August 1952
1805
INDUSTRIAL AND ENGINEERING CHEMISTRY
period of 5 minutes after each casting, These data indicate t h a t the optimum properties are obtained a t a temperature of 370" C. with a 5-minute baking period. With higher sintering temperatures, Le., above 400" C., the films showed a marked decrease in tensile and dielectric strength. With sintering times longer than 5 minutes, optimum properties were obtained a t kmperatures slightly lower than 370" C. The properties obtained in the optimum sintering temperature ranges were equivalent t o those of the polymer fabricated by molding techniques (7). MISCELLANEOUS APPLICATIONS
In addition t o the preparation of thin films, Teflon tetrafluoroethylene resin dispersions wcre found suitable for impregnating woven fabrics and mat structures t o render them chemically more resis tant. These were further laminntcd by compression molding a t temperatures above the transition point a t 327' C. into useful products having excellent resistance t o corrosive
chemicals and excellent dielectric properties. Combinations of the dispersion? with stable fillers, such as titanium dioxide, carbon, silica. talc, calcium silicate, and mica, were also found useful foi coating and impregnating applications. Dispersions of the Teflon resin could also be applied by spraying onto various surfaces within the limitations of the critical thickness disrussed above, but for arceptable adhesion to metal and ceramic suifaces special formul~tionsare recommended. The various properties of the Teflon tetrafluoroethyl~neresin in its dispersed form as described in this paper involv? numeroue theoretical considerations bearing on colloid chemistry which warrant further investigation for this unueually inert fluorine polymer. RECEIVED for review October 26, 1951. ACCEPTED April 3, 1952. Presented as part of the Svmposium on Fluorine-Containing Polvmers before the Division of Polymer Chemistry a t the 120th Meeting of t h e AhfERIcaN CHEMICAL SOCIETY, New York, September 1951.
(Teflon Tetrafluoroethylene Resin Dispersion)
EXTRUSION PROPERTIES OF LUBRICATED RESIN FROM COAGULATED DISPERSION JOHN F. LONTZ, JOSEPH A. JAFFE, LESTER E. ROBB, AND WILLIAM B. HAPPOLDT, JR. Polychemicals Department, E. I . d u Pont d e Nemours & Co., Inc., Wilmington, Del.
T
HE extrusion of Teflon above its transition temperature at ability of these lubricants depends upon the extrusion conditions 327 C. involves certain mechanical and thermal problems t o which the compositions are subjected. Thus, the boiling which limit to some extent the rates by which such fabricated point of the organic compound must be above room temperature t o avoid evaporation from exposed surfaces. The boiling point articles as rods, tubes, sheets, and wire coverings can be made. I n an attempt to circumvent some of the difficulties involved in high temperature extrusion, a n investigation of the extrusion properties of the TABLE I. EXTRUSION QUALITYOF VARIOUSORGANICLIQUIDSAS lubricated polymer resin was undertaken. LUBRICANTS FOR COAGULATED DISPERSION POLYMER In studying the properties of