US2789926A

US2789926A - Process of insulating wire with polytetrafluoroethylene

Info

Publication number: US2789926A
Application number: US495944A
Authority: US
Inventors: Robert W Finholt; William J Wunch
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1955-03-22
Filing date: 1955-03-22
Publication date: 1957-04-23
Anticipated expiration: 1974-04-23

Description

April 23, '1957. R. w. FINHOLT Er AL 2,789,926

PROCESS oF IINSULAT'ING WIRE.L wrm POLYTETRAFLUCROETHYLENE 2 sheetssheet 1 I'Filed'uamn 22, 195,5 A 1 April 23, 1957 R. w. FlNHoLT Er AL 2,789,926

.PROCESS OFYINSULATING WIRE WITH POLYTETRAFLUOROETHYLENE Filed Maron 22, 1955 -2 sheets-sheet 2 y- WMM United tates haten.

PROCESS F NSULATHNG WERE WKTH PLYTETRAFLURUETHYLENE Robert W. Finholt, Erie, and William Si. Wunch, Wesleyvilie, Pa., assignors to General Electric Company, a corporation of New York Application March 22, 1955, Serial No. 495,944

13 Claims. (Cl. 117-213) This invention relates to wire provided with an electrically insulating coating of solid polytetrauoroethylene and to a process of producing such insulated wire, and more particularly to a process of producing a tough llexible coating of this insulating material on electrically conducting wire in which the material is initially pressed on the wire in powdered form, 'as by means of calender rollers. Such insulated wire is especially useful in armature coils for large electric motors.

The use `of calender rollers for applying coatings of various insulating materials, such as rubber, to wire is well-known in the art and the process is known as a calendering process. This process has the advantage of high speed and low cost. In accordance with this process, the wire is passed downward between grooved calender rollers and powdered insulating material is fed to the rollers so as to be pressed onto the wire as it passes between the rollers, which are suitably driven in a direction to follow the wire as it passes between them.

This calendering process was also proposed prior to our invention for use in applying a coating of solid polytetrauoroethylene insulating material to wire, the material being fed to the rollers in powdered form and pressed on the wire by the rollers. Thereafter the pressed coated wire was heated to sinter or fuse the material thereby to form a tough, llexible coating.

However, prior to our invention, the coating of solid polytetrafluoroethylene formed on wire by the calendering process was unreliable and unpredictable both physically and electrically, and was characterized by persistent flaws, the sources of which were unknown. As a result, this insulated electrically conducting wire was of such inferior quality because of the low dielectric strength of its insulating coating that it was unsuitable for use in commercial electric apparatus. Its average dielectric strength was approximately 180 volts per .001" of insulation, with low dielectric strength in the regions of the numerous flaws. In addition such insulated wire was unsatisfactory from the physical standpoint because of the inability of the insulating coating to withstand the high crushing pressures existing in some electric apparatus. More specifically, it was found that the insulating coating completely failed, i. e., collapsed, when subjected to a crushing pressure of approximately 3000 p. s. i. with resulting failure of its dielectric Strength, whereas in certain electric apparatus, such as the armature coil conductors of large7 high-speed electric motors, the crushing pressure applied to the insulated wire is substantially as great and sometimes greater than this value of 3000 p. s. i., so that no adequate margin of safety was provided.

Therefore, an object of our invention is an electricaliy insulated wire provided with an insulating coating of polytetrafluoroethylene having a high crush pressure strength.

Another object is a simple and inexpensive process for producing electric wire on an insulation of solid polytetralluoroethylene which has a uniform dielectric strength.

A further object is a simple and inexpensive calendering process for producing on electrically conducting wire e ig@ an insulating coating of solid polytetrauoroethylene which is uniform and strong physically and electrically, and suitable for use in high-power, high-speed motors, as well as in other electric apparatus.

