I
NDESTRIAL applications of the rapidly groxTing chem-
istry of fluorine are achieving prominence ( 7 ) . Some of these developments-for example, the use of chlorofluorohydrocarbons as refrigerants, of anhydrous hydrofluoric acid as an alkylation catalyst in the petroleum industry, and of dichlorodifluoromethane as a propellant in insecticidal bombs-have been widely publicized. Less well known is the development of fluorine-cont,aining high polymers (29). The first of these polymeric products to reach the commercial stage is poly tetrafluoroethylene, in which all of the atoms attached t’o the carbon chain are fluorine. This comhination results in a material with unusual properties. MONOMER
Nore than fifty years ago chemists began to work on fluorinesubstituted ethylenes. Papers published in 1890 (8, 81, 22, 32) describe attempts to prepare tetrafluoroethylene by the reaction of fluorine with carbon, fluorine v i t h chlorinated niethanes, or silver fliioritle wit11 tetrachloroethylene. From the meaycr data
furnished, it is not certain whether these experimenters reached their goal. Humiston (13) reported the preparation of tetrafluoroethylene by the reaction of fluorine and charcoal, but the only physical constant listed is not correct for tetrafluoroethyle m ; hence, there is some doubt as to the identity of his product. Tetrafluoroethylene, an odorless nontoxic gas, was found later to have the properties listed in Table I. Ruff and Bretschneider (26) gave the first reliable and relatively complete description of the monomer, which they prepared by decomposing tetrafluoromethane in an electric arc. Bromination, followed by dehalogenation with zinc, was employed in separating pure tetrafluoroethylene from the pyrolysis products. Thornton, Burg, and Schlesinger (30) thought that they might have obtained tetrafluoroethylcne from the action of a highvoltage discharge on dichlorodifluoromethane, but made no attempt to identify it. Subsequently, Locke, Brode, and Henne (20) prepared the monomer by the action of zinc on tetrafluoroclichloroethane. Henne and llidgley (12) a1.o employed this
September, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
871
method in studies of the reactivity of fluorine in aliphatic compounds, More recently, Ruff and Willenberg (87) reacted fluorine with silver cyanide to obtain hexafluoroazomethane, which breaks don-n in an elwtric arc t o yield some tetrafluoroethylene, among other products. Another method of preparation is by the pyrolysis of chlorodifluoromethane or bromodifluoromethane according t o Downing, Benning, and IlcHarness :9) and Torkington and Thompson (51). PO LYMERI ZATIOIV
Polyniers of tetrafluoroethylene were first disclosed in 1941 by Plunkett (24). Although the polymerization of various halogenated ethylenes had been studied for some time, there had been no indication previously that high polymers might be formed
XIanufacture of Polgtetrafluoroethylene Shapes for Various 4pplications. ( T o p ) Rods .&re Produced by Extrusion in an ExperiInental Unit. (Center) RIolded Cylinders Are Shated on a Lathe to Yield Tape. (Below) Seamless Cotered &ire Is the Product of 'This Operation; Bare Wire on the Spool at the Bottom Passes up Through a Vertical Extrusion Chamber Where the Plastic Sheath Is ipplied. View on the Opposite Page Shows Products Fabricated of Polytetrafluoroethylene for Inrlustrial Use.
Polytetrafluoroethylene is a new plastic being ntanufuctured o n a n experimental p l a n t scale. I t is insoluble i n all solcents tried so f a r and is n o t attacked below i t s m e l t i n g p o i n t by any c o m m o n corrosice agent except m o l t e n alkali metals. I t withstands temperatures u p to 300" C. f o r long periods without serious degradation and is n o t brittle a t low temperatures. .-in outstanding property is i t s combination of low power factor and low dielectric cons t a n t . T h e principal current u s e s forpoIytetraJluoroethq.1ene are as gaskets and packing in e q u i p m e n t f o r handling h o t corrosive liquids a n d as electrical insulation, particular13 u t high frequencies and under strenuous environm e n t s . T h e plastic is being sold i n s m a l l quantities f o r development purposes, i n t h e f o r m of sinzple shapes such as tupe. sheets, rods and tubes, gaskets. and insrclated wirv.
