Final Technical Report COMBUSTION KINETICS OF TETRAFLUOROETHYLENE Richard A. Matula Fluid Dynamics Laboratory Department of Mechanical Engineering The University of Michigan Supported by: AIR FORCE OFFICE OF SCIENTIFIC RESEARCH GRANT NO. AF-AFOSR- 1144-67 July 1968

TABLE OF CONTENTS Page LIST OF TABLES iv LIST OF FIGURES v ABSTRACT vi I. INTRODUCTION 1 II. ANALYTICAL DEVELOPMENTS 3 Ao Introduction 3 B. Experimental 3 C. Results and Discussion 4 III. PYROLYSIS OF C2F4 AND C3F6 10 A. C2F4 Pyrolysis 10 1. Introduction 10 2. Experimental 11 3. Results 13 4. Discussion 19 B. C3F6 Pyrolysis 23 1. Introduction 23 2. Experimental 23 3. Results and discussion 23 IV. PYROLYSIS OF CF20 30 A. Introduction 30 B. Experimental 30 C. Results and Discussion 31 V. PRELIMINARY C2F4 OXIDATION RESULTS 35 A. Introduction 35 B. Experimental 36 C. Results and Discussion 36 VI. REFERENCES 39 iii

LIST OF TABLES Table Page I. Relative retention volumes of several fluorocarbons on porapak type T 5 II. Relative retention volumes of several fluorocarbons on porapak type N 6 III. Relative retention volumes of several fluorocarbons on porapak types R and S IV. Relative retention volumes of several fluorocarbons on porapak type Q 8 V. Second order rate constart for tetrafluoroethylene pyrolysis at 725 K 15 VI. Average values of the first-order rate constant, k4 25 iv

LIST OF FIGURES Figure Page 1. Gas-solid chromatogram of fluorocarbons. 9 2. Arrhenius temperature dependence of k2. 16 3. Arrhenius temperature dependence of kh. 17 4. Arrhenius temperature dependence of k". 18 5. Arrhenius plot of k4 vs. 1/T. 26 6. Perfluorobutene-2 production in perfluoropropene pyrolysis. 27 7. Perfluoroisobutene production in perfluoropropene pyrolysis. 28 8. Arrhenius plot for kCF2O. 33 9. Arrhenius plot for kCo2. 34 10. Oxidation of C2F4. 38

ABSTRACT The pyrolysis of C2F4, C3F6, and CF20 have been studied in the temperature and pressure ranges 300-4550C, 25-760 torr; 550-675~C, 50-410 torr; and 330 -480~C, 25-600 torr, respectively. The rate equations and appropriate Arrhenius rate constants for these three reactions are reported. The oxidation kinetics of C2F4 are also being investigated and preliminary results in the temperature and pressure ranges 175-300~C and 25-200 torr are discussed. Finally the application of gas-solid chromatography techniques to the separation of low molecular weight fluorocarbons and the analysis of the C2F4 oxidation products are discussed. vi

I. INTRODUCTION In many practical applications the detailed flow fields about reentry vehicles can only be calculated if the kinetics of the flow field are known. The flow field chemistry may be particularly important when combustion reactions are possible. Frequently, the ablation products of reentry vehicles may, undergo exothermic reactions in the boundary layer or the wake, and therefore, the combustion kinetics of ablation products are important in determining the state of the flow field including the wake electron densities. Teflon (C2F4)n which is classified as a subliming ablator has been employed as an ablation material in a number of investigations.1-4 Under the action of the aerodynamic heating, the Teflon surface begins to depolymerize into the monomer (C2F4), that has a very high vapor pressure, after the surface reaches its ablation temperature. Under most conditions, the monomer flashes directly into the vapor phase without passing through the liquid state. The Teflon ablation is given by (C2F4) polymer (C2F4)vapor + 750 B/lbm (C2F4) (1) polymer vapor The C2F4 vapor may participate in the following combustion reaction in the boundary layer or vehicle wake (C2F4)vapor + 02 - 2CF20 + 3,200 B/lbm (C2F4). (2) vapor Comparing Eqs. (1) and (2) it is seen that the energy liberated by the combustion process is approximately four times the heat of ablation. Therefore, if the exothermic combustion reaction takes place to any extent in the boundary layer or wake, the flow field will be strongly influenced. The above discussion indicates that a general understanding of chemical kinetics of the C/F/O/N reaction system, including the oxidation of C2F4 and the pyrolysis and further reactions of the various oxidation products, is necessary before a complete understanding of the flow field in the region near a reentry vehicle using a Teflon ablation shield can be obtained. The purpose of this continuing research project is to study the thermal oxidation of C2F4 and the thermal stability of the various fluorocarbons and oxygenated fluorocarbons that may be important in this reaction system. During the past year, the thermal stability of C2F4 and C3F6 have been studied. Since CF20 is one of the primary oxygenated products of the C2F4 oxidation system,

its thermal stability has also been considered. The results of the preliminary C2F4 oxidation experiments are also reported. 2

II. ANALYTICAL DEVELOPMENTS A. INTRODUCTION During the course of these studies, gas chromatography techniques have been utilized as the primary analytical method for both the qualitative and quantitative determination of fluorocarbon mixtures. Therefore considerable effort has been expended to develop columns which can be employed to efficiently separate the various compounds of interest. The chromatographic separation of the major C2F4 oxidation products has been previously reported by Matula and co-workers.5,6 During the past year the relative retention times of a number of low molecular weight fluorocarbons on Poropak columns (Waters Associates, Inc.) have been determined and these results are reported below. B. EXPERIMENTAL An Aerograph model No. 202-B gas chromatograph employing a thermal conductivity detector was used for all of the separations. Mixtures were introduced into the gas chromatograph through a gas sampling valve used in conjunction with a 2 ml sample volume. The chromatograph was equipped with a linear temperature programmer which was capable of maintaining isothermal column operation in the temperature range 30 to 4000C. The fluorocarbons utilized in this study were obtained from a number of sources. The perfluoromethane (CF4), perfluoroethane (C2F6) and a mixture of cis- and trans- C4F8-2 were purchased from the Matheson Company, East Rutherford, New Jersey. The 2-trifluoromethylpropene (C4F1o), perfluorobutane (C4Fo1), perfluorobutadiene-l, 2 (C4F6), perfluorobutyne-2 (C4F6), perfluorocyclobutene (c-C4F6), perfluorocyclobutane (c-C4F8), perfluoropropane (C3F8), and perfluoropropene (C3F6) were purchased from Penninsular Chem-Research Inc., Gainesville, Florida. The perfluoroethylene (C2F4) was purchased from Columbia Organic Chemicals, Inc., Columbia, South Carolina, and the iso-C4F8 was produced by pyrolysing perfluoropropene at 700'C for 15 min in a Vycor reactor vessel. A number of variable length GSC columns were constructed by packing 1/4" O.D. copper tubing with 50/80 mesh Poropak (Waters Associate, Inc.). The separation capabilities of Types N, P, Q, R, S, and T Poropak were studied. Before final installation in the chromatgraph, each of the columns was heated to 200~C and purged with helium (60 ml/min) for 2 hr. The retention volumes of all compounds were determined from the analysis of both pure compounds and fluorocarbon mixtures that had been prepared in the laboratory. The separations were obtained by operating the columns isothermally in the temperature range 75 to 175~C while maintaining a constant helium carrier gas flow rate of 60 ml/min.

C. RESULTS AND DISCUSSION The relative retention volumes of the various fluorocarbon compounds as a function of column material, length and temperature are listed in Tables 1-4. All of these results are based on a helium carrier gas flow rate of 60 ml/min. If the relative retention volume of a compound is not listed in the tables the retention time was greater than 25 min, and a notation of n.a. implies that a compound was not tested. Poropak Type P does not effectively separate the compounds of interest and hence results for this column are not listed. A 10 ft column of Poropak Type T maintained at 150'C was found to be the most effective for the separation of a mixture containing air and a large number of low molecular weight fluorocarbons. A GSC chromatogram of a complex, gaseous fluorocarbon mixture obtained with the aid of a 10 ft, Poropak Type T column is shown in Fig. 1. The column temperature was maintained at 150~C and the separation was completed in approximately 17 min. The perfluorocyclobutane and perfluorobutane were not resolved on this column.