We have found that the successful production of a uniform high quality insulating coating of solid polytetrafluoroethylene on wire by the calendering process is dependent upon certain critical factors, including temperature of the powdered material, size of the particles of powdered material, calender roller diameter, surface conditions of the wire and the rollers, and the maintenance of the powdered material substantially free from :static electric charges. In addition, we have found that the crush strength of the insulating coating can be increased to a satisfactory safety operating value by a heat tempering treatment following the sintering heat treatment.

Briefly, in carrying out our invention we use a fine polytetrafluoroethylene powder having a particle diameter size of .005" to .015, which powder is maintained during application to the wire by the calendering process at a temperature of approximately F. We utilize grooved calender rollers having a diameter equal approximately to 2.5 plus 250 times the thickness of the insulating coating. Preferably the wire is polished or lubricated and the rollers machined to have a surface roughness of 30-40 micro-inches. Means are provided for preventing static electric charges on the powdered material. After a sintering heat treatment to form a tough, flexible coating, the coated wire is heated to increase its crush strength at a temperature of approximately 300 C. for a period of three hours, which heat treatment we have found substantially doubles the crush strength of the insulating coating.

Further objects and advantages of this invention will become apparent and this invention will be better understood from the following description taken in connection with the accompanying drawings. The features of novelty which characterize this invention will be pointed out with particularity in the claims annexed to and forming part of the specification.

Fig. 1 shows an expanded perspective view of calendering apparatus for applying the powdered material to two rectangular wires in parallel;

Fig. 2 is an enlarged fragmentary plan view of the rollers of Fig. 1 showing the shape of the grooves for a rectangular wire;

Fig. 3 is an enlarged fragmentary view similar to Fig. 2 but showing grooves shaped for coating round wire, four grooves in each roller for coating four wires in parallel being shown;

Fig. 4 is an enlarged fragmentary cross-sectional view of the static eliminators;

Fig. 5 :shows curves of the rate of feed of the powdered material and the dielectric strength of the coating plotted against temperature in degrees F. as abscissa; and

Fig. 6 is a curve showing the variation of roller size with the thickness of the coating.

Referring to Fig. 1, we have shown a motor 1 attached by a chain 2 to a sprocket 3 which is connected to drive one of the calender rollers 4. The roller 4 is supported on the stationary bearings 5. A similar roller 6 is mounted on similar bearings '7 in such a position that the grooves in the peripheries of the two rollers mate to form two orifices 8a and 3b between them (Fig. 2) of a suliicient size and suitable shape to accommodate the two rectangular wires 8 and the coating. Fig. 3 shows a fragmentary view of grooved calender rollers for coating four round wires. For our purposes the term calender rollers is defined as mating rollers having peripheral grooves which define an orifice between the rollers. In order to synchronize the calender rollers 4 and 6, they afsaeae are drivingly connected by the attached gear members 9 and 10. In this process, the calender rollers d and 6 are thrust vapart by the reaction to the compression of the insulating material. This thrust force may exceed a ton, so that calender rollers 4 and 6 must be mounted rigidly to prevent ruinous deflections.

The wires 8 to be coated are pulled downwardly by suitable means (not shown) between these rollers d and 6 through the orifices 8a and 8b and then through a suit- Aable oven 11 for sintering the coating. The supply of polytetraiiuoroethylene powder 12 is positioned over the rollers in a hopper 13. The polymer powder 12 passes through the chute 14 and the sieve 15 into -a vibrating inclined trough 16 which feeds the powder to the rollers 4 and 6 in the region of the formed orifices 3a and Sb. It should be noted that the grooves are so placed as to provide orifices that are balanced to prevent the rollers from warping in use. The powder is kept in the region of the orifices by an open box formed by the calender rollers 4 and 6 and the retaining plates 17 and 18 (Figs. l, 2 and 3).

