Journal of Research 01 the National Bureau of Standards Vol. 51, No.6, December 1953 Research Paper 2461

Thermal Degradation of Tetrafluoroethylene and Hydrofluoroethylene Polymers in a Vacuum!

S. L. Madorsky, v. E. Hart, S. Straus, and V. A. Sedlak *

Te fl on and tetrafluoroethylene photopolymers, on py rolysis in a vacuum at 423.5 0 to 513.00 C, yield almost J 00 perce nt of monomer. The rate of formation of monomer at a n.,· given temperat ure fo llows a first-order react ion and is independent of t he method of prepara­ tion of poly mer or its initial average molecular weight. The activation energy was deter­ mined by a pressure method and a weigh t method, and a value of 80.5 kcal was found b.'· both methods. A prelimina ry heating of Teflo n in air at 400 0 to 470 0 C did not change appreciably its rate of degradation into monomer when it was subse quently heated in a yacuum. Polyvinyl fluoride, 1,I-polyvin y li dene fluoride, and polytrifluoroethylene were pyrolyzed in t he range 372 0 to 5000 C. The volatiles consisted in all cases of HF and a wax-like material consisting of chain fragments of low volatility. Polyvinyl fluoride and polytrifl uoroethylene degrade to co mplete volatilization, whereas 1,1-poly vinyli dene fluoride becomes stabilized at about 70-percent loss of we ig ht. The rate-of-volat ilization curves indi­ cate a fi l'st-order reaction for polyvinyl fluo ri de, a zero-order reaction fo r t rifluoroethylene, alld an undeterm in ed order for 1,1-polyvinyli dene fluoride. The order of t hermal stabili ty for t hese poly mers, as com pared with polymethylene, is as follows: Poly vinyl f1uoride < polymeth,vlene < polytrifluoroethylene< ] . J -polyvi nylidene f1uoridc < polytetrafluoroeth ylene.

1. Introduction TAB I,E 1. Che mical analysis of hydTojluorocaTbon polymers

There is very little in t Il e literature on the therma l AnalysiS degradation of fluorocarbons, in general, or on Carbon Ilydrogen F luorine rr otal fluorocarbon polymers, ill par ticular. Swarts [1 ),2 fo u nd Rogers and Cady [2], and Steunenberg and Cady [3) ~'heo. , --- ~'~~0~1-- Theo· . pyrolyzed a number of low molecular weigh t fluoro­ rrtical I'ound re ti ea l Found retiral l' ound carbons in tbe presence of a glowing platinum filament. Lewis and Naylor [4l pyrolyzed polytet­ 0/,1 % % % % % % 41. 2 41. 0 99.5 0 0 Pol Y\'inyl flu ori de .. 52.2 52.0 0.0 6.5 rafluoroethylene at 600 and 700 C and at press ures Po l ~' vi ny l id e n c fluoride ...... 37.5 3i.4 3.2 3. 2 59.3 58.5 99. I varying from 5 to 760 mm. Hg. The volatiles consisted l'olytrifluoro ethyl· of C2F 4 , C3F 6 , and C4Fg . In this work a study was ene. ______29.0 29.7 1.5 1. 5 69.5 68. 0 99.2 made of the thermal degradation of a series of fluoropolymers to determine their r elative thermal 3. Apparatus and Experimental Procedure stability, the nature and relative amounts of the volatiles given off, and th e rates of thermal degrada­ The investigation of thermal d eCTradation of this tion. This series includes polytetrafluoroethylene series of polym ers was carried out along two lines. (Tefl on) [- C2F 4- ln , polyvinyl fluoride [- CzHaF- l., 1. Pyrolysis in a vacuum and .fj·actionation and analysis of the 1,1-polyvinylidene fluoride [- C H2F2- ln, and poly­ volatile pTodllcts. This procedure was followed for all of the z polymers, except Tefl on, using a Dewar-like molecular still , trifluoroethylene [- CzHF a- ln. which has been described in previous papers [6, 7). A 20- to 30-mg sample, either in solution or in fin ely divided from, was spread on a platinum t ray. Size of t he sample was limited so 2 . Materials Used as to prevent loss of materi a l by spattering during p yrolysis. The sample was first subjected t o a prelimi nary hea ting in a The polytetrafluoroe(;hylene was a commercial vacuum for 2 hr at about 1500 C in order to eliminate th e Teflon tape, 0.07 mm tbick. The polyvinyl fluoride solven t and adsorbed gases. It was t hen brought to the temperature of pyrolysis by heatin g fO l' 45 min, and this and the polyvinylidene fluoride were prepared by temperature was then maintain ed for 20 min. The following E . 