TABLE 1 RELATIVE RETENTION VOLUMES OF SEVERAL FLUOROCARBONS ON PORAPAK TYPE T (C2F6 = 1.00) 6 ft at 100~C 6 ft at 1500C 6 ft at 1750C 10 ft at 1500C 10 ft at 175~C Compound tm(a) RRV(b) tm RRV tm RRV tm RRV tm RRV Air 0.70 o.49 0.65 0.68 0.66 0.82 1.03 O.64 1.03 0.70 CF4 0.82 0.57 0.71 0.74 0.66 0.82 1.17 0.72 1.13 0.77 C2F6 1.44 1.00 0.96 1.00 0.81 1.00 1.62 1.00 1.47 1.00 C2F4 1.91 1.33 1.09 1.14 0.92 1.14 1.89 1.17 1.68 1.14 C3F8 3.53 2.45 1.55 1.62 1.19 1.47 2.73 1.67 2.21 1.50 CF3-C=C-CF3 5.91 4.10 1.92 2.00 1.539 1.72 3.51 2.17 2.75 1.87 C3F6 6.61 4.59 2.10 2.19 1.47 1.82 3.84 2.37 2.96 2.01 c-C4F8 10.6 7.36 2.90 3.02 1.94 2.39 5.40 3.33 3.90 2.65 k1 CF3-CF2-CF2-CF3 10.6 7.36 2.90 3.02 1.94 2.39 5.40 3-33 3 90 2.65 trans-C3F8-2 16.9 11.74 3.50 3.65 2.18 2.69 6.48 4.00 4.98 3.39 cis-C4F8-2 n.a. n.a. n.a. n.a. n.a. n.a. 7.30 4.51 n.a. n.a. c-C3F6 18.0 12.50 3.95 4.12 2.44 3-.01 7.45 4.60 5.02 3.42 CF2=CF-CF=CF2 22.3 15.49 4.55 3.74 2.73 3537 8.60 5.31 5.63 3.83 iso-C3F8 n.a. n.a. n.a. n.a. n.a. n.a. 10.4 6.42 n.a. n.a. CF3-C=CF2 --- --- 8.08 8.42 4.55 5.62 15.4 9.51 9.25 6.29 CF3 (a) tm = retention time in minutes to peak. (b) RRV = relative retention volume with respect to perfluoroethane. Helium Carrier Gas Flow Rate: 60 ml/min Column Material: 50/80 mesh

TABLE 2 RELATIVE RETENTION VOLUMES OF SEVERAL FLUOROCARBONS ON PORAPAK TYPE N (C2F6 = 1.00) 5 ft at 100~C 5 ft at 150~C 10 ft at 175~C Compound tm(a) RRV(b) tm RRV tm RRV Air o.60 0.49 0.58 0.76 1.17 0.81 CF4 0.71 0.58 0.58 0.76 1.17 0.81 C2F6 1.22 1.00 0.76 1.00 1.44 1.00 C2F4 1.56 1.27 0.86 1.12 1.62 1.12 C3F8 2.87 2.535 1.19 1.57 2.03 1.41 CF3-C=C-CF3 4.62 3.79 1.48 1.95 2.35 1.63 C3F6 4.99 4.09 1.57 2.07 2.95 2.05 c-C4F8 7.84 6.13 2.15 2.83 3.13 2.17 CF3-CF2-CF2-CF3 8.44 6.92 2.24 2.93 3.13 2.17 trans-C4D8-2 12.53 10.27 2.56 3537 3.46 2.40 cis-C4F8-2 n.a. n.a. n.a. n.a. 3.70 2.57 c-C4F6 12.53 10.27 2.83 3.72 3.90 2.71 CF2=CF-CF=CF2 16.2 13.28 3.29 4.33 4.35 3.02 iso-C4F8 n.a. n.a. n.a. n.a. 4.65 3.23 CF3-C=CF2 --- 5.90 7.76 7.32 4.o8 CF3 tm = retention time in minutes to peak. b)RRV = relative retention volume with respect to perfluoroethane. Hleium Carrier Gas Flow Rate: 60 mil/min Column Material: 50/80 mesh

TABLE 5 RELATIVE RETENTION VOLUMES OF SEVERAL FLUOROCARBONS ON PORAPAK TYPES R AND S (C2F6 = i.00) Type R Type R Type S TypeS 6 ft at 1000C R 6 ft at 15000 S 6 ft at 1000C s 6. ft at 1500C C ompoound t()RVb tm(a) RRV(b) tm RRV tm RRV tm RV Air 0.85 0.57 0.86 0.80 0.80 0.57 0.75 0.75 CF4 0.95 0.64 0.86 0.80 0.90 0.64 0.80 0.80 C2F6 1.48 1.00 1.07 1.00 1.41 1.00 1.00 1.00 C2F4 i.86 1.26 1.17 1.10 1.71 1.21 1.10 1.12 C3F8 3.00 2.03 1.54 1.44 2.98 2.11 1.42 1.42 CF3-C=C-CF3 4.30 2.91 1.81 1.69 2.98 2.11 1.42 1.42 C3F6 4.33 2.93 1.80 1.68 4.19 2.97 1.70 1.70 C-C4F8 6.88 4.65 2.59 2.23 6.90 4.89 2.50 2.50 CF3-CF2-CF2-CF3 7.56 5.11 2.57 2.40 7.65 5.41 2.45 2.45 trans-C4F8-2 8.80 5.95 2.57 2.40 8.8o 6.24 2.45 2.45 C-C4F6 9.62 6.50 2.87 2.68 9.98 7.08 2.70 2.70 CF2=CF-CF=CF2 12.50 8.31 5.51 3.09 12.5 8.86 3.20 5.20 CF3-C=CF2 --- --- 6.10 5.70 --- --- 5.69 5.69 CF3 (a)t = retention time in minutes to peak. Helium Carrier gas flow rate: 60 mi/mm (b)RRV relative retention volume with respect to perfluoroethane. Column Material: 50/80 mesh

TABLE 4 RELATIVE RETENTION VOLUMES OF SEVERAL FLUOROCARBONS ON PROAPAK TYPE Q (C2F6 = 1.00) 6 ft at 100~C 6 ft at 150~C Compound tm(a) RRV(b) tm RRV Air 0.79 0.52 0.83 0.75 CF4 0.90 0.60 0.85 0.77 C2F6 1.51 1.00 1.11 1.00 C2F4 1.74 1.15 1.22 1.10 C3F8 3.32 2.20 1.56 1.51 CF3-C=C-CF3 4.55 3.01 1.80 1.73 C3F6 4.20 2.78 1.95 1.76 c-C4F8 7.49 4.96 2.81 2.53 CF3-CF2-CF2-CF3 n.a. n.a. n.a. n.a. trans-C4F8-2 n.a. n.a. n.a. n.a. c-C4F6 n.a. n.a. n.a. n.a. CF2=CF-CF=CF2 n.a. n.a. n.a. n.a. CF3-C=CF2 n.a. n.a. n.a. n.a. CF3 (a) tm = retention time in minutes to peak. b)RRV = relative retention volume with respect to perfluoroethane. Helium Carrier gas flow rate: 60 ml/min Column Material: 50/80 mesh

c-C4F8 C2F4 air LLi CF -C=C-CFc z cr.-c cF L~d I,3g ro., rr ~~~~~~~~~C41- 2 ~=CF'C-C~C F2 LD U) ~~3 Li CF transF y C3F8 C4F8_2 CF2:CF-CFxCF. Cr_ LU CF3 C F5;-C:C F2 0 2 4 6 MINUTES 10 12 14 16 18 Fig. 1. Gas-solid chromatogram of fluorocarbons.