INDUSTRIAL AND ENGINEERING CHEMISTRY
872 TIR1.E
I.
Vol. 38, No. 9
S o cvidcnce of reaction of the nornial polymer n i t h chlorine,
l'.ROPERTIES
Of
Color Odor Boiling point, ' C. Freezing point, ' C . Critical ternoerature. O C. Critical pres'sure. Ih. i s q . ~ i n , Vapor pressure (log p ) . atni. -70.3" to 00 C. 00 t o 33.35 C. Liquid density (d), pranis/cc. -1000 to -400 c. - 40' t o 8' C. 8' to 30' C. Critical density, grarns/cc. Dielectric constant a t 28' C. 15 Ih./sq. in. ahs. 126 Ib./sq. in. alw. Thermal conductivity a t 30' C., cal./ sq. cm./sec./' C./cni. a D a t a supplied by A. F. Benning, pan?.
TETRAFLTOROETHTLENCU bromine, or iodine has bFen detected. I t appears, honever, that Sone rone 3 -1142.5 33.3 372 -76
5.G210 -- 8 7 3 . 1 4 ' T 5,5906 - 8GG.84/T
1,0017 1.015
0.000037 Jackson Laboratory, D u Pont Coin-
from tetrafluoroethylene. I n fact, a conclusion widely accepted for a long time was that' the participation of an umubst,ituted methylene group \vas essential for addition polymerization (10). Foreign patents (fd,15,.16, B), hoxever, disclosed the polymerization of substitut.ed vinyl compounds which contain fluorine and a t least onc other halogen atom substituted for hydrogen in the vinyl radical, such as chlorodifluoroethylene and chlorotrifluoroethylene. Thesc patents (15, 16, 28) implied that the polymerization of fluorinated ethylenes lacking a t least one other halogen would be difficult, whereas actually tetrafluoroethylene, under superatmospheric pressure and with suitable catalysts, is polymerized to useful products n-ith reasonable facility (5, 19). Kevertheless, the erroneous impression has tended t o persist that ethylenes with all hydrogens substituted cannot be polymerized. In the polymerizat,ion of tetrafluoroethylene, safety demands that adequate precautions be taken t o dissipate the heat of reaction. The monomer, prior t o its polymerization, is thermodynamically unstable to such a degree that the heat generated during polymerization is likely to initiate the disproport,ionat,ionof monomer t o carbon and carbon tetrafluoride, a n exothermic reaction which can proceed with considerable violence. (The heat of polymerization has been estimated as 20,000-25,000 calories per mole, but experimental values obtained in small plant equipment have been somewhat higher.) POLYMER
On the other hand, polymerized tetrafluoroethylene is a remarkably stable substance, unique among organic compounds in chemical inertness, resistance to change a t high temperatures, and extremely lo^ dielectric loss factor. The dense solid exhibits none of the instability of the monomer. This fact is demonstrated in Tables 11, 111, and IS', which list some of the properties of polytetrafluoroethylene. RESISTAKCE TO REAGENTS AND SOLVESTS.Polytetrafluoroethylene is extremely resistant t o attack by corrosive reagents and to dissolution by all solvents tried. KOsubstance has been found m-hich will dissolve or even swell the polymer (Table 11). Several hundred solvents have been tried, including a great variety of halogenated hydrocarbons, ketones, esters, etc. Many high-boiling liquids, such as dibutyl phthalate, have been tested at their boiling points without evincing any sign of attack. Table 111 demonstrates the outshnding chemical inertness of the material; such potent reagents as aqua regia, chlorosulfonic acid, acetyl chloride, boron trifluoride, hot nitric acid, and boiling solutions of sodium hydroxide do not affect the polymer. Of all the common reagents tested a t atmospheric pressure and temperatures below 300 ' C., only molten allcali metals have the ability t o attack polytetrafluoroethylene appreciably. Evident,ly, molten sodium will remove fluorine atoms from carbons in the polymer chains.