1. du Pont de Nemours & Co. The polytri­ fractions were co ll ected: I, residue; II, a waxlike material, fluol'oethylene was prepared from the monomer by nonvolatile in a vacuum at room temperature ; III, a fractiOn photopolymerization at - 20 ° C in the presence of volat ile at room temperature; IV, a gaseous fraction noneOll­ densable at the temperat ure of li quid nitrogen. \Veights of all di-tert-bu tyl peroxide and then heated ovel'llight at four fractions were determined, and, in t he case of fractions 105° C .3 Analyses for C , H , and F in th e hydro­ III and IV, chemi cal composition was determined by means of fluorocarbon polymers are given in table 1. t he mass spectrometer. T o facilitate mass-spectrometric a nalysis, fraction III was further subdivided by distillation ' Present address: U. S. Public H ealth, A tlanta, Ga. into a li gh t fraction, lIlA, and a heavy fraction IlIB. For I ~rhig work was performed as a part of the research project on high-temperaLure­ T ef1 on, which requires a higher temperature of pyrolysis and r~si s tant pol ymers sponROred by the Ordnance Corps, D epartment of the Army. T he paper was presented at the !24th meeting of the American Chemical Socict)' , which yields almost 100 percent of monomer , a different type Polymer Chemistry Division, September 1953. of apparatus was used. , Figures in brackets in dicate the literature references at tbe end of this paper . 2. Rate oj volatilization oj polymers in a vacuum. This • The monomer and polymer were prepared by R. E. Florin and D. W . Brown, of the Polymer Structure Section of t),e National Bureau of Standards. The property was investigated in t he case of all poly mers by a monomer was prepared by the method of Park, Sharrah, and Lacher [51. weight method, and in the case of Teflon , also by a pressur c 327 method. The spring-balance apparatus u'sed in the weight Rates of volatilization of Teflon were determined from method is described in a previous paper [8]. The sample was the pressure developed in the calibrated volume. The limited to 5 to 8 mg in order to avoid loss by spattering during pressure was measured at time intervals by means of a multi­ the heating. It was placed in a platinum crucible, and the plying manometer, F, containing a low-vapor-pressure silicone ) crucible was suspended by means of a 3-mil tungsten wire from oil in the left arm of the manometer on top of a mercury a sensitive tungsten-spring balance, enclosed in a Pyrex column. The manometer was calibrated by means of a housing, which could be evacuated to about 10- 6 mm Hg. three-scale McLeod gage, reading to 25 mm Hg, with a The crucible was heated externally, and loss of weight of the precision of 0.2 percent. The scale on the multiplying sample was determined by observing a crossline on an exten­ manometer could be read with a precision of 0.02 mm. sion of the spring. Pressures up to 6 mm were measured. An apparatus for the study of rate of thermal degradation A sample of the fraction noncondensable at the temperature of Teflon by the pressure method is shown diagrammatically of liquid nitrogen, corresponding to fraction IV when t he in figure 1. In this figure, A is a quartz tube, 3 cm long, D ewar-like apparatus is used for pyrolysis, was obtained in 6-mm inside diameter, and 8-mm outside diameter, closed the following manner. Liquid nitrogen was placed around at one end. This tube fits into a larger quartz tube, B, trap L. The condensable material, corresponding to fraction closed at one end. Tube B is sealed to a Pyrex ground joint, III, condensed in this trap, while fraction IV remained sus­ C, by means of a graded seal. Samples weighing from 5 to pended in the space between pump E and stopcock D. 345 mg were used in this apparatus. If spattering occurred Stopcock H over tube G was closed, and the tube was sealed during pyrolysis, it would not result in loss of material in off at K. The contents of tube G were analyzed in the mass this apparatus. This apparatus was also used in t he study spectrometer. The weight of fraction IV was determined of pyrolysis of Teflon. from its pressure, total volume, and analysis. A cylindrical electric heater, not shown in the figure, was To obtain a sample of fraction III for analysis, the system designed to maintain a uniform temperature at its center for was evacuated while trap L was still immersed in liquid nitro­ a distance of about 4 em. This heater could be moved