III. PYROLYSIS OF C2F4 AND C3F6 A. C2F4 PYROLYSIS 1. Introduction The vapor phase dimerization of tetrafluoroethylene (C2F4) in the temperature and pressure ranges 290-4700C and 100 to 700 torr have been studied by Lacher, Tompkin, and Park.7 Their experiments were conducted in a one-liter Pyrex vessel, and the kinetic results were based on total pressure measurements as a function of reaction time. The rate of consumption of C2F4 was found to be second order with respect to C2F4 and. the second order Arrhenius rate constant (k2) was reported to have an activation energy of 26.299 kcal/ mole and a frequency factor of 16.5 x 10 cc/mole-sec-1. Atkinson and co workers8'9 studied the decomposition of C2F4 at temperatures from 300 to 800~C. At temperatures below 600~C, they reported that the second order dimerization of C2F4 to octafluorocyclobutane (c-C4F8) and the first order back reaction were much faster than any of the other reactions which were taking place. In the temperature range 600 to 800~C hexafluoroethane (C2F6) and octafluorobutenes were also formed, and at temperatures above 800~C C2F6 and tars were the primary products. The dimerization of C2F4 in the temperature and pressure range 300-550'C and 200 to 550 torr respectively was studied in a static Pyre reactor. The kinetic data were based on the measurement of total pressure as a function of reaction time, and the second order Arrhenius rate constant (k2) for the dimerization of C2F4 in this temperature range was reported to have an activation energy of 25.4 kcal/mole and a frequency factor of 10.3 x 1010 cc/mole-sec-l. 10 Butler studied the first order thermal decomposition of c-C4F8 to C2F4 in the temperature range 360 to 560~C. These experiments were conducted in a one-liter Pyrex flask, and initial reactant pressures between 0.003 and 600 torr were considered. Based on the measured equilibrium composition in this system and the numerical value of the first order rate constant for the decomposition of c-C4F8, Butler calculated that the second order rate constant (k2) for the rate of decrease of C2F4 and an activation energy of 24.0 kcal/ mole and a frequency factor of 1010.4 cc/mole-sec-1. Butler also found that during the course of c-C4F8 decomposition that a slow parallel decomposition forming perfluoropropene (C3F6) occurred with a first order rate constant (k3). The Arrhenius parameters for k3 were determined to be 1017.2 sec-1 and 87.2 kcal/mole. Atkinson and Atkinson9 also showed that the formation of C3F6 from c-C4F8 is a first order reaction. Their experiments were conducted in a nickel pyrolysis tube and covered the temperature range 550 to 650~C. Based on these experiments, the first order rate constant (k3) was reported to have an activation energy of 79.0 kcal/mole and a frequency factor of 3.9 x lo6 Sec-l 10

11 Lifshitz et al., have studied the thermal decomposition of c-C4F8 behind reflected shock waves in a single-pulse shock tube over the temperature range 770 to 9300C. During the course of these experiments, shock waves were driven into highly diluted c-C4F8 argon mixtures and the initial partial pressures of c-C4F8 behind the reflected shock waves were approximately 7 mm. The rate of decomposition of c-C4F8 was found to be first order with respect to c-C4F8, and the first order Arrhenius rate constant (kl) was reported to have an activation energy of 74.300 kcal/mole and a frequency factor of 2.1 x 1016 sec.- These results can be used in conjunction with the known temperature dependence equilibrium constant for the c-C4F8, _2 C2F4 equilibrium system (i.e., see Ref. 10) to estimate k2. In order to evaluate the second order rate constant (k2) for the dimerization of C2F4 based on the experimental determination of total pressure as a function of reaction time, the previous investigators7,8 were forced to make certain assumptions. The present investigation was undertaken in order to determine three independent numerical values of the rate constant (k2). The three independent values of k2 were determined by simultaneously measuring the C2F4 concentration, the c-C4F8 concentration, and the total pressure as a function of reaction time. The concentrations of C2F4 and c-C4F8 were determined with the aid of GSC chromatography. The numerical values of k2 based on total pressure are compared directly to the values reported in Refs. 7 and 8, and the values of k2 based on the concentration measurements were used to check the assumptions which were made in order to determine k2 from the total pressure measurements. The present series of experiments were conducted in the temperature and pressure ranges 300-4550C and 25-760 torr. 2. EXPERIMENTAL The experiments were conducted in a cylindrical, Vycor reactor which was enclosed in a horizontal wire-wound furnace. The vessel was approximately 250 mm long and had a volume of 455 ml. Prior to instillation in the furnace the reactor was cleaned with a 5% HF-H20 solution. Power was supplied to the furnace from a commercially available temperature controller which was capable of controlling the temperature to within +0.50C over a period of several hours. In order to insure that temperature gradients along the furnace cavity were negligible, a manually controlled guard heater was installed at each end of the furnace cavity. The temperature of the reactor vessel was monitored by four chromel-alumel thermocouples which were placed in contact with the reactor wall and equally spaced along the longitudinal axis of the vessel. The sampling tube which extended to the geometric center of the vessel and the pump tube which was sealed flush with the reactor wall were both made of 6 mm Vycor tubing. Both the sampling and pump tubes were terminated outside of the furnace by 2 mm greaseless vacuum stopcocks. The dead volume between the furnace and the stopcocks was approximately 0.6 ml. Since large gas samples were extracted from the reactor, this small dead volume did not have a significant effect on the results. The experimental facilities utilized for these experiments are described in detail in a previous publication.6 11

An Aerograph model 202-B dual column, hot wire, thermal conductivity gas chromatograph was used to identify and quantitatively determine the gaseous products as a function of reaction time. A Beckman IR-10 infrared spectrophotometer with a spectral range 300-4000 cm-1 was used as a back up instrument for the identification of any species which escaped detection by the gas chromatograph. A 4 ft column of 50/80 mesh Poropak (Waters Associate, Inc.) Type N maintained at 100'C was used to separate the C2F4 pyrolysis products. The column was packed in 1/4 in. O.D. type 316 stainless steel tubing, and the helium carrier gas flow rate was maintained at 75 ml/min. Prior to final installation in the chromatograph, the column was activated by heating it to 200~C while purging with helium (75 ml/min) for 2 hr. The concentrations of the various products were determined by comparing the electrical output of the chromatograph from the unknown sample to the output from a calibration mixture of known component concentrations. The C2F4 used in this study was purchased from Columbia Organic Chemicals, Inc., Columbia, South Carolina, and it was stored in a steel cylinder as a liquified gas under its own vapor pressure of approximately 20 atm. at 200C. The manufacturer stabilized the liquid phase, by adding 1% by weight of alphapinene to the liquid. The supplier specified that the minimum purity of the gas phase was 99%. Subsequent gas chromatographic analysis of the C2F4 indicated that the major gas phase impurity was c-C4F8 and that traces of C02,CF4, and C2F6 were also present. The mole fraction of the c-C4F8 impurity was determined to be approximately 9 x 10-3. In order to determine if the rate of C2F4 pyrolysis was effected by residual inhibitor which may have been present in the gaseous C2F4 supplied from the cylinder, a number of preliminary experiments were conducted in which both purified C2F4 and C2F4 taken directly from the cylinder were pyrolysed. Purified C2F4 was obtained by withdrawing a sample of C2F4 from the cylinder and collecting that fraction of the sample which was volatile at -126~C and condensible at -196~C. Heicklen and Knight12 report that this purification technique yields C2F4 with less than 0.1% of any impurity. In all cases the experimental results were identical for both purified and cylinder C2F4. Therefore in all experiments the C2F4 was taken directly from the cylinder and used without further purification. The c-C4F8 and C3F6 used in these experiments were purchased from the Matheson Company, East Rutherford, New Jersey and Air Products and Chemicals, Inc., Allentown, Pennsylvania, respectively. Both of the gases had impurities of less than 1% and were used directly without further purification. Reactants and calibration mixtures were introduced into the reactor and gas chromatograph through a glass manifold equipped with greaseless vacuum stopcocks. A gas sampling valve was used in conjunction with a 2 ml sample volume to inject samples into the gas chromatograph. All pressure measurements were made with a Wallace and Tiernan Type 145 Precision Dial Manometer which has a range of O to 30 in. of Hg vacuum and a least count of 0.05 in. of Hg. A mechanical vacuum pump, vented through a standard laboratory fume hood, was capable of evacuating the system to a pressure of approximately 103 torr. 12