The variable speed motor 19 drives an agitator or paddle 20 which causes the polytetrauoroethylene powder to flow to the vibrating trough 16. The trough 16 is shown as being mounted on an electromagnetic vibratory platform 22. After the supply of powder reaches the calender rollers d and 6, it is compacted on the wires by the rollers under great pressure, 100 to 10,000 p. s. i. with about 2,000 p. s. i. preferred, whereby the particles of the powder cohere to each other to form a solid but brittle coating on the wire. The motor 1 drives the rollers at a synchronized slightly slower peripheral speed than the speed of the wires S being coated. Obviously, this may be a continuous process which is limited only by the length of the wire used. It will be understood that a `suitable tension is applied to the wires at some point below the oven 11, as by winding the wires on a power driven reel, so as to draw the wires through the orifices between the rollers. The speed of the wire is limited by the heating capacity of the oven 11 and by the ability t-o feed the polytetrafluoroethylene powder 12 to the rollers. The thickness of the coating maybe controlled within narrow limits by adding or removing

shims

23 and 24 between the

bearing holders

25, 26 and 27, and 2S respectively to vary the spacing of the calender rollers 4 and 6. We prefer that the powder retaining plates 1'7 and i8 -be secured to the

bearing holders

26 and 28 only to facilitate this adjustment. To change the thickness of the coating more than a few thousandths of an inch, the calender rollers 4 and 6 should be replaced by `other rollers having a 'suitably different diameter as pointed out hereinafter.

In order that the rectangular wires 3 are presented to the calender rollers in the proper alignment with respect to the orifices Sa and 8b, the wires are passed through the wire guide 29 before passing through the orices 8a and 8b defined by the peripheiies of the calender rollers. The wire guide 29 is provided with a reservoir 29a to store a suitable lubricant as hereinafter defined.

The

static eliininat'ors

30, 31, 32 and 33 are positioned to remove any static charge from the powder 12 and maintain it free from such charges. Any conventional static eliminators may be used, but the particular ones shown are high voltage discharge-corona type, shown in detail in Fig. 4, wherein a plurality of needles 3dextend respectively to the plurality of orifices 35 in each of the

cylindrical members

32 and 33. A voltage is applied between the needle members 34 of the two static eliminato'rs to cause the electric discharge of any charged powder particles flowing :between these lstatic eliminatiors. if the static eliminatol's are not used, a series of static discharges takes place between the wires and the calender lrollers d and 6 and uneven ow of powder 12 results. The unevenvow causes small physical faults vwith consequent dielectric faults.

When practicing this process with powdered polytetraiiuoroethylene, we have found that the electrical properties of the insulating coating are erratic and unsatisfactory unless the temperatures of the powder and rollers are maintained at approximately 75 F. In Fig. 5 a shaded temperature range i's shown wherein this process can be carried out with satisfactory results. The line AB shows the rate of feed of the powdered polymer with the top plateau at the left being maximum capacity of the machine, the line CD shows the average quality, i. e. dieiectric strength, of the coating and the line EF shows the dielectric strength minimum. This process provides a coating having uniform dielectric characteristics only when carried out within the rather narrow temperature Vlimits from 68 F. to 84 F. with the best result at about 75 F. This is remarkable since polytetrafluoroethylene when once applied to the wire is satisfactory in insulation service over a temperature range of from approximately C. to 280 C. Prior to this discovery of the critical temperature, the process gave unreliable results because of wide variations in room temperature and hence powder temperature, producing an insulation that was unuseable.

lt is preferred to have the machine shown in Fig. l enclosed in an air conditioned room maintained at a temperature of approximately 75 F. It was found that below 68 F. the coating quality was unuseable. At lower temperatures the feed of the powdered polytetraiuoroethylene was easily controlled, but the powder was so dry that it would not cohere sufficiently to produce a good compacted coating. Numerous physical imperfections causing dielectric flaws occur in the coating with the process carried out at a temperature below 68 F.