in gen. With stopcock D closed, fraction III was then allowed a fixed horizontal position, so that tube A fitted approxi­ to expand into the apparatus by removing liquid nitrogen mately in the center of the heater muffle. An a-c 110-v from around trap L. Fraction III was then sampled in tube current was fed to the heater through a voltage stabilizer G' by closing stopcock H' and placing liquid nitrogen around . J and was controlled by variable resistances. The temperature G' while the sample tube was sealed off at K'. Weight of was measured by means of a platinum versus platinum-10 fraction III was determined from its pressure, total volume percent rhodium thermocouple fixed permanently in the and analysis. muffle of the heater, so that when the heater was moved in In some experiments it was found expedient to collect the 1 position for pyrolysis, the junction of the t hermocouple entire fraction III and to weigh it. This was done by collect­ came in contact with the closed end of tube B. The tem­ ing it in one of the weighed tubes, M, provided with ground perature inside of tube A was calibrated by means of an joints, O. Liquid nitrogen was placed around the lower part additional platinum versus platinum-10 percent rhodium of M and held there until condensation was complete. The thermocouple placed temporarily inside of this t ube. There tube was then sealed at N, without melting it off, and sub­ was no appreciable temperature gradient throughout the sequently weighed. R epeated experiments showed a good length of t ube A. Temperature fluctuations of both thermo­ check between the two methods of determining total weight couples were within about + 0.5 deg C. of fraction III. Tube A, containing a weighed specimen of T eflon, was The residue, fraction I, was weighed in tube A. A wax-like placed in tube B. The apparatus was evacuated to about material, fraction II, appeared in some experiments as a de­ 10- 3 mm Hg by means of an oil pump, not shown in t he posit in the cold part of tube B, just outside the heater. The figure, and a liquid-nitrogen trap, P. Preheating of a sample weight of t his fraction was determined by subtracting the of Teflon in a vacuum at about 3000 to 3500 C for 48 hr sum of weights of fractions III and IV from the total loss of resulted in no appreciable loss in weight. The heater, which weight of the sample. In cases where fraction II did not had been maintained overnight at the operating temperature, appear, there was a good balance between total weight of all was then moved into position around tube B. It took 3 to 5 fractions and original weight of sample. min for the temperature of t he sample to reach a constant value. At the termination of an experiment, the heater was 4 . Pyrolysis of Teflon removed quickly from the apparatus. During pyrolysis t he apparatus was cut off from the Results of pyrolysis of Teflon are shown in table 2. evacuation pump by means of stopcock D , while a mercury­ This table gives also, in the last column, the results diffusion pump, E, remained in operation. This pump was of studies of rates of thermal degradation. The ex­ effective in removing the volatiles from the pyrolysis zone, against a back pressure of 25 mm. The volume between periments were carried out in the pressure apparatus this pump and stopcock D was calibrated. shown in figure 1 and are marked P in column 1 of

D K------K A C B p 0 0 N- N

L M ' M G1 Go F E FIGURE 1. Apparatus for the study of Tates of thermal degradation oj polymers by the p1·essure method. 328 T A BLE 2. R'C perimental data on thennal degradation of Teflon by the p1'eSSW'e and weight methods 100 90 A./ POLYVINYL,;" Method /'r e mpcra~ Duration Weight of T otal loss FLUORIDE Hate of vola· 80 lV .L employed ' ture sa mple in wei ght tilizatioll v-{'0LYTRIFLUORO- ETHYLENE 70 L ~ ,==! °0 min mq % %/min / { p 423. 5 337 305.7 0.5 0.00152 60 / /' f P 434.5 252 345. 3 . 9 .00368 / V P 450.0 332 99. 7 4.4 .0136 50 Lt L '--- P 454.0 108 210.5 1.8 . 0170 TEFLO~ P 474. 5 82 57.4 6.4 .0806 40 I-- / ! POLYVINYLIOENE W 480.0 420 tf J.. FLUORIDE 6. 45 46. 7 .U8 30 VL JL P 480.5 142 49.6 17.6 .1364 W 490. 0 360 7. Ol 61. 8 .240 496.5 20 L L 1 L P 56 140.3 19.6 .3887 6 P 499. 0 220 7.73 63.4 .465 1 ~OLYMETHY~ ~ 10 / W 500.0 445 7.05 93.1 . 490 ~ Iif" W 510.0 230 6.93 91. 