During any series of experiments, the reactants were introduced into the reactor, which was maintained at a controlled temperature, and the time dependence of the total pressure, and the concentration of both reactants and products was determined by withdrawing a large sample from the reactor at various reaction times and analyzing the sample with the aid of the gas chromatograph and the IR spectrophotometer. The experimental data at the lowest temperature were determined for reactions in which the extent of reaction, with respect to c-C4F8, ranged from approximately 10 to 35%. At the highest temperature the extent of reaction varied from approximately 20 to 75%. These data which were obtained with temperature, initial pressure, and reactant composition as independent variables were used for the evaluation of the necessary rate equations and the appropriate Arrhenius parameters. 3. RESULTS The results of previous investigators have indicated that the rate of C2F4 pyrolysis, in the temperature range 300-5500C, can be represented by C2F4] = -k2[C2F4]2 + kl[c-C4F8] (3) dt Butler has shown that if the temperature is less than 5000C the second term on the right-hand side of Eq. (3) is insignificant with respect to the first term. Therefore the rate expression for the pyrolysis of C2F4 at temperatures below 500~C can be represented by the equation d[C2F4 ]2 = -k2[CaF4]. (4) dt The integrated form of Eq. (4) yields the rate constant (k2) as a function of parameters which were experimentally determined k2 = CF ]- [C2F4 ] cc/mole-sec (5) where t is the reaction time in seconds and [C2F4]t and [C2F4J0 are the measured concentrations of C2F4 (moles/cc) at time t and t = 0 respectively. Assuming that the back reaction is insignificant and that the only important products are C2F4 and c-C4F8, Eq, (3) can be rewritten in terms of the c-C4F8 concentration. d[-C4F] = k2 (([C2F4] -2[C-C4F8]O)-2[C-C4F8]t)cc/mole-sec-l. (6) 13

The numerical value of the second order rate constant for C2F4 pyrolysis (k2) can be determined by integrating Eq. (6)., 1 g 2[c-C4Fs]t - [c-C4F8])0 cc/mole-sec k1 = 2 t [C2F4]o([C2F4]o-2([c-C4FIt [C-C4F' ]O) The numerical value of k2 as calculated from Eq. (7) should be equal to the value of k2 calculated from Eq. (5). However, since the two k2's are based on independent experimental measurements,the "prime" nomenclature is used to differentiated between the two experimental values of the rate constant k2. Since the stoichiometry of the reaction has been assumed and the only products are C2F4 and c-C4F8, the numerical value of k2 can also be determined by measuring the total pressure of the products as a function of time. The numerical value of k2 based on total pressure measurements is given the symbol k2 = RT(t)) -1CFo cc/mole-sec-. (8) 2 t (2Pt)-(PCaF4)o (PCaF o where R is the universal gas constant, T is the reaction temperature, Pt is the total system pressure at time t and Po is the initial pressure. The order of the C2F4 pyrolysis reaction was determined at 3650C by applying the half-life method. During the course of these experiments, the initial C2F4 pressure was varied from 740 to 175 torr, and the C2F4 half-life was approximately 2 x 103 sec when the initial C2F4 pressure was 175 torr. A least mean square fit to the data indicated that the order of reaction was 1.98, and hence for all practical purposes the rate equation for the pyrolysis of C2F4 is given by Eq. (4). Once the reaction was established to be second order, the pyrolysis of C2F4 was studied over the temperature and initial pressure ranges 300 to 450'C and 50 to 200 torr, respectively. The three independent rate constants given by Eqs. (5), (7), and (8) were calculated from the experimentally determined time dependence of the C2F4 concentration, c-C4F8 concentration and total pressure. The numerical values of these rate constants based on a series of experiments with a reaction temperature and initial C2F4 pressure of 4520C and 50.8 torr are listed in Table 5. Arrhenius plates of the three second order rate constants k2, k2, and k2' are given in Figs. 2-4. Each of these curves is based on 31 data points. The Arrhenius curves for k" as calculated from the results given in Refs. 7 and 8 are also given in Fig. 4. 14

TABLE 5 SECOND ORDER RATE CONSTANT FOR TETRAFLUOROETHYLENE PYROLYSIS AT 7250K t k2 k' k' (sec) (cc/mole- sec-1) 300 2240 1947 2520 300 2278 1904 2520 600 2379 1791 2380 600 2355 1865 2380 900 2295 1871 2250 900 2260 1886 2250 1200 2250 2502 1800 2381 2561 Average 2305 1877 2420 15

m 7 \. - - 6 X Y.\ 5 \ 4 3 I I I, I I 1.40 1.50 1.60 1.70 x o3, (K)' I Fig. 2. Arrhenius temperature dependence of k2. 16

8;s 7 I' 6 ' CJ 5 4 3\ 1.40 1.50 1.60 1.70 x 103 (~K)' T Fig. 3. Arrhenius temperature dependence of k;. 17

,,X jo aouapuadap Ganqa9aduIaX snrTuaGqI.V *' 'TI, i.(>to) ' 01 X OL.'1 09'1 OS' O V ',, S % 'S AOanJ.LS.I. N3$3:ad Z,

The Arrhenius expressions for the temperature dependence of the three rate constants obtained by a least-mean-squares fit of the experimental data are given by k2 = 1011.07 +.03 exp(-25635+~9) cc/mole-sec-1 (9) RT kt = 10o0.81. 07exp( 25,l4o+200) cc/mole-sec2l (10) RT k = 10ll.36 + 07exp(-26, 50+200)cc/molesec-. (11) RT The thermal decomposition of c-C4F8 was studied in the temperature and pressure ranges 452 to 552~C and 100 to 200 torr. These experiments were conducted in order to confirm the results of previous investigators concerning the relative rates of the second order production of c-C4F8 from C2F4 and the first order decomposition of c-C4F8. The results of these experiments confirmed the validity of the assumption that the term representing the first order back reaction in Eq. (3) is insignificant with respect to the contribution of the second order term for temperatures less than 460~C. It has been previously shown9'0 that both C2F4 and C3F6 are formed by parallel, unimolecular reactions during the course of c-C4F8 pyrolysis. These unimolecular reactions are given by c-C4F8 - 5 2 C2F4 (12) C-C4F8 3 i C3F6 + CF2. (13) 10 Butler has shown that both kl and k3 can be determined if k2 is known and the concentrations of c-C4F8, C2F4, and C3F6 are measured during the course of c-C4F8 pyrolysis. The numerical values of kl and k3 based on our limited c-C4F8 pyrolysis data at both 452 and 552~C are in reasonable agreement with the results given by Butler.10 4. DISCUSSION The pyrolysis of C2F4 has been studied in the temperature and pressure ranges 290 to 470~C and 100 to 700 torr. This reaction was shown to be second order with respect to the concentration of C2F4 and the numerical value of the second order rate constant based on three independent experimentl measurements has been determined. These results were obtained by simultaneously measuring the C2F4 concentration, the c-C4F8 concentration and the total pressure as a function of reaction time. 19

