Radiolysis of CF,-C,F,
Mixtures
William C. Askew1and Thomas M. Reed I l l 2 Department of Chemical Engineering, University of Florida, Gainesville, Fla. 5,9601
Mixtures of CF4 with C2F6were exposed to cobalt-60 gamma rays. Mixtures were also irradiated b y neutrons and gamma rays in the Low Intensity Training Reactor at Qak Ridge. These materials were converted to mixtures of higher molecular-weight perfluoroalkanes containing no unsaturated molecules. The predominant structures in the products were C3F8 and branched-chain perfluoroalkanes, i-CaFIO, i-CbFl?, 2,3(CF3)&F8. Chain-transfer processes and termination by radical combination were the predominant processes in perfluorocarbon radiolysis.
s o m e of the processes which can occur in pure C2F6 (e.g., carbon-carbon bond rupture and fluorine atom abstraction) do not occur in pure CFI under ionizing radiation. The interaction of the two systems of processes may be studied by the radiolysis of CF4-C2F6 mixtures. The results elucidate some of the details of the radiolysis reactions. Experimental
CF4-CzF6 mixtures viere prepared in a vacuum system, and qamples of approximatrly 0.5 gram were sealed under vacuum in pure aluminum tubing. The starting materials were chromatographically pure. Water and carbon dioxide were removed by pascing the gases through magnesium perchlorate and ascarite, and air was removed by freeze-pump-thaw cycles in a vacuum system. Samples mere irradiated a t 120'F for one week in the Low Intensity Training Reactor (LITR) a t Oak Ridge, in which the neutron flus had the following energy distribution: thermal neutrons, 3.46 X 1013 n/cmz sec; 3 meV neutrons (by 5 6 ~ to , Co), 3.48 X 10l2 n/cm2 see; and 4 meV neutrons (by Fe to X n ) , 1.78 X 10l2n/cmz see. Other samples were exposed to gamma irradiation in a cobalt-60 source which has been previously characterized (Askew et al., 1968). The absorbed dose rate was approximately 3000 rad/ min as measured by a benzenewater dosimeter solution. Analyses of products were obtained by gas-liquid chromatography (-\skew et al., 1968). The total original sample weight was recovered from the irradiated sample tubes as volatile material.
The presence of a small amount ( l O ~ o ) of C2F6initially in the mixture reverses the signs of these G values; there is a net gain of CF4 (G = +0.6) and a net loss of C2F6(G = -0.9). As the initial ratio of C2F6to CF4 in the mixture is increased, the G for C F I decreases to negative values, whereas that' for C2F6 remains essentially constant a t G = -0.8 to -0.9 up to 60 or 70 mol % C2F6. At this composition the CF4 production begins to rise again, and the C2Fsin the irradiated material falls rapidly to G = - 4 for initially pure C2F6. The sum of the G values for CF4 and C2F6is linear in initial mole fraction, always negative, and extrapolat'es to zero as t'he initial C2F6 mole fraction goes to zero. The trend in G values for CF,, C2F6, and their sum est,rapolates to t,he respective values observed in initially 100% CzFs, but they do not extrapolate to the observed values in initially iO070 C,F,. It thus appears that the processes which occur in the mixtures also occur in initially pure CrF6but not, in pure C'F,. Radiolysis of CF, produces CF,' and F' radicals
-
CF,
1 Present address, Chemical Engineering Department, Auburn University, Auburn, Ala. 36830. T o whom correspondence should be addressed.
F.
(f123.9kcal)
(1)
These radicals can form C2Fs, F2,or CFI. Parenthetical numbers are enthalpy changes calculated from values of Bryant' (1962). When C2F6 is present, a t,ransfer reaction occurs with the formation of C2Fj' and regeneration of CF,:
+ C2F6
CF3'
+
C2F5'+ CF,
-
(-0.6 kcal)
(2)
which leads to C3F8 and n-C4Floby chain termination steps:
+ CF3'
Results and Discussion
G values (molecules/100 eV absorbed) of the products from the cobalt-60 gamma-ray irradiations are plotted in Figures 1 and 2 vs. mol yo of C2Fs in the original mixtures. The G values were calculated from the weight of each species found in the radiolysis products. Weight percentages reported in the analysis are estimated to be correct to better than i l % of the value reported, and G values are estimated to be accurate to i . l O ~ o , . Complete tabulations of analyses are given in Reed (1969). The G values for CF, and CLF6in the radiolysis products from the mixtures are shown in Figure 1. Pure CF4 is depleted (G= -1.1) in gamma rays to produce C2F6 (G = +0.2).