5 .952 o U P 510.5 149. 5.58 76. 0 .956 370 390 410 430 450 470 490 510 530 P 513. 0 173 5.25 90.1 1. 254 TEMPERATURE. ·C FIGUR E 2. R elative thennal stability of Teflon, polymethylene • P and W refer to experiments ca rried out by the pressure and weight methods, respectively. and hydrojl1i01'oethylen e polymers. table 2. Ther e was no wax-like (fraction II) deposit in any of these experimen ts. Fraction I V, on th e TABLE 3. P yrolysis of Tejlon in co nsecutive 60-minute steps , average, amoun ted to 0.1 mole percen t of the total and mass-sp ectrometer analysis of the volatile products volatilized par t and consisted of CO. Thus fraction Cumu· Fractions based III is practically equal to total loss in weigh t given lative on volatilized Mass·spectrometer analysis of fractions III in column 5 of table 2. T eflon retains it original volatil· part for each step and IV co mbined, for each step 'r e m ~ izution shape until about 50 percen t of volatilization . pera- based B eyond this point it softens and slumps. ture on or i ~ina l II I n IV C2F4 C3F, SiF, CO, co A method of comparing the thermal stability of a sam ple number of polym ers is described in previous papers ------°c % % % % Mole % Mole % Mole % Mole % Mole % [7 , 9]. In this work the thermal tability of T efl on 504 23.2 0 99.97 0.03 96.8 2.9 0 0.2 0.11 is shown in figure 2, compared with that of hydro­ 509 47. 9 0 99.98 .02 96.9 2.7 .1 .2 . 07 517 71. 4 0 99.95 . 05 96.0 3.0 .4 .4 . 18 fluorocarbon polymers and polymethylene. In the 53 93.7 8 91. 73 .27 86.8 6.4 3.2 2.6 1. 03 case of T eflon, the time required to heat a sample to the operating temperature was only 3 to 5 min as compared with 45 min fo r the oth er polym ers that were pyrolyzed in the D ewar-like apparatus. In the last experiment, at 513.0° C, where volatiliza­ In order to put the thermal-stability curve for T eflon tion was 90.1 pOl'cen t, the weigh t of the sample was on a comparable basis with the other curves, it was small ; and, if a fraction II appeared, it was too sm all moved 15° C to the left. This adjustmen t was made to be detected. on the basis of experiments with polystyrene and R esults of mass-spectrometer analysis are also polytrifluoroethylene in the Dewar-like apparatus shown in table 3. The CO shown in this table ap­ and the pressure apparatus. peared as fraction IV, and the r est appeared as frac­ In one experim en t a sample of about 100 mg of tion III. As seen from this table, composition of the T eflon was hea ted in foul' consecutive steps to almost volatiles did not vary up to at least 7l.4 percent of complete volatilization . In view of the large size volatilization. The SiF4, CO2, and CO migh t have of the sample, the volatiles were condensed during resulted from oxygen in the polym er 0 1' , most likely, pyrolysis in tm p Q, fi gure 1, by m eans of liquid from a reaction between T efl on and quar tz [10]. nitrogen to insure their complete removal from the hot zone. H owever, when collecting samples for weighing or for analysis, or when evacuating the 5. Rates of Volatilization of Teflon apparatus between steps, trap Q was maintained at room temperature. After each step , samples from 5.1. Determination by the weight method fractions III and IV were collected for analysis in tubes G and G', respectively (fig. 1). Total weight of E xperimental data on rate experiments with fraction III was ob tained by collecting it in on e of T eflon tape in the spring-balance apparatus are the tubes M an d weighing. Allowance was m ade for shown in table 2. The weigh t experiments are the amount of fraction III collected in tube G. marked W in the first column of this table. In R esults of the step-experiment are hown in table 3. figure 3 the rates are plotted in percen tages of the Fraction II appears only in the last step, where the original sample per minute against percentages vol­ cumulative volatilization was 93.7 percent . In all atilized. By extrapolating th e straight lines to zero the pressure experimen ts shown in table 2, except volatilized, th e initial rates ar e obtained . These the last one, m aximum volatilization was 76 percent, initial rates are shown in the last column of table 2 and fraction II was not observed in these experiments. (for experimen ts m arked W ). 