temperatures shown. The broadcomponent near 20 = 1 8 is due to the amorphous component of polyethylene.

(Fig. 4). The corresponding rate of mass loss curves a gainst temperature are shown inFig. 5. Since the mass loss is indicative of the scission of the chemical bonds leading tovolatilization, it is clear from Fig. 4 and Fig. 5 that even a t the lowest heating rate, thisprocess becomes noticeable only above 300C. This is consistent with the results fromthe XRD studies described above. These results can be made more quantitative by thecalculation of the associated activation energy E using the temperature T, = 480, 505and 5 15C where the rate of mas s loss ( - dm/dt) is maxim um (Fig. 5) for the heatingrate p = 5, 10 and 20C min- respectively. A plot of ln(P/T,) against l/T, shouldyield a straight line with the slope = -E/R [6], where R is the gas constant. This plot,shown in Fig. 6, yields E = 37 kcal mol - for the thermal decomposition of the CP


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M .M. Ibr ahim et al. Fuel Processing Technology 49 (1996) 65-73 69

140001 I

t Comingled Plastics i12000 : 0 0 Amorphous )>b , 0 Total

o -- -a- QQ n CrystallineI10000 1\

8000 -

6000

4000 -

2000

0 ~a*_r?_~______n______n--____a______*-______n ..I1 I, 1 I, I I I, I, I, I1 I,, I,, I,, I,,

10090

50 8b40 as30 g

20 5oc1

50 100 150 200 250 300 350 400 450 500Temoerature (Cl

Fig. 3. Temperature dependence of the crystalline, amorphous and total (amorphous plus crystalline)components of CP as determined from diffractograms of Fig. 2.

o ,, , , , , , , , ,x>300 350 400 450 500 5

T (C)i 0

Fig. 4. Remaining weight versus temperature for the commingled plastics in argon flow for the three heatingrates of 5, 10 and 20C min- . The horizontal lines represent constant decom position fraction cx = 0.5 andQ = 0.7.


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70 M. M. brahi m et al. Fuel Processing Technology 49 (I 996) 65-73

T (C)Fig. 5. The rate of weight change (dm/dr) versus temperature data for the commingled plastics in argon forthree heating rates. T,, is the temperature of maximum weight loss = -dm/dt.

sample. The data in Fig. 4 can also be used to determine by the constant (Y fractionof decomposition) method of Flynn a nd W all [7] where

1 din/3-- =--R b d(l/T) (1)

where b is a constant = 1. By retaining higher-order terms in the calculations, terms

-14.0E _ 37 kcal / molc

- 16. 0 L1. 25 1. 30 1.35l / Tm10- 3/ K)

Fig. 6. Plot of In@/T i) versus l / T for the heating rates p = 5, 10 and 20C min- . The solid line is theleast squares fit, the slope yielding E = 37 kcal mol- .


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M .M. Ibr ahim et al. Fuel Pr ocessing Technology 49 (I 996) 65-73 71

- 1. 0

E - 42 kcal/mol

Q2 - 2. 0 -sF-

- 2. 5 -

J1. 25 1. 30 1. 35 1. 40

1/ T 10- 3/)Fig. 7. Plot of In p versus l/T for a = 0.5 and 0.7, the solid lines being the least-squares tit through thepoints. See text for activation energies.

which are weakly dependent on temperature, we have derived the following expressionfor b

2T T2--b=l+ ~,R) (2)

The plots of In B versus l/T for (Y= 0.5 and (Y= 0.7 are shown in Fig. 7. For E = 37kcal mol - and temperatures used in Fig. 7, we find b = 1.08. Using this value of b, wefind E = 42 kcal mol - for (Y= 0.7 and E = 36 kcal mol- for OL 0.5 yielding anaverage value of 39 kcal mol- determined above by a different method, although usingthe same data. For comparison, for pure PE, values of E from 45 to 70 kcal mol- havebeen reported by different investigators [1,2]. We also measured thermogravimetricweight loss data in air. This yielded uneven curves most likely due to interaction withoxygen. C onsequently, we were not able to determine the relevant activation en ergiesusing the above procedures.W e next consider the results of the ESR experiments. First, it is noted that the CPsample does not give a detectable E SR signal at room tem perature. In Fig. 8, we showthe results of careful ESR experiments carried out under 500 psig of H, pressure for twocases: (i) CP alone and (ii> CP plus 1 0% nanoscale Al,O, plus 1 0% elemental sulfur.The nanoscale Al,O, was obtained from Nanop hase Technologies C orporation (453Comm erce St. Burr Ridge, IL 60521, USA ) and it has a particle size of 20 nm with 45m2 g-r surface area. Elemental sulfur was added to simulate the recent successful ESRexperiments with Fe-based catalysts [8], and because with coal liquefaction, sulfur


