Pyrolysis and Oxidative Pyrolysis of Polypropylene - Advances in

15 Pyrolysis and Oxidative Pyrolysis of

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0169.ch015

Polypropylene JAMES C . W . C H I E N and JOSEPH K. Y. K I A N G Department of Chemistry, Department of Polymer Science and Engineering, Polymer Research Institute, Materials Research Laboratories, University of Massachusetts, Amherst, M A 01003

Amorphous and semi-crystalline polypropylene samples were pyrolyzed in He from 388°-438°C and in air from 240°-289°C. A novel interfaced pyrolysis gas chromato graphic peak identification system was used to analyze the products on-the-fly; the chemical structures of the produ were determined also by mass spectrometry. Pyrolysis of polypropylene in He has activation energies of 5-1-56 kcal mol and afirst-orderrate constant of 10 sec at 414°C. The olefinic products observed can be rationalized by a mechanism involving intramolecular chain transfer processes of primary and secondary alkyl radicals, the latter being of greater importance. Oxidative pyrolysis of polypropylene has an activation energy of about 16 kcal mol ; the first -order rate constant is about 5X10 sec at 264°C. The main products aside from CO , H O, acetaldehyde, and hydrocarbons are ketones. A simple mechanistic scheme ha been proposed involving C-C scissions of tertiary alkoxy radical accompanied by Η transfer, which can account for most of the observed products. Similar processes for sec ondary alkoxy radicals seem to lead mainly to formaldehyde Differences in pyrolysis product distributions reported here and by other workers may be attributed to the rapid re moval of the products by the carrier gas in our experiments -1

-3

1 -

-1

-3

2

-1

2

s p h e r e has been surprisingly little fundamental work reported chemistry of the process involved in the burning of polymers inhibition, despite the great technological and socio-economical tance of the subject. I n our laboratories w e have undertaken a 0-8412-0381-4/78/33-169-175$05.75/l © 1978 American Chemical Society In Stabilization and Degradation of Polymers; Allara, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

o n the and its impor funda-


176

STABILIZATION

A N D D E G R A D A T I O N

O F

P O L Y M E R S

mental research of this problem. The effort is divided into the study of the chemistry involved i n a polymer flame and the effect of flame retardants and the study of the chemistry involved i n the solid polymer phase and the effect of flame retardants. This report is concerned with the second topic. W h e n various polymers were burned i n air i n a candle-like manner ( I ) , the maximum flame temperatures were 490°-740°C and the tem peratures of the melt surface were 230°-540°C. Burge and Tipper (2) found the temperature of polyethylene to be 400°-500°C at the burning surface and decreasing to 200°-300°C at 1 cm below the surface; the temperature being greater when probed from above the sample and smaller when probed from below. Whether the degradative processes for the polymers are pure pyrolysis or oxidative pyrolysis is still largely unsettled. It has been shown that the oxygen concentration 1 m m above the surface of a burning polyethylene rod is only ca. 1 % (2). Similar results were reported for polypropylene (3). F r o m this it may be inferred that the polymer i n the melt surface may be oxygen-depleted. O n the other hand, the polymer further below the surface probably contains the usual amount of dissolved oxygen. It is likely that oxidative pyrolysis contributes significantly or even predominantly to the liberation of vola tile and combustible fragments from the polymer. The relative impor tance of pure pyrolysis and oxidative pyrolysis can only be established by experimentation. F o r instance, if an efficient flame retardant sup presses markedly the oxidative pyrolysis but has no effect at all on the thermal pyrolysis, then further search for new flame retardants should be emphasized on compounds which could interrupt peroxy radical chain reactions. O n the other hand, if nonoxidative pyrolysis dominates, then heat dissipating or intumescent additives would be effective. In this chapter we describe a novel system for the study of pyrolysis and oxidative pyrolysis of polymers. The results of such a study on amorphous and semicrystalline polypropylene are presented and com pared with other reported works. The effects of known flame retardants on the rates of pyrolysis and oxidative pyrolysis of the polymer and the product distribution have been investigated and w i l l be published elsewhere. Experimental Materials. The amorphous polypropylene ( A P P ) used is that of unstabilized Eastobond from Tennessee Eastman. The semicrystalhne polypropylene ( I P P ) is the Profax 6501 from Hercules Incorporated. It has a crystallinity of 6 1 % as determined from its density (4). Interfaced Pyrolysis Gas Chromatographic Peak Identification System (IPGCS). The versatile I P G C S (5) incorporates instrumentation

In Stabilization and Degradation of Polymers; Allara, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.