The temperature dependence of the second order rate constant (k2) based solely on total pressure measurements, has been previously reported in the literature.7'8 In the temperature range of interest, the values of k' reported in Refs. 7 and 8 vary by approximately 26%. The numerical values of k2 and k' evaluated in the present study are within approximately 5 to 12% of each other and they generally fall between the results of the previous investigators. However, the second order rate constant (k2) based on the measured c-C4F8 concentration, is approximately 20% lower than k2. A number of possible reasons for the variance between k2 and k2 and k2' were considered. An error analysis was made in order to determine if the variance in the rate constants was due to experimental errors. This analysis revealed that the expected deviation in the rate constants based on the estimated experimental errors were not large enough to account for the measured deviations. To experimental determination of the second order rate constants k2, k2, and k2' were based on two assumptions: (1) C2F4 and c-C4F8 were the only important products; (2) the first order rate of decomposition of c-C4F8 was negligible with respect to the second order rate of formation of c-C4F8. Careful gas chromatographic analysis of the reaction products indicated that no significant side reactions were occurring when the reaction temperature was less than or equal to 452~C. As discussed earlier, the validity of the second assumption was confirmed by studying the thermal decomposition of c-C4F8 at 4520C. The possible effects of any heterogeneous effects were also considered. The previous studies7,8 were conducted in Pyrex reactor vessels in which the surface to volume ratios were varied by a factor of 100 with no significant effect on the rate of reaction. The present investigation was conducted in a Vycor reactor with yet another surface to volume ratio. Since heterogeneous reactions are strongly influenced by both the reactor material and surface to volume ratio it was oncluded that the consistency of the results obtained by similar methods over a broad range of experimental conditions preclude any significant surface effects. The frequency factor associated with the second order rate constant k2 can be estimated from either simple kinetic theory considerations or the theory of absolute reaction rates. The molecular diameter of C2F4, which is required for the kinetic theory calculations, is assumed to be 5.12A.13 The molecular diameter reported in Ref. 13 was determined by correlating viscosity measurements with the aid of the Lennard-Jones(6-12) intermolecular potential energy function. The calculated frequency factor, based on kinetic theory, at 455~C is 1.35 x 1014 cc/mole-sec-l. This result is approximately one thousand times the experimentally determined frequency factor, and hence the steric factor for this reaction is approximately 10-3. Since C2F4 has a number of internal degrees of freedom, a steric factor of 10-3 is reasonable. The theory of absolute reaction rates can be employed to estimated the theoretical bimolecular rate constantlh 20

kT Q- -Eo/RT (14) kth - Q2C2F4 e (14) where k is Boltzmann s constant, h is Planck's constant, Eo is the activation energy, at absolute zero and Q+ and QC F are the partitfion functions of the activated complex and CaF4 respectivefy. The relationship between the experimental activation energy, Ea, and Eo is determined by equating the logarmithic differential of Eqo (14) to Ea/RT2. d(en k) ERT2 _ d(en kth) dT dT (1) Comb-ining Eqs. (14) and (15) with 'the temperature dependence of the rectant and act-ivated complex partition function yields xi xj Eo Ea - RT 9Xi -2 2 Xj - 1 (16) a ei"-l j e j-1 hcw x kT (17) where u is the molecular vibration wave numnber and xi and. xo are calculated. from the kvo wn vibrational wave numbers of the activated complex and C2F4 respectively. The experimental frequency factor can be estimated by combining the results of Eqs. (14) and (16). kT Q- x. 7 A = Q2F exp X. e - - - 11 (18) A Q C2Fc e 1 -1 ZX J-( The partition function for C2F4 is readily calculated with the aid of -the molecular parameters listed in the JANAF Tables.15 The partition functicon of the activated complex is calculated based on the assumption that it has the same geometrical arrangement as c-C4F8. The 23 fundamental vibrational frequencies of c-C4F8 are available in the literature.16 However, 'the numerical. valiue of a number of these frequencies are not known accurately. Electron diffractioin studiesl17 have been used t;o evaluate the spatial configuration of c-C, Fs, and -these results have indicated that edC4F8 is nonplanar. Therefore the symmetry number of this molecule is tw'o. The frequency factor, as calculated from Eq. (18) at 455~C, is approximately 1.5 x 107 cc/mole-sec-lQ This result is approximately 104 lower than the experimentally observed frequency factor. The statistical evaluation of the equilibrium constant for ~he reaction c-C4F8 ~ 2 C2F4 requires that the partition funct;ions of both C2F4 and c-C4F8 be kinown. Since the partition xncticns of both C2F4 and c-C4F8 had been evaluated at 455~C, the equilibrium constant of the reaetion mentioned above is 21

readily evaluated at 455~C. The calculated equilibrium constant is in excellent agreement with the experimental value,10 and hence the discrepency between the measured and calculated frequency factors can not be attributed to numerical details. Since the partition function of C2F4 is accurately known, the theoretical frequency factor can only approach the measured value if the partition function of the activated complex is increased by approximately 104 The rotational and translational contributions to Qc-C4F8 are relatively well known, and hence any increase in the total partition function would most probably have too come from vibrational contributions. The discrepency between the experimental and calculated frequency factors could be reduced significantly if the actual activated complex is geometrically similar to c-C4F8 and. is loosely bound. The loosely bound complex could have a number of low vibrational frequencies which would increase Q~ significantly. The hydrocarbon system analogous to the fluorocarbon system of interest has also been studied. c-C4H8 ) 2 C2H4 (19) The heat of reaction at- 4270C and the equilibrium constant at 4550C for the dissociation of cyclobutane, see Eq. (19), are 18.92 Kcal/mole and 7.43 x 10-2 moles/cc respectively.l8 The corresponding heats of reaction and equilibrium constant for the dissociation of c-C4F8, see Eq. (12), are 49.9'2 Kcal/mole and 4o12 x 10-11 moles/cc respectively.l18 These thermochemical data indicate that the c-C4F8 is considerably more stable than c-C4H8o The pyrolysis of C2H4 has been studied by a number of investigatorsl1-22 Since the pyrolysis of C2H4 is considerably more complex than the analogous fluorocarbon reaction, a direct comparison between the present results and the C2H4 system can not be made. The thermal decomposition of c-C4H8 has been investigated by Walters and co-workers23-25 and Pritchard and co-workerso26 The first-, order Arrhenius rate constant, ko, for the decomposition of c-C4F8 is reported to have a frequency factor of 1015.71 sec-1 and an activation energy of 62.8 Kcal/mole.l4 Butler10 has reported. that the frequency factor and acti.vati.on energy of the unimolecular rate constant for the decomposition of c-C4F8 are 1016.0 sec-1 and '74.3 Kcal/mole respectively. The two unimolecular frequency factors are in good, agreement. However, the activation energy for the perfluoro compound is 112 5 Kcal/mole higher than the analogous hydrocarbon. Since the c-C4F8 is more stable than c-C4H8 this result is reasonable. 22

B. C3F6 PYROLYSIS 1. Introduction Atkinson and Atkinson9 have studied the pyrolysis of perfluoropropene and perfluoroisobutene in the temperature ranges 600 to 675 and. 700 to '7500C respectively. They reported that their data could be represented by a reaction of order 1. 5. The Arrhenius rate content for this reaction was reported to have an activation energy of 53.5 Kcal/mole and a frequency factor of 7.2 x 1010 -1/2-molel/2-sec-1. The purpose of the present investigation was to study the thermal decomposition of perfluoropropene in the temperature and pressure ranges 550-675~C and 50-410 torr respectively. 2o Experimental The experiments were conducted in a cylindrical Vycor reactor vessel (250 mm long by 60 mm I.D.) which was maintained at a constant temperature by an electrically heated furnace. The reactor temperature, which was measured with the aid of four chromel-alumel thermocouples, was controlled to within ~1/2~C over a period of several, hours. An Aerograph model 202-B dual column, hot wire, thermal conductivity gas chromatography and a Beckman IR-10 infrared spectrophotometer with a spectral range 300-4000 cm-1 were used to identify and quarntitatively determine the gaseous products as a function of reaction time. A five foot column of 50/80 mesh Poropak Type N maintained at 1355C was used in conjunction with the gas chromatograph. The perfluoroporpene used in these experiments was purchased from Air Products and Chemicals, Inc. Gas chromatographic analysis of the C3Fe indicated that it had a minimum purity of 99.8%. 3. Results And Discussion The pyrolysis of perfluoropropene (C3F6) was studied over the temperature and ini-tial pressure ranges 552-6760C and 50-410 torr respectively. The order of the reaction with respect to perfluoropropene was determined by measuring the half-life of perfluoroporpene as a function of initial perfluoropropene concentration at 5990C. The half-lives for initial perfloropropene pressures of 51,102 and 204 torr were 70, 64 and 58 minutes respectively. Based on these data, the reaction order was calculated to be 1o 01. Therefore the rate equation for the pyrolysis of perfluoropropene can be represented by the first order expression r[r. k4[C3F6] (20j 25