329 1.0 9 W cr ::> , , (J" --oO 8 , ~ ~ A>-- ::l'" .9 7 cr , , k .cr--<:r- Q. , .0-- , , fa-- 6 :s , .8 lD, .. <5 -- '" 5 ffi" >- " ~ 4 ~ .; .7 V ~ 0 P< 3 w i 0 ' . "" !;j Cf '<...... cr ...J w 2 ::> ...6 ~ () p ':l z ~ .1 o , 8"" ~ + w .d - 0 ::> ~ .5 \ ~ 0 :::; inw ----,.~ ~. -.l ~ .1 cr ~ ,0 ... .J ~ , ~- .2 0 ~ .4 gC> I~ o p" .3 o... ~ ~ o 20 40 60 80 100 120 140 160 180 ~ .3 TIME FROM START. MINUTES cr ~ ---- ~ kl FIGURE 4. Rate of thermal degradation of T eflon at 513. 0° C •2 1'\ I (J """0- r-o--o ~ by the pressure method . 480' ~ ~ •...... • Time versus pressure plot; __• time versus log of residue plot. .1 -_\ ~~ ~ v -v ~ the following experiment. A 1.3-mm cube of o 0 o 10 20 30 40 50 60 70 80 90 100 Teflon was pyrolyzed at 514.5 for 133 min. Total PERCENTAGE VOLATILIZED volatilization was 78.3 percent, and the rate was the same as for tape Teflon pyrolyzed under similar FIGURE 3. Rate of thermal degradation of Teflon by the weight method, as a f unction of percentage of voiati l-i zation. conditions. In another experiment, a 1.9-mm cube of Teflon was pyrolyzed at 495°. Initially, the rate was slow, but it soon reached the value for tape 5.2. Determination by the pressure method Teflon. Some photopolymers of tetrafluoroethylene were Inasmuch as Teflon yields, on pyrolysis, mainly prepared [12] by irradiating the monomer with the monomer and a small per centage of other prod­ ultraviolet light in the presence of (CFa)2Hg, benzoyl ucts, all in constant proportion and volatile at room peroxide, CFaI, and di-tert-butyl peroxide as cata­ temperature, the rate of volatilization can be ob­ lysts; also in the absence of a catalyst. On pyrolysis tained from a curve in which pressure of the gas in a in a vacuum, these polymers had the same rates of fixed volume is plotted against time. Results of a volatilization as the tape Teflon. series of rate experiments in the pressure apparatus The activation energy is obtained by plotting are shown in the last column of table 2. An example logarithms of rate versus inverse of absolute tempera­ of a pressure-time curve is shown by the curved line ture. This plot is shown in figure 5 for the weight in figure 4 for a rate experiment made at 513° C. and the pressure experiments. All the points fall on This experiment is shown in table 2 as the last a straight line. Multiplying the slope of this line by experiment of the series. 2.303R, where R is the gas constant, 1.987 cal/deg, a The condition for a first-order reaction is that the value of 80.5 kcal is obtained , which is the activation plot of In(a-x), where (a -x) is the residue, against energy for the thermal degradation of Teflon. The time, t, is a straight line [11] . In such a plot, rate of frequency factor, A , as calculated by means of reaction, k, is the slope of the straight line, and can Arrhenius' equation, has a value of 4.7 X I0 18 , when thus be evaluated. The straigh t line in figure 4 was rates are expressed in fraction per second. obtained by plotting log of residue, calculated in terms of pressure, against time for the 513.0° experi­ 6. Pyrolysis of Polyvinyl Fluoride, Polyviny­ ment. Using such curves as shown in figure 4, the lidene Fluoride, and Polytrifluoroethylene rates shown in the last column of table 2 were ob­ tained for the experiments marked P. Pyrolysis of these polymers was carried out in the To determine the effect of heating Teflon in air on D ewar-like molecular still. Experimental details rates of volatilization, samples were heated in air at are shown in table 4. The gaseous fraction, IV, 400°, 425°, and 450° C. The amounts volatilized amounted in all cases to less than 0.1 percent of the were about the same as in a vacuum. When the total volatilized part. Mass-spectrometer analysis samples were heated at 470 °, the amount volatilized showed it to consist of hydrogen and carbon monox­ was slightly greater in air than in a vacuum. The ide. The less volatile fraction, III, was found on residues from air-heated samples were pyrolyzed in a mass-spectrometer analysis to consist, in the cases of vacuum. The behavior of these residues, with polyvinyl and polyvinylidene fluorides, mainly of regard to rate and products of volatilization, was SiF4, H20 , and some unidentified hydrofluorocarbon about the same as