15.

C H I E N

A N D

KiANG

177

Pyrolysis of Polypropylene

for thermal degradation of polymer under slow or ultra-rapid temperature rise and under inert or reactive atmosphere conditions. Evolved volatiles are transferred to a master trap manifold where precolumn procedures may be applied prior to gas chromatographic separation. Identification and analysis of individual peaks are then performed quickly by rapid scan vapor phase i r spectrophotometry, elemental analysis for carbon, hydrogen, oxygen, nitrogen, etc., functional group fingerprinting by vapor-phase thermal cracking, and molecular weight determination by differential gas density measurement (mass chromatography). Figure 1 is a block diagram of the I P G C S system. A n interfaced laboratory computer provides for data acquisition, reduction, and control. In order to make clear the operation of the system, a brief discussion of individual instruments follows. SPEX MP-3 THERMAL ANALYZER

TRAP § SELECTOR MANIFOLD

MASTER

NOR00N

G.C.

PDP

201 1 R

11/10E LAB

J

SYSTEM CDS 820 REACTION SYSTEM

Figure 1.

CDS GC-2

1200

PEAK IDENflFIER

SPEX NC-2 MASS CHROMYIOGRAJHl

Block diagram of the interfaced pyrolysis gas chromatographic peak identification system

The M P - 3 multipurpose thermal analyzer is manufactured by the Spex Industries. It pyrolyzes sample at programmed rates from 4°/min40°C/min up to 1000°C. It was modified by the addition of two laminar flow controllers ( H G C 187; Analabs) which provide controlled atmos pheres of up to four gaseous mixtures. The C D S 820 (Chemical Data Systems) consists of a Pyroprobe 100 capable of delivering to a polymer sample a maximum heating rate of 20,000°C/sec; the C D S 820 provides a controlled atmosphere for the Pyroprobe. The master gas chromatograph is a Varian 2760 instrument with thermal conductivity and flame ionization detection. A second Varian 2760 gas chromatograph ( G C - 2 ) serves for analysis of samples from two sources, pyrolysis products from the C D S 820 and from the struc tural determination function of the C D S 1200. The latter instrument (Chemical Data System) is a functional group and elemental analyzer which generates a vapor-phase thermolytic" dissociation pattern for functional group analysis and also performs elemental analysis. The effluent from the master G C is split so that 10% of it is directed to the detector and 90% of it to the C D S 1200. A stop-flow valve admits one


178

STABILIZATION A N D DEGRADATION

OF

POLYMERS

peak at a time, thus enabling the analysis of all the components of the pyrolysates without appreciable peak spreading. The use of C D S 1200 i n organic analysis has been discussed by Liebman et al. (6). The Norcon 201 rapid-scan, vapor-phase spectrophotometer is a double beam grating instrument which scans a spectrum from 4000 to 670 c m " i n either 6 or 30 sec, w i t h recycle times of 8 and 40 sec, respectively. Its wavelength accuracy is ± 0.05/A w i t h =b 0.025/x, repro ducibility. The sample cell has a volume of 5 m L and is constructed of gold-coated glass (4 X 4-mm cross section, 30 cm i n length). The path length to volume ratio is 6.0 cm" . The cell is enclosed i n an oven regulated to ± 2.5°, from 120°-250°C. The Norcon 201 is connected to the master G C through heated transfer line. A n effluent fraction of 0.025 λ is sufficient to yield a good spectrum. The Norcon 201 is interfaced with a P D P 11/10E computer system. This system has a R T 11 foreground/background operating software, F O R T R A N , and L a b Applications Program L i b r a r y V 0 3 w h i c h contains S P A R T A and T H R U programs for data acquisition and a I R F L A V program for the acquisition of ir spectral data w i t h digital filtering. Another routine, I R S P B A , performs background subtraction and spectrum print out.