The average values of the rate constant (k4) calculated from the integrated form of Eq. (20) as a function of initial pressure and temperature are given in Table 6. An Arrenius plot of the first order rate constant (k4) as determined from the data is given in Fig. 5. A least mean squares fit to the experimental data yields k = 18.08~+0.26 -4e7,2000+1, 000 -1 k4q = 10.8exp( RT ) I)sec (21) Gas chromatographic and infrared analysis of the reaction products indicated that perfluorobutene-2 and perfluoroisobutene were the major fluorocarbon reaction product s The perfluorobutene-2 was identified by its gas chromatographic retent ion time. Since pure perfluoroisobutene was not available for calibration of the gas chromatograph, the perfluoroisobutene in the reaction products was iderntified by its uniquely characteristic infrared absorption line at 10.05 microns.9 In addition to the above mentioned fluorocarbons traces of perfluorobutene-l were detected under certain operating conditions. The perfluorobutene-l1 was detected by both its characteristic retention time and its IR spectra. The concentration of perfluorobutene-2 as a function of reaction time for initial perfluoropropene pressures of 204 torr and various temperatures is shown in Fig. 6. In all cases the rate of perfluorobutene-2 production increased, and the order of the rate of production of perfluorobutene-2 with respect to the perfluoropropene concentration was between one and two., The concentratbion of perfluorolsobutene as a function of reaction time for initial perfluoropropene pressures of 204 torr and various temperatures is shown in Fig. 7. Since pure perfluoroisobutene was not available for calibration of the gas chromatograph, the quantitative results for this compound are based on the assumption that the thermal conductivity detector had the same sensitivity to perfluoroisobutene as the average of the perfluorobutene-2 and. octafluorocyclobutane sensitivities. The rate of production of perfluoroisobutene with respect to perfluoropropene also had an order between one and two. In addition to the fluorocarbons mentioned above, the gas phase reaction products contained carbon monoxide, carbon dioxide and silicon tetrafluoride. ButlerlO also noticed side reactions with the wall when he studied the pyrolysis of octafluorocyclobutane 'in a Pyrex vessel in the temperature range 360-560~C. In all cases these three compounds were significant products. A mass balance on ItJhe carbon, including all of the gaseous carbon containing compounds, at 650'C indicated that a small carbon mass loss of 10 to 2~5 occurred. This mass loss was attributed to the small flakes of white dust that condensed in the cooler parts of the system. The present experimental results indicate that the pyrolysis of perfluoropropene, in the temperature and initial pressure ranges 550-675~C and 50-410 torsr, can be represented by a first order reaction. When the pyrolysis of perfluorupropene was carried out in a nickel vessel iEn the same temperature 24

TABLE 6 AVERAGE VALUES OF THE FIRST-ORDER RATE CONSTANT, k4 T Initial Number of x Pressure Data Points (sec-1) (~C) (torr) 676 204 7 141 676 102 9 149 650 204 11 73.5 650 102 6 71.2 650 51 6 73.2 599 2o4 6 20.5 599 102 7 18 7 599 51 10 18.5 552 408 10 3.73 552 204 9 2.67

300 200 100 50 1. 1 0I 0 m 10 -- 1.0 L.I 1.2 1.3 x 103 K'-I Fig. 5. Arrhenius plot of k4 vs. 1/T. 26

3 599~ C 650~C 676~C 2 p t E c C /552 OC 0 0 30 60 90 120 t, min Fig. 6. Perfluorobutene-2 production in perfluoropropene pyrolysis.

650 OC 7 6 5P. = 204 torr h.o ~ ~ ~ I / ~~~~~~599 OC o 1 1 /+ Ob 676 OC -- I I I. 1 o I.! l52-C 0 30 60 90 120 t, min Fig. T. Perfluoroisobutene production in perfluoropropene pyrolysis.

range, Atkinson and Atkinson9 reported that their data could be represented by a reaction of order 1. 5. In their paper Atkinison and Atkinson9 do not specify the range of initial C3F6 concentrations which were considered. However, these authors reported that the perfluoropropene half-life was approximately 60 min when the reaction temperature and. initial C3F6 pressure were approximately 600~C and 400 torr respectively. As discussed earlier, the half-life of the perfluoropropene pyrolysis at 5990C determined in the present investigation varied between 58 and 70 minutes when the initial perfluoropropene pressure was varied from approximately 50 to 200 torr. A reaction of order 1. 5 requires that the half-life be approximately doubled when the initial reactant concentration is decreased by a factor of four. Considering these comments and. the fact that the half-life as measured by Atkinson and Atkinson9 is approximately the same as determined in this investigation it is reasonable to suggest that the pyrolysis of perfluoropropene is a first order reaction. 29

IV. PYROLYSIS OF CF20 Ao INTRODUCTION Both the thermodynamic and. kinetic properties of carbonyl fluoride, CFO, have been studied by several investigators. The heat of formation has been evaluated based on both experimental results27-29 and theoretical bond energy considerations.30 The equilibrium constant for the reaction 2 CF2QiCF4 + CO2 has been experimentally determined over the temperature range 300 to 11000C.29,31 Modica and La Graff2 have investigated the pyrolysis of CF20 in excess Ar behind reflected shock waves. Mixtures containing ~ CF20 and. 95 percent Ar were shock heated. to temperatures between 2430 and. 33300C at total pressures near 0.8 atmo Assuming that the rate law was given by -d.[CF2O] dt Ar= k [Ar][CF20] (22) the second order rate constant kAr was found. to be kAr = 4.29 x 1011 T1/2 exp(-55,575/RT) cc/mole-sec - (23) The purpose of the present investigation was to study the thermal decomposition of CF20 in the temperature and, pressure ranges 330-480C and 25-600 torr respectively. B. EXPERIMENTAL The experiments were conducted in a cylindrical fused quartz reactor vessel (170 mm long by 60 mm I.D. ) with a surface-to-volume ratio of approximately 0.76 cm-l. The reaction temperature, maintained by an electrically heated furnace, was measured with the aid of four chromel-alumel thermocouples. The temperature was controlled to within +1/20C. over a period of several hours. An Aerograph model 202-B dual column thermal conductivity gas chromatograph and a Beckman IR-10 infrared spectrophotometer were used to identify and quantitatively determine the gaseous products as a function of reaction time. A six foot composite column, consisting of two feet of 50/80 mesh Poropak Type T followed by four feet of 50/80 mesh Poropak Type N was used in conjunction with the gas chromatcograph.5 The CF20 used in this investigation was purchased from the Matheson Company, Inc. The manufacturer indicated that the minimum purit;y of the CF20 was 98 percent. However, subsequent analysis in our laboratory, using both gas chromatographic and infrared spectroscopic techniques, 3o

showed that the CF20O had only trace CO and CF4 inpurities, but the gas also had a CO2 impurity of approximately 10%. This CF20 was used without further purification, and in all cases the initial C02 impurity was taken into account. All of the experimental data were obtained with C02 production less than 50 percent of its equilibrium value. C. RESULTS AND DISCUSSION The pyrolysis of CF20 was studied in a fused quartz reactor vessel in the initial pressure temperature and pressure ranges 330-480~C and 25-600 torr respectively. The reaction was found to be heterogeneous and the major gas phase products were CF20, CO2 and. SiF4. The order of reaction was determined by using both the differential method and the method of integration. 33 Both the rate of production of CO2 and the rate of consumption of CF20 were found to be half-order with respect to the CF20 concentration. Thus the heteros geneous rate equation for the pyrolysis of carbonyl fluoride can be represented by the half-order expression -d[CF20] = k5[CF20]1/ (24) dt and similarly for the production of carbon dixoide d = k6[C20] (25) dte Arrhenius plots of the half-order rate constants k5 and k6 are given in Figs. 8 and 9. A least mean square fit to the experimental data yielded k5 = (10-2.21+0.23) exp[-14,360+140 IRT]( cc ) (26) sec-1 k6 = (10-1.41~0.22) exp[-16,6+680 -IRT](mole(/)) (27) Since silicon tetrafluoride (Sif4) was consistently detected as a reaction product, it is suggested that the pyrolysis of CF2O in a fused quartz reactor vessel is heterogeneous in nature. A possible reaction scheme, which accounts for the experimentally observed kinetics, is 31