Also, it was found that above 80 F. the powder is sticky, the particles adhering together so as to produce an uneven feed of the powder to the calender rollers. Above F. this problem appears to be insurmountable.

Since this critical temperature discovery, we have learned that solid polytetraiiuoroethylene goes through a crystalline phase change with change in its temperature and above 84 F. is in one crystalline phase which is sticky and below 68 F. is in another crystalline phase which is very dry. For the best results the process should be carried out in the temperature region between 68 F. gto 84 F. where the powder is a mixture of these two crystalline phases.

Referring to Fig. 6, it was found that the calender roller overall diameter should be a function of the coating thickness desired. This function shown in the curve of Fig. 6 is very nearly represented by the formula X=250Y12.5, where X is the abscis'sa measured in inches of the over-all roller diameter and Y is the ordinate measured in inches of the coating thickness. This formula was derived empirically when it was found that rollers of 5 diameter provided the best coating for a thickness of .01" and rollers of l2 diameter resulting in the best coating at .04. It was also found that roller diameters of 8 produce the best results for an insulation thickness of .024 and roller diameter of l0" produced the best results for thicknesses of .028". The use of rollers that are too small causes too small a flow of the polytetraiiuoroethylene, leaving bare wire spots or low density spots in the coating which are weak dielectrically. Rollers that are too large tend to pack the powder undertoo high a pressure and cause slip plane flaws in the coating which also cause dielectric flaws.

The size of the polytetrafluoroethylene powder particles used in this process is very important. A powder size TF-l marketed under the registered trademark of Teion by E. I. du Pont de Nemours and Company is much too large to work satisfactorily in this process. When this powder was divided into a finer powder marketed as Teiion TF-S, it is more useful but we have found that TF-5 is too large. 4A special powder ground nely in liquid nitrogen produces a much better coating arsasas 1.

in -this process. The approximate diameter of these powder particles is for TF-1, approximately .04"; TF-5, .03; and liquid nitrogen ground .01". A powder size that is used in enamels is .001 diameter or less, but this very tine enamel particle size does not operate as satisfactorily in this process. The region of best operation for particle size has been found to be between .005" and .015 particle diameter. Larger particles produce a granular insulation that is weak in its dielectric properties, While the enamel particle size is so line that as to be difficult to feed to the calender rollers.

The relative roughness of the calender roller and wire is important. In general it is desirable that one be rough and the other be smooth. After much experimentation we believe that it is best to have smooth wires and rough rollers. We use wires which have been polished by very line emery or crocus cloth and calender rollers which havel been machine ground. Applying a thin film of a lubricant such as a silicone oil to the wire from the reservoir 29a will produce the same results as polishing. Often the rollers have their grooved mating surfaces prepared by a machining process which leaves a surface roughness of about 30 to 40 micro inches. Using these unpolished rollers will usually be satisfactory until they become polished by use. This roughness on one tends to pull the powder into the compacting region and smoothness on the other allows the How of the polytetratiuoroethylene powder under pressure to even out the powder distribution and give coatings of uniform thickness on the wire which coatings are essential to have a flawless insulation coating on the wire.

After the Wire is coated, it is passed through the oven where the coating is heated to a fusing temperature above 327 C., the fusing temperature of the polytetrafluoroethylene. An oven temperature of 400 C. has been found to be satisfactory, but temperatures between'350 C. and 500 C. may be used. Solid polytetrafluoroethylene forms a gel above 327 C. but has no liquid state. In the fusion oven the decomposition rate is a function of temperature which doubles about every 12 C. over 327 C. Thus, the temperature and the fusing time are dependent. Most of the time in the oven is spent in bringing the copper and the solid polytetrafluoroethylene up to the temperature above 327 C. After the fusing, the coated wires may be separated by means such as rotary knives (not shown) or other cutters (not shown) and allowed to cool. Without the fusing or sintering step, the particles do not cohere sufficiently and consequently the coating is brittle and has a low resistance to abrasion and bending. The sintering step coalesces the particles into a tough exible coating which will allow the coated wires to be twisted or be bent to form coils.