these halogens arc absorbed to some extent by the molded polymer. On the orher hand, polytetrafluoroethylene under certain conditiolis is subject to attack by fluorine. On prolonged contact n-itli fluorine a t atmospheric pressure the moldings are merely bleached, and not othenvise affected, but, under buprratmospheric pressure a t room temperature they are strongly attacked by this halogen. I n such cascs the presence of reactive contaminants-e.g., water or grease-has been suspected. Presumably, in combining 11-ith these impurities the fluorine may generate high enough temperatures locally t o initiate attack on polytetrafluoroetliykne. The product of this highly exothermic reaction is carbon tetrafluoride, Tvhich is removed as a gas so that the polymer disappears completely except,, sometimes, for a small carbonaceous residue. K h e n the fluorine is diluted with an equal volume of nitrogen, the incidence of this attack is greatly reduced, and moldings consistently h a w been heated in the gaseous mixture at one atmosphere and temperatures up to 150" C. Tvithout dcgradation. The absorption of water by nioldeil polytetrafluoroethylene is very low and is rated as zero by the standard 24hour A.S.T.M. test, although samples immersed for long intervals show a small but measurable gain in weight. Apparently the absorption is influenced to a fair degree by the manner in which t>hemolded specimen has been prepared. This factor also influences the permeability of polytetrafluoroethylene films to moisture, but the moisture transmission of continuous films is alviays low. Water alone docs not wet the polymer, but water containing surface active agents or water-ethanol mixtures will spread on the surface of the polymer.
TABLE11. REAGEXTS WHICHDo KOT DISSOLVE OR SWELL AT TEXPERAT~RES UP TO THEIR POLYTETRAFLUOROETHYLENE BOILING POINTS Water Methanol Ethanol Bedzyl alcohol Higher alcohols Diethyl ether Higher ethers Cellosolve Benzaldehyde Acetone Methyl ethyl ketone Cy c 1oh ex anon e Acetophenone Other ketones Aliphatic hydrocarbons Benzene Toluene Xylene Other aromatic hydrocarbons Trichloroethylene Perchloroethylene Hexachloroethane Other halogenated hydrocarbons
Phenol Cresols Naphthols Ethyl acetate Ethyl hexoate Dibutyl phthalate Dibutyl sebacate Diisobutyl adipate Other esters Formic acid Acetic acid Propionic acid Trichloroacetic acid Benzoic acid Aniline Nitrobenzene Benzonitrile Benzoyl chloride Acetic anhydride Dioxane Furan Pyridine Piperidine
For all practical purposes, pol~tetr-fluoroethylene is unaffected by water. Hon-ever, a measurable, but extremely slow, reaction of water ivith finclly divided polymer a t temperatures above 200" C. has been detected. While this reaction is sufficiently slow to have no bearing upon the uses contemplated thus far for the product, it is mentioned on the chance that for some special case the minute traces of evolved hydrogen fluoride might have significant effect. T m R x t I . PRomxrIm. Pol3meric substances resistant to proiongeil henting a t elevated temperat'ures are no longer a novclty ( 2 , 25). However, tlic s t a b i l i t ~of polytetrafluoroetiiylene a t high temperxiurcs is sufficientl>- outstanding to demand special attention. hlthough the polymer ha' an appreciably 10n.cr len-
873
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1946
peratures and are easily fractured, they are form-stable and do not flow under their onm xveight. The polymer can be baked for ninny hours a t temperatures up t o 390" C. with only a minute loss in ncight, although prolonged heating a t these temperatures does lead to degradation. BS the temperature is raised above 400" C., tlw loss in n-eight occurs a t an increasingly higher rate (3). The identity of all the volatile products is not knonn ivith certainty, hut sonic are toxic; there-