+
CFJ'
C2F5'
C3F8 (-86.3 kcal)
2C2F5'+ n- C4F10 (- 81.5 kcal)
(3) (4)
K h e n C2F6is present, chain initiation steps in addition to Reaction 1 are C,F, C,F,
-
2CF;
(f91.1kcal)
C2F5'
+
F'
(f123.3kcal)
(5)
(6)
The net result of Reaction 5 may also be obtained by an energy transfer from excited perfluoromethane CFI': CF,
CF," C2F6*
+ -
CF4*
CZF6
CF4
f
C~FG*
2CF3'
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 4, 1972
447
0
I 0
I
I
I
I
60 80 20 40 INITIAL MOLE % C2F6
Figure 1. G for CFd and for
c2F6
I
100
20 40 60 80 INITIAL MOLE Yo C2Fs
100
yo
Figure 3. CF4 and CZF6 wt in products from CFrCzF6 mixtures after 1 week in Low Intensity Training Reactor 0 From CF4 in contact with aluminum fllings, 1 week in LlTR
vs. % CZFS
0.8
IO-
5K (3
(3
09 0 0 20
40 60 80 3 INITIALMOLE % C2F6
0.1
0
20 40 60 80 INITIAL MOLE % C2F6
I1
yo
Figure 2. G value of primary products vs. mole of c2F6in mixture from cobalt-60 iroriginal sample CFd-czF6 radiations
Such a process is supported by the substantial negative G values for C2F6in dilute mixtures. Reactions 5 or 7, in which 2CF3' radicals are produced for each Cd?6 consumed, together with Reaction 2 in which one CF4 is produced for each CF,' consumed, are suppmted by the observed net production of CF4 in mivtures dilute in C2F6. As the initial concentration of CzF6 is increased, CF,' 1s taken up by CzF5'to produce C3Fs (Reaction 3), resulting in negative G values for CF, when the initial composition is in the range 30-9070 C2F6. Apparently in this composition range, a good deal of the CFs' radicals originate from radiolysis of CF4, Reaction 1, rather than from radiolysis of CzF6, Reaction 5 . The latter source of CF,' would not result in a net disappearance of CF,. Of the total G = -0.9 for C2F6 disappearance a t 509.', initial C2F6, 0.7 appears in products containing C?Fj radicals [C3F8, C4Fla, C3Fa0, (C2F5)20].Thus, 448
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 4 , 1972
Figure 4. Radiolysis products of cF4-c2F6 mixtures with straight-chain structures after 1 week in Low Intensity Training Reactor X From CFd in contact with aluminum filings, 1 week in LlTR 0 From C*F6,4 weeks in LlTR
only G = -0.2 corresponds to C2F6 disappearance by radiolytic carbon-carbon bond scission of Reaction 5 and/or 7 . (Oxygen in the perfluorocarbon ethers derives from radical reaction \vith aluminum oxide a t the tube wall. Total G for ethers is approvimately 0.2.) As the initial CzFs concentration is increased above 70 mol yo,the initiation of chains by Reaction 1 becomes less important than initiation by Reactions 5 and 6. The chaintransfer Reaction 2 results in a n increase in G for CF, and a precipitous decrease in G for C2F6 as 100% CzF6 is approached. In 1 0 0 ~ CzF6 o there are the two initiation processes: namely, Reactions 5 and 6. Each CF, produced in the chain-transfer Reaction 2 from CF,' obtained by Reaction 5 should correspond to a product containing a CzF;' radical [CIFI,
C4F10, C3Fs0, (CzF&O]. The excess of G for these products over that for CF, gives G for CzF6 disappearing and producing C Z F ~by ' the initiation process (Reaction 6). This excess was essentially zero, indicating that in pure CzF6 the initiation process in radiolysis is Reaction 5. Mixtures of CF4 and CzF6 were exposed in the L I T R to large doses (1 week) of unknown amounts of radiation. According to the above discussion of the results from known doses, we would expect that large doses would deplete CzF6 by the radiolysis processes 5 and 6 and the transfer process 2 and further that the last would maintain a high level of CF4 in radiolysis product. The products from the LITR exposures bear out this expectation as shown in Figure 3. I n the mixtures under radiation, CF3' radicals are ubiquitous and chain-transfer processes occur frequently. C3F8 and n-C4F10, formed by termination Reactions 3 and 4, enter chain-transfer reactions with CF3' and CzFs'. The more stable C3 and Cd radicals formed in these transfer reactions are the secondary radicals (Bryant, 1962). For example, the heats of the following reactions are typical for the abstraction of F from C3Fs:
+ C3Fs CF3' + C3Fs CZFS'+ C3Fs C Z F ~+ ' C3Fs
CF3'
+
+ CF, CF3CFCF3 + CF, CFSCFzCFz' + CzFs CF3CFCF3 + CzFe
CF3CFzCFz'
+ + +
(- 0.6 kcal)
(- 14.0 kcal)
"'m 0 .I
0
INITIAL MOLE % %Fe
Figure 5. Radiolysis products of cF4-czF6 mixtures with branched structures after 1 week in Low Intensity Training Reactor
(+ 0.5 kcal) (- 12.9 kcal)
Relative heats of reaction fo1loLv a similar pattern for primary and secondary C4F9' radicals obtained by abstraction of F from n-C4FI0in transfer reactions. This preference for secondary radicals predicts that the preponderance of molecules formed by termination reactions involving C3 and Ca radicals will be branched chain structures. Comparing the product analyses summarized by Figures 4 and 5 shows that i-C4F10, i-CdF12, and branched C6F14isomers are produced in considerably greater quantities than are the normal structures. Conclusions
This investigation of radiolysis in CF4-CzF6 mixtures offers strong support for the importance of chain-transfer processes in irradiation of fluorocarbons. The results interpreted in
terms of chain transfer are consistent with the treatment of Bryant (1962) for the thermodynamics of perfluorocarbons and their radicals. Although Bryant's heats of formation show that termination by disproportionation (in which two radicals form one perfluoroalkane and one perfluoroalkene) is exothermic, no unsaturated molecules were produced by radiolysis. Termination by formation of a single perfluoroalkane molecule is still more exothermic and excludes disproportionation. literature Cited
Askew, W. C., Reed, T. M., Mailen, J. C., Radiat. Res., 33, 28291 (1968).
Bryant, W. M ,D., J.Polym. Sci., 56,277-96 (1962). Reed, T. M ,U.S. At. Energy Comm. Rep. 0R0-3632-2, 1969. RECEIVED for review June 9, 1972 ACCEPTEDSeptember 8, 1972
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No.
4, 1972 449
