This report has been prepared

for information and record

purposes and is not to be referenced

in any publication.

NATIONAL BUREAU OF STANDARDS REPORT

10 691

/ /

FLASH PYROLYSIS OF POLYTETRAFLUOROETHYLENE

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

4219223 January 17, 1972 10 691

FLASH PYROLYSIS OF POLYTETRAFLUOROETHYLENE

'by

Maya Paabo and J. J. Comeford

Fire Research Section

Building Research Division

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

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U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

FLASH PYROLYSIS OF POLYTETRAFLUOROETHYLENE

by

Maya Paabo and J. J. Comeford

Abstract

Polytetrafluoroethylene was thermally decomposed using high

energy fluxes from a molecular gas laser emitting energy at

10.6 ym for exposure times of up to 1.5 minutes. Two modes

of decomposition were noted as evidenced by a change in

stable decomposition products as determined by gas chromatography. Nine different low molecular weight (four or less carbon atoms) fluorocarbons have been detected. In

vacuo the primary reaction is depolymerization to the

monomer, C

2

F

4

. In nitrogen and helium atmospheres chain

rupture is more complete to yield significant amounts of

carbon and fluorocarbons CF^, C^F^, and ^

6

' Decomposition in nitrogen atmospheres and to a lesser extent in

helium is accompanied by a stream of incandescent carbon

emitting radiation in the visible region of the spectrum.

It is postulated that in nitrogen atmospheres higher

temperatures result with the formation of significant

amounts of trifluoromethyl radical.
%
FLASH PYROLYSIS OF POLYTETRAFLUOROETHYLENE

by

1.0 Maya Paabo and J. J. Comeford

INTRODUCTION

The thermal decomposition of polytetrafl uoroethyl ene has been

studied extensively and most recently reviewed by W. W. Wright in

considerable detail (1). These studies have been carried out in

inert atmospheres or vacuum. Most research has emphasized the

kinetics of decomposition in vacuum. Pyrolysis generally has been

studied under isothermal conditions or with a programmed rate of

temperature increase. Decomposition in vacuum was shown to

yield predominantly monomer decreasing from 95% at 500 °C to about

75% at 1200 °C. Increasing pressure appears to favor the dimer,

C^Fg, P resumabl y formed from the gas phase dimerization of the

monomer. The major product, tetrafl uoroethyl ene , is assumed to

form by chain unzipping. Isothermal pyrolysis in inert atmospheres

studied in the 500-600 °C range, appears to proceed in the same

way.

The present investigation extends the range of pyrolysis conditions

into the high temperature region for inert atmospheres. The

molecular gas laser provides a reproducible source of energy

variable over a wide range of energy densities. The energy fluxes

available allow pyrolysis studies up to and through the temperature

ranges encountered in polymer combustion. Elucidation of the

chemical mechanisms of pyrolysis requires a knowledge of the

details of the polymer pyrolysis process under high temperature and

oxidizing conditions encountered in combustion. Knowledge of the

mechanism of pyrolysis of a "simple" polymer such as polytetrafl uoroethyl ene should lead to an improved understanding of ignition,

flashover, and smoke formation.

2.0

EXPERIMENTAL METHOD

2.1

Pyrolysis Cell

The pyrolysis experiments were carried out in a modified 10 cm x 2 cm pyrex cylindrical infrared gas cell. The cell was modified by addition

of a 2 cm diameter side arm fitted with a 25 mm NaCl window to transmit

the laser radiation. A rigid disc of polytetrafl uoroethyl ene was

positioned in the side arm approximately 1 cm from the NaCl window and

cell evacuated to 0.13 N/m2 (10“ 3 Torr) or less. For experiments

involving inert atmospheres the cell was next filled with He or N^ to

a pressure of 67000 N/m2 (500 Torr) . This pressure was arbitrarily

1
chosen so that the final pressure inside the cell would not be much

above atmospheric pressure.

2.2 Radiation Source

A 130 watt molecular gas laser (CO^) operated CW (continuous wave)

provided a source of 10.6 ym radiation for the sample pyrolysis.

The unfocused laser beam of 1.3 cm diameter was employed at a power

density of 41 watts/cm2

.

