We have studied the thermal degradation of polytetrafluoroethylene (PTFE) in wet air, dry air and nitrogen gas at temperatures ranging from 310 to 520 C. The principal gas product was tetrafluoroethylene (TFE). The nitrogen experiments yielded only TFE, but experiments in dry and wet air also produced several other minor gas products. The dry air experiments produced the most smoke and the least gas products, with the nitrogen experiments giving no smoke and the highest amounts of gas products. Wet air experiments gave intermediate results for smoke and gas formation. The rate of degradation in wet air showed an apparent activation energy of 103 kca/mol. We also performed FTIR analyses of the smoke particulate, determining that the particulate formed in dry air is different than that formed in wet air

Duan, H. M.

Mckinnon, J. Thomas

English

NASA Technical Reports Server

N96- 15574 
Thermal Degradation of Polytetrafluoroethylene in Tube Reactors 
J. Thomas McKinnon and H. M. Duan 
Deprtment of Chemical Engineering and Petroleum Refining 
Colorado School of Mines, Golakn, CO 80401 
We have studied the thermal degradation of polytetrafluoroethylene (PTFE) in wet air, 
dry air and nitrogen gas at temperatures ranging from 3 10°C to 520°C. The principal gas 
product was tetrafluoroethylene (TFE). The nitrogen experiments yielded only TFE, but 
experiments in dry and wet air also produced several other minor gas products. The dry air 
experiments produced the most smoke and the least gas products, with the nitrogen 
experiments giving no smoke and the highest amounts of gas products. Wet air 
experiments gave intermediate results for smoke and gas formation. The rate of 
degradation in wet air showed an apparent activation energy of 103 kcal/mol. We also 
performed FTIR analyses of the smoke particulate, determining that the particulate formed 
in dry air is different than that formed in wet air. 
Polytetrafluoethylene (PTFE) is widely 
used by industry in such applications as 
electric insulation and coatings. The 
chemical and thermal stability of PTFE is the 
primary reason for its widespread use. 
However, once the PTFE starts to 
decompose at high temperatures, the gas 
products and the smoke particulate formed in 
the degradation are known to be very toxic 
[l], causing symptoms known as “fume 
fever” [2-41. It is believed that much of the 
toxicity is due to ultrafine particles (de100 
nm) fiom the degradation process. This 
problem becomes particularly serious on 
space craft where no venting is possible [5]. 
In this paper we present the results of the 
first part of our ongoing efforts to obtain an 
understanding of the effect of gravity on the 
toxicity of ultrafme particles and gas phase 
materials produced when fluorocarbon 
polymers are thermally degraded or burned. 
The motivation for the project is to provide a 
basic technical foundation on which policies 
for space craft health and safety with regard 
to fire and polymers can be formulated. 
Figure 1 illustrates the experimental 
apparatus we constructed to study the 
pyrolysis and burning of PTFE. Nitrogen, dry 
air, or wet air flowed into the steel tube of 2.5 
cm inner diameter and 80 cm long. The 
furnace and the Monel tube were preheated to 
a desired temperature before a glass boat 
containing 1 gram PTFE powers (Aldrich cat. 
No. 18,247-8) was put into the center of the 
furnace. The smoke produced in the reaction 
was collected by adsorption onto densely- 
packed glass wool. The gas products were 
directed to either a cryogenic trap, a mass 
spectrometer (MS) or a gas chromatograph 
