DECOMPOSITION OF POLYTETRAFLUOROETHYLENE
2671
The Decomposition of Polytetrafluoroethylene in a Glow Discharge
by Eckart Mathias and Glenn H. Miller Department of Chemistry, University of California, Santa Barbara, California (Received February 9 , 1967)
The decomposition of polytetrafluoroethylene in a glow discharge was studied using helium, helium plus oxygen, or oxygen as a carrier gas. I n the absence of oxygen, the products were GF4, CF4, C2F6, C3F6, C3F8, C, and polymer. With oxygen present, the reaction produced COF2, CzF4, CF4, GF6, C3F6, c3F8, CO, and C02. The data support a hypothesis which includes unzipping via CF2 radicals, polymer formation via C2F4 excited species, CF2 radical disproportionation to account for the F species, and oxidation of C2F4 excited species to give COF2.
The kinetics, mechanism, and thermodynamics of the thermal decomposition of polytetrafluoroethylene (PTFE) has been studied by a number of The results of these investigations can be briefly summarized by saying that in the temperature range from 360 to 600", the reaction is of the first order with tetrafluoroethylene being the primary product. The mechanism involves rupture of carbon-carbon bonds followed by the unzipping of the resultant radicals. Only a few of the cited references give analytical data for the composition of the gaseous products resulting from the depolymerization rea~tion.'-~J The products reported include CF4,C2F4,CaF6,C4F8, SiF4,C02, and CO. Of these, only C2F4 and C3F6 are confirmed by all investigators. In the present work, PTFE was decomposed in the presence of a microwave-excited glow discharge. Helium, helium plus oxygen, or oxygen was used as a carrier gas in the flow system. I n the absence of oxygen, the principal products were CzF4, CF4, CZFB, CaFs, C3F8, C, and polymer. I n the presence of oxygen, the reaction proceeds with the formation of COF2, CF4, C2F4, C2F6, C3F6, C3F8, CO, and C02. The evidence obtained supports a reaction mechanism involving CF2radical reactions.
Experimental Section Apparatus. The electrodeless discharge was sustained by means of a Type 2A cavity described by Fehsenfeld, Evenson, and Broida." Power was supplied from a PGM 10 Raytheon microwave power generator. The reaction cell is illustrated in Figure 1. The
PTFE samples consisted of 11-cm sections of heavy wall tubing (1.0 cm o.d., 0.6 cm i.d.) which were positioned in the 13-mm 0.d. Pyrex tube and held by the slip-fit connection illustrated. The microwave cavity was positioned axially around the tube in the position indicated by the two lines. The cell was connected to the apparatus via 12/30 glass joints. A schematic of the reaction system is shown in Figure 2. The helium was dried by means of P4OI0(A) and traces of oxygen were removed by passage over hot copper (B). The blow-off tubes (C) permitted the gases to be supplied to the mass flowmeters (D) at (1) 5. L. Madorsky, V. E. Hart, S. Straus, and V . A. Sedlak, J . Res. Natl. Bur. Std., 51, 327 (1953). (2) E. E. Lewis and M. A. Naylor, J . Am. Chem. Soc., 69, 1968 (1947). (3) J. C. Siegle, L. T . Muus, T.-P. Lin, and H. A. Larsen, J . PoZy?ner Sci., A2, 391 (1964). (4) R. E. Florin, L. A. Wall, D. W. Brown, L. A. Hymo, and J. D. Michaelsen, J . Res. Natl. Bur. Std., 53, 121 (1954). (5) R. E. Florin, M. S. Parker, and L. A. Wall, ibid., 70A, 115 (1966). (6) L. A. Wall and J. D. Michaelsen, ibid., 56, 27 (1956). (7) R. E. Kupel, M. Nolan, R. G. Keenan, &I.Hite, and L. D. Scheel, Anal. Chem., 36, 386 (1964). (8) B. Carroll and E. P. Manche, J . Appl. Polymer Sci., 9, 1895 (1965). (9) H. C. Anderson, MakromoE. Chem., 51, 233 (1962). (10) C. D. Doyle, J . Appl. Polymer Sci., 5 , 285 (1961). (11) C. L. Rosen and A. J. Melveger, J . Phys. Chem., 68, 1079 (1964). (12) H. L. Friedman, Technical Information Series No. R59SD385, Missile and Space Vehicle Department, General Electric Co., 1959. (13) W. M. D. Bryant, J . Polymer Sci., 56, 277 (1962). (14) F. C. Fehsenfeld, K . M. Evenson, and H. P. Broida, National Bureau of Standards Report No. 8701, U. S. Government Printing Office, Washington, D . C., 1964.