CF20 wall FCO + R (28) R+CF20 - RF+FCO (29) wall FCO -- R+C02: K9 (30) 2FCO — d CF20+CO: K (31) 10 where R is an intermediate radical. When a steady-state analysis is performed on the R and FCO radicals, the pyrolysis of carbonyl fluoride and. the production of carbon dioxide are given by _d[CF20 2k7 1/2 1/2 d[C] -= -k9() [CF2] (32) dt k d[C02] 2k7 1/2 1/2 dt +k (ek=) [CF20] (33) dt k1o Inspection of the two rate Eqs. (32) and. (33), which were derived. based on the assumed mechanism, indicates that the two experimentally determined overall rate constants should. be equal. The least mean square results given in Eqs. (26) and (27) are in reasonable agreement, but the Arrhenius curve for k5 lies below the corresponding curve for k6. The numerical values of k5 and k6 are equal within the experimental errors at 367 and 413lC, and k5 tends to fall slightly below k6 at 425 and 462~C. However, even at the higher temperatures the average values of the two experimentally determines overall rate conr stants very by only approximately thirty percent. The proposed. reaction mechanism is consistent with the experimentally determinent reaction orders within the experimental error, the theoretical and experimental rate constants are also in agreement. 32

-14.5 --15.0 -15.5 ' -16.0\ -16.5 -- 1.40 1.45 1.50 1. 'I I 1.35 1.40 1.45 1.50 1.55 (I/T) 10O Fig. 8. Arrhenius plot for kCF0 33

-14.5 -0 -15. 0 -15.5 -c0 C -16.0 -16.5 1.35 1.40 1.45 1.50 155 (I/T) x I 0 Fig. 9. Arrhenius plot for kC Co

V. PRELIMINARY C2F4 OXIDATION RESULTS A. INTRODUCTION A number of authors have considered reaction systems involving both C2F4 and 02' Caglioti et al.,34 have studied the radiation induced oxidation of C2F4 at room temperature. Their experiments were carried out with 1/1 mixtures of C2F4/02 initially, at a total pressure of one atmosphere. The primary reaction products were shown to be: CF20, (CF2)20 and a liquid polyperoxide. The authors did not attempt to explain -the experimental data. Heicklen, et al.,35 have studied the mercury-photosensitized oxidation of C2F4 at room temperature. Their results led them to conclude that CF2 radials did not react to any significant extent with 02 molecules. They also concluded that the primary source of CF20 was a reaction between 02 and an electronically excited C2F4 molecule. Modica and LaGraf,06 have studied the high temperature oxidation of C2F4-02-Ar mixtures. These experiments were conducted in the temperature range 1225 to 2125~C, and the time dependence of the various product concentrations were determined with the aid of both TOF mass spectrometric and UV spectroscopic techniques. Their experimental results led them to suggest that the oxidation reaction was taking place between CF2 radicals and 02 molecules rather than as a direct molecular reaction between C2F4 and 02. In this temperature range the oxidation products included CO and CF4, and. the mass spectrometic data indicated that CF20 was not a reaction product. Bauer, Ho and Resler37 have studied the thermal oxidation of C2F4 in a single-pulse shock tube. Mixtures of C2F4 and 02, highly diluted in argon, were shock heated for periods of about 1.5 milliseconds to temperatures in the range 900-1725~C. Based on mass spectrometric analysis of the shocked mixtures, the major reaction products were found to include CF20, CO, CF4 and C3F6, and small amounts of C02 and C3F6 were also produced. A plausible mechanism for the oxidation reaction has also been suggested. The thermal explosion limits of C2F4/02 mixtures at both high and moderate pressures have been studied. 8-9 Heicklen and.ightO have considered the thermal oxidation of perfluoroolefins. Mixtures of 02 and C2F4 were heated in a flow system at temperatures from 225-7250C. In the C2F4-02 system, the major product was CF20 and smaller amounts of c-C3F6, C3F6 and c-C4F8 were also formed. The analytical measurements were made with the aid of an infrared spectrophotometer. Their results were not conclusive, but they have made a number of suggestions. They presume that the initiating step is C2F4 + 02 - CF202 + CF2 (34) 35

where reaction (34) may occur on the wall. It is further suggested. that the major product, CF20, is probably produced by a chain. CF2 + 02- CF202 (35) or CF2 + 02 — CF20 + 0 (36) and C2F4 + CF202 --- CF2 + 2CF20 (37) or C2F4 + 0 — ) CF2 + CF20 (38) Even though the extent of reaction was kept low the rate constants varied with contact time, and therefore no rate equations or Arrhenius rate constants were suggested by the authors. The thermal oxidation of C2F4 in the temperature range 150 to 3000C is presently under investigation. Preliminary experimental results are reported. in the subsequent paragraphs. B. EXPERIMENTAL The C2F4 oxidation studies were conducted. in the same basic facility that was employed for the pyrolysis of C2F4. Preliminary data have been obtained in the temperature and total pressure ranges 150-3000~C and. 25-250 torr respectively, and a number of C2F4/02 mixtures with C2F4 mole fractions from 0.1 to 0.9 have been studied. The gas phase reaction products have been analyzed with the aid of GSC chromatographic techniques, and. the various reaction products were separated with the aid of the columns described in Reference 6. C. RESULTS AND DISCUSSION In all cases, the major reaction product was found to be CF20. During a number of the experiments, small amounts of C02 and c-C3F6 were also mf

detected. A typical curve representing the concentration of the rectants and CF20 as a function of reaction time is shown in Fig. 10. These data correspond to an equimolar mixture of C2F4 and 02 reacting at a temperature and initial reactant pressure of 175~C and 100 mm of Hg. respectively. The shape of the CF20 curve indicates that there is an initial induction period during which the rate of production of CF20 is relatively low followed. by a relatively linear region. Finally the rate of production of CF20 decreases as the C2F4 and 02 are consumed. Since the rate of consumption of C2F4 is higher than the rate of consumption of 02, the simple stoichiometric reaction C2F4 + 02 — 2CF20 (39) is not obeyed. In order to estimate the relative composition of any unknown products, atomic carbon, atomic fluorine and atomic oxygen mass balances were evaluated. When the mass balances were evaluated for the data shown in Fig. 10, all of the oxygen was accounted for and. deficiencies in both carbon and fluorine were found in the gaseous products. Assuming that all of the carbon and, fluorine were combined in a single molecule the molecule must have the chemical formula (CF)n. The carbon and fluorine deficiencies were approximately 4. 7 and 10.5% at reaction times of 60 and 180 min respectively. The trace amounts of c-C3F6 in the reaction products wNere not included in the mass balances, but these contributions were not significant in any of the experiments. An attempt was made to correlate the kinetic data with the aid. of the following overall rate expressions d[CF20] k 1[C2F4]u [02 v (40) dt d[C2F4] - -kl2[C2F4] [O~2] (41) dt d0] - -kl3[C2F4]y [0]Z (42) dt The kinetic data were not ammenable to consistent analysis with the aid of Eqs. (34)-(36). The same general problem was encountered by Heicklen and Knight. At the present time, supplementary experiments are in progress, and additional data analysis techniques are under consideration. It is anticipated that more light will be shed on this reaction system in the future. 37

0.5 C2,4 + 0.5 02 20 T = 175 ~C Po -100mm Hg 18 16 13 CF20 114 ~~0~~~~~~~02 0 E F 1c ]2 -z 0 20 40 60 80 100 120 140 ISO 180 Fig. 10. Oxidation of C2F4.