2.3 Ga_s Analysis

In a typical experiment the sample disc of smaller , diameter than

the laser beam was irradiated for a specified time, usually less

than 1 minute, and the cell was transferred to a double-beam

infrared spectrophotometer and the absorption spectrum recorded in

the range 650-4000 cm" 1

.

A gas chromatograph equipped with a thermal conductivity detector

was employed for separation and identification of gaseous pyrolysis

products. A sampling valve with 2 ml loop was used in conjunction

with a vacuum manifold for sample introduction. Fluorocarbons

containing two or more carbon atoms were separated at 0 °C on a 540 cm column packed with Chromosorb W

1 (30-60 mesh) coated with

30 Wt% of 1H, 1H, 7H-dodecafluoroheptyl acrylate (2). Due to the wide

range in boiling points an additional column was required to resolve

the tetrafluoromethane from nitrogen and traces of air. CF^ and the

fluorocarbons were separated at 22-24 °C on a 150 cm Porapak R

1

(80-100 mesh) column. Helium carrier gas (60 ml/min at 64 psig) was

used with injector and detector temperature maintained at 22-23 °C.

Detector current was 100 ma and recorder sensitivity was 1 mv full

scale.

Qualitative identification of the pyrolysis products was made from

the infrared spectrograms and a comparison of retention data for

reference gases. The gases CF^, C

2

F 6

, CgFg, cyclo - C^Fg, ^Fg-2

(mixture of cis and trans) and polytetrafluoroethylene, were of

commercial origin. CgF. was prepared by controlled heating of

polytetrafluoroethylene in vacuum (3).

1 Certain commercial materials are identified in this paper in

order to adequately specify the experimental procedure. In no case does such identification imply recommendation or

endorsement by the National Bureau of Standards ,. nor does it

imply that the material identified is necessarily the best

available for the purpose.

2
Quantitative determination of the components from peak areas of

the chromatograms was evaluated with an electromechanical integrator. The area normalization method was used for estimating

the concentration of individual components. This method assumes

that the ratio of one peak area to the sum of all peak areas is

proportional to the mole percent of a particular component.

Individual response factors were not used (4). The ratio of the

peak area for CF^ to the areas of C^F^ and C^Fg resolved on Porapak R was used to correlate CF^ concentration with the data

obtained with the fluoracrylate column.

2.4 Ultraviolet Measurements

Ultraviolet spectra of the incandescent sample were obtained by

igniting the sample in a quartz cell fitted with a NaCl window

to admit the laser beam. A medium dispersion quartz spectrograph was employed and the spectra were photographi cally

recorded on IF and IN 4 x 10 inch plates.

3.0 EXPERIMENTAL RESULTS

A series of typical laser pyrolyses are summarized in Table 1.

Pyrolysis was carried out in vacuum, helium, and nitrogen. A

representative gas chromatogram of a pyrolysis mixture in helium

is shown in Figure 1. The identity of the two minor peaks, due

to C^F^ and iso - C^Fg, were not confirmed due to lack of

reference gases (2). Duplicate chromatograms for identical

sample mixtures were reproducible within ±1.5% for major

Components. To reduce thermal cleavage of the rock salt window

due to the laser beam the longer exposures were not continuous.

Sample weight loss ranged from 0.2 g to 0.3 g.

Infrared spectrograms obtained for all pyrolysis gas mixtures

supplemented the gas chromatographic data. The structural

similarity of the components and their large number limited

the usefulness of the infrared spectrograms.

Comparison of the pyrolysis in nitrogen and vacuum shows a dramatic change both in appearance and composition of the products.

Vacuum laser pyrolysis results in evolution of some white fluorocarbon deposit and over 90% C^F^. This agrees with earlier

vacuum pyrolysis work where the method of heating was different (5)

.

Only 1% of the dimer cyclo - C^Fg was formed and CgFg was observed

in higher concentrations than previously reported which suggests

somewhat higher temperatures during the laser pyrolysis. No Si

F

4 or oxygen containing compounds were observed.

3
Laser pyrolysis in nitrogen consistently results in a decrease in

C

2

F^ from over 90% to 35% (Table 1).. In addition large increases

occur in CF^, and C^F^, from 0 to 30%, 0 to '30%, and 7 to

20% respectively. Also the higher molecular weight fl uorocarbons

,

cyclo - C^Fg, trans - C^Fg-2, cis - C^Fg-2, and iso - C^Fg appear.