1

2

The M C - 2 mass chromatograph from the Spex Industries determines molecular weight of G C eluent through differential gas density measure ments. A sample is split into two equal fractions; they are carried by two different gases, C 0 and Freon 115, through two matched G C columns into density-balance detectors. The molecular weight of the unknown is obtained from 2

mol wt =

( A i / A ) Κ * mol w t c G i mol w t Q (Αχ/Α,) Κ - 1 —

2

C

2

(D

where the instrument constant Κ is calibrated w i t h a substance of known molecular weight, mol w t , st

(2) In these equations m o l w t o i and mol w t G 2 are the molecular weights of the two carrier gases, and A i / A is the ratio of peak height response of a standard for the two detectors. The accuracy for the determination of molecular weight w i t h M C - 2 depends upon the instrument constant Κ (Equation 2 ) . The best results are obtained for values of Κ determined w i t h known compounds having molecular weights nearly the same as the unknown. L l o y d , et al. (7) has developed a least-square, curve-fitting program to fit the calibration Κ values to a polynomial w h i c h was then used i n an iterative procedure C

C

2


15.

CHIEN AND K i A N G

179



to calculate the molecular weights of the unknown. The molecular weights thus obtained are accurate to dbl mass unit as judged by com parison with mass spectrometric results. The advantages of the simple M C - 2 mass chromatography are offset by the loss of information about geometric isomerism. Therefore, the technique must be used together with mass spectrometry. In addition to the above I P G C S system, other thermoanalytical tech niques were used also i n this work. This includes a conventional pyrolysis-GC-mass spectrometry system which consists of a C D S Pyroprobe 100, a Perkin-Elmer 990 G C , and a H i t a c h i - P e r k i n - E l m e r R M V 66 mass spectrometer, as well as a duPont 900 thermal analyzer. Procedures for Pyrolysis. In the pyrolysis-GC-mass spectrometry experiments, about 1 mg of polymer was weighed into a quartz tube which was inserted into the heating coil of the Pyroprobe. The latter fitted directly into the injection port of the P e r k i n - E l m e r 990 G C . The G C was operated at a manifold temperature of 220°C, injector tempera ture of 210 °C, interface temperature of 255°C, H e flow rate of 83 m L m i n , and F I D detection. Samples were pyrolyzed at 600°, 650°, 700°, 750°, 800°, 850°, 900°, and 950°C at a heating rate of 20,000°C sec" . A l l samples were held for 20 sec at the final temperatures. T w o columns were used to separate the pyrolysis products pro grammed from 1°C m i n " to 6 ° C min" . A Porapak Q column (4 ft X % i n . O D ) was used to separate low boiling compounds and a 4 % SE30 on A B S column (6 ft X % i n . O D ) resolves higher boiling products. The gas chromatogram was obtained first. This was followed by an identical run whereas a peak of interest was just emerging, the interface splitter valve was opened to admit the sample into the mass spectrometer. The latter was operated at an electron energy of 70 eV and a filament current of 3.4 A . F o r pyrolysis with the I P G C S system, about 2 m g of polymer was weighed into a quartz tube. It was placed into the oven of the M P - 3 apparatus. The polymer was pyrolyzed at 388°, 414°, and 438°C at a heating rate of 40°C m i n " under a H e flow rate of 25 m L min" . The weight of the residue was obtained with a microbalance. The products, after passage through the master trap manifold, were collected i n a 1 ft X Vs i n . O D glass bead column at —195°C. W h e n all of the products have been collected, the column was heated rapidly to 300°C and the pyrolyzates were back-flushed into the master G C using F I D detection. L o w boiling hydrocarbons were separated with a Chromosorb 102 column (12 ft X % i n . O D ) ; higher boiling products were resolved with an 8% Dexsil 300 G C on Chromosorb W - H P column (6 ft X % i n . O D ) . A t a H e flow rate of 14 m L m i n ' , the column was kept first at 37 °C for 3 min, then programmed at 4 ° C m i n " to a final temperature of 250°C for the Chromosorb 102 column and to 300°C for the Dexsil column. The mo lecular weight of the product was determined with the M C - 2 instrument calibrated with n-alkanes. Weight percentage of the products were cal culated from the integrated area of the G C peaks taking into consideration the attenuation factors. Dietz (8) had shown previously that F I D is equally sensitive to all aliphatic hydrocarbons. -1