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72 MM . I brahi m et al./Fuel Processing Technol ogy 49 (19 ) 65-73Commingled Plastics (CP) CP +lO A&O, +lO S

244OC

360C351C

.- 361c--- -

Fig. 8. The recordings of the electron spin resonance spectra for CP and for CP plus AI,O, and sulfur, close tothe degradation temperatures resulting in an ESR signal.

appears to inhibit sintering of the particles and enhance liquefaction [9]. In any case, theESR results of Fig. 8 show the following dramatic results: with CP alone, the first hintof an ESR signal due to its degradation is seen near 36 O C, with a clear sign al at 381Cwhose intensity then increases with increase in temperature. On the other hand, with CPplus A&O , and sulfur, we see a clear ESR signal at 28O C, nearly 100C lower th an thethermal case. At higher temperatures, the signal again increases. By carrying outexperiments with Al,O, and with sulfur loading alone, we have determined tha t Al,O,plays no role in the degradation of CP and that the degradation at the lower temperatureis only due to elemental sulfur. In addition, H , pressure is not necessary for thisdegradation since it is observed even in a vacuum-sealed sample as long as sulfur ispresent. Additional experiments for determining the mechanism of this effect due tosulfur are now in progress. In recent experiments by other researchers [lo- 131, a numberof acid catalysts have been found to be effective in the depolymerization and liquefac-tion of polymers and CP. We will test these catalysts in the near future by our ESRmethodology described above.In summary, the results presented here show that PE in the CP samples begins todegrade thermally near 380C and this degradation temperature is lowered by about100C due to the action of the elemental sulfur. Additional experiments with this andother catalysts are in progress.


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MM . Ibr ahim et al./ Fuel Pr ocessing Technology 49 (1996) 65-73 73

cknowledgementsThis work was supported in part by the U.S. Department of Energy through the

Consortium for Fossil Fuel Liquefaction Science, Grant No. DE-FC22-93PC93053.

References[1] Encyclopedia of Polymer Science and Technology, Vol. 4, Interscience, New York, 1966.[2] L.L. Wall, S.L. Madorsky, D.W. Brown, S. Straus and R. Simha, J. Am. Chem. Sot., 76 (1954)

3430-3437.[3] M.M. Ibrahim and M.S. Seehra, Energy Fuels, 5 (1991) 74-78.[4] M.M. Ibrahim and M.S. Seehra, Am. Chem. Sot., Div. Fuel Chem., Prepr., 37 (1992) 1131-1140.[5] S.L. A ggarwal an d G.P. Tilley, J. Polym. Sci., 18 (19.55 ) 17-26.[6] M.K.I. Ismail and S.L. Rogers, Carbon, 30 (1992) 229-239.[7] J.H. Flynn and L.A. Wall, Polym. Lett, 4 (1%6) 323-328.[8] M.M. Ibrahim and M.S. Seehra, Energy Fuels, 8 (1994) 48-52.[9] F. Derbyshire and T. Hager, Fuel, 73 (1994) 1087-1092.

[lo] P. Sivakumar, Heon Jung, J.W. Tiemey and I. Wender, Fuel Process. Technol., 49 (1996 ) 219-232.[I 11 Z. Feng, J. Zhao, J. Rockwell, D . Bailey, and G. Huffman, Fu el Process. Technol., 49 (199 6) 17-30 .1121W.B. Ding, W. Tuntawiroo n, J. Liang and L.L. Anderson, Fuel Process. Technol., 49 (199 6) 49-63.[13] K. Liu, and H.L.C. Meuzelaar, Fuel Process. Technol., 49 (1996) I-15.

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Thermal and Catalytic Degradation of Cmmingled Plastics, 1996