the aqueous dis[Die, 0.010-inch (lO-mi!) tape die, 1% inches wide: lubricant concentration, 18 to 20% by persions of the resin described in the preceding weight of mixture; temperature, 20' t o 25' C. (room temperature)] Viscositya paper, it was discovered t h a t the colloidal form of Pressure Temp., Range, Extrusion polymer forms a polymer-in-oil composition when ' C. Lb./Sq. 1n.b Qualityc Organic Compounds Cps. coagulated in the presence of organic lubricants. Hexane 0.326 20 18,000 Poor, does not extrude Further work indicated that these polymer-in-oil Octane 0.542 20 Fair 18,000 (approx.) compositions could also be prepared by mixing Decane 0.77 22.3 Excellent 6,000-10,000 Dodecane 1.26 23.3 4,000 t h e dry coagulated polymer with a variety of Hexadecane (cetane) 3.59 22.2 2,000- 4,000 Evcellent Excellent Hydrocarbon oil (Primof C)d .. 2,000- 4,000 organic compounds, some of which act as lubriBenzene 0 . bb4 20 Excellent 2,000- 5,000 cants. These compositions can be extruded into Toluene 20 0.590 2,000- 5,000 Excellent 20 0.648 5,000- 9,000 Excellent Fair various forms at room temperature at rates apKxiY 25 2.0 4,500- 5,000 Decalin 25 2.41 5,500- 6,000 Fair preciably higher than those attainable by the Chlorobenzene 0.80 20.1 12,000-15,000 Fair high temperature extrusion of t h e nonlubricated Chlorinated biphenyl (Aroclor 1242) 6 .. 2,0004,000 Excellent polymer. Ethanol 1.20 20 4,000- 7,000 Excellent Fair O
LUBRICANT COMPOSITIONS
Teflon tetrafluoroethylene resin particles obtained from the coagulated aqueous dispersion are readily wetted by organic compounds. It was found that compounds which act as lubricantb are not limited t o any specific chemical structure. Thus, representative members of such organic classes as hydrocarbons, both aliphatic and aromatic, halogenated hydrocarbons, alcohols, glycols, esters, ethers, ketones, silicone oils, tricresyl phosphate, and fluorocarbon oils are effective lubricants for the coagulated polymer. The suit-
tert-Butyl alcohol Ethylene glycol But lacetate MetK yl benzoate Djbutyl phthalate Di-n-propyl tetrachlorophthalate Anis o1e .Tricresyl phosphate (tech.) Silicone DC-500 (50 cts.)f Perfluoro lube oil, FC-3328
2.95 2.18
20 0
2.067
..
20
1.'ii
20
..
..
..
vi'shous oil
..
..
..
11,000-14,000 4,000- 6,000 6,000-1 1,000 14 000-18 000 4:OOO-
6,'OOO
4,000- 8,000
6,000-12.000 4,000- 6,000 10,000-12,000
16,000-18,000~
Fair Evcellent Fair Excellent Fair Excellent Excellent Exrellent Fair
Taken from "International Critical Tables" and other reference sources. b Force applied on a ll/r-inch ram (Carver press). 0 Rated in the order: excellent. fair, and poor, as a oomposite rating of smoothness, absence of strains, and flexibility. d Standard Oil Co. of New Jersey. Monsanto Chemioal Co. f Dow-Corning Corp. 9 Organic Chemicals Department, E. I. du Pont de Nemours 8. Co., Inc. Requires a minimum temperature of 160" C. t o extrude a t this pressure range,
1806
INDUSTRIAL AND ENGINEERING CHEMISTRY
LUBRICANT: CETANE DIE
2
0
2
; IO-MIL TAPE I V 2 IN. WIDE
300C
'
crj
m
A
2000 \
K
a
fn fn W K 0
z IO00
2
u)
3
a kX W
C
Figure 1.