1

1

1

1

1

1

1


180

STABILIZATION

A N D D E G R A D A T I O N

O F

P O L Y M E R S


Procedures for Oxidative Pyrolysis. A l l oxidative pyrolysis experi ments were performed with the I P G C S system. About 5 mg of polymer were pyrolyzed i n an air atmosphere at 240°, 264°, and 289°C. The same Chromosorb 102 column was used for the separation of low molecu lar weight products, but a Carbowax 2 0 M on Chromosorb Ρ column ( 12 ft X % i n . O D ) replaces the Dexsil column for the separation of high molecular weight products. The products were characterized primarily by the Norcon 201 spectrophotometer and mass spectrometry. Results Pyrolysis of Polypropylene. For the measurement of rates of pyroly sis, the temperature range is limited to that of conveniently measurable rates. Figure 2 shows the thermograms of A P P and IPP which provides the choice of temperature for pyrolysis.

150

Figure 2.

250

It

350

TGA of PP in He: , IPP

450

, APP;

The rate of pyrolysis was obtained b y weighing the polymer sample before and after a preset hold time i n M P - 3 . The results are given i n Figure 3. The process follows first-order kinetics as shown i n Figure 4 and are semilog plots of wt % of polymer residue vs. t. The rate con stants and activation energies for pyrolysis of polypropylene are given i n Table I. B y comparison, I P P decomposes faster than A P P at 388°C but more slowly at 438°C. This is reflected i n a lower activation energy of 51 kcal mol" for the semicrystalline polypropylene, the accuracy for the activation energy is about 10%. The chromatograms of the pyrolyzates of I P P at 438°C are shown i n Figure 5. The products and their distributions at three temperatures are summarized i n Tables II and III. The chemical structures of the products were obtained from mass spectrometry; Table I V presents the 1



CHIEN AND K i A N G

Ρ


3

?

3

4

b—ft—go—3o 4ô—ëo 6b HOLD TIME(MIN) Figure 4.

First-order plot of PP pyrolysis in He: , APP; ,ΙΡΡ


182

STABILIZATION A N D DEGRADATION O F P O L Y M E R S

Table I.

Kinetics of Pyrolysis of Polypropylene in He Rate Constants,

Temp., °C

APP

388 414 438 Activation energy, kcal mol"

3.65 X 10" 1.38 Χ 10" 7.60 X 10" -


1

Sec'

1

IPP 4

3 3

4.0 X 10' 1.05 Χ 10" 6.18 Χ 10"

Λ

4 3 3

Κ 1

5 b

0 1

fragmentation patterns of the major flash pyrolysis products of polypropyl ene. Identical products were obtained at temperatures from 600°-950°C. Tables II and III showed that essentially the same products were produced by pyrolysis at much lower temperatures except for C H and C H i . Figures 6 and 7 show the rates of formation of individual products at 414°C, each normalized to the amount at complete pyrolysis. Oxidative Pyrolysis. The thermograms of polypropylene obtained in air are shown i n Figure 8, which suggests the temperatures to be used for oxidative pyrolysis. The results of oxidative pyrolysis at 240°, 264°, and 289°C are given i n Figure 9; Figure 10 shows the first-order kinetic plots. The rate constants and activation energies are given i n Table V . L i k e the pyrolysis i n inert atmosphere (Table I I ) , the oxidative pyrolysis of I P P at low temperature is slightly faster than A P P and slower at high temperature resulting i n a somewhat smaller activation energy for the semicrystalline polypropylene (Figure 11). The chromatograms for the oxidative pyrolyzates obtained at 363 °C for I P P are given i n Figure 12. The same products were formed at the 2

Figure 5.

e

Low-boiling GC pyrogram of IPP in He at 438°C


4

0

15.