Visual observation indicates intense incandescence of the black

carbon stream having the appearance of a flame. Pyrolysis in a helium atmosphere resembles the nitrogen pyrolysis but with a somewhat higher concentration of C

2

F

4

and less incandescent

carbon. Concentrations are somewhat variable due to slight

variations in irradiation conditions.

Ultraviolet emission spectra of the incandescence indicated a

o continuum from about 9000 o'A (the upper limit of the plate and

optics) to about 4000 A closely resembling spectra of diffusion

flames rich in radiating carbon. Since the maximum duration of

the emission is a few seconds other species possibly present in . the flame in much lower concentrations would not be detected.

4.0 DISCUSSION ,

The details of the degradation mechanism for polytetrafluoroethylene in inert atmospheres has yet to be determined. The

generally favored mechanism of random chain scission with

unzipping of the polymer to form the monomer accounts for the

laser pyrolysis products in vacuum (1,6). This mechanism

assumes formation of , the monomer C

2

F 4 directly rather than by

dimerization of CFg. On the basis of calculations of bond

dissociation energies Errede favors a primary reaction to form

CF 2> The CF

2

radical on the basis of spectroscopic studies has

been shown to be exceptionally stable with a lifetime in the

millisecond range (8).

The laser pyrolysis in inert atmospheres results in a large

decrease in C^ with the formation of an almost equal amount of

C

2

Fg and significant amounts of CF^, CgFg, and iso - C^Fg. The

intense incandescence in inert atmospheres and carbon formation

suggest much higher temperatures are attained. These products

not found in the vacuum pyrolysis could readily form from the

CFg radical produced by fluori nation of CF

2 produced as a primary step in the decomposition. The much higher temperatures

would provide the necessary fluorine.

1 . CF

2 + F + CF3

2. 2CF

3

-C

2

F

6

CF

3 + F + CF

4

3 . 4
Some of the C^Fg, reaction 2, could result from direct fluorination

of C^F^ formed by dimerization of CF^. Infrared spectroscopic

studies at low temperatures have shown that reactions 1, 2, 3

readily occur with negligible activation energies (9, 10).

5.0 REFERENCES

1. W. W. Wright, in Thermal Stability of Polymers, Vol . 1,

R. T. Conley, Ed., Marcel Dekker Inc., New York, 1970,

pp. 289-293.

2. S. A. Greene and F. M. Wachi , Anal. Chem., 35, 928 (1963).

3. S. L. Madorsky, V. E. Hart, S. Straus, and V. Z # Sedlak,

J. Res. Nat. Bur. Stand., 51_, 327 (1953).

4. R. E. Kupel and L. D. Scheel , Am. Ipd. Hyg._ Assoc. J.,

28, 27 (1968).

5. S. Straus and S. L. Madorsky, J. Res. Nat. Bur. Stand.,

66A, 401 (1962).

6. S. L. Madorsky, Thermal Degradation of Organic Polymers,

Interscience Publishers, New York, 1964, p. 136.

7. L. A. Errede, J. Org. Chem., 27^, 3425 (1962).

8. S. H. Bauer, K. C. Hou, and E. L. Resler, Or., Phys.

Fluids, 12, (Ft. 2), 1-125 (1969).

9. D. E. Milligan, M. E. Jacox, and J. J. Comeford, 0. Chem.

Phys., 44, 4058 (1966).

10. D. E. Milligan and M. E. Oacox, J. Chem. Phys., 48, 2265,

(1968).

5
Table

1

Laser

Pyrolysis

Products

of

Polytetrafluoroethylene

6

Results

are

expressed

in

mole

percent.
490

0000

®d*D —OSI

®d*0 - SID

O

oo

®i*D -SNVyi

®i*0 -010AD

bJ2

00 p

» , s J D

<X>

(Xfr) *J*D

(Xtr) ®J 3D

+ HIV 3

<3- o

ooo

(AW) 3SNOdS3d yOi0313Q

Figure

1

Chromatogram

of

polytetrafl

uoroethylene

pyrolysis

products

in

helium