\
20 30 % LUBRICANT
40
Effect of Lubricant Content on Extrusion Pressure
must not be so high as to preclude the removal by volatilization of the lubricant before sintering the shaped object. The proportion of the organic compound used in the composition is important. Generally, the higher the amount of lubricant employed, the lower is the pressure required for extrusion. For optimum extrudability it has been found that a concentration of 18 to 20% is the most suitable. A list of typical polymerorganic liquid mixtures that can be extruded is presented in Table I. A lubricant concentration of 18 to 20% was used in all cases, and a tape, 10 mils thick and l l / z inches wide, was extruded a t room temperature. The first column of the table represents some of the organic compounds tested as lubricants, and the third column represents the pressure, in thousands of pounds per square inch, exerted on a l'/a-inch ram in extruding the polymer mixture through the die. The extrusion quality in the fourth column is a composite rating of smoothness, absence of strains and tears, and flexibility. Among the aliphatic compounds, decane, dodecane, hexadecane, and Prim01 C arc considered as excellent lubricants. The extrusion pressures associated with these materials are reasonably low and yield high quality tapes. Of the aromatic compounds, benzene, toluene, and p-xylene are rated as excellent lubricants, whereas Tetralin and Decalin, even with their higher viscosities, are not as good lubricants. Of the alcohols, ethanol and ethylene glycol give mixtures that can be extruded into good tape a t low pressures; tert-butyl alcohol, on the other hand, is not as effective. Among the miscellaneous compounds investigated, dibutyl phthalate, tricresyl phosphate, and Silicone DC-500 are considered t o be excellent lubricants. Certain of the organic compounds, notably the higher paraffin hydrocarbons and aromatic hydrocarbons, seem toimpart extrudability a t comparatively low pressures. The higher aliphatic hydrocarbons, particularly those in the range of CE to C ~ O appear , t o be most suitable because of their low evaporation rate a t room temperature. Furthermore, after formation of the extruded article, this type of lubricant can be removed readily, either by extraction or by volatilization a t moderately elevated tempera-
Vol. 44, No. 8
tures. These hydrocarbons have been found to be excellent lubricants for extrusion compositions because they can be mixed readily with the coagulated polymer by several methods and, after extrusion, can be removed from the shaped article by volatilization below the sintering temperature of the polymer. Certain of the chemical types listed have a low order of inflammability, as in the case of the chlorinated biphenyl and tricresyl phosphate. The silicone lubricants offer useful compositions which could be utilized without removing the lubricants which, in themselves, have excellent dielectric properties. For most extrusions, 18 to 20% of the paraffin hydrocarbons gives a mixture that can be forced and shaped through constrictions down t o 0.005 inch in cross sections. At these lubricant concentrations, the pressures needed to extrude the lubricated mixture are within the range attainable in commercial hydraulic equipment. Compositions with less than 18 to 20% lubricant require correspondingly higher pressures, whereas those with more lubricant extrude a t lower pressures, as is evident from the data shown in Figure 1. This curve illustrates the effect of lubricant content on the extrusion pressure, using cetane as the lubricant and a 10-mil tape die, l1/2 inches wide. From this it can be seen that the extrusion pressure increases inversely in a linear relationship with lubricant concentrations (within the range illustrated here). The pressure can be decreased from 3000 to 280 pounds per square inch by increasing the lubricant concentration from 18 to 30% cetane. Compositions containing 18 to 20% cetane yield extruded tapes which are free of splits and other strain marks after removal of the lubricant and subsequent fusion above 327" C. Tapes prepared with less lubricant usually have rough and grainy surfaces. Moreover, higher proportions of lubricant cause splits and tears, presumably because, on removal of lubricant, the resulting highly porous extrudate fails to fuse completely on sintering. PROPERTIES OF LUBRICAST-POLYMER MIXTURES. The mixtures vary in appearance from dry, white powders a t low luloii-
Figure 2.
Equipment for Coagulating Polytetrafluoroethylene Dispersions 1. Dispersion 2. One-gallon vessel 3. Turbine agitator with baffles 4. R.p.m. indicator 5. Air motor
August 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
1807