sile strength a t 300" C. than a t room temperature, it can be held for long periods of time a t the higher temperature with relatively little change. I n one month a t 300' C. the tensile strength of molded bars will drop only 10-2070; a t 250" C. the loss in tensile strength is negligible over long intervals. When polymer is heated a t continuously increasing temperatures, R solid-phase transition is observed in the range 320" to 327" C. .-it these temperatures heavy moldings become trans-
6 350
~
_
2 0. _
TTHYLENF
_ --_ --
60 80 TIME- M I N U T E S
40
too
14
I
20
40
60 TIME- MINUTES
80
60 TIME- MINUTES
80
~
100
-_
140
c U
a IOOw a 9
S
/
I
1'
,/
20
I
__-_-
' II
40 60 80 TIME- MINUTES
100
20
40
100
Figure 1. Time-Temperature Heating and Cooling Curves for Polytetrafliroroethylene and Polythene parent, the coefficient of thermal expansion increases sharply, :md the tensile strength of moldings drops precipitously. Evidently the polymer loses its microcrystallinity and tends t o become amorphous. Heating and cooling curves (Figure 1, A and B ) show the profound effect obtained in passing a block of polymer through this transition point. Hen-ever, it is remarkable that even though moldings lack strength above these tern-
fore, caution should bc: exercised in timdling these off-gases. For complete safety it is recommended that adequate ventilation be provided whenever polytetrafluoroct~ivleneis subjccted to temperatures above 200" C. (Care also should be taken to avoid dust or vapors given off during the niechanical working of polytetrafluoroethylene, particularly 7;ihere frictional heat may result in high local temperatures.)
874
INDUSTRIAL AND ENGINEERING CHEMISTRY
2
Figure 2. X-Ray Photograph of Wire Insulated with Polytetrafluoroethylene before (1) and after (2) Mandrel Cutting Test
.It the other extreme of temperature, polytetrafluoroethylene is unusual in t h a t it does not tend to be embrittled by chilling. Films may be flexed easily a t temperatures below - 100 O C., although samples chilled in liquid air to -185" C. always can be cracked. ELECTRICAL PROPERTIES. Table IVB demonstrates t h a t polytetrafluoroethylene combines the unique advantages of low power factor and low dielectric constant. These result in a dielectric loss through the polymer which is a t least as low as, or lower than, that of polystyrene or polythene (polyethylene), which previously were pre-eminent in this field. Moreover, this low loss is constant over a very wide range of frequencies extending from 60 cycles to 3000 megacycles. With regard to dielectric strength, samples of polytetrafluoroethylene sheeting have tested as high as 4000 volts per mil a t single points, but the average will run only 1000 to 2000 volts per mil, dependipg on thickness and on method of preparation. I n t,ests to determine arc resistance it was found that the unfilled polymer does not "track"; that is, no conducting carbonaceous decomposition products are formed, and hence electrical leaks will not continue across the path of the arc after the voltage has been dropped. -kt the extremely high temperatures produced by a sustained arc, the material will suffer degradation to volatile products, which causes a gradual disappearance of the insulation, but in this respect polytetrafluoroethylene is far superior to any other organic insulation product. MECHASICAL PROPERTIES. Table TVC shows that polytetrafluoroethylene possesses many advantages but is also subject to certain physical limitations which, in part, arise from the difficulties connected with molding it. Because of its abnormally high melt Triscosity, the polymer never forms a flo\Table melt, and therefore it is not possible to employ conventional t,echniques. The alternative methods devised for fabricating articles of polytetrafluoroethylene have not been adequate in all cases to permit full realization of the potentialities of