Volume 7 1 , Number 8 J u l y 1967
2672
ECKART MATHIASAND GLENNH. MILLER
U
Figure 2. Schematic of apparatus.
Figure 1. Reaction cell.
atmospheric pressure. Flow was controlled by means of Whitey 3RS4 stainless steel needle valves (E). All flow lines were made of 0.25-in. copper tubing with Swagelock brass connectors. The reaction cell was connected to the metal system via brass tapered joints, and flexibility was provided by a stainless steel sylphon insert (F). A modified Perkin-Elmer vpc valve (G) was used to direct reaction products into the column. Since it was impossible to maintain a good vacuum with this valve in the system, the valve was surrounded with a metal sleeve which was flushed with helium; thus only small amounts of helium entered the system and leakage of air was prevented. Samples for vpc analysis were collected in a trap (H) which had a volume of 12.58 ml. A copper trap (J) was used to protect the pump and to obtain large samples of reaction products. A Wallace and Tiernan model FA 160 absolute pressure gauge (Ilode1 154D vpc equipped with thermal conductivity detectors was used with a 4-m silica gel column. A helium flow of 84 cc/min and a temperature of 75" resulted in the retention times tabulated in Table I. The retention times of all the listed products were checked by using known samples, and the identity of all samples was confirmed by mass spectrographic analysis. Quantitative determinations were made with a Perkin-Elmer Model 194 digital The Journal of Physical Chemistry
Table I : Relative Retention Times for Reaction ProducW 0.53 0.64 1.00 3.00 3.62 3.86 9.94 20.15 32.14 41.08 Times are normalized to CF4 = 1.00. The C4 compounds were proved to be absent in the reaction products by mass spectrographic analysis.
integrator which gave 6000 counts/min for full scale deflection. The PTFE tubing was obtained from the R. S. Hughes Co. The helium was commercial grade 99.99% purity. The oxygen was obtained from The Linde Co. Procedwe. Prior to beginning a run, the apparatus was evacuated to a pressure of approximately 0.1 torr and degassed for at least 1 hr. The helium was then admitted and its flow (usually 20 cc/min) was regulated by a needle valve and measured by a mass flowmeter. Before applying the discharge, the cavity and reaction cell were cooled with a strong jet of Dry Ice cooled air which was left on throughout the experiment. The discharge was then initiated with a Tesla coil and maintained by the microwave power generator which was set at 70% power, equivalent to about 60 w radiofrequency.
DECOMPOSITION OF POLYTETRAFLUOROETHYLENE
2673
When the pressure of the system reached its maximum of about 11.6 torr, a sample of the degradation products was trapped in the liquid nitrogen cooled tube and transferred directly into the vpc apparatus.
Results The initial color of the discharge was pink, but this changed rapidly to a light blue. Immediately following the appearance of the blue color, the pressure of the system increased. As the degradation products accumulated, the color changed to a darker blue. The degradation of the PTFE proceeded very rapidly and the pressure of the system increased to approximately 11.5 torr and remained there for the duration of the experiment. Figure 3 shows a plot of the total reaction pressure us. time for an extended run in which only helium waa3 used as the carrier gas. The pressure at zero time corresponds to the time before the plasma turned light blue. 4 t the time indicated by the first vertical line of Figure 3, the PTFE had decomposed to such an extent that a hole appeared in the PTFE tube and the plasma came in contact with the glass. The reaction with the glass was very vigorous; the glass became hot, and the glass tube then imploded. The data reported in this paper, however, are for experiments in which the runs were stopped before the plasma had a chance to contact the glass. Table I1 shows the yields of the individual compounds Table 11: Reaction Products from the Glow Discharge Decomposition of PTFE (Data Are Reported as Mole Per Cent) He
0.37 6.43 85.22 :2.08 *5.90
He
+
0 2
02
02
0.31 1,77 1.23 11.22 3.32 70.60 4.62 6.93
0.22 0.64 0.97 9.24 5.99 72.22 5.10 5.62
0.85 3.97 1.96 16.10 12.10 52.62 5.39 7.01
43.62 ___ 4.43
26.63 3.03
109.42 16.20
produced during the degradation reaction. Column 1 gives the average mole fractions determined from six experiments in which only helium was used as the carrier gas. Good reproducibility was obtained. Columns 2, 3, and 4 give the yield data for experiments in which helium plus oxygen or just oxygen was used as the carrier gas. The oxygen ratio indicated in the bottom row shows the amount of oxygen in the carrier
I'
xx
0
20
40
80
60
100
120
140
Time, 8ec.
