INDUSTRIAL AND ENGINEERING CHEMISTRY
852
like residue. The gaseous fraction amounted, on the average, t o 0.lY0 by weight of the original sample of polystyrene and consisted chiefly of carbon monoxide. The liquid fraction consisted of 94.3 mole yo styrene and 5.7 mole yo toluene. T h e waxlike fraction consisted of a mixture of dimer, trimer, and some tetramer, with a n average molecular weight of 264 * 2. A hard tan-colored residue, which was obtained in the experiments a t lower temperatures, mas found to have an average molecular weight of 2182 * 70. Weight ratio of t h e liquid fraction t o the waxlike fraction in all the experiments above 350" C. is close t o a constant, independent of amount of original polystyrene used, of duration or temperature of pyrolysis. Composition of each fraction is also independent of these variables. Maximum yield of styrene was obtained a t 420" C. and amounted to 42%. by Lveight of the original polystyrene. A comparison of this work which was carried out under most favorable conditions of molecular distillation, with the work of Staudinger and Steinhofer,. who pyrolyzed a 208-gram sample of polystyrene a t a pressure of 0.1 mm. of mercury and at a temperature of 290' to 320 ' C., shows a remarkable similarity of results.
TABLE V. PYROLYTIC FRACTIONATION OF POLYSTYRENE Experimental Conditions , W t . of sample g. Press., mm. H g Temp., C. Duration, hrs. Component
co Styrene Ethyl benzene Toluene Total Dimer *Trimer Tetramer Total
Staudinger and Steinhofer
-_
Present Work Expt. 7 Expt. 9 208 208 0.02398 0.02332 760 10-1 10 -6 10 -5 310-350 290-320 397.5 400 6 12 0.5 0.5 Gaseous Fraction (V) Weight Yo of Original Sample ..... ..... 0.16 0.13 Fraction Volatile a t Room Temp. (111) 62.5 38.46 40.32 37.00 ..... ..... 0.02 0.04 ..... ..... 1.48 1.80 62.5 88.46 41.82 38.84 Fraction Nonvolatile at Room Temp. (I1 IV)
I
19.32 3.85 0 23.17
I1
19.32 23.08 3.85 46.25
+
Expt. 10 0.02552 10 -8 400 0.5
0.13
0.14
38.58 0.00 1.74 40.32
38.64 0.02 1.68 40.34
.....
.... .. ...
..... .....
..... .....
..... .....
49.10
49.50
54.40
50.97
11:53
5.25
8.57
.
.
I
.
.
..I..
Residue (I) 9.62
11.54
8.92
Vol. 40, No. 5
ACKNOWLEDGMENT
experiments 7, 9, and 10. I n experiment I the amounts of monomer, dimer, and trimer collected were 62.5, 19.32, and 3.857,, respectively, of the original weight of polystyrene used. In experiment I1 the fractions collected were a monomer, dimer, trimer, and tetramer in amounts of 38.46, 19.32, 23.08, and 3.85%, respectively. Total of the dimer, trimer, and tetramer in the latter case was 46.15%. I n the case of the present work the averages for experiments 7, 9, and 10 were 40.33% for the monomer and 50.97% for the sum of the dimer, trimer, and tetramer. I n addition t o this, the average molecular weight of fraction I1 of the authors' work was 264 as compared with the mean molecular xeight of 263 for the dimer, trimer, and tetramer, in experiment 2 of Staudinger and Steinhofer. The fact that only small fragments, no larger than the tetramer, were obtained in the present work, in spite of the favorable conditions for molecular distillation, is in qualitative agreement with the view expressed by Frenkel (1) who states that linear or chainlike macromolecules cannot exist in the gas phase, €or instead of evaporating they must be disintegmted or depolymerized into smaller units, mainly monomeric or dimeric. Frenkel further points out that the vapor phase will consist of fragments in which the number of units is equal t o W,/Crl, where W 1is dissociation energy and U1 is evaporation energy of the monomer. I n the case of polystyrene, W,, the energy required t o rupture a carbon latent heat of to carbon bond, is approximately 80 kg.-cal. and U,, vaporization of styrene, is about 9 kg.-cal. Therefore, W1/UI = 80/9 = 9. The polystyrene chain having a uniform structure throughout its length (except for the ends), fragments from a monomer t o a nonomer should be equally possible. The average molecular weight of the mixture of all the fragments, except the monomer, should, therefore, be much higher than 264. The fact that the average molecular weight of fraction I1 was only 264 could be explained by assuming that largep fragments split off along with the smaller fragments, but that the former remain entangled in the macromolecules and split further into smaller fragments. This assumption should explain the fact that pyrolysis of polystyrene is a slow process. SUMMARY
Pyrolysis of small samples of polystyrene, of molecular weight
of about 230,000, was carried out in a vacuum of 10-6 mm. of mer,cury and at temperatures between 350" and 420' C. Time of pyrolysis was varied from 0.5 to 4 hours. Results indicate that
pyrolysis begins a t about 350" and is almost complete a t 400" C. The volatile products of pyrolvsis were separated into one gaseous fraction; one liquid fraction, volatile a t room temperature; one waxlikp fraction, nonvolatile at room temperature; and a horny-
The authors wish to thank Robert M. Reese, mho made the mass spectrometric anal3 ses, and Dorothy Thompson and Laura Williamson, n h o made the computations of these analyses in connection with this work. The authors are indebted to Arthur Chemical Company for samples of poIystyrene Roche of the DOTV used in the experiments. LITERATURE CITED
(1) F r e n k e l , J., " K i n e t i c T h e o r y of Liquids," p. 451, Oxford, C l a r e n d o n P r e s s , 1946. (2) L a n g m u i r , I.,Phgs. Rev., 2,329 11913). (3) S t a u d i n g e r , H., and Steinhofer, A,, Ann., 517, 35 (1935). RECEIVED November 28, 1947. Presented before the Division of Cellulose Chemistry, a t the 112th >Ieetlng of the A i ? f E R I c A N CHEVICALSOCIETY, Kern York, N. Y.
Vacuum Drying of Paper-Correction In the paper "Vacuum Drying of Paper" [ N . $1. Foote, IND. ENG.CHEW,39,1642 (1947)], the curve for 100' C. in Figure 6, " ~ ~ o l u m e t r iLoads c Presented to the Vacuum System by the Water Present' on 1800 Grams of Paper in a %-Liter ank," is in error. The corrected figure is shown.
2000
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ISO'C. IO
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PRESSURE
30
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