cant concentrations t o soft, somewhat greasy, puttylike materials a t moderatelubricant concentrations. At lubricant concentrations above 50% the mixtures acquire the consistency of grease or paste but are too soft for extrusion into form-stable articles, such as tapes, monofilaments, and wire coatings. These polymer-inoil compositions have the property of coalescing under pressure. Thus, these mixtures can be compacted without heating or fusion and also rolled into tough, flexible sheeting which can be stretched further into even thinner film. The compacted lubricated mixtures do not have the strength of t h e ultimate fused polymer, and, upon tearing, show a fibrous structure. PREPARATION OF EXTRUSION MIXTURES
The lubricated polymer mixtures have been prepared by three methods. Two of these methods involve the preparation of the coagulated polymer followed by compounding t h e dry powder with an organic compound. The third method involves simultaneous coagulating and mixing with a water-insoluble organic lubricant. Prolonged mechanPREPARATION OF COAGULATED POLYMER. ical agitation of t h e aqueous dispersion causes irreversible coagulation of the polymer This property has been useful in t h e preparation of ubricated compositions. T h e equipment for this coagulation procedure is relatively simple and, as illustrated in Figure 2, consists of a 1-gallon vessel, a motor-driven air agitator equipped with baffles, and a speed indicator. With such a n assembly, the coagulation of the dispersed polymer is usually complete within 5 minutes at moderate speeds (200 t o 600 r.p.m.) and the resulting coagulated polymer collects at the top of the container. After filtration, it has a somewhat grainy appearance. The polymer must be dried before mixing with the lubricant. The product may be dried in a number of ways. The most efficient method is by drying in trays made from 10-mesh screens which allow good air circulation through the powder. The owder is spread t o a bed depth of approximately l / ~inch a n f the drying trays are then placed in a steam-heated oven a t 150' C. for a minimum of 12 hours. The depth of the bed is important for best drying results, since the top layer tends t o pack together and reduce the diffusion of the water vapor from the lower regions. After thorough drying, the coagulated polymer is ready for lubrication. LUBRICATION. Misting has been found t o give the most uniform lubricated composition. Furthermore, it is possible to control the lubricant content t o within f0.5%. The method consists of misting or spraying the lubricant onto the dried coagulated powder in a rotating container. This can be carried out in a suitable mechanical blender of which there are several commercial types. The second method of lubrication involves lubrication of coagulum by slurrying. When the unlubricated coagulated polymer is suspended in a large volume of low boiling organic liquid, particularly methanol, it has a strong affinity for lubricating liquids of higher viscosity. For example, lubrication of the dried granules can be achieved by slurrying 500 grams of the coagulum in 1000 ml. of methanol to which 110 t o 125 grams of the hydrocarbon are added. The entire mixture is agitated at 500 r.p.m. for 1 t o 2 minutes. The resulting lubricated coagulum is then filtered and dried. This method produces a uniformly lubricated, free-flowing composition. The third method of lubrication involves simultaneous coagulation and lubrication. This method consists of adding the lubricant t o the aqueous dispersion and coagulating the mixture by the addition of polar solvents or electrolytes with stirring. Acetone, methanol, ethyl alcohol, and salts, such as ammonium acetate and ammonium carbonate, are effective coagulating agents. By this technique, the lubricant displaces the water on the surface of the polymer particles and complete transfer of the lubricant t o the particle surfaces is accomplished. The grain sizeof the resulting lubricated coagulum depends upon the speed of agitation. Thus, a variety of grain sizes can be produced in the equipment shown in Figure 2. Very fine powders are made a t
Figure 3.
Disassembled Extruder for 10-Mil Tape, 1.5 Inches Wide 1,2. Die sections 3. Connecting flow tube 4. Polymer cylinder 5. Plug (movable)
agitation speeds of 14,000 r.p.m., while larger grains held by 10mesh screens are formed at speeds of 500 r.p.m. EXTRUSION PROCESS
The lubricated Teflon compositions are readily shaped in a ram-type extruder, in which there is a minimum of shearing action. Where appreciable shearing action is applied, the polymer assumes a fibrous, discontinuous form. For this reason, the use of a screw-type extruder is not practicable. Lubricated compositions of Teflon have been shaped into continuous tape and coatings on wire. The complete process consists of extruding the lubricated mixture below the 327" C. transition point of the polymer, removing the lubricant by solvent extraction or by volatilization, subjecting the lubricant-free extrudate t o temperatures above the transition point, and finally either annealing or quenching. The essential features of the
Figure 4.
Extruder for 10-Mil Tape (1.5 Inches Wide) Assembled and Mounted in Press
1808
INDUSTRIAL AND ENGINEERING CHEMISTRY
SIN TERl N O
c,OVEN
Figure 5.