for original T eflon. fragments. In the case of polytrifluoroethylene, The effect of thickness of the T eflon specimen on this fraction, in addition to SiF4 and H 20 , contained rate of volatilization in a vacuum was determined by some CO!. 330 TE MPERATURE : C under co nditions of vacuum and insufficient amount 520 510 500 490 480 470 4 60 4 50 440 430 .4 of H 20 , hydrofluorosilicic acid tend to decompose .2 back into HF and SiF4• Pyrex also contains ome + B 20 3 , which might react with HF, but there was no o evidence in th e mass-spectrometer analysis of the .2 presence of BF3 . Aluminum oxide, ordinarily pres­ .4 ent in the glass, does not react appreciably 'with HF ~ ,6 at room temperature. ~ The CO 2 found in fraction III from polytrifiuol"O­ ~ .8 -' ethylene could not be explained on the basis of a ~ 1.0 -'o reaction between fluorocarbon and Si02 of the glass. > 1.2 "- According to White and Rice [10] such a reaction o 1.4 w takes place at elevated temperatuJ"es, and in this case ~ 1.6 the polymer was in contact during pyrolysis with "- o 1. 8 platinum, and the polymer fragments were in contact with glass at low temperatures. It is possible that '" 2.0 g the polytrifluoroethylene contained some oxygen as 2.2 a part of its structure. This oxygen, on reacting 2.4 with the polymer, could give CO2, No analysis fol' 2 .6 oxygen in the original polymer was made because of a nal~ ' s i s 2.8 the difficulty involved in such in the presence 3.0 '--_--'-_----'-__'---_--'-_---'- __'--_--'-_---'-_ ---' of fluorine. The fluorocarbon tends to react with 1.26 1.28 1.30 1.32 1.34 1.36 1.38 lA O 1.02 1.44 the glass container at the elevated temperature LOG liT ,,03 employed in analysis, and CO 2 r es ults from such a FIGURE 5. A ctivatl:on energy slope fo?' the th ermal degradat-ion reaction. The CO detected in fraction. IV from of 'Teflon. pyrolysis of hydrofluorocarbon polymers could come e, W eigh t m ethod; 0 , pressure method. from oxygen as an impurity. Fraction III was calculated in thc case of all three polymers to HF on the basi of SiF4 content in the TABLJ, 4 . PyrolysIS of hydTO.fluoromrbon polY lll eI's " volatiles. This fraction is shown in the last two columns of table 4 in weight percent of the total FraC'tio n III ( II F) volatilized part and in weight percent of sample. 'W eigh t Fraction II consisted of a nonvolatile light-brown E xpcr il1l('11L of 'I'('m"ol ' vO ;~~\ ~~d sa lllple ntU!'l' part Oasf'd on IBa sed on wax-like deposit, soluble in acetone. Because frac­ volatilir,cd I original tion IV was only about 0.1 percent, fraction II, in P:1rt sa III pIc percentage of total volatilized part, can be taken as P oIY" in,'l flu ori de the difl'e l" en ce between 100 and the percentage of fraction Ill, in terms of volatilized part, given in my 0 C % % % table 4. :\'[ass-speetrometer analysis of fraction III l. .. _ .. _--_.------12. 8 :li 2 16.5 49.7 8.2 2 ...... 21. 8 :385 40. 7 37. 3 15.2 indicated the presence of hydrofluorocarbon mole­ 3 ...... 26.7 400 66.4 28.2 18. i cules of molecular weight up to 150. However, in 4 --+. -_. ------_. 18. R 420 9 ~. 'I 5 . 0_, __ .' _ _ __ ----- 26.2 180 95.:; 27.5 26. 2 calculations of HF as fraction III, these hydrofluoro­ ------. carbon fragments were included in fraction II be­ J> o l~·\' inylid(lll c fiuo l' icll" cause their natu1"C could not be identified. Judging ------_._,------~ from results of pyrolysis of other polymers, in the 26. I 380 3. 7 0 0 21. ;J 433 J5.2 41. 4 G.3 I temperature range of 370 to 430 C [7, 9], fraction L::::::::::::::::::I 19. 6 444 47. (i 51. 5 24.5 29. G 456 65.5 II from the hydrofluoropolymers should have an 29.2 484 67.5 average molec ular weight of about 600 to 700. The t ::::::::::::::::::1 27. 1 530 71. 0 48. 0 34 .2 residue, fraction I , from all three polymers appeared Polytrilluo roethylene light brown during the early stages of degradation and dark brown toward the end. In the case of 1...... 23. 7 '100 27.4 8. 0 9.0 polyvinylidene fluoride, the residue, above 50 per­ 2 ...... 25. 5 415 56.5 5. 3 J2.3 :1...... 17. 1 425 94. <\ cent of volatilization, appeared black. 4 ..•...... 23.5 432 97. 7 .5. 7 23. 0 5. ______. ______As can be seen from table 4, polyvinyl fluoride and 34. 8 475 98. 7 polytrifluoro ethylene volatilize almost 100 percent.