Figure 3. Plot of reaction pressure us. time.
gas before the start of a run compared to the amount present during the reaction. The CBFBcompound found in this work was hexafluoropropene and not the cyclic hexafluoropropane which has been reported from thermal degradation studies. When oxygen was added to the system, the oxygenated products CO, COZ, and COFz were obtained. I n addition, a minute peak at the relative retention time of 15.9 (scale of Table I) appeared. This was tentatively identified as CF3CF0 from the mass spectrum analysis. The highest molecular weight gaseous product found was CSF8. No higher C4 fluorocarbons were detected by either the vpc or mass spectrograph. A considerable amount of solid yellow polymer was formed in all runs where helium alone was the carrier gas. It was deposited in an area about 1 cm below the plasma. This polymer was 23.2% soluble in carbon tetrachloride. Free carbon was also deposited immediately below the plasma region and some was mixed with the solid polymer. No carbon was detected when oxygen was present in the carrier gas and only a very small amount of a white solid polymer appeared. Calculations of a material balance (helium as a carrier gas) failed to account for approximately 38% of the PTFE lost during the degradation. S o liquid polymer could be detected, however, and no additional polymer was found in any other location in the system. A possible explanation is the formation of a material that reacted with the silica gel of the column and was not detected.
Discussion The decomposition of PTFE in a glow discharge proceeds rapidly as a result of both thermally and radiation-induced reactions. The color of the initiating helium glow changes rapidly to a light blue and then Volume 71, .Yumber 8 July 1967
ECKART R I A T H I A S AND GLENNH.
2674
3IILLER
to a darker blue as the decomposition products become PTFE. Siegle, et aL13found small amounts of CF4 the species responsible for sustaining the discharge. in all of his experiments but reported the presence of The initial reaction step involves the formation of C3Fs only when the pyrolysis was carried out at 480" ~ free-radical fragments via random chain ~ l e a v a g e . ~ ~ ~ Junder autogenous pressures restricted to 35 torr (COz and Nz were also present). Kupel, et uL17studied the (CFZ), +RICFz. RzCFz. (1) cracking pattern of the complete reaction mixtures This is followed by depropagation which undoubtedly resulting from decompositions at temperatures as high occurs via CF? radical eliminati~n.'~ as 875". At this temperature they reported the presence of CzF5+ ion. The presence of CF4 in Siegle's RICF?. +R1. CF2 work could possibly be accounted for by an oxidation RzCF?. +Rz. CF2 reaction since SiF4 was also reported. The appearance (2) of these products in the present investigation can be hlost authors have considered the thermal depropagaattributed to either a higher reaction temperature tion step to be the elimination of CzF4monomer units. (plasma temperatures are equivalent to several If C2F4 fragments were the unzipped units, however, thousand degrees) or the presence of ionic species one would expect more complete oxidation and loss of which are absent in the straight thermal deconiposiCnFZn+? product yields when oxygen is added to the tion. The latter explanation is favored since the system. The reported inertness of CFz radicals toward presence of oxygen has no appreciable effect on their oxygen16 is consistent with our results and leads us synthesis during the reaction. to favor the CF? elimination mechanism. No carbon dioxide, carbon monoxide, or other oxyThe CF? radicals are produced in large concentragenated products were detected when PTFE was detions and the following reactions occur composed in the presence of helium as a carrier gas. The addition of oxygen to the system resulted in the CF2 CFz + CzF4 (3) formation of both COS,CO, and a trace of another oxyCF? +C F + F 2e(4) genated product