Vol. 44, No. 8
REMOVAL OF LUBRICANT.Following the extrusion of the tape or wire coating, it is essential to remove the lubricant to prevent its ignition on exposure to the high fusion temperature. The lubricant can be removed either by extraction with a solvent or by volatilization in a suitable chamber heated below the 327" C. transition temperature of Teflon. EXTRACTIOK. The hydrocarbon lubricants are readily extracted by aliphatic and aromatic hydrocarbon solvents. T h e extraction can be carried out either batchwise or continuously, by passing the tape through an extraction bath. A diagrammatic sketch of the extraction and sintering operations is shown in Figure 5. The extraction bath is located below the sintering oven through which the tape is passed t o fuse t h c polymer. An experimental laboratory setup of these operation6 is illustrated in Figure 6. The extruded tape shown here is passed through a multiple-pass extraction bath and then up through t h e sintering oven and finally wrapped around the take-up roil. The efficiency of the lubricant extraction depends upon the extraction temperature and the thickness of the tape. In Figure 7 are shown the extraction rates obtained vr-ith boiling p-xylene and tape prepared from a composition containing 18.4% cetane. The curves indicate that with increasing thickness the extraction time increases correspondingly. For example, a 95% removal of oil was effected in about l1/2 minutes from a IO-mil thick tape, and the extraction time increased to about 5 minutes for a 20-mil thick tape. It is apparent from these data that the solvent extraction method is practicable for the making of 0.01-inch thick tape but requires prolonged treatment for greater thicknesses.
Continuous Extraction and Sintering of Teflon Tape
extrusion process for making tape and coating wire will be described briefly. EXTKUSION OF TAPE. Continuous lengths of tapes varying in thickness from approximately 0.005 to 0.125 inch have been made in an extruder, the components of which are shown in Figure 3. With greater tape thicknesses, difficulties have been encountered in removing the lubricant. The extruder consists of a cylindrical chamber from which the lubricated polymer is forced through die sections, 11/z inches wide, by the application of pressure. The extruder assembly mounted in a laboratory press is illustrated in Figure 4. This assembly is provided with a simple take-off roll. The cylinder has a capacity of approximately 100 grams. Successive batches knit satisfactorily t o give a single continuous tape. In operation the lubricated mixture is placed in the cylinder and forced through the die by the movable plug which is actuated by the press. Increasing pressure is applied gradually until the tape emerges from the die a t a rate varying from 5 to 20 feet per minute. This rate depends upon the applied pressure and the constriction imposed by the die. Sormally, pressures of 800 t o 2400 pounds per square inch on the material are sufficient for the extrusion of 0.01-inch thick tape, using hydrocarbon lubricants. During the extrusion, sufficient tension is applied by the take-off roll to smooth out any bowing or curling in the center of the tape. The resulting nonfused tapes are soft and extensible showing little tensile strength; they draw easily and ultimately tear like fibrous materials. ROLLING. The thickness of the extruded tape can be decreased by passing it through a pair of rolls. This rolling procedure has been found to be useful in preparing various thicknesses of tape and in correcting variations in thickness caused by uneven extrusion pressure or drawoff tension. By employing several passes through the rolls the thickness of tapes has been reduced from 0.02 down t o 0.001 inch, Like the extruded tapes, the rolled tapes are soft and extensible, showing similar tearing characteristics.
Figure 6.
Continuous Extraction Bath and Sintering Oven E. Extraction bath S.
Sintering oven
T . Take-up roll
VOLATILIZATION. The optimum operating temperature for volatilizing cetane is between 300 and 325 C. This volatilization can be effected b y passing the tape through a single heating chamber, consisting of a pair of electrically heated plates spaced approximately 1 inch apart. Essentially, complete volatilization from a 0.01-inch thick tape can be obtained in such an oven in 35 seconds. SINTERING. The lubricant-free tape must be fused at or above its 327" C. transition temperature in order t o acquire i t s maxiO
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1952
% E XTRACTLD
1809
TABLE11. PROPERTIES OF WATER-QUENCHED AND SLOWCOOLEDTAPES
IO0 90
(Thickness, 8 mil) Tensile
40 MIL
Type of Tape Slow-cooled Quenched, 25" C. water Quenched, ice water
80 a
70
Strengths
Elongation,
(Lb./Sq. I;.) 3370 4150 4540
270 295 220
%
ASTM Method D 412-41.