a. Duration of oac h,cxperimcnt- 45 minutes of heat ing LO pyrolysis temperature, Polyvinyliclene fluoride, however , seems to become fo llo wed by 30 minutes of pyrolysis at th is tom perature. stabilized at around 65 percent of volatilization. vVhen the temperature of pyrolysis was raised from The SiF4 and H 20 arc due to a reaction between 456 0 to 530 0 C, the additional loss i.n weight was only HF, resulting from pyrolysis, and Si0 2 in the glass 5.5 percent. In one experiment in the pressure apparatus. This reaction can take place at room apparatus a 69.3-mg sample of polyvinylidene fluo­ temperatme in the presence of a trace of H 20 . For­ ride was heated in a vacuum from room temperature mation of H 2SiF6 from HF and SiF4 is possible, but, to 650 0 C in 80 min. Total loss by volatilization was 331 ~ . 8 ,---,---,---,---,---,--,,--,---,---,---, ffi 60 I I VV- :i ~ ~~~~ i o" "- 50 5 · 6~--~--~~~~~--~~~~--~--~--~ lIV~ )~ "­ ... ~vt., z "I ~ .4 1---+-1----+...., ~ 4 0 V N.. .. :::; :! V !;i .2 ~ 30 ... o..J o V ...> .. ~ L--o o " 20 • ~ 10 20 30 4 0 50 60 70 80 90 100 o / W ~~n a: PERCENTAGE VO LATILIZED N --0 ::J 10 I---~/ t u- FIGURE 7. Rates of thermal degradation of hydrofluoroethylene ..J --- polymers, as a function of percentage of volatilization . ~ 0 o 10 20 3 0 40 50 60 70 8 0 90 100 PERCENTAGE VOLATILIZED tion resembles that of a first-order reaction, whereas FIGU RE 6. Yield of HF, in percentage of total available HF in the polymer, as a function of total percentage of volatiliza­ the polytrifluoroethylene curve resembles that of a tion from the polymer. zero-order reaction, at least in the range 25 to 80 percent of volatilization. The rate curve for poly­ vinylidene fluoride is conditioned primarily by the 76.1 percent. The residue was further h eated in the stabilization effect, above 50 percent of volatilization. apparatus for 30 min by means of a MeleeI' burner, applied to the outer tube B (fig. 1). Additional loss in weigh t was l.5 percent of the original sample. 8. Discussion of Results The residue resembled coke in hardness and appear­ ance. In another experiment a 78.9-m g sample of 8 .1. Teflon polyvinylidene fluoride was heated gradually in the same apparatus from room temperature to 500 0 In considering the mechanism of thermal degrada­ during 2 hI' and then kept at 500 0 for 1 hr. Loss of tion of T eflon, the following experimental facts should weight by volatilization was 70.3 percent. Chemical be borne in mind: The monomer yield is almost 100 analysis showed that the residue still contained l.7 percent; the material softens and slumps above about percent of Hand 12.6 percent of F by weight. 50 percen t of volatilization; the reaction is of first R elative thermal stability curves for the three order; and the rate of volatilization is very likely in­ hydrofluorocarbon polymers are shown in figure 2, dependent of chain length , because T eflon, tetra­ in comparison with similar curves for Teflon and fiuoroethylene photopolymers, and air-heated Teflon polymethylene. The polymethylene curve is based all had the same rates of volatilization and the same on pyrolysis experiments made in the Dewar-like volatile products. apparatus at five temperatures. E xperimental con­ On the basis of these facts, a mechanism involving ditions were about the same as in the case of the unzipping of monomer units at free-radical ends of hydrofluorocarbon polymers. The polymethylene chains is assumed. The absence of fraction II would was a nonbranched, high-molecular weight polymer seem to indicate that the kinetic chain length is very of the same stock that was used by Mandelkern and long, so that once unzipping is initiated, it continues associates [13] in their study of intrinsic viscosity. in most cases to the end of the molecule. -Initiation Loss of HF from the three hydrofluorocarbon may take place when free radicals form either through polymers during pyrolysis, in percentage of available breaking off of foreign elements, or groups of ele­ HF in the polymer, is plotted against percentage of ments, at the ehain ends, or when a break occurs in volatilization in figure 6. the chain due to thermal agitation. Such thermal breaks are more likely to occur in the