tentatively identified as CF3CF0. eThe addition of oxygen stopped the formation of both eCzF4-+ C?F4+ 2e(5) carbon and polymer. There was no apparent loss of CF2 +CF+ F(6) any product other than a decrease in the CzF4yield. Reactions 4 and 5 are probably responsible for susWall and llfichaelsena found only SiF4, CO2, and CO taining the discharge. C2F4+ is undoubtedly a major when PTFE was decomposed in an oxygen atmosphere contributor due t u the low energy requirement of 10.3 at 457", and Kupel, et aL17recorded only very small ev . amounts of CzF4 at 670" and negligible quantities at Weisz17 reported that no reaction occurred between 875". The fact that CzF4 is still the predominant SiOz and decomposition products when CzF4 was subproduct in the glow discharge oxidation is probably jected to an electric discharge, and he concluded that due to the presence of large quantities of C2F4+ which neither F nor F2 was present in his system. I n this is less liable to oxidation than the excited species. work, we found that the reaction products react very Heicklen, Knight, and GreeneIg found that oxygen rapidly with SiOz and thus conclude that the species had no effect on the c-C3F6 yield during the mercuryF or v- is certainly present. These are probably photosensitized reaction of C2F4. They proposed a formed via reaction 6 and the free-radical reaction scheme involving the reaction of an electronically excited C2F4 molecule. CFz -+ C F F (7) CzF4 0 +2COFz (91 The presence of free carbon interspersed with the yellow-orange polymer deposit downstream of the reacSince the addition of oxygen to the glow discharge systion zone can best be explained by the reaction protem produced no apparent diminution in the amount of posed by ;\lastrangelo'* CnF2n+2 low molecular weight products, we are led to conclude that a similar mechanism must predominate 2CF +CF, C (8)
+
+ +
+
+ +
+ + + +
+
+
+
The compounds CF4, C2F6, and C3F8 were prominent reaction products found in the reported experiments; however, none of these was reported in the work of Illadorsky, et uLll Lewis and Naylor12 or Wall and 31ichaelsen6 on the straight thermal decomposition of The JOUTTUZ~ of Physical Chemistry
(15) L.A. Errede, J . Org. Chem., 27, 3425 (1962). (16) F.W.Dalby, J. Chem. Phys., 41, 2297 (1964). (17) P.B . Weisz, J . Phys. Chem., 59, 464 (1955). (18) 5. V. R.Mastrangelo, J , Am. Chem. Soc., 84, 1122 (1962). (19) J. Heicklen, V. Knight, and S. A. Greene, J. Chem. Phys., 42, 221 (1965).
DECOMPOSITION OF POLYTETRAFLUOROETHYLENE
here. The oxidation mechanism must be primarily a free-radical reaction. The oxygen (0, 02,excited) could also add to the C2F4 via CF3CFO as an intermediate CzFii
+ 0 +CF3CFO (CzF40)
(10)
followed by CF3CFO +COF2
+ CF2
(11)
The COF2 formed decomposes quantitatively on the chromatographic column to give 2COF2 ---) C02
+ CFd
(12)
The excess COa and the CO formed in the system result from the direct reaction of oxygen with the carbon formed from reaction 8. The addition of oxygen quenches the formation of high molecular weight polymers; hence, these polymers
2675
must form from reactions involving CzF4excited species rather than directly from CF2 additions. The following schemes account for all the observ* tions. (1) Depolymerization is initiated by random chain cleavage. (2) The unzipping occurs via CF2 radicals. (3) The discharge is sustained by C2F4+ and CF+ ion formation reactions. (4) The major gaseous product, CzF4, is formed from the combination of CF2 radicals. ( 5 ) Radical disproportionation reactions account for the presence of carbon and the F or F-. The latter are responsible for the low molecular weight saturated fluorocarbons produced. (6) Oxygen reacts with C2F4 excited species to give COF2. (7) Polymer formation occurs via C2F4 rather than CF2 addition reactions.
Acknowledgment. The authors are indebted to the National Science Foundation for its support of this work.
Volume 71 12'umber 8 J u l y 1967 ~