60 50 40
30 20
I
I
EXTRACTION RATE FOR R E M O V A L OF L U B R I G A N T BY B O I L I N G P-XYLENE
10 0
I 2 3 4 Figure 7.
5 6 7 8 Q I O I I I2 T I M E- M I N U T L S
Extraction Rate for Removal of Lubricant by Boiling p-Xylene
mum strength. The sintering is accomplished readily by passing the tape through an electrically heated oven similiar to that used in the volatilization step. For continuous operation, the oven should be constructed to avoid any tension in the fused tape, to allow for approximately 18 t o 20% lengthwise shrinkage during sintering, and to maintain even heating. QUENCHING AND COLD DRAWING.To obtain maximum tensile strength and elongation it is desirable to quench the fused tape in cold water immediately after sintering. Quenched tape is more transparent than slowly cooled tape. Typical variations in physical properties are shown in Table JI. A tensile strength of 3370 pounds per square inch for an annealed tape can be increased t o 4540 pounds per square inch by quenching in ice water. The electrical properties, ~ u c has dielectric constant, dissipation factor, and volume resistivity, are not affected by the type of cooling. The toughness of the tape can be greatly increased by cold drawing, which also serves to correct any wrinkling or buckling in the tape. An illustration of the pronounced effect of fourfold drawing is given in Table 111. It is e v i d e n t t h a t high tensile
amount of lubricant residue is all that i6 required to attain the inherent dielectric quality of the polymer. Illustrative of the fusion and cold-drawing effects are the data shown in Table IV. An extruded and rolled tape that has not been extracted or sintered had a dissipation factor a t 1000- of 0.094, a dielectric constant of 2.7, a volume resistivity of 1.6 X 10'2 ohmcm., and a dielectric strength of 615 volts per mil. ExEXTRUSION tracting the lubricant decreased the dissipation f a c t o r t o 0.0045 and increased the volume resistivity fourfold, w i t h a s l i g h t increase in dielectric VOLATlLlZAT! ON strength. After f u s i o n f o r 30 minutes a t 350" C . , t h e dissipation factor d e c r e a s e d SlNTE RING some tenfold with a substantial increase i n v o l u m e resistivity and die l e c t r i c strength. These p r o p e r t i e s remain s u b s t a n Figure 8. Laboratory Equipment tially the same on for Extruding Lubricated Teflon cold drawing. Mixtures
-7 p
OF COLDDRAWING ON PHYSICAL TABLE111. EFFECT PROPERTIES OF EXTRUDED AND FUSED TAPE
Undrawn 0.0035 0.375 5760 220
Thickness, inch Width, inch Tensile strength, lb./sq. in. Elongation, % Q
Drawn 4 X a 0.0025 0.219 16,800 60
Drawn lengthwise in direction of extrusion.
> TAPEAND COLDDRAWINGOF FUSED :c PROPERTIES ELECTRICAL PROPERTI E S
The electrical quality is dependent solely on the thermal treatment. Thus, complete fusion d t h Sufficient thermal dwell to bake out the relatively small
Tape Processing
Width, In. 2.0
tracted Fused ( 3 5 0 O C.,30minutesf Fused and cold drawn
2.0 1.8 1.7
Thickness, In. 0.002
DissipaDielection trio Factor Constant (1000-) (lOOO-) 0.094 2.7
0.002
0.0045 0.0005
~=::~l~l,a:~lf~~~dand ex0.002
0.0012
0.0005
1.8 2.2 2.2
Volume Resistivity, Ohm-Cm.
Dielectric Strength, Volts/Mil
1.6 X 10'2
615
6.6 X 10'2
950 1890 2200
>lo'* >lo14
INDUSTRIAL AND ENGINEERING CHEMISTRY
1810
Figure 10. 1.
2.
Figure 9. 1. 2. 3.