(GC). The gas flow was -90 d m i n .  In the 
dry and wet air reaction atmospheres, there 
was suilicient oxygen in the reactor for the 
complete oxidation of PTFE. 
The experimental temperatures ranged 
from 310°C to 520°C. Gas products were 
separated by a 30 meter HP-1 capillary 
column before entering the flame ionization 
detector. Figures 2a and 2b show the typical 
GC results for the gas products. There were 
major peaks at retention time 0.93 minutes. 
For reactions in the air atmospheres, several 
147 
https://ntrs.nasa.gov/search.jsp?R=19960008408 2020-06-16T05:22:41+00:00Z
small peaks were also observed at longer 
retention times. 
Figure 3 shows the major peaks (peaks 
with retention time 0.93 minutes) for 
reactions at different temperatures. The 
areas of the peaks are listed in Table 1. For 
temperatures lower than 37OoC, peak areas 
are negligible. At temperatures higher than 
about 37OoC, the peak start to be evident. 
Figure 4a shows the peak area A as a 
knction of the temperature. In figure 4b, a 
plot of logA vs. lOOO/T, the data points form 
a straight line. The slope of the line 
corresponds to an apparent activation energy 
of 103 kcal/mol. The value is close, perhaps 
merely coincidentally, to the bond 
dissociation energy of a C-F bond. 
Flow 
Meter 
F A - '  Liquid 11111N 
Figure 1: Experimental set up. 
18ooo Nitrogedl5 
- 
T484C 
f 
- - .  IC 
0 1 2 3 4 5 6 7  
Time (min) 
Figure 2a: Gas chromatograms of the gas 
products. Samples were taken 40 minutes 
after the PTFE powders were put into the 
reactor. 
3600 
2400 
a 
l m m  0 0 1 2 3 4 5 6 7  
Time (min) 
Figure 2b: Peaks other than the major 
peak are very small. T=484OC. 
148 
A I  
16Ooo - 
14ooo 
12ooo 
loo00 
P --- 
6ooo 
4ooo-- 
0 8  
1 
- -  
- *  
406&&:'
- -  
386C - -  394c 
-316C 
378- 
340C 
3 13C 
0.6 1.6 2.6 
Tune 
Figure 3: Temperature dependency of the 
major peaks. 
T("C) 313 3 16 340 378 
Area trace trace trace 497 
3 86 394 397 406 41 1 
1308 4418 8945 16863 36503 
Table 1: Areas of the major peak at  
various temperatures. 
6 5 0 6 6 0 6 7 0 6 8 0 6 9 0  
Temperature (K) 
Figure 4a: Temperature dependency of 
the peak areas. 
5 
4.5 
4 
a g 3.5 - 
3 
2.5 
2 
1.45 1.5 1.55 
1OoOTT 
Figure 4b: The data point follows 
Arrhenius relation with E,=103kcal/mol. 
Gas products fiom reactions in wet air were 
analyzed by mass spectrometry. Figure 5 shows 
one of the spectra. There is no peak above 100 
amu, the mass of C2F4 molecule. So the 
concentrations of chemicals having fragments 
heavier than 100 amu (such as C3F4, C~FS.. . ) are 
negligible. Table 2 lists the mass peaks (less 
those peaks contributed by air). All fragments in 
Table 2 correspond to fiagments of CzF4. We 
therefore believe that the gas appearing at 0.93 
minutes in the gas chromatograms is c2F4. 
0 40 80 120 160 200 240 280 
Mass (amu) 
Figure 5: MS of the gas products. No mass 
heavier than 100 amu was seen (T=51OoC). 
fiagment C CF CF2 CF3 CzFj CzFj 
mass 12 31 50 69 81 100 
~~ 
1000*I/I(31) 325 1000 205 405 250 195 
~~ 
Table 2: The fragments of the gas products. 
149 
I -  1’ NtrogenT=465C I 
0.000 2.000 4.000 6.000 
Retention lime (min) 
0.000 4.000 8.000 
Retention lime (min) 
Figure 6a: Chromatogram of the emission 
from reaction of 1 gram PTFE powder in 
pure nitrogen. The numbers denote the 
minutes after the sample was put into the 
reactor. 
Wet air T=476C 
80000 
60000 
a 
79 
0 2 4 6 
Retention mme (min) 
I 
~~ 
Figure 6b: Chromatogram of the emission 
from reaction of 1 gram PTFE powder in 
wet air. The numbers denote the minutes 
after the sample was put into the reactor 
I -1 1 
Dry air T=476C 
33dfi 60 I 