We have found that the insulation produced by this process has a marginal mechanical crush strengthfor use in heavy duty high speed rotating electric machiney. It is not uncommon to have a pressure in the rotor of a machine in excess of 2500 p. s. i. in certain regions, whereas the insulation coating produced by this calendering process often fails, i. e., collapses completely at 3,000 p. s. i- Of course, this margin is a much too small safety factor so that the properties of the coated wire produced by this method as thus far described prevent its use in heavy duty machines of this type. However, we have found that a three-hour tempering at approximately 300 C., as by placing the wire in an open coil in an oven maintained at this temperature, will more than double the crush strength and give an insulation coating on the wire that will withstand crushing pressures of 6500 p. s. i. with an elastic limit of 2400 p. s. i. This insulated Wire can be bent easily and can be bound into an armature with conventional steel wire. The tempering to be satisfactory must be done at a temperature within the limits of 280 C. to 327 C. The wire must not be heated to a temperature as high as 327 C. but the closer to 327 C. it is kept the quicker the action becomes. Practically it is risky ltoV heat the tempering oven to a temperature above 310 C. because of oven control overshoot of temperature, and at 280 C. the process is too slow for effective use. With this added step, we are able to use the insulation for the complete insulation of the wire to be wound into armature coils in heavy duty dynamoelectric machines with a safe operating pressure limit of 5000 p. s. i., the usual grounding' insulation in the core slots or in the end winding not being required, nor is any outside protection of the coils necessary, such as with glass tape.

We have insulated wire in this continuous process at 12 ft. per minute. Any further increase beyond this speed is governed only by the ability to feed the powder to the calender rollers, and the ability of the oven 11 to fuse the coating.

ln summary, we have produced wire having a tough flexible coating of polytetrafluoroethylene which coating is on the order of .01 to .04" in thickness by a process of calendering. For the best results, the process should be carried out Within the temperature range of 68 to 84 F., the powder size be within the range of .005 to .015, the roller size be approximately in the ratio 2.5 plus 250 times the thickness of the insulation coating, the insulation being sintered by being heated in an oven at approximately 400 C. The crush strength of the insulation may be improved by tempering the polymer for three hours at approximately 300 C. A wire insulated in this manner provides an insulation having an average dielectric strength of 535 volts per .001 and a minimum dielectric strength of 430 volts per .001 of coating. The coated wire will safely withstand a crushing pressure of 5000 p. s. i. with an elastic limit of 2400 p. s. i.

While we have shown and described the particular embodiment of this invention, further modifications and improvements will occur to those skilled in the art. For instance, the coating thickness may be more or less than that we have actually provided. We desire it to be understood, therefore, that this invention is not limited to the form shown, and we intend by the appended claims to cover all modifications which do not depart from the true spirit and scope of this invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. The process of insulating a wire with a tough tiexible coating of solid polytetrauoroethylene material, which comprises pressing the material in powdered form on the wire while maintaining the material at a temperature within the range of 68 F. to 84 F. thereby to form a coating of compacted powdered material on the wire, and heating the coated wire to a temperature above 327 C. to fuse the material into a tough flexible coating.

2. The process of insulating wire with a tough iiexible coating of solid polytetrauoroethylene material which comprises pressing the material in powdered form on the wire while maintaining the material at a temperature Within the range of 68 F. to 84 F. thereby to form a coating of compacted powdered material on the wire, heating the coated wire to a temperature above 327 C. to fuse the material into a tough flexible coating and thereafter heat treating the coated wire at a temperature within the range of 280 C. to 327 C. to increase the crush strength of the coating.