C H I E N

A N D

Table II.

183


KiANG

Pyrolysis Products of Polypropylene at 4 3 8 ° C

wt%°


Peak 1* 2* 3" 4 5 6 7 8 9 10 11 12 13 14

Hydrocarbons

Mol Wt

C H (methane) * C H (ethane) C H (propylene) C H (isobutylene) C H i o (2-pentene) C e H i (3-methyl-l-pentene) C7H12 (3-methyl-3,5-hexadiene) CgHie (4-methyl-3-heptene) C H i e (2,4-dimethyl-heptadiene) 4

2

55.7 70.0 81.2 96.8 110 123.5 125.9 139.5 157.5 168.2 180.6

e

3

e

4

8

5

2

9

C9H18 (2,4-dimethyl-heptene) C10H20 C11H22

(4,6-dimethyl-3-nonene) C12H24 (2,4,6-trimethyl-8-nonene) Cl3H24

APP

IPP

0.1 1.2 12.2 3.2 15.8 11.4 2.0 d d 39.3 2.4 1.3 9.5 1.7

0.09 1.4 10.7 2.5 15.5 10.2 1.7 d d 42.6 2.1 1.6 10.2 1.6

"Identified by retention time; molecular weight determination by M C - 2 was thwarted by instability in base line during first several minutes of operation of this instrument. Structure determined by mass spectrometry. Average of two determinations. * Incompletely resolved. 6 0

other temperatures, the main difference being that the product distribu tions are not the same (Figure 13). Most of the products were identified by the Norcon 201 ir spectro photometer. The spectra were compared w i t h the atlas compiled by Table III.

Pyrolysis Products of Polypropylene at 3 8 8 ° and 4 1 4 ° C * Pyrolysis 414°C wt % of Products

Temperature 388°C wt % of Products

Hydrocarbons

APP

IPP

APP

IPP

CH C2H6 C3H6 C4H8 C5H10

0.08 1.0 10.6 2.7 16.5 10.3 1.6 41.3 2.2 1.7 11.0 1.3

0.12 1.6 11.7 2.1 18.9 11.9 1.4 36.0 1.5 2.0 12.3 1.2

0.09 0.9 9.0 2.7 15.0 12.0 1.1 43.0 1.7 2.4 11.3 1.4

0.08 0.07 6.9 1.8 14.0 10.5 1.0 45.2 1.5 3.1 13.5 1.6

4

CeHl2

C7H12 C9H18 C10H20 C11H22

C12H24 C13H24

* Same footnotes as in Table II.


184

STABILIZATION A N D DEGRADATION O F POLYMERS

Table IV.

Identification of Flash Pyrolysis Products of Polypropylene by Mass Spectrometry


Product

Fragmentation

Methane Ethylene Propylene Isobutane 2- Pentene 3- Methyl-l-pentene 3- Methyl-3,5-hexadiene 4- Methyl-3-heptene 2,4-Dimethyl-heptadiene 4,6-Dimethyl-3-nonene 2,4,6-Trimethyl-8-nonene 2,4,6,8,10-Pentamethyl3,9-undecadiene 2,4,6,8,10-Pentamethyl1-undecene

Patterns in Mass Numbers

16,15 28, 27,26 42,41,27 58,43 70, 55,42,41,39 84, 69 56,41 96,81,67, 55,41,39 112, 69, 55,41,39,27 124,123,109,95,82, 67, 55,41,39 154,111,85, 69, 55,43,41,39 168,153,125, 111, 97, 83,69,57,43 222,207,179,166,151,137,123,109,95,83, 69, 55,41 224,210,168,153,141,125, 111, 97, 83, 69,57, 43

10

. 20 H O L D TIME(MIN)