VI. REFERENCES 1. Economos, C., ARS J., 1074, July, (1962). 2. Steg, L., and Lew, H., AGARD Hypersonic Conference TCEA, Rhode-St. Genese, Belgium, April 3-6, (1962). 35 Steg, L., ARS J., 815 September, (1960). 4. Adams, M., ARS J., 625, September, (1959). 5. Drennan, G. A. and Matula, R. A., J. Chromatogo, 34, 77 (1968). 6. Matula, R. A., Final Technical Report Air Force Office of Scientific Report, Grant No. AFOSR.1144-66, Univ. of Mich., September, (1967). 7. J. Lacher, G. Tompkin and J. Park, J. Am. Chem. Soc., 74, 1693 (1952). 8. B. Atkinson and A. Trenwith, J. Chem. Soc. (London) 2082 (1953). 9. B. Atkinson and V. Atkinson, J. Chem. Soc. (London) 2086 (1957). 10. J. N. Butler, J. Am. Chem. Soc., 84, 1393 (1962). 11. A. Lifshitz, H. F. Caroll and S. H. Bauer, J. Chem. Phys., 39, 1661 (1963). 12. J. Heicklen and V. Knight, U.S. Air Force Report No. SSD-TR-66-105 (June, 1966). 135 J. H. Simmons, Fluorine Chemistry, Vol. 5, Academic Press, New York, New York, 1964, p. 150. 14. S. Glasstone, K. J. Laidler, and. H. Eyring, The Theory of Rate Processes, McGraw-Hill Book Co., Inc., New York, N.Y. (1941). 15. W. H. Jones (Chairman), JANAF Interim Thermochemical Tables, Dow Chemical Company, Midland, Michigan (Sept. 30, 1964). 16. H. H. Claassen, J. Chem. Phys., 18, No. 4, 543 (1950). 17. H. P. Lemaire, R. L. Livingston, J. Chem. Phys., 18, No. 4, 569 (1950). 18. D. R. Stull, E. F. Westrum, Jr. and G. C. Sinke, The Chemical Thermodynamics of Organic Compounds, John Wiley and Sons, Inc. New York, New York (1968 in press). 9

19. F. Fischer and H. Pichler, Brennstoff-Chem., 13, 381, 406, 435 (1932). 20. R. E. Burke, B. G. B ldwin, and C. H. Whitacre, Ind. Engr. Chem. 29, 326 (1937). 21. E. F. Greene, R. L. Taylor and W. L. Patterson, Jr., J. Physo Chem., 62, 238 (1958). 22~ G. B. Skinner and E. M. Sukolski, J. Phys. Chem., 64, 1028 (1960). 23. F. Kern and W. D. Walters, Proc. Natl. Acad. Sci., 43, 937 (1952). 24. C. T. Genaux and W. D. Walters, J. Am. Chem. Soc., 73, 4497, (1951). 25. C. T. Genaux and W. D. Walters, J. Am. Chem. Soc., 75, 6196 (1953). 26. H. Pritchard, R. Sowden and A. Truaman-Dickenson, Proc. Roy. Soc. (London), A218, 416 (1953). 27. J. J. Ball, et. al., "Methane-Oxygen-Fluorine Flames; Spectral and Calorimetry Studies", U.S. Gov't. Research Reports, 31, 228, (1959). 28. H. C. Duus, Ind. Eng. Chem., 47, 1445, (1955). 29. S. C. Li, J. Chinese Chem. Soc., 11, 14, (1944). 30. P. Altman and M. Farber, Biblis, Tech. Reports, 12, 43 (1949). 31. O Ruff and S. C. Li, Z. Anorg. Allgem. Chem., 342, 272, (1939). 32. A. Modica and J. LaGraff, Tech. Memorandum, RAD-TM-65-29, AVCO Corp., Wilmington, Mass. (1965). 33. K. J. Laider, Chemical Kinetics, McGraw-Hill Book Co., New York, (1965)> 34. V. Caglioti, M. Lennzi and A. Mele, Nautre, 201, 610, (February 8, 1964). 35. J. Heicklen, V. Knight and S. Greene, J. Phys. Chem., 42, No. 1, 221 (1965)o 36. A. P. Monica and J. E. LaGraff, J. Phys. Chem., 43, No. 9, 3383 (1965). 37. S. H. Bauer, K. C. Hou and E. L. Resler Jr., "Single Pulse Shock Tube Stuides of the Pyrolysis of Fluorocarbons and the Oxidation of Perfluoroethylene,," 6th International Shock Tube Symposium (1967). 40

38. R. Kiyama, J. Osugi and S. Kusuhara, Rev., Phy. Chem. Japan, 27, 22 (1957). 39. H. Teranishi, "Studies of Explosions Under High Pressure IV," 28, 9, (1958)> 40. J. Heicklen, and V. Knight, "The Thermal Oxidation of Perfluoroolefins, " SSD-TR-65-120, Aerospace Corporation (July 1965). 41

Unclas sified Security Classification DOCUMENT CONTROL DATA -R&D (Security claesificatton of title, body of abatract and indexing annotation must be entered when the overall report is claasfled) 1 ORIGINATING ACTIVITY (Corporate author) 2. REPORT SECURITY C.LASSIFICATION The University of Michigan Unclassified Department of Mechanical Engineering 2. GROUP Ann Arbor, Michigan 3. REPORT TITLE COMBUSTION KINETICS OF TETRAFLUOROETIYLENE 4. DESCRIPTIVE NOTES (Typo of report and Inclusive dates) Final Technical Report S. AUTHOR(S) (Last name. first name, initial) Matula, Richard A. 6-. REPORT -DATE 7a. TOTAL NO. OF PACES 7b. NO. OF REFS July 1968 41 40 Ba. CONTRACT OR GRANT NO. 9a. ORIGINATOR'S REPORT NUMBER(S) AF-AFOSR- 1144-6T b. PROJECT NO. C. Sg. OTHER REPORT NO(S) (Any other numbers that may be aesigned Othi report) d. 1 0. A V A IL ABILITY/LIMITATION NOTICES Copies are available from the originator. 11. SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY Air Force Office of Scientific Research 13. ABSTRACT The pyrolysis of C2F4. C3F6, and CF20 have been studied in the temperature and pressure ranges 300-4550C, 25-760 torr; 550-675~C, 50-410 torr; and 330 -480~C, 25-600 torr, respectively. The rate equations and appropriate Arrhenius rate constants for these three reactions are reported. The oxidation kinetics of C2F4 are also being investigated and preliminary results in the temperature and pressure ranges 175-3000C and 25-200 torr are discussed. Finally the application of gas-solid chromatography techniques to the separation of low molecular weight fluorocarbons and the analysis of the C2F4 oxidation products are discussed. DD D*JAN64' 1473 Unclassified Security Classification

Unclassified Security Classification 14. LINK A LINK B LINK C KEY WORDS ROLE WT ROLE WT ROLE WT Tetrafluoroethylene Oxygen Perfluoropropene Carbonyl Fluorine Perfluorocyclobutane Reaction rates Gas phase reactions Reaction kinetics Fluorocarbon Pyrolysis Combustion kinetics Gas chromatography INSTRUCTIONS 1. ORIGINATING ACTIVITY: Enter the name and address imposed by security classification, using standard statements of the contractor, subcontractor, grantee, Department of De- such as: fense activity or other organization (corporate author) issuing (1) "Qualified requesters may obtain copies of this the report. report from DDC." 2a. REPORT SECUIKTY CLASSIFICATION: Enter the. over- (2) "Foreign announcement and dissemination of this all security classification of the report. Indicate whether report by DDC is not authorized. "Restricted Data" is included. Marking is to be in accordr ance with appropriate security regulations. (3) "U. S. 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UNIVERSITY OF MICHIGAN i 9 1111111111111111111111111 1 11111111 3 9015 03483 7529