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Thermal Degradation of Tetrafluoroethylene and Hydrofluoroethylene Polymers in a Vacuum!

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Thermal Degradation of Tetrafluoroethylene and Hydrofluoroethylene Polymers in a Vacuum! 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.
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