this material. For the present, moldings have high impact strengt'h and are tough, but they are easily strained beyond the point of elastic recovery. I n measuring the tensile strength of test specimens, it is found that the material begins to draw when loads of 15002000 pounds per square inch are reached (at no-load cross-head speeds of 1 t'o 20 inches per minute). The material can be extended 30045070 before breaking occurs at. 25004500 pounds per square inch of the original cross section of the specimens. I n oriented tapes, tensile strengths as high as 20,000-25,000 pounds per square inch have been measured along the line of orientation. It is not possible to establish a modulus of elasticity for
Vol. 38, No. 9
the material in the normal sense. but at a no-load cross-head speed of 0.05 inch per minute, values around 1700 pounds per square inch have been recorded a t deformations of 0.1% under compression. Like conventional thermoplastic materials, polytetrafluoroethylene under stress is subject to cold flow, which increases in degree as the ambient temperature is raised. However, this tendency t'oward deformation is not excessive for many services over a fairly wide range of conditions. *is Figure 2 shows, when wire insulated with polytetrafluoroethylene was looped oveI a 2.75-inch mandrel and maintained under a load of 50 pounds for 42 hours a t 235" C., i t did not migrate through the coating to any appreciable extent. With conventional high-softening insulation materials, such as polyvinyl formals or polydichlorostyrene, the vire cuts through the coating a t a markedly greater rate. Another characteristic of polytetrafluoroethylene which must be considered in evaluating the property of cold flow is the fact that, the material strongly resists attempts to force i t through narrow openings. Whereas heavy moldings of the polymer can be compressed easily by loading the flat face, very thin sheets are less subject to flow. This is of some importance in many of the industrial applications of the polymer. The property of cold flow is responsible for the low values reported for the heat-distortion temperature in Table IV-4, which does not give a true picture of the serviceability of the material a t elevated temperatures. This test is strictly applicable only to rigid plastics, whereas polytetrafluoroethylene is somewhat flexible. Although polytetrafluoroethylene can be easily deformed beyond the limits for complete recovery, it does exhibit some delayed elasticity. Figure 3 shows the extent of deformation and recovery in tests of small molded cylinders of the polymer, inch in diameter and 3,'s inch in height. At loads below 1000 pounds per square inch there was no difference between the degree of recovery of samples unconfined and those held i n a tightfitting yoke with only 1/18 inch protrudiag. Above 1000 pounds per square inch, the partially confined specimens shonwi the lesser deformation. At loads of 2500-3000 pounds per square inch, there was a greater strain than a t 1000 pounds per square inch and the degree of recovery wa9 less. Variation in the temperature of the test over the range 25-95' C. effected only a minor change in the amount of deformation for a given load or in the degree of recovery on removal of the stress. As indicated earlier, polytetrafluoroethylene can be oriented by cold drawing or rolling. AUso. orientation sometimes is induced during the fabrication of shapes and articles from the material. Moldings which are oriented are subject to elastic niemory and, on heating t o elevated temperatures, undergo Some re~~
TABLE
111. EFFECT OF CORROSIVE
R E a G E s T S OK POLYTETRA-
FLUOROETHYLENE Reagent 1ICl. concentrated Aqua regia H S O t , concentrated H N O i , fuming HCIOI 2Hz0 € L S O r , concentrated HlSOi fuming HF d n c e n t r a t e d HF: liquid anhydrous Organic acids
%%Hk%?