Extrusion Equipment for Coating Wire
Uncoated wire Hydraulic p u m p Extrusion r a m
5:
6.
Extrusion cylinder Die Coated unbaked wire
Vol. 44. No. 8
Oven and Take-up Rolls Used for Baking Extruded Insulation
Vaporizing zone Sintering zone
3.
4.
Spark tester Oven temperature control
the physical and electrical properties characteristic of the unlubricated, sintered polymer. This method of coating wire is a t present suited for applying insulation with wall thicknesses of from 0.0005 to 0.075 inch at speeds up to 50 feet per minute. Figures 9 and 10 illustrate a laboratory setup for insulating conductors a t extrusion speeds of 4 feet per minute. ACKNOWLEDGMENT
EXTRUSION OF WIRE INSULATION
The general method for coating metallic conductors with the lubricated polymer is the same as that described for the preparation of tape. The lubricated Teflon is extruded onto the conductor after which the coated wire is passed into an oven where the lubricant is volatilized and the polymer fused viith n corresponding shrinkage of 187, in diameter. In coating wire, the lubricated composition is forced around the wire in a concentric manner rather than across the wire as is the case with extruders equipped with cross-head dies. The essential components of laboratory scale equipment suitable for extruding the lubricated polymer onto conductors are shown in Figure 8. The stock is first pressed into a cylindrical preform having a hole through its longitudinal axis. The preform is then inserted into the extrusion cylinder. The conductor to be coated is threaded through a hole in the piston of a ram extruder and is led through the preform and through the die opening. From the die opening, the wire runs downward through the ovens, quench bath, and finally to the drawoff rolls. The die is heated to a surface temperature of about, 90" C. Extrusion is accomplished by applying pressure to the preform by means of the piston while the conductor is drawn through the take-off rolls a t constant speed. The coated wire, on emerging from the die, passes through an oven divided into two zones. The top zone, maintained a t a temperature of 300" C., serves t o volatilize the lubricant from the polymer. The lower zone, a t a temperature of 350" t o 400' C., fuses the polymer. The fused coating is then quenched or annealed to yield insulation having
The authors wish to express their indebtedness to P. A . Dahleri and J. T. Nolen, whosupplied theaqueous polytetrafluoroethyleiie dispersions used in this investigation, and to acknowledge thc major contributions of K. L. Berry and R. M. Joyce of the cheniical department of bhe Du Pont Co. in the development of thr: dispersions. The authors also wish t o acknowledge the assistance of C. E. Willoughby and C. G. Wortz, also of the chemical department, in the preparation of electron photomicrograplis and in the determination of the specific surface of the polymer. Certain method&and applications referred t o in this paper have been included in U. S. patent applications. LITERATURE CITED
(1) Berry, K. L., U. S. Patent 2,478,229 (rlug. 9, 1949). (2) Dalla Valle, J . M., "Micromeritics-The Technology of Fine Particles," 2nd ed., chap. 17, New- York, Pitman Publishing
Co., 1948.
(3)
Hanford, W. E . , and Joyce, R. AI., J . Am. Chem. Soc., 68,
2082
(1946). 15) (6)
Lontz, J. F., Jaffe, J. -4., Robb, L. E., and Happoldt, W. B.. Jr., IND. EXG.CHEM., 44, 1800 (1952). Plunkett, R. J., U. S.Patent 2,230,654 (Feb. 4 , 1941). Renfrew, 11. M., U. S. Patent 2,534,058; Brit. Patent 631.570
(7)
Renfrew, M. M., and Lewis, E. E., IXD.ENG. CHEM.,38,
(4)
(Nov. 4, 1949). 870
(1946). (8)
Watson, J. H. L., Anal. Chem., 20, 576
RECEIVED for review October 26, 1951.
(1948).
ACCEPTED April 3, 1952. Presented as part of the Symposium on Fluorine-Containing Polymers before the Division of Polymer Chemistry at the 120th Meeting of the AXERICAN New York, September 1951. CHEXICAL SOCIETY,