20000 
15ooo 
loo00 
P 
Figure 6c: Chromatogram of the emission 
from reaction of 1 gram PTF’E powder in 
dry air. The numbers denote the minutes 
after the sample was put into the reactor 
The formation of C2F4 was very sensitive 
to the reaction atmosphere. In pure nitrogen 
gas, no smoke was formed and C2F4 was the 
only degradation product. The reaction 
lasted longer and the concentration of C2FJ 
gas in the emission was higher for the 
nitrogen environment than for the wet air or 
dry air environments (see figures 6% 6b, and 
6c). Degradation in dry air yielded the least 
C2F4 gas, but the most smoke. Visual 
observations indicated that the amount of 
smoke formed by reactions in dry air was 
much higher than that formed in wet air. 
Figures 7% 7b, and 7c show the results of 
FTIR analyses of pure PTFE and the smoke 
powders formed by reactions in wet and dry 
air atmospheres. The three IR spectra are 
clearly different. Pure PTFE powder has two 
relatively sharp peaks around 1212 cm-’ and 
1156 cm-’. The IR spectrum for powder 
formed by reaction in wet air has a major 
peak at about 1101 cm-’, with several 
smaller peaks located at 1637 cm”, 
1421 cm-’, 934 cm-’, and SO5 cm”. The IR 
spectrum for the powder formed by reaction 
in dry air has very large peaks at 1101 cm-’ 
and 76Scm-’. These results show that the 
chemical formula of the powder formed by 
150 
reaction in wet air is different from that of the 
powder formed by reaction in dry air. 
8 
35 
Figure 7a: IR spectrum of pure PTFE 
powder. 
lowest c2F4 concentrations, the shortest 
reaction times, and large amounts of smoke. 
Wet air experiments gave results intermediate 
to the nitrogen and dry air experiments. IR 
spectra indicate differences between smoke 
formed in wet air and dry air environments. 
We are in the process of obtaining more 
information on the mechanisms of the 
formation of smoke. Various experiments, 
including measurements of smoke 
concentration, reaction at higher temperature, 
TEM and SEM experiments, CJL+2 (n=6, 7, 
and 8) thermal degradation and computer 
simulations wil take place in the near future. 
We thank NASA for supporting this 
research under contract number NAG3- 1628. 
1 5 s s 1 o s z I . D z m o r p o t a a D  
w-r *.I) 
Figure 7b: IR spectrum of powder formed by 
PTFE reaction in wet air. 
Figure 7c: IR spectrum of powder formed by 
PTFE reaction in dry air. 
In summary, we have studied the thermal 
degradation of PTFE in pure nitrogen gas, 
wet air, and dry air at temperatures ranging 
from 31OOC to 510OC. The primary gas 
product is C2F4. The reaction rate follows 
the Arrhenius formula r,Ae"EdkT, with 
E,-103 kcal/mol. The nitrogen experiments 
yielded the highest c2F4 concentrations, the 
longest reaction times, and no observable 
smoke. The dry air experiments gave the 
References: 
1. B. B. Baker and M. A. Kaiser, AmZyticczl 
Chemistry 63. 79A-83A (1991) 
2. G. Oberdoster, J. Ferin,. Gelein an R. S. 
Soderholm, Eviron. Health Perspectives, 
3 .  G. Oberdoster, J. Ferin, J. Finkelstein, P. 
Wade and N. Corson, J. Aerosol Sci. 21, 
4. G. Oberdoster, J. Ferin, J. Finkelstein, R. 
Baggs, D. M. Stavert and B. E. Lehnert, 
22nd International Conference on Envirn. 
Systems, Seattle, WA, July 1992, Paper 
92 1388, SAE International, Warrendale, 
PA 
5 .  H. D. Ross, Overview of NASA's 
microgravity combustion science and fire 
safety program. 2nd International 
Microgravity Workshop, Sept. 15-1 7, 
(1991) 
3 84-3 87 (1 990) 
1992, NASA CP-10113. 
151 


Thermal degradation of polytetrafluoroethylene in tube reactors

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