30

Figure 6. Product distribution of pyrolysis of PP in He at 414°C: ·, CH ; • , C , H ; O , C H ; A , C H ; ^> C H ; Δ, C H ; Θ , C H 8

k

S

6

6

12

2

6

7

S

10

J2

W e l t i (9). The quality of the spectra is uniformly good as exemplified retention times are: C 0 , H 0 , formaldehyde, acetaldehyde, methanol, propylene, isobutylene, acetone, hexene, and other unsaturated aldehydes and ketones. The actual chemical structures of these ketones and of several other products are being determined by mass spectrometry. 2

Discussion

of

2

Results

Pyrolysis of Polypropylene. There have been several published studies of pyrolysis of polypropylene. They are more i n discord than i n



15.

CHIEN AND K i A N G

185


Figure 7. Product distribution of pyrolysis of PP in He at 414°C: ·, C H ; •, C H ; O, C H ; Δ, 9

18

J2

H

10

20

agreement. Different kinetics, activation energies, and products were reported. W a l l and Straus (JO) found the rate of volatilization of polypropyl ene at 375°C to increase rapidly to a maximum at 40% conversion and then decrease rapidly with further heating. Similar behaviors were ob served also for polyethylene (10). However, branches longer than a methyl group were found to eliminate the maxima i n the rate curves even when present i n quite low concentration ( I I ) . O n the other hand, M a dorsky and Straus (12) and this work found the kinetics of pyrolysis to be first order.

TX Figure 8.

TGA of PP in air: , IPP

, APP;



186


Ό

1

Figure 9.

2

3

A

5 6 ? 8 HOLD TIME(MIN)

9~~Î0

Overall % pyrolysis of PP in air: APP; , IPP

,

E v e n though the pyrolysis of polypropylene is mechanistically com plicated (vide infra), the kinetics is first order because the rate deter mining step is the homolysis of the C - C bond describable b y a welldefined rate constant. W e found the activation energy for pyrolysis to be 51 and 56 kcal mol" , respectively, for I P P and A P P . Other literature values are 60 kcal m o l measured at 3 5 0 ° - 4 0 0 ° C b y W a l l and Straus (10), 58 kcal mol" i n the range 3 3 6 ° - 3 6 6 ° C reported b y Madorsky and Straus (12) and 55 kcal mol" found b y Moissev et al. (13) i n the tem perature range 3 2 0 ° - 4 2 0 ° C . W e found the pyrolysis products i n our work to be predominantly olefins. A small amount of methane was found at all temperatures. M i n o r 1

- 1

1

1

HOLD TMECMIN)

Figure 10. First-order plot of PP pyrolysis in air: , APP; , IPP


15.

Table V .

Kinetic Results for Oxidative Pyrolysis of Polypropylene Rate Constants,

Temp., °C

APP

240 264 289 Energy of activation, kcal mol"

2.27 Χ 10" 5.20 Χ 10" 1.02 χ 10" --

1


187


CHIEN AND K i A N G

Sec'

1

IPP 3 3

2

2.52 Χ 10" 5.35 Χ 10" 9.2 Χ 10" ~

1 7

3 3

3

Λ

1 5

quantities of ethane and C n H 2 4 were observed at low temperatures and isobutane was found i n flash pyrolysis. The major products shown i n Tables II, III, and I V peaks were: 3, propylene; 4, isobutylene; 5, 2-pentene; 6, methylpentene; and 10, dimethylheptene. Significant amounts of trimethylnonene (13) were also identified. These results are most similar to those of Tsuchiya and Sumi (14) execpt for certain prod ucts. They found more methane than we d i d . Their major Ce product was assigned to be 2-methyl-l-pentene. However, the most important discrepancy is that Tsuchiya and Sumi found pentane to be the major pyrolysis product. The main products were found by Moiseev et al. (13) to be propylene, isobutylene, and pentene. A t the other end of the spec trum, Bailey and Liotta (15, 16) found that at 340°C pyrolysis of poly propylene yielded 80-90% propane and at 380°C, 6 5 % was n-pentane. Màdorsky (17) studied the pyrolysis product over the widest tem perature range from 3 8 0 ° - 1 2 0 0 ° C . A t the low temperature regions pro pylene, isobutylene, butane, pentene, pentane, and hexene are the major

Figure 11.