"a, liquid Brr (at 1 atm.) Cln (at 1 atm.) Fz (at 1 atm.) KMnOa, 5% HzOI, 30% PClr BFsRF Ua
Nil02 CISOIH Oaone
Temp. 8' Testing, C. 2.5 a n d 50 25 and fi0 25 25 Fnd 80 25 and 25 25 and 25 a n d 25 and 25 25 and 25 and 150 25 and 25 100 25 200 100 2.5 25
100 85 300
100 100 100 100
Actloll None None None Solie Sone h-one None Sone None None Sone None
100 100 100 ?;one
Sone .Ittacked s l o u l r h?one Sone None
September, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
traction. Hence, articles intended for use at temperatures approaching the transition point should have their stresses relieved by preheating slightly above the temperatures of service, or must be made from stock which is not oriented. Molded polymer is not "hard" by normal qualitative standards, and the surface is easily dented and scratched. However, because of its slippery character it is more resistant to abrasion than might a t first be expected. I t s surfaces are self-lubricating to some extent. The coefficient of friction for polytetrafluoroethylene on metals is lox, and it is even lower for the material against itself. Vhen pieces of polymer are rubbed across one another, they do not gall or bind, even under comparatively heavy loads. Similarly, smooth metal surfaces do not bite into the polymer. This means that the material has possibilities as a self-lubricating bearing for systems operating under light loads. However, in such bearings, shafts must be operated slowly enough to avoid the build-up of temperatures which are sufficiently high t o degrade the polymer. Since the thermal conductivity of the material is very low, the removal of frictional heat from such a system becomes a serious problem. RELATION O F MOLECULAR STRUCTURE TO PROPERTIES
A complete explanation of the unusual properties of polytetrafiuoroethylene is still to be worked out. However, a number of observations have been made which may be helpful in arriving at a better understanding (IOA). CRYsuL STRUCTURE. .klthough no xork has been donc on the exact determination of crystal lattices, dimensions of the unit cell, etc., some preliminary x-ray studies have indicated a high degree of crystallinity in the normal molded polymer. This is not altogether surprising since the simplicity of the molecule (the repeating unit is -CFPCFs-) would be expected t o lead to ready crystallization, despite comparatively Tveak forces between thc chains. This simplicity of structure, irith freedom from side groups and consequent low geometrical bulkiness, lends itself easily to close packing of the chains. Owing to the great length of the molecules and to the inherent disorder oi such a system. crystallizatioii presumably is never complete. The crystallites must be separated by regions of amorphous or disordered molecules, and a single molecule may pass alternately through several amorphous and crystalline regions. Examination of polytetrafluoroethylene tapes under a polarizing microscope with high magnification shows up evidence of spherulitic structure similar to that observed in polythene. 111 studying the crystalline character of polytetrafluoroethylene, time-temperature curves were drawn from the data obtained Tvhen a block of the polymer was heated and cooled through its "melting" point. These curves (Figure 1, A and B ) , indicate that the change in state is relatively rapid, and the two bresks in the curves also suggest that there are two crystalline phases of different melting point. A similar study (Figure 1, C and D ) was made of polythene (same rates of heating and cooling and same size and shape of polymer blocks). Again, relatively rapid changes in state werc observed, but there was only a faint suggestion of a second break in the cooling curve arid only a single break was I'iiund in the heating curve. Both polytetrafluoroethylene and polythene exhibited supercooling to the extent of 6-7" C. X-ray diffraction photographs of unoriented polytetrsfluoroethylene film show only slight changes in degree of crystallinity between room temperature and 300" C. However, the sharp diffraction rings disappear when the t'emperature is increased t,o 340" C. When drawn to 300y0 of its original length, the film gives 3. simple x-ray pattern of discrete bright spots such as is commonly associated with highly oriented fibers having a minimum of substitution in the chains. At 310" C. this orientation slowly disappears as the thermal agitation of the molecules gradually overcomes the polymer's resistance to flow. Polymer v1iic.h ha. bren he:it,ed xbove its trnnsitioii point nrid tiiwi
Property
a15
Value
A.S.T.JI. JIethod of Test
A . Thermal Properties Deformation under load (at 50' C.), % 50 Heat-distortion temp., O C . High load 62 L o a load 130 Sp. heat (38' t o 128' C.), cal./gram/" C. 0.25 Coefficient of expansion per C. (25' t o 60' C.) 1 . 0 x lo-& Thermal conductivity (0.46 cm.), cal./sq. cm./hr./' C./cm. 2.1 Brittleness temp., C. < -80
B. Electrical Properties Dielectric strength, short-time, volts/mil 0,080 inch 500 0.005 inch 1500 Dielectric constant 60 cycles 2 0 108 cycles 2 0 10' cycles 2 0 10' cycles 2.0 3 X 10' cycles 2 0 Power factor 60 cycles 1 0 0002 103 cycles