3. The process of insulating a wire with a tough flexible coating of solid polytetrauoroethylene material by means of grooved calender rollers which comprises passing the wire downward between the rollers and feeding the material in powdered form to the rollers while maintain.- ing the material wire and the rollers at a temperature of approximately 75 F. thereby to form a coating of com# pacted powdered material on the wire, heating the coated wire to a temperature of approximately 400 C. to fuse the material into a tough tiexible coating and thereafter heating the coated wire at a temperature of approximately 300 C. for a period of approximately three hours to iucrease the crush strength of the coating.

4. The process of insulating wire with a tough exible coating of solid polytetrauoroethylene material by means of groove calender rollers which comprises providing the material in the form of a powder having a particle size of .005" to .015", pressing the powdered material on the wire while maintaining the material at a temperature within the range of 68 F. to 84 F. thereby to form a coating of compacted powdered material on the wire, heating the coated wire to a temperature above 327 C. to fuse the material into a tough exible coating and thereafter heating the coated wire at a temperature within the range of 280 C. to 327 C. to increase the crush strength of the coating.

5. The process of insulating wire with a tough ilexble coating of solid polytetrafluoroethylene material by means of grooved calender rollers'which comprises providing the material in the form of a powder having a particle size of .005" to .015, pressing the powdered material on the wire while maintaining the material substantially free from static electricity and at a temperature within the range of 68 F. to 84 F. thereby to form a coating of compacted powdered material on the wire, heating the coated wire to a temperature above 327 C. to fuse the material into a tough exible coating and thereafter heating the coated wire at a temperature within the range of 280 C. to 327 C. to increase the crush strength of the coating.

6. The process of insulating wire with a tough flexible coating of polytetrauoroethylene material by means of grooved calender rollers which comprises providing the material in the form of a powder having a particle size of .005 to .015", providing grooved calender rollers having a diameter in inches equal to 2.5 plus 250 times the desired coating thickness, positioning the rollers to give the desired coating thickness, passing the wire between the rollers and feeding the powdered material to the rollers while maintaining the material at a temperature within the range of 68 F. to 84 F. thereby to form a coating of compacted powdered material on the wire, heating the coated wire as it leaves the rollers to a temperature above 327 C. to fuse the material into a tough exible coating and thereafter heating the coated wire at a temperature within the range of 280 C. to 327 C. to increase the crush strength of the coating.

7. The process of insulating wire with a tough tlexible coating of polytetrauoroethylene material by means of grooved calender rollers which comprises providing the material in the form of a powder having a particle size of approximately .01, providing grooved calender rollers having roughened groove wall surfaces and having diameters in inches equal to 2.5 plus 250 times the desired coating thickness, positioning the rollers to give the desired coating thickness, maintaining the wire, material and rollers at a temperature of approximately 75 F., applying a coating of lubricant tothe wire, removing static electricity from the wire and the powder, passing the wire downward between the rollers and feeding the powdered material to the rollers thereby to form a coating of compacted powdered material on the wire, heating the coated wire as it leaves the rollers to a temperature of approximately 400 C. to fuse the material into a tough exible coating and thereafter heating the coated wire for three hours at `a temperature of approximately 300 C. to increase the crush strength of the coating.

8. rThe process for applying to a wire la tough flexible coating of Q Jolytetraiicoroethylene insulating material by means of gro-overl calender rollers comprising the steps ot grinding the me. rial into a powder having a particle size ot appronim...ely selecting calendering rollers having diameter in inches equal to 2.5" plus 250 times the desired coating thickness, positioning the rollers to give the desired coating thickness, maintaining the wires, powder and apparatus at a temperature of approximately 75 F., applying a thin coating of -lubricant to the wire, retrieving static electricity from the wire and the powder,

passing the wire downward between the rollers and feed-` ing the powdered material to the rollers thereby to form a coating' of compacted powdered material on the wire, heating the coated wire as it leavesV the rollers to a temperature above 327. C. to fuse the material into a tough flexible coating and'thereafter heating the coated wire for three hours at a temperature of 300 C. to increase the crush strength of the coating.