Arrhenius plot of PP pyrolysis in air: APP; , IFF

,



188

STABILIZATION A N D DEGRADATION O F POLYMERS

Figure 12b.

High-boiling GC pyrogram of IPP in air at 289°C

HOLD TIME(MIN)

Figure 13. Product distribution of pyrolysis of PP in air at 289°C: O , CH CHO; ·, acetone; Δ , C ; Θ, C ; •, C ; •, C s

7

s

8

9



15.

189


CHIEN AND K i A N G

products. However, the amount of pentane found was only 11 mol % as compared with 6 5 % and 3 1 % reported by Bailey (16) and Tsuchiya and Sumi (14), respectively. Madorsky (17) observed that between 400° and 800°C, the major products are produced i n nearly the same amounts except for propylene, whose yield increases suddenly suggesting unzip ping of the polymer. Finally, A P P and I P P seem to be slightly different i n their pyrolysis behavior as had been noted also by Bresler et al. (18). However, the differences are not too great and their significance needs further verification. In order to reconcile to some degree the results cited here, the mecha nism for pyrolysis of polypropylene needs to be considered. The mecha nism commonly accepted is based on those proposed for the gas-phase degradation of simple hydrocarbons (19, 20). C H A I N INITIATION.

H

H

H

^ww* C-CH -C~CH -C--CH2-^^ 2

4H

2

CH

3

CH

3

3

H

H

H

^ .•w/'x/vC—CH —C—CH · + 2

I

l

CH3

2

H

/ux/v/vC—CH^C ·

CH3

CH3

(D

(3)

[ I

CH3

(n)

where I is a primary aliphatic radical and I I is a secondary radical. In highly stereoregular and pure polypropylene initiation is the result of thermal homolysis. However, most commercial polymers contain small percentages of impurities and structural irregularities. Chain initiation via scission of weak links cannot be ruled out as a possibility. UNZIPPING

(DEPOLYMEMZATION).

H I or I I - » Γ or I I ' + C H - C = C H 3

2

(propylene)

(4)

F o r polymers with low ceiling temperature, such as poly(a-methylstyrene) and poly(methylmethacrylate), unzipping is the predominant degradative process. However, this is unimportant for pyrolysis of poly propylene at low temperature. Unzipping becomes more important above 800°C but still is not the dominant reaction. C H A I N TRANSFER.

Intramolecular Chain Transfer. Intramolecular hydrogen transfer (20) leads to many of the observed products. I n the equations below,



190


Figure 14.

A 6-sec scan of vapor phase ir of acetone

the numbers above the arrow indicate the modes of hydrogen transfer. Not a l l of the possible products were found i n our experiments; the italicized products correspond to those found i n significant amounts experimentally. F r o m the primary radical:

I

1.2 1.3 1.4

> II +

isobutylene

> I + 4-methyl-2-pentene » II + 2,4-dimethylpentene

1.5 > I+ 1.6

4,6-dimethyl-heptene

> II + 2,4,6-trimethylheptene

1.7 » I+ 1.8

4,6,8-trimethyl-nonene

» II + 2,4,6,8-tetrametnylnonene


15.

191


CHIEN AND K i A N G

F r o m the secondary radical: 1.2 II

> I + 2-pentene 1.3 > II + 2-methylpentene 1.4


> I+

4-methyl-heptene

(6)

1.5 > II + 2,4-dimethylheptene 1.6 > I + 4,6-dimethyl-nonene 1.7 > II + 2,4,6-tnmethylnonene Transfer.

Intermolecuhr H

H

^~vC-CH l CH

H

H

· + - ^ ^ C M D H g - C - C H z - C

Pyrolysis and Oxidative Pyrolysis of Polypropylene - Advances in

Recommend Documents