long chains than in the short chains, and result in free-radical 7. Rates of Volatilization of Hydrofluoro­ chain ends. The general trend during pyrolysis is car bon Polymers in a Vacuum for the average chain length to become shorter, as indicated by the fact that the r esidue softens at about In view of the complex nature of the thermal 50 percent volatilization. On the other hand, the degradation of polyvinyl fluoride, polyvinylidene shorter the chains, the greater will be the tendency fluoride, and polytrifluoroethylene, involving split­ for their recombination at their free-radical ends. ting off of HF, in addition to scissions of th e chain, activation energies would be of little significance. 8 .2. Hydrofluorocarbon polymers However, the shape of the rate curves might be of interest as r evealing details of the mechanism of The substitution of one or more hydrogen atome thermal degradation. One rate curve for each of for fluorine on the chain changes radically the nature these polymers is shown in figure 7. The important of the polymer. Unlike polytetrafluoroethylene, none aspect of these curves is not their relative rate, for of the hydrofluorocarbon polymers yield any appre­ this is shown in figure 2, but their shape. The poly­ ciable amount of monomer. Instead of monomer, vinvl-fluoride curve beyond 19 percent of volatiliza- the volatiles consist of HF and chain fragments of 332 Yariolls sizes. The foll O\\" ing m echanism is suggested. 9. References When HF breaks oIr, a double-bond forms in the chain at that point. A break in the chain may then [1] F. Swarts, Bu!. soc. chim. Belg. 42, 114 (1933). [2] G. C. Roger and G. H. Cady, J . Am . Chcm. Soc. 73, occur at a C- C bond in ,a-position to this double 3513 (1952). bond. This break, as in polyethylene, results in one [3] R. K. Steunenberg and G. H. Cady, J . Am . Chcm. Soc. end of the break becoming saturated and the other 14, 4165 (1952). end forming a double bone1. In poly vinylidene [4] E. E. Lewis and H. A. Naylor, J. Am. Chcm. Soc. 69, 1968 (1947). fluoride, where there is more available HF, double [5] J. D . P ark, M. L. Sharrah, and J. R. Lachcr, J . Am . bonds will form in the chain at an accelerated rate Chem. Soc. 71, 2339 (1949). until they appear in conjugated posi tion. This [6] S. L. Madorsky and S. Straus, J . Rescarch NBS 40, causes a lesser degree of chain scissions and the chain 41 7 (1948) RP1886; Ind. Eng. Chem. 40,848 (1948). [7] S. L . Madorsky, S. Straus, D. Thompson, ann L. becomes stabilized. The yield of fraction III (HF) , Williamson, J . Research NBS 42, 499 (1949) RP1989; in percentage of volatilized part or in percentage of J . P olymer Sci. 4, 639 (1949). sample (table 4), is greater from polyvinylidene [8] s. L. Madorsky, J. Polymer Sci. 9, ]33 (1952). fluoride than from polyvinyl fl~lOride , which is to be [9] S. Straus and S. L. Madorsky, J . Research NBS 50, 165 (1953) RP2405. expected. However, in percentage of total available [10] L. White, Jr., and O. K . Rice, J. Am. Chem. Soc. 69, HF on the chains, the yield of HF is abou t the same 267 (1947). (fig. 6). . [11] S. Glasstone, Text book of physical chemistry, 2d ed., In the case of polytriflu oroethylene, the amount of p. 1046 (D. Van Nostrand Co., New York, N. Y., 1946). [12] R. E. Florin, L. A. Wall, D . W. Brown, and L. Hymo HF liberated is small as compared with polyvinyl (paper in preparation). and polyvinylidene fluorides (fig. 6). It seems [13] L. Mandelkern, M. H ellmann, D . W. Brown, D. E. that an abundance of fluorine on the polymer chain Roberts, and F. A. Quinn, Jr., J . Am. Chcm. Soc. 75, is less favorable to splitting off of HF than a similar 696 (1953). [14] Donald Drucsedow and C. F. Gibbs, Symposium on abundance of hydrogen. Discoloration of fraction Polymer D cgradation Mcchanisms, National Bureau II and of the residue in all three polymers is probably of Standards, 1953. due to runs of conjugated double bonds, the same as was fo und in Lhe pyrolysis of polyvinyl chloride [14]. Discoloration was found more pronounced in the case of polyvinyliclene fluoride than in the cases of the other two hydrofl uorocarbon polymers. IVASHING'l'ON, July 23, 1953.

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