9. A process for applying to a wire a tough tiexible ting of solid polytetrauoroethylene insulating mateb" means of grooved calender rollers comprising the steps of providing the material in powder for bearing a particle size of .005 to .015", polishing the wire to be coated, maintaining the wire, powder and apparatus to a temperature within the range of 68 F. to 84 F., passing the wire to be coated downwardly between the rollers having their periphery machine ground to a predeterlic;

mined roughness, removing static electricity from the powder, feeding the powdered material to the rollers, heating the coated wire as it leaves said rollers to a temperature above 327 C. to fuse the material into a tough exible coating and thereafter heating the coated wires in a tempering oven heated to a temperature within the range of 280 C. to 310 C.

10. The process of insulating a wire with a tough iiexible coating of solid polytetrauoroethylene material,.

which comprises removing static electricity from the material and. wire to be coated, pressing the material in the form of a powder on the wire while maintaining the material, wire and compacting apparatus at a temperature of approximately F. thereby to form a coating of compacted material on the wire, and heating the coated wire to a temperature above 327 C. to fuse the material into a tough exible coating.

l1. The process of insulating wire with a tough flexible coating of solid polytetrauoroethylene material which comprises, lubricating the wire, pressing the material in the form of a powder on the wire while maintaining the materialat a temperature of approximately 75 F. thereby to form a coating of compacted material on the wire, and heating the coated wire to a temperature above 327 C. to fuse the material into a tough flexible coating.

12. The process of insulating wire with a tough flexible coating of solid polytetrauoroethylene material by means of grooved calender rollers which comprises, providing the material in the form of a powder having a particle size of .005 to .015, pressing the powdered material on the wire while maintaining the material substantially free lfrom static-electricity and at a temperature of approximately 75 F. thereby to form a coating of compacted powdered material on the wire, and heating the coated wire to a temperature above 327 C. to fuse the material into a tough flexible coating.

13. A.- process Vfor applying to a wire a tough liexible coating of solid tetrauoroethylene polymer insulation by means of grooved `calender rollers, comprising, polishing the wireto-be coated, maintaining the wire, polymer and rollers toa temperature of approximately 75 F., passing the wire to be coated downwardly between the grooved rollers having their grooves machine ground to a predetermined roughness, removing static electricity from the polymer in the form of a powder, conveying the polymer to the rollers, and heating the coated wire as it leaves the rollers to a temperature above 327 C. to fuse the polymer into a tough flexible coating.

eerences Cited in the tile of this patent UNTED STATES. PATENTS 2,392,338 Joyce Ian. 8, 1946 2,427,183 Berry Sept. 9, 1947 2,456,621 Cheney Dec. 21, 1948 2,485,691 Bogese Oct.,25, 1949 2,538,808Y Swiss Ian. 23, 1951 2,547,047 Saums et al Apr. 3, 1951

Claims

13. A PROCESS FOR APPLYING TO A WIRE A TOUGH FLEXIBLE COATING OF SOLID TETRAFLUOROETHYLENE POLYMER INSULATION BY MEANS OF GROOVED CALENDER ROLLERS, COMPRISING, POLISHING THE WIRE TO BE COATED, MAINTAINING THE WIRE, POLYMER AND ROLLERS TO A TEMPERATURE OF APPROXIMATELY 75*F., PASSING THE WIRE TO BE COATED DOWNWARDLY BETWEEN THE GROOVED ROLLERS HAVING THEIR GROOVES MACHINE GROUND TO A PREDETERMINED ROUGHNESS, REMOVING STATIC ELECTRICITY FROM THE POLYMER IN THE FORM OF A POWDER, CONVEYING THE POLYMER TO THE ROLLER, AND HEATING THE COATED WIRE AS IT LEAVES THE ROLLERS TO A TEMPERATURE ABOVE 327*C. TO FUSE THE POLYMER INTO A TOUGH FLEXIBLE COATING.